Mid-infrared optical coherence tomography with a stabilized OP-GaP optical parametric oscillator.

We demonstrate mid-infrared time-domain optical coherence tomography (OCT) with an orientation-patterned GaP optical parametric oscillator. Instantaneous broadband mid-infrared spectra provide reduced scattering for OCT applications including cultural heritage, quality assurance, and security. B-scan calibrations performed across the wavelength tuning range show depth resolutions of 67 µm at 5.1 µm and 88 µm at 10.5 µm. Volumetric imaging inside a plastic bank card is demonstrated at 5.1 µm, with a 1 Hz A-scan rate that indicates the potential of stable broadband OPO sources to contribute to mid-infrared OCT.

Automated control and stabilization of ultrabroadband laser pulse angular dispersion.

We present an innovative automatic control of angular dispersion for high-power laser systems. A novel, to the best of our knowledge, diagnostic has been developed to visualize angular dispersion in ultrashort near-infrared laser pulses for on-shot analysis. The output of a commercial ultrabroadband oscillator was prepared with an arbitrary chromatic dispersion and sent through a compensation system composed of 4° glass wedges in motorized mounts. These wedges were rotationally controlled in discrete steps about the beam axis in accordance with the diagnostic, via an automated feedback loop, to successfully eliminate angular dispersion to a precision of 5 nrad/nm. The system can be implemented to maintain a zero or nonzero target dispersion for experiments.

Multi-target two-photon dual-comb LiDAR.

By substituting two-photon cross-correlation in a wide-bandgap photodiode for the coherent gating conventionally used in dual-comb ranging, two-photon dual-comb LiDAR exchanges data-intensive interferometric acquisition for a single time-stamp from which an absolute distance can be inferred. Here, we report the application of two-photon dual-comb LiDAR to obtain real-time ranging to three independent targets with only a single silicon-photodiode detector. We show precisions of 197-255 nm (2 seconds averaging time) for static targets, and real-time simultaneous ranging to three dynamic targets driven by independent sinusoidal, saw-tooth and square waveforms. Finally, we demonstrate multi-target ranging to three points on a rigid body to provide simultaneous pitch and yaw angular measurements with precisions of 27.1 arcsec (130 µrad) on a 25 mm baseline.

GPU-accelerated full-field modelling of highly dispersive ultrafast optical parametric oscillators.

We demonstrate GPU-accelerated modelling of ultrafast optical parametric oscillators (OPOs) via the χ(2) nonlinear envelope equation with 1265× improvement in execution time compared with a CPU-based approach. Incorporating an adaptive step-size algorithm and absorbing boundary conditions, our model is capable of simulating OPOs containing long (>10 mm) nonlinear crystals or significant intracavity dispersion with outputs generated in less than 1 minute, allowing the investigation of systems that were previously computationally prohibitive to explore. We implement real-world parameters such as optical coatings, material absorption, and non-ideal poling domains within quasi-phase matched nonlinear crystals, producing excellent agreement with the spectral tuning behaviour and average power from a previously reported prism-based OPO. Our digital twinning approach provides a low-cost iterative development platform for ultrafast OPOs.

Misalignment-free, Kerr-lens-modelocked Yb:Y2O3 2.2-GHz oscillator, amplified by a semiconductor optical amplifier.

We present a fully bonded, misalignment-free, diode-pumped Yb:ceramic (Yb:Y2O3) oscillator producing 190-fs pulses at a repetition frequency of 2.185 GHz. Self-starting Kerr-lens-modelocked operation was obtained from both outputs of the ring cavity with an average combined output power of 14-30 mW for pump powers from 380-670 mW. The fully bonded design provided self-starting, turnkey operation, with a relative intensity noise of 0.025% from 1 Hz-1 MHz. Tuning of the pulse repetition rate over a 120 kHz range was demonstrated for a 2°C change in temperature. Chirped-pulse amplification in a semiconductor optical amplifier was shown to increase the pulse average power to 69 mW and the pulse energy (peak power) from 2.5 pJ (12 W) to 32 pJ (71 W).

Fiber-delivered heterodyne spectroscopy with a mid-infrared frequency comb.

