CONTINUOUS-WAVE, ALL-SOLID-STATE, YELLOW ...
Australian Institute of Physics 17th National Congress 2006 – Brisbane, 3-8 December 2006 RiverPhys
CONTINUOUS-WAVE, ALL-SOLID-STATE, YELLOW LASER SOURCE AT 588 nm P. Dekker, H.M. Pask, D. Spence, J. A. Piper Centre for Lasers and Applications, Department of Physics, Macquarie University, NSW 2109, Australia
Abstract We report continuous-wave powers up to 704 mW and quasi-cw powers up to 1574 mW (50% duty cycle) at 588 nm from a diode-pumped Nd:GdVO4 laser with intracavity Raman-shifting (in KGd(WO4)2) and intracavity frequencydoubling (in LBO), with diode-to-588 nm conversion efficiencies of up to 7%. We also present a rate-equation model of the laser which is used to outline the key design issues for cw intracavity doubled Raman lasers, in particular the roles played by thermal loading in the Nd:GdVO4 laser crystal, resonator mode sizes in the gain and nonlinear media and passive resonator losses. Introduction There is significant current interest in continuous-wave (cw) solid-state yellow laser sources for a diverse range of applications including ophthalmology, biomedicine and visual displays, and a variety of approaches have been taken to develop promising devices. These include an extra-cavity, frequency-doubled diode-pumped Yb fibre laser generating 40 mW at 575 nm with a diode-yellow conversion efficiency of 7% [1], a 3 W, 589 nm source based on extracavity frequency-doubling of the 1179 nm output of a Raman fibre laser pumped by a diode-pumped Yb fibre laser (estimated overall optical efficiency ~3%) [2], and a 0.75 W, 593.5 nm laser source based on sum frequency mixing of the 1064 and 1342 nm lines of two Nd:YVO4 lasers with a diode-yellow conversion efficiency ~11% [3]. Our approach to developing yellow sources uses diode-end-pumped Nd lasers incorporating intracavity Raman-shifting and second-harmonicgeneration in crystalline materials; we believe this approach could yield practical, cost-effective devices at low-medium powers (up to ~1 W) due to the small number of components, small physical size, and the low complexity to assemble. Crystalline Raman lasers, based on a variety of Nd-doped pump sources, have significantly increased the number of devices that can access the spectral region between 1.1 and 1.5 µm and also to the yellow-orange spectral region through frequency doubling of the Stokes output (or by direct pumping the Raman material with frequency-doubled Nd lasers [4]). A wide variety of experimental configurations have been employed to achieve Raman conversion, including intracavity Raman generation, external resonator based lasers and Raman generators (primarily used with ultrashort pulse pump lasers). A recent review [5] provides an overview of the field. Prior to 2004, all reported crystalline Raman lasers had been pulsed devices, and it was believed that the high peak powers required to reach threshold for SRS necessitated a Q-switched fundamental laser as the pump source for SRS. The first continuous-wave (cw) crystalline Raman laser was reported by Grabitchikov, using an external Raman resonator pumped by an Ar+ laser at 514 nm [6] Three subsequent papers [7,8,9] reported cw Raman laser action using intracavity resonator configurations, and for the first time diode-pumped laser systems. In this paper, we report a 704 mW cw intracavity frequency-doubled Raman laser which operates in the yellow at 588 nm. To the best of our knowledge this is the first demonstration of an all solid-state cw yellow laser based on crystalline media. We also outline the key design considerations required for efficient cw intracavity Raman laser action, and use numerical modelling to illustrate the interplay and trade-offs between the various design parameters. Laser Design The Raman laser configuration is illustrated in Figure 1. The resonator was bounded by a pair of flat mirrors having high transmission at the diode (808 nm) and yellow (588 nm) wavelengths, and high reflectivity at fundamental (1063 nm) and Stokes (1176 nm) wavelengths. Two sets of mirrors were used in the experiments. Mirror set A, for operation at the first Stokes wavelength, and also for the yellow, had