Adjusting measurement values of signals

Abstract

Methods and apparatus for adjusting at least one measurement value associated with a signal are disclosed. An example method includes determining, by processing, at least one measurement value from an output signal wherein the output signal is derived from an electrical signal which is representative of the optical signal, and is amplified using an amplifier. A first value indicative of at least one of a measure of optical power of the optical signal or a level of the output signal may be determined. When the first value is indicative that the saturation condition of the amplifier is met, a measure of optical power of the optical signal is used to determine a correction factor. When the first value is indicative that a saturation condition of the amplifier is not met, it is determined that the correction factor corresponds to a configured value. The method further comprises adjusting the at least one measurement value using the determined correction factor.

Description

ADJUSTING MEASUREMENT VALUES OF SIGNALSTECHNICAL FIELD

[0001] The present disclosure relates to adjustment of at least one measurement value of a signal and in particular to methods, apparatus and machine readable medium encoding instructions to adjust measurement values associated with signals.BACKGROUND

[0002] Optical signals may be used for communication and sensing. In some cases, amplification of a signal is performed during the processing thereof (in some examples after the optical signal has been converted into an electrical signal), for example to provide current to voltage conversion and / or to boost a signal, and accurate measurements are desirable. However, saturation of the amplifier can cause inaccuracy.

[0003] For example, optical signals may be used to assess characteristics of an optical fiber by measuring a round-trip travel time of an optical pulse reflected and / or backscattered within the fiber. The optical pulse may be backscattered from scattering sites due to imperfections in the fiber and / or may be reflected from points along the fiber, which may result from, for example, an optical fiber splice or even a breakage in the fiber.

[0004] Such methods may use optical time domain reflectometry (OTDR). This may for example comprise 'standard' OTDR, which utilizes relatively wide pulses, or correlation OTDR which utilizes coded sequences of relatively narrow pulses. Examples of correlation OTDR are described in WO2023031439A1, which is incorporated herein by reference. Time resolution of below 1 ns has been demonstrated by correlation OTDR, which allows precise measurement of fiber latency characteristics of optical fibers and supports fiber latency asymmetry calibration, which are in turn both used in communication techniques such as 5G Radio Area Network (RAN) applications where nanosecond accuracy is desired.

[0005] Whilst OTDR techniques described above can achieve sub-ns resolution, there can be issues in accurately measuring characteristics of optical signals. This is due to a number of reasons. Firstly, receivers (in some examples, transceivers) often convert the optical signal into an electrical signal, and the electrical signal is then amplified. This allows use of relatively low-cost receiver apparatus, in particular if amplifiers such as variable gain amplifiers are used. However, the amplifier can in some examples introduce inaccuracy. Secondly, some receivers are AC-coupled so the continuous contribution from Rayleigh backscattering is removed. This can obscure the real absolute amplitude of the returned signals, making it difficult to evaluate power levels accurately.

[0006] It may be noted that while OTDR provides a particular example of a situation where accurate measurements of optical power are useful, there are other examples of optical sensing applications which could benefit from the techniques described herein, which may include sensor applications with a high dynamic range of the expected signal.SUMMARY

[0007] Certain aspects and embodiments described herein may provide a simple solution for enabling adjustment of at least one value of a measurement made of an optical signal while reducing or obviating problems with existing solutions. For example, a trace representing a received signal (either directly or following processing thereof) may be adjusted to better reflect the level of optical power received.

[0008] According to a first aspect of the present disclosure, there is provided a method of adjusting at least one measurement value of an optical signal. The method comprises determining, by processing circuitry, the measurement value(s) from an output signal. The output signal is derived from an electrical signal, which in turn is representative of the optical signal, and is (or has been) amplified using an amplifier. The method comprises determining, by processing circuitry, a first value. In some examples, the first value is indicative of at least one of a measure of optical power of the optical signal. In other examples, the first value is indicative of a level of the output signal. The method further comprises determining, by the processing circuitry, if the first value is indicative that a saturation condition of the amplifier is met. The method further comprises, when the first value is indicative that a saturation condition of the amplifier is met, using, by the processing circuitry, a measurement of optical power of the optical signal represented by the electrical signal to determine a correction factor. The method further comprises, when the first value is indicative that a saturation condition of the amplifier is not met, determining, by the processing circuitry, that the correction factor corresponds to a configured value. The method further comprises adjusting the measurement value(s) using the determined correction factor.

[0009] In such examples, the output signal may be derived directly from measurement of the amplified electrical signal (for example, the output signal may comprise a conventional OTDR trace), or may be derived therefrom for example by cross correlating the amplified electrical signal and a numerical sequence used to encode transmitted optical signal from which the received optical signal is derived (e.g. the output signal may comprise a correlation OTDR trace). A measurement value may comprise any value from output signal. For example, a measurement value may comprise a voltage level indicative of a return from a specific portion of an optical fibre in an OTDR method.

[0010] According to a second aspect of the present disclosure, there is provided an optical receiver apparatus comprising a receiver and processing circuitry. The receiver comprises an optical power monitor, a photosensor and an amplifier. The receiver is configured to receive an optical signal and measure an optical power of the optical signal using the optical power monitor. In some examples, the optical power monitor may receive a portion of the optical signal while in other examples, it may receive a measure of the optical power once the signal has been converted to an electrical signal, for example as a photocurrent. The receiver is further configured to convert the optical signal to an electrical signal using the photosensor and amplify the electrical signal using the amplifier, and to derive an output signal from the amplified electrical signal. The processing circuitry is configured to determine a first value indicative of at least one of a value indicative of the level of the output signal or the measure of the optical power and when the first value is indicative that a saturation condition of the amplifier is not met, use a configured value to determine a correction factor. When the first value is indicative that a saturation condition of the amplifier is met, the processing circuitry is configured to use the measurement of optical power to determine the correction factor. The processing circuitry is further configured to adjust at least one value associated with the output signal using the correction factor.

