Laser communication network implemented with multi-chroic filters

A laser communication network implemented with multi-chroic filters that are able to partition signals from a band of wavelengths into different sub-bands that enable more than one relay terminal to interconnect users (i.e., sources and sinks of traffic) is provided. A band of wavelengths may be partitioned to enable users to communicate with relay satellites, and relay satellites to communicate with one another, using a predefined set of transmission and reception wavelengths regardless of which particular relay is in the communication session. In other words, embodiments support both relay satellites and user satellites by constructing distinct passbands for relay-borne terminals and the same passbands for users.

FIELD

The present invention generally relates to communications, and more specifically, to a laser communication network implemented with multi-chroic filters.

BACKGROUND

Relay satellites and user satellites need to deconflict their use of wavelengths for laser communications. Not only must relay satellites communicate with users in a user band, but they also must communicate with each other at wavebands distinct from the user band. Relay satellites, which are not the sources or sinks of substantial network traffic, may host different types of terminals that use different wavebands, such as Type A terminals and Type B terminals.

By definition, a relay must include more than one terminal. For example, a relay may include one Type A terminal and one Type B terminal, two Type A terminals and one Type B terminal, one Type B terminal and two Type A terminals, two of each type of terminal, or more than two of either or both Type A terminals and Type B terminals. One may regard a relay-type as a relay having a number and mix of terminals. These include the terminal mixes listed above.

By definition, a Type A terminal can transmit a laser communications signal to be received at a Type B terminal at a wavelength in a first waveband. Similarly, a Type B terminal can transmit to a Type A terminal at a wavelength in a second waveband distinct from the first waveband. Each terminal type uses a separate transmit and receive waveband. Thus, in monostatic terminal designs (i.e., those that use a single telescope or optical module to both transmit and receive optical signals), the higher power transmit signals are readily distinguished from lower power received signals.

User terminals, which are the sources or sinks of substantial network traffic, must be able to communicate with either a Type A terminal or a Type B terminal. Therefore, user terminals cannot themselves be a Type A terminal or a Type B terminal. A user waveband distinct from the first and second wavebands is necessary to include data sources and sinks. Prior networks avoid this wavelength conflict either by operating point-to-point (i.e., without relays, and therefore, not really a network) or by not including more than one relay. Accordingly, an improved technique for performing waveband de-confliction in multi-relay free space optical networks may be beneficial.

SUMMARY

Certain embodiments of the present invention may provide solutions to the problems and needs in the art that have not yet been fully identified, appreciated, or solved by conventional communications technologies. For example, some embodiments pertain to a laser communication network implemented with multi-chroic filters. Such embodiments may add a user waveband that de-conflicts the wavelength plan and enables relay-able traffic (i.e., traffic flowing through one or more relays) from source to sink. Per the above, this is not done in existing systems, which operate point-to-point or only with a single multi-terminal relay.

In an embodiment, a system includes at least one user device including a user terminal with a multi-chroic filter. The user terminal includes a user transmit filter and a user receive filter. The system also includes two or more relays. Each relay includes at least two terminals having different types from one another, as well as from the user terminal. Each relay terminal includes a relay transmit filter and a relay receive filter. The multi-chroic filters are configured to partition signals from bands of a wavelength into a plurality of sub-bands using a predefined set of transmission and receiving wavelengths.

In another embodiment, a relay includes a first terminal of a first type that includes a first multi-chroic filter. The relay also includes a second terminal of a second type that includes a multi-chroic filter. The multi-chroic filters of the first terminal and the second terminal are configured to partition signals from bands of a wavelength into a plurality of sub-bands using a predefined set of transmission and receiving wavelengths.

In yet another embodiment, a relay includes a Type A terminal that includes a first multi-chroic filter. The relay also includes a Type B terminal that includes a multi-chroic filter. The multi-chroic filters of the Type A terminal and the Type B terminal are configured to partition signals from bands of a wavelength into a plurality of sub-bands using a predefined set of transmission and receiving wavelengths.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Some embodiments pertain to a laser communication network implemented with multi-chroic filters that are able to partition signals from bands of wavelengths into different sub-bands that enable more than one relay terminal (e.g., a set of series-coupled relays) to interconnect users (i.e., sources and sinks of traffic). Indeed, a band of wavelengths may be partitioned to enable users to communicate with relay satellites, and relay satellites to communicate with one another, using a predefined set of transmission and reception wavelengths regardless of which particular relay is in the communication session. In other words, embodiments support both relay satellites and user satellites by constructing distinct passbands for relay-borne terminals in the same passbands for users.

