Μm Pulses Research Articles

Mid-infrared supercontinuum generation in a fluoroindate fiber with 1.4 W time-averaged power

A mid-infrared supercontinuum generation in a step-index fluoroindate fiber pumped by a Tm-doped fiber amplifier seeded with 1.55 µm pulses is reported for the first time. By pumping a 9 µm core fluoride fiber with 3.7 W of power, a broadband supercontinuum with a total output power of 1.41 W and a spectrum extending to 4.1 µm was demonstrated. It is believed that this is the broadest mid-infrared supercontinuum spectrum with Watt-level time-averaged power generated out of a fluoroindate fiber. The impurities incorporated in the fiber are identified as the main limitation in terms of output power scaling up.

Generation and parametric amplification of broadband chirped pulses in the near-infrared

We demonstrate generation and optical parametric amplification of broadband chirped pulses in the range of 1.8-2.5μm. The setup is built around Ti:sapphire oscillator as a seed source and 1 kHz Nd:YAG laser system as a pump source. Visible broadband seed pulses are temporally stretched and amplified in a non-collinear optical parametric amplifier before being mixed with fundamental harmonic of the pump laser. Difference frequency generation between positively-chirped broadband pulses centered at 0.7μm and non-chirped narrowband pulses at 1064 nm produces negatively-chirped wide spectral bandwidth pulses in the infrared. After subsequent parametric amplification, pulses with more than 0.5 mJ energy were obtained with spectral bandwidth supporting transform-limited pulse durations as short as 23 fs.

Efficient continuous-wave and passively Q-switched pulse laser operations in a diffusion-bonded sapphire/Er:Yb:YAl3(BO3)4/sapphire composite crystal around 1.55 μm.

A composite crystal consisting of a 1.5-mm-thick Er:Yb:YAl3(BO3)4 crystal between two 1.2-mm-thick sapphire crystals was fabricated by the thermal diffusion bonding technique. Compared with a lone Er:Yb:YAl3(BO3)4 crystal measured under the identical experimental conditions, higher laser performances were demonstrated in the sapphire/Er:Yb:YAl3(BO3)4/sapphire composite crystal due to the reduction of the thermal effects. End-pumped by a 976 nm laser diode in a hemispherical cavity, a 1.55 μm continuous-wave laser with a maximum output power of 1.75 W and a slope efficiency of 36% was obtained in the composite crystal when the incident pump power was 6.54 W. Passively Q-switched by a Co2+:MgAl2O4 crystal, a 1.52 μm pulse laser with energy of 10 μJ and repetition frequency of 105 kHz was also realized in the composite crystal. Pulse width was 315 ns. The results show that the sapphire/Er:Yb:YAl3(BO3)4/sapphire composite crystal is an excellent active element for 1.55 μm laser.

Broadband Degenerate Femtosecond OPO around 2060 nm

We demonstrate a broadband degenerate femtosecond optical parametric oscillator (OPO) around 2060 nm. Synchronically pumped by a commercially available 75.6 MHz Yb:KGW femtosecond laser, it is able to generate self-phase-locked 2 μm pulses with 82 mW output power and 3 dB bandwidth of 105 nm. The degenerate operation is naturally stable in free-running condition, with only 0.8% rms fluctuation in 3.5 hours. We also obtain broadband spectrum extending from 1600 nm to 2500 nm with the 3 dB bandwidth of 452 nm by increasing the intracavity power density.

EDM machinability and parametric optimisation of 2014Al/Al<SUB align="right">2O<SUB align="right">3 composite by RSM

The present work investigates the machinability and influence of process parameters on the machining characteristics in EDM of AA2014/Al2O3 composite. The composite was fabricated through stir casting technique. Process parameters considered are current, voltage, pulse on time and wt.% reinforcement for the purpose of optimisation. Machining characteristics viz. MRR, TWR and SR are investigated. Experiments were designed by adopting rotatable centred composite design based on RSM. The machined surface is further analysed by SEM-EDS. Results revealed that MRR, TWR and SR increase with an increase in the current and pulse on time. However, less TWR and good surface finish are obtained at the lower setting of current, pulse on time and optimum setting of voltage and wt.% reinforcement. Performance measures recorded experimentally at optimum values of process parameters, i.e., 12 A current, 60 V voltage, 60 µm pulse on time and 5 wt.% Al2O3/AA2014 reinforcement are 0.1501 g/min MRR, 0.0032 g/min TWR and 9.367 µm SR.

