Coexistent optical parametric oscillation (OPO) and stimulated Raman scattering (SRS) are demonstrated in an X-cut KTiOAsO4 (KTA) crystal. The 30-mm-long KTA crystal is placed within a diode-end-pumped acousto-optically (AO) Q-switched Nd:YAG laser cavity to construct an intracavity optical parametric oscillator. Coexistent Raman conversion of the fundamental wave is observed from the KTA crystal. With a diode power of 7.43 W and a pulse repetition rate (PRR) of 20 kHz, a signal (1535.0 nm) power of 0.92 W is obtained, corresponding to a diode-to-signal conversion efficiency of 12.4%. A first-Stokes (1091.4 nm) power of 0.17 W is obtained.
©2008 Optical Society of America
Laser sources with different wavelengths are required in many practical applications, e.g. optical microscopy, spectroscopy, medicine and remote sensing. Nonlinear optical frequency conversion technologies are reliable and efficient ways to obtain diverse and versatile coherent sources especially with emitting wavelengths that are difficult or impossible to access with laser gain media. Optical parametric oscillation (OPO) and stimulated Raman scattering (SRS) are two efficient kinds of frequency converting processes.
OPOs have been widely studied to obtain coherent sources covering from visible to far-infrared band [1–3]. KTiOPO4 (KTP) and its isomorphs have played important roles in OPOs, especially those emitting in the eye-safe band [4–7]. SRS, a third order nonlinear effect, is another research interest for efficient frequency conversions. Crystalline Raman lasers based on SRS have got a rapid escalation in the past decade [8–15]. Those emitting at around 1.18 µm [10,12], 0.59 µm  and 1.5 µm  have attracted most attention. Coexistent OPO and SRS can extend the spectral range of a laser source and hence is of great interest [16–19].
KTiOAsO4 (KTA), one of the isomorphs of KTP, has established itself as an efficient and reliable material for OPOs [6,7,20]. 1534.7-nm signal power of 33 W was obtained in an extracavity KTA OPO  and both high-power eye-safe signal and mid-infrared idler outputs were obtained in [7,20]. Besides the attractive performances in parametric processes, KTA has potential to be an efficient Raman crystal. Its polarized Raman spectra were studied and the strongest phonon line was found to be located at 234 cm-1 [21,22]. KTA was recently proved to be efficient for nanosecond SRS applications . Within a diode-end-pumped intracavity Raman laser cavity, a diode-to-Stokes conversion efficiency of 17% was obtained from a KTA Raman crystal in X(ZZ)X configuration .
Here we report on coexistent optical parametric oscillation and stimulated Raman scattering in one KTA crystal. The 30-mm-long KTA crystal is cut in X-axis direction to satisfy both type II non-critical phase matching (NCPM) condition for OPO and the X(ZZ)X configuration for SRS. An intracavity singly resonant optical parametric oscillator configuration is adopted with a diode-end-pumped acousto-optically (AO) Q-switched Nd:YAG laser as the pumping source. In former investigations in coexistent OPO and SRS, SRS occurred in the resonators of the OPOs, i.e. Raman conversions of the parametric signals were observed [16–19]. Although novel laser lines were obtained, this kind of Raman conversions would limit the efficiencies of the parametric conversions, as mentioned in . Here in our experiment, SRS shares the same cavity with the fundamental wave. Raman conversion of the fundamental wave, not the signal, is observed together with OPO, which is the first time to our knowledge. With a diode power of 7.43 W and a pulse repetition rate (PRR) of 20 kHz, a signal (1535.0 nm) power of 0.92 W is obtained, corresponding to a diode-to-signal conversion efficiency of 12.4%. A first-Stokes (1091.4 nm) power of 0.17 W is obtained. The total conversion efficiency from diode power to signal and Stokes powers is 14.7%.
2. Experimental setup
The experimental arrangement for investigating the coexistent OPO and SRS in KTA is shown in Fig. 1. The rear mirror was a plano–concave mirror with a curvature radius of 3000 mm. Its plane face was coated for antireflection (AR) at 808 nm (T>99.8%), the concave face was coated for high-transmission (HT) at 808 nm (T>97%) and high-reflection (HR) at 1064–1100 nm (R>99.9%). The output coupler (OC) (plano–plano, BK7) had a dichroic coating that was highly reflective at 1064 nm (R>99.9%), highly reflective at 1091.4 nm (R R=99.8%) and partially reflective (PR) at 1535 nm (R S=63%). The cavity for the fundamental laser was formed by the concave face of the rear mirror and the plane output coupler. Its overall cavity length was 90 mm. The Raman laser shared the same cavity.
