Electron beam pattern generators are nowadays used extensively for the production of masks and for direct writing on wafers. For obvious reasons electron beam pattern generators are optimized for integrated circuit fabrication. However there is also considerable potential for the use of electron beam lithography in other areas. In this paper recent trends in the development of electron beam pattern generators are described and the problems encountered in the application of electron beam machines in other areas e.g. integrated optics are discussed.
Gas lasers have shown to be capable of delivering tens of terrawatt aspeak power or tens of kilowatt as average power. The efficiencies of most high power gas lasers are relatively high compared with other types of lasers. For instance molecular lasers, osci 1 lating on low lying vibrational levels, and excimer lasers may have intrinsic efficiencies above 10%. The wavelengths of these gas lasers cover the range from the far infrared to the ultra-violet region, say from 12000 to 193 nm. The most important properties are the scalability, optical homogeneity of the excited medium, and the relatively low price per watt of output power. The disadvantages may be the large size of the systems and the relatively narrow line width with limited tunability compared with solid state systems producing the same peak power.
The use of 'V' and 'U' folds in waveguide resonators has been studied experimentally and compared with simple theory. The potential for size reduction and lower loss ( < 3% single pass) of the 'U' fold configurations is exploited in the construction of a 50 watt, compact device with heterodyne quality output mode. Intracavity modulation techniques applicable to this technology are also discussed.
The CO2 Waveguide laser is a small, rugged source of 10.6 micron radiation. Ferranti have optimised the design of this device into a miniature CO2 laser, with long shelf life, long active life, high efficiency and excellent mode, which is ideal for hand held applications both military and commercial.
A number of different designs of CO2 wavegulue lasers have been built in order to establish the perforkiance of rugged, reliable devices ah.i.d to test their suitability for a number of different applications. In particular the need to demonstrate long active and passive life times has led to the adoption ot fluxless, controlled atmosphere brazing and welding processes during a number of the construction phases. Fully developed lasers have been built which have demonstrated 5000 hours active life and 7 years shelf life to date. The further need for more compact, lower cost and higher power lasers has resulted in the development of new ceramic bonding techniques- Prototype lasers have been built to establish the design parameters and technology for folded waveguide paths leading to increased power from compact, smaller dimensioned devices-In this paper the technology advances which have now made the waveguide laser a viable tool for military, medical and industrial applicatioo.s will be described and discussed.
Frequency stabilization of TEA CO2 lasers has been achieved using a phase conjugate mirror in the CO2 gain cell itself. The mirror is obtained by degenerate four wave mixing with cw pumping. Numerical analysis adaptated to our set up allows us to precise the optimum parameters and is found to agree with the experimental results.
A wide-aperture, single-mode TEA-CO2 laser has been developed using a graded-reflectivity mirror to discriminate against higher-order modes. The laser frequency was stabilized by an active cavity-length control plus injection from a low-pressure, CW CO2 laser. The use of a graded-reflectivity output coupler allowed discrimination against higher-order transverse modes while producing a near diffraction limited far-field beam pattern. Because the transverse mode dimension is large, the frequency "chirp" was low.
We report the results of studies intended to characterise the amplifying behaviour of rf-excited CO2 discharge modules in the context of assessing their potential application as optical preamplifiers. The dependences of the small-signal gain coefficient and the amplified spontaneous emission noise power on rf input power density have been measured simultaneously for various gas mixtures and pressures.
Theoretical predictions and experimental measurements of the transmission characteristics of a 1.0 mm I.D. hollow core silica waveguide have been made as a function of wavelength across the 9-11 μm waveband. Good qualitative agreement is observed between the theoretical and experimental data. Significantly lower attenuation coefficients are attained at the lower end of the waveband as a result of dramatic changes in the complex refractive index of silica brought about by dispersion phenomena associated with a strong absorption band centered at = 9.25 μm.
