March 15, 2014 / Vol. 39, No. 6 / OPTICS LETTERS

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Optical noise of stabilized high-power single frequency optically pumped semiconductor laser Alexandre Laurain,1,* Cody Mart,1 Jörg Hader,1 Jerome V. Moloney,1 Bernadette Kunert,2 and Wolfgang Stolz2 1

College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA 2 Department of Physics, Philipps Universität, Marburg 35032, Germany *Corresponding author: [email protected]

Received January 3, 2014; revised February 11, 2014; accepted February 12, 2014; posted February 12, 2014 (Doc. ID 203681); published March 12, 2014 We present a study of an actively stabilized optically pumped semiconductor laser operating single frequency at a wavelength of 1015 nm. In free running operation, the laser exhibits a single frequency output power of 15 W with a linewidth of 995 kHz for a sampling time of 1 s. The intensity and the frequency of the laser were independently stabilized to reach a laser linewidth of only 4 kHz for the same sampling time. To identify and reduce the different sources of noise, the relative intensity noise and frequency noise spectral density are investigated under various conditions. © 2014 Optical Society of America OCIS codes: (140.3570) Lasers, single-mode; (140.3425) Laser stabilization; (140.5960) Semiconductor lasers; (140.7270) Vertical emitting lasers; (030.1640) Coherence. http://dx.doi.org/10.1364/OL.39.001573

The spatial and temporal coherence of lasers along with the power and the long-term stability are of paramount importance for numerous scientific applications, such as gravity-wave detectors, high-performance optical gyroscopes, high-resolution spectroscopy, lidar application, laser cooling, and atomic-clock research. Lasers with stable frequency and intensity are also required for long-distance optical communications, where a stable local oscillator laser is needed for heterodyne-detection schemes. To fully understand and optimize the performance of such laser sources, it is necessary to have a detailed and quantitative knowledge of the physical or technical processes that limit the performances. The purpose of this Letter is to investigate these limits in the case of an actively stabilized optically pumped semiconductor laser (OPSL), also referred to as a vertical external cavity surface emitting laser (VECSEL). VECSELs have shown great potential for the generation of high-power, highbrightness laser output with high temporal coherence. The semiconductor active medium combined with the external cavity geometry offers enormous flexibility for generating TEM00 beams at wavelengths ranging from mid-infrared [1–3] to deep ultraviolet via efficient intracavity harmonic generation [4], as well as terahertz radiation via difference frequency generation [5]. To date, the maximum power obtained with a single chip VECSEL was demonstrated around 1 μm with more than 100 W in multimode operation [6,7], and recently we demonstrated an output power of more than 15 W in single frequency operation with a sub-MHz linewidth [8]. In this Letter, we demonstrate a robust active stabilization of this single frequency laser source and we investigate the intensity and frequency noise under free running and stabilized operation. The theoretical and technical limits are also discussed. The schematic of the high-power single frequency VECSEL is shown in Fig. 1. The OPSL structure consists of a half-wavelength InGaP confinement layer, a resonant periodic gain region composed of 10 InGaAs quantum wells placed at the antinodes of the optical field and surrounded by pump-absorbing GaAs(P) barriers, and 0146-9592/14/061573-04$15.00/0

a high-reflectivity (HR) Bragg mirror composed of 22.5 AlAs and GaAs quarter wavelength layer pairs. The structure was grown in reverse order on a (100) GaAs substrate by metal-organic vapor phase epitaxy and was designed to emit around λ  1020 nm at room temperature. The grown structure was then processed as described in [8]. The Z-shaped VECSEL cavity is folded with one HR mirror with a radius of curvature of 30 cm, and a small flat HR mirror mounted on piezoelectric transducer (PZT). The end mirror is a flat 3% output coupler. The selection of the longitudinal and polarization mode is realized with a 500 μm thick YAG etalon and a 2.5 mm thick quartz birefringent filter placed in the cavity at Brewster’s angle. The entire OPSL device was placed in a thermally and acoustically insulated box and set on an air-floated optical table in order to reduce intensity and frequency fluctuations. In order to stabilize the laser frequency, a fraction of the laser beam is optically isolated and mode matched into a Fabry–Perrot interferometer (FPI), which consists of two fixed dielectric mirrors spaced by a zerodur material (low thermal expansion glass). The measured Fabry–Perrot cavity frequency bandwidth is 11.2 MHz. A fraction of the beam was also collected by a photodiode to stabilize the laser intensity by modulating the pump intensity. We should note that Fiber-coupled Pump Diodes

TEC CVD Diamond

Pump driver PD

OPSL ET

PZT

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OC=3%

Copper Heatsink

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Mode-matching lens

Fig. 1. Schematic setup of the stabilized single frequency OPSL device. © 2014 Optical Society of America