[0011] According to a third aspect of the present disclosure, there is provided a machine readable medium encoding instructions for adjusting a value associated with a measurement of a signal. The instructions, when executed by a computer, cause the computer to determine a first value indicative of at least one of a level of an output signal derived from an electrical signal or a measure of optical power of an optical signal, wherein the electrical signal is representative of the optical signal, and is (or has been) amplified using an amplifier. The instructions further cause the computer to determine if the first value is indicative that asaturation condition of the amplifier is met. The instructions further cause the computer to, when the first value is indicative that a saturation condition of the amplifier is met, use a measurement of optical power of the optical signal represented by the electrical signal to determine a correction factor. The instructions further cause the computer to, when the value is indicative that a saturation condition of the amplifier is not met, determine that the correction factor corresponds to a configured value. The instructions further cause the computer to adjust at least one value associated with the output signal using the determined correction factor.

[0012] Certain embodiments of the present disclosure may provide one or more of the following technical benefits. Certain embodiments may allow an amplitude of an optical signal to be measured more accurately. Certain embodiments may allow accurate estimation of optical power levels of OTDR events using standard pluggable transceivers used for telecommunication. Certain embodiments may make use of readily available apparatus (e.g., standard pluggable transceivers) to provide an effective solution at a relatively low cost which allows for easy integration into existing systems.

[0013] This summary is not an extensive overview of all contemplated embodiments and is not intended to identify key or critical aspects or features of any or all embodiments or to delineate the scope of any or all embodiments. In that sense, other aspects and features will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying Figures.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Exemplary embodiments will be described in more detail with reference to the following Figures, in which:

[0015] FIG. 1 is a schematic diagram illustrating the relationship between current and output voltage of an amplifier according to an embodiment.

[0016] FIGs. 2 to 4 are flowcharts of methods of adjusting measurement value(s) of an optical signal according to embodiments.

[0017] FIG. 5 is a schematic diagram illustrating a receiver apparatus according to an embodiment.

[0018] FIG. 6A illustrates an output signal comprising a raw trace with unadjusted amplitude values according to an example.

[0019] FIG. 6B illustrates an adjusted trace using the values from Fig. 6A according to an example.

[0020] FIG. 7 illustrates an output signal comprising a raw trace with unadjusted amplitude values according to an example.

[0021] FIG 8. Illustrates an apparatus according to an example.DETAILED DESCRIPTION

[0022] The embodiments set forth below represent information to enable those skilled in the art to practice the embodiments. Upon reading the following description in light of the accompanying Figures, those skilled in the art will understand the concepts of the description and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the description.

[0023] In the following description, numerous specific details are set forth. However, it is understood that embodiments may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques have not been shown in detail in order not to obscure the understanding of the description. Those of ordinary skill in the art, with the included description, will be able to implement appropriate functionality without undue experimentation.

[0024] References in the specification to "one embodiment," "an embodiment," "an example embodiment," etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0025] As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes," and / or "including" when used herein, specify the presence of stated features, integers, steps, operations, elements,and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0026] As already discussed, existing solutions for providing measurement of optical signals, and in particular those with receivers which include amplifiers, may suffer from inaccuracies. Certain embodiments described herein may offer a solution to one or more of the problems highlighted herein.

[0027] Figure 1 is a schematic diagram illustrating the relationship between current and output voltage of an example amplifier. The relationship between an input current (which in examples herein is a photo-current, which is proportional to a power of a received optical signal) and an output voltage is linear for low values until a saturation point is reached, i.e. when the voltage is above the saturation voltage (Vsat) or the power is above the saturation power (Psat). For example, this may be the point at which an Automatic Gain Control (AGC) starts controlling the gain to keep the output voltage constant.

[0028] The saturation point may be a characteristic of the apparatus (or of the amplifier thereof). To put it another way, Vsat and Psat are generally static values associated with the particular amplifier. These values may for example be measured during calibration of the amplifier, for example by a manufacturer thereof, and may be stored in a memory associated with the amplifier. For example, the voltage and power values of the saturation point may be stored in a memory accessible to the receiver apparatus (e.g. a transceiver memory) or to processing circuitry associated therewith. The memory may for example comprise a read-only memory (ROM), and / or may comprise non-volatile memory. In a particular example, the transceiver memory may be EEPROM.

[0029] Considering as an example an application from the field of OTDR, and more particularly correlation OTDR, an optical signal representing a bit signal may be launched into an optical fiber and backscattered or reflected from within the optical fiber to be received at a receiver. A photosensor (e.g. a photodiode) may be used to convert the optical signal in to an electrical signal, which may in turn be converted by an amplifier to a voltage signal. This in turn may be converted to a digital signal by an analogue to digital converter, and processed by processing circuitry such as a FPGA and / or processors such as microprocessors. A crosscorrelation between the digital coded bit sequence and the measured signal may be performed in order to derive a raw 'trace' representative of the signal returned from each portion of the fiber. The values of Vsat and Psat may be considered as configured values. Thevalues may be stored separately as two values, or as a single value based on Psat and Vsat, e.g. a ratio of Psat and Vsat.

[0030] It may be noted that in 'conventional' OTDR, an output signal (or trace) derived from a measurement the electrical signal may comprise a time series of measurement values which form a series of peaks corresponding to reflections from reflections sites received over time, wherein the time at which the peak occurs is indicative of a distance to the reflection site(s). In a correlation OTDR, a similar trace is derived as an output of the cross-correlation process. However, the electrical signal as detected does not comprise a simple series of peaks relating to individual reflection sites. Rather, that signal is first digitized by means of an Analog-to- Digital converter as described above, and is sampled at the same rate as the transmitted sequence bit-rate, before the cross-correlation is carried out (for example using software or in an FPGA) to derive the trace. It will be appreciated that the derived trace is still indicative of a level of the electrical trace output by the amplifier.

[0031] Referring again to the graph of Figure 1, it may be noted that when the amplifier is in the saturated condition, it is not possible to directly invert the function to translate the cross correlated electrical values back to the corresponding optical power values. As a consequence, the digital signal representing the OTDR return does not indicate the correct amplitude of the reflection events.

[0032] While this could be addressed by ensuring that the optical power is low, this may be associated with a loss in range of the OTDR inspection of the fiber. Moreover, as described in greater detail below, it may be beneficial to operate such that reflected light is significantly higher in power than the light backscattered through Rayleigh scattering.

