WO2023060452A1 - Émetteur-récepteur optique pour terminal de communication optique - Google Patents

Émetteur-récepteur optique pour terminal de communication optique Download PDF

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Publication number
WO2023060452A1
WO2023060452A1 PCT/CN2021/123380 CN2021123380W WO2023060452A1 WO 2023060452 A1 WO2023060452 A1 WO 2023060452A1 CN 2021123380 W CN2021123380 W CN 2021123380W WO 2023060452 A1 WO2023060452 A1 WO 2023060452A1
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WIPO (PCT)
Prior art keywords
optical
communication terminal
telescope
edfa
optical communication
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PCT/CN2021/123380
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English (en)
Inventor
Yu Qin
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Beijing Sinaero Information And Communication Technology Limited
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Publication date
Application filed by Beijing Sinaero Information And Communication Technology Limited filed Critical Beijing Sinaero Information And Communication Technology Limited
Priority to PCT/CN2021/123380 priority Critical patent/WO2023060452A1/fr
Publication of WO2023060452A1 publication Critical patent/WO2023060452A1/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
    • H04B10/112—Line-of-sight transmission over an extended range
    • H04B10/1123—Bidirectional transmission
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/0006—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 with means to keep optical surfaces clean, e.g. by preventing or removing dirt, stains, contamination, condensation
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B7/00—Mountings, adjusting means, or light-tight connections, for optical elements
    • G02B7/18—Mountings, adjusting means, or light-tight connections, for optical elements for prisms; for mirrors
    • G02B7/182—Mountings, adjusting means, or light-tight connections, for optical elements for prisms; for mirrors for mirrors
    • G02B7/198—Mountings, adjusting means, or light-tight connections, for optical elements for prisms; for mirrors for mirrors with means for adjusting the mirror relative to its support
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
    • H04B10/112—Line-of-sight transmission over an extended range
    • H04B10/1123—Bidirectional transmission
    • H04B10/1125—Bidirectional transmission using a single common optical path
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
    • H04B10/118—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum specially adapted for satellite communication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/40—Transceivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/504—Laser transmitters using direct modulation

Definitions

  • the invention relates to an optical transceiver for an optical communication terminal for optical communication using laser beams to transport optical communication signals or Quantum Keys for Quantum Key Distribution (QKD) .
  • QKD Quantum Keys for Quantum Key Distribution
  • a communication system comprises two or multiple optical communication terminals which communicate between each other. These optical communication terminals are assembled on a stationary platform, on mobile platforms or on manned or unmanned flight platforms.
  • the communication link between the optical communication terminals can be an end-to-end connection or a network of optical communication terminals in which some optical communication terminals may act as a relay station.
  • Optical communication does complement and/or substitute communication based on radio frequencies. Optical communication provides higher data rates than conventional radio frequency transmission. Compared to conventional radio frequency terminals optical communication terminals OCTs inherently support low size, weight and power (SWaP) . Accordingly, a huge amount of data can be effectively transmitted via broadband channels of optical communication terminals.
  • SWaP weight and power
  • Figure 1 shows schematically a communication system SYS including different kinds of terminals located on ground and adapted to communicate with non-ground communication terminals integrated in different types of manned or unmanned flying platforms.
  • Telecommunication systems can also comprise interplatform links.
  • High-altitude platform systems HAPS can communicate with each other through associated communication terminals.
  • Aeroplanes can communicate either directly with a ground terminal or indirectly via relay communication terminals as illustrated in figure 1.
  • Ground based terminals can be either located at fixed positions or integrated in movable objects or vehicles such as trains, vehicles, such as cars as shown in figure 1.
  • a communication between handheld mobile user equipment UE and ground terminals via near ground flying communication terminals is also possible as illustrated schematically in figure 1.
  • Flight platforms in a communication system SYS may also provide communication access points for any location on earth even when no land-based telecommunication networks are available at the respective location.
  • An alignment of the optical communication terminal can be provided by means of a coarse pointing assembly CPA.
  • a mirror of the coarse pointing assembly is used as a beam pointing mechanism.
  • the rotation axis of the mirror is positioned around its center of gravity to minimize a gravity torque which a motor of the coarse pointing assembly must counteract.
  • this configuration results in a large CPA structure.
  • the optical surface of the mirror of the coarse pointing assembly as well as the receiving telescope are largely exposed during non-operational phases of the optical communication terminal. This in turn increases the contamination rate for the sensitive optical surfaces resulting in a decrease of the operation lifetime and performance of the equipment.
  • an object of the present invention to provide an optical communication terminal for optical communication adapted to overcome the above-mentioned drawbacks of a conventional coarse pointing assembly with a mirror having a rotation axis at its center of gravity.
  • optical communication terminal for optical communication using laser beams to transport optical communication signals.
  • the invention provides according to a first aspect an optical coarse pointing assembly of an optical communication terminal, said optical coarse pointing assembly being adapted to establish and/or maintain an optical communication link between the optical communication terminal and an external optical flight platform, wherein the optical coarse pointing assembly has a CPA mirror adapted to reflect an optical beam received from the external optical flight platform into a telescope of an optical head unit of said optical communication terminal and/or to reflect an optical beam received from the telescope of the optical head unit of said optical communication terminal to the external optical flight platform, wherein the CPA mirror of said optical coarse pointing assembly is pivotable around an elevation axis located off a center of gravity of said CPA mirror.
  • An advantage of the optical coarse pointing assembly according to the first aspect of the present invention resides in that the size of the CPA structure is minimal facilitating transport of the optical communication terminal. Further, the optical reflection surface of the CPA mirror and other optical components can be protected during non-operational phases of the mission. A CPA mirror in its closed position diminishes the contamination rate of the sensitive optical surfaces and increases the operation lifetime and performance of the optical communication terminal.
  • the CPA mirror is pivotable around the elevation axis by a controllable drive mechanism.
  • a pivot angle of the CPA mirror around the elevation axis is adjustable by a control unit adapted to control the drive mechanism of the CPA mirror.
  • controllable drive mechanism of the CPA mirror comprises a Capstan drive mechanism or a gear drive mechanism.
  • the CPA mirror comprises a mirror body having a reflecting optical surface layer located on a pivotable base element driven by the controllable drive mechanism of the CPA mirror.
  • the elevation axis is located at a lower edge of the mirror body of said CPA mirror.
  • the CPA mirror is pivotable around the elevation axis between a closed position and at least one open position.
  • the closed position the telescope of the optical head unit is covered by the CPA mirror.
  • an optical beam is reflected by the optical surface of the CPA mirror to establish and/or to maintain an optical communication link between the optical head unit and the external optical flight platform.
  • the CPA mirror is lockable in the closed position by a controllable locking mechanism of the optical coarse pointing assembly.
  • the coarse pointing assembly is placed as an exchangeable module in front of the telescope of the optical head unit of said optical communication terminal.
  • the coarse pointing assembly is rotatable around an Azimuth axis and/or around an elevation axis to establish and/or to maintain an optical communication link between the optical head unit and the external optical flight platform.
  • the optical surface layer of the mounted CPA mirror faces a front surface of the telescope of the optical head unit of said optical communication terminal.
  • the optical surface layer of the CPA mirror is made of a highly reflective material, in particular gold.
  • the pivotable base element of the CPA mirror is made of a physically light material, in particular aluminium or Zerodur.
