JP2003511965A - Reconfigurable laser communication terminal - Google Patents

Abstract

SUMMARY OF THE INVENTION A system and method for free space optical communication comprises a polarization switch for separating and steering a transmitted signal and a received signal between a corresponding transmitter 40 and receiver 48. And at least one reconfigurable terminal 30 that uses dual-wavelength operation to separate transmitted and received signals. Polarization-based switching provides a wavelength independent of beam steering to facilitate the reciprocal change in the wavelength of the transmitted and received signals. A controllable or passive polarization changing device 56, such as a waveplate or polarization rotator, allows the communication terminal 30 to reconfigure the beam steering optics 54, 62, 66 without the need for repositioning and associated precise alignment. Configurable.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a reconfigurable laser communication terminal particularly suitable for optical intersatellite links.

[0002]

[Prior art]

The optical inter-satellite link terminal is used for communication between two satellites. One major technical challenge in the design of communication networks containing these terminals is to separate the receiver channel in any terminal from other spurious emissions generated by transmit beams starting in the same optical terminal. is there. With typical inter-satellite distances and acceptable transmitter / receiver aperture sizes, the separation requirement is often greater than 90 dB, ie the spurious signal generated by the transmitter beam entering the receiver channel is probably at least 90 dB. Must not be smaller than the transmitter beam itself.

In communication networks that make up the Optical Inter-Satellite Link (OISL), one common separation method that achieves acceptable size and weight while producing the necessary separation between the transmit and receive beams is different. A method of operating a beam at a wavelength. Dual wavelength OISL networks are characterized by (at least) 2 operating wavelengths
You need one type of terminal. For example, a type A terminal transmits radiation at a first wavelength and receives radiation at a second wavelength, a type B terminal transmits at a second wavelength and a first wavelength
Receive at the wavelength of. Appropriate communication links require terminal A to communicate with terminal B. The operating wavelengths are typically separated by a wavelength difference of about 5-10 nanometers (nm), which is sufficient for an effective receiver bandpass filter to properly reject spurious transmitter emissions and meet the separation requirements. It is a large size. This method offers various advantages, including that the transmitter and receiver functions can share common telescope and pointing optics to reduce weight and cost. A dichroic beam splitter that transmits a second wavelength and reflects the first wavelength is used to separate the transmitted beam from the incoming received beam.

A second known separation method uses polarization-based switching techniques to separate the transmit and receive beams of the same wavelength but operating in orthogonal polarizations. This method provides only about 30-40 dB isolation based on the particular structure and is therefore not suitable for many OISL applications with more stringent isolation requirements.

Although physical separation of the optical paths provides acceptable separation, this third method requires separate telescope and pointing optics, thus increasing weight and cost compared to the previously mentioned methods. To do. Similarly, temporal separation, ie transmission and reception at different times, is a fourth method that can be used to provide sufficient separation, but imposes severe restrictions on the communication format. Furthermore, temporal separation is undesirable because it requires the system to adapt to changes in intersatellite distance.

Designers of space-based communication networks recognize that some parts of an OISL terminal will not run during the desired system life. To compensate, sufficient redundancy must be provided so that the entire system can function acceptably despite the loss of a significant portion of the terminal. A typical system design may specify 50% more terminals per satellite than needed to maintain the minimum acceptable performance. The implicit assumption when this method of reliability is applied with dual wavelength separation is that any given satellite maintains a link with complementary terminals of opposite type on one or more other satellites. Always have enough suitable type (A or B) terminals. In order to reduce costs while providing acceptable system redundancy, the terminals of the communication system must be reconfigurable so that when receiving the appropriate command from the network controller, A terminal will be B terminal. Must be convertible to.

[0007]

[Problems to be Solved by the Invention]

One way to provide this reconfiguration capability is to mechanically replace the dichroic beamsplitters so that the appropriate wavelengths for A or B terminals are directed to the receiver and from the transmitter. However, this method requires complex and expensive mechanical and electronic components to achieve the precise alignment required for repeated switching between types A and B throughout the life of the satellite network.

Accordingly, it is an object of the present invention to provide a reconfigurable communication terminal having an acceptable weight, price and isolation used in optical intersatellite links.

