Field of the Invention

This invention relates to a demodulator and an optical arrangement therefor. Particularly, but not exclusively, the invention relates to an optical arrangement for use in a polarisation diversity coherent demodulator, a method of configuring the optical arrangement thereof and a demodulator including the optical arrangement.

Background to the Invention

Phase-shift keying (PSK) is a known digital modulation technique that conveys data by changing (i.e. modulating) the phase of a carrier signal. Essentially, binary digits (bits) are encoded by associating a discrete set of phases of the carrier signal with a particular pattern of bits, known as a symbol. In differential phase-shift keying (DPSK) it is the change in successive phases of the signal that is used to determine the bit pattern, rather than the actual phase of the signal at any point in time.

Binary phase-shift keying (BPSK) makes use of two distinct phases separated by 180°. However, this technique only allows for 1 bit to be encoded per symbol (i.e. providing a total of 2 bits for each complete wavelength) and so it is not suitable for high data-rate applications. Quadrature phase-shift keying (QPSK) employs two discrete PSK signals multiplexed with one of the signals offset in phase by 90°, hence a QPSK signal encodes 2 bits per symbol. Thus, QPSK can be used to significantly increase a data rate when compared to BPSK. Both BPSK and QPSK can be implemented using differential PSK to form differential BPSK (DBPSK) and differential QPSK (DQPSK), respectively.

Traditionally, QPSK signals are demodulated by splitting the incoming signal into an in- phase wave component and a quadrature-phase wave component to determine the sequence of symbols encoded on the incoming signal. It will be realised that with this technique it is critical that the signal components associated with the in-phase wave and the quadrature-phase wave, respectively, are combined in the correct order so as to form the correct series of symbols. This can be problematic since any unforeseen delay in the time it takes for one of the waves to be decoded could disrupt the entire sequence so that the bits are not combined in the appropriate order. Such a delay could be inadvertently introduced by a slight misalignment or misplacement of one or more of the optical components provided in the demodulator.

Furthermore, as the decoding of wave components can now be more accurately performed by small hybrid optical chips such as those described in WO2010/095018, there are associated challenges in coupling the incoming signal (and reference signal, where employed) to the chips.

It is therefore an aim of the present invention to provide a demodulator (and an optical arrangement therefor) that addresses the above-mentioned problem.

Summary of the Invention

According to a first aspect of the present invention there is provided a demodulator for a polarisation diverse optical signal comprising:

a polarising beam splitter arranged to spilt an incoming optical signal into a first beam having a first polarisation state and a second beam having a second polarisation state;

a reference path for a coherent reference beam;

at least two balanced detectors, respectively configured to detect the difference between the first beam and the reference beam and the second beam and the reference beam; and

a path balancer configured to enable the first and second beams to have substantially equal optical lengths. The path balancer can be employed to ensure that the first and second beams have equal (or comparable) path lengths (e.g. with respect to a bit period). Accordingly, the signal need not be split symmetrically with respect to the two balanced detectors. The path balancer may therefore be used to ensure that the optical timing of the first and second beams is comparable so that the components of each beam that arrived together at the splitter will also arrive at the balanced detectors at substantially the same time, despite the fact that the signal input may be conveniently provided closer to one of balanced detectors. In the case of a coherent demodulator, this configuration provides a hugely flexible way of coupling the optical signal to the balanced detectors while enabling each beam to be resolved into its respective components independently of the other beam, before the corresponding components of the two beams are combined to determine the two bits of information that arrived together and that form a particular symbol. Embodiments of the invention therefore provide a demodulator in which the respective timing of the first and second beams can be accurately controlled by the path balancer so that errors resulting during the demodulation process can be minimised. In certain embodiments, the path balancer may also be employed to alter the path length of the first and/or second beams so as to accommodate particular demodulation schemes. Thus, the demodulator of the present invention can provide an accurate, efficient and compact optical receiver package, with resulting savings in manufacturing costs. The fact that the first and second beams may easily be equalised during manufacture is particularly advantageous because it allows for compensations to be made for individual differences resulting from manufacturing tolerances.

