A laser interferometer is the standard instrument for measuring the surface distributions and wave aberrations of different optical systems. As one of the most significant modules, the interferometric light source is used in conjunction with a beam expender, reference mirror, and imaging module to form stable and high contrast interferograms. The wide application of large-aperture optical devices has promoted the development of a large-aperture interferometer, and the wavelength tunable laser is the most popular light source for large-aperture interferometers. The high-power wavelength tunable interferometric light source with wide mode-hop-free (MHF) range requires a high-performance red gain chip. Domestic enterprises have made great progress in 650 nm semiconductor lasers. The wavelength tunable interferometric light source based on a 650 nm gain chip is crucial for the independent and controllable development of precision optical measurement equipment such as large-aperture interferometers.

We propose a wavelength tunable interferometric light source based on a 650 nm single angled facet (SAF) gain chip. The effects of the feedback strength at the output end and gain bandwidth of the gain chip on the laser stability and tuning characteristics are investigated based on the gain model of the external cavity laser. The influence of the waveguide tilt angle on the reflectivity of the chip output end is calculated. Relying on the mature technology, the 650 nm gain chip is rapidly verified. The Littman-type wavelength tunable laser is developed with this gain chip, and the main parameters, including the tuning range and resolution, are tested. We build a large-aperture interferometer consisting of the wavelength tunable interference light source, a Fizeau interferometer host, and a beam expansion system. The zero-order waveplates are used in the large-aperture interferometer to suppress the uneven contrast of the interferogram induced by the retardation differences at different wavelengths. Multiple interferometry measurements are conducted to study the repeatability of the large-aperture wavelength tunable interferometer.

The longitudinal modes with different reflectivity at the output end of the gain chip are shown in Fig.2. Figures 2 (a) and (b) indicate that the resonance effect of the internal cavity is weak when the reflectivity value r₂ is low. When the phase of the internal cavity does not match that of the external cavity, the longitudinal mode determined by the internal cavity can be ignored, as the longitudinal mode determined by the external cavity always dominates. When r₂ is high and the phase mismatch between the internal and external cavities gradually increases, the longitudinal mode determined by the internal cavity gradually dominates, causing a significant mode hop, as shown in Figs.2(c) and (d). The simulation results in Fig.3 show that the mode-hopping probability increases with current and temperature when r₂ exceeds 0.0041. The MHF range decreases dramatically when r₂ increases , as shown in Fig.3. Figure 4 depicts that the laser can reach a MHF range of 0.2 nm when the chip gain bandwidth is greater than 7 nm. Figure 9 shows that the 3 dB bandwidth of the gain chip remains at 8 nm as the pump current increases, indicating that the chip remains in the spontaneous emission state and that the gain bandwidth is wide, which meets the requirements of the wavelength tunable interference light sources. A Littman-type wavelength tunable light source based on a 650 nm chip is developed, with a laser power of 7.5 mW. Figure 11 shows that the laser outputs multiple longitudinal modes and that the beam quality factor in the y-direction jumps from 1.25 to 1.35 when the current exceeds 65 mA. A possible reason is that the increase in current causes the multiple longitudinal modes, which leads to an increase in the proportion of transverse modes corresponding to the longitudinal frequency. Consequently, the bias current of the laser should be limited to the range of the single longitudinal mode to ensure the contrast of the interference pattern. In addition, the laser has a MHF range of more than 100 pm and a wavelength resolution of 10 fm, as shown in Fig.13. The light source is applied in the 600 mm aperture interferometer. Figure 11 shows that the contrast of the phase-shifted interferogram is good. Additionally, the peak valley value of the surface shape is less than 0.1λ. Multiple measurements indicate that the RMS repeatability is better than 0.0001λ.

We theoretically investigate the requirements of the wavelength tunable interferometric light source for a gain chip. A 650 nm SAF gain chip with a bandwidth of 8 nm and a power of 5 mW is realized. We propose a wavelength tunable interferometric light source based on the above gain chip. The results show that the light source has a power of 7 mW, a MHF range of more than 100 pm, and a resolution of 10 fm. The root-mean-square repeatability of the 600 mm aperture interferometer with this light source is better than 0.0001λ, indicating that the light source satisfies the requirements of large-aperture and high-precision wavelength tunable interferometers.