## Single-Frequency operation of External-Cavity VCSELs: Non-linear multimode temporal dynamics and quantumlimit

Optics Express, Vol. 15, Issue 15, pp. 9403-9417 (2007)

http://dx.doi.org/10.1364/OE.15.009403

Acrobat PDF (9208 KB)

### Abstract

We present an experimental and theoretical investigation of the non-linear multimode dynamics of external–cavity VCSELs emitting at 1 and 2.3 *μ*m. We account for the stable single–frequency and linearly polarized emission by these laser sources, even in the presence of quantum noise and non-linear mode interactions originating from Four–Wave–Mixing via population pulsations in the quantum-wells. This fact is a consequence of the mode antiphase dynamics. Thanks to the high-Q external cavity configuration, the laser dynamics fall into the oscillation-relaxation-free class-A regime. The characteristic time to achieve single mode emission is ~1 ms for a 15 mm long cavity with an antireflection coated structure and no spectral filter, as for an “ideal” homogeneous gain laser. The side mode suppression ratio is as high as 40 dB, close to the quantum limit. The laser linewidth is at the quantum limit, and is ~1 Hz at 1mW output. An experimental value <20 kHz has been established. Under standard conditions, without spectral filtering, the optimum cavity length for highly coherent single mode operation is expected in the range 5 to 30 mm. Finally, for cavity lengths typically shorter than 5 mm, we rather have an “ideal” homogeneous gain class-B laser, exhibiting oscillation-relaxation of the intensity in the 0.1 GHz range. These properties contrast with the intrinsic strongly non-linear dynamics of conventional semiconductor lasers.

© 2007 Optical Society of America

## 1. Introduction

*µ*m. Semiconductor laser technology is maturing rapidly and is finding applications in areas such as high resolution spectroscopy (trace gas analysis, in-situ H

_{2}0 and CO

_{2}isotope ratio measurements), medicine, optical telecommunications and radar-lidar, metrology where ultra-narrow low noise sources are required. Thus there is a strong interest in developing high quality single frequency tunable lasers operating CW at RT with 1 to 1000 mW of output power in the

*λ*=0.8--2.7

*μ*m range, where III–V semiconductors (GaAs, InP, GaSb) are now available.

1. A. Garnache, A. Ouvrard, L. Cerutti, D. Barat, A. Vicet, F. Genty, Y. Rouillard, D. Romanini, and E. Cerda- Méndez, “2–2.7mm single frequency tunable Sb-based lasers operating in CW at RT: Microcavity and External- cavity VCSELs, DFB,” Proc. SPIE Photonics Europe, Semiconductor lasers and laser dynamics pp. 6184–23 (2006).

_{00}beam at high output power [2

2. S. Lutgen, T. Albrecht, P. Brick, W. Reill, J. Luft, and W. Spath, “8-W High-Efficiency Continuous-Wave Semiconductor Disk Laser at 1000 nm,” Appl. Phys. Lett. **82**, 3620–3622 (2003). [CrossRef]

1. A. Garnache, A. Ouvrard, L. Cerutti, D. Barat, A. Vicet, F. Genty, Y. Rouillard, D. Romanini, and E. Cerda- Méndez, “2–2.7mm single frequency tunable Sb-based lasers operating in CW at RT: Microcavity and External- cavity VCSELs, DFB,” Proc. SPIE Photonics Europe, Semiconductor lasers and laser dynamics pp. 6184–23 (2006).

3. A. Ouvrard, A. Garnache, L. Cerutti, F. Genty, and D. Romanini, “Single Frequency Tunable Sb-based VCSELs emitting at 2.3mm,” IEEE Photon. Technol. Lett. **17**, 128–134 (2005). [CrossRef]

4. M. Holm, D. Burns, and A. Ferguson, “Actively Stabilized Single-Frequency Vertical-External-Cavity AlGaAs Laser,” IEEE Photon. Technol. Lett. **11**, 1551–1553 (1999). [CrossRef]

1. A. Garnache, A. Ouvrard, L. Cerutti, D. Barat, A. Vicet, F. Genty, Y. Rouillard, D. Romanini, and E. Cerda- Méndez, “2–2.7mm single frequency tunable Sb-based lasers operating in CW at RT: Microcavity and External- cavity VCSELs, DFB,” Proc. SPIE Photonics Europe, Semiconductor lasers and laser dynamics pp. 6184–23 (2006).

3. A. Ouvrard, A. Garnache, L. Cerutti, F. Genty, and D. Romanini, “Single Frequency Tunable Sb-based VCSELs emitting at 2.3mm,” IEEE Photon. Technol. Lett. **17**, 128–134 (2005). [CrossRef]

*µ*m) stable optical cavity, where the contribution of amplified spontaneous emission is negligible. Previous publications reported on the stable single-frequency plane-concave short-cavity Quantum-Well (QW) VeCSEL s operating at RT at

*λ*=1

*µ*m (GaAs-based) [5

5. A. Garnache, A. Kachanov, F. Stoeckel, and R. Houdré, “Diode-pumped broadband Vertical-External-Cavity Surface-Emitting semiconductor Laser. Application to high sensitivity intracavity laser absorption spectroscopy,” J. Opt. Soc. Am. B **17**, 1589 (2000). [CrossRef]

*λ*=2.3

*µ*m (GaSb-based) [3

3. A. Ouvrard, A. Garnache, L. Cerutti, F. Genty, and D. Romanini, “Single Frequency Tunable Sb-based VCSELs emitting at 2.3mm,” IEEE Photon. Technol. Lett. **17**, 128–134 (2005). [CrossRef]

