Compared with traditional techniques, magnetic resonance imaging using hyperpolarized inert gas as the contrast agent has greatly improved the quality of lung images. The hyperpolarized inert gas is obtained by means of spin-exchange optical pumping, and pumping the alkali metal rubidium is the key to this process. However, the bandwidth of commercial high-power semiconductor lasers is wide, while the Doppler broadening linewidth of alkali metal atoms is narrow, which leads to low pumping efficiency. To match the pump spectrum width and atomic absorption spectrum width, researchers mainly use the surface grating or volume Bragg grating (VBG) as an external cavity feedback element to develop external cavity semiconductor lasers. In this way, the spectrum width can be narrowed, and the pumping efficiency can be improved to a certain extent. In the existing reports, however, few studies have realized the simultaneous output of lasers with narrow spectrum width and high power. Moreover, under the high power demand, most studies use the laser bar as the internal gain chip, which is exposed to problems such as poor beam quality and great coupling difficulty. Therefore, this work proposes an external cavity semiconductor laser based on a single-tube gain chip. The output performance of the laser can satisfy the narrow spectrum width and have high power in the meanwhile.

VBGs have the characteristics of a high damage threshold and polarization insensitivity. Therefore, this study uses a VBG as the external cavity feedback element and designs the external cavity structure given the performance characteristics of the element. The diffraction efficiency of the VBG is affected by the incident wavelength and the angle of the incident light. According to the diffraction efficiency diagram of the VBG (Fig. 2), a single-tube semiconductor laser with a suitable wavelength is selected as the gain chip. Meanwhile, the front cavity surface of the gain chip can resist reflection after treatment to suppress the influence of the internal cavity mode, and two lenses are added to the external cavity structure to collimate the beam for the best diffraction efficiency of the VBG. Secondly, the completed laser structure is tested, and the laser spectrum and power are monitored and recorded simultaneously. For the understanding of the wavelength stability of the external cavity semiconductor laser, the performance of the laser under different working currents and temperatures is monitored, and the data is analyzed.

The proposed VBG-based external cavity semiconductor laser can achieve the output power of 6.36 W and a spectrum of 0.036 nm at the operating current of 10 A and the operating temperature of 16 ℃ (Fig. 4), and the external cavity coupling efficiency reaches 88.5%. Compared with the single-tube semiconductor laser, the proposed semiconduction laser has significantly higher wavelength stability. The drift coefficient of the wavelength to current decreases from 1 nm/A to 0.01 nm/A (Fig. 6), and the temperature drift coefficient decreases from 0.350 nm/℃ to 0.005 nm/℃ (Fig. 7). In the experiment, it is found that when the difference between the central wavelength of the gain chip and that of the VBG feedback is within the tolerance range, the output from the external cavity semiconductor laser meets the characteristics of narrow spectral width, high power, and stable wavelength. Therefore, for the narrow spectral width and wavelength stability of the proposed laser in a wider operating current and temperature range, it is necessary to improve the maximum tolerance between the central wavelength of the gain chip and that of the volume Bragg grating feedback. The maximum tolerance of the proposed semiconductor laser is related to the feedback efficiency of the external cavity. It is affected by the reflectance of the front cavity surface of the gain chip, the diffraction efficiency of the VBG, the alignment position of each optical element, the reflectance of the lens, etc.

On the basis of the characteristics of VBGs, the structure of the external cavity semiconductor laser is designed in this study. The laser can output high power and narrow spectral width lasers, which solves the difficulty of matching spectral width with atomic absorption spectrum width in the pumping process and improves the pumping efficiency. The wavelength stability of the laser is tested under different operating currents and operating temperatures, and the results show that the laser has good wavelength stability within a certain operating current and temperature range. The relationship between the maximum tolerance between the central wavelength of the gain chip and that of the VBG feedback and the laser performance are presented. The research provides a theoretical and experimental basis for ensuring the stability of the external cavity semiconductor laser in a higher output power range, a larger operating current, and temperature range.