Divalent (Cr²⁺), trivalent (Cr³⁺), and tetravalent (Cr⁴⁺) chromium ion-doped tunable solid-state lasers operating in the near and mid-infrared spectral regions

Abstract

The chromium ion is among the most widely explored and used laser-active ions. Depending on the charge state of the laser-active chromium ion in the host, tunable chromium-doped lasers can be categorized under three groups as Cr²⁺, Cr³⁺, and Cr⁴⁺ lasers. In this paper, we provide a comprehensive overview of recent research conducted with chromium ion-doped tunable solid-state lasers, covering primarily the last 2 decades. After an introduction of the key physical parameters of tunable solid-state lasers, recent experimental results obtained with Cr³⁺, Cr⁴⁺, and Cr²⁺ ion-doped lasers are presented with an emphasis on alexandrite (Cr³⁺:BeAl₂O₄), Cr:LiCAF (Cr³⁺:LiCaAlF₆), Cr:LiSAF (Cr³⁺:LiSrAlF₆), Cr:LiSGaF (Cr³⁺:LiSrGaF₆), Cr⁴⁺:forsterite (Cr⁴⁺:Mg₂SiO₄), Cr⁴⁺:YAG (Cr⁴⁺:Y₃Al₅O₁₂), Cr²⁺:ZnSe, and Cr²⁺:ZnS lasers. For each laser system, recent developments involving power scaling, different excitation schemes, continuous-wave power performance, Q-switched/gain-switched operation, mode-locked operation, and emerging scientific/technological/biomedical applications are discussed. The paper is concluded with an overall assessment of these laser media and discussion about possible directions of research in future.

1 Introduction

Solid-state lasers comprise a large class of active media in which optical amplification is achieved by doping transition metal or rare-earth ions into insulating crystals, transparent ceramics, or glasses [1,2,3,4,5,6,7,8]. Population inversion in solid-state lasers is created via optical pumping, distinguishing them from semiconductor lasers which are electrically pumped via current injection. To date, many different ion–host combinations have been synthesized and demonstrated as solid-state laser media, resulting in the development of a large number of solid-state lasers, producing coherent radiation and spanning the ultraviolet, visible, and the infrared regions of the electromagnetic spectrum [9]. Although hundreds of ion–host combinations have been demonstrated as solid-state laser-active media, widely used systems are far fewer in number due to numerous stringent requirements that need to be met for the realization of practical laser systems. These include but are not limited to: (1) chemical and photostability of the ion–host combination, (2) resistance of the host to fracture during high-power operation, (3) resistance of the host to moisture, (4) ability to dope the host matrix at the desired laser-active ion concentration, (5) high luminescence quantum efficiency (QE) at room temperature, (6) high QE at the desired dopant concentration, (7) absence of absorption and scattering losses in the host at the lasing wavelengths, (8) absence of power degrading mechanisms such as excited-state absorption (ESA) or Auger-type energy-transfer upconversion (ETU) at the lasing wavelength [10,11,12], (9) absence of ESA at the pump wavelength[10, 13], (10) availability of practical pump lasers overlapping with the absorption band of the gain medium, and (11) repeatable synthesis protocol of the active medium. In practice, it may be next to impossible to find an ion–host combination that would satisfy all of these requirements. Nevertheless, it is possible to develop solid-state lasers with a desired set of favorable characteristics, making them ideal for a specific application. As a hypothetical example, a certain crystal may be hydroscopic and may have a low fracture resistance. However, at the same time, it may be ideal in terms of its low phonon nature as a host to generate broadly tunable infrared laser radiation in the low-power regime for numerous spectroscopic applications. As will be clear in the remainder of this review, the solid-state gain media that we review in this manuscript typically have trade-offs, since they may not simultaneously meet all of the stringent requirements listed above. However, in most cases, they possess the right set of favorable physical and optical properties that make them widely sought for numerous applications, requiring a specific wavelength or a specific regime of operation [for example, continuous-wave (cw), Q-switched, or mode-locked operation].

The chromium ion is among the most widely explored and used laser-active ions. Depending on the charge state of the laser-active chromium ion in the host, tunable chromium-doped lasers can be categorized under three groups as Cr²⁺, Cr³⁺, and Cr⁴⁺ lasers. Figure 1 shows the tuning range of a selected group of chromium lasers which we will review in this paper. As can be seen from Fig. 1, Cr³⁺ lasers cover the near-infrared region approximately between 700 and 1100 nm [14,15,16], whereas Cr⁴⁺ and Cr²⁺ operate over the wavelength ranges 1100–1600 nm [4] and 1850–3350 nm [17, 18], respectively. More detailed discussion of specific systems will be provided later. The spectroscopic properties and physical mechanisms underlying the laser operation of the ion–host combinations that are reviewed in this paper are well established, and have been extensively studied before. Our emphasis will therefore be on recent progress concentrating mainly on the last 2 decades and focusing on new regimes of operation, as well as novel resonator architectures that have been reported with these gain media.

Fig. 1

Tuning ranges of the chromium ion-doped solid-state lasers that will be reviewed in this work, along with the tuning range of the Ti³⁺:sapphire (Ti³⁺:Al₂O₃) laser

The review paper is organized as follows. Section 2 first introduces the key physical parameters that are needed in the characterization of tunable solid-state lasers, including the stimulated emission cross section, fluorescence lifetime, intrinsic slope efficiency, excited-state absorption cross section, and so on. Representative values of these parameters are later provided for each of the gain media reviewed in this paper. This is followed by an overview of Cr³⁺, Cr⁴⁺, and Cr²⁺ ion-doped lasers with an emphasis on alexandrite (Cr³⁺:BeAl₂O₄), Cr:LiCAF (Cr³⁺:LiCaAlF₆), Cr:LiSAF (Cr³⁺:LiSrAlF₆), Cr:LiSGaF (Cr³⁺:LiSrGaF₆), Cr⁴⁺:forsterite (Cr⁴⁺:Mg₂SiO₄), Cr⁴⁺:YAG (Cr⁴⁺:Y₃Al₅O₁₂), Cr²⁺:ZnSe, and Cr²⁺:ZnS lasers. Wherever possible, the laser performance of these gain media is compared with that of the Ti³⁺:sapphire laser and trade-offs are discussed. For each laser system, recent developments involving power scaling, different excitation schemes, cw power performance, Q-switched/gain-switched operation, mode-locked operation, and emerging scientific/technological/biomedical applications are overviewed. The paper is concluded with an overall assessment of these laser media and discussion about possible directions of research in future.

2 Key operational parameters of tunable solid-state lasers

Transition metal ion-doped solid-state lasers in general and the chromium ion-doped lasers, in particular, all belong to the class of tunable (or vibronic) solid-state lasers. In this section, we provide a concise overview of the key quantum electronic parameters which influence the power performance of tunable solid-state lasers. In this class of gain media, the electronic energy-level structure of the laser-active ion is determined by the local symmetry of the crystalline environment and strong electron–phonon coupling gives rise to broad tunability. The basic physical mechanisms underlying the spectroscopic properties of transition metal ion-doped solid-state gain media and electron–phonon coupling have been discussed in the previous works [1, 3, 4, 19, 20]. Using a suitable wavelength-selective intracavity element such a prism or a birefringent tuning plate, the output wavelength of a vibronic solid-state laser can be varied over a wide spectral range which is a substantial fraction of the peak emission wavelength. The fractional tuning range is defined as $\Delta \lambda /\lambda_{L}$, where $\lambda_{L}$ is the peak emission wavelength of the laser and $\Delta \lambda$ is the full-width-at-half maximum (FWHM) of the tuning range [17]. The fractional tuning range varies depending on the specific ion–host combination. For example, $\Delta \lambda /\lambda_{L}$ = 0.57 and 0.29 for Ti³⁺:sapphire and Cr³⁺:LiSAF lasers, respectively [21,22,23,24].

Rate equation formalism can be employed to analyze the power performance of tunable solid-state lasers in the presence of optical amplification, passive losses, and/or other power degrading mechanisms such as ESA at the pump/laser wavelength(s) [10, 12, 13, 25], ETU [10, 11], thermal loading [26,27,28,29], and other mechanisms. In general, the dependence of the output laser power on pump power could be quite complex. Even in the absence of thermal loading effects, power degrading mechanisms can lead to different types of output-input power dependence as has been classified in Ref. [10]. As a simple analytically tractable example, we consider a four-level end-pumped laser system subject to passive losses and ESA at the lasing wavelength. Neglecting thermal effects and diffractive beam spreading, it can be shown that in the continuous-wave (cw) regime, the laser output power $P_{L}$ varies nearly linearly with incident pump power $P_{p}$ above lasing threshold, as shown in Fig. 2. The incident threshold pump power $P_{th}$ required to attain lasing is given by [10, 12, 30]

$$P_{th} = \frac{{h\nu_{p} }}{{4\sigma_{e} \tau_{f} }}\frac{{\pi \left( {w_{p}^{2} + w_{L}^{2} } \right)}}{{\eta_{a} }}\frac{{\left( {L + T} \right)}}{{\left( {1 - f_{L} } \right)}},$$

(1)

