CA2273691A1 - System for the determination of alignment data in optical communications connections - Google Patents
System for the determination of alignment data in optical communications connections Download PDFInfo
- Publication number
- CA2273691A1 CA2273691A1 CA 2273691 CA2273691A CA2273691A1 CA 2273691 A1 CA2273691 A1 CA 2273691A1 CA 2273691 CA2273691 CA 2273691 CA 2273691 A CA2273691 A CA 2273691A CA 2273691 A1 CA2273691 A1 CA 2273691A1
- Authority
- CA
- Canada
- Prior art keywords
- optical
- output
- receiver
- amplifier
- transmitter
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
- H04B10/118—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum specially adapted for satellite communication
Landscapes
- Physics & Mathematics (AREA)
- Astronomy & Astrophysics (AREA)
- General Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Optical Communication System (AREA)
Abstract
The system is used for determining alignment data in optical communications connections between a receiver and a transmitter remote from the former. The spontaneously emitted output of the optical pre-amplifier in the receiver is used as the output signal for aligning the optical telescopic; device of the transmitter and/or the spontaneously emitted output of the optical output amplifier at the transmitter as the input signal for the alignment of the receiving telescope at the receiver. To this end, a filter (11) is provided in order to separate data signals (8) from the respective spontaneously emitted output (9), wherein means (12) are provided for processing this spontaneously emitted output as the output signal for aligning a telescope (10).
Description
SYSTEM FOR THE DETERMINATION OF ALIGNMENT DATA
IN OPTICAL COMMUNICATIONS CONNECTIONS
zo FIELD OF THE iIVVENTION
The invention relates to a system for the determination of alignment data in zs optical communications connections between a receiver and a transmitter, which is distant from the former and which is provided with an optical output amplifier, wherein the receiver has an optical telescopic device.
BACKGROUND OF THE INVENTION
One of the main problems with optical communications connections, for example between satellites (optical intersatellite links, OISL) is the exact alignment of the transmitting and receiving telescopes (pointing, acquisition, tracking, PAT) with each other (S. Arnon, N. S. Kopeika, Proc. IEEE, 8;i, 1646 to 7667, 999. Rough alignment is performed by means of the orbital data of the satellites. In concepts published up to now, the determination of the correction data for an exact alignment (pointing), or respectively for following (tracking), is accomplished either with the aid _ ,o _ of incidental strong laser sources (beacon laser), or by tapping of a portion of the optical output which transmits the data signal. The disadvantage of the first mentioned system lies in that additional components, and therefore additional energy, mass, complexity and costs are required. Moreover, an exact alignment of the beacon s telescope parallel with the optical axis of the transmitting, or respectively receiving telescope is required. With the second method, which can only be employed on the receiving end, while otherwise the system parameters and the transmission quality remain the same, it is necessary to increase the output of the modulated data signals, so that technological limits are rapidly reached.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore the object of the invention, which is described in what follows, to create a novel system of this type which is less elaborate.
In accordance with the invention, this object is attained by means of the characteristics disclosed in claim 1. One advantage of the system in accordance with the invention is that this method is not only suitable for communications between two satellites, but also for the optical free-space transmission in the atmosphere.
Advantageous embodiments of the invention are recited in the dependent claims.
zo The invention will be explained in greater detail in what follows by means of examples shown in the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
zs Fig. 1 is a schematic representation of a ifirst embodiment of a system in accordance with the invention with a receiver and a transmitter located a distance away, Fig. 2 shows the spectral output density of the data signal in such a system, Fig. 3 is a schematic representation of different embodiments of the system with a wavelength-dependent beam splitter, Fig. 4 shows a detail of a further embodirnent of the system in accordance with the invention, Fig. 5 shows a duplex system with a single transmitter/receiver telescope in accordance with the invention, and Fig. 6 shows a duplex system in accordance with the invention with separate transmitter and receiver telescopes.
DETAILED DESCRIPTION OF THE /'REFERRED EMBODIMENTS
Fig. 1 shows a transmitter 2 provided with an optical output amplifier 1, and a receiver 4 having a pre-amplifier 3. With this embodiment, the spontaneous emission ~0 5 and/or 6 unavoidably transmitted by the optical output amplifier 1 of the transmitter 2 and/or by the optical pre-amplifier 3 of the receiver 4, i.e. the so-called amplified spontaneous emission (ASE), which is always present in addition to the data signal 7, is used for PAT purposes. Because of this it is neither necessary to provide separate energy and hardware, and therefore mass, complexity and costs, for beacon lasers, nor is it necessary to split off output from the data signal 7, which is very weak anyway, at the receiver for PAT purposes. The elaborate adjustment of the beacon telescope parallel with the transmitter, or respective receiver telescope can also be omitted. The transmitter 2 of course includes an optical telescopic device 21 and an arrangement 22 for generating the optical message signal, and the receiver zo accordingly also has an optical telescopic device 41 and an arrangement 42 for processing the optical reception signal.
Since, in contrast to terrestrial fiber-optical networks for optical communications, intermediate amplifiers naturally cannot be usE:d with optical communications by 2s means of optical free-space links, the optical transmission output alone must be sufficient for establishing the connection with a required bit error probability. One possibility for generating the transmission output required for this reason is the optical amplification of the modulated light of lasers with low output power at the transmitter.
The existence of perfected optical traveling wave amplifiers, for example erbium-3o doped fiber amplifiers, EDFAs (J. C. Livas et ~~1., Proc. SPIE, vol. 2381, 38 to 47, 1995) of a wave length of 1.55 Nm is of benefit for the present invention, as well as the pre-amplifiers used to increase the sensitivity of the receiver (J. C.
