Experimental Results

β γ σ

σ

>> − +

+ 1) 1

ln(

1 ) ln(

1 ) ln(

02 02

s gs

A A R L

T

T (6-3)

where T₀ is the initial transmission of the saturable absorber, A /A_S is the ratio of the effective area in the gain medium and in the saturable absorber, R is the reflectivity of the output mirror, Lis the non-saturable intracavity round-trip dissipative optical loss, σ is the ground-state absorption cross-section of the saturable absorber, _gs σ is _γ the stimulated emission cross-section of the gain medium, γ is the inversion reduction factor with a value between 0 and 2 as discussed in [33], and β is the ratio of the excited-state absorption cross-section to that of the ground-state absorption in the saturable absorber. Since the σ value of the Nd:YVO4 crystal (25·10^-19cm²) is comparable to the σgs value of the Cr⁴⁺:YAG crystal (~(20±5)·10^-19cm² [34]),the ratio A/A_S =(ω₁/ω₂)² generally needs to be greater than 10 for good passively Q-switching. As seen in Fig. 6-2, this criterion can be satisfied in the present cavity for pump power higher than 10 W. In other words, the experimental result consists very well with the theoretical analysis.

6.5 Experimental Results

Fig.6-3 shows the average output power at 1573 nm with respect to the incident pump power. For all pump powers the beam quality M² factor was found to be less than 1.3. The average output power reached 1.56 W at an incident pump power of 14.5

was 10.8%. To the best of our knowledge, this is highest efficiency for average power conversion. The pulse temporal behavior at 1064 and 1573 nm was recorded by a LeCroy 9362 digital oscilloscope (500 MHz bandwidth) with a fast germanium photodiode. An oscilloscope trace of a train of the signal pulses is shown in the inset of Fig. 6-3. The pulse-to-pulse amplitude fluctuation was found to be within ±10%

.Fig.6-4 depicts the pulse repetition rate and the pulse energy at 1573 nm versus the incident pump power. It is seen that the pulse repetition rate is proportional to the incident pump power and up to 58.5 kHz at an incident pump power of 14.5 W. On the other hand, the pulse energy initially increases with pump power, and is almost saturated beyond 10 W of the incident pump power.

A typical temporal shape for the laser and signal pulses is shown in the inset of Fig.6-4. It can be seen that the several satellite peaks accompany the main pulse whose pulse width is approximately 2.5 ns. Although the present output coupler reflectivity (R_S= 85%) can lead to higher conversion efficiency, the stored energy is not fully extracted in a single output pulse. Since the remaining energy is sufficient to evolve the pump filed, the OPO threshold can be reached again and a second signal pulse is produced.

The energy in the satellites is about 30–40% of the total output energy. The output energy of the main pulse is estimated to be in the order of 15μJ at the pump power higher than 10 W. Therefore, the overall peak power can be higher than 5 kW. To produce a single pulse output, the OPO output reflectivity needs to be reduced to <75%.

As shown in Fig. 6-5, a single pulse can be generated with a signal reflectivity of 70%

on the output coupler. Experimental results reveal that the maximum conversion efficiency can be obtained with an output coupler of 85–90% at the sacrifice of peak power. If the high peak power is desired, the output reflectivity needs to be around 60–70%.

6.6 Summary

In summary, a high-power efficient diode-pumped passively Q-switched Nd:YVO4/KTP/ Cr4þ:YAG eye-safe laser has been demonstrated by using a saturable

absorber Cr4þ:YAG was coated as an output coupler of the OPO cavity to enhance the performance of passive Q-switching. Considering the thermal lensing effects, the cavity length was designed to allow mode matching with the pump beam and to provide the proper spot size in the saturable absorber. Consequently, the average output power at 1573 nm can amount to 1.56 W with a pulse repetition rate of 58.5 kHz and the peak power >5 kW at an incident pump power of 14.5 W.

References

[1] R.C. Stoneman, L. Esterowitz, in: Proceedings of the Conference on Advanced Solid State Lasers, Optical Society of America, Washington, DC, 1990, p. 176, paper WC5-2.

[2] P.F. Moulton, IEEE J. Quantum Electron. 21 (1985) 1582.

[3] P.F. Moulton, A. Mooradian, Appl. Phys. Lett. 35 (1979)838.

[4] S. K€uck, K. Petermann, U. Pohlmann, G. Huber, Phys. Rev. B 51 (1995) 17323.

[5] A. Sennaroglu, Prog. Quantum Electron. 26 (2002) 287.

[6] B. Schmaul, G. Huber, R. Clausen, B. Chai, P. LikamWa,M. Bass, Appl. Phys.

Lett. 62 (1993) 541.