Fig. 2

Variation of the laser output power as a function of the incident pump power for a hypothetical 4-level laser in the presence of passive resonator losses and excited-state absorption at the lasing wavelength

where $h\nu_{p}$ is the pump photon energy, $\sigma_{e}$ is the stimulated emission cross section expressed in cm² or m², $\tau_{f}$ is the fluorescence lifetime of the upper laser level, $\eta_{a}$ is the total absorption at the pump wavelength, $w_{p}$ is the pump spot size, $w_{L}$ is the laser spot size, $L$ is the round-trip passive loss of the resonator, $T$ is the output coupler transmission, and $f_{L}$ is the normalized strength of ESA, given by $f_{L} = \sigma_{esa} /\sigma_{e}$, with $\sigma_{esa} =$ excited-state absorption cross section at the lasing wavelength. Equation (1) is valid for $T{\text{ and }}L \ll 1$. If the gain medium is oriented at Brewster incidence, the spot sizes in the plane of incidence increase by the refractive index $n$ of the medium and the right-hand side of Eq. (1) should be multiplied by $n$ [30]. Note that when ESA is negligible, $P_{th}$ is inversely proportional to the product $\sigma_{e} \tau_{f}$, which is referred to as the emission cross section–lifetime product or sometimes, as the gain–lifetime product. Hence, gain media with large gain-lifetime product and small $f_{L}$ are clearly advantageous in achieving low-threshold cw laser operation. During gain-switched operation, assuming that the pump pulse is much shorter than $\tau_{f}$, the expression for the lasing threshold pump energy $E_{th}$ takes the form [30]

$$E_{th} = \frac{{h\nu_{p} }}{{4\sigma_{e} }}\frac{{\pi \left( {w_{p}^{2} + w_{L}^{2} } \right)}}{{\eta_{a} }}\frac{{\left( {L + T} \right)}}{{\left( {1 - f_{L} } \right)}}.$$

(2)

Returning to the cw regime of operation, the output power $P_{L}$ of the laser can be expressed as

$$P_{L} = \left\{ {\begin{array}{*{20}l} {0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{ for }}\,\,P_{p} < P_{th} } \\ {\eta \left( {P_{p} - P_{th} } \right)\,\,\,\,{\text{ for }}\,\,P_{p} \ge P_{th} \, } \\ \end{array} } \right.,$$

(3)

where $\eta$ is the slope efficiency with respect to the incident pump power and is given by

$$\eta = \frac{{\lambda_{p} }}{{\lambda {}_{L}}}\eta_{a} \left( {1 - f_{L} } \right)\frac{T}{T + L} = \eta_{a} \eta_{0} \frac{T}{T + L}.$$

(4)

In Eq. (4), $\eta_{0} = \left( {\lambda_{p} /\lambda_{L} } \right)\left( {1 - f_{L} } \right)$ and is referred to as the intrinsic slope efficiency. The intrinsic slope efficiency $\eta_{0}$ gives the slope efficiency of the laser with respect to absorbed pump power in the limit where the output coupling ($T$) is much larger than the resonator losses ($L$) and will be used in the comparison of different Cr³⁺ lasers in Sect. 3.

The large gain bandwidths of chromium ion-doped solid-state lasers make it possible to generate ultrashort pulses with duration in the picosecond and femtosecond time scales. Different mode locking initiation techniques such as Kerr-lens mode locking (KLM) or saturable absorber mode locking have been widely utilized for ultrashort pulse generation [31,32,33,34,35,36,37,38]. Recently explored saturable absorbers for mode locking include semiconductor saturable absorber mirrors (SESAMs) [33, 36], carbon nanotubes [39], and graphene [39, 40], among others. In a tunable laser with a gain bandwidth of $\Delta \nu$(FWHM), the temporal duration $\tau_{p}$(FWHM) of the transform-limited pulse that can be obtained is given by $\tau_{p} = K/\Delta \nu$, where the constant $K$ depends on the particular temporal pulse shape. In the case of passive mode locking, pulses with hyperbolic secant-squared (sech²) temporal profile are typically obtained and in this case, $K = 0.315$ [41]. Obtaining transform-limited pulses over the full available gain band requires careful mirror design with broadband reflectivity as well as dispersion management of the gain medium which is typically done using intracavity prism pairs [42], dispersion control mirrors [43, 44], and/or additional intracavity substrates.

Once pulse generation is initiated by KLM or saturable absorber mode locking, the pulses propagating inside the gain medium experience self-phase modulation (SPM) resulting from the Kerr nonlinearity of the medium. The strength of this Kerr effect is characterized by the nonlinear refractive index $n_{2}$ which gives the intensity-dependent contribution to the refractive index of the medium according to

$$n(r,t) = n_{0} + n_{2} I(r,t).$$

(5)

Here, n₀ is the refractive index of the medium at low intensities and I(r,t) is the intensity of the laser beam. Note that due to the spatio-temporal variation of the laser pulse intensity, the nonlinear contribution to the refractive index depends on time as well as position [45]. Nonlinear phase distortion of the propagating pulses for the case with $n_{2} > 0$ can be balanced by introducing negative group delay dispersion (GDD) into the resonator of the mode-locked laser, resulting in the formation of solitary pulses with sech² temporal profile. According to the soliton area theorem, the energy $E_{p}$ and temporal duration $\tau_{p}$(FWHM) of the steady-state solitary pulses obey

$$E_{p} \tau_{p} = 1.76\frac{{\left| D \right| \cdot \lambda_{L} \cdot A_{eff} }}{{\pi \cdot n_{2} \cdot d}},$$

(6)

where $D$ is the round-trip GDD, $d$ is the length of the medium, and $A_{eff}$ is the effective beam cross-sectional area within the gain medium [31]. The other parameters in Eq. (6) are as defined earlier. The larger the value of $n_{2}$, the easier it is to initiate pulse generation via KLM. However, as can be seen from Eq. (6), a larger amount of GDD will be necessary to stabilize the nonlinear phase distortions to produce the same pulse width. Equation (6) also shows that in media with low $n_{2}$, KLM operation may be difficult to initiate, since it may require a high pulse energy to stabilize solitary pulses. In such a case, an additional nonlinear medium may be added to the resonator or energy scaling schemes such as extended cavity architectures may be used to enhance the nonlinear phase modulation [46]. As the power of the propagating pulses increases, the strength of the Kerr effect-induced lensing also increases and self-focusing may cause pulse break up and mode locking instabilities. The critical power $P_{c}$ at which Kerr lensing balances the diffractive spreading of the beam is given by

$$P_{c} = \frac{{\alpha \lambda_{L}^{2} }}{{8\pi n_{0} n_{2} }},$$

(7)

where the dimensionless factor α has a value between 3.77 and 6.4 [47]. At pulse peak powers exceeding $P_{c}$, mode locking instabilities may arise and prevent further energy scaling during single-pulse operation.

3 Cr³⁺ ion-doped lasers

This section provides a review of recent research conducted in the development of Cr³⁺ ion-doped solid-state lasers. Cr³⁺ ion-doped active media are characterized by broad absorption and emission bands with peak emission wavelengths in the near-infrared region. Furthermore, the strong absorption bands of these gain media overlap well with the emission wavelengths of recently developed red diode lasers, resulting in renewed interest aiming at the development of compact, low-cost, and efficient diode-pumped tunable lasers. The broad emission bandwidths are also suitable for the generation of femtosecond pulses. To date, laser action has been demonstrated in numerous oxide and fluoride hosts doped with Cr³⁺ ions, with tunability in the 700–1100 nm range [2, 12, 48, 49]. The present review will focus mainly on alexandrite (Cr³⁺:BeAl₂O₄) [48] and Cr³⁺:colquiriite [12, 50] lasers in which intrinsic slope efficiencies in excess of 50% have been demonstrated. For most of the remaining members of Cr³⁺ ion-doped vibronic lasers, demonstrated slope efficiencies are typically lower, mainly due to excited-state absorption, with the exception of a few hosts such as emerald [49, 51]. In the following discussion of the Cr³⁺ ion-doped solid-state lasers, reference will be made to Table 1, which summarizes the important spectroscopic and laser characteristics of alexandrite (Cr³⁺:BeAl₂O₄) and Cr³⁺:colquiriite lasers (Cr³⁺:LiCaAlF₆, Cr³⁺:LiSrAlF₆, and Cr³⁺:LiSrGaF₆, also simply referred to as Cr:LiCAF, Cr:LiSAF, and Cr:LiSGAF, respectively), and compares them with those of the Ti³⁺:sapphire laser, which has the broadest tunability among the known vibronic solid-state lasers in the near-infrared. Note that whereas the tuning ranges of the Cr³⁺ lasers shown in Table 1 are not as broad as that for Ti:sapphire, they do provide efficient tunable laser radiation at important wavelengths suitable for a wide range of applications.

