CA1305523C

CA1305523C - Methods and devices for altering optical polarisation

Info

Publication number: CA1305523C
Application number: CA000555285A
Authority: CA
Inventors: Terence Geoffrey Hodgkinson; David William Smith
Original assignee: British Telecommunications PLC
Current assignee: IPG Photonics Corp
Priority date: 1986-12-29
Filing date: 1987-12-23
Publication date: 1992-07-21
Anticipated expiration: 2009-07-21
Also published as: DE3785662T2; DE3785662D1; JPS63174435A; HK130296A; JP2626896B2; AU8305987A; EP0277427B1; ATE88816T1; EP0277427A1; US5031236A; GB8630959D0; ES2040756T3; AU598619B2

Abstract

ABSTRACT
OPTICAL POLARISATION CONTROL

The invention provides methods and devices for processing an optical signal. In one embodiment an optical device (101) for scrambling an Input optical signal comprises an optical switch (10) which switches the optical signal between two intermediate optical paths (13,14) under control of a scrambling signal from A
control circuit (12). The polarisation of the signal in one of the intermediate optical paths (13) is rotated by a TE-TM converter (15). The polarisations of the two intermediate signals are thereby made mutually orthogonal. These mutually orthogonal signals are then recombined by a directional coupler (16) to provide a combined output optical signal with a scrambled polarisation alternating between othogonal states according to the frequency of the scrambling signal.
The invention may be applied to improve signal reception, for example, in coherent optical transmission systems.

Figure 3

Description

~ L3~5~23 BT CASE: A23567 WP NO: C400P

METHODS AND DEYICES FO~ ALTRING OPTICAL POLARISATION

The present i m ention relates to me~hods o~ processing an optical signal, and to optical devices for perfonmlng the processing, all ~or use in op~ical transmission systems. The inven~ion finds application in improving optical signal reception particularly, but not exclusively, in coherert optical transmission systems.
Coherent optical fibre transmission sys~ems using heterodyne and homodyne reception techniques of~er considerable ga~ns in receiver sensitivity over conventional direct detection syste~s. However, the perfonmance of a coherent receiver depends on matching the signal polarisation at the input to the receiver to the polarisation of the local oscillator. After transmission along a birefringent opt~cal fibre, the state of polarisation of the signal at the receiver input is generally indeterminate~ The signal is detected by mixing o~ received and local oscillator fiel~s on the surfaçe of a square law optical detecto~. If ~he polarisa~ions of the received signal and ~he local oscillatnr signal ~re not ma~ched there is an effective receiver performance penalty which shows up as fading in ~he detected signal.
In the extreme case of orthogonal received and locat oscillator signal polarisations, the signal is completely l~st.
To reduce the problem of polarisation sensitiYity a so-called polarisation diversity receiver has been proposed by Takanori Okoshi, Shiro Ryu and Kazura Kikuchi in "Polarisation diversity receiver for heterodyne coherent optical fibre communications", ICOC 1983 Payer 30C3-2, Tokyo, June 1983.

~30~iSi;~3 In ~he polarisation d~vers~ty receiver, two orthogonally polarised componen~s of ~he inpu~ signal are separated and heterodyne detected indlvidually befor2 being summed in an output stage. The polarisation diversity receiver limits ~he ~ximum fade loss to 3dB.
However, one ~isadvantage is that the conventional receiver electronics mus~ be dupli~ated in such a receiver. The Okoshi et al design also requires a Faraday rotator for polarisation changing. Faraday rotators are inconvenient for the high cperat~ng currents wh~ch ~hey need and for the powerful magnetic fields which they must generate in order to influence the polarisa~ion.
An al~ernative solution to the problem of coherent receiver design is provided by so-called polar~sation tracking receivers which employ sophisticated feedback loops to actively control the loc~l osc711a~or polarisation in order to match it to the signal polarisation (or vice-versa). Conveniently~ the local oscillator polarisdtion is altered by using piezoelectric transducers, rctatable phase pl~tes or si~ilar bulk optical devices. An example polarisation tracking receiver is described in "New polar~sation-control scheme for optical heterodyne receiver using two Faraday rotators", Okoshi e~ al, Electronics Letters, Yol. 21 No.
18, 29 August l9B5. One drawback of this type o~ recefver is the necessity for a complex and sensitive feedback control system. The disadvantages of Faraday rotators have already been mentioned above. Whilst piezoelectric transducers are frequently considered as alternatives, these too have disadvantages. For exampleJ they are mechanical and consequently relatiYely slow in operation.
Furthermore, the high voltages of the order of lkY which are nePded to operate these devices are potentially hazardous to associated electronic circuits and consequently demand speciat protection.

