WO2009062237A1 - In-band osnr monitor with pmd insensitivity - Google Patents

Abstract

An optical signal is processed so as to isolate at least one signal impairment for measurement, by exploiting whether or not signal elements arising from a given impairment exhibit statistical dependence between polarised signal components. The optical signal is split to produce first and second optical signal components which are distinctly polarised, e.g. orthogonally polarised. The first and second optical signal components are separately converted to the electrical domain to produce first and second electrical signal components. The first and second electrical signal components are processed to separate correlated signal impairment elements, such as PMD impairments, which are correlated between the first and second electrical signal components from uncorrelated signal impairment elements, such as ASE noise, which are uncorrelated between the first and second electrical signal components.

Description

"In-band OSNR monitor with PMD insensitivity"

Cross-Reference to Related Applications

The present application claims priority from Australian Provisional Patent Application No 2007906201 filed on 12 November 2007, the content of which is incorporated herein by reference.

Technical Field

The present invention relates to measurement of signal impairments in an optical network, and in particular relates to in-band measurement of the signal to noise ratio in the presence of polarisation mode dispersion.

Background of the Invention

In order to monitor the performance of wavelength division multiplexed (WDM) optical networks it is useful to have a measurement of the signal to noise ratio (SNR) of each wavelength channel at various points throughout the network. In networks that are predominantly degraded by amplified spontaneous emission (ASE) noise the SNR is correlated with the optical signal to noise ratio (OSNR). The optical signal to noise ratio (OSNR) is a key physical layer performance measure of optical communication systems.

The OSNR has traditionally been measured with an optical spectrum analyser (OSA). The in-band OSNR can be estimated by interpolating ASE noise floor measured at points between adjacent wavelength channels. However, the OSA method fails for WDM systems with high spectral efficiency, in which the modulation sidebands between closely spaced channels mask the true OSNR level, and reduction of the resolution bandwidth of the OSA will be of no benefit. The OSA method also fails in reconfigurable networks such as high speed reconfigurable optical add drop multiplexer (ROADM) based systems where different channels may traverse through different optical paths, and in which the effects of optical filtering and the broad overlapping signal sidebands make traditional out-of-band approximations inapplicable. It is therefore preferable to obtain an in-band measure of OSNR by directly measuring the noise within the signal bandwidth. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Summary of the Invention According to a first aspect, the present invention provides a method of isolating at least one impairment of an optical signal, the method comprising: splitting the optical signal to produce a first optical signal component and a second optical signal component of distinct polarisation to the first optical signal component; converting the first optical signal component to the electrical domain to produce a first electrical signal component; converting the second optical signal component to the electrical domain to produce a second electrical signal component; and electrically processing the first and second electrical signal components to separate correlated signal impairment elements which are correlated between the first and second electrical signal components from uncorrelated signal impairment elements which are uncorrelated between the first and second electrical signal components.

According to a second aspect, the present invention provides a device for isolating at least one impairment of an optical signal, the device comprising: a polarisation beam splitter for splitting the optical signal to produce a first optical signal component and a second optical signal component of distinct polarisation to the first optical signal component; a first photodetector for converting the first optical signal component to the electrical domain to produce a first electrical signal component; a second photodetector for converting the second optical signal component to the electrical domain to produce a second electrical signal component; and a signal processor for processing the first and second electrical signal components to separate correlated signal impairment elements which are correlated between the first and second electrical signal components from uncorrelated signal impairment elements which are uncorrelated between the first and second electrical signal components.

According to a third aspect the present invention provides a computer program product comprising computer program code means to make a computer execute a procedure for isolating at least one impairment of an optical signal, the computer program product comprising: computer program code means for causing splitting of the optical signal to produce a first optical signal component and a second optical signal component of distinct polarisation to the first optical signal component; computer program code means for causing conversion of the first optical signal component to the electrical domain to produce a first electrical signal component; computer program code means for causing conversion of the second optical signal component to the electrical domain to produce a second electrical signal component; and computer program code means for processing the first and second electrical signal components to separate correlated signal impairment elements which are correlated between the first and second electrical signal components from uncorrelated signal impairment elements which are uncorrelated between the first and second electrical signal components. The present invention thus recognises that different signal impairments can be separated and isolated by exploiting whether or not signal elements arising from a given impairment exhibit statistical dependence between polarised signal components. Impairments causing signal elements to arise which exhibit such statistical dependence can be cancelled out to allow direct measurement of uncorrelated (statistically independent) signal elements, or vice versa. Herein, correlated signal elements include signal elements which are anti-correlated between the first and second electrical signal components, such as PMD-induced noise elements in the electrical signal components.