“Multi-chroic” is a neologism for a set of network-enabling filters having specific waveband-allocation properties. This term is used herein to distinguish some embodiments from dichroic filters, which separate light into a one lowpass band and one highpass band (i.e., two wavebands, with one being a stopband and the other being a passband). Dichroic filters are commonly used. However, the use of multi-chroic filters allows for multiple passbands and multiple stopbands (e.g., at least two passbands and/or at least two stopbands for Type A terminals and Type B terminals, as in the embodiment ofFIGS. 3A and 3B). In some embodiments, the multi-chroic filters may include one or more dichroic filters. The user filter inFIGS. 3A and 3Bcan be a bandstop filter or a dichroic filter, for example. However, it is not proper to describe the whole set of filters as dichroic even if one filter is dichroic.

It should be noted that while two types of terminals and their corresponding wavebands, i.e., Type “A” and Type “B,” are described herein, additional terminal types, relay types, and passbands may be accommodated without deviating from the scope of the invention. It should also be noted that while satellite systems are discussed herein, non-satellite users may be included in some embodiments. For instance, high speed relay networks in some embodiments may include airborne users, shipborne users, users in areas that are under-served by other broadband technologies, deep space probes, rovers, communication devices on the moon, other planets, or other bodies than the Earth, or any other suitable land, sea (whether on the surface or underwater), air, or space-based users without deviating from the scope of the invention. Indeed, some embodiments offer a mechanism to extend planetary networks beyond terrestrial and undersea networks.

Some embodiments use Consultative Committee for Space Data Systems (CCSDS) Orange Book-compatible waveband assignments. Referring to the Center Frequency Specification in section 4.1.1 thereof, the center frequency of the optical carrier shall be 191.6+n×0.1 THz, where n is an integer ranging from 0 to 33. It should be noted that these center frequencies in the optical C-band are a subset of those defined in the ITU-T G.694.1 frequency grid with 100 GHz channel spacing. The frequencies range from 191.6 THz to 194.9 THz and correspond to wavelengths in vacuum ranging from 1538.19 nm to 1564.68 nm. Center frequencies between 191.6 THz and 193.2 THz, inclusive, (i.e., n=0 to 16) are designated as Data Communications Band A and shall be used in this specification by transmitters in Type A terminals. Center frequencies between 193.3 THz and 194.9 THz, inclusive, (n=17 to 33) are designated as Data Communications Band B and shall be used in this specification by transmitters in Type B terminals. However, it should be noted that any suitable center frequency specifications may be used without deviating from the scope of the invention.

In some embodiments, N bands are at a given wavelength. For instance, consider the scenario with four bands 1, 2, 3, and 4. Transmission from a Type A terminal to a Type B terminal (either or both of which may be a geosynchronous relay satellite) is at a center wavelength of λ1. Transmission from a Type B terminal to a Type A terminal is at a center wavelength of λ2. Transmission from either a Type A terminal or a Type B terminal to a user terminal is at center wavelength of λ3. Transmission from a user terminal to either a Type A terminal or a Type B terminal is at center wavelength of λ4. All four wavelengths are distinct, and are part of distinct wavebands.

In order to facilitate communications, filters of optical modules (e.g., telescopes with embedded filtering, signal routing, and related capabilities in laser communication systems) for each terminal are optically coated for the same passbands for users and for complementary passbands for Type A versus Type B terminals, as shown inFIGS. 3A, 3B, 4A, and 4B, for example. The coatings may be or include hafnium oxide(s), tantalum oxide(s), silicon dioxide, or any combination thereof in any number of layers without deviating from the scope of the invention. Indeed, in some embodiments, more than 100 layers may be used in an optical coating, as is frequently the case for coarse wavelength division multiplexing (CWDM) applications. However, CWDM is typically used in fiber optic networks, and not in free-space networks. Some nonlimiting examples of coating vendors include, but are not limited to, REO™, CVI Laser Optics™, and Andover Corporation™.

