Zilun Chen

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.

Numerical analysis of the effects of aberrations on coherently combined fiber laser beams

Numerical analysis of the effects of aberrations on coherently combined fiber laser beams is presented. We prove that traditional beam quality criteria, such as the M2 factor and the Strehl ratio, do not consider necessary information to evaluate the quality of a coherently combined laser beam. The beam propagation factor (BPF) is introduced and studied as a proper beam quality factor for the coherently combined beam. Two main categories of aberrations, geometry and nongeometry factors, are numerically studied to investigate their effect on beam quality by using the BPF criterion. For a ring-distributed fiber laser array with certain vacancy factor and a RMS value of tilt error, we obtain a semianalytical equation to evaluate their effect on beam quality. We present a brief discussion of those aberrations at the end of this paper. Our generalized methodology offers a good reference for investigating coherent combining of fiber laser beams in a comprehensive way.

A compact seven-core photonic crystal fiber supercontinuum source with 42.3 W output power

We have fabricated a new type of seven-core photonic crystal fiber and demonstrated its all-fiber supercontinuum source pumped with a ytterbium-doped fiber master oscillator power amplifier. The generated supercontinuum covers the wavelength range from 720?nm to beyond 1700?nm, with an output power of 42.3?W. It is the first high-power all-fiber SC source based on seven-core photonic crystal fiber.

High-power fiber lasers based on tandem pumping

Power scaling of fiber lasers is challenged by several factors, such as the brightness of the pump source, the nonlinear effect, modal instability, and so on. Pumping active fibers with high-brightness fiber lasers instead of laser diodes is a promising solution for the brightness limitation and modal instability. In this paper, for the first time to our knowledge, we present a general review on the achievements of various kinds of high-power fiber lasers based on the tandem pumping scheme in the past few years. The requirements for tandem pumping ytterbium (Yb), erbium (Er), thulium (Tm), and holmium (Ho)-doped fibers are analyzed, and corresponding achievements are summarized. Hundreds of watts of fiber lasers at ∼1020, ∼1500, and 1900 nm and hundred-watt-level fiber lasers at ∼1150 and ∼1180  nm have been successfully achieved. Then, these powerful fiber lasers with high brightness can be employed as pump sources for Yb-, Er-, Tm- and Ho-doped fibers. Moreover, a recent experimental result of a 3.5 kW Yb-doped fiber amplifier in an all-fiber format is reported in addition to previous typical achievements. The underlying challenges for further power scaling, including the nonlinear effect suppression and special fiber design, are briefly discussed. Exploring the tandem pumping scheme in novel application fields is discussed as well.

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%.

Hundred-Watt-Level, All-Fiber-Integrated Supercontinuum Generation from Photonic Crystal Fiber

A photonic crystal fiber (PCF) based supercontinuum source with hundred-watt-level average power output is presented in this paper. The output delivery fiber of a 120 W picosecond master-oscillator power-amplifier (MOPA) laser with a 15 ?m fiber core is directly spliced with a 2.6-m-long PCF to form the all-fiber-integrated SC source. Using the controlled air-hole collapse technique to expand the core diameter of the PCF, a super-low splice loss (?0.2 dB) between the delivery fiber and the PCF has been demonstrated. A 92.5 W SC spanning from about 700 nm to beyond 1700 nm is obtained.

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.

Mode-field expansion in photonic crystal fibers

We propose two novel methods to make mode-field expanders using photonic crystal fibers (PCFs). By heating and collapsing the center rings of airholes around the core using the selective airhole collapse technique, we enlarge the core of the fibers and get mode-field expanders. Meanwhile, by heating and controlled shrinking all airholes of the PCFs, power confinement in the core will decrease and the mode-field diameter of the fibers will be enlarged, too. Both methods are studied in theory and experiment.

Ultraviolet-extended flat supercontinuum generation in cascaded photonic crystal fiber tapers

Cascaded photonic crystal fiber (PCF) tapers in a monolithic fiber obtained by post-processing techniques such as hole inflation and tapering have been manufactured. An ultraviolet-extended supercontinuum (SC) generation down to 352 nm wavelength pumped at 1064 nm in cascading PCF tapers has been obtained. High spectral flatness (5 dB) has been achieved in the entire visible window.

All-Fiber-Integrated High-Power Supercontinuum Sources Based on Multi-Core Photonic Crystal Fibers

The obstacles of power scaling the supercontinuum (SC) source based on single-core photonic crystal fiber (PCF) are analyzed. The combination of high-power fiber lasers and multi-core PCFs would be a feasible method to obtain an all-fiber-integrated high-power broadband SC source (covering visible range). In this paper, we present a comprehensive study of high-power SC generation in multi-core PCFs. Comparative experiments are performed by using a high-power pulse-repetition-rate-tunable picosecond fiber laser to pump two kinds of home-made seven-core PCFs. The influences of PCF structure (fiber dispersion property) and pulse repetition rate (pulse peak power) on the SC generation in multi-core PCFs are investigated in detail. When the picosecond fiber laser at a pulse repetition rate of 1.9 GHz is adopted as the pump, 116 W SC spanning from 800 to 1700 nm is generated in 1# seven-core PCF. Also 64 W visible SC spanning at least 500-1700 nm is demonstrated in 2# seven-core PCF at a pump pulse repetition rate of 480 MHz. The potential of extending the spectral range and scaling the output power for the SC source based on multi-core PCFs are analyzed and discussed.