Livas et al., Proc. SPIE, vol. 2381, 38 to 47, 1995).
35 Fig. 2 shows the spectral output density S~(f) 8 of the data signal 7, as well as the spectral output density of the amplified spont2~neous emission NASE 9 of the optical output amplifier 1 at the input of the receiver ~1.. At the same time, 9 represents the _y-spectral output density of the spontaneous emission 6 radiated by the optical pre-amplifier 3, which can also be encountered at the output of the transmitter 2.
Fig. 3 shows a possible system realization containing, inter alia, an optical s telescopic device 10, a wavelength- dependent or polarization-dependent beam splitter 11 and a PAT system 12, wherein the (latter can contain as components, for example, an angle-amplifying optical system 13, a 4-quadrant detector 14 and an electronic control device 31 for telescope alignment. The separation of the data signal and the PAT signal can take place not only via the wavelength, but also via the ~o polarization. Since the wavelength is reversely proportional to the frequency, the terms "wavelength-dependent" or "polarization- dependent" are synonymous in this connection. A beam splitter is employed in both cases, which lets light through or reflects it as a function of the polarization or of the wavelength. The ASE
output is spectrally wider by orders of magnitude than the data signal, so that a separation by ~s means of the wavelength is easily possible. Furthermore, the data signal typically exists in a single polarization, while the ASE output occupies both orthogonal polarizations. This makes a separation by means of the polarization easily possible.
Fig. 4 shows a modified version of the PAT system 15 which, besides the Zo components from Fig. 3, additionally contains a waveguide bundle, for example a monomode optical fiber bundle 16, and a CCD element (charge-coupled device) 17, which is connected to an electronic control device 18 for the telescope alignment.
Besides the desired output signal, which ins amplified in respect to the input, optical travelling wave amplifiers emit background light, i.e. the mentioned amplified zs spontaneous emission (ASE), which has two properties which are important for understanding the invention:
1. The ASE output 5, 6, 9 is spectrally wider by orders of magnitude than the data signal 7 (Fig. 1 ) or 8 (Fig. 3). Per spatial mode, the spectral output density Na,se so 9 is NASE = hf(G -1 )nsp, (A) wherein hf is the energy of a photon of the considered frequency, G is the (output) 35 amplification of the optical amplifier, and nsp is the (frequency-dependent) spontaneous emission factor. With high-output EDFAs, which are now available, nsp ties between 1 and 3.2. Because of the broadband character of the ASE output 5, 6, 9 in comparison with the data signal 7, 8, it is possible to perform an almost complete spectral separation of the received data signal 7 and the ASE output 5, 6 with the aid of wavelength-dependent beam splitters 11. Because of this, the entire output of the data signal is available for data transmission, while the ASE output 9 (Fig.
IN OPTICAL COMMUNICATIONS CONNECTIONS
zo FIELD OF THE iIVVENTION
The invention relates to a system for the determination of alignment data in zs optical communications connections between a receiver and a transmitter, which is distant from the former and which is provided with an optical output amplifier, wherein the receiver has an optical telescopic device.
BACKGROUND OF THE INVENTION
One of the main problems with optical communications connections, for example between satellites (optical intersatellite links, OISL) is the exact alignment of the transmitting and receiving telescopes (pointing, acquisition, tracking, PAT) with each other (S. Arnon, N. S. Kopeika, Proc. IEEE, 8;i, 1646 to 7667, 999. Rough alignment is performed by means of the orbital data of the satellites. In concepts published up to now, the determination of the correction data for an exact alignment (pointing), or respectively for following (tracking), is accomplished either with the aid _ ,o _ of incidental strong laser sources (beacon laser), or by tapping of a portion of the optical output which transmits the data signal. The disadvantage of the first mentioned system lies in that additional components, and therefore additional energy, mass, complexity and costs are required. Moreover, an exact alignment of the beacon s telescope parallel with the optical axis of the transmitting, or respectively receiving telescope is required. With the second method, which can only be employed on the receiving end, while otherwise the system parameters and the transmission quality remain the same, it is necessary to increase the output of the modulated data signals, so that technological limits are rapidly reached.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore the object of the invention, which is described in what follows, to create a novel system of this type which is less elaborate.
In accordance with the invention, this object is attained by means of the characteristics disclosed in claim 1. One advantage of the system in accordance with the invention is that this method is not only suitable for communications between two satellites, but also for the optical free-space transmission in the atmosphere.
Advantageous embodiments of the invention are recited in the dependent claims.
zo The invention will be explained in greater detail in what follows by means of examples shown in the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
zs Fig. 1 is a schematic representation of a ifirst embodiment of a system in accordance with the invention with a receiver and a transmitter located a distance away, Fig. 2 shows the spectral output density of the data signal in such a system, Fig. 3 is a schematic representation of different embodiments of the system with a wavelength-dependent beam splitter, Fig. 4 shows a detail of a further embodirnent of the system in accordance with the invention, Fig. 5 shows a duplex system with a single transmitter/receiver telescope in accordance with the invention, and Fig. 6 shows a duplex system in accordance with the invention with separate transmitter and receiver telescopes.
