Vertical External Cavity Surface Emitting Laser Research Articles

Wavelength and Pump-Power Dependent Nonlinear Refraction and Absorption in a Semiconductor Disk Laser

We characterize both nonlinear refraction and absorption in a vertical-external-cavity surface-emitting laser (VECSEL) as a function of pump irradiance over the whole range of lasing wavelengths. We observe an approximately linear decrease in magnitude for both nonlinear refraction and nonlinear absorption with optical pumping when the thermally induced shift of the lasing wavelength is considered. Our results are of particular significance for understanding self-modelocking of VECSELs which might be driven by nonlinear lensing.

Design of gain region of high-power vertical external cavity surface emitting semiconductor laser and its fabrication

<sec> The vertical external cavity surface emitting laser (VECSEL) is one of the hottest research fields of semiconductor lasers, due to its high power and good beam quality. However, there are few reports about how to systematically design the active region of VECSEL. In this paper, the gain design of quantum wells, which are the most important region within the VECSEL, is carried out. </sec><sec> To achieve low power consumption under high temperature condition, epitaxial structure of the VECSEL is optimized by using the commercial software PICS3D. Firstly, the relationship between the structure of quantum well and the gain is simulated by the <i>k</i>·<i>p</i> method. Then, the gain spectra of quantum wells at different carrier densities and temperatures are compared with each other, and the optimal composition and thickness of quantum well are thus determined. The temperature drift coefficient is 0.36 nm/K, obtained by simulating the drift of the gain peak wavelength at the working temperature. Finally, the gain spectra of quantum wells with five different barriers are compared with each other. The slight blue shift of the gain peak in the quantum well with five different barriers accommodates the different emission thermal drifts of the quantum well at high temperature operation. With the GaAsP barriers on both sides of quantum well the gain characteristics of quantum wells can be improved efficiently. </sec><sec> The designed structure is deposited by the MOCVD system. According to the reflection spectrum of the gain chip, measured by ellipsometer, the stop-band over 100 nm is centered at the about 970 nm wavelength, confirming accurate growth of the VECSEL. The 808 nm pump laser is focused on the surface of VECSEL chip at an incident angle from 30° to 50°. The VECSEL light-light characteristics are tested under the output coupling mirror with different reflectivity. The output power of VECSEL with a 97.7% reflectance output coupling mirror reaches 9.82 W at the pumping power of 35 W, without saturating the power curve. By using the external mirrors with different reflectivity, there appears the wavelength shift with the pumping power changing from 0.216 nm/W to 0.16 nm/W. Thus, the internal heating effects are different for VECSEL with different mirrors. The divergence angles at two orthogonal directions are 9.2° and 9.0°, respectively. And the circle profile of optical field shows good symmetry. </sec>

Orthogonally polarized frequency combs in a mode-locked VECSEL

We introduce a spin–flip model for a vertical-external-cavity surface-emitting laser (VECSEL) with a saturable absorber. We demonstrate the possibility, due to the spin–flip dynamics, to generate two orthogonally linearly polarized frequency combs in the mode-locked regime. The two combs are shifted in wavelength due to the birefringence in the VECSEL gain and/or saturable absorption mirror. We show that the polarization degree of freedom may also lead to several pulses being generated per roundtrip in the two orthogonal linear polarizations and to more complicated dynamics with both linear polarizations excited.

Thermal phase fluctuations in optically pumped dual-frequency vertical external-cavity surface-emitting lasers for cesium clocks based on coherent population trapping

A fully analytical model is established for the thermal fluctuations of the beatnote phase of an optically pumped dual-frequency vertical-external-cavity surface-emitting laser (VECSEL). This model starts with the resolution of the heat equation inside the semiconductor chip structure and follows with the evaluation of the induced thermo-optic phase shift. Both the fluctuations of the heat induced by the optical pumping and the thermodynamic fluctuations at room temperature are considered. On the one hand, the thermal response of the structure is investigated and a significant thermal lens effect caused by the pump is deduced. On the other hand, the power spectral density of the frequency noise is calculated in the presence of diffusion spatial anisotropy. The present model is in very good agreement with the phase noise measured for a dual-frequency VECSEL at 852 nm for application to metrology and the validity of the usual low-pass filter model is discussed.

Monolithic High Contrast Grating Nitride-Based VECSEL

This paper proposes a new design for a nitride-based vertical-external-cavity surface-emitting laser (VECSEL) emitting at 394 nm. The structure contains a monolithic high contrast grating (MHCG) instead of a conventional distributed Bragg reflector (DBR), makes the vertical design of the laser more compact. In a numerical analysis, we consider several designs for MHCG VECSELs and show that replacing the DBR with an MHCG improves laser performance by 50%, in terms of output energy and slope efficiency. Further optimization of the MHCG VECSEL design achieves an almost threefold improvement in both values.