The first optical signal component and the second optical signal component are preferably orthogonally polarised.

In preferred embodiments, the electrical processing comprises: sampling the first and second electrical signal components to retrieve a sample set; retrieving a plurality of such sample sets over time; and assessing a distribution of the sample sets for indications of correlated signal components and uncorrelated signal components. In such embodiments, and in cases where the optical signal is modulated by differential phase-shift keying (DPSK) format, the assessing preferably comprises at least one of, and preferably all of: determining a ratio of average powers which we will term the (direct current) DC power ratio between the samples of the sample sets; determining a principal component angle of the distribution; and determining a deviation from an expected relation between DC power ratio and principal component angle. Such embodiments recognise that uncorrelated impairment-induced signal elements have a consistent relationship between DC power ratio and principal component angle, and that correlated impairment-induced signal elements introduce deviations to that relationship, from which deviations the correlated impairment-induced signal elements may be measured and/or cancelled. The assessing may additionally or alternatively comprise determining a variance of a minor principal component axis; and/or determining a beat noise variation with principal component angle. In preferred embodiments of the invention, a measure of an impairment having correlated signal elements, such as PMD, includes compensation for an input polarisation angle relative to the polarisation splitter. Such embodiments recognise that even when the impairment is present, negligible correlated signal elements may arise at certain input polarisation angles. Such embodiments may thus provide for polarisation control to align the input polarisation angle with the axis of a polarisation beam splitter and thus substantially remove such impairments, allowing for isolation of and direct measurement of impairments which produce uncorrelated signal elements in the first and second optical signal components. Alternative embodiments may provide for polarisation control to align the input polarisation angle at 45 degrees to the axis of a polarisation beam splitter and thus substantially maximise the presence of correlated signal elements in the first and second optical signal components so as to maximise sensitivity of measurement of underlying impairments. Polarisation control may be effected by active feedback, or by simply varying the input polarisation state, for example by a rotating quarter waveplate, and monitoring for minimum and/or maximum levels of correlated signal elements. Polarisation control may additionally or alternatively be effected by obtaining measurements at two or more distinct input polarisation states, or by passing the input optical signal to two separate polarisation beam splitters orientated at 45 degrees to each other and performing the present invention in respect of each polarisation beam splitter.

Embodiments of the invention may additionally or alternatively provide for isolation of impairments which induce uncorrelated signal elements from impairments which induce correlated signal elements, by optically filtering the optical signal prior to splitting in order to inhibit rapid phase changes during optical symbol transitions. Such embodiments recognise that sensitivity of the present invention to impairments such as PMD noise is related to a rate at which optical phase changes during symbol transitions, so that minimising this rate reduces sensitivity to differential group delay allowing isolation of other impairments such as OSNR. The filtering could be provided by an optical filter or a delay line interferometer. The correlated signal elements may arise from PMD. The uncorrelated signal elements may arise from ASE.

In alternative embodiments, correlated signal elements in the first and second optical signal components may be substantially cancelled by: equalising a power of the correlated signal element in the first electrical signal component with a power of the correlated signal element in the second electrical signal component; and subtracting the equalised first electrical signal component and second electrical signal component. Such embodiments may thus output only those signal components which are substantially uncorrelated between the first and second optical signal components. For example, such embodiments may substantially remove data and PMD-induced noise, which are correlated signal elements, while outputting ASE-induced noise, which is not correlated. The equalising may be effected by variable optical attenuators controlled by feedback from the electrical domain, and/or by passing the optical signal through a polarisation controller configured at substantially 45° to the input of the polarisation beam splitter, and/or by waiting for polarisation changes in the optical signal to cause equalisation from time to time for example by providing a rotating quarter wave plate prior to splitting, and/or by electrical domain signal processing.