~3q~i23 Polar~sat~on sensitive devices may also be employed in optical transmission sys~ems whieh use direct detection rather than coherent techniques. For example, planar waveguide dev~ces, which generally operate with a preferred input polarisation, may be used to switch signals at a receiver or for signal distribution at network nodes. Therefore, conventionally some fon~ of polarisation contro1 1 s al so requ~ red i n di rect detecti on systems which use such components if the potential problems of extreme signal loss which can occur from misalignment of signal polarisation are to be avoided.
I~ is one object of the present invention ~o provide a method of processing an optical signal which enables the problems of polarisation mismatch which may occur at polarisation sensitive parts of a transmission system to ~e overcome or at least mitigated.
Another object of the present inYention is to provide an optieal device for use in optical transmission systems which operates according to the method and removes the need for polarisat~on matching at polarisation sensitive parts of a system.
It is a further object of the present invention to provide a version of the aforesaid optical device which may be used both to process an optical signal by a method according to the method of the invention and which may also be used more generally as a polarisation controller.
Such a device is suitable for use in a polarisation tracking receiver, for example, and may be implemented as an integrated optical device. The device does not require, and avoids the associated disadvantages of, bulk optical components as described above.
According to the invention in a first aspect a method of processing an optical signal comprises altering the polarisation state of the optical signal in sequential L3~ 3 time interYals under control of a control signal dt a prede~ermined scrambling frequency to provide a processed optical signdl with substan~ially orthogonal polarisation states alternating sequentially according to the scrambling frequency.
Using this method dn optical signal with d single, substantially constant, initial polarisation state is transformed into a signal with alternating orthogonal polarisation states This ensures that, if such a processed optical signal is input to a polarisation sensitive device with one of the alterna~ing polarisation states wholly unmatched or badly ma~ched to the device, the other will be wholly or at least better matched, and consequently the processed signal will be passed by the polarisation sensitive device and, more particularly, complete sisnal loss, which might otherwise occur with a single polarisation signal, ~s avoicled.
Conveniently the me~hod ~ncludes splitting the optical signal into two intermPdiate optical signals, altering the polarisation states of the inter~ediate op~ical signals relative to each other and recombining the intermediate opt~cal signals to produce the processed optical signal.
; The relative proportions of the optical signal which are split into the intermediate optical signals may be controlled by switching of the optical signal under control of the control signal.
The optical signal is preferably completely switched in sequential time intervals to alternately provide each of the intermediate optical signals and the po1arisation states of the switched intermediate optical signals are - rendered mutually orthogonal. The processed signal is then conveniently formed from a sequential recombination of the alternating, intermediate optical signals.
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Alternatively, for example, the method may include rendering the polarisation sta~es of the ~ntermedia~e optical signdls mutually orthogonal and phase modulating at least ~ne of the intermediate optical signals under control of the control signal. In this case the vector sum of the polarisation states of ~he recombined intermediate optical signals alternates to provide the required alternating orthogonal polarisation sta~es of the processed optical signal.
Preferably, the phase of one of the intermediate optical signals is modulated to impose step phase shifts of substant~ally 180 during alternate half-cycles o~
the control signal. In this way i~ is possible to implemænt the method using only one phase modulator.
When the method is used for altering the polarisation state of an optical signal used for infor~ation transmission the control signal is set at a frequency sufficient such that infonmation carried on the signal can be made available a~ a detector using the output optical signal in one of the alternating orthoaonal states ~ndependent of the other. 3y providing a transmitted information signal with a1ternating othogonal polarisation states, the possibili~y of total signal loss which may otherwise occur at pGlarisation sensitive parts of a system if polarisation states are not well matched is avoided. Alternating the polarisation, at a sufficient frequency as defined above, eFfectively duplicates the transmitted signal such that the information in the signal may be independently reconstructed by examination of the signal in either polarisation state alone, if necessary.
The processed signal comprising alternating polarisation states may be optically coupled to a birefringent medium such as an optical fibre for transmission to a detector. However, whilst d birefringent transm~ssion medium may rotate a s~ngle input polarisation in an arbitrary manner, it can be shown that two orthogonally polarised inputs, although each subject ~o some absolute arbitrary rotation, will nevertheless remain orthogonal relative to each other when output from the birefringent medium. Thus, even if one of the orthogonal states output fro~ the birefringent medium is completely mismatched at some polarisation sensitive part of the transmission system, the other will invariably be well matched, and consequently, the Infon~ation availab1e from the signal w~ll not be lost.
The method according to the i mention may be employed in transmission systems using coherent or direct detection techniques.
It will be appreciated that, since the polarisation of the 10cal oscillator in a coherent receiver can never be simultaneous1y orthogonal to both components of the processed optical signal, scrambling of the polarisation of a ~ransmitted siqnal using ~he method of the present i mention eliminates the poss~bili~y of complete signal loss at the receiver which can occur in a conventional coherent transmission syste~.
In a coherent optical system, as an alternative to scramblLng the polarisation of the transmit~ed information signal directly, an equiYalent resul~ may be achieved by scrambling the local oscillator signal by the method of the invention. In this case, the control signal is set a~ a frequency sufficient such that the information on the information signal is independent1y detectable if the polarisation of one of the alternatinc orthogonal states of the processed local oscillator signal alone matches the polarisation state of the information signal arriving at the receiver.

~3~ 3 ", - 7 ^

This alternative may be especially su~tabte~ for example, where se~/eral distributed transmitters are transmitting ~o one central receiver, as onle polarlsation scrambler will then suffice to ensure detection of all the transmltted signals.
The method may also be used to process an optkdl signal comprising a plurality of signals at substantially separate wavelengths, for example, as in a frequency mult~plex transmission syste~.
Where the optical signal comprises one or more disital optical signals, the frequency of the control signal should be no~ less than the minimum o~ the bit rates of the digital optical signals. Any information in the d~g~tal signal will then be availab1e from eîther polarisation state of the processed signal. This is one feature of the invention which is especially useful in optical transmission systems.
According to a further aspect o~ the present invention an optical device for processing an op~ical signal by a method of the invention comprises control ~eans for providing a con~rol signal at a predetermined scrambling frequency and means for altering the polarisation state of an input op~ical signal under control of the con~rol means to provide an output optical signal wi~h substantially orthogonal polarisation states alterna~ing sequentially according to the scrambling frequency.
Conveniently~ the altering means includes means for splitting ~he input optical signal from an input signal path into two intermediate signal paths, polarisation changing means disposed in at least one of the intermediate optical:paths to render the polarisation states of the intermediate optical signals orthogonal relative to each other and means for recombining the intermedi~ate optical signals to provide the desired output OptiCdl signal.

The means for splitting the op~ical signal ~ay comprise an optical switch for switching sele table proportions of the input optical signal from the input signal path into the ~wo in~enmediate optical si~nal paths, the switched proportions being selected under control of the control mæans.
Conveniently~ the optical sw~tch comprises a voltdge controlled coupler (VCC)O
In this case~ the cortrol means provides the control sign~l for operating the switch. The proportions of the input signal which are switched into each of the intermediate signal paths ~s deter~ined by the voltage applied to the VCC. Preferably the control signal comprises d squarewave voltage such that the input signal is 100/o switched alternately between each of the ~ntermediate signal paths at the scramblin~ frequency.
Optical devices according to the invention may be used in transmission systems to scramble the polarisat~on state of an optical signal for information transmission to ensure tha~ the infor~a~ion is available from either of ~he al~ernating orthogonal sta~es of the output signal independent of the other. In a coherent optical ~ransmission system such devices nay be used to scramble the local oscillator signal rather than the transmitted information signal itself. Various advantages of these arrangements have already been noted in the discussion of the method of the invention aboYe.
Compared with the ideal conditions for heterodyne detection where local oscillator ard received information signal polarisations are identical, S i 5na 1 scrambling introduces a constant 3dB sensitivity reduction at the receiver. However9 the scrambled signal detection is independent of the relative states of ~olarisation. The 3dB scrambling loss is a constant and a maximum, . ~

