1 Introduction

Mixed vanadate laser crystal technology (for example Nd:Gd x Y1−x VO4) has recently received increasing interest due to the potential customisation of the fluorescence spectrum [1–4] compared to the fixed spectra of neodymium-doped pure vanadate hosts (e.g. Nd:YVO4 and Nd:GdVO4). Generally, this mixing lowers the emission cross section, meaning high average power operation can be more problematic, but can also provide significantly larger spectral bandwidths. Increased spectral bandwidth offers particular attraction for mode locking, with creation of shorter pulse durations. Previously, the highest power obtained from a mixed vanadate laser was achieved by Omatsu et al. [5], with ∼20 W of average power in continuous-wave (CW) operation and ∼8 W in passively Q-switched operation [6], in the bounce geometry. Further power scaling was limited by available pump power.

The bounce geometry laser is a high gain laser system that takes advantage of the intensely diode-pumped region just below the pump face, particularly in Nd-doped gain media [7]. The laser mode takes a path of grazing-incidence, total internal reflection at the high-gain pump face, leading to excellent spatial averaging of gain and refractive-index non-uniformities. By utilising increased pump power, it is thought that power scaling beyond the levels reported in [5, 6] can be achieved.

Mode locking of mixed vanadate lasers has only thus far been achieved either actively or via semiconductor saturable absorber mirrors (SESAMs) [2, 8] and the highest average power mode-locked operation is around 7 W [9]. Nonlinear mirror (NLM) mode locking [10, 11] has produced high-power mode-locked operation in the bounce geometry [12] and its simple design has been proven to produce robust and reliable picosecond pulses from other Nd- and Yb-doped gain media [13, 14]. In its most elementary form, NLM mode locking involves the use of an intra-cavity nonlinear crystal (NLC) and a dichroic output coupler in a double-pass, phase-matched regime. A detailed explanation of the process can be found in [10] and [12]. The benefits of using the NLM include an order of magnitude higher damage threshold than the current state-of-the-art SESAMs (damage threshold for BiBO measured at >48 GW cm−2 under illumination with 56 ps pulses at 1064 nm [15], compared with 300 MW cm−2 stated by BATOP for the SESAM product SAM-1064-13-x-500fs [16]). Our previous work [17] involved the use of SESAMs to mode lock a Nd:YVO4 bounce geometry laser with success, but gradual deterioration and catastrophic damage to the SESAM were experimentally observed, especially under Q-switched mode-locking conditions. Further to the benefit of higher damage thresholds, the NLM can be easily implemented at any wavelength laser, just through choice of NLC.

In this paper, we demonstrate the highest average power operation of a mixed vanadate (Nd:Gd0.6Y0.4VO4) oscillator in CW operation, with 27.5 W in multimode and 23 W in TEM00 operation. Furthermore, we demonstrate the first NLM mode locking of a mixed vanadate laser, achieving a record 16.8 W of average power at a repetition rate of 100 MHz and a pulse duration of 12.7 ps. Throughout operation, no damage was observed to the BiBO NLC.

2 Experimental mixed vanadate laser system in continuous-wave operation

The laser gain medium was a 20 × 5 × 2 mm3 slab of 1.5 at.% doped Nd:Gd0.6Y0.4VO4, contact cooled via the 20 × 5 mm2 faces. The 20×2 mm2 face was anti-reflection (AR) coated for the pump wavelength of 808 nm and a fast-axis collimated pump diode was used. The 5 × 2 mm2 faces were AR coated for the lasing wavelength of ∼1064 nm and it was operated in the bounce geometry [7].

The cavity was set up as shown in Fig. 1. A vertical cylindrical lens (VCLD) of focal length 12.7 mm was used to bring the pump radiation to a line focus on the 20 × 2 mm2 face; two further VCLs (of focal length 50 mm) were incorporated into the cavity for efficient matching of the vertical dimension of the laser TEM00 mode propagating in the cavity to the ‘size’ of the pump region [7]. An output coupler of reflectivity R=60 % was used.

Fig. 1
figure 1

Experimental laser system for continuous-wave operation

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2.1 Compact multimode cavity

A compact, symmetric cavity was initially used with cavity arm lengths L 1=L 2=70 mm. This produced a spatially multimode output in the horizontal, but single mode in the vertical, direction. The power curve for this system is shown in Fig. 2 by the filled red circles. The maximum output power obtained was 27.5 W for 50 W pumping, which is the highest output power from a mixed vanadate oscillator, to the best of our knowledge. The optical-to-optical efficiency (\(\eta_{\text{opt--opt}}\)) was 55 %. The power curves in Fig. 2 do not appear to roll over with increasing pump power, suggesting that further power scaling can be achieved through use of higher power pump diodes.

Fig. 2
figure 2

Power curves for continuous-wave (CW) and mode-locked operation. The filled red circles are data for the CW multimode cavity; the open blue circles are data for the CW TEM00 cavity; the green squares are data for the mode-locked (TEM00) cavity—the dashed black circle indicates the range over which CW mode locking was observed

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2.2 TEM00 cavity

The cavity arms were extended to L 1=100 mm and L 2=270 mm for TEM00 operation to obtain a match of the TEM00 mode size to the horizontal size of the gain region. The power curve obtained from this cavity is also shown in Fig. 2. The ‘dip’ in the power around 30–40 W pumping is a characteristic of the asymmetric cavity entering an unstable region due to pump-induced changes in the thermal lens [7, 18]. The maximum output power obtained at 50 W pump power was 23 W, corresponding to \(\eta_{\text{opt--opt}}\) of 45 %. The spatial profile obtained from this system is shown in Fig. 3. The M 2 value for this system was 1.5 in the vertical and 1.6 in the horizontal directions.

