JP2008145406A - Optical transmission path measuring device and impulse response measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission path measuring device capable of improving measurement accuracy of an impulse response. <P>SOLUTION: This device includes a pulse generation part 2 formed to be able to generate a pseudo random pulse signal Sp having a prescribed length continuously as long as one period or longer; a delay part 6 for delaying a specific pseudo random pulse signal Ss having a prescribed length from a prescribed bit in the middle of the first period of the pseudo random pulse signal Sp to a bit just before the prescribed bit of a pseudo random pulse signal Sp in the next second period generated in succession with the pseudo random pulse signal Sp, and outputting it as an internal delayed pulse signal S0-Sn (the internal delayed pulse signal S0 is also the specific pseudo random pulse signal Ss); an operation part 7 for operating each mutual correlation between the internal delayed pulse signal S0-Sn and a return signal Sr, and determining correlation coefficient values D0-Dn; and a processing part 8 for measuring an impulse response characteristic of an optical transmission path 10 based on each correlation coefficient value D0-Dn. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical transmission line measuring apparatus and an impulse response measuring method for measuring an impulse response of an optical transmission line such as an optical fiber by using a pseudo random pulse signal.

  As this type of measuring apparatus, a measuring apparatus disclosed in Japanese Patent Laid-Open No. 9-26376 is known. This measuring apparatus transmits a random optical pulse, a random pulse generator that generates a pseudo random pulse signal, a random optical pulse generator that receives a pseudo random pulse signal from the random pulse generator, and generates a random optical pulse. Reflected light detection means for detecting a reflected light pulse from an optical transmission line, a variable delay circuit for receiving a pseudo random pulse signal from a random pulse generator and delaying it for an arbitrary time, and a pulse signal from the reflected light detection means and variable The cross-correlation detector that obtains the cross-correlation with the pulse signal from the delay circuit and the delay time of the variable delay circuit when the correlation coefficient value obtained by the cross-correlation detector is the maximum are input to find the failure point of the optical transmission line A distance calculator and configured as an OTDR (Optical Time Domain Reflectometer)

  According to this measuring apparatus, a light pulse is generated as a light pulse by modulating a light wave with a pseudo random pulse signal composed of an irregular bit string such as a PN sequence (PseudoNoise Sequence) signal. By measuring the impulse response by taking the cross-correlation between the pseudo-random pulse signal transmitted through the optical transmission line and variable delay, and the pulse signal obtained by photoelectrically converting the reflected light pulse, Since the reflected light pulse can be found, it is possible to accurately measure the position and distance of the obstacle point.

  As shown in FIG. 5, in this type of measuring apparatus, a random light pulse LP generated by modulating a light wave with a pseudo random pulse signal Sp is transmitted to an optical transmission line as shown in FIG. The random pulse signal Sp is changed to Δt × 0 (= 0), Δt × 1, Δt × 2,..., Δt × i (i is an integer of 0 or more),. Internal delay pulse signals S0, S1, S2,..., Si,..., Sn that are delayed by a predetermined time Δt (a slight time) as shown in FIG. The cross-correlation between the internal delay pulse signal and the pulse signal (return signal) obtained by photoelectrically converting the reflected light pulse (return light) is calculated. In this case, the return signal Sr includes a return signal Sr0 based on the random optical pulse LP reflected at the incident end of the random optical pulse LP on the optical transmission line, and a random optical pulse LP (Fresnel) reflected at the end of the optical transmission line. A return signal Srt based on reflection, a return signal Srg based on ghost, and a return signal based on a random optical pulse LP reflected at a part other than the incident end or the end of the optical transmission line (for example, sequentially separated from the incident end) Return signals Sr1, Sr2, Sr3... Based on random light pulses LP reflected at the respective positions P1, P2, P3.

