CA1295059C - Optical demultiplexing - Google Patents
Optical demultiplexingInfo
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- CA1295059C CA1295059C CA000571241A CA571241A CA1295059C CA 1295059 C CA1295059 C CA 1295059C CA 000571241 A CA000571241 A CA 000571241A CA 571241 A CA571241 A CA 571241A CA 1295059 C CA1295059 C CA 1295059C
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Abstract
ABSTRACT
Optical Multiplexing An optical signal is multiplexed for use in a network including a control station (1), a high data rate optical path (2) connecting the control station (1) to a node (3,4) and low data rate optical paths (8, 20) extending from the node (3, 4) to receiving stations (5).
The method of multiplexing includes repeatedly sweeping the source (6) of the optical signal through a wide wavelength range such that a distinct wavelength channel is provided for each receiving station (5). The repetition rate is equal to the low data rate. At the node (3, 4) the signal is resolved into constituent wavelength channels occupying distinct time slots and transmitted via respective low data rate optical paths (8, 20) to the receiving stations (5).
Figure 1
Optical Multiplexing An optical signal is multiplexed for use in a network including a control station (1), a high data rate optical path (2) connecting the control station (1) to a node (3,4) and low data rate optical paths (8, 20) extending from the node (3, 4) to receiving stations (5).
The method of multiplexing includes repeatedly sweeping the source (6) of the optical signal through a wide wavelength range such that a distinct wavelength channel is provided for each receiving station (5). The repetition rate is equal to the low data rate. At the node (3, 4) the signal is resolved into constituent wavelength channels occupying distinct time slots and transmitted via respective low data rate optical paths (8, 20) to the receiving stations (5).
Figure 1
Description
BT CASE NO. A23638 ~L~ $~9 WP N0. 083~P
OPTICAL ~ULTIPLEXING
The present invention relates to a method of dividing an optical signal in~o a number of distinct channels for transmission to different receiving stations and a network employing such a system. Such a me~hod may be used for telephony or for other forms of data transmission.
A typical network to which the present invention might be applied comprises a central control station, a single ~ibre transmission line linking the control s~ation to a o node and a large number of fibre transmission lines extending from the node to individual subscriber stations. To spread the capital cost of the optical system up to and including the node as widely as possible it is desirable to spread the equipment cost between users and where necessary maximize the number of subscribers connected to a given node. In practice time division multiplexing can be used to connect as many as a thousand subscribers to one node. As a resul~ even though the data transmission rates between one subscriber and the node may be comparatively low the data transmission rate between the node and the control station needs to be very much higher if the node is not to act as a data "bottlenec~".
With conventional methods of time division multiplexing the complexity o~ the equipment required at the node and/or at each subscriber station to cope with the assembly/disasse~bly of many multiplexed channels and the conversion be~ween high and low data rates is such that it brings severe disadvantages in terms of cost and reliability. This significantly reduces and in some cases .
:
.. .. . . ... . . .. . . .. ... .. .....
`" ~295~9 eliminates altogether the advantages achieved in the first place by connecting many customers to a single node.
According to one aspect of the invention, there is provided an optical ~ncoder for an optical communications system comprising an optical source for g~nerating an optical signal corresponding to an input time-domain multiplexed signal of two or more information channels separated in the time-domain and having a tuning means arranged to vary the wavelength of the optical signal as a function of time so that those portions of the optical signal corresponding to a respective distinct time-domain multiplexed channel are generated at a predetermined wavelength.
The present invention provides a time division multiplexed optical signal in which each channel is encoded into a wavelegth multiplex using only one optical sourse for all the channels avoiding the requirement for multiple optical sources, i.e. one per channel as required, ~or example, in the ~ystem described b~ Miller in US Patent ~,~67,468. ~ further advantage of the present invention is that it does not require the channels to be interleaved bit by bit but can be employed to wavelength encode asynchronous channels of variable bit length section.
An encoder according to the first aspect of the present invention finds application in an optical communications system according to the second aspect of the present invention, namely in an optical communications system having a control station, a first optical path connecting the control station to a node and secondary optical paths connecting the node to two or more receiving stations including an optical encoder: according to the ~irst aspect of the invention; a first transmitting means for transmitting the optical signal from the controlstation via the first optical path to the node: a passive wavelength analyser at the node arranged ,.~
' ~,7 ~Z~5~5~
to direc~ respective wavelength channels via the secondary optical paths to the receiving stations.
Where the time domain multiplex signal has channels which are bit interleaved the encoder is preferably arranged to repeatedly sweep the optical frequency through a preselected wavelength range. The rate of sweep ischosen such that each channel bit is coded to a distinct wavelength, the process being repeated each set of inter:Leaved channel hits. However, the present invention in its broader o aspect does not require the wavelengths for each successive channel to be in any order. The tuning means may select wavelength in any order as will be required for the more general asynchronous time domain multiplex described above.
