1. Introduction

Laser technology is maturing rapidly and is finding applications in areas such as high resolution spectroscopy, medicine, optical telecoms, radar-lidar, metrology…where highly coherent tunable high power light sources are required. Thus there is a strong interest to develop high quality laser operating in the continuous-wave (cw) regime at room temperature (RT) in the λ=0.5−3µm range. Single frequency tunable solid-state lasers rely on intracavity filtering, in a commonly bulky complex optical system. A highly coherent tunable laser design can be achieved using an extended-cavity quantum-well (QW) semiconductor surface emitting laser in a stable optical cavity, so called External-cavity VCSELs (VeCSELs). In addition to compactness and low power consumption, semiconductor lasers have the advantage to cover a wide emission wavelength range, from the blue to the infrared at RT exploiting III-V semiconductor technologies. Diode-pumped VeCSELs combine the approach of diode-pumped solid-state lasers and engineered semiconductor lasers, generating both low divergence circular diffraction limited output beams (TEM₀₀) and multiwatt powers in cw at RT [1]. Indeed this high-Q vertical cavity design boosts the light coherence, generating continuously-tunable kHz-linewidth single-frequency high efficiency laser with a linear polarization state, without any intracavity filter, as reported in [2, 3, 4] at power levels ≤20mW. These features are intrinsic to the use of a high-Q air-gap relatively ”long” stable cavity with a homogeneous gain - as with QW’s -, where amplified spontaneous emission and non-linear mode interactions are negligible [4]. Other works reported on high power single frequency VeCSEL operation at RT using intracavity filters [5, 6, 7], up to 0.5W [7] at 283 K, but in bulky and relatively complex laser systems with low efficiency <8%, no continuous tunability, and a broad free running linewidth >100 MHz.

In this paper, we demonstrate high power high efficiency single frequency operation of a low noise compact QW VeCSEL with a broad continuous tunability, optically pumped by a multimode diode laser at RT in cw. No thermal management was required to get similar power levels as published in [5, 6, 7]. We took advantage of thermal lens–based stability to develop a short external cavity without the need of any intracavity filter. The beam quality factor (M²), the side-mode-suppression-ratio (SMSR), the polarization state, the relative intensity noise (RIN) and the frequency noise are studied. The design principles can be extended to any wavelength.

2. High power VeCSEL device technology

2.1. 1/2–VCSEL structure design: the gain mirror

The GaAs-based 1/2-VCSEL structure emitting at λ=1µm designed for ~800 nm pumping was grown by MOCVD in a D180-Veeco TurboDisc reactor using TMGa, TMAl, TMIn, and AsH3 at 60 mTorr and at a temperature of 700° C. It is composed of an epitaxial high-reflectivity (99.9%) bottom AlAs/GaAs Bragg mirror (27.5 pairs) and an active layer on top, designed with 6 strained balanced InGaAs/GaAs(P) QWs in a 13λ/2 long active region, avoiding crystal darklines (fast traps). A low excitation carrier lifetime >10 ns was measured (and ~3 ns at laser threshold as calculated), ensuring complete bleaching of residual absorption in weakly-pumped laser areas. The QW’s are distributed among the optical standing-wave antinodes with a distribution function 11101010100 (from air to bragg mirror) such as the excited carrier density is almost equal in the QW’s, to ensure a low laser threshold and a homogeneous gain [4]. The QW number is optimized for low threshold and large differential gain, adapted for 1–2% cavity losses. A 30 nm AlAs confinement layer and a 8 nm GaAs caping layer are added on top. A λ/4 thick Si₃N₄ antireflection coating is evaporated on top to suppress microcavity effect and pump reflection. Thus 88% of the incident pump power P_p is absorbed in the active region thick GaAs barriers. For this simple technology device, no post-growth processing is required as no thermal management technology was applied here. Figure 1(a) shows the reflectivity and low excitation photoluminescence spectra of the as-grown structure.

 

Fig. 1. a) Reflectivity and photoluminescence of the 1/2-VCSEL structure grown on GaAs emitting at 1µm. b) High power single frequency tunable VeCSEL design

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2.2. Short stable cavity principle and setup

The VeCSEL device is formed by the gain mirror (1/2-VCSEL), a millimeter air gap L, and a commercial flat mirror with a reflectivity of 1–T_oc⋍99% (Fig. 1(b)). The chip is soldered on a Peltier element to stabilize the chip temperature with a precision of 10⁻³ K. A Piezoelectric Transducer (PZT) is used to tune the cavity length over 5µm, thus the laser frequency. The external mirror is held by an ultra-stable mirror mount (New Focus 9882). The 1/2-VCSEL structure is optically–pumped in cw by a commercial fibre–coupled high power multimode GaAs laser diode (JDSU, P_p=1W at 830 nm, 60mm core diameter). The pump beam was focused on a circular spot size of 2w_p=80µm diameter (FWHM) with two commercial achromat lenses at a incidence angle of ~5°. The components are glued on a home made breadboard and inserted in a metallic box for thermal and acoustic noise isolation.

