We employed an HgGa2S4 nonlinear crystal to achieve the highest average power from a high-repetition rate mid-infrared femtosecond optical parametric oscillator (OPO), synchronously pumped near 1 μm. The continuous tuning of the nonresonated idler extended from below 4.4 to beyond 12 μm. The OPO was pumped by a 6.5-W, 1.035-μm, 81-MHz, 220-fs Yb-fiber laser. The idler output reached an average power of over 355 mW at 5.2 μm (at a signal power of > 2 W at 1.25 μm). The peak quantum efficiency of the OPO approached 40%. The signal output was diffraction limited with M2 < 1.1. No crystal damage was observed at average pump intensities as high as 60 kW/cm2.
We employed mercury thiogallate (HgGa2S4 or HGS) to achieve the highest average output power from a 1 µm synchronously pumped optical parametric oscillator (SPOPO) generating femtosecond idler pulses continuously tunable in the mid-IR. HGS is one of the most promising wide band-gap non-oxide nonlinear crystals without two-photon absorption (TPA) losses at such pump wavelengths. The SPOPO was pumped by an 80 MHz mode-locked Yb-fiber laser with 6.5 W of average power at 1.035 µm and a pulse duration of 220 fs. Continuous tuning of the non-resonated idler was achieved from 4.4 to beyond 12 µm with an average output power of over 350 mW at 5.2 µm (>2 W at 1.25 µm for the signal) and tens of mW at the edges of the idler tuning range (hundreds of mW for the signal). The quantum efficiency of the device exceeded 20 % up to 8 µm (> 30% in the maximum). The output of the OPO was diffraction limited with a beam quality factor (M²) below 1.1. No crystal damage was observed at average power densities as high as 60 kW / cm² although using lower power densities was necessary to achieve the best possible beam quality. The only other crystals that were successfully employed in lower power SPOPOs in the past, CdSiP2 (CSP) and OPGaP, lack either the tuning range due to limited mid-IR transparency or the capability for continuous tuning.
We present an efficient coherent source widely tunable in the mid-infrared spectral range consisting of a commercial picosecond Yb-fiber laser operating at 80 MHz repetition rate, a synchronously-pumped OPO (SPOPO) and differencefrequency generation (DFG) in AgGaSe2. With an average input pump power of 7.8 W at 1032 nm and at 80 MHz, the SPOPO outputs are tunable from 1380 to 1980 nm (Signal) and from 2.1 to ~4 μm (Idler) with pulse durations between 2.1 and 2.6 ps over the entire tuning range. After temporally overlapping Signal and Idler through a delay line, the two beams are spatially recombined with a dichroic mirror (reflecting for the Signal in s-polarization and transmitting for the Idler in p-polarization), and focused by a 150 mm CaF2 lens to a common focus. For DFG we employ an AR-coated 10- mm thick AgGaSe2 nonlinear crystal cut for type-I interaction at θ =52°. The generated mid-infrared picosecond pulses are continuously tunable between 5 and 18 μm with average power up to 130 mW at 6 μm and more than 1 mW at 18 μm. Their spectra and autocorrelation traces are measured up to 15 μm and 11 μm, respectively, and indicate that the input spectral bandwidth and pulse duration are maintained to a great extent in the nonlinear frequency conversion processes. The pulse duration slightly decreases from 2.1 to 1.9 ps at 6.7 μm while the spectral bandwidth supports ~1.5 ps (~10 cm-1)durations across the entire mid-infrared tuning range. For the first time narrow-band mid-infrared pulses with energy exceeding 1 nJ are generated at such high repetition rates.