High-Power Higher Order Hermite–Gaussian and Laguerre–Gaussian Beams From Vertical External Cavity Surface Emitting Lasers

We report a vertical external cavity surface emitting laser (VECSEL) with an optical pumping scheme for the generation of higher order Hermite–Gaussian transverse modes. The VECSEL chip with emission around 1068 nm in a linear cavity configuration is capable of delivering a variety of higher order modes with Watt-level output power. A very good agreement between the modeling and experimental results validate the ideas underpinning the method. Additionally, simultaneous lasing of two independent and collinear transverse modes from a single laser cavity is demonstrated. An external astigmatic mode converter is finally utilized for the conversion of Hermite–Gaussian modes into corresponding Laguerre–Gaussian modes.

Flip-Chip Wafer-Fused OP-VECSELs Emitting 3.65 W at the 1.55-μm Waveband

Optically pumped vertical external cavity surface emitting lasers (VECSELs) based on flip-chip gain mirrors emitting at the 1.55 μm wavelength range are reported. The gain mirrors employ wafer-fused InAlGaAs/InP quantum well heterostructures and GaAs/AlAs distributed Bragg reflectors fixed on a diamond heat-sink substrate in a flip-chip geometry, incorporated in a V-cavity configuration. A maximum output power of 3.65 W was achieved for a heatsink temperature of 11°C and employing a 2.2% output coupler. The laser exhibited circular beam profiles for the full emission power range. This demonstration represents more than five-fold increase of the output power compared to the state-of-the-art flip-chip VECSELs previously reported at the 1.55 μm wavelength range. It opens new perspectives for developing practical VECSEL-based laser systems operating at a wavelength range widely used in many applications.

Generation and Stabilization of Continuous-Wave THz Emission From a Bi-Color VECSEL

We present a laser system for the generation and stabilization of continuous wave emission at 1.9 THz. The system is based on difference frequency generation (DFG) from a bi-color vertical external cavity surface emitting laser (VECSEL). The intracavity DFG process enables THz generation with milliwatt power level at room temperature with a clear quadratic dependence red on the intracavity power. The two colors are spectrally selected via an intracavity etalon while the laser cavity geometry is engineered to minimize mode competition between the two colors via spatial hole burning, improving the stability of the THz emission. The intensity noise of the THz output is further reduced by 16 dB/Hz with an active stabilization scheme. The origin of residual fluctuations is discussed and we found that the asymmetrical amplification of one color reduced the RIN even further, down to 21 dB/Hz, comparing to the free-running symmetrical lasing.

Investigation of an electron-beam pumped VECSEL based on an InGaAs/AlGaAs heterostructure

We report the results of a study of an e-beam pumped vertical-external-cavity surface-emitting laser (VECSEL) based on an InGaAs/AlGaAs heterostructure. Metalorganic chemical vapour deposition (MOCVD) is employed to grow two structures of different design, which contain 10 quantum wells (QWs) and a built-in distributed Bragg reflector (DBR) mirror. Under repetitively pulsed electron-beam excitation (50 Hz, 250 ns), a peak output power of 5.5 W is achieved at a wavelength of 2.5 W and an output power of 2.5 W at 1.013 μm with a total convergence angle no larger than 20 mrad.

Broadband continuous single-mode tuning of a short-cavity quantum-cascade VECSEL

Changing the length of a laser cavity is a simple technique for continuously tuning the wavelength of a laser but is rarely used for broad fractional tuning, with a notable exception of the vertical-cavity surface-emitting laser (VCSEL)1,2. This is because, to avoid mode hopping, the cavity must be kept optically short to ensure a large free spectral range compared to the gain bandwidth of the amplifying material. Terahertz quantum-cascade lasers are ideal candidates for such a short cavity scheme as they demonstrate exceptional gain bandwidths (up to octave spanning)3 and can be integrated with broadband amplifying metasurfaces4. We present such a quantum-cascade metasurface-based vertical-external-cavity surface-emitting laser (VECSEL) that exhibits over 20% continuous fractional tuning of a single laser mode. Such tuning is possible because the metasurface has subwavelength thickness, which allows lasing on low-order Fabry–Perot cavity modes. Good beam quality and high output power are simultaneously obtained. Terahertz quantum-cascade lasers with over 20% continuous fractional tuning of a single laser mode are now possible thanks to metasurfaces.

Narrow-linewidth operation of folded 1178nm VECSEL with twisted-mode cavity.

