WO2000079690A2

WO2000079690A2 - Receiving multiple wavelengths at high transmission rates

Info

Publication number: WO2000079690A2
Application number: PCT/US2000/016648
Authority: WO
Inventors: Robert J. Smith
Original assignee: Ball Aerospace & Technologies Corp.
Priority date: 1999-06-23
Filing date: 2000-06-15
Publication date: 2000-12-28
Also published as: WO2000079690A3; AU7824300A

Abstract

An apparatus for receiving data through the atmosphere at rates greater than 1 gigabit/sec. is provided (Figure 1). The apparatus includes a holographic unit (reference numeral 116 in Figure 1) and a detector assembly (reference numeral 106 in Figure 1). The holographic unit receives light having different wavelengths representative of data at different frequencies. The holographic unit focuses the light on the detector assembly, preferably, a plurality of pick-off mirrors spaced from each other (reference numeral 120 in Figure 1). Each pick-off mirror collects light at a predetermined wavelength. Each pick-off mirror directs the light at its associated wavelength to a very fast, high refractive index focusing lens (reference numeral 124 in Figure 1). The detector assembly also includes a plurality of detector units (reference numeral 128 in Figure 1). Each detector unit is in contact with one of the lenses. The detector units are able to receive tightly focused light having the transmitted data for subsequent processing.

Description

RECEIVING MULTIPLE WAVELENGTHS AT HIGH TRANSMISSION RATES

FIELD OF THE INVENTION The present invention relates to laser communication receivers, and more specifically to an apparatus and method to receive high data rate signals transmitted through atmospheric distortion using a holographic unit.

BACKGROUND INFORMATION Various communication systems are known for transmitting through the atmosphere. Most commonly, microwave communication devices are used for this communication. Additionally, various optical techniques for communication are known. Microwave technologies used for data links through the atmosphere generally suffer from low data rates. For example, an 18 gigahertz (GHz) carrier is sometimes used to transmit between a satellite and a ground station. However, modulating 1 gigabit/sec. data rates on the 18 GHz carrier is not currently practically accomplished.

By increasing the carrier frequency, the data rate may also be increased. For example, a 60 GHz carrier could support modulation data rates of 1 gigabit/sec. or higher.

However, the atmosphere itself attenuates the 60 GHz carrier such that transmission through the atmosphere is not practical.

Terrestrial based optical transmission systems are known. However, these systems cannot transmit through the atmosphere at high data rates. These systems generally enable data to travel short distances between a ground-based transmitter and a ground-based receiver at very low data rates. At present, large telescopes are used to allow optical transmission of data through the atmosphere. An optical carrier is modulated with data to transmit information through the atmosphere. Adaptive optics are used to focus the light to improve beam quality. These systems are large and expensive and can generally be operated for small intervals of time (e.g., 15 minutes). Additionally, these systems do not operate acceptably in daylight where sunlight can saturate the system.

SUMMARY OF THE INVENTION In accordance with the present invention, apparatus and method for high- bandwidth data transmission through the atmosphere are disclosed. The apparatus includes a holographic unit and a detector assembly. The holographic unit receives light including one or more wavelengths that have been modulated by the high frequency data. The detector assembly is responsive to the output light from the holographic unit. The detector assembly detects the data and includes one or more focusing elements having a high refractive index that reduces a focal spot size carrying the data so that the data can be properly detected.

With regard to practicing the invention, first data having a first wavelength is transmitted through atmospheric distortion at a transmission rate of greater than 1 gigabit/second. The holographic unit provides a focal cone of light that is applied to the detector assembly. In the case of the first data having the first wavelength, a suitably positioned pick-off mirror gathers the light having the first wavelength and directs it to a focusing element. The focusing element reduces the focal spot size of the light so that it can be satisfactorily collected by a detector unit for subsequent processing.

Based on the foregoing summary, a number of salient features of the present invention are readily discerned. Transmission of data or other information through the atmosphere at high data rates exceeding one gigabit/sec. is accomplished. There is essentially no delay in detecting the received data, preferably, using a detector assembly. In that regard, the detector assembly includes a lens or other focusing element having a large refractive index for use in reducing a spot size of the light having the transmitted information. Substantial information can be detected at different carrier frequencies using different wavelengths in relatively short periods of time. The information is prepared for the detector assembly using a holographic unit. The invention properly functions even when direct sunlight is present.

Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a block diagram of a laser communication system which communicates at high data rates through the atmosphere; Fig. 2 is a schematic view of four focal cones interacting with a sorter block;

Fig. 3 is a perspective view which schematically illustrates a volume hologram; Fig.4 is a cross sectional view which schematically illustrates a parabolic volume hologram;

Fig. 5 is an enlarged cross sectional view which schematically illustrates a portion of a holographic mirror; and Fig.6 is a cross sectional view which schematically illustrates a parabolic volume hologram focusing incident laser light.

DETAILED DESCRIPTION

With reference to Fig. 1, an embodiment of a laser communication system 100 is shown in block diagram form. The laser communication system 100 includes a laser communication transmitter 104 and a laser communication receiver 102. Included in the communication receiver 102 is a holographic unit 1 16 and a detector assembly 106. The detector assembly 106 includes a linking device 120, focusing element 124 and a detector unit 128 for each modulated laser beam 1 12. In this embodiment, the transmitter 104 is located in a satellite and the receiver 102 is located in a ground station.

However, other embodiments could put the transmitter in the atmosphere as well.

Atmospheric distortion 108 along the transmission path causes wavefront distortion in modulated laser beams 112. In other words, atmospheric distortion 108 from thermal currents or atmospheric turbulence bends the tightly focused laser light of the laser beams 1 12. As can be appreciated by those skilled in the art, atmospheric distortion 108 makes it difficult to focus the modulated laser beams 112.

Data is modulated onto a laser light carrier and is demodulated later from the laser light carrier in order to transport data great distances. In the laser communication transmitter, an optical transmitter 132 modulates data on a laser light carrier to form a modulated laser beam 112. The laser beam 112 is modulated with data at a rate greater than 1 gigabit/sec. and preferably, 2 gigabit/sec. In order for the laser beam 112 to reach the laser communication receiver 102, the beam 112 travels through atmospheric distortion 108. In the laser communication receiver 102, a holographic unit 116 focuses the laser beam 112 into a focus area on a linking device 120, such as a pick-off mirror. A single holographic unit 116 focuses each laser beam 112 on its respective linking device 120 which is positionally offset from the other linking devices 120. The linking device 120 redirects the laser beam 1 12 to a focusing element 124, such as a lens. After reduction of a spot size by the focusing element 124, a detector demodulates the data from the carrier.

The laser communication transmitter 104 includes a number of optical transmitters 132. The optical transmitters 132 each transmit a modulated laser beam 112 having a different wavelength λ. In one embodiment, there are four modulated laser beams 112 having wavelengths near 1550 nanometers where each wavelength differs by approximately 4 nanometers. Each laser beam 112 has a spot size which is increased by the atmospheric distortion 108. The spot size is a cross-sectional diameter of the laser beam 1 12 at its focus. The holographic unit 116 is a primary optic for the laser communication receiver

102. The modulated laser beams 1 12 are aimed at the holographic unit 116, whereafter, the laser beams 112 are focused by the holographic unit 116. In this embodiment, the holographic unit 116 is a reflecting volume hologram which is parabolic so as to focus the laser beams 112 into a cone shape. Because each laser beam 1 12 has a different wavelength λ, the focal area (i.e., the tip of the each cone-shaped focus) is at a different distance from the holographic unit 116. After passing though the atmospheric distortion 108 and being focused by the holographic unit 116, the spot size at the tip of the cone- shaped focus of this embodiment is greater than 100 microns or approximately 200 microns in diameter. The holographic unit 116 is a passive element and does not allows reshaping of the volume hologram to improve focus. Adaptive optics which allow reshaping of the optics can reduce this spot size with resultant improved focus, but they are expensive and complex.

The linking device 120 is placed near the tip of the cone-shaped focus or focal area to redirect the laser beams 112 to their respective focusing element 124. Each laser beam 112 has a focal area which is near its respective linking device 120. In this embodiment, the linking device 120 is a pick-off mirror. The pick-off mirror is angled to redirect the laser beam 112 to the focusing element 124.