In embodiments where the terminal includes an interferometer, a Fabry-Perot (FP) etalon may be used for optical communications. An FP etalon is the simplest form of FP interferometer. Its primary optical property is that if a monochromatic light ray travels back and forth between two mirrors and the back-and-forth optical distance between the mirrors equals an integral number of wavelengths (λ, 2λ, 3λ, etc.), then the light passes through the etalon. A given etalon coating layer may have a thickness of between 157 nm to 3 μm in some embodiments. An etalon is typically one of the following: (1) two very flat, very parallel mirrors (planar etalon—most are of this type); or (2) two identical spherical mirrors with their concave sides facing each other and with the distance between the mirrors equal to each mirror's radius of curvature (confocal or spherical etalon).

A multi-chroic filter set may be implemented in some embodiments with optical coatings, fiber Bragg gratings (FBGs), prisms, other wavelength-selective components, or any combination thereof. Optical coatings may be applied by e-beam evaporation, which directs an electron beam at one or more crucibles containing the desired coating materials (e.g., silicon dioxide, titanium dioxide, silicon, hafnium oxide, etc.). In certain embodiments, other coating deposition or growth techniques may be employed, such as chemical vapor deposition, plating, etc.

Optical coatings in some embodiments may include sophisticated layer structures of aggregates of sub-filter units (e.g., multi-layer etalons) and spacer layers. Spacer layers need not be of the same thickness or material composition in some embodiments. For coatings, the slope of the filter skirts or passband/stopband edge slopes and shape are a function of the total number and type(s) of the coating layers, well as the number and type(s) of spacer layers, coating film density, and other design choice and process variables. By way of nonlimiting example, a filter in the multi-chroic filter set may have a coating that includes 10 etalons and 11 spacer layers. For FBGs, one may program the desired filter characteristics into the fiber by exposing the fiber to an intense beam pattern using a short wavelength laser.

FIG. 1illustrates a satellite network100that includes both Type A and Type B terminals on multiple relays and multiple users having at least one respective terminal, according to an embodiment of the present invention. Network100includes satellites110, submarines111, ships112, computing devices113, vehicles114, and drones115as users. These users may communicate with relays120, which each have Type A and Type B terminals, per the above. Relays120may be used to transport communications from a user terminal via one or more relays120, for example. For instance, satellites110, submarines111, ships112, computing devices113, vehicles114, and drones115may be a user terminal. It should be appreciated that any desired number and types of terminals may be used by the relays in some embodiments without deviating from the scope of the invention.

FIG. 2illustrates multi-chroic filters in an optical module200, according to an embodiment of the present invention. Optical module200includes a primary mirror205and a secondary mirror210that is configured to focus light that is sent by/received from optical module200. Optical module200also includes a relay transmit filter220, a user transmit filter230, a relay receive filter240, and a user receive filter250. Filters220,230filter light transmitted by mirrors205,210and filters240,250filter light received by optical module200.

FIGS. 3A and 3Billustrate a transmit subset300and a receive subset310of a communication scheme implemented in a laser communication network with multi-chroic filters, according to an embodiment of the present invention. The transmission (Tx) path filters are shown inFIG. 3A. The receive (Rx) path filters are shown inFIG. 3Band have the opposite stopband/passband characteristics (e.g., from coatings, the wavelength-selective pattern of an FBG, or other wavelength-selective elements). InFIGS. 3A and 3B, the Type A terminal transmits to the Type B terminal on band Tx1and transmits to the user on band Tx3. The Type A terminal also receives from the Type B terminal on band Rx2and receives from the user on band Rx4.

Like the Type A terminal, the Type B terminal transmits to and receives from the user on bands Tx3and Rx4, respectively. However, the Type B terminal transmits to the Type A terminal on band Tx2and receives from the Type A terminal on band Rx1. The user does not care about bands 1 and 2 since these are for relay communications. However, the user transmits to both types of terminals on band Tx4and receives from both types of terminals on band Rx3.

