JP2003329545A - Optical fiber measuring apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately obtain a change in time of the intensity of return light in an optical fiber. <P>SOLUTION: The optical fiber measuring apparatus comprises: an acoustic optical element 22 for switching continuous light to zero-order light and high- order light according to the signal level of a pseudo random pulse signal for outputting; a multiplexer 24 for multiplying the zero-order light and high-order light outputted from the acoustic optical element for applying to the optical fiber as measurement light; a detector 27 for multiplying return light from the optical fiber and continuous light from the light source for executing optical heterodyne detection; a signal extraction section 30 for extracting a signal that changes with time for indicating a change in time of the intensity of return light from the detection signal outputted from the detector; a correlator 34 for calculating the correlation between the signal that changes with time and the pseudo random pulse signal for outputting as a correlation signal; and a characteristic calculation section 36 for calculating the characteristics of the optical fiber from the correlation signal. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of making measurement light incident on an optical fiber to be measured, and returning light from this optical fiber due to backscattering and Fresnel reflection. And an optical fiber measuring apparatus for measuring characteristics of the optical fiber from the returned light. 2. Description of the Related Art In an optical communication system, an optical fiber measuring apparatus for measuring characteristics such as transmission characteristics and attenuation characteristics of an optical fiber as a transmission medium of an optical signal and detecting occurrence of abnormality is generally shown in FIG. It is configured as shown in FIG. In the optical fiber measuring apparatus 1, a pulse generator 2 generates a pulse a having a pulse width T ₁ and sends it to a light source 3 at a constant repetition period T ₀ . For example, the light source 3 composed of a semiconductor laser outputs an optical pulse b having a pulse width T ₁ corresponding to the input pulse a and _a repetition period T ₀ . The frequency (or wavelength) of the light of this light pulse b
Is set to a single frequency f equal to the frequency (or wavelength) of light used in an actual optical communication system. The light pulse b emitted from the light source 3 is applied to the optical circulator 4
And is output from the optical fiber measurement device 1,
The light is incident on one end 5a of the optical fiber 5 to be measured. The light pulse b incident on the optical fiber 5 to be measured propagates through the optical fiber 5 toward the other end 5b. In the propagation process, the light pulse b is generated due to back scattering or Fresnel reflection. Return light d whose traveling direction is opposite to b is generated. This return light d is applied to one end 5 of the optical fiber 5.
a returns to the optical fiber measuring apparatus 1, is separated from the incident optical pulse b by the optical circulator 4, is converted into an electric signal return signal e by the optical receiver 6, and is input to the characteristic calculation unit 7. I have. [0005] A trigger synchronized with the pulse a is input from the pulse generator 2 to the characteristic calculator 7. Then, the characteristic calculator 7 displays on the display screen the time change characteristic 8 of the intensity of the return light d including the backscattered light and the Fresnel reflected light from the output time of the optical pulse b of the return signal e. Then, from the obtained time change characteristics 8 of the return light d, various characteristics such as the transmission characteristics and the attenuation characteristics of the optical fiber 5 to be measured are calculated. [0006] However, the optical fiber measuring apparatus 1 shown in FIG. 7 has the following problems to be solved. That is, in order to measure various characteristics of the optical fiber 5 such as the transmission characteristic and the attenuation characteristic with high accuracy, it is necessary to obtain the above-mentioned time change characteristic 8 of the intensity of the return light d with high accuracy. In order to obtain the time change characteristic 8 of the intensity of the return light d with high accuracy, the optical fiber 5
To increase the intensity of the light pulse b, which is the measuring light applied to
It is necessary to extend the dynamic range by increasing the intensity of the corresponding return light d itself. In order to increase the intensity of the light pulse b, it is considered that the peak level of the light pulse b is increased or the pulse width of the light pulse b is increased. However, there is a certain limit in increasing the peak level of the light pulse b output from the light source 3 composed of a semiconductor laser. Further, if the pulse width of the optical pulse b is increased, the measured optical fiber 5
The spatial resolution is reduced. In order to eliminate such inconvenience, the measurement light incident on the optical fiber 5 to be measured is not a single light pulse b but a pseudo-random pulse signal having a predetermined length converted into a pseudo-random pulse signal into an optical signal. It is conceivable that a measuring light composed of a random optical pulse train and composed of the pseudo random optical pulse train is incident on the optical fiber 5 to be measured. Then, the return light obtained from the optical fiber 5 is converted into an electric signal to obtain a time change signal of the light intensity of the return light. Then, the correlation between the time-varying signal and the pseudo-random pulse signal is calculated to obtain a correlation signal. This correlation signal indicates the degree to which the pseudo-random pulse signal at each time of the pulse width unit in the time change signal of the return light is included. As a result, the time resolution of the time change signal of the return light, that is, the spatial resolution is pulsed. Can be improved to the width unit. Further, the pulse width of the measurement light can be expanded equivalently, so that the dynamic range of the return light can be expanded. However, the following problems still exist in an optical fiber measuring apparatus employing a pseudo-random light pulse train as the measuring light. For example, when the optical fiber 5 is laid on the sea floor, as shown in FIG. 8, since the optical fiber 5 has a long distance, the upstream optical fiber 5c and the downstream optical fiber 5d For example, a repeater 9 is provided for every 50 km in the optical cable 5e composed of
Each repeater 9 has a pair of optical amplifiers 10a and 10b incorporated therein. Further, in each of the optical amplifiers 10a and 10b, an automatic intensity control circuit (APC) 11 for keeping the signal intensity of the amplified optical signal constant is incorporated. further,
The output terminals of the optical amplifiers 10a and 10b are connected by a loop optical fiber 12. When a test for one optical fiber 5c in such an optical cable 5e is carried out using the above-described optical fiber measuring device 1, this one optical fiber 5c is used.