Another object of the present invention is to provide a system for separating a transmit beam from a receive beam in a dual wavelength optical communication system that exhibits similar performance characteristics for both types of terminals to facilitate reconfigurability. Is to provide a method.

Yet another object of the present invention is to provide a method of reconfiguring an optical communication terminal while reducing or eliminating mechanical movement of optical elements.

Another object of the present invention is to provide a polarization-based reconfigurable communication system and method that combines the optical paths of the transmit and receive beams with improved isolation.

An additional object of the present invention is to provide a system and method for reconfiguring an optical communication terminal in response to control commands by exchanging the types of transmitted and received signals.

Yet another object of the present invention is to provide a system and method for reconfiguring an optical communication terminal that reduces or eliminates repositioning of beam steering components.

[0014]

[Means for Solving the Problems]

The systems and methods for optical communications separate the transmitted and received signals between corresponding receivers and transmitters, respectively, in combination with polarization switching to steer the transmitted signals. It includes at least one reconfigurable terminal that uses dual wavelength operation to separate received signals. Polarization-based switching provides beam steering independent wavelengths to facilitate the interchange of wavelengths of transmitted and received signals. A controllable or passive polarization-changing device, such as a waveplate or polarization rotator, with a selectable or tunable bandpass filter allows the communication terminal to reposition the beam steering optics and the associated precise alignment. It can be reconfigured without need.

In one embodiment, an optical communication terminal according to the present invention comprises a transmitter for generating an optical signal having a first polarization and a first wavelength, a second polarization and a second polarization. A receiver for receiving an optical signal having a wavelength. Polarization beam splitter is the first
The optical signal having the polarized wave and the first wavelength is guided from the transmitter, and the optical signal having the second polarized wave and the second wavelength is guided to the receiver. A reconfigurable polarization changing device that operates to change the polarization of the optical signal passing through responds to the command signal and changes the output signal polarization (from changing from the fourth polarization to the second polarization). ) The first state is changed from the first polarization to the third polarization, and the output signal polarization is (while changing from the third polarization to the second polarization).
Select at least one of the second states that is changed from the first polarization to the fourth polarization.

The optical communication method according to the present invention comprises the steps of transmitting an optical communication signal having a first wavelength and a first polarization and receiving an optical communication signal having a second wavelength and a second polarization. Contains. The transmitted and received signals propagate along an optical path and pass through at least one optical element common to both signals. The first and second wavelengths are selected to provide a predetermined isolation level between transmitted and received optical communication signals within a particular communication terminal. The method also spatially separates the transmitted signal from the received signal that is based on the first and second polarizations but is substantially independent of the first and second wavelengths, and the first and second The method also includes the step of selectively reconfiguring the communication terminal by interchanging two wavelengths and polarizations, whereby the optical communication signal is transmitted from the terminal at the second wavelength and the second polarization, Received by the terminal at a wavelength of 1 and a first polarization.

One embodiment of an optical communication system according to the present invention transmits an optical signal of a first wavelength having a first polarization and receives an optical signal of a second wavelength and a second polarization. It includes a first satellite equipped with a first optical communication terminal. The at least one additional satellite having a second optical communication terminal transmits an optical signal of a second wavelength having a second polarization and receives an optical signal of the first wavelength and the first polarization. To do. At least a first optical communication terminal is responsive to the command signal to transmit and receive wavelengths and polarizations, preferably without any beam steering repositioning and / or realignment. It is selectively reconfigurable to interchange with each other.

The present invention provides many advantages of optical communication systems. For example, the present invention provides superior performance as compared to other methods as measured by price, size, weight, complexity, and reliability. The present invention uses a conventional beam splitter to achieve the required light switching or beam steering, avoiding realignment of the beam steering optics and the associated precise alignment. The present invention comprises commercially available polarization components that enable a low cost solution to the switching problem while providing a reconfigurable communication terminal to improve system flexibility and overall reliability. You can

[0019]

DETAILED DESCRIPTION OF THE INVENTION

The foregoing and other objects, features, and advantages of the present invention will be readily appreciated by those skilled in the art from the following detailed description of the best mode for carrying out the invention in connection with the accompanying drawings. Referring to FIG. 1, there is shown a block diagram illustrating one application of an optical satellite communication system according to the present invention. As those skilled in the art will appreciate, the present invention is generally applicable to any free space optical communication system, an example of which is a satellite based communication system as illustrated and described in detail below. Similarly, while the following illustrated embodiments are described with reference to communication signals, the present invention must often use common optics to receive the signals and solve similar separation problems discussed above. It is equally applicable to acquisition and tracking systems and / or subsystems.