  • the CPA assembly is adapted to provide a basic angular motion of the optical beam in an acquisition mode of the optical communication terminal to establish a communication link between the optical communication terminal and the external flight platform and/or to compensate a relative motion between the optical communication terminal and the external flight platform to maintain an already established communication link between the optical communication terminal and the external flight platform by controlling a position of the CPA mirror of said CPA assembly.
  • the invention further provides a method for establishing and/or for maintaining an optical communication link.
  • the invention provides accordingly a method for establishing and/or maintaining an optical communication link between an optical communication terminal and an external optical flight platform by pivoting a CPA mirror of an optical coarse pointing assembly of said optical communication terminal around an elevation axis located off a center of gravity of said CPA mirror.
  • Conventional laser communication terminals can use a shared telescope aperture to transmit and receive data and tracking laser beams.
  • the laser beams can be generated and coupled into a shared beam path, for instance by means of a chromatic beam splitter.
  • a transmission light can exit through a telescope having optical surfaces to widen the collimated transmit laser beam to the target laser beam diameter.
  • a fraction of the transmitted light is backscattered or reflected into the optical system which may cause self-blinding effects, especially on an extended tracking sensor. Since the transmit laser beam is typically many orders of magnitude stronger than the light received from the external counter communication terminal, this will cause negative effects on the tracking and communication performance.
  • an optical communication terminal comprising an optical telescope adapted to expand a diameter of an optical beam received from an optical transmitter unit of said optical communication terminal and to reduce a diameter of an optical beam received from an external optical flight platform for an optical receiver unit of said optical communication terminal, wherein at least one central obscuration coating is provided on a surface of an optical component of said optical receiver unit and is adapted to suppress light backscattered and/or reflected by said optical telescope of said optical communication terminal.
  • the central obscuration coating comprises a material which is non-transparent at least in a predefined communication frequency range of the optical communication terminal.
  • the central obscuration coating comprises an absorbing or a reflective material.
  • the central obscuration coating comprises a circular shape with a predefined radius.
  • the central obscuration coating is provided at a center of an optical component of said optical receiver unit.
  • the optical component comprises an optical lens of the optical receiver unit of said optical communication terminal.
  • the optical telescope of the optical communication terminal comprises an on-axis lens telescope without a central obscuration.
  • the on-axis lens telescope comprises a single piece optical lens or multiple lenses made of a transparent material in the predefined communication frequency range of the optical communication terminal.
  • the material of the single piece optical lens or of the multiple optical lenses are made of a homogeneous material.
  • the material of the single piece optical lens or of the multiple optical lenses may comprise silicon or germanium.
  • the optical telescope lens of the telescope comprises an aspherical convex front surface and a spherical or aspherical concave rear surface.
  • the optical telescope lens is designed to expand a laser beam transmitted by the optical transmitter unit of the transceiver and going through the rear surface of the optical telescope lens and is designed to reduce a laser beam in diameter received through the front surface of the optical telescope lens wherein the reduced laser beam is output through the rear surface of the optical telescope lens to the optical receiver unit of the transceiver via a chromatic filter unit.
  • the central obscuration coating provided on the surface of the component of said optical receiver unit is adapted to shield a tracking sensor of the optical receiver unit of the transceiver of said optical communication terminal against light backscattered and/or reflected by the optical telescope of said optical communication terminal.
  • the central obscuration coating provides an attenuation depending on a ratio between a radius of the central obscuration coating and a beam radius of a received optical beam.
  • a lateral surface of a conical section of the optical telescope lens comprises a number of suppression steps being adapted to suppress backscattered and/or reflected optical light.
  • the front surface of the telescope lens is adapted to receive a laser beam from a coarse pointing assembly of said optical communication terminal.
  • a conventional optical communication system can use a shared telescope assembly for the transmit and receive laser beams to save size, weight and power consumption of the optical communication terminal.
  • the amplified laser light can be generated at a booster Erbium doped fiber amplifier EDFA present on the transmitter side.
  • EDFA Erbium doped fiber amplifier
  • unwanted amplified spontaneous emission (ASE) noise is also generated.
  • the amplified light signal is directed towards a point ahead assembly and to a dichroic beam splitter having the task to guide the transmission light to the telescope assembly after reflecting it at a fine pointing assembly FPA.
  • not all transmission light is guided out of the telescope. Some portion of the light is reflected back to the optical system.
  • the reflected light arrives at the dichroic beam splitter and gets passed towards the receiving path where filters can be present to suppress the background light. These filters are centered at the receiving wavelength and can suppress the transmission light, however, the generated amplified spontaneous noise generated by the booster EDFA is present at the receiving wavelength and can get through these filters and arrive at the highly sensitive tracking sensor. Accordingly, the tracking sensor can get saturated which does hinder the normal operation of tracking. In a worst case, the tracking sensor can even get damaged if the intensity of the transmission light is too high. Another issue caused by the back-reflected ASE noise is that it can drastically interfere with the weak received communication signal.
  • an optical transceiver being able to reduce the amplified spontaneous emission (ASE) noise such that the ASE noise does no longer blind sensors provided an the receiver unit and/or such that the ASE noise does not interfere with the received communication signals.
  • ASE amplified spontaneous emission
  • an optical transceiver of an optical communication terminal having an optical transmitter unit adapted to transmit optical communication signals within at least one predefined communication frequency range and an optical receiver unit adapted to receive optical communication signals within the predefined communication frequency range, wherein the optical transmitter unit comprises a laser diode adapted to generate a laser beam amplified by a booster Erbium Doped Fiber Amplifier (EDFA) of said optical transmitter unit, wherein an amplification gain of said booster EDFA is lowered to reduce a broadband EDFA Amplified Spontaneous Emission, ASE, noise generated by the booster EDFA, wherein the lowered amplification gain of the booster EDFA is compensated by a corresponding increase of the output power of the laser beam generated by the laser diode.
  • EDFA Erbium Doped Fiber Amplifier
  • the optical transmitter unit is adapted to transmit an optical transmission signal at a predefined transmission wavelength within the predetermined communication frequency range of the optical communication terminal.
  • the optical receiver unit is adapted to receive an optical reception signal at a predefined reception wavelength within the predetermined communication frequency range of the optical communication terminal.
  • the optical transmission signal of the optical transmitter unit is supplied via a point ahead assembly, a dichroic beam splitter and via a fine pointing assembly to a telescope of said optical communication terminal.
  • the optical receiver unit of the optical communication terminal comprises at least one optical bandpass filter adapted to suppress light outside a reception filter frequency band centered around a reception wavelength of the optical receiver unit.
  • the optical bandpass filter of the optical receiver unit is adapted to suppress reflected light at the transmission wavelength of the optical transmitter unit of said optical communication terminal.
  • the EDFA ASE noise generated by the booster EDFA of the optical transmitter unit comprises a spectral overlap with the reception filter frequency band of the at least one optical bandpass filter of the optical receiver unit of said optical communication terminal.
  • an optical reception signal having passed through the optical bandpass filter of the optical receiver unit is split by a beam splitter into a first optical signal supplied to a tracking sensor of the optical receiver unit and into a second optical signal supplied via a single mode fiber and an EDFA preamplifier to an optical signal demodulator of the optical receiver unit of said optical communication terminal.
  • an adjustment of the amplification gain of the booster EDFA of the optical transmitter unit and the corresponding adjustment of the output power of the laser diode of the optical transmitter unit to reduce the EDFA ASE noise is performed by a control unit of the optical transmitter unit in a calibration mode of the optical communication terminal.