  In the measuring apparatus, the cross-correlation of each of these return signals with the internal delay pulse signals S0, S1, S2,..., Si,. For example, only the levels of the return signals Sr0 and Srt at the entrance end and the end of the optical transmission line and the level of the return signal Srg based on the ghost are increased, and other parts of the optical transmission line such as the return signals Sr1 and Sr2 In an optical transmission line in which the level of the return signal is low, the internal delayed pulse signals S0, S1, S2,..., Sn for the return signals Sr0, Srt, Srg having a high signal level. Among the return signals Sr0, Srt, Srg of the signal, and the internal delay pulse signal having the same phase (the bit strings of each other match), the correlation coefficient value increases, but the signal at other parts of the optical transmission line For return signals with low levels, the correlation coefficient value is small even in the calculation of these return signals and the internal delay pulse signal having the same phase. On the other hand, when a failure point such as a break exists in the optical transmission line, the level of the return signal based on the random light pulse reflected at this failure point is different from the level of the return signal before and after that. In such a case, the return signal at this point of failure coincides with the internal delay pulse signals S0, S1, S2,..., Si,. The correlation coefficient value obtained by calculation with the delayed pulse signal is also a unique value. That is, the correlation coefficient value obtained in this way depends on the level of the return signal.

Therefore, for the return signal of a predetermined level, the internal delay pulse signal (this return signal) for which the correlation coefficient value corresponding to the level of the return signal is obtained for the distance from the incident end of the generation site in the optical transmission line. It can be calculated based on the delay time of the internal delay pulse signal whose phase matches that of the signal. Specifically, for the return signal generation portion whose phase matches that of the internal delay pulse signal Si (delay time: Δt × i), the expression (Vc × Δt × i / 2) indicates that from the incident end of the optical transmission line. The distance can be determined. Here, Vc is the propagation speed of the random optical pulse in the optical transmission line. Also, the level of the return signal can be obtained from the correlation coefficient value.
JP-A-9-26376 (page 2-3, FIG. 1)

  However, the above measuring apparatus has the following problems. That is, in this type of measuring apparatus, as described above, for all return signals Sr0 included in the return signal Sr, the internal delay pulse signals S0, S1, S2,..., Si,. The cross correlation is calculated with reference to FIG. 6, and the return signal Sr0 will be described as an example in more detail. The correlation coefficient value for the return signal Sr0 is the return signal Sr0, Sr1, Sr2, Sr3,... Included in the output period of the internal delay pulse signal S0 (intercorrelation target section with the internal delay pulse signal S0). And the total value of the cross-correlation between the internal delay pulse signal S0. In this case, the correlation coefficient value with the internal delay pulse signal S0 having the same bit string increases, and the correlation coefficient value with the other return signals Sr1, Sr2, Sr3,.

  However, the other return signals Sr1, Sr2, Sr3,... Have a smaller correlation coefficient value than the return signal Sr0, but as shown in FIG. 6, they are cross-correlated with the internal delay pulse signal S0. In the section (the output period of the internal delay pulse signal S0), there are sections A1, A2, A3,. A2 is filled with pseudorandom pulse signals (pseudorandom pulse signals included in the sections B1, B2, B3,...) Protruding from the output period of the internal delay pulse signal S0. It becomes a large value compared with the correlation coefficient value when the ideal cross-correlation is performed. As a result, the internal delay pulse signal S0 and the internal value represented by the sum of the correlation coefficient values of the return signals Sr0, Sr1, Sr2, Sr3,... Included in the output period of the internal delay pulse signal S0. The correlation coefficient value between the delayed pulse signal S0 and the return signal Sr is not an ideal value but a value including an error. This error is included in the correlation coefficient values of the return signals Sr1, Sr2, Sr3,... Included in the return signal Sr, and is not so problematic for measurement of a return signal having a high signal level such as the return signal Sr0. However, it has a great influence on the measurement of a return signal with a low signal level. Therefore, the conventional measuring apparatus has a problem that it is difficult to measure the impulse response of the optical transmission line with high measurement accuracy (high S / N ratio).

  The present invention has been made in view of the above problems, and has as its main object to provide an optical transmission line measurement device and an impulse response measurement method that can improve the measurement accuracy of impulse responses.

