Robert J. Freiberg

Laser injection locking

The basic principles of laser injection locking are discussed and the status of recent experimental work is reviewed. The subject is introduced by means of appropriate lower frequency and microwave injection-locking circuits. These are used to explore the basic physical phenomena involved and to illustrate the essential role of gain saturation in laser injection locking. Oscillators and below-threshold regenerative amplifiers are discussed to provide a more complete and cohesive picture. Experimental results with a variety of CW and quasi-CW injection-locked amplifiers including unstable resonator geometries are presented. Preliminary results with a novel injection-locked passively Q-switched CO 2 amplifier are also given.

An experimental study of unstable confocal CO 2 resonators

The results of an experimental study to determine the spatial properties of unstable confocal resonator modes and their influence upon both the fractional output coupling and the far-field angular divergence are presented. The radial intensity distributions of the lowest order circulating mode within CO 2 unstable resonators and the corresponding near- and far-field patterns have been measured. The radial profiles exhibit a significantly nonuniform intensity distribution consistent with the results of recent theoretical analysis. The far-field intensity distribution as well as the fractional output coupling have been investigated for a number of unstable confocal resonator geometries. For modes contained within the uniform-gain region of the laser tube, the experimental results are in excellent agreement with Siegmans unstable resonator analysis.

Asymmetric unstable traveling-wave resonators

Experimental data pertaining to the intracavity propagation characteristics, the mode volume dissimilarity, and the fractional output coupling Characteristics associated with the oppositely traveling waves in CO 2 asymmetric unstable ring resonators are reported. Equivalence relations are derived, extending Siegmans unstable resonator analysis to encompass asymmetric traveling-wave geometries. Optical feedback techniques to suppress the oscillation associated with one of the traveling waves are demonstrated to be an effective means of achieving unidirectional ring laser operation.

A recirculating, self-contained DF/HF pulsed laser

The operation of the first self-contained, recirculating, electrically initiated, pulsed chemical laser is reported. A unique combination of scrubber beds selectively removes ground-state DF(HF) and other deleterious gases from the recirculating gas flow. By providing a small gas replenishment, it is possible to operate the laser for extended periods of time with little loss in power. The processes that degrade the power output in the absence of chemical scrubbing are discussed.

Magnetic stabilization of the plasma column in flowing molecular lasers

A technique of magnetically stabilizing the position of the plasma column and the resulting inversion region in lower pressure flowing molecular lasers is described. A transverse magnetic field, mutually perpendicular to the discharge axial electric field and the gas flow velocity, is employed to maintain the electrical discharge column parallel with the optic axis. The magnetic stabilization of the discharge is analyzed in terms of the Lorentz forces and the resulting ambipolar drifts of the plasma. A comparison with magnetically stabilized arcs is presented. Supporting experimental data obtained for pure gases and typical laser gas mixtures are given. The operating characteristics of the cross-field CO 2 laser which employs a premixed gas flow channeled transverse to the stabilized discharge to provide convective gas cooling are discussed. The experimental results of a parametric study on the dependence of gain and power output upon gas flow velocities, magnetic field, gas mixtures, and pressure are presented. The operation of the first premixed CW electrically initiated chemical laser has been achieved using cross-field magnetic discharge stabilization. Preliminary operating characteristics of this chemical laser for the 3-µ rotational-vibrational transitions in HF and the 4-µ transitions in DF are presented.

Recirculating DF/HF chemical lasers suitable for portable system applications

gated using a scanning Fabry-Perot interferometer. The data consisted of multiple scans of a single Fabry-Perot fringe which were photographically recorded over various exposure times. The frequency variations exhibited by these multiple traces indicate that over a period of 1 sec, long term frequency stabilities of better than 3 parts in 10’ are achievable with a relatively straight forward resonator design. Similar data obtained over shorter time durations indicated an approximate inverse square root dependence upon the observation time. For times shorter than 100 msec, the frequency stability was less than 1 part in 10’ and could not be resolved by the present scanning FabryPerot technique. I The frequency spectrum produced by heterodyning two identical H F mixing lasers, both operating on P,-1 (6) , was displayed on a low frequency spectrumaqalyzer. Thedataindicated that the frequency stability of the present device is predominantly limited by the low frequency disturbances coupled to the mechanical resonator structure from the environment and not by the mixing within resonator medium. Consequently, significantly better frequency stability should be attainable by further sophistication of the resonator’s mechanical properties before the influence of the active medium imposes limitations upon the device’s performance. To evaluate the suitability of these chemical lasers for applications such as laser communications, where frequency and amplitude stability is required over very long time periods, active stabilization techniques wereinvestigated. A particularly attractive technique which was developed is that of locking the HF laser output to the minimum of the Lamb dip by using active dither stabilization. The laser cavity was tuned in frequency such that the dither was symmetrically applied about the minimum of the Lamb dip which was located at the center of the 300 MHz Doppler broadened gain curve. A comparison of the open and close loop performance of the chemical laser was made. In the open loop mode when the cavity length was changed due to thermal or mechanical influences, substantial variations in both the intensity and frequency of the laser were observed. However, when the laser was actively locked to the Lamb dip by means of an electronic servo system, the laser intensity and frequency remained constant within the limits associated with the dither stabilization over periods in excess of an hour and for a wide range of conditions. Data will also be presented comparing the performance of a premixed, CW electricallyinitiated, chemical laser* with that of the chemical mixing laser and the suitability of these CW chemical lasers as frequency-stable local oscillators for mid-IR heterodyne applications will be discussed.