- Email: [email protected]
Efficient intra-cavity second harmonic generation by a diode-pumped actively Q-switched Nd:YAG laser
Optics & Laser Technology,
Vol. 29, No. 6. pp. 317-319. 8
Printed
in Great
1998 Elsevier Britain.
All rights reserved
0030.-3992/97 PII:
1997
Science Ltd $17.00 + 0.00
0030-3992(97)00022-4
ELSEVIER
Efficient intra-cavity second harmonic generation by a diode-pumped actively Q-switched Nd :YAG laser A. AGNESI, G. C. REALI, C. SOLCIA, P. G. GOBBI A compact and efficient intra-cavity doubled, actively Q-switched diode-pumped Nd :YAG laser has been developed. It generates as much as 1.55 W at 532 nm, with oulses of 400 ns at a reoetition rate of 15 kHz. with an overall ootical-to-ootical efficiency of 15.5%. A beam quality factor M2 ‘e 2.35 has been measured at this power level. @ 1998 Elsevier Science Ltd. KEYWORDS:
lasers (diode-pumped),
active Q-switching,
Introduction
intra-cavity
doubling
Nd :YAG laser, pumped by a fibre coupled 10 W diode array, which generates up to 1.55 W at 532 nm.
Considerable interest in building either CW or pulsed, compact, visible all-solid-state lasers for medical applications, covering a range from low to 2-3 W average power, has developed over the last few years (see, for example, Ref. 1 for a review). In fact, this power range seems to be the most appropriate to cover several demands for ophthalmic, dentistry, pneumology and dermatology applications. Replacing low-efficiency and cumbersome high-power CW ion lasers, which are required for some of these applications, with robust and compact semiconductor laser-pumped, frequency doubled solid-state lasers of comparable power is one of the aims of the current medical laser source development. Recently, high power laser sources based on the concept of intracavity frequency doubling have been reported. Lasers delivering more than 5 W CW optical power2.3 are, in fact, commercially available now. A 30 W diode-pumped Q-switched laser, giving 3.5 W average power in the green wavelength has also been reported recently4. A difficulty faced in the realization of these sources is the problem of carefully controlling the pump-induced thermal effects in the active medium, which are a main cause of laser spatial instability, and thus of small frequency conversion.
Experimental
results
and discussion
We designed the resonator to meet the following criteria: high efficiency, compactness, low misalignment sensitivity and good beam quality. The layout of the laser is shown schematically in Fig. 1. We employed a fibre-coupled diode array (Spectra Diode Labs, model SDL-3450-P5) emitting 10 W at 808 nm to pump longitudinally a Nd : YAG crystal, 5 mm diameter x 7mm length, through a dielectric coating acting as a high reflectivity mirror at 1064 nm and with high transmission at 808 nm. The second face of the Nd:YAG rod was antireflection coated at 1064 nm. The acousto-optic modulator (AOM) was a fused-silica Brewster slab 23 mm long (NEOS Technologies, model N36027-5-0.8-BR). The cavity was folded at the concave mirror Mt (radius of curvature ROC = 100 mm), which had high reflectivity at 1064 nm, and a transmittance ~90% at 532 nm. The small, ~7” folding angle did not introduce a significant astigmatism of the cavity. The flat
For ophthalmic surgery, a laser power exceeding 2 W is not generally required; instead, great concern is given to compactness and efficiency features. In this paper we present the design and the characterization of an efficient kHz actively Q-switched and intracavity doubled AA, GCR and CS are in INFM-Dipartimento di Elettronica, Universita di Pavia, Via Ferrata I-271 00 Pavia, Italy. Fax : +39-382.422583; e-mail: [email protected]. PGG is at Laser Medicine Research, lstituto Scientific0 Ospedale S. Raffaele, Via Olgettina 60-20132 Milano, Italy. Received 18 February 1997. Accepted 7 May 1997.
Fig. 1 Schematic view of the laser. PL: fibre-coupled laser diode-pump; FO: focusing optics: LC: Nd:YAG laser crystal; AO: acusto-optic modulator; Mr : concave mirror, radius of curvature = 100 mm, R z- 99.8% at 1064 nm, T =z 90% at 532 nm; M2: flat end-mirror, HR at 1064 and 532 nm
317
318
lntracavity
doubled
Q-switched
diode-pumped
Nd: YAG laser: A. Agnesi et al.
mirror MZ was coated for high reflectivity at both 1064 nm and 532 nm. We chose a 5 mm long KTP crystal, cut for Type II second harmonic generation (SHG) at 532 nm, as the non-linear doubling crystal because it has a high non-linear coefficient, it does not show photorefractive damage at our power levels, and it is relatively inexpensive.
means that the usable green power was ~62% available CW infrared power.