By exploiting the excellent short-term phase stability between consecutive pulses from a free-running optical parametric oscillator frequency comb, we report the first example of hollow-core fiber-delivered heterodyne spectroscopy in the 3.1-3.8 µm wavelength range. The technique provides a means of spectroscopically interrogating a sample situated at the distal end of a fiber, with all electronics and light sources situated at the proximal end and with an inherent capability to suppress spectroscopically interfering features present in the free-space and in-fiber delivery path. Using a silica anti-resonant, hollow-core delivery fiber, we demonstrate high quality transmission and attenuated total reflectance spectroscopy of a plastic sample for fiber lengths of up to 40 m, significantly exceeding the few-meter lengths typically possible using solid-core fibers. The technique opens a route to implementing multi-species spectroscopic monitoring in remote and / or hostile industrial environments and medical applications.

Feed-forward stabilization of a single-frequency, diode-pumped Pr:YLF-Cr:LiCAF laser operating at 813.42†nm.

We introduce a simple and compact diode-pumped Pr:YLF-Cr:LiCAF laser, operating at 813.42 nm and providing a 130-mW, single-frequency output tunable over a 3-GHz range. The laser has a short-term intrinsic linewidth estimated to be 700 Hz (ß-separation method), while exhibiting a free-running wavelength stability of below 1 pm in one hour. Using a feed-forward technique we demonstrate the integration of the laser output into a fully stabilized, 1-GHz Ti:sapphire laser frequency comb, resulting in a heterodyne beat note between the laser and the comb with a bandwidth of 65 kHz. Combining feed-forward control with a low-bandwidth servo feedback loop permits stable long-term locking with an rms beat note variation of 15 kHz over 2 minutes. This performance makes the laser a potential candidate for the lattice laser in a 87Sr optical lattice clock.

Three-element, self-starting Kerr-lens-modelocked 1-GHz Ti:sapphire oscillator pumped by a single laser diode.

We present a Kerr-lens-modelocked, three-element, diode-pumped Ti:sapphire laser producing 111-fs pulses at a repetition frequency of 1.02 GHz. Self-starting soliton-modelocked operation with an output power of 106 mW was obtained when the laser was pumped at 1.0 W with a single 527-nm laser diode. The output exhibits a relative intensity noise of 0.06% (1 Hz - 1 MHz) and locking of the repetition rate to an external reference is demonstrated with a phase error of 1.7 mrad (1 Hz-1 MHz). The simplicity of the laser makes it an attractive candidate as a module for integration into larger systems.

Compressive hyperspectral imaging in the molecular fingerprint band.

Spectrally-resolved imaging provides a spectrum for each pixel of an image that, in the mid-infrared, can enable its chemical composition to be mapped by exploiting the correlation between spectroscopic features and specific molecular groups. The compatibility of Fourier-transform interferometry with full-field imaging makes it the spectroscopic method of choice, but Nyquist-limited fringe sampling restricts the increments of the interferometer arm length to no more than a few microns, making the acquisition time-consuming. Here, we demonstrate a compressive hyperspectral imaging strategy that combines non-uniform sampling and a smoothness-promoting prior to acquire data at 15% of the Nyquist rate, providing a significant acquisition-rate improvement over state-of-the-art techniques. By illuminating test objects with a sequence of suitably designed light spectra, we demonstrate compressive hyperspectral imaging across the 700-1400 cm-1 region in transmission mode. A post-processing analysis of the resulting hyperspectral images shows the potential of the method for efficient non-destructive classification of different materials on painted cultural heritage.

Hollow-core fiber delivery of broadband mid-infrared light for remote spectroscopy.

High-resolution multi-species spectroscopy is achieved by delivering broadband 3-4-µm mid-infrared light through a 4.5-meter-long silica-based hollow-core optical fiber. Absorptions from H37Cl, H35Cl, H2O and CH4 present in the gas within the fiber core are observed, and the corresponding gas concentrations are obtained to 5-ppb precision using a high-resolution Fourier-transform spectrometer and a full-spectrum multi-species fitting algorithm. We show that by fully fitting the narrow absorption features of these light molecules their contributions can be nulled, enabling further spectroscopy of C3H6O and C3H8O contained in a Herriott cell after the fiber. As a demonstration of the potential to extend fiber-delivered broadband mid-infrared spectroscopy to significant distances, we present a high-resolution characterization of the transmission of a 63-meter length of hollow-core fiber, fully fitting the input and output spectra to obtain the intra-fiber gas concentrations. We show that, despite the fiber not having been purged, useful spectroscopic windows are still preserved which have the potential to enable hydrocarbon spectroscopy at the distal end of fibers with lengths of tens or even hundreds of meters.