Mode-locked Vertical External-cavity Research Articles

Nonlinear reflectivity of AlGaInP SESAMs for mode locking in the red spectral range.

Mode-locked vertical external-cavity semiconductor lasers (VECSELs) are a wavelength-versatile laser that relies on a semiconductor saturable absorber mirror (SESAM) to initiate pulsed emission while simultaneously significantly influencing the pulse's properties. A SESAM can be characterized using a nonlinear reflectivity setup, realized here in the red spectral range around 660 nm and achieving a moderate peak-to-peak variation of 0.17%. We use our home-built mode-locked VECSEL to reach a high maximum fluence up to 430 µJ/cm2 with strongly chirped 7.5 ps pulses. This allows the first-of-its-kind characterization of GaInP quantum well SESAMs, thereby revealing saturation fluences of 38 µJ/cm2 and 34 µJ/cm2 with modulation depths of 5% and 10.3% for SESAMs comprising one or two active quantum wells, respectively. For all structures, a nonsaturable loss of 2.8% is found and attributed to scattering loss.

All-intracavity fourth harmonic generation in a passively mode locked VECSEL for ultrafast UV emission

We demonstrate all-intracavity fourth harmonic generation in an ultrafast passively mode locked vertical external cavity surface emitting laser (VECSEL). This novel laser cavity design utilizes a high Q Z-cavity resonator to buildup the circulating power of the second harmonic for frequency quadrupling the fundamental 1060 nm to 265 nm. The demonstrated UV output has an average power of 525 μW, pulse width of 3 ps, and a repetition rate of 510 MHz. This results in a peak pulse power of 250–300 mW over a 30 min duration without active stabilization equipment.

Wiggling instabilities of temporal localized states in passively mode-locked vertical external-cavity surface-emitting lasers.

We analyze the emergence of wiggling temporal localized states in a passively mode-locked vertical external-cavity surface-emitting laser composed by a gain chip and a resonant saturable absorber mirror. We show that the wiggling instability stems from the interplay between the third-order dispersion induced by the micro-cavities and their frequency mismatch. The latter is identified as an experimentally crucial parameter defining the range of existence of stable emission. We reveal the homoclinic scenario underlying the wiggling phenomenon, and we show how it allows us to control the oscillation parameters.

Microscopic modeling of non-normal incidence vertical external cavity surface-emitting laser cavities

The optimization of a V-cavity geometry to obtain intense ultrafast pulses for a modelocked vertical external-cavity surface-emitting laser is studied using an expanded form of the transverse Maxwell semiconductor Bloch equations. The influence of the incidence angle and relative cavity arm lengths is considered with respect to both the pump-probe computed instantaneous gain and group delay dispersion and the converged modelocked state. Changes in the angle are seen to lead to modest changes in dispersion but significant deformations of the modelocked pulse. Large changes in relative arm lengths are seen to lead to modest changes in the modelocked pulse with optimal pulses being observed with a 1:1 arm length ratio. The underlying microscopic dynamics are shown to drive these behaviors. This work provides a theoretical means to optimize experimental cavity geometry for desirable modelocking behaviors.

Phase-Incoherent Photonic Molecules in V-Shaped Mode-Locked Vertical-External-Cavity Surface-Emitting Semiconductor Lasers

Passively mode-locked vertical external-cavity surface-emitting semiconductor lasers (VECSELs) composed of a gain chip and a semiconductor saturable absorber have been drawing much attention due to their excellent performance figures. In this work we investigate how localized structures and incoherent, non-locally bound, pulse molecules emerge in a long cavity VECSELs using a V-shaped cavity geometry. We show that these states are bistable with the laser off state and that they are individually addressable. Using a model based upon delay differential equations, we demonstrate that pulse clusters result from the cavity geometry and from the non-local coupling with the gain medium; this leads to locally independent, yet globally bound phase, incoherent photonic molecules. Using a multiple time-scale analysis, we derive an amplitude equation for the field that allows us to predict analytically the distance between the elements of a cluster.

Microscopic modeling of transverse mode instabilities in mode-locked vertical external-cavity surface-emitting lasers

The generation and control of higher order transverse modes within a mode-locked vertical external-cavity surface-emitting laser with a semiconductor saturable absorber mirror are studied using a numerical solver for the two dimensional Maxwell Semiconductor Bloch Equations. In this work, the complex spatiotemporal evolution of the pulse toward a mode-locked state depends sensitively on the pumping level and pump to pulse spot size ratio. Microscopic physics sourced effects such as kinetic hole burning and filling, occurring on femtosecond timescales, play a central role. In particular, unsaturated charge carriers, both spectrally and transversally, initiate the development of asymmetric pulse profiles that transform over various characteristic time scales in a carrier chasing behavior.

Temporal localized structures in mode-locked vertical external-cavity surface-emitting lasers.

Temporal localized states (TLSs) are individually addressable structures traveling in optical resonators. They can be used to obtain bits of information and generate frequency combs with tunable spectral density. We show that a pair of specially designed nonlinear mirrors, a 1/2 vertical-cavity surface-emitting laser and a semiconductor saturable absorber, coupled in self-imaging conditions, can lead to the generation of such TLSs. Our results indicate how a conventional passive mode-locking scheme can be adapted to provide a robust and simple system emitting TLSs and paves the way towards the observation of three dimensional confined states, the so-called light bullets.

