JPH0325732B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <<Industrial Application Field>> The present invention relates to improvement of an optical fiber testing device using the OTDR method.

<<Prior Art>> Optical fibers have scattering inherent to glass. Light waves propagating through an optical fiber core undergo Rayleigh scattering due to scattering sources such as dopants within the core. In particular, among this scattered light, the scattered light that is guided toward the rear of the fiber core (in the direction of the light source) is called backscattered light.

OTDR (Optical Time Domain
This is called the Reflectometry method.

FIG. 6 is a block diagram showing the basic configuration of an optical fiber testing device using the conventional OTDR method. The output light from the light source LD1 made of a semiconductor laser is separated into light for transmission and local oscillation light by a directional coupler CP1 made of a fused coupler. The transmission light is pulse-modulated by shifting the optical frequency by a predetermined frequency in the acousto-optic element AO1 excited by the drive circuit DR1, and is sent to the fiber under test via the directional coupler CP2 similar to CP1 and the optical connector CN1. Inject to FB1. The received light consisting of Fresnel reflected light and backscattered light returning from the fiber under test FB1 is sent to the directional coupler CP2.
After being multiplexed with the local oscillation light at , it enters the light receiving element PD1, which is a PIN photodiode. Beat components of the received light and local oscillation light appear in the output of the light receiving element PD1, which is narrowed by the bandpass filter BF1 and detected after improving the S/N ratio. Since it uses an optical heterodyne detection method, it is possible to detect signals even from received light levels that would be buried in thermal noise using a direct detection method.

≪Problems to be solved by the invention≫ However, in the optical fiber testing device configured as described above, the frequency shift is given by the acousto-optic element AO1, but in addition to this, a fused coupler is also used as a directional coupler. This makes the configuration complicated.

Another problem was that the light receiving element and amplifier were saturated by excessive light due to Fresnel reflection at the input end of the optical fiber.

The present invention was made in order to solve the above problems, and is equipped with an optical mask function to prevent the saturation of the light receiving element and amplifier due to unnecessary excessive light input in an optical fiber testing equipment using an optical heterodyne detection method. The purpose is to realize an optical fiber testing device with a simple configuration.

<Means for Solving the Problems> The present invention relates to an optical fiber testing device that observes the state of the fiber under test by inputting light from a light source into the fiber under test and detecting backscattered light from the fiber under test. It is characterized by an acousto-optic element that deflects the output light from the light source so that it enters the fiber under test and shifts its frequency, and a backscattered light from the fiber under test and the light from the light source. The optical system includes an optical system that multiplexes O-order transmitted light generated when the light enters the acousto-optic element, and a light receiving element that detects the combined output light from this optical system.

≪Example≫ The present invention will be explained in detail below using the drawings.

FIG. 1 is a block diagram showing an embodiment of an optical fiber testing apparatus according to the present invention. LD2 is composed of a semiconductor laser etc., frequency stabilized,
IS1 is a dynamically single-mode coherent light source with a spectral linewidth suppressed to at least 500 kHz or less, and is unnecessary for semiconductor lasers by transmitting the output light of this light source LD2 and blocking the light in the opposite direction returning to the light source side. FB2 is an optical fiber that inputs the output light of this isolator IS1, LSI is a lens that inputs the output light of this optical fiber FB2, and PS1 inputs the output light of this lens LS1. End face (upper side of the diagram)
The prism that enters AO2 is this prism PS1
DR2 is a drive circuit that excites this acousto-optic element AO2, PS2 is a prism that inputs the deflection output of the acousto-optic element AO2 into one end face (upper side of the figure), and LS2 is a prism that makes the deflection output of the acousto-optic element AO2 incident. A lens into which the reflected light from prism PS2 enters; FB3 is an optical fiber into which the output light from this lens LS2 enters;
CN2 is an optical connector into which the output light of this optical fiber FB3 is input, FB1 is an optical fiber to be measured to which this connector CN2 is connected, and M1 is the prism PS1.
The reflected light from one end surface causes the acousto-optic element AO2 to
A mirror that passes through as transmitted light and is reflected on the other end surface (lower side of the figure) of the prism PS2 and then enters the mirror;
AT1 is an attenuator into which the light reflected from this mirror is incident, and HM1 is an attenuator into which the light passing through this attenuator AT1 is incident, and HM1 is the attenuator into which the light passing through this attenuator AT1 is incident, and which is connected to the optical fiber to be measured.
The return light from FB1 is connected to connector CN2, optical fiber.
FB3, a half mirror that is reflected at one end surface of the prism PS2 via the lens LS2, transmitted through the acousto-optic element AO2 as O-order transmitted light, reflected at the other end surface of the prism PS1 (lower side in the figure), and then enters from the other side. , LS3 is a condensing lens that inputs the output light of this half mirror HM1, and PD2 is this lens.
A light-receiving element receives the light passing through LS3, A1 is an amplifier circuit that receives the electrical output of this light-receiving element PD2,
SP1 is a signal processing circuit that inputs the output of this amplifier circuit A1 and controls the light source LD2 and drive circuit DR2, and CP1 is a computer that inputs the output of this signal processing circuit SP1.