DETAILED DESCRIPTION OF THE /'REFERRED EMBODIMENTS
Fig. 1 shows a transmitter 2 provided with an optical output amplifier 1, and a receiver 4 having a pre-amplifier 3. With this embodiment, the spontaneous emission ~0 5 and/or 6 unavoidably transmitted by the optical output amplifier 1 of the transmitter 2 and/or by the optical pre-amplifier 3 of the receiver 4, i.e. the so-called amplified spontaneous emission (ASE), which is always present in addition to the data signal 7, is used for PAT purposes. Because of this it is neither necessary to provide separate energy and hardware, and therefore mass, complexity and costs, for beacon lasers, nor is it necessary to split off output from the data signal 7, which is very weak anyway, at the receiver for PAT purposes. The elaborate adjustment of the beacon telescope parallel with the transmitter, or respective receiver telescope can also be omitted. The transmitter 2 of course includes an optical telescopic device 21 and an arrangement 22 for generating the optical message signal, and the receiver zo accordingly also has an optical telescopic device 41 and an arrangement 42 for processing the optical reception signal.
Since, in contrast to terrestrial fiber-optical networks for optical communications, intermediate amplifiers naturally cannot be usE:d with optical communications by 2s means of optical free-space links, the optical transmission output alone must be sufficient for establishing the connection with a required bit error probability. One possibility for generating the transmission output required for this reason is the optical amplification of the modulated light of lasers with low output power at the transmitter.
The existence of perfected optical traveling wave amplifiers, for example erbium-3o doped fiber amplifiers, EDFAs (J. C. Livas et ~~1., Proc. SPIE, vol. 2381, 38 to 47, 1995) of a wave length of 1.55 Nm is of benefit for the present invention, as well as the pre-amplifiers used to increase the sensitivity of the receiver (J. C.
Livas et al., Proc. SPIE, vol. 2381, 38 to 47, 1995).
35 Fig. 2 shows the spectral output density S~(f) 8 of the data signal 7, as well as the spectral output density of the amplified spont2~neous emission NASE 9 of the optical output amplifier 1 at the input of the receiver ~1.. At the same time, 9 represents the _y-spectral output density of the spontaneous emission 6 radiated by the optical pre-amplifier 3, which can also be encountered at the output of the transmitter 2.
Fig. 3 shows a possible system realization containing, inter alia, an optical s telescopic device 10, a wavelength- dependent or polarization-dependent beam splitter 11 and a PAT system 12, wherein the (latter can contain as components, for example, an angle-amplifying optical system 13, a 4-quadrant detector 14 and an electronic control device 31 for telescope alignment. The separation of the data signal and the PAT signal can take place not only via the wavelength, but also via the ~o polarization. Since the wavelength is reversely proportional to the frequency, the terms "wavelength-dependent" or "polarization- dependent" are synonymous in this connection. A beam splitter is employed in both cases, which lets light through or reflects it as a function of the polarization or of the wavelength. The ASE
output is spectrally wider by orders of magnitude than the data signal, so that a separation by ~s means of the wavelength is easily possible. Furthermore, the data signal typically exists in a single polarization, while the ASE output occupies both orthogonal polarizations. This makes a separation by means of the polarization easily possible.
Fig. 4 shows a modified version of the PAT system 15 which, besides the Zo components from Fig. 3, additionally contains a waveguide bundle, for example a monomode optical fiber bundle 16, and a CCD element (charge-coupled device) 17, which is connected to an electronic control device 18 for the telescope alignment.
Besides the desired output signal, which ins amplified in respect to the input, optical travelling wave amplifiers emit background light, i.e. the mentioned amplified zs spontaneous emission (ASE), which has two properties which are important for understanding the invention:
1. The ASE output 5, 6, 9 is spectrally wider by orders of magnitude than the data signal 7 (Fig. 1 ) or 8 (Fig. 3). Per spatial mode, the spectral output density Na,se so 9 is NASE = hf(G -1 )nsp, (A) wherein hf is the energy of a photon of the considered frequency, G is the (output) 35 amplification of the optical amplifier, and nsp is the (frequency-dependent) spontaneous emission factor. With high-output EDFAs, which are now available, nsp ties between 1 and 3.2. Because of the broadband character of the ASE output 5, 6, 9 in comparison with the data signal 7, 8, it is possible to perform an almost complete spectral separation of the received data signal 7 and the ASE output 5, 6 with the aid of wavelength-dependent beam splitters 11. Because of this, the entire output of the data signal is available for data transmission, while the ASE output 9 (Fig.
2), or portions thereof, can be used for the PAT system. In accordance with the invention, a s separation of the signal from the ASE output is also possible by means of the polarization properties of the two optical fields" Similar conditions apply for the ASE
output of the receiver at the associated transmitter.
2. Unless appropriate spatial filtering is performed, the ASE output can be found ~o in all spatial modes of the optical system, so that in general the equation (A) needs to be multiplied by the number of modes present. In the actual OISL, however, the number of the actually emitted modes, both of the transmitter ASE output 5 as well as the receiver ASE output 6, lies between 1 and 2, because diffraction of the optical telescopic device of the transmitter 2, as well as that of the receiver 4, is made as i5 limited as possible.
Since the ASE output occupies the same spatial modes as the data signal, the same propagation conditions apply to both. The lacking chronological coherence of the ASE output is not important for this. At the: receiver it is therefore possible in a simple way to make a first estimate of the ASE: output of the transmitter.