[0033] Figure 2 is a flowchart illustrating a method for adjusting at least one measurement value of an optical signal which may address this issue. The method is carried out by processing circuitry, which may be associated with receiver apparatus for an optical signal. In some examples, the processing circuitry may be provided as part of the receiver apparatus.

[0034] Prior to being processed, in some examples, the optical signal may be received at a receiver apparatus (which may be a transceiver, for example the original source of the optical signal). The received optical signal may be converted to an electrical signal (for example by a photodiode) and the optical powerthereof is measured. The electrical signal may be amplified by at least one amplifier, for example a transimpedance amplifier, and / or a linear amplifier. The resulting electrical signal is measured to determine an output signal. In some examples,deriving the output signal comprises measuring the electrical signal while in other examples it comprises processing the electrical signal, for example using cross correlation. An adjustment is applied to the value(s) of the output signal derived from the electrical signal in order to better represent the power of the received optical signal. The adjustment applied thereto depends on whether it is determined that the amplifier is operating in a linear region, in which the signal gain is consistent (i.e. the ramped portion of Figure 1), or has reached a saturation condition in which the signal gain is reduced, or zero (in some examples, by Automatic Gain Control (AGC)). This means that, when fully saturated, the input voltage and the output voltage may be the same.

[0035] At block 200, at least one measurement value is determined from an output signal. The output signal is derived from an electrical signal, and the electrical signal is representative of the optical signal, and has been amplified using an amplifier. The output signal may for example comprise a time series voltage signal representing the output of the amplifier, and the measurement value(s) may comprise a voltage value from this signal. In some examples, the output signal is derived by processing such a voltage signal, for example using cross correlation, to represent the returns from each portion of an optical fiber.

[0036] At block 202, a first value is determined. The first value is indicative of at least one of (i) a measure of optical power of a received optical signal or (ii) a level of the output signal (e.g. voltage output from an amplifier, or a processed version thereof). Either of these measures may be indicative of the amplifier being in a saturation condition (i.e. operating in a non-linear manner). For example, an input power level above a threshold may cause saturation of the amplifier, or, as the output voltage is limited to a maximum output voltage, an output signal indicative that the output voltage is at or approaching that limit is indicative that the amplifier is partially or fully saturated.

[0037] When the first value is a measure of optical power, this may be determined by summing the power of the optical signal returned for a single pulse (or for a series of pulses if correlation OTDR is used). In some examples, the optical power is determined by an optical power monitor which monitors an average optical power. For example, this may be associated with a time constant such that it integrates the optical power received over a time period (which may include the time over which a return signal is expected). The measure of optical power may update over time.

[0038] In some examples, the optical signal may be a trace of an optical signal received over time, for example an OTDR trace in which case the optical power may comprise the returned optical power associated with reflection events.

[0039] In some examples, the optical power may for example be determined at the point of conversion of the received optical signal into an electrical signal. For example the optical power may be determined based on the current output when the input signal is converted into electrical current (e.g. the photocurrent output by a photodiode). In other examples, the optical power may for example be measured by an optical power monitor, which may measure a known proportion of a received optical signal. In some examples of Correlation OTDR (for example as described in WO2023031439), a threshold may then be applied to this output.

[0040] When the first value is a measure of the level of an output signal, this may be an output voltage of the amplifier. In other examples, the level may be determined following processing thereof, such that the first value is derived from a processed output trace, e.g. as a result of a cross correlation analysis, as described in greater detail below. The output signal may be representative of voltage levels. In some such examples, where the output signal comprises a number of peaks, the voltage level associated with the peaks may be summed to determine the first value. In other examples, the voltage of a single peak may provide the measure of the level of an electrical signal, if the single peak exceeds the saturation voltage of the amplifier. This may be particularly effective if a single peak is dominant. In some examples, the largest n peaks may be summed, where n is an integer.

[0041] In OTDR, for example, the voltage of the signal output by the amplifier in response to a single pulse (or a series of pulses for OTDR) may be summed. In some examples, the peaks in the signal may be summed (for example, voltages above a threshold value may be summed). As mentioned above, in correlation OTDR, processing is used to compute a cross-correlation between a transmitted optical signal (which may comprise a digital coded bit sequence) and the returned optical signal in order to derive or compute an estimate of the return from each portion of the fiber, and this derived output signal may be used to determine the first value, e.g. by summing the peaks therein.

[0042] At block 204, it is determined if the first value is indicative that a saturation condition of an amplifier is met.

[0043] For example, if the first value is a level of an output signal which is indicative of a measured or derived voltage, this may comprise comparing the value to a value indicative ofa saturation voltage of the amplifier Vsat, which may be stored in a memory accessible to the processing circuitry carrying out the method. Comparing the value may comprise determining if the total amplitude of the detected events is below or above a receiver amplifier saturation voltage. In some examples, determining if the value is indicative that a saturation condition of the amplifier is met comprises comparing the value to a value which is less than the saturation voltage of the amplifier by a predetermined factor. In some examples, the value which is less than the saturation voltage of the amplifier by a predetermined factor may be calculated by dividing Vsat by a factor, e. Providing such a factor allows for a small error in the saved saturation voltage (which may be due to a lack of precision, e.g. a number of decimal places), while still ensuring that a saturation state is detected. In other examples, the first value may be indicative of a power output by the amplifier.

[0044] In further examples, if the first value is indicative of optical power, then this may be compared to a value indicative of a saturation power Psat of the amplifier. While a factor relating to a margin of error could also be considered for this measure, this may be less relevant in the power domain since there are a continuum of values within this range.