In an alternative scheme, bands 3 and 2 may be swapped. Such a scheme is shown inFIGS. 4A and 4B, which illustrate a transmit subset400and a receive subset410thereof. This scheme corresponds with D2 of Table I and design #35 of Table II. These designs use the CCSDS Orange Book Center Frequency Specification, where the frequency is 191.6+n×0.1 THz and n is an integer ranging from 0 to 33. Here, Tx1is n=0 to 7, Tx2is n=8 to 16, Tx3is n=17 to 24, and Tx4is n=26 to 33. The wavelength is on the abscissa.

Referring toFIG. 2, optical module200may implement communication schemes300,310,400,410ofFIGS. 3A, 3B, 4A, and 4B, respectively. For instance, relay transmit filter220may transmit on bands Tx1or Tx3for Type A and on bands Tx2or Tx3for Type B. User transmit filter230may transmit on band Tx4for both Type A and Type B. Relay receive filter240may receive on bands Rx2or Rx4for Type A and on bands Rx1or Rx4for Type B. User receive filter250may receive on band Rx3for both Type A and Type B. The wavebands for a given subscript “i”, i.e., Txiand Rxi, are the same (i.e., i=1, 2, 3, 4).

FIG. 5illustrates a satellite network500where each satellite has more than one terminal, according to an embodiment of the present invention. In this embodiment, relay satellites510have antipodal Type A and Type B terminal pairs, i.e., terminal pairs not co-located or adjacent on a spacecraft. However, any desired number and/or types of terminals may be used without deviating from the scope of the invention.

In satellite network500, relay satellites510communicate with opposing terminals of other relay satellites510that are in view, i.e., line-of-sight. Each Type A terminal of a relay satellite510communicates with a corresponding Type B terminal of another satellite510, and vice versa. Earth-local user satellites520(e.g., low Earth orbit (LEO), medium Earth orbit (MEO), geosynchronous Earth orbit (GEO), geostationary orbit, etc.), or users on Earth, as well as deep space users530(e.g., probes, colonies or installations on other planets or bodies, rovers, etc.), can communicate with either terminal type. Advantages of including multiple terminals on relay satellites510include, but are not limited to, providing network redundancy, providing a double full dual ring, etc.

FIG. 6illustrates a satellite network600with a non-dual-ring structure, according to an embodiment of the present invention. In this embodiment, relay satellites610only have a single type A and Type B terminal (i.e., one of each). User satellites620can communicate with both terminal types, but relay satellites610communicate with one another via opposite terminals (i.e., Type A to Type B and vice versa).

Various multi-chroic filter designs are possible for Type A and Type B terminals in some embodiments. A nonlimiting example of a waveband design table is shown below in Table I. D1 is per the communication scheme ofFIGS. 3A and 3Band D2 is per the communication scheme ofFIGS. 4A and 4B.

In Table I, “P” stands for Pass (i.e., the transmitter passband is the receiver stopband), “B” stands for Block (i.e., the transmitter stopband is the receiver passband), and “G” stands for Gap (i.e., block). Also, 1 is Tx1(from a Type A terminal), 2 is Tx2(from a Type B terminal), 3 is Tx3(from either a Type A terminal or a Type B terminal to a user terminal), and 4 is Tx4(from a user terminal). Note that wavebands 1 and 4 are kept fixed in this example.

Table II below shows a nonlimiting example of an extended waveband design table.

For the Table II solutions substantially highlighted in dark gray (i.e., D2, D9, D12, and D13), the Type A and Type B labels can be swapped, and low/high wavelengths can be swapped (left-to-right). Consolidated passbands (e.g., in the D2 design) need not be used in some embodiments. Rather, filter designs for which the adjacency count is zero or one may be allowed.

The adjacency count only applies to the filters for Type A and Type B terminals, and not user terminals. If both Type A and Type B have two adjacent passbands, the adjacency count is two (i.e., both Type A and Type B). Adjacent passbands may make it easier for a multi-layer optical coating to achieve a desired stopband-to-passband rejection ratio or skirt steepness. There are 120 total feasible designs when four bands and a gap are used.