coatings with 85%T at 808 nm, 0.09%T at 1063 nm, 0.4%T at 1176 nm and 93%T at 588 nm). Mirror set B, used to obtain the highest yellow powers, had coatings with 96%T at 808 nm, 0.006%T at 1063 nm, 0.004%T at 1176 nm and 95%T at 588 nm). Resonator stability was achieved by way of the strong positive thermal lens in the laser gain medium (the thermal lens in the Nd:GdVO4 laser crystal for a incident pump power of 21 W had focal length estimated from measurements of resonator stability to be +66 mm). Cavity length was kept to a minimum, and was ~45 mm for operation at 1176 nm (no LBO in cavity) and ~62 mm for operation at 588 nm. Paper No. WC016
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Australian Institute of Physics 17th National Congress 2006 – Brisbane, 3-8 December 2006 RiverPhys
The pump source was a 30 W fiber-coupled 808 nm diode laser (φ=400 µm, NA ~ 0.22), imaged with unity magnification onto an AR-coated (1064-1200 nm) a-cut 0.3at. % Nd:GdVO4 crystal (3x3x10 mm). Raman shifting was obtained using a KGd(WO4)2 (KGW) crystal with dimensions of 5x5x25 mm, AR-coated for the near-infrared and cut and oriented for propagation along the Np axis with the plane of polarization parallel to Nn. KGW was selected for its superior thermal properties, good Raman gain coefficient and high damage threshold [10]. Second harmonic generation (SHG) of the 1176 nm Stokes line was obtained using a temperature controlled (~45 °C), 3x3x10 mm non-critically phase matched (NCPM, θ=90°, φ=0°) LBO crystal coated AR at 1064-1200 nm. Fig. 1. Schematic of cw Raman laser Nd:GdVO4
LBO
KGd(WO4)2
Laser diode
M2
M1
CW Laser Operation at 1176 nm (mirror set A) Threshold for lasing at the fundamental occurred for 0.7 W power from the laser diode, and we obtained Raman threshold for 6.6 W of incident pump power. Figure 2 shows the Stokes output as a function of pump power, together with the residual fundamental which becomes strongly depleted above threshold for SHG. Above threshold the 1176 nm firstStokes Raman power increased linearly with pump power, reaching 1563 mW from the output mirror for a maximum pump power of 20.4 W, limited by the onset of coating damage. Note also that the transmission of the input mirror was equal to that of the output mirror (0.4%T at 1176nm), thus approximately 1.5 W Raman power was lost from the input mirror. The low output coupler transmission at 1176 nm (0.4%) in comparison with the other resonator losses (estimated to be at least 1%) substantially limited the Raman laser output that could be obtained; clearly optimization of output coupling in relation to other losses would increase the output powers. CW Laser Operation at 588 nm (mirror set B) Nonlinear output coupling through frequency-doubling of the Stokes optical field is particularly well suited to extracting the Stokes optical field efficiently. Low thresholds are possible due to the high-Q cavity at both the fundamental and Stokes wavelengths), while at higher circulating fundamental and Stokes powers, losses are dominated by the nonlinear conversion to the visible, which is coupled from the resonator through a dichroic end-mirror.
1.6
Residual 1063nm Raman 1176nm
1.4 Output power ( W )
Instantaneous 588 nm power ( mW )
The cw yellow output power at 588 nm as a function of diode-pump power incident on the laser crystal in shown in Figure 3 (solid squares). At 15.5 W pump power the cw visible output was stable at 704 mW, with amplitude noise of approximately 12%. At higher pump powers, the visible output decreased and became unstable as the resonator approached the stability limit. We note a similar yellow power would have been lost either through the input mirror or would have been absorbed in the Nd:GdVO4 laser crystal (α588nm>2cm-1 ).
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Incident pump power ( W )
Fig. 2 CW Raman and residual fundamental output power as a function of diode pump power.
Paper No. WC016
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Instantaneous incident pump power ( W )
Fig. 3. CW and quasi-cw 588 nm output powers as a function of instantaneous diode power for CW and modulated (50% duty cycle) excitation.
