Kim G. Jespersen

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.

A stretcher fiber for use in fs chirped pulse Yb amplifiers

A newly developed fiber for use in pulse stretchers for chirped pulse amplifiers working in the 1 mum wavelength range of Yb fiber amplifiers is reported. The fiber has a record high numerical third order to second order dispersion beta(3)/beta(2) ratio of -7.7 fs. The fiber has very good dispersion match to a grating compressor for second, third, and fourth order dispersion. By combining the stretcher fiber with an anomalous dispersion fiber working in a higher order mode, even higher beta(3)/beta(2) ratio of -16.8 fs is demonstrated. The combined module shows very good dispersion match to a grating compressor.

A higher-order-mode fiber delivery for Ti:Sapphire femtosecond lasers

We report the first higher-order-mode fiber with anomalous dispersion at 800nm and demonstrate its potential in femtosecond pulse delivery for Ti:Sapphire femtosecond lasers. We obtain 125fs pulses after propagating a distance of 3.6 meters in solid-silica fiber. The pulses could be further compressed in a quartz rod to nearly chirp-free 110fs pulses. Femtosecond pulse delivery is achieved by launching the laser output directly into the delivery fiber without any pre-chirping of the input pulse. The demonstrated pulse delivery scheme suggests scaling to >20meters for pulse delivery in harsh environments not suited for oscillator operation or in applications that require long distance flexibility.

Generation of Cerenkov radiation at 850 nm in higher-order-mode fiber

We demonstrate Cerenkov radiation generation in a higher-order-mode fiber with anomalous dispersion from approximately 700 to 800 nm. Cerenkov radiation at 850 nm with 0.6 nJ pulse energy is generated with 60% power conversion efficiency.

Generation of high fidelity 62-fs, 7-nJ pulses at 1035 nm from a net normal-dispersion Yb-fiber laser with anomalous dispersion higher-order-mode fiber

We present a mode-locked 24 MHz Yb-doped fiber oscillator with a higher-order mode fiber for dispersion compensation. The oscillator operates in the net normal dispersion regime and generates clean 6 nJ pulses that can be dechirped down to 150 fs.

Optimization of higher order mode fibers for dispersion management of femtosecond fiber lasers

Femtosecond fiber lasers are currently of great interest due to their small size, stable operation, long lifetime and low cost compared to bulk lasers. However, for operation in the 1 µm wavelength range of Yb lasers, a major obstacle has been the lack of suitable fibers with anomalous dispersion that can compensate for the normal dispersion of the conventional active and passive fibers used. However, a new promising fiber device using a higher order mode (HOM) with anomalous dispersion in the 1 μm range has recently been demonstrated. The device comprises integrated all fiber mode converters based on long period gratings (LPG), and hence has the potential to be low loss and easy to splice, while offering a large effective area, and the possibility of third order dispersion compensation. In this paper, optimization of HOM fibers with anomalous dispersion in the 1 μm range has been investigated theoretically and experimentally. Fibers with dispersion coefficients ranging from +50 to +300 ps/(nm·km) at 1060 nm have been fabricated and devices including integrated LPG mode converters have been characterized. Modeled and measured properties of the modules, such as dispersion, grating bandwidth etc., are found to correlate well. It is shown that there is a tradeoff between a high dispersion coefficient and the bandwidth of LPG mode converters. The characteristics of such HOM devices have been studied in a linear, passively mode-locked laser-cavity using SESAM as saturable absorber.

High peak-power monolithic femtosecond ytterbium fiber chirped pulse amplifier with a spliced-on hollow core fiber compressor

We demonstrate a monolithic Yb-fiber chirped pulse amplifier that uses a dispersion matched fiber stretcher and a spliced-on hollow core photonic bandgap fiber compressor. For an output energy of 77 nJ, 220 fs pulses with 92% of the energy contained in the main pulse, can be obtained with minimal nonlinearities in the system. 135 nJ pulses are obtained with 226 fs duration and 82 percent of the energy in the main pulse. Due to the good dispersion match of the stretcher to the hollow core photonic bandgap fiber compressor, the duration of the output pulses is within 10% of the Fourier limited duration.

A dispersion compensating fiber with a factor 5 improvement in figure of merit and a factor 4.5 improvement in effective area

A Dispersion compensating fiber working in the LP<inf>02</inf> mode with a figure of merit of 2200 ps/(nm·dB), an effective area of 90 µm², and MPI of −37 dB is reported.

Stretcher fibers for chirped pulse amplifiers at 1030 nm and 1550 nm

Chirped pulse amplification is a durable and widely used scheme for producing short pulses with duration less than 1ps and pulse energies from μJ to even mJ levels. The compressor unit needs to be able to handle high peak powers and is therefore traditionally made out of free space diffraction gratings. The stretcher unit, on the other hand, only has to handle low peak power and can therefore be realized with a dispersion managed fiber. We review our work on stretcher fiber design and dispersion managed modules in the Ytterbium gain band and Erbium gain band and present a modified stretcher fiber for managing numerically higher third and fourth order dispersion. The modified design allows for a lower incident angle on the compressor gratings and thereby reducing the grating separation and compressor size.

Optical low-coherence interferometry for reconstruction of the modal-content in few-mode fibers

A novel method for reconstructing the dispersion-corrected modal content of fibers is experimentally demonstrated. It provides the modal weights, the profiles, relative group-delays, and dispersion of all the modes without assuming any optical properties of the fiber under test.