The resonator was designed principally to provide an easy matching of the cavity mode to the pump volume (the pump radiation was confined into a spot of ~300 urn radius) through a proper adjustment of the position of MI, which necessarily has to be changed with the pump power due to the power-dependent thermal lensing in the Nd : YAG rod. A detailed characterization of the thermal lensing produced by our pumping system has been reported elsewhere’.
The beam quality, measured by knife edge scanning, was close to the diffraction limit for an output power up to ~1.3 W (M* < 1.5), and only moderately deteriorates at the highest pump powers (M* z 2.35). However, this detail is not critical considering that this source has been developed for medical applications and the laser output is generally fibre-coupled for easy handling and delivery.
With the chosen lengths of the cavity arms, 1 = 15 cm and d z 5 cm, the laser mode radii ( l/e2 of the intensity peak) in the rod and on mirror M2 were ~300 urn and ~70&80 urn, respectively. The cavity was always operated close to the centre of its stability region, which, for each pump power level, allowed it to have a low misalignment sensitivity. The KTP crystal was located ~8 mm from Mz, still within the Rayleigh range of the focused cavity mode. With the laser operating CW, we achieved about 340 mW in a nearly TEMoo mode (the KTP crystal did not have the optimal length for the best CW doubling with the chosen resonator geometry), and furthermore we could select a single longitudinal mode because of the Lyot filter realized by the birefringent non-linear crystal and the Brewster cut AOM5,6. The green output power notably improved when the AOM was turned on, allowing for kHz repetition rate Q-switching. In this case, the laser operated with many longitudinal modes (due to the pulse build-up time being too short to select effectively the single axial mode). Either external injection seeding or an electronically controlled Q-switch injection can be provided whenever single longitudinal mode operation becomes an issue. We observed that the KTP orientation for phasematching in the Q-switching operation was significantly altered with respect to the CW regime. This can be attributed to the different average power level circulating in the cavity, which produces a different bias of the angular orientation due to the small crystal absorption at 1064 nm and at 532 nm. We got the best results operating at 15 kHz pulse frequency. Figure 2 shows the output power and the measured beam quality factor at each pump level. As much as 1.55 W were obtained at 10 W pump power through mirror Ml, with an overall 15.5% optical-tooptical conversion. (Eliminating the measured 10% Fresnel losses at 532 nm of this not optimized component, we could estimate a performance z 1.7 W of green radiation from the KTP crystal, enhancing the conversion efficiency to a significant 17%.) We notice that the best CW power at 1064 nm that could be extracted by our laser when properly coupled (~90% reflectivity output coupler), was ~2.5 W: this
of the
The pulse-to-pulse short term stability (over a few minutes timescale) is cl + lo!, while the fluctuations of the average power over hours of operation have been measured to be f 3%.
The Q-switched pulse duration increased from ~200 ns to ~400 ns at the maximum pump power. Both the pulse lengthening and the increased beam divergence are likely to be due to the significantly increased non-linear loss experienced by the pulse peak at the highest powers, and to back-conversion processes. Indeed, this explanation is also suggested by the fact that the beam quality deteriorates with respect to CW operation, where the diffraction-limit is maintained for the whole pumping range. We did not observe any crystal damage in these experiments, and the laser performance was reproducible, on a day-to-day base, without cavity readjustments. Some different applications, like ultraviolet-generation or pumping of optical parametric oscillators, would require shorter and more intense pulses, maintaining the average power available with this design, and also possibly improving the beam quality at high power levels. Detailed analysis of the resonator Q-switching spatio-temporal dynamics, considering the intra-cavity SHG process and jts angular selectivity under strong depletion conditions, would be required, and this represents the next step in our research.
Conclusions We have developed and characterized a compact intra-cavity doubled, diode-pumped Nd : YAG laser generating x1.55 W average power at 532 nm, with Q-switching pulses at a 15 kHz rate. The high efficiency (15.5% optical-to-optical) and compactness of this 2.0~..,.,....,....,....
1.5
z
3
,,...
