N13A-T005 TITLE: Ultra-Wideband, Low-Power Compound Semiconductor Electro-optic Modulator

TECHNOLOGY AREAS: Air Platform, Information Systems, Sensors

ACQUISITION PROGRAM: PMA 290

OBJECTIVE: Develop and demonstrate a compound semiconductor external electro-optic modulator for ultra-wideband RF/analog signal transmission on aircraft

DESCRIPTION: New military communications, sensing and surveillance systems require ever-faster real time acquisition and transmission of electronic signals to achieve continuous sensing of electromagnetic spectrum. For the development and utilization of such systems ultra-wide bandwidths, low power operation, immunity to interference and survival under high input signals are essential. Transmission of ultra-wide band digital data over fiber optic transmission lines is another essential application for next generation military communications and data centers. Such links provide immunity to interference and can survive large input signals and operate at moderate power levels. As wider and wider portions of the electromagnetic spectrum are accessed and utilized, wider operational bandwidths are needed. In these regards electro-optic modulators that require drive voltage less than 1 volt (V), broadband operation in excess of 40 gigahertz (GHz), loss <5 decibels (dB) and able to operate at optical powers up to 100 milliwatt (mW) are essential. Furthermore the impedance of the modulator electrode should be as close to 50 ohms as possible which eliminates impedance matching issues and reduces the return loss. It is also highly desirable for such modulator designs to be scaled up to wider bandwidths approaching 100 GHz, possibly at the expense of drive voltage. At present, there is not an existing technology that can deliver such a modulator.

Presently the most commonly used electro-optic modulator material system is lithium niobate (LiNbO3). This is a mature technology and can provide the required bandwidth using traveling wave designs. However velocity matching requires electrical signal to go faster than the optical signal. Furthermore electrode length is restricted due to precise velocity matching needed. These requirements make the drive voltage rather high, at 5 V or higher level, even for advanced designs using micro machining techniques. Polymers offer better velocity matching but drive voltages are also higher.

External electro-optic modulators provide distinct advantages. Such devices are also key components for fiber optic links, delay lines, transmitters and signal processing. For example broadband analog links with gain is possible using low drive voltage modulators that can transmit moderate optical powers. Compound semiconductor electro-optic modulators have lower electro-optic coefficients compared to LiNbO3 and polymers but have high refractive indices that show very little dispersion from microwave to optical frequencies. High refractive index improves electro-optic efficiency and low index dispersion allows traveling wave devices using the loaded line approach. Electro-optic efficiency can be increased further using multi quantum well cores. Modulators in such materials also benefit from advanced device processing techniques. Such techniques allow the fabrication of highly confined electro-optically active optical waveguides and nanowires. A tightly confined optical mode overlapping very well with externally applied electric fields can create very efficient electro-optic modulation enabling very low drive voltages. Compound semiconductors can enable development of electro-optic modulators with very low drive voltage and ultra-wide bandwidth operation. Major challenges include uncooled operation over -40 to +80 degrees Celcius (minimum), low thermal noise, compatibility with moderate power (100mW), low relative intensity noise laser diode sources, and compact packaging with bend insensitive single-mode fiber coupling.

PHASE I: Design an ultra-wideband semiconductor electro-optic modulator that provides very efficient electro-optic modulation. Establish proof of concept. Develop a modulator fabrication process and a modulator test plan.

PHASE II: Optimize electrical and optical design and fabricate low voltage high speed packaged modulator prototype. Demonstrate modulator electrical and optical performance for high speed and high frequency range operation. Demonstrate single-mode fiber pigtailed electro-optic modulator packaging.

PHASE III: Transition the demonstrated modulator technology to radar systems, electronic warfare systems, and communication systems on Naval Aviation Platforms.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The technology would find application in commercial systems such as fiber-optic networks and telecommunications, where photonic integration offers compelling advantages over board-level processing.

REFERENCES:
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