AU647273B2 - Continuous polarization regulation - Google Patents

Description

4' V 1 I 14 X

WI

S F Ref: 198003

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

a Name and Address of Applicant: Actual Inventor(s): Address for Service: Invention Title: Siemens Aktiengesellschaft Wittelsbacherplatz 2 D-8000 Munich 2

GERMANY

Chandan Das Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Continuous Polarization Regulation a. 9 9* a a. a a a.

The following statement is a full description of this Invention, including the best method of performing it known to me/us:- 5845/3 91 P 8032 DE Siemens Aktiengesellschaft Continuous polarization regulation The invention relates to a process for continuous polarization regulation according to the preamble of Claim 1.

EP-Al 0,248,234 discloses a process for the regulation of the polarization of a light signal, in which the light signal passes through three birefringent elements of a polarization converter and the birefringences of the three elements El, E2 and E3 are 10 continuously altered as a function of actuating signals.

The arrangement, known from the aforementioned EP-Al 0,248,234, for polarization regulation is diagrammatically represented in Fig. 1 of the present application. In the known arrangement, the actuating signals are generated by comparison of the polarization of at least one coherent light signal with a reference polarization condition by means of an analyser or of a directional coupler RK disposed in a superheterodyne receiver. In this case, the reference polarization condition is the polarization condition of a second light signal at the entrance LO, but it is also possible to use the polarization condition of a polarization-selective element. In this case and also hereinbelow, birefringence of a light signal is understood as referring to the phase retardation which can be detected between the two principal axis components of a light signal passing through a bifringent element; these two components, which are also designated as characteristic modes, differ from one another in birefringent elements by their propagation velocity. The retardation D is considered hereinbelow as a relative retardation and is indicated as a fraction of the wavelength of the light or of the phase of the light.

In the case where optical superheterodyne 2 reception is employed, the problem consists in that a received light signal having unknown polarization and extremely low power is to be heterodyned with the second light signal, generated by a local laser, having the same polarization. On account of the low optical power of the light received, it is in this case expedient to pass the light of the local laser through the polarization converter, since this light is available, at an adequate level. Accordingly, the function of the polarization regulation is in this case to transform a light signal having a specified polarization into a light signal having a desired polarization as a function of the polarization of the received light signal.

The known arrangement, shown in Fig. 1, for polarization regulation exhibits a first entrance ES, at which the received light signal is present, which received light signal is passed on to the associated entrance of a directional coupler RK. A second entrance LO of the arrangement is connected to a local laser and receives from the latter a light signal having a known polarization and relatively high power, which signal is fed via a polarization converter PT with a first, a second and a third birefringent element ES1, ES2 and ES3 to a further entrance of the directional coupler RK. In addition to the first optical exit Al for measurement purposes, the directional coupler RK includes a second optical exit A2 for a downstream detector DET, which includes a photodiode with, connected thereto, a photocurrent amplifier. Further arrangements for signal processing and a squarer Q are connected to the detector output A2E, as part of the arrangement for polarization regulation. The output of the squarer Q is connected via a low-pass filter TPF to a control input of a regulator R, which drives at least [lacuna] of the three birefringent elements El, E2 and E3 contained in an actuator device. In addition to the actuating signals, generated by the regulator R, for setting the working point, relatively small square wave signals are heterodyned therewith. This gives rise to a square wave 3 modulation of the birefringences and thus to small deviations from the birefringent working points, for example, of the first and of the second birefringent element El, E2, which have the effect of small changes in intensity and thus changes in power of the superheterodyne signal. The squaring gives rise to evaluatable amplitude changes of the input signal of the regulator R. The regulator R forms approximately therefrom all first and second order derivatives of the intensity with respect to the birefringences for example of the first and of the second birefringent element El, E2. The regulator R emits corresponding actuating signals, by which the birefringent elements El, E2 are shifted in the direction cf the gradient derived from the 15 change in intensity and the birefringence. By the evaluation of the second derivatives of the intensity with respect to the birefringence, it is in this case possible to detect saddle points and points of minimum intensity of the superheterodyne signal and to depart from them in the direction of a maximum under conditions of undisturbed operation and far removed from the limits of the range of the birefringence of the elements. At maximum intensity of the superheterodyne signal, the polarization conditions of the received light signal and 25 of the locally generated light signal are in agreement.

