1 Introduction

Tunable continuous-wave (CW) multiwavelength solid state lasers are important for various scientific and industrial instruments such as dual-wavelength laser probe, two-wavelength interferometer, differential lidar, and THz-wave difference frequency generator [1–3]. Since the active ion of Nd3+ has mainly allowed three transitions of 4F3/2→4I9/2, 4F3/2→4I11/2, and 4F3/2→4I13/2, corresponding to the emitting wavelengths around 0.9, 1.06, and 1.3 μm, respectively, it is possible to achieve single and multiwavelength operations of an Nd3+ laser through a proper design of the laser. For example, several groups have reported simultaneous dual-wavelength lasers at 1.06 and 1.3 μm, utilizing the 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions in Nd-doped YVO4 and YAG crystals with dual-band coated mirrors or multiple mirrors of different single-band coatings [4–6].

Owing to the crystal field, each of the three primary transitions of the Nd3+ ion is split into closely packed Stark levels, permitting laser radiation from the intermanifold transitions. Multiwavelength operation is possible for intermanifold transitions with comparable emission cross sections. Simultaneous dual-wavelength CW operation based on the 1.3-μm intermanifold transitions in Nd:YVO4 and Nd:YAG crystals was previously demonstrated without wavelength tunability [7–9]. In this paper, we present a novel and yet convenient scheme to achieve wavelength tunability and single/multiwavelength selectivity at the same time through the 4F3/2→4I11/2 intermanifold transitions of a c-cut Nd:YVO4 laser.

Normally, an a-cut Nd:YVO4 crystal is used as a laser gain medium because of the four-time higher emission cross section and polarized laser output compared with a c-cut Nd:YVO4 laser. However, the much higher emission cross section of the main transition at 1064 nm in an a-cut Nd:YVO4 crystal makes it difficult to perform multiwavelength emission and wavelength tuning in the intermanifold group of the 4F3/2→4I11/2 transition. One major advantage of a c-cut Nd:YVO4 crystal is its relatively smooth intermanifold emission spectrum for the 4F3/2→4I11/2 transition. The emission cross sections at 1062.6, 1064.4, 1066.5, 1083.3, and 1087.1 nm in the 4F3/2→4I11/2 transition are comparable to each other and were determined to be about 2.26×10-19, 2.6×10-19, 2.95×10-19, 1.36×10-19, 0.88×10-19 cm2, respectively [10]. This broad and smooth emission spectrum is advantageous of realizing a broadly tunable single or multiwavelength laser. We report in the following a discretely tunable single and multiwavelength CW diode-end pumped laser based on the 4F3/2–4I11/2 intermanifold transitions in a 0.25-at. % c-cut Nd:YVO4 crystal by using an intracavity etalon with a sub-THz free-spectral range (FSR) as a wavelength tuning element. As can be seen below, the wide free-spectrum range is favorable for selecting just a narrow emission line or several widely separated emission lines at a time from the laser.

2 Experimental setup

Figure 1 shows the experimental arrangement of the diode-laser pumped Nd:YVO4 laser with a sub-THz intracavity etalon to select the wavelength. The laser crystal is a c-cut, 0.25-at. % Nd3+ doped Nd:YVO4 crystal with a length of 10 mm and an aperture of 3×3 mm2. The pump source is a fiber-coupled laser diode at 808 nm producing a maximum power of ∼19 W. The coupling fiber has a core diameter of 800 μm and a numerical aperture of 0.18. The pump radiation was coupled into the laser crystal by an optical focusing system with a 35-mm focal length with a 97.6 % power coupling efficiency. The radius of the pump beam inside the laser crystal w p was calculated to be about 0.358 mm, deduced from a measured thermal focal length for the crystal [11]. The absorption coefficient of the Nd:YVO4 crystal at 808 nm was measured to be 2.21 cm-1 or absorption efficiency of 89 %, corresponding to an absorption cross section of 7×10-20 cm2. The input concave mirror M1, having a radius of curvature of 250 mm, was coated with anti-reflection (AR) dielectric layers at 808 nm (T>99.5 %) and high-reflection (HR) dielectric layers between 1000–1200 nm (>99.9). The M2 mirror is a flat mirror with reflectance of 99.38 %, 99.20 %, 94.20 %, and 86.85 % at 1087, 1084, 1067, and 1062 nm, respectively. The gain crystal was placed closely to M1 to enhance the overlap between the pump and cavity modes. The pumping side (S1) of the laser crystal has high-transmission coating at 808 nm (T>99.5 %) and both sides were AR coated at 1067 nm (R<0.5 %). The laser crystal was wrapped in an indium foil with 0.2 mm thickness and was placed in a copper holder cooled to 20 °C favorable for maintaining the thermal stability of the laser.

