JPH022907A - Optical pulse tester - Google Patents

Abstract

PURPOSE:To separate reflected light and back scattering light and to make it possible to decrease a dead zone caused by the reflected light to the same degree of the distance resolution of the back scattering light by providing the difference in beat frequency between the reflected light and the back scattering light after light/electricity transducing action. CONSTITUTION:When emitted lights from a signal light source 1 and a local oscillating light source 11 are inputted into directional couplers 13 and 14, the difference in light paths of the propagating lights is corrected with a control system 12. It is considered that a light path in this apparatus is 0. The light sources 1 and 11 undergo frequency modulation and emit lights at constant amplitudes. The relationship among the light emitting frequencies is f3>f2>f4>f1. The beat frequency of the reflected light Lsb' at a connector 6 and the locally oscillated light Lo is f2-f1. In the beat frequency, the component of f3-f1 is not present. When only the component of f3-f1 is filtered through a BPF 15 for the beat frequency, the reflected light Lsb' can be completely blocked. The component part whose beat frequency is f3-f4 in the back scattering light Lsa' is blocked. Therefore, a dead zone can be drastically decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Purpose of the invention [Field of industrial application] The present invention is designed to simplify the measurement of optical fibers for people! ) The present invention relates to an optical pulse tester which can be manufactured at a low price because the time period during which the circuit of the optical pulse tester is temporarily saturated optically or electrically by the reflected light at one end is extremely short.

[Prior Art] In an optical pulse tester that uses backscattered light to measure the position of the fiber break point and fiber loss, the input end of the optical fiber under test (mainly between the optical pulse tester and the optical fiber under test) is used. The optical pulse tester circuit is temporarily saturated optically or electrically by the reflected light generated at the connector connection part of the optical fiber under test, creating an unobservable area called a dead zone near the input end of the optical fiber under test. , this was a big problem. Therefore, as an optical pulse tester A equipped with an optical pulse processing system AL for the purpose of eliminating the dead zone, one using an optical switch as shown in FIG. 7 is well known. In the figure, 1 is a light source for signal light;
2 is an optical switch, 3 is a control system that synchronizes the operation timing of the signal light source 1 and the optical switch 2, 4 is an optical/electrical converter, 5 is an electrical signal processing system, 6 is a connector, and 7.8.9 is a coupling. 10 is a single mode optical fiber to be measured. The waveforms stored in the memory by the electrical signal processing system 5 are averaged to improve the S/N ratio.

This uses a light source that uses pulse intensity modulation, and takes advantage of the fact that the backscattered light is delayed in time relative to the reflected light at the end of the optical fiber.By switching the optical path for a certain period of time, the reflected light and the backscattered light are separated. aiming to separate. Here, the amplitude of the light from the signal light source 1 changes in a pulsed manner as shown in FIG. A single optical pulse l-p emitted by the signal filling light f21 passes through the optical switch 2, is excited to the optical fiber to be measured 10, and propagates through the optical fiber 10. Backscattered light L p generated during propagation of optical pulses l and p
a' and the reflected light Lpb' propagate back to the input end of the optical fiber 10 to be measured, and the optical switch 2 converts the backscattered light L
Oa' is branch-coupled to an optical/electrical converter 4. After the signal is converted into an electrical signal ES by the optical/electrical converter 4, the electrical signal processing system 5 performs averaging processing to improve the S/N.

The optical switch 2 electrically switches the path of the light to guide the returned light to the light/N air converter 4 which is the light receiving section.
Any part of the returned waveform can be extracted. Therefore, an excessive reflected signal L pb generated at the input end of the optical fiber 10
' can be masked, so saturation of the light receiving section and electrical system can be prevented, and dead zones can be reduced.