The detector unit 128 converts the modulated laser beam 112 into an electrical signal which contains the modulation data. At data rates above 1 gigabit/sec, wide band detectors are required, such as a PIN diode or the like. However, wide band detectors with a large diameter are currently unavailable. As detectors get larger their capacitance increases. If large enough, this capacitance obscures the modulated data. Accordingly, for data rates above 1 gigabit/sec, currently available PIN diodes are limited to a diameter of approximately 40 microns.

The focusing element 124 receives the laser beam 1 12 from the linking device 120 and focuses the light to reduce the spot size diameter. This focusing reduces to less than 50 microns the greater than 100 micron spot size. Preferably, the spot size is reduced to 40 microns or less. Reducing the spot size allows focusing more light on the detector unit 128 which improves data transmission efficiency. For example, if a 40 micron detector were used with a spot size of 200 microns, only 4% of power would reach the detector. To enable focusing, the speed of the laser light may be reduced in the focusing element 124 to 30% or less of the speed through free space. The focusing element 124 has an index of refraction of 2 or more. Preferably, the focusing element 124 could be made of Silicon or Germanium which respectively have indices of refraction of 3.5 and 4. Silicon is preferred for wavelengths around 1500 nm and Germanium is preferred for wavelengths around 2000 nm.

With reference to Fig. 2, a channel pick-off assembly or sorter block 200 is shown with four focal cones 204 incident thereon. Each focal cone 204 corresponds to a slightly different wavelength λ and has a focal area 208 offset from the focal areas 208 for the other wavelengths λ. The sorter block 200 includes a glass rod 212, four linking devices 120, four focusing elements 124, and four detector units 128. Each focal cone

204 corresponds to a different modulated laser beam 112 which carries different data. Although not shown in Fig.2, the holographic unit 116 produces the four focal cones 204 which are spaced apart because each results from laser light of slightly different wavelengths λ. Each focal cone 204 has at its apex a focal area 208. In this embodiment, the focal cone is 20° to provide good separation between the four modulated laser beams 112.

The linking device, such as pick-off mirror 120, is positioned at the focal area 208 to redirect the laser light to a focusing element 124. The pick-off mirrors 120 are small to reduce the interference with other focal cones 204. The focusing element 124 focuses the laser light to reduce the spot size for a detector unit 128. Preferably, the detector unit 128 is in direct contact with the focusing element 124, although, it need not be. Referring to Fig. 3, a holographic unit 116 is schematically shown. In this embodiment, the holographic unit 116 is a volume hologram which reflects predetermined wavelengths of an incident laser beam 1 12. The holographic unit 116 includes an emulsion layer 300 and a flat glass disc 304. The emulsion layer 300 reflects predetermined wavelengths of interest while the others pass through the emulsion layer

300 and out the glass disc 304.

With reference to Fig. 4, a cross section of the holographic unit 116 is schematically shown with laser light 112 incident thereon. The emulsion layer includes a number of weakly-reflecting parabolic surfaces 400. Each surface 400 reflects and focuses only a small fraction of the laser light 112 incident thereon and allows the balance of the light to continue through.

Referring to Fig. 5, a cross-sectional portion of the emulsion layer 300 is schematically shown. Seven weakly-reflecting parabolic surfaces 400 are shown, however, it is to be understood that there could be many more parabolic surfaces 400. Additionally, even though the parabolic surfaces 400 appear to be drawn as straight lines, it is to be understood that they are generally parabolic shaped. The weak reflections from each parabolic surface 400 combine to form a strong reflection 500. For example, incident light 112 passing though 15 surfaces could be 90% reflected. The surface spacing of the parabolic surfaces 400 is the half wavelength of the incident light 112. Wavelengths which differ more than 1% from the half wavelength are generally not focused by the parabolic surfaces 400 and pass through the emulsion layer 300 with 85% efficiency. Because other wavelengths pass through, sunlight is not focused by the holographic unit 116. This allows receiving information from the laser communication transceiver 104 even if it is located in-line with the sun. With reference to Fig. 6, the reflection of an incident laser beam 112 off the holographic unit 116 is schematically shown. In this embodiment, the holographic unit 116 is a volume hologram which parabolically reflects incident light. The reflected light of a desired wavelength forms a focal cone 204. At an apex of the focal cone 204 is a focal area or focus 208. If the surface spacing of the parabolic surfaces 400 deviates from the half wavelength spacing, the parabolic surfaces will focus 208 at slightly different distances from the emulsion layer 300. The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill and knowledge of the relevant art, are within the scope of the present invention. By way of example only, the invention need not be limited to a holographic mirror because a holographic lens could alternatively be used. The embodiments discussed hereinabove are further intended to explain the best mode known of practicing the inventions and to enable others skilled in the art to utilize the inventions in such, or in other embodiments and with the various modifications required by their particular application or uses of the inventions. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.