The operation of the optical fiber testing apparatus configured as described above will now be described. FIG. 2 is a time chart for explaining the operation of the apparatus shown in FIG. Light source LD2
Pulsed light with time width T _p output from (Figure 2 A)
is incident on one end face of prism PS1 via isolator IS1, optical fiber FB2 and lens LS1, is reflected, and is incident on acousto-optic element AO2.
Since the acousto-optic element AO2 is excited with ultrasonic waves by the drive circuit DR2 in the section _TAO (FIG. 2B), it deflects the incident light by diffraction (first-order diffracted light here) and at the same time shifts the frequency by Δf. acousto-optic element
The polarized light output from AO2 is reflected by one end face of prism PS2 and is sent to the fiber under test FB via lens LS2, optical fiber FB3, and optical connector CN2.
1. The Fresnel reflected light generated at the input end of the fiber to be measured is connected to the optical connector CN2 and the optical fiber.
Reverse FB3 and lens LS2, prism PS
The light is reflected from one end face of 2 and enters the acousto-optic element AO2. Since the acousto-optic element AO2 is excited in the interval τ (Fig. 2B), the Fresnel reflected light is deflected, reflected at one end face of the prism PS1, and enters the isolator IS1 via the lens LS1 and the optical fiber FB2. do. The isolator IS1 blocks Fresnel reflected light from entering the light source LD2 and affecting its operation (optical mask function). Next, backscattered light from the fiber under test FB1 (Figure 2C)
is incident on the acousto-optic element AO2 as above, but
At this time, since the acousto-optic element AO2 is not excited (FIG. 2B), the light is transmitted as O-order transmitted light, reflected by the other end face of the prism PS1, and then incident on the half mirror HM1. In addition, the light from light source LD2 (Fig. 2 A) output in section T _LO is also transmitted through the prism.
After being reflected by PS1, it enters the acousto-optic element AO2,
It is transmitted as O-order transmitted light. This transmitted light is reflected by the other end face of the prism PS2, then reflected by the mirror M1, passes through the attenuator AT1, and then enters the half mirror HM1 as locally oscillated light. The backscattered light and the local oscillation light mixed by the half mirror HM1 interfere with each other, and are then focused by the lens LS3 and incident on the photodetector PD2, where a beat signal with a frequency △f, which is the difference between the two lights, is detected (heterodyne detection). ). The electrical output of the photodetector PD2 is amplified by the amplifier circuit A1,
After A/D conversion, averaging processing, etc. are performed in the signal processing circuit SP1, the signal is input to the computer CP1.
The attenuator AT1 adjusts the power of the locally oscillated light to a sufficient extent to increase the S/N ratio to the quantum limit in heterodyne detection.

According to the optical fiber testing device having the above configuration, the configuration can be simplified by providing the acousto-optic element with both the functions of an optical frequency shift and an optical directional coupler.

