We have demonstrated the highest reported output power from a mid-IR ZGP OPO. The laser is a cascaded hybrid system consisting of a thulium fibre laser, Ho:YAG solid state laser and a Zinc Germanium Phosphide parametric oscillator. The system produces 27 W of output power in the 3-5 μm wavelength range with an M2 = 4.0 when operating in a repetitively q-switched mode, and a modulated peak output power of 99 W at a reduced duty cycle of 25%.
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The mid-IR spectral region is of interest for a range of applications due to the presence of low loss atmospheric transmission windows in the 3-5 µm wavelength region. This property coupled with the presence of molecular transitions useful for spectroscopy makes this wavelength band of particular interest for remote sensing and defence applications.
High power laser sources targeting these wavelengths have typically been based on frequency conversion of 2 µm lasers using ZnGeP2 (ZGP) optical parametric oscillators (OPO’s). ZGP exhibits excellent thermal, mechanical and non-linear properties suitable for scaling to high average powers. Previous power-scaling results in this spectral region have demonstrated a pump power limited output of up to 22 W  from a single ZGP OPO with no evidence of roll over or loss of efficiency. These results demonstrate the potential for further power scaling by developing higher power 2 µm lasers.
A number of 2 μm sources have been developed which are suitable for pumping ZGP OPOs . These include diode pumped Nd:YAG laser pumped KTP OPOs [3, 4], diode pumped Tm:YAP [5, 6] and Ho:YAG or Ho:YLF lasers resonantly pumped by; diodes , Tm:YLF [8-10] or thulium fibre lasers [1, 11-15].
Resonant pumping of Ho:YAG results in a small quantum defect and enables power scaling of the solid-state laser without degrading the beam quality. This approach has the added advantage of enabling the ZGP OPO to be pumped well above 2 µm, below which defect related absorption losses in ZGP limit the OPO power scaling potential. 1.9 μm diodes, Tm:YLF and thulium fibre lasers have all been used to resonantly pump high power Ho:YAG lasers. Diode pumping has advantages in terms of simplicity and an output of 55 W at 2.12 μm has been demonstrated but with only 35% conversion efficiency and poor beam quality . In addition, the electrical efficiency of diodes at 1.9 μm is lower than that of 0.79 µm diode pumped Tm fibre lasers. A Tm:YLF pumped Ho:YAG source has been demonstrated with output power of 103 W, a slope efficiency of 68% and an M2 < 2.0 .
Fibre lasers have a much larger surface area than their solid state counterparts offering advantages for thermal management at high average powers. They also have a near diffraction limited beam quality aiding end pumping applications. Initial demonstrations of a thulium fibre laser pumped Ho:YAG achieved an output power of 6.4 W with a slope efficiency of up to 80%, a conversion efficiency of 67% and an M2 = 1.1 . Recently thulium fibre laser pumped Ho:YAG has been power scaled to 42 W with an M2 = 1.7 demonstrated from a single rod . Further power scaling will require linearly increasing the Ho:YAG module count, thereby increasing the size and complexity of the system, as has been demonstrated for thulium fibre laser pumped Ho:YLF system .
The system described here follows from our previous work  which utilized a hybrid thulium fibre laser pumped single rod Ho:YAG laser. This architecture produces excellent output beam quality despite the potential limitations of thermal lensing and thermal and stress birefringence inherent with end-pumped laser designs. In this work we describe the power scaling of a Ho:YAG laser consisting of a single oscillator utilizing two Ho:YAG rods which is capable of producing more than 60 W of average output power with beam quality of M2 < 1.2. The ZGP OPO pumped using this source demonstrated a maximum output of 27 W in the mid-IR with a conversion efficiency of 62% and an M2 = 4.0.
The continuous wave (CW) thulium fibre lasers that are used to pump the Ho:YAG laser can also be operated quasi-CW with similar average output powers. This results in a higher modulated peak output power with a reduced duty cycle which is advantageous for some applications. We present results from the system operating at 1 kHz with a 25% duty cycle which resulted in a modulated peak output power in the 3-5 μm mid-IR band of 99 W. The average mid-IR output power is currently limited by the optical isolator between the Ho:YAG laser and ZGP OPO.