Fig. 1
figure 1

Experimental setup of the diode-pumped single and multiwavelength Nd:YVO4 laser with an intracavity sub-THz etalon as a wavelength tuning element

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3 Experimental results and discussions

3.1 Single-wavelength operation at 1083.9 nm

In this subsection, we present single-wavelength operation of the laser at 1083.9 nm without the intracavity etalon. This wavelength has never been reported as a lasing wavelength from a Nd:YVO4 laser in the past. Figure 2 shows the output spectrum of the free-running laser without an intracavity etalon, indicating a spectral peak at 1083.9 nm with a spectral bandwidth (FWHM) of ∼2.3 cm-1. The result was recorded for a 10-W input pump power and 1.57-W output power from an 85-mm long laser cavity. When we were increasing the pump power, we observed a red shift of 9 pm/W in the output laser spectrum, which could be attributable to some change of the stimulated emission cross section with temperature [12]. It is worth mentioning that the stimulated emission cross section at this wavelength is approximately half that of the popular transition wavelength at 1067 nm for a c-cut Nd:YVO4 crystal. In our experiment, we achieved lasing at 1083.9 nm with an output coupling loss of 6∼13 % between 1067–1062 nm and <1 % at 1084 nm.

Fig. 2
figure 2

The output spectrum of the free-running laser without an intracavity etalon, indicating a spectral peak at 1083.9 nm

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The alignment for the laser cavity and the pump beam was carefully adjusted to maximize the laser output and minimize the laser threshold for a TEM00 output mode. Figure 3 shows the laser output power as a function of the incident pump power for several cavity lengths. It can be clearly seen from Fig. 3 that, at first, the laser output power increases linearly with the pump power and then, in the case of the long cavity, the output power decreases to zero; whereas, in the case of the short cavity, the output power continues to grow, and finally saturates at a high pump power. The laser dynamics shown in Fig. 3 is closely related to the thermal effect in the cavity [13]. For a long cavity, thermal lensing causes the laser cavity to become unstable when the pump power increases to a critical value, beyond which the laser output power starts to decrease. However, in the case of short cavity, the thermal effect could excite some high-order laser modes in the cavity, deteriorate the laser beam quality, but results in an increased output power.

Fig. 3
figure 3

The laser output power at 1083.9 nm as a function of the incident pump power for several cavity lengths without an intracavity etalon. A long cavity is susceptible to thermal-lensing induced instability

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With a cavity length of 40 mm, we measured a maximum laser output power of 4.33 W under an incident pump power of 18.8 W, which corresponds to an optical-to-optical conversion efficiency of 23 %. The measured slope efficiency is 28 %, which could be further improved by optimizing the output coupling of mirror M2. From Fig. 3, the laser threshold increases from 0.867 to 1.5 W as the cavity length increases from 40 to 100 mm. The measured pump threshold of 0.867 W agrees with the following theoretical model derived from the space-dependent rate equation for an ideal four-level laser material [14]:

$$ P_{th} = \frac{\pi h\nu _{p}(w_{p}^{2} + w_{l}^{2})[ - \ln (R) + L]}{4\eta _{p}\sigma _{e}\tau}$$
(1)

by using σ e =1.3×10-19 cm2 for the emission cross section [10], R=99.2 % for the reflectance of the output-coupling mirror at 1084 nm, L=0.1 % for the internal roundtrip loss, τ=92.4 μs for the laser’s upper-level lifetime, hν p =1.53 eV for the pump photon energy at 808 nm, α=2.21 cm-1 for the absorption coefficient at 808 nm, l=10 mm for the crystal length, w p =0.358 and w l =0.250 mm for the waist radii of pump and laser beams, respectively, and η p =66 % for the pumping efficiency. We measured the far-field divergent angle of the output laser beam and substituted it into the Gaussian beam propagation law to obtain the laser waist w l .

3.2 Multiwavelength operation

In this subsection, we report broadly, discretely tunable laser operations between 1.06 and 1.09 μm for the laser depicted in Fig. 1. In our experiment, uncoated glass etalons with thickness of 114 and 150 μm, corresponding to FSR of 0.88 and 0.67 THz, respectively, were utilized to balance gain and loss in the cavity and make a multiwavelength laser. Figure 4 shows the tuning range of the laser over three discrete spectral regions, 1066.5–1066.8, 1083.1–1084.6, and 1087.2–1088.2 nm, when the laser was tuned by the 150-μm thick etalon over the intermanifold groups of the 4F3/2→4I11/2 transition of the crystal. At certain etalon tuning angles, the laser can oscillate at dual wavelengths of (1084.6, 1087.4), (1066.5, 1088.0) and (1066.8, 1083.3) nm, as shown in Fig. 5, which was recorded at an incident pump power of 10 W and a fixed cavity length of 86.6 mm. We observed that the thresholds of the dual-wavelength lasers are different for different wavelength groups. The [threshold pump power, maximum output power] for the wavelength groups of (1084.6, 1087.4), (1066.5, 1088.0) and (1066.8, 1083.3) nm was measured to be [3.5, 0.90], [7.5, 1.33], and [6.8, 1.24] W, respectively. The maximum output powers for the three dual-wavelength groups were obtained at input pump powers of 17.5, 18.3, and 13 W, respectively. Beyond the maximum output power, the laser output became unstable due to gain competition from different spectral lines. As shown in Fig. 6, the laser can also oscillate simultaneously at four wavelengths of (1066.9, 1082.9, 1085.7, 1088.3) nm with fine adjustment on the angle of the intracavity etalon. A total output power of 1.2 W in the four spectral components was obtained at a pump power of 15 W. However, gain competition among the two groups of wavelengths (1066.9, 1088.3) and (1082.9, 1085.7) nm makes the four-wavelength operation unstable.