As a result of the alteration of the polarization condition of the received light signal, the working point of one of the two regulated birefringent elements may move S: to the setting limit, so that this element in question must be reset. In order to avoid undesired polarization conditions and thus losses of intensity at the exit of the directional coupler RK, during the resetting process, the resetting process is combined with an adjustment of the hitherto unregulated third birefringent element E3.

The prerequisite for the resetting without loss of intensity is, in this case, that the polarization of the entrance light, i.e. the theoretical polarization of the polarization regulation arrangement, undergoes virtually no alteration during the resetting phase. Only after 4 completion of the resetting phase are any changes which occur stabilised, so that losses of intensity may indeed occur where the polarization of the entrance light alters relatively rapidly. The function of the above-described known continuous polarization regulation is moreover based on the assumption that three ideal birefringent elements can be employed and that the range limits and the reset points are set with precision. If these conditions are not met, difficulties in regulation may be encountered, even with constant theoretical polarization, in the resetting phase.

Accordingly, the object of the present invention consists in providing a continuous polarization regulation which can take account of rapid alterations of the polarization of the entrance light, even during the phase of resetting one of the birefringent elements; moreover, the intention is also to be able to use non-ideal bire- 0S*o *fringent elements, and an adequate tolerance range is to be available as well for the setting of the range limits 20 and of the reset points.

In the initially mentioned process for continuous polarization regulation, this object is achieved according to the invention in that this process is further developed by the features indicated in the characterising clause of Patent Claim 1.

Particularly advantageous in the case of the process according to the invention are the relatively small ranges of actuation for the individual birefringent elements, which increase the accuracy of scaling and the reliability of operation. Accordingly, when using the fibre grips which are frequently employed as birefringent elements there is soon a risk of destruction of the fibre when applying relatively large compressive forces; liquid crystal elements frequently require a large thickness where the ranges of actuation are relatively large, so that the consequence is a greatly reduced actuator speed.

As a result of the smaller ranges of actuation, the required drive powers also become smaller. The process according to the invention further has the advantage that 5 only the birefringent element situated directly downstream of the polarization converter entrance requires a relevant scaling with respect to the points H and V for the linear polarization on the Poincar6 sphere.

This is easy insofar as the first birefringent element is not influenced by the further birefringent elements and manages with only one minimal range of actuation. The process according to the invention has only one resetting procedure, which is common to all birefringent elements; in this case, the resetting process is self-optimised, so that on the one hand the effort expended on the design of the regulator R is relatively small and on the other hand only slight adjustments are made to the birefringent elements, which may possibly have to be reversed during the resetting procedure. Furthermore, the process according to the invention offers the possibility of additionally applying the measures described. in the German Patent Application P 41 04 366.9 against a 4* blocking of the polarization regulation.

S 20 Expedient refinements of the process according to the invention are described in greater detail in Patent Claims 2 to 4.

The invention is to be explained in greater detail hereinbelow with reference to an illustrative embodiment shown in the drawing. In the drawing: Fig. 1 shows a diagrammatic representation of a known arrangement for polarization regulation for Soptical superheterodyne reception, Fig. 2 shows the construction of a polarization converter consisting of four fibre grips as linearly birefringent elements, Fig. 3 shows the possibilities for adjustment from an entrance polarization into a different exit polarization for the first birefringent element in the polarization converter according to Fig.

2, Fig. 4 shows the range limits for the exit polarization condition following the second birefringent element of the polarization converter according 6 to Fig. 2, Fig. 5 shows examples of alterations of the exit polarization of the first birefringent element of Fig. 2 following the resetting procedure, and Fig. 6 shows an explanation of the possibilities for further regulation during the resetting process for the third and fourth birefringent elements of Fig. 2.

Fig. 1 has already been described in detail in the description of the prior art, so that no further discussion of this figure will be presented at this point.

