Blue-Green Lasers: Revolutionizing Anti-Submarine Warfare and Network-Centric Operations

Recent breakthroughs have demonstrated the viability of UOWC systems with both impressive data rates and transmission distances. Notably, in 2016, researchers achieved a 1.5 Gbps data rate over a 20-meter underwater channel using a 450 nm LD and NRZ-OOK modulation. Other experiments pushed the boundaries further: a 9.6 Gbps data rate at 8 meters using 16-QAM OFDM; a 2.2 Gbps link over 12 meters; and even 16 Gbps over 10 meters using four-level pulse amplitude modulation (4-PAM) and advanced optical feedback techniques. In deeper underwater environments, Japan’s “Kaiko” system demonstrated 20 Mbps over 120 meters at 700 meters depth using avalanche photodiodes and photomultipliers. These achievements mark a significant transition from experimental setups to viable systems ready for operational use in defense, oceanographic research, and subsea infrastructure monitoring.

Blue Green Laser Enabling Technologies

Blue-green lasers, operating within the 450–550 nm spectral window, are vital for undersea communications because seawater exhibits minimal light absorption at these wavelengths. For defense applications—particularly submarine communication—lasers must meet stringent criteria: compactness, low power consumption, ruggedness, and the ability to pulse in the nanosecond regime at high repetition rates. Research across Europe, the U.S., and Asia has accelerated, driving the development of next-generation blue-green sources that combine high power, efficiency, and reliability.

In Germany, a collaboration among the Institute of Laser-Physics, DESY’s Center for Free-Electron Laser Science, and the Hamburg Centre for Ultrafast Imaging, alongside Università di Pisa, successfully demonstrated a green laser based on a holmium-doped LiLuF₄ crystal. Pumped by a blue laser diode, it emitted pulses at 549.4 nm with a repetition rate of 5.3 kHz and an average power of 7.7 mW. Meanwhile, in Taiwan, researchers achieved a breakthrough in underwater wireless optical communication (UWOC) using a 450-nm GaN laser diode carrying 16-QAM OFDM-modulated data. They demonstrated 12.4 Gbps data transmission in tap water over 1.7 meters and 7.2 Gbps over 6.8 meters in seawater—highlighting the trade-off between bandwidth and range in different underwater conditions.

The rapid advancement in semiconductor technology—particularly in the development of high-power, narrow-beam laser diodes (LDs)—has enabled high-speed, long-range underwater communication links. While light-emitting diodes (LEDs) have been used in earlier systems, their wide beam divergence and limited modulation bandwidth (typically around 10 MHz) result in high geometric losses and relatively low data throughput. In contrast, LDs offer tightly collimated beams with divergence anglhes in the milli-radian range and support modulation bandwidths in the GHz range, enabling gigabit-class data rates over significantly longer distances.

Across the Atlantic, the European Union’s KETmaritime project, funded by a €1 million ERDF grant, advanced the field by developing a continuous-wave blue laser with a record-setting output of 7.5 W at 452 nm. This fiber-based system, designed by France’s CIMAP lab, is optimized for long-range penetration in seawater, ideal for underwater LiDAR, imaging, and secure high-resolution communications. Blue-green laser systems outperform traditional acoustic systems by offering higher directionality, range resolution, and immunity to interception—making them crucial for next-generation anti-submarine warfare and undersea sensing.

In the U.S., companies like Aculight (now part of Lockheed Martin) have developed scalable hybrid laser architectures suitable for naval deployment. Their proposed system, supported by Navy SBIR funding, uses a directly modulated 1064 nm semiconductor laser amplified in a Yb-doped fiber and frequency-doubled via a non-critically phase-matched LBO crystal to reach 532 nm. With a projected output of over 10 W, pulse durations between 0.5–5 ns, and repetition rates from 100 kHz to 10 MHz, such lasers meet the Navy’s stringent requirements for stealthy, high-bandwidth undersea links. These advancements not only promise enhanced communication for submarines and unmanned underwater vehicles (UUVs) but also open doors for dual-use applications in medicine, materials processing, and secure optical networks.

