Overview
editAccording to the U.S. Navy's - Naval Research Laboratory, a Modulating Retro-reflector (MRR) system combines an optical retro-reflector and an electro-optic shutter to allow two-way optical communications. [1]
Free space optical communication technology has emerged in recent years as an attractive alternative to the conventional Radio Frequency (RF) systems. This emergence is due in large part to the increasing maturity of lasers and compact optical systems that enable exploitation of the inherent advantages (over RF) of the much shorter wavelengths characteristic of optical and near-infrared carriers:
- Larger bandwidth
- Low probability of intercept
- Immunity from interference or jamming
- Frequency spectrum allocation issue relief
- Smaller, lighter, lower power
Figure 1. Modulating retro-reflector
Technology
editAn MRR couples an optical retro-reflector with a Multiple Quantum Well (MQW) modulator to direct optical signals to receiver providing communications device.[2][3]
The MRR with its related electronics are mounted on a convenient platform. A remotely located optical transmitter/receiver system consisting of a laser, telescope, and detector provides and optical signal to Modulating Retro-reflector. The incident light from the transmit system, is modulated by the electro-optical shutter, and then reflected to the receiver. Figure 1 illustrates the concept.
The NRL modulating retro-reflector uses a semiconductor based MQW shutter capable of modulation rates up to 10 Mbps, depending on link characteristics. (See "Modulating Retro-reflector Using Multiple Quantum Well Technology", U.S. Patent No. 6,154,299, awarded November, 2000.)
The technology enables the use of near-infrared frequencies that provide communications that are not susceptible to issues related to frequency allocation. The Multiple Quantum Well modulating retro-reflector has the added advantages of being compact, lightweight, and requires very little power. The small array MRR provides up to an order of magnitude in consumed power savings over an equivalent RF system.
The concept of a modulating retro-reflector is not new dating back to the 1940's. However, it wasn't until 1993 the enabling technologies were on hand to turn concept into reality[4]. Specifically, modulators available that could support a link at reasonable communications data rates.
In addition, an electro-optical shutter, a key component in MRR technology, must have a high switching speed, low power consumption, large area, wide field-of-view, and high optical quality. It must also function at certain wavelengths where appropriate laser sources are available, be radiation-tolerant (for non-terrestrial applications), and be rugged. Mechanical shutters and ferroelectric liquid crystal (FLC) devices, for example, are too slow, heavy, or are not robust enough. Further, modulating retro-reflector systems must are desired to operate at data rates of megabits per second (Mbps) and higher, and over large temperature ranges characteristic of installation out-of-doors and in space. NRL has pioneered use of a different type of electro-optic shutter, developing a semiconductor-based optical switch based on GaAs Multiple Quantum Wells (MQW). Figure 1 illustrates the concept of a modulating retro-reflector.
MULTIPLE QUANTUM WELL MODULATORS
Semiconductor MQW modulators are one of the few technologies that meet all the requirements need for practical applications. When used as a shutter, MQW technology offers many advantages: it is robust solid state, operates at low voltages (less than 20 V) and low power (tens of milliWatts), and is capable of very high switching speeds. MQW modulators have been run at Gbps data rates in fiber optic applications.
Figure 2. Absorbance vs. Frequency When a moderate (~15V) voltage is placed across the shutter in reverse bias, the absorption feature changes, shifting to longer wavelengths and dropping in magnitude. Thus, the transmission of the device near this absorption feature changes dramatically. Figure 2 shows absorbance data for an InGaAs MQW modulator designed and grown at NRL for use in a modulating retro-reflector system. The figure illustrates how the application of a moderate voltage shifts the transmittance. Hence, a signal can be encoded in an On-Off-Keying format onto the carrier interrogation beam.
This modulator consists of 75 periods of InGaAs wells surrounded by AlGaAs barriers. The device is grown on an n-type GaAs wafer and is capped by a p-type contact layer, thus forming a P-I-N diode. This device is a transmissive modulator designed to work at a wavelength of 980 nm, compatible with many good laser diode sources. These materials have very good performance operating in reflection architectures. Choice of modulator type and configuration architecture is application-dependent.
Figure 3.a A schematic of a MQW modulator.
Figure 3.b A fabricated .5cm transmissive device.
Once grown, the wafer is fabricated into discrete devices using a multi-step photolithography process consisting of etching and metallization steps. The NRL experimental devices have a 5-mm aperture, though larger devices are possible and are being designed and developed. It is important to point out that while MQW modulators have been used in many applications to date, modulators of such a large size are uncommon and require special fabrication techniques. Figure 3 shows a block diagram and photo of a wide aperture MQW shutter designed, grown, and fabricated at NRL.
