Waveform observation system, in particular for an optical time domain reflectometer

Abstract: According to this invention, a waveform data input section (100) inputs sequential discrete waveform data to be observed. A level restricting section (3, 7) determines whether a level of the other data in a predetermined data period including a predetermined number of data continuous to one data of the discrete waveform input data is existed or not within a predeter mined restriction value using a level of the one data as a reference level, directly outputs the data when the level of the data is existed within the predetermined restriction value, and restricts the level of the data within the predetermined restriction value to output the data when the level of the data is not existed within the predetermined restriction value. A averaging section (8) averages the predetermined number of data output by the level restricting section (3, 7). A control section (104) sequentially shifts a sequence of the one data having the reference level in a determining operation of the level restricting section (3, 7), thereby causing to output smoothed data for the waveform data to be observed from an averaging section (8). A display section (102) displays, under the control of the control section (104), the smooth data output from the averaging section (8).

Description: The present invention generally relates to a waveform observation system and, more particularly, to a waveform observation system such as an optical time domain reflectometer (to be referred to as an OTDR hereinafter) used in a noise reduction device by a smoothing process. Such an OTDR is known, for example, from EP-A-0 318 043. In recent years, in the field of measuring equipment, various waveform observation apparatuses have been realized. A high-precision waveform observation apparatus having accurate observation is strongly demanded. An OTDR sends an optical pulse into an optical fiber to be measured and converts a reflection optical pulse reflected from a fault point of the optical fiber and backscattering light generated in the optical fiber into an electric signal at a light-receiving section so as to detect the electric signal. A predetermined operating process is executed to the detected signal to measure the optical loss or fault point of the optical fiber to be measured. In an OTDR of this type, since the backscattering light is represented as a very small detection signal, an S/N ratio of the detection signal must be increased by a digital averaging method using an A/D converter, and the signal buried in noise must be detected without waveform distortion. Since an increase in S/N ratio by a noise reduction technique using a digital averaging process depends on the number of averaging processes and the number of bits of the A/D converter, the time for the measuring process is practically restricted, and an actual increase in S/N ratio is limited due to actual restriction of a measuring processing time and quantization noise. For this reason, in an OTDR using this technique, especially, it may be difficult to accurately measure a long-distance optical fiber. Of operating techniques for reducing a noise component included in a waveform to be measured without influence of restriction of a measuring time by a smoothing process to extract a target signal component, a technique called a shift averaging method is known. This shift averaging method performs the following processes to digital waveform data to reduce noise. When the waveform data has n discrete values x(i) (i = 1, 2, 3, ..., n), a resultant value of x(i), i.e., an average value y(i) is calculated using a "weighting function" w(j) (j = -m, ..., -1, 0, 1, ..., m) constituted by N (= 2m + 1) discrete points as follows: When the weighting function w(j) to be used has a rectangular shape shown in Fig. 2A, a method using this weight function is called a simple shift averaging method. When the weighting function w(j) to be used is a function shown in Fig. 2B, a method using this function is called a polynomial adaptation method. As is apparent from equation(1), in the shift averaging method, a weighted averaging process is performed to the N data having the value x(i) as a center by the function w(j), thereby calculating the averaged value y(i). In the shift averaging method for performing the above process, when a waveform level is rapidly changed in a smoothing period N having data x(i) at a middle portion thereof obtained by the width of the weighting function, the change disadvantageously becomes moderate at the positive or negative edge of the rectangular pulse waveform, i.e., the waveform becomes rounded. In order to solve the above drawback of the shift averaging method, an adaptive smoothing method is used. That is, in the adaptative smoothing method, a variance σ n 2 of noise of waveform data and a variance σ x 2 (i) of waveform data in a smoothing period N are calculated, thereby calculating a smoothing value y(i) as follows: where x (i) is an average value obtained by performing the simple shift averaging method to the N waveform data. The above adaptative smoothing method has characteristics as follows. If σ x 2 (i) = σ n 2 , then y(i) = x (i), and if σ x 2 (i) >> σ n 2 , then y(i) = x(i). That is, according to the adaptative smoothing method, due to the above characteristics, when a waveform level is rapidly changed in the smoothing period N (i.e., σ x 2 (i) >> σ n 2 ), y(i) = x(i) is obtained. An original waveform is directly used as a smoothing value, and the waveform is not rounded. In this case, however, a noise component is directly output as the smoothing value, noise is not advantageously reduced. When a