New progress has been made in vertical external cavity surface emitting semiconductor lasers

Dual-wavelength output semiconductor lasers have important application prospects in the fields of interferometry, spectral analysis, interferometric imaging and difference-frequency terahertz. Vertical External Cavity Surface-Emitting Laser (VECSEL) combines the characteristics of high beam quality of solid-state laser and rich wavelength of semiconductor laser. It can achieve high output power, good beam quality and large coverage wavelength range at the same time. At the same time, VECSEL has a flexible external cavity type, which can realize filtering, tuning, mode-locking, frequency doubling and other functions by adding optical elements into the cavity. In recent years, VECSEL scheme has become a hotspot in the field of laser dual-wavelength output.

A team led by Professor Jianwei Zhang from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, has proposed an optically pumped vertical external cavity surface emitting semiconductor laser (VECSEL) that uses a single gain chip to achieve a dual-wavelength output, according to Myms. The maximum laser power of the VECSEL can reach 560 mW at the stable dual-wavelength output. The light spot presents symmetrical Gaussian morphology in the orthogonal direction, and the divergence angles in the orthogonal direction are 6.68° and 6.87°, respectively. The research results have been published in the journal Infrared and Millimeter Wave.

The VECSEL system proposed in this study consists of a pumped laser, a heat sink and temperature control TEC, a gain chip and an output coupled mirror. The luminous region of the gain chip is composed of two sets of quantum Wells with different luminous wavelengths. One set of quantum Wells with short luminous wavelength is pumped by the absorption region, and the other set of quantum Wells with long luminous wavelength is pumped by the inner well. As the VECSEL operates, the heat sink temperature at the bottom of the gain chip is controlled by a semiconductor cooling sheet TEC.

(a) epitaxial structure diagram of dual-wavelength VECSEL gain chip; (b) Band map of source region of VECSEL gain chip

Diagram of the VECSEL system

During VECSEL operation, the short wavelength quantum well pumped by the absorption region takes the lead in firing. Due to the intensity modulation effect of the quantum well with longer luminous wavelength on the short wavelength quantum well, the spectral peak intensity of the two wavelengths can be observed to oscillate periodically with time. At this time, the output laser of VECSEL can be observed in the form of pulse output by using a highly sensitive detector. With the further increase of pump power, VECSEL's output laser presents a stable dual-wavelength output. When the substrate temperature is -10℃, the maximum dual-wavelength output power reaches 560 mW, and the laser wavelength is 967.5 nm and 969.8 nm, respectively. In addition, the pulse modulation phenomenon of VECSEL can be realized with the proposed scheme when the pump power is low. The dual-wavelength VECSEL has Gaussian symmetrical spot morphology, and the laser divergence angles in the two orthogonal directions are only 6.68° and 6.87°.

Laser spectra of VECSEL at different pump power when TEC temperature control is -10℃

At -10℃, VECSEL's output far-field distribution when the pump power is 5 W is illustrated as a two-dimensional color map of output light spot

In summary, this study proposes a VECSEL gain chip structure using two sets of quantum Wells with different luminescence wavelengths as the luminescence layer, which achieves stable dual-wavelength output by changing the external optical pump power level. This new gain chip structure will have a good prospect in the realization of dual-wavelength laser and optical modulation applications.

Source: MEMS