  In order to achieve the above object, an optical transmission line measuring apparatus according to claim 1, wherein a pulse generation unit that generates a pseudo-random pulse signal having a predetermined length, a random optical pulse based on the pseudo-random pulse signal, and optical transmission An optical pulse generator that emits light to the path, a photoelectric converter that receives the return light of the random optical pulse from the optical transmission path and converts it into an electrical signal, and a delay that delays the pseudo-random pulse signal by a predetermined time A delay unit that generates and outputs a random pulse signal; a calculation unit that calculates a cross-correlation between the pseudo-random pulse signal and the delayed random pulse signal and the electrical signal; A processing unit that measures an impulse response of the optical transmission line based on the obtained correlation coefficient value, and the pulse generation unit includes the pseudo-random pulse signal. The delay unit is formed so as to be continuously generated for one period or more, and the delay unit is generated following a predetermined cycle from a predetermined bit in the middle of one cycle in the pseudo-random pulse signal generated by the pulse generation unit. In the subsequent pseudo-random pulse signal of a predetermined period, the specific pseudo-random pulse having the predetermined length up to the bit immediately before the predetermined bit is delayed and output as the delayed random pulse signal.

  The impulse response measurement method according to claim 2 generates a random optical pulse based on a pseudo-random pulse signal having a predetermined length and emits the random optical pulse to an optical transmission path, and returns the random optical pulse from the optical transmission path. Is obtained by calculating each cross-correlation between the pseudo random pulse signal and the delayed random pulse signal obtained by delaying the pseudo random pulse signal by a predetermined time and the electrical signal. An impulse response measurement method for measuring an impulse response of the optical transmission line based on the method, wherein the pseudo random pulse signal is continuously generated for one period or more, and a predetermined bit in the middle of one period in the pseudo random pulse signal 1 of the predetermined bit in the pseudo-random pulse signal having a predetermined period after the next generated following the one period. Delaying the specific pseudo-random pulses of the predetermined length to the previous bit and the delayed random pulse signals.

  In the optical transmission line measuring apparatus according to claim 1 and the impulse response measuring method according to claim 2, the pseudo random pulse signal is generated from a predetermined bit in the middle of one period in the generated pseudo random pulse signal. A specific pseudo-random pulse signal having a predetermined length up to a bit immediately before a predetermined bit in a pseudo-random pulse signal having a predetermined period after the next is delayed by a predetermined time to detect a cross-correlation with a return signal. Therefore, according to the optical transmission line measuring apparatus and the impulse response measuring method, the pseudo random pulse signal is included before and after the bit string corresponding to the specific pseudo random pulse signal in any of the return signals from each part of the optical transmission line. Therefore, in the cross-correlation with each delayed random pulse signal, the ideal correlation coefficient value can be detected even when the bits do not completely match (the phase is shifted). Each correlation coefficient value between the random pulse signal and the return signal can be detected with very little error. Therefore, according to the optical transmission line measuring apparatus and the impulse response measurement method, the measurement accuracy of the impulse response characteristic for the optical transmission line can be sufficiently improved.

  The best mode of an optical transmission line measuring apparatus (hereinafter also referred to as “measuring apparatus”) and an impulse response measuring method according to the present invention will be described below with reference to the accompanying drawings.

  As shown in FIG. 1, the measuring device 1 includes a pulse generation unit 2, an optical pulse generation unit 3, a branching unit 4, a photoelectric conversion unit 5, a delay unit 6, a calculation unit 7, a processing unit 8, and a display unit 9. The characteristic (impulse response characteristic) of the optical transmission line 10 can be measured by executing the impulse response measurement method according to the present invention using a pseudo random pulse signal. Therefore, this impulse response measuring method will be described together with the description of the measuring apparatus 1.