Table 1 Spectroscopic, laser, and thermal properties of alexandrite (Cr³⁺:BeAl₂O₄), Cr:LiCAF (Cr³⁺:LiCaAlF₆), Cr:LiSAF (Cr³⁺:LiSrAlF₆), Cr:LiSGaF (Cr³⁺:LiSrGaF₆), and Ti³⁺:Sapphire (Ti³⁺:Al₂O₃) gain media

3.1 Alexandrite (Cr³⁺:BeAl₂O₄) lasers

Historically, alexandrite (Cr³⁺-doped chrysoberyl, Cr³⁺:BeAl₂O₄) was the first room-temperature, tunable solid-state laser reported in the mid-1970s (Morris and Cline, “Chromium-doped beryllium aluminate lasers,” (US3997853A, 1976), [14, 48, 55, 82, 83]. In alexandrite, the presence of broad absorption bands in the visible region makes it possible to optically excite the gain medium with flash lamps or visible diode and/or solid-state lasers. The vibronically broadened upper laser level ⁴T₂(lifetime = 6.6 μs) remains populated at room temperature, since it is close to the lower metastable level ²E (lifetime = 1.54 ms) with an energy difference of only 800 cm⁻¹ (approximately 4 $k_{B} T$ at room temperature, where $k_{B}$ = Boltzmann constant and $T$ = absolute temperature). The resulting ${}^{4}T_{2} \to {}^{4}A_{2}$ transition yields broadly tunable laser emission in the 701–758 nm wavelength range. The effective upper state lifetime of alexandrite is 260 μs at room temperature, which is larger than those of the Cr³⁺:colquiriite lasers (Table 1), making the alexandrite gain medium particularly useful for Q-switched operation. The close proximity of the ⁴T₂ and ²E levels further leads to temperature-dependent emission properties which were recently investigated in detail for the E//b lasing polarization. In this study, analytical fitting formulas were developed based on temperature-dependent emission spectra and numerical extrapolation showed that the small-signal gain of alexandrite at 760 nm peaks above room temperature, approximately around 85 °C [84]. Favorable laser properties of alexandrite include (see Table 1): (1) high $\sigma_{e} \tau_{f}$ product, making it possible to obtain low-threshold cw operation, (2) high intrinsic slope efficiency ($\eta_{0}$) of 65%, comparable to that of Ti³⁺:sapphire and Cr³⁺:colquiriite lasers, (3) negligible Auger-type ETU, (4) high thermal conductivity comparable to that of Ti³⁺:sapphire, (5) high fracture toughness and thermal shock resistance which allows power scaling and durability against optical damage, and (6) high nonlinear refractive index which can be employed to initiate Kerr-lens mode-locked operation to generate ultrashort optical pulses.

Following the recent developments in high-power red/blue diode lasers, numerous studies focused on the development of next-generation, diode-pumped cw alexandrite lasers and power scaling. Beyatli et al. demonstrated low-cost tapered diode-pumped operation of an alexandrite laser excited via the R₂ line at 678.5 nm, yielding 200 mW of output power at 755 nm with power slope efficiency of 38% [52]. Yorulmaz et al. reported cw and self-Q-switched operations of an efficient, single-mode diode-pumped alexandrite laser with a lasing threshold of 13 mW. During Q-switched operation, stable pulse trains at kHz repetition rates and with duration of 5–15 μs were obtained [15]. Multi-10 watt laser operation of an alexandrite laser via direct red diode pumping was reported by Teppitaksak et al. [85]. In this study, as high as 26.2 W of output power was obtained with a power slope efficiency of 49%, demonstrating the great potential of this gain medium for power scaling. Q-switched operation of the alexandrite laser was also demonstrated with 0.7 mJ of output pulse energy at 1 kHz repetition rate [85]. Fiber-coupled diode pumping of alexandrite was further demonstrated by Arbabzadah et al. who obtained a maximum output power of 1 W with 44.2% efficiency and tunability in the 737–796 nm wavelength range [86]. By optimizing the crystal temperature across the tuning range, Kerridge-Jones and Damzen obtained the record lowest cw lasing wavelength (714 nm) and record tuning range (104 nm) with a cw diode-pumped alexandrite laser [59]. Sheng et al. demonstrated single-frequency operation of a diode-end-pumped tunable alexandrite laser in cw regime using a bow-tie cavity design in a unidirectional ring configuration [87]. Fibrich et al. reported cw alexandrite micro-chip lasers pumped by an InGaN diode laser at 445 nm, generating 210 mW at 680 nm (78 K) and 570 mW at 750 nm (354 K) with slope efficiencies of 15% and 39%, respectively [88].

Spectroscopic studies have been conducted in recent years to investigate the gain dynamics and the power scaling potential of alexandrite lasers. Fibrich et al. investigated the temperature dependence of the spectroscopic and lasing properties of alexandrite over the 78–400 K temperature range. In this study, the alexandrite laser operated at the non-vibronic R line at 680 nm at cryogenic temperatures. Vibronic lasing near 750 nm corresponding to the ${}^{4}T_{2} \to {}^{4}A_{2}$ transition occurred above 260 K [89]. The effects of ground-state absorption and pump ESA on the power efficiency and threshold pump power of alexandrite lasers were further investigated by employing analytical modeling [10, 90].

Pump sources other than blue/red diodes have also been utilized to investigate different regimes of operation of alexandrite lasers. Dual-wavelength operation of an alexandrite laser was demonstrated by Ghanbari and Major who employed 532-nm pumping, generating a maximum output power of 870 mW at 745.2 nm and 756.2 nm with a slope efficiency of 24% [91]. In another study, Pichon et al. demonstrated the first LED-pumped alexandrite laser, which was also the first ever LED-pumped chromium ion-doped laser. Using 2200 LEDs at 450 nm together with a Ce:YAG concentrator, the alexandrite laser was excited at 550 nm, generating 2.9 mJ at the wavelength of 748 nm at a repetition rate of 10 Hz [92].

Numerous recent studies focused on further development of Q-switched alexandrite lasers. Thomas et al. reported Q-switched operation of a cw diode-pumped alexandrite laser up to 10 kHz repetition frequency with pulse energy of 150 μJ at 759 nm. Cavity-dumped Q-switching of a diode-pumped alexandrite laser was also demonstrated for the first time, producing 3-ns pulses with 170 μJ energy at 4 kHz [93]. Diode side pumped alexandrite slab lasers in linear and bounce geometry were further reported with as high as 23.4 mJ energy at the repetition rate of 100 Hz [94]. Munk et al. reported the first demonstration of single longitudinal mode operation of a diode-pumped alexandrite laser excited at 636 nm, generating 1.1 mJ pulses at the repetition frequency of 150 Hz at 770 nm with a linewidth less than 10 MHz [95]. Further refinement of single longitudinal mode operation of diode-pumped alexandrite lasers resulted in the generation of 1.7 mJ pulses at the pulse repetition frequency of 500 Hz [96]. Parali et al. reported on a tunable alexandrite laser, passively Q-switched with a semiconductor saturable absorber mirror (SESAM), generating 550-ns pulses with 41 mW of average power at the repetition rate of 27 kHz [97]. Highest pulse repetition frequency of 135 kHz was reported using a red diode-pumped, self Q-switched alexandrite laser generating 660-ns pulses [98].

The broad emission bandwidth of the alexandrite gain medium can be utilized for the generation of ultrashort optical pulses, as well. Ghanbari et al. reported the first experimental demonstration of Kerr-lens mode locking in alexandrite lasers, producing 170-fs pulses at 755 nm with an average power of 780 mW and pulse repetition frequency of 80 MHz [99]. Cihan et al. later used an extended, multipass-cavity design to lower the repetition rate of a KLM alexandrite laser to 5.6 MHz and generated 70-fs pulses near 750 nm [100]. The extended cavity design further eliminated the Q-switching/spectral instabilities, in agreement with previous simulations which show that chaotic behavior of alexandrite lasers depends on the resonator length [101]. An InP/InGaP quantum-dot semiconductor saturable absorber mirror (SESAM) was used to initiate passive mode locking of an alexandrite laser, resulting in 380-fs pulses with an average output power of 295 mW at 775 nm [102]. Graphene mode locking of a 532-nm pumped alexandrite laser was first demonstrated by Cihan et al. who used a multipass-cavity design to eliminate spectral instabilities and generated 65-fs pulses with 1.4 nJ energy at a pulse repetition frequency of 5.56 MHz [61]. Figure 3 shows the schematic of the graphene mode-locked, extended cavity femtosecond alexandrite laser, and the interferometric autocorrelation and spectrum of the generated 65-fs pulses. More recently, Miao et al. reported a high-repetition rate self-mode-locked alexandrite laser pumped by a 658-nm diode laser and obtained 201-fs pulses with a pulse repetition rate of 7.5 GHz at 754 nm [103].

Fig. 3

(Reprinted with permission from Ref. [61] © The Optical Society)

a Schematic of the graphene mode-locked, extended cavity femtosecond alexandrite laser. b Interferometric autocorrelation and spectrum of the generated 65-fs pulses near 750 nm

Alexandrite lasers have been widely used in medical imaging (such as 3D photoacoustic mammography [104] and high-contrast vascular imaging [105]), dermatology [106,107,108], lidar systems [109], and ultraviolet generation, among others. In a recent experiment, Wang et al. reported efficient second harmonic generation with a diode-pumped cw alexandrite laser and obtained record 2.55 W of second harmonic power at 378 nm [110]. Coney and Damzen reported a diamond-shaped slab amplifier architecture for the development of high-energy Q-switched alexandrite lasers for lidar applications [111].

