S. P. Stark

14 GHz visible supercontinuum generation: calibration sources for astronomical spectrographs

We report the use of a specially designed tapered photonic crystal fiber to produce a broadband optical spectrum covering the visible spectral range. The pump source is a frequency doubled Yb fiber laser operating at a repetition rate of 14 GHz and emitting sub-5 pJ pulses. We experimentally determine the optimum core diameter and achieve a 235 nm broad spectrum. Numerical simulations are used to identify the underlying mechanisms and explain spectral features. The high repetition rate makes this system a promising candidate for precision calibration of astronomical spectrographs.

Fiber chirped pulse amplifier at 2.08 μm emitting 383-fs pulses at 10 nJ and 7 MHz.

An all-polarization maintaining (PM) fiber chirped pulse amplifier system at 2.08 μm based on thulium:holmium codoped gain fibers is reported. An inhouse built oscillator emits pulses at a repetition rate of 7 MHz with a spectral full width at half-maximum (FWHM) bandwidth of 23.5 nm at 2.8 mW average output power. The pulses are temporally stretched and subsequently amplified in a double-stage amplifier setup. The stretched pulses are compressed to 383 fs by use of a Martinez-style setup at an output pulse energy of 10.2 nJ. By neglecting temporal stretching, high peak powers in a single amplifier stage led to Raman soliton formation at 2.3 μm.

Nonlinear amplification of side-modes in frequency combs.

We investigate how suppressed modes in frequency combs are modified upon frequency doubling and self-phase modulation. We find, both experimentally and by using a simplified model, that these side-modes are amplified relative to the principal comb modes. Whereas frequency doubling increases their relative strength by 6 dB, the growth due to self-phase modulation can be much stronger and generally increases with nonlinear propagation length. Upper limits for this effect are derived in this work. This behavior has implications for high-precision calibration of spectrographs with frequency combs used for example in astronomy. For this application, Fabry-Pérot filter cavities are used to increase the mode spacing to exceed the resolution of the spectrograph. Frequency conversion and/or spectral broadening after non-perfect filtering reamplify the suppressed modes, which can lead to calibration errors.

Relative stability of two laser frequency combs for routine operation on HARPS and FOCES

We report on the installation of a laser frequency comb (LFC) at the HARPS spectrograph, which we characterize relative to a second LFC that we had brought to HARPS for testing. This allowed us for the first time to probe the relative stability of two independent astronomical LFCs over an extended wavelength range. Both LFCs covered the spectral range of HARPS at least from 460 to 690 nm. After optimization of the fiber coupling to HARPS to suppress modal noise, a relative stability of the two LFCs in the low cm/s range was obtained. In combination with the results of our four earlier LFC test campaigns on HARPS, the available data now cover a time span of more than six years.

Spectral flattening of supercontinua with a spatial light modulator

We demonstrate the generation of broad spectra with a flat intensity distribution from originally highly structured supercontinua, obtained with femtosecond pulses in a photonic crystal fiber. This is accomplished by truncating the spectra at a constant level using a liquid crystal based spatial light modulator. The technique is useful for astronomical spectrograph calibration using frequency combs, where it allows to equalize the optical power of the calibration lines. This enables an improved calibration accuracy by maximizing each line’s signal-to-noise ratio.

Spectrally flattened, broadband astronomical frequency combs

We demonstrate the generation of broadband, visible astronomical frequency combs with flattened spectral envelopes. The flat-top region of the spectrum ranges from about 450 to 730 nm, at mode spacings of 18 and 25 GHz.

Suppressed mode recovery in nonlinear fibers of a Fabry-Perot-filtered frequency comb

A Yb-fiber based frequency comb spanning more than 150 nm with a multi-GHz mode spacing was set up. Dynamic reamplification of suppressed modes in a nonlinear fiber after the filter cavities was observed and analyzed.

kW picosecond thin-disk regenerative amplifier

TRUMPF Scientific Lasers provides ultrafast laser sources for the scientific community with high pulse energies and high average power. All systems are based on the industrialized TRUMPF thin-disk technology. Regenerative amplifiers systems with multi-millijoule pulses, kilohertz repetition rates and picosecond pulse durations are available. Record values of 220mJ at 1kHz could be demonstrated originally developed for pumping optical parametric amplifiers. The ultimate goal is to combine high energies, <100mJ per pulse, with average powers of several hundred watts to a kilowatt. Based on a regenerative amplifier containing two Ytterbium doped thin-disks operated at ambient temperature pulses with picosecond duration and more than 100mJ could be generated at a repetition rate of 10kHz reaching 1kW of average output power. This system is designed to operate at different repetition rates from 100kHz down to 5kHz so that even higher pulse energies can be reached. This type of ultrafast sources uncover new application fields in science. Laser based lightning rods, X-ray lasers and Compton backscatter sources are among them.

High-average power picosecond thin-disk regenerative amplifiers

Today thin-disk lasers routinely provide high pulse energies at picosecond pulse durations and kHz repetition rates. Systems with more than 200mJ per pulse are commercially available. After the introduction of the Dira 200-1, providing 200mJ at 1kHz, TRUMPF Scientific Lasers complements its thin-disk regenerative amplifier product portfolio by systems with a few hundred Watts of average output power. Still based on a single disk a flexible laser system with more than 500W was realized. Originally, it was designed for a 50kHz operation, delivering 10mJ pulses, but it also can be set-up for different repetition rates like 10kHz or 100kHz. TRUMPF Scientific Lasers regenerative amplifiers show an excellent long-term performance. The 500W system has a power stability of 0.5%. Scientific applications often require higher average output powers, even with high pulse energies. Based on the extensive experience with highest average power continuous wave laser systems by TRUMPF a more powerful regenerative amplifier system is currently under development by TRUMPF Scientific Lasers. This laser uses two disk laser heads inside the same cavity to provide more than 1kW average output power. First results show an average power of more than 1kW at repetition rates of 10kHz and higher. A pulse duration below 1ps could be reached.

Ultrafast Lasers and Frequency Combs for Industrial Applications

We have developed compact ultrafast laser sources for a wide variety of applications, including quality control and optical frequency combs for precision measurements. Fields ranging from astronomical observatories to spaceflights, optical clocks or long distance links are served with this technology.