未來展望

我們已經成功的使用 two-mirror 在調節 PPLN 的波長，接下來可以再將 two-mirror 運用在調節 KTP 的角度，也就是從溫度的調節進一步到用角度的調 節。同樣是 two-mirror 架構，預期由於 KTP 上不需鍍上高反射率的膜，故對於 晶體的傷害將會降低，同樣的對於穩定度來說，也將會是一大的優勢，請參見圖 6.2-2。

另一方面，許多關於光學參數振盪器的背後物理意義更有許多待以討論研 究。

„

1.gain medium

Nd:YVO4& Nd:GdVO4

„

2.cavity

two-mirror & three-mirror

To our knowledge, using the two-mirror construction is

realized forthe first time

1. To our knowledge, two-mirror construction to tune PPLN is realized for the first time 2.Nd:Gd0.7Y0.3VO4 has the better efficiency than Nd:GdVO4 crystal

We get the much better efficiency by using the Nd:GdVO4 crystal

0.25%Nd:YVO4

0.27%Nd:GdVO4

0.2%Nd:Gd0.7Y0.3VO4

„

3.tunable laser by tuning temperatue

Nd:GdVO4

two-mirror

圖 6.2-1 實驗結果總結做總結。

Coupling lens

LaLasseerr && OPO ccaavviittyy

808nm 1:1

LD

Cr:YAG Nd:GdVO⁴ KTP

z

x y

圖 6.2-2 延著 xz 平面來轉動 KTP，就相當於改變了 KTP 的夾角θ ，由於相位匹 配的原因，轉動角度的過程也牽動了訊號光的波長輸出。

October 1, 2004 / Vol. 29, No. 19 / OPTICS LETTERS 2279

Diode-pumped passively Q-switched picosecond Nd:GD

Y

VO

self-stimulated Raman laser

Y. F. Chen, M. L. Ku, L. Y. Tsai, and Y. C. Chen

Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan

Received April 16, 2004

An efficiency of 8.2% is demonstrated for a diode-pumped passively Q-switched self-stimulated Raman laser with an a-cut mixed vanadate crystal, Nd:Gd0.8Y0.2VO4. At 2.2 W of incident pump power, the self-stimulated Raman laser produces pulses as short as 660 ps at a Stokes wavelength of 1175 nm with 2.7 mJ of energy per pulse at a 66-kHz repetition rate. © 2004 Optical Society of America

OCIS codes: 140.3480, 140.3550.

Although stimulated Raman scattering (SRS) in crystalline media has been proposed for more than 40 years,¹ the study of all-solid-state Raman lasers has undergone something of a renaissance in recent years owing to the discovery and development of new Raman crystals.^{2 – 7} Currently, the commonly used crystals for SRS are Ba共NO3兲2,⁸LiIO3,⁹ KGd共WO4兲2,¹⁰ and PbWO4.¹¹ Kaminskii et al.¹² recently proposed yttrium orthovanadate共YVO4兲 and gadolinium ortho-vanadate 共GdVO⁴兲 crystals as promising candidates for eff icient SRS. Combinations of their stimulated-emission and SRS properties make Nd-doped YVO4

and GdVO4crystals attractive self-SRS laser media.

More recently, a compact diode-pumped passively Q-switched self-stimulated Raman laser was success-fully demonstrated by use of a Cr⁴¹:YAG saturable absorber with a c-cut Nd:YVO4 or a c-cut Nd:GdVO4

crystal as the gain medium.^13,14 Experimental re-sults revealed that conventional a-cut Nd:YVO4 and Nd:GdVO4 crystals do not result in successful SRS because their large emission cross sections signif i-cantly limit energy storage capacities in passive Q-switching operation. However, the thermal lens power in c-cut Nd-doped vanadate crystals is ap-proximately three times larger than that in a-cut crystals because the thermo-optic coefficients dn兾dT and thermal expansion coefficients of the former are strongly dependent on orientation of the crystal axes. In other words, c-cut vanadate crystals are not appropriate for power scaling.¹⁵ To solve this problem, a new Nd-doped mixed vanadate crystal, Nd:GdxY12xVO4, was demonstrated by Liu et al. in 2003.^16,17 The passive Q-switching performance based on Nd:GdxY12xVO4 crystal was found to be substantially superior to that with either Nd:YVO4

or Nd:GdVO4 crystal. The signif icant improvement reveals the possibility that an a-cut Nd:GdxY12xVO4

crystal can be used as a self-SRS active medium in a passive Q-switching operation.

