Laser ranging and burst illumination imaging (BIL) in the atmospheric transmission window at 1.5μm are made difficult by speckle effects which are observed when a rough surface is illuminated by a laser beam with a coherence length greater than the characteristic surface feature size. For the narrowband pulsed lasers currently used, this length is of the order of a few millimetres which leads to observable speckle effects for many common surfaces. In this context we describe progress towards the development a short-coherence length laser source operating at 1.5μm and based on optical parametric amplification of broadband seed pulses from a modelocked femtosecond erbium-doped fibre laser.
Our optical parametric amplifier (OPA) system comprises a compact actively Q-switched 1047nm Nd:YLF laser, operating at ~1kHz repetition frequency, to which a 54MHz femtosecond 1.55μm Er:fibre laser is synchronised. The fibre laser produces bandwidth-limited 100fs pulses which are stretched by chromatic dispersion in a spool of SMF28 fibre to match the 3.5ns duration of the Q-switched pulses. Pulses from the Nd:YLF and Er:fibre lasers act as the pump and seed respectively for an OPA based on an aperiodically-poled crystal of MgO:PPLN containing a single linearly chirped grating. The chirp grating enables broadband parametric amplification across a wavelength range comparable with the spectral bandwidth of the seed pulses, amounting to ~150nm in the wings of the spectrum. Early results from this system have demonstrated output energies of 2.55μJ and a single-pass gain ~51dB and are expected to be increased with continued development of the project.
Conventionally optical parametric oscillators (OPOs) have been used in high-resolution absorption-spectroscopy as narrow-band tuneable sources where the measurement resolution is determined by the OPO output linewidth, rather than the wavelength resolution of the detector. In contrast, the absorption spectroscopy of gases and other media has for many years been carried out using instruments such as Fourier-transform infrared (FTIR) spectrometers or high-resolution diffraction-grating-based tuneable monochromators. These techniques commonly utilise broadband thermal sources with highly-divergent illumination beams limiting their use in remote sensing or fibre delivery applications.
The work presented here reports a new approach to FTIR spectroscopy based around a novel Ti:sapphire pumped, signal-resonant OPO that uses a 10mm crystal of aperiodically-poled lithium niobate (APPLN) as the gain medium producing an idler output covering a 3.2-3.85μm tuning range with a typical full-width-half-maximum bandwidth of 85nm. Methane was used to demonstrate the technique since the OPO tuning range almost completely covers the strongest mid-infrared absorption lines of methane from 3.0-3.7μm (limited only by the available resonator optics). A double-beam Michelson interferometer was built around the OPO idler beam using a helium-neon laser as the second beam to self-calibrate each trace. Course tuning of the OPO resulted in the measurement of absorption data across the 3.2-3.85μm tuning range using methane held at pressures ranging from 2000mbar down to 25mbar. A maximum resolution of around 1cm-1 was achieved using a simple rapidly scanning mirror assembly indicating that with further development this approach could yield very high-resolution measurements.
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