Hollow optical waveguides have some unique properties compared with their solid core counterparts. These include very broad waveband, high power transmission characteristics in conjunction with extremely low interfacing and propagation losses. Such characteristics provide considerable potential for novel electro-optic and infrared devices and systems. In conjunction with discrete micro-optical components hollow waveguides have been used to demonstrate integrated circuits which are the optical analogue of the electronic PCB. The underlying physics and technology of hollow optical waveguides will be discussed in the context of a wide range of applications.
We describe the development of a time dependent thulium laser model. The model is used to predict both the CW and temporal behaviour of a Tm:YAG laser. Experimental results from a diode-pumped Tm:YAG laser are obtained and the model is used to obtain good agreement with these observations for both the CW and temporal behaviour of the laser. Particular results relate to switch-on time delays and the effect of pump diode modulation on Tm laser efficiency. The laser model has been extended to the case of the Ho:YAG laser where other important effects due to ground state depletion and self re-absorption must be taken into account. The holmium laser model has recently been used to predict reported experimental results from a thulium fibre laser pumped Ho:YAG laser.
Non-linear optical wavelength conversion of near-infrared lasers within optical parametric oscillators (OPOs) offers a
route to powerful tunable sources in the mid-infrared (mid-IR). Engineered quasi-phasematched (QPM) non-linear
optical materials based on gallium arsenide (GaAs) offer an alternative to conventional birefringently phasematched
single-crystal materials such as ZnGeP2, which are currently used in mid-IR OPOs. QPM GaAs crystals have been
assembled from commercially available, high-optical quality 100-micron thickness gallium arsenide (GaAs) wafers
using a novel glass-bonding (GB) process. This uses thin layers of an infrared transmitting glass (refractive index
matched to GaAs) deposited onto each GaAs wafer, which, when heated under pressure, fuse the wafers together to
form a monolithic structure. By varying the thickness of the deposited glass layers, the dispersion in the glass can be
used to compensate for variations in GaAs wafer thickness and to fine tune the phasematching wavelengths of the QPM
crystal. GBGaAs crystals with up to 100 layers have been designed and built for wavelength conversion from 2 &mgr;m into
the mid-IR. We report the performance of these crystals used as optical parametric amplifiers (OPAs) in the mid-IR,
when pumped by a 2.094 &mgr;m source, and compare these results to measurements for a ZGP OPA. In addition, the
dependence of conversion within GBGaAs crystals on the polarisation state of the amplifier seed beam has been
investigated along with the temperature dependence of the optimum operating wavelength. Good agreement between
experimental results and performance predictions obtained from a numerical model is observed.
Optical parametric oscillators (OPOs) offer a route to powerful tunable output in the mid-infrared (mid-IR). Mid-IR OPOs exploit wavelength conversion of near-infrared lasers within non-linear optical materials. A new approach to engineering suitable non-linear OPO materials is being developed as an alternative to conventional chalcopyrite crystals such as ZnGeP2. These new materials use commercially available, high-optical quality gallium arsenide (GaAs) wafers and a novel glass-bonding (GB) process to assemble quasi-phase matched (QPM) multilayer structures. The assembled QPM GaAs stack must have low optical loss and a large useable aperture and needs to be produced reliably with a minimum of 50 layers. Results from a recent sequence of 50-layer GBGaAs stack fabrication will be presented. Of the six stacks successfully bonded two had a useable aperture of approximately 20 mm2 (40% of the maximum available). Of these, one has the lowest absorption and transmission loss per layer (0.07% measured at 2 μm) of any multi-layer glass-bonded QPM GaAs stack produced to date. By adjusting the load distribution at the edges of the stack during bonding the useable optical aperture was increased to nearly 90%. Results from non-linear wavelength conversion experiments into the mid-infrared using multi-layer GBGaAs crystals will be presented.
The remote detection and identification of liquid chemical contamination is a difficult problem for which no satisfactory solution has yet been found. We have investigated a new technique, pulsed indirect photoacoustic spectroscopy (PIPAS), and made an assessment of its potential for operation at stand-off ranges of order 10m. The method involves optical excitation of the liquid surface with a pulsed laser operating in the 9-11μm region. Pulse lengths are of order 3μs, with energy ~300μJ and repetition rates ~200Hz. Rapid heating of the liquid by the laser pulse produces acoustic emission at the surface, and this is detected by a sensitive directional microphone to increase the signal-to-noise ratio and reduce background clutter. The acoustic pulse strength is related to the liquid's absorption coefficient at the laser wavelength; tuning allows spectroscopic investigation and a means of chemical identification. Maximum coverage rates have been examined, and further experiments have examined the specificity of the technique, allowing a preliminary assessment of false-alarm and missed-signal rates. The practical aspects of applying the technique in a field environment have been assessed.