A fiber coupled diode laser (30 W, NA=0.22, d core=400 µm) was used as the pumping source. The 1 at.%-Nd-doped YAG rod was of size ∅4 mm×5 mm with AR coatings at both 808 nm and 1064–1100 nm (T>99.8%) on both faces. The 40-mm-long AO Q-switch (Gooch and Housego) had AR coatings on both faces at 1064 nm (T>99.8%) and was driven at 40.68 MHz center frequency with an rf power of 15 W. The KTA crystal of size 5×5×30 mm3 was cut along X-axis (θ=90°, ϕ=0°) to satisfy type II NCPM condition for the parametric conversion. For SRS considerations, this X-cut KTA crystal can satisfy the X(ZZ)X configuration . One face of it was coated to have high transmission at 1064–1100 nm (T>99%) and high reflectivity at the signal wavelength of 1535 nm (R>99.8%). The other face was AR coated at both 1535 nm and 1064–1100 nm (T>99.8%). The singly resonant OPO cavity was comprised by the HR coating of the KTA crystal and the PR coating of the OC. Its cavity length was 33 mm. Both the Nd:YAG and KTA crystals were wrapped with indium foil and mounted in water-cooled copper blocks. The water temperature was maintained at 19 °C.
3. Experimental results and discussions
The wavelength of the signal wave was measured by using a wide-range optical spectrum analyzer (AQ 6315A, Yokogawa). Typical optical spectra of the fundamental, the first-Stokes and the signal waves are shown in Fig. 2.
The signal wavelength of 1535.0 nm is in accordance with the case of type II NCPM KTA pumped by 1064.2-nm laser. According to the phase matching condition, fundamental wave with a polarization direction along Y-axis of KTA contributed to the parametric process. As a result, the main residual fundamental-wave photons after parametric conversions polarized along Z-axis of KTA. These residual photons satisfied the X(ZZ)X configuration, which led to the coexistent SRS. The wavelength (1091.4 nm) of the first-Stokes line corresponded to the frequency shift of 234 cm-1 [21,22].
Output powers were measured by an EPM 2000 power meter (Coherent Inc.). A band-pass filter (BPF) and a dichroic mirror (DM) were used when powers and pulse shapes were measured. The BPF could block off a wide band from 300 to 1200 nm except a 5-nm band near 1064 nm. The DM was coated to have high transmission at 1535 nm (T>99.5%) and high reflectivity at 1064–1100 nm (R>99.8%). Both the BPF and the DM were made of BK7 glass. Results at PRRs of 15, 20 and 25 kHz are shown in Fig. 3.
Because both the OC and the DM were made of BK7 glass, the idler wave at around 3.5 µm was strongly absorbed. As a result, we didn’t observe it in the experiments. From Fig. 3(a), OPO threshold at 15 kHz was found to be less than 1.5 W. The highest signal power of 0.92 W was obtained at a pump power of 7.43 W and a PRR of 20 kHz. This corresponded to a diode-to-signal conversion efficiency of 12.4%. As shown in Fig. 3(b), a first-Stokes power of 0.17 W was obtained at the pump power of 7.43 W and the PRR of 20 kHz. The total conversion efficiency from diode power to signal and Stokes powers was 14.7%, improved by 2.3 percentage points compared with the diode-to-signal conversion efficiency of 12.4%. The average power stability of the signal was better than 1% during 1h operation, so was that of the Stokes. The output polarizations of the signal and the Stokes line were found to be both along Z-axis of the KTA crystal, which was in accordance with the type II NCPM condition of OPO and X(ZZ)X configuration of SRS.