EDM machinability and parametric optimisation of 2014Al/Al<SUB align="right">2O<SUB align="right">3 composite by RSM

The present work investigates the machinability and influence of process parameters on the machining characteristics in EDM of AA2014/Al2O3 composite. The composite was fabricated through stir casting technique. Process parameters considered are current, voltage, pulse on time and wt.% reinforcement for the purpose of optimisation. Machining characteristics viz. MRR, TWR and SR are investigated. Experiments were designed by adopting rotatable centred composite design based on RSM. The machined surface is further analysed by SEM-EDS. Results revealed that MRR, TWR and SR increase with an increase in the current and pulse on time. However, less TWR and good surface finish are obtained at the lower setting of current, pulse on time and optimum setting of voltage and wt.% reinforcement. Performance measures recorded experimentally at optimum values of process parameters, i.e., 12 A current, 60 V voltage, 60 µm pulse on time and 5 wt.% Al2O3/AA2014 reinforcement are 0.1501 g/min MRR, 0.0032 g/min TWR and 9.367 µm SR.

Generation of high-energy narrowband 205 μm pulses for seeding a Ho:YLF laser

We experimentally demonstrate efficient generation of high-energy (82 μJ) narrowband 2.05 μm pulses pumped with 1 mJ broadband Ti:sapphire laser pulses, utilizing dual-chirped optical parametric amplification (DC-OPA) in a BBO crystal. The narrowband 2.05 μm pulses will be primarily used for seeding an Ho:YLF laser, which solves the synchronization issue when Ti:sapphire and Ho:YLF lasers are needed for developing midinfrared lasers. The narrowband 2.05 μm pulse from the unique DC-OPA design can seed the Ho:YLF laser much more efficiently than the broadband 2.05 μm pulse from traditional OPA technology.

Cascaded gain-switching in the mid-infrared region

In this report, we demonstrate mid-infrared dual-waveband (i.e., ~3 μm and ~2 μm) pulses from a cascaded gain-switched Ho3+-doped ZBLAN fiber laser by the use of hybrid pumping of 1150 nm CW and pulse LDs for the first time. Stable ~3 μm gain-switched pulses with the maximum output power 262.14 mW and shortest pulse duration of 0.824 μs were first gained at the repetition rate of 80 kHz and wavelength of 2928.5 nm. Then stable ~2 μm gain-switched pulses at 2068 nm were achieved at a switchable repetition rate between 40 kHz and 80 kHz. The maximum output power and shortest pulse duration were 75.23 mW and 0.787 μs, respectively (not simultaneously). Between them, there is a power-dependent μs-order time delay. This dual-waveband laser source has great potential in laser surgery, material processing.

Ultraviolet out-of-band radiation studies in laser tin plasma sources

Out-of-band long wavelength emission measurements from high power, high-repetition-rate extreme-ultra-violet lithography (EUVL) laser plasma sources are imperative to estimating heat deposition in EUV mirrors, and the impact of short wavelength light transported through the imaging system to the wafer surface. This paper reports a series of experiments conducted to measure the absolute spectral irradiances of laser-plasmas produced from planar tin targets over the wavelength region of 124 to 164 nm by 1.06 μm wavelength, 10 ns full-width-at-half-maximum Gaussian laser pulses. The use of spherical targets is relevant to the EUVL source scenario. Although plasmas produced from planar surfaces evolve differently, there is a close similarity to the evolution of current from 10.6 μm CO2 laser EUVL sources, which use a pre-pulse from a lower energy solid-state laser to melt and reform an initial spherical droplet into a thin planar disc target. The maximum of radiation conversion efficiency in the 124–164 nm wavelength band (1%/2πsr) occurs at the laser intensity of 1010 W cm−2. A developed collisional-radiative model reveals the strong experimental spectra that originate mainly from the 4d105p2-4d105s5p, 4d105p-4d105s resonance lines, and 4d95p-4d95s unresolved transition arrays from Sn III, Sn IV, and Sn V ions, respectively. The calculated conversion efficiencies using a 2D radiation-hydrodynamics model are in agreement with the measurements. The model predicts the out-of-band (100–400 nm) radiation conversion efficiencies generated by both 1.06 and 10.6 μm pulses. The 10.6 μm laser pulse produces a higher conversion efficiency (12%/2πsr) at the lower laser intensity of 109 W cm−2.

Carrier-envelope phase stable, 5.4 μJ, broadband, mid-infrared pulse generation from a 1-ps, Yb:YAG thin-disk laser.

