Patent Description:
Various embodiments are described herein that relate to optical communication systems, and in particular, to an optical communication system using a photonic lantern.

Free space optical (FSO) communication links can be established between various optical communication terminals. For example, FSO links can occur between one or more satellites (i.e., inter-satellite FSO links), between satellites and ground-terminals, as well as between different ground-terminals. <CIT> relates to an orbital angular momentum (OAM) photon lantern. <CIT> relates to a fiber for space-division multiplexed optical communications and method of use. <CIT> relates to a quantum communication system and a quantum communication method.

The following introduction is provided to introduce the reader to the more detailed discussion to follow. The introduction is not intended to limit to define any claim or as yet unclaimed invention. One or more inventions may reside in any combi nation or sub-combination of elements or process steps disclosed in any part of this document including its claims and figures.

In accordance with a broad aspect of the teachings herein, there is provided an optical system comprising: an optical transmitting unit coupled to a signal transmitting path; an optical receiving unit coupled to a signal receiving path; a photonic lantern, the photonic lantern extending between a first open end and a second open end, the first open end comprising an opening to a single multi-mode fiber, and the second open end comprising a plurality of single mode fibers that are adiabatically coupled to the multimode fiber, the plurality of single-mode fibers include a single-mode fiber adapted to carry a fundamental optical mode and the remaining single-mode fibers adapted to carry higher-order optical modes, wherein, the single-mode fiber adapted to carry the fundamental mode is coupled to the signal transmitting path, the remaining single-mode fibers are coupled to the signal receiving path, and the first end is coupled to an external optical signal path.

In at least one embodiment, the optical system further comprises a collimator lens located between the external optical signal path and the first end of the photonic lantern.

In at least one embodiment, the signal receiving path comprises a first receiving path portion and a second receiving path portion, the first receiving path portion comprising a plurality of mode-specific paths that are coupled to each single-mode fiber that receives the higher-order optical modes.

In at least one embodiment, the plurality of mode-specific receiving paths comprise optical links that are passively spliced together to form the second receiving path portion, the second receiving path portion also comprising an optical link.

In at least one embodiment, the optical system is adapted to receive an optical signal through the multi-mode fiber, the optical signal being received off-axis wherein the received optical signal travels in an inward direction through the photonic lantern and exists from one or more of the single-mode fibers at the second end adapted to carry the higher-order optical modes.

In at least one embodiment, optical system further comprises an optical directional coupler located along the signal transmitting path, between the transmitting unit and the opening to the single-mode fiber at the second end adapted to carry the fundamental mode.

In at least one embodiment, a first transmission path portion extends between the transmitting unit and the optical directional coupler, and a second transmission path portion extends between the optical directional coupler and the opening of the single-mode fiber at the second end adapted to carry the fundamental mode.

In at least one embodiment, the optical system is adapted to receive optical signals through the first end of the photonic lantern, the optical signals being received on-axis, wherein the received optical signal travels in an inward direction through the photonic lantern and exits the second end of the photonic lantern through the single-mode fiber adapted to carry the fundamental mode, and the optical directional coupler is configured to route the received optical signal onto the signal receiving path.

In at least one embodiment, the optical directional coupler is an optical circulator, the optical circulator comprising: a first port coupled to the optical transmitter via the first transmission path portion; a second port coupled, via the second transmission path portion, to the opening of the single-mode fiber adapted to carry the fundamental mode at the second end of the photonic lantern; and a third port coupled to the receiving unit via the optical receiving path, and wherein optical signals received in the first port are directed to exit the second port, and optical signals received in the second port are directed to exit the third port.

In at least one embodiment, the optical directional coupler comprises a wave division multiplexer (WDM), and wherein, the WDM is configured to receive transmitted optical signals having a first range of wavelengths and travelling in an outward direction along the first transmission path portion, and the WDM is configured pass the transmitted optical signals to the second transmission path portion, and the WDM is configured to receive received optical signals having a second range of wavelengths and travelling in an inward direction along the second transmission path portion, and the WDM is configured to pass the optical signals to the signal receiving path, and the second range of wavelengths being different from the first range of wavelengths.

In accordance with another broad aspect of the teachings herein, there is provided an optical system comprising: an optical transmitting unit coupled to a signal transmitting path; an optical receiving unit coupled to a signal receiving path; a modified photonic lantern, the modified photonic lantern extending between a first open end and a second open end, the first open end comprising a single multi-mode fiber and a central-single mode fiber, the multi-mode fiber surrounding the central single-mode fiber, and the second end comprising a plurality of single mode fibers that are adiabatically coupled to the multi-mode fiber, each single-mode fiber adapted to carry a higher order optical mode, the second end further comprising the central single-mode fiber, wherein the central single-mode fiber extends between the first open end and the second open end and is adapted to carry a fundamental optical mode, wherein, the central single-mode fiber is coupled, at the second end, to the signal transmitting path, and the plurality of single-mode fibers are coupled, at the second end, to the signal receiving path, and the first end of the photonic lantern is coupled to an external optical signal path.

In at least one embodiment, the optical receiving path comprises a first receiving path portion and a second receiving path portion, the first receiving path portion comprising a plurality of mode-specific paths that are coupled to each single-mode fiber.

In at least one embodiment, the optical system is adapted to receive an optical signal through the multi-mode fiber, the optical signal being received off-axis wherein the received optical signal travels in an inward direction through the modified photonic lantern and exists from one or more of the single-mode fibers at the second end of the modified photonic lantern.

