Multiple Quantum Well Modulators Semiconductor

 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.

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.