For the bit interleaved time domain multiplex signal using a combination o~ time division multiplexing and wavelength division multiplexing the present invention can provide a high data rate between the control station and the node through the use of a wide bandwidth wi~h many wavelength channels and a low data rate between the node and each receiving station through the use of a narrow optical bandwidth and a single wavelength channel without requiring active processing to convert ~etween different sampling rates. Since the time division multiplexing/demultiple~ing occurs at the control station and the time slots are wavelength encoded the node need only have passive wavelength resolving equipment. The use of a method in accordance with the present invention therefore allows the construction of an optical network with significant advantages in terms of cost and reliability. It can he used simply to provide each receiving station with a signal for modulation and retransmission but may readily be adapted to transmit data to the receiving stations in addition~
. .
" .i ,, ,., , - , ,:
~2~
~ 4 --Preferably the system further comprise~ ~odulatinq the received signal at one or more of the recei~ing stations at the low data rate and returning the modulated signal via the node through the high data rate optical path to the control station. Preferably one or more of the channels is pulse code modulated at the cont:rol station with non-zero logic levels and at the respective subscriber station the received optical po~er is split, part of the power being directed to a demodulator and o part of the power being modulated and retransmitted by the receiving station. ~lternatively the method may further comprise transmitting a signal from one or more of the receiving stations to the node to modulate the respective one or more wavelength chann01s at the node for reSransmission from the node to the control s~ation.
B~ modulating the signal to be retransmitted at the j node rather than at the receiving station the present invention not only reduces the complexity and cost of the equipment required by each receiving station but also eliminates the difficulty of retainin~ different channels in their correct time slots when different receiving stations are at different distances from the node. This problem does not arise when the channels are modulated and reflected at the node at a single fixed distance from the controlling station. The present invention offers further advantages in terms of data security since each receiving station has access only to the data intended for it.
According to a further aspect of the present invention a ~ethod of optically encoding a received time domain multiplexed signal of two or more in~ormation channels comprising generating an optical signal corres~onding to the received signal by means of a wavelength tunable optical source while varying the ~aveleng~h of the optical signal so that those portions of the optical signal corresponding to rPspective distinct channels are generated at distinct wavelengths.
According to a yet further aspect of the present invention a method of communication comprises optically encoding a time domain multiplexed signal of two or more information channels as an optical signal at a control station;
o transmitting the optical signal to a node, resolving the optical signal into its constituent wavelength portions at the node, transmitting the different wavelength portions to respective receiving stations.
lS Preferably the node ~urther includes a spatial light modulator and a mirror arranged in the passi~e wavelength analyser to modulate and reflect bac~ to the control station one or more wavelength channels in response to signals from the respective one or more receiving stations. Preferably the source of light is a semiconductor laser and the means to sweep the light periodically through a wide wavelength range includes a ~una~le external optical cavity.
A method and a system in accordance with the present invention are now descrlbed in detail with reference to the accompanying drawings, in which:-Figure 1 is a diagram illustrating a method and system in accordance ~ith the present invention;
Figure 2 is a diagram illustrating modulation/demodulation methods for use in a method or system in accor~ance with the present invention; andr Figure 3 is a diagram illustrating modulatorstanalysers for use in a method or sys~em in accordance with the present invention.
f~
~Z95~5~
In the system shown in Figure l; a controlling station 1 is connected via a ~ide bandwidth opkical ~ibre 2 to a node 3~ The node 3 is in turn connested to a series of further nodes 4 from vhich fibres carrying narrow optical bandwidths 20 extend to receiving stations 5"
The control station 1 includes a semiconductor laser 6 which provides the optical po~er for the system. The semiconductor laser 6 is modulated to provide time-division multiple~ed channels in a conv~tional ~
fashion. In addition an external optical cavity is used to sweep the output of the laser 6 through a range of frequencies such that each successive time~division channel occupies a distinct wavelength channel (~ 2 ~ ~n). The frequency sweep is repeat~d at a rate equal to the data rate desired for transmission along the low bit rate fibres, 64 KHz in the case of the system illustratedO Although handling only a narrow optical bandwidth the low bit rate fibres have a sufficiently broad elec-~ical bandwidth to trans~mit narrow pulses without significant spreading. A delay may be included before each successive sweep to allow for differential delays within the system as a result of dispersion : a 15 nsec delay is sufficient for a total signal path of 7 km. It is Eound to be possible to resolve channels with a wavelength spacin~ of 0,1 nanometers: sweeping the laser through a wavelength range of 100 nanometers can therefore provide more than 1000 distinct wavelength channels. Thus although the system uses a relatively low sampling rate of 64 KHz the output from the control station 1 which is joined to the fibre 2 by an optical coupler 7 can have a data rate as high as ~5.5 Mbit/s (1,024 x 64 kbit/s) in the case of a 1,024 channel system.