We report on a simple scheme to generate broadband, μJ pulses centered at 2.1 μm with an intrinsic carrier-envelope phase (CEP) stability from the output of a Yb:YAG regenerative amplifier delivering 1-ps pulses with randomly varying CEP. To the best of our knowledge, the reported system has the highest optical-to-optical efficiency for converting 1-ps, 1 μm pulses to CEP stable, broadband, 2.1 μm pulses. The generated coherent light carries an energy of 5.4 μJ, at 5 kHz repetition rate, that can be scaled to higher energy or power by using a suitable front end, if required. The system is ideally suited for seeding broadband parametric amplifiers and multichannel synthesizers pumped by picosecond Yb-doped amplifiers, obviating the need for active timing synchronization. Alternatively, this scheme can be combined with high-power oscillators with tens of μJ energy to generate CEP stable, multioctave supercontinua, suitable for field-resolved and time-resolved spectroscopy.

Towards Terawatt Sub-Cycle Long-Wave Infrared Pulses via Chirped Optical Parametric Amplification and Indirect Pulse Shaping

We present an approach for both efficient generation and amplification of 4–12 μm pulses by tailoring the phase matching of the nonlinear crystal Zinc Germanium Phosphide (ZGP) in a narrowband-pumped optical parametric chirped pulse amplifier (OPCPA) and a broadband-pumped dual-chirped optical parametric amplifier (DC-OPA), respectively. Preliminary experimental results are obtained for generating 1.8–4.2 μm super broadband spectra, which can be used to seed both the signal of the OPCPA and the pump of the DC-OPA. The theoretical pump-to-idler conversion efficiency reaches 27% in the DC-OPA pumped by a chirped broadband Cr2+:ZnSe/ZnS laser, enabling the generation of Terawatt-level 4–12 μm pulses with an available large-aperture ZGP. Furthermore, the 4–12 μm idler pulses can be compressed to sub-cycle pulses by compensating the tailored positive chirp of the idler pulses using the bulk compressor NaCl, and by indirectly controlling the higher-order idler phase through tuning the signal (2.4–4.0 μm) phase with a commercially available acousto-optic programmable dispersive filter (AOPDF). A similar approach is also described for generating high-energy 4–12 μm sub-cycle pulses via OPCPA pumped by a 2 μm Ho:YLF laser.

Near- and mid-IR ultrashort laser pulse filamentation in a molecular atmosphere: a comparative analysis

Self-focusing and filamentation of high-power pulsed laser radiation along a spectrally selective absorbing air path are numerically simulated in three spectral ranges centered at 0.8, 3.9, and 10.6 μm. A detailed comparative analysis of the main pulse parameters in the filamentation area is performed to examine the potential of various laser sources for atmospheric optics. We found that the radiation of mid-IR lasers (3.9 and 10.6 μm) forms the longest filamentation region and the widest supercontinual spectrum in comparison with near-IR radiation. Filamentation of the 3.9 μm laser pulse results in the best spatially continued plasma channel, while the 10.6 μm pulse retains the widest spectral composition under the conditions of strong molecular absorption in the atmosphere.

Spatio-temporal characterization of intense few-cycle 2 μm pulses.

We present a variant of spatially encoded spectral shearing interferometry for measuring two-dimensional spatio-temporal slices of few-cycle pulses centered around 2 μm. We demonstrate experimentally that the device accurately retrieves the pulse-front tilt caused by angular dispersion of two-cycle pulses. We then use the technique to characterize 500-650 μJ pulses from a hollow fiber pulse compressor, with durations as short as 7.1 fs (1.3 optical cycles).

1 J, 0.5 kHz repetition rate picosecond laser.

We report the demonstration of a diode-pumped chirped pulse amplification Yb:YAG laser that produces λ=1.03 μm pulses of up to 1.5 J energy compressible to sub-5 ps duration at a repetition rate of 500 Hz (750 W average power). Amplification to high energy takes place in cryogenically cooled Yb:YAG active mirrors designed for kilowatt average power laser operation. This compact laser system will enable new advances in high-average-power ultrashort-pulse lasers and high-repetition-rate tabletop soft x-ray lasers. As a first application, the laser was used to pump a 400 Hz λ=18.9 nm laser.

Bandwidth control in 5 μm pulse generation by dual-chirped optical parametric amplification

We extend the technique of difference-frequency mixing of broadband chirped near-infrared (IR) pulses to longer wavelengths, thereby generating high-energy, bandwidth-tunable laser pulses in the mid-IR spectral region. Numerical modeling has been performed to examine the influence of pump and seed chirp on the generated idler bandwidth and pulse duration. The amplifier is realized using zinc-germanium phosphide crystals pumped by a 2 μm ultrafast parametric source. Idler pulses at a center wavelength of 5 μm with adjustable bandwidth are produced by equally chirping the pump and seed pulses using material dispersion. Using this simple approach, efficient generation of narrowband, short, mid-IR laser pulses is achieved without the use of high-loss dispersive gratings or prism pairs.