To the best of our knowledge this is the highest published 3-5 µm output power achieved by a solid-state laser system in both repetitively q-switched and reduced duty cycle modes of operation. These results, showing no sign of roll over in output power from the OPO, demonstrate the potential for further power scaling of mid-IR sources using ZGP OPO’s.
The schematic layout of the mid-IR laser system is shown in Fig. 1 and consists of a pair of thulium fibre lasers pumping a dual rod Ho:YAG laser. The output of this laser is then frequency converted into the mid-IR using a ZGP OPO.
The thulium fibre lasers are similar to those reported previously , and their design is shown in Fig. 2 . The thulium fibre lasers are co-pumped through a fibre taper which converts the 400/480 µm/0.22 NA multimode diode delivery fibre to the 250 µm outer diameter of the double clad fibres used for the fibre Bragg gratings (FBG’s) and active thulium fibre. The laser cavity is formed by high reflector (HR) (R > 95%) and output coupler (OC) (R ~ 10%) FBG’s written in 25/250 µm germanium doped fibre, and 1.8 m of 25/250 µm 2.2 wt% thulium doped fibre with an effective core NA of 0.1. The laser is end-capped with coreless fibre to allow the output beam to expand at the exit of the fibre enabling a dichroic mirror to be butt-coupled to the fibre whilst avoiding catastrophic optical damage. This mirror retro-reflects the transmitted pump power and increases the electrical-optical conversion efficiency of the thulium fibre laser. Each of the thulium fibre lasers is then focused by a 12 mm AR coated (T > 99% @ 1.908 µm) Infrasil lens to produce a 200 µm 1/e2 radius beam in the Ho:YAG rods.
The thulium and hence Ho:YAG lasers could be operated at a variable pump diode duty cycle of between 0 and 100%, typical operation was at 25% and 100%. There was sufficient peak diode pump power available from the 350 W, 0.79 µm fibre-coupled pump diodes to enable operation at similar average power levels at both the 25% and 100% duty cycles.
The Ho:YAG laser contains two 50 mm 0.7 wt% doped Ho:YAG rods, end-capped with 5 mm of un-doped YAG. Each of the Ho:YAG rods is single-pass pumped by a thulium fibre laser. The laser cavity is formed by 100 mm radius of curvature (ROC) high reflector (HR) (R > 99% @ 2.09 µm) and output coupler (OC) (R = 50% @ 2.09 µm) mirrors with an anti-reflection (AR) coated 50 mm focal length plano-convex lens in the centre of the cavity. The total cavity length is 276 mm. Q-switched operation is achieved by the use of an RTP q-switch in conjunction with a quarter wave-plate and dichroic thin film polarisers (TFP) (R > 99% s-pol @ 2.09 µm, R < 5% p-pol @ 2.09 µm and R < 2% r-pol @ 1.908 µm). The RTP q-switch operates at a quarter-wave voltage of 1.25 kV at repetition rates up to 140 kHz with operation demonstrated at circulating power levels beyond 150 W. An angle tuned 100 µm thick fused silica etalon was used to stabilize the output spectrum of the Ho:YAG laser to a single spectral feature at 2.090 µm.
The output from the Ho:YAG laser is collimated by a 110 mm plano-convex lens. A half-wave plate and polarizer are used to vary the ZGP OPO input pulse energy to characterise the ZGP OPO. A 40 W high power isolator was used to prevent OPO back reflections and back conversion from causing instability in the Ho:YAG oscillator. The Ho:YAG output is focused to a 180 µm 1/e2 radius beam in the ZGP OPO using a plano-convex lens with the centre of the OPO cavity positioned at the beam waist.The type-I ZGP OPO consists of two 16 mm long AR coated ZGP crystals (T > 95% @ 2.09 µm, T > 98% @ 3.5-5.2 µm), arranged in a walk-off compensated geometry. The absorption co-efficients of the two ZGP crystals at 2.090 µm was measured to be α = 0.04, 0.08 cm-1. Both single and double pass OPO configurations were investigated with a physical cavity length of 36 mm. The cavity configurations used are detailed in Table 1 .
