Passively synchronized mode-locked fiber lasers for coherent anti-Stokes Raman imaging.

We have proposed and implemented a polarization-maintaining passively synchronized fiber laser system, which could deliver tunable dual-color picosecond pulses by including a frequency-doubling module and a spectral broadening module. Specifically, the output from the involved Er-doped fiber laser were used to generate second-harmonic pulses at 790 nm with a quadratic nonlinear crystal. In parallel, the amplified pulses from the synchronized Yb-doped fiber laser were launched into a 150-m single mode fiber, which resulted in not only substantial spectral bandwidth broadening from 0.1 to 20.1 nm, but also a significant Raman-induced signal around 1080 nm. Consequently, narrow spectra from 1018-1051 nm and 1070-1095 nm could be continuously tuned via a tunable bandpass filter, corresponding to Raman bonds from 2835-3143 cm-1 and 3312-3525 cm-1. Finally, the achieved tunable synchronized pulses enabled us to microscopically examine mouse ear samples based on coherent anti-Stokes Raman and second harmonic generation imaging. Therefore, our tunable passively-synchronized fiber laser system would be promising to provide a simple and compact laser source for subsequent coherent Raman microscopy.

Fiber-based optical parametric oscillator with flexible repetition rates by rational harmonic pumping.

We demonstrated a fiber-based synchronously pumped optical parametric oscillator (SPOPO) with flexible repetition rates while retaining the cavity length. In contrast to conventional free-space SPOPO, the repetition rate of output signal pulses was solely determined by the repetition rate of the pump source in harmonic, fractional and rational operations. The relevant mechanism relies on synchronous pumping and intrinsic losses in our fiber resonator. The novel scheme enabled us to flexibly tune the repetition rate from 0.5 to 6.0 MHz without altering the resonator configuration. The resulting pulse properties were systematically analyzed at various operation conditions, and particularly showed that a wavelength tuning range of 157 nm was obtained. Such rational harmonic resonance implemented in our SPOPO provides not only a simple yet effective way to tune the repetition rate, but also a feasible approach to narrow down the spectral bandwidth. The presented SPOPO could be useful in nonlinear biomedical imaging by offering a convenient approach to optimize the pulse repetition rate for different biomedical samples with minimum photodamage.

Polarization switch of four-wave mixing in a lawtunable fiber optical parametric oscillator.

We reported the simultaneous generation and selective manipulation of scalar and cross-phase modulation instabilities in a fiber optical parametric oscillator. Numerical and experimental results show independent control of parametric gain by changing the input pump polarization state. The resonant cavity enables power enhancement of 45 dB for the spontaneous sidebands, generating laser pulses tunable from 783 to 791 nm and 896 to 1005 nm due to the combination of four-wave mixing, cascaded Raman scattering and other nonlinear effects. This gain controlled, wavelength tunable, fiber-based laser source may find applications in the fields of nonlinear biomedical imaging and stimulated Raman spectroscopy.

Low-repetition-rate all-fiber integrated optical parametric oscillator for coherent anti-Stokes Raman spectroscopy.

All-fiber optical parametric oscillator (OPO), offering advantages like robustness, compactness and low lost, has attracted intense interest in coherent anti-Stokes Raman scattering spectroscopy. In typical fiber-based OPO configurations, detrimental nonlinear effects due to intense pump field in fiber coupling devices would inevitably degrade the spectral purity and conversion efficiency, especially when the OPO operated at low repetition rates. Here we demonstrated a new OPO design by placing the main amplifier inside the cavity, where the amplified pump pulses were directly coupled into the nonlinear fiber. Consequently, lower threshold, higher output power and narrower spectrum were obtained. In particular, effective suppression of spectral noise was experimentally observed, resulting in threshold reductions of 37.5%, 17.2%, and 5.2% with a comparison to a conventional OPO operating at repetition rates of 1, 2 and 3 MHz, respectively. Furthermore, the generated synchronized two-color laser sources at a low repetition rate were then employed to detect CH vibrational bands in an ethanol sample. This spectral tailored cavity design is expected to greatly promote the spread of compact all-fiber laser source to nonlinear biomedical imaging.