waveform is rounded or noise is not effectively reduced in an OTDR, a marker cannot be desirably adjusted to a predetermined waveform point and cannot accurately detect a fault point of an optical fiber. As described above, in the noise reduction technique by a smoothing process free from an influence of restriction of measuring time, although a simple shift averaging method and a polynomial application method included in a shift averaging method is effective to noise reduction, a signal waveform is rounded. In addition, in the adaptative smoothing method, although the signal waveform is not rounded, when the level of the waveform is rapidly changed, noise is not reduced at the rapidly changed portion of the waveform. Therefore, the noise reduction technique by a smoothing process cannot be directly applied to an OTDR. It is, therefore, an object of the present invention to provide a new and improved waveform observation system which can perform accurate measurement for a short measuring time using a noise reduction device by a smoothing process capable of suppressing roundness of a waveform and effectively reducing noise. According to the present invention, there is provided an optical time domain reflectometer having the features defined in the appended claim 1. Further embodiments of the invention are specified in the dependent claims. This invention can be more fully understood from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing an arrangement of a main part of the first embodiment of the present invention; Fig. 2A is a graph illustrating a weighting function used in a smoothing process using a simple shift averaging method; Fig. 2B is a graph illustrating a weighting function used in a smoothing process using a polynomial adaptation method; Figs. 3A and 3B are views for explaining a principle of a noise reduction technique using a smoothing process of the present invention; Fig. 4 is a view in which smoothed data obtained by smoothing an original waveform using the smoothing process of the present invention is compared with smoothed data obtained by smoothing the original waveform using only a conventional simple shift averaging method; Fig. 5 is a block diagram showing an arrangement of a main part of the second embodiment of the present invention; Figs. 6 to 10 are views in which the waveform displayed according to the second embodiment is compared with an original waveform and waveforms displayed by smoothing the original waveform using various smoothing processes. Reference will now be made in detail to the presently preferred embodiments of the invention as illustrated in the accompanying drawings, in which like reference characters designate like or corresponding parts throughout the several drawings. First, a smoothing method and a principle of a noise reduction device based on the smoothing method used in the present invention will be described below. In a noise reduction technique based on the smoothing process used in the present invention, the following process is executed to waveform data. In order to give a predetermined restriction value to a predetermined number of sequential discrete waveform input data, i.e., waveform data in smoothing period, a noise variance σ n 2 of e.g., the waveform data is calculated. Absolute values |x(i) - x(j)| of differences between waveform data x(i) to be smoothed at a specified point and (N - 1) waveform data x(j) except for the data x(i) in a smoothing period N are calculated. The value |x(i) - x(j)| is compared with a value K√σ n 2 (K is a constant) given as a predetermined restriction value. In this case, if the comparison value is give by: |x(i) - x(j)| ≥ K√σ n 2 3 the x(j) is set to be, e.g., x(j) = x(i) 4 If the value is given by: |x(i) - x(j) | < K√σ n 2 5 the waveform data x(j) is set to be x(j) = x(j). Then, the weighted averaging process used in the above-described known simple shift averaging method (or a polynomial adaptation method) is performed to the N waveform data, and the resultant data is given as a smoothed value of the data x(i). The feature of the noise reduction technique using the smoothing method of the present invention is described as follows. A variation in waveform data in the smoothing period N is restricted to, e.g., the value K√σ n 2 , and the shift averaging process is performed to the waveform data. The symbol K represents a constant for determining a degree of noise reduction and is set to be a desired value, from 4 to 7. Fig. 3A is a view for explaining the above equations. When an absolute value of a level difference between the waveform data x(i) to be smoothed and waveform data x(j) does not exceed the restriction value K√σ n 2 , the waveform data x(j) is used as the waveform data x(j). When the absolute value exceeds the restriction value, the level of the waveform data x(j) is decreased to that of the data x(i). Thus, the weighted averaging process used in the simple shift averaging method is performed to the waveform data x(i). Fig. 3B is a view for also explaining the above equations. When absolute values of the level differences between the waveform data x(i) to be smoothed and N - 1 (in this case, 6) waveform data x(j) exceed the restriction value K√σ n 2 , all the levels of, waveform data x(j) are increased to the levels of the waveform data x(i), and the weighted averaging process used in the simple shift averaging method is performed to the data x(i). Note that, in these cases, the data x(j) need not be set to be the data x(i), but may be set to be arbitrary data smaller than in the restriction value K√σ n 2 . The first embodiment of the present invention will be described below on the basis of the above principle. Fig. 1 