[0045] In some OTDR examples, or for similar applications, the amplifier may be configured such that the saturation power of the amplifier is above any expected Rayleigh backscattered power. The measured optical power may for example include Rayleigh backscattered power, but this power will not generally be received at the amplifier due to an intervening AC coupling. To ensure that the measured optical power and the value indicative of a saturation power of the amplifier are directly comparable, the saturation power may be set to be higher than the expected Rayleigh backscattered power by a reasonable margin. For example, if the backscattered Rayleigh power is expected to be in a region of -30 decibel-milliwatts (dBm), then a suitable amplifier may have a saturation power rating of -25dBm. Moreover, the Rayleigh back-scattered power Pbs may be estimated using the formula:

[0046] Pbs= Ptx - b*(l-exp(-2aL)) / 2a

[0047] Where b is the Rayleigh backscattering coefficient (typically 5e-8 1 / m), a is the attenuation coefficient (typically 0.2 dB / km), Ptx is the transceiver transmitted power and L is the fiber length (which may be known, or can be estimated directly from the OTDR trace). This value may then be subtracted from the optical power measurement to determine thepower from reflection sites. However, in some examples, this may only be performed if the estimated Rayleigh back-scattered power is significant relative to the Psat.

[0048] In some examples, and more generally, and as mentioned above, determining if the first value is indicative that a saturation condition of the amplifier is met may comprise determining if the amplifier is operating in a linear state in which a relationship between a photocurrent of the received optical power and the output voltage of the amplified electrical signal is linear, or a saturated state. In some examples, in the saturated state, Automatic Gain Control, AGC, controls the output voltage.

[0049] If it is determined in block 204 that the saturation condition is met, the method proceeds to block 206 and a measurement of optical power, Prx, is used to determine a correction factor, k. In some examples, in such cases, k=Prx / Vsum, where Vsum is the sum total amplitude of the events in the digital signal as identified using the amplifier (and / or estimated using correlation OTDR). In some aspects, Vsum is the total of the amplitudes of a voltage signal representing the reflections received by the OTDR.

[0050] In other examples, k=Prx / Vsat, where Vsat is the saturation voltage of the amplifier. It will be appreciated that, since the saturation condition is met, it may be expected in this case that Vsum and Vsat are approximately equal to one another. Moreover, it may be appreciated that photodiodes may provide a relatively more accurate measurement of received power above a certain threshold. As this state relates to a saturation condition, it is relatively likely that such a threshold is exceeded.

[0051] If it is determined in block 204 that the saturation condition is not met, the method proceeds to block 208 and a configured value is used as the correction factor. In some examples, the configured value is determined based on a ratio of a power and a voltage at which, or before, the amplifier becomes saturated. The configured value may for example be equal to Psat / Vsat, where Psat is the optical input power to a receiver at the saturation point of the amplifier and Vsat is the saturation voltage of the amplifier. Psat, Vsat and / or the ratio Psat / Vsat may be stored in a memory accessible to the processing circuitry carrying out the method. It will be appreciated that, since the behaviour below the saturation point is linear, this same value could be provided by any P / V ratio below the saturation point, and therefore Psat and Vsat may not be used explicitly in determining the correction factor k. In some aspects, the correction factor is applied after the OTDR cross-correlation is made between the received signal and the transmitted signal. This adjustment may therefore effectively performa conversion of an output signal comprising a voltage trace into a power trace representative of the optical power, wherein the conversion is calibrated to the particular apparatus in use.

[0052] In some examples, the correction factor when the saturation condition is not met is expressed as the inverse of an equivalent amplifier gain (AMP_eq_gain), wherein the correction factor, k, is equal to l / AMP_eq_gain. This configured value AMP_eq_gain may be predetermined as a characteristic of the apparatus and stored in a memory accessible to the processing circuitry. In examples herein, the amplifier is an electrical amplifier (e.g. a transimpedance amplifier (TIA)) with an electrical input and electrical output. The AMP_eq_gain value estimates an "equivalent" gain where the input is the optical power, which is equivalent to assuming an amplifier with an optical input and an electrical output which operates with a photodiode with unit responsivity.

[0053] The configured value is therefore a static value, being associated with attributes of, or inherent to, the apparatus (e.g. attributes of the amplifier) which may be expected not to change over time and / or may be stored in a memory thereof. The optical power measurement is however determined 'on the fly', in response to an optical signal being received.

[0054] At block 210, the at least one output signal value (and in some examples, the output signal as a whole) is adjusted using the correction factor. In some examples, this adjusted value, or adjusted output, may be output to a screen or as data for a user. In some examples, a number of additional correction factors may also be used. The additional factors may take into account at least one of the gain / loss of electrical elements within the receiver. In other examples, a factor of two may be applied, depending on whether the measured voltage is a peak to peak voltage or the absolute value for examples with only positive peak values.

[0055] In summary this method solves the amplitude calibration problem by utilizing two different values as correction values. The values used are reliable in the operating region where they are utilised. Viewed another way, the use of different correction factors may account for a state in which an applied gain is unknown (i.e. a saturated state).

[0056] Figure 3 is a flowchart illustrating a particular method for receiving an optical signal and adjusting at least one measurement value thereof, and may provide an example of a method as set out in Figure 2. At least part of the method is carried out by processing circuitry, which may be associated with receiver apparatus for an optical signal. In some examples, theprocessing circuitry may be provided as part of the receiver apparatus. In this example, the method is a method for performing Correlation OTDR.

[0057] The method comprises, at block 300, receiving an optical signal. In this example, the optical signal may be reflected light, for example light reflected from scattering points or reflection points within an optical fiber.

[0058] At block 302, an optical power of the optical signal, Prx, is measured. In this example, the optical signal is measured using an optical power meter (which may be a power monitor which receives part of the optical input, or which may receive a photocurrent directly or indirectly from a photo sensor which is used to convert the optical signal into an electrical signal in block 304) and used to determine the power level of the signal as a whole. As such, the same photodiode may be used for both optical power measurement and for converting the received optical signal to an electrical signal. In other examples, the optical power measurement is carried out by a different device, e.g. a separate photodiode.

[0059] At block 304, the optical signal is converted to an electrical signal. This may be achieved using a photodiode, for example an Avalanche PhotoDiode (APD).

[0060] At block 306, the electrical signal is amplified. This may be achieved using an amplifier such as one or more of a linear amplifier, a limiting amplifier and / or a transimpedance amplifier (TIA). Linear amplifiers may be effective in maintaining the analog nature of the optical signal. The amplification may provide a current to voltage conversion from the output of the photodetector. As photodetectors such as photodiodes may have a current response which is more linear than their voltage response, the current may be converted to a measurable voltage by the amplifier. Thus, the use of a photodiode in conjunction with an amplifier can result in a relatively low-cost apparatus which performs reliably over a range of input values.