3.2 Cr³⁺:colquiriite lasers

Some of the key spectroscopic, laser, and thermo-mechanical properties of the three most widely investigated Cr³⁺:colquiriite lasers (Cr³⁺:LiSAF, Cr³⁺:LiCAF, and Cr³⁺:LiSGAF) are listed in Table 1. A recent review article by U. Demirbas provides a thorough overview of the general spectroscopic and thermo-mechanical properties as well as the power performance of these lasers [53]. Historically, laser action in this class of Cr³⁺ lasers was first reported in late 1980s and early 1990s [12, 50, 57]. Different from ruby and alexandrite gain media, the ²E level in Cr³⁺:colquiriites overlaps with the vibronically broadened ² T level, resulting in broadly tunable emission in the near-IR region between 700 and 1100 nm, the specific range depending on the particular host crystal. Examination of Table 1 shows that the Cr³⁺:Colquiriite laser family possesses numerous favorable spectroscopic and laser properties including: (1) high intrinsic slope efficiency greater than 50% for Cr³⁺:LiSAF, Cr³⁺:LiCAF, and Cr³⁺:LiSGAF, (2) large $\sigma_{e} \tau_{f}$ product on the order of $(20 - 30) \times 10^{ - 19}$ cm²–μs, which allows low-threshold lasing, and (3) the presence of broad emission bands for the generation of sub-20-fs pulses. In particular, refinement in dispersion management resulted in the generation of 9-fs, 10-fs, and 14-fs pulses from Cr³⁺:LiCAF, Cr³⁺:LiSAF, and Cr³⁺:LiSGAF lasers, respectively [62, 63]. However, three limitations pose challenge for power scaling beyond several watts of cw output power. These include (1) ESA at the lasing wavelength with $f_{L} = 0.18 - 0.33$, (2) Auger-type ETU, and (3) poor thermo-mechanical properties. In the case of Auger-type ETU, the population $N_{2}$ in the upper laser level varies as [10]

$$\frac{{dN_{2} }}{dt} = - \frac{{N_{2} }}{{\tau_{f} }} - 2\gamma N_{2}^{2} ,$$

(8)

where $\tau_{f}$ is the fluorescence lifetime at low levels of inversion and $\gamma$ is the Auger ETU parameter. Physically, the presence of ETU reduces the effective upper state lifetime and the achievable optical gain as the pumping intensity is increased, and limits power scaling. Looking at Table 1, Auger-type ETU is more pronounced in Cr³⁺:LiSAF and Cr³⁺:LiSGAF than in Cr³⁺:LiCAF. Regarding the thermo-mechanical properties of colquiriites listed in Table 1, we see that thermal conductivity, tensile fracture strength, thermal shock resistance, and fracture toughness of colquiriites are in general far poorer in comparison with alexandrite and sapphire hosts. This has limited the cw output powers of Cr³⁺-doped colquiriite lasers to the few-watt range. In the case of Cr³⁺:LiSAF, the highest cw output power of 3 W was obtained using a multipass-slab amplifier geometry [112]. Multi-mode diode pumping was further employed to generate about 2.5 W of cw power at the free running wavelength and about 1 W of second-harmonic (blue) power with Cr³⁺:LiSAF and Cr³⁺:LiCAF lasers [113, 114]. In the case of the Cr³⁺:LiCAF laser, crystal fracture was observed at pump powers under 6 W [114]. Nevertheless, low-to-medium power Cr³⁺-colquiriite lasers have been demonstrated as robust sources of femtosecond pulses in the near-infrared as discussed below.

With the advent of high-brightness single-mode red diodes, low-threshold, low-cost Cr³⁺-colquiriite lasers have been developed and used for femtosecond pulse generation and power scaling. Demibras et al. demonstrated efficient tunable cw lasing and mode-locked operations of Cr³⁺-doped LiSAF, LiSGaF, and LiCAF lasers using low-cost, single-mode pump diodes. Continuous-wave tuning ranges of 782–1042 nm, 777–977 nm, and 754–871 nm were obtained with Cr³⁺:LiSAF, Cr³⁺:LiSGAF, and Cr³⁺:LiCAF lasers, respectively. Up to 2.5 nJ of pulse energies were further generated with sub-100 fs pulse duration and 100 MHz repetition rate during mode-locked operation using SESAMs [24]. Possibility of thin-disk operation of Cr³⁺:LiSAF and Cr³⁺:LiCAF lasers was further investigated using numerical methods [115].

A variety of other pump sources have also been utilized to investigate different regimes of operation and different architectures of Cr³⁺-colquiriite lasers. Kunpeng et al. demonstrated tunable dual-wavelength operation of a Cr:LiSAF laser pumped at 671 nm with a frequency doubled Nd:YVO₄ laser. By keeping one wavelength fixed at 862 nm, the other could be tuned between 840 and 882 nm using external diffraction grating feedback, with potential applications in THz generation [116]. Biasetti et al. employed femtosecond laser inscription to fabricate stress-induced (type II) waveguides in a Cr:LiSAF crystal, with propagation loss of less than 2 dB/cm at 633 nm [117]. Pichon et al. demonstrated cavity-dumped LED-pumped operation of a Cr:LiSAF laser, generating 1.1 mJ pulses having pulse duration of 8.5 ns at the central wavelength of 840 nm and at a repetition rate of 10 Hz. Second and third harmonic generation was obtained in an LBO crystal, resulting in 108 μJ and 13 μJ pulses at 420 nm and 280 nm, respectively [118].

Historically, active and passive mode locking techniques were employed as early as in 1990s to generate ps and fs pulses from Cr³⁺-colquiriite lasers [119,120,121], followed by experiments that aimed at the optimization of cavity dispersion and pumping schemes, leading to the generation of sub-15-fs pulses from Cr:LiSAF, Cr:LiCAF, and Cr:LiSGAF lasers [62,63,64]. In more recent development of ultrashort pulse Cr³⁺-colquiriite lasers, emerging mode locking techniques and fast saturable absorbers have been utilized. Because the nonlinear refractive index of colquiriites is low, gain filtering poses a serious limitation to stable mode locking during KLM operation. One possible route to overcome this limitation involves use of extended cavity to scale up the pulse energy and semiconductor saturable Bragg reflectors to initiate mode locking, as was demonstrated by Prasankumar et al. who obtained 39-fs pulses from a diode-pumped multipass-cavity (MPC) Cr:LiSAF laser at a repetition rate of 8.6 MHz [122]. Mode locking using an SESAM-containing MPC resonator was also applied to Cr:LiCAF lasers and 10-nJ, 98-fs pulses were obtained at 10-MHz repetition rate [123]. To cancel the gain filtering effect, gain matched output couplers (GMOCs) with a transmission profile that matches the gain spectrum of the laser medium have also been introduced [124]. Using GMOCs, stable KLM operation of Cr:LiSAF lasers has been reported with output pulse duration as short as 13 fs [125, 126]. In addition to SESAM mode locking of single-mode diode-pumped Cr³⁺-colquiriite lasers [24, 127], single-walled carbon nanotube (SWCNT) and monolayer graphene saturable absorbers have been used to mode lock Cr:LiSAF lasers. Agnesi et al. demonstrated the first SWCNT mode-locked operation of a Cr:LiSAF laser and generated as short as 106-fs pulses by employing single-mode diode pumping. The SWCNT mode-locked Cr:LiSAF laser could be tuned over a 14-nm window [128]. Optimization of SWCNT parameters and cavity dispersion led to the generation of pulses as short as 21 fs from diode-pumped Cr:LiSAF lasers [129]. In other studies, Canbaz et al. used a monolayer graphene saturable absorber to generate 68 fs pulses with 11.5 mW of output power with 270 mW of pump power from a single-mode diode-pumped Cr:LiSAF laser at 850 nm [130]. By optimizing the cavity optics and dispersion, 19-fs pulses could be generated from the Cr:LiSAF at a repetition rate of 107 MHz and with peak power of 4.2 kW [131]. Figure 4a, b shows the experimental setup of the graphene mode-locked Cr:LiSAF laser and the interferometric autocorrelation trace of the 19-fs pulses generated with this laser. Recent studies have shown that even far from the gain peak, SESAM mode locking of Cr:LiSAF lasers is possible. Using a tapered diode-pumped Cr:LiSAF laser, 110-fs pulses were generated near the wavelength of 1 μm [132]. To scale the pulse energy of low-average-power SESAM mode-locked femtosecond Cr:LiSAF lasers, Demirbas et al. employed cavity dumping with a fused silica acousto-optic cavity dumper and generated 120-fs pulses at 825 nm with 112 nJ of energy per pulse and 0.93 MW peak power at the dumping frequency of 10 kHz using only 600 mW of pump power [133]. Single-mode diode-pumped femtosecond Cr³⁺-colquiriite lasers can be used as low-cost alternatives to Ti³⁺:sapphire lasers, especially in biomedical imaging applications such as multi-photon microscopy [134].