The satellite communication system 10 includes a plurality of satellites 12, 14 in an orbit centered on the earth 16. Satellites 12 and 14 are capable of optical communication in free space according to the present invention 1
The above optical communication terminal 18 is included. The satellites 12, 14 also include a directional communication system that transmits information to the ground station 20 via a downlink 22, which is typically an RF link. Similarly, ground station 20 may communicate data, including control commands, to satellites 12, 14 via uplink 24.

The communication system 10 is preferably a dual wavelength system with at least two types of terminals 18 featuring selected operating wavelengths. For example, a terminal 26 called type "A" transmits radiation at a _first wavelength λ ₁ and polarization P ₁ and receives radiation at a _second wavelength λ ₂ and polarization P ₂ . A supplementary terminal called Type "B" 28
Transmits radiation at a _second wavelength λ ₂ and polarization P ₂ and receives radiation at a _first wavelength λ ₁ and polarization P ₁ . A suitable communication link requires an "A" terminal to communicate with a "B" terminal. The two operating wavelengths are typically separated by 5-10 nanometers. This is large enough for a readily available receiver bandpass filter to adequately reject transmitter emissions while providing the required isolation of typically 90 dB.

The two satellites preferably transmit and receive communication signals along a common line of sight or communication beam axis, which is shown separately in FIG. 1 as a transmit beam and a receive beam, merely for clarity and ease of illustration. Has been done. As used throughout this description, a beam or signal is "transmitted" when it leaves a communication terminal and "received" when it arrives at the communication terminal. The transmitter "generates" a signal or beam having an associated wavelength and polarization. The generated signal or beam passes through various optical elements in the communication terminal including the polarization changing device, and the beams transmitted thereby have the same wavelength but different polarization with respect to the generated beam. Is characterized. Similarly, a "received" signal having an associated wavelength and polarization has previously passed through a polarization changing device within the communication terminal, which is referred to as the "detected" signal or beam passing to the receiver. Therefore, the "detected" beam or signal has the same wavelength but different polarization with respect to the "received" beam arriving at the communication terminal.

In order to meet the desired system life, redundant communication terminals may be included to compensate for terminals that degrade or fail to perform during the desired life span. This method is most prevalent in satellite-based systems, since repairing or replacing terminals is very difficult or impossible. However, a sufficient number of supplemental type terminals must exist to maintain the required links. The present invention provides one or more reconfigurable terminals 30 to reduce the number of terminals required, as will be described in more detail below.

As shown in FIG. 1, the satellite 12 transmits an optical signal of a _first wavelength λ ₁ having a polarization P ₁ of a second wavelength λ ₂ and a second polarization P ₂ . It includes a first communication terminal 26 for receiving an optical signal. Satellite 14 preferably includes a second optical communication terminal 28 that transmits an optical signal of _second wavelength λ ₂ having polarization P ₂ . The communication terminal 28 receives the signal of the first wavelength λ ₁ having the first polarization P ₁ . The optical communication terminal 30 is selectively reconfigurable so as to mutually change the transmission wavelength and polarization and the reception wavelength and polarization in response to the command signal from the ground station 20. The satellite 14 may include various other optical communication terminals 18 that transmit optical signals of a first wavelength λ ₁ and a first polarization P ₁ . Similarly, satellite 12 may include various other optical communication terminals that transmit optical signals of the second wavelength λ ₂ and the second polarization P ₂ .

Referring to FIG. 2, there is shown a partial block diagram of an optical communication terminal according to the present invention. Those skilled in the art will understand that this block diagram only includes elements that facilitate the description of the present invention, and that actual structures may include additional elements and / or equivalent elements without departing from the scope of the present invention. You will recognize good things. Similarly, the present invention may be comprised of discrete, commercially available components, custom integrated components, or any combination.