The pulse generator 2 is configured to generate a pseudo-random pulse signal Sp having a predetermined length in one cycle and to continuously output the generated pseudo-random pulse signal Sp for one cycle or more (in this example, two cycles). ing. As the pseudo-random pulse signal Sp, any of an M sequence (Maximum linear linear recurring sequence) of a PN sequence signal, a Legendre sequence, a Hall sequence, and a twin-prime sequence may be used. In this example, the pulse generation unit 2 includes, as an example, a shift register of k (k is an integer of 4 or more, for example, 27) and a feedback circuit of an EXOR circuit (both not shown), and has one cycle. M-sequence signal having a predetermined code length q (= 2 ^k −1) is output as a pseudo-random pulse signal Sp. In addition, the pulse generator 2 generates the pseudo random pulse signal Sp from a predetermined bit (m bits, m is an integer of 10,000 or more) in the middle (middle) of the pseudo random pulse signal Sp of one cycle (first cycle). Next to a predetermined bit (a bit having the same bit position as the predetermined bit in the first period) in the pseudo-random pulse signal Sp generated in the following predetermined period (in this example, the next second period) subsequently to A bit string having a predetermined length q up to (m−1) bits is output as a specific pseudo-random pulse signal Ss (see FIG. 2). In this example, as an example, the 1-bit length Tb of the pseudo random pulse signal Sp is set to 20 ns. Also, the time T0 (= m × Tb; see FIG. 2) from the start of output of the pseudo random pulse signal Sp in the first cycle to the start of output of the specific pseudo random pulse signal Ss is, for example, up to the return signal Srg based on ghost. When measuring the above characteristic, the length is set to a time longer than the time from the start of generation of the pseudo random pulse signal Sp until the return signal Srg based on the ghost returns to the incident end of the optical transmission line 10. Further, the pulse generator 2 generates a pseudo-random pulse signal Sp of the second period until at least the output of the internal delay pulse signal Sn described later from the delay unit 6 is completed.

  The optical pulse generator 3 generates a random optical pulse LP based on the pseudo random pulse signal Sp, that is, by optically modulating light with the pseudo random pulse signal Sp. The optical pulse generator 3 emits the generated random optical pulse LP from the one end (incident end) into the optical transmission line 10 via the branching unit 4. As an example, the optical pulse generator 3 is configured to include a laser light source (not shown), and this laser light source is directly turned on / off (optically modulated) by a pseudo random pulse signal Sp. Output LP. As an example, the branching unit 4 is composed of a directional coupler. Further, the branching unit 4 emits the random optical pulse LP generated by the optical pulse generating unit 3 to the optical transmission line 10 as described above, and is generated in the optical transmission line 10 due to the incidence of the random optical pulse LP. In addition, the return light Lr emitted from the incident end of the optical transmission line 10 is output to the photoelectric conversion unit 5.

  The photoelectric conversion unit 5 includes a photodiode such as an APD (Avalanche photodiode), converts the return light Lr into a return signal Sr (electrical signal in the present invention), and outputs it. The delay unit 6 includes a plurality of (n) delay circuits 6a that are set to have the same delay time (Δt, 20 ns as an example) and are connected in series to each other. With this configuration, the delay unit 6 is based on the specific pseudo random pulse signal Ss output from the pulse generation unit 2 and has an internal delay with a delay time of zero (Δt × 0 = 0) with respect to the specific pseudo random pulse signal Ss. The internal delay pulse signal S1 having a delay time (Δt × 1) with respect to the pulse signal S0 and the specific pseudo-random pulse signal Ss,..., And the delay time (Δt × i) with respect to the specific pseudo-random pulse signal Ss Internal delay pulse signal Si,..., For specific pseudorandom pulse signal Ss, for internal delay pulse signal Sn-1 having a delay time (Δt × (n−1)), and for specific pseudorandom pulse signal Ss An internal delay pulse signal Sn having a delay time (Δt × n) is generated and output.

