The present disclosure generally relates to communication systems; more specifically, to a receiver for omnidirectional free space optical communications.
Free Space Optical (FSO) communication is the fusion of wireless technology and optical fiber communications systems. It has the potential of providing fiber optic data rates without the physical restraints of optical fiber cables. FSO communication has been an area of interest in providing high speed data links for various applications such as satellite communications and short range point-to-point optical networks. Tracking and auto-alignment techniques are often used to establish optical links for point-to-point networks. Lately, there has been a push toward developing non-line-of-sight (NLOS) or Omnidirectional FSO (O-FSO) links for military and other applications. Currently, O-FSO technologies range from a simple lens to telescopes to NLOS solar blind ultraviolet scattering. Spatial Domain Multiplexing (SDM) methods have been used in conjunction with FSO communication to encrypt data for transmission via helical ring patterns. Some other reported technologies include modulated retroreflectors, optical concentrators, spherical structures and direct detection schemes. However, these technologies are limited to data rates of less than 1 Mb/s with practical link ranges varying between 10 and 100 meters. Therefore, there is a need for FSO links that provide better omni-directionality, higher bandwidth and longer ranges.
In accordance with the teachings disclosed herein, embodiments related to a free space optical communications receiver and related system are disclosed.
In an embodiment, a FSO receiver comprises a plurality of optical fibers and a photodetector. Each of the plurality of optical fibers have a first and a second end. The first ends of the plurality of optical fibers are splayed apart to receive FSO energy. The photodetector is communicatively coupled to the second ends of the plurality of optical fibers and is positioned to receive the FSO energy from the second ends of the plurality of optical fibers. Each of the first ends of the plurality of optical fibers may have an acceptance cone and these first ends may be splayed apart such that the acceptance cones of the plurality of optical fibers overlap to form an omnidirectional acceptance zone.
In another embodiment, an optical communication system comprises a transmitter and receiver. The transmitter is configured to transmit FSO energy and the receiver is configured to receive FSO energy. The receiver comprises a plurality of optical fibers and photodetector. The optical fibers each have a first and second end. The first ends of the plurality of optical fibers are splayed apart to receive FSO energy. The photodetector is communicatively coupled to the second ends of the plurality of optical fibers and is positioned to receive the FSO energy from the second ends of the plurality of optical fibers. Each of the first ends of the plurality of optical fibers may have an acceptance cone and these first ends may be splayed apart such that the acceptance cones of the plurality of optical fibers overlap to form an omnidirectional acceptance zone.
In a further embodiment, an optical communication system comprises a first transmitter, a second transmitter and a receiver. The first transmitter is located at a first position and is configured to transmit a first FSO energy beam. The second transmitter is located at a second position and is configured to transmit a second FSO energy beam. The receiver is configured to receive the first and second FSO energy beams. The receiver comprises first and second optical fibers and a photodetector. The first and second optical fibers each have a first and a second end. The first ends of the first and second optical fibers are splayed apart and are positioned to receive the first and second FSO energy beams, respectively. The photodetector is communicatively coupled to the second ends of the first and second optical fibers and is positioned to receive the first and second FSO energy beams from the second ends of the first and second optical fibers.
A detailed description of the embodiments for a receiver for O-FSO communications will now be presented with reference to
An embodiment of O-FSO receiver 100 is shown in
Container 110 is optional and the size and shape of container 110 is exemplary. In this embodiment, container 110 is a hollow cone-shaped holder used to support fiber bundle 105. Container 110 may also contain the photodetector. Other forms of support can also be used. For example, a dome-shaped clear plastic support connected to the optical fibers of fiber bundle 105 at or near their splayed ends could be used to hold fiber bundle in its hemispherical shape. Alternatively, a fiber bundle could be 3D printed into a hemispherical shape using known techniques.
The number of optical fibers needed in an optical fiber bundle in order to achieve omni-directionality depends on the numerical aperture of the optical fibers used. The numerical aperture determines the acceptance cone or field of view that the optical fiber will couple photons from. The size of the acceptance cone, in turn, determines the distance apart the splayed ends of the optical fiber can be. The distance between the splayed ends of the optical fibers determines how many optical fibers are needed to cover, for example, a hemispherical shape for full omni-directionality. The relationship between the numerical aperture and the number of optical fibers is illustrated in
The splayed ends of four optical fibers (205, 215, 225 and 235) belonging to a fiber bundle (such as, for example, fiber bundle 105 of
The splayed ends of five optical fibers (250, 260, 270, 280 and 290), each having a smaller numerical aperture than those shown in
As more optical fibers are added to the optical fiber subset shown in
Although each subset of optical fibers illustrated above has like numerical apertures, optical fibers of varying numerical apertures can be used; however, the separation distance of the splayed ends of optical fibers of varying numerical apertures will need to be adjusted to accommodate the different sized acceptance cones.
