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EP-4346124-B1 - FREE SPACE OPTICAL COMMUNICATIONS TERMINAL

EP4346124B1EP 4346124 B1EP4346124 B1EP 4346124B1EP-4346124-B1

Inventors

  • QUINTANA SANCHEZ, Crisanto
  • ERRY, GAVIN
  • THUEUX, Yoann

Dates

Publication Date
20260506
Application Date
20230928

Claims (14)

  1. A free space optical communications terminal (100) comprising along a principal axis thereof: a quarter-wave plate (105); a Faraday rotator (110); a polarisation splitter (115); a first arm (120) comprising a receiver (126) and a transmitter (128); and a second arm (130) comprising a beam position tracking detector (132) ; wherein: for a received circularly polarised beam (101a), received by the free space optical communications terminal and incident on the quarter-wave plate: the quarter-wave plate is configured to convert the received circularly polarised beam to a received linearly polarised beam (101b); the Faraday rotator is configured to rotate the polarisation of the received linearly polarised beam to produce a rotated received linearly polarised beam (101c); and the polarisation splitter is configured to provide a first portion (101e) of the rotated received linearly polarised beam to the first arm and a second portion (101d) of the rotated received linearly polarised beam to the second arm; and for a transmitted beam (102e), produced by the transmitter of the first arm and incident on the polarisation splitter: the polarisation splitter is configured to provide a transmitted linearly polarised beam (102c) to the Faraday rotator; the Faraday rotator is configured to rotate the polarisation of the transmitted linearly polarised beam to produce a rotated transmitted linearly polarised beam (102b); and the quarter-wave plate is configured to convert the rotated transmitted linearly polarised beam to an output beam (102a) with an output polarisation; wherein the Faraday rotator is controllable to vary the ratio of light between the first portion and the second portion.
  2. The free space optical communications terminal according to any previous claim, wherein the quarter-wave plate, the Faraday rotator, and the polarisation splitter are configured such that the output polarisation of the output beam is circular.
  3. The free space optical communications terminal according to any previous claim, wherein the quarter-wave plate, the Faraday rotator, and the polarisation splitter are configured such that the first received portion is equal to the second received portion.
  4. The free space optical communications terminal according to any previous claim, wherein the beam position sensing detector is a quadrant tracking detector.
  5. The free space optical communications terminal according to any previous claim, wherein the first arm further comprises an optical circulator (124), the transmitter configured to address a first port of the optical circulator; the polarisation splitter configured to address a second port of the optical circulator; the receiver configured to address a third port of the optical circulator. Such that, in use, light received by the first arm is directed by the optical circulator to the optical detector; and light produced by the optical source is directed by the optical circulator to the polarisation splitter, optionally wherein at least one of: the polarisation splitter is a polarising beam splitter, the receiver is an optical detector, and the transmitter is an optical laser source.
  6. The free space optical communications terminal according to any previous claim, wherein the quarter-wave plate is configured such that the rotational alignment of a fast axis of the quarter-wave plate can be varied, optionally wherein the quarter-wave plate is rotatably mounted such that the fast-axis alignment can be varied.
  7. The free space optical communications terminal according to any previous claim, wherein the Faraday rotator comprises an electromagnetic element (112) and the strength of a magnetic field applied by the electromagnetic element can be varied such that a rotation imparted by the Faraday rotator on incident light is varied.
  8. The free space optical communications terminal according to claim 7 when dependent on claim 6, wherein the fast axis of the quarter-wave plate is maintained at a fixed rotational displacement from the output polarisation of the Faraday rotator.
  9. The free space optical communications terminal according to claim 8, wherein the fast axis of the quarter-wave plate is aligned 45 degrees from the output polarisation of the Faraday rotator.
  10. The free space optical communications terminal according to any previous claim, further comprising beam steering means (150) configured to modify a beam path through the terminal, wherein optionally the free space optical communication terminal is according to claim 4 and the beam position sensing is configured to provide steering information, the beam steering means configured to use the steering information to modify the beam path through the terminal.
  11. A free space optical communications system (900), comprising a plurality of the free space optical communications terminal of any preceding claim.
  12. A vehicle (1100, 1000, 1200) comprising the free space optical communications terminal according to any one of claims 1 to 10, or a free space optical communications terminal of the free space optical communications system of claim 11, optionally wherein the vehicle is an aircraft and/or spacecraft.
  13. A method for free space optical communications, the method for use with a terminal comprising a quarter-wave plate (105); a Faraday rotator (110); a polarisation splitter (115); a first arm (120) comprising a receiver (126) and a transmitter (128); and a second arm (130) comprising a beam position tracking detector (132); the method comprising: generating, using the transmitter, an output beam of light encoding information to be communicated, the output beam having a first linear polarisation; rotating, using the Faraday rotator, the output beam from a first linear polarisation to a second linear polarisation; and transforming, using the quarter-wave plate, the output beam from a second linear polarisation to a third polarisation to be transmitted to a target; and receiving an input beam of light encoding information to be communicated, the input beam having a fourth polarisation; transforming, using the quarter-wave plate, the input beam from a fourth polarisation to a fifth polarisation; rotating, using the Faraday rotator, the input beam from a fifth polarisation to a sixth polarisation; splitting, using the polarisation splitter, the input beam between the first arm and the second arm and receiving a portion of the input beam at the receiver; wherein the method further comprises transforming, using the quarter-wave plate, and rotating, using the Faraday rotator, the input beam to change the ratio of input beam split between the first arm and the second arm.
  14. A method according to claim 13, wherein the third polarisation is a circular polarisation, and/or the fourth polarisation is a circular polarisation, and/or the fifth and sixth polarisations are linear polarisations.