In at least one embodiment, the optical system further comprises an optical directional coupler located along the signal transmitting path, between the transmitting unit and the opening to the central single-mode fiber at the second end of the modified photonic lantern.

In at least one embodiment, a first transmission path portion extends between the transmitting unit and the optical directional coupler, and a second transmission path portion extends between the optical directional coupler and the opening of the central single-mode fiber at the second end of the photonic lantern.

In at least one embodiment, the optical system is adapted to receive optical signals through the opening of the central fiber at the first end of the modified photonic lantern, the optical signals being received on-axis, wherein the received optical signal travels in an inward direction through the modified photonic lantern and exits the opening of the central fiber at the second end of the photonic lantern, and the optical directional coupler is configured to route the received optical signal onto the signal receiving path.

In at least one embodiment, the optical directional coupler is an optical circulator, the optical circulator comprising: a first port coupled to the optical transmitter via the first transmission path portion; a second port coupled, via the second transmission path portion, to the opening of the central single-mode fiber at the second end of the photonic lantern; and a third port coupled to the receiving unit via the optical receiving path, and wherein optical signals received in the first port are directed to exit the second port, and optical signals received in the second port are directed to exit the third port.

In at least one embodiment, the optical system further comprises a fine pointing assembly interposed between the first end of the modified optical lantern and the external optical signal path.

Other features and advantages of the present application will become apparent from the following detailed description taken together with the accompanying drawings.

For a better understanding of the various embodiments described herein, and to show more clearly how these various embodiments may be carried into effect, reference will be made, by way of example, to the accompanying drawings which show at least one example embodiments, and which are now described. The drawings are not intended to limit the scope of the teachings described herein.

Further aspects and features of the example embodiments described herein will appear from the following description taken together with the accompanying drawings.

Reference is now made to <FIG>, which shows an example environment <NUM> for operating free space optical (FSO) communication systems, in accordance with the teachings provided herein.

FSO communication systems are often located within communication terminals installed on satellites <NUM>, <NUM> orbiting a ground (e.g., earth) <NUM> reference, as well as on airborne vehicles (i.e., aircrafts) and various ground-based terminals <NUM> (e.g., mobile or stationary). The communications systems are adapted to receive and/or transmit optical signals across free space mediums, including air mediums and/or vacuums (i.e., space). Each FSO system may include a transmitter to transmit outgoing optical signals and/or receivers to receive incoming optical signals. To this end, FSO communication links can be established between neighboring optical communication terminals to allow for exchange of data. For example, FSO links can be established between two or more satellites (i.e., inter-satellite FSO links 112a, <NUM>b), between satellites and ground-terminals (i.e., FSO links 114a, <NUM>b), as well as between various ground-terminals. Each FSO link can include downlinks (112a, 114a) as well as uplinks (<NUM>b, <NUM>b).

Reference is now briefly made to <FIG>, which shows a simplified block diagram of an example arrangement <NUM> for free space optical (FSO) communication, according to some embodiments.

As shown, an FSO link <NUM> may be established between at least two optical communication terminals <NUM>, <NUM>, i.e., terminals installed on satellites, aircrafts or on ground. The FSO link <NUM> can allow data to be exchanged between these terminals <NUM>, <NUM> over a free space medium <NUM>. In some cases, a communication terminal may only transmit optical signals, receive optical signals, or otherwise both transmit and receive optical signals (i.e., a transceiver terminal).

Reference is now made to <FIG>, which shows an example FSO communication system <NUM> that may be located within a communication terminal, i.e., terminals <NUM>, <NUM>. The system <NUM> is an example of a conventional design for an optical communication system which allows for both transmission and reception of optical signals.

As shown, the FSO communication system <NUM> may include an optical signal transmitting pathway <NUM>, an optical signal receiving pathway <NUM>, and an external pathway <NUM>. Interposed between the pathways <NUM>, <NUM>, <NUM> is a beam splitter <NUM>. In the illustrated example, the beam splitter <NUM> is a dichroic mirror that splits the transmitting (Tx) and receiving (Rx) pathways along a central wavelength. For example, the dichroic beam splitter can pass outgoing optical signals having a first range of wavelengths, while reflecting incoming optical signals having a second range of wavelengths. In other cases, rather than being a dichroic mirror, the beam splitter <NUM> can comprise a polarizing beam splitter which separates optical signals based on their polarization. In still other cases, aperture splitting or mode splitting methods can also be used to separate the transmitting pathway <NUM> from the receiving pathway <NUM>.

Transmitting pathway <NUM> may include a first fiber optic link or cable <NUM> for carrying transmitted optical signals <NUM>, i.e., generated by an upstream transmitter, such as a laser light source. The fiber optic cable <NUM> carries the optical signal <NUM>, and transmits the optical signal <NUM>, via an internal aperture <NUM>, towards the beam splitter <NUM> (i.e., from an open end of the optical fiber <NUM>). In this example, the transmitted optical signal <NUM> is within the first range of wavelengths that passes directly through the dichroic beam splitter <NUM>. The transmitted signal <NUM> continues onwards from the beam splitter <NUM> to the external pathway <NUM>. External pathway <NUM> includes, for example, an external optical assembly <NUM>, which comprises various mirrors, lenses etc. that magnify the outgoing signal, as well as direct the outgoing signal along a particular direction, i.e., via a coarse pointing assembly. The transmitted signal may then continue further onwards to other external communication terminals.