Light pulse b is applied to c. Return light d resulting from backscattering and Fresnel reflection generated in the section between the repeaters 9 in the optical fiber 5c is transmitted to the optical fiber test apparatus 1 via each loop optical fiber 12 and the other optical fiber 5d. Return. Here, the automatic intensity control circuit (APC)
11 has a function of making the signal intensity of the amplified optical signal constant, so that when a pseudo-random light pulse train is adopted as the measurement light applied to one optical fiber 5c, the pseudo-random light pulse train is As shown in FIG. 9A, the lighted section and the non-lighted section are arranged irregularly, and the section length between the lighted section and the non-lighted section is not constant. Therefore, when such a pseudo-random optical pulse train is subjected to a process for keeping the signal intensity constant, the optical amplifier 1 in the repeater 9 which has been in the excited state without any signal until then is executed.
0 indicates a transient response characteristic, and the pulse peak power changes as shown in FIG. As a result, there is a concern that when the correlation between the pseudo-random pulse signal and the time-varying signal of the return light in which the pulse peak power is changed, a correct correlation signal cannot be obtained. As a result, the measurement accuracy of the characteristics for the optical fiber to be measured decreases. FIG. 10 shows a case where a pseudo-random optical pulse train having a fixed length (pseudo-random pulse train length) is applied to one optical fiber 5c from an optical fiber measuring device, and the light intensity of the return light obtained from the other optical fiber 5d is measured. It is a figure showing a characteristic. As indicated by the solid line characteristic of the simulation waveform, the light intensity of the return light increases from the output start time of the pseudo random optical pulse train to the end of the output of the pseudo random optical pulse train. Then, when the output of the pseudo-random optical pulse train is completed, the light intensity of the return light maintains a constant value for a period corresponding to the transmission time of the pulse train, and the pulse train decreases because it passes through the fiber to be measured sequentially from the beginning of the pulse train. . However, when the automatic intensity control (APC) is performed, the system does not respond linearly due to the effect of the pseudo-random light pulse train as shown by the broken line characteristics in FIG. 10, and the light intensity of the return light continues to increase. I will. Therefore, when a simple pseudo-random light pulse train is adopted as the measuring light and the optical fiber including the repeater 9 on which the automatic power control (APC) is performed is measured, an accurate time change signal of the light intensity of the returning light can be obtained. Can not be obtained. The present invention has been made in view of such circumstances, and employs a pseudo-random light pulse train in which measurement light incident on an optical fiber to be measured is substantially continuous light, thereby reducing return light. The spatial resolution of the time-varying signal can be improved to the pulse width unit, and the pulse width of the measurement light can be expanded equivalently, so that the dynamic range of the return light can be expanded and the measurement accuracy of the characteristics of the optical fiber to be measured can be improved. An object of the present invention is to provide an optical fiber measuring device that can be significantly improved. In order to solve the above-mentioned problems, the present invention is directed to a method in which a measuring light is made incident on an optical fiber to be measured and is caused by back scattering and Fresnel reflection from the optical fiber. In an optical fiber measuring device that detects return light and measures characteristics of an optical fiber from the return light, a light source that generates continuous light of a constant frequency and a pseudorandom pulse signal of a predetermined length are generated at a constant repetition period. The pulse generator receives the continuous light and the pseudo random pulse signal, and sets the continuous light to 0 according to the signal level of the pseudo random pulse signal.
