A reliable long-distance and high-speed underwater communication link plays an important role in the undersea exploration and data collection. At present, the most widely used underwater communication method is still dominated by underwater acoustic communication. Although it can reach the transmission distance of several kilometers and more, the data rate is very low, typically on the order of tens of kilo-bits per second. As another option, the use of radio frequency (RF) communication is limited by the skin effect of seawater, and the transmission distance is generally below 10 m.
NetCentric warfare requires persistent, rapid, and preferably covert data communication among all platforms. Submarine to submarine, ship to submarine and satellite to submarine communication has emerged as one of the most challenging and necessary technologies in the present network centric warfare.
Submarine communications have always been a challenge because radio waves can’t penetrate sea water. Ultra-low frequency electromagnetic waves have been used but are a slow method of communicating. Submarines are completely reliant on satellites for communications and orders from their commanders ashore. However, Satellite communications require submarines to briefly surface and the use of towed antennae compromise the ability of the vessel to remain stealthy.
However, challenge is communications in command and control denied environment (C2DE), the area in which communications are jammed or degraded. There is no technology currently available that allows submarines to conduct communications in a C2DE. The only method currently available is for the submarine to navigate to unaffected waters, conduct all of its communications, and then to travel back to the C2DE, wasting valuable time and possibly compromising the submarine’s mission.
Blue-green lasers is a potential technique for high bandwidth underwater wireless communication because of its high data transfer rate, reasonably large range, small size, low power consumption, immunity to interference and jamming and covertness of transmission. The laser works in much the same way as a fiber optic cable, with the medium for data transfer being the air instead of the cable. Blue and blue-green laser wavelengths can penetrate sea water so offer the potential of improved submarine communications. Blue and blue-green laser wavelengths can penetrate sea water so offer the potential of improved submarine communications.
As long as there is a clear LOS between the transmitter and receiver, high data transfer rates are available.The technology also works under water, but the range of transmission is greatly diminished. Data transfer rates of between 7 and 10 Mbps with a 99.99% success rate were observed, but only in the 10 to 20 meter range.
“Fibertek has developed for the U.S. Navy’s Space and Naval Warfare Systems Command (SPAWAR) a green underwater laser system capable of multirate high- bandwidth communication up to 1 Gbs in clear or littoral waters at 10 to 100 meters range. The system was evaluated in simulated and representative waters, over a variety of link geometry and water conditions,” according to US Navy SPAWARS.
Navy requirements for Blue-Green lasers: Laser communication could provide critical data link between submarines and command center
High power lasers have many potential applications in DOD systems. With the rapid increase in UAVs and UUVs, the requirements for compact, efficient and lightweight laser sources has increased rapidly. In addition to communications, Navy missions such as mine detection and other optical technologies that utilize blue-green lasers could also benefit from the development of the laser system.
The Navy is investing heavily in the use of unmanned underwater vehicles (UUVs) to help in areas including mine warfare, oceanography, salvage, and rescue operations. Used in conjunction with the blue-green laser, the UUV would be able to meet all of the submarine’s communication needs without the submarine ever coming to PD. The laser-fitted UUVs relay information from anchored data nodes to a sensor in the submarine’s sail.
The modeled UUVs will patrol a linear area recharging at the completion of each patrol at an undersea garage, according to Forest B. Mclaughlin of the Naval Post Graduate School who carried out the research. The garage will receive continuous updates from a sensor placed outside of the C2DE, but tethered to the garage. The garage will update the UUVs while they recharge and relay its continuous data feed to data links spaced along the patrol route of the UUVs.
The UUVs will then download updates while passing by the data links to refresh their current information.When a submarine comes in contact with one of these UUVs, it will slow down and allow the UUV to approach. The submarine will then receive the UUVs broadcast via LOS blue-green laser transmission from above.
DARPA’s TRITON research program for Blue Green laser communications between Aircraft and Submarine
DARPA had launched in June 2012,Tactical Relay Information Network (TRITON) research program to develop a blue-laser submarine communications system able to link submerged submarines with nearby aircraft for anti-submarine warfare (ASW).
The TRITON project was to build on technology, DARPA developed in the 1990s under the Tactical Airborne Laser Communications (TALC), which tested blue-green laser communications to link submerged submarines with Navy P-3 maritime patrol aircraft. TALC matched a blue laser to a cesium atomic line resonance receiver at 455.6 microns; the downlink was a green diode-pumped laser compatible with existing submarine receivers at 532 microns.
