Two-Photon Microscopy (TPM) can provide three-dimensional morphological and functional contrast in vivo. Through proper staining, TPM can be utilized to create virtual, HE equivalent images and thus can improve throughput in histology-based applications. We previously reported on a new light source for TPM that employs a compact and robust fiber-amplified, directly modulated laser. This laser is pulse-to-pulse wavelength switchable between 1064 nm, 1122 nm, and 1186 nm with an adjustable pulse duration from 50ps to 5ns and arbitrary repetition rates up to 1MHz at kW-peak powers. Despite the longer pulse duration, it can achieve similar average signal levels compared to fs-setups by lowering the repetition rate to achieve similar cw and peak power levels. The longer pulses lead to a larger number of photons per pulse, which yields single shot fluorescence lifetime measurements (FLIM) by applying a fast 4 GSamples/s digitizer. In the previous setup, the wavelengths were limited to 1064 nm and longer. Here, we use four wave mixing in a non-linear photonic crystal fiber to expand the wavelength range down to 940 nm. This wavelength is highly suitable for imaging green fluorescent proteins in neurosciences and stains such as acridine orange (AO), eosin yellow (EY) and sulforhodamine 101 (SR101) used for histology applications. In a more compact setup, we also show virtual HE histological imaging using a direct 1030 nm fiber MOPA.
An ultrafast ytterbium-doped fiber laser system based on coherent beam combination of 16 amplifier channels is presented. The system delivers 1.83 kW average power at 2.3 mJ pulse energy and 240 fs pulse duration. The combining efficiency of 82% and the beam M2-value of 1.8 currently is limited by thermal lensing in some optical components, which were identified and are to be replaced.
Trapping nanoscopic objects to observe their dynamic behaviour for extended periods of time is an ongoing quest. Particularly, sub-100nm transparent objects are hard to catch and most techniques rely on immobilisation or transient diffusion through a confocal laser focus. We present an Anti-Brownian ELectrokinetic trap1–7 (pioneered by A. E. Cohen and W. E. Moerner) to hold nanoparticles and individual FoF1-ATP synthase proteins in solution. We are interested in the conformational dynamics of this membrane-bound rotary motor protein that we monitor using single-molecule FRET. The ABELtrap is an active feedback system cancelling the nano-object’s Brownian motion by applying an electric field. We show how the induced electrokinetic forces confine the motion of nanoparticles and proteoliposomes to the centre of the trap.
State-of-the-art ultrafast fiber lasers currently are limited in peak power by excessive nonlinearity and in average power by modal instabilities. Coherent beam combination in space and time is a successful strategy to continue power scaling by circumventing these limitations. Following this approach, we demonstrate an ultrafast fiber-laser system featuring spatial beam combination of 8 amplifier channels and temporal combination of a burst comprising 4 pulses. Active phase stabilization of this 10-armed interferometer is achieved using LOCSET and Hänsch-Couillaud techniques. The system delivers 1 kW average power at 1 mJ pulse energy, being limited by pump power, and delivers 12 mJ pulse energy at 700 W average power, being limited by optically induced damage. The system efficiency is 91% and 78%, respectively, which is due to inequalities of nonlinearity between the amplifier channels and to inequality of power and nonlinearity between the pulses within the burst. In all cases, the pulse duration is ~260 fs and the M2-value is better than 1.2. Further power scaling is possible using more amplifier channels and longer pulse bursts.
Among other modern imaging techniques, stimulated Raman Scattering (SRS) requires an extremely quiet, widely wavelength tunable laser, which, up to now, is unheard of in fiber laser systems. We present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber. We employ an all-fiber frequency and repetition rate tunable laser in order to enable wideband conversion in the linear OPO cavity arrangement, the signal and idler radiation can be tuned between 764 and 960 nm and 1164 and 1552 nm at 9.5 MHz. Thus, all biochemically relevant Raman shifts between 922 and 3322 cm-1 may be addressed in combination with a secondary output, which is tunable between 1024 and 1052 nm. This ultra-low noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging. We measure the relative intensity noise of this output beam at 9.5 MHz to be between -145 and -148 dBc, which is low enough to enable high-speed SRS imaging with a good signal-to-noise ratio. The laser system is computer controlled to access a certain energy differences within one second. Combining FWM based conversion, with all-fiber Yb-based fiber lasers enables the construction of the first automated, turn-key and widely tunable fiber laser. This laser concept could be the missing piece to establish CRS imaging as a reliable guiding tool for clinical diagnostics and surgical guidance.