In Fig. 2 a polarization converter according to the invention is described, having four actuating elements which comprise four so-called fibre grips, which act as linearly birefringent elements over a sufficiently Swide range of actuation. The fibre grip, which is known per se, consists of a relay in which an optical waveguide Sis disposed between the movable armature and a stationary part, on which optical waveguide the armature exerts a variable mechanical pressure, as a function of the electric current flowing through the relay. As a result of this pressure, those parts of the optical waveguide which are acted upon become birefringent, the maximum S 25 pressure which can be exerted and thus the maximum attainable birefringence being limited on strength grounds. For the generation of the pressure it is also possible to employ, in place of the relays, piezoelectric elements by means of which higher setting speeds are attainable. The invention can also be performed in conjunction with polarization converters in which the required linearly birefringent elements are produced using integrated optics by means of lithium niobate applied to a substrate.

In Fig. 2, PO designates the polarization condition at the entrance of the polarization converter, i.e.

upstream of the first birefringent element El. The polarization condition at the exit of the first linearly birefringent element El is designated by P1; this is the 7 entrance polarization condition for the second birefringent element E2, which is rotated in a direction through 45° in relation to the first birefringent element El, so that the force exerted on the optical waveguide acts at the corresponding angle. The exit polarization condition of the second birefringent element E2 is designated by P2; this is, at the same time, the entrance polarization condition of the third linearly birefringent element E3, the pressure direction of which coincides with that of the first birefringent element El. The exit polarization condition of the third birefringent element E3 is designated by P3; this is the entrance polarization condition for the fourth linearly birefringent element E4, the pressure direction of which coincides with that of the second birefringent element E2 and which emits a light signal having a polarization condition P4 to the exit of the polarization converter. As a result of the rotation of the second and of the fourth birefringent element E2, E4 through 45", there is created a differing 20 position of the axes of rotation of the birefringence, related to the Poincar6 sphere, of 90" for two adjacent birefringent elements in each case.

In normal operation, the first and the second birefringent element El, E2 are acted upon by actuating 25 signals from the regulator R, while the third and the fourth birefringent element E3, E4 are firmly clamped in their neutral position, for example at the centre of their range of adjustment in the case of odd multiples of i/2, so that the pertinent relay windings have a fixedly set electric current flowing through them.

The Poincar4 sphere representations in Figures 3 to 6 serve to explain the function of the polarization converter according to Fig. 2.

Fig. 3 shows an example of the possibilities for adjustment of the entrance polarization PO of the first birefringent element El into its exit polarization P1. On the Poincar6 sphere various points are shown, which correspond to specific polarizations. The two points at the poles of the Poincar6 sphere correspond to circularly 8 polarized light waves; in this case, a left-rotating circular polarization corresponds to the point L and a right-rotating circular polarization corresponds to the point R. The point H designates horizontal polarization and the point V vertical polarization, while the points P and Q represent points of penetration of the axes of rotation DRE1, DRE3 of the first and of the third birefringent element El, E3 through the surface of the Poincar6 sphere. In this case, linear polarization occurs at -45" at the point Q, and linear polarization with a polarization angle of +450 at the point P.

The first birefringent element El is selected so that the entrance polarization PO of this element, which polarization lies on the great circle through the points L, H, R and V between the points L and H, is converted along the great circle as far as a position between the *0S0 points V and L. Accordingly, in the case of the range of actuation of the first birefringent element El it can be 0* stated that its minimm settable retardation D1,n is a 0 20 and the difference between the maximum settable and the minimum settable retardation D1m, D1ln is in the range between r and 2r. In this case, it is desirable that the S. maximum and the minimum settable retardation Dl. and Dlmin should be situated closer to the points V and H respectively, i.e. closer to the horizontal plane, since this favours the capability of further regulation of the third and of the fourth birefringent element E3, E4 during the resetting process of the first and of the second birefringent element El, E2. The hysteresis of the first 30 birefringent element between successively following reset initiations is increased by wider ranges of actuation of the retardation D1 of the first birefringent element El.