[0061] At block 308, the output signal is derived and processed. This may comprise converting the amplified electrical signal to a digital signal using an analogue to digital converter and determining a cross correlation of the digital signal with a numerical sequence used to encode a transmitted optical signal from which the optical signal received in block 300 was derived. This allows an output signal representative of the returns from each portion of fibre to be derived, which may be represented as an output signal comprising a voltage trace mapping the distance to reflection sites within a fiber. However, in other examples, such processing need not be performed, and the output trace may comprise a measurement of theamplified electrical signal. At least one measurement value may be determined from the output signal. For example, the measurement value(s) may comprise a voltage value associated with the total returned optical power from a particular portion of the fiber.

[0062] At block 310, a value indicative of a voltage of the (amplified) electrical signal, Vsum is determined. As will be further set out below, this value may comprise a sum of a number of events, which in this example comprises the response of a fiber to the series of pulses input thereto. This may comprise the sum of peaks in the output signal, for example values which are above a threshold value.

[0063] At block 312, the value indicative of the voltage of the electrical signal is compared to a value indicative of a saturation voltage of the amplifier. In this example, this comprises comparing Vsum to Vsat / e, where e is a predetermined factor providing an error margin which ensures that all values of Vsum which may place the amplifier in the saturation condition are considered. In some examples, the use of e may be omitted.

[0064] Block 314 comprises determining if the saturation condition is met, or more particularly in this example if Vsum > Vsat / e.

[0065] If it is determined in block 314 that the saturation condition is not met (Vsum < Vsat / e), the method proceeds to block 316 and in this example, a ratio of Psat and Vsat is used as the correction factor k. This configured value may be static and / or predetermined, being associated with characteristics of the amplifier, and for example being stored in a memory which is accessible by processing circuitry carrying out the method of block 316.

[0066] If it is determined in block 314 that the saturation condition is met (Vsum > Vsat / e), the method proceeds to block 318, and a ratio of the measurement of optical power Prx, measured in block 302 and the measured voltage Vsum is used as the correction factor k. As noted above, in some examples, the saturation voltage of the amplifier Vsat could be used in place of Vsum, and in practice they should be similar to one another when the amplifier is in the saturated state, but as the value Vsum is available in any event, it may provide a more accurate reflection of the system in that instance. This correction factor is therefore responsive to the amount of optical power entering the system at that time, and will vary depending on the amount of optical power received.

[0067] In summary then, in this example:If Vsum < Vsat / e, the saturation condition is not met and k = Psat / VsatIf Vsum > Vsat / e, the saturation condition is met and k = Prx / Vsum

[0068] As noted above, in other examples, when the saturation condition is met, k could instead be determined as Prx / Vsat, and / or when the saturation condition is not met, k could be determined as l / AMP_eq_gain.

[0069] At block 320, measurement value(s) of the output signal is or are adjusted using the correction factor, k, suitable for the determined state. For example, this may comprise scaling the voltage(s) associated with peak(s) in the output signal, and / or multiplying effectively multiplying an output signal as a whole (i.e. all the values making up the output signal) by the correction factor.

[0070] It may be noted that cross correlation is a linear process, and therefore this step could be performed after the output signal has been adjusted. In such an example, the voltage could be estimated as a sum of voltage measurements of the amplified electrical trace.

[0071] Figure 4 is a flowchart illustrating a particular method for adjusting at least one measurement value of an optical signal, and may provide an example of a method as set out in Figure 2. The method is carried out by processing circuitry, which may be associated with receiver apparatus for an optical signal. In some examples, the processing circuitry may be provided as part of the receiver apparatus.

[0072] The method comprises, at block 402, determining an optical power, Prx, of the optical signal, as described above.

[0073] At block 404, the optical power, Prx, is compared to the saturation power, Psat. In some examples, an estimated power associated with Rayleigh backscatter Pbs is subtracted from Prx priorto performingthe comparison. However, in this example, Pbs is small compared to Psat, so this may be ignored.

[0074] At block 406, it is determined whether Prx > Psat, indicating that the saturation condition is met.

[0075] If the saturation condition is met, the method proceeds to block 408, and optical power, Prx, is used to determine a correction factor. For example, the correction factor may be Prx / Vsat, or Prx / Vsum.

[0076] If the saturation condition is not met, a configured value is used as a correction factor, as shown in block 410. The correction factor k may comprise Psat / Vsat or l / AMP_eq_gain in such examples.

[0077] In summary then, in this example:If Prx < Psat, the saturation condition is not met and k = Psat / Vsat or l / AMP_eq_gainIf Prx > Psat, the saturation condition is met and k = Prx / Vsat or Prx / Vsum

[0078] At block 412, the output signal measurement value(s) is / are adjusted using the correction factor. The output signal may be obtained as set out above.

[0079] Figure 5 is a schematic diagram illustrating receiver apparatus 500. In this example, the receiver apparatus comprises a digital Optical Time Doman Reflectometry (OTDR) receiver, although in other examples, the apparatus 500 may be configured for different types of sensing and / or may be a transceiver with a transmission facility. In particular examples, the apparatus 500 may be a standard receiver / transceiver such as a small form factor pluggable (SFP) module.

[0080] The receiver apparatus 500 is configured to receive an optical signal 502 and comprises an optical power monitor 504 configured to measure an optical power of the optical signal 502. In an example the optical power monitor 504 receives a photocurrent directly or indirectly from the photosensor 506 described below. In some examples, the photocurrent is received by the optical power monitor 504 via a pin within the amplifier 510, which is also described below.

[0081] The receiver apparatus 500 further comprises a photosensor 506. The photosensor 506 may for example be a light sensor such as a photodiode, for example, an avalanche photodiode. In use of the apparatus 500, the photosensor 506 converts the optical signal 502 into an electrical signal 508. In some examples, the photosensor may have a high responsivity and / or a low noise rating.