2.5
-
l.O-
a” 0.5
r
0.0
Fl.0
0
2
4
6
8
10
pin WI Fig. 2 Output power at 532 nm versus input pump power at 808 nm (as delivered from the diode). In the same figure: beam quality factor M2 versus input pump power
lntracavity
doubled
Q-switched
diode-pumped
all-solid-state source make it attractive as a tool for ophthalmic surgery, but it could be useful in other applications as well. 3
Further improvements are still possible, for example optimizing the intra-cavity optics (mirror MI), by a more accurate nonlinear crystal arrangement, or taking the more radical action of replacing the Nd:YAG laser material with the more efficient Nd:YVOd and using a more efficient non-linear crystal.
4
5
References 1 Pratesi, R. (Ed). Luser L’lucc in chirurgicr e nwdicinrr ed irz 2
hiotecnologiu, CNR, Firenze (1995) Selker, M.D., Johnston, T.J., Frangineas, G., Nightingale, J.L., Negus, D.K. > 8.5 watts of single frequency 532 nm light from a
6
Nd: YAG laser: A. Agnesi
et al.
319
diode pumped intra-cavity doubled ring laser, in Cmferencr on Lusers unci Electra-Optics, Vol 9. 1996 OSA Technical Digest Series (Optical Society of America, Washington DC), paper CPD2 I Nighan, W.L., Hutchinson, S.B., Dudley, D., Keirstead, M.S. Highly efficient, diode-bar-end-pumped Q-switched Nd: YAG laser with I1 W TEMoo output. in Conjinwce on Lasers md EIecrro-Optics, Vol 9. 1996 OSA Technical Digest Series (Optical Society of America. Washington DC) paper CWN?. Hemmati, H., Lesh, J.R. 3.5 W Q-switched 532 nm Nd:YAG laser pumped with fiber-coupled diode lasers. Opt Lrtt, 19 (1994) 1322-1324 Agnesi, A., Dell’Acqua, S., Reali, G.C., Gohhi, P.G., Ragazzi, D. Design and characterization of a diode pumped. single longitudinal and transverse mode. intra-cavity doubled CW Nd:YAG laser, Appl Opt, 36 (1997) 597-601 Nagai, H., Kume, M., Ohta, I., Shimizu, G., Kazumura, M. Lownoise operation of a diode-pumped intracavity-doubled Nd : YAG laser using a Brewster plate, IEEE J Qum/ Electrm. 28 (1992) 1164-1167
Optics & Laser Technology
Vol. 29, No. 6. pp. 317-319. 8
Printed
in Great
1998 Elsevier Britain.
All rights reserved
0030.-3992/97 PII:
1997
Science Ltd $17.00 + 0.00
0030-3992(97)00022-4
ELSEVIER
Efficient intra-cavity second harmonic generation by a diode-pumped actively Q-switched Nd :YAG laser A. AGNESI, G. C. REALI, C. SOLCIA, P. G. GOBBI A compact and efficient intra-cavity doubled, actively Q-switched diode-pumped Nd :YAG laser has been developed. It generates as much as 1.55 W at 532 nm, with oulses of 400 ns at a reoetition rate of 15 kHz. with an overall ootical-to-ootical efficiency of 15.5%. A beam quality factor M2 ‘e 2.35 has been measured at this power level. @ 1998 Elsevier Science Ltd. KEYWORDS:
lasers (diode-pumped),
active Q-switching,
Introduction
intra-cavity
doubling
Nd :YAG laser, pumped by a fibre coupled 10 W diode array, which generates up to 1.55 W at 532 nm.
Considerable interest in building either CW or pulsed, compact, visible all-solid-state lasers for medical applications, covering a range from low to 2-3 W average power, has developed over the last few years (see, for example, Ref. 1 for a review). In fact, this power range seems to be the most appropriate to cover several demands for ophthalmic, dentistry, pneumology and dermatology applications. Replacing low-efficiency and cumbersome high-power CW ion lasers, which are required for some of these applications, with robust and compact semiconductor laser-pumped, frequency doubled solid-state lasers of comparable power is one of the aims of the current medical laser source development. Recently, high power laser sources based on the concept of intracavity frequency doubling have been reported. Lasers delivering more than 5 W CW optical power2.3 are, in fact, commercially available now. A 30 W diode-pumped Q-switched laser, giving 3.5 W average power in the green wavelength has also been reported recently4. A difficulty faced in the realization of these sources is the problem of carefully controlling the pump-induced thermal effects in the active medium, which are a main cause of laser spatial instability, and thus of small frequency conversion.
