N24A-T003 TITLE: High-Frequency 40 GB/s MWIR and LWIR Metamaterials-based Electro-Optical Modulators for Free-Space Optical Communications

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Integrated Network Systems-of-Systems; Sustainment; Trusted AI and Autonomy

OBJECTIVE: Develop tunable metamaterials that enable narrow-linewidth multi-watt laser transmitter, which operate with ultrafast modulation (40 GHz) and high-beam quality in the 4 to 12 µ spectral region, to provide optical communications in RF-denied environments.

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 RF-denied and/or contested environments. Most current proposed or deployed FSO systems are in the short wave Infrared (SWIR) regime at around 1.55 µm 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 and satellite uplinks have seen some success, but the 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 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 verified that there are advantages to using long-wave infrared (LWIR) wavelengths for FSO links through the atmosphere [Ref 3]. When the channel transmission is affected by fog, clouds, haze, dust, or turbulence, a near-IR (~ 1.55 µm wavelength) FSO link suffers significantly more attenuation relative to LWIR systems. With the exception of fog and clouds, mid-wave infrared (MWIR) systems share the advantages of LWIR and benefit from higher performance lasers and detectors, as well as reduced diffraction relative to LWIR. Unfortunately, the high-cost, low-bandwidth, and low-output power of otherwise suitable MWIR and LWIR sources has prevented their adoption for this application.

Quantum Cascade Lasers (QCLs) and Intraband Cascade Lasers (ICLs) have seen significant development in the past decade, to the point where devices with watt-level output are now feasible at wavelengths covering most of the MWIR, 3-5 µ and LWIR, 8–14 µ spectral regions. Unfortunately, the functionality of these devices is limited, particularly for laser communications, due to the lack of an appropriate modulator. The carrier population in a QCL may be modulated to ~ 100 GHz due to ultrafast carrier dynamics, but the optical modulation bandwidth is significantly less than 10 GHz due to the photon lifetime in the cavity. As the cavity is made longer to produce more power, the modulation speed is reduced by increased photon lifetime, as well as the difficulty of modulating the large current, > 1 A, required in a multi-watt device. Furthermore, high-power devices experience beam pointing instability under large-signal modulation [Ref 4]. These problems are solved in near-IR communications through external modulation and optical amplification, but optical amplification that is compatible with a modulated signal is not available over the majority of the spectral ranges of interest, nor has an appropriate modulator been demonstrated.

Optical metamaterials (MMs), sub-wavelength electromagnetic structures that exhibit optical properties not readily found in natural materials [Ref 5], have huge potential for use in LWIR devices where sub-wavelength structures are > 1 µm and so may be readily fabricated with well-established lithography. These materials are presently being explored for numerous applications including tunable filters [Ref 6], multicolor IR imaging [Ref 7], ultracompact IR optical components [Ref 8], and optical switching [Ref 9]. However, to be useful as an active device such as a modulator, the metamaterial must be tunable at speeds greater than 10 GHz. Extremely compact plasmonic devices have been demonstrated for on-chip collimation, as well as other optical functions [Ref 8], but so far appear to be difficult to fabricate and lack tunability.

Recently, tunable metamaterial devices based on carrier depletion have been demonstrated with the fastest published result reaching 1.5 GHz (750 MHz 3-dB bandwidth) and projected operation up to 10 GHz [Ref 10]. Further development of these materials is needed to achieve low-insertion loss, high-modulation depth, and modulation to speeds > 40 GHz. The metamaterial-based optical modulator is a viable and feasible technology and the following metrics are the Higher efficiency, longer FSO link for this project:

	Threshold	Goal	Primary Benefits
Signal bandwidth (3-dB)	> 10 GHz	> 40 GHz	Larger data rate, lower cost per bit
Insertion loss	< 3 dB	< 1 dB	Higher efficiency, longer FSO link
Optical output power	> 0.5 W	> 4 W	Longer FSO link
Optical aperture	> 50 µ	> 500 µ	Lower cost (simpler alignment)
Optical bandwidth	> 10 nm	> 100 nm	Lower cost (wavelength control)
Modulation depth	> 5%	> 90%	Higher efficiency, longer FSO link

PHASE I: Develop concepts for a tunable metamaterial-based optical modulator capable of providing dynamic narrow linewidth tunable properties within the MWIR/LWIR spectral range. It is expected that these concepts will be breadboard demonstrated in order for the tunable metamaterial-based optical modulator to be optimized to operate with a laser transmitter (e.g., QCLs or ICLs).

Demonstrate the feasibility of the proposed tunable metamaterial-based optical modulator concept through numerical simulation and breadboard demonstration of the basic physics of the device compatible with achieving the above topic description threshold performance requirements.