is a view schematically showing an arrangement of a waveform observation apparatus according to the first embodiment of the present invention, and is a block diagram of a main part functionally showing the arrangement of a smoothing process section (a noise reduction device) 101. The waveform observation apparatus is mainly constituted by a measuring section 100, a smoothing section 101, and a display section 102. As the measuring section 100 an OTDR is used. The measuring section 100 outputs digital waveform data of a predetermined number of bits for each measuring point. A variance σ n 2 of noise included in waveform data 1 of a predetermined number of bits applied from the measuring section 100 to an input terminal IN of the smoothing section 101 and subjected to smoothing is calculated by a σ n 2 calculating section 2. In a predetermined averaging period N , an absolute value |x(i) - x(j)| of level differences between ith waveform data x(i) to be smoothed and (N - 1) (= 2m) waveform data x(j) from j = i - m to j = i + m having the data x(i) as a center, are calculated in a waveform level difference calculating section 3. Note that the smoothing period N is input from an N value input section 4. By using a constant K for determining a degree of noise reduction input from a K value input section 5 and the variance σ n 2 obtained from the σ n 2 calculating section 2, a restriction value K√σ n 2 is calculated by a restriction value calculating section 6. The restriction value K√σ n 2 is compared with the absolute values |x(i) - x(j)| of the level difference between the data x(j) and x(i) at a waveform level difference restriction section 7. During this comparison, the waveform level difference restriction section 7 outputs the data x(j) when the absolute value |x(i) - x(j)| does not exceed the restriction value K√σ n 2 . When the absolute value |x(i) - x(j)| exceeds the restriction value K√σ n 2 , the difference restriction section 7 replaces the level of the data x(j) with the level of the data x(i) and outputs it. A simple shift average of the data x(i) and the (N - 1) data x(j) including the replaced data x(j), i.e., the N waveform data are calculated by a simple shift averaging section 8. The resultant value is output from an output terminal OUT to a display section 102 as a smoothed data 9. The above sequential processes are controlled by a controller 104 including a CPU and peripheral circuits (a ROM, a RAM, an I/O interface, and, a keyboard), so as to obtain the smoothed value for all of the waveform data. As shown in the display section 102, smoothed data B obtained when the smoothing period N = 3 is applied to original data A according to the present invention does not include waveform roundness (represented by reference numerals 5' to 8' in Fig. 4) which occurs in smoothed data C obtained by a smoothing process using a simple shift averaging method, thereby effectively increasing an S/N ratio. Fig. 5 shows the second embodiment of the present invention which is applied to an OTDR. The OTDR of this embodiment substantially comprises a timing generating section 11, a light-emitting section 12, a directional coupler 13, a light-receiving section 14, an amplifying section 15 including first and second amplifiers 15a and 15b and a variable attenuator 15d, a simple adding section (digital averaging section) 17, a smoothing section 19, a logarithmic converting section 20, a display section 21, a ROM 22, a RAM 23, and a CPU section 28. This OTDR receives and detects a backscattering beam and a Fresnel beam reflected by an optical fiber 30 to be measured by supplying an optical pulse to the optical fiber 30 to be measured, and the received signals are amplified by the amplifying section 15 and then A/D-converted by the A/D converting section 16, thereby obtaining digital waveform data. A simple adding process (a digital averaging process) and a smoothing process are performed to the digital waveform data of each measuring point (e.g., each of 250 to 5,000 points), and the resultant waveform is displayed on the display section 21. An amount of attenuation of the variable attenuator 15d of the amplifying section 15 is manually or automatically controlled to have an optimal value such that saturation does not occur in the displayed waveform. The CPU section 28 receives a command from the keyboard 28b through the I/O interface 28a and controls the timing generating section 11, the display section 21, the simple adding section 17, the RAM 23, the smoothing section 19, the logarithmic converting section 20, and the ROM 20. The timing generating section 11 generates and outputs a trigger signal for causing a light-emitting element (e.g., a laser diode; LD) of the light-emitting section 12 to oscillate and a sampling pulse for driving the A/D converting section 16 and the simple adding section 17. The light-emitting section 12 outputs an optical pulse on the basis of the trigger signal supplied from the timing generator 11, and the optical pulse is emitted to the optical fiber 30 to be measured through the directional coupler 13. The directional coupler 13 splits the backscattering beam and the Fresnel reflection beam on the light-receiving section 14 side. These beams are reflected from the optical fiber 30 by supplying the optical pulse from the light-emitting section 12. The light-receiving section 14 is constituted by, e.g., an avalanche photodiode (APD) and receives and detects the backscattering beam and the Fresnel reflection beam split by the directional coupler 13 and generated from the optical fiber 30 to be measured. The amplifying section 15 is constituted by including the first amplifier 15a, the variable attenuator 15d, and