Analogous zo considerations apply to the estimate of the ASE output of the receiver pre-amplifier at the transmitter. If the diameters of the telescopes at the transmitter and the receiver are designated as DTX and DRX, the communications distance, which is large in respect to the telescope diameter, as R, and the wavelength of the transmitter as , the fraction of the ASE output density, which is coupled into the receiver system from zs the transmitter, can be estimated as NASE,RX ~ [DRxDTx /R~,]Z hf(G - 1 )nsp (B) Realistically assuming an erbium-doped output amplifier at the transmitter (~, _ so 1.55Nm), with G = 45dB and a noise factor of l~ = 6dB, the diameter of a transmitter telescope as well as a receiver telescope of 10 cm and a transmission distance of, for example, 6000 km, Na,se,Rx results as 9,310-2' W/Hz. If an optical bandwidth of 20 nm of the PAT system is assumed (the entire amplification bandwidth of an EDFA
is approximately 30 nm), an output for PAT purposes of 2,4~10-e W (_ -46 dBm) results, 3s which appears to be sufficient in comparison vvith other systems (R.
Cockshott, D.
Purll, Proc. SPIE, vol. 2381, 206 to 214, 1995,1.
_6._ Fig. 3 shows a possible realization of the system for the case of using the transmitter ASE output at the receiver of an OISL. The impinging optical wave is focused by means of an optical telescopic deviice 10. As the schematic representation of the spectra in Fig. 3 shows, the majority of i:he ASE output is branched off by s means of a generally known frequency-depenclent beam splitter 11, as indicated in Fig. 3, with a band-pass/band- stop characteristic, or also with low bandpass/high bandpass characteristics (N. Kashima, "Passive Optical Components for Optical Fiber Transmission", Artech House, Boston, 1995). This output can be conducted, for example, to a 4-quadrant detector 14 in accordance with the prior art via an angle-~o amplifying optical system 13, for example a telescope, which is used for increasing the angular resolution of the PAT system 12, wherein a CCD sensor, or respectively discrete photodetectors, can also be employedl. The control signals for the alignment of the optical telescopic device 10, or respectively for its tracking, are obtained from the output signals of the 4-quadrant detector (Irl/. Auer, SPIE Milestone Series, vol.
15 MS 700 (D. Begley, ed.), 275 to 280, 7994, and D. M. Southwood, Proc. SPIE, vol.
7 635, 286 to 299, 1992).
It is also remarkable that the PAT system in accordance with the invention can be employed, with small modifications, under strong background illumination. The zo background radiation of Venus, for example, is approximately 410-25 W/Hz per spatial mode, that of the sun approximately 410'2° W/Hz per spatial mode (W. R.
Leeb, Applied Optics, 3443 to 3449, 1989). Since these sources emit simultaneously in many modes, the total background radiation is greater than the ASE output and would impair the PAT system. The use of a mode filter, which limits the received radiation to z5 one mode, provides a remedy here. The ASE output received for PAT purposes is therefore only insignificantly weakened, because the ASE output only occupies 1 to 2 spatial modes anyway, as already mentioned above. The multi-mode background radiation, however, is greatly attenuated and in accordance with this realization lies in its output below the ASE output.
Fig. 4 shows a possible realization of the processing system for the ASE
output used in this case. Instead of a direct illumination of a 4-quadrant detector, a monomode waveguide bundle or optical fiber bundle 16 is used in combination with, for example, a CCD element 17. This optical fiber bundle 16, which for example can 3s be an integrated optical device or an optical finer bundle, has been inserted between the angle-amplifying optical device and the detector, and operates as a mode filter.
The multi-mode background radiation is limited to one mode by the coupling in monomode fibers, and in its output therefore lies below the ASE output of the transmitter coupled into the receiver which, as mentioned above, only occupies one or two modes anyway and, in comparison with thE: background radiation, is therefore only weakly attenuated when being coupled in. A C:CD element is represented in Fig.
4 as the detector, however, a 4-quadrant detector, a focal plane array, or discrete s photodiodes could also be employed. The mode filter need not necessarily be produced by means of optical fiber technology" since every type of optical waveguides, for example as integrated optical devices, is suitable for this.
Therefore the presence of optical travelinca wave amplifiers in the transmitter ~o and/or the receiver is characteristic for the system in accordance with the invention for determining the alignment data in an optical free-space transmission system, wherein the transmitter, or respectively receiver telescope is aligned or tracked by means of the ASE output. For example, erbium-doped fliber amplifiers, EDFA, in a wavelength range of 1.55 Nm, Nd- or respectively Pr-doped fiber amplifiers of a wavelength of 1.3 ~s Nm, or DC-NDFA at 1.06 Nm, or every other optical traveling wave amplifier, can be employed. If such an amplifier is provided, the: alignment data for the respectively oppositely located telescope can be obtained by means of the ASE output transmitted by the traveling wave amplifier.
zo The duplex system in accordance with Fic~. 5 has two terminals 51 and 56, each with an optical telescopic device 511, or respectively 561, to each of which a transmitter unit 512, or respectively 562, and a receiver unit 513, or respectively 563, are connected. The two optical message-transmitter units 512 and 562 are each provided with an optical amplifier 514, or respE:ctively 564, and/or the two optical z5 message-receiver units 513 and 563 are each provided with an optical pre-amplifier 515, or respectively 565. In accordance with the present invention, not only is the transmitter signal 52 transmitted by the optical telescopic device 511 of the terminal 51, and the transmitter signal 57 by the optical telescopic device 561 of the terminal 56, but also transmitted are an ASE output 53 of the optical transmission amplifier 514 3o and/or an ASE output 54 of the optical pre-amplifier 515 of the terminal 51, as well as an ASE output 58 of the optical transmission simplifier 564 and an ASE output 59 of the optical pre-amplifier 565 of the terminal 56.