High-power fiber lasers

High-power fiber lasers are emerging as a transformative platform across defense, scientific, and industrial domains due to their superior beam quality, energy efficiency, and compact design. Unlike traditional chemical or solid-state lasers, fiber lasers offer unmatched thermal management, robustness, and scalability by leveraging long interaction lengths within ultra-pure, waveguide-structured optical fibers. Historically, most fiber lasers have operated at infrared wavelengths (above 1 micron), driven by materials limitations and the needs of industrial applications. However, cutting-edge research—particularly at Lawrence Livermore National Laboratory (LLNL)—is pushing the boundaries of fiber laser technology into the visible spectrum. This expansion unlocks new frontiers in remote sensing, underwater optical communication, adaptive optics, and high-resolution spectroscopy, where shorter wavelengths like blue and green are essential.

A recent Laboratory Directed Research and Development (LDRD) project at LLNL led by physicist Paul Pax has demonstrated breakthrough performance in a neodymium-doped fiber laser system. By engineering a novel waveguide design that suppresses the dominant 1060 nm emission line, researchers successfully shifted laser output to the rarer 925 nm band. This achievement lays the foundation for efficient harmonic frequency conversion into the blue-green spectrum—crucial for applications such as submarine communication and oceanic LIDAR. The laser has already achieved a record-setting 27 W of output power at 925 nm, with potential for scaling via larger-core fiber designs. The team is also investigating other rare-earth dopants like samarium and terbium, which can lase directly at visible wavelengths (651 nm and 545 nm, respectively). With the recent availability of powerful short-wavelength pump diodes (400–450 nm), fiber lasers are poised for a new era of compact, high-power light sources, enabling precision beam delivery in underwater environments and next-generation directed energy systems.

Supporting these advances are cutting-edge innovations in both photonics and detection systems. Programs like the EU’s KETmaritime initiative and China’s state-backed research efforts have demonstrated significant progress in generating stable, continuous-wave blue-green lasers with sufficient power for maritime environments. A major milestone was achieved by CIMAP, which developed a 7.5-watt CW laser at 452 nm—an ideal wavelength for penetrating seawater efficiently. These sources are not only vital for underwater optical communication but also serve dual-use roles in bathymetric LIDAR and autonomous underwater vehicle (AUV) navigation.

Bridging the Green Gap: NIST’s Microresonator Breakthrough

A recent advance from the National Institute of Standards and Technology (NIST) has the potential to revolutionize blue-green laser communications by addressing the long-standing “green gap”—the difficulty of producing compact, high-efficiency lasers in the green and yellow spectrum. NIST researchers developed a ring-shaped silicon nitride microresonator that converts infrared laser light into visible wavelengths through optical parametric oscillation (OPO). This chip-scale resonator circulates light thousands of times until new, stable wavelengths are generated, including over 150 distinct colors across the green spectrum.

This innovation is particularly promising for underwater laser communication systems, where water’s transparency is optimal in the 450–550 nm range. Compact, tunable green light sources will significantly enhance UUV payload miniaturization, enable more efficient underwater data links, and support next-gen quantum communication systems. As efforts continue to improve energy efficiency and output power, NIST’s microresonator technology could become a cornerstone in the future of blue-green laser-based defense and maritime networks.

Superconducting Nanowire Single-Photon Detectors (SNSPDs)

Complementing these laser sources are next-generation detectors, particularly superconducting nanowire single-photon detectors (SNSPDs), which offer over 90% efficiency, sub-50 picosecond timing jitter, and extremely low dark count rates. These detectors are crucial for capturing weak or scattered laser signals in turbid marine environments, enabling secure and reliable communication even at depth. Combined with sophisticated modulation formats like OFDM and photon-efficient coding schemes, these technologies are laying the foundation for multi-domain interoperability—where submarines, ships, aircraft, and satellites can operate as a unified, data-driven force in the most challenging and contested maritime zones.

Superconducting Nanowire Single-Photon Detectors (SNSPDs) have become the gold standard for ultra-sensitive, high-speed photon detection in cutting-edge optical systems. These detectors are built from nanometer-scale superconducting wires—typically around 5 nm thick and 100 nm wide—cooled well below their critical temperature and biased just below their superconducting threshold. When a single photon is absorbed, the local temperature rise momentarily disrupts superconductivity, producing a measurable voltage pulse. Among the most widely used materials for SNSPDs is niobium nitride (NbN), which provides an optimal balance of high critical temperature (~10 K), low timing jitter (as low as 50 picoseconds), and low dark count rates. NbN-based SNSPDs have achieved quantum efficiencies up to 67% at near-infrared wavelengths (e.g., 1064 nm), with photon counting rates in the hundreds of megahertz, making them ideal for time-resolved applications in quantum optics and laser communications.