Fig. 4
figure 4

Continuous tuning over three discrete spectral regions of the c-cut Nd:YVO4 laser obtained by rotating the 0.67-THz etalon inside the laser cavity at 10-W diode pump power

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Fig. 5
figure 5figure 5figure 5

Spectra of dual-wavelength operation of the laser at (a) (1087.4, 1084.6), (b) (1088.0, 1066.5), and (c) (1083.3, 1066.8) nm at 10-W diode pump power and with an intracavity etalon having a FSR=0.67 THz

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Fig. 6
figure 6

Spectrum of the four-wavelength laser achieved by fine adjustment of an intracavity etalon having a FSR=0.67 THz

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It should be pointed out that, in the dual-wavelength laser, because the two paired transitions share a common upper level 4F3/2 of an Nd3+ ion, the gain competition between the two wavelengths is not negligible. For the dual-wavelength operation of the three paired wavelengths (1084.6, 1087.4), (1066.5, 1088.0) and (1066.8, 1083.3) nm, we observed that oscillations at 1084.6, 1088.0, and 1066.8 nm built up first, respectively, and then their powers decreased and fluctuated when the other components began to oscillate. Gain competition from different spectral lines becomes more evident at a high pump power. As the pump power increases, the second spectral component starts its oscillation as soon as the pump reaches its oscillation threshold. The oscillation of the second spectral component drains the pump power from the first spectral component and causes the power of the first spectral component to decrease. To optimize simultaneous multiwavelength lasing, a balance is needed between the gain and loss for each of the lasing wavelengths at the same time. We also observed a red shift of the transition lines when increasing the pump power. The red shift could be related to the change of the crystal temperature under different pump powers.

A different group of wavelengths from the laser can be selected by using an etalon with a different FSR. For example, by using the other etalon with 0.114 mm thickness, the laser achieved dual-wavelength oscillations at (1066.7, 1083.7), (1065.2, 1086.1), and (1064.7, 1085.0) nm, and four-wavelength oscillation at (1062.8, 1066.7, 1082.8, 1086.3) nm.

4 Summary

In summary, we have reported a discretely tunable, single and multiwavelength CW diode-end-pumped laser based on the 4F3/2–4I11/2 intermanifold transitions in a c-cut Nd:YVO4 crystal. By using an intracavity 0.67-THz free-spectral-range etalon, the laser can be easily tuned in the three discrete regions of 1066.5–1066.8, 1083.1–1084.6, and 1087.2–1088.2 nm. Also, the laser can achieve dual-wavelength operation at (1084.6, 1087.4), (1066.5, 1088.0) and (1066.8, 1083.3) nm, and four-wavelength operation at (1066.9, 1082.9, 1085.7, 1088.3) nm. In addition, with a 0.88-THz free-spectral-range etalon, the laser can achieve dual-wavelength operation at (1066.7, 1083.7), (1065.2, 1086.1), and (1064.7, 1085.0) nm, and four-wavelength operation at (1062.8, 1066.7, 1082.8, 1086.3) nm. Without the intracavity etalon, the laser generates an output power of 4.33 W at 1083.9 nm with a pump power of 18.8 W, slope efficiency of 28 %, and optical-to-optical conversion efficiency of 23 %.

With the laser wavelengths and tunability demonstrated in this work, a number of applications can be made possible. For instance, when tuned to the dual-wavelength emissions, this laser can be used as a source to generate THz waves through difference frequency generation in a suitable nonlinear optical material. Also, the fourth-harmonic of the 1083.9-nm line can be utilized to excite norfloxacin around 272 nm for sensitive determination of yeast double-stranded DNA (dsDNA) from its fluorescence at 415 nm [15]. Furthermore, second harmonic generation of the 1084 and 1066-nm lines can provide laser radiations for absorption spectroscopy of oxy-hemoglobin (HbO2) around 542 and 533 nm [16].