In the reverse case, an incoming optical signal, i.e., received from another external communication terminal, is received along the external pathway <NUM> via the external optical assembly <NUM>. From the external optical assembly <NUM>, the signal travels towards the beam splitter <NUM>. The received signal <NUM> may be within a second range of wavelengths that is reflected by the beam splitter <NUM> towards the receiving pathway <NUM>, and away from the transmission pathway <NUM>. The received signal <NUM> travels through the receiving pathway <NUM> and, via an internal aperture <NUM>, is received into a second fiber optic cable or link <NUM>. The fiber optic cable <NUM> carries the received signal towards various receiving modules (i.e., modules for signal processing and demodulation, etc.).

As illustrated, each internal aperture <NUM>, <NUM> may also include a corresponding fine pointing optical assembly <NUM>, <NUM>, as well as an actuator <NUM>, <NUM> for controlling the respective fine pointing assembly. The fine pointing assemblies <NUM>, <NUM> couple to the respective fiber optic link <NUM>, <NUM> and either, (i) receive outgoing optical signals therefrom (i.e., assembly <NUM>), or (ii) transmit incoming optical signals thereto (i.e., assembly <NUM>). The fine pointing assemblies <NUM>, <NUM> may comprises, for example, fast steering mirrors, and the actuators <NUM>, <NUM> may comprises motors that rotate the fast steering mirrors. In some cases, only one of the fine pointing assemblies <NUM>, <NUM> and corresponding actuators <NUM>, <NUM> may be provided in the system. In some embodiments, a fine point assembly <NUM> (and corresponding actuator <NUM>) may also be interposed between the beam splitter <NUM> and the external optical assembly <NUM>.

The fine pointing assemblies <NUM>, <NUM> may be adapted to provide fine beam steering of the corresponding optical signal. For example, this may involve jitter stabilization to maintain accurate directional beam steering notwithstanding vibrational forces. The fine pointing assemblies <NUM>, <NUM> may also used for point ahead or point behind offset corrections. Point ahead and behind offset corrections compensate for nonnegligible time-of-flight considerations when the FSO system <NUM> communicates with an external terminal having a high relative velocity (see e.g., satellites <NUM>, <NUM> in <FIG>). For example, during transmission of optical signals, as between the time the optical signal is transmitted by the FSO system <NUM>, and the time the optical signal is received at an external terminal - the receiving terminal may have shifted its position owing to its high relative velocity (see e.g., satellite <NUM> in <FIG> shifting positions from position "A" to "B"). Accordingly, the fine pointing assembly <NUM> corrects the outgoing direction of the outgoing signal to accommodate for this positional shift. In the reverse case, when an optical signal is received from an external terminal, the fine pointing assembly <NUM> can effect small corrective deflections to the incoming signals so as to properly route the received signal into the optical link <NUM>.

To this end, a number of disadvantages have been appreciated in the conventional design of FSO communication systems as shown by way of example in <FIG>. One significant disadvantage is that the conventional design requires separating the transmitting and receiving channels (i.e., transmitting channel <NUM> and receiving channel <NUM>). Each separate channel includes separate fiber coupling links <NUM>, <NUM>, as well as separate optical systems for each link (i.e., separate fine pointing assemblies <NUM>, <NUM>). Accordingly, to realize the conventional design, at least twice the system components (i.e., fine pointing assemblies and fiber optic links) and control systems (i.e., for controlling actuators <NUM>, <NUM>) are required to accommodate each separate channel. This, in turn, increases the mass and power consumption of the communication system. In many cases, free space optic (FSO) applications (i.e., satellites) require low mass and low power consumption for effective operation.

A further appreciated disadvantage is that the transmit and receive signals must have different wavelengths, or otherwise, different polarizations. This is to enable the beam splitter <NUM> to effectively separate between the transmit and receive channels. As such, techniques such as wavelength division multiplexing (WDM) - which allow for increased information transfer in the transmitted or received optical signals - may not operate well with the conventional system design.

In view of the foregoing, there is a desire for an optical communication system that can overcome at least some of the aforementioned disadvantages.

In accordance with at least some embodiments provided herein, there is provided an optical communication system that incorporates transmission and receiving channels (or transmission and receiving pathways) into a shared, or common fiber optic assembly. The optical communication system is realized through the use of nascent photonic lantern technology. Embodiments of the provided optical communication system may overcome at least some of the aforementioned disadvantages of conventional FSO communication systems.

Reference is now made to <FIG>, which show an example photonic lantern <NUM>. The photonic lantern <NUM> is an example of a photonic lantern that may be incorporated into an optical communication system in accordance with the teachings provided herein.

In general, photonic lanterns operate by adiabatically merging several single-mode fiber optic cores into a single multi-mode optic fiber core, or vice versa. Various optical lantern constructions and architectures will be known to those skilled in the art, and include photonic lanterns that use, for example, aperiodic single-mode fiber Bragg gratings (see also e.g., <NPL>), which provides a review of known photonic lantern designs).

As best shown in <FIG>, the conventional photonic lantern extends between a first end <NUM> and a distal opposed second end <NUM>. The first end <NUM> includes an opening into a single multi-mode optical fiber <NUM> (<FIG>), while the second end <NUM> includes a plurality of openings corresponding to a plurality of single-mode optical fibers (see e.g., 408a - <NUM> in <FIG>). A mid-portion <NUM> extends between the first end <NUM> and second end <NUM> and adiabatically merges the single multi-mode fiber <NUM> to the plurality of single-mode fibers <NUM>. More particularly, the mid-portion <NUM> maps various free space optical modes (e.g., gaussian, Hermite-gaussian and Laguerre-gaussian modes) entering the multi-mode fiber at the first end <NUM> into individualized singularized modes corresponding to each single-mode fibers <NUM>. At least one of the single-mode fibers carries a fundamental optical mode from the multi-mode core (e.g., HE<NUM> or LP<NUM> modes, as known in the art), while the remaining single-mode fibers carry other higher order optical modes. In some cases, the single-mode fiber, carrying the fundamental mode, may be located at the radial center of the second end <NUM> of the lantern (e.g., <NUM> in <FIG>).

