Sub-kHz-Linewidth Wavelength-Tunable Single-Frequency Ring-Cavity Fiber Laser for C- and L-Band Operation

In this paper, a sub-kHz-linewidth wavelength-tunable single-frequency ring-cavity fiber laser operating in C- and L-band is demonstrated experimentally. In our design, a single-longitudinal mode is achieved in a wide tuning range by a narrow-band filtering device composed of a fiber Fabry-Pérot interferometer and a 1.6-m-long un-pumped Er³⁺-doped fiber as a dynamic grating. Under the control of a fiber Fabry-Pérot tunable filter, a wavelength tuning range of 76.69 nm from 1535.50 to 1612.19 nm with a tuning interval of about 0.2 nm is finally achieved. In the entire tuning range of the fiber laser, the laser linewidth is less than 310 Hz. The measured optical signal-to-noise ratio is over 40 dB and the relative intensity noise is lower than -143 dB/Hz at frequencies above 1 MHz. Our results suggest that such fiber laser with narrow-linewidth and superior wavelength tunability can serve as a promising candidate applied in optical communication system.

I. Introduction

Tunable single-frequency fiber laser (TSFFL) is able to realize a narrow linewidth, low noise, and tunable wavelength output. Therefore, it is extensively used in many fields, e.g., optical communication [1], [2], Lidar [3], fiber sensing [4], [5], and spectroscopy [6]. With the rapid development of dense wavelength division multiplexing technology, the number of channels required by an optical communication system also significantly increases [7]. One of the technical approaches is to adopt multi-channel laser array composed of a sequence of single-frequency lasers with fixed operating wavelengths, but it is not only expensive but also conducts a rather bulky system [8], which is limited in application. In contrast, a TSFFL is able to provide enough communication channels without sacrificing the system complexity, which also effectively facilitates the maintenance. At present, with regard to the available TSFFL, the number of C-band channels is not enough to meet the request in application, while the L-band (1565- 1620 nm) with a wide range of wavelengths is an attractive solution to be used in such systems [9]. Therefore, it is of great need to design and implement a narrow-linewidth single-frequency fiber laser with a better performance in wavelength scalability which can cover both C- and L-band to expand the channels of the optical communication system.