OPTICS LETTERS / Vol. 39, No. 6 / March 15, 2014

Output power characteristic.

the use of optical isolators is necessary, as the feedback from the FPI or the photodiode could destabilize the laser, resulting in multimode operation. The output power characteristic is plotted in Fig. 2. We reached a maximum output power of 15.1 W for an incident pump power of 112 W at a temperature of 10°C. We should note that about 30% of the incident pump power is reflected by the surface of the uncoated chip. To demonstrate single frequency operation, the laser spectrum was recorded at high power with a grating spectrometer [Fig. 3(a)] and with a high-resolution scanning FPI to resolve the longitudinal modes [Fig. 3(b)]. In order to identify and reduce the sources of noises limiting the performance of the VECSEL, the intensity and the frequency noise of the laser were investigated. First, the relative intensity noise (RIN) of the VECSEL was measured in free running condition, while the optical cavity was exposed to the laboratory environment. Figure 4 shows a 1∕f α noise below 1 kHz, which is characteristic of acoustic and thermal noise. The VECSEL cavity was then enclosed in a thermally and acoustically insulated box, and this contribution was reduced by more than 20 dB at 10 Hz. The remaining noise peaks around 60 Hz are due to the electrical noise of the pump current driver that is directly transferred to the VECSEL via the pump intensity fluctuation. The pump RIN was measured at high power in free running condition; the relative rms intensity noise in the 1 Hz–5 MHz range is 0.15%. This relatively high value is partly due to the partition noise in the highly multimode pump fiber, and partly due to the switching-mode power supply used here that causes strong intensity fluctuations at the switching frequency and its harmonics. These peaks are greatly reduced when the VECSEL intensity is stabilized and the RIN spectrum is nearly flat up to 6 kHz at a level of −95 dB. The small peak at 6 kHz is due to the resonance of the feedback circuit. In all these cases, the RIN spectrum decreases after 600 kHz until the fluctuation power

Fig. 3. (a) Laser spectrum recorded at maximum power. (b) Transmission through a 55 MHz FWHM confocal scanning FPI (free spectral range of 1.5 GHz).

Fig. 4. RIN of the VECSEL at maximum power under various conditions.

spectral density reaches the shot noise, as the pump intensity fluctuation is filtered by the high-Q VECSEL cavity. Indeed, the photon lifetime is well above the carrier lifetime, leading to a relaxation-oscillation-free dynamic. Finally, when the intensity and frequency were both stabilized, the RIN spectrum was further reduced below 1 kHz and above 10 kHz. This contrasts with monolithic lasers where a stabilization of the wavelength generally results in an increase of the intensity noise. In our case, the intensity noise is partly coupled to the frequency noise of the VECSEL. We attribute this effect to the spectral filtering of the cavity, which modulates the optical losses when the frequency fluctuates. The resulting relative rms fluctuation in the 1 Hz–5 MHz frequency range has been reduced from 2% to 60dB

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for comparison. We can see a reduction of the frequency noise of about 60 dB at 10 Hz. The bandwidth of the servo loop is limited by the PZT resonance, which is at 25 kHz. This value has been optimized by using a small mass mirror on the PZT, as the resonance frequency is inversely proportional to the root of the total mass. The resulting rms frequency noise has been reduced from 720 to 30.2 kHz for a sampling time of 1 s. We also plotted in Fig. 6 the pump-induced thermal fluctuations calculated with the measured pump RIN [Eq. (1)]. The experimental spectrum shows relatively good agreement with the simulated one, with additional noise peaks around 200 Hz that are due to mechanical vibrations generated by the water cooling system. Since the main frequency noise limitation appears to come from the pump intensity fluctuations, it is highly beneficial for linewidth narrowing to stabilize the intensity noise of the VECSEL, as it directly follows the pump fluctuations up to the cutoff frequency of the optical cavity. The strong frequency noise reduction obtained here is then a result of both the intensity and the frequency stabilization. We should note that the use of a linear power supply would also be beneficial, as they are usually quieter, and in some case their use can result in a frequency noise dominated by mechanical noise [12]. Above 10 kHz, the frequency noise reaches a plateau at 2.103 Hz2 ∕Hz, which is about two orders of magnitude higher than the level expected from the pump-induced carrier fluctuations. This discrepancy might come from an underevaluation of the linewidth enhancement factor or Henry factor αh , since the VECSEL operates at a relatively high carrier density [13]. Again, the frequency noise is only limited by technical noise, as the fundamental quantum noise or Schawlow– Townes–Henry limit is far below here (Fig. 5). Finally, the linewidth associated with these two regimes was deduced from integral computation of the frequency noise spectral density [14]. When the VECSEL is enclosed in the insulating box, the laser spectrum exhibits a quasi-Voigt profile with a FWHM of 995 kHz in free running operation (Fig. 7). In stabilized operation, the spectrum presents a Gaussian-shaped central peak of 4 kHz FWHM with Lorentzian-like wings containing about 40% of the total power. We should note that in the latter case, the estimated linewidth corresponds to the upper limit of the actual linewidth, as the frequency noise spectrum also contains the electronic noise from the servo loop. We presented a study of the intensity and frequency noise properties of a 15 W single frequency VECSEL in