Fig. 4

(Reprinted with permission from Ref. [131] © The Optical Society)

a Experimental setup of the graphene mode-locked Cr:LiSAF laser and b interferometric autocorrelation trace of the 19-fs pulses

4 Cr⁴⁺ ion-doped lasers

Since the first demonstration of lasing in Cr⁴⁺:forsterite (Cr⁴⁺:Mg₂SiO₄) and Cr⁴⁺:YAG (Cr⁴⁺:Y₃Al₅O₁₂) in late 1980s [135, 136], numerous Cr⁴⁺-doped crystals have been investigated as tunable lasers or saturable absorbers. Detailed review of the early work on the spectroscopy as well as laser and saturable absorber characteristics of Cr⁴⁺-doped crystals may be found in Refs. [2, 4, 5]. Briefly, tetrahedrally coordinated Cr⁴⁺ ion with 3d² configuration possesses broad emission bands in the near-infrared, originating from the optical transition between the phonon coupled $^{3} T_{2}$ and ${}^{3}A_{2}$ states. Similarly, the absorption bands are also broad, enabling the pumping of Cr⁴⁺-doped crystals with lasers operating in the red and near-infrared spectral regions. In the particular case of Cr⁴⁺:forsterite, for example, pumping was achieved with alexandrite, krypton-ion, Ti:sapphire, as well as Nd:YAG lasers [137, 138]. Other favorable properties of Cr⁴⁺-doped gain media include a four-level energy scheme of the active ion, which makes it possible to obtain low-threshold laser operation and the possibility to generate ultrashort optical pulses in the femtosecond regime. To date, lasing has been demonstrated in a variety of crystalline hosts doped with Cr⁴⁺ ions. Examples include Cr⁴⁺:forsterite [135], Cr⁴⁺:YAG [136], Cr⁴⁺:Y₂SiO₅ [139], Cr⁴⁺:Ca₂GeO₄ [140], Cr⁴⁺:Y₃Sc_xAl_5-xO₁₂ [141], and Cr⁴⁺:LuAG[5], to name a few. We will mainly focus on Cr⁴⁺:forsterite and Cr⁴⁺:YAG lasers, since the highest power efficiencies were reported with these gain media. Table 2 lists some of the spectroscopic and laser properties of Cr⁴⁺:forsterite and Cr⁴⁺:YAG gain media. There is a notable spread in some of the previously reported parameter values such as the absorption and emission cross sections. Those values marked with “*” in Table 2 denote the average of the reported values from different studies. The tuning ranges of Cr⁴⁺:forsterite and Cr⁴⁺:YAG lasers extend from 1130 to 1367 nm and from 1309 to 1596 nm, respectively, with corresponding fractional tuning ranges of close to 20%. In comparison with Cr³⁺:colquiriites, the $\sigma_{e} \tau_{f}$ product is smaller for Cr⁴⁺:forsterite and Cr⁴⁺:YAG, but resonators with tight focusing can be designed to achieve low-threshold laser operation [142]. We note in passing that Cr⁴⁺-doped crystals, in particular, Cr⁴⁺:forsterite [143, 144] and Cr⁴⁺:YAG [5] crystals, have also been employed as saturable absorbers to generate Q-switched and/or mode-locked pulses from visible and near-IR lasers. However, Cr⁴⁺:YAG has by far been the most widely employed Cr⁴⁺-doped saturable absorber for the Q-switching of 1-μm lasers [5, 145, 146], since, in comparison with Cr⁴⁺:forsterite, Cr⁴⁺:YAG has a larger absorption cross section ($23 \times 10^{ - 19} cm^{2}$ versus $6 \times 10^{ - 19} cm^{2}$, see Table 2) and is less susceptible to pump ESA at 1 μm (average normalized pump ESA cross section for Cr⁴⁺:forsterite and Cr⁴⁺:YAG is 0.31 and 0.16 at 1.064 μm, respectively) [147]. In recent years, remarkable developments in resonator architecture led to the emergence of compact passively Q-switched 1-μm lasers, one example being the diffusion bonded Nd³⁺:YAG/Cr⁴⁺:YAG micro-chip design which was demonstrated to generate peak powers greater than 30 MW with sub-ns pulse durations [148, 149].

Table 2 Spectroscopic and laser properties of Cr⁴⁺:forsterite (Cr⁴⁺:Mg₂SiO₄) and Cr⁴⁺:YAG (Cr⁴⁺:Y₃Al₅O₁₂) gain media

Coming back to Cr⁴⁺:forsterite and Cr⁴⁺:YAG lasers, one major obstacle to power scaling is the phenomenon of lifetime thermal loading [29]. To explain this effect, we note that, similar to most vibronic lasers, the fluorescence lifetime of Cr⁴⁺-doped lasers decreases with increasing temperature as the strength of non-radiative decay rate increases. If the thermal conductivity of the medium is also low, this leads to excessive internal heating of the gain medium and causes a local reduction in the fluorescence lifetime and, hence, reduction in the overall available optical gain. In an earlier investigation, a suitable figure of merit $\Phi_{\tau }$ was defined to quantify the effect of lifetime thermal loading for end-pumped solid-state lasers according to [29]

$$\Phi_{\tau } = \frac{{\tau_{f} (T_{b} )}}{{\Delta \tau_{f} }} = \frac{4\pi }{{\ln 2}}\frac{{\tau_{f} (T_{b} )}}{{\tau_{fT} }}\frac{\kappa }{{\eta_{h} \alpha_{p0} P_{p} \left( {1 + 2\ln \left( {\frac{{r_{0} }}{{\omega_{h} }}} \right)} \right)}},$$

(9)

where $T_{b}$ is the crystal boundary temperature, $\Delta \tau_{f}$ is the maximum change in the fluorescence lifetime due to 1 W of incident pump power (in other words, the incident pump power $P_{p}$ appearing in Eq. (9) is equal to 1 W), and $\tau_{f} (T_{b} )$ equals the value of the fluorescence lifetime at the crystal boundary temperature $T_{b}$. In addition, $r_{0}$ is the radius of the crystal rod, $\omega_{h}$ is the radial width of the heat load (assumed to be constant), $\kappa$ is the heat conductivity of the crystal, $\tau_{fT}$ is the linear slope of the lifetime-temperature data near $T_{b}$, $\alpha_{p0}$ is the differential absorption coefficient (taken as 1 cm⁻¹) at the pump wavelength, and $\eta_{h}$ is the heating fraction. In other words, $\Phi_{\tau }$ is related to the inverse of the maximum fractional change in the fluorescence lifetime due to internal heating of the gain medium. The heating fraction $\eta_{h}$ further gives the fraction of the absorbed optical power converted to heat inside the crystal and, in the limit of low intracavity laser intensities (for example, near lasing threshold), is given by [29]

$$\eta_{h} = 1 - \frac{{\lambda_{p} }}{{\lambda_{L} }}\frac{{\tau_{f} (T_{b} )}}{{\tau_{r} }}.$$

(10)

In Eq. (10), $\tau_{r}$ is the radiative lifetime at low temperatures and the effect of local heating on $\tau_{f}$ is neglected. As an example, the room-temperature fluorescence lifetimes of both Cr⁴⁺:forsterite and Cr⁴⁺:YAG gain media are considerably lower than the corresponding radiative lifetimes (see Table 3), resulting in low-intensity $\eta_{h}$ values above 0.9. This indicates that a major fraction of the pump is converted to unwanted heat and increases the thermal loading of the gain medium. This is in contrast to the Ti³⁺:sapphire gain medium for which $\eta_{h} = 0.44$(Table 3). Using the spectroscopic and thermal parameters given in Table 3, $\Phi_{\tau }$ comes to 6.7 and 29 for Cr⁴⁺:forsterite and Cr⁴⁺:YAG, respectively. Physically, relatively low values of $\Phi_{\tau }$ indicate that internal heating due to excessive unused pump power reduces the effective fluorescence lifetime and hence the available population inversion to achieve optical gain. In the case of Ti³⁺:sapphire, $\Phi_{\tau }$ comes to 232, indicating that lifetime thermal loading is negligible in this gain medium. Similarly, $\Phi_{\tau }$ comes to 740 and 150 for Cr³⁺:LiCAF and Cr²⁺:ZnSe, respectively, again showing that typically, lifetime thermal loading is not a major factor for power degradation in Cr³⁺ and Cr²⁺ lasers[29]. Another important factor that limits power scaling is the presence of ESA at the lasing wavelength with $f_{L} = 0.16 \, \;{\text{and }}\;0.31$ for in Cr⁴⁺:forsterite and Cr⁴⁺:YAG. As a result of lifetime thermal loading and ESA, only watt-level operation has been possible with cw Cr⁴⁺:forsterite and Cr⁴⁺:YAG lasers near room-temperature [73, 156, 160, 165,166,167,168,169]. In the low-power regime, Evans et al. demonstrated direct diode pumping of Cr⁴⁺:forsterite and Cr⁴⁺:Ca₂GeO₄ lasers, and obtained tuning in the 1236–1300 nm and 1390–1475 nm ranges, respectively, using a semiconductor master oscillator-power amplifier (MOPA) at 980 nm [170]. In the case of Cr⁴⁺:forsterite and Cr⁴⁺:YAG lasers, most of the studies focused on ultrashort pulse generation near 1250 and 1500 nm, respectively, and investigation of lasing with different pump sources, as will be discussed in the next sections.

Table 3 Spectroscopic and thermal properties of Cr⁴⁺:forsterite, Cr⁴⁺:YAG, and Ti³⁺:sapphire used in the calculation of the figure of merit $\Phi_{\tau }$ for lifetime thermal loading

A recent and promising direction of research focused on the fabrication and development of Cr⁴⁺:YAG single-crystal fiber lasers. Reference [171] provides a detailed state-of-the-art review of the fabrication and applications of single-crystal fibers. Huang et al. fabricated a 10-μm-core Cr⁴⁺:YAG single-crystal fiber inside a sapphire tube and obtained broadband amplified spontaneous emission (ASE) with a bandwidth of 265 nm, centered at 1400 nm, using a 1064-nm Yb-fiber as the pump source [172]. In a series of lasing experiments reported by Lai et al., low-threshold laser operation (78 mW of threshold pump power) of a single-crystal Cr⁴⁺:YAG fiber laser having a double-clad structure was demonstrated with a slope efficiency of 34% and output power of 100 mW [157, 173]. Near-field lasing dynamics of the single-crystal Cr⁴⁺:YAG fiber laser was further investigated [174]. Lin et al. demonstrated the fabrication of high-quality single-crystal Cr⁴⁺:YAG fibers drawn with the laser-heated pedestal growth (LHPG) method and analyzed the microstructure formation with transmission electron microscopy [175]. Broadly tunable operation of a single-crystal Cr⁴⁺:YAG fiber laser was further demonstrated between 1353 and 1509 nm by Jheng et al. [176]. Recently, Lin et al. investigated the fabrication and emission characteristics of Cr⁴⁺-doped yttrium orthosilicate (Cr⁴⁺:Y₂SiO₅) crystal fiber which exhibited a broad emission peaking at 1257 nm with a bandwidth of 246 nm [177].

