JPS6235657B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical branch circuit suitable for use in measuring backscattered light of an optical fiber.

When light propagates through an optical fiber, backscattered light is produced by reflection or scattering. To measure this backscattered light as an evaluation of the optical fiber, an optical branching circuit is required to separate the incident light and the backscattered light.

Conventional optical branching circuits of this type include (1) a beam splitter type that uses a half mirror, (2) a polarization splitter type that uses the birefringence of crystals to separate light, and (3) an optical deflector. There are methods such as an optical deflector type that digitally separates light using

Method (3) above removes the very large level of Fresnel reflection that occurs at the input end of the optical fiber under test by electrically controlling the on/off time of the optical deflector, and This method can be said to be superior to methods (1) and (2) because it is possible to extract only the backscattered light that is significant.

However, the conventional optical branch circuit using the method (3) above also has problems, so a detailed explanation will be given with reference to FIG. FIG. 1 shows a conventional optical branching circuit using an ultrasonic deflector as an optical deflector. In the figure, a light pulse L _f emitted from a light source 1 passes through an ultrasonic deflector 2 while it is off, and then enters an optical fiber 4 to be measured through a lens 3 . The light scattered backward within the optical fiber 4 returns to the ultrasonic deflector 2 through the lens 3 again. At this time, when the ultrasonic deflector 2 is operated (turned on) by the oscillator 5, the backscattered light L _b deviates from the original optical path. The backscattered light L _b thus separated is guided to a detector 7 via a lens 6 and subjected to measurement.
Note that 2a is a grating generated by ultrasonic waves.

In this case, it should be noted that the deflection angle of the ultrasonic deflector 2 is only 2 to 4 degrees at most.
Therefore, in order to separate the deflected backscattered light L _b from the original optical path sufficiently, the distance between the light source 1 and the ultrasonic deflector 2 and between the detector 7 and the ultrasonic deflector 2 should be approximately
It needed to be about 30cm to 1m, and the optical branch circuit had become large. Furthermore, since the spacing is wide as described above, if there is a slight positional deviation among the light source 1, ultrasonic deflector 2, lens 3, and optical fiber 4 to be measured, or a slight deviation in the deflection angle of the ultrasonic deflector 2, the optical path will be distorted. Since this changes greatly, it is necessary to attach a fine movement table to the ultrasonic deflector 2, the optical fiber 4 to be measured, and the detector 7 in order to optimally receive the backscattered light _Lb. As a result, the optical branch circuit has become complicated and large, and has poor operability.

SUMMARY OF THE INVENTION In view of the above-mentioned prior art, it is an object of the present invention to provide an optical deflector type optical branching circuit that is compact and has stable characteristics. To achieve this objective, in the present invention, in order to separate backscattered light that is deflected by an optical deflector and backscattered light that travels straight without being deflected, both backscattered lights are separated from the light from the light source. It was decided that the light would be focused by the lens itself used to make the light incident on the light deflector. Furthermore, the light from the light source is applied to a lens through a transmitting optical fiber and enters an optical deflector, and one of the lights separated by the lens is received by a receiving optical fiber. Below, Figure 2~
The present invention will be explained based on FIGS. 5a and 5b.

FIG. 2 shows an embodiment of the present invention, in which light emitted from a light source 1 is input into a light transmitting optical fiber 8 and guided to the vicinity of an ultrasonic deflector 2. In FIG. The light emitted from the optical fiber 8 is collimated by the first lens 9, passes through the ultrasonic deflector 2, is condensed by the second lens 3, and enters the optical fiber 4 to be measured. The light reflected or scattered within the optical fiber 4 to be measured and propagated backward, that is, the backscattered light, is collimated by the second lens 3 and reaches the ultrasonic deflector 2 .
Now, suppose that the parallel light from the first lens 9 is passed through the ultrasonic deflector 2 in an off state, and then the ultrasonic deflector 2 is operated, that is, turned on, by the oscillator 5. The traveling direction of the backscattered light changes, it is focused by the first lens 9, enters the light receiving optical fiber 10 near the ultrasonic deflector 2, and is guided to the detector 7.

On the other hand, even if the ultrasonic deflector 2 is operated, its diffraction efficiency is not 100%, so some backscattered light travels straight without being deflected.
As a result, the light is focused on the end face of the light transmitting optical fiber 8 which is arranged at a distance D, which will be described later, with respect to the light receiving optical fiber 10. In this way, the first lens 9
Accordingly, the deflected backscattered light and the straightly traveling backscattered light can be separated into a focused beam spot.

