We investigated in-band pumping of Tm,Ho,Lu:CaGdAlO4 (CALGO) using a Raman shifted Er-fiber laser (1678 nm) in the continuous-wave (CW) and mode-locked (ML) regimes. The 6-mm long, antireflection-coated, a-cut CALGO was doped with 4.48at.% Tm (sensitizer), 0.54at.% Ho (emission) and 5.51at.% Lu (compositional disorder). For mode-locking we employed a GaSb SESAM and chirped mirrors (round-trip group-delay dispersion: -1250 fs2). Pumping with 5.5 W (unpolarized), the average output power (0.2% output coupler) was 148 mW at ⁓96 MHz. The spectrum was centered at 2071.5 nm with a FWHM of 21.5 nm (sigma-polarization) and the pulse duration was 218 fs (time-bandwidth product: 0.327).
We demonstrate that thick (3-mm) Periodically-Poled LiNbO3 (PPLN) enables energy scaling of a non-resonant optical parametric oscillator (NRO) operated in the narrowband mode with a Volume Bragg grating (VBG) at the signal wavelength. Utilizing the full available pump power at 1064 nm we obtained maximum average powers of 2.25 and 2.08 W for the signal (1822 nm) and idler (2383 nm) at 10 kHz, at a conversion efficiency of 32.8%, i.e. a two-fold increase in terms of pulse energies. The signal and idler linewidths were ⁓1 nm, the pulse lengths ⁓6 ns and the idler beam propagation factor ⁓5.
By combining high beam quality picosecond pulsed optical parametric amplifiers at 2.94 μm with Rapid Evaportive Ionisation Mass Spectrometry (REIMS), we have demonstrated record spatial pixel resolutions for ambient mass spectrometry (MS) imaging of < 10 μm. In this contribution, we introduce our work in this area, demonstrating the platform workflow and highlighting recent results of metabolic imaging at the single cell resolution level.
We present a nanosecond-pulsed 655 nm laser source based on frequency-doubling a Raman-shifted fiber laser. At a repetition rate of 1.5 MHz, the source generates an average power of 3.3 W, corresponding to a pulse energy of 2.2 μJ, with a pulse duration of 1.8 ns. The fundamental Raman-shifted fiber laser operating at 1310 nm has a novel configuration where the first Raman shift is performed in an Yb-doped fiber amplifier and the second Raman shift is performed in a phosphosilicate fiber. Both Raman shifting stages are seeded with narrow linewidth CW signals, enabling the temporal properties of an amplified 1064 nm modulated laser diode to be transferred to narrow-band light at 1310 nm with very high conversion efficiency. The resulting micro-Joule-level, nanosecond pulses at 1310 nm are frequency-doubled to 655 nm in a double LBO crystal setup with a conversion efficiency of 51%. The multi-Watt, micro-Joule-level red pulses have near diffraction limited beam quality (M2 ≤ 1.04), making this source ideally suited to biomedical imaging applications such as super-resolution and photoacoustic microscopy.
We present a novel photonic Crystal Fiber (PCF), designed for degenerate four-wave mixing (FWM), with a Yb-doped core to amplify the FWM pump light via stimulated emission. Using a 1030 nm Q-switched microchip laser as the FWM pump, the generation of anti-Stokes light at 691 nm was enhanced by using a 976 nm CW laser diode to create a population inversion in the Yb-doped core of the PCF, which amplifies the 1030 nm pulses. For a 1030 nm incident average power of 15 mW (4 kW peak power), the 691 nm anti-Stokes power generated increased from 0 to 1.15 mW when the incident 976 nm power was increased from 0 to 287 mW. FWM was not observed for this 1030 nm input power level in a similar length of a conventional PCF with the same phase-matching properties. Hence, we demonstrate that amplification of the FWM pump pulse through stimulated emission boosts the generated anti-Stokes power, providing a promising route to increasing the pump to anti-Stokes conversion efficiency beyond what is possible with non-rare-earth-doped FWM PCFs.
