Zejin Liu

Stable all-fiber dual-wavelength thulium-doped fiber laser and its coherent beam combination

A stable all-fiber dual-wavelength thulium-doped fiber laser (TFL) based on cascaded fiber Bragg grating array is proposed and demonstrated. The TFL operates at 1940.9 and 1948.8 nm simultaneously with a 20 dB optical signal-to-noise ratio (OSNR) ratio. As a proof-of-concept experiment, two dual-wavelength thulium-doped fiber amplifiers are coherently combined. The fringe contrast is improved from nearly 0 to 91% when the system evolves from open-loop to close-loop. The feasibility of application of dual-wavelength TFL in coherent beam combination is verified.

20 W all fiber supercontinuum generation from picosecond MOPA pumped photonic crystal fiber

An all fiber high power supercontinuum (SC) source is demonstrated by pumping a section of photonic crystal fiber (PCF) with a picosecond MOPA laser. The core of the PCF is enlarged at the input end through a serious of PCF post processing method to match the output fiber of the picosecond laser, to ensure low loss splicing, hence high power operation of the whole system. The supercontinuum output spectrum covers the wavelength range from 650 nm to beyond 1700 nm. Limited by available pump power, 20 W super-continuum output power is obtained under 29.5 W picosecond pump power, giving a high optical to optical conversion efficiency of 67.8%.

Coherent beam combining of pulsed fiber lasers with hybrid phase control

We demonstrate a scalable architecture for coherent combining of all-fiber pulsed lasers with hybrid phase control involving passive phasing and active phasing for the first time. Synchronized and passive phased pulsed laser array is built by intra-activity phase modulation. Active phase control on the passive phased pulsed laser array using stochastic parallel gradient descent algorithm provides stable in-phase coherent beam combining pattern in a turbulent atmosphere. The fringe visibility is increased from 0 two 0.43 and the power encircled in the main-lobe is 1.616 times increased when the system evolves from passive phasing to hybrid phasing. The architecture can be easily scaled up to high power by increasing power of each individual laser, number of laser elements and introduction of power amplifiers, which will lead a promising way for scaleable high power coherent beam combining of pulsed lasers.

Optical parameters and upconversion fluorescence in Tm3+/Yb3+ codoped tellurite glass

Tm3+/Yb3+ codoped tellurite glass has been prepared. Density, refractive index, optical absorption, Judd-Ofelt parameters and spontaneous transition probabilities of Tm3+ have been measured and calculated, respectively. Intense blue three-photon upconversion fluorescence and S-band (1470 nm) fluorescence were investigated under the excitation of a 980 nm diode laser at room temperature. Judd-Ofelt parameters, strong blue three-photon upcoversion emission of Tm3+ in glass indicate that Tm3+/Yb3+ codoped tellurite glass is a promising blue color upconversion optical and laser material. In addition, experiment results showed the 980 nm laser was more efficient than 808 nm laser when pumping Tm3+/Yb3+ codoped tellurite glass, Tm3+/Yb3+ codoped tellurite glass also could be a promising material for S-band amplification.

29.2 W, 20 kHz linewidth all fiber single-frequency MOPA laser

We demonstrate a high power all fiber single-frequency MOPA laser. The center wavelength of the single-frequency fiber laser seed is 1064 nm with a linewidth narrower than 20 kHz. A two-stage power amplification configuration is employed. A single-frequency fiber laser with an output power of 29.2 W is obtained, and the optical to optical efficiency is 78%.

Mutual injection locking and coherent beam combining of two thulium doped fiber lasers

We demonstrate which we believe the first investigation on mutual injection-locking and coherent combining of two thulium-doped fiber lasers. Mutual coherence between the two fiber lasers is established by means of mutual coupling through two 3 dB couplers. High combining efficiency of 99% for two fiber laser has been obtained, and the fringe contrast of the intensity pattern at the receiving plane is as high as 93%. The solution to scale to high power coherent beam combining is briefly discussed.

Spectroscopic properties of Tm3+ ions in Tm3+/Yb3+ codoped tellurite glass

Absorption coefficience of Tm³⁺ ions in Tm³⁺/Yb³⁺ codoped tellurite glass have been analyzed by using the J-O theory. The J-O parameters are calculated to be Ω<inf>2</inf> = 11.11×10⁻²⁰ cm², Ω<inf>4</inf> = 3.5×10⁻²⁰ cm² and Ω<inf>6</inf> = 3.6×10⁻²⁰ cm². Spontaneous radiative transition probabilities, branching ratios and radiative life time of Tm³⁺ ions are also calculated, the calculation results indicate that it is difficult to get S-band amplification through single 808 nm laser pump in this glass, this is tested by 808 nm single wavelength pump experiment. The Yb³⁺ codoping with Tm³⁺ and energy transfer process between them make 980 nm single wavelength pump scheme possible to get S-band amplification, experiment results indicate that 980 nm pump energy absorbed by Yb³⁺ is transferred to Tm³⁺, consequently, the low cost 980 nm pump scheme could make it become promising material for S-band amplification.