It has been appreciated herein that photonic lanterns may have a unique and novel application in designing FSO communication systems.

Reference is now made to <FIG>, which shows an example embodiment of an FSO communication system <NUM> which incorporates a modified photonic lantern, according to at least one embodiment.

As shown, the system <NUM> includes the modified photonic lantern <NUM>, which extends between a first photonic lantern end <NUM> and a second photonic lantern end <NUM>. The modified photonic lantern <NUM> is generally analogous to the photonic lantern <NUM> of <FIG>, with the exception that the lantern <NUM> has been modified to include a central single-mode fiber <NUM> extending between the first and second ends <NUM>, <NUM>. The central single-mode fiber <NUM> carries a fundamental optical mode through the photonic lantern and is a separate optical fiber from the remaining photonic lantern.

To better clarify the modification to the photonic lantern <NUM>, <FIG> shows a view of the first end <NUM> of the modified photonic lantern <NUM>. As illustrated, the first end <NUM> may include the outer cladding <NUM>, as well as an opening for a single multimode fiber core <NUM>. The first end <NUM> also includes an opening into the central single-mode fiber <NUM> which is positioned (i.e., nested) in the radial center of the circular multimode fiber core. <FIG> shows a view of the second end <NUM> of the modified photonic lantern <NUM>. The second end <NUM> includes openings for a plurality of single-mode fibers 610a - <NUM>, which are adiabatically coupled to the multi-mode fiber <NUM> at the first end <NUM>. The second end <NUM> also includes an opening to the central single-mode fiber <NUM>. In this embodiment, at the second end <NUM>, the central single-mode fiber <NUM> may replace the single-mode fiber that ordinarily carries the fundamental mode in the conventional lantern (i.e., <NUM> in <FIG>). That is, the single-mode fiber, normally carrying the fundamental mode, may be removed to accommodate the central single-mode fiber <NUM> that is extending between the first end <NUM> and the second end <NUM>. The modified photonic lantern <NUM> therefore comprises the central single mode-fiber <NUM> surrounded by a structure corresponding to an otherwise conventional photonic lantern structure but with the fundamental single-mode fiber being removed to accommodate central fiber <NUM>. In some embodiments, the first end <NUM> may have a multi-mode core that can carry <NUM> to <NUM> different optical modes, and the second end <NUM> may include <NUM> to <NUM> single-mode fibers <NUM> that map to each mode. The operation of the modified photonic lantern <NUM> within the optical system <NUM> is clarified in greater detail herein.

Continuing with reference to <FIG>, the first end <NUM> of the photonic lantern <NUM> may be coupled to an external communication path <NUM>. The external communication path <NUM> may be analogous to the external pathway <NUM> in <FIG> and may include, for example, an external optical assembly (not shown). The external optical assembly may be similar to the assembly <NUM> and may be used to communicate with other optical communication terminals (i.e., located on other satellites).

In some cases, a fine pointing assembly 505a and a corresponding actuator 505b may be interposed between the first end <NUM> of the lantern <NUM> and the external communication path <NUM>. The fine pointing assembly 505a may receive or transmit optical signals via an internal aperture <NUM>. The actuator 505b may be controlled, for example, by a controller <NUM>.

The remainder of the optical system <NUM> is now explained in greater detail with reference to <FIG>. <FIG> illustrates an example case where optical signals are transmitted through the optical system <NUM>, while <FIG> and <FIG> illustrate example cases where optical signals are received through the optical system <NUM>. For ease of description, not all elements of the optical system <NUM> as shown in <FIG> are reproduced in each of <FIG>, however it will be appreciated that these elements may still be included in the optical system.

Reference is now made to <FIG>, which shows an example case where the optical communication system <NUM> is used for transmitting optical signals.

As shown, the optical system <NUM> includes a transmitting unit <NUM>. Transmitting unit <NUM> can convert outgoing signals from an alternate communication and/or processing format (e.g. Ethernet) into optical signals carrying data. The transmission unit <NUM> can be configured to modulate outgoing signals for transmission as an optical laser signal along a signal transmission path <NUM>. For example, in some cases, the transmission unit <NUM> may include a laser light source. In some cases, the transmission unit <NUM> may include an external or integrated optical modulator such as an electro-absorption modulator (EAM) or a Lithium Niobate Mach Zehnder external modulator for example. The optical modulator may be operable to modulate the laser light source to generate an outgoing optical laser signal which is transmitted along the signal transmission path <NUM> to the photonic lantern <NUM>. For example, the light source may be modulated such as by phase modulating the carrier optical signal (e.g., the laser beam) such that a modulated transmitted optical signal is generated which includes a sequence of multi-photon pulses with varying phase shifts, each phase shift corresponding to a unique data symbol (e.g., one more bits of information). In various cases, the amplitude of the carrier signal can also be varied, i.e., in addition to the phase, to encode a wider array of data. Examples of phase modulation schemes, and related variants, include n-PSK (phase-shift key) modulation, quadrature phase shift keying (QPSK), dualpolarization quadrature phase shift keying (DP-QPSK), offset phase shift keying (OPSK) modulation and n-QAM (quadrature amplitude modulation). In at least one embodiment, the transmitting unit <NUM> is coupled (i.e., electrically coupled) to the controller <NUM>, which can include a processor with executable instructions that can control operation of the transmitting unit <NUM> (i.e., controlling time of transmission, data to be modulated into carrier signal, etc.).