*µ*m and 2.3

*µ*m. We will then present a modeling of the spectro-temporal dynamics of the QW VeCSEL including quantum noise. We demonstrate why theses sources operate stable single-frequency and linearly polarized, even without any intracavity selective element, in the presence of Four-Wave-Mixing (FWM) in the QWgain during the build-up transient. This is the most relevant non-linear mode interaction occurring in this laser source and is a consequence of pulsations of the carrier density at the intermode beat frequencies [6

6. S. Kovalenko, S. Semin, and D. Toptygin, “Influence of the Raman mode interaction on the lasing kinetics of a wide-band ring laser,” Sov. J. Quantum Electron. **21(4)**, 407–411 (1991). [CrossRef]

*λ*. It is valid for any linear or ring cavity of length shorter than typically 3m if the beam incidence angle on the 1/2-VCSEL is

*<*λ/w

_{0}(w

_{0}the laser beam radius).

## 2. Experimental study

### 2.1. Setup and device principles

*λ*thick active region (depending on the pump absorption coefficient). Details of the GaAs- and GaSb-based structures, for the 1

*µm*(InGaAs/GaAs QWs) and 2.3

*µm*(InGaAsSb/AlGaAsSb QWs) spectral range can be found in [5

5. A. Garnache, A. Kachanov, F. Stoeckel, and R. Houdré, “Diode-pumped broadband Vertical-External-Cavity Surface-Emitting semiconductor Laser. Application to high sensitivity intracavity laser absorption spectroscopy,” J. Opt. Soc. Am. B **17**, 1589 (2000). [CrossRef]

7. L. Cerutti, A. Garnache, A. Ouvrard, and F. Genty, “High Temperature CW Operation of Sb-Based Vertical External Cavity Surface Emitting Laser near 2.3mm,” J. Cryst. Growth **268**, 128 (2004). [CrossRef]

_{3}N

_{4}(

*λ*/4 thick) antireflection coating was added at the air-semiconductor interface to suppress the residual microcavity effect. Even without this coating coupled-cavities effects are not relevant to the laser dynamics [9

9. S. Hodges, M. Munroe, J. Cooper, and M. Raymer, “Multimode laser model with coupled cavities and quantum noise,” J. Opt. Soc. Am. B **14**, 191–199 (1997). [CrossRef]

5. A. Garnache, A. Kachanov, F. Stoeckel, and R. Houdré, “Diode-pumped broadband Vertical-External-Cavity Surface-Emitting semiconductor Laser. Application to high sensitivity intracavity laser absorption spectroscopy,” J. Opt. Soc. Am. B **17**, 1589 (2000). [CrossRef]

10. A. Garnache, S. Hoogland, A. Tropper, I. Sagnes, G. Saint-Girons, and J. Roberts, “Sub-500-fs soliton-like pulse in a passively mode-locked broadband surface-emitting laser with 100-mW average power,” Appl. Phys. Lett. **80**, 3892–3894 (2002). [CrossRef]

*µ*m thick). Thus the typical mode number within the gain bandwidth (full-width at half-maximum, FWHM) for a 15(4)mm long cavity is 500(130) with a smooth gain profile: the laser cavity is strongly multimode. Without a wedge, single mode operation was observed to be more stable thanks to the substrate acting effectively as a weak intracavity etalon [5

**17**, 1589 (2000). [CrossRef]

^{2}at RT. The pump beam was focused on a spot size ranging from 20 to 40

*µ*m (FWHM). In order to match this to the cavity mode waist, the cavity length must be close to the edge of stability given by the mirror radius of curvature. Typically a lasing region was obtained in the range 20 to 400

*µ*m below this length. It is interesting to remark that the use of a multimode-fibre-coupled laser diode as a pumping source is observed to hinder single mode operation of a VeCSEL in CW, unless a selective spectral element is inserted in the cavity (e.g. a 50

*µ*m thick Fabry-Perot glass etalon) [8]. This is due to the large intensity fluctuations of this source, which generate strong enough QW temperature perturbations and thus modulate the gain spectral position. Thanks to the external cavity transverse stability, the emission can be forced TEM

_{00}using a tight pump beam size with respect to the laser cavity mode waist [1

### 2.2. Single frequency operation and transient spectral dynamics

*µ*m and 2.3

*µ*m (Fig. 1) [1

**17**, 128–134 (2005). [CrossRef]

**17**, 1589 (2000). [CrossRef]

*M*<1.2 [1

^{2}*µ*m and 5mW at 2.3

*µ*m in this configuration. Using a Fagbry-Perot interferometer, we measured a laser linewidth below 20 kHz (apparatus function limited; 20

*µs*integration time) with the 2.3

*µ*m device [1

**17**, 128–134 (2005). [CrossRef]

*µ*m of one of these laser systems is shown in Fig. 2. Mode hopping arose due to temperature drift of the optical bench. This on-the-bench device showed high stability over hours, as good as commercial single-frequency laser diodes or solid-state lasers.