As described above, the demodulator is preferably of the type known as a coherent demodulator and is configured for polarisation diversity coherent detection. More specifically, the demodulator may be configured for polarisation diversity quadrature phase shift keying (QPSK) or even higher order phase shift keying such as that described in US7,546,041 . It will be understood that the demodulator of the present invention may be adapted for differential phase shift keying (DPSK), differential quadrature phase shift keying (DQPSK) or higher order homodyne operation. Thus, depending on the demodulation technique being employed, the signal may be compared with itself (e.g. at a different phase or with a one symbol delay) or with a reference signal.

The demodulator may be configured to operate at any desired data rate, for example, 40Gbps or 100Gbps.

The balanced detectors may comprise one or more semiconductor demodulator 'chips' such as those described in WO2010/095018. Each detector may therefore comprise a hybrid structure (e.g. a planar lightwave circuit PLC), comprising waveguide paths which are balanced in insertion loss and birefringence, plus photo-diode detectors. Two or more such detectors may be employed and may be provided on separate substrates or on a single, integrated substrate (e.g. chip). Furthermore, each balanced detector may comprise a plurality of discrete components or may be provided as a single component (e.g. chip).

In certain embodiments, the balanced detectors may be configured to combine the first and second beams with respective first and second reference beams. In which case, the balanced detectors may comprise a first detector comprising a first input for the first beam, a first reference input for the first reference beam, a first (lightwave) interferometer for combining the first beam with the first reference beam, and a first photo-detector for generating electrical signals representative of bits encoded on the first beam. The balanced detectors may further comprise a second detector comprising a second input for the second beam, a second reference input for the second reference beam, a second (lightwave) interferometer for combining the second beam with the second reference beam, and a second photo-detector for generating electrical signals representative of bits encoded on the second beam. The demodulator may further comprise a digital signal processor (DSP) for decoding the respective electrical signals from the first and second detectors.

The path balancer may be implemented using free-space optics (rather than using co- planar waveguide circuits) in order to minimise manufacturing costs. However, particularly where the balanced detectors employ waveguide circuits, the coupling of the incoming optical signal to the balanced detectors can be difficult because accurate placement of the free-space optics (without occupying a significant area of the demodulator package) becomes critical. It is therefore a significant advantage that the present invention allows for adjustment of the path lengths of the first and/or second beams so that they can be equalised even if they are arranged to take significantly different routes between the splitter and the balanced detectors.

In particular embodiments, the path balancer may be constituted by a path length adjustment (path equalization or path matching) mechanism which may comprise a loop in at least one of a first or second optical path along which the respective first and second beams are arranged to propagate. The loop may be adjustable so that the length of the at least one of the first or second optical paths can be altered to match the other of the first or second optical paths. The loop may be open or closed (i.e. the loop may comprise a start position and an end position that substantially coincide or that are spaced apart).

The loop may be provided by one or more retroreflectors. The retroreflectors may be configured to reflect a beam of light orthogonally to a source of the beam. In a particular embodiment, two retroreflectors are provided in one or both of the first and second optical paths. The retroreflectors may be configured to form an Optical trombone' in which the path length of the loop can be varied by moving one of the retroreflectors with respect to the other. Moving one retroref lector towards another may shorten the path length while moving one retroreflector away from another may lengthen the path length.

One or more of the retroreflectors may be constituted by a corner prism. The corner prism may comprise two mutually perpendicular planar surfaces capable of bracketing a beam of light therebetween. At least one of the planar surfaces may be arranged at an angle of approximately 45° to the incident beam of light. The use of one or more corner prisms is advantageous since they can be introduced into at least one of the first or second optical paths without changing the optical alignment or coupling of the first or second beams (i.e. the angular alignment of the prisms may be independent of the beam angular alignment). Accordingly, the alignment of the first and second beams can be performed to achieve high coupling with the balanced detectors, prior to the inclusion of the path balancer. In these particular embodiments, the optical path lengths of the first and second beams can be adjusted to within the placement precision of the (or each) prism, which is likely to be of the order of 1 mm or so (which may correspond to be a fraction of a bit period).

