Mode-locked Vertical External Cavity Surface Emitting Lasers Research Articles

Over 10 kW peak-power low repetition rate passively mode-locked VECSEL using a saturable Bragg reflector.

Reducing the repetition rate is one of the effective ways to increase the peak-power of the mode-locked pulses. However, for a vertical-external-cavity surface-emitting laser (VECSEL), the carrier lifetime in the nanosecond regime limits the further reduction of the pulse repetition rate, or in other words, limits the average output power of the mode-locked laser at low repetition rates, and ultimately restricts the peak-power of the pulses. This work uses a specially designed saturable Bragg reflector to start the mode-locking, and both low repetition rate and high average power are achieved simultaneously in a passively mode-locked VECSEL. When the repetition rate of the pulses is 78 MHz, a peak power of 12.9 kW, twice the reported highest value, is achieved. The pulse repetition rate is further reduced to the lowest value of 67 MHz, and peak-powers of 2.33, 3.00 and 1.23 kW under the fundamental, second-harmonic, and fourth-harmonic mode-locking are demonstrated, respectively. In addition, a theoretical analysis of the influence of the saturation fluence of the saturable absorber on the evolution of the mode-locked pulses is also provided.

Improved underwater wireless optical communication using a passively mode-locked VECSEL

This paper presents improved underwater wireless optical communication (UWOC) based on a 490 nm passively mode-locked vertical-external-cavity surface-emitting laser (VECSEL) with the repetition rate of 1.46 GHz and the pulse width of 2.25 ps. After conducting a comprehensive evaluation of the performance characteristics of quasi-cyclic low-density parity-check (QC-LDPC) coding and pulse-position modulation (PPM) in relation to bit-error-rate (BER) and communication capacity across different signal-to-noise ratios (SNRs), a passively mode-locked VECSEL based UWOC system is designed and experimentally performed. The UWOC system employs the QC-LDPC encoding technique and PPM modulation scheme that are most suitable for the system. Compared with the case without QC-LDPC, the minimum received optical power with coding is improved by 3.5 dBm, and the SNR with coding is improved by 0.4 dB under condition of 256-PPM and forward error correction (FEC) threshold of 3.8×10−3. Meanwhile, compared with the UWOC system using a continuous-wave (CW) VECSEL as the light source, the attenuation coefficient of the UWOC system based on the mode-locked VECSEL is decreased by 0.04 m-1, and the BER of the mode-locked VECSEL based UWOC system is decreased by 0.3 dB.

324-fs Pulses From a SESAM Modelocked Backside-Cooled 2-μm VECSEL

We present the first modelocked backside-cooled GaSb VECSEL (Vertical External Cavity Surface Emitting Laser) operating above <inline-formula> <tex-math notation="LaTeX">$2 ~\mu \text{m}$ </tex-math></inline-formula>. Using a two quantum well InGaSb SESAM (SEmiconductor Saturable Absorber Mirror) in a V-shaped cavity arrangement we obtain femtosecond modelocking at a center wavelength of 2061 nm, with pulses as short as 324 fs, an average output power as high as 65 mW at a repetition rate of 3 GHz. An operation in a picosecond regime is further demonstrated with an even higher average output power of up to 260 mW and 4.5-ps pulses. We perform a characterization in both the femto- and picosecond regimes with measurements of the pulse duration, the optical spectrum, and the pulse repetition rate (frequency and time domain analysis) at low and high power. In this context, the influence of intracavity group delay dispersion (GDD) is investigated. We find that GDD has a strong influence on the pulse duration (fs or ps regime) and thus on the performance of our SESAM-modelocked 2-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> GaSb VECSEL.

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.

Simulation of the influence of gain and SESAM on characteristics of mode-locked VECSELs

Vertical external cavity surface-emitting lasers (VECSELs) have unique properties such as excellent beam quality, high average output power, and high repetition rate. Short pulses from VECSELs, especially picosecond and femtosecond, can be achieved by a passive mode-locking mechanism with a semiconductor saturable absorber mirror (SESAM). We theoretically simulate the pulse evolution dynamics in VECSELs and investigate the influences of different parameters of gain and SESAM on pulse characteristics. To the best of our knowledge, this is the first time that the influences of fast recovery time and two-photon absorption coefficient on pulse characteristics have been studied.

High peak power, sub-ps green emission in a passively mode locked W-cavity VECSEL.

We report on the experimental results of a passively mode-locked vertical external cavity surface emitting laser (VECSEL), implemented in a W-cavity configuration, using a lithium triborate (LBO) crystal for intra-cavity second harmonic generation (SHG) at 528 nm. The W-cavity configuration allows separation of the crystal from the semiconductor saturable absorber mirror (SESAM), enabling independent control over the Gaussian beam sizes at the crystal, chip, and SESAM. This optimized cavity demonstrated a second harmonic pulse width of ~760 fs at a frequency of 465 MHz and 230 mW average output power, resulting in a peak pulse power of 580 W.

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.

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.

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.

Coherent beam combining and noise analysis of a colliding pulse modelocked VECSEL.