**17**, 1589 (2000). [CrossRef]

*η*above threshold were tested (Fig.3). The VeCSEL transient observed on the photodiode was oscillation-relaxation-free, showing that this high-Q external cavity source worked in the class-A regime. An Acousto-Optic-Modulator, triggered on the pump pulse, allowed deflecting the laser beam into a monochromator during a short time Δ

*t*at a generation time

*t*. Spectra were recorded with a 2.5

_{g}*µ*m extended InGaAs linear-array photodiode (Xenics, 512 pixels). Special care was taken to avoid thermal effect induced spectral shift or broadening. Absorption lines also slightly perturbed the spectral narrowing. As shown in Fig.4, the narrowing of the laser spectral width (FWHM) Δσ slowed down at short generation times for higher

*η*. But at long

*t*(≫100ms), VeCSEL emission always became single mode. In the absence of non-linear mode interactions (i.e. for an “ideal” homogeneous gain laser), Δ

_{g}*σ*

^{-2}should be proportional to

*t*[5

_{g}**17**, 1589 (2000). [CrossRef]

*η*. On the other hand increasing spectral broadening with the pump rate at short generation times is a signature of non-linear mode interactions in a multimode laser [6

6. S. Kovalenko, S. Semin, and D. Toptygin, “Influence of the Raman mode interaction on the lasing kinetics of a wide-band ring laser,” Sov. J. Quantum Electron. **21(4)**, 407–411 (1991). [CrossRef]

14. M. Yamada, “Theoretical analysis of nonlinear optical phenomena taking into account the beating vibration of the electron density in semiconductor lasers,” J. Appl. Phys. **66**, 81–89 (1989). [CrossRef]

*t*of 1 s [5

_{g}**17**, 1589 (2000). [CrossRef]

### 3. Multimode VeCSEL non-linear dynamics theoretical study and the quantum limit

#### 3.1. Laser equation formulation

*k*≃ 200cm

_{B}T^{-1}at 300K [15]. Here the top of the gain profile is approximated by a parabolic function. Note that these assumptions become invalid if one considers a Quantum-Dot active region, which exhibits an inhomogeneous gain broadening due to non uniform dot size distribution. In this particular case, the laser spectrum would be multi-mode and the laser would manifest a more complex multi-mode dynamics [11, 16

16. M. Grundmann, “How a quantum-dot laser turns on,” Appl. Phys. Lett. **77**, 1428–1430 (2000). [CrossRef]

^{-1}

*≃10*

_{g}^{-4}ns is much shorter than the carrier lifetime

*τ*~ns and the photon lifetime. Thus the dynamics are governed only by the mode field amplitude and the carrier density. The assumptions done in this paper, as well as the parameters given values, are valid for typical (GaAs, InP, GaSb)-based VeCSELs emitting in the 0.8–3

*µ*m range, optically or electrically pumped (assuming uniform injection over the laser beam profile).

6. S. Kovalenko, S. Semin, and D. Toptygin, “Influence of the Raman mode interaction on the lasing kinetics of a wide-band ring laser,” Sov. J. Quantum Electron. **21(4)**, 407–411 (1991). [CrossRef]

9. S. Hodges, M. Munroe, J. Cooper, and M. Raymer, “Multimode laser model with coupled cavities and quantum noise,” J. Opt. Soc. Am. B **14**, 191–199 (1997). [CrossRef]

14. M. Yamada, “Theoretical analysis of nonlinear optical phenomena taking into account the beating vibration of the electron density in semiconductor lasers,” J. Appl. Phys. **66**, 81–89 (1989). [CrossRef]

17. P. A. Khandokhin, I. V. Koryukin, Y. I. Khanin, and P. Mandel, “Influence of carrier diffusion on the dynamics of a two-mode laser,” IEEE J. Quantum Electron. **31**, 647–652 (1995). [CrossRef]

*L*. Here

*A*is the complex amplitude of the

_{q}*qth*cavity mode (|

*A*|

^{2}is the photon number), Δ

_{q}=

*ω*-

_{q}*ω*=

_{0}*q*Δ,

*ω*is the bare-cavity frequency of mode

_{q}*q*, and Δ=

*πc*/

*L*is the cavity mode frequency separation, assuming negligible group delay dispersion in the cavity (typical calculated value ≪1000

*fs*[10

^{2}10. A. Garnache, S. Hoogland, A. Tropper, I. Sagnes, G. Saint-Girons, and J. Roberts, “Sub-500-fs soliton-like pulse in a passively mode-locked broadband surface-emitting laser with 100-mW average power,” Appl. Phys. Lett. **80**, 3892–3894 (2002). [CrossRef]

*ω*is the central frequency.

_{0}*λ*and QWs are usually located at the antinodes of the laser electric field, all the modes are in phase on the gain medium. This prevents from longitudinal spatial-hole-burning [9

9. S. Hodges, M. Munroe, J. Cooper, and M. Raymer, “Multimode laser model with coupled cavities and quantum noise,” J. Opt. Soc. Am. B **14**, 191–199 (1997). [CrossRef]

14. M. Yamada, “Theoretical analysis of nonlinear optical phenomena taking into account the beating vibration of the electron density in semiconductor lasers,” J. Appl. Phys. **66**, 81–89 (1989). [CrossRef]

17. P. A. Khandokhin, I. V. Koryukin, Y. I. Khanin, and P. Mandel, “Influence of carrier diffusion on the dynamics of a two-mode laser,” IEEE J. Quantum Electron. **31**, 647–652 (1995). [CrossRef]

17. P. A. Khandokhin, I. V. Koryukin, Y. I. Khanin, and P. Mandel, “Influence of carrier diffusion on the dynamics of a two-mode laser,” IEEE J. Quantum Electron. **31**, 647–652 (1995). [CrossRef]

*D*is the heavy hole diffusion coefficient and t the carrier lifetime). Note that in the case of a ring resonator or if the 1/2-VCSEL is located in the middle of the cavity, where the structure is used as a plane folding mirror with an incidence angle for the beam smaller than typically

_{h}*θ*<

_{i}*λ*/2

*w*

_{0}≃3° (for a 2

*w*

_{0}=20

*µ*m laser spot diameter at

*λ*=1

*µ*m), the model below is valid (replacing 2

*L*by

*L*′ the cavity perimeter). For larger angles, an interference pattern is created in the plane of incidence across the beam, due to standing wave distribution of each mode