Description

Technical Field The present invention relates to free space optical communications terminals and methods of operating free space optical communications terminals. Background Free space optical (FSO) communications use light propagating in free space to transmit data. In the context of FSO communications, 'free space' refers to, for example, air, space, vacuum, or similar and is in contrast with communications via solids such as a fibre-optic cable. FSO communications ordinarily rely on direct line of sight between transmitter and receiver and so rely on directing an optical beam between FSO communication terminals FSO communications can be useful for example in cases where communication via physical connections, such as fibre optic cables or other data cables, is impractical. One such case is, for example, communications between an aircraft such as a drone and a ground-based terminal. FSO communications can offer higher data rates and improved security as compared to other wireless communication techniques. For example, FSO communications can achieve higher data rates, and can be less susceptible to jamming and interception compared to radio frequency (RF) communications. It is desirable to allow for a reduction in the size, weight, and power (SWaP) requirements of a FSO communications terminal. US11190293B1 relates to a polarization multiplexed free space optical communication system. CN210780813U relates to space laser communication, a high-isolation and samefrequency space laser for transmitting and receiving a same polarization state. WO2022/178430A1 relates to optical communications, and, more particularly, to integrated optical communication systems including optical communication transceivers. Summary According to a first aspect of the present invention, there is provided a free space optical communications terminal comprising, along a principal axis thereof: a quarter-wave plate, a Faraday rotator, a polarisation splitter, a first arm comprising a receiver and a transmitter, and a second arm comprising a beam position tracking detector. The quarter-wave plate, the Faraday rotator, and the polarisation splitter are configured such that: for a received circularly polarised beam incident on the quarter-wave plate, the quarter-wave plate converts the received circularly polarised beam to a received linearly polarised beam; the Faraday rotator rotates the polarisation of the received linearly polarised beam to produce a rotated received linearly polarised beam; and the polarisation splitter provides a first portion of the rotated received linearly polarised beam to the first arm and a second portion of the rotated received linearly polarised beam to the second arm. For a transmitted beam produced by the transmitter of the first arm and incident on the polarisation splitter, the polarisation splitter provides a transmitted linearly polarised beam to the Faraday rotator; the Faraday rotator rotates the polarisation of the transmitted linearly polarised beam to produce a rotated transmitted linearly polarised beam; and the quarter-wave plate converts the rotated transmitted linearly polarised beam to an output beam with an output polarisation. The Faraday rotator is controllable to vary the ratio of light between the first portion and the second portion. In use, the terminal may receive circularly polarised light and split the received light between two arms of the terminal, and send light, produced by a transmitter in the first arm of the terminal, having an output polarisation such as circular polarisation. Accordingly, the beam position tracking detector in the second arm can utilise the received beam, but the transmitter and the receiver are provided in the same, first, arm. The transmitter and receiver being provided in the same arm can, in tum, allow for the size, weight, and power requirements of the terminal to be reduced, as, for example, optical elements such as collimation optics and relay optics can be common to both the transmitter and receiver, and may be shortened to improve system integration even further. For example, beam tracking and steering control, such as fine steering provided by a fast-steering mirror, can be common to both the transmitter and receiver if the received beam and the transmitted beam follow substantially similar paths. In combining the transmitter and receiver arms such that they utilise a single aperture and thereby share parts of the optical steering system, the system size, weight, and power requirements of the terminal can be minimised. In examples, the quarter-wave plate, the Faraday rotator, and the polarisation splitter are configured such that the output polarisation is circular. The output polarisation being circular means that the incoming polarisation for a receiving terminal is invariant with the spatial orientation of the receiving terminal, which can permit flexibility and robustness for a free space optical communications network. In examples, the receiv