JP2676045B2 - Optical pulse tester - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Object of the Invention [Industrial field of application] The present invention is a simple optical pulse tester that is optically or electrically driven by reflected light at the incident end of an optical fiber to be measured. The present invention relates to an optical pulse tester that can be manufactured at a low cost with a very short time in which the circuit is temporarily saturated.

[Prior Art] In an optical pulse tester that measures the position of a fiber break point and fiber loss using backscattered light, an incident end of an optical fiber to be measured (mainly an optical pulse tester and an optical fiber to be measured is used). Optically or electrically the circuit of the optical pulse tester is temporarily saturated by the reflected light generated at the connector connection part of the optical fiber, and a dead zone called an unobservable area is created near the incident end of the optical fiber under test. This was a big problem. For this reason, as the optical pulse tester A provided with the optical pulse processing system AL for eliminating the dead zone, one using an optical switch as shown in FIG. 7 is well known in the related art. In the figure, 1 is a signal light source, 2
Is an optical switch, 3 is a control system for adjusting the operation timings of the signal light source 1 and the optical switch 2, 4 is an optical / electrical converter,
Reference numeral 5 is an electric signal processing system, 6 is a connector, 7, 8 and 9 are coupling single-mode optical fibers, and 10 is a measured single-mode optical fiber. The waveform stored in the memory by the electric signal processing system 5 is
Averaged to improve S / N ratio.

This is to separate the reflected light and the backscattered light by switching the optical path for a certain time by utilizing the fact that the backscattered light is delayed with respect to the reflected light at the incident end of the optical fiber using a light source by pulse intensity modulation. I am aiming for. Here, the light amplitude of the signal light source 1 changes in a pulse shape as shown in FIG. The single optical pulse Lp emitted by the signal light source 1 passes through the optical switch 2 and is fed to the optical fiber under test.
It is excited by 10 and propagates in the optical fiber 10. Light pulse Lp
The backscattered light Lpa 'and the reflected light Lpb' generated during the propagation of the light propagate backward to the incident end of the optical fiber 10 to be measured, and the backscattered light Lpa 'is branched and coupled to the optical / electrical converter 4 by the optical switch 2. After being converted into the electrical signal Es by the optical / electrical converter 4, the electrical signal processing system 5 performs averaging processing for improving S / N. The optical switch 2 guides the return light to the optical / electrical converter 4, which is a light receiving section, by electrically switching the path of the light, so that an arbitrary portion of the return waveform can be extracted. Therefore, the excessive reflection signal generated at the incident end of the optical fiber 10
Since Lpb ′ can be masked, saturation of the light receiving part and the electric system can be prevented, and the dead zone can be reduced.

However, the switching time of the optical switch 2 in the optical pulse tester A using the conventional optical switch 2 is about 1 μs.
Therefore, the area of the measured optical fiber 10 which is unobservable due to the switching of the optical path by the optical switch 2 is about 100 m or more from the near end, and even if the influence of the reflection at the incident end of the measured optical fiber 10 can be avoided. The dead zone based on the switching time of the optical switch 2 exists regardless of the resolution of the signal light source 1, and there is a drawback that highly accurate loss measurement cannot be performed. There is a semiconductor optical switch as an optical switch that has a switching speed higher than that of an optical switch using an acousto-optic effect using ultrasonic waves, but since it has a large waveguide loss and a large crosstalk, it cannot be used in an optical pulse tester. Have difficulty.

[Problems to be Solved by the Invention] An object of the present invention is to provide an optical pulse tester capable of significantly reducing the dead zone due to the reflected light of the present invention to the same extent as the distance resolution of the backscattered light in the optical fiber under test. Is.

(2) Configuration of the Invention [Means for Solving the Problems] The present invention adopts an FSK heterodyne detection method that changes the light emission frequency of the light source in a pulsed manner, and the beat frequency after optical / electrical conversion by reflected light and backscattered light. The main feature is that the dead zone is reduced to the same level as the distance resolution of the optical pulse tester by providing a difference between the two and achieving complete separation of reflected light and backscattered light without using an optical switch. .

Example 1 A first example of the present invention will be described with reference to FIG.

FIG. 1 is a schematic configuration diagram of the present invention B having an optical pulse processing system BL, and the same elements as those in the conventional example A of FIG.

In the figure, 11 is a light source for local light, 12 is a control system for synchronizing the operation timing of the two light sources 1 and 11, 13 and 14 are directional couplers, 15 is a bandpass filter, and 16 and 17 are single couplings. It is a mode optical fiber.

Since the control system 12 corrects the optical path difference of the respective propagating lights when the outgoing lights of the signal light source 1 and the local light source 11 are incident on the direction control coupler 14, in the present embodiment, The optical path inside the device is considered to be zero.

Generally, in heterodyne detection, the power of the local light L0 is
P _L , the power of the signal light Ls is P _S , the frequency of the local light L0 is f _L , the frequency of the signal light Ls is f _S , and the phase difference between the local light L0 and the signal light Ls is φ. The output current i _C (t) of is given by the following equation.

i _C (t) = aP _L P _S cos {2π (f _L −f _S ) + φ} (1) In the above equation, a is a proportional constant peculiar to the light source element, and f _L −f _S is a beat frequency. .