Ultrafast optical parametric oscillator pumped by a vertical external-cavity surface-emitting laser (VECSEL)

We report the first optical parametric oscillator synchronously pumped by a SESAM modelocked vertical external-cavity surface-emitting laser (VECSEL). As a nonlinear medium, we use a periodically poled MgO:PPLN crystal. The VECSEL operates at a wavelength of 982 nm and a repetition rate of 198 MHz. The pump radiation is converted to signal and idler wavelengths tunable in the ranges of 1.4-1.8 µm and 2.2-3.5 μm, respectively, simply by a change of the poling period and crystal temperature. The signal pulses have a duration between 2 ps to 4 ps and an average output power up to 100 mW.

High-power amplification of a femtosecond vertical external-cavity surface-emitting laser in an Yb:YAG waveguide.

We present the amplification of a mode-locked vertical external-cavity surface-emitting laser (VECSEL) using an Yb:YAG crystalline waveguide as gain medium. The VECSEL seed laser operates at a center wavelength of 1030 nm and generates 300-fs pulses at a repetition rate of 1.77 GHz. An average seed power of 60 mW was launched onto a 8.3 mm long fs-laser written Yb:YAG waveguide pumped by 7.7 W from a 969-nm continuous-wave VECSEL. The amplifier achieves an average output power of up to 2.9 W, corresponding to an amplification factor of 17 dB. Due to gain narrowing, the pulse duration increases to 629 fs. Our results show that crystalline waveguides are a promising technique for the realization of compact multi-watt ultrafast amplifier systems.

High-power 100 fs semiconductor disk lasers

Optically pumped passively modelocked semiconductor disk lasers (SDLs) provide superior performance in average output power, a broad range of operation wavelengths, and reduced complexity. Here, we present record performance with high average power and pulse durations as short as 100 fs with a semiconductor saturable absorber mirror (SESAM) modelocked vertical external-cavity surface-emitting laser (VECSEL) at a center wavelength of 1034 nm. A comprehensive pulse characterization confirms fundamental modelocking with a close to transform-limited output pulse of 128 fs and with negatively chirped output pulses as short as 107 fs, which are externally compressed to 96 fs with a single path through a 2-mm-thick ZnSe plate. For the “96 fs result” the pulse repetition rate is 1.6 GHz, the average output power is 100 mW, and the pulse peak power is 560 W. The transform-limited optical spectrum could in principle support pulses as short as 65 fs with higher order dispersion compensation. We measured the most relevant spectral and nonlinear VECSEL and SESAM parameters and used them as input parameters for our pulse formation simulations. These simulations agree well with our experimental results and provide an outlook for further performance scaling of ultrafast SDL technology.

Timing Jitter Reduction of a Mode-Locked VECSEL Using an Optically Triggered SESAM

A technique for actively stabilizing the repetition rate of a mode-locked vertical external-cavity surface-emitting laser is presented using an optically triggered semiconductor saturable absorber mirror. This method enables a significant reduction of the timing jitter measured in the frequency range [100 Hz, 10 MHz] and makes this kind of externally locked laser suitable for high-speed sampling of microwave signals. The experimental demonstration shows jitter reduction by a factor of 16 from 8 ps down to 423 fs for a 1.68-GHz repetition rate pulse train. The hold-in locking bandwidth and stabilization efficiency are shown to be a function of the average power and modulation index of the optical control signal.

Optimization of Ultrashort Pulse Generation in Passively Mode-Locked Vertical External-Cavity Semiconductor Lasers

We present a genetic algorithm based method to optimize the gain medium design of a passively mode-locked vertical external-cavity surface-emitting laser (VECSEL) for ultrashort pulse generation. Beginning with a given standard VECSEL design, we show that a simple modification to the DBR within the active mirror can significantly reduce both the duration and the chirp of the generated pulse. We demonstrate, using realistic numerical simulations, that the full-width at half-maximum of the pulse can be reduced by more than a factor of two. The required design changes come at the cost of increased sensitivity to perturbations in the active chip growth. Methods of counteracting this trend are presented.

All-GaInNAs ultrafast lasers: Material development for emitters and absorbers

Defect engineering is a key feature in material development for active and passive laser devices. Active devices such as surface emitting lasers require excellent material quality with low defect concentration and good strain management. In contrast, passive devices such as saturable absorbers benefit from nonradiative recombination via defect states. Different molecular beam epitaxy (MBE) growth conditions and annealing parameters were developed to optimize GaInNAs for both active and passive devices. We have demonstrated for the first time an all-GaInNAs modelocked vertical external-cavity surface-emitting laser (VECSEL) at 1.3 μm. We combined a GaInNAs VECSEL with a GaInNAs semiconductor saturable absorber mirror (SESAM) in a laser cavity. The VECSEL was optically pumped by an 808 nm semiconductor diode laser. The intracavity GaInNAs SESAM self-starts stable modelocking and generates a pulse duration of 18.7 ps with a pulse repetition rate of 6.1 GHz at 57 mW of average output power at a center wavelength of 1308 nm. In this paper, we briefly review the modelocking result and then focus on the MBE growth and fabrication of both active and passive GaInNAs devices.

10 GHz passively mode-locked external-cavity semiconductor laser with 1.4W average output power

We present a 10GHz passively mode-locked vertical external-cavity surface-emitting semiconductor laser (VECSEL) with 1.4W average output power in 6.1ps pulses. The output features a very good pulse quality with a time–bandwidth product of 0.42 in a nearly diffraction-limited beam. This demonstrates that passively mode-locked VECSELs are suitable for generating high powers in high-repetition-rate pulse trains.