Optical signals <NUM>, generated by the transmitting unit <NUM>, may travel in an outward direction along the signal transmission path <NUM>. The signal transmission path <NUM> may extend between the transmitting unit <NUM> and the opening of the central optic fiber <NUM> at the second photonic lantern end <NUM>. In at least one embodiment, the signal transmission path <NUM> may comprise an optical fiber cable or link. As used herein, the outward direction (i.e., outward signal direction) may refer to a direction that includes a signal travel path that includes signals travelling from the second end <NUM> of the lantern to the first end <NUM> of the lantern, while an inward direction (i.e., inward signal direction) may refer to a direction which includes a signal travel path extending from the first end <NUM> to the second end <NUM> of the lantern <NUM>.

Travelling in the outward direction, the optical signal can travel through the signal transmission path <NUM> and onwards through the photonic lantern <NUM>, via the central fiber <NUM>, i.e., travelling from the second end <NUM> to the first end <NUM> of the modified photonic lantern <NUM>. To this end, in order to travel within the central fiber <NUM>, the optical signal generated by the transmitting unit <NUM>, may be of a fundamental optical mode (i.e., the transmitting unit <NUM> may generate the carrier optical signal having the fundamental mode that is adapted to travel through the central fiber <NUM>). Once the transmitted optical signal <NUM> exits the central fiber <NUM> at the first lantern end <NUM>, the optical signal <NUM> may continue along the external signal path <NUM>. In the illustrated embodiment, the first end <NUM> of the lantern <NUM> may be an open end. In at least some embodiments, the transmitted signal may exit the single-mode fiber <NUM> and may be directed by the fine pointing assembly 505a to accommodate for point ahead or point behind offsets.

In at least some embodiments, as illustrated, the signal transmission path <NUM> may be interposed by an optical directional coupler <NUM>. The optical directional coupler <NUM> segments the signal transmission path <NUM> into a first transmission path portion 522a and a second transmission path portion 522b. The first portion 522a extends between the transmitting unit <NUM> and the optical directional coupler <NUM>, while the second portion 522b extends between the optical directional coupler <NUM> and the opening of the central fiber <NUM> at the second lantern end <NUM>. The operation of the optical directional coupler <NUM> is explained in greater detail herein with reference to <FIG> and <FIG>.

Reference is now made to <FIG> and <FIG>, which show example cases where the optical communication system <NUM> is used for receiving optical signals. <FIG> illustrates an example case where optical signals are received off-axis, while <FIG> illustrates an example case where optical signals are received on-axis.

Reference is initially made to <FIG>, which shows the optical communication system <NUM> receiving off-axis optical signals.

As shown, when received optical signals 516a are subject to a point ahead or point behind offsets, they may be received off-axis. That is, the received optical signal may be received by an offset angle corresponding to the point ahead or point behind angle as between the transmitting optical terminal and the receiving optical terminal. When a signal is received off-axis, it has been appreciated that the offset may appear (i.e., manifest) as an optical mode offset in the received optical signal. In other words, the angled reception of the optical beam may "distort" the optical beam at the receiving optical terminal such that the received optical signal is now characterized by one or more higher order optical modes that exclude the fundamental optical mode. In <FIG>, the reception of off-axis optical signals is expressed by arrows that are angled away from a central axis <NUM> that runs through a radial center (or otherwise, a center point) of the first end <NUM> of the photonic lantern <NUM> (i.e., corresponding to the location of the central fiber <NUM>).

Accordingly, as the received optical signal comprises one or more higher optical modes the signal 516a may not enter the photonic lantern <NUM> through the central fiber <NUM> (i.e., corresponding to a fundamental mode) (i.e., the optical signal not carried by the central fiber <NUM>). Rather, the received optical signal is received through (or otherwise carried by) the surrounding multi-mode fiber core (<NUM> in <FIG>). In this manner, the design of the modified photonic lantern <NUM> allows the received higher-order mode off-axis optical signal 516a to be diverted away from the transmitting unit <NUM>, which is coupled to the central fiber <NUM>. The modified photonic lantern <NUM> then adiabatically couples the received multi-mode optical signal (also referred to herein as a received higher-order mode optical signal), such that as the signal travels in the inward direction, and the various constituent signal modes exit the photonic lantern <NUM> at the second end <NUM>, and via the one or more corresponding single-mode optical fibers (i.e., <NUM> in <FIG>). Here it will be understood that the each of the single-mode received optical signals, carried by each single-mode fiber <NUM>, can map to one of the modes in the received multimode optical signal.

At the second end <NUM> of the lantern <NUM>, the single-mode fibers <NUM> may each be coupled to a signal receiving path <NUM>. The signal receiving path <NUM> may comprise, for example, one or more fiber optic links or cables that couple each of the single-mode fibers <NUM> to a receiving unit <NUM> (i.e., coupled to the openings of the single-mode fibers <NUM>).

