Zongfu Jiang

Coherent combining technology of master oscillator power amplifier fiber arrays

Coherent beam combination of fiber laser array is an important technology of realize high-power, high-radiance fiber laser system. In this paper, Master Oscillator-Power Amplifier scheme is used to realize phase controlling of three ytterbium fiber amplifiers, the experiment results of both two and three fiber amplifiers are given and compared. Far-field patterns with different fill factor are studied experimentally. We perform optical phase-noise measurements of a commercial 1-W ytterbium fiber amplifier using our phase control electronics, the dominant phase noises of the 1-W fiber amplifier are at frequencies below one kilohertz.

Mutual Injection-Locking and Coherent Combining of Two Individual Fiber Lasers

In this paper, mutual injection-locking and coherent combining are demonstrated with two individual erbium-doped fiber lasers that were coupled by two fiber splitters. Mutual injection-locking theory of two lasers is analyzed. In the free-running state, the far-field beam profile is a simple intensity superposition, as expected of two incoherent beams. Under mutual injection-locking, interference fringes with high contrast ratio are obtained, and the two fiber lasers lase at the same wavelength with a stable output power. We have found that the two fiber lasers are always in out-of-phase mode, which is consistent with theoretical analysis. Coherent beam combining by mutual injection-locking is realized without the need for length or amplitude control. This method can be easily scaled to combine more beams.

Mid-IR supercontinuum generation in Tm/Ho codoped fiber amplifier

We demonstrate all-fiber-integrated mid-IR supercontinuum generation in a Tm/Ho codoped fiber amplifier, where the codoped fiber is both nonlinear and a gain medium. The observed supercontinuum spectrum ranges from 1760 to 2600 nm and has an extremely high spectral flatness, with a 3 dB bandwidth ~635 nm (from 1845 to 2480 nm). In the Tm/Ho codoped fiber, as well as the nonlinear optical processes, the Tm3+ and Ho3+ ions also play important roles in mid-IR supercontinuum generation. The 3F4–3H6 transition for Tm3+ radiates a laser in the 1.8–2.1 μm spectral region, the 5I7–5I8 transition for Ho3+ radiates a laser around 2.1 μm and the 3H4–3H5 transition for Tm3+ radiates a laser in the 2.2–2.5 μm spectral region. All of these energy level transitions contribute towards supercontinuum generation.

Low-Loss Fusion Splicing Photonic Crystal Fibers and Double Cladding Fibers by Controlled Hole Collapse and Tapering

We demonstrate a novel method for low-loss splicing photonic crystal fibers (PCFs) and double cladding fibers (DCFs) by controlled hole collapse and tapering using a conventional fusion splicer. Through controlled hole collapse, two center rings of holes were collapsed to expand the core of the PCF, while the cladding diameter of the PCF was almostly not changed. The mode field diameter (MFD) and inner cladding diameter of the DCFs were decreased by tapering. So an optimum mode field and cladding match at the interface of PCF-DCF and an adiabatic mode field variation in the longitudinal direction of the PCF and DCF can be achieved. This is the first time, to our knowledge, to realize low-loss splicing between the DCFs and PCFs. Splice losses as low as 0.65 dB were achieved between a PCF and a DCF with MFDs of 5.8 and 26.5 μm, and with the inner cladding diameters of 125 and 250 μm, respectively.

Fast and accurate modal decomposition of multimode fiber based on stochastic parallel gradient descent algorithm

A new approach for the complete modal decomposition of the optical fields emerging from the multimode fiber is presented in this paper. Based on the stochastic parallel gradient descent algorithm, mode coefficients for all the bound modes in the multimode fiber can be exactly calculated by utilizing one intensity profile of the beam. Numerical simulation validates the feasibility, and the reconstructed error is below 0.1%. In the case of six modes within the fiber, the running time is about 2 s.

Hierarchical structure of the optical path length of the supersonic turbulent boundary layer

The optical path length (OPL) of supersonic turbulent boundary layer of Mach number 3.0 is obtained with the nanoparticle-based planar laser scattering technique, and its structure is analyzed within the framework of hierarchical symmetry assumption. Our result offers reasonable evidence for that the OPL obeys this assumption with parameter β depending on q. The scaling exponent ζ(q) of structure function is computed and compared with the theoretical prediction of She-Leveque model. The curve ζ(q) we obtained is convex and smaller than the theoretical value for small q, which is attributed to the large scale structure of the OPL.

0.4 μJ, 7 kW ultrabroadband noise-like pulse direct generation from an all-fiber dumbbell-shaped laser.

We report the direct generation of 0.4 μJ, 7 kW ultrabroadband picosecond noise-like pulses from an Yb-doped all-fiber oscillator based on dual nonlinear optical loop mirrors (NOLMs). Under the highest pump power, the average power of the main output port reached 1.4 W, and the 3 dB spectral bandwidths reached 76 nm and 165 nm from the two output ports, respectively. The design of dual-NOLMs shows both exceptional compactness in construction and distinct flexibility on the engineering of the mode-locking behaviors. To the best of our knowledge, this is the first demonstration of a watt-level dual-NOLM-based fiber laser. Based on this laser, the pulse energy and peak power of picosecond noise-like pulse from an all-fiber oscillator have been elevated by an order of magnitude.

General analysis of SRS-limited high-power fiber lasers and design strategy

In this paper, a spectral model by incorporating SRS effect is proposed and established, which is feasible for analyzing the SRS effect both in high-power fiber oscillator and master oscillator power amplifier (MOPA) system. The theoretical results show that the SRS effect is tightly related to the bandwidths of the fiber Bragg gratings (FBGs) and it can be efficiently suppressed by optimizing the bandwidth of the FBGs. Besides, the established theoretical model is also feasible for analyzing the influence of seed power on the SRS effect. The theoretical predictions agree well with the previous experimental results.

Investigation of stimulated Raman scattering effect in high-power fiber amplifiers seeded by narrow-band filtered superfluorescent source

In this paper the stimulated Raman scattering (SRS) effect in high-power fiber amplifiers seeded by the narrow-band filtered superfluorescent source (SFS) is firstly analyzed both theoretically and experimentally. Spectral models for the formation of the SFS and the spectral evolution in high-power fiber amplifiers seeded by filtered SFS are proposed. It is found that the SRS effect in high-power fiber amplifiers depends on the spectral width of the filtered SFS seed. The theoretical predictions are in qualitative agreements with the experimental results.

Versatile long cavity widely tunable pulsed Yb-doped fiber laser with up to 27655th harmonic mode locking order

We report flexible dissipative soliton generation from an all-fiberized all-normal-dispersion (ANDi) long cavity actively mode locked ytterbium doped fiber laser based on improved harmonic mode locking technique. The laser is featured with unusually wide and fine tunabilities in repetition rate and operating wavelength, meanwhile superior stability is maintained. The repetition rate of the laser can be flexibly controlled from 226 kHz to 6.25 GHz (corresponding to the highest 27655th order harmonic mode locking) in the interval of 226 kHz, while supermode suppression is confined above 50 dB and the pulse duration is retained in the range of 38 ps~80 ps. As high as 4.3 nJ pulse energy can be achieved with a low pump power of 160 mW when operating at the fundamental mode locking regime. The operating wavelength of the laser can be tuned in the wide range of 1005 nm~1100 nm. As far as we know, it is the first demonstration of up to ten thousands order stable harmonic mode locking in ANDi fiber laser, which manifests the capability of generating both high energy pulse and ultra-high repetition rate pulse in a single ANDi cavity. The destabilization of dissipative soliton under strong pump is also demonstrated.