The Navy is interested in submarine laser communications to reduce reliance on towed-buoy receivers, to enhance the communications reliability and data throughput to ballistic missile submarines, and to enhance coordination among aircraft and fast attack submarines for ASW. TRITON sought to overcome the limitations that sunlight poses to submarine laser communications, which requires a high peak power laser and an optical filter with narrow spectral bandwidth, high transmission, and wide-field-of-view.
The U.S. Defense Department had awarded QinetiQ North America a contract to develop a blue laser communications system. QinetiQ under Its Submarine-Enabling Airborne Data Exchange and Enhancement Program (SEADEEP) has already demonstrated communications through the air-water interface equivalent to data rates available with wideband Internet communications at home, according to the company. In recent years, the interest in free space optical communication has renewed from advancements in blue-green sources and detectors.
Blue Green laser communication sytem
Optical waves can serve as new information carriers in underwater communication. The seawater has a wavelength absorption window of blue-green light , which provides the feasibility for underwater optical wireless communication (UOWC). Research on various underwater optical channel characteristics and models has made breakthroughs in recent years. In addition, the rapid development of the semiconductor technology offers a possibility of system implementation. Light-emitting diodes (LEDs) and laser diodes (LDs) have been used as light sources for building UOWC systems.
The performance of an UOWC system depends on the optical source, detector, optical channel and communication techniques to deal with system imperfectness.
The large divergence angle of the commercial LED results in severe geometric losses in the water, so the transmission distance is limited. Moreover, its limited bandwidth of the 10 MHz level makes it difficult to achieve a higher data rate. A collimated laser beam has very small divergence on the order of milli-radians, and is able to propagate along a long-distance underwater channel. Meanwhile, the modulation bandwidth of an LD is relatively high, which is the key to realization of a high-speed UOWC system.
Recently much attention has been paid to the long-distance or high-speed UOWC based on blue or green LDs and LEDs. Key performances of the UOWC systems with data rate at least 1.5 Gbps or distance longer than 100 m reported in the literature. In 2016, a 1.5 Gbps UOWC link over 20 m underwater channel using a 450 nm LD and non-return-to-zero on-off keying (NRZ-OOK) modulation was demonstrated. In the same year, an innovative UOWC system based on a two-stage injection-locked 405 nm LD transmitter with 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal was proposed and demonstrated, achieving up to 8 m/9.6 Gbps.
In 2017, a Japanese team designed an underwater optical communication detector called “Kaiko”, which used avalanche photodiode (APD) and photomultiplier (PMT) as receiver to successfully realize 120 m and 20 Mbps data transmission at a depth of 700 m . By using NRZ-OOK modulation, a 2.2 Gbps UOWC link at the underwater transmission distance of 12 m was reported. Data rate of 16 Gbps at 10 m transmission distance was achieved by using four-level pulse amplitude modulation (4-PAM), in which the light injection and optoelectronic feedback techniques were utilized to enlarge the modulation bandwidth. In addition, a UOWC system with the maximum rate of 5.6 Gbps and a distance of 10.2 m was reported. The modulation method used was 16-QAM-OFDM. A high-speed communication system of 5.5 Gbps operated through a water-air channel with 21 m water sub-channel and 5 m air sub-channel was realized. The modulation format of 32-QAM-OFDM was adopted.
By using NRZ-OOK modulation, 2.7 Gbps UOWC communication at the underwater transmission distance of 34.5 m was demonstrated. In 2018, 3.32 bits per photon were delivered by an UOWC system using 256-pulse-position modulation (PPM) and different rate Reed-Solomon (RS) and Low-density Parity-check (LDPC) codes. In 2019, a commercial underwater optical communication product called BlueComm-200 was released. The combination of a 450 nm LED and a PMT yielded the maximum reach of 150 m/10 Mbps. Recently, a team also implemented a 60 m UOWC system with the maximum data rate of 2.5 Gbps based on NRZ-OOK and a 450 nm LD.
Underwater BG laser Communication System
Fibertek has developed a green underwater laser system capable of multi-rate high-bandwidth communication in clear or littoral waters at 10-100 meters range. The transmitter and receiver are packaged in 12″ diameter UUV compatible water-tight canisters for submersible operation.
The flexible fiber-laser based transmitter is capable of data transmission up to 1 Gbps and can also be operated in a programmable RF-modulated pulsed mode for underwater imaging applications. With the high-bandwidth photomultiplier tube based receiver, back-to-back communication has been demonstrated at up to 250 Mbps.