In this Letter we report, for the first time to our knowledge, an efficient picosecond diode-pumped pas-sively Q-switched self-stimulated Raman laser based on an a-cut Nd:GdxY12xVO4 crystal. At an incident pump power of 2.2 W, an average output power of 180 mW was obtained from a Nd:Gd0.8Y0.2VO4 crystal

tition rate of 66 kHz. The pulse width was generally shorter than 800 ps; consequently the maximum peak power was greater than 3.4 kW.

The experimental configuration for the diode-pumped passively Q-switched Nd:GdxY12xVO4兾 Cr⁴¹:YAG laser with self-frequency Raman conversion is depicted in Fig. 1. The cavity mirrors have special dichromatic coatings for efficient conversion at the first Stokes component. The active medium was a 1.0-at. % Nd³¹, 6-mm-long Nd:GdxY12xVO4 crystal.

Both sides of the laser crystal were coated for antire-f lection at 1064 nm 共R , 0.2%兲. The pump source was a 2.5-W 808-nm fiber-coupled laser diode with a core diameter of 200 mm and a numerical aperture of 0.16. A focusing lens with 16.5-mm focal length and 90% coupling eff iciency was used to reimage the pump beam into the laser crystal. The pump spot radius was ⬃100 mm. The input mirror was a 15-mm radius-of-curvature concave mirror with an-tiref lection coating at the diode wavelength on the entrance face 共R , 0.2%兲, a high-ref lection coating at the lasing wavelength 共R . 99.8%兲, and a high-transmission coating at the diode wavelength on the other surface 共T . 90%兲. The Cr⁴¹:YAG crystal had a thickness of 2 mm, with 70% initial transmission at 1064 nm. Both sides of the Cr⁴¹:YAG crystal were antiref lection coated at the fundamental wavelength 共R , 0.2%兲. The f lat output coupler had ref lectivities R . 99.8% at 1064 nm and R苷 55% at 1175 nm. The overall laser cavity length was approximately 10 mm.

Fig. 1. Schematic of a diode-pumped passively Q-switched self-stimulated Raman laser: HR, highly ref lective; HT,

2280 OPTICS LETTERS / Vol. 29, No. 19 / October 1, 2004 Spectral information on the laser was monitored by an optical spectrum analyzer (Advantest Q8381A). The spectrum analyzer, which employs a diffraction lattice monochromator, can be used for high-speed measure-ment of pulse light with a resolution of 0.1 nm. The pulse temporal behavior was recorded by a LeCroy digital oscilloscope (Wavepro 7100, 10 Gsamples兾s, 1-GHz bandwidth) with a fast p –i – n photodiode.

A series of Nd:GdxY12xVO4 crystals with several Gd compositions of x 苷 0, 0.2, 0.4, 0.6, 0.8, 1.0 were used in an experiment to investigate the qualif ica-tions for intracavity Raman conversion. First the cw performance of Nd:GdxY12xVO4 crystals was investigated by use of an output coupler with ref lec-tivity R 苷 80%. The difference in cw efficiency was found to be within 65%. Figure 2 shows the ratio s兾s^YVO4 as a function of Gd composition x, where s and sYVO₄ are the stimulated-emission cross sections of Nd:GdxY12xVO4 and Nd:YVO4, respectively. The solid curve is a least-squares fit to the experimental data by use of a polynomial function. It can be seen that there is a minimum stimulated-emission cross section somewhere in the range x 苷 0.6 0.8.

In an intracavity SRS experiment, however, only Nd:Gd0.8Y0.2VO4 and Nd:Gd0.6Y0.4VO4 crystals were found to result in self-stimulated Raman conversion;

no Raman conversion was observed in the other mixed vanadate crystals 共x 苷 0, 0.2, 0.4, 1.0兲. Even though the Stokes shift decreased from 882 to 890 cm²¹ when Gd composition x increased from 0 to 1.0, this small variation did not have a noticeable inf luence on SRS results. The variation of s with composition factor x, however, is believed not to be related to the crystal lattice constants because x-ray diffraction results¹⁸ have revealed that the lattice constants are linear with Gd composition x. As discussed in Ref. 16, the bowing effect of s in the Nd:GdxY12xVO4 crystals almost certainly comes from a modification of the local crystal-field environment. As the reduction of stimulated-emission cross section can eff iciently enhance passive Q-switching performance,^19,20 the superiority of Nd:Gd0.8Y0.2VO4 and Nd:Gd0.6Y0.4VO4

crystals for intracavity SRS comes mainly from the improvement of the energy storage capacity.

