级联掺Yb增益光纤提高拍频信号信噪比的实验研究

Experimental study on increasing signal-to-noise ratio of a beat note by cascading an Yb-doped fiber in an Er-fiber comb

飞秒光学频率梳的出现使对未知激光的绝对频率测量成为可能,极大地简化了激光绝对频率的量值溯源和比对工作.为了保证测量数值的准确性,飞秒光学频率梳与未知激光的拍频信号fb的信噪比要求大于30 d B.针对碘稳频532 nm激光绝对频率测量的特定需求,以532 nm激光的基频光1064 nm激光的绝对频率测量为着眼点,本文采用303 MHz重复频率的掺Er光纤光学频率梳,首先通过激光放大和光谱展宽技术使光谱覆盖到1μm波段,然后采用级联掺Yb增益光纤技术,将扩谱后1μm波段的激光功率进行放大,提高了掺Er光纤光学频率梳扩谱后1μm波长附近的激光强度.采用碘稳频532 nm激光的基频光作为待测光源与飞秒光学频率梳进行拍频.实验表明,与未经过光谱增强的激光相比,光谱增强后的激光与1064 nm激光拍频信号的信噪比提高了5 d B,保持在35 d B附近.该技术有效地缓解了采用掺Er光纤光梳测量1064 nm激光绝对频率时对直接扩谱所获得的1μm波长激光的强度要求.

The harmonic optical frequency chain is the only tool for measuring optical frequency till the advent of a femtosecond optical frequency comb（FOFC）. However, its disadvantages are obvious, such as high cost, difficult construction, complex usage, and complicated maintenance. The emergence of femtosecond optical frequency combs（FOFCs） makes it possible to measure the absolute frequency of a laser, which greatly simplifies the quantity traceability of the absolute frequency value and comparison, and allows the length unit ＂m＂ to be directly traced back to the time unit ＂s＂. The beat note（fb） between an FOFC and a test laser is one of the most important data in measuring absolute frequency of the test laser. In order to ensure the accuracy and reliability of the measurement, the signal-to-noise ratio（SNR） of fb should be above 30 d B at 300 k Hz resolution bandwidth. Among the wavelength standards recommended to replicate＂meter＂（SI）, iodine-stabilized 633 nm lasers and iodine-stabilized 532 nm lasers have been widely used. Compared with iodine-stabilized 633 nm lasers, iodine-stabilized 532 nm lasers have the advantages of high stability, high output power,no modulation and fiber coupled output. Therefore, it is of great importance to measure and monitor the absolute frequency of an iodine-stabilized 532 nm laser. Aiming at the specific requirements for absolute frequency measurement of an iodine-stabilized 532 nm laser, the absolute frequency measurement of its fundamental 1064 nm laser has been studied. In this paper, a high-repetition-rate Er-doped femtosecond fiber laser is adopted as an optical source in the system. The repetition rate of the fiber laser is 303 MHz, the output power in the continuous-wave state is 130 mW and the average output power in the mode-locking state is 80 mW. The highest SNR of fbbetween the comb light and a 1064 nm laser generated by an iodine-stabilized 532 nm laser is only 30 d B due to the low intensity at 1 μm wavelength in the supercontinuum, which just reaches the SNR threshold meeting the counter’s working condition. In order to improve the accuracy and reliability of absolute frequency measurement, the technique of cascading an Yb-doped fiber amplifier after spectral broadening is adopted to enhance the spectral intensity at 1 μm wavelength. The experimental results indicate that the SNR of fbbetween a 1 μm laser after spectral enhancement and a 1064 nm laser is increased by 5 d B and kept at 35 d B for several days, meeting requirements for long-term continuous monitoring. This method can effectively reduce the intensity requirements at 1 μm wavelength when the spectrum is directly broadened in the Er-FOFC.