  The calculation unit 7 includes a plurality (n + 1) cross-correlators 7a having the same configuration. The first cross-correlator 7a among these cross-correlators 7a detects the cross-correlation between the internal delay pulse signal S0 and the return signal Sr and outputs the correlation coefficient value D0. Since the correlation coefficient value D0 is calculated based on the internal delay pulse signal S0 having a delay time of zero with respect to the specific pseudo random pulse signal Ss, the return signal based on the return light Lr at the incident end of the optical transmission line 10 This is the correlation coefficient value for Sr. The second cross-correlator 7a detects the cross-correlation between the internal delay pulse signal S1 and the return signal Sr and outputs the correlation coefficient value D1. Since this correlation coefficient value D1 is calculated based on the internal delay pulse signal S1 having a delay time Δt with respect to the specific pseudo-random pulse signal Ss, the distance (Vc × Δt / 2) from the incident end of the optical transmission line 10 This is a correlation coefficient value for the return signal Sr based on the return light Lr at positions separated by a distance. Similarly, each cross-correlator 7a detects the cross-correlation between the corresponding internal delay pulse signal and the return signal Sr, and outputs the correlation coefficient value. In this case, the i-th cross-correlator 7a detects the cross-correlation between the internal delay pulse signal Si and the return signal Sr and outputs the correlation coefficient value Di. Since the correlation coefficient value Di is calculated based on the internal delay pulse signal Si having a delay time (Δt × i) with respect to the specific pseudo random pulse signal Ss, the distance (Vc × This is a correlation coefficient value for the return signal Sr based on the return light Lr at a position separated by Δt × i / 2).

  The processing unit 8 measures the impulse response characteristics of the optical transmission line 10 by executing a measurement process based on the correlation coefficient values D0 to Dn output from the calculation unit 7. Further, the processing unit 8 causes the display unit 9 to display the characteristics of the optical transmission line 10 measured by outputting the display data Dd.

  Next, the operation of the measuring apparatus 1 will be described. It is assumed that one end of an optical transmission line 10 (specifically, an optical fiber) as a measurement target is connected to the branching unit 4 in a state where the other end (termination) is open. In order to facilitate understanding of the invention, the delay time from the input of the pseudo-random pulse signal Sp to the output of the random light pulse LP in the optical pulse generator 3 and the return from the input of the return light Lr in the photoelectric converter 5 The delay time until the output of the signal Sr and the delay time in the branch unit 4 are zero.

  In this measuring apparatus 1, in the operating state, first, the pulse generator 2 starts generating and outputting the first (first period) pseudo-random pulse signal Sp as shown in FIG. 3 starts generating the first (first period) random light pulse LP and outputs to the branching unit 4 based on the pseudo-random pulse signal Sp. As a result, the first random light pulse LP starts to enter the optical transmission line 10 via the branching unit 4.

  Further, the pulse generation unit 2 starts from the middle m bit (predetermined bit in the present invention) of the first pseudo random pulse signal Sp (from the time when the time T0 has elapsed from the start of the output of the first bit). Along with the output to the pulse generator 3, the pseudo-random pulse signal Sp is started to be output to the delay unit 6 as the specific pseudo-random pulse signal Ss. Further, the pulse generation unit 2 has the same code string as the first pseudo-random pulse signal Sp so as to be continuous with the first pseudo-random pulse signal Sp after the generation of the first pseudo-random pulse signal Sp is completed. The second generation (second period) of the pseudo random pulse signal Sp is started and output to the optical pulse generation unit 3 and also output to the delay unit 6 as the specific pseudo random pulse signal Ss. As a result, the second (second cycle) random light pulse LP starts to enter the optical transmission line 10. The pulse generation unit 2 stops outputting the specific pseudo-random pulse signal Ss up to the second (m−1) -th bit, while stopping the output of the pseudo-random pulse signal Sp as an example. Stops output when generation of is completed.

  As shown in FIG. 2, the delay unit 6 converts the input specific pseudo-random pulse signal Ss into Δt × 0, Δt × 1, Δt × 2,..., Δt × i,. 1), internal delay pulse signals S0, S1, S2,..., Si,..., Sn−1, Sn delayed by Δt as Δt × n are generated and output to the arithmetic unit 7.