  • the telescope comprises an optical telescope lens made of a homogeneous material being transparent in the communication frequency range of the optical communication terminal and having a high refractive index.
  • the optical telescope lens is designed to expand a laser beam transmitted by the optical transmitter unit entering through a rear surface of the optical telescope lens and is designed to reduce a laser beam in diameter received through a front surface of the optical telescope lens, wherein the reduced laser beam is output through the rear surface of the optical telescope lens to the optical receiver unit of said optical communication terminal.
  • the optical telescope lens of the telescope comprises an aspherical convex front surface and a spherical or aspherical concave rear surface.
  • the optical telescope lens of the telescope is made of a homogeneous material.
  • This homogeneous material can comprise in a possible implementation silicon or germanium.
  • the optical telescope lens of the telescope comprises a conical section and/or a cylindrical section, wherein a lateral surface of the conical section of the optical telescope lens comprises a number of suppression steps being adapted to suppress backscattered or reflected optical signals.
  • the optical receiver unit comprises at least one optical component having a central obscuration coating adapted to suppress light backscattered and/or reflected by an optical telescope of the optical communication terminal.
  • Optical communication terminals are sensitive against vibrations and mechanical forces applied to the hardware as they arise on flying platforms. Further, transportation space for transporting an optical communication terminal is very limited.
  • optical communication terminal which is resilient against mechanical forces during transport and which occupies not much transportation space during transport.
  • the present invention provides according to a fourth aspect an optical communication terminal for optical communication using laser beams to transport optical communication signals, said optical communication terminal comprising: an optical transceiver having an optical transmitter unit adapted to transmit optical communication signals within at least one predefined communication frequency range and an optical receiver unit adapted to receive optical communication signals within the predefined communication frequency range and an optical head unit having a telescope provided for collection of optical power and adaption of a laser beam diameter, wherein the telescope comprises a single piece optical telescope lens made of a homogeneous material being transparent in the communication frequency range and having a high refractive index.
  • the optical telescope lens of the telescope comprises a front surface and a rear surface.
  • the optical telescope lens comprises an aspherical convex front surface.
  • the optical communication terminal further comprises in a possible embodiment a spherical or aspherical concave rear surface.
  • the optical telescope lens is designed to expand a laser beam transmitted by the optical transmitter unit entering through the rear surface of the optical telescope lens and is further designed to reduce a laser beam in diameter received through the front surface of the optical telescope lens, wherein the reduced laser beam is output through the rear surface of the optical telescope lens to the optical receiver unit.
  • the homogenous material of the optical telescope lens can comprise a high thermal conductivity.
  • high thermal conductivity can be achieved.
  • the homogenous material of the optical telescope lens can comprise absorptive properties that reject optical signals having a frequency outside the predefined communication frequency range to minimize optical noise in the optical receiver unit.
  • the optical telescope lens can comprise at least one rotationally symmetrical section with a surrounding lateral surface.
  • the optical telescope lens can comprise a conical section and/or a cylindrical section.
  • the lateral surface of the conical section of the optical telescope lens can comprise a number of suppression steps adapted to suppress backscattered or reflected optical signals.
  • the homogeneous material of the optical telescope lens comprises silicon.
  • the homogeneous material of the optical telescope lens can comprise germanium.
  • the homogeneous material of the optical telescope lens can also comprise glass.
  • the optical telescope lens is manufactured in a turning manufacturing process.
  • the optical telescope lens can also be manufactured in a grinding process.
  • the optical telescope lens is manufactured in an additive manufacturing process.
  • the optical receiver unit can comprise a single piece or composite optical filter and a beam splitter lens to supply a received optical signal to a tracking sensor of said optical transceiver.
  • the aspherical convex front surface of the optical telescope lens is adapted to receive a laser beam from a coarse pointing assembly CPA of said optical communication terminal.
  • a frame structure used for mounting the optical transceiver and/or the optical head unit of the optical communication terminal is manufactured in an additive manufacturing process.
  • the optical head unit can comprise a point ahead assembly PAA, a fine pointing assembly FPA and/or a chromatic beam splitter CBS.
  • the chromatic beam splitter is designed to separate the received optical beam and the optical beam from the transmitting unit by the respective wavelength properties.
  • the optical communication terminal is integrated in a mobile flying station comprising a high-altitude platform, an aero-plane or a drone or other or can be integrated in a ground station for providing tower-to-tower communication.
  • Figure 1 illustrates a communication system using communication terminals to transport communication signals
  • Figure 2 shows a block diagram illustrating a possible exemplary embodiment of an optical communication terminal OCT according to an aspect of the present invention
  • Figure 3 shows a further exemplary embodiment of an optical communication terminal OCT according to an aspect of the present invention
  • Figure 4 shows a block diagram for illustrating a possible exemplary embodiment of an optical communication terminal OCT according to an aspect of the present invention
  • Figure 5 shows a schematic diagram for illustrating a possible embodiment of an optical system within an optical communication terminal OCT according to an aspect of the present invention
  • Figures 6A, 6B illustrate an exemplary embodiment of an optical telescope lens used in an optical communication terminal OCT according to an aspect of the present invention
  • Figure 7 illustrates a possible exemplary embodiment of an optical system with a coarse pointing assembly CPA of an optical communication terminal OCT according to an aspect of the present invention
  • Figure 8 shows a top view on an optical system implemented in an optical head unit of an optical communication terminal according to an embodiment of the present invention
  • Figure 9 shows a top view on an optical system as used in an optical communication terminal OCT according to an aspect of the present invention.
  • Figure 10 shows an exemplary embodiment of an optical filter and beam splitter lens used in an optical communication terminal OCT according to an aspect of the present invention
  • Figures 11A, 11B, 11C show an exemplary embodiment of a frame structure of an optical communication terminal OCT according to an aspect of the present invention
  • Figures 12A, 12B illustrate a possible exemplary embodiment of an optical coarse pointing assembly of an optical communication terminal according to an aspect of the present invention
  • Figure 13 shows a schematic diagram for illustrating the operation of an optical coarse pointing assembly of an optical terminal as illustrated in figures 12A, 12B;
  • Figures 14A, 14B illustrate the operation of an optical communication terminal including a central obscuration coating according to a further aspect of the present invention
  • Figure 15 illustrates a technical effect of a central obscuration coating in a possible exemplary embodiment of an optical communication terminal
  • Figure 16 shows a schematic diagram for illustrating a possible exemplary embodiment of an optical transceiver with reduced EDFA amplified spontaneous emission noise reduction according to a further aspect of the present invention.
  • An optical communication terminal (OCT) 1 can be used in a communication system SYS as illustrated in figure 1.
  • the optical communication terminal 1 can be integrated in a mobile flying station comprising a high-altitude platform, an aero-plane or a drone or other as shown in figure 1.
  • An Optical communication terminal OCT can also be integrated in a ground station for providing tower-to-tower communication.
  • the optical communication terminal 1 can be used to supplement or to substitute a radio frequency communication link between a base station and mobile flying platforms.
  • the optical communication terminal 1 provides one or more broadband optical communication channels by using laser beams to transport or carry modulated optical communication signals via at least one optical communication link OCL.
  • Figure 2 shows a block diagram of an optical communication terminal 1 according to an aspect of the present invention.
  • the optical communication terminal 1 comprises an optical transceiver 2 and an optical head unit 3.
  • the optical transceiver 2 has an optical transmitter unit 2A adapted to transmit optical communication signals within at least one predefined communication frequency range.