Embodiments of the present invention may utilise some or all of the techniques set out in International Patent Publication No. WO2006/116802, the content of which is incorporated herein by reference

The present invention thus recognizes that a key requirement of an OSNR monitor is that it be accurate even in the presence of other impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD). Preferred embodiments of the invention may thus provide for measurement of OSNR in the presence of PMD, and/or may provide for PMD to be measured independently of OSNR. Brief Description of the Drawings

An example of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic of a signal processing technique for the OSNR monitor; Figure 2 shows a scatter plot of the two beam splitter outputs for a typical level of OSNR;

Figure 3 shows calibration curves of measured beat noise versus the ratio of DC powers in the two receivers for various input polarization states and levels of OSNR;

Figure 4 shows the OSNR determined from the beat noise and DC power measurements versus the measurement of OSNR with an optical spectrum analyzer, both in the presence and absence of PMD;

Figure 5 shows alternative calibration curves comprising plots of ASE beat noise vs. and angle of the major principal component;

Figure 6 illustrates a calibration curve of beat noise vs. the angle of the major principal component axis, for varying levels of OSNR, with and without PMD;

Figures 7a and 7b are plots of calibration curves of beat noise vs. DC power ratio for a 40 Gb/s signal, in the absence and presence of PMD, respectively;

Figures 8a and 8b are plots of intensity and phase for a 10 Gbit/s NRZ DPSK signal in the absence of PMD, while Figures 8c and 8d are plots of intensity and phase for a 10 Gbit/s NRZ DPSK signal in the presence of PMD;

Figures 9a and 9b show plots of beat noise versus AC angle with and without PMD respectively;

Figures 10a and 10b show experimental measurements of a polarizer output for a 10 Gbit/s A-DPSK signal, in the absence and presence of PMD, respectively; Figures 11a and l ib are plots of the DC power ratio vs. the angle of the principal component axis, in the presence and absence of PMD, respectively;

Figures 12a and 12b are plots, from slightly differing perspectives, of a three dimensional calibration curve for correcting OSNR measurements to allow for beat noise, DC power split ratio and the principal component angle; Figure 13 shows beat noise measurements before and after PMD compensation using signal processing; Figure 14a is a plot of the sampled outputs of the polarization beam splitter ports when PMD is not aligned with the PBS axis, and Figure 14b is a plot of the sampled outputs of the polarization beam splitter ports when PMD is aligned with the PBS axis;

Figures 15a and 15b are calibration curves of a 40 Gbit/s DPSK signal with an OSNR of 21 dB and 10 ps of PMD when subjected to 80 GHz optical filtering and 30 GHz optical filtering, respectively;

Figure 16 illustrates a general-purpose computing device that may be used in an exemplary system for implementing the invention.

Description of the Preferred Embodiments

A preferred implementation of a polarization based in-band optical signal to noise ratio (OSNR) monitor is shown in Figure 1. We note the absence of any polarization control. The wavelength channel is tapped from a through-line and input into a polarization beam splitter (PBS) and then detected by a pair of matched low speed receivers. The AC and DC receiver outputs are then fed to a signal processing module to determine the OSNR.

Figure 2 is a scatter plot or two dimensional histogram of the two beam splitter outputs for a typical level of OSNR. The beat noise is obtained from the variance of the projection of points along the minor principal component axis. A tapped WDM channel is fed into a polarization beam splitter and then detected by a pair of low speed matched detectors. The detector outputs are then sent to a digital signal processing module that removes any common signal component and measures the amplifier spontaneous emission (ASE) beat noise. We note that the beat noise will in general be a combination of signal-spontaneous (sg-sp) beat noise and spontaneous-spontaneous (sp-sp) beat noise. In the absence of polarization mode dispersion (PMD) the beat noise together with a measurement of the average (DC) power in each of the two channels gives an unambiguous measurement of the OSNR. The OSNR can be determined by applying principal component analysis to the two dimensional dataset. The ASE beat noise is obtained from the variance of the dataset along the minor principal component axis. In Figure 3 we show calibration curves of measured beat noise versus the ratio of DC powers (dB) in the two receivers, for various input polarization states and levels of OSNR. We note the characteristic peak at an equal DC power split between the two receivers, in this case the beat noise is dominated by signal-spontaneous (s-sp or sig- spon) beat noise. At larger (asymmetric) DC split ratios the beat noise diminishes and becomes dominated by spontaneous-spontaneous (sp-sp) beat noise.