  Further, with the incidence of the random light pulse LP, the return light Lr resulting from the incidence of the random light pulse LP starts to be emitted from the incident end of the optical transmission line 10. The branching unit 4 outputs the return light Lr to the photoelectric conversion unit 5, and the photoelectric conversion unit 5 converts the return light Lr into a return signal Sr and outputs it. In this case, as shown in FIG. 2, the return signal Sr includes a return signal Sr0 based on the random optical pulse LP reflected at the incident end of the optical transmission line 10 and a random optical pulse reflected at the end of the optical transmission line 10. A return signal Srt based on LP (Fresnel reflection), a return signal Srg based on a ghost, and a return signal based on a random optical pulse reflected at a part other than the incident end and the end of the optical transmission line 10 (for example, from the incident end) ..) Based on random light pulses reflected at sequentially spaced positions P1, P2,...

In the calculation unit 7, each cross-correlator 7a detects the cross-correlation between the corresponding internal delay pulse signal S0,..., Sn among the internal delay pulse signals S0,. ..., Dn is output. The operation of detecting the cross-correlation between the internal delay pulse signals S0,..., Sn and the return signal Sr in each cross-correlator 7a will be described by taking the first cross-correlator 7a as an example. As shown in FIG. 3, in the cross-correlation between the internal delay pulse signal S0 and the return signal Sr, each return signal Sr0, included in the return signal Sr and included in the output period of the internal delay pulse signal S0, The cross-correlation between Sr1, Sr2, Sr3,... And the internal delay pulse signal S0 is detected. In this case, the internal delayed pulse signal S0 and the bit string (shaded part) corresponding to the specific pseudo-random pulse signal Ss included in the return signal Sr0 are input to the cross-correlator 7a at the same timing. Therefore, as a result of the complete matching of the bits, the correlation coefficient value becomes a large value (a value ^obtained by multiplying the numerical value “2 ^k −1” by the signal strength (amplitude) of the signal Sr0).

  On the other hand, since the other return signals Sr1, Sr2, Sr3,... Are delayed with respect to the return signal Sr0, as shown in FIG. 3, the internal delay pulse signal S0 and each return signal Sr1 , Sr2, Sr3,..., And the bit string corresponding to the specific pseudo-random pulse signal Ss (the hatched portion) is input to the cross-correlator 7a with a bit shift. As a result, the correlation coefficient value becomes a small value as a result of the bits not matching each other. In particular, in this example, as shown in the figure, in the calculation unit 7, a pseudo-random pulse signal for one cycle starting from the middle m-th bit of the first pseudo-random pulse signal Sp is used as a specific pseudo-random pulse signal Ss. Since it is configured to detect cross-correlation, the sections (cross-correlation target sections) C1, C2, C3 that are cross-correlated with the internal delay pulse signal S0 in the other return signals Sr1, Sr2, Sr3,. Are pseudo-random pulse signals, and the pseudo-random pulse signals in each section are obtained by circulating a bit string constituting the internal delay pulse signal S0 (specific pseudo-random pulse signal Ss). It has become. Therefore, in the first cross-correlator 7a, the cross-correlation between each return signal Sr1, Sr2, Sr3,... And the internal delay pulse signal S0 is detected in an ideal state. , A theoretical minimum value (a value obtained by multiplying the numerical value “−1” by the signal strength (amplitude) of each return signal) when the M-sequence pseudo-random pulses are cross-correlated and shifted, and each return signal Sr0, The error included in the correlation coefficient value D0 between the internal delay pulse signal S0 and the return signal Sr represented by the sum of correlation coefficient values for Sr1, Sr2, Sr3,... Is extremely small. Similarly, the correlation coefficient values D1, D2,..., Dn are detected in the second to nth cross-correlator 7a with very little error.

