A prototype mid-infrared spectral imaging system for rapid assessment of cells for cytological diagnosis is reported. Based on a fibre optic super-continuum source that has large spectral brightness and is coupled in to an acousto-optic tuneable filter that can rapidly scan over a set of wavelengths that are chosen to give a high level of selectivity for a specific skin disease. The system has the potential to collect an image cube of 100 wavelengths and 300k pixels in 2 seconds so that cells on living people could be analysed. The system has been evaluated with colon cells over 2700- 3100 cm-1.
We report a singly resonant optical parametric oscillator (SRO) based on a ZnGeP2 crystal directly-pumped by
a lamp-pumped Q-switched CrTmHo:YAG laser. The IR wavelength was tuneable from 4.7 μm to 7.8 μm via
crystal angle tuning. A maximum optical to optical efficiency of 56% was obtained from pump (2.09 μm) to
total IR at a pump energy of 6.5 mJ. The corresponding idler energy was 1.45 mJ. The SRO was measured to
have a slope efficiency of 64% and a threshold of 1 mJ. The spatial beam quality of the idler, characterized by
the M2 parameter, was 1.38 when the SRO was pumped at 2.5 times threshold. These results show that ZnGeP2
OPOs directly pumped by a CrTmHo:YAG laser can be operated efficiently, while maintaining good IR beam
quality. The wide tuning range and a high pulse energy makes this SRO particularly suitable for spectroscopic
applications, and, tests in the field medical application, e.g., for cutting of soft tissue during surgery or corneal
corrections.
Frequency modulation (FM) techniques are well known methods for improving signal-to-noise ratios in laser
spectroscopy. Such techniques have proven particularly effective with diode lasers due to the ease with which they can
be frequency modulated via their injection current. Although singly-resonant optical parametric oscillators (OPOs) are
flexible, powerful and widely-tunable sources for mid-infrared laser spectroscopy, the utilization of FM techniques with
OPOs suffers from the inconvenience of requiring an external mid-IR modulator. As a consequence, FM techniques have
not been implemented with such devices. In this paper we describe the implementation of wavelength-modulation
spectroscopy (WMS) and frequency-modulation spectroscopy (FMS) using a singly-resonant OPO pumped by a fiber-amplified
diode laser. The OPO was capable of producing output powers of up to 1W in the 3.15-3.8 &mgr;m range with
continuous tuning over >100GHz on millisecond timescales. Frequency modulation, via injection current, of the diode
laser transferred directly to the OPO's idler output, allowing mid-IR FM to be achieved without external modulation
devices. WMS and FMS spectra of methane were then recorded, clearly demonstrating that this approach provides a
means of implementing these important techniques with powerful, widely tunable, mid-IR sources while retaining the
simple, flexible modulation properties of diode lasers.
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