Experimental
results
and discussion
We designed the resonator to meet the following criteria: high efficiency, compactness, low misalignment sensitivity and good beam quality. The layout of the laser is shown schematically in Fig. 1. We employed a fibre-coupled diode array (Spectra Diode Labs, model SDL-3450-P5) emitting 10 W at 808 nm to pump longitudinally a Nd : YAG crystal, 5 mm diameter x 7mm length, through a dielectric coating acting as a high reflectivity mirror at 1064 nm and with high transmission at 808 nm. The second face of the Nd:YAG rod was antireflection coated at 1064 nm. The acousto-optic modulator (AOM) was a fused-silica Brewster slab 23 mm long (NEOS Technologies, model N36027-5-0.8-BR). The cavity was folded at the concave mirror Mt (radius of curvature ROC = 100 mm), which had high reflectivity at 1064 nm, and a transmittance ~90% at 532 nm. The small, ~7” folding angle did not introduce a significant astigmatism of the cavity. The flat
For ophthalmic surgery, a laser power exceeding 2 W is not generally required; instead, great concern is given to compactness and efficiency features. In this paper we present the design and the characterization of an efficient kHz actively Q-switched and intracavity doubled AA, GCR and CS are in INFM-Dipartimento di Elettronica, Universita di Pavia, Via Ferrata I-271 00 Pavia, Italy. Fax : +39-382.422583; e-mail: [email protected]. PGG is at Laser Medicine Research, lstituto Scientific0 Ospedale S. Raffaele, Via Olgettina 60-20132 Milano, Italy. Received 18 February 1997. Accepted 7 May 1997.
Fig. 1 Schematic view of the laser. PL: fibre-coupled laser diode-pump; FO: focusing optics: LC: Nd:YAG laser crystal; AO: acusto-optic modulator; Mr : concave mirror, radius of curvature = 100 mm, R z- 99.8% at 1064 nm, T =z 90% at 532 nm; M2: flat end-mirror, HR at 1064 and 532 nm
317
318
lntracavity
doubled
Q-switched
diode-pumped
Nd: YAG laser: A. Agnesi et al.
mirror MZ was coated for high reflectivity at both 1064 nm and 532 nm. We chose a 5 mm long KTP crystal, cut for Type II second harmonic generation (SHG) at 532 nm, as the non-linear doubling crystal because it has a high non-linear coefficient, it does not show photorefractive damage at our power levels, and it is relatively inexpensive.
means that the usable green power was ~62% available CW infrared power.
The resonator was designed principally to provide an easy matching of the cavity mode to the pump volume (the pump radiation was confined into a spot of ~300 urn radius) through a proper adjustment of the position of MI, which necessarily has to be changed with the pump power due to the power-dependent thermal lensing in the Nd : YAG rod. A detailed characterization of the thermal lensing produced by our pumping system has been reported elsewhere’.
The beam quality, measured by knife edge scanning, was close to the diffraction limit for an output power up to ~1.3 W (M* < 1.5), and only moderately deteriorates at the highest pump powers (M* z 2.35). However, this detail is not critical considering that this source has been developed for medical applications and the laser output is generally fibre-coupled for easy handling and delivery.
With the chosen lengths of the cavity arms, 1 = 15 cm and d z 5 cm, the laser mode radii ( l/e2 of the intensity peak) in the rod and on mirror M2 were ~300 urn and ~70&80 urn, respectively. The cavity was always operated close to the centre of its stability region, which, for each pump power level, allowed it to have a low misalignment sensitivity. The KTP crystal was located ~8 mm from Mz, still within the Rayleigh range of the focused cavity mode. With the laser operating CW, we achieved about 340 mW in a nearly TEMoo mode (the KTP crystal did not have the optimal length for the best CW doubling with the chosen resonator geometry), and furthermore we could select a single longitudinal mode because of the Lyot filter realized by the birefringent non-linear crystal and the Brewster cut AOM5,6. The green output power notably improved when the AOM was turned on, allowing for kHz repetition rate Q-switching. In this case, the laser operated with many longitudinal modes (due to the pulse build-up time being too short to select effectively the single axial mode). Either external injection seeding or an electronically controlled Q-switch injection can be provided whenever single longitudinal mode operation becomes an issue. We observed that the KTP orientation for phasematching in the Q-switching operation was significantly altered with respect to the CW regime. This can be attributed to the different average power level circulating in the cavity, which produces a different bias of the angular orientation due to the small crystal absorption at 1064 nm and at 532 nm. We got the best results operating at 15 kHz pulse frequency. Figure 2 shows the output power and the measured beam quality factor at each pump level. As much as 1.55 W were obtained at 10 W pump power through mirror Ml, with an overall 15.5% optical-tooptical conversion. (Eliminating the measured 10% Fresnel losses at 532 nm of this not optimized component, we could estimate a performance z 1.7 W of green radiation from the KTP crystal, enhancing the conversion efficiency to a significant 17%.) We notice that the best CW power at 1064 nm that could be extracted by our laser when properly coupled (~90% reflectivity output coupler), was ~2.5 W: this
of the
The pulse-to-pulse short term stability (over a few minutes timescale) is cl + lo!, while the fluctuations of the average power over hours of operation have been measured to be f 3%.
