Secure Optical Quantum Communications

Classical laser communication gimbals are coupled to 105um multimodal receiving fibers for the high-power transmission of data, fine pointing, and tracking. These fibers cannot be used in free space optical communication applications using Quantum Key Distribution (QKD) since polarization state information encoded by QKD photons is not retained. To accommodate low energy QKD photons and high energy data streams necessary for encryption of optical links, the inventor adopted a space-and-wave (SAW) division de-multiplexing approach. The SAW division method uses a double clad fiber with a 9um core and a 105um 1st cladding. This arrangement captures 1590nm wavelength QKD photons in the core channel and a 1555.75nm wavelength data channel in the 1st cladding. By defining wavelength separation between 30-40nm, a single focusing lens can be used to focus only one wavelength to a diffraction limited spot (see figures included). Using this method, a QKD channel is focused to a diffraction limited spot on the 9um core of the double clad fiber. The chosen wavelength separation generates a defocused diffraction pattern with a hollow center, and with remaining optical power in concentric rings outside of the 9um core, yet inside the 105um core. The QKD signal is directed into the 9um core, and the data channel is coupled into the 105um secondary core for traditional data demodulation.

NASA Glenn's researchers have developed a means of transporting multiple radio frequency carriers through a common optical beam. In contrast to RF infrastructure systems alone, this type of hybrid RF/optical system can provide a very high data-capacity signal communication and significantly reduce power, volume, and complexity. Based on an optical wavelength division multiplexing (WDM) technique, in which optical wavelengths are generated by a tunable diode laser (TDL), the system enables multiple microwave bands to be combined and transmitted all in one unit. The WDM technique uses a different optical wavelength to carry each separate and independent high-frequency microwave band (e.g., L, C, X, Ku, Ka, Q, or higher bands). Since each RF carrier operates at a different optical wavelength, the tunable diode laser can, with the use of an electronic tunable laser controller unit, adjust the spacing wavelength and thereby minimize any crosstalk effect. Glenn's novel design features a tunable laser, configured to generate multiple optical wavelengths, along with an optical transmitter. The optical transmitter modulates each of the optical wavelengths with a corresponding RF band and then encodes each of the modulated optical wavelengths onto a single laser beam. In this way, the system can transmit multiple radio frequency bands using a single laser beam. Glenn's groundbreaking concept can greatly improve the system flexibility and scalability - not to mention the cost of - both ground and space communications.

Initially developed for missions to Mars, Teletenna integrates RF and optical communication technologies to transmit data from deep space to Earth at extremely high speeds. The system combines a co-boresighted telescope and a Ka-band RF antenna to minimize system mass and enhance performance. Designed with an optimal focal length-to-diameter ratio, the apparatus features a classical Cassegrain geometry, including a sub-reflector in front of the RF feed which acts as a mirror for the optical signal while being transparent to the RF signal. The apparatus also mechanically and thermally isolates the RF reflector system from the optics to offer maximum stability. Teletenna was created to overcome two significant challenges to DSOC: 1) laser inefficiency due to poor alignment during spacecraft disturbances and 2) performance degradation due to lack of rigidity in vibrational environments (such as space). The first challenge is addressed by the telescope portion of this technology, which facilitates the acquisition and maintenance of the link with ease - even in less than ideal conditions. The second challenge is addressed by rigidly fixing the RF reflector to the spacecraft body and attaching the optical section to a vibration isolation platform. The result is a device that can point to within 0.5 degrees of the sun (traditional optical systems are limited to 3 degrees), allowing for approximately 20 extra days of contact time between Earth and Mars. By combining RF and optical communications, this breakthrough innovation has the power to transform communications as we know it. Glenn welcomes co-development opportunities.

The new NASA LaRC Pulsed 2-Micron Laser Transmitter for Coherent 3-D Doppler Wind Lidar Systems is an innovative concept and architecture based on a Tm:Fiber laser end-pumped Ho:YAG laser transmitter. This transmitter meets the requirements for space-based coherent Doppler wind lidar while reducing the mission failure risks. A key advantage of this YAG based transmitter technology includes the fact that the design is based on mature and low-risk space-qualified YAG host crystal. The transmitter operates at a 2096 nm wavelength using Ho:YAG, resulting in high atmospheric transmission (>99%), versus a transmitter operating at 2053 nm using co- doped Tm:Ho:LuLiF, which suffers limited transmission (90%) due to water vapor interference. In-band pumping through Tm:Fiber pump Ho:YAG architecture offers lower quantum defect from 1908 to 2096 nm (9.1%) compared to traditionally used co-doped Tm:Ho:LuLiF of 792 to 2051 nm (61%). The transmitter has an efficient pump compared to LuLF, since YAG has 27% higher pump absorption and 52% lower reabsorption of the emitted 2-micron, resulting in higher efficiency and lower heat load. Being isotropic, YAG is amenable for spatial-hole burning mitigation which supports linear cavity architecture without compromising injection seeding quality. This attribute is important in designing a compact, stable, high seeding efficiency laser. A folded linear cavity for long pulse (>200 ns), transform limited line-width (2.2 MHz) and high beam quality (M2 = 1.04) - the most critical parameters for coherent detection - are easier to achieve using YAG compared to LuLF. Lower heat load results in high repetition rate (>300 Hz) operation, which allows higher probability of wind measurements through broken clouds, off clouds, and below clouds, thus reducing errors and increasing science data product quantity and quality.

The NASA receiver is specifically designed for use in coherent LiDAR systems that leverage high-energy (i.e., > 1mJ) fiber laser transmitters. Within the receiver, an outgoing laser pulse from the high-energy laser transmitter is precisely manipulated using robust dielectric and coated optics including mirrors, waveplates, a beamsplitter, and a beam expander. These components appropriately condition and direct the high-energy light out of the instrument to the atmosphere for measurement. Lower energy atmospheric backscatter that returns to the system is captured, manipulated, and directed using several of the previously noted high-energy compatible bulk optics. The beam splitter redirects the return signal to mirrors and a waveplate ahead of a mode-matching component that couples the signal to a fiber optic cable that is routed to a 50/50 coupler photodetector. The receiver’s hybrid optic design capitalizes on the advantages of both high-energy bulk optics and fiber optics, resulting in order-of-magnitude enhancement in performance, enhanced functionality, and increased flexibility that make it ideal for long-distance or low-backscatter LiDAR applications. The related patent is now available to license. Please note that NASA does not manufacturer products itself for commercial sale.