The axis of rotation DRE2 of the second birefringent element E2 penetrates the surface of the Poincare sphere at the points H and V, and thus is perpendicular to the axis of rotation DRE1 for the first birefringent element El. Accordingly, the adjustment paths of the second birefringent element E2 are on the Poincare sphere parallel to the great circle defined by 9 the points L, P, R and Q. In the case of the range of actuation of the second birefringent element, i.e. of the difference between the maximum settable retardation designated by D2,, and the minimum settable retardation designated by D2.in, the following is applicable: D2, D2mi n 2r and for D2min 7/4 (2 KI 1) and for D2ma n/4 (2 K 2 1), it only being necessary for these conditions to be observed approximately and K, being an integer greater than zero or zero and K 2 being an integer greater than zero. Having regard to the most reliable adjustments possible of the third and of the fourth birefringent element in the resetting phase, it should be ensured that 15 both in the case of the minimum retardation D2in of the second birefringent element E2 and also in the case of 'the maximum retardation D2ma the polarization P2 is converted more closely to the great circle of the Poincar6 sphere which passes through the points LhVR than to the great circle passing through the points HPVQ.

During regulation, progressive adjustments of the :first or of the second birefringent element in the same direction may lead to the approaching and to the exceeding of a range limit for the adjustment.

25 Fig. 4 shows the range limits of the polarization condition P2 of the optical wave at the exit of the second birefringent element according to Fig. 2. Thus, Fig. 4 shows the overrun and underrun positions respectively for the retardation D1 of the first birefringent element, which positions comprise circles, parallel to the great circle LPRQ, in the vicinity of the points H and V respectively; in this case, the retardation D2 of the second birefringent element has any selectable value within its range limits. Corresponding range limits may be indicated for the retardation of the second birefringent element E2.

Selectively upon reaching or upon exceeding the predetermined range limits, a resetting procedure for the first and the second birefringent element El, E2 is 10 initiated by the regulator R. In this case, in the first instance the third and the fourth birefringent element E3, E4 are no longer acted upon by the actuating signal for the respective neutral position, but by a corresponding regulating signal. The neutral positions D 3 mid for the third and D 4 mid for the fourth birefringent element are normally situated at approximately

D

3 mid D 4 mid /2 (2 k 3 1) where k 3 is an integer greater than zero.

In the case of the illustrative embodiment shown in Fig. 4, the following dimensioning is applicable: Dlmi 0, D1, 01.51, D2,n ir/4, D2m, 2.75w, D3mi d 1.5r and D 4 mid In this example, the retardation D2 of the second birefringent element is within the range between 0.

2 and 2 7 5w at any selectable position.

,o In the case of the further embodiments, it is in the first instance assumed that the first birefringent 20 element El exceeds its predetermined range limit; the considerations are also similarly transferable to the second birefringent element E2.

Upon initiating the resetting procedure, Dltarse~ and Dlnew and D2n,, respectively are determined from the retardations D1 and'D2 respectively which are in existence at this time and which are designated hereinbelow as D1lod and D 2 o1d respectively. In general, Dlg,,,,t is determined as the retardation of the first birefringent element El, in the case of which the polarization P1 of the optical wave emitted by this element passes to the nearest equatorial point H or V of the Poincar6 sphere for the path of adjustment of the first birefringent element. In the illustrative embodiment, Dltargst is set to the value 0.

2 5n if the retardation is Dl<Dlidf and a value of 1.

2 5r is determined for Dls,,t if D1 is greater than or equal to D1,ia.

If, at the time of resetting, the polarization P1 of the light emitted by the first element El differs from Dliad by more than w/2, then Dln,,, DlOld is set; otherwise, 11 D1ln W Dlold is set. In the first case, the retardation of the first element El is such that the polarization Pl following the first birefringent element El is reflected by it onto the other half of the Poincar6 sphere, i.e.

the polarizations Plold and Plnew occur on different halves of the Poincar6 sphere; in this case, the plane of separation is defined by the points H, P, V, Q for the two halves of the sphere.

Using the selected dimensioning, the following is obtained: If /2 Dl1d for Dlold T/4 Dnew Dlold for 4 Dl 0 .d 5 1.257

S

2 .5r Dl d for 1.

25 r Dll d Fig. 5 shows the adjustments of D1old into the associated Dlnew for two cases. In the case 5a, the entrance polarization PO and also the exit polarization Plod are situated on the great circle arc of the Poincar6 sphere which is situated between the points L and H. The nearest equatorial point is the point H, via which the S 20 resetting of the first birefringent element El to a new exit position Pin., then takes place; the resultant retardation Dlnew is obtained from the difference between Pinew and PO.

