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Study of Amplitude Noise in a Continuous-Wave Intracavity Frequency-Doubled Raman Laser Jipeng Lin, Helen M. Pask, Andrew J. Lee, and David J. Spence
Abstract— We study the temporal amplitude stability of a continuous-wave (CW) intracavity frequency-doubled selfRaman Nd:GdVO4 laser (yellow laser), and compare this with that of a CW intracavity frequency-doubled Nd-doped laser (green laser). Both experimental and theoretical results indicate that, while at high pump powers the yellow laser shows strong amplitude fluctuations (peak to peak >20%) similar to that in the simple green laser, for lower powers there is a regime where the yellow laser exhibits low (peak to peak >100), which averages out the competition effects [7]–[9]. One might expect a similar “yellow problem” to exist for CW intracavity frequency-doubled Raman lasers (yellow
Manuscript received July 28, 2010; revised September 24, 2010; accepted October 1, 2010. Date of current version February 24, 2011. The authors are with MQ Photonics, Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia (e-mail:
[email protected];
[email protected]; andrew.lee@mq. edu.au;
[email protected]). Digital Object Identifier 10.1109/JQE.2010.2086048
lasers) in which another nonlinear process, namely stimulated Raman scattering (SRS), is involved in addition to secondharmonic generation (SHG). The fundamental wavelength is first shifted to the first Stokes wavelength by SRS, and then the first Stokes optical field is frequency-doubled by an intracavity nonlinear crystal [such as lithium borate (LBO) or potassium titanyl phosphate]. The influence of the additional SRS process on the amplitude stability of an intracavity frequency-doubled Raman laser could make the laser behave quite differently, and this has not been studied before. In this paper, we study the amplitude noise of a CW intracavity frequency-doubled self-Raman laser both experimentally and theoretically. In the experiment, we set up a CW yellow laser system, and compared its noise properties with those of a conventional CW intracavity frequency-doubled green laser. We also use two-mode rate equations to describe the coupling and competition of adjacent longitudinal modes in the cavity, and compare and contrast the output stability of the green laser with the yellow laser. Both experimental and theoretical results show that, at high pump powers, the yellow laser exhibits large amplitude noise analogous to the green laser. However, unlike the green laser, there is a regime at lower powers where the yellow laser has low amplitude noise. The existence of this stable window makes it possible to build very simple yellow lasers that do not suffer from amplitude instability. II. E XPERIMENT A. Experimental Setup We set up a Nd:GdVO4 laser which can be operated as either a green laser or a self-Raman yellow laser, as illustrated in Fig. 1. The green laser was obtained by SHG of the fundamental 1064 nm, using a 4 × 4× 15 mm LBO (θ = 90°, φ = 11.4°) antireflection (AR) coated at 1064–1200 nm. For the self-Raman yellow laser, both fundamental at 1064 nm and the first Stokes at 1173 nm resonated, and a 4 × 4 × 10 mm LBO crystal cut for non-critical phase matching (θ = 90°, φ = 0°) was used for yellow generation by SHG of the first Stokes at 1173 nm. Both green and yellow laser resonators were bound with the same mirror set, a flat input mirror M1 and a 500-mm radius of curvature concave output coupler M2. Both mirrors have the same coating (T = 96% at 808 nm, R = 99.994% at 1063 nm, R = 99.996% at 1173 nm, T = 93% at 586 nm, T = 97% at 532 nm). A high-brightness fiber-coupled 30-W 880-nm diode (LIMO ∼200 μm, N.A. ∼0.22, unpolarized) was imaged onto an AR-coated (1064– 1200 nm) a-cut 0.3 at.% Nd:GdVO4 crystal which was placed
0018–9197/$26.00 © 2011 IEEE
LIN et al.: AMPLITUDE NOISE IN A CW INTRACAVITY FREQUENCY-DOUBLED RAMAN LASER
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close to the input mirror. The pump spot’s radius was 170 μm to match the TEM00 mode in the cavity. The overall optical length of the resonator was 80 mm. Filters were used to separate the residual infrared beam and green/yellow laser beams. The waveform of the amplitude noise was monitored with a PIN (Thorlabs-DET10A/M 1 GHz) photodiode and an oscilloscope (Tektronix-3052-500 MHz). B. Amplitude Noise in a Green Laser System The threshold of the green laser was 0.19 W. Strong amplitude noise was observed, as shown in Fig. 2. The peakto-peak amplitude noise (2σ /av) was measured to be >20%, which is characteristic of the typical green problem. When the green output was about 100 mW, the spectral linewidths for the fundamental and green lines were measured to be 0.3 and 0.1 nm, respectively, with an optical spectrum analyzer (Ocean-Optics-HR4000, 0.02 nm resolution), from which we inferred that there were about 20 longitudinal modes for the fundamental wavelength in the cavity. C. Amplitude Noise in a Self-Raman Laser with Intracavity SHG (Yellow Laser) For the yellow laser, the threshold for the fundamental (1064 nm) was 0.18 W, and threshold for the first Stokes (1173 nm) and yellow emission (586.5 nm) was 2.5 W. We studied the amplitude behavior of the Raman laser both with and without the intracavity LBO crystal. Without the LBO crystal, the laser ran stably at 1173 nm with a peakto-peak amplitude noise 50%. With uniform pumping, the simulated yellow laser remains noisy indefinitely. However, for a real laser, there are always perturbations from the environment such as mechanical vibration. If we perturb the model by briefly increasing the losses, the laser can “reset” into a stable mode before the exponentially growing oscillation restarts the noisy behavior. We believe that such perturbations account for the experimental observation of short stable periods interspersed within extended noisy periods.