  Finally, the processing unit 8 measures the impulse response characteristics of the optical transmission line 10 by executing measurement processing based on the correlation coefficient values D0 to Dn output from the calculation unit 7. In this measurement process, the processing unit 8 uses the delay times Δt × 0, Δt × 1, Δt × 2,... Sn for the internal delay pulse signals S0, S1, S2,. , Δt × i,..., Δt × n, distances (Vc × Δt × i / 2) corresponding to the correlation coefficient values D0, D1, D2,. Is calculated. Subsequently, the correlation coefficient values D0, D1, D2,..., Di,..., Dn and the correlation coefficient values D0, D1, D2,. Based on the distance, the impulse response characteristic of the optical transmission line 10, specifically, how the correlation coefficient value changes with respect to the distance from the incident end of the optical transmission line 10 is calculated (measured). Thereby, the measurement process is completed. Finally, the processing unit 8 outputs the display data Dd and causes the display unit 9 to display the measurement result as indicated by the thick line E2 in FIG. Thereby, the measurement of the impulse response characteristic of the optical transmission line 10 by the measuring apparatus 1 is completed. The operator measures the occurrence state of Fresnel reflection, Rayleigh scattering, ghost, etc. about the optical transmission line 10 by confirming the measurement result (impulse response characteristic) displayed on the display unit 9.

As described above, in the measurement apparatus 1 and the impulse response measurement method executed by the measurement apparatus 1, the pseudo random pulse signal is generated from m bits in the first period in the pseudo random pulse signal Sp generated by the pulse generator 2. The specific pseudo of a predetermined length q (= 2 ^k −1) up to the bit immediately before m bits, that is, (m−1) bits, in the pseudo random pulse signal Sp of the next second period generated following Sp. The cross-correlation between the internal delay pulse signals S0,..., Sn generated by delaying the random pulse signal Ss and the return signal Sr is detected. Therefore, according to the measurement apparatus 1 and the impulse response measurement method, any of the return signals (return signals Sr0, Sr1, Sr2, Srt, Srg, etc.) from each part of the optical transmission line 10 that constitutes the return signal Sr. Since the pseudo-random pulse signal is included before and after the bit string corresponding to the specific pseudo-random pulse signal Ss, the bits are not completely matched in the cross-correlation with the internal delay pulse signals S0,. (Even when the phase is shifted), the ideal correlation coefficient value can be detected, so that each correlation coefficient value D0, D1 between the internal delay pulse signals S0,..., Sn and the return signal Sr. , D2,..., Dn can be detected with very little error. Therefore, according to the measurement apparatus 1 and the impulse response measurement method, the measurement accuracy of the impulse response characteristic for the optical transmission line 10 can be sufficiently improved.

  Specifically, FIG. 4 shows the result of measuring the impulse response characteristics of an optical fiber of about 500 m as the optical transmission line 10 with the conventional measuring apparatus and the measuring apparatus 1 according to the present invention. In the same figure, the thin line E1 shows the result measured with the conventional measuring apparatus, and the thick line E2 shows the result measured with the measuring apparatus 1 according to the present invention. In the figure, the measurement result of the measurement apparatus 1 is displayed with a slight shift in the long distance direction with respect to the measurement result of the conventional measurement apparatus so that the measurement results of both apparatuses can be easily compared. According to this measurement result, in the impulse response characteristic measured with the conventional measuring apparatus, Fresnel reflection near 500 m can be measured without any problem, but it can be confirmed that the noise level is generally increased. On the other hand, in the impulse response characteristic measured by the measuring apparatus 1 according to the present invention, it can be confirmed that the noise level is lowered as a whole, and the conventional measuring apparatus is buried in noise along with Fresnel reflection near 500 m. It is also possible to measure a low level signal (for example, ghost or Rayleigh scattering) that cannot be measured.

  Note that the present invention is not limited to the embodiment of the invention described above, and can be modified as appropriate. For example, in the above-described embodiment, the delay unit 6 is configured by the n delay circuits 6a and the calculation unit 7 is configured by (n + 1) cross-correlators 7a. 6. Arithmetic unit 7 and processing unit 8 are configured by a DSP to delay the specific pseudo-random pulse signal Ss by Δt, and to correlate each internal delayed pulse signal S0,..., Sn with the return signal Sr. It is also possible to adopt a configuration in which software is used.