the second amplifier 15b. The first amplifier 15a provided in the previous stage amplifies the detection signal from the light-receiving section 14 with a predetermined degree of amplification. The variable attenuator 15d attenuates the signal amplified by the first amplifier 15a, and the signal is amplified by the second amplifier 15b of the sequential stage with a predetermined degree of amplification again. The amplified signal is output to the A/D converting section 16. The A/D converting section 16 A/D-converts a signal output from the second amplifying section 15b on the basis of a sampling pulse supplied from the timing generating section 11 and outputs it to the simple adding section 17 as, e.g., digital data having 6 bits. A signal is A/D-converted on the basis of the sampling pulse supplied from the timing generating section 11 as in the A/D converting section 16. The simple adding section 17 causes a digital averaging process to be sequentially applied to the converted signal 2¹⁰ to 2²⁶ times at every measuring point of the optical fiber 30 to be measured and outputs it to the smoothing section 19 as data of 32 bits per point. The smoothing section 19 serves as the noise reduction device of the smoothing section 101 in the first embodiment. The smoothing section 19 further reduces noise by a smoothing process so that the unsatisfactory noise reduction performed by the digital averaging process of the simple adding section 17 is compensated, thereby supplying output data of the smoothing section 19 to the logarithmic converting section 20. The logarithmic converting section 20 sequentially converts the output data from the smoothing section 19 into logarithmic data, and the data is supplied to the display section 21 to be displayed. Fig. 6 is a graph showing waveform data obtained when the original waveform (in this case the number of measured points is 500) shown in Fig. 7 to which a smoothing process is applied and measured by the OTDR according to the second embodiment. Fig. 6 shows waveform data measured (the number of measured points is 500) when a smoothing period N = 21 and a constant for determining a degree of noise reduction K = 4. As is apparent from Fig. 6, no roundness of the waveform occurs at the positive and negative edges of the pulse waveform, and noise is effectively reduced. In the noise reduction technique using the smoothing process of the present invention, when a signal level is lower than a noise level, the signal waveform is distorted. When the noise level is lower than the desired signal level by averaging using a simple adding process, smoothing according to the noise reduction technique by the smoothing process of the present invention is performed. At this time, the noise can be effectively reduced without rounding the signal waveform. Fig. 8 shows a display waveform obtained when a original waveform is smoothed by a conventional simple shift averaging method. In Fig. 8, although noise is satisfactorily reduced, the positive and negative edges of the pulse waveform are rounded, and the pulse width of the waveform is larger than that of the original waveform (Fig. 6). Fig. 9 shows a display waveform obtained when the original waveform is smoothed by a conventional polynomial adaptation method. In Fig. 9, although the width of the pulse waveform is smaller than that of the pulse waveform in Fig. 8, a degree of noise reduction is degraded. Fig. 10 shows a display waveform obtained when the original waveform is smoothed by a conventional adaptative smoothing method. In Fig. 10, although the width of the waveform is equal to that of the original waveform, noise remains in the positive edge of the pulse waveform. Note that, in any cases in Figs. 8 to 10, a number of measured points is 500, and a smoothing period N is given by N = 21. As is apparent from the above result, the noise reduction technique by the smoothing process of the present invention is effective much more than a noise reduction technique by other smoothing processes. As described above, in the noise reduction technique using the smoothing process of the present invention, a variance value σ n 2 of noise included in a waveform is calculated by a smoothing section in advance, and a variation in waveform data in a smoothing period N is restricted by a predetermined value K√σ n 2 (K is a constant), thereby performing a smoothing process using a shift averaging method. Therefore, according to the present invention, the following drawbacks can be solved. That is, roundness of a signal waveform is included in a noise reduction apparatus using a smoothing process by a conventional simple shift averaging method, and a degree of noise reduction is degraded at a portion wherein a level of the waveform is rapidly changed. In an OTDR to which the above smoothing technique is applied, the following advantages can be obtained. (1) Since a display waveform having a higher S/N ratio can be obtained without rounding the positive edge of a Fresnel reflection beam emitted from a fault point or the like for a measuring time shorter than the measuring time for obtaining the same effect as described above by only simple adding process, a marker is rightly adjusted to a desired waveform point, and the fault point can be accurately measured. (2) Since the negative edge of the Fresnel reflection beam and the negative edge of a step waveform of a backscattering beam at a fiber connection point are not rounded, a resolution between two points can be improved. The present invention is not limited to the above embodiments shown in the accompanying drawings, and various changes and modifications may be effected without departing from the scope of the invention.

