All-normal dispersion ytterbium-doped fiber laser mode locked by nonlinear multimode interference.

We report a mode-locking operation using a nonlinear multimode interference (MMI)-based saturable absorber in an ytterbium-doped fiber laser under an all-normal dispersion configuration. The MMI is observed in a step-index single mode-multimode-single mode (SMS) fiber structure. The intensity and wavelength-dependent transmission of the SMS structure has been studied numerically to understand the Kerr-induced saturable absorber properties of MMI in a step-index fiber. Numerical investigations show that presence of a narrow-band wavelength-tunable spectral filter is necessary for a nonlinear MMI-based saturable absorber to be effective for mode locking. A single-pulse mode-locking operation in the laser is realized at a very low pump power of ${\sim}{56.3}\;{\rm mW}$ at which the laser generates 180 ps duration pulses at ${\sim}{9}\;{\rm MHz}$ repetition rate with measured signal-to-noise ratio of ${\sim}{50}\;{\rm dB}$. This all-fiber integrated laser operates in multi-pulse and noise-like rectangular pulse regimes at increased pump power.

Generation of stable clean ultrashort pulses in a simple all-fiber, all-normal dispersion ytterbium-doped mode-locked laser.

We report an all-fiber, all-normal dispersion ytterbium (Yb)-doped fiber oscillator mode locked by nonlinear polarization evolution. The mode locking is realized in different pump power regimes by adjusting the polarization controllers. The average output power, pulse energy, and compressed pulse duration varies from 60 to 150 mW, 1.4 to 3.5 nJ, and 350 to 146 fs, respectively, when the pump power to the oscillator is varied from 250 to 500 mW. Particularly at 500 mW pump power, the oscillator generates highly chirped 5 ps duration pulses at 42 MHz that are compressed to a 146 fs duration with a clean temporal profile. The signal-to-noise ratio of the train of mode-locked pulses is 85 dB, indicating stable mode-locking operation. The generalized nonlinear Schrödinger equation is solved numerically to model the passive mode locking in a Yb oscillator, supporting the experimental results. In a chirped pulse Yb amplifier, the signal power is scaled to a 10 W level, and the amplified pulse is compressed to 355 fs duration.

Chair-like pulses in an all-normal dispersion Ytterbium-doped mode-locked fiber laser.

We report, for what we believe is the first time, generation of stable chair-like pulses (a pulse shape with an initial long flat portion followed by a short high peak power portion resembling the shape of a chair) by mode locking of a Ytterbium (Yb)-doped fiber laser. Chair-like pulse shapes are achieved by implementing dual saturable absorbers, one based on a nonlinear optical loop mirror (NOLM) and the other based on nonlinear polarization rotation (NPR) inside the cavity. The transmission characteristics of the NOLM-NPR pair leading to the formation of chair-like pulses are numerically investigated. We also report the amplification characteristics of chair-like pulses in an external multistage Yb-doped fiber amplifier setup at different repetition rates of the pulse train. It was found that the chair-like pulses are suitable for amplification, and more than 10 W of average power at 460 kHz repetition rate have been obtained at total pump power of ∼20 W coupled to the power amplifier. At a lower repetition rate (115 kHz), ∼8 W of average power were obtained corresponding to ∼70 µJ of pulse energy with negligible contribution from amplified spontaneous emission or stimulated Raman scattering. We believe that such an oscillator-amplifier system could serve as an attractive tool for micromachining applications.

Switchable Q-switched and modelocked operation in ytterbium doped fiber laser under all-normal-dispersion configuration.

We have constructed an Yb-doped fiber laser in all-normal-dispersion configuration which can be independently operated in Q-switched or modelocked configuration with the help of a simple fiber optic ring resonator (FORR). In the presence of FORR, the laser operates in Q-switched mode producing stable pulses in the range of 1 µs-200 ns with repetition rate in the range of 45 kHz-82 kHz. On the other hand, the laser can be easily switched to mode-locked operation by disjoining the FORR loop producing train of ultrashort pulses of ∼5 ps duration (compressible to ∼150 fs) at ∼38 MHz repetition rate. The transmission characteristics of FORR in combination with the nonlinear polarization rotation for passive Q-switching operation is numerically investigated and experimentally verified. The laser can serve as a versatile seed source for power amplifier which can be easily configured for application in the fields that require different pulsed fiber lasers.

Note: Broadly tunable all-fiber ytterbium laser with 0.05 nm spectral width based on multimode interference filter.

A multimode interference filter with narrow transmission bandwidth and large self-imaging wavelength interval is constructed and implemented in an ytterbium doped fiber laser in all-fiber format for broad wavelength tunability as well as narrow spectral width of the output beam. The peak transmission wavelength of the multimode interference filter was tuned with the help of a standard in-fiber polarization controller. With this simple mechanism more than 30 nm (1038 nm-1070 nm) tuning range is demonstrated. The spectral width of the output beam from the laser was measured to be 0.05 nm.

Note: amplification characteristics of all-normal-dispersion mode-locked Yb-doped fiber laser: influence of input pulse shape.

The amplification properties of the pulses at and after the nonlinear polarization rejection (NPR) port of an all-normal-dispersion Yb-doped mode-locked fiber laser are studied. The experimental results show that the spectra of the output pulses after the NPR port are considerably resistant to distortions on amplification and can be compressed in the femtosecond regime without any significant side-lobes and hence can serve as an excellent seed source for further power amplification. The experimental results are substantiated by numerical analysis of the amplifier setup.

Note: self Q-switched Nd:YVO4 laser at 914 nm.

By exploiting the saturable nature of re-absorption loss under the quasi-three-level laser transition self Q-switching operation in Nd:YVO(4) laser at 914 nm is demonstrated. The stable self-pulsing operation was observed with Nd:YVO(4) crystal with doping concentration of 0.3 at.%. At an incident pump power of 30 W around 600 mW of average power was obtained. Individual pulse energy and FWHM pulse duration were experimentally measured to be 10 µJ and 460 ns, respectively. The performance of the laser was analyzed with the help of a rate equation model. The simulation results show good agreement with the experiment.