4.1 Ultrashort pulse generation with Cr⁴⁺:forsterite lasers

Focusing first on Cr⁴⁺:forsterite lasers, following the early demonstrations of active mode locking and KLM operation [178,179,180,181,182], refinements in cavity optics and dispersion management resulted in the generation of pulses as short as 14 fs [159]. In pulse amplification experiments, Jonusauskas et al. developed a regeneratively amplified Cr⁴⁺:forsterite laser, generating 30-fs pulses near 1.22 μm by employing the chirped-pulse amplification technique. Pulses with as high as 1GW peak power and 54-fs pulse duration were generated at a repetition rate of 1-kHz [183]. Togashi et al. further demonstrated a regeneratively amplified Cr⁴⁺:forsterite laser at 1.24 μm laser with an output energy of 0.4 mJ and pulse duration of 77 fs at the repetition rate of 10 Hz, corresponding to a peak power of 5.2 GW [184]. Figure 5 shows the experimental setup of the regeneratively amplified Cr⁴⁺:forsterite laser with peak power of 5.2 GW. Gordienko et al. reported on a repetitively pulsed femtosecond Cr⁴⁺:forsterite laser in GW regime with repetition rates of 1–50 Hz, achieving focused beam intensity of greater than 10¹⁶ W/cm² [185]. An amplified TW class Cr⁴⁺:forsterite laser was developed by Agranat et al. who generated 79-fs pulses with 90 mJ of pulse energy near 1.2 μm at a repetition rate of 10 Hz [186]. In later experiments reported by Lozhkarev et al., a Cr⁴⁺:forsterite master oscillator was used to seed an optical parametric chirped-pulse amplifier (OPCPA), resulting in the generation of pulses with peak power reaching 560 TW [187, 188]. Extended cavity architectures were also used to scale up the pulse energy of mode-locked Cr⁴⁺:forsterite lasers at MHz repetition rates. Shcheslavskiy et al. used an extended cavity laser architecture to obtain 40-fs pulses with 17 nJ pulse energy at the repetition rate of 26.5 MHz [189]. Cankaya et al. demonstrated KLM operation of a chirped-pulse Cr⁴⁺:forsterite laser in a q-preserving multipass configuration operated at 4.9 MHz, generating 81 nJ of pulse energy. The pulses were compressed from 5.5 ps to 607 fs using a diffraction grating pair [190]. More recently, Ivanov et al. operated a self-mode-locked Cr⁴⁺:forsterite laser below the soliton blow-up threshold at repetition rates of 15–20 MHz, generating sub-100-fs pulses with pulse energies as high as 33 nJ and peak powers exceeding 0.3 MW [191].

Fig. 5

Experimental setup of the regeneratively amplified Cr⁴⁺:forsterite laser with peak power of 5.2 GW (After Ref. [184])

Improved resonator designs and various types of next-generation saturable absorbers were used to optimize the mode-locked operation of low-average-power femtosecond Cr⁴⁺:forsterite lasers. Sennaroglu et al. demonstrated a low-threshold KLM Cr⁴⁺:forsterite laser, generating 86-fs pulses at a repetition rate of 200 MHz with 85 mW of average output power. During cw operation, the absorbed threshold pump power was as low as 290 mW [142]. McWilliam et al. utilized a low-loss GaInNAs saturable Bragg reflector to generate 62-fs pulses near 1.3 μm with a Cr⁴⁺:forsterite laser operated at a repetition rate of 180 MHz [192]. Zolotovskaya et al. demonstrated mode-locked operation of a Cr⁴⁺:forsterite laser with adjustable pulse duration using a voltage controlled p–n junction quantum-dot semiconductor saturable absorber mirror. Pulse durations between 17.4 to 6.4 ps could be obtained by varying the applied voltage between 0 and − 4.5 V [193]. Cho et al. used single-walled carbon nanotube saturable absorbers (SWCT-SAs) to initiate mode-locked operation of a Cr⁴⁺:forsterite, generating 80-fs pulses with up to 295 mW of output power and 78 MHz repetition rate [194, 195]. Baylam et al. demonstrated energy scaling of a SWCNT mode-locked Cr⁴⁺:forsterite laser using a multipass cavity operating at a repetition rate of 4.51 MHz. Pulses with 121 fs duration were generated near 1.25 μm with a pulse energy of 10 nJ and peak power of 84 kW. Attempts at further scaling of the output pulse energy resulted in multi-pulse operation due to self-focusing in the gain medium [162].

Monolayer graphene with its ultrabroadband saturable absorption band is also suitable for initiating mode-locked operation of Cr⁴⁺:forsterite lasers. Monolayer graphene was used by Cho et al. to initiate mode-locked operation of a Cr⁴⁺:forsterite laser, yielding sub-100 fs pulses near 1.25 μm, at a repetition rate of 75 MHz and with 230 mW of average power [196]. Ozharar et al. later used an extended, multipass-cavity configuration to produce 100-fs pulses with a maximum peak power of 53 kW from a graphene mode-locked Cr⁴⁺:forsterite laser [163]. One drawback of graphene is that even in monolayer form, it introduces about 5% loss into the laser resonator due to its intrinsic broadband absorption and increases the lasing threshold. Several approaches have been explored to reduce the overall insertion loss of monolayer graphene. Baylam et al. used voltage reconfigurable graphene super capacitor structures and showed that, by varying the bias voltages in the range of 0.5–1 V, the absorption and hence the insertion loss of graphene could be reduced, while maintaining a sufficient amount of saturable absorption to initiate mode locking. Pulses with sub-100 fs duration could be produced from a multipass-cavity Cr⁴⁺:forsterite laser operating near 1255 nm, at a pulse repetition rate of 4.51 MHz. [197, 198]. More recently, Morova et al. used zebra-patterned graphene saturable absorber (ZeGSA), formed by femtosecond laser ablation and demonstrated that each region with a different duty cycle of ablated stripes can be used as a saturable absorber with adjustable loss. Use of ZeGSA improved the power performance of a mode-locked Cr⁴⁺:forsterite laser and compared to conventional graphene mode locking, ZeGSA increased the output power from 68 to 114 mW, with a corresponding reduction in pulse duration from 62 to 48 fs [199]. Figure 6 shows the femtosecond laser ablation setup and the images of the zebra-patterned graphene samples. Figure 7 further shows the optical spectrum, autocorrelation, and radio-frequency spectrum of the 48-fs pulses generated with the ZeGSA mode-locked Cr⁴⁺:forsterite laser.

Fig. 6

(Reprinted with permission from Ref. [199] © The Optical Society)

Femtosecond laser ablation setup and the images of the zebra-patterned graphene samples used for mode locking the Cr⁴⁺:forsterite laser

Fig. 7

(Reprinted with permission from Ref. [199] © The Optical Society)

a Optical spectrum, b autocorrelation, and c radio-frequency spectrum of the 48-fs pulses generated with the ZeGSA mode-locked Cr⁴⁺:forsterite laser

Cr⁴⁺:forsterite lasers have been used in a diverse range of applications, including synchronization of independently mode-locked lasers with attosecond timing jitter [200, 201], generation of stabilized frequency combs [202], THz generation [203,204,205,206], supercontinuum generation in tapered fibers [207] and pressurized molecular nitrogen [208], third harmonic generation in carbon nanotubes[209], strong-field harmonic generation in xenon[210], micro-structuring of silicon [211], and gas spectroscopy [212]. Deep tissue penetration near 1.3 μm also makes Cr⁴⁺:forsterite lasers ideal for biomedical applications such as optical coherence tomography [213] and nonlinear optical microscopy of biological systems [214,215,216]

4.2 Ultrashort pulse generation with Cr⁴⁺:YAG lasers

Historically, French et al. reported on the first actively mode-locked Cr⁴⁺:YAG laser and generated 26-ps pulses, tunable between 1396 and 1482 nm, using a lead molybdate acousto-optic modulator [217]. Kerr-lens mode-locked operation of a femtosecond Cr⁴⁺:YAG laser was first reported by Sennaroglu et al. who obtained 120-fs pulses at 1.52 μm at a pulse repetition frequency of 81 MHz and with 360 mW of output power [218]. Further developments in femtosecond pulse generation with Cr⁴⁺:YAG lasers involved sub-100-fs pulse generation [219], generation of 44-fs pulses at 1.52 μm via SESAM mode locking [220], pumping with a Yb-fiber laser to generate 26-fs pulses from a self-starting Cr⁴⁺:YAG oscillator [221], leading to the generation of pulses as short as 20 fs from a prismless Cr⁴⁺:YAG laser [160]. A compact resonator architecture was used by Leburn et al. to operate a KLM Cr⁴⁺:YAG at a repetition frequency of 4 GHz and generated 82-fs, nearly transform-limited pulses near 1.5 μm [222]. Naumov et al. reported a directly diode-pumped KLM Cr⁴⁺:YAG laser, which generated as short as 65-fs pulses at the repetition rate of 100 MHz and with 30 mW of output power near 1.5 μm [223]. Cho et al. reported the first SWCNT mode-locked Cr⁴⁺:YAG laser and obtained 92-fs pulses near 1.5 μm at the repetition rate of 85 MHz and with 110 mW of output power [224]. Sub-50-fs, nearly transform-limited pulses were later generated from an SWCNT mode-locked Cr⁴⁺:YAG by Cafiso et al. [225]. Monolayer graphene mode locking of a Cr⁴⁺:YAG laser has also been demonstrated, yielding 91-fs pulses near 1.5 μm [226]. More recently, a high-repetition rate, SWCNT mode-locked Cr⁴⁺:YAG was further reported, generating 110-fs pulses at a repetition rate of 550 MHz [227].