MQW modulators are inherently quiet devices, accurately reproducing the applied voltage as a modulated waveform. An important parameter is contrast ratio, defined as Imax/Imin. This parameter affects the overall signal-to-noise ratio. Its magnitude depends on the drive voltage applied to the device and the wavelength of the interrogating laser relative to the exciton peak. The contrast ratio increases as the voltage goes up until a saturation value is reached. Typically, the modulators fabricated at NRL have had contrast ratios between 1.75:1 to 4:1 for applied voltages between 10 V and 25 V, depending on the structure.
There are three important considerations in the manufacture and fabrication of a given device: inherent maximum modulation rate vs. aperture size; electrical power consumption vs. aperture size; and yield.
INHERENT MAXIMUM MODULATION RATE vs. APERTURE SIZE
The fundamental limit in the switching speed of the modulator is the resistance-capacitance limit. A key trade is area of the modulator vs. area of the clear aperture. If the modulator area is small, the capacitance is small, hence the modulation rate can be faster. However, for longer application ranges on the order of several hundred meters, larger apertures are needed to close the link. For a given modulator, the speed of the shutter scales inversely as the square of the modulator diameter.
ELECTRICAL POWER CONSUMPTION vs. APERTURE SIZE
When the drive voltage waveform is optimized, the electrical power consumption of a MQW modulating retro-reflector varies as:
Where Dmod is the diameter of the modulator, V is the voltage applied to the modulator (fixed by the required optical contrast ratio), B is the maximum data rate of the device, and RS is the sheet resistance of the device. Thus a large power penalty may be paid for increasing the diameter of the MQW shutter.
YIELD
MQW devices must be operated at high reverse bias fields to achieve good contrast ratios. In perfect quantum well material this is not a problem, but the presence of a defect in the semiconductor crystal can cause the device to break down at voltages below those necessary for operation. Specifically, a defect will cause an electrical short that prevents development of the necessary electrical field across the intrinsic region of the PIN diode. The larger the device the higher the probability of such a defect. Thus, If a defect occurs in the manufacture of a large monolithic device, the whole shutter is lost.
Figure 4. Amplitude vs time for a nine-"pixel" modulator.
To address these issues, NRL has designed and fabricated segmented devices as well as monolithic modulators. That is, a given modulator might be "pixellated" into several segments, each driven with the same signal. This technique means that speed can be achieved as well as larger apertures. The "pixellization" inherently reduces the sheet resistance of the device, decreasing the resistance-capacitance time and reducing electrical power consumption. For example, a one centimeter monolithic device might require 400 mW to support a one Mbps link. A similar nine segmented device would require 45 mW to support the same link with the same overall effective aperture. A transmissive device with nine "pixels" with an overall diameter of 0.5 cm was shown to support over 10 Mbps. A representative trace is shown in Figure 4. A Photograph of a modulator segmented into 9 pixels is shown in Figure 5.
Figure 5. Segmentation of MQW modulator into 9 pixels.
This fabrication technique allows for higher speeds, larger apertures, and increased yield. If a single "pixel" is lost due to defects but is one of nine or sixteen, the contrast ratio necessary to provide the requisite signal-to-noise to close a link is still high. There are considerations that make fabrication of a segmented device more complicated, including bond wire management on the device, driving multiple segments, and temperature stabilization.
Figure 6. An interferogram of an InGaAs transmissive MQW modulator.
An additional important characteristic of the modulator is its optical wavefront quality. If the modulator causes aberrations in the beam, the returned optical signal will be attenuated and insufficient light may be present to close the link. In Figure 6, an infrared interferometric measurement of a one-cm piece of the InGaAs modulator is shown. As can be seen, the optical quality of the device is very good and should not deleteriously impact system performance.
Applications
edit- Ground-to-Air Communications
- Ground-to-Satellite Communications
- Internal Electronics Bus Interaction/Communication
- Inter, Intra-Office Communications
- Vehicle-to-Vehicle Communications
- Industrial Manufacturing
See Also
editReferences
edit- ^ http://mrr.nrl.navy.mil/overview.html
- ^ http://www.nrl.navy.mil/fpco/publications/2000United%20States%20Patent_%206,154,299.pdf
- ^ https://drum.umd.edu/dspace/handle/1903/6807
- ^ Fritz et al., Appl. Phys. Lett. 63(4), Jul. 26, 1993