Ee-Leong Lim

Cladding pumped few-mode EDFA for mode division multiplexed transmission

We experimentally demonstrate a few-mode erbium doped fiber amplifier (FM-EDFA) supporting 6 spatial modes with a cladding pumped architecture. Average modal gains are measured to be >20dB between 1534nm-1565nm with a differential modal gain of ~3dB among the mode groups and noise figures of 6-7dB. The cladding pumped FM-EDFA offers a cost effective alternative to core-pumped variant as low cost, high power multimode pumps can be used, and offers performance, scalability and simplicity to FM-EDFA design.

High-energy, in-band pumped erbium doped fiber amplifiers

We have demonstrated and compared high-energy, in-band pumped erbium doped fiber amplifiers operating at 1562.5 nm under both a core pumping scheme (CRS) and a cladding pumping scheme (CLS). The CRS/CLS sources generated smooth, single-peak pulses with maximum pulse energies of ~1.53/1.50 mJ, and corresponding pulse widths of ~176/182 ns respectively, with an M2 of ~1.6 in both cases. However, the conversion efficiency for the CLS was >1.5 times higher than the equivalent CRS variant operating at the same pulse energy due to the lower pump intensity in the CLS that mitigates the detrimental effects of ion concentration quenching. With a longer fiber length in a CLS implementation a pulse energy of ~2.6 mJ is demonstrated with a corresponding M2 of ~4.2. Using numerical simulations we explain that the saturation of pulse energy observed in our experiments is due to saturation of the pump absorption.

Optimizing the pumping configuration for the power scaling of in-band pumped erbium doped fiber amplifiers

A highly efficient (~80%), high power (18.45 W) in-band, core pumped erbium/ytterbium co-doped fiber laser is demonstrated. To the best of our knowledge, this is the highest reported efficiency from an in-band pumped 1.5 µm fiber laser operating in the tens of watts regime. Using a fitted simulation model, we show that the significantly sub-quantum limit conversion efficiency of in-band pumped erbium doped fiber amplifiers observed experimentally can be explained by concentration quenching. We then numerically study and experimentally validate the optimum pumping configuration for power scaling of in-band, cladding pumped erbium doped fiber amplifiers. Our simulation results indicate that a ~77% power conversion efficiency with high output power should be possible through cladding pumping of current commercially available pure Erbium doped active fibers providing the loss experienced by the cladding guided 1535 nm pump due to the coating absorption can be reduced to an acceptable level by better coating material choice. The power conversion efficiency has the potential to exceed 90% if concentration quenching of erbium ions can be reduced via improvements in fiber design and fabrication.

Minimizing differential modal gain in cladding-pumped EDFAs supporting four and six mode groups

We employ a Genetic Algorithm for the purpose of minimization of the maximum differential modal gain (DMG) over all the supported signal modes (at the same wavelength) of cladding-pumped four-mode and six-mode-group EDFAs. The optimal EDFA designs found through the algorithm provide less than 1 dB DMG across the C-band (1530-1565 nm) whilst achieving more than 20 dB gain per mode. We then analyze the sensitivity of the DMG to small variations from the optimal value of the erbium doping concentration and the structural parameters, and estimate the fabrication tolerance for reliable amplifier performance.

Gain equalization of a six-mode-group ring core multimode EDFA

We propose a 6-mode-group ring core multimode erbium doped fiber amplifier (RC-MM-EDFA) capable of providing almost identical gain among the six mode groups within the C band using either core- or cladding-pumped implementations.

The Multipeak Phenomena and Nonlinear Effects in

We investigate control of the Q-switch rise time as a means to eliminate the undesirable formation of multipeak structure within actively Q-switched fiber lasers. Our experiments show that the development of pulse substructure is solely due to gain dynamic effects and that nonlinearity does not play a decisive role in the process. Using a spectrally resolved optical time gating technique, we investigate nonlinear pulse evolution for both smooth and structured Q-switch pulses of otherwise similar characteristics in Q-switch cavity comprising anomalously dispersive fiber. Useful insight is obtained into the complex interplay of the gain dynamic and various nonlinear effects governing the detailed pulse evolution. The measurements highlight the value and potential of this characterization technique for studying such lasers.

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We theoretically demonstrate the performance of a step index multimode (two mode-group) erbium-doped fiber amplifier with a localized erbium doped ring distribution for Space Division Multiplexed (SDM) transmission.

-Switched Fiber Lasers

We demonstrate an inband, core-pumped master oscillator power amplifier (MOPA) with a maximum pulse energy of 1.56 mJ at a repetition rate of 1.25 kHz, seeded by an actively Q-switched Erbium/Ytterbium-codoped fiber (EYDF) ring laser, producing 150-ns pulses at 1562.5 nm. To maximize energy extraction whilst minimizing signal saturation effects, a 40μm Er3+-doped larger mode area (LMA) fiber was used as the gain medium. A 1535 nm single mode fiber laser was used for in-band pumping of the LMA fiber. The output beam quality (M2) was measured to be ~1.6. This is to the best of our knowledge is the highest reported pulse energy for a pulse fiber laser at 1.5 μm with M2~1.6.

Modal gain control in a multimode erbium doped fiber amplifier incorporating ring doping

An efficient and stable laser action from a double-clad erbium/ytterbium-doped fiber (EYDF) is demonstrated using a fiber Bragg grating at room temperature. The fiber laser operates at wavelength of 1552.3 nm with a slope efficiency of 38% when pumped by multi-mode 915 nm laser diodes. However, the slope efficiency is higher in the EYDF laser configured with double-sided output due to the high splicing loss between the multi-mode combiner and double-clad EYDF. The laser also has a spectral bandwidth of 0.2 nm and signal to noise ratio of more than 25 dB. The threshold power to achieve lasing is measured to be approximately 500 mW for this laser.