Here, the light sources 1 and 11 undergo frequency modulation as shown in FIGS. 2A and 2B, and emit light with a constant amplitude. In the figure, there is a mutual relationship of f ₃ > f ₂ > f ₄ > f ₁ . In this method, the backscattered light Lsa ′ is observed by paying attention to the component of the frequency f ₃ . Here, if only the component of the frequency f ₃ is detected, it can be considered that the light emission power from the signal light source 1 is a single optical pulse having an amplitude only from time t ₁ to t ₂ . Therefore, t ₁
-T ₂ corresponds to the pulse width in the normal optical pulse tester A, and determines the distance resolution of the optical pulse tester A. Before time t ₁ , the frequency of the local light L 0 is f ₁ , and the frequency of the signal light Ls is f ₂ for both the reflected light Lsb ′ and the backscattered light Lsa ′.
Therefore, as shown in FIG. 3 (a), the reflected light from the connector 6
The beat frequencies of Lsb ′ and backscattered light Lsa ′ are both f ₁
It becomes −f ₁ .

The frequency of the local light L0 between times t ₁ and t ₂ is f ₄ , the frequency of the signal light Ls is f ₃ of the reflected light Lsb ′, and the time t
_The backscattered light Lsa 'from before the reflection point generated before _{1 and} received after t ₁ is f ₁ , and the backscattered light Lsa' generated after time t ₁ and received is f ₃ . As shown in FIG. 3B, at this time, the beat frequency of the reflected light Lsb ′ is f ₃ −f ₄ , and the beat frequencies of the back scattered light Lsa ′ are f ₂ −f ₄ and f ₃ −f.
₄

Frequency of time t ₂ after the local light L0 is f _1, the signal light Ls
The reflected light Lsb ′ is f ₂ and the backscattered light Ls from the point beyond the reflection point that is generated before time t ₁ and received after time t ₂
a ′ is f ₂ and the backscattered light Lsa ′ generated between time t ₁ and t ₂ and received after t ₂ is f ₃ and the backscattered light generated after time t ₂ and received Lsa ′ is f ₂ . As shown in FIG. 3C, at this time, the beat frequency of the reflected light Lsb ′ is f ₂ −f ₁ and the beat frequency of the back scattered light Lsa ′ is f ₂ −f ₁
And f ₃ −f ₁ .

Among the light-receiving component, the beat frequencies including components of the reflected light Lsb 'frequency f ₃ is f ₃ -f _4, backscattered light frequency f ₃
Lsa 'beat frequency comprising components of an f ₃ -f ₄ and f ₃ -f _1, also the reflected light Lsb at any time from above' beat frequency between the local light L0 and the f ₃ -f ₁ component Since the beat frequency does not exist, the reflected light Lsb ′ can be completely blocked by filtering only the f ₃ −f ₁ component by the bandpass filter 15. The dead zone newly generated by blocking the component of the backscattered light Lsa ′ whose beat frequency is f ₃ −f ₄ is the dead zone existing in the prior art for masking the reflected light Lsb ′. It can be significantly reduced compared to.

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

d = c _f (t ₂ −t ₁ ) (3) In the equation, c _f is the speed of light in the optical fiber. In this method, the resolution is almost the same as the dead zone.

Example 2 A second example of the present invention will be described with reference to FIG.

FIG. 4 is a schematic configuration diagram of the present invention C provided with an optical pulse processing system CL, and the same elements as those in the first embodiment of FIG. 1 are designated by the same reference numerals.

The signal light sources 1 in the figure each undergo frequency modulation as shown in FIG. 5 and emit light with a constant amplitude. In the figure, mutual f ₃ > f ₂
There is a relation of> f ₁ .

In this embodiment, since the intensity of the reflected light Lsb 'at the incident end of the optical fiber 10 to be measured is much higher than the intensity of the backscattered light Lsa', the local light source 11 of the first embodiment is omitted. Consider the reflected light Lsb ′ as the local light L0.

Before time t ₁ , the frequency of the reflected light Lsb ′, which is local light, is f
_2, the frequency of the signal light Ls is backscattered light Lsa 'is f _2. Therefore, the beat frequency of the backscattered light Lsa 'becomes zero.

The frequency of the reflected light Lsb ′, which is the local light between time t ₁ and t _2, is f ₃ , and the frequency of the signal light Ls is the backscattered light Lsa ′ generated before time t ₁ and received after t _1. Is f ₂ , and the backscattered light Lsa ′ generated and received after time t ₁ is f ₃ . At this time, the beat frequency of the backscattered light Lsa ′ is f ₃ −f ₂
And 0.

