The remote detection and identification of liquid chemical contamination is a difficult problem f... more 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.
Conference Digest. 2000 Conference on Lasers and Electro-Optics Europe (Cat. No.00TH8505)
Summary form only given, as follows. The technique of pulsed photoacoustic (PA) spectroscopy has ... more Summary form only given, as follows. The technique of pulsed photoacoustic (PA) spectroscopy has recently been applied to the examination of free liquid surfaces at short range (a few cm). The mechanism involves expansion of air resulting from heat conduction from the laser heated surface of the sample under investigation. This drives an acoustic pulse that is detected by a
ABSTRACT The remote detection and identification of liquid chemical contamination is a difficult ... more ABSTRACT 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.
Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications VI, 2007
Non-linear optical wavelength conversion of near-infrared lasers within optical parametric oscill... more 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
Hollow optical waveguides have some unique properties compared with their solid core counterparts... more 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
ABSTRACT Sub-nanosecond duration eyesafe pulses at 1572 nm with energies of up to 10 mJ are produ... more ABSTRACT Sub-nanosecond duration eyesafe pulses at 1572 nm with energies of up to 10 mJ are produced for high precision ranging applications by cavity-dumped operation of an intracavity-pumped KTP OPO.
An integrated optic approach, using hollow waveguides, has been evaluated for a compact, rugged, ... more An integrated optic approach, using hollow waveguides, has been evaluated for a compact, rugged, high efficiency heterodyne optical mixing circuit in the middle infrared. The approach has involved the creation of hollow waveguides and alignment features for a beam combiner component in a glass-ceramic substrate. The performance of the integrated beam combiner was tested as part of a full laser heterodyne spectro-radiometer in which a quantum cascade laser local oscillator emitting at 9.7 µm was mixed with incoherent radiation. The performance has been evaluated with both cryogenically-cooled and peltier-cooled photomixers demonstrating consistent detection limits of two and five times the shot noise limit, respectively. The hollow waveguide mixer has also shown advantages in temporal stability, laser spatial mode cleansing, and reduced sensitivity to optical feedback.
A method for the remote detection and identification of liquid chemicals at ranges of tens of met... more A method for the remote detection and identification of liquid chemicals at ranges of tens of meters is presented. The technique uses pulsed indirect photoacoustic spectroscopy in the 10-microm wavelength region. Enhanced sensitivity is brought about by three main system developments: (1) increased laser-pulse energy (150 microJ/pulse), leading to increased strength of the generated photoacoustic signal; (2) increased microphone sensitivity and improved directionality by the use of a 60-cm-diameter parabolic dish; and (3) signal processing that allows improved discrimination of the signal from noise levels through prior knowledge of the pulse shape and pulse-repetition frequency. The practical aspects of applying the technique in a field environment are briefly examined, and possible applications of this technique are discussed.
The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free... more The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free liquid surfaces, and the prospects are assessed for remote detection and identification of chemical species in a field environment. A CO 2 laser ͑tunable within the 9-11-m region͒ provides pulsed excitation for a variety of sample types; the resulting photoacoustic pulses are detected at ranges of the order of a few centimeters. The phenomenon is investigated as a function of parameters such as temperature, sample depth, laser-pulse energy, pulse length, and beam diameter. The results are in good agreement with a theoretical model that assumes the mechanism to be expansion of air resulting from heat conduction from the laser-heated surface of the sample under investigation. Signal and noise processing issues are discussed briefly, and the possible extension of the technique to ranges of the order of 10 m is assessed.
Diffusion bonded (DB) walk-off compensated KTP crystals offer an alternative nonlinear medium for... more Diffusion bonded (DB) walk-off compensated KTP crystals offer an alternative nonlinear medium for efficient 1 to 2 m conversion within optical parametric oscillators (OPOs) at low pulse energies. Spatial variations in optical absorption and transmission values measured at 2 m are reported for two DB-KTP crystals. Finally, a comparison is made between the conversion efficiency obtained from a degenerate 1 m pumped OPO using a single 20 mm KTP crystal and an equivalent length DB-KTP crystal consisting of two bonded 10 mm crystals.
ABSTRACT The remote detection and identification of liquid chemical contamination is a difficult ... more ABSTRACT 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.
Optically Based Biological and Chemical Sensing, and Optically Based Materials for Defence, 2005
Optical parametric oscillators (OPOs) offer a route to powerful tunable output in the mid-infrare... more 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)
We describe the development of a time dependent thulium laser model. The model is used to predict... more 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.
Solid State Laser Technologies and Femtosecond Phenomena, 2004
Zinc germanium phosphide (ZGP) is well suited to use in optical parametric oscillators (OPOs) for... more 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 mum). Typical OPO applications seek to exploit pump wavelengths close to 2 mum so that both the output wavelengths fall within the mid-infrared waveband. However, the material typically suffers optical loss arising from
Optical parametric oscillators (OPOs) offer a route to powerful tunable output in the mid-infrare... more 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)
Solid State Laser Technologies and Femtosecond Phenomena, 2004
One route to generating mid-infrared (mid-IR) radiation is through a two-stage non-linear convers... more 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 (d eff ~ 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.
We describe the development of a time dependent thulium laser model. The model is used to predict... more 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.
Solid State Laser Technologies and Femtosecond Phenomena, 2004
Optical parametric oscillators (OPOs) using zinc germanium phosphide (ZGP) crystals as the active... more 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 splitstep 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.