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Australian Institute of Physics 17th National Congress 2006 – Brisbane, 3-8 December 2006 RiverPhys
Quasi-cw operation at 588 nm (50% duty cycle, mirror set B) Given that the minimum resonator length (62 mm) is set by the physical size of the crystals, maximum pump power is limited by the thermal lensing in the laser crystal. We have therefore undertaken experiments to test the potential for generating higher cw yellow powers by operating the diode pump at reduced (50%) duty cycle (specifically using a mechanical chopper inserted in the pump beam path giving a 200 Hz square-wave pump train). The reduced thermal load in the laser crystal resulted in a thermal lens of approximately twice the focal length for the same instantaneous power in fully-CW mode. Figure 3 also shows 588 nm output in this regime (50% duty cycle); instantaneous power of 1574 mW was obtained for an instantaneous diode pump power of 22.4 W. Operation at 588nm using mirror sets A and B Laser performance at 588 nm has been fully investigated using both mirror sets A and B. Mirror set B provided a cavity with much higher cavity Q for both the fundamental and Stokes optical fields, and resulted in substantially lower thresholds and higher output powers, the results of which are summarised in Table 1 for the cases of cw and quasi-cw operation. The output powers obtained using mirror set B are approximately twice those obtained with mirror set A. From transmission spectra of the three crystals, we estimate the round-trip fundamental and Stokes resonator losses to be around 1.4 and 2.0% using mirror set A, and around 1.2% and 1.2% using mirror set B. However we also note that the true resonator loss is hard to determine in such a short cavity where the elements are aligned at near-normal incidence in order to maximize the output power. Table 1: Diode powers for SRS threshold and maximum yellow output power using mirror sets A and B
Mirrors Set A Set B
Cw threshold (W) 6.9 2.5
Max cw power (mW) 320 (17.6W from diode) 704 (15.5W from diode)
Quasi-cw threshold 7.0 (W) 2.5 (W)
Max quasi-cw power (mW) 700 (21.6W from diode) 1574 (22.4W from diode)
Numerical Modelling of the cw yellow laser The cw yellow laser involves three simultaneous optical processes; generation of fundamental radiation in the laser crystal, SRS in the Raman crystal, and second harmonic generation (SHG) is a doubling crystal. Efficient laser operation requires each of these optical processes to be optimized simultaneously. There are a lot of complex effects that determine the subtle behaviour of the system, including high order transverse modes, thermal lensing, spectral effects, and polarization effects amongst others. In the work presented here, we use a simple model in order to determine the trends that underlie this complex behaviour. Such a model does not aim to give perfect agreement with experimental measurements, but aims to aid our understanding of the basic interplay between the key design parameters of the Raman laser. As such, it is a valuable tool for exploring the regimes in which efficient operation is feasible. The model solves the rate equations which describe fundamental, first Stokes and second harmonic optical fields in the steady-state regime. We model beams with top hat transverse profiles; no account of cavity modes is made, but rather the behaviour of the laser is calculated for specific spot radii in each of the active elements in the cavity. The spot radius can be different in different elements, but is assumed to be constant within each element. The pump radiation is also considered to be focused to a constant radius in the laser crystal. By calculating the efficiency of the laser operation as all of these parameters are varied, we can study the importance and sensitivity of parameters and sets of parameters. The key assumptions are as follows. We assume that the pump energy is completely absorbed by the laser crystal (an accurate assumption for most experiments) and a perfect overlap of the pumped volume with the laser mode. We also ignore any phase mismatch in the doubling crystal and ignore any depolarisation effects between the gain and nonlinear media. It should also be noted that the doubled stokes