Hollow-waveguide integrated optics for 10.6-μm lidar

Field analysis with a mode-tunable local oscillator

“…Inpractice both the waveguide interferometers were based on 2.0 mm square cross section hollow dielectric waveguides formed from polycrystalline alumina plates. The large waveguide dimensions compared with the 10.6 tm wavelength of the light, and the choice of polycrystalline alumina as the wall material, ensured low attenuation coefficients even for high order modes [1]. Theoretical predictions [1], supported experimental measurements, confirmed that the mode jo suffers < 5% percent attenuation in propagating through the analyser structure.…”

“…The main reason for basing the practical implementation of the concept on a hollow multimode waveguide interferometer was that we had demonstrated the use of hollow waveguides as the basis of 10.6 pm laser radar systems in earlier work [1] and had used the same technology to prove the concept of a mode tuneable local oscillator [2]. As illustrated in figure 1 the waveguide interferometer both allows radiation from the laser source to be transmitted to the target and facilitates the analysis of radiation returned from it in terms of the hybrid EH,,, modes of the multimode waveguides from which the interferometer is formed.. …”

“…Because of the required multimode nature of the two interferometers the whole structure was designed and manufacture to tolerances which ensured high fidelity propagation of multimode fields. In practice this meant the attainment of angular alignment of components and waveguides to better than 0.5 mrad and lateral location of waveguides, one with respect to another, to better than 0.1 mm [1].…”

“…In order to make a complete measurement of the input field F(x,y), the moduli and phase measurements are repeated for each individual waveguide mode that the mode tuneable local oscillator generates. The field F(x,y) is then computed by substitution ofthe measured values of A,,, and 4 in equation (1). With a suitable computer, intensity and phase contours of the input field may then be generated.…”

“…Further research and development of the concept will involve a number of different facets of work, including: (1) improvement in the fidelity and range of modes generated by the mode tuneable local oscillator, including the generation of two-dimensional modes e.g. El-I56, (2) automation and computer control of the mode generation and mode spectra measurement processes, (3) operation based on a high speed mercury cadmium telluride detector in conjunction with a frequency offset mode tuneable local oscillator, (4) use of a refmed analyser to make moduli and phase measurements of the waveguide mode spectra produced by fields returned from a wide range ofreal targets.…”