Optically-pumped SESAM-modelocked semiconductor disk lasers have become interesting ultrafast lasers with gigahertz pulse repetition rates, high average power and adjustable lasing wavelength. It is well established that colliding pulse modelocking (CPM) can generate both shorter pulses and improved stability. These improvements however typically come at the expense of a more complex ring cavity and two output beams. So far similar modelocking results have been obtained with CPM vertical external-cavity surface-emitting lasers (VECSELs) and with SESAM-modelocked VECSELs or modelocked integrated external-cavity surface-emitting lasers (MIXSELs) in a linear cavity. However coherent beam combining of the two output beams of a CPM VECSEL could result in a significantly higher peak power. This is interesting for example for applications in biomedical microscopy and frequency metrology. Here we demonstrate with a more detailed noise analysis that for both output beams of a CPM VECSEL the pulse repetition rates and the carrier envelope offset frequencies are locked to each other. In contrast to standard SESAM-modelocked VECSELs in a linear cavity, we only have been able to actively stabilize the pulse repetition rate of the CPM VECSEL by cavity length control and not by pump-power control. Furthermore, a first coherent beam combining experiment of the two output beams is demonstrated.

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.

Multi-Angle VECSEL Cavities for Dispersion Control and Peak-Power Scaling

We present a novel cavity design for vertical external cavity surface emitting lasers (VECSELs) enabling multiple interactions with the gain structure under different angles in a single round trip. This allows for a low round-trip group delay dispersion (GDD) despite using high-gain resonant VECSEL structures possessing pronounced resonances in their reflective GDD profile. Femtosecond-regime pulses with an average output power of 1.14 W and record peak intensities for mode-locked VECSELs of 6.3 kW are presented with simulations demonstrating the GDD compensating mechanism employed in this scheme.

Pulse interactions in a colliding pulse mode-locked vertical external cavity surface emitting laser

The passive mode locking of vertical external cavity surface emitting lasers (VECSELs) enables the generation of high brightness ultrashort pulses at high repetition rates with unmatched performance. The peak power achievable with sub-200-fs pulse duration is mostly limited by the stability of the fundamental mode-locking regime as side pulses or harmonic mode locking emerges at high pump power. Here, we study a colliding pulse mode-locked VECSEL generating a pulse duration as short as 128 fs, with an average power of 90 mW per beam and a repetition rate of 3.27 GHz. The relevant laser parameters under different pumping regimes before and after the emergence of a side pulse are then used as input parameters for the simulation of the pulse interactions in the saturable absorber. We present a new comprehensive model for the calculation of saturable losses in the saturable absorber mirror and we study the energy transfer between the two counter-propagating pulses. This study reveals how a colliding pulse scheme reduces the saturation fluence of the absorber by a factor 2.9 and suppresses the mode competition between the two counterpropagating pulses of the ring cavity.

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.

Single-photon emission at a rate of 143 MHz from a deterministic quantum-dot microlens triggered by a mode-locked vertical-external-cavity surface-emitting laser

We report on the realization of a quantum dot (QD) based single-photon source with a record-high single-photon emission rate. The quantum light source consists of an InGaAs QD which is deterministically integrated within a monolithic microlens with a distributed Bragg reflector as back-side mirror, which is triggered using the frequency-doubled emission of a mode-locked vertical-external-cavity surface-emitting laser (ML-VECSEL). The utilized compact and stable laser system allows us to excite the single-QD microlens at a wavelength of 508 nm with a pulse repetition rate close to 500 MHz at a pulse width of 4.2 ps. Probing the photon statistics of the emission from a single QD state at saturation, we demonstrate single-photon emission of the QD-microlens chip with g(2)(0) &amp;lt; 0.03 at a record-high single-photon flux of (143 ± 16) MHz collected by the first lens of the detection system. Our approach is fully compatible with resonant excitation schemes using wavelength tunable ML-VECSELs, which will optimize the quantum optical properties of the single-photon emission in terms of photon indistinguishability.

Nonequilibrium and thermal effects in mode-locked VECSELs

Ultrafast femtosecond timescale dynamics in Vertical External Cavity Surface Emitting Lasers (VECSELs) have recently been employed to achieve record average power and duration mode-locked pulses by employing different types of saturable absorbers and Kerr Lens elements. Microscopic many-body dynamics are expected to dominate when attempting to push pulse durations below 100 fs. We present a preliminary microscopic simulation of ultrafast mode-locking in order to expose the role of hot carrier distributions in establishing ultrafast mode-locking.

Recent advances in development of vertical-cavity based short pulse source at 1.55 μm

This paper reviews and discusses recent developments in passively mode-locked vertical external cavity surface emitting lasers (ML-VECSELs) for short pulse generation at 1.55 μm. After comparing ML-VECSELs to other options for short pulse generation, we reviewed the results of ML-VECSELs operating at telecommunication wavelength and point out the challenges in achieving sub-picosecond operation from a ML-VECSEL at 1.55 μm. We described our recent work in the VECSELs and semiconductor saturable absorber mirrors (SESAMs), their structure design, optimization and characterization, with the goal of moving the pulse width from picosecond to sub-picosecond.

Exploring ultrafast negative Kerr effect for mode-locking vertical external-cavity surface-emitting lasers

We present analytical considerations of "self-mode-locked" operation in a typical vertical external-cavity surface-emitting laser (VECSEL) cavity geometry by means of Kerr lens action in the semiconductor gain chip. We predict Kerr-lens mode-locked operation for both soft- and hard-apertures placed at the optimal intra-cavity positions. These predictions are experimentally verified in a Kerr-lens mode-locked VECSEL capable of producing pulse durations of below 500 fs at 1 GHz repetition rate.