radiation is not resonated in the cavity. Figure 4 shows the calculated conversion efficiency of the yellow laser with 1.2% round-trip loss for the fundamental and stokes fields based loosely on the laser design described above. The diode pump power incident on the laser crystal was 15 W. For the estimated spot sizes of 120 µm in the KGW crystal and 100 µm in the LBO crystal the model predicts an efficiency of 8%, somewhat higher than the experimentally measured 4.5%. The discrepancy is attributed to the modematching assumptions of the model and uncertainty in spot sizes. More importantly, it can be seen in figure 4, that to achieve more efficient operation, we need to decrease the spot size in both the Raman and doubling crystals as much as possible. A balance between the two is also required: for example if the conversion through SHG is too high compared to the Raman conversion we see a decrease in efficiency, as the doubling crystal presents too high a loss to the stokes field, preventing sufficient build up of the stokes field for efficient conversion from the fundamental field to the stokes field. On the other hand if the conversion through SRS is far too high, we see a decrease in the efficiency as the output coupling Paper No. WC016
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Australian Institute of Physics 17th National Congress 2006 – Brisbane, 3-8 December 2006 RiverPhys
presented to the stokes field exceeds its optimum value. Increasing the Raman and doubling parameters together allows us to increase the output coupling for the Stokes radiation whilst still maintaining an appropriate output coupling for fundamental photons. Figure 5 shows the calculated efficiency of the yellow laser as a function of losses for stokes and fundamental fields (including mirror transmission and reflection losses from the three AR-coated crystals). The approximate operating points using mirror sets A and B are shown in the figure, and are predicted to have efficiencies of around 3% and 8% respectively, once again somewhat higher than measured efficiencies of around 1.8% and 4.5%. Nevertheless, the trend clearly agrees with experimental observations. The modelling shows that substantial improvements to efficiency can be anticipated through further reduction in resonator losses, in particular at the Stokes wavelengths as well as through a decrease in the mode size in the Raman and doubling crystals. 4.0
Fundamental loss (%)
Raman spot radius (µm)
200
2%
175 150 125
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10%
50 25
14% 24% 25
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100
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9%
15%
1.5 1.0
21%
+B
+A
0.5
20%
50
3%
3.5
125
150
175
200
D o u b lin g s p o t ra d iu s ( µ m ) Figure 4: Calculated yellow laser efficiency vs spot sizes in the KGW and LBO crystals. (1.2% rt losses for fundamental and Stokes fields, 150 um spot in laser crystal, 15 W pump power.)
27% 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Raman loss (%) Figure 5: Calculated yellow laser efficiency vs the fundamental and Stokes round trip losses. (150 µm spot in the laser crystal, 120 µm spot size in KGW Raman crystal, 100 µm spot size in LBO, 15 W pump power.)
Conclusions We have demonstrated an all-solid-state continuous-wave yellow laser which generates 1.56 W output at 1176 nm, and, by intracavity frequency doubling with LBO, 680 mW at 588 nm. Major improvements in output power were obtained by reducing the fundamental and Stokes resonator losses using new mirrors, and numerical modelling of the laser shows that further improvements to efficiency would result with further reductions, particularly to the Stokes losses. These could be achieved for example by applying the HR coatings directly to the laser and doubling crystals, or by diffusion bonding the crystals. Quasi-cw yellow powers as high as 1.5 W were achieved when the diode was operated at 50% duty cycle. This, combined with the dependence of laser efficiency on pump power, suggests that with an improved resonator design in particular incorporating the thermal loading of the laser crystal in order to achieve smaller spot sizes in the Raman and doubling crystals, would strongly improve the laser efficiency. An improved resonator design would also collect the yellow photons generated in both directions, as in the present design, around half the generated yellow photons are lost by absorption by the laser crystal or through the input mirror. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
