Compact harmonic cavity optical parametric oscillator for optical parametric amplifier seeding.

We present a broadly tunable highly efficient frequency conversion scheme, based on a low-threshold harmonic cavity optical parametric oscillator (OPO) followed by an idler-seeded power amplifier. By choosing the cavity length of the OPO equal to the 10th harmonic of its 41 MHz Yb:KGW solid-state pump laser, a very compact optical setup is achieved. A singly-resonant cavity without output coupler results in a low oscillation threshold of only 28-100 mW in the entire signal tuning range of 1.37-1.8 µm. The 2.4-4.15 µm idler radiation is coupled out at the 41 MHz pump frequency and employed to seed a post amplifier with nearly Watt-level output power. In addition, the seeder plus power amplifier concept results in clean signal and idler pulses at the fundamental repetition rate of 41 MHz with a time-bandwidth product below 0.4 and a relative intensity noise 10 dB lower compared to the solid-state pump laser.

Alignment-free difference frequency light source tunable from 5 to 20†µm by mixing two independently tunable OPOs.

Tunable mid-infrared ultrashort lasers have become an essential tool in vibrational spectroscopy in recent years. They enabled and pushed a variety of spectroscopic applications due to their high brilliance, beam quality, low noise, and accessible wavelength range up to 20 µm. Many state-of-the-art devices apply difference frequency generation (DFG) to reach the mid-infrared spectral region. Here, birefringent phase-matching is typically employed, resulting in a significant crystal rotation during wavelength tuning. This causes a beam offset, which needs to be compensated to maintain stable beam pointing. This is crucial for any application. In this work, we present a DFG concept, which avoids crystal rotation and eliminates beam pointing variations over a broad wavelength range. It is based on two independently tunable input beams, provided by synchronously pumped parametric seeding units. We compare our concept to the more common DFG approach of mixing the signal and idler beams from a single optical parametric amplifier (OPA) or oscillator (OPO). In comparison, our concept enhances the photon efficiency of wavelengths exceeding 11 µm more than a factor of 10 and we still achieve milliwatts of output power up to 20 µm. This concept enhances DFG setups for beam-pointing-sensitive spectroscopic applications and can enable research at the border between the mid- and far-IR range due to its highly efficient performance.

Low drift cw-seeded high-repetition-rate optical parametric amplifier for fingerprint coherent Raman spectroscopy.

We introduce a broadly tunable robust source for fingerprint (170 - 1620 cm-1) Raman spectroscopy. A cw thulium-doped fiber laser seeds an optical parametric amplifier, which is pumped by a 7-W, 450-fs Yb:KGW bulk mode-locked oscillator with 41 MHz repetition rate. The output radiation is frequency doubled in a MgO:PPLN crystal and generates 0.7 - 1.3-ps-long narrowband pump pulses that are tunable between 885 and 1015 nm with >80 mW average power. The Stokes beam is delivered by a part of the oscillator output, which is sent through an etalon to create pulses with 1.7 ps duration. We demonstrate a stimulated Raman gain measurement of toluene in the fingerprint spectral range. The cw seeding intrinsically ensures low spectral drift.

Ultra-stable high average power femtosecond laser system tunable from 1.33 to 20 µm.

A highly stable 350 fs laser system with a gap-free tunability from 1.33 to 2.0 µm and 2.13 to 20 µm is demonstrated. Nanojoule-level pulse energy is achieved in the mid-infrared at a 43 MHz repetition rate. The system utilizes a post-amplified fiber-feedback optical parametric oscillator followed by difference frequency generation between the signal and idler. No locking or synchronization electronics are required to achieve outstanding free-running output power and spectral stability of the whole system. Ultra-low intensity noise, close to the pump laser's noise figure, enables shot-noise limited measurements.

Multi-Watt femtosecond optical parametric master oscillator power amplifier at 43 MHz.

We present a high repetition rate mid-infrared optical parametric master oscillator power amplifier (MOPA) scheme, which is tunable from 1370 to 4120nm. Up to 4.3W average output power are generated at 1370nm, corresponding to a photon conversion efficiency of 78%. Bandwidths of 6 to 12nm with pulse durations between 250 and 400fs have been measured. Strong conversion saturation over the whole signal range is observed, resulting in excellent power stability. The system consists of a fiber-feedback optical parametric oscillator that seeds an optical parametric power amplifier. Both systems are pumped by the same Yb:KGW femtosecond oscillator.

High-power mid-infrared high repetition-rate supercontinuum source based on a chalcogenide step-index fiber.