  • the optical transceiver 2 further comprises an optical receiver unit 2B adapted to receive optical communication signals within the predefined communication frequency range.
  • the optical communication terminal 1 further comprises the optical head unit 3.
  • the optical head unit 3 comprises a telescope TLA provided for collection of optical power and adaption of a laser beam diameter of a laser beam.
  • the telescope TLA of the optical head unit 3 can comprise a single piece optical telescope lens 3A made of a homogenous material being transparent in the communication frequency range and having a high refractive index n.
  • the optical telescope lens 3A of the telescope TLA within the optical head unit 3 can comprise a convex front surface FSUR and a concave rear surface RSUR.
  • the convex front surface of the optical telescope lens 3A of the telescope TLA within the optical head unit 3 is formed aspherical.
  • the concave rear surface of the optical telescope lens 3A of the telescope TLA can comprise a spherical or aspherical concave rear surface.
  • the telescope TLA of the optical head unit 3 is adapted to receive an incoming communication signal carried via a laser beam and can reduce the size of a diameter of the laser beam to match the size of optical components of said optical communication terminal 1.
  • the optical telescope TLA of the optical head unit 3 is also adapted to operate as a beam expander of a transmitted laser beam. In this way the aperture and secondary collimating lenses can be combined in a single component to reduce the assembly and integration effort by minimizing the number of required components within the optical head unit 3 of the optical communication terminal 1.
  • the optical telescope lens 3A can be made of silicon. Silicon comprises a high refractive index n.
  • n a further advantage of using silicon resides in the fact that silicon has also a high thermal conductivity. Further the absorptive properties of silicon make it a good long pass filter that can reject visible wavelength and other signals outside the transmission frequency range and consequently allows to minimize the optical noise in the receiver system. Further, silicon can protect the sensitive receiver and tracking sensors from thermal damages and direct exposure to sunlight. Due to the large heat conductive properties of silicon the temperature can stay uniform through the full body of the lens containing a uniform refractive index. Further, the high refractive index n of silicon can allow to keep the system length short by still maintaining a reasonable radius of curvature for the aperture of the optical telescope lens.
  • the optical telescope lens 3A can also be fabricated from other homogenous materials than silicon.
  • the optical telescope lens of the optical telescope of the optical head unit 3 is designed to expand a laser beam transmitted by the optical transmitter unit 2A and entering through the rear surface RSUR of the optical telescope lens.
  • the optical telescope lens is further designed to reduce a laser beam in diameter received through the front surface FSUR of the optical telescope lens. The reduced laser beam is then output through the rear surface RSUR of the optical telescope lens to the optical receiver unit 2B of the optical transceiver 2.
  • the homogenous material of the optical telescope lens comprises absorptive properties that reject optical signals having a frequency outside the predefined communication frequency range, in particular visible light, to minimize the optical noise in the optical receiver unit 2B.
  • the optical telescope lens 3A can comprise different shapes. Possible embodiments of the optical telescope lens 3A can comprise at least one rotationally symmetrical section with a surrounding lateral surface.
  • the optical telescope lens of the optical head unit 3 can also comprise a conical section. In a still further possible embodiment the optical telescope lens of the optical head unit 3 can also comprise a cylindrical section.
  • the lateral surface of the conical section of the optical telescope lens can comprise a number of suppression steps which are adapted to suppress any kind of backscattered or reflected optical signals.
  • the optical telescope lens can be manufactured in a manufacturing process.
  • the manufacturing process can comprise a turning manufacturing process, grinding process or an additive manufacturing process.
  • the frame structure used for mounting the optical transceiver 2 and/or the optical head unit 3 of the optical communication terminal 1 can in a possible embodiment be manufactured in an additive manufacturing process.
  • the optical head unit 3 as illustrated in the block diagram of figure 4 can comprise different sub-units including a point Ahead Assembly, PAA, a Fine Pointing Assembly, FPA, and a chromatic beam splitter, CBS.
  • the chromatic beam splitter CBS can be designed to separate the received optical beam and the optical beam received from the transmitter unit assembly by their respective wavelength properties.
  • Figure 3 shows a possible embodiment of an optical communication terminal (OCT) 1 integrated in a frame structure.
  • the optical communication terminal 1 comprises a coarse pointing assembly (CPA) 4 in front of the optical head unit 3.
  • the optical head unit 3 is connected to an optical transmit and receive system 2 forming an optical transceiver.
  • the optical transceiver 2 can also be integrated in the optical head unit 3.
  • the optical transceiver 2 can further be connected to an electronic unit 5 of the optical communication terminal 1.
  • the electronic unit 5 can comprise different electronic components including an FEC codec system or a terminal micro controller TMC.
  • the electronic unit 5 can further include an FPA/PAA controller FPC and an EPS and supervisory entity.
  • the total length L of the optical communication terminal 1 comprises length L1 of the CPA 4, length L2 of the optical head unit 3 and integrated optical transceiver 2 and the length L3 of the electronic unit 5 within the frame as illustrated in figure 3.
  • Figure 4 illustrates the optical head unit 3 of the optical communication terminal 1 in more detail.
  • the optical head unit 3 comprises the integrated optical transmit and receive system 2, i.e. the optical transceiver 2.
  • the optical communication terminal 1 can provide bidirectional communication to transport data at high bit rates of more than 1 Gbps in both directions.
  • the transmit and receive path are both sharing the same telescope TLA.
  • Signal separation can be achieved by polarization and by the frequency of the signal.
  • a coarse alignment of the optical communication terminal 1 can be performed by means of the coarse pointing assembly (CPA) 4.
  • the coarse pointing assembly 4 is adapted to steer and align the optical beam both for reception and transmission of light.
  • the coarse pointing assembly 4 can be used to provide the basic angular motion of the laser beam required for an acquisition mode and also to anticipate on the relative motion between different optical communication terminals.
  • the coarse pointing assembly (CPA) 4 provides a relatively large angular range. In this way it is possible to establish and maintain an optical communication link OCL in particular with partnering optical flight platforms to accommodate for a large relative motion between two neighboring flight platforms or between a flight platform and a ground station with minimal impact on a beam quality of the laser beam.
  • the coarse pointing assembly (CPA) 4 is provided in front of the telescope TLA of the optical head unit 3.
  • the telescope TLA of the optical head unit 3 is provided for collection of optical power and for adaption of a laser beam diameter of the respective laser beam.
  • the telescope TLA of the optical head unit 3 can comprise a single piece optical telescope lens 3A made of a homogenous material. Further, the optical telescope lens 3A is transparent in the respective communication frequency range and comprises a high refractive index n.
  • the telescope TLA can be connected to a BS ACU Unit 3B as shown in figure 4 which can be used as alignment and calibration tool.
  • the optical head unit 3 further comprises a fine pointing assembly (FPA) 3C as shown in figure 4.
  • the optical head unit 3 further includes a chromatic beam splitter (CBS) 3D.
  • the optical head unit 3 comprises in the illustrated embodiment of figure 4 a point ahead assembly (PAA) 3E.
  • the transceiver 2 is integrated in the optical head unit 3.
  • the transceiver 2 comprises an optical transmitter unit 2A and an optical receiver unit 2B.
  • the optical transmitter unit 2A is adapted to transmit optical communication signals within at least one predefined communication frequency range.
  • the optical receiver unit 2B is adapted to receive optical communication signals within the predefined communication frequency range.
  • the optical transmitter unit 2A includes a transmitter and modulator, a booster EDFA and a transmission collimator.