The pulse temporal behavior was monitored by a digital phosphor oscilloscope (TDS 5052B, Tektronix) and a fast p-i-n photodiode. Pulse shapes obtained with a pump power of 7.43 W and a PRR of 20 kHz are given as an example, as shown in Fig. 4. The upper lines (channel 2) display the pulse shapes of the fundamental wave. The lower line (channel 1) in Fig. 4(a) shows the pulse shape of the signal wave. The lower line in Fig. 4(b) represents the first-Stokes wave. It should be pointed out that the longitudinal axes in Fig. 4 are with arbitrary units. And the peak intensity of the signal wave was much stronger than that of the first-Stokes wave. For the signal, the pulse-to-pulse amplitude fluctuation was found to be within ±10% and it was around ±15% for the Stokes line.
Here, pulse-series phenomenon in the OPO as described in  was also observed, as shown in Fig. 4(a). The first and second depletions of the fundamental wave led to generations of the two signal pulses. The fundamental pulse shape in Fig. 4(a) differed from that shown in , since rapid depletion of the fundamental wave occurred again after the parametric conversion process. The third depletion resulted in generation of the first-Stokes wave, as shown in Fig. 4(b). The little pulse before the first-Stokes pulse was from the residual fundamental wave through the BPF.
Fig. 4 shows evident time difference between the OPO and SRS processes. The time delay between the signal and Stokes pulses was about 50 ns. Since the parametric gain was large, signal pulse was built up rapidly before the generation of Stokes pulse, as is shown clear in Fig. 4. Because SRS was not able to block the parametric conversion during the formation of the signal pulse, we could obtain the high diode-to-signal conversion efficiency, i.e. 12.4%. Meanwhile, the fast build-up signal pulse consumed lots of fundamental photons. The amount of residual fundamental photons for Raman conversion was relatively small, which resulted in low Raman gain. So the Stokes wave needed long time accumulating inside the cavity, as shown in Fig. 4(b). Anyway, the nonlinear optical effects in our work made better use of the fundamental waves, so total conversion efficiency of up to 14.7% was obtained.
Other two factors would influence the Stokes conversion efficiency. First, although Nd:YAG is isotropic without pumping, thermal loads in practical operations induce birefringence. This resulted in elliptical distribution of the fundamental-wave amplitude. So, placement of the KTA crystal, i.e. the angle between Y-axis of KTA and the major axis of the ellipse, influenced the distribution of the fundamental-laser intensity between OPO and SRS. In our experiments, this influence was not considered, which accounted partly for the inefficient SRS. Second, the OC’s reflectivity at 1091.4 nm was too high (99.8%). If this reflectivity is optimized, the output Stokes power is believed to increase.
Pulse widths of the signal and first-Stokes waves at PRRs of 15, 20 and 25 kHz are shown in Fig. 5. Each point was obtained by averaging arbitrary 20 pulse-width values. It should be pointed out that the values in Fig. 5(a) are for the first (or the main) pulse of the pulse series in the OPO.
Take the case at 20 kHz as an example, the signal pulse widths decreased from 7.6 ns to 3.5 ns with the pump power tuned from 1.57 W to 7.43 W. Because of the relatively low gain of the SRS and high reflectivity of the OC, the circulating time of the Stokes pulse was too long. As a result, the widths of the Stokes pulses were much longer than those of the signal pulses. Also at 20 kHz, the Stokes pulse widths were 173.3 ns and 56.3 ns at pump powers of 2.29 W and 7.43 W, respectively.
Coexistent optical parametric oscillation and stimulated Raman scattering have been observed in one KTiOAsO4 (KTA) crystal. An intracavity singly resonant optical parametric oscillator configuration was adopted with a diode-end-pumped acousto-optically (AO) Q-switched Nd:YAG laser as the pumping source. The Raman oscillation shared the same cavity with the fundamental wave. With a diode power of 7.43 W and a pulse repetition rate (PRR) of 20 kHz, a signal (1535.0 nm) power of 0.92 W was obtained, corresponding to a diode-to-signal conversion efficiency of 12.4%. A first-Stokes (1091.4 nm) power of 0.17 W was obtained. The total conversion efficiency from diode power to signal and Stokes powers was 14.7%. This experimental configuration improved the efficiency of nonlinear optical frequency conversions by making better use of the fundamental waves. And this work is helpful to extend the wavelength range of a Nd:YAG laser.
This work was supported by the Science and Technology Development Program of Shandong Province (No. 2007GG10001026), the National Natural Science Foundation of China (No. 60677027), and the Research Fund for the Doctoral Program of Higher Education of China (No. 20060422025).
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