}S~3 g regardless of any di~ferences ln po1arisatlonO In pract1cal trans~ission systems where the received s~gnal polarlsation is generally indeterminate, thls represents a considerable ad~antage over the conven~ional case whe~e, in the absence of complex and specialised prior ar~
receiver designs, total signal loss may occur.
The minimum desirable scrambling frequency for use in an information transmission system has been defined above in relation to the information carried on the relevant signal. It will be appreciated that ~he maximum desirdble serambling frequency ~ay be constrained by other system parameters. For example9 for noise filtering considerat~ons, for digital signals it is generally preferable to l~mit the in~ermediate frequency (IF) bandwidth in non-synchronous IF detectors to within a factor of ten times the slgnal bit-rate. In such instances, the scrambling frequency is preferably llkewise restricted to be no more than ten times the bit rate.
Similar limiting factors appropriate to alternative receiver designs will be apparent to those skilled in ~he art.
As an alternative ~o step-switching the input signal it is possible to sweep the signal smoothly between the intermediate signal paths. For example, the scrambling signal may comprise a sinusoidal vol~age. In these circumstances, however, it is preferable that phase modulating means are disposed in at least one of the intermediate signal paths for adjusting the relative phase of the intermediate optical signals. Preferably, the phase modulating means is controlled by the control means to impose alternating phase shifts on the intermediate signal in one path, such that the ~ector sum of the polarisation states of the intermediate optical signals alternates to provide the alternating, orthogonal polar~sation states of the output s~gnal. Con~eniently, the phase modulating means imposes a 180 phase shift onto the signal in one of the intenmediate xignal paths for the duraticn of every second half cycle of the scrambling sinusoid. As îs explained in the detailed description below, the introduction of this phase shift means that the polarisation state of the cornbined output signal alternates dS desired and the detected signal amplitude at a recei~er is rendered independen~ of the signal polarisation. As for the step-switched case above, again there is a constant and maximum loss in sensitivity of 3d5 regardless of ~he relative states of polarisation of the received and local oscilla~or signals.
Preferably, the polarisation changing means is adapted to introduce a fixed rotation of 90 in the polarisation of a sig~al in one of the intermediate optical paths relative to the polarisation of the signa7 in the other path. This is the most straightforward way to achieve the orthoQonal polarisations required. It is not necessary to use compl kated tunable bulk optlcal polarisation rotators, such as the aforementioned Faraday rotators, for example.
Conveniently, the polarisation changing means comprises a TE-TM converter, which ~ay be implemented as a planar integrated optical device. The polarisation changing means may further comprise an integrated optical phase shifter in association with the TE-TM converter.
The phase shifter may be used to adjust the phase difference between TM and TE modes of ~he input signal in the first intermediate optical path to 90 so that the TE-TM converter may operate for all incoming states of polarisation. If the polarisation of the input signal is predetermined and fixed then the phase shifter is not needed.

~3~ 3 Convenient7y, therefore, the input siynal path w~ll comprise a polarisation holding optical fibre to enable the s~ate of polarisation of the input signal to the optical dev~ce to be fixed. Where the device components are integrated on a lithium niobate substrate, for example~ ~he input polarisation will preferably be fixed in TE mode.
As an alternative to using an integr~ted TE-TM
converter, the polarisation rotation may also be simply effected using a suitable birefringen~ medium such as, for example, an appropriate length of birefringent optkal fibre. The fibre might ~hen perfonn the funct~ons of an intenmediate optical path and polarisation changing means together. Other variations will be apparent to those skilled in the art.
The input signal will normally come frcm a semiconductor laser. The input signal may be provided via an optical modulator, for example, a planar waveguide device formed on a substrate with the optical device.
The recombining means is conYeniently a 1:1 directional coup1er. Most preferably ~he coup1er is a polarisation selec~ive device rranged to cross couple only one of the ~wo orthogonal polarisations to- maximi~e the output signal power. Using a polarisation selective coupler the 3dB reduction in transmiss~ble power which is other~ise introduced by an ordinary coupler is avoided.
In order to balance the contributions to the output signal amplitude from each of the intermediate signal paths it may be desirable tc introduce an optical attenuator into one or both of these paths.
In a further aspect cf he present invention an optical device is provided ~hich can both process an optical signal by the methcc of the inventicn and additionally alter the pol~risation state of an input signal frGm any given sta e so any other desired state.