However, in the optical pulse tester A using the conventional optical switch 2, the switching time of the optical switch 2 is approximately 1 μs, and the area of the optical fiber 10 under test that becomes unobservable due to switching of the optical path by the optical switch 2 is Even if the distance from the near end of the optical fiber to be measured is approximately 100 m or more and the influence of reflection at the 4th end of the optical fiber to be measured can be avoided, the optical switch 2
A dead zone based on the switching time exists regardless of the resolution of the signal light source 1, which has the drawback that highly accurate loss measurement cannot be performed.

Semiconductor optical switches are optical switches that use ultrasonic waves and have faster switching speeds than optical switches that use acousto-optic effects, but they have large waveguide loss and crosstalk, so they cannot be used in optical pulse testers. It is difficult.

[Problems to be Solved by the Invention] An object of the present invention is to provide an optical pulse tester that can significantly reduce the dead zone caused by reflected light to the same extent as the distance resolution of backscattered light in an optical fiber to be measured. It is.

(2) Structure of the invention [Means for solving the problems] The present invention incorporates an FSK heterodyne detection method that changes the emission frequency of a light source into a pulse, and uses reflected light and backscattered light to generate a beat frequency after optical/electrical conversion. The main feature is that the reflected light and backscattered light are completely separated without using an optical switch, and the dead zone is reduced to the same level as the distance resolution of the optical pulse tester.

Implementation example 1] A first embodiment of the invention will be described with reference to FIG.

FIG. 1 is a schematic diagram of the present invention B, which is equipped with an optical pulse processing system B. The same elements as those of the conventional example A shown in FIG. 7 are given the same reference numerals.

In the figure, 11 is a local light source, and 12 is two light sources 1.11e.
13.14 is a directional coupler, 15 is a bandpass filter, and 16.17 is a single mode optical fiber for coupling.

In the control system 12, the optical path difference between the respective propagation lights when the respective output lights of the signal light light source 1 and the local light source 11 are incident on the directional bamboo coupling 14 is corrected. The optical path inside the device is assumed to be 0.

Generally, in heterodyne detection, the power of the local light LO is PL, and the power of the signal light LS is P.

Set the frequency of the local light LO to f4. The frequency of the signal light Ls is fS
When one station emits light and the phase difference between O and the signal light ls is φ, the output current i c (1) after optical/electrical conversion is given by the following equation.

i (1) = aP1 p, cos (27r (fL-
f,)+φ) ・・・・・・・・・・・・
(1) In the above equation, a is a proportionality constant specific to the light source element, and f.

fS is the beat frequency.

Here, the lights 11 and 11 are respectively shown in FIG. 2(a).

The receiver receives frequency modulation as shown in (b) and emits light with a constant amplitude. In the figure, the relationship is fS>f2>f4>fl. In this method, the backscattered light Lsa' is observed by focusing on the component of frequency "3".Here, if only the component of frequency f3 is detected, the light emitted by the signal light source 1 will emit light from time t1 to t2. Therefore, 11-12 corresponds to the pulse width in a normal optical pulse tester A, and determines the distance resolution of the optical pulse tester A. Before time t1, the frequency of the local light O is fl, and the frequency of the signal light LS is f2 for both the reflected light Lsb' and the backscattered light Lsa'.

Therefore, as shown in FIG. 3(a), the beat frequencies of the reflected light LSb' and the backscattered light Lsa' at the connector 6 are both f2-fl.

The frequency of local light LO from time t to t2 is f4
The frequency of the signal light Is is that the reflected light L sb' is f
, the backscattered light L sa' from the reflection point that is generated before time t1 and received after t1 is f2, and the backscattered light L sa' that is generated and received after time t1 is f
It is S. At this time, as shown in FIG. 3(b), the reflected light L
The beat frequency of sb' is f - f4, and the beat frequency of backscattered light sa' is f - f and f
It is S-f4.