A distributed feedback approach to CO2 waveguide laser resonator design based on the use of wall gratings has a number of potential advantages. In particular a move away from the need for bulky, accurately aligned, multidielectric coated mirror resonators, could have a significant impact on both manufacturing time and overall size leading to a new generation of compact low cost 10μm waveguide lasers. Distributed feedback (DFB) from wall gratings in hollow dielectric waveguides has been treated theoretically, initially by Marcuse 1, and further extended by Miles and Grow 2. The critical result of the latter analysis being that in order to attain sufficient feedback from a wall grating the waveguide wall separation should be of the order of 100Pms. Unfortunately waveguide attenuation is proportional to the inverse cube of the waveguide wall separation, with the result that commonly used waveguide materials such as Alumina, whilst having low enough attenuation for the fabrication of 1.5-2.0 mm bore lasers 5, have prohibitively high losses at 100um bore sizes. In this paper, following a brief review of hollow waveguide distributed feedback phenomena, the parameters affecting both waveguiding losses and feedback coefficients will be considered in the context of achieving highly transmitting small cross-section waveguides which will provide an effective basis for experimental studies of DFB phenomena. The results of theoretical predictions of potentially attractive waveguide materials and configurations will be presented and discussed. Finally the details of an experimental study of attenuation and distributed feedback phenomena in very small cross-section hollow dielectric waveguides will be described, with preliminary measurements being compared with computed data and the potential for a DFB waveguide laser being reviewed in the light of the results.
Proc. SPIE 0806, Loss Of Infrared Optical System MTF Resulting From Damage Caused By High Power Continuous Wave And Low Average Power Repetitive Pulsed Visible - UV Lasers, 0000 (24 August 1987); doi: 10.1117/12.941397
Infrared optical systems are, fundamentally, very sensitive devices which use rather extraordi optical electronics systems to process optical signals. Unfortunately, this very sensitivity may be excite by high power continuous wave visible - UV lasers and low average powered repetitive visible - UV pulsed lasers. Fundamentally, the lasers interact with the optical thin film dielectric coatings causing them to shift the working wavelength band and, thereby, taking away the optical signal from the sensor. In this paper are described analytical expressions for evaluating the temperature transient in the optical thin films and the subsequent diminution of the Modulation Transfer Function (MTF) of the optical system. Additionally, expressions are provided for evaluating reflective metal surfaces destruction through slip failure which will diminish the Strehl ratio of the system and subsequent loss of MTF.
The infrared transmission properties of uniformly curved hollow dielectric guides, usually found by solving the field equations in toroidal coordinates, are studied here in terms of the modes of the straight guide. The results are in encouraging agreement with experiments at 10.6 μm on short bent silica tubes. Mode output quality and guide dissipative losses are seen to depend sharply on curvature and guide geometry.
A continuous wave (CW) CO2 laser radar configured for Doppler imaging is described. The system produces a velocity image by scanning the laser beam across the scene, coherently detecting the scattered radiation and measuring any Doppler shift present. Examples of the images achieved are shown and an indication of the potential applications given.
Since 1982, SFENA has been developing a FM-CW Laser Radar first designed for terrain-following and terrain-avoidance of combat aircraft. In order to prove the effectiveness of this concept, a compact experimental engineering model of the sensor underwent flight evaluation trials on a PUMA helicopter at the French Flight Test Center in 1985. Altimetric data of the terrain flown over by the aircraft were acquired and batch proces-sed, allowing the creation of 10 x 10 kilometer altimetric maps centered on the aircraft and divided into 40 meter square elementary cells. The altimetric files were processed in various ways : 3-D projections, extraction of altimetric profiles, production of level contours. They were compared with various references such as DLMS data files, radio-altimeter profiles and geographical maps. The results are presented according to two types of analysis : - visual correlation between 3-D projections, - standard-deviation between laser radar and radio-altimeter profiles. Some other in-flight performances such as : probability of detection on typical obstacles, false alarm levels, scanning influence on detection, are also discussed.
The results of performance measurements of 10.6μm pulsed coherent laser rangefinders are described and compared with theory. An assessment is made of the implications on performance of heterodyne mixing efficiency, speckle and target return statistics.
A number of experimental high p.r.f. CO2 TEA lasers have been demonstrated in the past in laboratory or tripod-mounted tests, but have not been sufficiently compact or rugged for incorporation into rangefinders for military. aircraft. Suitable forward-looking positions in aircraft are so confined that only Nd:YAG based rangefinders have found application to date and these pose much too high an eye-hazard to be used for routine terrain-following.
The effects of atmospheric turbulence, nature of the target and receiver design on the performance of a 10.6μm pulsed coherent laser radar are evaluated and the predictions compared with experimental data.