An acousto-optic element that switches to and outputs the next-order light and higher-order light, a multiplexer that combines the zero-order light and the higher-order light output from the acousto-optic element and enters the optical fiber as measurement light, and an optical fiber. An optical heterodyne detector that performs optical heterodyne detection by multiplexing return light and continuous light from the light source,
A signal extracting unit that extracts a time-varying signal indicating a temporal variation of the intensity of the return light from the detection signal output from the optical heterodyne detector, and a time-varying signal extracted by the signal extracting unit and a pseudo-random pulse signal The apparatus includes a correlator that calculates a correlation and outputs it as a correlation signal, and a characteristic calculator that calculates characteristics of the optical fiber based on the correlation signal output from the correlator. In the optical fiber measuring apparatus thus configured, a pseudo random pulse signal of a predetermined length is output from the pulse generator at a constant repetition period. An acousto-optic device is incorporated in the optical fiber measuring device. As is well known, when an acousto-optic device applies high frequency vibration from the outside, an elastic wave is generated inside. When light having a certain frequency (or wavelength) is incident in this state, the elastic wave acts as a grating to diffract the incident light, and the frequency (or wavelength) is shifted by the frequency applied from the outside. I do. Light whose frequency (or wavelength) is shifted is referred to as higher-order light. In a state where no high-frequency vibration is applied from the outside, the incident light is transmitted without being diffracted. The light that is not diffracted, ie, the frequency (or wavelength) of which is not shifted, is referred to as zero-order light. Therefore, by controlling on / off the high-frequency vibration applied to the acousto-optic element according to the signal level of the pseudo-random pulse signal, continuous light can be converted into zero-order light and higher-order light according to the signal level of the pseudo-random pulse signal. And output from the acousto-optic element is possible. The zero-order light and the higher-order light output from the acousto-optic element are multiplexed and incident on the optical fiber as measurement light. In this measurement light, the high-level (ON) period of the pseudo-random pulse signal is high-order light, and the low-level (OFF period) and the signal-free period of the pseudo-random pulse signal are zero-order light. Therefore, the measurement light is substantially continuous light that is frequency (or wavelength) modulated by the pseudo random pulse signal. The return light obtained from the optical fiber to be measured also includes zero-order light and higher-order light. By heterodyne detecting this return light with continuous light from the light source, light having a frequency (or wavelength) of the difference between the continuous light from the light source and the higher-order light, and the sum frequency of the continuous light and the higher-order light from the light source (or (Wavelength) is obtained as a detection signal. From this detection signal, for example, a time change signal of the return light corresponding to the original pseudo random pulse signal is obtained by a signal extraction unit including a filter or the like. The obtained correlation signal between the time-varying signal of the return light and the original pseudo-random pulse signal indicates the degree to which the pseudo-random pulse signal at each time of the pulse width unit in the time-varying signal of the return light is included. Therefore, as a result, the spatial resolution of the time-varying signal of the return light can be improved to a pulse width unit. Therefore, various characteristics of the optical fiber can be obtained based on the correlation signal. An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of an optical fiber measuring device according to an embodiment of the present invention. For example, the light source 20 composed of a semiconductor racer
Sends out continuous light g of a constant frequency (or wavelength) f to the optical amplifier 21 as shown in FIG. The optical amplifier 21 amplifies the input continuous light g and makes it incident on the acousto-optic element 22.
On the other hand, as shown in FIG. 4, the pulse generator 23 generates a pseudo random pulse signal (PN) having a predetermined length (predetermined pulse number) T _1.
And outputs the signal) h at a constant repetition period T _0. The pseudo random pulse signal (PN) output from the pulse generator 23
The signal h) is also applied to the acousto-optic element 22. FIG. 2 is a diagram showing the principle of operation of the acousto-optic device 22. The acousto-optic element 22 includes an acousto-optic element main body 22a, an ultrasonic vibrator 22b attached to one end surface of the acousto-optical element main body 22a,
and an oscillator 22c for applying a high-frequency signal to the signal b. The oscillator 22c starts during a period when the pseudo random pulse signal (PN signal) h input from the pulse generator 23 is at a high level (ON) and stops during a period during which the pseudo random pulse signal is low (OFF). As shown in FIG. 3, during the period when the pseudo random pulse signal (PN signal) h is at the high level (ON), the ultrasonic transducer 22b is placed in the acousto-optic element body 22a.