S. Sinha, C. Langrock, M. J. F. Digonnet, M. M. Fejer and R. L. Byer, Opt. Lett., 31, 347 (2006). D. Georgiev, V. P. Gapontsev, A. G. Dronov, M. Y. Vyatkin, A. B. Bulkov, S. V. Popov and J. R. Taylor, Opt. Express, 13, 6772 (2005). J. Janousek, S. Johansson, P. Tidemand-Lichtenberg, S. H. Wang, J. L. Mortensen, P. Buchhave and F. Laurell, Opt. Express, 13, 1188 (2005). H. M. Pask, Prog. Quantum Electron., 27, 3-56 (2003). R. P. Mildren, H. M. Pask, H. Ogilvy and J. A. Piper, Opt. Lett., 30, 1500 (2005). A. S. Grabtchikov, V. A. Lisinetskii, V. A. Orlovich, M. Schmitt, R. Maksimenka and W. Kiefer, Opt. Lett., 29, 2524 (2004). A. A. Demidovich, A. S. Grabtchikov, V. A. Lisinetskii, V. N. Burakevich, V. A. Orlovich and W. Kiefer, Opt. Lett., 30, 1701 (2005). V. A. Orlovich, V. N. Burakevich, A. S. Grabtchikov, V. A. Lisinetskii, A. A. Demidovich, H. J. Eichler and P. Y. Turpin, Laser Phys. Lett., 3, 71 (2006). H. M. Pask, Opt. Lett., 30, 2454 (2005). I. V. Mochalov, Opt. Eng., 36, 1660 (1997)
Paper No. WC016
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CONTINUOUS-WAVE, ALL-SOLID-STATE, YELLOW LASER SOURCE AT 588 nm P. Dekker, H.M. Pask, D. Spence, J. A. Piper Centre for Lasers and Applications, Department of Physics, Macquarie University, NSW 2109, Australia
Abstract We report continuous-wave powers up to 704 mW and quasi-cw powers up to 1574 mW (50% duty cycle) at 588 nm from a diode-pumped Nd:GdVO4 laser with intracavity Raman-shifting (in KGd(WO4)2) and intracavity frequencydoubling (in LBO), with diode-to-588 nm conversion efficiencies of up to 7%. We also present a rate-equation model of the laser which is used to outline the key design issues for cw intracavity doubled Raman lasers, in particular the roles played by thermal loading in the Nd:GdVO4 laser crystal, resonator mode sizes in the gain and nonlinear media and passive resonator losses. Introduction There is significant current interest in continuous-wave (cw) solid-state yellow laser sources for a diverse range of applications including ophthalmology, biomedicine and visual displays, and a variety of approaches have been taken to develop promising devices. These include an extra-cavity, frequency-doubled diode-pumped Yb fibre laser generating 40 mW at 575 nm with a diode-yellow conversion efficiency of 7% [1], a 3 W, 589 nm source based on extracavity frequency-doubling of the 1179 nm output of a Raman fibre laser pumped by a diode-pumped Yb fibre laser (estimated overall optical efficiency ~3%) [2], and a 0.75 W, 593.5 nm laser source based on sum frequency mixing of the 1064 and 1342 nm lines of two Nd:YVO4 lasers with a diode-yellow conversion efficiency ~11% [3]. Our approach to developing yellow sources uses diode-end-pumped Nd lasers incorporating intracavity Raman-shifting and second-harmonicgeneration in crystalline materials; we believe this approach could yield practical, cost-effective devices at low-medium powers (up to ~1 W) due to the small number of components, small physical size, and the low complexity to assemble. Crystalline Raman lasers, based on a variety of Nd-doped pump sources, have significantly increased the number of devices that can access the spectral region between 1.1 and 1.5 µm and also to the yellow-orange spectral region through frequency doubling of the Stokes output (or by direct pumping the Raman material with frequency-doubled Nd lasers [4]). A wide variety of experimental configurations have been employed to achieve Raman conversion, including intracavity Raman generation, external resonator based lasers and Raman generators (primarily used with ultrashort pulse pump lasers). A recent review [5] provides an overview of the field. Prior to 2004, all reported crystalline Raman lasers had been pulsed devices, and it was believed that the high peak powers required to reach threshold for SRS necessitated a Q-switched fundamental laser as the pump source for SRS. The first continuous-wave (cw) crystalline Raman laser was reported