Since each channel in the data stream leaving the control station 1 occupies a distinct wavelength channel the data stream may be demultiplexed using a simple passive wavelength analyser and without the use of active electronic devices. Figure 1 shows a two stage demultiplexing system. The high data rate ~broad bandwidth) data stream on the fibre 2 is taken first to the node 3 which analyses the wide-bandwidth signal into eight reduced bandwidth groups each containing 128 of the original channels. Each of the eight groups is output from the node 3 to one of eight reduced bandwidth fibres 8. Each of the reduced bandwidth fibres 8 leads to a further node 4 (all but two of the reduced bandwidth fibres 8 and the nodes 4 are omitted from Figure 1 for clarity). Each of the nodes 4 contains an analyser which splits the reduced bandwidth group into its constituent wavelength channels and outputs each channel along a separate narrow bandwidth (low data rate) fibre 20 to one of 1,024 receiving stations 5 (all but one of the receiving stations 5 are omitted for clarity).
Each of the receiving stations is arranged both to detect a signal modulated on its respective channel using a receiver operating at the low data rate and to impose further modulation on the respective channel for transmission bac~ to the control station. The modulation/demodulation scheme used is illustrated diagrammatically i~ Figure ~. ~ signal is imposed upon one or more of the channels at the control station using binary pulse code modulation techniques. Since the receiving stations do not themselves contain any source of optical power it is necessary to maximize the power transmitted to them. To this end ~he pulse code modulation uses two non-zero logic levels rather than conventional "on-off" coding to avoid the loss of power which would otherwise occur as a result of the modulation to the zero level. Each receiving station 5 includes an additional fibre coupler 9 which splits the light in two, part of the light being used for detection of the modulated signal using a threshold detector 10 the remaining optical power being used for modulation by a modulator 12, reflection by a mirror 13 and retransmission to the control station 1. At the control stati~n 1 the optical signal is received by a detector 11 and the signal decoded by a receiver having a : da'a rate of at least 65.5 Mbit/s. This use of pulse code modulation followed by splitting at the receiving station 5 results in a 6 dB power loss in the path from the .. . .. . ......... . . ........ .... .... .... ...
s~
_ 9 control station 1 to the receiving station detector 10: 3 dB due to the use of the half power level for the "off"
state plus a further 3 dB loss at the coupler. In a signal reflected ~o the control station from the receiving station there are two passes through the extra coupler 9 and hence a 9 dB loss overall assuming a 50:50 coupler ratio. This additional loss of 9 d~ may be reduced by employing a coupler with an unequal coupling ratio since less attenuation is incurrecl in the path from the receiving station 5 to the control station 1 and the receiving station has in any case higher sensitivity.
Other elements of the system also contribute to power losses: tables 1 and 2 show overall power budgets for transmission from the control station 1 to the receiving station 5 and transmission from the control station 1 to the receiving station 5 and back to the control station 1 respectively.
DOWNSTREAM POWER BUDGET
-Launch Power 0 dB
Sensitivity at 64 ~bit/sec -70 dB
Available system loss 70 dB
coupler (50:50) 3 dB
25 Fibre (3.5 km) 2.5 dB
1st stage MUX 6 dB
2nd stage MUX 6 `\dB
Coupler ~90:10~ 10 dB
Penalty for non-zero off-state 3 dB
Signal reduction through 64KHz filter 30 dB
System loss 60.5 dB 60.5 dB
Sys~em margin . g.5 dB
, ~35g~
UPSTREAM POWE~ ~UDGET (INDIVIDUAL MODULATORS PER
, ~ ~
CUSTOM~R) Launch power 0 dB
5 Sensitivity at 65.5 Mbit/sec ~49 dB
Available system loss 49 dB
Coupler (50:50) 3 dB
Fibre 2.5 dB
1st stage MVX 6 dB
10 2nd stage MUX 6 dB
Coupler l90:10) 0.5 dB
Modulator/Reflector 2 dB
2nd stage MUX 6 dB
1st stage MUX 6 dB
15 Fibre 2.5 dB
Coupler (50:50) 3 dB
Penalty for non-zero off-state 3 dB ¦~
System loss 40.5 dB 40.5 dB ' System margin 8.5 dB
In an alternative embodiment of the present invention modulation of the light for retransmission to the control station 1 takes place not at the individual receiving stations 5 but at the nodes 4. In this case the wavelength analysers in the nodes 4 include optical modulators as shown in Figure 3. In common with the simple analysers discussed above the analyser/modulators include a reflection diffraction grating 14 and a concave mirror 15 arranged to direct each distinct wavelength to a unique point in space for coupling to a particular fibre. However in addition the analyser/modulator of Figure 3 includes a spatial light modulator (SLM) 18.
The SLM 18 may be transmissive (eg. a magneto-optic cell or liquid crystal structure) in which case a mirror (not shown) is positioned adjacent its rear surface to reflect ~4~
the incident light. Alternatively the SLM 18 may be inherently reflective (e.g. a silicon micro-machined surface~. A beam splitter 19 directs part of the light to fibres connected to the receiving stations and th~
remainder to the SLM 18. Incoming rays from the control station 1 are analysed and directed to the appropriate receiving stations as normal. However instead of the receiving stations being arranged to modulate and retransmit a portion of the light received each receiving station is connected electrically to the point on the array of the SLM 18 corresponding to its wavelength channel and as a result controls the modulation of that portion of the light which is reflected back to the control station 1 from the SLM 18.