Blue Green Laser technologies
Lasers must operate in the blue-green wavelength region and be pulsed with nanosecond durations at rates ranging from 10’s to 1000’s of kHz. The nature of the operating platforms requires the lasers be low in power consumption, small, lightweight, rugged and reliable.
Group of scientists from the Institute of Laser-Physics, the Center for Free-Electron Laser Science at DESY, and the Hamburg Centre for Ultrafast Imaging (all in Hamburg, Germany) and Università di Pisa (Italy) has developed a green-emitting laser based on doping by the rare-earth metal holmium (Ho), in the form of a doped lithium lutetium fluoride (Ho3+:LiLuF4) crystal. Pumped by a blue laser diode, the room-temperature pulsed laser emitted at a 549.4 nm wavelength with an average output power of 7.7 mW, a pulse-repetition rate of 5.3 kHz, and pulse duration of 1.6 μs.
Blue Laser Diode Enables Underwater Communication at 12.4 Gbps
Researchers from Taiwan Wu, T.C. and others demonstrated high-speed underwater wireless optical communication (UWOC) in tap-water and seawater environments over long distances. The 450-nm blue GaN laser diode (LD) directly modulated by pre-leveled 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data was employed to implement its maximal transmission capacity of up to 10 Gbps.
The 450-nm blue LD was controlled at a room temperature of 25 °C for maintaining high external quantum efficiency. The divergent blue laser beam carrying 16-QAM OFDM data was collimated to a parallel laser beam through an objective lens (Newport, F-LA22) with 5.5-mm aperture and 11-mm focal length, and launched into a 1.7-m water tank filled with tap water or seawater.
The proposed UWOC in tap water provided a maximal allowable communication bit rate increase from 5.2 to 12.4 Gbps with the corresponding underwater transmission distance significantly reduced from 10.2 to 1.7 m, exhibiting a bit rate/distance decaying slope of −0.847 Gbps/m. When conducting the same type of UWOC in seawater, light scattering induced by impurities attenuated the blue laser power, thereby degrading the transmission with a slightly higher decay ratio of 0.941 Gbps/m. The blue LD based UWOC enables a 16-QAM OFDM bit rate of up to 7.2 Gbps for transmission in seawater more than 6.8 m.
High-Power Fiber Lasers
Fiber lasers have several advantages over traditional chemical, gas, and solid-state high-power lasers: unequalled beam quality, good heat dissipation, high efficiency, and robust reliability. So far, most fiber lasers operate at wavelengths longer than one micron, as dictated by the application in which they’re used and developments in materials.
In a Laboratory Directed Research and Development (LDRD) project, LLNL researchers are exploring the use of new materials, fabrication methods, and fiber designs with the goal of extending fiber laser technology to shorter wavelengths. Such sources would benefit a variety of applications, including spectroscopy, remote sensing (LIDAR, or light detection and ranging, with water lines), adaptive optics systems (laser guide stars) to correct for atmospheric distortions in ground-based telescopes, underwater communications, and beam delivery (machining and directed energy).
“Fiber laser sources are unmatched in terms of brightness and efficiency due to the combination of waveguiding, long interaction lengths, excellent thermal management and ultrapure materials,” said NIF & Photon Science physicist Paul Pax, the lead researcher on the project. “New short-wavelength pump diodes in the 400- to 450-nanometer (violet light) range are becoming readily available, and their powers are increasing,” he said. “This opens up new avenues for short-wavelength fiber lasers that were not available before.”
A fiber laser typically consists of a dual-core optical fiber, with one core nested inside the other; the inner core is doped with a rare-earth element such as erbium, ytterbium, or neodymium. Light from a pump laser is fired into the end or side of the fiber and guided along the fiber by the undoped outer core. As the pump light passes through the inner core, the dopant is stimulated to emit radiation, or lase, at one of the rare-earth element’s characteristic wavelengths.
Pax said an early success in the program has been the development and fabrication of a neodymium-doped fiber laser with a novel filtering waveguide—the structure that supports well-defined modes, or transmission paths, in the laser—that enables it to operate at a wavelength of 925 nanometers (nm) instead of neodymium’s otherwise-strongest characteristic wavelength of about 1060 nm.
“The strong 1060-nm line has to be suppressed or it depletes all the gain,” he said. “This waveguide allows us to do just that, which makes operation at 925 possible. This wavelength is useful for remote sensing and, with harmonic frequency conversion, for blue-green-light underwater communications. The laser is operating with “very good efficiency and good beam quality,” Pax said. “Power is already a record for this type of laser—27 watts at 925 nanometers—limited by available pump power. And the waveguide design allows for scaling the power by increasing the core size.