The residual pump power and the mid-IR signal and idler were separated by a dichroic mirror and monitored using thermal power meters. A CaF2 wedge was used to sample the output beams which were monitored using a pyro-electric camera (Pyrocam III, Spiricon).
The beam quality of the thulium fibre laser, Ho:YAG laser and the ZGP OPO was measured by profiling the laser beam through a waist using a pyro-electric camera. Beam radii were then calculated using a 4-sigma method and these were then fitted to a hyperbolic function using a least squares fit to determine the M2 parameter of the beam.
3.1 Repetitively Q-switched operation
The individual thulium fibre lasers were demonstrated at more than 80 W of average output power, and were typically operated at ~55 W. The lasers achieved a slope efficiency of 36% versus launched diode power as shown in Fig. 3(a) , and the beam quality was measured to be M2 = 1.4-1.6. Typically the spectrum consisted of two features corresponding to multiple transverse mode operation of the large mode area fibre.
Repetitively q-switched operation (RQSW) of the Ho:YAG laser at 35 kHz produced up to 60 W of output power with excellent beam quality and a slope efficiency of 59% as shown in Fig. 3(b). The pulse width was 50±5 ns and a typical pulse shape is shown in the Fig. 3(c). The beam quality was measured to be M2 = 1.1 at 44 W, corresponding to the maximum power that was used to pump the ZGP OPO. At higher powers some degradation of the beam quality was observed in the near field profiles due to non-optimal pump mode size and the beam quality was measured to be M2 = 1.2 at 60 W. The dependence of the beam quality on output power is shown in Fig. 3(d) with a typical near field beam profile at 44 W of output power. The laser was typically operated at 1.1 mJ per pulse to avoid damage to the resonator optics.
The maximum pump power incident on the ZGP OPO was limited by the optical isolator which was specified for operation up to 40 W. Operating at levels as high as 44 W did not impair system reliability and this formed a practical upper limit above which thermal lensing in the Faraday rotator was observed. The resulting output power from the two ZGP OPO configurations are shown in Fig. 4(a) . At 100% duty cycle using the double pass OPO configuration a mid-IR output power of 27.1 W was achieved from an input of 43.2 W with a slope efficiency of 67%. The overall conversion efficiency was 62% and the beam quality of the OPO was measured to be M2 = 4.0. The single pass OPO configuration produced less output power, 24.2 W, with 7.9 W of residual Ho:YAG pump power transmitted through the OPO. The output displayed a typical doubly resonant OPO output spectrum as shown in Fig. 4(b). The single pass beam quality was measured to be M2 = 3.1 and was found to vary as a function of ZGP OPO output power, Fig. 4(c).
3.2 Reduced duty cycle operation
The system was operated at 25% output duty cycle with a repetition rate of 1 kHz. Due to the three-level nature of the thulium and holmium laser transitions a higher pump diode duty cycle is required to achieve a 25% output duty cycle from the Ho:YAG and hence from the ZGP OPO. This extra pump energy restores the inversion to the level present at the end of the previous pump cycle, which has subsequently been depleted by spontaneous emission. In this regime we define the modulated peak power as the average output power divided by the output duty cycle, in contrast to the pulse peak power of the individual q-switched pulses which is typically ~ 22 kW from the Ho:YAG and ~ 14 kW from the ZGP OPO.
At the maximum operating current of the pump diodes the diode duty cycle required to achieve a 25% output duty cycle from the Ho:YAG laser and hence the ZGP OPO was 27%. At maximum current the modulated peak power launched from each pump diode was 400 W resulting in up to 152 W modulated peak power from a single thulium fibre laser, and 286 W from the two thulium lasers combined. The thulium modulated peak output power is plotted in Fig. 5(a) showing a slope efficiency of 39%.
