How to control the dual-wavelength vertical external cavity surface emitting semiconductor laser (VECSEL)
Dual-wavelength laser is a light source with great application potential. Its application range includes nonlinear frequency conversion, dual-wavelength interference technology, optical communication, photodynamic medicine and other fields, so it has attracted the attention of many researchers at home and abroad. Since semiconductor lasers have the advantages of high efficiency, light weight, wide wavelength range, and high reliability, if dual-wavelength laser output can be achieved through semiconductor lasers, it will be very conducive to promoting the practical process of dual-wavelength light sources.
Vertical-External-Cavity Surface-Emitting Laser (VECSEL) combines the advantages of solid-state lasers and semiconductor lasers, and can obtain high output power and high beam quality at the same time. In addition, VECSEL has a flexible external cavity structure, and various optical elements can be added in the cavity, so as to realize functions such as wavelength tuning, mode locking and nonlinear frequency conversion. Therefore, VECSEL has the potential to become an ideal dual-wavelength semiconductor laser, and researchers have been working on the realization of dual-wavelength VECSEL.
Recently, the research team of Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, University of Chinese Academy of Sciences, and Changchun Zhongke Changguang Space-Time Optoelectronics Technology Co., Ltd. jointly proposed a compact VECSEL, which can control pumping The optical power realizes the mutual conversion between the two laser wavelengths output by the VECSEL, and the interval between the two wavelengths is close to 50 nm. At 0°C, the maximum output power of each lasing wavelength is above 1.5 W. With the change of the pump power, the lasing wavelength can be switched between 950 nm and 1000 nm, and dual-wavelength simultaneous lasing can be achieved at a power level above 1.5 W. This kind of VECSEL device with switchable wavelength and dual-wavelength simultaneous lasing has great application potential in the fields of optical modulation and difference frequency. Relevant research results have been published in the journal "Journal of Infrared and Millimeter Waves".
Compared with the previous dual-wavelength VECSEL structure design, in this study, the researchers mainly achieved wavelength conversion through thermal modulation of the gain spectrum. The gain chip of VECSEL adopts a bottom-emitting surface-emitting laser structure, and a barrier layer, a multi-quantum well active region with a resonant periodic structure and a Distributed Bragg Reflector (DBR) are successively grown on an undoped GaAs substrate. structure. GaInP is grown between the active region and the substrate as an etching barrier layer, which is mainly used to imitate the damage to the chip structure caused by the substrate removal process, and it has a higher potential barrier than the pump absorption region, which can also prevent the loading The flow particles diffuse to the surface to form non-radiative recombination. The active area of this gain chip consists of 9 InGaAs/GaAsP/AlGaAs strained quantum wells. In the overall structure design, the position of the 9 quantum wells is located at the center of the standing wave peak, forming a resonant periodic structure, avoiding the spatial hole burning effect, and obtaining higher optical gain.
Schematic diagram of the working of the VECSEL system, the inset is the standing wave light field of the gain chip of the device.
The researchers further tested the performance of the VECSEL device. By increasing the gain chip structure design of the gain-cavity mode pre-offset, under different pump powers, the gain spectrum is tuned to one of the two modes determined by the down-tilt angle of the reflectivity spectrum, realizing the gain chip The lasing wavelength is switchable between 950 nm and 1000 nm. In addition, at a specific pump power, simultaneous dual-wavelength lasing of VECSEL can be achieved. At 0°C, the maximum output power corresponding to the two lasing wavelengths exceeds 1.5 W, and the frequency difference between the two wavelengths is 15.66 THz. The relative intensities of the two oscillation modes in the dual-wavelength VECSEL can be tuned by fine-tuning the pump power, and both oscillation modes exhibit good stability.
The PL spectrum of the luminous area and the reflection spectrum of the overall structure of the VECSEL gain chip
Output performance of the VECSEL device at 0°C (a) and its far-field divergence angles (b) at pump powers of 8 W and 20 W
At 0°C, the relationship between the output spectrum of the VECSEL device and the pump power (a), and the relationship between the output wavelength of the VECSEL device and the temperature when the pump power is constant (b)
At 0°C, the beam quality M² (a) of the VECSEL device when outputting at a single wavelength, and the beam quality M² of the VECSEL device when operating at two wavelengths, the inset is the shape of the output spot (b)
The dual-wavelength laser proposed in this study can avoid complex gain chip structure design and complex epitaxial growth, and does not need to insert and tune optical components in the external cavity. In addition, it is small, strong, stable, and practical. It has high practical value in fields such as difference frequency.
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