Influence of electro-optic modulator on Er-doped fiber femtosecond laser

Abstract

Narrow-linewidth femtosecond optical frequency comb plays an important role in the fields, such as optical clock comparison, time frequency transfer, ultrastable microwave generation, absolute distance measurement, high precision spectroscopy, etc. Due to the influence of the lifetime of the upper energy level in the gain medium, the linewidth of Er-fiber combs is generally on the order of several hundred kilohertz. In order to narrow the linewidth of comb teeth, an effective method is to insert a fast response electro-optic modulator (EOM) into the laser cavity, so that the servo bandwidth of fiber comb is extended to several hundred kilohertz, which provides a feedback mechanism for fast servo locking. Among them, a high quality femtosecond laser is the core. Based on this, the influence of the EOM on the parameters of Er-fiber femtosecond laser is studied in this paper. By calculating the refractive index, group velocity dispersion, and phase delay of the electro-optic crystal, the influence of the introduction of the EOM on the laser performance is analyzed. A LiNbO₃ (LN) crystal with a length of 3 mm and <i>x</i>-cut is selected as the EOM and inserted into the laser cavity. The influence of the applied voltage of the EOM on the repetition rate and carrier envelope offset frequency of the laser are obtained experimentally. When the voltage on the LN crystal changes from -200 to 200 V, the adjustment of repetition rate is 60 Hz and the carrier envelope offset frequency is 25 MHz. Then the two parameters are phase locked through the EOM. Furthermore, by phase locking the beat note between the fiber comb and a narrow-linewidth continue wavelength laser at 1542 nm, it is verified that the introduction of the EOM can expand the servo bandwidth of the laser to more than 236 kHz, which provides a technical basis for establishing narrow linewidth femtosecond optical frequency combs. The following work will verify the performance of comb line, that is, when the comb is locked to a narrow-linewidth laser (such as 1542 nm), the performance of comb line at wavelength (such as 698, 729 nm, and so on) of distant place will be analyzed in detail.