The duplex system in accordance with Fig. 6 has two terminals 61 and 66, each 3s with a transmitter optical telescopic device 61'1, or respectively 661, and a receiver optical telescopic device 621, or respectively fi71. Respectively one transmitter unit 612, or respectively 662, is connected to the transmitter optical telescopic device 611, or respectively 661, and a receiver unit 613, or respectively 663, is connected to the -~3-receiver optical telescopic device 621, or respectively 671. The two optical message transmitter units 612 and 662 are each provided with an optical amplifier 614 and 664, and the two optical message receiver units 613 and 663 are each provided with an optical pre- amplifier 615, or respectively 665. Therefore, in accordance with the present invention, not only is the transmitter signal 62 transmitted by the optical telescopic device 611 of the terminal 61, and t:he transmitter signal 67 of the optical telescopic device 661 of the terminal 66, but also an ASE output 63 of the optical transmitter amplifier 614 of the terminal 61, and an ASE output 64 of the optical pre-amplifier 615 and/or an ASE output 68 of the transmission amplifier 664 and an ASE
~o output 69 of the pre- amplifier 665 of the terminal 66.
In Figs. 5 and 6, the arrangements 523 and 623 are intended as transmitters for an optical message signal A, the arrangements 578 and 678 as transmitters for an optical message signal B, the arrangements 504 and 604 as receivers for an optical data signal B and the arrangements 509 and Ei09 as receivers for an optical data signal A.
Since such optical amplifiers, or respectively pre- amplifiers are provided in the terminals 51, 56 (Fig. 5) and 61, 66 (Fig. 6), ii: is possible to receive the alignment zo data for the respectively oppositely located telescope by means of the ASE
output transmitted by the traveling wave amplifier. Tlhis means that, with the availability of one of the amplifiers 514 or 515, or respectively 564, or respectively 614 or 615, the alignment data for the receiving telescope 561, or respectively 661 and 671 in the opposite terminal can be obtained and, with the availability of one of the amplifiers 564 z5 or 565, or respectively 664 or 665, the alignment data for the transmitting telescope 511, or respectively 611 and 621 in the opposite terminal can be attained.
In all of these cases it is important that the ASE output of the optical traveling wave amplifier is not suppressed by technical measures, for example filtered, so that it 3o is actually transmitted by the transmitter, or respectively receiver, such as was explained above by means of Figs. 5 and 6. In Fig. 5 a system is represented in which the transmitter and receiver use the sarne telescope 511, or respectively 561.
In contrast thereto, Fig. 6 represents a system, in which the transmitter and the receiver use separate telescopes 611, 621, or respectively 661, 671. The systems 35 differ from the simplex system of FIg. 1, in which an optical data transmission takes place in only one direction. In case the ASE output is used for PAT purposes, no additional telescope is required in all these cases for transmitting, or respectively receiving, the tracking signal. This is in contrast to conventional systems with beacon _g._ lasers, in which often special telescopes are present for this purpose. If respectively only one optical amplifier is employed in the communications terminals, either a transmission amplifier or a pre-amplifier, in a duplex system (Figs. 5 and 6) both terminals can still always be aligned by means of the ASE output, since at least one s amplifier, which transmits an ASE output, is still located at each end of the transmission path. But in a simplex system only one of the communications terminals can be aligned by means of the ASE output if one amplifier fails.
Fig. 3 also shows other realization of the aystem, wherein in accordance with the ~o first variation 71 the beam splitter has a band-~pass/band-stop characteristic in such a way that a narrow frequency range around the data signal is passed, and the remaining ASE output is reflected. In the further variations 72 and 73, a beam splitter with high band-pass/low band-pass characteristics is used in such a way that, either in accordance with the one variation 72, the range of low frequencies is reflected and the remainder is passed or, in accordance with variation 73, the opposite takes place, because such beam splitters are easier to realize, but wherein the output available for PAT purposes is also less than with variation T1. The fourth variation relates to the use of a polarization-dependent beam splitter. In this case, use is made of the effect that the data signal normally only takes on one: polarization state, but the ASE output zo is evenly distributed over both possible polarization states. With a polarization-dependent beam splitter it is therefore possiblE: to separate half the ASE
output from the data signal and from the other half of the ASE output.
output of the receiver at the associated transmitter.
2. Unless appropriate spatial filtering is performed, the ASE output can be found ~o in all spatial modes of the optical system, so that in general the equation (A) needs to be multiplied by the number of modes present. In the actual OISL, however, the number of the actually emitted modes, both of the transmitter ASE output 5 as well as the receiver ASE output 6, lies between 1 and 2, because diffraction of the optical telescopic device of the transmitter 2, as well as that of the receiver 4, is made as i5 limited as possible.
Since the ASE output occupies the same spatial modes as the data signal, the same propagation conditions apply to both. The lacking chronological coherence of the ASE output is not important for this. At the: receiver it is therefore possible in a simple way to make a first estimate of the ASE: output of the transmitter.
Analogous zo considerations apply to the estimate of the ASE output of the receiver pre-amplifier at the transmitter. If the diameters of the telescopes at the transmitter and the receiver are designated as DTX and DRX, the communications distance, which is large in respect to the telescope diameter, as R, and the wavelength of the transmitter as , the fraction of the ASE output density, which is coupled into the receiver system from zs the transmitter, can be estimated as NASE,RX ~ [DRxDTx /R~,]Z hf(G - 1 )nsp (B) Realistically assuming an erbium-doped output amplifier at the transmitter (~, _ so 1.55Nm), with G = 45dB and a noise factor of l~ = 6dB, the diameter of a transmitter telescope as well as a receiver telescope of 10 cm and a transmission distance of, for example, 6000 km, Na,se,Rx results as 9,310-2' W/Hz. If an optical bandwidth of 20 nm of the PAT system is assumed (the entire amplification bandwidth of an EDFA
is approximately 30 nm), an output for PAT purposes of 2,4~10-e W (_ -46 dBm) results, 3s which appears to be sufficient in comparison vvith other systems (R.
