Widely tunable multi-wavelength thulium-doped mode-locked all-fiber laser
Date of Issue2017-05-24
School of Electrical and Electronic Engineering
All-fiber lasers have attracted tremendous research interests due to the advantages of minimum alignment requirement, high energy efficiency, low maintenance, low sensitivity and wavelength independent to the temperature. The all-fiber lasers with wavelength-tunable operation and multi-wavelength emission are of much interest because the emission wavelength can be tuned within a broad spectral range and multiple lasing wavelength can be emit simultaneously, which is useful for applications in spectroscopy, wavelength division multiplexing communication, optical signal processing, optical instrument and system diagnostics, etc. Furthermore, if this kind of laser is with pulsed output and high pulse energy, it will become an irreplaceable light source for time-domain and high-intensity applications. Now the operating wavelength of most tunable multi-wavelength all-fiber pulsed lasers is in the 1 µm and 1.5 µm regime and a few in 2 µm regime, in which thulium- (Tm-) or holmium-doped fibers are used as the gain medium. Since 2 µm regime is the eye-safe regime, lasers operating in this regime is particularly important for medical and military applications. There are various methods that can realize tunable operation and multi-wavelength emission in fiber laser. The basic principle is to form a special spectral filter with a tunable center wavelength, which acts as a wavelength selection element to enable the tunable operation. The filter is with multiple maxima in the spectral domain, and the maxima are with equal amplitude. With this filter the multi-wavelength emission can be enabled. This filter can be formed by many methods, such as using multimode fibers, fiber tapers, photonics crystal fibers, fiber Bragg gratings, Fabry-Perot filters, nonlinear polarization evolution (NPE) technique, nonlinear amplified loop mirror technique, etc. The tunable operation realized by fiber tapers, photonics crystal fibers, Fabry-Perot filters, etc. is based on applying stress or tension on the fibers, which makes tunable range limited. The multi-wavelength operation realized by fiber Bragg gratings is based on incorporating multiple pieces of gratings, which makes the cavity complex. NPE is chosen because its potential tunable range covers the whole emission spectrum, rather than a specific range, and the number of emission wavelength is easily controlled by the length of the birefringent fiber inside the cavity, without additional cost on the cavity complexity. A Tm-doped fiber is chosen to be the gain medium because it has broad emission spectra, the eye-safe emission regime, commercially available pumping diode, and high pumping efficiency. Bidirectional pumping method is used to broaden the gain bandwidth by power enhancement and red-shifting the emission wavelength towards the operation wavelength regime of components used inside the cavity, where the loss of components is minimized. The gain bandwidth here is different from the commonly used definition, which is the full width at half maximum. The gain bandwidth used here is the bandwidth of the gain which can let the laser reach the mode-locked threshold. NPE is used to enable mode locking, tunable operation and multi-wavelength emission. The broadband transmissions of other optical components inside the cavity such as isolator and coupler are also studied in order to identify their effects on the gain bandwidth. We have developed a widely tunable multi-wavelength Tm-doped mode-locked all-fiber laser. The main characteristics of this laser are the single-wavelength tunable operation, multi-wavelength tunable operation, and multi-wavelength switchable operation. The tunable range of single-wavelength mode locking is 136 nm (from 1842 to 1978 nm). The tunable range of dual-, tri-, and four-wavelength mode locking is 52 nm (from 1864 to 1916 nm), 49 nm (from 1863 to 1912 nm), and 55 nm (from 1860 to 1915 nm), respectively. The tunable ranges above are the widest in such kind of laser, to the best of our knowledge. For the multi-wavelength switchable operation, we can achieve the full binary control, which is firstly reported in such kind of laser. This could act as an optical binary system, which has potential applications in optical signal processing, optical switching devices and optical communication. Other phenomena are also observed with this setup, including the quasi-five-wavelength Q-switched mode locking, the peaks and dips in the soliton sidebands, and the noise-like pulses. The demonstrated laser is with a compact structure, with easy operation by adjusting the polarization controllers to realize the tunable operation and multi-wavelength emission, and may find applications in optical communications, spectroscopy, and time-resolved applications.
DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics