Realization of frequency calibration for 532 nm wavelength laser based on spectral enhancement technology

Abstract

The iodine frequency stabilized 532 nm Nd:YAG laser plays an important role in realizing the reproduction unit of length “meter (m)”, absolute gravity measurement, gravitational waves detection, precision spectroscopy, distance metrology, etc. Absolute frequency measurement and calibration of the laser are of great significance for evaluating the performance of laser. The previous method of extending the erbium-doped fiber optical frequency comb (Er-FOFC) to the wavelength of 532 nm was to first amplify the seed light, then realize frequency-doubled with a periodic polarization lithium niobate crystal, and finally couple it into a photonic crystal fiber to expand the spectrum to the 532 nm band. With such a technique, the a signal-to-noise ratio (SNR) of the beat signal between the iodine-stabilized 532 nm Nd:YAG laser and the Er-FOFC was approximately 30 dB. Moreover, the SNR of the beat signal was unstable, resulting in the errors in frequency measurement with a counter. This is not conducive to the long-term frequency measurement of the iodine-stabilized 532 nm Nd:YAG laser. Therefore, a method that can obtain both high SNR and long-term stable beat signals is required. In this paper, an Er-FOFC is developed. The spectral enhancement of its broadening at 1 μm is carried out, and then expanded to the wavelength at 532 nm by using a frequency-doubling crystal. The output power of the Er-FOFC is 20 mW, which is first amplified to 370 mW by an Er-fiber amplifier and then compressed to a pulse width of 45.7 fs. Subsequently, the spectrum is extended to cover the wavelength at 1 μm with a highly nonlinear fiber, resulting in an output power of 180 mW. The broadened spectrum at 1 μm is amplified to 601 mW by a Yb-fiber amplifier, and the compressed power increases to 420 mW. Using an MgO:PPLN crystal, the compressed laser is frequency-doubled to produce a 532 nm laser output with 155 mW power and a doubling efficiency of 36%. Utilizing this system, the absolute frequency measurements are conducted on the fundamental frequency light at 1064 nm and the doubled frequency light at 532 nm from the iodine-stabilized 532 nm Nd:YAG laser, yielding a beat signal with an SNR of greater than 40 dB. This SNR represents a 13 dB improvement compared with the result obtained when an amplified seed light is frequency-doubled by using PPLN and then coupled into a PCF for direct spectral broadening to cover the 532 nm band. Over several days of continuous monitoring, there is no observed risk of SNR degradation. Moreover, subsequent frequency measurements are carried out continuously for over several hours, with the results maintaining consistency with recommended values.