The Q-switched pulse duration increased from ~200 ns to ~400 ns at the maximum pump power. Both the pulse lengthening and the increased beam divergence are likely to be due to the significantly increased non-linear loss experienced by the pulse peak at the highest powers, and to back-conversion processes. Indeed, this explanation is also suggested by the fact that the beam quality deteriorates with respect to CW operation, where the diffraction-limit is maintained for the whole pumping range. We did not observe any crystal damage in these experiments, and the laser performance was reproducible, on a day-to-day base, without cavity readjustments. Some different applications, like ultraviolet-generation or pumping of optical parametric oscillators, would require shorter and more intense pulses, maintaining the average power available with this design, and also possibly improving the beam quality at high power levels. Detailed analysis of the resonator Q-switching spatio-temporal dynamics, considering the intra-cavity SHG process and jts angular selectivity under strong depletion conditions, would be required, and this represents the next step in our research.
Conclusions We have developed and characterized a compact intra-cavity doubled, diode-pumped Nd : YAG laser generating x1.55 W average power at 532 nm, with Q-switching pulses at a 15 kHz rate. The high efficiency (15.5% optical-to-optical) and compactness of this 2.0~..,.,....,....,....
1.5
z
3
,,...
2.5
-
l.O-
a” 0.5
r
0.0
Fl.0
0
2
4
6
8
10
pin WI Fig. 2 Output power at 532 nm versus input pump power at 808 nm (as delivered from the diode). In the same figure: beam quality factor M2 versus input pump power
lntracavity
doubled
Q-switched
diode-pumped
all-solid-state source make it attractive as a tool for ophthalmic surgery, but it could be useful in other applications as well. 3
Further improvements are still possible, for example optimizing the intra-cavity optics (mirror MI), by a more accurate nonlinear crystal arrangement, or taking the more radical action of replacing the Nd:YAG laser material with the more efficient Nd:YVOd and using a more efficient non-linear crystal.
4
5
References 1 Pratesi, R. (Ed). Luser L’lucc in chirurgicr e nwdicinrr ed irz 2
hiotecnologiu, CNR, Firenze (1995) Selker, M.D., Johnston, T.J., Frangineas, G., Nightingale, J.L., Negus, D.K. > 8.5 watts of single frequency 532 nm light from a
6
Nd: YAG laser: A. Agnesi
et al.
319
diode pumped intra-cavity doubled ring laser, in Cmferencr on Lusers unci Electra-Optics, Vol 9. 1996 OSA Technical Digest Series (Optical Society of America, Washington DC), paper CPD2 I Nighan, W.L., Hutchinson, S.B., Dudley, D., Keirstead, M.S. Highly efficient, diode-bar-end-pumped Q-switched Nd: YAG laser with I1 W TEMoo output. in Conjinwce on Lasers md EIecrro-Optics, Vol 9. 1996 OSA Technical Digest Series (Optical Society of America. Washington DC) paper CWN?. Hemmati, H., Lesh, J.R. 3.5 W Q-switched 532 nm Nd:YAG laser pumped with fiber-coupled diode lasers. Opt Lrtt, 19 (1994) 1322-1324 Agnesi, A., Dell’Acqua, S., Reali, G.C., Gohhi, P.G., Ragazzi, D. Design and characterization of a diode pumped. single longitudinal and transverse mode. intra-cavity doubled CW Nd:YAG laser, Appl Opt, 36 (1997) 597-601 Nagai, H., Kume, M., Ohta, I., Shimizu, G., Kazumura, M. Lownoise operation of a diode-pumped intracavity-doubled Nd : YAG laser using a Brewster plate, IEEE J Qum/ Electrm. 28 (1992) 1164-1167
Optics & Laser Technology