Within the last two decades, vertical-external-cavity surface-emitting lasers (VECSELs) have attracted rising interest from both industry and science. They have proven to be versatile lasers which can be specifically designed for research and applications that require a particular regime of operation. Various emission schemes ranging from narrow-linewidth emission, pulsed light or multimode emission to a frequency-converted output are feasible owing to remarkable device features. Being composed of a semiconductor gain mirror and an external cavity, not only is a unique access to high-brightness output and a high-beam quality is provided, but also wavelength flexibility. Moreover, the exploitation of intra-cavity frequency conversion further extends the accessible spectral range from the ultraviolet (UV) to the terahertz (THz). In this work, recent advances in the field of VECSELs are highlighted.

https://iopscience.iop.org/article/10.1088/2040-8978/18/9/093003/pdf

Recent advances in VECSELs

This paper will review the recent advances in the field of ultrashort pulse generation from optically pumped vertical-external-cavity surface-emitting lasers (OP-VECSELs). In this review, we will summarize the most significant results presented over the last 15 years, before highlighting recent breakthroughs related to mode-locked VECSELs by different research groups. Different mode-locking techniques for OP-VECSELs are described in detail. Previously, saturable absorbers, such as semiconductor saturable absorber mirrors—external, or internal as in mode-locked integrated external-cavity surface emitting lasers (MIXSEL)—, and recently, novel-material-based carbon-nanotube and graphene saturable absorbers have been employed. A new mode-locking method was presented and discussed in recent years. This method is referred to as self-mode-locking or saturable-absorber-free operation of mode-locked VECSELs. In this context, we particularly focus on achievements regarding self-mode-locking, which is considered a promising technique for the realization of high-power, compact, robust and cost-efficient ultrashort pulse lasers. To date, the presented mode-locking techniques have led to great enhancement in average powers, peak powers, and repetition rates that can be achieved with passively mode-locked VECSELs.

Mode-locked semiconductor disk lasers

The incorporation of the material-based saturable absorber (SA) to generate mode-locked fiber laser (MLFL) has been extensively explored and demonstrated. However, the material-based SAs have some challenges in terms of optical damage threshold, design complexity, robustness, and stability due to several factors. Presently, several works have been demonstrated to address material-based SA fabrication issues. Therefore, this paper aims to comprehensively review the pros and cons of material-based SA in terms of material synthesis techniques, material characteristics, material-polymer composite, SA structure, and deposition techniques, along with current issues and challenges, and conclude with concrete recommendations. All the highlighted insights of this review will contribute to the increased efforts toward the development of the material-based SAs for the MLFL.

Recent research and advances of material-based saturable absorber in mode-locked fiber laser

We compare the heat extraction efficiency for a standard Vertical External Cavity Surface-Emitting Laser and the distributed Bragg reflector (DBR)-free structure employing a single and double diamond heat-spreaders, respectively. Both heterostructures grown by Molecular Beam Epitaxy employ two identical active regions designed for emission at 980 nm. We show that the thermal resistance has been decreased 15 times when there is no DBR and heat is extracted from both side of active region. For DBR-free laser no thermal rollover of power conversion characteristic was observed in the range of considered input powers.

/pdf/highly-efficient-heat-extraction-by-double-diamond-heat-532wke6e43.pdf

Highly efficient heat extraction by double diamond heat-spreaders applied to a vertical external cavity surface-emitting laser

We report the mode-locked operation of an AlGaInP-based semiconductor disk laser without a saturable absorber. The active region containing 20 GaInP quantum wells is used in a linear cavity with a curved outcoupling mirror. The gain chip is optically pumped by a 532 nm laser, and mode-locking is achieved by carefully adjusting the pump spot size. For a pump power of 6.8 W, an average output power of up to 30 mW is reached at a laser wavelength of 666 nm. The pulsed emission is characterized using a fast oscilloscope and a spectrum analyzer, demonstrating stable single-pulse operation at a repetition rate of 3.5 GHz. Intensity autocorrelation measurements reveal a FWHM pulse duration of 22 ps with an additional coherence peak on top, indicating noise-like pulses. The frequency spectrum, as well as the Gaussian beam profile and the measured beam propagation factor below 1.1, shows no influence of higher order transverse modes contributing to the mode-locked operation.

Self-mode-locked AlGaInP-VECSEL

Comparison of AlGaInP-VECSEL gain structures

Semiconductor disk lasers (SDLs), also known as vertical-external-cavity surface-emitting-lasers (VECSELs) are an ideal choice to reach high output powers. Further advantageous characteristics [1] of this laser type, such as superior beam quality with a radial symmetric TEM00 beam profile, are provided by the external cavity, which also enables easy implementation of optical intra-cavity elements like frequency filters or frequency converting crystals. A further big advantage of VECSELs is given by the possibility of bandgap engineering. By proper adjustment of the semiconductor material and their composition, many different wavelength areas can be covered. The optimizaion of the quantim well (QW) arrangement should further lead to improved performance e.g. low threshold and high output power.

High-power (> 400 mW) laser emission at 332 nm of frequency-doubled, optically pumped AlGaInP disk laser with an optimized quantum well structure

Optically pumped semiconductor (OPS) vertical external-cavity surface-emitting lasers (VECSELs) are an important category of power scalable lasers with a wide range of applications in biophotonics, medicine technologies, spectroscopy, projector technologies and lithography. The open laser resonator allows to implement frequency selective and converting intra-cavity elements. The possibility of bandgap engineering, laser emission in the fundamental Gaussian mode and the technical simplicity leads to ongoing growth of the area of applications for these tunable laser sources. We present degradation studies of metal-organic vapor-phase epitaxy (MOVPE) grown, optically pumped, red-emitting AlGaInP-VECSELs with quantum wells (QWs) as active layers. Laser performance in continuous operation, pumped with a 532nm Nd:YAG laser and recorded over several hours, will be shown. Surface investigations of the gain structure via large-area photoluminescence maps show the possible consequences of optical pumping. A comparison of barrier-pumped performance data with the data of an in-well pumped VECSEL device is shown.

Degradation studies and pump optimization of optically pumped red-emitting AlGaInP-VECSELs

We present a passively mode-locked semiconductor disk laser (SDL) emitting at 650nm with intra-cavity second harmonic generation to the ultraviolet (UV) spectral range. Both the gain and the absorber structure contain InP quantum dots (QDs) as active material. In a v-shaped cavity using the semiconductor samples as end mirrors, a beta barium borate (BBO) crystal is placed in front of the semiconductor saturable absorber mirror (SESAM) for pulsed UV laser emission in one of the two outcoupled beams. Autocorrelation (AC) measurements at the fundamental wavelength reveal a FWHM pulse duration of 1.22ps. With a repetition frequency of 836MHz, the average output power is 10mW per beam for the red emission and 0.5mW at 325nm.

Intra-cavity frequency-doubled mode-locked semiconductor disk laser at 325 nm.

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Papers

Comparison of AlGaInP-VECSEL gain structures

High-power (> 400 mW) laser emission at 332 nm of frequency-doubled, optically pumped AlGaInP disk laser with an optimized quantum well structure

Degradation studies and pump optimization of optically pumped red-emitting AlGaInP-VECSELs

Intra-cavity frequency-doubled mode-locked semiconductor disk laser at 325 nm.