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The underlying cause for generating a stable window in the yellow laser which is not seen for the green laser is hard to explain beyond attributing it to the intrinsic behavior of the coupled equations. For the green problem, Mandel and Wu used a bifurcation analysis of the rate equations of a twomode green laser system, revealing this intrinsic nature of the instability [12], [13]. They used the “stable region diagram” to show that the green laser enters an unstable region immediately above the threshold. Similar analytic studies could be carried out on the far more complex two-mode rate equations for the yellow laser, we anticipate that in the “stable region diagram” for yellow laser, there will be a stable region between the laser threshold and the transition pump power Ptr . C. Discussion of Ptr While at high pump powers the yellow laser exhibits the “yellow problem” noise analogous to “green problem” noise in simple green lasers, the yellow laser shows stable behavior below a transition pump power. This behavior is not seen in green lasers, and presents a simple way to generate stable yellow emission that is not available for green lasers. To make the yellow laser run stably across a wider power range and obtain higher yellow emission, it is necessary to increase Ptr and understand which system parameters influence Ptr most. Using the model, we numerically determined that Ptr is primarily dependent on two main system parameters: the SRS threshold PS R S and cavity optical length L. Next we consider how changing these parameters might be useful in turn. Our present simulation results predict that the transition power Ptr is proportional to PS R S , which in turn is determined by the length and Raman gain of the Raman crystal, the mode sizes in the Raman crystal, and intracavity losses for both the fundamental and Stokes optical fields [6]. The ratio Ptr /PS R S is about two in our experimental laser. Although increasing PS R S can provide a wider stable range, the overall efficiency would be compromised accordingly [6] and the maximum stable yellow output power would not necessarily increase. Increasing the cavity optical length L is a better route to
increase the transition power Ptr as shown in Fig. 8. For a CW laser, the SRS threshold and the efficiency do not depend explicitly on L [6], and so extending the optical length will strongly increase the maximum yellow output power of low amplitude noise, shown in Fig. 8. Unfortunately, the selfRaman laser showed an extremely strong thermal lens effect [14], which limited the maximum cavity length for stable operation, the longest cavity that we have built to date is 80 mm. More careful cavity design and thermal management will be required in the future to increase the cavity length as well as the yellow output power in the stable regime. We note that one solution to the green problem is to move to a far longer cavity, which relies on a very large number of modes averaging out the amplitude noise [7]–[9]. In contrast, for the yellow laser, extending the cavity does not reduce the noise but increase the range of operation where there is no noise problem, even for two modes. IV. C ONCLUSION We have studied the amplitude noise in a yellow CW intracavity Raman frequency-doubled laser system both experimentally and theoretically, and compared it with that in a CW green laser. At high pump powers, the green and yellow lasers both exhibit strong amplitude noise, showing a yellow analogue of the well-known green problem. However, in contrast to the green laser, we have found that a regime exists for pump powers below a transition pump power Ptr for which the yellow laser runs with very low amplitude noise, and up to 100 mW yellow emission was demonstrated. This behavior offers a simple route to generate stable output from yellow Raman lasers, sidestepping the need for more complex solutions to the green problem. The simulation results predict that, by extending the cavity optical length L, the maximum stable yellow power can be substantially increased. R EFERENCES [1] L. Fan, Y. X. Fan, Y. Q. Li, H. Zhang, Q. Wang, J. Wang, and H. Wang, “High-efficiency continuous-wave Raman conversion with a BaWO4 Raman crystal,” Opt. Lett., vol. 34, no. 11, pp. 