Infrared (IR) imaging is important in many disciplines but is limited by inefficient, noisy and expensive cameras. Nonlinear interferometers (NLI) enable imaging with undetected photons, where correlated visible-IR photon pairs convey information about an object illuminated in the IR but detected by a visible camera. We introduce compact PPLN based Michelson-style NLI sand discuss their operation in the context of a comprehensive model, exploring the influence of internal losses, IR seeding, and parametric gain on interferometer contrast and visibility. We show that NLI performance can be enhanced for samples with low transmission even in the presence of significant experimental losses.
Cadmium silicon phosphide, CdSiP2 (CSP), exhibits the highest d-coefficient (d36 = 85 pm/V) among all practical nonlinear optical crystals. Its large band gap of 2.45 eV allows for 1-micron pumping with widely-available Nd- and Yb-based laser sources, and its dispersion properties are such that a 1-um pump yields non-critically phase-matched temperature-tunable output between 6.2-6.5 um (an attractive range for minimally-invasive laser surgery). However, residual 1-um absorption losses in CSP are not insignificant (0.16-0.2 cm-1). In this work we focused on identifying, and ultimately minimizing, the point defects responsible for these losses by correlating EPR spectra with polarized absorption near 1-um.
We present a nanosecond, non-resonant optical parametric oscillator (NRO) based on a 20 mm long periodically-poled LiNbO3 (PPLN) crystal operating at 30-70 kHz. Pumping with a nanosecond Nd:YVO4 laser at 1064 nm in a double-pass configuration, the signal plus idler average output power reached 1.2 W for a pump level of 4 W (at 35 kHz repetition rate). Narrowband seeding with a Tm-fiber laser is employed to narrow the linewidths of the signal to 0.8 nm and the idler to 2 nm. Continuous-wave seed levels as low as 2 mW were sufficient to produce the effect which means that this technique could be useful for single-frequency operation using DFB seed laser diodes. At higher pump power levels > 4 W, the linewidth narrowing effect produced by the seeding was insufficient to prevent broader linewidth operation of the NRO signal and idler outputs. Pumping the NRO at higher repetition rates to scale the average output power of the NRO whilst remaining in the narrow linewidth operation mode is discussed.
We apply mode transformation to an Yb fiber laser for direct generation of a first order vortex mode (LG01), yielding LG01 power of 5W at 96 % purity (from modal decomposition) with 16W pumping. The laser used standard single mode Yb doped fibers operating at 1064 nm. A free-space Sagnac interferometer formed one reflector of the cavity by feeding back the internal Gaussian mode of the fiber laser and output coupling a LG01 via interferometric mode transformation. It was stable over hours of operation and days of inactivity, and was insensitive to polarisation. The maximum output power was only limited through heating of a optical element, which could be mitigated with thermal management. We also show that additional spiral phase plates (SPPs) are a route to higher purity, higher order vortex modes than with SPPs alone due to improved intensity matching between LG01 and higher order states.
We report a single-cell level resolution (≤10 µm), laser desorption-based mass spectrometry imaging platform. An optical parametric amplifier is used to generate ∼100 ps, 200 nJ pulses at around 3 µm with a maximum repetition rate of 500 kHz. The pulses are tightly focussed on to fresh frozen animal tissue samples with a thickness of 10 µm. Small volumes of tissue are readily ablated by the laser and are subsequently chemically analyzed using a Rapid Evaporative Ionization Mass Spectrometry (REIMS) source installed on a time of flight mass analyzer. Raster scanning the samples through the laser focus enables the acquisition of mass spectrometry data which can be processed into images with pixel size 10 µm without oversampling, corresponding to cellular level resolution.
We report a mid-infrared (MIR) source emitting at 3 μm, employing a novel χ(3)/χ(2) cascaded nonlinear conversion architecture. Picosecond pulses from a 1.064 μm mode-locked Yb:fiber pump laser are used to generate 1.65 μm signal pulses through χ(3) based four-wave mixing in photonic crystal fiber (PCF). The output of the PCF is then directly focused into a periodically poled lithium niobate crystal to generate idler radiation around 3 μm through χ(2) based three-wave mixing between the pump and signal pulses.
