Kazi S. Abedin

Highly nondegenerate femtosecond four-wave mixing in tapered microstructure fiber

We demonstrate efficient, highly nondegenerate four-wave mixing of femtosecond pulses, with a frequency shift of ∼6000 cm−1, in an 18 cm tapered microstructure fiber. Using a pump at 810 nm and a signal at 1540 nm, light is generated at wavelengths between 535 nm and 570 nm with 10% efficiency. Due to the walk-off between pump and signal pulses in the fiber, the interaction length in the tapered fiber is only 1.4 cm. Ten percent efficiency is achieved in this short length because of the enhanced nonlinearity of the tapered fiber and its unique dispersion characteristics.

Seven-core erbium-doped double-clad fiber amplifier pumped simultaneously by side-coupled multimode fiber

We demonstrate a seven-core erbium-doped fiber amplifier in which all the cores were pumped simultaneously by a side-coupled tapered multimode fiber. The amplifier has multicore (MC) MC inputs and MC outputs, which can be readily spliced to MC transmission fiber for amplifying space division multiplexed signals. Gain over 25 dB was obtained in each of the cores over a 40-nm bandwidth covering the C-band.

Self-stabilized passive, harmonically mode-locked stretched-pulse erbium fiber ring laser

We have studied a passive, harmonically mode-locked stretched-pulse erbium fiber ring laser with net positive dispersion that is self-stabilized by gain depletion and electrostriction. Periodic pulses with supermode suppression of >75 dB and picosecond jitter are achieved. The pulses are compressible to 125 fs by external chirp compensation. The repetition rate is 220 MHz, and the average power is as high as 80 mW.

Higher order FM mode locking for pulse-repetition-rate enhancement in actively mode-locked lasers: theory and experiment

We present a novel higher order FM mode-locked technique for active mode-locked lasers which utilizes the higher order sidebands generated by an intracavity phase modulator to establish the mode locking. The resulting mode-locked output exhibits an enhancement of the pulse repetition rate over the modulation frequency by an integral multiple. The higher order FM mode locking is studied theoretically in a laser with a homogeneous gain medium, and simple analytical expressions are obtained to characterize the output pulses. It is shown that the scheme not only enhances the pulse repetition rate but also ensures chirp-free pulses and is effective in eliminating the output pulse phase-state instability, which are commonly observed in conventional FM mode-locked lasers. The effect of group velocity dispersion and cavity nonlinearity is also investigated through numerical solution of the self-consistency equation applied to a fiber ring laser. Finally, detailed experimental results on repetition rate enhancement in fiber lasers are presented and shown to be in good agreement with the theoretical results.

Single-frequency Brillouin distributed feedback fiber laser

We demonstrate a single-frequency Brillouin distributed feedback laser (DFB). The DFB laser cavity was a 12.4 cm long fiber Bragg grating with a π-phase shift offset from the grating center. It exhibited a threshold of 30 mW and conversion efficiency from pump to Stokes wave as high as 27%. Higher-order Stokes waves were suppressed by more than 20 dB. The Stokes output of the laser could be obtained in either the forward or backward direction, simply by changing the orientation of the offset of the discrete phase shift with respect to the pump propagation direction. The DFB laser operated over a pump frequency range of 1.2 GHz, more than 60 times larger than the SBS gain bandwidth.

Repetition-rate multiplication in actively mode-locked fiber lasers by higher-order FM mode locking using a high-finesse Fabry–Perot filter

We demonstrate an FM mode-locking technique for multiplying the pulse repetition rate of lasers by several times above the modulation frequency. The mode locking of the laser is based on the generation of higher-order FM sidebands by an intracavity phase modulator and the periodic selection of the sidebands by an intracavity high-finesse Fabry–Perot filter at spacings of free spectral range which is a harmonic of the modulation frequency. We show the generation of stable, uniformly spaced, equal amplitude optical pulses from a fiber ring laser with two-, three-, and fourfold enhancement in the repetition rate.

Raman fiber distributed feedback lasers.

We demonstrate fiber distributed feedback (DFB) lasers using Raman gain in two germanosilicate fibers. Our DFB cavities were 124 mm uniform fiber Bragg gratings with a π phase shift offset from the grating center. Our pump was at 1480 nm and the DFB lasers operated on a single longitudinal mode near 1584 nm. In a commercial Raman gain fiber, the maximum output power, linewidth, and threshold were 150 mW, 7.5 MHz, and 39 W, respectively. In a commercial highly nonlinear fiber, these figures improved to 350 mW, 4 MHz, and 4.3 W, respectively. In both lasers, more than 75% of pump power was transmitted, allowing for the possibility of substantial amplification in subsequent Raman gain fiber.

Pulse repetition frequency multiplication via intracavity optical filtering in AM mode-locked fiber ring lasers

We demonstrate pulse repetition frequency multiplication in AM mode-locked fiber ring lasers using optical filtering realized via an intracavity fiber Fabry-Perot filter (FFP) and show that the generated optical pulses are highly stable in amplitude noise and timing jitter. A 3.477-GHz optical pulse train is generated using a modulation signal of 869.284 MHz, a fourth subharmonic multiple of the 3.48-GHz free spectral range of FFP. The generated optical pulses exhibit a high degree of pulse stability in terms of a large suppression of supermode noise, a low amplitude noise of 0.93 %, and a timing jitter of 1.2 ps.

Wavelength tunable high-repetition-rate picosecond and femtosecond pulse sources based on highly nonlinear photonic crystal fiber

We demonstrate an actively mode-locked dispersion-managed erbium fiber laser and a Raman soliton wavelength converter that use photonic crystal fibers (PCFs) as the nonlinear medium. The high nonlinearity and large anomalous dispersion of the PCF resulted in a significant reduction in the length. We generated 1.0-ps pulses tunable over 1535-1560 nm at a 10-GHz repetition rate and 1.3-ps pulses at a 40-GHz repetition rate from a laser that was only 36 m long. Furthermore, we obtained 10-GHz femtosecond solitons, tunable over a 90-nm range, by means of soliton self-frequency shift of the mode-locked laser pulses in a 12.6-m-long PCF.

Widely tunable femtosecond soliton pulse generation at a 10-GHz repetition rate by use of the soliton self-frequency shift in photonic crystal fiber

We demonstrate a soliton self-frequency shift of approximately 120 nm in a fiber with 1.56-microm pulses generated at a 10-GHz repetition rate by an actively mode-locked laser. A highly nonlinear photonic crystal fiber with a length of only 12.6 m and a nonlinear coefficient of 62 W(-1) km(-1) is used to achieve such broadband operation. The wavelengths of the resulting sub-300-fs solitons can be tuned effectively by adjusting the input power. The maximum output power of the solitons exceeds 200 mW.