DOI: 10.1007/s00340-003-1279-3 Appl. Phys. B 77, 493–495 (2003)
Lasers and Optics
Applied Physics B
y.f. chen1,u s.w. chen1 y.c. chen1 y.p. lan1 s.w. tsai2
Compact efficient intracavity optical parametric
oscillator with a passively Q-switched
Nd : YVO
4
/Cr
4
+
: YAG laser in a hemispherical
cavity
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan
Received: 16 June 2003/Revised version: 10 July 2003 Published online: 30 September 2003 • © Springer-Verlag 2003
ABSTRACTA compact eye-safe optical parametric oscillator (OPO) using a noncritically phase-matched KTP crystal intra-cavity pumped by a passively Q-switched Nd : YVO4laser is experimentally demonstrated. To enhance the performance of passive Q-switching, a Cr4+: YAG saturable absorber crystal is coated as an OPO output coupler in a nearly hemispherical cavity. With an incident pump power of 2.5 W, the compact in-tracavity OPO cavity, operating at 62.5 kHz, produces average powers at 1573 nm up to 255 mW and peak powers higher than 1 kW.
PACS42.60.Gd; 42.65.Yj; 42.55.Xi
1 Introduction
Compact pulsed lasers with emission at the eyesafe wavelength region (1.5–1.6 µm) are of great interest for many applications such as telemetry and range finders [1]. The need for high-peak-power eyesafe laser sources has stimulated much interest in intracavity optical parametric oscillators (OPO’s). Although intracavity OPO’s have been proposed for over 30 years [2–4], only recently have their merits been appreciated, with the advent of high-damage-threshold non-linear crystals and diode-pumped Nd-doped lasers [1, 5–7].
Diode-pumped Q-switched microchip lasers are compact efficient solid-state lasers with a diffraction-limited output beam. Saturable-absorber Q-switching has the advantages of potentially lower cost and simplicity in fabrication and operation. In recent year, Cr4+: YAG crystals have been successfully used as passive Q-switches for a variety of gain media such as Nd : YAG [8], Nd : YVO4 [9–11], and Nd : GdVO4 crystals [12], etc. Nd : YVO4 and Nd : GdVO4 crystals have several advantages over Nd : YAG crystals, in-cluding higher absorption cross section, wider absorption bandwidth, and a polarized output. The linearly polarized laser output is beneficial not only to non-linear wavelength conversion, but also to avoiding of undesired birefringent effects. It is, however, usually difficult to operate a diode-pumped passively Q-switched Nd : YVO4 and Nd : GdVO4 lasers with Cr4+: YAG saturable absorbers because of their
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large emission cross-sections. For good passive Q-switching, absorption saturation in the absorber must occur before gain saturation in the laser crystal (the second threshold condi-tion) [13, 14]. Even though passively Q-switched Nd : YVO4 and Nd : GdVO4 lasers have been demonstrated [9–12], the output pulse energy and peak power are obviously lower than those of Nd : YAG laser. Therefore, so far the pumped sources for passively Q-switched intracavity OPO’s are mostly com-posed of Nd : YAG and Cr4+: YAG crystals. The relatively narrow absorption band of Nd : YAG crystal, however, sets stringent requirements on the spectrum of the pump diodes.
In this work we report a compact, efficient scheme for gen-erating 1573-nm laser based on intracavity OPO of a diode-pumped passively Q-switched Nd : YVO4/Cr4+: YAG laser. With an incident pump power of 2.5 W, the compact intra-cavity OPO intra-cavity, operating at 62.5 kHz, produces average powers at 1573 nm up to 255 mW and peak powers higher than 1 kW.
2 Experimental setup
Figure 1 is a schematic of the passively Q-switched intracavity OPO laser. The novelty is that a saturable absorber Cr4+: YAG is coated as an output coupler of the OPO cavity and a nearly hemispherical cavity is used to en-hance the performance of passive Q-switching. The active medium was an a-cut 1.0 at. % Nd3+, 2-mm-long Nd : YVO4 crystal. Both sides of the laser crystal were coated for an-tireflection at 1064 nm (R< 0.2%). The pump source was a 2.5-W 808-nm fiber-coupled laser diode with a core diam-eter of 200µm and a numerical aperture of 0.16. Focusing
FIGURE 1 Schematic of the intracavity OPO pumped by a diode-pumped passively Q-switched Nd: YVO4/Cr4+: YAG laser
494 Applied Physics B – Lasers and Optics
lens with 12.5 mm focal length and 95% coupling efficiency was used to re-image the pump beam into the laser crystal. The average pump-spot radius,ωp, was around 150µm. The input mirror, M1, was a 50 mm radius-of-curvature concave mirror with antireflection coating at the diode wavelength on the entrance face (R< 0.2%), high-reflection coating at las-ing wavelength (R> 99.8%) and high-transmission coating at the diode wavelength on the other surface (T> 95%). Note that the laser crystal was placed very near (0.5 ∼ 1 mm) the input mirror. The OPO cavity was formed by a coated KTP crystal and a coated Cr4+: YAG crystal. The 20-mm-long KTP crystal was used in type II noncritical phase-matching configuration along the x axis (θ = 90◦ andϕ = 0◦) to have both a maximum effective nonlinear coefficient and no walk-off between the pump, signal, and idler beams [15–18]. The KTP crystal was coated to have high reflectivity at the signal wavelength of 1573 nm (R> 99.8%) and high transmission at the pump wavelength of 1064 nm (T> 95%). The other face of the KTP crystal was antireflection coated at 1573 nm and 1064 nm. The Cr4+: YAG crystal has a thickness of 2 mm with 80% initial transmission at 1064 nm. One side of the Cr4+: YAG crystal was coated so that it was nominally highly reflecting at 1064 nm (R> 99.8%) and partially reflecting at 1573 nm(Rs= 80%). The remaining side was antireflection coated at 1064 and 1573 nm. The overall Nd : YVO4 laser cavity length was approximately 55 mm and the OPO cavity length was about 23 mm.