The optical communication terminal 30 includes a transmitter 40 for generating an output optical signal having an associated polarization.
Is included. The transmitter 40 may be composed of any number of known coherent radiation sources with or without associated optics. In this embodiment, transmitter 40 includes laser assembly 41, beam shaping optics 42, and optional polarization device 43.
Is included. In one embodiment, the transmitter 40 produces coherent radiation at a first wavelength, such as 1550 nanometers, having a _first linear polarization (LP ₁ ). The transmission wavelength λ _T is λ ₁ or λ ₂ depending on whether terminal 30 is a type “A” or “B” terminal, respectively. The first linearly polarized light is indicated by dots representing vertically polarized light, i.e. light polarized perpendicular to the plane of the drawing.

In a laser that produces a selectable tunable wavelength in response to a control command,
Only one laser is needed. As an example, for semiconductor lasers operating in the wavelength range near 1550 nanometers, a convenient way to achieve the necessary wavelength change from λ ₁ to λ ₂ is to change the temperature of the diode laser. . instead of,
If the required temperature change magnitude of the diode laser is unacceptable, or if the system has a different type of laser that produces a substantially fixed wavelength, the transmitter 40 is associated with a second laser assembly 44. Beam shaping optics 45 and optional polarization device 46 may be included. The second laser assembly 44 includes a laser operating at a _second wavelength λ ₂ . The beams produced by the first laser assembly 41 and the second laser assembly 44 have the same polarization state (LP ₁ ). Beam combiner 47 directs the two beams along the same optical path 49. Many methods for realizing the function of the beam combiner 47 are known in the existing art. For many types of lasers, gratings or dichroic mirrors may be used. For lasers operating near 1550 nanometers, other optical elements well known in the art including wavelength division multiplexers (WDM) or fiber-based optical switches are also available.

Also shown in FIG. 2, terminal 30 includes a receiver 48 for receiving an incoming optical signal having an associated polarization. Receiver 48 includes a suitable detector 50 and associated optics such as a selectable bandpass filter 52 that provides the required isolation. Suitable bandpass filters are available in existing technology. Widely separated wavelengths (eg λ ₁ and λ ₂ with a difference greater than 15 to 20 nanometers)
In, thin film bandpass filters are sufficient and can be used to replace filters with bandpass wavelengths that require mechanical switching. For narrow wavelength spacing, a dielectrically coated Fabry-Perot etalon aligned perpendicular to the beam propagation direction can be used. Since the etalon passes through a periodic set of wavelengths, thin film filters are used in series to eliminate all etalon bandpass wavelengths except the desired wavelength. For maximum accuracy, etalons require temperature stabilization to avoid thermally induced drift of bandpass wavelengths. This same temperature controller is used to tune the bandpass of the etalon from λ ₁ to λ ₂ , thereby providing the required selectability of the received wavelength without the need for any mechanical exchange. Receiver 48 detects an optical signal having a _second linear polarization (LP ₂ ), which is preferably indicated by an arrow perpendicular to the direction of propagation and which represents a horizontal linear polarization.

A polarization beam splitter 54 in the terminal 30 guides an optical signal having a _first linear polarization (LP ₁ ) from the transmitter 40 and outputs an optical signal having a second linear polarization (LP ₂ ). Receiving machine
Operable to direct to 48. The reconfigurable polarization switching device 56 is operable to change the polarization of the passing optical signal.

In one embodiment, polarization changing device 56 selects one of a first state and a second state in response to a command signal received from command processor 58. In this first state, the signal polarization generated is changed from the first polarization (LP ₁ ) to the first polarization (P ₁ ) (while the received signal polarization P ₂ is the second Of the linear polarization LP ₂ of the same). In this second state, the signal polarization generated is changed to a _second polarization (P ₂ ), where the received signal polarization P ₁ is simultaneously changed to a _second linear polarization LP ₂ . ). In one embodiment of the invention, the first and second linear polarizations (generated and detected) are orthogonal linear polarizations and the first and second polarizations (transmitted and received) are orthogonal circles. It is polarized wave. For example, the first linear polarization (LP ₁ ) is a vertical linear polarization, and the second linear polarization (LP ₂ ) is a horizontal linear polarization. In this case, the polarization changing device 56 changes the first linear polarization (LP ₁ ) into a circular polarization (P ₁ ) such as left circular polarization (LCP). Similarly, the received signal is right circularly polarized (RCP) and is converted by the polarization changing device 56 into a horizontally linearly polarized signal. Upon receiving a command from the command processor 58, the polarization switching device 56 converts the first linear polarization (LP ₁ ) into the second polarization (RCP) instead of the first polarization (LCP). The reconfiguration is coordinated by the network controller (ground station) and therefore the communicating terminal is of the appropriate type. Alternatively, the unique control of the type of terminal may be performed by the processor onboard the satellite. Preferably, the reconstruction also changes the wavelength associated with the transmitted optical signal and the received optical signal to provide the predetermined separation. In this way, the selectable bandpass filter 52 is adjusted to exchange the transmit and receive wavelengths.