~3~

According to this further aspect of the invention an optical devlce comprises means for splitting selectable propor~ions o~ an input optical signal from an input signal path into two intermediate optical signal paths, polar7sation changing means disposed in at least one of the inten~ediate optical signal paths and arranged to render the polarisation s~ates of the intermediate optical signals substantially mutually orthogonal, controllable phase modulating means in at least one of the ~ntenmediate optical paths, control ~eans for controlling the splitting means to select the split proportions and for controlling the phase ~odulating means to control the phase relationsh~p between the split signals, and means for recombining the two intermediate optical signals for providing a combined output signal of any desired polarisation under control of the first and second control means.
As an alterna~iYe to employing such an optical deYice to scramble an optical signal in a ~ransmission system accord~ng to the method of the invention ~s described above, the device can be used as a simplified polarisation controller, for example, for use in polarisation tracking receivers.
It will be observed that by appropriately controlling the relative proportions of two orthogonally polarised signals and by controlling the phase of these signals relative to each other a device according to this further aspect of the invention can construct a combined output signal with any desired polarisation. Thus, to process an optical signal by a method of the invention, the intermediate optical signal phase relationship is controlled to alternate as required. Alternatively, for use as a polarisation controller, the device may be inserted in the local oscillator path of a coherent ~3~ S ~ 3 receiver, for example, to provide a csntinuous (as opposed to an al~ernat~ng) adjustmen~ of signal polarisation ~o enable the local osclllator polarisation ~o track an incoming s~gnal polarisation as is required in a polarisation tracking receiver.
It is a particular advan~age of ~his more generally applicable device that all the major optical components, for example, splitting means (switch), phase modulator~s), polarisation changing means and recombining means toutput coupler) may be fabrica~ed as integrated optical devices and on a common substrate if desired.
It will be apprecia~ed that preferred device features defined with respect to the previously described embodiments of devices according to the invention are applicable, where appropriate, to embodiments of the device according to ~he invention in th~s further aspect mutatis mutandis. For exa~ple, the splitt~ng means may conveniently comprise an optical switch such as a voltaqe controlled coupler (YCC).
Various aspects of ~he invention will now be described in detail, by way of example only, with reference ~o ~he accompanying F~gures, in which:
Figure 1 is a schematic diagram of an optical transmission system using a method of processing an optical signals according to the present invention;
Figure 2 is a schematic diagram of a coherent optical receiver using the method ~o process a local oscillator signal;
Figures 3 and 4 are schematic diagrams of embodiments of an optical device according to the present invention;
Figures 5 to 8 show the results of simulations of processing a variety of signals according to the method of the present invention; and Figures 9 and lû are schematk e7~odiments of optlcal devlces accord~ng to the fur~her aspect of the invention ~hich can process signals according ts the method and also provide more general polarisa~ion control functions.
The transmission system illustrated in Figure 1 comprises a transmitter 1 connec~ed to ~ransmit a signal via op~ical waYeguides 5,6 to a receiver 4. An optical device 100 for processing the signal is positioned in the optical path between the transmitter 1 and receiver 4.
The processing device 100 itself com~rises a polarisation al~tering device 2 and a oontroller 3. In this case the altering device 2 comprises a voltage o~ current controllable birefringent material. (Electro-optic materials exhib~ting Kerr and/or Pnckel effects;
magneto-optic ~aterials for hraday rotation; or materials with stress-dependent bire~ringence are examples known in the art.) The altering device 2 is arranged to receive an input signal ~rom the trans~itter via the waveguide 5 and to output a processed (or "scrambled") s~gnal via the waYe9UIde 6 tG the receiver 4. The signal at the input 7 of the altering device 2 is arranged ~o have a linear, substantially constant, polarisation state. In practice9 the polarisation of a s~gnal from the transmit~er 1 may drift slowly over a long period (eg days), but this is not particularly relevant when compared ~o the (very) high frequencies at which optical transmission systems operate. However, the input polarisation state may be assured using conventional means, if required. For example, the waveguide 5 may be made of polarisation holding optical fibre. The altering device 2 is aligned with its birefringent material axes at 45 to the axis of polarisation of the input signal.

3~ 3 The operat~on of the polarisat~on alter~ng device 2 ~s controlled by the controller 3. A control signal, the "scrambling" signal, from the controller 3 is applied to the birefringent material in the altering devke 2 so that the birefringence is altered along one axis only. The amplitude of th~ scrambling signal is set ~o produce a birefringence change sufficient to cause an alternating 180 phase shift in a signal component along that axis.
The polarisation of the processed signal at the output 8 of the altering device 2 then alternates be~:ween two orthogonal, l~near states according to the frequency of the scramb1ing signal.
Whatever the bircfringence of the second connecting waveguide 6, as will be explained in more detail below, it can be shown that the signal which arrives at the input 9 of the receiver 4 will then also have alternating othogonal polarisation states talthough those states may not necessarily be l~near). Therefore, even if one of those orthogona7 polarisation states is completely m~s-matched to the receiver 4, the complcmentary, orthogonal polariisat~on will nevertheless be passed and the processed signal will be detected.
~ n Figure 1 the polarisation scrambling is perfonmed direct1y on the ~ransmitted signal. In coherent optical transmission systems it is possibleg as an alternative, to scramble the local oscillator signal to achieYe similar advantages. Figure 2 shows a schematic diagram of a coherent optical receiver illustra~ing how the scrambling may be carried out. The receiver comprises a balanced coherent detector 30 which receives an input ~ransmitted information signal ~arriving on an input fibre 31) after mixing with a local oscillator signa1 in an input optical coupler 32. In this case~ the processing device 100 Fomprlsing the polarisation altering device 2 and ' ;- ~3~

controller 3, as used ~n the embod~ment of F~gure 1, is arranged to process (scramble) the signal from ~he local oscillator source 33 to provide a scrambled local oscillator signal with alternat~ng orthogonal polarisat70ns to the mixing coupler 32. For convenience, the detec~ed signal output and the local oscillator frequency control prov~ded by the detector 30 are also ;~ indicated in Figure 2.
To ensure that the input information signal is fully ~* 2~ detectable even ~f one of the orthogonal local oscillator polarisations is completely mis~atched ~o the informatiQn s~gnal polarisat~on it is necessary to ensure that the scrambling is carried ou~ at a sufficiently high frequency (for example, at least as high as ~he blt rate of a digital information signal). If the infonmation rate changes, it may then be desirable to correspondingly alter the control signal scrambling frequency. The control7er 3 of Figure 2 may be prov~ded with the option to make such adjustment in response to an appropriate feedback 34 from the detector 30. (If desired, a s~milar provision cauld also be made in ~he arrangement of Flgure 1 to enable ~he ; controller 3 to respond appropriately to changes in the infonmat~on rate of the signal from the transmltter 4~
The basic processing device 100 described in rela~ion ~o Figures 1 and 2 may be provided in other fonms.
Alternative optical devices for carryinq out the processing will be described below. Yarisus aspects of ~he present invention will also be treated in ~ore detail with particular reference to these alternative devices.
: Referring now to Figure 3, an optical device 101 for polarisation scrambling comprises a Yoltage controlled coupler (YCC) 10 configured to switch a signal from an input optical fibre 11 alternately between two intermediate optical paths 13, 14. The signal ;