*q*(with induced grating spatial frequencies sin(

*θ*)(

_{i}*w*/

_{0}*πc*+

*q*/

*L*)). It will induce spatial inhomogeneity of the carrier density in the QWplane. Thus non-linear mode interactions due to “transverse” Spatial- Hole-Burning would arise, preventing from single mode operation. Heavy hole carrier diffusion has to be taken into account as well, as it would smooth the population grating [14

**66**, 81–89 (1989). [CrossRef]

**31**, 647–652 (1995). [CrossRef]

*A*and the population inversion

_{q}*N*(carrier number:

*N*=

*n*×

*V*where

_{a}*n*is the carrier density,

*V*=

_{a}*πw*

^{2}

_{0}

*L*is the active volume), valid for Δ≫

*γ*, are then given by

_{q}*γ*=ln(1-

_{q}*T*)

_{q}*c*/2

*L*is the cavity loss rate for mode

*q*(

*T*are the total optical losses),

_{q}*N*the effective transparency carrier number, assuming a linear gain dependence with N,

_{t}*α*the linewidth enhancement factor,

*q*, where Γ

_{g}is the modal gain bandwidth. Γ

*is reduced by a factor ~3 typically with a resonant λ -long micro-cavity design without coating. Δ*

_{g}*=(mq) Δ. The peak stimulated emission rate is approximated as,*

_{mq}*≃*

_{c}*n*/2×|

_{o}*EQW*/

*E*|2 is the longitudinal confinement factor (

_{incident}*E*and

_{QW}*E*are the electric field on the QWs and incident on the VeCSEL structure, respectively),

_{incident}*dg/dn*the differential gain [15] taken as constant and calculated around the carrier density threshold value,

*w0*is the pump/laser radius (to simplify the pump shape is assumed to be a top hat). We calculated Γ

*=2 for an antireflection coated 1/2-VCSEL structure, and Γ*

_{c}*=7 for a resonant structure design (microcavity length set to a multiple of λ/2) without any coating [5*

_{c}**17**, 1589 (2000). [CrossRef]

**14**, 191–199 (1997). [CrossRef]

^{2}). Finally,

*P*is the absolute pump rate.

*τ*in the QW [12, 15], as the carrier density

*n*is almost constant above threshold and the pump rate is not too high above threshold.

*F*and

_{q}*F*are the normalized random Langevin forces for the electric field and the carrier number respectively [9

_{N}**14**, 191–199 (1997). [CrossRef]

18. S. A. Kovalenko, “Quantum intensity fluctuations in multimode cw lasers and maximum sensitivity of intracavity laser spectroscopy,” Sov. J. Quantum Electron. **11**, 759–762 (1981). [CrossRef]

**66**, 81–89 (1989). [CrossRef]

^{-1}

*≃0.1*

_{g}*ps*. This effect occurs even when the number of injected carriers is constant. For a typical VeCSEL cavity Γ

*≫ Δ, the active region is very thin compared to the wavelength (in contrast to an edge emitting laser) and the electric field value is small compared to the saturation value. Thus the non-linear gain is weak and uniform over the laser bandwidth. The ratio*

_{g}*R*

_{3/1}between the third order gain and the linear gain can be evaluated [14

**66**, 81–89 (1989). [CrossRef]

*R*≃5.3×10

_{cv}^{-29}m×C (for GaAs) is the absolute value of the dipole moment of the QW,

*n*≃3.5 is the optical index,

_{o}*λ*= 1

*µ*m is the free space laser wavelength,

*η*=2 is the excess of gain over the threshold value and

*N*is the number of oscillating modes (~10).

_{mode}*τ*

^{-1}with respect to

*γ*, which leads to oscillatory transients in the GHz regime for usual devices. In the single-mode limit, the unique frequency of relaxation oscillations depends on the excess pump parameter η and on both relaxation constants, in the form

_{0}**31**, 647–652 (1995). [CrossRef]

*γ0*is smaller than

*τ*

^{-1}allowing for the adiabatic elimination of

*N*from the dynamics and leading to a class-A laser without relaxation oscillations of the total photon number for single mode operation. This laser system is thus of great interest for the realization of low intensity-noise sources, as is confirmed experimentally (Fig. 3, bottom) and theoretically (Fig. 6). However, FWM via population pulsation could modulate slightly the carrier density and the total intensity, as shown below, at frequencies in the sub-MHz range.

*τ*

^{-1}≥

*γ*for a typical VeCSEL (i.e. cavity lengths 3 m≫

_{0}*L*≥ 5 mm), Eqs. (2,3) can be written in third order perturbation form by expanding the population inversion as a Fourier time series around the equilibrium value (

*N*)≃

^{st}-N_{t}*γ0/B*. We assume that the population oscillates slightly at the intermode beat frequencies (harmonics amplitude ≪

_{0}*N*), and that the population inversion as well as the fields do not change much on a time scale of Δ

^{st}-N_{t}^{-1}. We thus obtain the VeCSEL equations for the complex field amplitude,

*M*=1/τ

_{s}*B*is the saturation photon number. In a nonselective resonator (

_{0}*γ*=

_{q}*γ*), the following relationship is valid for the saturated gain in the

_{0}*qth*mode

*η*=(

*τP-N*) is the excess of gain over threshold. The

_{t})/(τP_{th}-N_{t}*C*’s given by

_{m}*A*, they can be determined experimentally since they are the components of an expansion of the total field intensity as a Fourier time series, where

_{q}*C*

_{0}corresponds to the total photon number. At the onset of pumping, it represents the spontaneous radiation intensity. Then