Claims: An optical time domain reflectometer comprising: optical pulse emitting means (12, 13) for causing an optical pulse to be incident on an optical fiber (30) to be measured; light-receiving means (13, 14) for receiving a light beam reflected by the optical fiber (30) ; signal converting means (16) for converting a signal output from the light receiving means (13, 14) into a predetermined number of sequential discrete waveform data points (x(i)); means for smoothing the levels of the waveform data; and display means (21, 102) for displaying waveform data whose levels have been smoothed, characterized in that the reflectometer further comprises means (19, 101) for smoothing the sequential discrete waveform data including a) level restricting means (2 to 7) for determining whether the levels of adjacent data points x(j) within a predetermined data range (period N) about a selected data point x(i) fall within the range of a width defined by the amount of a predetermined restriction value (K√σ n 2 ) from the selected data point x(i) as a reference value and i) if the levels of the adjacent data points x(j) fall within the range of the restriction value (K√σ n 2 ), the adjacent data points x(j) are directly output by the level restricting means (2 to 7) without restriction; whereas ii) if the level of any one of the adjacent data points x(j) falls outside of the range of the restriction value (K√σ n 2 ), the level or levels of such adjacent data points x(j) are restricted to fall within the said range and the restricted adjacent data points are output by the level restricting means (2 to 7); and b) shift averaging means (8) for averaging the predetermined number of data output by the level restricting means (2 to 7); and further comprises control means (28, 104) for controlling the level restricting means (2 to 7) and the shift averaging means (8) to repeatedly perform the smoothing operation on successive selected data points x(i) of the sequential discrete waveform data, wherein the display means (21, 102) display the smoothed data output from the shift averaging means (8) under the control of the control means (28, 104). The reflectometer according to claim 1, characterized by further comprising simple adding means (17), arranged between the signal converting means (16) and the level restricting means (2 to 7), for averaging each of the sequential discrete waveform data points x(i) by a digital averaging process before the smoothing operation, the averaging being carried out a predetermined number of times at each measuring point along the optical fiber (30). The reflectometer according to claim 1 or 2, characterized in that the level restricting means (2 to 7) includes means (2, 5, 6) for calculating the predetermined restriction value (K√σ n 2 ) in relation to a variance value of noise (σ n 2 ) of the sequential discrete waveform data. The reflectometer according to claim 3, characterized in that the calculating means (2, 5, 6) comprises: noise variance value calculating means (2) for calculating a variance value (σ n 2 ) of noise of the sequential discrete waveform data; constant value input means (5) for inputting a constant (K) determining the degree of noise reduction of the sequential discrete waveform data; and restriction value calculating means (6) for calculating a restriction value (K√σ n 2 ) of a variation of the sequential discrete waveform data on the basis of the variance value (σ n 2 ) of noise. The reflectometer according to any of claims 1 to 4, characterized in that level restricting means (2 to 7) further comprises: range value input means (4) for inputting a value (N) indicating a smoothing range (N = 2m + 1) defined by a range of adjacent data points (x(j) ; j = -m..., -1, 0, +1,... +m) about the selected data point x(i) of the discrete waveform data supplied from the signal converting means (16), waveform level difference calculating means (3) for calculating the absolute value of the level difference |x(i) - x(j)| between the selected data point x(i) at the respective specific point (i) to be smoothed and each of the (N-1) adjacent data points (x(j); j = -m, ..., -1, +1,..., +m) in the smoothing range, excluding the data point x(i) at the selected point, and waveform level difference restricting means (7) for comparing the absolute values |x(i) - x(j)| of the level differences with the amount of the restriction value (K√σ n 2 ) and for outputting the adjacent data points x(j) as follows: if |x(i) - x(j)| ≥ K√σ n 2 then x(j) = x(i) ; and if |x(i) - x(j)| < K√σ n 2 then x(j) = x(j).

Inventors: MAKITA SATOSHI, SAKAMOTO TAKASHI, MAKITA, SATOSHI, SAKAMOTO, TAKASHI
Applicants: ANRITSU CORP, ANRITSU CORPORATION
Application year: 1991
Main classification: G01M11
IPC codes: G01M11