  • the generated optical signal is applied by the optical transmitter unit 2A via the point ahead assembly 3E to the chromatic beam splitter 3D as shown in figure 4.
  • the optical receiver unit 2B comprises at least one optical filter receiving the optical communication signal from the chromatic beam splitter 3D.
  • the optical receiver unit 2B further comprises a BS beam splitter 16 adapted to split the received optical beam into two signal components.
  • the first split optical signal component is supplied to a preamp EDFA 18 and an RFE and demodulation unit 19 of the optical receiver unit 2B.
  • the second split optical signal component is supplied in the illustrated embodiment to a four-quadrant detector (4QD) 8.
  • Fine pointing functionality can be provided to mitigate high frequency disturbances in the common optical signal path. These disturbances may originate from various vibration sources on a flight platform. Further for ground-based communication terminals atmospheric turbulence can be corrected.
  • Figure 5 illustrates a top view on an optical system within the optical communication terminal 1 according to the present invention.
  • the telescope TLA of the optical head unit 3 comprises in the illustrated exemplary embodiment a single piece optical telescope lens 3A which is made of a homogenous material.
  • the single piece optical telescope lens 3A of the telescope TLA is transparent in the predefined communication frequency range.
  • the optical telescope lens 3A of the telescope TLA comprises a high refractive index n.
  • the optical telescope lens 3A is designed to expand a laser beam transmitted by the optical transmitter unit 2A via the point ahead assembly 3E entering through a rear surface RSUR of the optical telescope lens 3A.
  • the optical telescope lens 3A of the telescope TLA is further designed to reduce a laser beam in its diameter received through a front surface FSUR of the optical telescope lens 3A.
  • the reduced laser beam is output through the rear surface RSUR of the optical telescope lens 3A and travels through the chromatic beam splitter 3D to the optical receiver unit 2B.
  • the optical telescope lens 3A of the telescope TLA is made in a possible embodiment of a single piece and forms a uniform body. This makes the optical system as illustrated in figure 5 resilient against mechanical vibrations or mechanical forces. Since the telescope TLA comprises a single piece optical lens misalignments caused by mechanical forces or vibrations can be avoided.
  • the optical system using the single piece optical telescope lens 3A does not require a complex interaction between different components such as different lenses or mirrors so that the complexity is reduced and the resilience and robustness of the optical system is increased.
  • the optical system with the single piece optical telescope lens 3A does operate more reliably because misalignments or failures different optical components cannot occur.
  • the use of a single piece optical telescope lens 3A allows a reduced size of the optical system and as a consequence a reduced size of the optical head unit 3. In this way the length and total size of the optical communication terminal 1 can be minimized thus saving transport space.
  • the optical telescope lens 3A of the telescope TLA comprises a front surface FSUR and a rear surface RSUR as illustrated in figure 5.
  • the front surface FSUR of the optical telescope lens 3A comprises an aspherical convex front surface.
  • the rear surface RSUR is a concave surface.
  • the concave rear surface RSUR can comprise a spherical concave rear surface or an aspherical concave rear surface.
  • the optical telescope lens 3A expands a laser beam transmitted by the optical transmitter unit 2A entering through the rear surface RSUR of the optical telescope lens 3A and is also designed to reduce a laser beam in its diameter when received through the front surface FSUR. The reduced laser beam is then output through the rear surface RSUR of the optical telescope lens 3A and supplied to the optical receiver unit 2B of the transceiver 2.
  • the homogenous material of the optical telescope lens 3A as illustrated in the embodiment of figure 5 can comprise absorptive properties that reject optical signals having a frequency outside the predefined communication frequency range to minimize any kind of optical noise in the optical receiver unit 2B of the transceiver 2.
  • the optical telescope lens 3A of the telescope TLA can comprise at least one rotationally symmetrical section with a surrounding lateral surface.
  • the optical telescope lens 3A comprises a conical section.
  • the optical telescope lens 3A can also comprise other shapes and forms as also explained in context with figures 11A, 11B, 11C.
  • the conical section of the optical telescope lens 3A comprises in a preferred embodiment a number of suppression steps adapted to suppress backscattered or reflected optical signals.
  • Figure 6A illustrates a conical design of the optical telescope lens 3A with a number of suppression steps to suppress backscattered or reflected optical signals.
  • Figure 6A shows a front view and a side view and a perspective rear view on the optical telescope lens 3A of the telescope TLA.
  • Figure 6B shows a view on a further possible exemplary embodiment of an optical telescope lens 3A which can be used in an optical communication terminal 1 according to the present invention.
  • the number N of suppression steps may vary depending on the use case and size of the lens.
  • the front surface FSUR of the optical telescope lens 3A comprises an aspherical convex front surface FSUR.
  • Figure 6B shows a further possible exemplary embodiment of an optical telescope lens 3A having not only a conical section but also a cylindrical section.
  • the lateral surface of conical section of the optical telescope lens 3A in figure 6B can also comprise suppression steps 6 to suppress backscattered or reflected optical signals.
  • the size and length of the different sections of the optical telescope lens 3A can vary depending on the use case.
  • Figure 7 shows a cross section of an optical system with a coarse pointing assembly (CPA) in front.
  • the coarse pointing assembly (CPA) comprises an X/Y pointing system including a so-called Risley prism pair.
  • Figure 8 shows a further top view on an optical system implemented in an optical head unit 3 of an optical communication terminal 1 according to the present invention.
  • the optical receiver unit 2B can comprise a single piece optical filter and beam splitter lens 7 adapted to supply a received optical signal to a tracking sensor 8 of the optical transceiver 2.
  • the optical filter and beam splitter lens 7 can also be assembled from different components which can be glued together. It may comprise a first lens 7A, a folding mirror prism 7B, a lens 7C and a beam splitter cube 7D.
  • Figure 9 shows different views of a possible implementation of the optical filter and beam splitter lens 7.
  • the telescope TLA of the optical head unit 3 is adapted to receive an incoming communication signal and to distribute the received power to tracking and data sensors 8.
  • the optical filter and beam splitter lens 7 can be implemented by individual components such as lenses and beam splitters combined into a single component. In this way it is possible to reduce the assembly and integration effort by minimizing further the number of required components within the optical system of the optical communication terminal 1.
  • Figure 10 shows different views on frame structures for mounting the optical transceiver 2 and/or for mounting the optical head unit 3 of the optical communication terminal 1.
  • the frame structure 9 as illustrated in figure 10 can be manufactured in an additive manufacturing process.
  • the frame structure 9 can hold the CPA 4 in front of the optical head unit 3 and the electronic unit 5 of the optical communication terminal 1 as shown in figure 10.
  • the optical head unit 3 in turn can comprise a point ahead assembly PAA, a fine pointing assembly FPA as well as a chromatic beam splitter CBS.
  • Figures 11A, 11B, 11C show cross sections through different exemplary embodiments of an optical telescope lens 3A as used in a possible embodiment of the optical system of the optical communication terminal 1.
  • Figure 11A shows an optical telescope lens 3A having only a conical section which may comprise in a possible implementation a lateral surface with a predefined number N of suppression steps 6.
  • Figure 11B illustrates a possible embodiment of the optical telescope lens 3A comprising a conical section and a cylindrical section.
  • Figure 11C shows a further exemplary embodiment of an optical telescope lens 3A comprising only a cylindrical section.
  • the optical telescope lens 3A comprises a convex front surface FSUR and a concave rear surface RSUR as illustrated in figures 11A, 11B, 11C.