The frequency of the local light beam O after time t is fl, the frequency of the signal light LS is f2, and the reflected light L sb' is f2, and the backscatter from the reflection point that occurs before time t and is received after t2 The light Lsa' is f, the backscattered light Lsa' generated between time t and 12 and received after t is f3, and the backscattered light 1sa' generated and received after time t2 is is fl. Figure 3 (C
), the beat frequency of the scraped reflected light L sb' is fl-f, and the beat frequencies of the backscattered light L sa' are f -f and f3-fl. Among the received light components, the beat frequency that includes the component of the reflected light L sb' with the frequency f3 is f3-f4, and the beat frequencies that include the component of the backscattered light L sa' with the frequency f3 are f3-f4 and f3-fl. From the above, at any point in time, the beat frequency between the reflected light L sb' and the local light O is f3-
Since there is no f1 component, the beat frequency is f3-f1
If only the component is filtered by the bandpass filter 5, the reflected light Lsb' can be completely blocked. In the above, the beat frequency of the backscattered light Lsa' is f3-f4
The dead zone newly created by blocking the component can be significantly reduced compared to the dead zone that existed in the prior art to mask the reflected light 1sb'.

Here, the dead zone d is given by the following equation.

(j=C, (t2-tl)...(3
) where Cf is the speed of light in the optical fiber. Note that in this method, the resolution is comparable to that of the dead zone.

[Implementation example 2] A second embodiment of the invention will be described with reference to FIG.

FIG. 4 is a schematic diagram of the present invention C equipped with an optical pulse processing system C[, and the same elements as those in the first embodiment shown in FIG. 1 are given the same reference numerals.

Each of the signal light sources 1 in the figure receives frequency modulation as shown in FIG. 5 and emits light with a constant amplitude. In the same figure, mutually f'
There is a relationship of >fl>f.

In this embodiment, since the intensity of the reflected light L sb' at the input end of the optical fiber 10 to be measured is extremely large compared to the intensity of the backscattered light 1sa, the local light source 1 of the first embodiment
1 is omitted, the reflected light is reflected, and sb' is considered to be the local light and O.

Before time t1, the frequency of the reflected light L31)' which is the local light is fl, and the frequency of the signal light Ls is the frequency of the backscattered light I.
Sa' is fl. Therefore, the beat frequency of the backscattered light L sa' becomes 0.

Reflected light L s that is local light from time t to t2
The frequency of b' is f3, and the frequency of the signal light [S is the backscattered light Ls generated before time t1 and received after t1.
a' is fl, and backscattered light L sa' generated and received after time t1 is f3. At this time, the beat frequencies of the backscattered light Lsa' are f3-fl and O.

The frequency of the reflected light L sb' which is the local light after time t2 is fl, the frequency of the signal light LS is time t, and the frequency of the backscattered light L sa' generated previously and received after t2 is f.
The backscattered light Lsa' generated between time t1 and t2 and received after t2 is local light, and the frequency of the reflected light Lsb' is f3, and the frequency is f3, which is generated and received after time t2. The backscattered light 1 sa' is fl. At this time, the beat frequency of the backscattered light 1 sa' is t'
-r, t'-f'1 and i1 Here, if only the component of the beat frequency f -fl is filtered with a bandpass filter, the reflected light L sb' can be completely separated, and the resolution ability t2 It is possible to significantly narrow the dead zone to the same extent as -t1.

Here, the dead zone d is given by the above equation (3), and the resolution is approximately the same as the dead zone.

[Implementation Example 3] A third embodiment of the present invention will be described with reference to FIG.

FIG. 6 is a schematic configuration diagram of the present invention equipped with an optical pulse processing system D[, and the same elements as those in the second embodiment shown in FIG. 4 are given the same reference numerals.

In the figure, 18 is a reflection plate, and 19 is a single mode fiber for coupling. Here, the reflection plate 18 has the reflected light L in the second embodiment.
Since sb' is considered to be local light, it is essential to have a reflected light L sb' with a magnitude above a certain level.
When cutting the incident end of 0 with a cutter, if the cut is not well made into a mirror-like cut surface, sufficient reflected light Lsb'
Directional coupler 13 to solve the problem of not being able to secure
branched from through the coupling fiber 19, and always connected to the reflection plate 1.
This is specially provided to introduce a certain level of reflected light L sb' from the light source 8.