Figure 3 shows the average output power at a Stokes wavelength of 1175 nm with respect to the incident pump power from the laser diode. At 2.2 W of incident pump power, the maximum average output power reached 180 and 110 mW for Nd:Gd0.8Y0.2VO4

and Nd:Gd0.6Y0.4VO4 crystals, respectively. The conversion efficiency from diode laser input power to Raman output power was approximately 8.2% and 5% for Nd:Gd0.8Y0.2VO4and Nd:Gd0.6Y0.4VO4crystals, respectively. The present conversion efficiency is considerably higher than the ⬃0.7% obtained with Nd:KGd共WO4兲2 crystal.¹⁰

Figure 4 depicts the pulse-repetition rate versus the incident pump power for both self-stimulated Raman lasers. It can be seen that increasing the pump power increases the pulse repetition rate. The maximum repetition rate was as much as 66 and 38 kHz for Nd:Gd Y VO and Nd:Gd Y VO

energy versus the incident pump power for both self-stimulated Raman lasers. As the self-stimulated-emission cross section of Nd:Gd0.8Y0.2VO4 crystal is somewhat larger than that of Nd:Gd0.6Y0.4VO4 crystal, the SRS pulse energy produced by use of Nd:Gd0.8Y0.4VO4

Fig. 2. Ratio s兾sYVO4 as a function of Gd composition:

experimental results from Ref. 16 (open squares) and the present study (open triangles); a least-squares f it to experi-mental data by use of a polynomial function (solid curve).

Fig. 3. Average output power and pulse repetition rate at a Stokes wavelength of 1175 nm with respect to the inci-dent pump power.

Fig. 4. Pulse repetition rate versus incident pump power

October 1, 2004 / Vol. 29, No. 19 / OPTICS LETTERS 2281

Fig. 5. Pulse energy versus the incident pump power for self-stimulated Raman lasers.

Fig. 6. Typical oscilloscope traces for fundamental and Raman pulses.

crystal is slightly lower than that by Nd:Gd0.6Y0.4VO4

crystal. Even so, Nd:Gd0.8Y0.4VO4crystal is superior to Nd:Gd0.6Y0.4VO4 crystal in overall Raman conver-sion eff iciency, as shown in Fig. 3.

Typical time traces for the fundamental and Raman pulses are shown in Fig. 6. The width of the Raman pulse was found to be nearly the same for both lasers and to be insensitive to the pump power; its value was approximately 600 –800 ps. As a result, the maximum peak power was generally greater than 3.4 kW.

A picosecond diode-pumped passively Q-switched self-SRS laser has been efficiently demonstrated by use of Nd:GdxY12xVO4 crystals with x 苷 0.6 and x 苷 0.8. With an a-cut Nd:Gd0.8Y0.2VO4 crystal the conversion eff iciency for the average power is 8.2%

from pump diode input to self-Raman output, and the slope efficiency is as much as 15%. Consequently the

average output power can amount to 180 mW with a pulse repetition rate of 66 kHz and a peak power of .3.4 kW at an incident pump power of 2.2 W. These results provide the incentive for scaling a self-SRS laser by use of a Nd:Gd0.7Y0.3VO4 crystal with low Nd-dopant concentration.

The authors thank the National Science Council for their financial support of this research under contract NSC-92-2112-M-009-013. Y. F. Chen’s e-mail address is [email protected].

References

1. G. Eckhardt, D. P. Bortfeld, and M. Geller, Appl. Phys.

Lett. 3, 137 (1963).

2. H. M. Pask, Prog. Quantum Electron. 27, 3 (2003).

3. R. G. Zevrev, T. T. Basiev, and A. M. Prokhorov, Opt.

Mater. 11, 335 (1999).

4. J. T. Murray, W. L. Austin, and R. C. Powell, in Ad-vanced Solid-State Lasers, W. R. Bosenberg and M. J.

Fejer, eds., Vol. 19 of OSA Trends in Optics and Pho-tonics Series (Optical Society of America, Washington, D.C., 1998), pp. 129 – 135.

5. J. T. Murray, W. L. Austin, and R. C. Powell, in Ad-vanced Solid-State Lasers, M. J. Fejer, H. Injeyan, and U. Keller, eds., Vol. 26 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washing-ton, D.C., 1999), pp. 575 – 578.