Once light is coupled into the splayed ends of the optical fibers of a fiber bundle, it travels through the optical fibers to the non-splayed ends. A portion of receiver 300, showing the non-splayed ends of fiber bundle 305 is shown in
As used herein, a focusing device is any device that focuses the optical energy to a narrower area. The lens shown in
Embodiments of the present invention may receive optical energy from multiple sources. An embodiment of FSO communication system 400 is shown in
Embodiments of the present invention can be used for transmission ranges up to about 5 kilometers; however, embodiments can be adapted for use over longer ranges and with fading channels. Embodiments of the present invention can work with standard FSO transmitters that utilize a laser or LED source to act as an omnidirectional source. Transmission wavelength is only dependent on the material that is used to make the device and can be designed according to the requirements of the system. Therefore, embodiments of the present invention can be integrated into preexisting FSO systems. Embodiments may also be used in new optical sensors and in optical wireless internet/networks.
Exemplary Simulation
An exemplary embodiment of a fiber bundle of an O-FSO receiver was modeled using an OSLO engine by Lambda Research Corporation. The exemplary OSLO model of the fiber bundle is shown in
where Io is scalar initial intensity, and θd is the divergence angle of the source. The divergence angle can be used to calculate the solid angle, Ω, of the transmitting source.
Ω=∫02π∫0θ
Solid angle can also be related to the beam width of the source. The solid angle of the transmitters in O-FSO equations are generally modeled as:
where A is the active area of the photodetector, D. is the solid angle of the transmitter source, R is the transceiver range, Pr is the received power at the detector, and Pt is the transmitted power. The power gain of the link, PG, can be calculated as the ratio of power received divided by power transmitted. Also, path loss, PL, is the negative of power gain (PL=−PG,) in decibel scale. The OSLO extended source LED transmits a circular distribution where the semi-height and semi-width (spot size) are defined at the
point of the Gaussian distribution. Rays traced through the exemplary receiver model were organized probabilistically within the intensity profile of the source. The result of the simulation was the fraction of received power, or the ratio of the number of rays that successfully propagated through the exemplary receiver model divided by the total number of rays transmitted. The results for the FOV of the system are provided in
The analysis of the exemplary O-FSO receive model involves two key parameters—omni-directionality and the received power as a function of range. Two sources, referred to herein as the omni-directional source and the poignant source, were utilized to test the omni-directionality of the exemplary receiver model. The omni-directional source has a divergence angle θd=90° with a semi-height and semi-width of 200 mm, whereas the poignant source has θd=22.5° with a semi-height and semi-width of 0.1 mm. The sources were swept relative to the contour of the exemplary receiver model. The results are shown in
Multiple fiber structures side-by-side would qualitatively collect spill-over from their adjacent fiber structures, as illustrated in
The source was compared to the mathematical model represented by power gain, PG, equation above to develop a baseline for the subsequent sections. This was accomplished by utilizing an independent OSLO model, different from that presented in herein. The results from the simulation are shown in
It can be noted from
As mentioned, the exemplary O-FSO receiver is simulated with the two different sources then calibrated to predict a realistic power gain for the receiver for ranges shown in
where y is the power gain of the system at one meter, m is the slope in dB/dec, d is the distance, and d0 is the reference distance (one meter). For the omni-directional source: y=−70.0 dB and m=−13.8 db/dec and for the poignant source: y=−58.7 dB and m=−16.1 db/dec. The equation is valid for the linear region where d is greater than one meter. The data from the simulations as well as the equation for both sources are plotted in
A quantum limited or amplified spontaneous emission limited receiver with on/off key modulation was used to develop a benchmark capable of predicting the feasibility of the exemplary receiver model for ranges between 100 m and one kilometer. This leads to a received power of 15.8 dB photons/bit and system performance with Bit Error Rate (BER) of 10−9. The conversion from watt to photons per bit,
is given by:
where DR is the data rate of the signal that is being sent.
Having now described the invention, the construction, the operation and use of preferred embodiments thereof, and the advantageous new and useful results obtained thereby, the new and useful constructions, and reasonable mechanical equivalents thereof obvious to those skilled in the art, are set forth in the appended claims.
This application claims the benefit of U.S. Provisional Application No. 61/985,744 entitled “Omnidirectional Free Space Optical Communications Receiver” filed Apr. 29, 2014, which is hereby incorporated by reference in its entirety.
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5165774 | Windross | Nov 1992 | A |
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Number | Date | Country | |
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20150333828 A1 | Nov 2015 | US |
Number | Date | Country | |
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61985744 | Apr 2014 | US |