1687–1689, Jun. 2009. [2] A. J. Lee, H. M. Pask, P. Dekker, and J. A. Piper, “High efficiency, multi-Watt CW yellow emission from an intracavity-doubled self-Raman laser using Nd:GdVO4 ,” Opt. Exp., vol. 16, no. 26, pp. 21958–21963, Dec. 2008. [3] T. Baer, “Large-amplitude fluctuations due to longitudinal mode coupling in diode-pumped intracavity-doubled Nd:YAG lasers,” J. Opt. Soc. Am. B, vol. 3, no. 9, pp. 1175–1180, Sep. 1986. [4] T. Y. Fan, “Single-axial mode, intracavity doubled Nd:YAG laser,” IEEE J. Quantum Electron., vol. 27, no. 9, pp. 2091–2093, Sep. 1991. [5] M. D. Selker, T. J. Johnston, G. Frangineas, J. L. Nightingale, and D. K. Negus, “8.5 watts of single frequency 532-nm light from a diode pumped intra-cavity ring laser,” in Proc. CLEO/OSA, vol. 9. Washington D.C., 1996, no. CPD-21. [6] D. J. Spence, P. Dekker, and H. M. Pask, “Modeling of continuous wave intracavity Raman lasers,” IEEE J. Sel. Topics Quantum Electron., vol. 13, no. 3, pp. 756–763, May–Jun. 2007. [7] W. Nighan, J. Cole, and T. Baer, “Diode pumped, multiaxial mode, intracavity doubled laser,” U.S. Patent 5 446 749, Aug. 29, 1995. [8] V. Magni, G. Cerullo, S. De Silvestri, O. Svelto, L. J. Qian, and M. Danailov, “Intracavity frequency doubling of a CW high-power TEM00 Nd:YLF laser,” Opt. Lett., vol. 18, no. 24, pp. 2111–2113, Dec. 1993. [9] X. Peng, L. Xu, and A. Asundi, “High-power efficient continuous-wave TEM00 intracavity frequency-doubled diode-pumped Nd:YLF laser,” Appl. Opt., vol. 44, no. 5, pp. 800–807, Feb. 2005.
LIN et al.: AMPLITUDE NOISE IN A CW INTRACAVITY FREQUENCY-DOUBLED RAMAN LASER
[10] A. E. Siegman, Lasers. Mill Valley, CA: University Science, 1986, ch. 25, pp. 962–964. [11] R. W. Boyd, Nonlinear Optics, 3rd ed. New York: Academic, 2008, ch. 10, pp. 473–508. [12] X.-G. Wu and P. Mandel, “Second-harmonic generation in a multimode laser cavity,” J. Opt. Soc. Am. B, vol. 4, no. 11, pp. 1870–1877, 1987. [13] P. Mandel and X.-G. Wu, “Second-harmonic generation in a laser cavity,” J. Opt. Soc. Am. B, vol. 3, no. 7, pp. 940–948, 1986. [14] P. Dekker, H. M. Pask, D. J. Spence, and J. A. Piper, “Continuouswave, intracavity doubled, self-Raman laser operation in Nd:GdVO4 at 586.5 nm,” Opt. Exp., vol. 15, no. 11, pp. 7038–7046, May 2007.
Jipeng Lin received the B.E. degree in physical engineering from Tsinghua University, Beijing, China, in 2005. He is currently pursuing the Ph.D. degree in MQ photonics, Department of Physics and Astronomy, Macquarie University, Sydney, Australia. His current research interests include solid-state Raman lasers and visible laser systems.
Helen M. Pask received the B.Sc. and Ph.D. degrees in physics from Macquarie University, Sydney, Australia, in 1987 and 1992, respectively. She was a Post-Doctoral Researcher working in the field of fiber lasers at the University of Southampton, Southampton, U.K. In 1995, she joined
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Macquarie University, where she is a Vice-Chancellor’s Innovation Fellow in the Department of Physics and Astronomy. Her current research interests include crystalline Raman lasers, solid-state lasers, Raman spectroscopy, and terahertz lasers and applications.
Andrew J. Lee received the Bachelors degree (Honors First Class) in optoelectronics and the Ph.D. degree in physics from Macquarie University, Sydney, Australia, in 2001 and 2007, respectively. He is currently a Research Associate at Macquarie University, and is working on the development of new Raman laser technologies. His current research interests include development of high power continuous-wave crystalline Raman laser sources, fabrication of optical micro- and nano-structures using laser-based processes, and terahertz generation.
David J. Spence received the M.Phys. degree and the D.Phil degree in the field of laser-plasma interactions from Oxford University, Oxford, U.K., in 1997 and 2001, respectively. He is currently a Senior Lecturer at Macquarie University, Sydney, Australia. His current research interests include lasers and nonlinear optics, with emphasis on ultrafast lasers and Raman lasers. He was awarded an Australian Post-Doctoral Fellowship from the Australian Research Council to study ultraviolet lasers at Macquarie University in 2006, after continuing as a PostDoctoral Fellow at Oxford University.