We demonstrate a tunable and robust femtosecond supercontinuum source with a maximum output power of 550 mW and a maximum spectral width of up to 2.0 µm, which can cover the mid-infrared region from 2.3 µm up to 4.9 µm by tuning the pump wavelength. As2S3 chalcogenide step-index fibers with core diameters of 7 and 9 µm are pumped at different wavelengths from 2.5 µm up to 4.1 µm with femtosecond pulses by means of a post-amplified optical parametric oscillator pumped by an Yb:KGW laser. The spectral behavior of the supercontinuum is investigated by changing the pump wavelength, core diameter, fiber length, and pump power. Self-phase modulation is identified as the main broadening mechanism in the normal dispersion regime. This source promises to be an excellent laboratory tool for infrared spectroscopy owing to its high brilliance as demonstrated for the CS2-absorption bands around 3.5 µm.

Compact, low-noise, all-solid-state laser system for stimulated Raman scattering microscopy.

We present a highly stable and compact laser source for stimulated Raman scattering (SRS) microscopy. cw-seeding of an optical parametric amplifier pumped by a bulk femtosecond Yb-oscillator and self-phase modulation in a tapered fiber allow for broad tunability without any optical or electronic synchronization. The source features noise levels of the Stokes beam close to the shot-noise limit at MHz modulation frequencies. We demonstrate the superior performance of our system by SRS imaging of micrometer-sized polymer beads.

Watt-level optical parametric amplifier at 42 MHz tunable from 1.35 to 4.5 µm coherently seeded with solitons.

We report on an optical parametric amplifier at high repetition rate of 41.7 MHz seeded by an optical soliton from a tapered fiber. Gap-free signal tuning from 1.35 µm to 1.95 µm with corresponding idler wavelengths from 2.2 µm to 4.5 µm is demonstrated. The system provides up to 1.8 W average power at 1.4 µm, more than 1.1 W up to 1.7 µm, and more than 400 mW up to 4.0 µm with a signal pulse duration of 200 to 300 fs. It is directly pumped by a solid-state oscillator providing up to 7.4 W at 1.04 µm wavelength with 425 fs pulse duration. Soliton-seeding is shown to lead to excellent pulse-to-pulse stability, but it introduces a timing-jitter on the millisecond timescale. Using a two-stage concept the timing-jitter is efficiently suppressed due to the passive synchronization of both conversion stages.

Combining cw-seeding with highly nonlinear fibers in a broadly tunable femtosecond optical parametric amplifier at 42 MHz.

We report on a precisely tunable and highly stable femtosecond oscillator-pumped optical parametric amplifier at a 41.7 MHz repetition rate for spectroscopic applications. A novel concept based on cw-seeding of a first amplification stage with subsequent spectral broadening and shaping, followed by two further amplification stages, allows for precise sub-nanometer and gap-free tuning from 1.35 to 1.75 µm and 2.55 to 4.5 µm. Excellent spectral stability is demonstrated with deviations of less than 0.008% rms central wavelength and 1.6% rms bandwidth over 1 h. Spectral shaping of the seed pulse allows precise adjustment of both the bandwidth and the pulse duration over a broad range at a given central wavelength. Transform-limited pulses nearly as short as 107 fs are achieved. More than half a Watt of average power in the near- and more than 200 mW in the mid-infrared with power fluctuations less than 0.6% rms over 1 h provide an excellent basis for spectroscopic experiments. The pulse-to-pulse power fluctuations are as small as 1.8%. Further, we demonstrate for the first time, to the best of our knowledge, that by using hollow-core capillaries with highly nonlinear liquids as a host medium for self-phase modulation, the signal tuning range can be extended and covers the region from 1.4 µm up to the point of degeneracy at 2.07 µm. Hence, the idler covers 2.07 to 4.0 µm.

Broadly tunable femtosecond near- and mid-IR source by direct pumping of an OPA with a 41.7 MHz Yb:KGW oscillator.

We generate over half a watt of tunable near-IR (1380-1830 nm) and several hundred milliwatts in the mid-IR (2.4-4.2 µm) as well as milliwatt level mid-IR (4.85-9.33 µm) femtosecond radiation by pumping an optical parametric amplifier directly with a 7.4 W Yb:KGW oscillator at 41.7 MHz repetition rate. We use 5 mm PPLN and 2 mm GaSe as downconversion crystals and seed this process by a supercontinuum from a tapered fiber. The system is extremely simple and very stable and could replace more complex OPOs as tunable light sources.

Milliwatt-level mid-infrared (10.5-16.5 µm) difference frequency generation with a femtosecond dual-signal-wavelength optical parametric oscillator.

We demonstrate the generation of mid-infrared radiation using a femtosecond dual-signal-wavelength optical parametric oscillator and difference frequency generation in an extracavity gallium selenide or silver gallium diselenide crystal. This system generates up to 4.3 mW of average mid-infrared power. Its spectra can be tuned to between 10.5 µm and 16.5 µm wavelength (952 cm(-1)-606 cm(-1)) with more than 50 cm(-1) spectral bandwidth. We demonstrate that the power and spectra of this system are temporally very stable.

High-power widely tunable sub-20 fs Gaussian laser pulses for ultrafast nonlinear spectroscopy.