The mode beam radiiω1on the laser crystal andω2on the saturable absorber can be given by
ω1= λLc π Lc Rc− Lc, ω 2= λLc π Rc− Lc Lc , (1) whereλ is the lasing wavelength, Lc is the effective cavity length, and Rcis the radius of curvature of the input mirror. The effective cavity length is given by Lc= L∗c+l(1/n −1)+ lKTP(1/nKTP−1), L∗cis the cavity length, n is the refractive in-dices along the c axis of the Nd : YVO4crystal, l is the length of the Nd : YVO4crystal, lKTPis the length of the KTP crys-tal, and nKTPis the KTP refractive index for the output laser beam. For the present cavity length of L∗c= 55 mm, ω1 and ω2can be calculated to be 225µm and 71 µm, respectively. Withω1= 225 µm and ωp= 150 µm, the ratio between the mode and pump area,α = (ω1/ωp)2= 2.25, satisfies the de-sign criterion of mode-matching optimization [19]. On the other hand, the ratio of the mode area in the gain medium and in the saturable absorber, A/As= (ω1/ωp)2= 10, satisfies the criterion for good passively Q-switching [13, 14].
3 Result and discussion
Figure 2 shows the average output power and the pulse repetition rate at 1573 nm with respect to the absorbed pump power. For all pump powers the beam quality M2 fac-tor was found to be less than 1.3. The average output power reached 255 mW, and the pulse repetition rate was 62.5 kHz at an incident pump power of 2.5 W. The threshold power and the slope efficiency were 1.1 W and 18.3%, respectively. The conversion efficient from diode laser input power to OPO sig-nal output power was 10.2%. To the best of our knowledge,
FIGURE 2 Dependence of the average output power and the pulse repeti-tion rate at 1573 nm on the absorbed pump power. An oscilloscope trace of a train of the signal pulses is shown in the inset
this is highest efficiency for average power conversion re-ported to date.
The pulse temporal behavior at 1573 nm was recorded by a LeCroy 9362 digital oscilloscope (500 MHz band-width) with a fast germanium photodiode. An oscilloscope trace of a train of the signal pulses is shown in the in-set of Fig. 2. The pulse-to-pulse amplitude fluctuation was found to be within ±10%. Figure 3 shows typical tempo-ral shapes for the laser and signal pulses. The relatively short signal pulse indicates that the OPO effectively cav-ity dumps the laser energy. Experimental results reveal that the signal pulse width decreases from 6.0 ns at threshold to 3.8 ns at 2.5 W of incident pump power. Figure 4 depicts the peak power and the pulse energy at 1573 nm versus the absorbed pump power. It is seen that the pulse energy ini-tially increases with pump power, and is almost saturated
FIGURE 3 Typical temporal shapes for the laser and signal pulses with a signal reflectivity of 80% on the output coupler
CHENet al. Compact efficient passively Q-switched intracavity optical parametric oscillator 495
FIGURE 4 Dependence of the peak power and the pulse energy at 1573 nm on the absorbed pump power
beyond 2 times the OPO threshold. The striking feature is that with the maximum pump power of 2.5 W the sig-nal peak power can exceed 1 kW at a pulse repetition rate of 62.5 kHz.
Finally, it is worthwhile to mention that the temporal char-acteristics of the present cavity highly depend on the laser alignment, pump spot size and mirror reflectivity. As shown in
FIGURE 5 Typical temporal shapes for the laser and signal pulses with a signal reflectivity of 90% on the output coupler
Fig. 5, a train of laser and signal pulses is usually produced for a higher OPO reflectivity on the output coupler (Rs= 90%). Under the normal mode-matching circumstances, the output optimization of the present cavity mainly consists in the de-sign of the output reflectivity. 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%.
4 Summary
In summary, operation of a singly resonant pulsed KTP intracavity OPO pumped by a diode-pumped passively Q-switched Nd : YVO4/Cr4+: YAG laser has been demon-strated. 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 (2.5 W). 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 kWwith a pulse repetition rate of 62.5 kHz at an incident pump power of 2.5 W.
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