At 25% duty cycle and maximum pump diode current the modulated peak power from the Ho:YAG laser reached 162 W with a pulse repetition rate of 140 kHz, see Fig. 5(b). As expected, the pulse width and beam quality of the Ho:YAG laser were dependent on pulse energy and average power but independent of the duty cycle. Figure 5(c) depicts the typical temporal dynamics of the laser system in QCW mode. The inset to Fig. 5(c) shows the strong relaxation oscillations from the thulium fibre lasers operating at reduced duty cycle.
The corresponding results from the ZGP OPO are also shown in Fig. 5(d). In the double-pass OPO configuration the modulated peak output power was 99 W giving an average mid-IR output power of 24.7 W with a slope efficiency of 69%. This was limited by the modulated peak output power of the 0.79 µm laser diodes rather than the optical isolator which was the case for RQSW operation.
The use of a dual rod architecture has been demonstrated to be a simple and reliable method to power scale a Tm fibre laser pumped Ho:YAG laser. Excellent beam quality of M2 < 1.1 and a high slope efficiency of 59% has been demonstrated. Operation at 25% duty cycle has shown the flexibility of the system to produce modulated waveforms with similar average powers to those produced at 100% duty cycle. The single pass pump geometry of the Ho:YAG resulted in a slope efficiency that is lower than other reported Ho:YAG lasers [13, 16]. Birefringence effects in the Ho:YAG have not been observed at the power levels used in this study, with only up to 30 W extracted from each rod. Such effects would manifest as depolarized power leaking through the polarised dichroic mirrors in Fig. 1.
The OPO efficiencies reported here are comparable to other high power ZGP OPO’s . Several demonstrations have made use of cavity length matching between the OPO and Ho:YAG oscillators to improve efficiency . This technique was not utilized in our system as we found that the shortest OPO cavity length gave the best performance. This corresponded to a ratio of 1:3.45 between the round trip times of the OPO and the Ho:YAG oscillators. No roll over is observed in the ZGP OPO output indicating that further power scaling is achievable.
To investigate the origin of the ZGP OPO beam quality degradation the M2 of the OPO output was measured at three thermal loads. This was done by passing the Ho:YAG pump through a mechanical chopping disc with either a 10% or 50% duty cycle to vary the thermal load while leaving the Ho:YAG running at 100% duty cycle. In these tests a single pass OPO was used, the maximum OPO pump power was 40.7 W and the pump 1/e2 waist radius in the ZGP OPO was 0.18 mm. The results shown in Fig. 4(c) demonstrate that the M2 of the ZGP OPO output degrades as the average power increases. This degradation is thus believed to be due to the increase in thermal lensing as a function of pump power resulting in a reduction in the signal cavity mode size relative to the pump mode size. This mode mismatch increases the gain present for the higher order modes, degrading the beam quality.
We have to the best of our knowledge demonstrated the highest reported solid-state mid-IR laser source with 27.0 W of output power. An efficient high power electro-optic q-switched Ho:YAG laser capable of producing 62 W at 2.09 µm with M2 < 1.2 is used to pump the ZGP OPO with a slope efficiency of 69% and conversion efficiency of 62%. The mid-IR OPO output is currently limited by the optical isolator between the Ho:YAG laser and ZGP OPO.
The system is also able to produce waveforms with higher modulated peak powers at a reduced duty cycle whilst maintaining high average output power. At 25% output duty cycle a mid-IR modulated peak power of 99 W has been achieved.
At high average powers thermal lensing in the ZGP crystals degrades the OPO beam quality from M2 = 2.5 at 10.6 W mid-IR output down to M2 = 4.0 at 27.0 W, and also increases the risk of optical damage.
Future work will investigate the use of a ring OPO to remove the requirement for the optical isolator which is currently the limiting component to further power scaling.
The authors would like to thank Len Corena and Dmitrii Stepanov (DSTO) for grating fabrication and Mark Hughes and Phil Davies (DSTO) for mechanical design and fabrication.
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