Further, it is possible to prevent the light receiving element and the amplifier from becoming saturated due to Fresnel reflection occurring at the input end of the fiber to be measured.

In the above embodiment, a stabilized continuous output light source LD2 is used, and a modulator consisting of an electro-optical element or the like is inserted between the isolator IS1 and the optical fiber FB2 to perform optical pulse modulation. Good too.

Also, a prism may be used instead of the half mirror HM1.

FIG. 3 is a block diagram showing a second embodiment of the optical fiber testing apparatus according to the present invention. The same parts as those in the apparatus shown in FIG. 1 are given the same symbols and the explanation will be omitted. LS4 is a condensing lens that receives the light reflected from the other end surface of the prism PS1, and FB4 is an optical fiber that receives the light that has been condensed by this lens LS4, CP.
3, the output light of this optical fiber FB4 enters from one end thereof, and the output light from one end of the optical fiber FB4 enters the light receiving element.
A fiber coupler (fused coupler) that enters PD2, AT2 is an attenuator similar to AT1 that enters the light reflected on the other end surface of prism PS2, and LS
Reference numeral 5 denotes a condensing lens into which the light passing through the attenuator AT2 is incident, and FB5 is an optical fiber through which the output light of this lens LS5 is incident and enters the fiber coupler from the other side. The backscattered light that has passed through the optical fiber FB4 and the local oscillation light that has passed through the optical fiber FB5 are mixed by the fiber coupler CP3 and then enter the light receiving element PD2.

Although the operating principle of the apparatus configured as described above is the same as that of the apparatus shown in FIG. 1, it has the advantage of a simpler configuration.

FIG. 4 is a block diagram showing a third embodiment of the present invention. The same parts as those in the apparatus shown in FIG. 1 are given the same symbols and the explanation will be omitted. PS3 is Prism PS
M2 is a Wollaton prism that inputs the light reflected from the other end surface of HM1 and makes the transmitted P-polarized light enter one end of the half mirror HM1.
A mirror that receives the S-polarized light output reflected by PS3,
AT2 is an attenuator that receives the locally oscillated light reflected from the other end surface of the prism PS2, PS4 is a polarizing prism that receives the light that has passed through the attenuator AT2, and HM2 is the S-polarized light that is reflected by the polarizing prism PS4 and the S-polarized light that is reflected by the mirror M2. A half mirror that receives S-polarized light from different end faces, LS4 is a condensing lens that receives the output light of this half mirror HM2, PD3 is a second light receiving element that receives the output light of this lens LS4, and CM1 is the prism.
The P-polarized light transmitted through PS4 is reflected by mirror M1 and then enters the Babinet-Soleil compensator, and the output light enters the other end of the half mirror HM1. Lens incident on PD2, A2 is light receiving element PD
2. Input the electrical output of PD3 to the signal processing circuit SP
This is an amplifier circuit that outputs to 1.

The operation of the apparatus having the above configuration will be explained below. The backscattered light reflected on the other end surface of prism PS1 is separated into P polarized light that passes through Wollaton prism PS3 and S polarized light that is reflected, and the locally oscillated light reflected on the other end surface of prism PS2 passes through attenuator AT2 and becomes polarized light. The light is separated into P-polarized light that passes through the prism PS4 and S-polarized light that is reflected. polarizing prism
P-polarized light transmitted through PS4 is reflected by mirror M1 and then passed through Babinet Soleil compensator CM1;
What is the P-polarized light that passes through the Wollaton Prism PS3?
The light is mixed by the half mirror HM1 and enters the light receiving element PD2 via the lens LS3. The S-polarized light reflected by the Wollaton Prism PS3 is the polarizing prism PS4.
The S-polarized light reflected by the mirror HM2 is mixed with the S-polarized light reflected by the half mirror HM2, and enters the light receiving element PD3 via the lens LS4. The electrical outputs of the light receiving elements PD2 and PD3 are summed by the amplifier circuit A2, and then processed in the same manner as in the apparatus of FIG. The Babinet Soleil compensator CM1 is used to match the polarization plane of the locally oscillated light with the polarization plane of the backscattered light, and is used as necessary. The ratio of P-polarized light and S-polarized light whose polarization planes are separated by the polarizing prism PS4 is set to 1:1 to improve the S/N ratio.