An elastic wave 22d is generated due to the vibration of the above, and the continuous light g of the frequency f incident in this state becomes the higher-order light i of the frequency (f + Δf).
Is output as On the other hand, a pseudo random pulse signal (P
Since the elastic wave 22d is not generated in the acousto-optic element main body 22a during the period when the N signal) h is at the low level (OFF), the continuous light g having the frequency f incident in this state is not shifted to the original frequency. Output as the zero-order light j having f. The zero-order light j output from the acousto-optic element 22
And the higher-order light i are multiplexed by the multiplexer 24, and as a measurement light k, a pair of optical fibers 26a and 26 constituting the optical cable 26 are output from an output terminal 25a of the optical fiber measurement device.
b is applied to the optical fiber 26a to be measured. As shown in FIG. 3, the measurement light k is light having a frequency f when the pseudo-random pulse signal (PN signal) h is at a low level, and the frequency (f) when the pseudo-random pulse signal (PN signal) h is at a high level. f + Δf) is substantially continuous light. As shown in FIG. 8, a number of repeaters 9 are interposed in the optical cable 26. Therefore, return light m resulting from back scattering or Fresnel reflection of the measurement light k propagating through the optical fiber 26a to be tested is input from the other optical fiber 26b to the input terminal 25b of the optical fiber measurement device. The return light m incident from the input terminal 25b into the optical fiber measuring device is incident on one end of the multiplexer 28 of the optical heterodyne detector 27. A continuous light g having a frequency f output from the light source 20 is provided at the other end of the multiplexer 28.
Is incident. Return light m includes light of frequency f (zero-order light)
And light of a frequency (f + Δf) (higher-order light). Therefore, as shown in FIG. 4, the continuous light g and the return light m are multiplexed, and the detected signal o has (f + Δf−f) = Δf
And a signal having a frequency of (f + Δf + f) = (2f + Δf). In this detection signal o, Δf and (2f
+ Δf) is a state in which a beat is generated. Specifically, the frequency of the continuous light g is about 192 T (tera) Hz, and Δf applied as a modulation signal to the acousto-optic element 22 is about 100 MHz. This detection signal o is Δf
Are input to the signal extraction unit 30. In the signal extracting section 30, filters 31, A
A / D converter 32 and a square addition circuit 33 are incorporated.
The filter 31 extracts only a component of Δf of about 100 MHz included in the detection signal o input to the signal extraction unit 30 and sets it as a time change signal p of the intensity of the return light. That is, as shown in FIG. 4, the time-varying signal p of the intensity of the return light includes only a signal component corresponding to the pseudo random pulse signal h output from the pulse generator 23. In other words, a time change signal of the intensity of the return light corresponding to the state applied to the optical fiber 26a is obtained using the light of only the pseudo random pulse signal h as the measurement light. The time change signal p of the intensity of the return light is A /
A / D conversion is performed by the D converter 32, converted to a (+) component by the square adder 33, averaged over a plurality of repetition periods T _0, and a time change signal p _{1 of the} final return light intensity. Is input from the signal extraction unit 30 to the correlator 34. The pseudo random pulse signal (PN signal) h output from the pulse generator 23 is input to the correlator 34. The correlator 34 calculates the correlation between the time change signal p ₁ of the intensity of the return light and the pseudo random pulse signal h and outputs a correlation signal q. The correlation signal q shown in FIG.
Shows a time-varying signal of the return light with the spatial resolution improved to the pulse width unit as a result. The correlation signal q, which is output from the correlator 34 and indicates the highly accurate return light time-varying signal, is calculated by the characteristic calculator 36.
The logarithmic conversion is performed by the LOG converter 37 in, and the data is converted into decibels and displayed on the display 38. Further, the characteristic calculating unit 36 uses the correlation signal q indicating the highly accurate return light time-varying signal to calculate the optical fiber 2 to be measured.