Required Phase I deliverables will include a report with a modeling plan, device designs, and performance goals.

The Phase I effort will include prototype plans to be implemented under Phase II.

PHASE II: Fabricate and demonstrate a prototype system having a laser transmitter operating with a tunable metamaterial-based optical modulator operating in a MWIR or LWIR atmospheric window. The prototype system will be evaluated to determine its capability in meeting the performance goals defined in the Phase I report. System performance will be demonstrated through prototype evaluation and modeling or analytical methods over the required range of parameters. Evaluation results along with military specification considerations that were not addressed in the Phase I concept design will be used to refine the prototype into a design that will meet this STTR topic description requirements.

PHASE III DUAL USE APPLICATIONS: Finalize packaging for transition to military and commercial applications. Develop a plan and demonstrate capability to fabricate and package devices for military platforms and outline design for typical avionic ruggedness requirements. Perform final avionics integration activities and qualification testing. Demonstrate plan for device manufacturing. Provide support for operational testing and validation and qualify the system for Navy use.

Commercial applications for this technology could include telecommunications, imaging, sensing, satellite communications, fiber-optic networks, wireless networking, terrestrial optical links, infrared dynamic labels, and object identifiers.

REFERENCES:

1. Rensch, D. B., & Long, R. K. (1970). Comparative studies of extinction and backscattering by aerosols, fog, and rain at 10.6 µ and 0.63 µ. Applied Optics, 9(7), 1563-1573. https://doi.org/10.1364/AO.9.001563
2. Juarez, J. C., Souza, K. T., Nicholes, D. D., O'Toole, M. P., Patel, K., Perrino, K. M., Riggins, J. L. II, Tomey, H. J., & Venkat, R. A. (2018, February). Testing of a compact 10-Gbps Lasercomm system at Trident Warrior 2017. In Free-Space Laser Communication and Atmospheric Propagation XXX (Vol. 10524, p. 105240E). International Society for Optics and Photonics. https://doi.org/10.1117/12.2290143
3. Delga, A., & Leviandier, L. (2019, February). Free-space optical communications with quantum cascade lasers. In Quantum sensing and nano electronics and photonics XVI (Vol. 10926, pp. 140-155). SPIE. https://doi.org/10.1117/12.2515651
4. Bewley, W. W., Lindle, J. R., Kim, C. S., Vurgaftman, I., Meyer, J. R., Evans, A. J., Yu, J. S., Slivken, S., & Razeghi, M. (2005). Beam steering in high-power CW quantum-cascade lasers. IEEE journal of quantum electronics, 41(6), 833-841. https://doi.org/10.1109/JQE.2005.846691
5. Cheben, P., Halir, R., Schmid, J. H., Atwater, H. A., & Smith, D. R. (2018). Subwavelength integrated photonics. Nature, 560(7720), 565-572. https://doi.org/10.1038/s41586-018-0421-7
6. Jun, Y. C., Gonzales, E., Reno, J. L., Shaner, E. A., Gabbay, A., & Brener, I. (2012). Active tuning of mid-infrared metamaterials by electrical control of carrier densities. Optics express, 20(2), 1903-1911. https://doi.org/10.1364/OE.20.001903
7. Montoya, J. A., Tian, Z. B., Krishna, S., & Padilla, W. J. (2017). Ultra-thin infrared metamaterial detector for multicolor imaging applications. Optics express, 25(19), 23343-23355. https://doi.org/10.1364/OE.25.023343
8. Yu, N., Blanchard, R., Fan, J. A., Wang, Q. J., Kats, M., & Capasso, F. (2010, January). Wavefront engineering of semiconductor lasers using plasmonics. In 2010 3rd International Nanoelectronics Conference (INEC) (pp. 70-71). IEEE. https://doi.org/10.1109/INEC.2010.5424528
9. Sharkawy, A., Shi, S., Prather, D. W., & Soref, R. A. (2002). Electro-optical switching using coupled photonic crystal waveguides. Optics Express, 10(20), 1048-1059. https://doi.org/10.1364/OE.10.001048
10. Pirotta, S., Tran, N. L., Jollivet, A., Biasiol, G., Crozat, P., Manceau, J. M., Bousseksou, A., & Colombelli, R. (2021). Fast amplitude modulation up to 1.5 GHz of mid-IR free-space beams at room-temperature. Nature communications, 12(1), 1-6. https://doi.org/10.1038/s41467-020-20710-2

KEYWORDS: Laser; Modulator; Optical Transceiver; Metamaterial; Tunable; Communications

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