According to the optical fiber testing device having such a configuration, as explained above, each polarization component is divided into electrical signals, and then both components are added together.
Since it uses a polarization diversity reception method,
Not affected by fading phenomenon. The fading phenomenon is a phenomenon in which the backscattered light is observed to fluctuate significantly because the sensitivity of heterodyne detection is polarization dependent.When backscattering occurs, the polarization state changes depending on the location on the fiber under test. This is thought to occur because the state of polarization changes before it returns to the input end, causing interference. The polarization diversity reception method can prevent the detection efficiency from decreasing due to variations in the polarization state.

Note that in the above embodiments, not only those that utilize birefringence such as the Wollaton prism, but also
Any means capable of separating the planes of polarization, such as a polarizing beam splitter, can be used.

FIG. 5 is a block diagram showing a modification of the device shown in FIG. 4. The same parts as those in the apparatus shown in FIG. 4 are given the same symbols and the explanation thereof will be omitted. HW1 is polarizing mirror M
a half-wave plate that makes the S-polarized light reflected by the S-polarized light transmitted through the half mirror HM2 and then enters the half-mirror HM2, and converts it into P-polarized light;
HM3 is P transmitted through Wollaton prism PS3
It is a half mirror that mixes polarized light and P-polarized light that is the output of the half-wave plate. P-polarized light and S-polarized light, which are a mixture of backscattered light and locally oscillated light, are detected by photodetectors PD2 and PD3, respectively, and processed in the same manner as in the apparatus shown in FIG. 4.

<<Effects of the Invention>> As described above, according to the present invention, in an optical fiber testing apparatus using an optical heterodyne detection method, an optical fiber tester equipped with an optical mask function that prevents saturation of a light receiving element and an amplifier due to unnecessary excessive light input can be used. A fiber testing device can be realized with a simple configuration.

[Brief explanation of drawings]

FIG. 1 is a configuration block diagram showing an embodiment of an optical fiber testing device according to the present invention, FIG. 2 is a time chart for explaining the operation of the device shown in FIG. 1, and FIG. The second part of the test equipment
FIG. 4 is a block diagram showing a third embodiment of the optical fiber testing device according to the present invention, and FIG. 5 is a block diagram showing a modification of the device shown in FIG. 4. , FIG. 6 is a block diagram showing an example of a conventional optical fiber testing device. LD2...Light source, FB1...Fiber under test,
AO2...Acousto-optic element, PS1, PS2...Prism, HM1, HM2, HM3...Half mirror, M1, M2...Mirror, M3...Polarizing mirror, CP3...Fiber coupler, PS3...Wolaton prism, PS4...Polarizing prism, PD2,
PD3...Photodetector.

Claims (1)

[Scope of Claims] 1. In an optical fiber testing device that observes the state of a fiber under test by inputting light from a light source into the fiber under test and detecting backscattered light from the fiber under test, the output light from the light source is an acousto-optic element that deflects the light so as to be incident on the fiber to be measured and shifts its frequency; and O-order transmission that occurs when the backscattered light of the fiber to be measured and the light from the light source enter the acousto-optic element. An optical fiber testing device comprising: an optical system that combines light; and a light receiving element that detects the combined output light from the optical system. 2 After outputting pulsed light from a light source, an acousto-optic element is excited for a predetermined period of time to deflect Fresnel reflected light returning from the fiber to be measured so that excessive input light does not enter the light receiving element. The optical fiber testing device according to item 1. 3 Separate each of the two O-order transmitted lights output from the acousto-optic element into orthogonal polarization components, combine the same polarization components, detect each with two light receiving elements, and add the two detection outputs. An optical fiber testing device according to claim 1, configured as follows.