*C*

_{0}increases exponentially before saturating and reaching the steady-state value

*C*after a build-up time

^{st}_{0}≈(η-1)M_{s}*t*≃ln[

_{b}*M*(η-1)]/(η-1)

_{s}*γ*. Neglecting the third term on the right hand side of Eq. 7, the response to a small perturbation of this class-A laser cavity is characterized by a first order low pass filter with a cut-off frequency

_{0}*f*easily calculated as

_{c}*F*since in a class-A laser the gain saturation damps instantaneously the population inversion fluctuations (adiabatic condition) [18

_{N}18. S. A. Kovalenko, “Quantum intensity fluctuations in multimode cw lasers and maximum sensitivity of intracavity laser spectroscopy,” Sov. J. Quantum Electron. **11**, 759–762 (1981). [CrossRef]

*F*should satisfy the following conditions:

_{q}*ζ*is the ratio of spontaneous emission to stimulated emission rate reduced to one photon [15], and is given by

*EF*are the quasi-Fermi levels for electrons and heavy holes. This ratio

_{c,v}*ζ*is close to unity for usual VCSEL lasers, as the carrier density at threshold is well above the transparency carrier density (>1.3) if the number of QW’s is not too large (<8).

**21(4)**, 407–411 (1991). [CrossRef]

*γ*and of the non-existence of spatial-hole-burning, except for the influence of the α parameter, which was experimentally studied in [13, 18

_{0}18. S. A. Kovalenko, “Quantum intensity fluctuations in multimode cw lasers and maximum sensitivity of intracavity laser spectroscopy,” Sov. J. Quantum Electron. **11**, 759–762 (1981). [CrossRef]

#### 3.2. Numerical simulations

*t*measured from the onset of pumping. The parameters of the equations were selected to be close to those of VeCSELs used in the experiment (Tab. 1). The results of numerical calculations indicated that after a generation time of 1

_{g}*µ*s (about twice the build-up time of the total intensity), the Gaussian width of the laser spectrum is about 33 modes which we took as the initial conditions. At that generation time, even if the total intensity is almost at its steady state value

*C*, calculations show that non-linear mode interactions did not yet modified significantly the mode dynamics (see section 3.3). The equations were initially solved for 121 modes. This number was reduced at longer

^{st}0*t*to accelerate the numerical integration. At all times, the field amplitudes for the modes at the edge of the spectrum were vanishingly small, which avoided edge effects associated with the finite number of coupled equations. Since spontaneous emission constitutes the seed for lasing, the field amplitudes in the various modes are initially statistically independent:

_{g}s**21(4)**, 407–411 (1991). [CrossRef]

*A*of the various modes had a Gaussian distribution in the complex plane with zero averages and variances 〈|

_{q}*A*|

_{q}^{2}〉, and it was assumed that ∑

*〈|*

_{q}*A*|

_{q}^{2}〉=

*C*.

^{st}_{0}*t*, whose duration satisfied the condition Δ

*t*<1/2

*π f*for the stability of the coupled equations. The random force

_{c}*F*simulating quantum fluctuations was added at each iteration using a two-dimensional Gaussian distribution in the complex amplitude space. The mean value of the added photon numbers is given by Eq. 12. In order to validate the third order perturbative model used, we checked that the amplitude of the carrier density oscillation harmonics were always small compared to the mean value (this ratio is ≪10

_{q}^{-2}for

*h*up to 3).

#### 3.3. Influence of FWM and α on the laser dynamics and the noise properties

*α*and quantum noise. As is experimentally observed (Fig. 4), non-linear mode interaction due to FWM slows down the spectral narrowing at short times (

*t*< 100

_{g}*µ*s), when the laser is still strongly multimode. At this stage, the stronger the pump the broader the spectrum is, and the spectral width does not follow the “ideal” monotonic narrowing proportional to

**17**, 1589 (2000). [CrossRef]

*C*. With or without FWM, the total photon number

_{m}*C*is almost constant and does not show any oscillation-relaxation, as expected for a class-A laser. In contrast to an “ideal” homogeneous laser showing a monotonic decay/increase of the mode amplitudes, and thus a monotonic decay of the

_{0}*C*, one can observe that the lasing intensities in the various modes are subject to random fluctuations when FWM is switched on. However, the variables

_{m≠0}*C*fluctuate almost harmonically in the frequency range 10–100 kHz (several order of magnitude lower than

_{m≠0}(t)*γ0*<τ

^{-1}<Δ), but with decaying amplitudes as for an “ideal” laser (Fig. 6). The fact that the variables

*C*decay almost to zero, indicates that phase relationships are established between the modes, in other words, the random phase distribution of the modes is lost, this decay also slows down the mode amplitude fluctuations. This effect is the opposite of passive mode-locking achieved by means of a saturable absorber [10

_{m≠0}10. A. Garnache, S. Hoogland, A. Tropper, I. Sagnes, G. Saint-Girons, and J. Roberts, “Sub-500-fs soliton-like pulse in a passively mode-locked broadband surface-emitting laser with 100-mW average power,” Appl. Phys. Lett. **80**, 3892–3894 (2002). [CrossRef]

*C*is due to the saturable amplifier; it can be called mode anti-locking and does not prevent spectral narrowing to occur. This can be considered as a typical signature of FWM in a saturable amplifier. Following the same procedure as in [6

_{m≠0}**21(4)**, 407–411 (1991). [CrossRef]

*f*and the decay time

^{NL}_{m}*τ*of the Fourier coefficient

^{NL}_{m}*C*are given by

_{m≠0}*f*≈21

^{NL}_{1}*µ*s (with

*τ*≈430

^{NL}_{m}*µ*s for

*η*=1.7), the free running approximation (“ideal” laser, no FWM term in Eq. 7) is valid for the rate equations: FWM does not yet modify the mode dynamics (Fig. 5- bottom and Fig. 6). The transition from pure quantum-noise driven dynamics to non-linear deterministic dynamics can be found by setting the time

*t*(Eq. 18) equal to the fastest period 1/

_{c}*f*

^{N}L_{1}, which leads to a non-linear threshold pump rate

*ηNL*

^{2}

*. If an intracavity filter is used to make Γ*

_{g}L^{3}*narrower (e.g.~1/3 for a resonant design) and the cavity is twice shorter, this threshold would increase up to*

_{g}*ηNL*≃2, and becomes thus observable.