Various characteristics such as transmission characteristics and attenuation characteristics for 6a are calculated. [0038] In such an optical fiber measurement apparatus constructed as above, the optical fiber 26 every predetermined repetition period T ₀
The pulse width of the light pulse applied to a can be substantially expanded as compared with the light pulse of the conventional light pulse test apparatus 1 shown in FIG. Therefore, the dynamic range of the obtained return light can be expanded, and the S / N ratio can be improved. Further, in the optical fiber measuring device thus configured, the measuring light k incident on the optical fiber 26a to be measured is substantially frequency-modulated by the pseudo random pulse signal h as shown in FIG. It is light that can be regarded as continuous light. Therefore, the measurement light k has a constant light intensity no matter what period is extracted. Therefore, even if the measurement light k passes through the automatic intensity control circuit (APC) 11 incorporated in each of the optical amplifiers 10a and 10b of the repeater 9 of the optical cable 5e shown in FIG. The level does not change. The components of the continuous light g other than the components of the pseudo random pulse signal h included in the return light m of the measurement light k are removed by the optical heterodyne detector 27 and the next signal extraction unit 30. The correlation between the time-varying signal p ₁ of the intensity of the return light after the removal of the extra component and the pseudo-random pulse signal h is calculated and becomes the correlation signal q. As a result, the correlation signal q indicates a time-varying signal of the return light whose spatial resolution has been improved to the pulse width unit. Therefore, it is possible to improve the measurement accuracy of the characteristic of the measurement target optical fiber 26a obtained from the correlation signal q. FIGS. 5 and 6 are diagrams showing a signal processing procedure when a 4-bit Golay code is used as a pseudo random pulse signal. As shown in FIG. 5A, a Golay code is composed of a code string A (1, 1, 1, -1) and a code string B (1, 1, -1) which is a complementary pair of the code string A. ,
1). If this Golay code is applied to an optical pulse train, “−1” cannot be expressed. Therefore, in this optical fiber measuring device, the optical pulse is expressed by the following four types of pulse trains. A ₊ = (1,1,1,0) A _ = (0,0,0,1) B ₊ = (1,1,0,1) B _ = (0,0,1,0) As shown in FIG. 5B, a 4-bit pulse train A ₊ , A as a pseudo-random pulse signal having a predetermined length T _1.
_, B ₊ , and B_ are applied to the optical fiber to be measured at a constant repetition period T ₀ . When each pulse train of 4 bits is applied to the optical fiber to be measured, theoretically, as shown in FIG. 5C, each applied bit is delayed by 1 bit.
Return light due to back scattering and Fresnel reflection appears. However, the actually observed return light has a waveform (time-elapsed waveform) obtained by adding (synthesizing) the four return lights as shown in FIG. 5D. The waveform of the return light (return light waveform) is obtained for each of the four types of pulse trains A ₊ , A_, B ₊ , and B_ described above. Actually, in order to improve the measurement accuracy, the corresponding pulse train is repeated a plurality of times for each pulse train A ₊ , A_, B ₊ , B_.
It is applied to the optical fiber to be measured, the return light waveform shown in FIG. 5D is measured, and the average value is calculated to obtain the return light waveform of the corresponding pulse train. FIG. 6 shows the four types of pulse trains A ₊ and A ₊ .
9 is a flowchart showing a procedure for obtaining a return light waveform for one pulse (a time change signal of return light intensity) from return light waveforms of _, B ₊ , and B_. Firstly, the pulse train _A + (1 by applying a pulse train _A + (1,1,1,0) to the optical fiber to be measured,
A return light waveform of (1, 1, 0) is obtained (S1). Similarly, A
_ (0,0,0,1) is applied to the optical fiber to be measured to obtain a return light waveform of pulse train A_ (0,0,0,1) (S2). Next, the return light waveform of the pulse train A_ (0,0,0,1) is subtracted from the return light waveform of the pulse train A ₊ (1,1,1,0) to obtain a difference return light waveform, that is, The return light waveform (time-change signal of the intensity of the return light) of Golay A (1,1,1, -1) is obtained (S3). This Golay A (1,
The return light waveform of (1,1, -1) and the original Golay A (1,1)
(1, 1, -1) (correlation signal) _RA is obtained (S
4). In a similar procedure, the pulse train B ₊ (1, 1,
0, 1) is applied to the optical fiber to be measured, and the pulse train B
₊ (1,1,0,1) of a return light waveform (S5). Similarly, a return light waveform of the pulse train B_ (0,0,1,0) is obtained by applying to the optical fiber to be measured B_ (0,0,1,0) (S6). Next, the return light waveform of the pulse train B_ (0,0,1,0) is subtracted from the return light waveform of the pulse train B ₊ (1,1,0,1) to obtain a difference return light waveform, that is, The return light waveform (time-change signal of the intensity of the return light) of the Golay B (1,1, -1,1,1) is obtained (S7). This Golay B (1,
1, −1, 1) and the original Golay B (1,
1, the correlation between -1,1) (correlation signal) of a R _B (S
8). [0052] Finally, the correlation correlated with (correlation signal) R _A (correlation signal) by adding the R _B, obtain one pulse of the return light waveform (time varying signal of the intensity of return light). As described above, by using a Golay code as a pseudo-random signal, it is possible to obtain a time-varying signal of the intensity of return light with an improved dynamic range with a spatial resolution corresponding to the pulse width. Specifically, the number of bits of the pseudo random pulse signal h is set to L (in the case of the above-mentioned Golay code, L = 3
2), the characteristic calculation unit 36 is compared with one pulse of the measurement light b in the conventional optical pulse test apparatus shown in FIG.