Cockshott, D.
Purll, Proc. SPIE, vol. 2381, 206 to 214, 1995,1.
_6._ Fig. 3 shows a possible realization of the system for the case of using the transmitter ASE output at the receiver of an OISL. The impinging optical wave is focused by means of an optical telescopic deviice 10. As the schematic representation of the spectra in Fig. 3 shows, the majority of i:he ASE output is branched off by s means of a generally known frequency-depenclent beam splitter 11, as indicated in Fig. 3, with a band-pass/band- stop characteristic, or also with low bandpass/high bandpass characteristics (N. Kashima, "Passive Optical Components for Optical Fiber Transmission", Artech House, Boston, 1995). This output can be conducted, for example, to a 4-quadrant detector 14 in accordance with the prior art via an angle-~o amplifying optical system 13, for example a telescope, which is used for increasing the angular resolution of the PAT system 12, wherein a CCD sensor, or respectively discrete photodetectors, can also be employedl. The control signals for the alignment of the optical telescopic device 10, or respectively for its tracking, are obtained from the output signals of the 4-quadrant detector (Irl/. Auer, SPIE Milestone Series, vol.
15 MS 700 (D. Begley, ed.), 275 to 280, 7994, and D. M. Southwood, Proc. SPIE, vol.
7 635, 286 to 299, 1992).
It is also remarkable that the PAT system in accordance with the invention can be employed, with small modifications, under strong background illumination. The zo background radiation of Venus, for example, is approximately 410-25 W/Hz per spatial mode, that of the sun approximately 410'2° W/Hz per spatial mode (W. R.
Leeb, Applied Optics, 3443 to 3449, 1989). Since these sources emit simultaneously in many modes, the total background radiation is greater than the ASE output and would impair the PAT system. The use of a mode filter, which limits the received radiation to z5 one mode, provides a remedy here. The ASE output received for PAT purposes is therefore only insignificantly weakened, because the ASE output only occupies 1 to 2 spatial modes anyway, as already mentioned above. The multi-mode background radiation, however, is greatly attenuated and in accordance with this realization lies in its output below the ASE output.
Fig. 4 shows a possible realization of the processing system for the ASE
output used in this case. Instead of a direct illumination of a 4-quadrant detector, a monomode waveguide bundle or optical fiber bundle 16 is used in combination with, for example, a CCD element 17. This optical fiber bundle 16, which for example can 3s be an integrated optical device or an optical finer bundle, has been inserted between the angle-amplifying optical device and the detector, and operates as a mode filter.
The multi-mode background radiation is limited to one mode by the coupling in monomode fibers, and in its output therefore lies below the ASE output of the transmitter coupled into the receiver which, as mentioned above, only occupies one or two modes anyway and, in comparison with thE: background radiation, is therefore only weakly attenuated when being coupled in. A C:CD element is represented in Fig.
4 as the detector, however, a 4-quadrant detector, a focal plane array, or discrete s photodiodes could also be employed. The mode filter need not necessarily be produced by means of optical fiber technology" since every type of optical waveguides, for example as integrated optical devices, is suitable for this.
Therefore the presence of optical travelinca wave amplifiers in the transmitter ~o and/or the receiver is characteristic for the system in accordance with the invention for determining the alignment data in an optical free-space transmission system, wherein the transmitter, or respectively receiver telescope is aligned or tracked by means of the ASE output. For example, erbium-doped fliber amplifiers, EDFA, in a wavelength range of 1.55 Nm, Nd- or respectively Pr-doped fiber amplifiers of a wavelength of 1.3 ~s Nm, or DC-NDFA at 1.06 Nm, or every other optical traveling wave amplifier, can be employed. If such an amplifier is provided, the: alignment data for the respectively oppositely located telescope can be obtained by means of the ASE output transmitted by the traveling wave amplifier.
zo The duplex system in accordance with Fic~. 5 has two terminals 51 and 56, each with an optical telescopic device 511, or respectively 561, to each of which a transmitter unit 512, or respectively 562, and a receiver unit 513, or respectively 563, are connected. The two optical message-transmitter units 512 and 562 are each provided with an optical amplifier 514, or respE:ctively 564, and/or the two optical z5 message-receiver units 513 and 563 are each provided with an optical pre-amplifier 515, or respectively 565. In accordance with the present invention, not only is the transmitter signal 52 transmitted by the optical telescopic device 511 of the terminal 51, and the transmitter signal 57 by the optical telescopic device 561 of the terminal 56, but also transmitted are an ASE output 53 of the optical transmission amplifier 514 3o and/or an ASE output 54 of the optical pre-amplifier 515 of the terminal 51, as well as an ASE output 58 of the optical transmission simplifier 564 and an ASE output 59 of the optical pre-amplifier 565 of the terminal 56.
The duplex system in accordance with Fig. 6 has two terminals 61 and 66, each 3s with a transmitter optical telescopic device 61'1, or respectively 661, and a receiver optical telescopic device 621, or respectively fi71. Respectively one transmitter unit 612, or respectively 662, is connected to the transmitter optical telescopic device 611, or respectively 661, and a receiver unit 613, or respectively 663, is connected to the -~3-receiver optical telescopic device 621, or respectively 671. The two optical message transmitter units 612 and 662 are each provided with an optical amplifier 614 and 664, and the two optical message receiver units 613 and 663 are each provided with an optical pre- amplifier 615, or respectively 665. Therefore, in accordance with the present invention, not only is the transmitter signal 62 transmitted by the optical telescopic device 611 of the terminal 61, and t:he transmitter signal 67 of the optical telescopic device 661 of the terminal 66, but also an ASE output 63 of the optical transmitter amplifier 614 of the terminal 61, and an ASE output 64 of the optical pre-amplifier 615 and/or an ASE output 68 of the transmission amplifier 664 and an ASE
~o output 69 of the pre- amplifier 665 of the terminal 66.