Figure 4 shows the OSNR determined from the beat noise and DC power measurements, versus the measurement of OSNR with an optical spectrum analyzer, in blue. This comparison allows a calibration curve to be constructed to allow an OSNR monitor to efficiently identify OSNR. Figure 4 also plots, in red, in-band OSNR measurements for a 10 Gbit/s NRZ signal measured in the presence of 23 ps of 1st order PMD, showing that the effect of PMD is barely discernible in such measurements.

Another calibration curve can be obtained by plotting the ASE noise versus the angle θ of the principal component axis (as defined in Figure 2). Examples of these calibration curves are shown in Figure 5. Whilst this approach avoids the need for separate DC power measurements the present invention recognises that it has increased sensitivity to PMD.

The effect of PMD on this in-band OSNR technique is now discussed in more detail. The in-band OSNR technique set out in figure 1 exhibits PMD robustness for a 10 Gbit/s NRZ signal with a differential group delay (DFG) of 23 ps with no modification of the method described above. These results are illustrated in the calibration curves and OSNR measurements inFigure 1 and Figure 4 respectively. In particular, Figure 1 shows experimental calibration curves across various power splits and at differing levels of OSNR, for a 10 Gbit/s NRZ signal produced by a standard zero-chirp transmitter measured in the presence of 23 ps of 1st order PMD. As can be seen PMD at this level has little discernible effect on the NRZ signal OSNR measures. In general however, alternative signal formats do show PMD sensitivity. This is clearly illustrated in the scatter of the calibration curves and OSNR measurements shown in Figures 7a and 7b which illustrate PMD sensitivity of a 40 Gbit/s A-DPSK signal. In particular, Figures 7a and 7b are calibration curves of beat noise versus DC power ratio for a 40 Gbit/s signal, in the absence and presence of 15ps of DFG, respectively, showing the effects of PMD.

We now consider the origin of PMD induced noise. The PMD sensitivity arises from an interferometric mixing of principal states induced by the polarization beam splitter. Components of the fast and slow signal PMD states are simultaneously projected onto both axes of the polarization beam splitter (PBS). The polarizer then acts as a Mach Zehnder demodulator (interferometer) with a delay equal to the differential group delay (DFG).

The result for a pure phase modulated system is illustrated in Figure 8 in which the evolution of intensity (Fig 8 a) and phase (Fig 8b) for an idealized (simulated) 10 Gbit/s NRZ DPSK signal are shown. The PBS outputs in the presence of 40 ps of 1st order PMD (assuming 7.5 GHz receiver bandwidth and no ASE noise) are shown in Fig 8c We note that PMD noise manifests as sequence notches and impulses corresponding to bit transitions. The electrically filtered receiver outputs are shown in Fig 8d. Note the anti-correlated out put of the two receivers. Fig 8 thus illustrates the effects of PMD- induced interferometric mixing of principal states. The effect is to produce a sequence of impulses at the bit transitions as seen in Fig. 8b. The RF power and spectrum of this PMD noise depends upon the input polarization state but in general can extend down to DC. A critical feature is that the PMD induced fluctuations in the two arms of the beam splitter are perfectly anti-correlated as is seen in Figs. 8a & 8b. The anti- correlation is analogous to that seen in the outputs of an interferometer and can be interpreted as a consequence of conservation of energy in that the sum of the intensity in PBS outputs is equal to the input. The signal intensity in each arm of the beam splitter can be written as x_x = ax_s + n_mD(t) + n_{λ ASE}(t)

*2 = (1 - Φ, - ⁿPMD (0 + «2,ΛSE (0 where xs is the input signal, a is the fraction of signal power in each arm, np_MD is the PMD noise and ΠA_SE is the ASE beat noise in each arm. It is to be noted that the PMD noise is equal in magnitude and opposite in sign in each arm of the beam splitter. In contrast the ASE noise in the two arms is uncorrelated.