*µ*s (few times 1/

*f*

^{NL}_{1}) when the laser spectrum width is only a few modes, the mode intensity fluctuations show decaying oscillations and typically only

*C*

_{1}and

*C*

_{2}still endure. Note also the spectrum red shift - or pulling - of the maximum amplitude, induced by the coupling between the parameter a and FWM. At longer times, one also sees clearly that the strong modes show an antiphase dynamics, due to FWMand the mode anti-locking process as explained above. Indeed, the strong adjacent mode oscillations have a relative phase shift of p. Thus, the fluctuations observed on the total photon number

*C*are due to white noise arising from quantum noise as in an “ideal” laser (Fig. 6, and 7-b and Eq. 19).

_{0}*η*. The characteristic time for this is about 1–2ms for

*η*=1.7. At these long times, FWM does not prevent from stable single mode operation, even taking into account quantum noise. FWM even accelerates slightly the dynamics to reach the steady state due to spontaneous emission, compared to an “ideal” homogeneous laser (Fig. 7). A large side-mode suppression ratio of 34 dB is achieved without any intracavity filter, similar to the “ideal” laser (Eq. 20) and in relatively good agreement with the experiment. Note however that, even if the

*C*coefficients almost vanish at the steady state, both phase sensitive interactions via FWM and spontaneous emission drive the mode dynamics in continuous wave. The side mode suppression ratio still increases at larger pump rates (40 dB for

_{m≠0}*η*=3) like for an “ideal” laser, but the time to reach the steady state only increases slightly (2–3ms for η=3). Thus any technical noise (acoustic, thermal) in the apparatus should not prevent from single mode operation for cavities shorter than typically L=30mm and an antireflection coated structure, as the acoustic noise frequency span stops below the kHz level for a standard optical bench.

**17**, 1589 (2000). [CrossRef]

*t*needed for the laser spectrum to collapse to a width of one mode separation D, after a strong perturbation (e.g. 100% pump modulation) is

_{c}^{2}

_{g}×

*L*

^{3}, assuming

*γ*. Note that reducing the gain or losses bandwidth has theoretically the same effect. Using an intracavity filter to narrow G

_{q}=γ_{0}*(e.g. ~1/3 for a resonant design) and a cavity twice shorter,*

_{g}*t*would increase up to 70 kHz, much faster than standard acoustic or thermal fluctuation frequencies. Note that the frequency response 1/

^{-1}_{c}*t*of this single mode system is several order of magnitude slower than the cutoff frequency

_{c}*f*(≃6.2MHz here, see Fig. 7-b and Eq. 10) of this class-A laser cavity.

_{c}*t*to

_{c}*f*, the emission will remain multimode in CW operation. In this case if any spectral filter is inserted in the cavity, reducing the modal gain bandwidth Γ

_{c}*, a single mode solution will arise more quickly and will be more stable, with a larger side-mode suppression-ratio. For a cavity longer than typically L=30mm, technical noise falling in the kHz range will prevent from single mode operation if no intracavity filter is inserted and/or large pump noise is present (e.g. when using a high power fiber-coupled pump laser diode) [8].*

_{g}*rms*relative photon noise for the total intensity is set by quantum noise as [11, 18

**11**, 759–762 (1981). [CrossRef]

*qth*side mode as

*SMSR*scales with

_{q}*P*×(Γ

_{e}^{2}

_{g}*L*)

^{-1}. These values are in agreement with the simulations. From the simulations of the mode phase dynamics at long

*t*with FWM (in the frequency range 100Hz to 40MHz), the power spectrum of the strong laser mode frequency (

_{g}*q*=-2) fluctuations displayed a white frequency noise at the quantum limit [15]. Thus the laser mode linewidth

*δνL*reach the quantum limit of about 1Hz for

*P*=1mW of total emitted power at 2.3 mm, as given by the modified Schawlow-Townes fundamental limit [15], which scales with

_{e}*P*

^{-1}

*×*

_{e}*L*

^{-2},

^{-1}), non-linear mode interactions are vanishing since the phase sensitive interaction terms in Eqs. (2,3), at frequencies

*D*≠0, are too fast for the gain medium. Thus the carrier equation can not be adiabatically eliminated from the dynamics. This situation leads to an “ideal” homogeneous multimode laser dynamics of class-B [11, 15], exhibiting relaxation oscillations of the intensity (in the 0.1GHz frequency range for L=1mm) without phase sensitive interactions. A class-B VeCSEL is thus theoretically single mode in CW. However, this class-B laser exhibits a resonant peak in the spectrum of intensity fluctuations, amplifying any noise source or fluctuations (in

_{mq}*N*or

*A*), and is much more sensitive to optical feedback.