6. C. He and T. H. Chyba, Opt. Commun. 135, 273 (1997).

7. P. ˇCerný and H. Jelínkova, Opt. Lett. 27, 360 (2002).

8. A. S. Eremenko, S. N. Karpukhin, and A. I. Stepanov, Sov. J. Quantum Electron. 10, 113 (1980).

9. E. O. Ammann and C. D. Decker, J. Appl. Phys. 48, 1973 (1977).

10. A. S. Grabtachikov, A. N. Kuzmin, V. A. Lisinetskii, V. A. Orlovich, G. I. Ryabtsev, and A. A. Demidovich, Appl. Phys. Lett. 75, 3742 (1999).

11. J. Findeisen, H. J. Eichler, and A. A. Kaminskii, IEEE J. Quantum Electron. 35, 173 (1999).

12. A. A. Kaminskii, K. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, and H. Weber, Appl. Phys. Lett. 83, 1289 (2003).

17. J. Liu, Z. Wang, X. Meng, Z. Shao, B. Ozygus, A. Ding, and H. Weber, Opt. Lett. 28, 2330 (2003).

18. L. J. Qin, X. L. Meng, L. Zhu, J. H. Liu, B. C. Xu, H. Z.

Xu, F. Y. Jiang, C. L. Du, X. Q. Wang, and Z. S. Shao, Chem. Phys. Lett. 380, 273 (2003).

19. A. E. Siegman, Lasers (University Science, Mill Valley, Calif., 1986), p. 1024.

20. G. H. Xiao and M. Bass, IEEE J. Quantum Electron.

33, 41 (1997).

DOI: 10.1007/s00340-004-1623-2 Appl. Phys. B 79, 823–825 (2004)

Lasers and Optics

Applied Physics B

y.f. chen^1,u s.w. chen¹ l.y. tsai¹ y.c. chen¹ c.h. chien²

Efficient sub-nanosecond intracavity optical parametric oscillator pumped with a passively Q-switched Nd:GdVO ₄ laser

1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, R.O.C.

2National Nano Device Laboratories, Hsinchu, Taiwan, R.O.C.

Received: 3 June 2004

Published online: 1 September 2004 • © Springer-Verlag 2004 ABSTRACTAn efficient diode-pumped passively Q-switched Nd:GdVO₄/Cr⁴⁺:YAG laser was employed to generate a high-repetition-rate, high-peak-power eye-safe laser beam with an intracavity optical parametric oscillator (OPO) based on a KTP crystal. The conversion efficiency for the average power is 8.3%

from pump diode input to OPO signal output and the slope effi-ciency is up to 10%. At an incident pump power of 14.5 W, the compact intracavity OPO cavity, operating at 46 kHz, produces average powers at 1571 nm up to 1.2 W with a pulse width as short as 700 ps.

PACS42.60.Gd; 42.65.Yj; 42.55.X

1 Introduction

Recently, neodymium-doped gadolinium ortho-vanadate (Nd:GdVO4) has proved to be an excellent gain medium due to its high absorption coefficient and large ther-mal conductivity [1–4]. Up to now, the output wavelengths of the researches involving Nd:GdVO4crystals were mostly fo-cused on 1.06, 1.34, 0.53, and 0.67 µm [5–9]. One area that demands particular attention is the so-called eye-safe region of the spectrum near 1.5–1.6 µm. Extremely short (< 1 ns) high-peak-power (> 10 kW) pulses of lasers at the eye-safe wavelength region are practically valuable for applications such as telemetry and range finders. One approach for high-peak-power eye-safe laser sources is based on intracavity op-tical parametric oscillators (OPOs) [10]. The advent of high-damage-threshold nonlinear crystals and diode-pumped Nd-doped lasers has led to a renaissance of interest in intracavity OPOs [11–13]. Recently, we demonstrated a compact effi-cient eye-safe OPO pumped by a diode-pumped passively Q-switched Nd:YVO4laser to produce peak powers at 1573 nm higher than 1 kW with pulse widths of 2.5 ns [14]. Compared with Nd:YVO4lasers, all the experimental results to date have revealed that Nd:GdVO4 crystals may be potentially more competent than Nd:YVO4 crystals in diode-pumped solid-state lasers. Even so, diode-pumped Nd:GdVO4 lasers have never been used to pump intracavity OPOs for generation of an eye-safe laser beam.