In Figs. 5 and 6, the arrangements 523 and 623 are intended as transmitters for an optical message signal A, the arrangements 578 and 678 as transmitters for an optical message signal B, the arrangements 504 and 604 as receivers for an optical data signal B and the arrangements 509 and Ei09 as receivers for an optical data signal A.
Since such optical amplifiers, or respectively pre- amplifiers are provided in the terminals 51, 56 (Fig. 5) and 61, 66 (Fig. 6), ii: is possible to receive the alignment zo data for the respectively oppositely located telescope by means of the ASE
output transmitted by the traveling wave amplifier. Tlhis means that, with the availability of one of the amplifiers 514 or 515, or respectively 564, or respectively 614 or 615, the alignment data for the receiving telescope 561, or respectively 661 and 671 in the opposite terminal can be obtained and, with the availability of one of the amplifiers 564 z5 or 565, or respectively 664 or 665, the alignment data for the transmitting telescope 511, or respectively 611 and 621 in the opposite terminal can be attained.
In all of these cases it is important that the ASE output of the optical traveling wave amplifier is not suppressed by technical measures, for example filtered, so that it 3o is actually transmitted by the transmitter, or respectively receiver, such as was explained above by means of Figs. 5 and 6. In Fig. 5 a system is represented in which the transmitter and receiver use the sarne telescope 511, or respectively 561.
In contrast thereto, Fig. 6 represents a system, in which the transmitter and the receiver use separate telescopes 611, 621, or respectively 661, 671. The systems 35 differ from the simplex system of FIg. 1, in which an optical data transmission takes place in only one direction. In case the ASE output is used for PAT purposes, no additional telescope is required in all these cases for transmitting, or respectively receiving, the tracking signal. This is in contrast to conventional systems with beacon _g._ lasers, in which often special telescopes are present for this purpose. If respectively only one optical amplifier is employed in the communications terminals, either a transmission amplifier or a pre-amplifier, in a duplex system (Figs. 5 and 6) both terminals can still always be aligned by means of the ASE output, since at least one s amplifier, which transmits an ASE output, is still located at each end of the transmission path. But in a simplex system only one of the communications terminals can be aligned by means of the ASE output if one amplifier fails.
Fig. 3 also shows other realization of the aystem, wherein in accordance with the ~o first variation 71 the beam splitter has a band-~pass/band-stop characteristic in such a way that a narrow frequency range around the data signal is passed, and the remaining ASE output is reflected. In the further variations 72 and 73, a beam splitter with high band-pass/low band-pass characteristics is used in such a way that, either in accordance with the one variation 72, the range of low frequencies is reflected and the remainder is passed or, in accordance with variation 73, the opposite takes place, because such beam splitters are easier to realize, but wherein the output available for PAT purposes is also less than with variation T1. The fourth variation relates to the use of a polarization-dependent beam splitter. In this case, use is made of the effect that the data signal normally only takes on one: polarization state, but the ASE output zo is evenly distributed over both possible polarization states. With a polarization-dependent beam splitter it is therefore possiblE: to separate half the ASE
output from the data signal and from the other half of the ASE output.
Claims (10)
1. A system for the determination of alignment data in optical communications connections between a receiver (4) and a transmitter (2), which is distant from the former and which is provided with an optical output amplifier (1 ), wherein the receiver (4) has an optical telescopic device (41), characterized in that the spontaneously emitted output (5) of the optical output amplifier (1) on the receiver end is used as the input signal for the alignment of the optical telescopic device (41, 10) of the receiver (4).
2. A system for the determination of alignment data in optical communications connections between a receiver (4) and a transmitter (2), which is distant from the former and which is provided with an optical telescopic device (21 ), wherein the receiver (4) has an optical pre-amplifier (3), characterized in that the spontaneously emitted output (6) of the optical pre-amplifier (3) is used in the receiver (4) as the input signal for the alignment of the optical telescopic device (21) of the transmitter (2).
3. A system for the determination of alignment data in optical communications connections between a receiver (56, 66) and a transmitter (51, 61), which is distant from the former, wherein the transmitter is provided with an optical output amplifier and an optical telescopic device and/or the receiver is provided with an optical telescopic device and an optical pre-amplifier, characterized in that the spontaneously emitted output (59, 69) of the optical pre-amplifier (565, 665) is used in the receiver (56, 66) as the input signal for the alignment of the optical telescopic device (511, 611, 621) of the transmitter (51, 61), and/or the spontaneously emitted output (53, 63) of the optical output amplifier (514, 614) at the receiver end is used as the input signal for the alignment of the receiving telescope (561, 661, 671) of the receiver (56, 66).
4. The system in accordance with one of claims 1 to 3, characterized in that a filter (11) is provided for separating the data signals (8) from the respective spontaneously emitted output, and that mean (13, 14, 31, 13, 16, 17, 18) are provided for processing this spontaneously emitted output as the output signal for aligning a telescope (10).
5. The system in accordance with claim 4, characterized in that this filter (11) is a wave-dependent beam splitter or a polarization filter.