_{q}## 4. Conclusion

_{00}beam, even without any intracavity selective spectral element, and in spite of the presence of FWMvia population pulsations in the QWs. FWMis slowing down spectral narrowing during the multimode transient, but does not prevent from single mode operation with high side mode suppression ratio and a linewidth <20 kHz. The physical explanation lies in the mode antiphase dynamics (called mode anti-locking). These highly coherent sources reach the quantum limit and the dynamics fall into the oscillation-relaxation-free class-A regime, similarly to an “ideal” homogeneous laser. The laser is single mode after typically

*t*=1ms for L=15mm without intracavity spectral filter.

_{g}*L*<30mm for typical parameters (

*T*~1%, τ~3ns). A simple intracavity spectral filter (a thin glass etalon) will allow using longer cavities for stable single mode operation [8], in the limit of

_{q}*L*≪3m, as it shortens the characteristic time

*t*(∝ Γ

_{c}^{2}

*×*

_{g}*L*

^{3}) needed to stabilize the single mode solution and plays against technical instabilities (acoustic, thermal, pump injection), having frequencies falling in/below the kHz range [4

4. M. Holm, D. Burns, and A. Ferguson, “Actively Stabilized Single-Frequency Vertical-External-Cavity AlGaAs Laser,” IEEE Photon. Technol. Lett. **11**, 1551–1553 (1999). [CrossRef]

**17**, 1589 (2000). [CrossRef]

## References and links

1. | A. Garnache, A. Ouvrard, L. Cerutti, D. Barat, A. Vicet, F. Genty, Y. Rouillard, D. Romanini, and E. Cerda- Méndez, “2–2.7mm single frequency tunable Sb-based lasers operating in CW at RT: Microcavity and External- cavity VCSELs, DFB,” Proc. SPIE Photonics Europe, Semiconductor lasers and laser dynamics pp. 6184–23 (2006). |

2. | S. Lutgen, T. Albrecht, P. Brick, W. Reill, J. Luft, and W. Spath, “8-W High-Efficiency Continuous-Wave Semiconductor Disk Laser at 1000 nm,” Appl. Phys. Lett. |

3. | A. Ouvrard, A. Garnache, L. Cerutti, F. Genty, and D. Romanini, “Single Frequency Tunable Sb-based VCSELs emitting at 2.3mm,” IEEE Photon. Technol. Lett. |

4. | M. Holm, D. Burns, and A. Ferguson, “Actively Stabilized Single-Frequency Vertical-External-Cavity AlGaAs Laser,” IEEE Photon. Technol. Lett. |

5. | A. Garnache, A. Kachanov, F. Stoeckel, and R. Houdré, “Diode-pumped broadband Vertical-External-Cavity Surface-Emitting semiconductor Laser. Application to high sensitivity intracavity laser absorption spectroscopy,” J. Opt. Soc. Am. B |

6. | S. Kovalenko, S. Semin, and D. Toptygin, “Influence of the Raman mode interaction on the lasing kinetics of a wide-band ring laser,” Sov. J. Quantum Electron. |

7. | L. Cerutti, A. Garnache, A. Ouvrard, and F. Genty, “High Temperature CW Operation of Sb-Based Vertical External Cavity Surface Emitting Laser near 2.3mm,” J. Cryst. Growth |

8. | M. Jacquemet, M. Domenech, G. Lucas-Leclin, P. Georges, J. Dion, M. Strassner, I. Sagnes, and A. Garnache, “Single-Frequency High-Power CW Vertical External Cavity Surface Emitting Semiconductor Laser at 1003 nm and 501nm by Intracavity Frequency Doubling,” Appl. Phys. B In press (2006). |

9. | S. Hodges, M. Munroe, J. Cooper, and M. Raymer, “Multimode laser model with coupled cavities and quantum noise,” J. Opt. Soc. Am. B |

10. | A. Garnache, S. Hoogland, A. Tropper, I. Sagnes, G. Saint-Girons, and J. Roberts, “Sub-500-fs soliton-like pulse in a passively mode-locked broadband surface-emitting laser with 100-mW average power,” Appl. Phys. Lett. |

11. | A. Garnache, “Study and realization of new types of near-IR lasers for high sensitivity intra-cavity-laser- absorption-spectroscopy application. Strongly multi-mode laser dynamics.” Ph.D. thesis, Joseph Fourier University, Grenoble (1999). |

12. | A. Garnache, A. Bouchier, E. K. Attarbaoui, A. Ouvrard, L. Cerutti, and E. Cerda-Méndez, “Sb-based type-I Quantum-Well Gain and Quantum Efficiency study. Application to 2.3mm VCSELs,” Proc. EOS annual meeting, Paris, Photonic Devices in Space (TOM 5) (2006). |

13. | S. E. Vinogradov, A. A. Kachanov, S. A. Kovalenko, and E. A. Sviridenkov, “Nonlinear dynamics of a multimode dye laser with an adjustable resonator dispersion and implications for the sensitivity of in-resonator laser spectroscopy,” JETP Lett. |

14. | M. Yamada, “Theoretical analysis of nonlinear optical phenomena taking into account the beating vibration of the electron density in semiconductor lasers,” J. Appl. Phys. |

15. | L. A. Coldren and S. W. Corzine, |

16. | M. Grundmann, “How a quantum-dot laser turns on,” Appl. Phys. Lett. |

17. | P. A. Khandokhin, I. V. Koryukin, Y. I. Khanin, and P. Mandel, “Influence of carrier diffusion on the dynamics of a two-mode laser,” IEEE J. Quantum Electron. |

18. | S. A. Kovalenko, “Quantum intensity fluctuations in multimode cw lasers and maximum sensitivity of intracavity laser spectroscopy,” Sov. J. Quantum Electron. |