15 The network can be upgraded ea~ily to continuous wave wideband operation in the same customer wavel~ngth channels by coupling in additional multiplexed laser signals.
Where the present invention is to be used to optically encode an asynchronous time domain multiplexed signal the tuning of the optical source 6 is controlled to vary with the timing of the various channels which may be irregular both in the periods of time the optical source is set to a given wavelength and in the order the wavelengths are set which order will depend on the order of the channels in the received time domain multiplex.
The secondary optical paths may be replaced by electrical connections between the node and the receiving stations.
: 35 ~ ," . .. ..
- . .
OPTICAL ~ULTIPLEXING
The present invention relates to a method of dividing an optical signal in~o a number of distinct channels for transmission to different receiving stations and a network employing such a system. Such a me~hod may be used for telephony or for other forms of data transmission.
A typical network to which the present invention might be applied comprises a central control station, a single ~ibre transmission line linking the control s~ation to a o node and a large number of fibre transmission lines extending from the node to individual subscriber stations. To spread the capital cost of the optical system up to and including the node as widely as possible it is desirable to spread the equipment cost between users and where necessary maximize the number of subscribers connected to a given node. In practice time division multiplexing can be used to connect as many as a thousand subscribers to one node. As a resul~ even though the data transmission rates between one subscriber and the node may be comparatively low the data transmission rate between the node and the control station needs to be very much higher if the node is not to act as a data "bottlenec~".
With conventional methods of time division multiplexing the complexity o~ the equipment required at the node and/or at each subscriber station to cope with the assembly/disasse~bly of many multiplexed channels and the conversion be~ween high and low data rates is such that it brings severe disadvantages in terms of cost and reliability. This significantly reduces and in some cases .
:
.. .. . . ... . . .. . . .. ... .. .....
`" ~295~9 eliminates altogether the advantages achieved in the first place by connecting many customers to a single node.
According to one aspect of the invention, there is provided an optical ~ncoder for an optical communications system comprising an optical source for g~nerating an optical signal corresponding to an input time-domain multiplexed signal of two or more information channels separated in the time-domain and having a tuning means arranged to vary the wavelength of the optical signal as a function of time so that those portions of the optical signal corresponding to a respective distinct time-domain multiplexed channel are generated at a predetermined wavelength.
The present invention provides a time division multiplexed optical signal in which each channel is encoded into a wavelegth multiplex using only one optical sourse for all the channels avoiding the requirement for multiple optical sources, i.e. one per channel as required, ~or example, in the ~ystem described b~ Miller in US Patent ~,~67,468. ~ further advantage of the present invention is that it does not require the channels to be interleaved bit by bit but can be employed to wavelength encode asynchronous channels of variable bit length section.
An encoder according to the first aspect of the present invention finds application in an optical communications system according to the second aspect of the present invention, namely in an optical communications system having a control station, a first optical path connecting the control station to a node and secondary optical paths connecting the node to two or more receiving stations including an optical encoder: according to the ~irst aspect of the invention; a first transmitting means for transmitting the optical signal from the controlstation via the first optical path to the node: a passive wavelength analyser at the node arranged ,.~
' ~,7 ~Z~5~5~
to direc~ respective wavelength channels via the secondary optical paths to the receiving stations.
Where the time domain multiplex signal has channels which are bit interleaved the encoder is preferably arranged to repeatedly sweep the optical frequency through a preselected wavelength range. The rate of sweep ischosen such that each channel bit is coded to a distinct wavelength, the process being repeated each set of inter:Leaved channel hits. However, the present invention in its broader o aspect does not require the wavelengths for each successive channel to be in any order. The tuning means may select wavelength in any order as will be required for the more general asynchronous time domain multiplex described above.
For the bit interleaved time domain multiplex signal using a combination o~ time division multiplexing and wavelength division multiplexing the present invention can provide a high data rate between the control station and the node through the use of a wide bandwidth wi~h many wavelength channels and a low data rate between the node and each receiving station through the use of a narrow optical bandwidth and a single wavelength channel without requiring active processing to convert ~etween different sampling rates. Since the time division multiplexing/demultiple~ing occurs at the control station and the time slots are wavelength encoded the node need only have passive wavelength resolving equipment. The use of a method in accordance with the present invention therefore allows the construction of an optical network with significant advantages in terms of cost and reliability. It can he used simply to provide each receiving station with a signal for modulation and retransmission but may readily be adapted to transmit data to the receiving stations in addition~
. .