Cr⁴⁺:YAG lasers have been used in numerous applications. Sorokin et al. investigated continuum generation near 1.5 μm with a KLM Cr⁴⁺:YAG laser and obtained 400-nm wide smooth broadening [228]. Kalashnikov et al. reported supercontinuum generation in an SF6 photonic crystal fiber by using a mode-locked Cr⁴⁺:YAG laser outputting 60-fs pulses and generated broadened spectra spanning more than one octave, as can be seen in Fig. 8 [229]. Gayen et al. investigated tissue welding applications of a Cr⁴⁺:YAG laser operating near 1.5 μm and obtained full-thickness tissue bonding with porcine aorta samples [230]. Madej et al. performed absolute frequency measurements with up to 1 × 10⁻¹¹ level accuracies, for 60 lines of the P and R branches for the υ1 + υ3 band of ¹³C₂H₂ with a Cr⁴⁺:YAG laser frequency comb near 1.5 μm [231]. Alcock et al. used a SESAM mode-locked Cr⁴⁺:YAG laser outputting sub-100 fs pulses to perform high precision measurements of the acetylene transition frequencies [232].

Fig. 8

Supercontinuum generation inside a photonic crystal fiber excited with a femtosecond Cr⁴⁺:YAG laser (after Ref. [229])

5 Cr²⁺ ion-doped lasers

Spectroscopic investigations carried out in 1960s and 1970s revealed that chalcogenides (examples including CdTe, ZnSe, and ZnS among others) doped with divalent transition metal (TM) ions such as Cr²⁺ and Fe²⁺ possess broad mid-infrared absorption and luminescence bands [233,234,235,236,237,238,239,240,241]. In the case of the Cr²⁺ ion having 3d⁴ configuration, these bands originate from the optical transitions between the phonon coupled ⁵T₂ and ⁵E states in a tetrahedral crystal field. In this class of Cr²⁺-doped gain media, laser action was first reported near 2500 nm in Cr²⁺:ZnSe and Cr²⁺:ZnS [242]. This was followed by laser demonstration in numerous other Cr²⁺-doped chalcogenides, including binary II–VI compounds such as CdSe and CdTe, as well as ternary II–VI compounds including CdMnTe, CdZnTe, and CdMgTe, to name a few [243].

In this section, we review some of the recent lasing work conducted with the two, most widely investigated members of the Cr²⁺-doped gain media, namely, Cr²⁺:ZnSe and Cr²⁺:ZnS. Further information about the early and recent development of transition metal ion-doped chalcogenide lasers can be found in Refs. [8, 244,245,246,247]. Some of the spectroscopic and laser properties of Cr²⁺:ZnSe and Cr²⁺:ZnS are summarized in Table 4, along with the properties of Ti³⁺:sapphire for comparison. Cr²⁺:ZnSe and Cr²⁺:ZnS possess many favorable spectroscopic properties which enable efficient laser performance, power scaling, and the ability to generate ultrashort laser pulses. These include: (1) negligible ESA at the lasing wavelength, (2) high luminescence quantum efficiency at room temperature, (3) high $\sigma_{e} \tau_{f}$ product to obtain low-threshold lasing, and (4) ultrabroad emission band extending over the 2–3 μm region, with fractional tuning bandwidths above 0.5, providing sufficient spectral bandwidth to generate mode-locked pulses with sub-20-fs duration near 2.4 μm. In the case of Cr²⁺:ZnSe, concentration quenching of luminescence is furthermore negligible for doping concentrations below 10¹⁹ cm⁻³ [248, 249]. The broad absorption band peaking at 1775 (1690) nm for Cr²⁺:ZnSe(Cr²⁺:ZnS) allows optical excitation of the gain medium with a variety of pump sources, including Co²⁺:MgF₂ [250, 251], Tm³⁺-doped solid-state [252], Tm³⁺-doped fiber[253], Er³⁺-doped fiber[254], color-center[255], hydrogen Raman[256], and diode lasers[257], among others. The exceptionally high nonlinear refractive index of the Cr²⁺:ZnSe and Cr²⁺:ZnS media on the order of 10^–14 cm²/W can be readily utilized to initiate KLM operation after introducing a sufficient amount of negative group delay dispersion to balance the nonlinear phase distortions induced by self-phase modulation. Due to all these favorable characteristics, Cr²⁺:ZnSe and Cr²⁺:ZnS lasers are often referred to as the “Ti³⁺:sapphire of the mid infrared region.” Looking at Table 4, we also note that the peak absorption cross section for both media is high, on the order of 10^–18 cm², further making these materials versatile saturable absorbers for passive Q-switching of lasers operating near 2 μm [258,259,260]. Furthermore, in addition to single-crystal growth methods [261], Cr²⁺ gain media can be synthesized by employing several low-cost methods such as diffusion doping of commercially available polycrystalline ZnSe/ZnS substrates [248, 262, 263] or hot-pressed ceramic fabrication [256, 264]. Mid-infrared emission was also reported in Cr²⁺:ZnSe thin films fabricated by pulsed laser deposition and molecular beam epitaxy [265, 266].

Table 4 Spectroscopic and laser properties of Cr²⁺:ZnSe, Cr²⁺:ZnS, and Ti³⁺:sapphire gain media

Novel resonator architectures were developed to operate Cr²⁺:ZnSe lasers in different regimes and to optimize the power performance. Schepler et al. employed analytical and experimental methods to investigate the strength of thermal lensing in face-cooled Cr²⁺:ZnSe laser disks and obtained 4 W of average power from a face-cooled Cr²⁺:ZnSe thin-disk laser [267]. Tuner et al. used an intracavity liquid crystal etalon driven with a square wave at 1 kHz to rapidly tune a Cr²⁺:ZnSe continuously between 2455 and 2650 nm. The Cr²⁺:ZnSe gain medium prepared by hot isostatic pressing could be operated with a narrow linewidth of less than 900 MHz [268]. Sennaroglu et al. experimentally investigated the effect of chromium-doping concentration on the power performance of Cr²⁺:ZnSe lasers and showed that for 2-mm-long samples, the optimum doping concentration for cw operation is 6 × 10¹⁸ ions/cm³ [249]. Kim et al. synthesized Cr²⁺:ZnSe powders having the grain sizes of 10 or 1 μm and demonstrated random powder lasing at 2350 nm with a minimum pumping threshold of 0.5 mJ (2.9 mm pump spot) [269]. Demirbas and Sennaroglu demonstrated intracavity pumping of a gain-switched Cr²⁺:ZnSe laser inside a KTP optical parametric oscillator operating at 1570 nm and demonstrated ultrabroad tuning between 1880 and 3100 nm [17]. Berry et al. employed femtosecond laser inscription to fabricate buried channel waveguides in polycrystalline Cr²⁺:ZnSe, generating a maximum output power of 1.7 W at 2500 nm [270]. Macdonald et al. further demonstrated waveguide laser operation of Cr²⁺:ZnS and obtained 101 mW at 2333 nm with a slope efficiency of 20% [271]. Moskalev et al. used a novel cavity design in which thermal effects were reduced by spinning the gain medium and obtained a record 140 W of cw output power from a Cr²⁺:ZnSe laser with 62% efficiency [272].