The polarization switching device 56 may comprise any number of known devices that reversibly control the properties of the polarized beam. For example, a slow wave device such as a quarter-wave plate rotates the quarter-wave plate at an angle of 90 ° about the optical beam axis to select the first or second polarized wave. It may be used in combination. Since the rotating quarter-wave plate (having planes with parallel planes) does not impose significant beam steering on the beam passing through it, the alignment tolerance is relatively large and easy to maintain in this way. . Alternatively, non-mechanical methods using electro-optic crystals may be used. As is well known, an electro-optic crystal driven by a DC voltage controlled by command processor 58 may be used to select the appropriate polarization state.

The polarization changing device 56 may also be constituted by an active or passive polarization rotator that rotates the output polarization by 45 ° but maintains the linear polarization. The received beam rotates an additional 45 ° as it passes through the polarization changing device (rotator) to give a total 90 ° rotation when needed. Polarization rotators are well known in the art,
It is usually manufactured from a quartz cut so that the beam propagates along the crystal optical axis (c-axis). This alternative means offers the advantage of preventing mechanical movement, but is sensitive to relative angular rotation along the line of sight or light beam axis between two communication terminals. If the two terminals undergo a relative angular rotation about the line of sight, the angular rotation induces a loss in the link. In particular, if the rotation angle is θ, then the loss is sin ² θ. In contrast, the quarter-wave plate structure is insensitive to the relative angular alignment of the terminals about the line of sight, and thus there is no loss due to angular orientation.

Various other beam steering optics may be used for acquisition, tracking, communication alignment purposes, such as fine pointing mirror 62, telescope 64, gimbal steering mirror 66. As shown in FIG. 2, the transmitted and received optical communication signals are transmitted along the optical path including the polarization beam splitter 54, the polarization changing device 56, the fine directional mirror 62, the telescope 64, and the gimbal steering mirror 66. Propagate through a plurality of common optical elements. Price as a result of this,
Complexity, weight is reduced. Thus, the present invention uses the dual wavelength method to achieve the desired separation between the transmitted and received signals, but separates them so that the two beams can share a common output and directional optics. To use polarization-based switching (which is essentially wavelength independent). This is possible because a set of polarisers works well over the 10-20 nanometer range for wavelengths near 1550 nanometers. This eliminates the need to replace any beam steering elements when the transmit and receive beams switch wavelengths, i.e. when the communication terminal is reconfigured from type "A" to type "B". In this way, once the polarization beam splitter is aligned, it need not be moved to accommodate different choices of operating wavelengths. The only optical elements that may be repositioned are those elements that do not adversely affect beam steering and have a relatively large tolerance for alignment. Therefore, rather than relying on very precise mechanical movements to maintain alignment during repeated exchange operations, careful adjustment and alignment need only be done once during final assembly and testing. .

Referring to FIG. 3, there is shown a flowchart illustrating an optical communication method according to the present invention. Those of ordinary skill in the art will recognize that the functions or steps represented by the flowcharts are not necessarily performed sequentially, one or more functions or steps may be performed simultaneously or substantially simultaneously. The block 80 receives the transmitted optical communication signal of the first wavelength (λ ₁ ) and the first polarization (P ₁ ), and receives the second wavelength (λ ₂ ) and the second polarization (P ₂ ). 2 represents the optical communication signal that has been generated. Preferably, the transmitted signal and the received signal propagate along an optical path in the communication terminal and pass through at least one optical element common to both. Also preferably, the first wavelength and the second wavelength are selected to provide a predetermined isolation level between the transmitted and received optical communication signals within a particular communication terminal. As represented by block 82, the transmitted and received signals are separated based on the first and second polarizations, but substantially independently of the first and second wavelengths. There is. As mentioned above, this is accomplished using a polarization beam splitter with appropriate performance characteristics over the wavelength of interest in connection with a polarization changing device.

