Duration: 17 months
Typical Optical Communication Terminals (OCTs) make use of the optical properties of polarization or wavelength to separate the transmit and receive channels. Polarization separation is effective if the polarization state of the receive channel is well-defined, but this is rarely the case in space-based OCTs. Additionally, this means of separation can be prone to self-blinding and feedback if foreign-object debris is present, even in small quantities. Wavelength selection resolves the issues associated with polarization, but at the cost of being unable to switch between the pre-selected transmit and receive wavelengths. Alternatively, it is possible to introduce a moving dichroic filter to actively select the required wavelengths, but this introduces additional cost, mass and design complexity. This proposal seeks to address all these issues by introducing a four-layer Volume Bragg Grating (VBG) directional filter. The primary advantage is significant mass savings because the VBG will be a single optical block (2-3 times the beam diameter). The second advantage of this design is that it will effectively separate and merge the transmit and receive channels without any moving parts and without resorting to polarization separation. Third, there is no requirement for any electronics or control to set the VBG position as would be required with wavelength separation. VBGs are transmissive diffraction gratings based on periodic layers of high and low refractive index. The gratings can be tuned to either transmit or reflect different beams on the basis of wavelength (spectral filter) or angle of incidence (angular filter). These properties can be combined to achieve simple multiplex gratings by judiciously selecting the four gratings to achieve a directional filter based on reciprocal optics. It is important to note there is always a small angle between the transmitted and received beams that is used to enable this invention.
