Performance of cat's eye modulating retro-reflectors for free-space optical communications

Abstract

Modulating retro-reflectors (MRR) couple passive optical retro-reflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing/acquisition/tracking system required on only one end of the link. In operation a conventional free space optical communications terminal, the interrogator, is used on one end of the link to illuminate the MRR on the other end of the link with a cw beam. The MRR imposes a modulation on the interrogating beam and passively retro-reflects it back to the interrogator. These types of systems are attractive for asymmetric communication links for which one end of the link cannot afford the weight, power or expense of a conventional free-space optical communication terminal. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large area MQW placed in front of the aperture of a corner-cube.For the MQW MRR, the maximum modulation can range into the gigahertz, limited only by the RC time constant of the device. This limitation, however, is a serious one. The optical aperture of an MRR cannot be too small or the amount of light retro-reflected will be insufficient to close the link. For typical corner-cube MQW MRR devices the modulator has a diameter between 0.5-1 cm and maximum modulation rates less than 10 Mbps. In this paper we describe a new kind of MQW MRR that uses a cat's eye retro-reflector with the MQW in the focal plane of the cat's eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 10 Mbps free space link over a range of 1 km is demonstrated. In addition a laboratory demonstration of a 70 Mbps MQW focal plane is described.

Modulating Retro-Reflector Devices and Current Link Performance at the Naval Research Laboratory

“…Maximum benefit is gained from CERs if the modulator array can react to the angle of the incoming light, switching off unilluminated pixels. This decreases both power consumption and self-heating [12], but requires more complex circuitry than for a CCR MRR. A benefit of the angle dependence of the focal point location is that it allows the MRR to simultaneously act as a transmitter and as an angle-of-arrival sensor.…”

“…CERs are best for links requiring high data rates at long distance, but with less restriction on space, weight, and power. An aspheric curved focal plane CER MRR weighing 410 g (including electronics) was used in a 7 km link at 45 Mb/s, and a new modulator has demonstrated 70 Mb/s operation [12]. It is being used in the flat focal plane CER in Figure 2(b) because the flat focal plane aids heatsinking, which is helpful at high data rates.…”

Progress in laser propagation in a maritime environment at the Naval Research Laboratory

“…Since the MQW devices are essentially resistance-capacitance (RC) devices whose speed is dependent on the area, the smaller units enable much faster modulation. We have reported (6) and anticipate the development of devices which can support greater than 100 Mbps. Graphics and photographs illustrating this type of retromodulator are shown in Figure 3.…”

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

“…However, it is only during the last decade that the field of retroreflector-based free-space optical (FSO) links has seen significant progress, enabled by the development of highspeed modulators. [1][2][3][4] Theses retro links consist of an active terminal (with laser source, beam steering, and receiver) on one end, and a retromodulator (typically corner cube or cat's eye 5,6 ) on the other end.…”