The remote detection and identification of liquid chemical contamination is a difficult problem f... more 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.
Conference Digest. 2000 Conference on Lasers and Electro-Optics Europe (Cat. No.00TH8505)
Summary form only given, as follows. The technique of pulsed photoacoustic (PA) spectroscopy has ... more Summary form only given, as follows. The technique of pulsed photoacoustic (PA) spectroscopy has recently been applied to the examination of free liquid surfaces at short range (a few cm). The mechanism involves expansion of air resulting from heat conduction from the laser heated surface of the sample under investigation. This drives an acoustic pulse that is detected by a
ABSTRACT The remote detection and identification of liquid chemical contamination is a difficult ... more ABSTRACT 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.
Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications VI, 2007
Non-linear optical wavelength conversion of near-infrared lasers within optical parametric oscill... more 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
Hollow optical waveguides have some unique properties compared with their solid core counterparts... more 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
ABSTRACT Sub-nanosecond duration eyesafe pulses at 1572 nm with energies of up to 10 mJ are produ... more ABSTRACT Sub-nanosecond duration eyesafe pulses at 1572 nm with energies of up to 10 mJ are produced for high precision ranging applications by cavity-dumped operation of an intracavity-pumped KTP OPO.
An integrated optic approach, using hollow waveguides, has been evaluated for a compact, rugged, ... more An integrated optic approach, using hollow waveguides, has been evaluated for a compact, rugged, high efficiency heterodyne optical mixing circuit in the middle infrared. The approach has involved the creation of hollow waveguides and alignment features for a beam combiner component in a glass-ceramic substrate. The performance of the integrated beam combiner was tested as part of a full laser heterodyne spectro-radiometer in which a quantum cascade laser local oscillator emitting at 9.7 µm was mixed with incoherent radiation. The performance has been evaluated with both cryogenically-cooled and peltier-cooled photomixers demonstrating consistent detection limits of two and five times the shot noise limit, respectively. The hollow waveguide mixer has also shown advantages in temporal stability, laser spatial mode cleansing, and reduced sensitivity to optical feedback.
A method for the remote detection and identification of liquid chemicals at ranges of tens of met... more A method for the remote detection and identification of liquid chemicals at ranges of tens of meters is presented. The technique uses pulsed indirect photoacoustic spectroscopy in the 10-microm wavelength region. Enhanced sensitivity is brought about by three main system developments: (1) increased laser-pulse energy (150 microJ/pulse), leading to increased strength of the generated photoacoustic signal; (2) increased microphone sensitivity and improved directionality by the use of a 60-cm-diameter parabolic dish; and (3) signal processing that allows improved discrimination of the signal from noise levels through prior knowledge of the pulse shape and pulse-repetition frequency. The practical aspects of applying the technique in a field environment are briefly examined, and possible applications of this technique are discussed.
The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free... more The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free liquid surfaces, and the prospects are assessed for remote detection and identification of chemical species in a field environment. A CO 2 laser ͑tunable within the 9-11-m region͒ provides pulsed excitation for a variety of sample types; the resulting photoacoustic pulses are detected at ranges of the order of a few centimeters. The phenomenon is investigated as a function of parameters such as temperature, sample depth, laser-pulse energy, pulse length, and beam diameter. The results are in good agreement with a theoretical model that assumes the mechanism to be expansion of air resulting from heat conduction from the laser-heated surface of the sample under investigation. Signal and noise processing issues are discussed briefly, and the possible extension of the technique to ranges of the order of 10 m is assessed.
Diffusion bonded (DB) walk-off compensated KTP crystals offer an alternative nonlinear medium for... more Diffusion bonded (DB) walk-off compensated KTP crystals offer an alternative nonlinear medium for efficient 1 to 2 m conversion within optical parametric oscillators (OPOs) at low pulse energies. Spatial variations in optical absorption and transmission values measured at 2 m are reported for two DB-KTP crystals. Finally, a comparison is made between the conversion efficiency obtained from a degenerate 1 m pumped OPO using a single 20 mm KTP crystal and an equivalent length DB-KTP crystal consisting of two bonded 10 mm crystals.
ABSTRACT The remote detection and identification of liquid chemical contamination is a difficult ... more ABSTRACT 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.
Optically Based Biological and Chemical Sensing, and Optically Based Materials for Defence, 2005
Optical parametric oscillators (OPOs) offer a route to powerful tunable output in the mid-infrare... more 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)
We describe the development of a time dependent thulium laser model. The model is used to predict... more 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.
Solid State Laser Technologies and Femtosecond Phenomena, 2004
Zinc germanium phosphide (ZGP) is well suited to use in optical parametric oscillators (OPOs) for... more 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 mum). Typical OPO applications seek to exploit pump wavelengths close to 2 mum so that both the output wavelengths fall within the mid-infrared waveband. However, the material typically suffers optical loss arising from
Optical parametric oscillators (OPOs) offer a route to powerful tunable output in the mid-infrare... more 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)
Solid State Laser Technologies and Femtosecond Phenomena, 2004
One route to generating mid-infrared (mid-IR) radiation is through a two-stage non-linear convers... more 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 (d eff ~ 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.
We describe the development of a time dependent thulium laser model. The model is used to predict... more 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.
Solid State Laser Technologies and Femtosecond Phenomena, 2004
Optical parametric oscillators (OPOs) using zinc germanium phosphide (ZGP) crystals as the active... more 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 splitstep 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.
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Papers by Brian Perrett