Wei Zung Chang

Coherent femtosecond pulse combining of multiple parallel chirped pulse fiber amplifiers

We report on femtosecond pulse combining with up to four parallel chirped-pulse fiber amplifier channels. Active phase locking is implemented using the LOCSET (Locking of Optical Coherence by Single-detector Electronic-frequency Tagging) single detector feedback technique, resulting in 96.4%, 94.0%, and 93.9% relative combining efficiency with two, three, and four channels respectively. Theoretical and experimental analysis of combining efficiency dependence on amplitude and phase noise shows convergence to a fixed value with increasing number of channels, indicating that multi-channel pulse combining with LOCSET feedback should be scalable to very large numbers of channels.

Array size scalability of passively coherently phased fiber laser arrays.

We explore, by means of experiments and simulation, the power combining efficiency and power fluctuation of coherently phased 2, 4, 6, 8, 10, 12, 14, 16-channel fiber-laser arrays using fused 50:50 single-mode couplers. The measured evolution of power combining efficiency with array size agrees with simulations based on a new propagation model. For our particular system the power fluctuations due to small wavelength-scale length variations are seen to scale with array size as N(3). Beat spectra support the notion that a lack of coherently-combined supermodes in arrays of increasing size leads to a decrease in combined-power efficiency.

Model for passive coherent beam combining in fiber laser arrays

We present a new model for studying the beam combining mechanism, spectral and temporal dynamics, the role of nonlinearity, and the power scaling issue of discretely coupled fiber laser arrays. The model accounts for the multiple longitudinal modes of individual fiber lasers and shows directly the formation of the composite-cavity modes. Detailed output power spectra and their evolution with increasing array size and pump power are also explored for the first time. In addition, it is, to our knowledge, the only model that closely resembles the real experimental conditions in which no deliberate control of the fiber lengths (mismatch) is required while highly efficient coherent beam combining is still attained.

Femtosecond pulse spectral synthesis in coherently-spectrally combined multi-channel fiber chirped pulse amplifiers

We demonstrate coherent spectral beam combining and femtosecond pulse spectral synthesis using three parallel fiber chirped pulse amplifiers, each amplifying different ultrashort-pulse spectra. This proof-of-concept experiment opens a path to simultaneously overcome individual-amplifier energy and power limitations, as well as limitations on amplified pulse spectra due to the gain narrowing in a single fiber amplifier.

Dynamical, bidirectional model for coherent beam combining in passive fiber laser arrays

We generalize the recently proposed model for coherent beam combining in passive fiber laser arrays [Opt. Express 17, 19509 (2009)] to include the transient gain dynamics and the complication of counterpropagating waves, two important features characterizing actual experimental conditions. The extended model reveals that beam combining is not affected by the population relaxation process or the presence of backward propagating waves, which only serve to co-saturate the gain. The presence of nonresonant nonlinearity is found to reduce the coherent combining efficiency at high power levels. We show that the array lases at the frequencies with minimum overall losses when multiple loss mechanisms are present.

Simultaneous passive coherent beam combining and mode locking of fiber laser arrays

We present a dynamic model of simultaneous passive coherent beam combining and passive mode locking for coupled fiber lasers. The presence of a saturable absorber in the composite cavity results in the generation of packets of mode locked pulse trains. Within each packet the repetition rate of the pulses is determined by the length difference between the fibers.

Dynamics of Passively Phased Ring Oscillator Fiber Laser Arrays

In this paper, we study fiber laser arrays coupled in a spatially filtered ring oscillator geometry that produces a tiled-array output. In particular, we examine the passive phasing dynamics, the efficiency of coherent combination, and the dependence of the systems behavior on nonlinearities in the fibers. For an array containing a small number of elements, we find that the fibers achieve a co-phased state within two round trips after a perturbation. Steady-state results agree with the previous work. We also find that the Kerr nonlinearity decreases the combining efficiency as determined from the on-axis intensity in the far-field output.

Femtosecond pulse coherent combining and spectral synthesis using four parallel chirped pulse fiber amplifiers

We demonstrate spectral synthesis of femtosecond pulses in a coherently combined four-channel fiber chirped pulse amplifier array. This approach offers a path to overcome gain narrowing limitations of individual fiber amplifier channels.

Simultaneous passive coherent combining and mode locking in fiber laser arrays

We present a detailed model of a multi-fiber interferometric resonator with a saturable absorber in the output arm. The results demonstrate coherent combining and the generation of mode locked pulses whose repetition rate depends on fiber length difference.

Femtosecond pulses from coherently combined parallel chirped pulse fiber amplifiers

Active coherent combining of femtosecond pulses from parallel chirped-pulse fiber amplifiers is demonstrated. This opens a new path for simultaneously increasing both energy and average power of ultrashort pulses from fiber based systems.