[7] M.V. Kuleshov, A.A. Lagatsky, A.V. Podlipensky, V.P.Mikhailov, A.A.

Kornienko, E.B. Dunina, S. Hartung, G.Huber, J. Opt. Soc. Am. B 15 (1998) 1205.

[8] I. Sok olska, E. Heumann, S. Kück, T. Lukasiewicz, Appl.Phys. B 71 (2000) 893. ! [9] A. Diening, P.E.-A. Möbert, G. Huber, J. Appl. Phys. 84(1998) 5900.

[10] S. Taccheo, P. Laporta, S. Longhi, O. Svelto, C. Svelto,Appl. Phys. B 63 (1996) 425.

[11] K.V. Yumashev, I.A. Denisov, N.N. Posnov, V.P. Mikhailov,R. Moncorg e, D. ! Vivien, B. Ferrand, Y. Guyot, J.Opt. Soc. Am. B 16 (1999) 2189.

[12] K. Spariosu, R.D. Stultz, M. Birnbaum, T.H. Allik, J.A.Hutchinson, Appl. Phys.

Lett. 62 (1993) 2763.

[13] R.D. Stultz, M.B. Camargo, M. Birnbaum, J. Appl. Phys.78 (1995) 2959.

[14] R.D. Stultz, M.B. Camargo, M. Lawler, D. Rockafellow,M. Birnbaum, in: W.R.

Bosenberg, M.M. Fejer (Eds.),Advanced Solid State Lasers, OSA Trends in Optics and Photonics Series, vol. 19, Optical Society of America,Washington, DC, 1998, p. 155.

[15] R.D. Stultz, M.B. Camargo, S.T. Montgomery, M. Birnbaum,K. Spariosu, Appl.

Phys. Lett. 64 (1994) 948.

[16] M. Birnbaum, M.B. Camargo, S. Lee, F. Unlu, R.D.Stultz, in: C.R. Pollock, W.R.

Bosenberg (Eds.), AdvancedSolid State Lasers, OSA Trends in Optics and Photonics Series, vol. 10, Optical Society of America, Washington, DC, 1997, p.

148.

[17] R. Fluck, R. Häring, R. Paschotta, E. Gini, H. Melchior,U. Keller, Appl. Phys.

Lett. 72 (1998) 3273.

[18] J. Falk, J.M. Yarborough, E.O. Ammann, IEEE J.Quantum Electron. 7 (1971) 359.

[19] R.S. Conroy, C.F. Rae, G.J. Friel, M.H. Dunn, B.D.Sinclair, J.M. Ley, Opt. Lett.

23 (1998) 1348.

[20] A.R. Geiger, H. Hemmati, W.H. Farr, N.S. Prasad, Opt.Lett. 21 (1996) 201.

[21] O.B. Jensen, T. Skettrup, O.B. Petersen, M.B. Larsen, J.Opt. A: Pure Appl. Opt. 4 (2002) 190.

[22] J.J. Zayhowski, C. Dill III, Opt. Lett. 19 (1994) 1427.

[23] Y.X. Bai, N. Wu, J. Zhang, J.Q. Li, S.Q. Li, J. Xu, P.Z.Deng, Appl. Opt. 36 (1997) 2468.

[24] C. Li, J. Song, D. Shen, N.S. Kim, J. Lu, K. Ueda, Appl.Phys. B 70 (2000) 471.

[25] Y.F. Chen, S.W. Chen, Y.C. Chen, Y.P. Lan, S.W. Tsai, Appl. Phys. B 77 (2003) 493.

[26] Y.F. Chen, T.M. Huang, C.F. Kao, C.L. Wang, S.C.Wang, IEEE J. Quantum Electron. 33 (1997) 1424.

[27] Y.F. Chen, IEEE J. Quantum Electron. 35 (1999)234.

[28] Y.F. Chen, T.M. Huang, C.C. Liao, Y.P. Lan, S.C. Wang,IEEE Photon. Technol.

Lett. 11 (1997) 1241.

[29] N. Hodgson, H. Weber, Optical Resonators, Springer-Verlag, Berlin, 1997.

1012.

[31] Y.F. Chen, S.W. Tsai, IEEE J. Quantum Electron. 37(2001) 586.

[32] Y.F. Chen, Y.P. Lan, H.L. Chang, J. Quantum Electron.37 (2001) 462.

[33] J.J. Degnan, D.B. Coyle, R.B. Kay, IEEE J. Quantum Electron. 34 (1998) 887.

[34] G. Xiao, J.H. Lim, S. Ynag, E.V. Stryland, M. Bass, L.Weichman, IEEE J.

Quantum Electron. 35 (1997)1086.

Fig.6-1 Schematic of the intracavity OPO pumped by a diodepumped passively Q-switched Nd:YVO4/Cr⁴⁺:YAG laser.