The block 84 includes a first wavelength and a second polarization so that the optical communication signal is transmitted at the second wavelength and the second polarization and is received at the first wavelength and the first polarization. Represents the step of selectively reconfiguring the communication terminal by exchanging the wavelengths and polarizations of the.
As described with reference to FIG. 2, block 80 of FIG. 3 may include generation of a signal having a first wavelength and a first linear polarization as represented by block 86. In the example shown in FIG. 2, (corresponding to the generated and detected beams)
The first and second linear polarizations are orthogonal linear polarizations, while the first and second polarizations (corresponding to transmitted and received beams) are orthogonal circular polarizations. Thus, the polarization is changed from the first linear polarization to one of the first or second (circular) polarizations selected, as represented by block 88. Similarly, the polarization of the received signal is from the first or second (circular) polarization to the second polarization as represented by block 90.
Is changed to the linear polarization of. As mentioned above, depending on the particular polarization changing device used, a "change" of polarization may include a change from linear polarization to circular polarization as well as rotation of the linear polarization. Thus, references to linear and circular polarization in the text and drawings are intended to clarify the description rather than limit the scope of the invention.

The optical communication terminal can be selectively reconfigured by exchanging the transmission polarization and wavelength and the reception polarization and wavelength. The transmit and receive polarizations may be changed by rotating the wave plate, as represented by block 94. Alternatively, this step may be performed by changing the voltage applied to the appropriate electro-optic crystal. The transmit and receive wavelengths (corresponding to generated and detected beams, respectively) can be adjusted by tuning the transmitter or by another source in the transmitter in connection with the selection of an appropriate bandpass filter in the receiver. May be exchanged by selecting.

Although the best mode considered for carrying out the invention has been described in detail, those skilled in the art to which the present invention pertains will appreciate various implementations of the invention as defined in the claims. Will recognize alternative designs and embodiments.

[Brief description of drawings]

[Figure 1]   FIG. 3 is a block diagram of an optical satellite communication system according to the present invention.

[Fig. 2]   FIG. 3 is a partial block diagram of an optical communication terminal according to the present invention.

[Figure 3]   6 is a flowchart showing an inter-satellite optical communication method according to the present invention.

Claims (8)

[Claims] 1. A transmitter (40) for generating an output optical signal having a first polarization and a first wavelength, and an incoming light having a second polarization and a second wavelength. A receiver (48) for receiving a signal, an optical signal having a first polarization and a first or second wavelength is guided from the transmitter (40), and a second polarization and a first or second wavelength are received. A polarization beam splitter (56) that operates to guide an optical signal to or from the receiver (48), and that operates to change the polarization of the optical signal that passes through, and the output signal polarization is Reconfigurable in response to a command signal to select one of a first state that is changed to three polarizations and at least a second state that the output signal polarization is changed to a fourth polarization Optical communication terminal equipped with a simple polarization changing device (56). 2. The first and second polarizations are linear polarizations perpendicular to each other, and the third polarization
The terminal according to claim 1, wherein the fourth polarized wave is a circular polarized wave. 3. The terminal according to claim 1, wherein the reconfigurable polarization changing device (56) comprises a wave plate. 4. The terminal according to claim 3, wherein the reconfigurable polarization changing device (56) further comprises a position setting motor (60) for rotating the wave plate in response to a command signal. 5. The terminal according to claim 1, wherein the reconfigurable polarization changing device (56) comprises an electro-optic crystal. 6. The terminal according to claim 1, wherein the reconfigurable polarization changing device (56) comprises a polarization rotator. 7. The terminal according to claim 6, wherein the polarization rotator is a passive polarization rotator. 8. A common pointing optical system (54, 62) for guiding a transmitted optical signal and a received optical signal along a common path between a polarization beam splitter (54) and another satellite.
, 66) and a telescope (64).