The researchers also will test other dopants, including samarium, which lases directly in the visible spectrum at 651 nm, and terbium, which emits at about 545 nm. “The two immediate applications (directed energy and submarine communications) aren’t the only reason to pursue visible fiber lasers,” Pax noted. “The possibilities are opening up because of the new pump diodes, and we want to be in a position to make use of them with novel active species (dopants) for fiber lasers.”
An EU maritime study has resulted in a record-breaking ‘blue-light’ laser with the capacity to transform underwater range-finding, imaging and communication across the maritime industry.
A €1m project supported by the European Regional Development Fund, KETmaritime involves seven partners across Europe whose goal is to find and use Key Enabling Technologies (KET) to bolster the Atlantic maritime industry. “In recent months, France-based multidisciplinary research laboratory CIMAP has been actively developing blue-light lasers in constant wave and pulsed regimes,” Vila said.
She continued: “It recently achieved a record 7.5W constant wave output at 452nm wavelength. This is understood to be by far the highest constant wave ‘pure blue’ power generated from a frequency-doubled fibre laser. “The absorption of light in pure water is lowest in the 400-450nm spectral range. Laser light set in this range can penetrate long distances with minimal reduction in strength. “These light sources can be used to determine distances, or by means of Lidar techniques record underwater objects, like submarines and archaeological sites.
“Conventional methods to detect underwater targets have employed acoustic waves. However, laser-based systems have clear advantages in high directionality and high range resolution. They also allow new methods of wide-band and interception-proofed communication.”
Blue-Green Laser for Undersea Communication Navy SBIR FY2006.1
Aculight has proposed the development of a blue/green laser system for underwater communications. The laser comprises a hybrid system in which a directly modulated 1064nm semiconductor laser is amplified in a multistage Yb-doped fiber amplifier and subsequently frequency doubled in non-critically phase match LBO crystal.
The company will demonstrate key specifications : >10W output power at or around 532nm with pulse durations in the range 0.5-5.0ns and pulse repetition frequencies (PRF) in the range 100kHz-10MHz. The proposed architecture, which overcomes the PRF, size, weight and efficiency limitations of current laser technologies, is ideally suited to the requirements of the solicitation and to the targeted deployment environment, according to the company.
The proposed high power green laser combining high power and high brightness with low signal noise at high repetition rates has a great potential to replace existing mode-locked DPSS lasers and directly modulated LDs for medical, material processing and defense applications, such as micromachining, marking, and semiconductor processing.
Superconducting nanowire single-photon detector (SNSPD)
Superconducting nanowire single-photon detector (SNSPD) has emerged as the fastest single-photon detector (SPD) for photon counting. The SNSPD consists of a thin (≈ 5 nm) and narrow (≈ 100 nm) superconducting nanowire. The nanowire is cooled well below its superconducting critical temperature and biased with a DC current that is close to but less than the superconducting critical current of the nanowire.
Most SNSPDs are made of niobium nitride (NbN), which offers a relatively high superconducting critical temperature (≈ 10 K) and a very fast cooling time (<100 picoseconds). NbN devices have demonstrated device detection efficiencies as high as 67% at 1064 nm wavelength with count rates in the hundreds of MHz. NbN devices have also demonstrated jitter – the uncertainty in the photon arrival time – of less than 50 picoseconds, as well as very low rates of dark counts, i.e. the occurrence of voltage pulses in the absence of a detected photon.
NASA’s Lunar Laser Communication Demonstration (LLCD), in collaboration with JPL’s optical communication group, have developed a ground receiver based on an MDL-fabricated 12-pixel Superconducting Nanowire Single-Photon Detectors (SNSPD) array. This receiver was fielded on the Optical Communications Telescope Laboratory (OCTL) at JPL’s Table Mountain Facility, to successfully downlink error-free data from lunar orbit at 79 megabits per second.
MDL’s tungsten silicides SNSPDs have demonstrated record-breaking efficiency (> 90%) in the infrared, with 150 ps timing jitter, 40 ns reset time, and subhertz intrinsic dark counts. They may be useful for a variety of future applications such as lidar, quantum communications, and high-time-resolution astrophysics.
References and Resources also include:
- Wu, T.-C. et al. Blue Laser Diode Enables Underwater Communication at 12.4 Gbps. Sci. Rep. 7, 40480; doi: 10.1038/srep40480 (2017)