feedback resonance at 25kHz

1k 10k Frequency (Hz)

100k

1M

Fig. 6. Frequency noise spectral density of a free running VECSEL compared to the intensity and frequency stabilized VECSEL. A simulation of the pump-induced thermal fluctuations is also plotted.

Fig. 7. Laser power spectrum density computed from the frequency noise spectrum density of (a) a free running and (b) intensity and frequency stabilized VECSEL.

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free running and actively stabilized conditions. We performed a simultaneous stabilization of the intensity and wavelength of the laser. We showed that a stabilization of the laser wavelength actually decreases the intensity noise, leading to a low-intensity noise source (0.22% rms over 1 s). We also showed that the frequency noise of the free running VECSEL is mainly limited by the pumpinduced thermal fluctuation such that a stabilization of the VECSEL intensity can reduce the frequency noise. With these relatively simple stabilization methods, we demonstrated up to 60 dB of frequency noise reduction in the low-frequency range, where the noise is the strongest, leading to a very stable, narrow linewidth (4 kHz over 1 s) and high-power laser source. These sources are of high interest for applications, such as spectroscopy, metrology, or remote sensing. Moreover, single frequency, high-power VECSEL master oscillators have marked potential use in free-space applications, as no further power amplification will be required and the amount of amplified spontaneous emission is negligible. This work was supported by the Air Force Office for Scientific Research under the grant FA9550-14-1-0062. References 1. A. Laurain, L. Cerutti, M. Myara, and A. Garnache, IEEE Photon. Technol. Lett. 24, 246 (2012).

2. B. Rösener, M. Rattunde, R. Moser, S. Kaspar, T. Töpper, C. Manz, K. Köhler, and J. Wagner, Opt. Lett. 36, 319 (2011). 3. A. Ouvrard, A. Garnache, L. Cerutti, F. Genty, and D. Romanini, IEEE Photon. Technol. Lett. 17, 2020 (2005). 4. Y. Kaneda, M. Fallahi, J. Hader, J. V. Moloney, S. W. Koch, B. Kunert, and W. Stoltz, Opt. Lett. 34, 3511 (2009). 5. J. R. Paul, M. Scheller, A. Laurain, A. Young, S. W. Koch, and J. Moloney, Opt. Lett. 38, 3654 (2013). 6. T.-L. Wang, B. Heinen, J. Hader, C. Dineen, M. Sparenberg, A. Weber, B. Kunert, S. Koch, J. Moloney, M. Koch, and W. Stolz, Laser Photon. Rev. 6, L12 (2012). 7. B. Heinen, T.-L. Wang, M. Sparenberg, A. Weber, B. Kunert, J. Hader, S. Koch, J. Moloney, M. Koch, and W. Stolz, Electron. Lett 48, 516 (2012). 8. A. Laurain, C. Mart, J. Hader, J. Moloney, B. Kunert, and W. Stolz, IEEE Photon. Technol. Lett. 26, 131 (2014). 9. A. Laurain, M. Myara, G. Beaudoin, I. Sagnes, and A. Garnache, Opt. Express 18, 14627 (2010). 10. M. Myara, M. Sellahi, A. Laurain, A. Michon, I. Sagnes, and A. Garnache, Proc. SPIE 8606, 86060Q (2013). 11. M. Reichling and H. Grönbeck, J. Appl. Phys. 75, 1914 (1994). 12. S. Kaspar, M. Rattunde, T. Topper, B. Rosener, C. Manz, K. Kohler, and J. Wagner, IEEE J. Quantum Electron. 49, 314 (2013). 13. J. Stohs, D. Bossert, D. Gallant, and S. Brueck, IEEE J. Quantum Electron. 37, 1449 (2001). 14. K. Petermann, Laser Diode Modulation and Noise (Kluwer, 1988).

Optical noise of stabilized high-power single frequency optically pumped semiconductor laser.

We present a study of an actively stabilized optically pumped semiconductor laser operating single frequency at a wavelength of 1015 nm. In free runni...
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