" .i ,, ,., , - , ,:
~2~
~ 4 --Preferably the system further comprise~ ~odulatinq the received signal at one or more of the recei~ing stations at the low data rate and returning the modulated signal via the node through the high data rate optical path to the control station. Preferably one or more of the channels is pulse code modulated at the cont:rol station with non-zero logic levels and at the respective subscriber station the received optical po~er is split, part of the power being directed to a demodulator and o part of the power being modulated and retransmitted by the receiving station. ~lternatively the method may further comprise transmitting a signal from one or more of the receiving stations to the node to modulate the respective one or more wavelength chann01s at the node for reSransmission from the node to the control s~ation.
B~ modulating the signal to be retransmitted at the j node rather than at the receiving station the present invention not only reduces the complexity and cost of the equipment required by each receiving station but also eliminates the difficulty of retainin~ different channels in their correct time slots when different receiving stations are at different distances from the node. This problem does not arise when the channels are modulated and reflected at the node at a single fixed distance from the controlling station. The present invention offers further advantages in terms of data security since each receiving station has access only to the data intended for it.
According to a further aspect of the present invention a ~ethod of optically encoding a received time domain multiplexed signal of two or more in~ormation channels comprising generating an optical signal corres~onding to the received signal by means of a wavelength tunable optical source while varying the ~aveleng~h of the optical signal so that those portions of the optical signal corresponding to rPspective distinct channels are generated at distinct wavelengths.
According to a yet further aspect of the present invention a method of communication comprises optically encoding a time domain multiplexed signal of two or more information channels as an optical signal at a control station;
o transmitting the optical signal to a node, resolving the optical signal into its constituent wavelength portions at the node, transmitting the different wavelength portions to respective receiving stations.
lS Preferably the node ~urther includes a spatial light modulator and a mirror arranged in the passi~e wavelength analyser to modulate and reflect bac~ to the control station one or more wavelength channels in response to signals from the respective one or more receiving stations. Preferably the source of light is a semiconductor laser and the means to sweep the light periodically through a wide wavelength range includes a ~una~le external optical cavity.
A method and a system in accordance with the present invention are now descrlbed in detail with reference to the accompanying drawings, in which:-Figure 1 is a diagram illustrating a method and system in accordance ~ith the present invention;
Figure 2 is a diagram illustrating modulation/demodulation methods for use in a method or system in accor~ance with the present invention; andr Figure 3 is a diagram illustrating modulatorstanalysers for use in a method or sys~em in accordance with the present invention.
f~
~Z95~5~
In the system shown in Figure l; a controlling station 1 is connected via a ~ide bandwidth opkical ~ibre 2 to a node 3~ The node 3 is in turn connested to a series of further nodes 4 from vhich fibres carrying narrow optical bandwidths 20 extend to receiving stations 5"
The control station 1 includes a semiconductor laser 6 which provides the optical po~er for the system. The semiconductor laser 6 is modulated to provide time-division multiple~ed channels in a conv~tional ~
fashion. In addition an external optical cavity is used to sweep the output of the laser 6 through a range of frequencies such that each successive time~division channel occupies a distinct wavelength channel (~ 2 ~ ~n). The frequency sweep is repeat~d at a rate equal to the data rate desired for transmission along the low bit rate fibres, 64 KHz in the case of the system illustratedO Although handling only a narrow optical bandwidth the low bit rate fibres have a sufficiently broad elec-~ical bandwidth to trans~mit narrow pulses without significant spreading. A delay may be included before each successive sweep to allow for differential delays within the system as a result of dispersion : a 15 nsec delay is sufficient for a total signal path of 7 km. It is Eound to be possible to resolve channels with a wavelength spacin~ of 0,1 nanometers: sweeping the laser through a wavelength range of 100 nanometers can therefore provide more than 1000 distinct wavelength channels. Thus although the system uses a relatively low sampling rate of 64 KHz the output from the control station 1 which is joined to the fibre 2 by an optical coupler 7 can have a data rate as high as ~5.5 Mbit/s (1,024 x 64 kbit/s) in the case of a 1,024 channel system.
Since each channel in the data stream leaving the control station 1 occupies a distinct wavelength channel the data stream may be demultiplexed using a simple passive wavelength analyser and without the use of active electronic devices. Figure 1 shows a two stage demultiplexing system. The high data rate ~broad bandwidth) data stream on the fibre 2 is taken first to the node 3 which analyses the wide-bandwidth signal into eight reduced bandwidth groups each containing 128 of the original channels. Each of the eight groups is output from the node 3 to one of eight reduced bandwidth fibres 8. Each of the reduced bandwidth fibres 8 leads to a further node 4 (all but two of the reduced bandwidth fibres 8 and the nodes 4 are omitted from Figure 1 for clarity). Each of the nodes 4 contains an analyser which splits the reduced bandwidth group into its constituent wavelength channels and outputs each channel along a separate narrow bandwidth (low data rate) fibre 20 to one of 1,024 receiving stations 5 (all but one of the receiving stations 5 are omitted for clarity).
