Stationary high-bandwidth beacon-tracking lasercom experiments and the evolution toward a mobile lasercom capability

2007

SPIE Proceedings

Cone tracking is a well known method to optimize the pointing of a beam, and has been previously reported in the case of direct mode free-space optical communication (FSOC) links 1, 2 , using a beacon 1 or a dedicated wavelength 2 to carry the feed-back. In a retro link, because the beam is reflected back to the transmitter, feedback data is locally available. We present here the results of an evaluation (both simulation and experiments) of the cone tracking technique applied to a retro link and how the implementation can be optimized for this specific case. We show that using only a small number of samples (as low as 16 with 625 Hz modulation frequency) we can retrieve the beam offset. We also show how the technique can be used to estimate the beam wander. We finally demonstrate cone tracking (closed loop maintaining the beam center on the retro-reflector) with modulation amplitude as low as 1% of the beam divergence.

Section: Theorymentioning

confidence: 99%

Evaluation of cone tracking for free-space optical communication with a retro-modulator

2007

SPIE Proceedings

“…Assuming that one can calculate the phase difference, the feedback signal has to be sent back to the other end (B), which creates a chicken and egg problem, unless one relies on out-of-band communication such as a RF link or an additional beam as proposed in prior work [3]. This question of the feedback path necessary to close the loop is addressed in section 2.…”

Section: Introductionmentioning

confidence: 99%

Evaluation of a cone tracking algorithm for free-space optical communication in direct mode

2009

SPIE Proceedings

The implementation of cone tracking to optimize the beam pointing in a retro-reflective link was reported in prior work [1]. One parameter required to close the loop is the phase difference between the scanning signal and the reflected modulated power, which gives the direction of the error. The phase difference is easy to measure in a retro-reflective link because the beam is reflected back to the transmitter making the feedback signal locally available. In a direct mode configuration, this phase difference is unknown. The problem was avoided in prior work [2] by using two detectors a few inches apart. This paper presents the results of the evaluation, using simulation, of an algorithm that estimates and tracks the phase difference between the two ends of the link making it possible to close the loop and insure convergence. It also shows how to implement the feedback path without using an additional beam as opposed to using an additional high-power wide-divergence beam.

Evaluation and implementation of cone tracking for free-space optical communication with a retromodulator

2010

Opt. Eng

Cone tracking is a well-known method to optimize the pointing of a beam, and has been reported previously in the case of direct-mode free-space optical communication links, using a beacon or a dedicated wavelength to carry the feedback. In a retro link, because the beam is reflected back to the transmitter, feedback data are locally available. We present here the results of an evaluation (both simulation and experiments) of the cone-tracking technique applied to a retro link and how the implementation can be optimized for this specific case. We show that, using only a small number of samples (as few as 16 with 625-Hz modulation frequency), we can retrieve the pointing error. We finally demonstrate cone tracking (a closed loop maintaining the beam center on the retroreflector) with modulation amplitude as low as 1% of the beam divergence. C 2010 Society of Photo-Optical Instrumentation Engineers.