by Grabitchikov, using an external Raman resonator pumped by an Ar+ laser at 514 nm [6] Three subsequent papers [7,8,9] reported cw Raman laser action using intracavity resonator configurations, and for the first time diode-pumped laser systems. In this paper, we report a 704 mW cw intracavity frequency-doubled Raman laser which operates in the yellow at 588 nm. To the best of our knowledge this is the first demonstration of an all solid-state cw yellow laser based on crystalline media. We also outline the key design considerations required for efficient cw intracavity Raman laser action, and use numerical modelling to illustrate the interplay and trade-offs between the various design parameters. Laser Design The Raman laser configuration is illustrated in Figure 1. The resonator was bounded by a pair of flat mirrors having high transmission at the diode (808 nm) and yellow (588 nm) wavelengths, and high reflectivity at fundamental (1063 nm) and Stokes (1176 nm) wavelengths. Two sets of mirrors were used in the experiments. Mirror set A, for operation at the first Stokes wavelength, and also for the yellow, had coatings with 85%T at 808 nm, 0.09%T at 1063 nm, 0.4%T at 1176 nm and 93%T at 588 nm). Mirror set B, used to obtain the highest yellow powers, had coatings with 96%T at 808 nm, 0.006%T at 1063 nm, 0.004%T at 1176 nm and 95%T at 588 nm). Resonator stability was achieved by way of the strong positive thermal lens in the laser gain medium (the thermal lens in the Nd:GdVO4 laser crystal for a incident pump power of 21 W had focal length estimated from measurements of resonator stability to be +66 mm). Cavity length was kept to a minimum, and was ~45 mm for operation at 1176 nm (no LBO in cavity) and ~62 mm for operation at 588 nm. Paper No. WC016
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Australian Institute of Physics 17th National Congress 2006 – Brisbane, 3-8 December 2006 RiverPhys
The pump source was a 30 W fiber-coupled 808 nm diode laser (φ=400 µm, NA ~ 0.22), imaged with unity magnification onto an AR-coated (1064-1200 nm) a-cut 0.3at. % Nd:GdVO4 crystal (3x3x10 mm). Raman shifting was obtained using a KGd(WO4)2 (KGW) crystal with dimensions of 5x5x25 mm, AR-coated for the near-infrared and cut and oriented for propagation along the Np axis with the plane of polarization parallel to Nn. KGW was selected for its superior thermal properties, good Raman gain coefficient and high damage threshold [10]. Second harmonic generation (SHG) of the 1176 nm Stokes line was obtained using a temperature controlled (~45 °C), 3x3x10 mm non-critically phase matched (NCPM, θ=90°, φ=0°) LBO crystal coated AR at 1064-1200 nm. Fig. 1. Schematic of cw Raman laser Nd:GdVO4
LBO
KGd(WO4)2
Laser diode
M2
M1
CW Laser Operation at 1176 nm (mirror set A) Threshold for lasing at the fundamental occurred for 0.7 W power from the laser diode, and we obtained Raman threshold for 6.6 W of incident pump power. Figure 2 shows the Stokes output as a function of pump power, together with the residual fundamental which becomes strongly depleted above threshold for SHG. Above threshold the 1176 nm firstStokes Raman power increased linearly with pump power, reaching 1563 mW from the output mirror for a maximum pump power of 20.4 W, limited by the onset of coating damage. Note also that the transmission of the input mirror was equal to that of the output mirror (0.4%T at 1176nm), thus approximately 1.5 W Raman power was lost from the input mirror. The low output coupler transmission at 1176 nm (0.4%) in comparison with the other resonator losses (estimated to be at least 1%) substantially limited the Raman laser output that could be obtained; clearly optimization of output coupling in relation to other losses would increase the output powers. CW Laser Operation at 588 nm (mirror set B) Nonlinear output coupling through frequency-doubling of the Stokes optical field is particularly well suited to extracting the Stokes optical field efficiently. Low thresholds are possible due to the high-Q cavity at both the fundamental and Stokes wavelengths), while at higher circulating fundamental and Stokes powers, losses are dominated by the nonlinear conversion to the visible, which is coupled from the resonator through a dichroic end-mirror.