A clear demonstration of the anti correlated nature of the polarisation noise (PN), also referred to as npMo(t), can also be seen in the 10 Gbit/s A-DPSK measurements in Figure 10. In more detail, Figure 10 shows experimental measurements of polarizer output for 10 Gbit/s A-DPSK system with 0 ps of PMD (Fig 10a) and 23 ps of PMD (Fig 10b), respectively. This further illustrates the effects of PMD-induced interferometric mixing of principal states. Here we have used high bandwidth receivers (30 GHz) to clearly demonstrate both the impulsive and anti correlated nature of PN. We note that low bandwidth receivers used in the preferred inexpensive monitor configuration integrate over a large number of these impulses but nevertheless give a Gaussian output that remains anti-correlated in the two receivers.

We now give an example of how signal processing can be used to separate PN from ASE noise in an A- DPSK system. The technique exploits the fact that the increases (errors) in beat noise due to PN are also accompanied by a rotation of the principal components axes. As an example, Figure 9 shows scatter plots of beat noise vs. angle for an A-DPSK signal. Figure 9a shows the case with 0 PMD and Figure 9b with 10 ps PMD. We note the rotation of the principal axis to angles outside the range of 0-90 degrees in the PMD case of Figure 9b. This rotation arises both due to the anti- correlated nature of the PMD noise and due to the correlation between the PMD noise and the signal. In particular for a DPSK format optical signal the PMD noise can be approximated by . n_PMD = kx_s(t) where the factor A; is a function of the PMD and the alignment of the signal polarisation. Also evident is the corresponding increase in the variance of the minor principal component, which gives rise to OSNR errors in the un- modified OSNR technique. To illustrate the more general case we refer to Figure 11a and l ib in which is plotted the DC power ratio (dB) versus the angle θ of the major principal component for 200 cases for a 40 Gbit/s A- DPSK system with random input polarization states (for a fixed OSNR 20 dB). Figure l ib shows the effects of PMD, once again including some measurements outside the range of 0-90 degrees. We note the smooth monotonic relationship in Figure 1 Ia in the absence of PMD, in contrast to the scattered results in Figure l ib in the presence of PMD. The present invention recognizes that the deviations in Figure l ib from the monotonic 0 ps DFG curve of Figure 11a is a signature for PN noise which can be used to: (1) identify cases which have increased noise (ASE error) due to PMD; (2) remove the PN noise component so as to measure only the ASE beat noise component; and (3) measure the PMD. In the following we refer to the 0 PMD curve of Figure 1 Ia as the PMD calibration curve.

It is further noted that the PMD calibration curves have an OSNR dependency which is evident at low (poor) levels of OSNR. This dependency is due to increasing DC contributions of the ASE noise. Therefore, in order to identify PN we generalize the calibration to another dimension so as to include (1) the beat noise, (2) DC power split ratio and (3) the principal component angle (PC) as the additional dimension. An example of the higher dimensional calibration surface for a fixed OSNR is shown from two differing perspectives in Figures 12a and 12b for a 40 Gbit/s A-DPSK signal. The axes are beat noise, DC power ratio and principle component angle. Cases for 0 PMD are marked by dots, while cases for 15 ps PMD are marked with crosses. The distance of the crosses from the surface of dots gives a measurable characteristic for measuring PMD. Preferably, a calibration set is generated for determining OSNR from noise and signal measurements, the calibration data including measurements of beat noise, AC angle (principal component angle), DC power ratio and received average power for a range of OSNR and received powers.

An alternative approach to identifying the presence of PMD, which uses the anti- correlated property of the PMD noise but which does not rely on the assumption that the PMD noise is correlated to the signal, is to compare the variance of the sum of the received channel powers (VaT(X₁) + Var(x₂)) with the variance of the sum of the channels (Var (x]+χ₂)). Due to cancellation of the PMD noise in the latter but not in the former, the difference between these results is indicative of PMD.

As mentioned above, as well as identifying points subject to PN, it is also possible to improve the OSNR accuracy by compensating for the PMD noise as illustrated in Figure 13. The lower line of Figure 13 shows a low OSNR error in the absence of PMD, while the upper line shows substantially greater error in the presence of 10 ps of PMD if PMD compensation is not carried out. The central line shows substantially improved reduced error in OSNR measurement in the presence of PMD when the measure illustrated in Figure 11 and 12 is applied for PMD compensation. In this example, a measure of the deviation from the PMD calibration surface of Figure 12 was used to compensate for PMD.