Each of the signal light sources 1 receives frequency modulation as shown in FIG. 5 and emits light with a constant amplitude.

In the figure, there is a mutual relationship of f3>f2>fl.

As in the second embodiment, it is possible to significantly reduce the dead zone to the same extent as the resolution.

(3) Effects of the invention Thus, the optical pulse testers B, C, and D of the present invention do not use optical switches in the optical systems of the optical pulse processing systems BL, CL, and OL, and the beat frequencies after heterodyne detection are different. Since the reflected light L sb' and the tangentially scattered light L sa' can be completely separated, the configuration of the optical system can be realized simply and at low cost, and the dead zone can be significantly reduced to the same level as the resolution. Qin effect.

[Brief explanation of the drawing]

FIG. 1 is a block diagram schematic diagram showing the first embodiment of the present invention, FIG. a)
(bHc) is a beat frequency characteristic diagram after the same hetero gain detection, FIG. 4 is a schematic block diagram showing the second embodiment of the present invention, and FIG. 5 is a modulation characteristic diagram of the signal light source. Fig. 6 is a schematic block diagram showing the third embodiment of the present invention, Fig. 7 is a block diagram showing a conventional example of an optical pulse tester, and Fig. 8 is the modulation characteristics of the light source. It is a diagram. A, B, C, D... Optical pulse tester AL BL, CL, DL... Optical pulse processing system 1...
Signal light source 3.12...Control system 4...Optical/electrical converter 5...Electrical signal processing system 6...Connector 7.8.9
．． 16.17.19...Single mode optical fiber for coupling 10...Optical fiber to be measured 11...Light source for local light 13.14...Directional coupler 15...Band pass filter 18 ... Reflector plate Fig. 1/ Fig. 2 (α) Fig. 2 (b) Time open (μS) Time (Ps) Fig. 8 (Q) Fig. 3 (C) Fig. 6 Fig. 4 Fig. 5 Figure 8

Claims (1)

[Claims] 1. A signal light source element whose emission frequency changes in the order of f_2, f_3, f_2, a first directional coupler, and an optical fiber to be measured are connected in this order through a single mode optical fiber for coupling. while optically coupling from one free end of the input port of the first directional coupler to the second directional coupler and the light receiver via a coupling single mode optical fiber. death,
On the other hand, the light emission frequency is f_ in synchronization with the signal light light source element.
An optical pulse processing system in which a local light source element that changes in the order of 1, f_4, and f_1 and one free end of the input port of the second directional coupler are optically coupled via a single mode optical fiber for coupling. , the relationship between the emission frequencies is |f_1-f_3|≠|f_1-f_2|, |f_1-
f_3|≠|f_2−f_3|, |f_1−f_3|≠
|f_3-f_4|, |f_1-f_3|≠|f_2-
f_4|, f_1≠f_3, and a bandpass filter having a center frequency of |f_1-f_3| is inserted between the light receiver and an electrical signal processing system electrically coupled thereto. The tester 2 optically couples a signal light source element whose emission frequency changes in the order of f_2, f_3, f_1, a directional coupler, and an optical fiber to be measured via a single mode optical fiber for coupling. In an optical pulse processing system in which one free end of the input port of the directional coupler is optically coupled to a light receiver via a coupling single mode optical fiber, the relationship between the emission frequencies is |f_1−f_3|≠ |f_1−f_2|, |f_1−
f_3 | An optical pulse tester 3 characterized by the following: A signal light source element whose emission frequency changes in the order of f_2, f_3, f_1, a directional coupler, and an optical fiber to be measured are connected in this order through a single mode optical fiber for coupling. In a light pulse processing system, one free end of the input port of the directional coupler is optically coupled to a light receiver and a reflector, respectively, via a coupling single mode optical fiber. The relationship between frequencies is |f_1-f_3|≠|f_1-f_2|, |f_1-
f_3 | Optical pulse tester featuring