Abstract: Methods of providing duplex free-space optical, communication comprising receiving a time-shift keying (TSK) encoded signal and selectively re-modulating—and optionally retro-reflecting —received TSK pulses so as to transmit an on-off keying (OOK) signal wherein modulation is achieved by operating a micro-opto-electronic mechanical system (MOEMS) device having a oscillation period, the difference in timing between logic 1 and logic 0 pulses of the TSK encoded signal being such that each pulse arrives at a time within a single MOEMS device oscillation period chosen to ensure high or low transmissivity through the MOEMS device independent of incident TSK encoded signal pulse value (0 or 1).

Abstract: Optically diverse coded aperture imaging (CAI) includes imaging a scene which is multi-spectrally diverse or polarimetrically diverse. A CAI system allows light rays from a scene to pass to a detector array through a coded aperture mask within an optical stop. The mask has multiple apertures, and produces overlapping coded images of the scene on the detector array. Detector array pixels receive and sum intensity contributions from each coded image. The detector array provides output data for processing to reconstruct an image. The mask provides for multi-spectral information to become encoded in the data. A linear integral equation incorporating explicit wavelength dependence relates the imaged scene to the data. This equation is solved by Landweber iteration to derive a multi-spectral image. An image with multiple polarisation states (polarimetric diversity) may be derived similarly with a linear integral equation incorporating explicit polarisation dependence.

Abstract: A method of reconstructing an image of a scene (4) from data output by a detector (8) in a coded aperture imaging system (2) comprises taking multiple frames of data using a different coded aperture array 6 for each frame. For each frame of data a decoding pattern is employed which is point source diffraction pattern corresponding to the coded aperture array used to acquire that frame. The decoding patterns are combined in a Gram matrix. A two-dimensional to one-dimensional mapping is applied to the frames of data. A solution to the data is constructed which is related to the scene (4) by an integral operator. The integral operator has an averaging kernel with a main lobe width which is smaller than the detector pixel size. Processing of the data yields an image which has sub-pixel resolution, i.e. resolution greater than the native resolution of the detector (8).

Abstract: Apparatus and methods for detecting angle of incidence of an optical beam. The apparatus employs two optical detectors, the first of which has placed in front of it a coating or layer which exhibits an angle-dependent optical transmission characteristic distinct from that of the light path in front of the second detector. The difference in a characteristic of the light received at the respective detectors therefore provides an indication of the angle of incidence of the light beam. The angle detector may be used particularly, though not exclusively, in conjunction with free space optical communications systems.

Abstract: Methods of providing duplex free-space optical, communication comprising receiving a time-shift keying (TSK) encoded signal and selectively re-modulating—and optionally retro-reflecting—received TSK pulses so as to transmit an on-off keying (OOK) signal. Related apparatus and signals are also provided.

Abstract: A phase locked phase conjugation system comprises a first (1) and a second (2) stimulated Brillouin scattering (SBS) cell. A laser beam is split into sub beams; one (9) is directed through the first SBS cell as a beam of collimated light and focused into the second SBS cell from which a phase conjugated beam is returned. One or more other sub beams (11, 12) are focused into the first SBS cell so that overlap occurs between focused sub beams and collimated beam. This provides a phase locked phase conjugated beam. Two or more sub beams are directed twice through laser amplifiers to provide an amplified phase locked phase conjugated beam. The second SBS cell may be a simple reflective cell operating on a focused laser sub beam, or may be an SBS loop arrangement having reflectors and lenses for causing optical feedback of the Brillouin scattered light.