A major advantage of the above embodiments is that the optical path length of the (or each) beam can be varied (i.e. tuned) during manufacture to ensure that each of the first and second optical paths has an appropriate optical length.

As mentioned above, in certain embodiments, a first and second reference signal may be provided so that the phase of the first and/or second beams can be compared to the phase of the reference signal in order to determine the symbol (i.e. bits) encoded on the optical signal. The coherent reference beam may be provided by a continuous wave (CW) laser configured to generate a reference signal. In certain embodiments, the reference signal may be split into a first and second reference beam to be compared, respectively, with the first and second beams of the incoming optical signal. Thus, a reference beam splitter may be provided to split the reference signal into the first and second reference beams.

The first and/or second detector may comprise a signal waveguide arranged to receive one of the first or second beams and/or a reference waveguide arranged to receive one of the first or second reference beams. The first and/or second detectors may further comprise an interference mechanism for combining the first or second beam with the respective first or second reference beam to determine a relative phase shift therebetween. A further path balancer may be provided to ensure that the first and/or second reference beams have paths that are of a desired (e.g. substantially equal) optical length. The further path balancer may be the same as that employed in the first and/or second optical paths or different thereto. It will, however, be understood that the further path balancer may not be required for normal operation of the demodulator but could be provided testing. Accordingly, the further path balancer may be included or removed as desired. The splitter may be configured such that the first beam will be orthogonally polarised with respect to the second beam.

Similarly, the reference beam splitter may be constituted by a polarising reference beam splitter such that the first reference beam has a first polarised state and the second reference beam has a second polarised state. The first reference beam may be orthogonally polarised with respect to the second reference beam.

A retarder may be provided in one of the first or second optical paths in order to alter the polarisation state of the beam. In a specific embodiment, the retarder is provided in the first optical path and is configured to alter the first polarised state of the first beam so that it is the same as the second polarised state of the second beam. In an alternative embodiment, the retarder is provided in the second optical path and is configured to alter the second polarised state of the second beam so that it is the same as the first polarised state of the first beam.

The path balancer may be configured to compensate for the inclusion of a retarder in one of the first or second optical paths so that the optical path lengths of the first and second beams remain substantially equal. According to a second aspect of the present invention there is provided an optical arrangement for a demodulator for a polarisation diverse optical signal, the demodulator comprising at least two balanced detectors, respectively configured to detect the difference between a first beam and a reference beam and a second beam and a reference beam, the optical arrangement comprising:

a polarising beam splitter arranged to spilt an incoming optical signal into a first beam having a first polarisation state and a second beam having a second polarisation state; and a path balancer configured to enable the first and second beams to have substantially equal optical lengths.

A signal input for the incoming optical signal may be disposed asymmetrically with respect to the balanced detectors.

According to a third aspect of the present invention there is provided a method of configuring an optical arrangement for a demodulator for a polarisation diverse optical signal comprising:

providing an optical arrangement in accordance with the second aspect of the present invention; and

employing the path balancer to ensure that the first and second beams have substantially equal optical lengths. As described above, the path balancer may be employed by active adjustment such as manipulation or operation (e.g. of a variable optical loop such as that comprising a so- called Optical trombone'). Thus, the present aspect of the invention provides a method which can be used for accurately ensuring that the first and second optical paths have substantially equal optical lengths and therefore the method helps to ensure that the first and second beams have comparable timing so that bits can be accurately decoded from the incoming signal in the correct order.