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~3~ 3 polar~sation in one of th~se paths 13 is rendered orthogonal to the signal polarisation in the o~her path 14 by a TE-TM converter 15 and the signals are then recombined via a 1:1 directional coupler :l6. The combined signal is then output at C into an opt~cal fibre 20.
The switching of the VCC 10 is controllled by a squarewave scrambling signal of frequency f generated by a control circuit 12. The VCC 10 is a planar elec~ro-optic waveguide device which pre~erentially transmits signals of a giYen polar~sation, in this case TE mode, only. The signal may be completely switched between the intermediate paths 13, 14 under cont~ol of the appropriate drive from the ~ontrol circuit 12.
TD achieve stability of the swi~ched si~nals, since the YCC 10 is polarisation sensitive, the polarisation of the input signal is constrained (in TE orienta~on) by a length of polarisat~on-hold~ng optical ~ibre 11 between the signa~ source, here a semiconductor laser 17, and the input of the YCC 10.
In operation, a signal from the laser 17 is alternately switched, at the scrambling frequency~ between the two interDediate signal paths 137 14 from the UU lO.
As the pol~risa~ion of the input signal is held matched to ~he selectivity of the VCC lG by the polarisation-holding fibre 11, the split signals leaYing the YCC are in the same polarisation. The signal polarisation in one path 13 alone is rendered orthogonal by the TE-TM converter 15 which is also a planar inte~rated optical deYiceD The signal loss in the TE-TM converter can generally be considered negligible. However, if necessary, the signals in the two intermediate optical paths can be balanced using an optical attenuator 21 optionally in one or both paths a s shown .

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After passage through the TE-TM converter She s~ynals in the two paths are thus orthogonally polarised relat~ve to each other. These orthogondlly polarised signals are ~hen combined by a 1:1 directional coupler 16 1nto a comblned output signal. The combined signal is shown as passing from one output C of the dirertional coupler 16 into an output fibre 20. The second output port D of the coupler 16 is shown terminated in a ron-reflectiYe manner~ However, i~ will be apparent tha~ with a conventional 1:1 coupler 16 the combined signal is potentially available for onward transmiSsiDn from each output port C,D. Consequently~ if the totil transmissible power available at the t~o output ports is directed ~o ~
single output a 3dB increase in transmissible signal power is achievable. Use of a polarisation selective coupler to selectively cross couple only one of the orthoQonal polarisatfons (in this case the TE mode), penmits this gain.
The ou~put signdl d~ C comprises a reconstruc~ion of the or~sinal input signal" but with alternatin~ orthogonal shifts of polarisa~on at the scrambling frequency, f. If the signal ~s digital, the scranbling frequency should be at least equal to the digital bit rate. With the embodiment of Figure ~, in order to make the received signal detection independent of the signal polarisation~
dS iS further explained below, the scrambling frequency is preferably squarewave.
Referring now to Figure 4, an al~ernative embodiment of the present invention will now be described. This embodiment arises from the inventor's appreciation that a phase modulaton can be used in place of the VCC to implement the signal polarisation scrambling. Tn this embodiment, the VCC is eliminated and is replaced by a passlve 1:1 directional coupler 24. One input A' of this coupler 24 receives the input s~gnal fro~ the laser 17.
The other input port B' is sui~ably provided w~th a non-re~lective termination. The input signal is continuously split in substantially e~ual proportions into the two intennediate signal paths 13,14 vi,a the output ports C',D' of the coupler 24. A phase modulator 22 is inserted Into one of these pa~hs 13,14. The phase modulator 22 is shown inserted in the path 14 separate from the path 13 which contains the TE-TM conYerter 15, but both these devices could equally well t)e inserted in the same path.
The scrambling is effected by switching the phase modulator 22 under control of a control circuit 2~ to introduce alterna~e 180 phase sh~fts into the signal in one of the intermediate signal paths at the scrambling frequency. The directional coupler 16 then produces a combined output signal from the two ~ntenmediate signals in the same manner as previously described. It will be appreciated that the combined output signal will again comprise a reconstruct~on of the origin~l input signal with alternating orthogon~l sh~fts of polarisatior at the scramb7ing frequency as for the embodiment of Figure 3.
In this case it is the vector sum of the polarisation states of the intenmediate optkal signals which alternates on recombination ~hereby providing the required alternating orthogonal polarisation shifts in the output signal.
Generally, when a signal is transmitted through a biref~ringent medium the polarisation of that signal will be a1tered. However, iF the transmitted signal comprises components~with mutually orthogonal poldrisations~ those component polarisa~ions, whilst they are individually altered during transmission, remain mutually orthogonal, as the present inventor has recognised to advantage.
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~3l3~ r~3 Th~s property Is demonstrated ~n ~igure 5 ~h~ch tllustrates the results of transmitting a signal wi~h orthogonally polarised components through a transmiss~on path comprlsing a series of birefringen~ sect~ons of transmission ~ed~um each with randomly oriented birefringent axes and random birefringent delay. The results relate to transmission through (a) 30 s~ch sections, (b) 50 such sections and (c) 100 such sections.
The upper plot in each case shows the trans~tted polarisation states and the lower the received polarisation states, which differ fram the transmitted states but which re~ain orthogsnal. The solid traces represent clockwise rotations and the dashed traces anticlockwise rotations o~ polarisation.
A signal with components of mutually orthogonal polarisations as produced by an optical device according to the present invention may be detected using a conventional coherent receiver. In this case the local oscillator signal in such a receiYer will have a single polarisation and there will be no sw~tching of the local oscillator slgnal between orthogonally polarised sta~es.
Consequently, the polarisation of the local oscillator signal itself can never be orthogonal to both components of the received, scrambled s~gnal. Therefore, the possibility of to~al fadins loss of the detected signal, whi~h can occur in conventional systems when the local oscillator signal polarisation is orthogonal to a received signal of single component polarisation, is avoided.