In the illustrated example, the optical receiving path <NUM> includes a first receiving path portion <NUM> and a second receiving path portion <NUM>. The first portion <NUM> may include multiple paths, i.e., 530a - 530n that connect to each respective opening of each single-mode fiber <NUM> (also referred to herein as mode-specific receiving paths <NUM>, or simply mode-specific paths). Each mode-specific path <NUM> receives a corresponding single-mode optical signal from the respective single-mode fiber <NUM>. The plurality of mode-specific paths <NUM> may then be combined into a single receiving path <NUM>. In at least one embodiment, the mode-specific paths <NUM> may be passively spliced together to combine into the single path portion <NUM> adapted to carry a single optical signal mode. For example, this may occur by way of known splicing techniques, such as via mechanical splicing or fusion splicing of optical links or cables corresponding to each of the mode-specific paths <NUM>. The received signa, that travels through the path portion <NUM>, may be referred to herein as a combined received single-mode optical signal. The combined received single-mode optical signal may then travel through the path <NUM>, and onwards toward the receiving unit <NUM>.

Receiving unit <NUM> can convert incoming the combined received single-mode optical signal into an alternate communication and/or processing format (e.g. Ethernet). The receiving unit <NUM> can be configured to demodulate incoming optical laser signal(s) received through signal reception path <NUM>. In at least one embodiment, the receiving unit <NUM> may be coupled to a controller <NUM>, which can control the operation of the receiving unit <NUM>. In some embodiments, the receiving unit <NUM> can include a heterodyne IQ (in-phase, and quadrature) demodulator photonic integrated circuit. The heterodyne IQ demodulator may include an amplified photodiode signal transducer and a local heterodyne laser source.

In some embodiments, the optical system <NUM> may include a signal processing unit <NUM>. The signal processing unit <NUM> may be interposed along the signal reception path <NUM> (i.e., along the second reception path portion <NUM>). The signal processing unit <NUM> can include one or more hardware sub-units for performing, for example, filtering, amplification, as well as for correcting for various time-varying and transmission-related errors in the received signal (i.e., to allow for proper decoding and/or demodulating of signal data).

In the illustrated example, the signal processing unit <NUM> can include a first sub-unit 536a for low noise pre-amplification and applying a tunable bandpass filter. In some embodiments, sub-units 536a may not include a tunable bandpass filter, which may be the case where the receiving unit <NUM> is a heterodyne receiver. The signal processing unit <NUM> may also include a second sub-unit 536b which performs digital signal processing (DSP) to compensate for phase shifts. In some cases, the DSP may comprise an electronic chip attached to the output of the optical receiving unit <NUM>. The signal processing unit <NUM> may be implemented using any known method known in the art. In some embodiments, one or more components of the signal processing unit <NUM> may be coupled to the controller <NUM>, such that the controller <NUM> can control operation and functioning of the unit <NUM>.

In some embodiments, phase offsets between the receiving single-mode fibers <NUM> may be compensated either actively or passively to minimize destructive interference between captured modes (see e.g., techniques as discussed in <NPL>). It also been appreciated that, in some cases, wavefront tilts in the incoming beam due to off-axis receive angles are likely to couple asymmetrically, resulting in a majority of the optical amplitude being captured by a single lantern output, thus limiting destructive interference losses.

Reference is now made to <FIG>, which shows the optical communication system <NUM>, and illustrates an example case where an on-axis signal is received.

In cases where an optical signal is not received with point ahead or point behind offsets, the optical signal 516b may be received on-axis. The on-axis signal is typically comprised of a single fundamental mode, and is therefore received back into the central fiber <NUM> via the first end <NUM> of the photonic lantern <NUM>. It will be appreciated that the positioning of the central fiber <NUM> in the radial center of the first lantern end <NUM> is to facilitate reception of on-axis signals (e.g., the axis <NUM>). The received optical signal 516b propagates in the inward direction through the central fiber <NUM> and exits the modified photonic lantern <NUM> at the second end <NUM>.

To prevent the received signal 518b from interfering with transmitting unit <NUM>, the optical directional coupler <NUM> can be positioned to re-route the received signal 518b away from the transmitting unit <NUM>, and back onto the signal recipient path <NUM> (i.e., directly, or via an intermediate signal path portion <NUM> comprising, for example, a fiber optic link). Accordingly, the optical directional coupler <NUM> can accommodate the unique case where the signal is not received off-axis. The optical directional coupler <NUM> is now explained in greater detail herein with reference to <FIG> and <FIG>.

<FIG> illustrates the optical system <NUM>, and illustrates an example embodiment of the optical directional coupler <NUM>. In this example, the optical directional coupler <NUM> comprises an optical circulator <NUM>. As shown, the optical circulator <NUM> can include a number of ports 524a, 524b, 524c: (i) port 524a may be coupled to the transmitting unit <NUM> via the first signal transmission path portion 522a, (ii) port 524b may be coupled to the opening of the central single-mode fiber <NUM> at the second end of the lantern <NUM> via the second signal transmission path portion 522b, and (iii) port 524c may be coupled to the receiving unit <NUM> via the receiving path <NUM> (i.e., port 524c may couple to the receiving path <NUM> either directly, or via the intermediate path portion <NUM> which can be passively spliced into the receiving path <NUM>).

Within the optical circulator <NUM>, first port 524a may be internally coupled to the second port 524b such that transmitted signals, i.e., from the transmitting unit <NUM>, are forwarded toward the central fiber <NUM> coupled to the second transmission path portion 522b. The second port 524b is further internally coupled to the third port 524c such that incoming on-axis received signals, i.e., from the central fiber (<FIG>), are routed to the signal receiving path <NUM>, and away from the transmitting unit <NUM>.

