MQW devices must be operated at high reverse bias fields

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.

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 Mbit/s 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 Mbit/s. A representative trace is shown in Figure 4. A Photograph of a modulator segmented into 9 pixels is shown in Figure 5.

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.

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.

October 15, 2011

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