To expla~n the form of s~gnal which ~s der~ved ~n coherent detection of a scrambled signdl it is helpful to consider a simplified ~athemat~cal analys~s. The following analysis considers heterodyne coherent detection by way of example. The transmitted optical field may be expressed generally as:
es=EsCos(~st~ (1 ) and the optical power in this field as:
P5=S/2 (2) In the general case, to take acco~nt of arbitrary polarisations, it is convenient tc resolve es into orthogonal field components eSx and esy wi~h associated phase sh~ts ~sx~ dsy esx=Es~KsxJ Cos(~st ~sx) esy=Es~Ksy]o 5cos(~st~dsy) where KSx and Ksy represent ~he fractions of the total rece~ved signal power ~n each of the respec~ive re~erence planes, and Ksy=l~Ksx, such that:

=Es[l-Ksx~o scos(~st~

The effects on the signal polarisation produced by transmission through a llnearly birefringent medium (haYing ~xes of birefringence x' and y') can be treated mathematically by a rotation of the reference axes through an angle o (x to x', y to y') together with a relative phase shift, ~, of the Y-field with respect to the X-field, such that:

~: :

' 3~5~

eSX ~ ~ Es tKSX ~ ] Co s( WS t Jsx ' ) ~ 6 ) Ksx~=o.5t~Ksx-o.5)cos2e~l~KsxKsy~o 5sln2ecos(dsy-~5x~ (7) ~'LKSx] 5CoseSin~sx~rK~jy]O SSin~Sin(dsy+~ ~
~SX =Tan- t~
({Ksx~0 5Cos~Cos0s~[Ksy30 5sinecos(~sy~3J

and: esyi=EstKsy~] CS(~st~sY') (9) Ksyl=0.5~0.5-Ksx]Cos2~-~KsxKsy] Sin2eCos(~sy-0sx~) (10) ~ rKsy~ 5CoseSin(~sy+~)-rKsx3n 5SinoSin~sx) dsy,-Tan ~rKSy]0 5cOs~cOS(~sy+~s)-[Ksx~0 5Sin~Cosdsx~

Thes~ equat~ons can be used iteratively where ~he signal is t~ansmitted through a series of different linearly bi ~-ringent transmiss~on paths. A si~ilar mathematical trea~ment may be develope~ for the genera? case of elliFtical birefringence. Alternatively the Poincare sphe~e representation may be used to illustra~e grap~ically the evolution of the polarisation. rSeeS for examcle, Rashleigh, "Origins and Control of Polaris~tion Effe:ts", Journal of Lightwave Technology, Yol.LT-1, No.2, June 1983 . ) ;ing the above equation:s it is ~ur~her possible to obta 1 expressions for the intermediate frequency (IF) SiSfi-,~ derived ~hen ~the received signal is subject to hete-~dyne detection. For comparison purposes it is conv nient to normalise the detected signal amplitude rela~ ve to the amplitwde which would be ob~ained in the conv-~tionaliy ideal case~where received and local osci~ ator signal:polarisations are perfectly matched. In :

:

. ~

-~ ~3~

5uch clrcumstances, in d digital system, for example, for a sequence of transmitted 'l's the amplitude of the demodulated IF s~gnal can be normalised to unity (ie. the mean vdlue of the IF envelope can be se~ to unity). Note that, for ideal amplitude sh~ft keying (ASK), there i5 no need to consider ~ransmitted 'O's because ther~ will be no resultant IF.
Taking ~he local oscillator signal with resolved components elX9 ely each carrying fractions of the local oscillator power K1~3 Kly respective~y, the demodulated IF signal, vIF can be expressed as:

YIF=ApCS(~IFt~ 12) where:
Ap= (KA~KB )0 5 ( 13) KA Kï xK~X ~ KlyKsy ~ ( 14) KB=[4(KlxKsx')(KlyKsyl)] CSr01y~~lx~~sy' dsx'] (15) and:
~p=Tan~ld(LS) (16) [KlXltsx,~Q 5Sin(~lx-~ ,3+[Kl It .]0 5sin(~ ~ ) ~( LS ~ 7 ) [I~lXKsX-] CS(dl~ 5X~)+[KlyKsyl]o-5cos( These equations are simplifications which are appropriate to one comp1ete cycle of a square~ave scrambled signal or to one particular instant if the scrambling signal is smoothly varying (eg sinusoida1).
Scrambling of the transmitted signal will intro~uce a time variance in Ksx'Ksy'~sx and ~sy dependent on the time variance of the scrambliny signal itself (eg. KSx will become KSx~S~t)], where S(t) is the scra~ling signal function).

ii23 , ...

In order to confirm the effects of signdl scrambllng as predicted by the above calculations, tests have been carried out simulating different co~blnations of loca7 oscillator and received s~gnal palar~sations.
~ xample results are lllustra~ed in Fi~ures 6 and 7 for a transmitted digital seq~ence of '1's scrambled with a squarewave scrambling signal at a frequency equal to the digital bit rate. The upper ~wo sets of plo~s in each case show the local oscillator polarisatlon and the relat~ve polarisa~ions of the scrambled, received signal components. The ?ower two sets of plots illustrate the demodulated IF characteristics of the received siqnals.
The vIF s~gnal ~s shown w~th its associated dmplitude envelope, Ap9 over a period of two '1' bits of the digital s~gnal in all the examples. The relative phase, ~p, of the IF signal over the same period is also shown.
F~gure 6 shows the results of tests using a linearly polarised local osc~llator signal, ~hilst Figure 7 shows results for an elliptically polarised local osc~llator signal .
In all the tests it was observed that the mean value of the IF envelope was 0~5. This value is normalised re1ative to ~unity for the convent~onal ideal of perfec~ly matched local oscillator and receiYed signal as mentioned above. The results confirm ~hat, relative to the ideal, the reduction in received signal power may be maintained at a constant and maximum of 3dB usiny squarewave scrambling. In contrast with conventional heterodynne detection, which frequently departs from the ideal, the scrambled signal is neYer entirely undetected.
Considering Figure 6~a), ~or example9 in conventional circumstances, in the absence of scrambling, if the recei~ed signal had the clockwise polarisation illustrated ~he signal would be completely missed. However, as the ~3~SS~3 ` 2~