Stimulated Raman Scattering requires an extremely quiet, widely wavelength tunable laser, which, up to now, is unheard of in fiber lasers. We present a compact and maintenance-free optical parametric oscillator based on degenerate four-wave mixing in a photonic crystal fiber. By employing an all-fiber frequency and repetition rate tunable laser as a seed source, we are able to generate tunable light between 1015 and 1065 nm. After amplification and subsequent conversion in the fiber OPO, signal and idler radiation between 785 and 960 nm and 1177 and 1500 nm may be generated with a repetition rate of 9 MHz. Therefore, we are able to address Raman shifts between 910 and 3030 cm-1. An additional output provides the Stokes radiation at 18 MHz required for the SRS process, which is passively synchronized to the tunable radiation. We measure the relative intensity noise of the Stokes beam at 9 MHz to be -150 dBc enabling high speed SRS imaging with a good signal-to-noise ratio. The combination of FWM based conversion, coupled with all-fiber Yb-based fiber lasers allows for the first turn-key, widely tunable and extremely compact laser systems developed for applications of CRS microscopy in clinics. This source could very well be the missing key instrument that CRS imaging requires for its real world transition.
Here, we present a passive 30-m long enhancement cavity that supports a steady-state enhancement of 198, which is the highest enhancement that has ever been reached in such a long cavity. Furthermore, we demonstrate the extraction of a short burst with a total energy of 53.6 μJ employing an acousto-optic modulator (AOM) as a switching device. The cavity was seeded with pulses of 1.49 μJ energy at 10 MHz repetition rate. The individual output coupled pulses showed an energy enhancement of up to 8.5 while the whole burst contained the entire energy of 36 input pulses. In the last section theoretical considerations for the single pulse extraction are presented and briefly discussed.
Over the last decade, the performance of femtosecond fiber laser systems has been rapidly improved. However, further improvements might be held back due to different physical limitations such as nonlinearities or optically induced damage. We demonstrate that with the coherent combination of four parallel fiber amplifiers record pulse energies and peak-powers of 5.7 mJ and 22 GW, respectively, could be achieved. These values could be realized with a chirped-pulse-amplification (CPA) laser system running at a repetition rate of 40 kHz and delivering a compressed average power of 230 W. A high combination efficiency of 89% was achieved demonstrating the scalability of the combining approach to a larger number of channels.
Coherent combination applied to state-of-the-art femtosecond fiber CPA systems has recently opened the route towards high energy and average power laser systems. The ~2 mJ, 340 fs, 196 W (100 kHz) pulses of such a system are coupled to an argon-filled hollow-core fiber for spectral broadening via self-phase modulation. Subsequent compression in a chirped mirror compressor leads to ~1 mJ, 45 fs, 96 W pulses. Under different conditions 580 μJ, 26 fs, 11.6 GW, 135 W (250 kHz) pulses are achieved. This is an unprecedented combination of average power and pulse energy.
The coherent combining of ultrashort pulses is a concept for scaling the pulse energy and average power of laser systems emitting ultrashort pulses. This concept has already been demonstrated for two-channel systems. In this contribution we report on a four-channel system that delivers 2.1 mJ pulse energies at a repetition rate of 100 kHz and 210 W of average power after compression. A combination efficiency of 88% was achieved and this demonstrates the scalability of the combining approach to a larger number of channels.
There is a great interest in obtaining laser pulses with a high average power as well as high pulse energies. Continuously pulsed systems face many problems to satisfy those requirements, independent on the amplifier concept. While many applications such as electron beam characterization and free-electron-laser seeding need high pulse energies at high repetition rates, they only need those laser pulses for a certain amount of time. Therefore, it is not necessary to run a laser system with continuous pulses at those parameters and a so-called burst mode might be sufficient and even essential in such cases. We report on a CPA-laser system, based on a large pitch fiber as a main-amplifier delivering bursts containing ultra-short, highly-energetic pulses. The burst rate is set to 20Hz, while each burst contains 2000 pulses at a pulse-repetition-rate of 10MHz and with a pulse-duration of 700fs. Hence the duty cycle D is 0.4%. To achieve a homogeneous pulse energy level between 27μJ and 31μJ after the compression, the main amplifier is pumped with a very high power of 1.6kW in a burst-mode (D=10%). By using an acousto-optical modulator (AOM) after the main-amp fiber, the residual output before and after the burst is removed to suppress ASE and any underground-pulses around the amplified burst. The limitations that could be observed during this experiment were mainly due to mode instabilities, which were detectable even on a very short time scale of a few hundred μs using a high speed camera.
Numerical and analytical considerations of nonlinear compression in noble-gas-filled hollow core fiber pulse
compressors are shown. Experiments are presented that enhance the peak power of existing fiber CPA architectures from
1.8 GW to 7.4 GW in a single stage. The pulses are shortened to 40 fs, which corresponds to a pulse shortening of more
than one order of magnitude. Sending these pulses into a second hollow fiber further shortens the pulses to 25 fs at a
pulse energy of 430 μJ at 30 kHz equaling an average power of 13 W. The peak power after the second stage is higher
than 10 GW. In addition, we present very recent experiments on average power scaling of hollow fiber pulse
compressors to the 100 W level. We compressed 200 W, 200 μJ, 500 fs pulses from a fiber CPA to 93 W, 93 μJ, 81 fs.