  • the optical telescope lens 3A can be made of different materials.
  • the optical telescope lens 3A is made of silicon. Silicon comprises absorptive properties to reject optical signals having a frequency outside the predefined communication frequency range.
  • the predefined communication frequency range can be for example an infrared frequency range or frequency band. In this way optical noise in the optical receiver unit 2B is minimized.
  • the communication frequency range or communication frequency band can comprise one or more communication channels CH transporting data in the same or opposing transmission directions.
  • Silicon Si comprises a relatively high refractive index n.
  • a further advantage of using silicon resides in the fact that silicon has a high thermal conductivity.
  • the optical telescope lens 3A of the telescope can be made of germanium Ge.
  • the optical telescope lens 3A of the telescope TLA can also be made of glass material.
  • the optical telescope lens 3A is made of a single uniform and homogenous piece.
  • the optical communication terminal 1 as illustrated in the block diagram of figure 4 comprises a coarse pointing assembly 4.
  • the coarse pointing assembly 4 can be mounted to the optical head unit 3 and is adapted to establish and/or to maintain an optical communication link OCL between the optical communication terminal 1 and an external optical flight platform.
  • the optical coarse pointing assembly 4 comprises in a preferred embodiment a CPA mirror 4A as illustrated in figures 12A, 12B and in the schematic diagram of figure 13.
  • the CPA mirror 4A is adapted to reflect an optical beam received from an external optical flight platform or from another optical communication terminal OCT and/or to reflect an optical beam received from the telescope TLA of the optical head unit 3 of the same optical communication terminal 1 to the external optical flight platform or to the external optical communication terminal OCT.
  • the CPA mirror 4A of the optical coarse pointing assembly 4 is pivotable around an elevation axis 4B located off a center of gravity (CoG) of the CPA mirror 4A.
  • the CPA mirror 4A is pivotable around the elevation axis 4B by a controllable elevation drive mechanism 4C of the optical coarse pointing assembly 4.
  • the controllable elevation drive mechanism 4C can comprise in a possible embodiment a Capstan drive mechanism as shown in figure 12B.
  • the elevation drive mechanism 4C for the CPA mirror 4A can also comprise a gear drive mechanism.
  • the coarse pointing assembly 4 further comprises an azimuth capstan drive mechanism 4D to adjust an azimuth angle.
  • the CPA mirror 4A comprises a mirror body having a reflecting optical surface layer located on a pivotable base element driven by the controllable drive mechanism 4C of the CPA mirror 4A.
  • the elevation axis 4B is located in a preferred embodiment at a lower edge of the mirror body of the CPA mirror 4A.
  • the CPA mirror 4A is pivotable around the elevation axis 4B between a closed position and between at least one open position.
  • Figures 12A, 12B illustrate the CPA mirror 4A in an open position where an optical beam is reflected by the optical surface of the CPA mirror 4A to establish or to maintain an optical communication link OCL between the optical head unit 3 of the optical communication terminal 1 and an external optical flight platform, in particular another external optical communication terminal OCT as also illustrated schematically in figure 13.
  • the telescope 3A of the optical head unit 3 is covered by the closed CPA mirror 4A.
  • the CPA mirror 4A can in a possible embodiment also be locked by a controllable locking mechanism of the optical coarse pointing assembly 4. This provides additional protection for the optical surfaces against damage and contamination.
  • the coarse pointing assembly 4 is rotatable around an Azimuth axis and/or around the elevation axis 4B to establish or to maintain an optical communication link OCL between the optical head unit 3 of the optical communication terminal OCT 1 and an external optical flight platform, in particular another optical communication terminal OCT 1.
  • the optical surface layer of the CPA mirror 4A faces a front surface of the telescope 3A of the optical head unit 3 within the optical communication terminal 1.
  • This optical surface layer of the CPA mirror 4A can in a possible embodiment be made of a highly reflective material, in particular gold.
  • the optical surface layer is placed on a pivotable base element of the CPA mirror 4A.
  • This pivotable base element can be made in a preferred embodiment of a physically light material, in particular aluminium or Zerodur.
  • the CPA assembly 4 of the optical communication terminal 1 is adapted to provide a basic angular motion of the optical laser beam in an acquisition mode of the optical communication terminal 1 to establish an optical communication link OCL between the optical communication terminal 1 and an external flight platform or external optical communication terminal OCT. Further, the basic angular motion of the optical laser beam can be performed to compensate a relative motion between the optical communication terminal 1 and the external flight platform or the external optical communication terminal OCT to maintain an already established optical communication link OCL between the optical communication terminal 1 and the external flight platform or external communication terminal OCT by controlling a position of the CPA mirror 4A of said CPA assembly 4.
  • the CPA mirror 4A of the coarse pointing assembly 4 is used as a laser beam pointing mechanism.
  • the CPA mirror 4A is pivotable around an elevation axis 4B being located off a center of gravity COG of the CPA mirror 4A.
  • the elevation axis 4B can be located in a possible embodiment at a lower edge of the CPA mirror 4A as shown in figures 12A, 12B and in the schematic diagram of figure 13. This unbalanced mounting is possible because the coarse pointing assembly 4 operates in a microgravity environment. In a microgravity environment, this off-COG configuration does not create extra torque.
  • the configuration of the optical coarse pointing assembly 4 as a pivotable CPA mirror 4A having an elevation axis 4B located off a center of gravity CoG of the CPA mirror 4A allows for a compact form factor during the transportation and commissioning of the optical communication terminal 1. Further, the CPA mirror 4A protects in its closed position the optical surfaces of the optical system against physical damage and reduces possible contaminations of the CPA mirror 4A and of the telescope TLA of the optical head unit 3. In some scenarios, commissioning of the equipment may be associated with large vibrations and shocks where the CPA mechanism according to the present invention can be secured to avoid potential damage. In a possible embodiment, the scanning CPA mirror 4A pivotable around the elevation axis 4B can be secured with a locking mechanism.
  • the off-center-of-gravity configuration of the CPA mirror 4A provides for a large amount of inertia around the rotational axis for the elevation axis 4B which may lower the angular acceleration for a given torque.
  • the moment of inertia around the Azimuth axis is decreased significantly. This results in an increase of angular acceleration around an azimuth axis comparable with a decrease of elevation axis acceleration.
  • the optical coarse pointing assembly 4 of the CPA mirror 4A pivotable around the elevation axis 4B being located off the center of gravity CoG of the CPA mirror 4A increases the compactness of the structures and consequently reduces the weight during transport of the optical coarse pointing assembly 4 mounted to the optical communication terminal 1. Further, the CPA mirror 4A being pivotable can provide additional protection against physical damage or contamination.
  • Figure 13 illustrates schematically the communication of two optical communication terminals 1, 1' via an optical communication link OCL where a laser beam is reflected by means of CPA mirrors 4A, 4A'.
  • the CPA mirrors 4A, 4A' are in the open position and are provided to maintain the optical communication link OCL between the optical head units 3 of the optical communication terminals 1, 1'.
  • the coarse pointing assembly 4 is rotatable around an Azimuth axis and around the elevation axis 4B to maintain the optical communication link OCL between the optical head units 3 of the optical communication terminals 1, 1'.