We report a CdSiP2 (CSP) based seeded optical parametric generator (OPG), emitting sub-nanosecond duration, 3 MHz repetition rate, wavelength tunable mid-infrared (MIR) light at 4.2-4.6 μm. We generate up to 0.25 W at 4.2 μm with a total pump conversion efficiency of 42%. The OPG is pumped by a 1.24 μm Raman fiber amplifier system. This is the first demonstration of pumping CSP with a Raman fiber source in this region, and we show that Raman fiber sources in the near-infrared (NIR) are ideal pump sources for non-critically phasematched (NCPM) CSP devices. Pumping CSP at 1.24 μm permits the use of NCPM whilst decreasing the negative effects of both two-photon absorption and linear absorption losses, when compared to conventional 1 μm pumping. This offers a potential advantage for MIR power scaling of CSP parametric devices due to a reduced thermal load in the crystal from residual pump absorption. The OPG is seeded with a continuous-wave fiber supercontinuum source emitting radiation in the 1.7 μm region, to lower the threshold pump intensity required for efficient conversion. NCPM and temperature tuning of the crystal allow for simple wavelength tuning of the idler radiation. We report on laser damage induced by elevated crystal temperatures, which we propose is linked to the decrease in CSP bandgap energy with increasing temperature.
PPLN based optical parametric oscillators pumped by high power lasers around 1 µm are well established sources for generating light in the 3-5 µm spectral region, of interest for a wide range of scientific, commercial and military applications. We have been investigating optical parametric amplification (OPA), or difference-frequency generation (DFG), single-pass alternatives to conventional resonant OPOs. This avoids the need for a cavity and the corresponding design constraints that this can impose; such as, fixed repetition rates, sensitive alignment and/or poor output beam qualities at high average power levels. In this paper, we review recent results on high average power ( > 6 W) nanosecond pulse generation in the 3.3-3.5 μm region at MHz repetition rates, employing Yb:fibre and Er:fibre master oscillator power amplifiers (MOPA) systems pumping PPLN OPAs. We use focused Gaussian beam theory to validate the experimental results. We will also discuss spectral extension further into the mid-infrared, using different nonlinear crystal and alternative rare-earth doped fibre MOPA and Raman shifted fibre laser combinations. Ongoing work aimed at the power scaling of the mid-infrared light in both the nanosecond pulsed and continuous wave regimes will be presented.
Second harmonic generation (SHG) is a ubiquitous technique for extending the spectral coverage of laser sources into regions that would otherwise be technologically challenging to access. SHG schemes typically rely on the use of bulk optical components, resulting in systems with large footprints requiring precise optical alignment. Integration of the SHG components into a single unit facilitates the implementation of compact, robust and turn-key sources, suitable for applications in biophotonic imaging, amongst others. We report on the development of fiber-coupled frequency doubling modules and their application to novel fiberintegrated picosecond pulse sources in the visible and near-visible. The modules employ a simple, single-pass configuration using a periodically-poled lithium niobate (PPLN) crystal as the nonlinear conversion medium. They are readily adaptable for different fiber pump laser configurations and are configurable with either fiber-coupled or collimated free-space outputs. Two sources using the modules are presented, operating at 780 nm and 560 nm. The 780 nm source utilizes an erbium master oscillator power fiber amplifier (MOPFA) scheme. SHG was performed in a 35 mm long crystal, generating 3.5 W of 780 nm radiation with a pulse duration of 410 ps at 50 MHz and conversion efficiencies exceeding 20%. Results of this source being used for parametric wavelength conversion in photonic crystal fiber are discussed. The 560 nm source was based on SHG of a Raman amplified CW diode pumped by a pulsed ytterbium-fiber MOPFA. This source generated 450 mW of average power with conversion efficiencies greater than 20%.
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