The method may be employed in a demodulator of the type known as a coherent demodulator and may be configured for polarisation diversity coherent detection. More specifically, the demodulator may be configured for polarisation diversity quadrature phase shift keying (QPSK) or even higher order phase shift keying such as that described in US7,546,041. It will be understood that the method may be employed in a demodulator adapted for differential phase shift keying (DPSK), differential quadrature phase shift keying (DQPSK) or higher order homodyne operation.

The method may further comprise the steps of coupling the first and/or second beams to a reference beam and tuning the phase and/or optical path lengths of at least one of the first, second or reference beams such that the first and/or second beam and the reference beam have comparable path lengths and optical timing.

The method may also facilitate biasing of the first and/or second beams during alignment of the optical components. The optional features described above in relation to the first aspect of the invention apply equally to the second and third aspects of the invention and vice versa. Brief Description of the Drawings

Some embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1A illustrates a coherent demodulator, in accordance with a first embodiment of the invention;

Figure 1 B illustrates an alternative coherent demodulator, in accordance with a second embodiment of the invention; and

Figure 2 illustrates a further coherent demodulator, in accordance with a third embodiment of the invention. Detailed Description of Certain Embodiments

With reference to Figure 1A, there is illustrated a coherent demodulator, in accordance with a first embodiment of the invention, comprising an optical arrangement 10. The optical arrangement 10 comprises a polarising beam splitter 12 in the form of a cubic prism made from two corner prisms 14, 16 arranged back-to-back. The beam splitter 12 is arranged to split an incoming signal 18 (sent from a signal source to the signal input 20) into a first beam 22 having a first polarised state and a second beam 24 having a second polarised state, which is orthogonal to the first polarised state. It will be understood that, in practice, the signal source will be located remotely from the optical arrangement 10 and the signal 18 will likely travel from the source to the optical arrangement 10 via an optical fibre (not shown).

The first beam 22 is essentially arranged to propagate straight through the beam splitter 12 and along a first optical path 25 that proceeds into a first balanced optical hybrid detector 28. The optical hybrid device 28 will be described in more detail below.

The second beam 24 is reflected by the beam splitter 12 by 90° with respect to the first beam 22. The second beam 24 is arranged to propagate along a second optical path 30 which passes through a retarder in the form of a half wave plate 32 before being reflected by a reflecting prism 34 so that the second beam 24 is parallel with the first beam 22 before it proceeds into a second balanced optical hybrid detector 38. As above, the second detector 38 will be described in more detail below. The half wave plate 32 is aligned within the second optical path 30 such that it rotates the polarisation state of the second beam 24 back to the same polarisation state of the first beam 22.

The optical arrangement 10 further comprises a path length adjustment (balancer) mechanism 40, which, in this embodiment is constituted by an optical trombone loop formed by a first retroreflective corner prism 42 provided in the first optical path 22 (between the beam splitter 12 and the first optical hybrid detector 28) and a second retroreflective corner prism 44 provided a distance from the first prism 42. The first and second prisms 42, 44 are triangular in plan view and comprise two mutually perpendicular planar surfaces 46, 48 and 46', 48' capable of bracketing a beam of light therebetween. The first prism 42 is orientated within the first optical path so that the external portion of the planar surface 46 is provided at an angle of approximately 45° to the first beam 22, to direct the first beam 22 in a direction substantially parallel to the second beam 24 as it exits the beam splitter 12. The second prism 44 is provided in the same orientation as the first prism 42 such that the first beam 22 is reflected from the internal portion of the planar surface 46' onto the internal portion of the planar surface 48' and back towards the first prism 42. The first beam 22 is then reflected from the external portion of the planar surface 48 of the first prism 42 towards the first optical hybrid detector 28. In this particular embodiment, the distance of the second prism 44 from the first prism 42 is variable so that the optical path length of the first beam 22 can be adjusted to substantially match the optical path length of the second beam 24. This may be done by taking absolute measurements of the first and second path lengths and adjusting the position of the second prism 44 so that the path lengths are equal to within an accuracy configured to suit the required symbol rate (e.g. within an accuracy of less than 1 mm).