test shows, whilst the srrambled sfgnal ts unde~ec~ed for one half of each bit perfod, ft fs una~bfguously de~eoted in the remafnder with a mean IF a~pl~tude over the whole bit-perfod at the 0.5 normalised level as expected.
For some purposes it may be preferred to sweep the signal smoothly between polarfsations~ rather than ~o ~ntroduce step changes with a s~uarewave scra~blin~ signal. It may be des~rable to use a sinusoidal scrambling frequency, for example. However, in such clrcumstances, if the scrambling signa1 alone ~s altered, the mean value of the de~ected IF (at a remo~e receiver) no longer rem~ins independent of the received signal polarisation. HoweYer, this preferable characteristic mdy be res~ored by introducing a 180 phase shift into one of the co~ponent fields (eg. d5x or ~sy) for the duration of alternate half-cycles of the scrambling s~gnal.
Ffgure 8 fllus~rates the results of tests using a sfnusofdal scrambling signal, wfth and without the relevan~ phase shif~. For these te:sts the ~ran~mitted signal with polar~sation states (i) ~as de~ected us~ng an e11iptically polarised local osc~llator signal ~ii). The results show, for transmission oYer two different ~r~nsmission lines (a) and (b), the received signal polarisations (ifi~, the IF ch~racterfs~ics in the absence of the phase shift (iv) and the IF characteristics w~h the phase shift (v). ~hen no phase shfft is fn~roduced into the scrambled signal, the mean IF level was 0.62 for transmiss~on line (a) and 0.33 for tra~smisssion line (b~. These re.~ults show that, in the absence of the phase shift, the ampli~ude of the detected signals is polarisation (ie. transmission line~ dependent. In contrast, however, when the appropriate 180 phase shift is introduced, as shown at (v), the mean IF level is 0.5 in both cases and the scrambled signal detection is again independent of the received polarisations as for the step-switched, squarewave case described above.

.. ,, ~ .

.

~31~5~

To achieve this independence usfng a s~othly varying scrambl~ng slgnal, such as a sinusoidal signal~ it is convenient to employ an al~ernative embodinen~ of a device according to the present invention as illus~rated~ ~or example, in Figure 9.
The device shown in Fi~ure 9 is a versioll of the processing devices described ~ith reference to Figure 3 and Figure 4 with phase modula~ors 22,22' in both intermediate optical paths. Equ~valent somponen~s of the device are therefore labelled wi~h equivalent reference numerals. The devke comprises a voltage controlled coupler 10 which sp1its proportions of an input optical signal from an input path 11 into ~wo intermediate signal paths 13,14. As with the Figure 3 deYice, a control circuit 12 controls the VCC 10 to determine the proportions which are split into each path 13,14. Each intermediate signal path 13,14 includes a phase modulator 22,22' in series with a polarisation rotator 15,15'. The phase modulators 22,22' are also controlled by the ~ontrol circ~it 12. If desired, ~he polarisation rotators 15~
may also be subject ~o control by the con~rol circu~t 12.
The output directional coupler 16 is again provided to recombine the signals from the in~ermediate signal paths int~ a combined output signal.
This device combines the features of the Fi~ure 3 and Figure 4 devices so that it is possible to scramble an input optical signal to produce an output s~gnal with alternating orthogonal polarisat~on states in either of the ways described with respect to those devices.
Thus, 3S one option, the controller 12 supplies the scrambling signal to the YCC whilst the polarisation rotatorls) 15915' are set to render the polarisation states of the intermediate optical signals mutually orthogonal (cf Figure 3 and related description~. In this ...

~S~3 case ~t may not be necess~r~ to introduce any relat~ve phase shif~9 so phase ~odulators 22,22' need not be drlven. In an alternative op~ion, the controller can set the YCC 10 to act as a 1:1 d~rect~onal coupler and can supply the scrambling signal to drive the phase modulators 22,22' to in~roduce alternating 180 relative phase shifts (cf Figure 4 and related description). The polarisation rstators 15,15' are set as for the first option.
It is possible to simplify ~he op~ical device of Figure 9 wh~lst still allowing the dual control of signal split (using VCC 10) and relative phase (using phase modulator 22)~ A si~pl~fied device is shown schematically In Figure 10, ~or example. In this embodiment, which is other~ise similar to the embodiment of Figure 9, a single phase modulator 22 is ~nserted in one of the two interm~d~ate optical paths 13914. The phase modulator 22 ~s a voltage controlled planar electro-optic waYeguide device which can be ~abricated on the same substrate as the VCC 1~. Both the VCC 10 and the phase modulator ar~
controlled by the control circuit 12. A TE-TM corverter 15 provides a fixed 90 polarisation rotation in one intermediate signal path only.
It will be appreciated that this device also can scramble an optical signal in a number of the various ~ays described ear7ier. For example9 if the scrambling signal is a smoothly varying signal the VCC, under control of the control circuit 12, likewise smoothly changes the proportions of the input signal which are delivered to each of ~he intermediate signal pa~hs 13,14. In these conditions, the switching of the YCC 10 is a continuous and smooth process in contrast with the alternating step-switching which occurs wi~h a squarewave scrambling signal. The phase modulator 22, however, then introduces ' , step changes ~n phase of 180 during alternate half cycles of the scrambl~ng s~gnal. The operation of the phase modulator 22 ~s synchronised with thle switchlng by the scrambling s~gnal and is also controlled by the control clrcuit 12.
The various forego~ng examples and device descriptlons demonstrate that using polarisation scrambling, a constant and maxlmum 3dB signal reduction, relative to the ideal~
Is incurred in coherent detection. In the case where signal polarisation is orthogonal to local oscillator polarisation, a conventional receiYer exper~ences complete signal fade. Polarisation scrambling clearly avoids this problem (see eg. Figure 6(a)). So long as the 3d8 reduction is within the receiver tolerance, no polarisation control would be required for the local oscillator.
A further advantage of the polarisation scrambling technique ls that signals of sufficiently differing wavelengths, for example, in a frequency multiplex system, may be scra~bled simul~3neously. In a mult~plex system it would only be necessary ~o have one scrambling unit per output transmission ~ibre serving several distinc~
transmitters. This represents a considerable potential advantage in equipment cost saving.
Practical devices for imple~enting polarisation scrambling have been described. However, the use of these devices is not necessarily restricted to such purposes.
In particular, the devices of Figures 9 and 10 offer greater flexibility of use. As an example, these devices may also be employed as improved polarisation controllers for use in polarisation tracking receivers. For example~
as noted earlier, they can conveniently be implemented as integrated optical devices and ds not need bulk optical components. For this purpose~ as the inventor has , recogn~sed, control o~ the signal proport~ons us~ng the VCC 10 and of signal phase us~ng the phase modulator(s) 22 together9 ~n combination w~th at least one polarisation rotator, can allow a comb~ned output s~gnal to be constructed wlth any desired po1arisation ~hatever. All ~hat is required is for the control circuit 12 to con~rol ~he VCC 10 ~o fix ~he relative proportions of ~he orthogonal s~gnals and to control the phase ~odula~or 22 to fix the relative phase shift between ~hese sign21s such ~hat the conbination of these stgnals produces a single combined output signal with the desired polarisation. In this case the control signal~s~ would nonmally be arranged to vary smoothly and slowly in contrast to the rapidly alternating signals used for scrambling.
In a polarisa~ion tracking receiver, the local oscillator would provide the most appropr~ate signal source. The control circu~t 12 would also conveniently comprise a feedback control circuit, for example a self-ac~uiring control loop, to maintatn the combined output s~gnal polarisa~ion ~atched to the received s~gnal polarisation.