Ultrashort pulse generation experiments have been conducted with Cr²⁺:ZnSe and Cr²⁺:ZnS lasers over the last 2 decades. Carrig et al. demonstrated the first mode locking of a Cr²⁺:ZnSe laser using an intracavity acousto-optic modulator and obtained 4.4-ps pulses at 2.47 μm at a repetition rate of 81 MHz [273]. Sorokina et al. reported on SESAM mode locking of Cr:ZnSe with InAs/GaSb based multiple quantum wells, generating 100 fs pulses at 200 MHz with 75 mW of output power near 2.45 μm [274]. Sorokin et al. demonstrated Kerr-lens mode locking of a Cr²⁺:ZnSe laser in two distinct regimes. In the case of high-power soliton regime, they generated 100-fs pulses with 300 mW of output power. In the case of chirped-pulse regime, 1-ps pulses with 170 mW of output power were obtained [275]. Cizmeciyan et al. reported on the KLM operation of a Cr²⁺:ZnSe laser pumped with a Tm:fiber laser at 1800 nm, generating 95-fs pulses near 2.4 μm at the repetition rate of 94 MHz and with an average output power of 40 mW [276]. Tolstik et al. reported KLM operation of a Cr²⁺:ZnS laser, pumped with an Er:Fiber laser, generating 69-fs pulses at 2.39 μm with pulse energy of as high as 3.8 nJ at the repetition rate of 145 MHz [277]. Cizmeciyan et al. further investigated various dispersion compensation methods, including prism pairs made of CaF₂ and MgF₂, as well as slabs of BK7 and YAG for the generation of femtosecond pulses from a KLM Cr²⁺:ZnSe laser. They reported as short as 92-fs pulses using a CaF₂ prism pair [278]. Figure 9 shows the spectrum and the autocorrelation of the 92-fs pulses obtained from the KLM Cr²⁺:ZnSe laser using a CaF₂ prism pair. Cizmeciyan et al. reported the first experimental demonstration of graphene mode locking of a Cr²⁺:ZnSe laser, generating 226-fs pulses at the repetition rate of 77 MHz with 70 mW of output power at the incident pump power of 1.6 W [40]. Tolstik et al. demonstrated the first graphene mode-locked Cr²⁺:ZnS laser at 2.4 μm and obtained 41-fs pulses with pulse energy of 2.3 nJ and pulse repetition rate of 108 MHz [279]. Recently, Barh et al. obtained the highest average output power (0.96 W) to date for a self-starting, SESAM mode-locked Cr²⁺:ZnS oscillator and generated 120-fs pulses at the wavelength of 2.37 μm with a pulse energy of ∼ 3.9 nJ and peak power of 32 kW. Type-I InGaSb/GaSb and type-II InAs/GaSb SESAMs were used in this study [280]. Nagl et al. demonstrated the first directly diode-pumped Kerr-lens mode-locked Cr²⁺:ZnSe laser, generating 45-fs pulses with more than 500 mW of output power at 2.4 μm. The Cr²⁺:ZnSe laser was pumped with a single InP diode laser at 1650 nm [281]. In a series of studies, Vasilyev et al. reported on the demonstration of octave-spanning femtosecond Cr²⁺:ZnS lasers at the central wavelength of 2.4 μm. External dispersion control in a YAG plate resulted in the generation of pulses as short as 19 fs and peak powers as high as 2.5 MW [254, 282]. Smolski et al. used an MOPA configuration to amplify 0.9-W, 80-fs pulses from a Cr²⁺:ZnS master oscillator (MO) up to an average power of 7.25 W (77 fs) using a Cr²⁺:ZnS power amplifier (PA). The MOPA output was used to pump an optical parametric oscillator, consisting of quasi-phase-matched GaAs crystal, and broadband mid-IR spectrum over the 3–8 μm region was generated. Figure 10 shows the experimental setup of the Cr²⁺:ZnS MOPA and the autocorrelation of the seed and output pulses [283]. To achieve high-average-power mode-locked operation, Vasilyev et al. used a spinning Cr²⁺:ZnSe gain medium and obtained 60-fs (8-cycle) pulses at the central wavelength of 2.4 μm with 27.5 W of average power [284]. Wang et al. further demonstrated an Er:fiber pumped Kerr-lens mode-locked Cr²⁺:ZnSe laser, outputting as short as 47-fs pulses (6-cycles) with an average output power of 250 mW and also observed second harmonic generation due to random quasi-phase matching in the polycrystalline Cr²⁺:ZnSe gain medium [285]. Slobodchikov and Moulton reported on the first demonstration of chirped-pulse amplification in Cr²⁺:ZnSe, generating 1-kHz, 300-fs pulses at 2475 nm with 0.3-mJ pulse energy and 1 GW of peak power [286]. Wu et al. and Leshchenko et al. reported on the development of multi-mJ Cr²⁺:ZnSe-based chirped-pulse amplifiers, with sub-50-fs pulse duration and reaching peak powers as high as 115 GW. Leshchenko et al. further used the amplified output for coherent soft X-ray generation up to 0.6 keV [287, 288].

Fig. 9

Spectrum and the autocorrelation of the 92-fs pulses obtained from a KLM Cr²⁺:ZnSe laser using a CaF₂ prism pair (after Ref. [278])

Fig. 10

Experimental setup of a femtosecond Cr²⁺:ZnS MOPA and the autocorrelation of the seed and output pulses (after Ref. [283])

One important application of mid-infrared lasers in general and Cr²⁺:ZnSe/Cr²⁺:ZnS lasers, in particular, is in high harmonic generation (HHG). As already mentioned in the previous paragraph, Leshchenko et al. used the output of a Cr²⁺:ZnSe chirped-pulse amplifier and focused it inside a gas jet containing argon/neon to generate coherent, soft X-ray radiation up to 0.6 keV of photon energy [288]. Vampa et al. used a femtosecond Cr²⁺:ZnS MOPA delivering 6-W, 26-fs pulses near 2.4 μm at a high-repetition rate of 76 MHz to generate high order harmonics in ZnO up to 11 eV [289]. Another application of Cr²⁺:ZnSe/ Cr²⁺:ZnS lasers is in infrared spectroscopy. As such, Vasilyev et al. developed a fully referenced Cr²⁺:ZnS optical frequency comb which operated between 1.79 and 2.86 μm with 3 W of average power at the repetition rate of 80 MHz with long-term stability [290]. Yadav et al. demonstrated cascaded second harmonic generation with a 2360-nm femtosecond Cr²⁺:ZnS and generated tunable second harmonic between 1137 and 1200 nm in MgO:PPLN crystal and fourth harmonic between 570 and 596 nm in a BIBO crystal [291]. On the infrared side, Zhang et al. used a Kerr-lens mode-locked Ho:YAG thin-disk laser to generate mid-infrared continuum in the spectral range of 2.7–20 μm in a ZnSe medium via difference frequency generation [292]. Vasilyev et al. further used a few-cycle femtosecond Cr²⁺:ZnS outputting 6-W, 78-MHz, 20-fs pulses to generate mid-infrared continuum between 5.8 and 12.5 μm via intra-pulse difference frequency generation in ZnGeP₂ crystal [293]. These experimental results show that Cr²⁺:ZnSe/ Cr²⁺:ZnS lasers are suitable for the generation of high harmonic, visible, near-infrared, and mid-infrared radiation, as well as direct generation of 2–3 μm radiation for numerous applications in spectroscopy.

6 Conclusions

In conclusion, we have provided a comprehensive overview of recent research that has been conducted in the development of three classes of chromium ion-doped tunable-solid-state lasers, focusing primarily on alexandrite (Cr³⁺:BeAl₂O₄), Cr:LiCAF (Cr³⁺:LiCaAlF₆), Cr:LiSAF (Cr³⁺:LiSrAlF₆), Cr:LiSGaF (Cr³⁺:LiSrGaF₆), Cr⁴⁺:forsterite (Cr⁴⁺:Mg₂SiO₄), Cr⁴⁺:YAG (Cr⁴⁺:Y₃Al₅O₁₂), Cr²⁺:ZnSe, and Cr²⁺:ZnS lasers. Based on the presented research results, the following conclusions may be drawn regarding the current state-of-the-art in chromium ion-doped solid-state lasers:

• (1) All of the eight gain media reviewed in this work can be used to generate broadly and smoothly tunable coherent radiation in different wavelength ranges from 700 to 3350 nm, with a narrow gap between 1600 and 1850 nm.

• (2) Following the emergence of high-brightness red diodes, low-cost femtosecond sources based on Cr³⁺-doped colquiriite gain media have been successfully developed in the 700–1100 nm range with up to watt-level average output powers [24]. Further power scaling in this class of lasers appears challenging due to several factors, including Auger-type ETU, ESA at the lasing wavelength, and weak thermo-mechanical properties of the host crystals.

• (3) Cr⁴⁺-doped lasers are suitable for the generation of femtosecond pulses in the near-infrared between 1200 and 1500 nm. Again, only watt-level cw output powers have been demonstrated near room temperature due to lifetime thermal loading and ESA at the lasing wavelength. Possible avenues for further power scaling may involve cryogenic operation [298] or single-crystal fiber geometries.

• (4) Recent experiments have demonstrated that Cr²⁺:ZnSe, and Cr²⁺:ZnS lasers are very promising in terms of power scaling, the ability to generate stable trains of few-cycle femtosecond pulses, and for very broad wavelength coverage from the visible to the mid-infrared [291, 293].

• (5) Alexandrite lasers also offer a great promise for cw power scaling due to the excellent thermo-mechanical properties of the host, and low levels of ETU and ESA. With further developments in red diode lasers, it should be possible to develop ultrashort pulse alexandrite lasers with sub-50-fs pulses and multi-watt average powers.

• (6) Our recent experiments have demonstrated that monolayer graphene can be used as a broadband saturable absorber for the initiation of mode locking across the full tuning range of chromium ion-doped solid-state lasers [40, 61, 131, 199].

We anticipate that future work will focus on (1) further power scaling in alexandrite, Cr²⁺:ZnSe, and Cr²⁺:ZnS lasers, (2) development of novel saturable absorbers for Q-switching and mode locking of the chromium ion-doped lasers, (3) development of single-crystal fiber lasers to eliminate thermal effects, and (4) applications of the chromium ion-doped lasers in metrology, biomedical imaging, high harmonic generation, and spectroscopy.