  • the CPA assembly 4 for each optical communication terminal 1, 1' is adapted to provide a basic angular motion of the optical laser beam in an acquisition mode of the optical communication terminal 1 to establish the optical communication link OCL between the optical communication terminal 1 and the external optical communication terminal 1'. Further, the angular motion can be used to compensate a relative motion between the optical communication terminal 1 and the external optical communication terminal 1' to maintain the optical communication link OCL established in the acquisition mode.
  • the CPA mirror 4A comprises an angle with respect to the front of the optical head unit 3 which can be adjusted by the controllable drive mechanism.
  • the pivot angle of the CPA mirror 4A around the elevation axis 4B is adjusted in a possible embodiment by a control unit of the optical communication terminal 1 adapted to control the drive mechanisms 4C, 4D of the CPA mirror 4A.
  • the pivot angle can be changed in a possible embodiment continuously to cause a basis angular motion of the optical laser beam to establish and/or to maintain the optical communication link OCL as shown in Fig. 13.
  • the current pivot angle of the CPA mirror 4A relative to the front surface of the optical head unit 3 can also be notified to the external flight platform and/or external optical communication terminal via the optical communication link OCL so that the external flight platform or external optical communication terminal can in turn adjust its own pivot angle accordingly. This results in a more robust optical communication link OCL between the optical communication terminal 1 and the external flight platform.
  • the optical communication terminal 1 as illustrated in the block diagram of figure 4 can use a shared telescope aperture to transmit and receive data and tracking laser beams.
  • the laser beam is generated and coupled into the shared (collimated) beam path by means of the chromatic beam splitter 3D.
  • the transmission light does exit through the telescope TLA to widen the collimated transmission laser beam to the target beam diameter.
  • the optical surfaces of the telescope lens 3A can be coated with an anti-reflection coating to minimize any kind of back-reflections into the receiver system of the transceiver 2.
  • the optical system can be manufactured in a clean-room environment to reduce backscattering caused by surface contaminations such as dust particles. A small fraction of e.g.
  • the transmission laser beam can comprise an intensity which is typically many orders of magnitude higher than the received light from the counter terminal. This might cause negative effects on the tracking and communication performance.
  • Figure 14A illustrates schematically a problem caused by back-reflections and backscattered light, especially at a receiving sensor such as a tracking sensor 8 of the receiver unit 2B.
  • Rays can be emitted from the transmit arm and back-reflected or backscattered at the telescope surface of the telescope lens 3A. Rays which hit a receiving sensor may potentially cause self-blinding effects.
  • the rays or beams pass through optical components 10-1, 10-2, 10-3, 10-4 such as optical filters and lenses.
  • the amount of returned light can be minimized by a high degree of cleanliness which results in few scattering particles and special absorbing measures like coatings or a specific geometry on the telescope sides of the optical telescope TLA.
  • this narrow laser beam of returned light will be rejected by a special spatial filter or central obstruction coating 11 at one of the optical surfaces of optical components 10-i within the optical receiver unit 2B such as the optical component 10-1 shown in Fig. 14B.
  • At least one central obstruction coating 11 is adapted to reduce the returned light by several orders of magnitude.
  • the received optical power is only marginally reduced by the at least one central obscuration coating 11, e.g. by around 10% (0.5dB) .
  • An embodiment of an optical communication terminal 1 comprising at least one central obscuration coating 11 on an optical component 10 is illustrated in figure 14B.
  • the invention provides according to a second aspect an optical communication terminal 1 comprising the optical telescope TLA comprising at least one lens 3A adapted to expand a diameter of an optical beam received from an optical transmitter unit 2A of the optical communication terminal 1 and to reduce a diameter of an optical beam received from an external optical flight platform or terminal for an optical receiver unit 2B of the optical communication terminal 1.
  • the optical communication terminal 1 as illustrated in the embodiment of figure 14B, at least one central obscuration coating 11 is provided on a surface of an optical component 10 of the optical receiver unit 2B. At least one central obscuration coating 11 is adapted to suppress light backscattered and/or reflected by the lens 3A of the optical telescope TLA of the optical communication terminal 1.
  • the central obscuration coating 11 is provided on a surface of an optical component 10 of the optical receiver unit 2B of the optical communication terminal 1.
  • the central obscuration coating 11 comprises a material which is non-transparent in a predefined communication frequency range of the optical communication terminal 1.
  • the central obscuration coating 11 provides on a front surface of the optical component 10 comprises an absorbing or a reflective material.
  • the central obscuration coating 11 can comprise a circular shape with a predefined radius r.
  • the central obscuration coating 11 is provided at a center of the optical component 10 of the optical receiver unit 2B.
  • the optical component 10 can for instance comprise an optical lens of the optical receiver unit 2B within the optical communication terminal 1.
  • the optical telescope TLA of the optical communication terminal 1 comprises an on-axis telescope lens 3A .
  • Acentral obscuration coating 11 is provided in the center of receiving optical signal path of the optical receiver unit 2B.
  • the on-axis telescope lens 3A can comprise in a possible embodiment a single piece optical lens.
  • the optical telescope TLA can comprise multiple optical lenses.
  • the telescope lens 3A is made of a transparent material being transparent in a predefined communication frequency range of the optical communication terminal 1.
  • the material of the optical telescope lens 3A comprises a homogeneous material.
  • the homogeneous material of the telescope lens 3A can for instance comprise silicon or germanium.
  • the optical telescope lens 3A of the telescope TLA can comprise a front surface FSUR and a rear surface RSUR.
  • the front surface FSUR of the optical telescope lens 3A is an aspherical convex front surface.
  • the rear surface RSUR can comprise a spherical or aspherical concave rear surface.
  • the optical telescope lens 3A illustrated in the embodiment of figure 14B is provided to expand a laser beam transmitted by the optical transmitter unit 2A and going through the rear surface RSUR of the optical telescope lens 3A and is further designed to reduce a laser beam in diameter received through the front surface FSUR of the optical telescope lens 3A. The reduced laser beam is then output through the rear surface RSUR of the optical telescope lens 3A to the optical receiver unit 2B via a folding mirror 12 and the chromatic filter unit 3D.
  • the central obscuration coating 11 is provided in the illustrated embodiment on a surface of an optical component 10-1 of the optical receiver unit 2B.
  • the central obscuration coating 11 is adapted to shield in the illustrated embodiment a tracking sensor 8 or any other optical sensor of the optical receiver unit 2B of the optical communication terminal 1 against light backscattered and/or reflected by the optical telescope TLA.
  • the central obscuration coating 11 shown in figure 14B provides an attenuation depending on a ratio between a central obscuration radius r and a beam radius R of a back-reflected optical beam as also illustrated schematically in figure 15.
  • Figure 15 illustrates the total power in Watt of the receiving sensor 8 for 1 Watt transmission power.
  • the central obscuration annulus radius is illustrated in millimeter.
  • the energy or power reflected onto the tracking sensor 8 is reduced with increased obscuration radius r.
  • a lateral surface of a conical section of the optical telescope lens 3A shown in figure 14B may comprise a number of suppression steps 6 being adapted to further suppress backscattered and/or reflected optical light.
  • the front surface FSUR of the telescope lens 3A can be adapted to receive a laser beam from a coarse pointing assembly 4 of the optical communication terminal 1 as illustrated in the block diagram of figure 4.
  • Figure 14B illustrates an isolation between the transmission laser beam and the tracking sensor 8 of about 60 dB without provision of a central obscuration.
  • the central obscuration coating 11 of 1.5mm radius assuming a 5mm receive-beam radius with a further attenuation of 25dB can be achieved at a loss of of the received beam.