It will also be noted that with the present embodiment of the invention, the alignment of the first and second beams 22, 24 can be performed so as to achieve high coupling with the respective balanced detectors 28, 38, prior to the inclusion of the path balancer 40. In addition, the optical path length of the first beam 22 can be tuned during manufacture so as to ensure that each of the first and second optical paths 25, 30 has a comparable optical length. Also included in the optical arrangement 10 of Figure 1A is a reference signal 50 which is provided for comparison with the incoming signal 18. A duplicate set of components (indicated by dashed reference numerals) are provided for splitting the reference signal 50 in a similar manner to the incoming signal 18.

The reference signal 50 is introduced to the optical arrangement 10 by reference source (or input) 52, which is linearly polarised in a horizontal direction to match the polarisation state of the first and second beams 25 and 30 as they reach the detectors 28, 38. However, it will be noted that, in practice, the reference source (e.g. a CW laser) may be located remotely from the optical arrangement 10 and may be introduced via an optical fibre, for example. The reference signal 50 is split by a non-polarising beam splitter 12' into a first reference beam 22' and a second reference beam 24' having the same polarisation state.

The first reference beam 22' is essentially arranged to propagate straight through the reference beam splitter 12' and along a first optical path 25' that proceeds into the second optical hybrid detector 38.

The second reference beam 24' is reflected by the beam splitter 12' by 90° with respect to the first reference beam 22'. The second reference beam 24' is arranged to propagate along a second optical path 30' which is reflected by a reflecting prism 34' so that the second reference beam 24' is parallel with the first reference beam 22' before it proceeds into the first optical hybrid detector 28.

In this embodiment, no retarder is provided in the either of the first or second reference beams 22', 24'. Furthermore, no path balancer is provided in either in the either of the first or second reference beams 22', 24', although this could be provided in other embodiments of the invention.

As explained above, the first and second balanced optical hybrid detectors 28, 38 each receive a portion (½E _S ) of the input signal 18 and a portion (½E ₀ ) of the reference signal 50. In each case, the signals ½E _S and ½E ₀ are combined in an optical hybrid interferometer 54 (all implemented on a planar waveguide interferometer chip in the present embodiment, although other optical mixers could be employed). The optical output in each case is a set of four signals: ½(E _S +E ₀ ); ½(E _S -E ₀ ); ½(E _s +jE ₀ ); and ½(E _S - jE ₀ ), where the portion of the input signal 18 has been combined with the four quadrature states associated with the portion of the reference signal 50 in complex- field space. The first two of the optical output signals are input to a first balanced photo-detector 56, the electronic output from which is then passed through an amplifier 58 before being relayed to a suitable digital signal processor (DSP) to decode the electronic signals. Similarly, the second two of the optical output signals are input to a second balanced photo-detector 60, the electronic output from which is then passed through an amplifier 62 before being relayed to the DSP. It should be noted that the output from both of the optical hybrid detectors 28, 38 is required to decode the signal 18 and therefore together they can be considered as a convertor configured to generate electrical signals representative of bits encoded on the optical signal 18.

Figure 1 B illustrates an alternative coherent demodulator, in accordance with a second embodiment of the invention, comprising an optical arrangement 70. The optical arrangement 70 is similar to that shown in Figure 1A except that the reference signal 72 is configured to travel along different paths to the first and second optical hybrid detectors 28, 38. Accordingly, like referenced numerals will be employed where appropriate.

As above, the reference signal 72 is introduced to the optical arrangement 70 by reference source 52, with the polarisation state at 45° to the horizontal. However, the reference signal 72 in the present embodiment is first reflected by 90°, towards the input signal 18, by a reflecting prism 74 before the reference signal 72 is split by a polarising reference beam splitter 76 into a first reference beam 78 having a first polarised state equal to that of 24 and a second reference beam 80 having a second polarised state equal to that of 25, which is orthogonal to the first polarised state.

