Intracavity coupling modulation proves to be useful for low-power nanosecond-pulse modulation. Some theoretical results are presented and it is shown that pulse distortions can be reduced by inserting an additional polarization coupler into the cavity. Experiments were carried out which support the theory.

CO 2 -laser traveling-wave intracavity GaAs coupling modulator for nanosecond pulses

Projections of the growth of earth-sensing systems for the latter half of the 1980's show a data transmission requirement of 300 Mbps and above. Mission constraints and objectives lead to the conclusion that the most efficient technique to return the data from the sensing satellite to a ground station is through a geosynchronous data relay satellite. Of the two links that are involved (sensing satellite to relay satellite and relay satellite to ground), a laser system is most attractive for the space-to-space link. The development of CO2 laser systems for space-to-space applications is discussed with the completion of a 300 Mpbs data relay receiver and its modification into a transceiver. The technology and state-of-the-art of such systems are described in detail.

Spaceborne CO2 laser communications systems

An analysis of intracavity coupling modulation is presented including the effects of laser gain, dispersion, and modulator position. We interpret the system as an active Fabry-Perot resonator containing a polarization modulator, and arrive at explicit expressions for the frequency response of coupled-out radiation. In addition to the expected distortions at cavity resonances, sideband amplitude enhancement was found for low modulation frequencies. Comparison of the theory with CO 2 laser experiments shows excellent agreement.

Single-mode laser intracavity coupling modulation

The experimental proof for the existence of frequency-response distortions due to cold cavity modes in the output of a coupling-modulated CO 2 laser is presented. As expected from theory, severe distortions occur if the retardation bias approaches a value where coupling loss and cavity loss are of comparable magnitude.

Frequency response of CO 2 laser coupling modulation near intermode spacing frequencies

Experiments are described in which a carbon dioxide laser oscillating at 28.4 THz has been modulated simultaneously by three commercial television signals with carriers at 54, 66, and 82 MHz. Signal-to-noise degradation in the system was measured to be less than 1 dB, corresponding to modulator drive power of 1 W. This combination of wide bandwidth and low power consumption was made possible by the application of coupling modulation. This technique employs an intracavity electrooptic crystal to which the modulating fields are applied. Despite the fact that the crystal is positioned inside the laser cavity, the maximum bandwidth attainable is independent of both the cavity Q and the laser transition linewidth, and is limited only by modulator characteristics and optical transit time. Rigrod's (1965) method has been adapted to obtain an expression for the power coupled out of the laser. Modulator frequency response and drive power requirements are also summarized. It is seen that the noise bandwidth performance of the system would allow simultaneous modulation by 17 color television channels, or equivalently, more than a 300-megabit/sec capacity.

C. J. Peruso

Papers

Multichannel television coupling modulation experiments using a CO2 laser.