<FIG> illustrates another example embodiment of the optical system <NUM>. In this embodiment, the optical directional coupler <NUM> comprises a wave division multiplexer (WDM). The WDM <NUM> is used to transmit (i.e., filter) signals emitted from the transmitting unit <NUM> at a first range of wavelengths, and to pass the signals from the first transmit path portion 522a to the second transmit path portion 522b, and towards the central fiber <NUM> (i.e., signals travelling in an outward direction). The WDM <NUM> can also be used to route received on-axis signals having a second wavelength range, along the signal receiving path <NUM>, i.e., to pass these signals - travelling in the inward direction - from the second signal transmit path portion 522b towards the signal receiving path <NUM>.

In will now be understood that the optical directional coupler is adapted to, (i) couple the second signal transmitting path portion 522b to the signal receiving path <NUM> in the case of received optical signals travelling in the inward direction, and further (ii) couple the first signal transmitting path portion 522a to the second signal transmitting path portion 522b in the case of transmitted signals travelling in the outward direction. Accordingly, the optical directional coupler is so-named for its function in coupling signals travelling in two opposite directions to two different signal paths. Further, it will be understood that the use of optical circulators and WDMs are only two non-limiting examples of devices or mechanisms that can act as optical directional couplers, and that the optical system <NUM> can include any other suitable device or mechanism that can similarly act as an optical directional coupler.

In view of the above, it will be appreciated that the use of the modified photonic lantern <NUM> in the FSO system <NUM> enables incorporating both the transmission and receiving channels into the same fiber optic assembly, i.e., comprising the modified photonic lantern. Accordingly, the transmit and receive channels can share, for example, a single fine steering assembly (i.e., <NUM> in <FIG>), thereby eliminating the dual controls for separate fine steering assemblies as shown in the conventional design in <FIG>. This, in turn, can allow the optical communication terminal to have a simpler design with lower mass and power consumption.

It will be further appreciated that, owing to the large multi-mode core at the first end <NUM> of the modified photonic lantern <NUM>, the lantern <NUM> is suited for communication terminals that receive over a large array of angles (i.e., owing to point ahead or point behind offsets), thereby allowing the FSO communication system <NUM> to have a large field of view. The multi-modes received at the first end <NUM> are then converted into a plurality of single-mode inputs, and via passive splicing, and are combined into one single-mode input. As explained above, the use of the optical directional coupler in association with a single central mode fiber that carries only a fundamental mode mitigates cases where an optical signal is received on-axis (<FIG>).

Reference is now made to <FIG>, which shows an optical communication system <NUM>, in accordance with some other embodiments. The system <NUM> is generally analogous to the system <NUM>, with the exception that a conventional (i.e., non-modified) photonic lantern <NUM> is used. For ease of description, not all components of the optical communication system <NUM> are illustrated (i.e., the fine steering assembly <NUM>, receiving unit <NUM>, transmitting unit <NUM>, controller <NUM>, and signal processing module <NUM>) - however, it will be understood that these components may be incorporated into the system <NUM> in an analogous manner as shown in system <NUM>.

In the illustrated embodiment, the photonic lantern <NUM> is placed in aperture space such that optical signals from different directions are collimated (i.e., by a collimator <NUM>) prior to entering the first end <NUM> of the photonic lantern <NUM>. During reception of optical signals (<FIG>), the collimator <NUM> ensures that light from different directions fill the entire internal aperture <NUM>, located at the first end <NUM> of the lantern <NUM>. Received optical signals that are off-axis will include multiple higher-order optical modes and will exit the photonic lantern from the second end <NUM> from one or more of the single mode fibers <NUM> that carry the higher-optical modes (e.g., 408a - <NUM> in <FIG>) (also referred to herein as higher-order mode fibers, or higher-order mode single-mode fibers). Received optical signals that are on-axis will include a fundamental mode that will propagate from the first end <NUM> to the second end <NUM>, and exit the single mode fiber that carries the fundamental mode (i.e., single mode fiber <NUM> in <FIG>) (also referred to herein as a fundamental mode fiber, or fundamental mode single-mode fiber). In this example embodiment, the single mode fiber 408c, which carries the fundamental mode, is coupled to the receiving-transmission pathway portion 522b.

In the reverse case, transmitted optical signals travel through the transmission pathway <NUM>, and into the single-mode fiber (i.e., single-mode fiber <NUM>) that is coupled to the transmission path <NUM> and which carries the fundamental mode. In this embodiment, as there is no central single-mode fiber as shown in the photonic lantern <NUM>, the transmitted optical signal exits from the multi-mode fiber at first end <NUM> of the lantern <NUM>. As a result, the transmitted optical signal emits across the full aperture <NUM>, and is directed towards the collimator <NUM>.

While not shown, the optical system <NUM> may also include other optical components and systems (i.e., other lenses, or a fine pointing assembly) rearward of the collimator <NUM> (i.e., in the outward direction), including the fine pointing assembly. For example, the collimator <NUM> may be interposed between the first end <NUM> of the photonic lantern <NUM> and a fine-pointing assembly 505a with an actuator 505b (not shown in <FIG>).

It will now be appreciated that the embodiments shown in <FIG> and <FIG> are joined by the common novel concept of using a photonic lantern as a common optic fiber link to receive and transmit signals, thereby precluding the need to use separate transmit and receive optical fiber pathways as between the transmitter/receiver and the external optical signal path (as well as fine pointing assembly) The embodiment of <FIG> modifies the photonic lantern to accommodate a single-mode fiber for the fundamental mode that extends between the two lantern ends, while the embodiment of <FIG> allows the use of a conventional lantern. In each case, the use of a photonic lantern is used, and positioned between an external signal path, a receiving signal path and a transmitting signal path.