Claims

What is claimed is:

1. A method for receiving high frequency signals, comprising: transmitting first data through atmospheric distortion, said transmitting first data step being conducted at a first rate that is greater than one gigabit/second and in which said first data that is being transmitted is associated with a first wavelength; receiving said first data at a first position of a detector assembly that collects said first data based on said first wavelength; and detecting said first data using a first detector unit, while avoiding delay in receiving any part of said first data by said first detector unit.

2. A method, as claimed in Claim 1 , wherein: said detecting first data step includes detecting at the same time, using said first detector unit, all of said first data that was received by said first position of said detector assembly at a particular instance in time.

3. A method, as claimed in Claim 1, wherein: said detecting first data step includes accepting a focal spot size diameter of greater than 40 microns and then reducing said focal spot size diameter.

4. A method, as claimed in Claim 3, wherein: said reducing step includes using a focusing element having a refractive index greater than 2.

5. A method, as claimed in Claim 4, wherein: said focusing element has a traverse length along and through which said first data having said associated first wavelength passes, and speed of said first data in passing along and through said traverse length is no greater than 30% of the speed of light in air.

6. A method, as claimed in Claim 3, wherein: said focal spot size diameter is greater than about 100 microns.

7. A method, as claimed in Claim 1, wherein: said detector assembly includes a linking device, a focusing element and a detector unit.

8. A method, as claimed in Claim 1, further comprising: focusing said first data with a holographic unit.

9. A method, as claimed in Claim 1 , wherein: said detecting first data step includes reflecting said first data associated with said first wavelength by a first mirror to a focusing element and with an output of said focusing element being in communication with said first detector unit.

10. A method, as claimed in Claim 1, further including: transmitting second data through the atmospheric distortion and with said second data that is being transmitted being associated with a second wavelength, different from said first wavelength, said transmitting second data step being conducted at a second rate of greater than one gigabit/second; receiving said second data at a second position of said detector assembly; and detecting said second data using a second detector unit.

11. A method, as claimed in Claim 10, wherein: said detecting said second data step includes accepting said second data using a linking device, directing said second data to a second focusing element and with the output of said second focusing element being in communication with said second detector unit.

12. An apparatus for receiving high frequency data associated with a first wavelength, comprising: a holographic unit that receives said data; and a detector assembly responsive to said holographic unit for detecting said data, said detector assembly including a focusing element having a refractive index that reduces a focal spot size associated with said data.

13. An apparatus, as claimed in Claim 12, wherein: said refractive index is greater than 2 and said focusing element reduces said focal spot size from a focal spot size diameter of greater than 100 microns to a focal spot size diameter of less than 50 microns.

14. An apparatus, as claimed in Claim 12, wherein: said detector assembly includes a linking device and a detector unit and with said focusing element being disposed between said linking device and said detector unit, with said focusing element receiving an input from said linking device and providing an output to said detector unit.

15. An apparatus, as claimed in Claim 12, wherein: said detector assembly includes a detector unit positioned at a distance from a focal area provided by said holographic unit and being associated with said data having said first wavelength.

16. An apparatus, as claimed in Claim 12, wherein: said detector assembly includes a sorter block for accepting said data and for providing a communication path for said first data from said sorter block.

17. An apparatus, as claimed in Claim 12, wherein: said detector assembly includes a linking device that accepts said first data associated with a focal spot size greater than 50 microns.

18. An apparatus, as claimed in Claim 12, wherein: said linking device includes a pick-off mirror, with said pick-off mirror being able to properly reflect a focal spot size greater than 200 microns.

19. An apparatus, as claimed in Claim 12, wherein: said holographic unit includes at least one of the following: a volume hologram, a holographic mirror and a passive element.