5 log on the display screen of the display 38 at
The dynamic range can be improved by [(L ^1/2 ) / 2] dB. As described above, in the optical fiber measuring apparatus of the present invention, the measuring light incident on the optical fiber to be measured is substantially continuous light into which a pseudo random optical pulse train is incorporated. Light is adopted. Therefore, in the optical fiber line having the repeater, the spatial resolution of the time-varying signal of the return light can be improved to one pulse width unit, and the pulse width of the measurement light can be expanded equivalently. The effect of the pseudo-random pulse train that can expand the dynamic range is sufficiently exhibited, and the measurement accuracy of the characteristics of the optical fiber to be measured can be greatly improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a schematic configuration of an optical fiber measuring device according to an embodiment of the present invention. FIG. 2 is a diagram showing an acousto-optical device incorporated in the optical fiber measuring device of the embodiment. FIG. 3 is a diagram for explaining an operation principle. FIG. 3 is a diagram showing a waveform of each light showing a procedure for obtaining measurement light incident on an optical fiber to be measured. FIG. 4 is a diagram showing an overall operation of the optical fiber measuring device of the embodiment. The time chart shown in FIG. 5: Golay (Gola)
y) A diagram for explaining the operation when a code is adopted. FIG. 6: Golay as the pseudo-random signal
FIG. 7 is a diagram showing a signal processing procedure when codes are used. FIG. 7 is a block diagram showing a schematic configuration of a conventional optical fiber measuring device. FIG. 8 is a diagram showing a relationship between an optical cable and a repeater. Built-in automatic intensity control circuit (AP
FIG. 10 is a diagram for explaining the operation of C). FIG. 10 is an automatic intensity control circuit (APC) for returning light.
20: Light source 21: Optical amplifier 22: Acousto-optic element 23: Pulse generators 24, 28 ... Combiner 26: Optical cables 26a, 26b ... Optical fiber 27 ... Optical heterodyne detector 29 ... Photoelectric converter 30 ... Signal extraction unit 31 ... Filter 32 ... A / D converter 33 ... Square adder 34 ... Correlator 36 ... Characteristic calculation unit 37 ... LOG converter 38 ... Display

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Tatsuyuki Maki             Henri 5-10-27 Minamiazabu, Minato-ku, Tokyo             Tsu Co., Ltd. (72) Inventor Shinji Hoshino             Henri 5-10-27 Minamiazabu, Minato-ku, Tokyo             Tsu Co., Ltd. (72) Inventor Kazuhiko Ishiguro             NEC Corporation 5-7-1 Shiba, Minato-ku, Tokyo NEC Corporation             In the formula company F term (reference) 2G086 CC03 KK01

Claims (1)

Claims 1. A measurement light (k) is made incident on an optical fiber (26a) to be measured, and return light (m) caused by back scattering and Fresnel reflection from the optical fiber is detected. Then, in the optical fiber measuring device for measuring the characteristics of the optical fiber from the return light, a light source (20) for generating continuous light of a constant frequency and a pseudo-random pulse signal of a predetermined length are generated at a constant repetition period. A pulse generator (23), receiving the continuous light and the pseudo random pulse signal,
An acousto-optic element (22) for switching the continuous light between zero-order light and high-order light according to the signal level of the pseudo-random pulse signal and outputting the same, and combining the zero-order light and high-order light output from the acousto-optic element. A multiplexer (24) that oscillates and enters the optical fiber as measurement light; and an optical heterodyne detector that multiplexes return light from the optical fiber and continuous light from the light source to perform optical heterodyne detection. (27), a signal extraction unit (30) for extracting a time-change signal indicating a time change of the intensity of the return light from the detection signal output from the optical heterodyne detector, and a time extracted by the signal extraction unit. A correlator (34) for calculating a correlation between the change signal and the pseudo-random pulse signal and outputting it as a correlation signal; and calculating a characteristic of the optical fiber based on the correlation signal output from the correlator. Characteristic calculation section for (36) and an optical fiber measuring device comprising a.