1.6
Residual 1063nm Raman 1176nm
1.4 Output power ( W )
Instantaneous 588 nm power ( mW )
The cw yellow output power at 588 nm as a function of diode-pump power incident on the laser crystal in shown in Figure 3 (solid squares). At 15.5 W pump power the cw visible output was stable at 704 mW, with amplitude noise of approximately 12%. At higher pump powers, the visible output decreased and became unstable as the resonator approached the stability limit. We note a similar yellow power would have been lost either through the input mirror or would have been absorbed in the Nd:GdVO4 laser crystal (α588nm>2cm-1 ).
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Fig. 2 CW Raman and residual fundamental output power as a function of diode pump power.
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Fig. 3. CW and quasi-cw 588 nm output powers as a function of instantaneous diode power for CW and modulated (50% duty cycle) excitation.
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Australian Institute of Physics 17th National Congress 2006 – Brisbane, 3-8 December 2006 RiverPhys
Quasi-cw operation at 588 nm (50% duty cycle, mirror set B) Given that the minimum resonator length (62 mm) is set by the physical size of the crystals, maximum pump power is limited by the thermal lensing in the laser crystal. We have therefore undertaken experiments to test the potential for generating higher cw yellow powers by operating the diode pump at reduced (50%) duty cycle (specifically using a mechanical chopper inserted in the pump beam path giving a 200 Hz square-wave pump train). The reduced thermal load in the laser crystal resulted in a thermal lens of approximately twice the focal length for the same instantaneous power in fully-CW mode. Figure 3 also shows 588 nm output in this regime (50% duty cycle); instantaneous power of 1574 mW was obtained for an instantaneous diode pump power of 22.4 W. Operation at 588nm using mirror sets A and B Laser performance at 588 nm has been fully investigated using both mirror sets A and B. Mirror set B provided a cavity with much higher cavity Q for both the fundamental and Stokes optical fields, and resulted in substantially lower thresholds and higher output powers, the results of which are summarised in Table 1 for the cases of cw and quasi-cw operation. The output powers obtained using mirror set B are approximately twice those obtained with mirror set A. From transmission spectra of the three crystals, we estimate the round-trip fundamental and Stokes resonator losses to be around 1.4 and 2.0% using mirror set A, and around 1.2% and 1.2% using mirror set B. However we also note that the true resonator loss is hard to determine in such a short cavity where the elements are aligned at near-normal incidence in order to maximize the output power. Table 1: Diode powers for SRS threshold and maximum yellow output power using mirror sets A and B
Mirrors Set A Set B
Cw threshold (W) 6.9 2.5
Max cw power (mW) 320 (17.6W from diode) 704 (15.5W from diode)
Quasi-cw threshold 7.0 (W) 2.5 (W)
Max quasi-cw power (mW) 700 (21.6W from diode) 1574 (22.4W from diode)
Numerical Modelling of the cw yellow laser The cw yellow laser involves three simultaneous optical processes; generation of fundamental radiation in the laser crystal, SRS in the Raman crystal, and second harmonic generation (SHG) is a doubling crystal. Efficient laser operation requires each of these optical processes to be optimized simultaneously. There are a lot of complex effects that determine the subtle behaviour of the system, including high order transverse modes, thermal lensing, spectral effects, and polarization effects amongst others. In the work presented here, we use a simple model in order to determine the trends that underlie this complex behaviour. Such a model does not aim to give perfect agreement with experimental measurements, but aims to aid our understanding of the basic interplay between the key design parameters of the Raman laser. As such, it is a valuable tool for exploring the regimes in which efficient operation is feasible. The model solves the rate equations which describe fundamental, first Stokes and second harmonic optical fields in the steady-state regime. We model beams with top hat transverse profiles; no account of cavity modes is made, but rather the behaviour of the laser is calculated for specific spot radii in each of the active elements in the cavity. The spot radius can be different in different elements, but is assumed to be constant within each element. The pump radiation is also considered to be focused to a constant radius in the laser crystal. By calculating the efficiency of the laser operation as all of these parameters are varied, we can study the importance and sensitivity of parameters and sets of parameters. The key assumptions are as follows. We