Intracavity-pumped Raman laser action in a fiber-laser–pumped, single-resonant, continuous-wave (cw) MgO:PPLN optical parametric oscillator with a high-Q linear resonator has been observed for the first time to our knowledge. Experimental results of this phenomenon investigation will be discussed.
©2006 Optical Society of America
Continuous-wave (cw), single-resonant optical parametric oscillators (SRO’s) based on periodically poled lithium niobate (PPLN) are practical and efficient sources of radiation in the mid-IR (3- to 5-µm) range . Using an Yb-doped fiber laser as a pump source  provides the combination of high pump power required for cw SRO’s, excellent pump beam profile, all-solid-state design, and compactness. Some concomitant nonlinear processes may be observed in PPLN owing to its high nonlinear gain coefficient. Simultaneous stimulated Raman scattering has been observed in a PPLN second-harmonic generator . Here we report on simultaneous parametric oscillator and intracavity-pumped Raman laser operation in a cw MgO:PPLN SRO, which has been observed for the first time to our knowledge.
2. Experimental setup
Our experimental setup is shown in Fig. 1. As a pump source we employed a cw fiber laser (YLD-10-LP, IPG Photonics, Oxford, MA) that produces 10 W of radiation at 1070 nm with ~0.6-nm FWHM bandwidth. The delivery fiber is a polarization-maintaining, single-mode fiber that is terminated with a collimator that produces a 1.6-mm-diam (1/e point) collimated beam. A Faraday isolator was used to prevent fiber laser damage due to back-reflection. A 15-cm-focal-length, AR-coated lens provided ~200-µm beam diameter inside the PPLN crystal. We used a MgO-doped PPLN crystal (HC Photonics Corp., Taiwan) to avoid photorefractive damage to the crystal at room temperature . The 1×8.2×50-mm crystal has seven different gratings with periods ranging from 28.5 µm to 31.5 µm with 0.5-µm increments to generate idler wavelengths covering the 3000- to 4000-nm spectral range. MgO:PPLN is AR coated on both sides for 1070/1440–1605/3160–4070 nm. The crystal is temperature stabilized at 35°C. A linear SRO resonator is formed by two identical mirrors that have high transmission at both the 1070-nm pump wavelength and 3000- to 4000-nm idler wavelength range and high reflectivity at 1500- to 1700-nm signal wavelength range. CaF2 meniscus mirror substrates with 10-cm radius-of-curvature were used. Two sets of mirrors were employed in the experiment: set #1 (Rocky Mountain Instrument Co., Lafayette, CO) provided 98.2%–99% reflectivity in the 1500- to 1700-nm range; set #2 (Quality Thin Films Inc., Oldsmar, FL) provided 99.4%–99.8% reflectivity in the same wavelength range (Fig. 2). The SRO resonator optical length was 120 mm. The output beam was collimated by an uncoated CaF2 lens. Dichroic beamsplitters directed residual pump and signal radiation to a beam block.
3. Experimental results and discussion
Idler output power and stability, idler beam profile, and signal spectra were recorded. Approximately 8 W of pump power reaches the PPLN crystal. An SRO with a 31-µm grating and mirror set #1 produced ~1.6 W of maximum idler power at 3260 nm with a 35% slope and 20% optical-to-optical efficiencies (see Fig. 3). In this case signal spectra consist of a single wavelength (signal) component at any pump power level. SRO behavior changes dramatically when mirror set #2 is employed: the threshold drops significantly from 3.3 W to 0.5 W, while the slope and optical-to-optical efficiencies decrease to 16% and 15%, respectively. The threshold of 0.5 W is the lowest reported for this kind of SRO. The idler beam profile corresponds to a TEM00 mode in both cases (mirror sets #1 and #2) at any pump power level and is shown in Fig. 4. Multiple Raman components appear in the signal spectrum when using mirror set #2, as shown in the inset of Fig. 3. The threshold of the Raman laser is observed at ~1.9 W of SRO pump power. Measured signal spectra for different signal operating wavelengths set by using different gratings are shown in Fig. 5. Five intense Raman lines with shifts that range from 46 to 631 cm-1 have been identified (see table in Fig. 5) that are roughly consistent with published values for lithium niobate , but comparable values are not available for MgO-doped PPLN. There were no additional spectral components observed near the idler wavelength when using a simple prism spectrometer.
The difference in SRO performance for the two mirror sets is explained by the order-of-magnitude difference in the Q factor of the SRO resonator for two sets of mirrors: set #1 provides Q ~ 108, while set #2 provides Q ~ 109. We have measured >200 mW of signal power leaking from the high-Q SRO resonator at the maximum pump power. Since the signal power circulating in the resonator is inversely proportional to mirror transmission, powers over 100 W are expected in the SRO with set #2. Given that the mirrors provide high reflectivity at the Raman wavelengths, the SRO mirrors also form a resonator for a Raman laser that is intracavity pumped by the intense SRO signal radiation. Not all the Raman components are generated at the various operating wavelengths set by each grating and are determined by a combination of the following factors: Raman gain cross section, Raman laser resonator losses, and pump power. The Raman gain cross section for each component is a material constant and does not change from grating to grating. Resonator losses do change because of variations in mirror reflectivity (see Fig. 2, mirror set #2). When the grating period decreases, the signal wavelength switches to shorter wavelengths; therefore, Raman wavelengths shift shorter as well. SRO mirror reflectivity decreases toward shorter wavelengths (Fig. 2), resulting in increased Raman laser resonator losses and reduced power for the SRO signal and Raman laser. The second Raman line (103 cm-1) illustrates this behavior while switching from grating #2 to grating #6: the amplitude of this spectral component decreases from grating #2 to grating #4 and this component disappears completely in the cases of grating #5 and #6.
We have also recorded idler output power stability in both cases. The stability of the pump fiber laser is very high with <1% rms variations. Idler output power stability in SRO with mirror set #1 [Fig. 6(a)] is relatively poor with 4.1% rms variations that is limited predominantly by mode hopping in the resonator . Simultaneous SRO and Raman laser operation in the SRO with mirror set #2 provides significantly better idler output power stability with 1.46% rms variations [Fig. 6(b)]. Raman laser action in an SRO works as a limiter for a signal, stabilizing intracavity signal power, and stabilizing idler output power. The idler output-power level of ~400 mW has been arbitrarily chosen. Improvement of output-power stability has been observed over all ranges of idler output powers above the Raman laser threshold.
We have demonstrated simultaneous MgO:PPLN SRO and Raman laser operation for the first time to our knowledge. The Raman laser acts as a limiter to an SRO signal power, increasing the idler output power stability (variations decrease from 4.1% to 1.46% rms).
This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.
References and links
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