[0082] The receiver apparatus 500 further comprises an amplifier 510 which is configured to amplify the electrical signal 508. The amplifier may act as a current to voltage converter, and / or may be configured to receive and to output an electrical signal. The amplifier 510 may comprise a transimpedance amplifier. In some examples, the amplifier comprises at least one of: a linear amplifier, a limiting amplifier, a variable gain amplifier and an output driver. In a particular example, the amplifier may comprise a linear Automatic Gain Control (AGC) amplifier, wherein the current may be provided to a transimpedance amplifier, and then to a variable gain amplifier before being passed to an output driver.

[0083] The receiver apparatus 500 further comprises processing circuitry 512 which is configured to derive an output signal from the amplified electrical signal. For example, this may comprise receiving a digital signal indicative of the output of the amplified electrical signal and in some examples performing processing such as cross correlation thereon. Theprocessing circuitry 512 is further configured to determine a first value indicative of at least one of a level of the output signal or the measure of optical power. A measure of optical power, Prx, may be provided by the optical power monitor 504 (shown as signal 514). A value indicative of the level of the output signal may comprise a sum value indicative of a total voltage of the amplified electrical signal over a period of time (Vsum), for example in response to a single pulse or a sequence of pulses in an OTDR signal. In some examples, this may be represented as a sum of peak voltages within an output signal representing a return trace in OTDR, or a trace derived therefrom, for example following a cross correlation operation.

[0084] The processing circuitry 512 is also configured to correct the measure of the amplified electrical signal, and to determine a correction factor k to use in correcting the signal, k is determined based on whether the first value indicates that the amplifier 510 is operating in a saturated condition or not. The processing circuitry 512 may for example compare the first value (e.g. the power or the voltage sum) to a threshold to determine if the amplifier 510 is operating in a saturated condition. When the amplifier 510 is not operating in a saturated condition, the processing circuitry 512 uses a configured value to determine a correction factor. For example, k may comprise a value of Psat / Vsat or l / AMP_eq_gain. Such values may be predetermined and / or static, being associated with intrinsic characteristics of the amplifier. The values may be stored in a memory, which may comprise a memory of the receiver apparatus 500.

[0085] When the value is indicative that the amplifier 510 is in a saturation condition, the measurement of optical power is used to determine the correction factor k. For example , k may comprise a value of Prx / Vsat or Prx / Vsum

[0086] The processing circuitry 512 is further configured to adjust values associated with the output signal using the correction factor k. This results in an adjusted output signal 516. For example, the processing circuitry 512 may comprise at least one processor.

[0087] Figure 6A is an example of an output signal, in this example an OTDR trace from which the locations of reflections along the optical fiber can be inferred. In this example, the OTDR trace was acquired from a 50 km point-to-point (P2P) fiber link. The trace is to be used to characterize the optical fiber, and the locations of reflections along the optical fiber are represented by peaks or spikes in the OTDR trace. In this example, the reflections are due to splices at connector junctions, and due to a terminal connector of the optical fiber.

[0088] In some examples, an optical fiber under test may be disconnected from its usual position within an optical network and connected to OTDR hardware for testing. Optical radiation is transmitted into the optical fiber and radiation backscattered and / or reflected from within the optical fiber is received at a photodetector of the OTDR hardware (e.g. a receiver apparatus, such as the apparatus 500 shown in Figure 5) and converted into an analogue electrical signal. ADC is then typically employed to convert the analogue electrical signal into a digital format for further processing.

[0089] Figure 6A shows a raw trace with uncalibrated amplitude values according to an embodiment. The vertical axis shows a voltage measurement in decibels (dB). The measurements are negative as each peak represents less than IV.

[0090] In this example, reflection events can be seen with amplitudes of -15.5dB (a reflection at the interface with the fiber), -41.5dB (from a first fiber splice), -50dB (from a second fiber splice) and -14.2dB from a terminal reflection.

[0091] The peaks are identified as values above a threshold. In some examples, other criteria may be used to identify a peak, such as analysis of the width or a shape of the peak, use of statistical functions such as Mean Square Error (MSE) or use of models, which may be models trained using Machine Learning techniques. The threshold may for example be a threshold above an expected noise signal, or in some examples an expected backscatter signal.

[0092] The values are converted from dB to volts and summed as follows:

[0093] Vsum = 10<15-5 / 10> + IQ(-415 / io)+ lo(-so / io)+ lo(-i4.2 / io)=Q.0663 V.

[0094] In this example, the saturation voltage of the amplifier, Vsat, is predetermined and stored in a memory which is accessible to processing circuitry carrying out the method.

[0095] Vsat = 10<-7 / 10> = 0.1995 V

[0096] Moreover, in this example a value e is set as 1.26 (which corresponds to 1 dB). This value may be chosen to be compatible with the level of accuracy with which the Vsat is stored, and is preferably small enough that the ratio remains close to Vsat.

[0097] To determine if the amplifier is operating in the saturated condition, Vsum is compared to Vsat / e = 0.1583 V

[0098] In this example, Vsum = 0.0663 V < Vsat / e = 0.1583 V, suggesting that the amplifier is operating in a linear (non- saturated) region.

[0099] Therefore, k may be assigned a value of Psat / Vsat.

[0100] k = Psat / Vsat = io(25 / 1°) / lO( 7 / lo)= 0.0158 (corresponding to - 18 dB)

[0101] The raw trace, in dBm, is therefore scaled by - 18 dB. In other words, each value in the trace is adjusted. The scaled trace is shown in Figure 6B.

[0102] A further example of an output signal is shown in Figure 7. In this second example a fiber of approximately 15km length has a single splice and an open connector at the end.

[0103] In this case, reflection events can be seen with amplitudes of -24dB (a reflection at the interface with the fiber), and -7.1dB (from the fiber splice). As there is an open connector, there is no significant end reflection. Therefore:

[0104] Vsum = 10<-7 1 / 10> + 10<-24 / 10> = 0.1990 V (-7 dB).

[0105] Vsat and e are as discussed above in relation to Figure 6A. Therefore, in this example, Vsum > Vsat / e and the amplifier is operating in a saturated condition.

[0106] The received optical power associated with the received optical signal can be read from a memory, and in this example is -18.4 dBm.

[0107] Therefore, the correction factor k is determined using the measured optical power as k = Prx / Vsat = io<18 4 / 10’ / 10<-7 / 10’ = 0.0724 V (-11.4 dB).