We demonstrate the generation of widely tunable sub-20 fs Gaussian-shaped laser pulses using a grating-based 4-f pulse shaper and a liquid crystal spatial light modulator. Our pump source is an Yb:KGW solitary mode-locked oscillator at 44 MHz repetition rate which is coupled into a large mode area microstructured fiber to generate a broad spectrum from below 900 nm to above 1150 nm. These pulses are precompressed by a prism sequence and subsequently sent into the pulse shaper. We use the multiphoton intrapulse interference phase scan (MIIPS) for phase shaping and iterative amplitude optimization to achieve Gaussian-like tunable sub-20 fs pulses with output powers of up to 142 mW as well as nontunable pulses with 310 mW output power as short as 11.5 fs.

Few-cycle OPCPA system at 143 kHz with more than 1 microJ of pulse energy.

We present an OPCPA system delivering 8.8 fs (3.3 optical cycles) pulses with 1.3 microJ of energy at 143 kHz repetition rate. Pump and seed for the parametric amplification are simultaneously generated by a broadband Ti:sapphire oscillator. The spectral components beyond 1000 nm are separated and amplified in an Yb:YAG thin-disk regenerative amplifier. The pulses are characterized using autocorrelation and SPIDER apparatus. With a pulse peak power of nearly 130 MW, the system is well-suited for future table top strong field experiments.

Sub-10-fs pulses from a MHz-NOPA with pulse energies of 0.4 microJ.

We present a non-collinear optical parametric amplifier (NOPA) delivering sub-10-fs pulses with 420 nJ of pulse energy. The system is driven by microjoule pulses from an Yb:KYW oscillator with cavity-dumping and a subsequent single-stage rod-type fiber amplifier at 1-MHz repetition rate. The ultrabroadband seed is based on stable white-light generation from 420 fs long pulses in a YAG plate.

Compact laser source for high-power white-light and widely tunable sub 65 fs laser pulses.

We demonstrate an Yb:KGW femtosecond solitary mode-locked laser oscillator, which is used as a pump source for tapered fibers to generate white-light laser pulses with an average output power of up to 2.5 W and a spectral bandwidth of over 1000 nm. By spectrally filtering these pulses and subsequent compression of the filtered pulses with a prism sequence, we are able to generate ultrashort laser pulses with durations between 26 and 65 fs that are tunable from 600 to 1450 nm and with tens to several hundreds of milliwatts of average power.

Double waveguide couplers produced by simultaneous femtosecond writing.

We report on a novel method to create waveguide coupler devices in fused silica by combining the technique of beam shaping with femtosecond laser writing. The method is based on a programmable phase modulator and a dynamic variation of the phase-pattern during the writing process. The major advantage is the possibility to create complex devices in a single sweep by simultaneously writing two or more waveguides with changing separation. The guiding properties and the coupling behavior between the waveguides are investigated.

Theoretical and experimental limits of cavity-dumping in passively mode-locked thin-disk oscillators.

The dynamical properties of a mode-locked thin disk laser with cavity-dumping in the solitary regime are studied using numerical simulations along with experimental data. Limitations of this system as well as their origin are identified. The results of these investigations agree very well with recently published experimental results. Based on these findings design criteria for future systems are deducted and estimates of possible pulse energies are made.

Passively mode-locked and cavity-dumped Yb:KY(WO(4))(2) oscillator with positive dispersion.

We demonstrate, what is to our knowledge the first passively mode-locked Ytterbium based solid state high energy laser oscillator operated in the positive dispersion regime. Compared to solitary mode-locking the pulse energy can be increased with even broader spectral bandwidth. With high speed cavity dumping the laser generates 2 muJ-pulses at a 1 MHz repetition rate. The chirped output pulses are compressible down to 420 fs.

Generation of few-cycle pulses directly from a MHz-NOPA.

We demonstrate the generation of 10-fs-pulses from a noncollinear optical parametric amplifier (NOPA). The NOPA is driven by microjoule pulses from a directly diode pumped Yb:KYW oscillator with cavity-dumping.

Synchronization-free all-solid-state laser system for stimulated Raman scattering microscopy.

We introduce an extremely simple and highly stable system for stimulated Raman scattering (SRS) microscopy. An 8-W, 450-fs Yb:KGW bulk oscillator with 41 MHz repetition rate pumps an optical parametric amplifier, which is seeded by a cw tunable external cavity diode laser. The output radiation is frequency doubled in a long PPLN crystal and generates 1.5-ps long narrowband pump pulses that are tunable between 760 and 820 nm with >50 mW average power. Part of the oscillator output is sent through an etalon and creates Stokes pulses with 100 mW average power and 1.7 ps duration. We demonstrate SRS microscopy at a 30-µs pixel dwell time with high chemical contrast, signal-to-noise ratio in excess of 45 and no need for balanced detection, thanks to the favorable noise properties of the bulk solid-state system. Cw seeding intrinsically ensures low spectral drift. We discuss its application to chemical contrast microscopy of freshly prepared plant tissue sections at different vibrational bands.