  • the illustrated embodiment allows to use an on-axis telescope lens design despite potential back-reflection and/or scattering effects. Different kinds of configurations of the lens telescope 3A can be used for this setup.
  • the provision of on-axis lens telescopes TLA provides advantages over off-axis lens telescopes or off-axis mirror telescopes. On-axis lens telescopes are easy to align and additionally, the manufacturing costs are significantly lower and the dimensions are more compact.
  • Figure 16 shows a block diagram to illustrate an operation of an optical transceiver 2 according to a further aspect of the present invention.
  • the invention provides according to a third aspect an optical transceiver 2 having an optical transmitter unit 2A adapted to transmit optical communication signals within at least one predefined communication frequency range and an optical receiver unit 2B adapted to receive optical communication signals within the predefined communication frequency range.
  • the optical transmitter unit 2A of the optical transceiver 2 according to the third aspect of the present invention comprises a laser diode LD of a transmitter and modulation unit 13 as illustrated in figure 16.
  • the laser diode of the transmitter and modulation unit 13 is adapted to generate a laser beam amplified by a booster Erbium Doped Fiber Amplifier, EDFA, 14 of the optical transmitter unit 2A.
  • EDFA Erbium Doped Fiber Amplifier
  • an amplification gain G of the booster EDFA 14 is lowered to reduce a broadband EDFA Amplified Spontaneous Emission, ASE, noise generated by the booster EDFA 14.
  • the lowered amplification gain G of the booster EDFA 14 is compensated at the same time by a corresponding increase of the output power of the laser beam generated by the laser diode LD of a laser and transmitter modulation unit 13.
  • the amplified light is generated by the booster Erbium Doped Fiber Amplifier, EDFA, 14 present at the optical transmitter unit 2A and can be supplied to a TX collimator 15 as also shown in Fig. 4.
  • the amplified signal is directed towards a point ahead assembly (PAA) 3E and then to the dichroic beam splitter (DBS) 3D.
  • the dichroic beam splitter (DBS) 3D has the task to guide the transmission light to the telescope lens 3A of the optical telescope TLA after reflecting it at the fine pointing assembly (FPA) 3A. Since not all of the transmission light is guided out of the telescope TLA, some portion of the light reflects back to the optical system.
  • the back-reflected light arrives at the dichroic beam splitter (DBS) 3D and gets passed towards the receiving path where the optical filters 10 can be provided to suppress the background light.
  • DBS dichroic beam splitter
  • These optical filters 10 are centered at the receiving wavelength, which is e.g. at 1565nm.
  • Optical filters 10 suppress the transmission light at e.g. 1540nm, however, the ASE noise generated by the booster EDFA 14 is still present at the receiving wavelength so it can get through these optical filters 10 and may arrive at the highly sensitive tracking sensor 8 of the optical receiver unit 2B after the wavelength independent intensity beam splitter IBS.
  • the amplification gain of the booster EDFA 14 is automatically lowered to reduce the broadband EDFA-ASE noise generated by the booster EDFA 14.
  • the output power of the laser beam generated by the laser diode 13 is increased to the same extent.
  • the suppression of the back-reflected ASE noise does also reduce the interference with the relatively weak received communication signals which are coupled into the single mode fiber (SMF) 17 of a preamplifier EDFA 18 of the receiver unit 2B.
  • SMF single mode fiber
  • the optical transmitter unit 2A is adapted to transmit an optical transmission signal at a predefined transmission wavelength within a predetermined communication frequency range of the optical communication terminal 1.
  • the optical receiver unit 2B is adapted to receive an optical reception signal at a predefined reception wavelength within the predetermined communication frequency range of the respective optical communication terminal 1.
  • the optical transmission signal of the optical transmitter unit 2A is supplied via the point ahead assembly (PAA) 3E, the dichroic beam splitter (DBS) 3D and via the fine pointing assembly (FPA) 3C to the telescope TLA of the optical communication terminal 1.
  • the optical receiver unit 2B of the optical communication terminal 1 comprises at least one optical bandpass filter BPF forming an optical component 10 which is adapted to suppress light outside a reception filter frequency band centered around the reception wavelength of the optical receiver unit 2B.
  • the optical bandpass filter BPF of the optical receiver unit 2B is adapted to suppress reflected light at the transmission wavelength of the optical transmitter unit 2A of the optical communication terminal 1.
  • the EDFA ASE noise generated by the booster EDFA 14 of the optical transmitter unit 2A can comprise a spectral overlap with the reception filter frequency band of at least one optical bandpass filter BPF of the optical receiver unit 2B within the optical communication terminal 1.
  • An optical reception signal having passed through the optical bandpass filter BPF of the optical receiver unit 2B is split by the intensity beam splitter (IBS) 16 into a first optical signal supplied to a tracking sensor 8 of the optical receiver unit 2B and into a second optical signal supplied via a single mode fiber (SMF) 17 and an EDFA preamplifier 18 to an optical signal demodulator 19 of the optical receiver unit 2B of the optical communication terminal 1 as illustrated in figure 16.
  • IBS intensity beam splitter
  • SMF single mode fiber
  • an adjustment of the amplification gain of the booster EDFA 14 of the optical transmitter unit 2A and the corresponding adjustment of the output power of the laser diode 13 of the optical transmitter unit 2A to reduce the EDFA ASE noise can be controlled by a control unit 20 of the optical transceiver 2 in a calibration mode of the optical communication terminal 1.
  • An ASE noise power coming out of the booster EDFA 14 is reduced such that it cannot blind the tracking sensor 8 and does not interfere with a communication signal which is in a possible implementation in a range of -43dbm for a 10Gbit/ssystem.
  • the ASE power is suppressed by reducing the gain G of the booster EDFA 14 by increasing at the same time the optical power from the seed laser diode LD.
  • the booster EDFA 14 needs to provide 30dB gain. If the input power is increased to +4dBm, then the booster EDFA 14 needs to provide only 26dB gain to achieve +30dBm output power.
  • a reduction of the gain of the booster EDFA 14 results in different benefits. It reduces the ASE noise power to diminish the self-blinding of the tracking sensor 8. Further, it reduces the interference between the ASE noise and the weak communication signals in the preamplifier EDFA 18 at the receiving side. Further, the booster EDFA 14 operating at a lower gain consumes less electrical power which provides a relief for the thermal management system and increases reliability and provides more structural integrity.

Abstract

Émetteur-récepteur optique (2) pour un terminal de communication optique, OCT (1), ayant une unité émetteur optique (2A) conçue pour transmettre des signaux de communication optique à l'intérieur d'au moins une plage de fréquences de communication prédéfinie, et une unité récepteur optique (2B) conçue pour recevoir des signaux de communication optique à l'intérieur de la plage de fréquences de communication prédéfinie, l'unité émetteur optique (2A) comprenant une diode laser conçue pour générer un faisceau laser amplifié par un suramplificateur optique à fibre dopée à l'erbium, EDFA, de ladite unité émetteur optique (2A), un gain d'amplification dudit suramplificateur EDFA étant abaissé pour réduire un bruit d'émission spontanée amplifiée, ASE, EDFA à large bande généré par ledit suramplificateur EDFA, le gain d'amplification abaissé du suramplificateur EDFA étant compensé par une augmentation correspondante de la puissance de sortie du faisceau laser généré par ladite diode laser.
PCT/CN2021/123380 2021-10-12 2021-10-12 Émetteur-récepteur optique pour terminal de communication optique WO2023060452A1 (fr)

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