elf-Mode-Locked Semiconductor Disk Lasers

Self-mode-locked quantum-well semiconductor disk laser: Green light is externally produced via second-harmonic generation with the infrared pulsed laser light. Inset: The autocorrelation trace shows the pulse width of 860 fs (top). The diagram of the long-span pulse train is shown at the bottom (left). The close-up of the pulse train reveals a 500 MHz repetition rate for the fundamental mode (right).

Vertical-external-cavity surface-emitting lasers (VECSELs), also called semiconductor disk lasers, are versatile lasers that serve as an excellent platform for the realization of various emission schemes due to their remarkable design flexibility and features. In recent years,researchers have achieved not only high-powermultimode or single-frequency continuous-wave operation, but also two-color and mode-locked emission. Furthermore, their external resonator can be exploited for intra-cavity frequency conversion via nonlinear elements, which drastically expands the accessible wavelength range. VECSELs have become particularly appealing sources of pulsed laser light, because they can provide an outstanding beam quality and a high pulse peak power.¹ Their potential to become compact, cost-efficient alternatives to commercial mode-locked lasers led to the pursuit of devices with ever shorter pulses, higher peak powers and enhanced tunability using resonator-integrated or even chip-integrated semiconductor saturable-absorber mirrors (SESAMs). However, the complex, power-sensitive and costly SESAMs, which have to be carefully designed for a certain wavelength range, naturally impose limitations on the performance of the device.

We have demonstrated SESAM-free VECSELs operated under self-mode-locking (SML) conditions. The SML scheme is not only applicable to quantum-well VECSELs,^2–4 but also to quantum-dot devices.⁵ Furthermore, SML can be used for passively harmonically mode-locked devices with sub-ps pulsed operation demonstrated at discrete power levels up to the third harmonic.⁴ We do not yet have a full understanding of the mechanism behind SML, which could allow the full utilization of its effects, and so this will be the subject of future studies. Nevertheless, an optimization of the dispersion and thermal management promises the achievement of shorter pulses in the 100 fs range and significantly higher peak powers exceeding the current values of up to 1 kW at repetition rates ranging from a few hundreds of MHz to a few GHz. We believe that in the near future, SML VECSELs, which combine the advantages of solid-state and semiconductor devices, can become robust, compact and low-cost sources of fs pulsed laser light.

Random Fiber Laser: Simpler and Brighter

Random fiber laser with ultimate efficiency. (a) Laser design with power distribution along the fiber; (b) typical beam profile; (c) input-output curve; and (d) quantum efficiency.

Raman fiber lasers provide versatile platforms to obtain high-brightness lasing over a broadwavelength range.¹ For a conventional Raman fiber laser with two mirrors at each end of the fiber cavity, the reflectivity and the reflection bandwidth of the mirrors should be chosen carefully to maximize laser output. However, lasing is possible in a long fiber without any point mirrors via random distributed feedback provided by Rayleigh backscattering, which is much simpler and more stable.²

In 2014 we showed that, despite its open laser cavity design, the generation efficiency of a random Raman fiber laser can be superior to those of conventional Raman fiber lasers. To achieve this, we designed the laser with a short (≤1 km) cavity.^3–5 The feedback mechanism comes from distributed Rayleigh scattering along the fiber and a fiber loop mirror formed by a 3 dB coupler, assuring intrinsically stable operation. Although the lasing threshold increased dramatically in such a short cavity, the differential generation efficiency demonstrates extraordinary behavior: just above the threshold, it amounts to several hundred percent, and more than 2 W of the lasing is generated from 0.5 W of pump excess over the lasing threshold. More important, the corresponding quantum efficiency can exceed 100 percent, due to the specific longitudinal power distribution along the cavity and lower loss for the generated wave compared to the pump wave. We define the quantum efficiency as the ratio of the Stokes photons, N_S^out = P_S^out/hν_S, to the pump photons in a loss-only regime at the cavity output, N_P^out = P_Pⁱⁿ e^–αPl/hν_P.Thus, the energy transfer to a new spectral band in the short random fiber laser is accompanied with the increase of the photon number by several percent N_S^out / N_P^out = 1.03.³ With shorter fiber length, the optical conversion efficiency could approach quantum limit (defined as hν_S/hν_P), and the power scaling of the laser can also be dramatically increased, due to the much higher threshold of the second-order random lasing.⁴ An even shorter cavity of 300 m helped to achieve a 73.7 W output, with a Gaussian profile in the fundamental transverse mode.⁵

The random fiber laser offers greater power performance compared to conventional counterparts in a simple design without environmentally sensitive components. A random fiber laser can be scaled to the kilowatt level of output power at the designated wavelength, creating a new direction for high-power optical sources.