  Further, the pulse generator 2 outputs the pseudo random pulse signal Sp continuously for two cycles, and from the m bits of the pseudo random pulse signal Sp of the first one cycle (first cycle), the next one cycle (first cycle). An example in which a bit string having a predetermined length q (a length of one period of the pseudo random pulse signal Sp) up to (m−1) bits of the pseudo random pulse signal Sp of (two periods) is output as the specific pseudo random pulse signal Ss. However, the pseudo-random pulse signal Sp is continuously output for three or more periods, and the next and subsequent predetermined values generated following this one period from the m bits of the pseudo-random pulse signal Sp of any one of these periods. For a predetermined length (q × j) up to (m−1) bits of the pseudo random pulse signal Sp of the period (for example, the j-th (j is an integer of 1 or more) generated following one period). It is also possible to employ a configuration of outputting the preparative column as specified pseudorandom pulse signal Ss. In this configuration, the delay unit 6 generates the internal delay pulse signals S0,..., Sn by delaying the specific pseudo random pulse signal Ss composed of the bit string of the predetermined length (q × j). In the calculation unit 7, each cross-correlator 7a detects the cross-correlation between the corresponding internal delay pulse signal of the internal delay pulse signals S0,..., Sn and the return signal Sr, and the correlation coefficient value. D0,..., Dn are output.

1 is a configuration diagram showing a configuration of a measuring device 1. FIG. 3 is a timing chart for explaining the operation of the measuring apparatus 1. It is a timing chart for demonstrating the cross correlation of internal delay pulse signal S0 and return signal Sr. It is a measurement result figure which shows each impulse response characteristic of the measuring apparatus 1 and the conventional measuring apparatus. It is a timing chart for demonstrating operation | movement of the conventional measuring apparatus. It is a timing chart for demonstrating the cross correlation of internal delay pulse signal S0 and return signal Sr performed by the conventional measuring apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Measuring apparatus 2 Pulse generation part 3 Optical pulse generation part 5 Photoelectric conversion part 6 Delay part 7 Calculation part 8 Processing part 10 Optical transmission line D0-Dn Correlation coefficient value LP Random light pulse q Predetermined length S0-Sn Internal delay pulse signal Sp Pseudo random pulse signal Sr Return signal Ss Specific pseudo random pulse signal

Claims (2)

A pulse generator that generates a pseudo-random pulse signal of a predetermined length; an optical pulse generator that generates a random optical pulse based on the pseudo-random pulse signal and emits the random optical pulse to the optical transmission line; and the random pulse from the optical transmission line A photoelectric conversion unit that receives return light of an optical pulse and converts it into an electrical signal, a delay unit that generates and outputs a delayed random pulse signal obtained by delaying the pseudo random pulse signal by a predetermined time, and the pseudo random pulse signal And a calculation unit for calculating a correlation coefficient value by calculating each cross-correlation between the delayed random pulse signal and the electrical signal, and measuring an impulse response of the optical transmission line based on the correlation coefficient value obtained by the calculation unit And a processing unit to
The pulse generator is formed so as to be able to generate the pseudo-random pulse signal continuously for one period or more,
The delay unit includes the pseudo random pulse signal having a predetermined period after the next generated from the predetermined bit in the middle of one period in the pseudo random pulse signal generated by the pulse generating unit. An optical transmission line measuring apparatus that delays the specific pseudo-random pulse having a predetermined length up to a bit immediately before a predetermined bit and outputs the delayed pulse signal as the delayed random pulse signal. A random optical pulse is generated based on a pseudo-random pulse signal of a predetermined length and emitted to an optical transmission path, and the return light of the random optical pulse from the optical transmission path is received and converted into an electrical signal, and the pseudo-random The impulse response of the optical transmission line is measured based on a correlation coefficient value obtained by calculating each cross-correlation between the pulse signal and the delayed random pulse signal obtained by delaying the pseudo random pulse signal by a predetermined time and the electrical signal. An impulse response measurement method,
The pseudo-random pulse signal is generated continuously for one period or more, and the pseudo-random pulse signal having a predetermined period after the next generated from the predetermined bit in the middle of one period in the pseudo-random pulse signal. The impulse response measurement method of delaying the specific pseudo-random pulse of the predetermined length up to the bit immediately before the predetermined bit in the signal to obtain the delayed random pulse signal.

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