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.
NASA inventors have developed a space-and-wave (SAW) division method to de-multiplex low energy QKD photons from high energy free space optical transmissions. The method exploits available double-clad fiber optic cable with 9um core fiber diameter and 105um 1st cladding outer diameter.
By optimizing wavelengths used for QKD photon and data transmission (wave division), a focusing lens can be used to create a diffraction pattern that focuses QKD photons on the core and the higher energy signal onto the 1st cladding (spatial division).
The SAW method enables de-multiplexing of simultaneously transmitted optical data and built-in photonic QKD encryption keys. The method was developed by NASA to send and receive encryption-keys using weak coherent pulsed light rather than entangled photons; it can be applied to the encryption of any free space optical communications.
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.
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.
There are several applications of this technology in telecommunications, including QKD-encrypted optical data links (satellite-to-ground, satellite-to-satellite, aircraft-to-ground, etc.).