assume that the pump energy is completely absorbed by the laser crystal (an accurate assumption for most experiments) and a perfect overlap of the pumped volume with the laser mode. We also ignore any phase mismatch in the doubling crystal and ignore any depolarisation effects between the gain and nonlinear media. It should also be noted that the doubled stokes radiation is not resonated in the cavity. Figure 4 shows the calculated conversion efficiency of the yellow laser with 1.2% round-trip loss for the fundamental and stokes fields based loosely on the laser design described above. The diode pump power incident on the laser crystal was 15 W. For the estimated spot sizes of 120 µm in the KGW crystal and 100 µm in the LBO crystal the model predicts an efficiency of 8%, somewhat higher than the experimentally measured 4.5%. The discrepancy is attributed to the modematching assumptions of the model and uncertainty in spot sizes. More importantly, it can be seen in figure 4, that to achieve more efficient operation, we need to decrease the spot size in both the Raman and doubling crystals as much as possible. A balance between the two is also required: for example if the conversion through SHG is too high compared to the Raman conversion we see a decrease in efficiency, as the doubling crystal presents too high a loss to the stokes field, preventing sufficient build up of the stokes field for efficient conversion from the fundamental field to the stokes field. On the other hand if the conversion through SRS is far too high, we see a decrease in the efficiency as the output coupling Paper No. WC016
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Australian Institute of Physics 17th National Congress 2006 – Brisbane, 3-8 December 2006 RiverPhys
presented to the stokes field exceeds its optimum value. Increasing the Raman and doubling parameters together allows us to increase the output coupling for the Stokes radiation whilst still maintaining an appropriate output coupling for fundamental photons. Figure 5 shows the calculated efficiency of the yellow laser as a function of losses for stokes and fundamental fields (including mirror transmission and reflection losses from the three AR-coated crystals). The approximate operating points using mirror sets A and B are shown in the figure, and are predicted to have efficiencies of around 3% and 8% respectively, once again somewhat higher than measured efficiencies of around 1.8% and 4.5%. Nevertheless, the trend clearly agrees with experimental observations. The modelling shows that substantial improvements to efficiency can be anticipated through further reduction in resonator losses, in particular at the Stokes wavelengths as well as through a decrease in the mode size in the Raman and doubling crystals. 4.0
Fundamental loss (%)
Raman spot radius (µm)
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175 150 125
6% 100 75
10%
50 25
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D o u b lin g s p o t ra d iu s ( µ m ) Figure 4: Calculated yellow laser efficiency vs spot sizes in the KGW and LBO crystals. (1.2% rt losses for fundamental and Stokes fields, 150 um spot in laser crystal, 15 W pump power.)
27% 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Raman loss (%) Figure 5: Calculated yellow laser efficiency vs the fundamental and Stokes round trip losses. (150 µm spot in the laser crystal, 120 µm spot size in KGW Raman crystal, 100 µm spot size in LBO, 15 W pump power.)
Conclusions We have demonstrated an all-solid-state continuous-wave yellow laser which generates 1.56 W output at 1176 nm, and, by intracavity frequency doubling with LBO, 680 mW at 588 nm. Major improvements in output power were obtained by reducing the fundamental and Stokes resonator losses using new mirrors, and numerical modelling of the laser shows that further improvements to efficiency would result with further reductions, particularly to the Stokes losses. These could be achieved for example by applying the HR coatings directly to the laser and doubling crystals, or by diffusion bonding the crystals. Quasi-cw yellow powers as high as 1.5 W were achieved when the diode was operated at 50% duty cycle. This, combined with the dependence of laser efficiency on pump power, suggests that with an improved resonator design in particular incorporating the thermal loading of the laser crystal in order to achieve smaller spot sizes in the Raman and doubling crystals, would strongly improve the laser efficiency. An improved resonator design would also collect the yellow photons generated in both directions, as in the present design, around half the generated yellow photons are lost by absorption by the laser crystal or through the input mirror. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
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