Each of the receiving stations is arranged both to detect a signal modulated on its respective channel using a receiver operating at the low data rate and to impose further modulation on the respective channel for transmission bac~ to the control station. The modulation/demodulation scheme used is illustrated diagrammatically i~ Figure ~. ~ signal is imposed upon one or more of the channels at the control station using binary pulse code modulation techniques. Since the receiving stations do not themselves contain any source of optical power it is necessary to maximize the power transmitted to them. To this end ~he pulse code modulation uses two non-zero logic levels rather than conventional "on-off" coding to avoid the loss of power which would otherwise occur as a result of the modulation to the zero level. Each receiving station 5 includes an additional fibre coupler 9 which splits the light in two, part of the light being used for detection of the modulated signal using a threshold detector 10 the remaining optical power being used for modulation by a modulator 12, reflection by a mirror 13 and retransmission to the control station 1. At the control stati~n 1 the optical signal is received by a detector 11 and the signal decoded by a receiver having a : da'a rate of at least 65.5 Mbit/s. This use of pulse code modulation followed by splitting at the receiving station 5 results in a 6 dB power loss in the path from the .. . .. . ......... . . ........ .... .... .... ...
s~
_ 9 control station 1 to the receiving station detector 10: 3 dB due to the use of the half power level for the "off"
state plus a further 3 dB loss at the coupler. In a signal reflected ~o the control station from the receiving station there are two passes through the extra coupler 9 and hence a 9 dB loss overall assuming a 50:50 coupler ratio. This additional loss of 9 d~ may be reduced by employing a coupler with an unequal coupling ratio since less attenuation is incurrecl in the path from the receiving station 5 to the control station 1 and the receiving station has in any case higher sensitivity.
Other elements of the system also contribute to power losses: tables 1 and 2 show overall power budgets for transmission from the control station 1 to the receiving station 5 and transmission from the control station 1 to the receiving station 5 and back to the control station 1 respectively.
DOWNSTREAM POWER BUDGET
-Launch Power 0 dB
Sensitivity at 64 ~bit/sec -70 dB
Available system loss 70 dB
coupler (50:50) 3 dB
25 Fibre (3.5 km) 2.5 dB
1st stage MUX 6 dB
2nd stage MUX 6 `\dB
Coupler ~90:10~ 10 dB
Penalty for non-zero off-state 3 dB
Signal reduction through 64KHz filter 30 dB
System loss 60.5 dB 60.5 dB
Sys~em margin . g.5 dB
, ~35g~
UPSTREAM POWE~ ~UDGET (INDIVIDUAL MODULATORS PER
, ~ ~
CUSTOM~R) Launch power 0 dB
5 Sensitivity at 65.5 Mbit/sec ~49 dB
Available system loss 49 dB
Coupler (50:50) 3 dB
Fibre 2.5 dB
1st stage MVX 6 dB
10 2nd stage MUX 6 dB
Coupler l90:10) 0.5 dB
Modulator/Reflector 2 dB
2nd stage MUX 6 dB
1st stage MUX 6 dB
15 Fibre 2.5 dB
Coupler (50:50) 3 dB
Penalty for non-zero off-state 3 dB ¦~
System loss 40.5 dB 40.5 dB ' System margin 8.5 dB
In an alternative embodiment of the present invention modulation of the light for retransmission to the control station 1 takes place not at the individual receiving stations 5 but at the nodes 4. In this case the wavelength analysers in the nodes 4 include optical modulators as shown in Figure 3. In common with the simple analysers discussed above the analyser/modulators include a reflection diffraction grating 14 and a concave mirror 15 arranged to direct each distinct wavelength to a unique point in space for coupling to a particular fibre. However in addition the analyser/modulator of Figure 3 includes a spatial light modulator (SLM) 18.
The SLM 18 may be transmissive (eg. a magneto-optic cell or liquid crystal structure) in which case a mirror (not shown) is positioned adjacent its rear surface to reflect ~4~
the incident light. Alternatively the SLM 18 may be inherently reflective (e.g. a silicon micro-machined surface~. A beam splitter 19 directs part of the light to fibres connected to the receiving stations and th~
remainder to the SLM 18. Incoming rays from the control station 1 are analysed and directed to the appropriate receiving stations as normal. However instead of the receiving stations being arranged to modulate and retransmit a portion of the light received each receiving station is connected electrically to the point on the array of the SLM 18 corresponding to its wavelength channel and as a result controls the modulation of that portion of the light which is reflected back to the control station 1 from the SLM 18.
15 The network can be upgraded ea~ily to continuous wave wideband operation in the same customer wavel~ngth channels by coupling in additional multiplexed laser signals.
Where the present invention is to be used to optically encode an asynchronous time domain multiplexed signal the tuning of the optical source 6 is controlled to vary with the timing of the various channels which may be irregular both in the periods of time the optical source is set to a given wavelength and in the order the wavelengths are set which order will depend on the order of the channels in the received time domain multiplex.