Fig. 3
figure 3

Spatial profile recorded at 21 W output power

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3 Experimental system for mode-locked operation

To incorporate the NLM for mode-locked operation, the cavity was further extended such that L 2∼1400 mm (schematic shown in Fig. 4). This was accomplished by the addition of two imaging lenses of focal lengths 300 and 200 mm. This also provided a demagnification factor of the laser mode of ∼2/3 and a spot size of ∼300 μm at the nonlinear crystal (NLC). The R=60 % output coupler was replaced with a dichroic output coupler, highly reflecting (HR) at 532 nm and R=70 % at 1064 nm. A 3×3×10 mm3 BiBO NLC was included in the cavity. BiBO was chosen as it has been successfully employed for NLM mode locking in previous studies [12]. The NLC was placed ∼25 mm away from the output coupler for correct phasing for back-conversion of the second harmonic to the fundamental on the return pass through the crystal. This condition leads to the formation of a nonlinear reflectivity, leading to mode locking.

Fig. 4
figure 4

Schematic of experimental mode-locked cavity

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The threshold for CW mode locking was at a pump power of ∼46 W and the system remained stably mode locked over the pump power range 46–50 W, indicated by the black, dashed circle in Fig. 2. Outside of this range, the laser operates in a CW regime. At the maximum pump power of 50 W, an output power of 16.8 W was measured—the highest power mode-locked mixed vanadate oscillator to date. The system was self starting and remained stably mode locked over the course of hours. The pulse-repetition rate was ∼100 MHz. The mode-locked pulse train is shown in Fig. 5.

Fig. 5
figure 5

Mode-locked pulse train recorded at 50 W pump power, 16.8 W output power

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3.1 Spectrum and pulse duration

During mode-locked operation, the spectrum broadened. The resolution of the spectrum analyser was 0.07 nm and detection of the bandwidth of the laser in CW operation was resolution limited. During mode-locked operation, the deconvolved measured bandwidth of the laser was 0.19 nm. The central wavelength was 1063.8 nm. At an output power of 16 W, the pulse duration was recorded with a laboratory-built background-free autocorrelator. The resulting trace from this is plotted in Fig. 6. The full-width at half-maximum from the autocorrelation trace was found to be 19.6 ps. Assuming a sech2 pulse shape, this corresponds to a pulse duration of 12.7 ps.

Fig. 6
figure 6

Autocorrelation trace of the mode-locked output. The autocorrelation FWHM is 19.6 ps, corresponding to a pulse duration of 12.7 ps

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The measured mode-locked bandwidth of 0.19 nm does not exploit the full available bandwidth that should be achieved with mixed vanadate gain media and thus further work is required to investigate this. The pulse duration measured from the output of the laser is expected to be longer than that of the pulse circulating in the cavity [13]; since the NLM reflects the peak of the pulse more than the wings, broader pulses are coupled out of the cavity. This could be addressed by utilising an adaptation of the NLM mode-locking technique: by operating the nonlinear crystal in a phase-mismatched regime, a cascaded-χ (2) lens evolves [19]. This can be considered to act on the pulse analogously to a Kerr lens [19–21], leading to a shorter pulse duration, with the correct cavity design.

4 Conclusion

The highest average power mixed vanadate oscillator has been presented, measured at 27.5 W from a compact, symmetric cavity and spatially multimode output. This corresponds to an optical-to-optical efficiency (\(\eta_{\text{opt--opt}}\)) of 55 %. An average output power of 23 W in TEM00 operation was demonstrated, by adapting the cavity design, corresponding to \(\eta_{\text{opt--opt}}\) of 45 %.

This TEM00 system was subsequently mode locked using the NLM technique in an extended cavity design. CW mode locking was demonstrated at an output power of 16.8 W and a repetition rate of ∼100 MHz. This is the first time the NLM technique has been used to mode lock a mixed vanadate laser; additionally, this is the highest power mode-locked mixed vanadate oscillator to date. The pulse duration was measured to be 12.7 ps. Throughout many hours of operation, no damage was observed to the BiBO crystal.

In comparison with prior NLM mode-locking publications of single vanadate crystals, this is the average highest power yet to be reported. Previously, the highest power was 12 W [12]. Regarding the pulse duration, however, though the 12.7 ps reported here shows a marginal improvement on the 14 ps in our previous work [12], pulse durations < 3 ps have been reported in [13, 19] and output powers of 15.4 W [21] in laser systems employing the cascaded-χ (2) lens approach. The lasing spectrum of the mixed vanadate system is greater that of the single vanadate lasers, so further work is required to exploit the broader fluorescence spectrum of mixed vanadate.

Future work will include further amplification, by means of increased pump power and a master-oscillator power-amplifier configuration, and methods of obtaining shorter pulse durations. This may include utilising the cascaded-χ (2) lens mode locking.