Automatic offset-frequency locking of external cavity diode laser in wide wavelength range

Abstract

To construct different synthetic wavelengths for multi-wavelength interferometric absolute distance measurements, an automatic offset-frequency locking method was proposed to lock the frequency of a tunable external cavity diode laser (ECDL) to a femtosecond optical frequency comb (OFC) in a wide wavelength range. First, based on the grating and double convex lens comb filtering, a beat signal detection unit was designed. In addition, the beat signal detection between the ECDL and target comb modes in a wide wavelength range was realized. A phase-frequency detector was then designed based on the principle of a lock-in amplifier; phase-locking with a large capture range, wide monotonic phase range, and high accuracy was achieved. Finally, multiple closed-loop controls were adopted to realize automatic adjustment and offset-frequency locking of the ECDL over a wide wavelength range. Experimental results show that the proposed method can automatically lock the ECDL to OFC in the range of 10 nm, and the average signal-to-noise ratio of the beat signal is approximately 35.9 dB. Over 4 h, the standard deviation of the ECDL laser frequency is approximately 1.49 kHz, and the relative Allan deviation is approximately 4.76 × 10-12 at 1 s. The proposed automatic offset-frequency locking method can satisfy the requirements of wide-range wavelength adjustment and high-precision frequency stabilization in precision interferometry.