In narrow-bandwidth coherent Raman scattering (CRS) microscopy, efficient signal generation is accomplished with two-color laser sources providing synchronized picosecond pulses whose frequency difference and spectral widths match the molecular Raman frequency and bandwidth, respectively. With vibrational bandwidths of typically 10 cm-1, the optimum laser pulse durations thus correspond to about 2 ps. Here, we present a new light source consisting of an amplified Yb-fiber oscillator providing 2-ps pulses at 1031 nm and a synchronously green-pumped optical parametric oscillator (OPO). The OPO slightly shortens the pulses to < 2 ps while maintaining a bandwidth of 10 cm-1. Output power levels of 1 W in both the 1031-nm and the OPO-branch with continuously tunable frequency differences between the two beams covering a broad range from 700 to 4500 cm-1 are achieved. In addition to CARS microscopy, this light source allows for SRS imaging via an integrated electro-optical modulation of the 1031-nm beam at 20 MHz with a depth of >95%, locked to the laser repetition rate of 80 MHz. The OPO noise at 20 MHz was found to be only 60% above the combined detector and laser noise of a conventional Nd:YVO pump source. This represents a significant reduction in laser noise when compared to other fiber-based laser sources previously proposed for SRS microscopy. When SRS imaging with this new light is compared with a Nd:YVO pumped OPO (delivering 7 ps and 5 ps pulses, respectively), a 5- to 6-fold increase in SRS signal strength and signal-to-noise ratio has been achieved. Video-rate SRS and the capability of multi-spectral SRS imaging are demonstrated.
We present an efficient coherent source widely tunable in the mid-infrared (mid-IR) spectral range consisting of a novel femtosecond Yb-fiber laser operating at ~50 MHz repetition rate, a synchronously-pumped OPO (SPOPO) and difference-frequency generation (DFG) in AgGaSe2. With an average input power of 5 W for ~260 fs pump pulses at 1034 nm, the SPOPO outputs are tunable from ~1710 to 1950 nm (signal) and from 2200 to 2600 nm (idler) with pulse durations between 200 and 250 fs over the entire tuning range. After temporally overlapping signal and idler through a delay line, the two beams are spatially recombined with a dichroic mirror and focused to a beam diameter of ~75 μm. For DFG we employ an uncoated 2-mm-thick AgGaSe2 nonlinear crystal cut for type-I interaction at θ=57°. The generated femtosecond mid-IR pulses are continuously tunable between 5 and 17 μm with average power of up to 69 mW at 6 μm and more than 1 mW at 17 μm. Their spectra and autocorrelation traces are measured up to 12 μm and 8 μm, respectively, and indicate that the input spectral bandwidth and pulse duration are maintained to a great extent in the nonlinear frequency conversion processes. The DFG pulse width measured at 7.2 μm amounts to ~300 fs (FWHM). The measured spectral bandwidth supports ~150 fs Gaussian pulse durations across the entire DFG tuning range. For the first time mid-IR pulses with energy exceeding 1 nJ are generated at such high repetition rates.
We investigated optical damage (surface and bulk) in one of the most promising wide bandgap nonoxide nonlinear crystals, HgGa 2 S 4 , that can be used in ∼1 -μm pumped optical parametric oscillators (OPOs) and synchronously pumped OPOs (SPOPOs) for generation of idler pulses above 4 μm without two-photon absorption losses at the pump wavelength. The optical damage has been characterized at the pump wavelength for different repetition rates using uncoated and antireflection-coated (mainly with a single layer for pump and signal wavelengths) samples. HgGa 2 S 4 is the most successful nonlinear crystal (both in terms of output energy and average power) for such OPOs, but optical damage inside the OPO has a lower threshold and represents at present the principal limitation for the achievable output. It is related to peak pulse and not to average intensity, and bulk damage in the form of scattering centers occurs before surface damage. Such bulk damage formation is faster at higher repetition rates. Lower repetition rates increase the lifetime of the crystal but do not solve the problem. The safe pump fluence in extracavity measurements is <1 J/cm 2 , which corresponds to ∼100 MW/cm 2 for the 8-ns pulse duration (both values peak on-axis). In the OPO, however, peak on-axis fluence should not exceed 0.3 J/cm 2 limited by the formation of bulk scattering centers in orange-phase HgGa 2 S 4 . In the nanosecond OPO regime, the damage resistivity of Cd-doped HgGa 2 S 4 is higher and that of the almost colorless CdGa 2 S 4 is roughly two times higher, but the latter has no sufficient birefringence for phase-matching. In SPOPOs operating in the ∼100 MHz regime, the damage limitations are related both to the peak pulse and the average intensities, but here HgGa 2 S 4 seems the best nonoxide candidate to obtain first steady-state operation with Yb-based mode-locked laser pump sources.
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