The frequency of the reflected light Lsb ′, which is the local light after time t _2, is f ₁
, And the signal light frequency of Ls are backscattered light Lsa received by the light generated at time t ₁ before t ₂ after 'is f _2, t from time t ₁
Backscattered light Lsa received by the light generated by t ₂ after until ₂ '
The frequency of the reflected light Lsb ′ that is the local light is f ₃ and
_The backscattered light Lsa ′ generated after ₂ and received is f ₁ . At this time, the beat frequency of the backscattered light Lsa ′ is f ₂ −f ₁ ,
f ₃ −f ₁ and 0.

Here, if only the component with the beat frequency f ₃ −f ₁ is filtered by a bandpass filter, the reflected light Lsb ′ can be completely separated, and the dead zone will be as large as the separation ability t ₂ −t _1. It is possible to narrow it.

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

Example 3 A third example of the present invention will be described with reference to FIG.

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

In the figure, reference numeral 18 is a reflector, and 19 is a coupling single mode fiber. Here, the reflector 18 is the reflected light Lsb 'in the second embodiment.
Since the reflected light Lsb ′ having a certain level or more is indispensable because it is considered to be local light, when the incident end of the optical fiber 10 to be measured is cut with a cutter to attach the connector 6, the cut end is a mirror surface. In order to solve the drawback that sufficient reflected light Lsb ′ cannot be secured if it is not cut well on the cut surface of the shape, it is branched from the directional coupler 13 via the coupling fiber 19 and always from the reflection plate 18. Reflected light Ls of a certain level
It is specially designed to introduce b '.

Here, each of the signal light sources 1 undergoes frequency modulation as shown in FIG. 5 and emits light with a constant amplitude. In the figure, mutual f ₃
There is a relation of> f ₂ > f ₁ . As in the second embodiment, the dead zone can be significantly reduced to the same level as the resolution.

(3) Effect of the Invention Thus, the optical pulse testers B, C and D of the present invention are reflected because the beat frequency after heterodyne detection is different without using an optical switch in the optical system of the optical pulse processing systems BL, CL and DL. Light Ls
Since b ′ and the backscattered light Lsa ′ can be completely separated, the optical system can be simply constructed at low cost, and the dead zone can be significantly reduced to the same extent as the resolution.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram according to a block diagram showing a first embodiment of the present invention, FIGS. 2 (a) and 2 (b) are modulation characteristic diagrams of the signal light source and the local light source, respectively, and FIG. a)
(B) and (c) are beat frequency characteristic diagrams after the same / heterodyne detection respectively, FIG. 4 is a schematic configuration diagram by a block diagram showing a second embodiment of the present invention, and FIG. 5 is the same / signal light source modulation FIG. 6 is a characteristic diagram, FIG. 6 is a schematic configuration diagram by a block diagram showing a third embodiment of the present invention, FIG. 7 is a schematic configuration diagram by a block diagram showing a conventional example of an optical pulse tester, and FIG. 8 is the same light source. 6 is a modulation characteristic diagram of FIG. 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 …… Connectors 7,8,9,16,17,19 …… Coupling single-mode optical fiber 10 …… Measured optical fiber 11 …… Local light source, 13,14… … Directional coupler 15 …… Bandpass filter 18 …… Reflector

Claims (3)

(57) [Claims] 1. A light source element for signal light whose emission frequency changes in the order of f ₂ , f ₃ , and f _2, a first directional coupler, and an optical fiber to be measured in order through a coupling single-mode optical fiber. While optically coupling with the first directional coupler, the second directional coupler and the optical receiver are optically coupled in this order from a free end of the incident port of the first directional coupler through a coupling single-mode optical fiber. On the other hand, the light source element for local light emission whose light emission frequency changes in the order of f ₁ , f ₄ , and f ₁ in synchronization with the light source element for signal light, and the free end of the incident port of the second directional coupler are connected to each other. In an optical pulse processing system optically coupled through 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 | ≠ | 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 the center frequency is between the photoreceiver and the electrical signal processing system electrically coupled to it. Optical pulse tester characterized by inserting a bandpass filter of | f ₁ −f ₃ | 2. A light source element for signal light, a directional coupler, and an optical fiber for measurement, whose emission frequencies change in the order of f ₂ , f ₃ , and f ₁ , are optically transmitted through a coupling single-mode optical fiber. In the optical pulse processing system in which the free end of the incident port of the directional coupler is optically coupled to the photodetector through the coupling single mode optical fiber, 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 and is electrically coupled to the photoreceiver An optical pulse tester characterized by inserting a bandpass filter having a center frequency of | f ₁ −f ₃ | between electrical signal processing systems. 3. A light source element for signal light, a directional coupler, and an optical fiber to be measured, in which the emission frequency changes in the order of f ₂ , f ₃ , and f ₁ , are optically transmitted through a coupling single-mode optical fiber. On the other hand, in the optical pulse processing system in which one end of the incident port of the directional coupler is optically coupled to the receiver and the reflector through the coupling single mode optical fiber, Is | f ₁ −f ₃ | ≠ | f ₁ −f ₂ |, | f ₁ −f ₃ | ≠ | f ₂ −f ₃ |, f ₁ ≠ f ₃ , and An optical pulse tester characterized by inserting a bandpass filter having a center frequency of | f ₁ −f ₃ | between electrically coupled electrical signal processing systems.