N211-015 TITLE: Long-Wave Infrared Transceivers for High Speed Free Space Optical Communications in Adverse Weather Conditions
RT&L FOCUS AREA(S): General Warfighting Requirements
TECHNOLOGY AREA(S): Information Systems
OBJECTIVE: Develop long-wave infrared transceiver components with high-data rate and low-bit error rate for use in free-space optical communications in adverse weather conditions.
DESCRIPTION: Free-space optical (FSO) communication links provide high-data rate, low latency, secure wireless, mobile communication that are difficult to jam or intercept and do not require spectrum management. FSO communication is an especially compelling alternative to a radio-frequency (RF) link with external RF Interference (RFI) in a RF-denied environment. Most current proposed or deployed FSO systems are in the short wave Infrared (SWIR) regime at around 1.55 micrometers due to ubiquity of the laser and optical components customized for fiber optical communications. Exceptionally high data rates at this wavelength range are possible when atmospheric effects are not present [Ref 1], and laser-based FSO communication is the leading solution for interconnecting new constellations of low-earth-orbit satellites. Terrestrial FSO links have seen some success, but link budget in the SWIR regime is often limited by optical obscurants such as haze, fog, clouds, atmospheric absorption, and turbulence presence in the atmosphere. SWIR links with stabilized telescopes have been demonstrated to achieve gigabit per second (Gb/s) communication between naval vessels in ship-to-ship and ship-to-shore configurations at ranges of 12 and 45 kilometers (km) [Ref 2], despite the link limitation to 1 km when the visibility was impaired by heavy fog. For FSO laser communications systems operating in the SWIR bands, including 1300 nm and 1550 nm, the photonic wavelength is comparable to the size of aerosols that scatter and attenuate the laser beam propagation in the channel.
Recent analysis has shown that operation in a more optimal long wave infrared (LWIR) wavelength range accessible from monolithic sources only via Quantum Cascade Lasers (QCLs) enables dramatically lower attenuation from a variety of atmospheric effects [Ref 3]. The attenuation due to the presence of optical obscurants, such as fog, haze, and maritime aerosols for 10-micrometer (µm) wavelength transmission, is strikingly over 300 times lower than that at 1550 nm. Furthermore, LWIR FSO communication link at 10 µm wavelength have much reduced Rayleigh scattering compared to the 1.55 µm counterpart. At the same time, the fast carrier dynamics of QCLs make high-speed direct modulation possible [Ref 4], thereby also reducing transmitter complexity.
The main goal of this SBIR topic is to develop the LWIR transceiver, including the laser for the transmitter and detector for the receiver to leverage the unique LWIR atmospheric transmission window that is more transparent than other wavelengths in adverse weather conditions. The adverse weather condition is defined as the atmospheric conditions where a 1.55 µm FSO link would suffer > 25dB attenuation due to multiple scattering caused by various hydrometeor types such as haze, clouds, fogs, and aerosols such as dusts, smoke, and pollens [Ref 1]. Current Fabry-Perot (FP) QCLs emitting in the 10-micron regime provide less than 1W single-facet continuous wave (CW) power with less than 5% efficiency [Ref 5]. Large QCLs have modulation bandwidths that are limited by the large device capacitance. Commercial distributed feedback (DFB) QCLs in this wavelength range emit less than 100 milliwatts, potentially limiting the FSO link budget. Innovative QCL designs are needed to increase the QCL room temperature CW output power while maintaining beam quality (M^2 < 1.5) and high reliability for the LWIR FSO system.
The Threshold and Objective parameters of QCL, detectors, and the transceivers are as follows:
Cost-effective FSO links must function with devices’ temperatures near ambient (25 degrees C) to minimize cooling system cost, size, and power. At these temperatures, thermally induced dark current unacceptably limits detectivity of conventional LWIR photodetectors needed for the receiver side of the FSO link. Reducing detector volume reduces the dark current, but also the area and responsivity. Recent research has shown that metal and dielectric resonators can enhance the collection area and responsivity, enabling high detectivity in the LWIR near room temperature [Ref 6]. High detectivity has been demonstrated in devices based on both inter-band and inter-subband absorption, but innovative designs are certainly required to achieve both high speed and high receiver sensitivity simultaneously.
FSO links based on LWIR QCLs and detectors operating at wavelengths optimized for highest system level performance will enable secure, mobile, naval communications in RF congested and denied environments. With the successful development of these critical LWIR components, a cost-effective and low space, weight, and power (SWaP) digital communication link that supports encryption with effective range over 100 km will be the objective of future development.
PHASE I: Design, develop, and demonstrate LWIR lasers and detectors needed for 10 Gb/s transmission for the adverse weather conditions [Ref 1]. The design should include plans for growth, fabrication, packaging processes, and a monolithic QCL transmitter emitting in the 10-micron wavelength region capable of 1W single facet CW operation and direct modulation bandwidth > 5 GHz. Detectors should have commensurate performance to enable the 10 Gb/s link. The Phase I effort will include prototype plans to be developed under Phase II.
PHASE II: Develop, demonstrate, and validate a prototype FSO link operating in the 10-micron region with at least 10 Gb/s data rate and BER of 1E-12 with 1 W average, single-spatial-mode transmitter launch power for the adverse weather conditions. Perform testing to explore the limits of operational speed and distance. Provide a production cost model.
PHASE III DUAL USE APPLICATIONS: Finalize development of the prototype based on Phase II results for transition and integration into a Navy operational test asset. Conduct risk management and mitigation.
Telecommunications and local, urban communications (communication nodes – line of sight) would benefit from this technology due to its high bandwidth capability even in adverse weather conditions.
KEYWORDS: long wave infrared; LWIR; photonics; lasers; photodetector; Free-space Optical; FSO Communications; adverse weather
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