The present embodiment thus provides for signal processing which allows the OSNR to be obtained, even in the presence of PMD, from measurements of the DC powers ratio, PC axes and the noise measured along the minor principal component. Importantly, the present invention recognizes that it is possible to exploit the differences in the statistical correlations of the PN noise (correlations between the orthogonal polarisation components of PN and the correlation of PN with the signal) with that of ASE noise (which is statistically independent- no correlations between the orthogonal polarisation components of ASE and no correlation of ASE noise with the signal )

It is noted that whilst the effects of PMD can be discriminated against with the technique of the presently described embodiment, it cannot unambiguously determine whether there is PMD in the signal, and can only give a lower bound of PMD. This is because PN is dependent upon the relative orientation of the principle states of the PMD with respect to the axes of the PBS. For the case where they are aligned there is no interferometric mixing, and hence there is negligible PN as illustrated in Figure 14. We note that the variance of the minor principal component still has a PMD component due to PMD induced time delay between receivers, but this is only significant at large RF frequencies and is negligible at MHz frequencies for realistic levels of DFG.

Therefore alternative embodiments may use polarization control to align the PSP with the axis of the beam splitter to afford an alternative means of negating the effects of

PMD, at the expense of a polarization control and possibly feedback control. The polarization control could be implemented with active feedback to ensure alignment or alternatively one could randomly scramble the input state and monitor minimum noise measure. It is noted that, instead of using a low speed receiver, use of a high speed receiver with this configuration would enable a measurement of the PN.

As stated previously PN noise is due to the interferometric mixing of fast and slow components of the signal. The sensitivity of the OSNR monitor to PN is related to the rate at which the optical phase changes during bit transitions: the slower the change, the less sensitive to DFG. One means of improving the performance is therefore to use an optical filter to inhibit the rapid phase changes during the bit transitions. The bandwidth of the filter will be less than signal line width. The benefits of an optical filter are illustrated in the improvement in the PMD robustness in a 40 Gb/s DPSK signal with 30 GHz optical filter, as shown in Figure 15. In general narrower bandwidths, for example 10 GHz on a 40 Gb/s increasingly improve the PMD tolerance but at a cost of decreasing optical power and sensitivity. Alternative embodiments may apply optical filtering in a manner to exacerbate PN, for example to improve PN measurements. We note that the optical filter could also be implemented as a delay line interferometer.

Preferred solutions may include a combination of optical filtering and signal processing.

It is further to be appreciated that in addition to PMD compensation when measuring ASE induced OSNR, the present invention may further provide for PMD measurement.

As noted in the preceding the measurement at a single input polarization state gives a lower bound on the PMD. The absence of PN does not reflect the absence of PMD since the PN depends upon the level of PMD and its orientation with respect to the PBS, as shown in Figure 14. Embodiments of the invention may therefore provide measurement at two (or more) input states, preferably rotated via a quarter waveplate oriented at 45 degrees. This will ensure that interferometric mixing will occur for one of the measurement states and that an unambiguous measure of the PMD can be obtained from the two measurements. An alternative but likely more expensive option is to have 4 simultaneous measurements from the output of two PBSs preferably orientated at 45 degrees to each other.

The present disclosure thus describes techniques that enable the OSNR monitor to measure OSNR in the presence of PMD and to alternatively or simultaneously measure PMD, relying on at least one of signal processing, optical filtering and polarization control. The depolarization induced by PMD may be interpreted as a result of the wavelength dependent state of polarization. That is, different optical frequency components of the signal have different states of polarization. Increasing the level of PMD increases the rate of polarization change with wavelength and hence decreases the degree of polarization of the signal. This invention recognizes that it is possible to use the statistical correlations of the PN to distinguish it from the uncorrelated ASE noise. We note that this requires detection of RF (non averaged) components of the signal.

Some portions of the detailed descriptions which follow are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as "processing" or "computing" or "calculating" or "determining" or "displaying" or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.

The present invention also relates to apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus.

The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will appear from the description below. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein.