6. The system in accordance with claim 4 or 5, characterized in that a PAT installation (Pointing, Aquisition, Tracking) (12) with an angle-amplifying optical system (13) is provided in order to receive the output, which results from the spontaneously emitted output (5) of the opposite side, filtered out by means of this filter (11), and that the PAT installation (12) includes a detector (14, 17) and an electronic control device (31, 18) for processing this filtered-out output.
7. The system in accordance with claim 6, characterized in that this detector is a 4-quadrant detector (14) or a CCD detector (charge-coupled device) (17).
8. The system in accordance with claim 6, characterized in that a focal plane array or discrete photodiodes are used as detectors.
9. The system in accordance with one of claims 1 to 8, characterized in that an optical traveling wave amplifier, for example an optical semiconductor amplifier, an erbium-doped fiber amplifier, or an Nd- or Pr-doped fiber amplifier is provided in the transmitter.
10. The system in accordance with one of claims 1 to 9, characterized in that spatial filtering by means of optical monomode waveguides (6) takes place for suppressing interfering background illumination.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AT97898A AT406311B (en) | 1998-05-06 | 1998-05-06 | ALIGNMENT, ACQUISITION AND TRACKING OF TELESCOPES IN OPTICAL COMMUNICATION CONNECTIONS BY MEANS OF THE SPONTANEOUS EMISSION OF OPTICAL AMPLIFIERS |
| ATA978/98-1 | 1998-06-05 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2273691A1 true CA2273691A1 (en) | 1999-12-05 |
Family
ID=3504149
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2273691 Abandoned CA2273691A1 (en) | 1998-05-06 | 1999-06-03 | System for the determination of alignment data in optical communications connections |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0952690A3 (en) |
| JP (1) | JP2000165328A (en) |
| AT (1) | AT406311B (en) |
| CA (1) | CA2273691A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7289737B2 (en) | 2003-04-10 | 2007-10-30 | Canon Kabushiki Kaisha | Communication optical system and free-space optics communication apparatus |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1990008433A1 (en) * | 1989-01-23 | 1990-07-26 | Massachusetts Institute Of Technology | Fiber-based receiver |
| JP3425964B2 (en) * | 1992-03-19 | 2003-07-14 | 富士通株式会社 | Optical signal generator and optical transmission system using stimulated Brillouin scattering |
| FR2691310B1 (en) * | 1992-05-12 | 1995-05-12 | Alsthom Cge Alcatel | Optical link device for mobile, in particular for earth satellite. |
-
1998
- 1998-05-06 AT AT97898A patent/AT406311B/en not_active IP Right Cessation
-
1999
- 1999-04-19 EP EP99107743A patent/EP0952690A3/en not_active Ceased
- 1999-06-01 JP JP11153677A patent/JP2000165328A/en active Pending
- 1999-06-03 CA CA 2273691 patent/CA2273691A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7289737B2 (en) | 2003-04-10 | 2007-10-30 | Canon Kabushiki Kaisha | Communication optical system and free-space optics communication apparatus |
Also Published As
| Publication number | Publication date |
|---|---|
| AT406311B (en) | 2000-04-25 |
| EP0952690A2 (en) | 1999-10-27 |
| JP2000165328A (en) | 2000-06-16 |
| EP0952690A3 (en) | 2000-03-01 |
| ATA97898A (en) | 1999-08-15 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US10313011B2 (en) | Free space optical receiver and free space optical receiving method | |
| JP2871702B2 (en) | Integrated fiber optical transceiver | |
| US20020171896A1 (en) | Free-space optical communication system employing wavelength conversion | |
| US20070053696A1 (en) | Optical amplifiers in a free space laser communication system | |
| Feng et al. | Experimental demonstration of bidirectional up to 40 Gbit/s QPSK coherent free-space optical communication link over∼ 1 km | |
| KR100324797B1 (en) | Wavelength-division-multiplexed free-space optical communication systems | |
| JPH0572777B2 (en) | ||
| US6330117B1 (en) | Optical isolator module and optical amplifier using the same | |
| EP1166469B1 (en) | Atmospheric optical communication | |
| US5436751A (en) | Analog optical transmission system and optical fiber amplifier | |
| CN1819501B (en) | High-code ratio communication system between relay satellite and low-track satellite | |
| Winzer et al. | Role of amplified spontaneous emission in optical free-space communication links with optical amplification: impact on isolation and data transmission and utilization for pointing, acquisition, and | |
| CN110855365A (en) | Relay-less optical fiber transmission system and relay-less optical fiber transmission method | |
| JPH03135081A (en) | Optical amplifier | |
| CA2273691A1 (en) | System for the determination of alignment data in optical communications connections | |
| JPH02241073A (en) | Redundancy system of light source for pumping | |
| Joshi et al. | Coherent optical receiver system with balanced photodetection | |
| JPH11242130A (en) | Light source module incorporating synthesizing function, optical amplifier using this module, and bidirectional optical transmission equipment | |
| Szajowski et al. | High power optical amplifier enable 1550 nm terrestrial free-space optical data-link operating@ 10 Gb/s | |
| US7308203B1 (en) | Free space optical data link for aircraft | |
| Shay et al. | 2.5-Gbps amplified retromodulator for free-space optical communications | |
| Winzer et al. | isolation and data transmission; utilization for pointing, acquisition, and tracking | |
| JPH0728106A (en) | Optical fiber amplifier and photo-signal transmitting system | |
| JPH04114524A (en) | Optical beam communication equipment and system | |
| JPH04186933A (en) | Wavelength multiplex transmitter |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Dead |