A vertical external-cavity surface-emitting laser (VECSEL) with a twisted-mode configuration is demonstrated. This architecture is particularly advantageous for power scaling of single-frequency VECSELs employing multiple gain mirrors in folded cavities. In such a configuration, some of the gain mirrors are inherently at the fold, and the lasing spectrum becomes unstable. This is caused by four waves interfering, destabilizing the standing wave pattern at the quantum wells. We show that the lasing spectrum can be narrowed by employing a twisted-mode configuration, which stabilizes the standing-wave pattern at the gain mirror. Furthermore, single-frequency output of more than 10 W at 1178 nm is demonstrated for a VECSEL employing two gain mirrors in a standing-wave cavity. In comparison, the output power for operation with one gain mirror only was 7.4 W when operating in single frequency. The choice of wavelength for the work reported in this paper is motivated by the opportunity to demonstrate compact VECSEL-based guide star lasers for adaptive optics via frequency doubling to the sodium D2 resonance at 589 nm.

Mode-locking in vertical external-cavity surface-emitting lasers with type-II quantum-well configurations

A microscopic study of mode-locked pulse generation is presented for vertical external-cavity surface-emitting lasers utilizing type-II quantum well configurations. The coupled Maxwell semiconductor Bloch equations are solved numerically where the type-II carrier replenishment is modeled via suitably chosen reservoirs. Conditions for stable mode-locked pulses are identified allowing for pulses in the 100 fs range. Design strategies for type-II configurations are proposed that avoid potentially unstable pulse dynamics.

In brief

PAGE 711 Researchers from China provide an experimental study of recovery time in a bulk gallium arsenide (GaAs) avalanche semiconductor switch. The switch, operating in a low energy-triggered mode, was found to have a recovery time reduction when its biased voltage was reduced. PAGE 705 Phillips Photonics researchers from both Germany and the Netherlands have produced a compact optically pumped vertical external-cavity surface-emitting laser (OP-VECSEL) which, consisting of an optically pumped semiconductor switch and surrounded by VCSEL chips mounted on a single heat sink, allows it to reach 10 Watts of power. External view of bulk avalanche semiconductor. PAGE 688 Conventional mobile devices have thermal management problems primarily managed with software-based techniques due to the need for compact size inhibited hardware solutions. Since software-based solutions inevitably inhibit device performance a team from Korea has developed a control algorithm which adaptively controls threshold performance based on CPU power efficiency in real time. Drawing scheme of VECSEL with compact design. PAGE 701 A pair of researchers from China present a multi-order bandpass filter based on unit cells. The fabricated filter features an ultra-wide stopband and impressive stopband rejection performance as confirmed by experimental results. The easily-tunable BPF topology is suitable for a variety of applications in microwave communication systems. Power consumption measurements. PAGE 707 An active clamping circuit for voltage-sharing among series-connected insulated gate bipolar translators during the turning-on process has been presented by a team of researchers in China. The design includes a time detection circuit to detect the pulse signal indicating the working time of the active clamping circuit for driving signal regulation. Bandpass filter unit cell. Proposed active clamping circuit for turning-on process.

VCSEL pumped VECSEL concept with compact design

The authors present a new compact optically pumped vertical external-cavity surface-emitting laser (OP-VECSEL), which consists of an optically pumped semiconductor chip surrounded by vertical-cavity surface-emitting laser (VCSEL) chips mounted together on one heat sink. This configuration allows to reach 10 W optical output power in single-mode operation with a very compact design. Furthermore, the authors reveal further improvements, which lead to a compact laser module with high power, highest beam quality and potentially low manufacturing costs.

Design and analysis of passively mode-locked all-quantum-dot electrically-driven VECSEL

Detailed characteristics of newly proposed passively mode-locked vertical-external-cavity surface-emitting laser (VECSEL) are investigated. The active material is a quantum dot (QD) InAs/GaAs lasing at a wavelength of 1.3μm. The QD-VECSEL is electrically-driven and is mode-locked by p-i-n QD based semiconductor saturable absorber mirror (SESAM). Time-domain travelling-wave models together with carrier concentration rate equations are employed for investigating the operation of the device. Different parameters that affect the mode-locking performance of the proposed device are examined. Such parameters include the VECSEL to SESAM beam area ratio, the reverse voltage applied to the p-i-n QD-SESAM, the injected current to the VECSEL, and the QD inhomogeneous broadening. The gain, absorption and pulse dynamics are studied, along with the spectral characteristics of the pulse train. The obtained results show that there is a possibility of obtaining mode-locked laser pulses from that device at a wavelength of 1.3μm with pulse repetition frequency of 45 GHz and average pulse width of approximately 10 ps.