A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium includes read only memory ("ROM"); random access memory ("RAM"); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc.

Turning to the drawings, wherein like reference numerals refer to like elements, the invention is illustrated as being implemented in a suitable computing environment. Although not required, the invention will be described in the general context of computer-executable instructions, such as program modules, being executed by a personal computer. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The invention may be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices.

The following description begins with a description of a general-purpose computing device that may be used in an exemplary system for implementing the invention, and the invention will be described in greater detail with reference to subsequent figures. Turning now to Figure 16, a general purpose computing device is shown in the form of a conventional personal computer 20, including a processing unit 21, a system memory 22, and a system bus 23 that couples various system components including the system memory to the processing unit 21. The system bus 23 may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes read only memory (ROM) 24 and random access memory (RAM) 25. A basic input/output system (BIOS) 26, containing the basic routines that help to transfer information between elements within the personal computer 20, such as during start-up, is stored in ROM 24. The personal computer 20 further includes a hard disk drive 27 for reading from and writing to a hard disk 60, a magnetic disk drive 28 for reading from or writing to a removable magnetic disk 29, and an optical disk drive 30 for reading from or writing to a removable optical disk 31 such as a CD ROM or other optical media.

The hard disk drive 27, magnetic disk drive 28, and optical disk drive 30 are connected to the system bus 23 by a hard disk drive interface 32, a magnetic disk drive interface 33, and an optical disk drive interface 34, respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the personal computer 20. Although the exemplary environment described herein employs a hard disk 60, a removable magnetic disk 29, and a removable optical disk 31, it will be appreciated by those skilled in the art that other types of computer readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories, read only memories, storage area networks, and the like may also be used in the exemplary operating environment.

A number of program modules may be stored on the hard disk 60, magnetic disk 29, optical disk 31, ROM 24 or RAM 25, including an operating system 35, one or more applications programs 36, other program modules 37, and program data 38. A user may enter commands and information into the personal computer 20 through input devices such as a keyboard 40 and a pointing device 42. Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit 21 through a serial port interface 46 that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB) or a network interface card. A monitor 47 or other type of display device is also connected to the system bus 23 via an interface, such as a video adapter 48. In addition to the monitor, personal computers typically include other peripheral output devices, not shown, such as speakers and printers.

The personal computer 20 may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer 49. The remote computer 49 may be another personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the personal computer 20, although only a memory storage device 50 has been illustrated in Figure 16. The logical connections depicted in Figure 16 include a local area network (LAN) 51 and a wide area network (WAN) 52. Such networking environments are commonplace in offices, enterprise- wide computer networks, intranets and, inter alia, the Internet.

When used in a LAN networking environment, the personal computer 20 is connected to the local network 51 through a network interface or adapter 53. When used in a WAN networking environment, the personal computer 20 typically includes a modem 54 or other means for establishing communications over the WAN 52. The modem 54, which may be internal or external, is connected to the system bus 23 via the serial port interface 46. In a networked environment, program modules depicted relative to the personal computer 20, or portions thereof, may be stored in the remote memory storage device. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used.

In the description that follows, the invention will be described with reference to acts and symbolic representations of operations that are performed by one or more computers, unless indicated otherwise. As such, it will be understood that such acts and operations, which are at times referred to as being computer-executed, include the manipulation by the processing unit of the computer of electrical signals representing data in a structured form. This manipulation transforms the data or maintains it at locations in the memory system of the computer, which reconfigures or otherwise alters the operation of the computer in a manner well understood by those skilled in the art. The data structures where data is maintained are physical locations of the memory that have particular properties defined by the format of the data. However, while the invention is being described in the foregoing context, it is not meant to be limiting as those of skill in the art will appreciate that various of the acts and operations described hereinafter may also be implemented in hardware.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope of the invention as broadly described. In particular it is to be appreciated that, except for techniques specified as being applicable only to DPSK, other techniques described here may be applied to other modulation formats. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A method of isolating at least one impairment of an optical signal, the method comprising: splitting the optical signal to produce a first optical signal component and a second optical signal component of distinct polarisation to the first optical signal component; converting the first optical signal component to the electrical domain to produce a first electrical signal component; converting the second optical signal component to the electrical domain to produce a second electrical signal component; and electrically processing the first and second electrical signal components to separate correlated signal impairment elements which are correlated between the first and second electrical signal components from uncorrelated signal impairment elements which are uncorrelated between the first and second electrical signal components.