The secondary optical paths may be replaced by electrical connections between the node and the receiving stations.
: 35 ~ ," . .. ..
- . .
Claims (17)
1. An optical encoder for an optical communications system comprising:
an optical source for generating an optical signal corresponding to an input time-domain multiplexed signal of two or more information channels separated in the time-domain and having a tuning means arranged to vary the wavelength of the optical signal as a function of time so that those portions of the optical signal corresponding to a respective distinct time-domain multiplexed channel are generated at a predetermined wavelength.
an optical source for generating an optical signal corresponding to an input time-domain multiplexed signal of two or more information channels separated in the time-domain and having a tuning means arranged to vary the wavelength of the optical signal as a function of time so that those portions of the optical signal corresponding to a respective distinct time-domain multiplexed channel are generated at a predetermined wavelength.
2. An optical encoder as in claim 1, in which the optical source is a semiconductor laser including a tunable external optical cavity.
3. An optical encoder for an optical communications system comprising an optical source for generating an optical signal corresponding to a time-domain multiplexed signal of two or more information channels having a tuning means arranged to vary the wavelength of the optical signal so that those portions of the optical signal corresponding to a respective distinct channel are generated at a distinct wavelength; and wherein the tuning means is arranged to repeatedly sweep the optical signal through a preselected wavelength range.
4. An optical communications system having a control station, a first optical path connecting the control station to a node and secondary optical paths connecting the node to two or more receiving stations including:
an optical encoder for an optical communications system comprising an optical source for generating an optical signal corresponding to a time-domain multiplexed signal of two or more information channels having a tuning means arranged to vary the wavelength of the optical signal so that those portions of the optical signal corresponding to a respective distinct channel are generated at a distinct wavelength;
a first transmitting means for transmitting the optical signal from the control station via the first optical path to the node; and a passive wavelength analyser at the node arranged to direct respective wavelength channels via the secondary optical paths to the receiving stations.
an optical encoder for an optical communications system comprising an optical source for generating an optical signal corresponding to a time-domain multiplexed signal of two or more information channels having a tuning means arranged to vary the wavelength of the optical signal so that those portions of the optical signal corresponding to a respective distinct channel are generated at a distinct wavelength;
a first transmitting means for transmitting the optical signal from the control station via the first optical path to the node; and a passive wavelength analyser at the node arranged to direct respective wavelength channels via the secondary optical paths to the receiving stations.
5. An optical communications system as claimed in claim 4, in which one or more of the receiving stations includes a modulating means whereby the optical signal received from the node can be modulated and returned to the control station via the node.
6. An optical communications system as claimed in claim 5, in which the optical encoder generates a pulse code modulated optical signal with non-zero logic levels.
7. An optical communications system according to claim 4, further comprising:
means to transmit a signal from one or more of the receiving stations to the node; and node modulating means at the node for modulating the respective one or more wavelength channels at the node for retransmission from the node to the control station.
means to transmit a signal from one or more of the receiving stations to the node; and node modulating means at the node for modulating the respective one or more wavelength channels at the node for retransmission from the node to the control station.
8. A method of optically encoding an input time-domain multiplexed signal of two or more information channels separated in the time-domain, said method comprising:
generating an optical signal corresponding to the input signal by means of a wavelength tunable optical source while varying the wavelength of the optical signal as a function of time so that those portions of the optical signal corresponding to respective distinct time-domain multiplexed channels are generated at predetermined wavelengths.
generating an optical signal corresponding to the input signal by means of a wavelength tunable optical source while varying the wavelength of the optical signal as a function of time so that those portions of the optical signal corresponding to respective distinct time-domain multiplexed channels are generated at predetermined wavelengths.
9. A method of optically encoding an input time-domain multiplexed signal of two or more information channels comprising:
generating an optical signal corresponding to said input signal by means of a wavelength tunable optical source by varying the wavelength of the optical signal so that those portions of the optical signal corresponding to respective distinct channels are generated at distinct wavelengths;
wherein the wavelength of the optical signal is repeatedly swept through a preselected wavelength range.
generating an optical signal corresponding to said input signal by means of a wavelength tunable optical source by varying the wavelength of the optical signal so that those portions of the optical signal corresponding to respective distinct channels are generated at distinct wavelengths;
wherein the wavelength of the optical signal is repeatedly swept through a preselected wavelength range.
10. A method of communication comprising:
optically encoding an input time-domain multiplexed signal of two or more information channels as an optical signal at a control station by generating an optical signal corresponding to said input signal by means of a wavelength tunable optical source while varying the wavelength of the optical signal so that those portions of the optical signal corresponding to respective distinct channels are generated at distinct wavelengths;
transmitting the optical signal to a node;
resolving the optical signal into its constituent wavelength portions at the node; and transmitting the different wavelength portions to respective receiving stations.
optically encoding an input time-domain multiplexed signal of two or more information channels as an optical signal at a control station by generating an optical signal corresponding to said input signal by means of a wavelength tunable optical source while varying the wavelength of the optical signal so that those portions of the optical signal corresponding to respective distinct channels are generated at distinct wavelengths;
transmitting the optical signal to a node;
resolving the optical signal into its constituent wavelength portions at the node; and transmitting the different wavelength portions to respective receiving stations.
