Richard S. Eng

Thermal lensing and frequency chirp in a heated CdTe modulator crystal and its effects on laser radar performance

The effects of optical and microwave heatings and thermally-induced birefringence in a CdTe modulator crystal on the performance of a linear FM CO2 laser radar are examined. This is conducted in terms of reductions in beam Strehl ratio and dynamic ranges of the Doppler shift and range for given optical and microwave powers. An analysis of the thermal lenses generated by these heatings is presented.

Broad-band multisection electrooptic modulators

A general solution has been derived for expressing the conversion of power from a base frequency to a modulation sideband using a multisection electrooptic modulator. The objective is to obtain broadband frequency modulation of a laser by the use of multiple collinear electrooptic crystals with microwave power levels well below the breakdown threshold. It is found that segmented structures lead to greater bandwidths, which increase with the number of modular sections. This is achieved by adjusting the phases of microwaves between sections to maximize the single-sideband conversion efficiency. It is shown that a 10.6- mu m CO/sub 2/ laser modulator with six geometrically identical CdTe sections can potentially achieve a 3-dB bandwidth of nearly 6 GHz using a 10-kW traveling-wave tube operating at 16-GHz center frequency. >

Dynamic speckle bandwidth and characterization of vibration frequency and amplitude using a CO2 laser vibrometer

The speckle bandwidth in a CO2 laser radar return is measured by applying a tilt motion to a diffuse target which is also driven with a piston motion by a loudspeaker. The speckle bandwidth vs. tilt frequency is reported. The frequency and amplitude characteristics of a glint target undergoing piston motion only are also described.

Performance of a sealed-off CO2-isotope laser amplifier for high resolution optical radar/lidar applications

The essential features and performance characteristics of a narrowband isotopic CO2 laser amplifier system are described. Sealed-off operation with over 30 dB net power gain, negligible frequency chirp and up to 70 microsec pulse duration, 300 mJ pulse energy, and 10 Hz pulse repetition rate have been achieved.

Tunable electro-optic modulators for laser radar applications - Experimental results

A tunable electro-optic single sideband modulator having a configuration consisting of a CdTe crystal surrounded by a ferrite placed inside a microwave waveguide can be used as a laser local oscillator to track the target translational Doppler frequency in a CO2 laser radar return. Results on measurements to determine the phase velocity matched bandwidth for the above composite modulator configuration are reported.

Design and construction of a wideband efficient electro-optic modulator

Modulators with a large instantaneous microwave bandwidth and high efficiency were developed for imposing frequency chirped waveforms on a high power CO2 laser output. These modulators may be cascaded to increase bandwidth and to increase efficiency. Details of the construction, together with the results, are presented.

Conversion Efficiency, Spectral And Mode Purities Of A Single Sideband Electro-Optic Modulator

The measured single sideband conversion efficiency of a 10.6 μm bulk-type CdTe electro-optic modulator over the 14-18 GHz modulation frequency range is shown to be in close agreement with the coupled-mode and segmented modulator theories. The paper addresses the effects of the rotational orientations of segmented crystals and indirectly proves that a broadband multisection modulator is feasible; it further shows that a modulator with crystals in rotatable segmented circular waveguides is spectrally widely tunable. The effects of mechanical pressure and off-axis beam propagation on conver-sion, mode purity, and beam quality are also discussed.

Recent Developments In SSB Modulators

A tutorial review of the principles of single sideband (SSB) electro-optic modulators is presented. Recent developments for broadbanding such modulators by multisection techniques and dual tandem arrangement is described. The discovery and use of crystal rotational orientation effects on the performance of multicrystal configurations is considered. The techniques for the theoretical treatment using coupled-mode equations for both multicrystal arrangements are developed. The results of theoretical calculations are presented in graphical form. These demonstrate the enhanced capabilities in terms of efficient conversion and increased bandwidths. Wherever possible, comparison of theory and experiment will be presented. The enhanced performance at 10.6 μm and at 1.06 μm available high quality crystals will be reviewed. The choice of crystals for each wavelength region is based on the physical properties such as symmetry, dielectric constant, electro-optic coefficient, optical and microwave losses. These parameters are incorporated in the theory and practical considerations of availability and quality required for optimum performance.

Theory of single and double sideband modulators

The concept of two magnetically tunable electro-optic (EO) modulators is described. A single-sideband in which the E-O crystal is surrounded by a hollow square tube of ferrite is axially magnetized in a square waveguide. The other is a double sideband device in a rectangular waveguide with the E-O crystal sandwiched between two ferrite slabs oppositely magnetized transverse to the waveguide. Each is tuned by a variable magnetic field in order to match the phase velocities of the varying microwave frequency and the optical velocity of the laser beam. Perturbation theory yields tuning curves of frequency versus magnetic field over a relatively broadband.

Signal isolation capability of rotating-wave-plate type electro-optic modulators

The signal isolation capabilities of rotating wave plate type single sideband electrooptic modulators operating in different configurations are analyzed. Discussion includes a broadband configuration in which the microwave pump wave travels in the backward direction. Experimental results confirming the feasibility of a simple configuration are presented.