Experimental Study of 450 nm Narrow-bandwidth Semiconductor Laser Based on Reflective Volume Bragg Grating

To explore the suppression effect of volume Bragg grating on the temperature drift and spectral line broadening caused by thermal effects during laser operation, a volume Bragg grating with a central wavelength of 449.7 nm was used as the external cavity reflector for blue semiconductor lasers. Under the external cavity mode-locking condition, the influence of different environmental variables on the laser output characteristics was studied. In the experiment, an optical system was built by using a blue light diode, a fast-axis collimating lens and a slow-axis collimating lens to achieve stable laser output. The control variable method was adopted to study the influence of a single variable. The water-cooling temperature, the excitation current and the distance between the slow-axis collimator and the volume Bragg grating were changed step by step, and the spectra in the free-running state and those under the external cavity mode-locking were compared and analyzed. Firstly, the wave-locking position of the volume grating was determined, and an adsorption device was used to modulate the position of the volume grating up and down to compare the free-running spectrum and the wave-locked spectrum under external cavity modulation. The experimental results showed that under the condition of constant water cooling at 22 degrees C, when a 3 A current was applied to the blue semiconductor chip, the central wavelength of the free- running laser output spectrum was at 447.4 nm, and the central wavelength of the laser output spectrum locked by the volume Bragg grating was at 449.7 nm, and the spectral line width was narrowed from 1.4 nm to 0.3 nm. When the current increased from 1 A to 3 A, significant wavelength drift and spectral line broadening caused by thermal effects were observed in the free-running spectrum. Moreover, as the current increased, the thermal effect of the semiconductor chip became more significant, and the central wavelength shifted towards the long wavelength at an accelerated speed. However, by adding the volume Bragg grating as the external cavity mirror of the semiconductor laser to form external cavity feedback, the feedback light participated in the competition of stimulated emission inside the chip, achieving the wave-locking effect. The central wavelength of the output light remained consistently at 449.7 nm, and no significant wavelength drift was detected. However, as the current increased, the competitiveness of the feedback light inside the chip weakened, and side lobes appeared in the free-running spectrum. Meanwhile, there was a slight widening of the spectral line width. When the current was fixed at 3 A and the temperature of the water-cooled copper heat sink increased from 16 degrees C to 22 degrees C, the central wavelength of the free-running spectrum increased from 446.9 nm to 447.4 nm, and the temperature drift coefficient was approximately 0.083 nm/degrees C. Under the same condition, by adding the volume Bragg grating, the central wavelength consistently stabilized at 449.7 nm, and the full width at half maximum was only 0.3 nm, with no obvious temperature drift phenomenon occurring. By changing the distance from the volume grating to the slow-axis collimating lens and analyzing the output spectrum, when the distance between the volume Bragg grating and the slow-axis collimating lens increased from 1 cm to 6 cm, the central wavelength remained unchanged, with a stable wave-locking effect. When the distance reached 6 cm, a weak side lobe appeared in the spectrum, and the position of the side lobe corresponded to the central wavelength of the free-running spectrum. After wavelength locking, there was still a linear relationship between the laser output power and the input current. This research provides a reference for the development of high- brightness, high-power, and high-color-purity blue semiconductor lasers.