11. A method of communication as claimed in claim 10, further comprising modulating the signal received at one or more of the receiving stations and returning the modulated signal via the node to the control station.
12. A method of communication as in claim 11, in which the portion of the optical signal corresponding to the information channels is pulse code modulated at the control station with non-zero logic levels and each corresponding receiving station includes a demodulator.
13. A method of communication as in claim 12, in which at each corresponding receiving station the received
14 optical power is split, part of the power being directed to the demodulator and part of the power being demodulated and retransmitted by the receiving station to the control station via the node.
14. A method of communication as in claim 10, further comprising transmitting a signal from one or more of the receiving stations to the node to modulate the respective one or more wavelength channels returned to a node for retransmission via the node to the control station.
14. A method of communication as in claim 10, further comprising transmitting a signal from one or more of the receiving stations to the node to modulate the respective one or more wavelength channels returned to a node for retransmission via the node to the control station.
15. An optical communications system having:
a control station;
a first optical path connecting the control station to a node and secondary optical paths connecting the node to two or more receiving stations;
said control station including an optical encoder having an optical source for generating an optical signal corresponding to a time-domain multiplexed signal of two or more information channels with a tuning means arranged to vary the wavelength of the optical signal so that those portions of the optical signal corresponding to a respective distinct channel are generated at a distinct wavelength;
a first transmitting means for transmitting the optical signal from the control station via the first optical path to the node; and a passive wavelength analyser at the node arranged to direct respective wavelength channels to optical detectors the electrical outputs of which are transmitted to the receiving stations.
a control station;
a first optical path connecting the control station to a node and secondary optical paths connecting the node to two or more receiving stations;
said control station including an optical encoder having an optical source for generating an optical signal corresponding to a time-domain multiplexed signal of two or more information channels with a tuning means arranged to vary the wavelength of the optical signal so that those portions of the optical signal corresponding to a respective distinct channel are generated at a distinct wavelength;
a first transmitting means for transmitting the optical signal from the control station via the first optical path to the node; and a passive wavelength analyser at the node arranged to direct respective wavelength channels to optical detectors the electrical outputs of which are transmitted to the receiving stations.
16. An optical encoder for an optical communications system, said encoder comprising:
a wavelength tunable optical signal source capable of generating optical output signals at n predetermined wavelengths .lambda.1, .lambda.2, . . . . .lambda.n;
said tunable optical signal source being driven by an input time-domain multiplexed signal having n time-domain multiplexed information channels TDC1, TDC2, ... TDCn such that the optical signal source operates at predetermined wavelength .lambda.1 during a time slot assigned to a respectively corresponding time-domain multiplexed channel TDCi, i being an integer in the range of 1 to n; and an optical output signal conduit disposed to carry optical output signals generated by said tunable optical signal source.
a wavelength tunable optical signal source capable of generating optical output signals at n predetermined wavelengths .lambda.1, .lambda.2, . . . . .lambda.n;
said tunable optical signal source being driven by an input time-domain multiplexed signal having n time-domain multiplexed information channels TDC1, TDC2, ... TDCn such that the optical signal source operates at predetermined wavelength .lambda.1 during a time slot assigned to a respectively corresponding time-domain multiplexed channel TDCi, i being an integer in the range of 1 to n; and an optical output signal conduit disposed to carry optical output signals generated by said tunable optical signal source.
17. A method for encoding n input signals onto an optical signal conduit, said method comprising the steps of:
time-division multiplexing said n input signals to produce a time-division multiplexed signal with n channels;
driving a wavelength tunable optical signal source with said time-division multiplexed signal to produce a wavelength-division and time-multiplexed optical output signal carrying the information content of a given predetermined one of said input signals in a predetermined time multiplexed channel at a corresponding predetermined optical wavelength.
time-division multiplexing said n input signals to produce a time-division multiplexed signal with n channels;
driving a wavelength tunable optical signal source with said time-division multiplexed signal to produce a wavelength-division and time-multiplexed optical output signal carrying the information content of a given predetermined one of said input signals in a predetermined time multiplexed channel at a corresponding predetermined optical wavelength.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| EP88305399.3 | 1988-06-13 | ||
| EP88305399A EP0295857B1 (en) | 1987-06-12 | 1988-06-13 | Optical multiplexing |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1295059C true CA1295059C (en) | 1992-01-28 |
Family
ID=8200100
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000571241A Expired - Fee Related CA1295059C (en) | 1988-06-13 | 1988-07-06 | Optical demultiplexing |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1295059C (en) |
-
1988
- 1988-07-06 CA CA000571241A patent/CA1295059C/en not_active Expired - Fee Related
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