2. The method of claim 1 wherein the processing comprises cancelling out correlated signal elements to improve measurement of uncorrelated signal elements.

3. The method of claim 1 or claim 2 wherein the processing comprises cancelling out uncorrelated signal elements to improve measurement of correlated signal elements.

4. The method of any one of claims 1 to 3 wherein the splitting produces substantially orthogonally polarised first and second optical signal components.

5. The method of any one of claims 1 to 4 wherein the electrical processing comprises: sampling the first and second electrical signal components to retrieve a sample set; retrieving a plurality of such sample sets over time; and assessing a distribution of the sample sets for indications of correlated signal components and uncorrelated signal components.

6. The method of claim 5 wherein the optical signal is modulated by a differential phase-shift keying (DPSK) format, and wherein the assessing comprises: determining a ratio of average powers (termed the DC power ratio) between the samples of the sample sets; determining a principal component angle of the distribution; and determining a deviation from an expected relation between DC power ratio and principal component angle.

7. The method of claim 5 or claim 6 wherein the assessing comprises determining a variance of a minor principal component axis of the distribution.

8. The method of any one of claims 5 to 7 wherein the assessing comprises determining a variation of beat noise with principal component angle.

9. The method of any one of claims 1 to 8 further comprising providing polarisation control to align the input polarisation angle with the axis of a polarisation beam splitter to isolate uncorrelated signal elements in the first and second optical signal components.

10. The method of any one of claims 1 to 8 further comprising providing polarisation control to align the input polarisation angle at 45 degrees to the axis of a polarisation beam splitter to maximise the presence of correlated signal elements in the first and second optical signal components.

11. The method of any one of claims 1 to 4 further comprising cancelling correlated signal elements in the first and second optical signal components by: equalising a power of the correlated signal element in the first electrical signal component with a power of the correlated signal element in the second electrical signal component; and subtracting the equalised first electrical signal component and second electrical signal component.

12. The method of claim 11 wherein the equalising is effected by variable optical attenuators controlled by feedback from the electrical domain.

13. The method of claim 11 or claim 12 wherein the equalising is effected by passing the optical signal through a polarisation controller configured at substantially 45° to the input of the polarisation beam splitter.

14. The method of any one of claims 11 to 13 wherein the equalising is effected by electrical domain signal processing.

15. The method of any one of claims 1 to 14 further comprising compensating a measure of an impairment having correlated signal elements for an input polarisation angle relative to the polarisation splitter.

16. The method of any one of claims 1 to 15 further comprising optically filtering the optical signal prior to splitting

17. The method of any one of claims 1 to 16 wherein the correlated signal impairment elements arise from PMD and wherein the uncorrelated signal impairment elements arise from ASE.

18. A device for isolating at least one impairment of an optical signal, the device comprising: a polarisation beam splitter for splitting the optical signal to produce a first optical signal component and a second optical signal component of distinct polarisation to the first optical signal component; a first photodetector for converting the first optical signal component to the electrical domain to produce a first electrical signal component; a second photodetector for converting the second optical signal component to the electrical domain to produce a second electrical signal component; and a signal processor for processing the first and second electrical signal components to separate correlated signal impairment elements which are correlated between the first and second electrical signal components from uncorrelated signal impairment elements which are uncorrelated between the first and second electrical signal components.

19. A computer program product comprising computer program code means to make a computer execute a procedure for isolating at least one impairment of an optical signal, the computer program product comprising: computer program code means for causing splitting of the optical signal to produce a first optical signal component and a second optical signal component of distinct polarisation to the first optical signal component; computer program code means for causing conversion of the first optical signal component to the electrical domain to produce a first electrical signal component; computer program code means for causing conversion of the second optical signal component to the electrical domain to produce a second electrical signal component; and computer program code means for processing the first and second electrical signal components to separate correlated signal impairment elements which are correlated between the first and second electrical signal components from uncorrelated signal impairment elements which are uncorrelated between the first and second electrical signal components.