Free-space NPR mode locked erbrium doped fiber laser based frequency comb for optical frequency measurement

TL;DR

This paper presents a stable, high-power, broadband frequency comb based on free-space NPR mode-locked erbium-doped fiber laser, enabling precise optical frequency measurements with enhanced spectral and power characteristics.

Contribution

The authors developed a free-space NPR mode-locked erbium fiber laser with significantly higher power and broader spectrum than traditional all-fiber lasers, achieving stable operation for over a year.

Findings

Achieved stable self-starting femtosecond fiber laser frequency comb.

Enhanced output power by 5 to 10 times compared to conventional designs.

Generated a supercontinuum spectrum suitable for optical frequency measurement.

Abstract

We have specifically investigated the free-space mode locking dynamics of erbium-doped fiber (EDF) mode-locked ultrafast lasers via nonlinear polarization rotation (NPR) in the normal dispersion regime. To do so, we built a passively mode-locked fiber laser based on NPR with a repetition rate of 89 MHz producing an octave-spanning spectrum due to supercontinuum (SC) generation in highly nonlinear fiber (HNLF). Most significantly, we have achieved highly stable self-starting NPR mode-locked femtosecond fiber laser based frequency comb which has been running mode locked for the past one year without any need to redo the mode locking. By using the free-space NPR comb scheme, we have not only shortened the cavity length, but also have obtained 5 to 10 times higher output power (more than 30 mW at central wavelength of 1570 nm) and much broader spectral comb bandwidth (about 54 nm) compared to conventional all-fiber cavity structure with less than 1 mW average output power and only 10 nm spectral bandwidth. The pulse output from the NPR comb is amplified through a 1 m long EDF, then compressed by a length of anomalous dispersion fiber to a near transform limited pulse duration. The amplified transform limited pulse, with an average power of 180 mW and pulse duration of 70 fs, is used to generate a supercontinuum of 140 mW. SC generation via propagation in HNLF is optimized for specific polling period and heating temperature of PPLN crystal for SHG around 1030 nm.