Optical parametric oscillators (OPOs) using zinc germanium phosphide (ZGP) crystals as the active non-linear medium are important devices for wavelength conversion into the 3 to 5 μm mid-infrared waveband. However, the presence of optical absorption within ZGP at the pump wavelength can lead to detrimental thermo-optic effects (thermal lensing and dephasing) when operated under high average power conditions. In order to characterise the strength of thermal effects within ZGP OPOs a theoretical model is under development based on the commercially available software package GLAD. Pump, signal and idler beams are represented by transverse arrays of complex amplitudes and propagated according to diffraction and kinetics algorithms. The ZGP crystal is modelled as a series of crystal slices, using a split-step technique, with the effects of non-linear conversion, absorption and thermal effects applied to each step in turn. We report modelling predictions obtained to date for the strength of the thermal lens induced in a ZGP crystal on exposure to a 5 Watt Q-switch pulsed high-repetition rate (10 kHz) wavelength doubled Nd:YLF laser at 2.094 μm. Predicted steady-state thermal focal lengths and time constants are compared to experimental results measured for two ZGP crystals, with high and low pump absorption levels. GLAD model predictions for a singly-resonant ZGP OPO in the absence of thermal effects are also compared to predictions from the widely available software package SNLO.
Zinc germanium phosphide (ZGP) is well suited to use in optical parametric oscillators (OPOs) for conversion of near-infrared laser output into the mid-infrared waveband (3 to 5 μm). Typical OPO applications seek to exploit pump wavelengths close to 2 μm so that both the output wavelengths fall within the mid-infrared waveband. However, the material typically suffers optical loss arising from growth defects that becomes significant at wavelengths below about 2.5 μm. We report the results of calorimetric studies that show that the loss can comprise both absorptive and scattering components. We have assessed the affect of loss at the pump wavelength on the conversion efficiency of a high pulse repetition frequency, doubly-resonant, ZGP OPO pumped with 2.094 μm radiation generated by a wavelength doubled Nd:YLF laser. The OPO used crystals having loss coefficients in the range 0.03 cm-1 to 0.3 cm-1. The reduction in slope efficiency for the conversion process was evaluated over a range of pump beam diameters (1/e2 intensity) from 0.12 mm to 0.30 mm. For the largest beam diameter a slope efficiency of 57% was measured for a ZGP OPO crystal having a loss coefficient of 0.03 cm-1. The slope efficiency reduced to about 30% when the loss coefficient was increased to 0.3 cm-1.
One route to generating mid-infrared (mid-IR) radiation is through a two-stage non-linear conversion process from the near-IR, exploiting powerful neodymium lasers operating at wavelengths close to 1 μm. In the first stage of this process non-linear conversion within a degenerate optical parametric oscillator (OPO) is used to double the wavelength of the 1 μm laser. The resultant 2 μm radiation is then used to pump a second OPO, based on a material such as ZGP, for conversion into the 3 to 5 μm mid-IR waveband. Periodically poled lithium niobate (PPLN) is a useful material for conversion from 1 to 2 μm due to its high non-linear coefficient (deff ~ 16 pm/V) and the long crystal lengths available (up to 50 mm). Slope efficiencies in excess of 40% have readily been achieved using a simple plane-plane resonator when pumped at 10 kHz with 3.5 mJ pulses from a 1.047 μm Nd:YLF laser. However, the OPO output was spectrally broad at degeneracy with a measured full-width-half-maximum (FWHM) linewidth of approximately 65 nm. This output linewidth is significantly broader than the spectral acceptance bandwidth of ZGP for conversion into the mid-IR. In this paper techniques for spectral narrowing the output from a degenerate PPLN OPO are investigated using two passive elements, a diffraction grating and an air spaced etalon. Slope efficiencies approaching 20% have been obtained using the grating in a dog-leg cavity configuration producing spectrally narrow 2 μm output with linewidths as low as 2 nm. A grating-narrowed degenerate PPLN OPO has been successfully used to pump a ZGP OPO.
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