In this work we report, for the first time to our know-ledge, the generation of a laser beam from an efficient sub-nanosecond intracavity OPO based on a diode-pumped pas-sively Q-switched Nd:GdVO4 laser. With an incident pump power of 14.5 W, the compact intracavity OPO cavity, oper-ating at 46 kHz, produces average powers at 1571 nm up to 1.2 W with pulse widths shorter than 700 ps and peak powers higher than 20 kW.

2 Experimental setup

A schematic of the passively Q-switched intracav-ity OPO laser is shown in Fig. 1. Here a saturable absorber Cr⁴⁺:YAGcrystal is coated as an output coupler of the OPO cavity and a nearly hemispherical cavity is used to enhance the performance of passive Q-switching. The OPO cavity was formed by a coated KTP crystal and a coated Cr⁴⁺:YAG crys-tal. The 20-mm-long KTP crystal was used in type II noncrit-ical phase-matching configuration along the x axis (θ = 90^◦ andφ = 0^◦) to have both a maximum effective nonlinear co-efficient and no walk-off between the pump, signal, and idler beams. The KTP crystal was coated to have high reflectivity at the signal wavelength of 1571 nm (R> 99.8%) and high transmission at the pump wavelength of 1063 nm (T> 95%).

The other face of the KTP crystal was antireflection coated at 1571 nm and 1063 nm. The Cr⁴⁺:YAGcrystal has a

thick-824 Applied Physics B – Lasers and Optics

ness of 3 mm with 80% initial transmission at 1063 nm. One side of the Cr⁴⁺:YAGcrystal was coated so that it was nom-inally highly reflecting at 1063 nm (R> 99.8%) and partially reflecting at 1571 nm (R= 80%). The remaining side was an-tireflection coated at 1063 and 1571 nm. The active medium was an a-cut 0.25 at. % Nd³⁺, 8-mm-long Nd:GdVO4 crys-tal. Both sides of the laser crystal were coated for antire-flection at 1063 nm (R< 0.2%). A Nd:GdVO4 crystal with low doping concentration was used to avoid the thermally in-duced fracture [15]. All crystals were wrapped with indium foil and mounted in water-cooled copper blocks. The water temperature was maintained at 25^◦C. The pump source was a 16-W, 808-nm fiber-coupled laser diode with a core diam-eter of 800µm and a numerical aperture of 0.2. A focusing lens with 12.5-mm focal length and 92% coupling efficiency was used to re-image the pump beam into the laser crystal.

The pump spot radius was around 350µm. The input mirror was a 50-mm radius-of-curvature concave mirror with an an-tireflection coating at the diode wavelength on the entrance face (R< 0.2%), a high-reflection coating at lasing wave-length (R> 99.8%) and a high-transmission coating at the diode wavelength on the other surface (T> 95%). The overall Nd:GdVO4laser cavity length was approximately 59 mm and the OPO cavity length was about 25 mm.

From the analysis of the coupled rate equations, the crite-rion for good passive Q-switching is given by [16–18]

ln

where T₀is the initial transmission of the saturable absorber, A/As is the ratio of the effective areas in the gain medium and in the saturable absorber, R is the reflectivity of the output mirror, L is the nonsaturable intracavity round-trip dissipative optical loss,σgs is the ground-state absorption cross section of the saturable absorber,σ is the stimulated emission cross section of the gain medium,γ is the inversion reduction fac-tor with a value between 0 and 2 as discussed in [19], andβ is the ratio of the excited-state absorption cross section to that of the ground-state absorption in the saturable absorber. Since the ratio A/Asin the present cavity is generally greater than 10, the criterion for good passive Q-switching can be satis-fied under the circumstance that theσ value of the Nd:GdVO4

crystal is comparable to theσgsvalue of the Cr⁴⁺:YAGcrystal.

3 Experimental results

Figure 2 shows the average output power at 1571 nm with respect to the incident pump power. For all pump powers the beam quality M² factor was found to be less than 2.0. The average output power reached 1.2 W at an incident pump power of 14.5 W. The conversion effi-ciency from diode laser input power to OPO signal output power was 8.3%. The pulse temporal behavior at 1063 nm and 1571 nmwas recorded by a LeCroy 9362 digital oscilloscope (500-MHz bandwidth) with a fast InGaAs photodiode. The

FIGURE 2 The average output power at 1571 nm with respect to the inci-dent pump power

Figure 3 depicts the pulse-repetition rate and the pulse en-ergy at 1571 nm versus the incident pump power. It is seen

Figure 3 depicts the pulse-repetition rate and the pulse en-ergy at 1571 nm versus the incident pump power. It is seen