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DARPA advances Microwave Photonics for RF signal processing and radars

The military/defense industry continues to encourage the advancement of multifunctional receivers. Yet the demands for higher-frequency operation, increased bandwidth, increased dynamic range, and enhanced sensitivity creates unique design challenges for present technology. Size, weight, power, and cost (SWAP-C) dynamics heavily influence the military/defense industry’s value for a particular technology.


DARPA has been investigating the development of microwave photonic technology that has the potential to dramatically reduce all of the SWAP-C parameters of a traditional RF telecommunications link. DARPA has identified three dominant components that could enable a photonic link: low-noise/high-power laser diodes, low-loss/low-drive-voltage electro-optic modulators, and high-power/highly linear photodiodes. DARPA-funded research efforts have led to many discoveries and advancements in microwave-photonic components, such as electro-optic modulators, high-power photodiodes, low-noise laser diodes, and microwave photonic link configurations.


With a lithium-niobate modulator configured as a Mach-Zehnder interferometer, an RF signal can be injected into a device. This step produces electro-optically modulated light signals—transmissible by optical fiber—that can later be converted back to RF signals.
Microwave photonic technology has the potential to dramatically reduce all of the SWAP-C parameters of a traditional RF telecommunications link. Thus, significant effort has been invested in replacing a link’s RF components with photonic ones. DARPA has identified three dominant components that could enable a photonic link: low-noise/high-power laser diodes, low-loss/low-drive-voltage electro-optic modulators, and high-power/highly linear photodiodes.


For laser diodes, high output power and low relative intensity noise (RIN) are essential for maintaining a low noise figure and significant dynamic range. DARPA research groups have demonstrated several of these technologies, including the following: a tunable, sampled-grating distributed Bragg grating laser, indium phosphide (InP) slab-coupled optical-waveguide external cavity lasers, and InP distributed feedback lasers.


For electro-optical modulators, the Mach-Zehnder modulator (MZM) has become the dominant technology for high-performance applications. Lithium-niobate modulators have been used for some time for MZM devices. Recently, however, gallium-arsenide (GaAs) and aluminum-gallium-arsenide (AlGaAs) MZM systems have been developed. In terms of photodiode technology, the uni-traveling-carrier (UTC) photodiode has become a favored architecture for high-performance receivers. A significant achievement of this technology was achieving an RF output power of 750 mW at 15 GHz with a third-order intercept point of less than +55 dBm.


Field Controllable Modulator Array (FCMA)

Microwave photonics is an important technology for military applications including point-to-point radio-frequency (RF) links, RF signal processing, radar, and RF spectrum management. The military microwave photonic systems deployed to date leverage past developments by the massive telecommunication industry, repurposing commercial components for specific military functions. The most recent developments in industrial telecommunications have been in specialized, application-specific photonic integrated circuits (PICs).


While these trends are expected to continue, military applications cannot benefit from these advancements because application-specific PICs cannot be repurposed. Furthermore, the volume of military systems is insufficient to support a dedicated PIC infrastructure at a bearable cost. DARPA launched SBIR  to solve this problem by developing field-configurable modulator arrays (FCMAs) that can be
purposed for military and commercial applications alike.


The FCMA concept is based on a set of electro-optic modulators that can be programmed for various functions. This SBIR will focus on lithium-niobate FCMAs. Though many materials are being considered for PICs, lithium niobate is mature, cost-effective, and provides the performance needed for military applications. Under this Direct-to-Phase 2 SBIR, performers will be required to design, fabricate, and demonstrate a FCMA that provides functionality for electronic protection, signals intelligence, radar beamforming, and communications. The progress and success of the SBIR will be measured by the following parameters. The FCMA must operate from 1 MHz to 18 GHz for all configurations.


The electronic-protection configuration shall utilize the nonlinear response of a Mach-Zehnder modulator to suppress a continuous-wave interference signal by 60 dB and suppress an interference signal with 10 MHz instantaneous bandwidth by 40 dB, both while reducing the largest intermodulation distortion by 30 dB. The signals-intelligence configuration shall improve the intrinsic third-order-limited spurious-free dynamic range of a Mach-Zehnder modulator by 10 dB. The radar-beamforming application requires the FCMA to provide 360 degrees of RF phase shift that can be modulated at 100 kHz. The communications configuration must support 10 Gb/s modulation on each of the in-phase and quadrature components of a lightwave.


Generating RF with Photonic Oscillators for low Noise (GRYPHON)

The DARPA Microsystems Technology Office is solicited innovative research proposals to develop compact, low noise microwave frequency synthesizers to enable advanced sensing and communication applications, in June 2021.


The GRYPHON program seeks to leverage the advantages of photonic microwave generation to develop integrated sources with phase noise performance that meets or exceeds that of the best discrete oscillator modules (e.g., oven-controlled crystal oscillators (OCXO), oven-controlled SAW oscillators, and dielectric resonator oscillators (DRO)), yet occupy a compact volume typical of far noisier chip-scale voltage controlled oscillators (VCO) (e.g., monolithic microwave integrated circuits (MMIC), Yttrium Iron Garnet (YIG), and uncompensated XOs). Moreover, by program end, GRYPHON microwave sources will operate as synthesizers with the ability to tune to any frequency from 1 – 40+ GHz during operation. This combination of features is unprecedented in today’s state of the art, and will establish a new regime of source technology that is expected to transform the types and capabilities of military and commercial microwave systems.


GRYPHON will pursue advanced prototype development as well as research studies into novel microwave generation techniques and components. In addition to the primary focus on performance and integration, later program phases will harden GRYPHON prototypes to basic environmental stresses sufficient to validate the technology for application-specific maturation after the conclusion of the program. DARPA expects that partnerships between the defense industrial base and the academic and small business research community may be necessary in order to achieve all program goals.


While DARPA is primarily interested in solutions that leverage photonic techniques, any approach that demonstrates a credible path to satisfying all metrics and goals of the proposed technical area will be considered for selection. Approaches that are compatible with future lowcost manufacturing at volumes of 1,000 to >10,000 units per year are preferred, and proposals should identify the technical elements that support this goal (e.g., microelectronics foundry fabrication of photonic components, automatable assembly of small numbers of microchips, etc.). Solutions that use domestic manufacturing capabilities to achieve program goals are preferred, as DARPA seeks to strengthen DoD access to differentiating technologies. Proposers should note that GRYPHON technologies may be subject to export control regulation, and refer to the Controlled Unclassified Information (CUI) Guide published alongside this BAA to determine how data and hardware will be safeguarded during the program.


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