To be in a single frequency light state, the laser has to operate on a single transverse and longitudinal mode, and light polarization state (linear along [110] here, see section 3) [2].

First, in order to stabilize only the fundamental transverse mode in this short free space cavity, we took advantage of pump induced thermal lens [2, 8], without here the need of a concave mirror (Fig. 2). Taking an average index change in the 1/2-VCSEL of dn/dT⋍2.7×10⁻⁴/K as measured in our structure, assuming a parabolic index profile (Fig. 2(a)), leads to a thermal lens value in the thin lens approximation of [9]

for an incident pump power P_p=1W. L_µc=2.4µm is the wave penetration length in the 1/2-VCSEL structure. $R_{\mathit{th}}\approx \eta \sqrt{\mathit{ln}\left(2\right)}⁄2\sqrt{\pi }\kappa w_p$ is the thermal impedance of the structure function of P_p, assuming here a gaussian pump beam and a homogeneous medium (precisely calculated with FEMLAB in our case);κ here is the thermal conductivity of GaAs, and η⋍0.35 is the fraction of heat power generated. Figure 2(b) shows the measured and calculated cavity stability diagrams, in good agreement, for a gaussian TEM₀₀ beam at the diffraction limit.

To stabilize only one longitudinal mode in the gain bandwidth, we took advantage of the ideal homogeneous gain behavior of QW VeCSEL [4], where non-linear mode interactions are weak. Then we chose a short cavity length L, without the need of any intracavity filter, to prevent from multimode operation due to technical perturbations, specially while using a multimode noisy pump [4, 7]. Indeed, although at the very beginning of the laser operation, several longitudinal cavity modes are amplified by the gain medium, the laser collapses to a single mode operation after a characteristic time t_c, given in [4, 7]. However if any process disturbs the laser dynamics before t_c (like gain frequency jitter), the emission remains spectrally multimode in cw operation. Note that mechanical fluctuations have negligible impact on gain jitter. Thermal fluctuations induce gain jitter at a rate of ~110 GHz/K, thus possible laser broadening: to develop a robust single mode laser the cavity has to be designed so that t_c is faster than any thermal fluctuations. Gain jitter is mainly generated from pump fluctuations via the thermal impedance (peltier fluctuations are slow and weak here). Thermal fluctuations are thus characterized by a short pass filter defined by thermal diffusion in the 1/2-VCSEL structure, with a cutoff frequency f^th_c<10 kHz (@–3 dB, see Fig. 5(b)). Assuming a 1D diffusion model in a homogeneous medium, we get f_thc ≈κ/πw ² _p ρC_T, where ρ is the semiconductor density and C_T is the heat capacity. To prevent from multimode operation and mode hopping, we thus get the hard condition 1/2πt_c≫f^th_c. This hard condition leads to a cavity length shorter than [4]

with our parameters. Γ_g⋍5 THz is the modal gain bandwidth (HWHM) [7] without any filter. For short enough L, slow thermal fluctuations will only generate possible mode hopping.

This short cavity design allows a broad continuous frequency tuning. These QW VeCSEL design principles are valid at any wavelength by using suitable semiconductor materials.

 

Fig. 2. a) Radial pump profile (experimental), temperature profile (FEMLAB simulation), and induced index profile (parabolic fit) in the 1/2-VCSEL. b) Thermal lens-based cavity stability: experimental/theoretical waist (@1/e ²) on the 1/2-VCSEL varying with (L ,P_p).

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3. Free running single frequency operation

Single frequency TEM₀₀ operation is observed at ~1.02µm and for cavity length ranging from L=0.2 to 1.6 mm. The maximum output power measured is 300mW at RT in cw (Fig. 3(a)) for P_p=0.92W and L⋍1 mm, limited by thermal roll over. The apparent high threshold is due to bad pump to laser overlapping without thermal lens, leading to a >100% actual slope efficiency as the laser waist reduces. QW absorption is negligible and quickly bleached, as laser waist is not much bigger than pump waist above threshold (~20× transparency density at peak intensity) and laser intensity rapidly stronger than the absorption saturation intensity respectively. The circular TEM₀₀ beam (Fig. 3(b)) shows a M ²<1.05 at maximum power very close to diffraction limit, in spite of a poor quality multimode pump diode beam, and a low divergence of 0.4° (FWHM). The SMSR is 60 dB, for L=1mm and maximum power (Fig. 4(a)), close to the quantum limit [4]. The light polarization is linear along the [110] crystal axis thanks to gain dichroïsm in QWs [2, 4]. The orthogonal polarization extinction ratio is >37 dB. We recorded a mode hope free tunability as wide as 840 GHz (or 2.9 nm) for L=400µm, varying L with the PZT voltage, by measuring the transmission through a GaAs Fabry-Perot etalon (Fig. 4(b)). The measured laser (gain) tuning rate with temperature is ~120 GHz/K.