Distributed Bragg Reflectors for GaN-Based Vertical-Cavity Surface-Emitting Lasers

A distributed Bragg reflector (DBR) is a key building block in the formation of semiconductor microcavities and vertical cavity surface emitting lasers (VCSELs). The success in epitaxial GaAs DBR mirrors paved the way for the ubiquitous deployment of III-V VCSELs in communication and mobile applications. However, a similar development of GaN-based blue VCSELs has been hindered by challenges in preparing DBRs that are mass producible. In this article, we provide a review of the history and current status of forming DBRs for GaN VCSELs. In general, the preparation of DBRs requires an optimization of epitaxy/fabrication processes, together with trading off parameters in optical, electrical, and thermal properties. The effort of epitaxial DBRs commenced in the 1990s and has evolved from using AlGaN, AlN, to using lattice-matched AlInN with GaN for DBRs. In parallel, dielectric DBRs have been studied since 2000 and have gone through a few design variations including epitaxial lateral overgrowth (ELO) and vertical external cavity surface emitting lasers (VECSEL). A recent trend is the use of selective etching to incorporate airgap or nanoporous GaN as low-index media in an epitaxial GaN DBR structure. The nanoporous GaN DBR represents an offshoot from the traditional epitaxial approach and may provide the needed flexibility in forming manufacturable GaN VCSELs. The trade-offs and limitations of each approach are also presented.

Offset-free mid-infrared frequency comb based on a mode-locked semiconductor laser.

We demonstrate a carrier-envelope offset-free frequency comb in the mid-wavelength infrared (MWIR) based on a passively mode-locked vertical external cavity surface emitting laser (VECSEL) operating at a 1.6GHz repetition rate. The 290mW output spanning 3.0-3.5μm is generated through difference frequency generation (DFG) in periodically poled lithium niobate. The VECSEL pulse train is centered at 1030nm and amplified up to 11W in a Yb fiber amplifier system. The output is split to generate a second pulse train at 1560nm through nonlinear broadening in a Si3N4 waveguide followed by amplification in an Er gain fiber. DFG between the 1030 and 1560nm pulse trains results in a coherent and offset-free MWIR frequency comb, verified with optical heterodyne beat note measurements. Active stabilization of the VECSEL repetition rate provides a fully stabilized high repetition rate frequency comb in the MWIR, uniquely suited for applications in molecular spectroscopy.

Comparison of single-side and double-side pumping of membrane external-cavity surface-emitting lasers.

We studied and compared single-side pumping (SSP) and double-side pumping (DSP) of a semiconductor membrane external-cavity surface-emitting laser (MECSEL). The MECSEL-active region was based on an AlGaAs quantum well structure embedded between two silicon carbide (SiC) wafer pieces that were used as transparent intra-cavity (IC) heat spreaders creating a symmetrical cooling environment. The gain structure targeted emission at 780 nm, a wavelength region that is important for many applications, and where the development of high-brightness high-power laser sources is gaining more momentum. By DSP at 20°C heat sink temperature, we could reduce the laser threshold from 0.79 to 0.69 W of absorbed pump power, while the maximum output power was increased from 3.13 to 3.22 W. The differential efficiency was improved from 31.9% to 34.4%, which represents a record value for SiC-cooled vertically emitting semiconductor lasers. The improvements are enabled by a reduced thermal resistance of the gain element by 9% compared to SSP. The beam quality was measured to be M2<1.09. Finally, we demonstrate a maximum tuning range from 767 to 811 nm. This wavelength range was not addressed by any MECSEL or vertical external-cavity surface-emitting laser device before and extends the available wavelengths for semiconductor based high-quality beam and high-power laser sources to a wavelength window relevant for quantum technology, spectroscopy, or medicine.

Non-equilibrium dynamics in the dual-wavelength operation of vertical external-cavity surface-emitting lasers.

Microscopic many-body theory coupled with Maxwell's equation is used to study dual-wavelength operation in vertical external-cavity surface-emitting lasers. The intrinsically dynamic nature of coexisting emission wavelengths in semiconductor lasers is associated with characteristic non-equilibrium carrier dynamics, which causes significant deformations of the quasi-equilibrium gain and carrier inversion. Extended numerical simulations are employed to efficiently investigate the parameter space to identify the regime for dual-wavelength operation. Using a frequency selective intracavity etalon, two families of modes are stabilized with dynamical interchange of the strongest emission peaks. For this operation mode, anti-correlated intensity noise is observed in agreement with the experiment. A method using effective frequency selective filtering is suggested for stabilization of genuine dual-wavelength output.

Tightly locked optical frequency comb from a semiconductor disk laser.

Ultrafast semiconductor disk lasers (SDLs) passively modelocked using semiconductor saturable absorbers mirrors (SESAMs) generate optical frequency combs (OFCs) with gigahertz line spacings - a regime where solid-state and fiber lasers struggle with geometrical and Q-switching limitations. We stabilized both the frequency comb spacing and the offset without any additional external optical amplification or pulse compression. The overall noise performance is competitive with other gigahertz OFCs. A SESAM-modelocked vertical external-cavity surface-emitting laser (VECSEL) at a center wavelength around 1 µm generates 122-fs pulses with 160 mW average output power and we only needed 17-pJ pulse energy coupled into a silicon nitride (Si3N4) waveguide for supercontinuum generation (SCG) and OFC offset stabilization.