Fig.6-2 Calculation results for the dependence of the mode size ratios ω₁/ω_p and

1 2/ω

ω on the pump power for the present cavity configuration.

Fig.6-3 Dependence of the average output power at 1573 nm on the incident pump power. An oscilloscope trace of a train of the signal pulses is shown in the inset.

Fig.6-4 Dependence of the pulse repetition rate and the pulse energy at 1573 nm on the incident pump power. A typical temporal shape for the laser and signal pulses is shown in the inset.

Incident pump power at 808 nm (W)

0 3 6 9 12 15

Average output power at 1573 nm (W)

0.0 0.3 0.6 0.9 1.2 1.5 1.8

Incident pump power at 808 nm (W)

0 3 6 9 12 15

Pulse repetition rate (kHz);Pulse energy (µJ)

0 10 20 30 40 50 60 70

pulse energy pulse repetition rate

Fig. 6-5 Typical temporal shapes for the laser and signal pulses with a signal reflectivity of 70% on the output coupler.

Chapter 7 Conclusion and Future Work

This dissertation experimentally and analytically investigated the OPO design of several eye-safe solid-state lasers. In the active eye-safe lasers region, we experimentally accomplished an high-repetition-rate OPO by using a non-critically phase-matched KTP crystal intracavity pumped by an AO Q-switched Nd³⁺:YVO4 laser.

Operation of a singly resonant pulsed KTP intracavity OPO pumped by an AO Q-switched Nd:YVO4 laser has been demonstrated. Using a type II non-critically phase-matched x-cut KTP crystal, eye-safe signal radiation at 1573 nm was generated in a plane-parallel to the intracavity OPO resonator. It was found that the maximum pulse energy may saturate beyond a given above-threshold intracavity OPO factor, but increasing the signal reflectivity of the output coupler can efficiently increases the average output power. The conversion efficiency for the average power is up to 10.6%

from pump diode input to OPO signal output. The effective cavity dump of intracavity OPO leads to a relatively short signal pulse width for repetition rates from 10 to 80 kHz. As a consequence, the peak power for signal output can be up to several kilowatts for the entire frequency range. The compact size and high efficiency of the present laser make it an attractive source for practical applications.

The next topic was the study of the output optimization of a high-repetition-rate diode-pumped Q-switched intracavity optical parametric oscillator at 1573 nm with a type-II non-critically phase-matched x-cut KTP crystal. Numerical calculations have also been performed to confirm the experimental results. It was found that the maximum conversion efficiency can be obtained with an output coupler of 85%–90%

at the cost of peak power. If high peak power is desired, the output reflectivity needs to be around 60%–70%.

In the passive eye-safe lasers region ,the operation of a singly resonant pulsed KTP intracavity OPO pumped by a diode-pumped passively Q-switched Nd :YVO4

/Cr⁴⁺ :YAG laser was demonstrated. A saturable absorber Cr⁴⁺ : YAG was coated as an output coupler of the OPO cavity to constitute a realistic, inexpensive source of eye-safe nanosecond laser. The low threshold power permits the use of a relatively

low-power laser diode (2.5W). The conversion efficiency for the average power is up to 10.2% from pump diode input to OPO signal output. The effective cavity dump of intracavity OPO leads to the relatively short signal pulse width with high repetition rates. As a consequence, the signal peak power can exceed 1 kW with a pulse repetition rate of 62.5 kHz at an incident pump power of 2.5W

In the last, an efficient high-power diode-pumped passively Q-switched Nd:YVO4

/KTP/Cr⁴⁺:YAG eye-safe laser was accomplished by considering of the thermal lensing effects. The cavity length was designed to allow mode matching with the pump beam and provided the proper spot size in the saturable absorber. Consequently, the average output power at 1573 nm can amount to 1.56 W with a pulse repetition rate of 58.5 kHz and the peak power above 5 kW at an incident pump power of 14.5 W.

As the passively QS experimental results had reviled that shared cavity configuration has better stability performance, it will be worthwhile to apply the similar configuration to the actively Q-switched system and to use this configuration with PPLN to demonstrate the wavelength tunable IOPO configuration.

These compact and high-efficient eye-safe lasers introduce many potential applications studying in the future :

First, since they are more safe to human eyes, they can be developed to replace the traditional solid state lasers which have been used in commercial range finder and laser radar applications.

Second, after scaling up the output power, they can be used to remand the insufficiency of laser diode in the high-repetition-rate high-power application such as in laser indicator and active-imaging system.

Third, after militarizing these prototype system, they could be used for defense applications such as army tank or missile-borne or navy range finder, imaging system, etc.. Furthermore, the future study could improve the defense industry and promote the self-defense ability.