 

Fig. 3. a) Single frequency VeCSEL output power in cw at RT. Conversion efficiency ~0.3W/A. b) Circular far field transverse distribution close to diffraction limit at high power.

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Fig. 4. a) Laser spectrum at high power recorded with an optical spectrum analyzer (15 GHz resolution). Solid vertical lines show the longitudinal mode positions. b) Laser frequency tunability (vs PZT voltage) recorded through a GaAs Fabry–Perot etalon (93 GHz FSR).

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4. Noise properties: intensity noise and linewidth

This high-Q external cavity approach leads to low RIN and frequency noise device, even pumping with a mutlimode high power diode. The free running single frequency VeCSEL was studied in terms of RIN (Fig. 5(a)) using a 50W high speed Si photodiode and an optical isolator (>40 dB). The VeCSEL RIN shows a laser cutoff frequency (@–3 dB) f^L_c~250MHz and a pump to laser RIN ratio close to 1 far above threshold, in agreement with the single mode laser theory [9, 10]. For small amplitude signal, large pumping and L~1 mm, this laser dynamics exhibits a relaxation-oscillation-free regime, even if in the transitional regime from class-B to class-A [4, 9, 10]. The emitted power is ultra-stable, leading to relative rms fluctuations ~0.15% at 0.3W output power. Above f^L_c the laser RIN reaches the shot noise level.

 

Fig. 5. a) Pump RIN_p, VeCSEL RIN and theoretical RIN transfer function spectra. The pump RIN_p spectrum is flat below 100 kHz down to 100 Hz. b) Frequency noise spectral density (experiment and theory) originating from pump noise (normalized to L ^-2).

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We performed direct frequency noise measurements using a home-made stable planoconcave Fabry-Perot interferometer (2 cm long, 850 finesse, 8.8MHz cutoff frequency) as frequency discriminator. The linewidth fundamental limit given by quantum noise is very low, at the Hz level here [4]. In our case we measured a rms laser frequency noise of 900 kHz (over 1 ms) for L⋍1.3mm without any active frequency stabilization. The laser frequency noise spectral density (Fig. 5(b)) exhibits a plateau below a low cutoff frequency <10 kHz, similar to the thermal diffusion cutoff frequency f^th_c in the 1/2-VCSEL structure, as simulated with FEMLAB (Fig. 5(b)). Therefore, we believe that the laser linewidth is pump noise induced thermal fluctuations limited. Note that mechanical noise generated almost a plateau (from 1 kHz down to 20 Hz) of frequency noise density ~5.10⁸ Hz ²/Hz×mm ², as measured in another experiment with the same prototype. The rms frequency noise, assuming a 2^nd order short pass filter for the thermal fluctuations power spectrum of amplitude RIN_pP ² _p R ² _th, is thus given by [10]

where Γ_T⋍dn/dT×c/λ×L_µc/L is the temperature to frequency fluctuations conversion coefficient average in the 1/2-VCSEL structure. The laser spectrum should exhibit a gaussian shape with a FWHM of $2\sqrt{\mathit{ln}\left(2\right)⁄\pi }\times \mathrm{\Delta }{\nu }_{\mathit{rms}}\simeq 850\mathrm{kHz}$ as Δν_rms is well greater than f^th_c [10]. This value is similar to what would be measured using a standard heterodyne technique. The linewidth obtained can be reduced by a decade optimizing L and wp using Eqs.(2,3).

5. Conclusion

We demonstrated a new scheme of QW semiconductor laser for high power (0.3 W) high efficiency low noise (sub-MHz linewidth) tunable single frequency source, based on a short cavity VeCSEL. We obtained diffraction limit beam even pumping with a low quality multimode diode laser. Pump properties define the cavity design and laser coherence. No special heat management was required here, but exploiting diamond bonded 1/2-VCSEL structure [1, 7] opens potential for multiwatt power level with similar properties, which can be extended to any wavelength by using suitable semiconductor materials. This low noise compact device shows higher quality than standard commercial laser diodes or solid-state lasers.

This work was supported by the French ANR MIREV program.