[0108] The raw trace, in dBm, should be scaled by - 11.4 dB

[0109] The measurement value associated with major peak then becomes = -7.1-11.4 = -18.5 dBm. This is a reasonable result since it is the only major peak, so its power corresponds to the majority of the received optical power. Moreover, it may be noted that a reflectance can be determined using:

[0110] Reflectance = -18.5 dBm - (2 dBm -2*15Km*0.2dB / Km) = -14.5 dB (where 0.2dB / Km is a typical value associated with losses and Rayleigh scattering. This is close to an open connector theoretical value, and thus shows that the scaled value provides a reasonable result.

[0111] Figure 8 shows an example of a computer readable medium 800 associated with a computer 802. The computer readable medium 800 stores instructions which, when executed by the computer 802, cause the computer 802 to determine a first value indicative of at least one of a measure of optical power of an optical signal or a level of an output signal derived from an electrical signal, wherein the electrical signal is representative of the optical signal, and is (i.e. has been) amplified using an amplifier.

[0112] The instructions, when executed by the computer 802, further cause the computer 802 to determine if the first value is indicative that a saturation condition of the amplifier is met. When the first value is indicative that a saturation condition of the amplifier is met, theinstructions cause the computer 802 to use a measurement of optical power of the optical signal to determine a correction factor. When the value is indicative that a saturation condition of the amplifier is not met, the instructions cause the computer 802 to determine that the correction factor corresponds to a configured value.

[0113] The instructions, when executed by the computer 802, further cause the computer 802 to adjust at least one output signal value using the determined correction factor.

[0114] In some examples, the first value is indicative of a voltage of the output signal. In such examples, the instructions to determine if the first value is indicative that a saturation condition of the amplifier is met comprise instructions to compare the first value to a value indicative of a saturation voltage of the amplifier.

[0115] In some examples, the first value is indicative of the power of the optical signal, and the instructions to determine if the first value is indicative that a saturation condition of the amplifier is met comprise instructions to compare the first value to a value indicative of a saturation power of the amplifier.

[0116] In some examples, when the first value is indicative that a saturation condition of the amplifier is met, the correction factor comprises a ratio of the measurement of optical power and a voltage value, wherein the voltage value comprises one of a measurement of the voltage of the electrical signal, and a voltage at which the amplifier becomes saturated.

[0117] The instructions 800 may further cause the computer to carry out any of the method steps set out above, and / or to control apparatus processing apparatus or receiver apparatus such as apparatus 500, to carry out such methods.

[0118] In some examples, the method is implemented using one or more processing circuitry and / or processors. For example, a first processing circuitry is configured to determine a value of the received optical power and / or store the configured values of the saturation voltage and power (Vsat and Psat), or one or more values indicating Vsat and Psat. In some examples, the configured values of Vsat and Psat are stored as a single value, e.g. a ratio of Vsat and Psat, or a value indicating separately or together the values of Vsat and Psat. For example, the first processing circuitry is implemented a non-volatile memory, e.g. an electrically erasable programmable read-only memory (EEPROM), e.g. integrated in a microcontroller. The photodiode may be a part of the first processing circuitry.

[0119] In some examples, a second processing circuitry is configured to cross-correlate the value of the received optical power with a transmitted signal, as part of a correlation OTDRprocess. In some examples, the received optical power is digitized using an analogue-to-digital converter, and the cross-correlation carried out by an integrated circuit (e.g. a Field Programmable Gate Array (FPGA). Alternatively, other types of circuit may be used, e.g. an Application-Specific Integrated Circuit (ASIC).

[0120] In some examples, the cross-correlated signal is processed in software operating on (or in combination with) any form of a third processing circuitry, e.g. an integrated circuit. The software, e.g. stored in a memory of the third processing circuitry, is configured to determine Vsum and / or Prx as described above. The third processing circuitry is further configured to determine the correction factor, according to whether the amplifier(s) are operating in a saturated or non-saturated domain. The third processing circuitry is further configured to apply the correction factor as described, in order to provide for a more accurate OTDR output.

[0121] In some examples, the first processing circuitry is implemented as part of a pluggable optical module, e.g. a Small Form-factor Pluggable (SFP) module. The second and third processing circuitry may be implemented in a host device, e.g. into which the SFP modules are insertable. The host device may provide for additional functions, e.g. providing for fronthaul in a radio access network. For example, the host device may be configured to transmit and receive radio data signals over one or more optical fibers. In some examples, the first processing circuitry is configured to output a value of the received optical power, and one or more values indicating Psat and / or Vsat (as a combined value or separately). In some examples, the second (or third) processing circuitry is configured to receive a value of the received optical power, and one or more values indicating Psat and / or Vsat (as a combined value or separately). The third processing circuitry is configured to output one or more values or data based on the OTDR, in which a correction factor has been selectively applied to the received optical measurements (e.g. applied to the cross-correlated values) based on a determination of whether one or more of the amplifiers is operating in a saturated range or a non-saturated range. The correction factor may be a pre-configured value (e.g. Psat / Vsat) when the amplifier is operating in a non-saturated range. The correction factor is based on received power, e.g. proportional or based on the received optical power measurement (e.g. Prx / Vsat), when the amplifier is operating in a saturated range (i.e. the amplifier output does not have a linear relationship to its input value).

[0122] The above examples of processing circuitry are merely examples, and one or more of the processing circuitry may be combined, located together or separately or carried out byother types of processing circuitry. In some aspects, the measurement value is not a value which is directly measured. Instead, the measurement value is based on a measurement, and further processed, e.g. by one or more of a summing, combining, processing or crosscorrelation. As such, the adjustment of the measurement value may be considered as a processing or correction of an output value or data.

[0123] Any element or functionality of a described embodiment may be combined with or replace a corresponding element or functionality of another described embodiment.

[0124] The above-described embodiments are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the description, which is defined solely by the appended claims.