In the case of Fig. 5b, the entrance polarization PO is approximately at the same value ae in Fig. 5a, but the exit polarization Pl 1 1d ahead of the resetting was on the great circle arc between L and V; the corresponding fe. retardation DioLd was then obtained from the difference between Plol d and PO. In this case, the equatorial point V is the closest point; the resetting proceeds in the direction of this point and, since Pl 1 od differs from Dlmid by more than r/2, to the other half of the Poincar6 sphere, so that Pln,, is now situated on the lower half of the Poincar6 sphere and D1n. represents a markedly smaller retardation than Dl1od.

Having regard to the various possibilities for setting the new value D2n,, of the retardation of the second birefringent element E2, there is a general rule applicable to the effect that that possibility is 12 selected which is closest to the centre of the range between D2,, and DGmin. In specific terms, if the exit polarization P1 of the first birefringent element El must be reflected into the lower half of the Poincar6 sphere, the following becomes applicable: S D 2 od x for D2old D2new

D

2 old i for D 2 old If the exit polarization P1 of the first birefringent element El is not to be reflected, i.e. Dlnew Dlod, the following is obtained:

D

2 old 2 r for D 2 old r/2 D2new D 2 Oid for n/2 D 2 ,od D2o.

1 27 for D 2 od Following the determination of Dltargset Dlnaw and D2new in accordance with the formulae specified hereinabove, the resetting procedure is executed in the following steps. In the first instance, the actual polarization regulation is switched over to the additional third and fourth birefringent elements E3 and E4.

In the next step, the retardation D1 of the first birefringent element is adjusted to Dl1,,,t. In the following step, the retardation of the second birefringent element is then adjusted from D2 to D2n,,. This step is noncritical, and takes place rapidly, since the entrance polarization P1 of the second birefringent element E2 is a characteristic mode thereof. Finally, the retardation D1 of the first birefringent element is adjusted to D1n..

Thereafter, the polarization of the exit signal of the 30 second birefringent element is again in the same condition as at the time of the triggering of the resetting procedure; during the resetting procedure, the third and the fourth birefringent element E3, E4 have also been reset if the exit polarization condition P4 of the polarization converter is unaltered.

The above-described dimensioning specifications guarantee the capability of the regulation of the polarization condition P4 at the exit of the polarization converter at any time. In addition to the measures explained 13 hereinabove, it is possible to enlarge the range of the guaranteed further regulability by modifications of the specifications for determining the retardations Dl1ne and D2n, respectively of the first and second birefringent element respectively; the exit polarization P4 of the polarization .converter may undergo further movement within this range during resetting. The first one of these measures consists in that the path of adjustment of Dltaroet to Dlnew during resetting is limited. The following rule may be used for this purpose: if D1,d, Dlnw is set to D1mid 0.4n, and if Dlnew a Dlm d then is set to Dlmid 0.

4 r. In place of the value 0.4r, it is in this case possible to select as an alternative another value which is more suitable in the specific case. By way of a further measure for enlarging the range of further regulability, it is possible to set D2n, to an integral multiple of i. These measures are to be explained hereinbelow in conjunction with Fig. 6. As has already been mentioned, the neutral positions for the retardation 20 of the third and of the fourth birefringent element have been selected at n/2 (2 k 2 since this guarantees that the third and the fourth birefringent element E3, E4 can accept the function of the regulation of the exit S polarization P4 of the polarization converter immediately after initiation of the resetting procedure. The exit range BP4 for further regulation during the resetting procedure by means of the third and the fourth birefringent element E3, E4 is shown in Fig. 6 in the region about the point P, since in the case of the 30 illustrative embodiment the retardations D3 and D4 were selected in each instance to have the value 1.5r. During the resetting procedure, the range BP3 of the exit polarization of the second birefringent element covers a narrowly defined region about the point H on the Poincare sphere. However, the range BP3 may also be situated about the point V on the Poincar6 sphere.

When using ideal birefringent elements and when applying appropriate dimensioning of the relevant parameters, the range BP3 may shrink as far as the respective 14 central point H or V after the first step of the resetting procedure, so that the third and the fourth birefringent element can generate the exit polarization P4 in any selectable polarization condition; this possibility is also available if D2n,, is selected to be an integral multiple of n.

