Carsten Krogh Nielsen

Supercontinuum generation in a photonic crystal fiber with two zero dispersion wavelengths

We demonstrate supercontinuum generation in a highly nonlinear photonic crystal fiber with two closely lying zero dispersion wavelengths. The special dispersion of the fiber has a profound influence on the supercontinuum which is generated through self-phase modulation and phasematched four-wave mixing and not soliton fission as in the initial photonic crystal fibers. The supercontinuum has high spectral density and is extremely independent of the input pulse over a wide range of input pulse parameters. Simulations show that the supercontinuum can be compressed to ultrashort pulses.

Self-starting self-similar all-polarization maintaining Yb-doped fiber laser

We report on the generation of self-similar pulses from an self-starting saturable absorber mirror (SAM) based environmentally stable fiber laser comprising only polarization maintaining (PM) fibers. Pulse energies of 1 nJ at a repetition rate of 17 MHz were obtained, which could be externally compressed to an autocorrelation width of 280 fs.

Continuous-wave wavelength conversion in a photonic crystal fiber with two zero-dispersion wavelengths.

We demonstrate continuous-wave wavelength conversion through four-wave mixing in an endlessly single mode photonic crystal fiber. Phasematching is possible at vanishing pump power in the anomalous dispersion regime between the two zero-dispersion wavelengths. By mixing appropriate pump and idler sources, signals in the range 500-650 nm are obtained in good accordance with calculated phasematching curves. The conversion efficiency from idler to signal power is currently limited to 0.3% by the low spectral density of the pump and idler sources at hand, but can be greatly enhanced by applying narrow line width lasers.

Fiber laser-based light source for coherent anti-Stokes Raman scattering microspectroscopy

We demonstrate an alternative light source for CARS microspectroscopy based on a fiber laser and a photonic-crystal fiber. The light source simultaneously delivers a near-transform-limited picosecond pump pulse at 1033.5 nm and a frequency-shifted, near-transform-limited femtosecond Stokes pulse, tunable from 1033.5 nm to 1400 nm. This corresponds to a range 0 - 2500 cm(-1), so that Raman-active vibrations in this frequency range can be probed. The spectral resolution is 5 cm(-1), given by the spectral width of the pump pulse. The frequency range that can be probed simultaneously is 200 cm(-1)-wide, given by the spectral width of the Stokes pulse. The achievable pulse powers are 50 mW for the pump and 2 mW for the Stokes pulse. The repetition rate is 35 MHz. We demonstrate the capability of this light source by performing CARS microspectroscopy and comparing CARS spectra with Raman spectra.

A 158 fs 5.3 nJ fiber-laser system at 1 microm using photonic bandgap fibers for dispersion control and pulse compression.

We demonstrate a 158 fs 5.3 nJ mode-locked laser system based on a fiber oscillator, fiber amplifier and fiber compressor. Dispersion compensation in the fiber oscillator was obtained with a solid-core photonic bandgap (SC-PBG) fiber spliced to standard fibers, and external compression is obtained with a hollow-core photonic bandgap (HC-PBG) fiber.

All-fiber mode-locked fiber laser.

An environmentally stable mode-locked fiber laser based on nonlinear polarization rotation is experimentally demonstrated. The laser is based on a novel laser configuration that has negligible low-power steady-state reflectivity from one side and, consequently, no CW gain. The laser is self starting and the configuration is implementable as an all-fiber laser with standard polarization-maintaining fiber-pigtailed components. A pulse duration of 5.6 ps is obtained at a repetition rate of 5.96 MHz and at an average power of 8 mW. As an application of the proposed laser configuration, 213 mW of supercontinuum (600-1750 nm) was demonstrated from a fiber laser system with no sections of free-space optics.

Stability analysis of an all-fiber coupled cavity Fabry-Perot additive pulse mode-locked laser

The coupled cavity Fabry-Perot additive pulse mode-locked (APM) design of an all-fiber mode-locked fiber laser is investigated theoretically. Experimental implementation of the design has already been demonstrated elsewhere, and in previous work, it was shown that contrary to a solid-state coupled cavity Fabry-Perot APM laser, an interferometric length match and stabilization of the two cavities is not needed. However, the pulse train seems to be located on top of a large CW background. In this paper, a theoretically analysis of the design based on the master equation derived by Haus is presented. Using the analytical theory developed for the master equation it is possible to determine and compare the round-trip loss for a mode-locked and for a CW solution and, hence, find the regimes where an all-fiber laser based on the design is expected to give a stable background free mode-locked output.

Dispersion compensation with solid-core photonic bandgap fiber in an Yb-doped mode-locked fiber laser

The recent development of photonic bandgap fibers with solid cores enables the construction of dispersion compensated all-fiber ultrashort mode-locked fiber lasers. Solid-core photonic bandgap fibers (SC-PBG) can be spliced to standard fibers with relative low loss and negligible Fresnel reflection due to the matching indexes of the cores. The fibers can provide significant anomalous dispersion with low nonlinearity and are therefore ideal for dispersion compensation in ultrafast fiber lasers. We demonstrate the use of a SC-PBG fiber for intra cavity dispersion compensation in an ytterbium based mode-locked fiber laser. The limitations on pulse duration due to the relative high third order dispersion of the SC-PBG fiber are discussed.

Fiber laser-based light source for CARS microspectroscopy

We demonstrate an alternative light source for CARS microspectroscopy based on a fiber laser and a photonic crystal fiber. The light source simultaneously delivers a picosecond pump pulse at 1033.5 nm and a frequency shifted femtosecond Stokes pulse, tunable from 1033.5 nm to 1400 nm. This corresponds to a range 0 - 2500 cm-1, so that Raman-active vibrations in this frequency range can be probed. The spectral resolution is 5 cm-1, given by the spectral width of the pump pulse. The frequency range that can be probed simultaneously is 200 cm-1-wide, given by the spectral width of the Stokes pulse. The achievable average powers are 50 mW for the pump and 2 mW for the Stokes pulse. The repetition rate is 35 MHz. We demonstrate the capability of this light source by performing CARS microspectroscopy and comparing CARS spectra with Raman spectra.

Investigations of the coupling between core modes and cladding modes in a double-clad Yb-doped photonic crystal fiber

In this work a new model based on the work with new double-clad Yb-doped photonic crystal fibers is presented. The model describes the effect of a coupling between core modes and cladding modes, causing a loss of power out of the core. The model agrees well with an experimental observed asymmetry in the output powers from the two ends of a pumped fiber due to pump depletion and the core-cladding coupling. A method to estimate the core-cladding coupling is included. The magnitude of the coupling depends on the geometry of the fiber. If an asymmetry is introduced between the coupling for the two orthogonal linear polarization directions, an polarization creating mechanism is predicted. This feature is investigated for a fiber where the asymmetric loss has been implemented by manufacturing the fiber to have an asymmetric transverse geometry.