Various apparatuses or processes have been described herein to provide an example of at least one embodiment of the claimed subject matter. No embodiment described limits any claimed subject matter and any claimed subject matter may cover processes, apparatuses, devices or systems that differ from those described. The claimed subject matter is not limited to apparatuses, devices, systems or processes having all of the features of any one apparatus, device, system or process described or to features common to multiple or all of the apparatuses, devices, systems or processes described. It is possible that an apparatus, device, system or process described is not an embodiment of any claimed subject matter. Any subject matter that is disclosed in an apparatus, device, system or process described that is not claimed in this document may be the subject matter of another protective instrument, for example, a continuing patent application, and the applicants, inventors or owners do not intend to abandon, disclaim or dedicate to the public any such subject matter by its disclosure in this document.

Furthermore, it will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the example embodiments described herein. However, it will be understood by those of ordinary skill in the art that the example embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the example embodiments described herein. In addition, the description is not to be considered as limiting the scope of the example embodiments described herein.

It should also be noted that the terms "coupled" or "coupling" as used herein can have several different meanings depending in the context in which the term is used. For example, the term coupling can have a mechanical or electrical connotation. For example, as used herein, the terms "coupled" or "coupling" can indicate that two elements or devices can be directly connected to one another or connected to one another through one or more intermediate elements or devices via an electrical element, electrical signal or a mechanical element such as but not limited to, a wire or a cable, for example, depending on the particular context.

It should be noted that terms of degree such as "substantially", "about" and "approximately" as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of the modified term if this deviation would not negate the meaning of the term it modifies.

Furthermore, the recitation of any numerical ranges by endpoints herein includes all numbers and fractions subsumed within that range (e.g. <NUM> to <NUM> includes <NUM> , <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>). It is also to be understood that all numbers and fractions thereof are presumed to be modified by the term "about" which means a variation up to a certain amount of the number to which reference is being made if the end result is not significantly changed (e.g., ±<NUM>%, ±<NUM>% ±<NUM>%, etc.).

The various embodiments of the devices, systems and methods described herein may be implemented using a combination of hardware and software. These embodiments may be implemented in part using computer programs executing on programmable devices, each programmable device including at least one processor, an operating system, one or more data stores (including volatile memory or non-volatile memory or other data storage elements or a combination thereof), at least one communication interface and any other associated hardware and software that is necessary to implement the functionality of at least one of the embodiments described herein. For example, and without limitation, the computing device may be a server, a network appliance, an embedded device, a computer expansion module, a personal computer, a laptop, a personal data assistant, a cellular telephone, a smart-phone device, a tablet computer, a wireless device or any other computing device capable of being configured to carry out the methods described herein. The particular embodiment depends on the application of the computing device.

In some embodiments, the communication interface may be a network communication interface, a USB connection or another suitable connection as is known by those skilled in the art. In other embodiments, the communication interface may be a software communication interface, such as those for inter-process communication (IPC). In still other embodiments, there may be a combination of communication interfaces implemented as hardware, software, and a combination thereof.

In at least some of the embodiments described herein, program code may be applied to input data to perform at least some of the functions described herein and to generate output information. The output information may be applied to one or more output devices, for display or for further processing.

At least some of the embodiments described herein that use programs may be implemented in a high level procedural or object oriented programming and/or scripting language or both. Accordingly, the program code may be written in C, Java, SQL or any other suitable programming language and may comprise modules or classes, as is known to those skilled in object-oriented programming. However, other programs may be implemented in assembly, machine language or firmware as needed. In either case, the language may be a compiled or interpreted language.

The computer programs may be stored on a storage media (e.g. a computer readable medium such as, but not limited to, ROM, magnetic disk, optical disc) or a device that is readable by a general or special purpose computing device. The program code, when read by the computing device, configures the computing device to operate in a new, specific and predefined manner in order to perform at least one of the methods described herein.

Furthermore, some of the programs associated with the system, processes and methods of the embodiments described herein are capable of being distributed in a computer program product comprising a computer readable medium that bears computer usable instructions for one or more processors. The medium may be provided in various forms, including non-transitory forms such as, but not limited to, one or more diskettes, compact disks, tapes, chips, and magnetic and electronic storage. In alternative embodiments the medium may be transitory in nature such as, but not limited to, wireline transmissions, satellite transmissions, internet transmissions (e.g. downloads), media, digital and analog signals, and the like. The computer useable instructions may also be in various formats, including compiled and non-compiled code.

Claim 1:
An optical system comprising (<NUM>):
a) an optical transmitting unit (<NUM>) coupled to a signal transmitting path (<NUM>);
b) an optical receiving unit (<NUM>) coupled to a signal receiving path (<NUM>);
c) a photonic lantern (<NUM>), the photonic lantern extending between a first open end (<NUM>) and a second open end (<NUM>), the first open end comprising an opening to a single multi-mode fiber (<NUM>), and the second open end comprising a plurality of single mode fibers (<NUM>) that are adiabatically coupled to the multi-mode fiber, the plurality of single-mode fibers include a single-mode fiber adapted to carry a fundamental optical mode and the remaining single-mode fibers adapted to carry higher-order optical modes,
characterized in that
the single-mode fiber adapted to carry the fundamental mode is coupled to the signal transmitting path,
the remaining single-mode fibers are coupled to the signal receiving path,
the first end is coupled to an external optical signal path, and
the signal receiving path (<NUM>) comprises a first receiving path portion (<NUM>) and a second receiving path portion (<NUM>), the first receiving path portion comprising a plurality of mode-specific paths that are coupled to each single-mode fiber that receives the higher-order optical modes.