These unique pulse parameters should be very well suited for high harmonic generation.
In this contribution we report on a novel approach for pump and stokes pulse generation in extremely compact all-fiber
systems using parametric frequency conversion (four-wave-mixing) in photonic-crystal fibers. Representing a
completely alignment-free approach, the all-fiber ytterbium-based short-pulse laser system provides intrinsically
synchronized tunable two-color picosecond pulses emitted from a single fiber end. The system was designed to address
important CH-stretch vibrational resonances. Strong CARS signals are generated and proved by spectroscopic
experiments, tuning the laser over the resonance of toluene at 3050cm-1. Furthermore the whole laser setup with a
footprint of only 30x30cm2 is mounted on a home-built laser-scanning-microscope and CARS imaging capabilities are
verified. The compact turn-key system represents a significant advance for CARS microscopy to enter real-world, in
particular bio-medical, applications.
We report on a novel 10 mJ-level diode-pumped Yb:KYW amplifier at 1040 nm, which generates picosecond pulses at a
repetition rate of 10 Hz. It will be used in the front end of a petawatt laser system for pumping an optical parametric
amplifier (OPA) for contrast enhancement. For synchronization purposes the amplifier is seeded by pulses that are
derived from the femtosecond oscillator. After stretching by a volume Bragg grating and amplification in a double stage
fiber amplifier the pulses are injected into the Yb:KYW regenerative cavity. Finally, the pulses are compressed to 1 ps
before preparing pump pulses for the OPA by second harmonic conversion.
We present second harmonic generation of a high average power, high energy femtosecond Yb doped fiber chirped pulse
amplifier. This system is operated at various repetition rates at a central wavelength of 1040 nm. After two pre-amplification
stages a main amplifier is used to achieve the required pulse energy for efficient second harmonic
generation. It is comprised of a 1.2 m long photonic crystal fiber with a mode field diameter of 45 μm. A dielectric
grating based compressor is used for compression to a pulse duration of about 406 fs. Second harmonic generation is
then achieved in a 500 μm thick BBO crystal. The conversion efficiency of the second harmonic generation remained
almost constant at >60 % for all repetition rates and average power levels. At 5.25 MHz the highest average power of
135 W at 520 nm was achieved. In addition this comes with an excellent beam quality which is validated by a measured
M2 < 1.2.
We present a high peak and average power laser system with ultrashort pulses at high repetition rates. Pulse shortening
and peak power enhancement of a state-of-the-art fiber laser system is achieved by utilizing nonlinearity, namely selfphase
modulation and subsequent compression in a chirped mirror compressor. The nonlinear interaction is achieved by
propagation in a noble gas filled hollow core fiber with an inner diameter of 200 μm and a length of 0.5 m. A total
second dispersion of -7000 fs2 is applied by a chirped mirror compressor resulting in ultrashort pulses of 71 fs duration.
This is achieved by coupling 400 MW, 800 fs pulses from the CPA system to the Xenon filled hollow core fiber. The
average power at the output of the compressor is measured to be 10 W at 50 kHz repetition rate resulting in 200 μJ pulse
energy. Hence, the compressed pulses have a peak power of more than 2 GW. Consequently, the pulses of the CPA
system are shortened by a factor of ten and the peak power is enhanced by a factor of 5. In addition this approach offers
further peak and average power scalability.
We report on an ytterbium-doped fiber CPA system delivering 325 W of average power at 40 MHz repetition rate
corresponding to 8.2 μJ pulse energy. The pulse duration is as short as 375 fs resulting in 22 MW of peak power.
We present a degenerated-parametric amplifier with gigawatt peak power operating at 1030 nm and 30 kHz repetition
rate. Pulses of a fiber chirped pulse amplification (FCPA) system with 650 fs pulse duration and 1 mJ pulse energy are
frequency doubled and used as pump source for a two stage optical parametric amplifier. Both the FCPA and the optical
parametric amplifier (OPA) are seeded by the same YB:KGW oscillator. Spectral broadening of the OPA seed signal in a
short-polarization-maintaining-step-index fiber creates enough bandwidth for sub 30 fs pulse generation, while temporal
synchronization of pump and signal is realized by means of a multipass cell in the OPA signal beam path. Parametric
amplification of the broadband signal takes place in two 1 mm BBO crystals. Pulse compression via chirped mirrors
yields 81 μJ pulses as short as 29 fs. The corresponding pulse peak power is estimated to be as large as 2 GW. Together
with the good beam quality (measured M2<1.8) this device enables high intensity experiments at high repetition rates.
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