Optical Retro-Directive Laser Link by Phase Conjugation

Paper number

IAC-07-B2.6.03

Author

Mr. Christian Schaefer, Kobe University, Japan

Coauthor

Dr. Nobuyuki Kaya, Kobe University, Japan

Coauthor

Dr. Osamu Matoba, Kobe University, Japan

Year

2007

Abstract

Optical communication links between satellites in low- and geostationary orbit and satellite-to earth links were demonstrated in the last years successfully. Nevertheless, a problem arises when the satellite experiences mechanical vibrations. Even small amplitude vibrations cause a significant increase in the bit-error rate, since the laser beam is directed by a small angular divergence towards the target.
Different methods for solving this problem were proposed in the past ranging from simply increasing the power to sophisticated feed-forward control loops. Furthermore, it was proposed to implement a non-mechanical fine steering device like an optical phased array (OPA) in order to make the system less sensitive to mechanical vibrations. OPAs steer a laser beam in a complete non-mechanical way that a beam gets spatially phase shifted in form of a phase-ramp by passing through the device.
There were only a few reports on establishing an all-optical feedback loop or retro-directive system using optical phase conjugation (OPC). We are examining this idea in this paper by comparing two different system designs. 
In the first design, a laser beacon from the receiver gets pre-amplified before it seeds a resonator in the transmitter and self-pumped phase conjugation takes place. The phase conjugated beam gets amplified before it automatically propagates back to the target. 
The second design uses double-phase conjugation to combine the phase conjugated beacon beam with a mutually incoherent second laser source on the transmitter. Since this process is angle critical, a fine steering device like an OPA is necessary to implement in this design.
Since OPC is typically achieved by holographic methods, the response time for writing the hologram is important. This paper gives an brief overview of materials where OPC can occur below the response times of mechanical vibrations and therefore will be potentially insensitive to mechanical vibrations. However, we believe that we need to adjust the beam’s direction due to the time delay of this process and the laser beam’s transmitter – receiver propagation runtime correction.
For that reason, we examine beam steering by a spatial light modulator based on twisted nematic liquid crystals as OPA. Furthermore, we implement this device into the retro-directive system design and examine the range for adjusting the phase conjugated beam.

Abstract document

IAC-07-B2.6.03.pdf

Manuscript document

IAC-07-B2.6.03.pdf (🔒 authorized access only).

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