Recent advancements have pushed SNSPD performance even further, with alternative materials such as tungsten silicide (WSi) demonstrating record efficiencies exceeding 90% in the infrared spectrum, alongside jitter of ~150 ps, rapid reset times of 40 nanoseconds, and intrinsic dark count rates below one count per second. NASA’s Lunar Laser Communication Demonstration (LLCD), in collaboration with JPL, successfully deployed a 12-pixel SNSPD array on the Optical Communications Telescope Laboratory (OCTL), achieving error-free optical downlinks from lunar orbit at 79 Mbps—a milestone for space-based optical communication. These breakthroughs position SNSPDs as a cornerstone technology for next-generation systems, ranging from satellite-to-Earth laser links and deep-space lidar to quantum-secure communications and astrophysical instrumentation requiring picosecond-scale timing precision.

Advances in laser packaging, such as compact canisters integrated into unmanned underwater vehicles (UUVs), have made deployment operationally feasible. These systems now support dual-mode operation—communication and imaging—and are suitable for integration into submarines, surface vessels, aerial platforms, and even satellites, enabling cross-domain communication frameworks.

Commercialization of these technologies has also begun. In 2019, Sonardyne introduced the BlueComm-200 system—a robust underwater optical modem combining 450 nm LEDs and PMT detectors to achieve up to 150 meters at 10 Mbps. Meanwhile, research teams have demonstrated 3.32 bits per photon using 256-pulse-position modulation and LDPC coding, signaling improvements not just in raw throughput but in photon efficiency. More recent implementations, such as a 2.5 Gbps UOWC system at 60 meters based on NRZ-OOK and a 450 nm LD, highlight the continued progression toward reliable, high-bandwidth submerged networks. Together, these developments position blue-green laser communication as a cornerstone of future undersea data exchange, enabling stealthy, high-speed links in both civilian and military domains.

Recent Operational Milestones: From Demonstrations to Deployment

Recent years have seen significant progress in blue-green laser systems transitioning from laboratory research to operational platforms. Taiwanese researchers set records by achieving 12.4 Gbps through tap water using 450-nm GaN laser diodes with 16-QAM OFDM modulation. Although seawater attenuates performance, data rates of over 7 Gbps were still achieved over several meters. Systems like Fibertek’s green laser modules deliver 1 Gbps over 10–100 meters in littoral zones and are now packaged for UUV compatibility. Commercial offerings like Sonardyne’s BlueComm-200 achieve 10 Mbps at 150 meters using LED-based transmitters paired with photomultiplier tubes.

DARPA’s Tactical Relay Information Network (TRITON) program, initiated in June 2012, was a major step toward enabling secure, high-bandwidth communication between submerged submarines and nearby aircraft. Building on foundational work from the earlier Tactical Airborne Laser Communications (TALC) initiative in the 1990s, TRITON aimed to overcome the limitations of conventional submarine communication by leveraging the blue-green optical window in seawater. TALC had successfully demonstrated the feasibility of matching a 455.6 nm blue laser with a cesium atomic line resonance filter for underwater signal reception, and used a 532 nm green diode-pumped laser for downlink transmission compatible with legacy submarine receivers. TRITON advanced this approach, focusing on higher data rates, improved signal clarity, and robust performance under operational conditions, including interference from solar background radiation.

The U.S. Navy’s growing interest in blue-green laser communications is tied to strategic needs in anti-submarine warfare (ASW) and ballistic missile submarine (SSBN) fleet operations. Traditional towed-buoy systems for communication require submarines to operate at periscope depth, compromising their stealth. TRITON and associated research sought to eliminate this vulnerability by enabling direct, line-of-sight laser communication from aircraft to submerged subs. QinetiQ North America was awarded a contract under this program and expanded its efforts through the Submarine-Enabling Airborne Data Exchange and Enhancement Program (SEADEEP). SEADEEP successfully demonstrated near-wideband internet-level communications through the air-sea interface, showcasing the viability of blue-green lasers for high-data-rate underwater optical links. Recent advancements in compact, high-power blue-green laser sources and ultra-sensitive detectors—such as atomic line filters and superconducting single-photon devices—have renewed global interest in free-space optical communication, reaffirming DARPA’s early foresight in undersea networked warfare.