.
:

Claims

1. A method of processing an optical signal comprising altering the polarisation state of the optical signal in sequential time intervals under control of a control signal at a predetermined scrambling frequency to provide a processed optical signal with substantially orthogonal polarisation states alternating sequentially according to the scrambling frequency.

2. A method according to claim 1 in which the polarisation state of the optical signal is altered by splitting the optical signal into two intermediate optical signals, altering the polarisation states of the intermediate optical signals relative to each other and recombining the intermediate optical signals to produce the processed optical signal.

3. A method according to claim 2 including controlling the relative proportions of the optical signal split into the intermediate optical signals by switching of the optical signal under control of the control signal.

4. A method according to claim 3 including the steps of rendering the polarisation states of the switched intermediate optical signals mutually orthogonal and recombining the intermediate optical signals such that the processed signal is formed from a sequential recombination of the alternating, intermediate optical signals.

5. A method according to claim 4 in which the control signal comprises a squarewave signal, and the optical signal is step-switched alternately to provide each of the intermediate optical signals.

6. A method according to claim 2 including the steps of rendering the polarisation states of the intermediate optical signals mutually orthogonal and phase modulating at least one of the intermediate optical signals under control of the control signal such that on recombining the intermediate optical signals the vector sum of the polarisation states of the intermediate optical signals alternates to provide the alternating orthogonal polarisation states of the processed optical signal.

7. A method according to claim 6 in which the phase of one of the intermediate optical signals is modulated to impose step phase shifts of substantially 180° during alternate half-cycles of the control signal.

8. A method according to any of claims 1, 3 or 6 for altering the polarisation state of an optical signal used for information transmission including the step of setting the control signal at a frequency sufficient such that information carried on the signal can be made available using the output optical signal in one of the alternating orthogonal states independent of the other.

9. A method according to any of claims 1, 3 or 6 for altering the polarisation state of a local oscillator signal used for information signal reception in a coherent optical transmission system including the step of setting the control signal at a frequency sufficient such that the information on the information signal is independently detectable if the polarisation of one of the alternating orthogonal states of the processed local oscillator signal alone matches the polarisation state of the information signal.

10. A method according to any of claims 1, 3 or 6 wherein the optical signal to be processed comprises a plurality of signals at substantially separate wavelengths.

11. A method according to any of claims 1, 3 or 6 wherein the optical signal comprises one or more digital optical signals and the frequency of the control signal is not less than the minimum of the bit rates of the digital optical signals.

12. An optical device comprising control means for providing a control signal at a predetermined scrambling frequency and means for altering the polarisation state of an input optical signal under control of the control means to provide an output optical signal with substantially orthogonal polarisation states alternating sequentially according to the scrambling frequency.

13. An optical device according to claim 12 in which the altering means includes means for splitting the input optical signal from an input signal path into two intermediate signal paths, polarisation changing means disposed in at least one of the intermediate optical paths to render the polarisation states of the intermediate optical signals mutually orthogonal and means for recombining the intermediate optical signals to provide the desired output optical signal.

14. An optical device according to claim 13 wherein the means for splitting the optical signal comprises an optical switch for switching selectable proportions of the input optical signal from the input signal path into the two intermediate optical signal paths, the switched proportions being selected under control of the control means.

15. An optical device according to claim 14 wherein the optical switch comprises a voltage controlled coupler.

16. An optical device according to claim 13 in which the altering means further includes phase modulating means in at least one of the intermediate optical paths for adjusting the relative phase of the intermediate optical signals.

17. An optical device according to claim 16 wherein the phase modulating means in one of the intermediate signal paths is controlled by the control means to impose alternating phase shifts on the intermediate signal in that path, such that the vector sum of the polarisation states of the intermediate optical signals alternates to provide the alternating, orthogonal polarisation states of the output signal.

18. An optical device according to claim 12, for altering the polarization state of an optical signal used for information transmission or reception in which the control means includes means for setting the control signal at a frequency sufficient such that the information can be made independently available using the output optical signal in one of the alternating orthogonal states independent of the other.

19. An optical device according to claim 13 wherein the means for splitting the input optical signal comprises a passive 1:1 directional coupler.

20. An optical device according to claim 13 wherein the recombining means comprises a passive 1:1 directional coupler.

21. An optical device according to claim 12 or claim 13 in which the altering means comprises a TE-TM converter.

22. An optical device according to any of claims 12, 13, or 14 comprising integrated optical waveguide devices.

23. on optical device comprising means for splitting selectable proportions of an input optical signal from an input signal path into two intermediate optical signal paths, polarisation changing means disposed in at least one of the intermediate optical signal paths and arranged to render the polarisation states of the intermediate optical signals substantially mutually orthogonal, controllable phase modulating means in at least one of the intermediate optical paths, control means for controlling the splitting means to select the split proportions and for controlling the phase modulating means to control the phase relationship between the split signals, and means for recombining the two intermediate optical signals for providing a combined output signal of any desired polarisation under control of the first and second control means.