Following completion of the resetting procedure for the first and the second birefringent element El, E2, the regulation of the exit polarization P4 is again switched over to these elements, while the third and the fourth birefringent element E3, E4 are adjusted until such time as they are again situated in their neutral position, i.e. the initial condition has been restored.

In the case of low quality regulation, i.e. relatively large deviations of the regulated variable from the theoretical value, the regulation of the third and fourth 99* birefringent elements E3, E4 can again be switched additicnally to that of the first and second elements El, E2.

A constant regulation using all four elements, in which 20 the third and fourth elements E3, E4 are adjusted within relatively narrow limits about their neutral positions, is advantageous in specific cases. Thus, a non-ideal entrance polarization or a non-ideal position of the axes Sof rotation of the first two elements El, E2 can accordingly be compensated by a 4-element regulation.

The continuous polarization regulation which has been described can be combined with the possibility, described in German Patent Application P 41 04 366.9, for increasing the reliability of regulation. Thus, on account of the regulation behaviour described in the patent application therein mentioned, there may be a blocking of the regulating system. The embodiments of the measures against an impending blocking of the regulating system are readily feasible when applying the continuous polarization regulation described in the present patent application, since this regulation can be interrupted at any time and can also be switched over to a regulation using all four birefringent elements.

Claims (4)

1. 2. Process according to Patent Claim 1, charac- terised in that the minimum retardation Dli, of the first birefringent element (El) is zero and the minimum retar- dation D2,j of the second birefringent element (E2) is r/4, in that the maximum retardation Dlm, of the first birefringent element is 1.5v and the maximum retardation D2,, of the second birefringent element is 2.75n, and in that the neutral positions and the mean retardations D 3 ,d 41 and D 4 mid respectively are
2. 3. Process according to Patent Claim 1, KP T 1,U) 17 0S *0 S 0* 0 *SOL 0* 0 characterised in that the required range of actuation for the retardation (Dl) of the first birefringent element 's (El) 4of 0 to 1.5r, the maximum retardation D2 of the second birefringent element (E2) and the required retardations D3, D4 of the third and fourth birefringent element (E3, E4) are 2
3. 4. Process according to Patent Claim 1, charac- terised in that on c, .encement of the resetting proce- dure the actuating signals for the polarization regula- tion are emitted not to the first and the second bire- fringent element (El, E2) but now to the third and the fourth birefringent element (E3, E4), in that in a first step the retardation of the first birefringent element (El) converts the entrance polarization (PO) at least 15 approximately to the vicinity of a characteristic mode of the second birefringent element (E2) and in a second step the retardation of the second birefringent element (E2) is adjusted approximately to the centre of the working range and subsequently in a third step the working range for the retardation of the first birefringent element (El) is adjusted so that, in spite of resetting of the third and of the fourth birefringent element (E3, E4) to their neutral positions, the desired exit polarization is achieved. 25 5. Process according to Patent Claim 1, charac- terised in that in the case of a non-ideal position of the axes of rotation of the first two birefringent ele- ments (El, E2) or of the entrance polarization (PO) in addition to these two elements the third and the fourth birefringent element (E3, E4) are also included in the constant regulation, but are adjusted only within rela- tively narrow limits about the predetermined neutral positions. SS 5 0055 S 5090 4* 5 00 5@ 4* 0 5S S. Siemens Aktiengese T P t orneys for the Applicant rny 18
4. 6. Process for the continuous regulation of the polarization of a coherent light signal, substantially as described herein with reference Figs. 2 to 6 of the drawings. DATED this TWELFTH day of JANUARY 1994 Siemens Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON *S* *o *o* *f IAD/5729W Abstract Continuous polarization regulation Especially in the case of optical superheterodyne receivers, it is necessary to reregulate the polarization of the locally generated light or of the received light. In this case, the polarization regulation takes place by adjustment of the retardation of birefringent elements which, following a number of similarly directed adjust- ment steps, may reach a limit of adjustment and have to be reset. In this case, the problem arises that rapid alterations of th, polarization of the entrance light must also be allowed for during the resetting phase; moreover, non-ideal birefringent elements are also to be used. According to the invention, four birefringent elements are used, of which the first two elements in the light path serve for normal regulation ana the following two elements serve for regulation during the resetting procedure. S. Fig. 2 a* 6 ae *e e 9