The first reference beam 78 is reflected by the beam splitter 76 by a further 90° so that it proceeds along a first optical path 82 into the second balanced optical hybrid detector 38.

The second reference beam 80 is arranged to propagate straight through the reference beam splitter 76 before being reflected by a further reflecting prism 84 so that the second reference beam 80 is essentially parallel with the first reference beam 82 before it proceeds into the first balanced optical hybrid detector 28.

Thus, although the optical components are arranged slightly differently in the embodiment of Figure 1 B, the first and second balanced optical hybrid detectors 28, 38 still receive the same respective portions (½E _S and ½E ₀ ) of the input signal 18 and reference signal 72 and so the bits are determined in the same manner as described above. Figure 2 shows a coherent demodulator in accordance with a further embodiment of the invention, comprising an optical arrangement 90 which is similar to that shown in Figure 1A, but wherein the optical arrangement 90 is illustrated in the reverse orientation to that shown in Figures 1A and 1 B. The main differences between the optical arrangement 10 of Figure 1A and that incorporated into Figure 2 are that the path balancer 40 is orientated in the opposite direction so that the loop in the first beam 22 is directed away from the second beam 24, the reflecting prism 34 is incorporated into an elongate beam splitter 12, the retarder 32 is provided after the reflecting prism 34 and lenses 92 are provided at each of the optical inputs to the first and second optical hybrid detectors 28, 38. Thus, as above, it is possible to vary the distance of the second prism 44 from the first prism 42 so that the optical path length of the first beam 22 can be equalled to the optical path length of the second beam 24. A reference path length adjustment (balancer) mechanism 40' is also provided in the first reference path 25' and, as above, is constituted by an optical trombone loop formed by a first retroreflective corner prism 42' and a second retroreflective corner prism 44' provided at a variable distance from the first prism 42'. Accordingly, the optical path length of the first reference beam 22' can be adjusted by varying the distance of the second prism 44' so as to equal the path length of the second reference beam 24'. However, in other embodiments, the reference path length adjustment mechanism 40' may not be required.

As for the input signal 18, the reference path length adjustment mechanism 40' is orientated so that the loop in the first reference beam 22' is directed away from the second reference beam 24'. In addition, the reflecting prism 34' (in the second optical path 30') is incorporated into an elongate beam splitter 12'.

It will be noted that as a result of the above, the first optical hybrid detector 28 receives the first beam 22 and the second reference beam 24' and the second optical hybrid detector 38 receives the second beam 24 and the first reference beam 22'. In other embodiments, the reference beams 22', 24' may be arranged to be received at the opposite detector to that shown. Although not shown in Figure 2, each of the first and second detectors 28, 38 are as described above in relation to Figure 1A and therefore comprise a waveguide interferometer configured to combine the two signals they receive (i.e. the first or second beams plus a reference signal) to determine the difference there-between so as to decode the bits encoded on the input signal 18.

During the manufacture of a coherent demodulator in accordance with an embodiment of the present invention, the path length adjustment (balancer) mechanism 40 may be employed to ensure that the first and second beams 22, 24 have substantially equal path lengths so that the two components of the incoming signal 18 can be demodulated with respect to a reference signal 50, 72 so as to generate the correct sequence of bits encoded on the incoming signal 18. It will be understood that the step of equalising the optical path lengths during the configuration and/or assembly of the optical arrangement 10, 70, 90 may be carried out by wire bonding free external connectors (i.e. pins) to a suitable power supply and monitor so that the results of the interference between the waveguides in each of the first and second balanced detectors 28, 38 can be assessed and appropriate adjustments made to the position of the second retroreflective corner prism 44. These pins may then be disconnected after the components have been set is in their desired positions.

It will be understood that the various embodiments described above can be utilised in order to provide an efficient and accurate optical arrangement for a coherent demodulator and that the features described above in relation to one embodiment could be incorporated into other embodiments.

It will also be appreciated by persons skilled in the art that various modifications may be made to the above embodiments without departing from the scope of the present invention.