Claims

CLAIMS1. A method of adjusting at least one measurement value comprising, by processing circuitry: determining the at least one measurement value from an output signal wherein the output signal is derived from an electrical signal which is representative of the optical signal, and is amplified using an amplifier; determining a first value indicative of at least one of a measure of optical power of the optical signal or a level of the output signal; determining if the first value is indicative that a saturation condition of the amplifier is met; when the first value is indicative that the saturation condition of the amplifier is met, use a measure of optical power of the optical signal to determine a correction factor; and when the first value is indicative that a saturation condition of the amplifier is not met, determining that the correction factor corresponds to a configured value; wherein the method further comprises adjusting the at least one measurement value using the determined correction factor.

2. The method of claim 1, further comprising: receiving the optical signal; measuring the optical power of the optical signal; converting the optical signal to an electrical signal; and amplifying the electrical signal.

3. The method of any preceding claim further comprising deriving the output signal by measuring the electrical signal output by the amplifier.

4. The method of claim 3, further comprising deriving the output signal by cross correlating the measured amplified electrical signal and a numerical sequence used to encode a transmitted optical signal from which the optical signal is derived.

5. The method of any preceding claim, wherein the first value is indicative of an output signal voltage.

6. The method of claim 5 wherein determining if the first value is indicative that a saturation condition of the amplifier is met comprises comparing the first value to a value indicative of a saturation voltage of the amplifier.

7. The method of claim 5 or claim 6, wherein the first value is derived by summing voltage values of the output signal.

8. The method of any of claims 5 to claim 7, wherein determining if the first value is indicative that a saturation condition of the amplifier is met comprises comparing the first value to a value which is less than the saturation voltage of the amplifier by a predetermined factor.

9. The method of any of claims 1 to 3, wherein the first value is indicative of the power of the optical signal, and determining if the first value is indicative that a saturation condition of the amplifier is met comprises comparing the first value to a value indicative of a saturation power of the amplifier.

10. The method of any preceding claim, wherein the configured value is determined based on a ratio of a power and voltage at which the amplifier becomes saturated.

11. The method of any preceding claim, wherein, when the first value is indicative that a saturation condition of the amplifier is met, the correction factor comprises a ratio of the measurement of optical power and a voltage value, wherein the voltage value comprises one of a measurement of the voltage of the output signal, and a voltage at which the amplifier becomes saturated.

12. The method of any preceding claim wherein determining if the first value is indicative that a saturation condition of the amplifier is met comprises determining if the amplifier isoperating in a linear state in which a relationship between a photo-current of the received optical power and the output voltage of the amplified electrical signal is linear, or a saturated state.

13. The method of any preceding claim, comprising a method of correlation Optical Time Domain Reflectometry, OTDR, wherein the received optical signal comprises a return signal from an optical fibre into which a transmitted optical signal was launched, wherein the transmitted optical signal comprises a series of optical pulses modulated based on a coded sequence, the method comprising: digitizing the amplified electrical signal; cross correlating the digitized signal with the coded sequence; deriving the output signal from the cross correlation, the output signal comprising a voltage trace indicative of the optical power returned from each portion of the optical fibre.

14. An optical receiver apparatus (500) comprising an optical power monitor (504), a photosensor (506) and an amplifier (510); and processing circuitry (512), wherein the receiver apparatus (500) is configured to: receive an optical signal; measure an optical power of the optical signal using the optical power monitor (504); convert the optical signal to an electrical signal using the photosensor (504); andAmplify the electrical signal using the amplifier (510), and the processing circuitry (512) is configured to: derive an output signal from the amplified electrical signal, determine a first value indicative of at least one of a level of output signal or the measure of optical power; and: when the first value is indicative that a saturation condition of the amplifier is not met, use a configured value to determine a correction factor;when the first value is indicative that a saturation condition of the amplifier is met, use the measurement of optical power to determine the correction factor; wherein the processing circuitry is further configured to adjust at least one value of the output signal.

15. The receiver apparatus (500) according to claim 14, wherein the amplifier (510) comprises at least one of a transimpedance amplifier, a linear amplifier; a limiting amplifier; a variable gain amplifier; an output driver.

16. The receiver apparatus (500) according to any claim 14 or 15 in which the receiver apparatus (500) comprises an Optical Time Domain Reflectometry (OTDR) receiver.

17. The receiver apparatus (500) according to claim 16 wherein the first value indicative of the level of the amplified electrical signal comprises a sum value indicative of (i) a total voltage of the amplified electrical signal over a period of time or (ii) a total voltage of a derived signal over a period of time, wherein the derived signal is derived by determining a cross correlation between the amplified electrical signal and a numerical sequence used to encode a transmitted optical signal from which the optical signal is derived.

18. The receiver apparatus (500) according to claim 16 or claim 17 wherein the amplifier (510) has a saturation power which exceeds an expected power level of Rayleigh backscatter.

19. A computer readable medium (800) comprising instructions which, when executed by a computer (802), cause the computer (802) to: determine a first value indicative of at least one of a measure of optical power of an optical signal or a level of an output signal derived from an electrical signal, wherein the electrical signal is representative of the optical signal, and is amplified using an amplifier; determine if the first value is indicative that a saturation condition of the amplifier is met;when the first value is indicative that a saturation condition of the amplifier is met, use a measurement of optical power of the optical signal which is represented by the electrical signal to determine a correction factor; when the value is indicative that a saturation condition of the amplifier is not met, determine that the correction factor corresponds to a configured value; and adjust at least one value associated with the output signal using the determined correction factor.

20. The computer readable medium (800) of claim 19, wherein the first value is indicative of a voltage of the output signal, and wherein the instructions to determine if the first value is indicative that a saturation condition of the amplifier is met comprise instructions to compare the first value to a value indicative of a saturation voltage of the amplifier.

21. The computer readable medium (800) of claim 19, wherein the first value is indicative of the power of the optical signal, and wherein the instructions to determine if the first value is indicative that a saturation condition of the amplifier is met comprise instructions to compare the first value to a value indicative of a saturation power of the amplifier.

22. The computer readable medium (800) of any of claims 19 to 21 wherein, when the first value is indicative that a saturation condition of the amplifier is met, the correction factor comprises a ratio of the measurement of optical power and a voltage value, wherein the voltage value comprises one of a measurement of the voltage of the output signal, and a voltage at which the amplifier becomes saturated.

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