**OCIS Codes**

(140.3430) Lasers and laser optics : Laser theory

(140.3570) Lasers and laser optics : Lasers, single-mode

(250.7260) Optoelectronics : Vertical cavity surface emitting lasers

(270.2500) Quantum optics : Fluctuations, relaxations, and noise

**ToC Category:**

Lasers and Laser Optics

**History**

Original Manuscript: February 26, 2007

Revised Manuscript: May 21, 2007

Manuscript Accepted: May 22, 2007

Published: July 16, 2007

**Citation**

A. Garnache, A. Ouvrard, and D. Romanini, "Single–Frequency operation of External–Cavity VCSELs: Non-linear multimode temporal dynamics and quantum limit.," Opt. Express **15**, 9403-9417 (2007)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-15-9403

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### References

- A. Garnache, A. Ouvrard, L. Cerutti, D. Barat, A. Vicet, F. Genty, Y. Rouillard, D. Romanini, and E. Cerda-Méndez, "2-2.7 μm single frequency tunable Sb-based lasers operating in CW at RT: Microcavity and External-cavity VCSELs, DFB," Proc. SPIE Photonics Europe, Semiconductor lasers and laser dynamics pp. 6184-23 (2006).
- S. Lutgen, T. Albrecht, P. Brick, W. Reill, J. Luft, and W. Spath, "8-W High-Efficiency Continuous-Wave Semiconductor Disk Laser at 1000 nm," Appl. Phys. Lett. 82, 3620-3622 (2003). [CrossRef]
- A. Ouvrard, A. Garnache, L. Cerutti, F. Genty, and D. Romanini, "Single Frequency Tunable Sb-based VCSELs emitting at 2.3 μm," IEEE Photon. Technol. Lett. 17, 128-134 (2005). [CrossRef]
- M. Holm, D. Burns, and A. Ferguson, "Actively Stabilized Single-Frequency Vertical-External-Cavity AlGaAs Laser," IEEE Photon. Technol. Lett. 11, 1551-1553 (1999). [CrossRef]
- A. Garnache, A. Kachanov, F. Stoeckel, and R. Houdre, "Diode-pumped broadband Vertical-External-Cavity Surface-Emitting semiconductor Laser. Application to high sensitivity intracavity laser absorption spectroscopy," J. Opt. Soc. Am. B 17, 1589 (2000). [CrossRef]
- S. Kovalenko, S. Semin, and D. Toptygin, "Influence of the Raman mode interaction on the lasing kinetics of a wide-band ring laser," Sov. J. Quantum Electron. 21(4), 407-411 (1991). [CrossRef]
- L. Cerutti, A. Garnache, A. Ouvrard, and F. Genty, "High Temperature CW Operation of Sb-Based Vertical External Cavity Surface Emitting Laser near 2.3 μm," J. Cryst. Growth 268, 128 (2004). [CrossRef]
- M. Jacquemet, M. Domenech, G. Lucas-Leclin, P. Georges, J. Dion, M. Strassner, I. Sagnes, and A. Garnache, "Single-Frequency High-Power CW Vertical External Cavity Surface Emitting Semiconductor Laser at 1003 nm and 501nm by Intracavity Frequency Doubling," Appl. Phys. BIn press (2006).
- S. Hodges, M. Munroe, J. Cooper, and M. Raymer, "Multimode laser model with coupled cavities and quantum noise," J. Opt. Soc. Am. B 14, 191-199 (1997). [CrossRef]
- A. Garnache, S. Hoogland, A. Tropper, I. Sagnes, G. Saint-Girons, and J. Roberts, "Sub-500-fs soliton-like pulse in a passively mode-locked broadband surface-emitting laser with 100-mW average power," Appl. Phys. Lett. 80, 3892-3894 (2002). [CrossRef]
- A. Garnache, "Study and realization of new types of near-IR lasers for high sensitivity intra-cavity-laser-absorption-spectroscopy application. Strongly multimode laser dynamics." Ph.D. thesis, Joseph Fourier University, Grenoble (1999).
- A. Garnache, A. Bouchier, E. K. Attarbaoui, A. Ouvrard, L. Cerutti, and E. Cerda-M endez, "Sb-based type-I Quantum-Well Gain and Quantum Efficiency study. Application to 2.3 μm VCSELs," Proc. EOS annual meeting, Paris, Photonic Devices in Space (TOM 5) (2006).
- S. E. Vinogradov, A. A. Kachanov, S. A. Kovalenko, and E. A. Sviridenkov, "Nonlinear dynamics of a multimode dye laser with an adjustable resonator dispersion and implications for the sensitivity of in-resonator laser spectroscopy," JETP Lett. 55, 581-585 (1992).
- M. Yamada, "Theoretical analysis of nonlinear optical phenomena taking into account the beating vibration of the electron density in semiconductor lasers," J. Appl. Phys. 66, 81-89 (1989). [CrossRef]
- L. A. Coldren and S. W. Corzine, Diode lasers and Photonic Integrated Circuits (Wiley, New York, 1995).
- M. Grundmann, "How a quantum-dot laser turns on," Appl. Phys. Lett. 77, 1428-1430 (2000). [CrossRef]
- P. A. Khandokhin, I. V. Koryukin, Y. I. Khanin, and P. Mandel, "Influence of carrier diffusion on the dynamics of a two-mode laser," IEEE J. Quantum Electron. 31, 647-652 (1995). [CrossRef]
- S. A. Kovalenko, "Quantum intensity fluctuations in multimode cw lasers and maximum sensitivity of intracavity laser spectroscopy," Sov. J. Quantum Electron. 11, 759-762 (1981). [CrossRef]

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