Photoconductivity of Er-doped InAs quantum dots embedded in strain-relaxed InGaAs layers with 1.5 µm cw and pulse excitation

We fabricated a photoconductive antenna structure utilizing Er-doped InAs quantum dot layers embedded in strain-relaxed In0.35Ga0.65As layers on a GaAs substrate. Mesa-shaped electrodes for the antenna structure were formed by photolithography and wet etching in order to suppress its dark current. We measured the photocurrent with the excitation of ∼1.5 µm cw and femtosecond pulse lasers. Compared with the dark current, the photocurrent was clearly observed under both cw and pulse excitation conditions and almost linearly increased with increasing excitation power in a wide range of magnitudes from 10 W/cm2 to 10 MW/cm2 order.

2D IR spectroscopy at 100 kHz utilizing a Mid-IR OPCPA laser source.

We present a 100 kHz 2D IR spectrometer. The system utilizes a ytterbium all normal dispersion fiber oscillator as a common source for the pump and seed beams of a MgO:PPLN OPCPA. The 1030 nm OPCPA pump is generated by amplification of the oscillator in cryocooled Yb:YAG amplifiers, while the 1.68 μm seed is generated in a OPO pumped by the oscillator. The OPCPA outputs are used in a ZGP DFG stage to generate 4.65 μm pulses. A mid-IR pulse shaper delivers pulse pairs to a 2D IR spectrometer allowing for data collection at 100 kHz.

Terahertz time-domain-spectroscopy system based on 1.55 μm fiber laser and photoconductive antennas from dilute bismides

We describe a terahertz time-domain-spectroscopy system that is based on photoconductive components fabricated from (GaIn)(AsBi) epitaxial layers and activated by femtosecond 1.55 μm pulses emitted by an Er-doped fiber laser. (GaIn)(AsBi) alloy grown on GaAs substrates contained 12.5%In and 8.5%Bi – a composition corresponding to a symmetrical approach of the conduction and valence band edges to each other. The layers were photosensitive to 1.55 μm wavelength radiation, had relatively large resistivities, and subpicosecond carrier lifetimes – a set of material parameters necessary for fabrication of efficient ultrafast photoconductor devices. The frequency limit of this system was 4.5 THz, its signal-to-noise ratio 65 dB. These parameters were comparable to their typical values for much bulkier solid-state laser based systems.

High-average-power, 50-fs parametric amplifier front-end at 1.55 μm.

An average-power-scalable, two-stage optical parametric chirped pulse amplifier is presented providing 90-μJ signal pulses at 1.55 μm and 45-μJ idler pulses at 3.1 μm at a repetition rate of 100 kHz. The signal pulses were recompressible to within a few percent of their ~50-fs Fourier limit in anti-reflection coated fused silica at negligible losses. The overall energy conversion efficiency from the 1030-nm pump to the recompressed signal reached 19%, significantly reducing the cost per watt of pump power compared to similar systems. The two-stage source will serve as the front-end of a three-stage system permitting the development of novel experimental strategies towards laser-based imaging of molecular structures and chemical reactivity.

Monostatic all-fiber scanning LADAR system.

A compact scanning LADAR system based on a fiber-coupled, monostatic configuration which transmits (TX) and receives (RX) through the same aperture has been developed. A small piezo-electric stripe actuator was used to resonantly vibrate a fiber cantilever tip and scan the transmitted near-single-mode optical beam and the cladding mode receiving aperture. When compared to conventional bi-static systems with polygon, galvo, or Risley-prism beam scanners, the described system offers several advantages: the inherent alignment of the receiver field-of-view (FOV) relative to the TX beam angle, small size and weight, and power efficiency. Optical alignment of the system was maintained at all ranges since there is no parallax between the TX beam and the receiver FOV. A position-sensing detector (PSD) was used to sense the instantaneous fiber tip position. The Si PSD operated in a two-photon absorption mode to detect the transmitted 1.5 μm pulses. The prototype system collected 50,000 points per second with a 6° full scan angle and a 27 mm clear aperture/40 mm focal length TX/RX lens, had a range precision of 4.7 mm, and was operated at a maximum range of 26 m.