Kai-Hsiu Liao

Generation of hard X-rays using an ultrafast fiber laser system

We report the first hard X-ray source driven by a femtosecond fiber laser. The high energy fiber CPA system incorporated a 65mum LMA fiber amplifying stage which provided 300-fs recompressed pulses and diffraction limited beam quality with M(2) < 1.07. A deformable mirror was used to optimize the wavefront and the spot size was focused down to 2.3 mum with an f/1.2 paraboloidal mirror. 50muJ was deposited on the nickel target with 2x10(15)-W/cm(2) focal intensity and a distinctive Ni K(alpha)-line (7.48 keV) emission was measured with 5x10(-8) energy conversion efficiency.

Broadly tunable carrier envelope phase stable optical parametric amplifier pumped by a monolithic ytterbium fiber amplifier

In an effort to develop a robust and efficient front end for a chirped-pulse parametric amplification chain, we demonstrate a broadband difference-frequency converter driven by a monolithic femtosecond Yb-doped-fiber amplifier and emitting carrier-envelope-offset-free pulses with the energy of tens of nanojoules tunable in the wavelength range from 1200 nm to beyond 2 mum. Next to providing these seed pulses, the system enables direct optical synchronization of Nd- and Yb-doped pump lasers for subsequent parametric amplification.

Dispersion and Nonlinear Phase-Shift Compensation in High-Peak-Power Short-Pulse Fiber Laser Sources Using Photonic-Crystal Fibers

Photonic-crystal fibers (PCFs) with a specifically designed dispersion profile and nonlinearity are shown to enable an accurate broadband compensation of the stretcher-compressor dispersion in fiber laser sources of high-peak-power ultrashort light pulses. We demonstrate that the nonlinear phase shift in such systems can partially compensate for the fourth-order dispersion, allowing the stretcher—compression group delay to be compensated up to the fourth-order dispersion terms by using a sequence of only two fibers—a standard optical fiber and a PCF.

A micromachined platform for localized index modulation in chirped fiber Bragg gratings and its application to ultrafast optical pulse shaping

This paper reports the development of a micromachined platform that locally modifies the refractive index of a chirped fiber Bragg grating (CFBG). This platform, composed of an array of electrothermal actuators, has been used to make an ultrashort optical pulse shaper. The micromachined array, with a footprint of 5 mm /spl times/ 1 mm, has 75 actuators spaced 60 /spl mu/m apart, and has been used to demonstrate pulse shaping both in the time and frequency domain. A normalized intensity peak of 1.5 at 1550 nm is obtained by applying about 2 mN of force using a single actuator. The intensity peak can be shifted by over 3 nm to the edge of the CFBG spectrum by using different actuators. The pulse width is modulated from 1.5 ps to 4 ps by application of 500 mW of power. System simulations track experimental data that show the individual actuators addressing distinct frequency components. Thermal experiments verify that heat generated by the electrothermal actuators has no measurable effect on the optical pulse spectrum obtained.

Versatile Yb-fiber-amplifier-based CEP-stable front-end for OPCPA

The rapidly rising technology of optical parametric chirped pulse amplification (OPCPA) [1] offers unique advantages, among others, such as high broadband gain, including frequency ranges inaccessible to direct laser amplification, and efficient energy conversion using narrowband picosecond pump pulses. The perennial problems of OPCPA, significantly slowing down its development into a mainstream amplifier technology, are pump-seed pulse synchronization and generation of a frequency-detuned compressible broadband seed pulse for the optical parametric amplifier (OPA). Demonstrated approaches to the synchronization of pump pulse sources include active locking of two master oscillators [2], nonlinear frequency shifting in a photonics crystal fiber (PCF) [3] and simultaneous seeding of the pump pulse laser and the OPA from an ultrabroadband Ti:sapphire oscillator [4]. IR OP(CP)A systems with passive carrier envelope phase (CEP) stabilization were realized using a broadband difference-frequency-generation (DFG) seed produced via spectral broadening of the output of full-fledged Ti:sapphire [5,6] and Yb [7] CPA systems. In this contribution we demonstrate a drastically simplified seeding/synchronization approach based on an environmentally stable µJ-energy 200-fs 100-kHz Yb-doped fiber amplifier (YDFA) that generates a white light continuum in bulk sapphire, pumps a DFG OPA and provides an optical seed compatible with most Nd and Yb ∼1-µm pump pulse amplifiers (Fig. 1).

Broadly tunable CEP-stable OPA pumped by a monolithic Yb-doped fiber amplifier

We demonstrate an efficient broadband difference frequency converter emitting carrier-envelope-offset-free seed pulses for chirped-pulse parametric amplification and pumped by a monolithic femtosecond Yb-doped fiber amplifier that simultaneously provides passive optical synchronization for an OPCPA pump pulse source.

An efficient Ni K α X-ray source driven by a high energy fiber CPA system

In this paper we report the first demonstration of efficient hard X-ray generation using a femtosecond fiber laser. The hard X-ray source has yield efficiencies similar to those obtained with solid state laser drivers with comparable pulse energies and intensities. Further experiments on increasing X-ray yield efficiency with higher pulse energies from the current system are in progress. Since this fiber laser technology is scalable to >100-W of average power this demonstration opens a path towards compact high-brightness hard X-ray sources for imaging and time resolved diffraction experiments.

Monolithic fiber-grating and MEMS based devices for controllable ultrafast pulse shaping

A new type of optical pulse shaper for arbitrary waveform generation is demonstrated, based on fiber Bragg grating and micro-electro-mechanical system (MEMS) technologies. This is an on-chip device which is compact, robust, monolithic, and programmable and can be used for a variety of applications such as higher order dispersion compensation in fiber communication links and high-energy pulse amplification.