At this time, as described above, by placing the optical fibers 8 and 10 for light transmission and light reception near the ultrasonic deflector 2 and inputting and outputting light, the light source 1, the detector 7 and the ultrasonic deflector 2 are connected to each other. The positional relationship with the optical branch circuit is not limited and the optical branch circuit can be made smaller. Further, since the distance between the optical fibers 8, 10 and the ultrasonic deflector 2 is narrow, even if there is some deviation in their position or deflection angle, it will hardly affect the reception of the backscattered light. That is, now, when the ultrasonic deflector 2 is turned on and off, the separation interval of the light beam on the focal plane of the first lens 9 is D, the focal length of the first lens 9 is f, and the ultrasonic deflector 2 is When the deflection angle is 2θ (radian), D=2θf...Equation (1). The separation distance D can be reduced to the fiber diameter when the optical fibers 8 and 10 are placed adjacent to each other as shown in FIG. Therefore, D=
Assuming 125μm and 2θ=2×π/180 (radians), f=3.6mm. Therefore, even considering the thickness of the lens 9 and the distance between the lens 9 and the ultrasonic deflector 2, the distance between the optical fibers 8, 10 and the ultrasonic deflector 2 is only 1 to 2 cm. , it is possible to fabricate an extremely miniaturized optical branch circuit.

Here, each optical fiber 8, 1 for transmitting and receiving light to be used is
0, the lenses 3 and 9 and the optical deflector will be explained. The optical fiber is generally a multi-mode optical fiber, but if the optical fiber 4 to be measured is a single-mode optical fiber, the light for light transmission is When a single mode optical fiber is used as the fiber 8, the incidence efficiency is higher, and a stable optical branch circuit can be produced which is not affected by the speckle pattern peculiar to multimode optical fibers. On the other hand, as the optical fiber 10 for receiving light, a multimode optical fiber with a large core diameter and a large relative refractive index difference is suitable in order to increase the tolerance to positional deviations and deflection angle deviations. Also,
A stepped multimode optical fiber is more desirable as the light receiving optical fiber 10 than a graded multimode optical fiber because the light receiving angle range is uniform and wide over the entire core region.

Although monocular lenses are shown as lenses 3 and 9 in the embodiment, rod lenses are suitable for miniaturization of the optical branch circuit and ease of assembly.

As the optical deflector, in addition to the ultrasonic deflector 2, a device utilizing an electro-optic effect can also be used. for example
These include prism-type optical deflectors made of electro-optic crystals such as KD ₂ PO ₄ , LiTaO ₃ , BaTiO ₃ , etc., and electro-optic optical deflectors that combine a modulation element made of electro-optic crystals and a birefringent prism for polarization separation. . In addition, the materials for the ultrasonic deflector include PbMoO ₄ , TeO ₂ , Pb ₅
(GeO ₄ ) (VO ₄ ) ₂ , α-HgS, Ge, Te glass,
LiNbO ₃ , As ₂ Se ₃ , etc. can be used.

FIG. 3 shows another embodiment, in contrast to FIG.
Light from light source 1 operates ultrasonic deflector 2 (on)
The backscattered light from the optical fiber 4 is guided to the detector 7 with the ultrasonic deflector 2 turned off. However, in this example,
Since the ultrasonic deflector does not necessarily have 100% deflection efficiency, the Fresnel reflected light from the end face of the optical fiber 4 to be measured is guided to the detector 7 while the ultrasonic deflector 2 is in operation. Please be careful when storing. That is,
Generally, Fresnel reflected light has a much larger signal than backscattered light (e.g. wavelength 0.85 μm, optical pulse width
If the optical fiber 4 is a multimode optical fiber with a relative refractive index difference of 1.0% at 100 ns, the signal difference is more than 30 dB), so the Fresnel reflected light saturates the detector 7 and the subsequent amplifier system. This will impair accurate measurement of backscattered light. In this respect, the embodiment shown in FIG. 3 can be said to be slightly inferior in characteristics to the embodiment shown in FIG.
Similar effects can be achieved in terms of stability against positional deviations and deflection angle deviations.

The embodiment shown in FIG. 4 differs from the embodiment shown in FIG. 2 in that the cutting and polishing angle α of the element of the ultrasonic deflector 2 is shifted from 90 degrees. This prevents the Fresnel reflected light from the incident end surface 2b of the ultrasonic deflector 2 from entering the light-receiving optical fiber 10, improving measurement accuracy. In Fig. 4, 2b' indicated by a broken line is the incident end face when α=90 degrees, A is the optical path of the Fresnel reflected light when α=90 degrees, and B is the Fresnel reflected light when α≠90 degrees. It is the optical path of This will be explained with reference to FIGS. 5a and 5b. FIGS. 5a and 5b show the trajectory of the incident light beam of the ultrasonic deflector 2 when the cutting and polishing angle of the ultrasonic deflector element is α=π/2−β (radians). In both cases of FIGS. 5a and 5b, the Fresnel reflected light B from the incident end surface 2b of the ultrasonic deflector 2 and the rear which enters the receiving optical fiber 10 from the optical fiber 4 to be measured via the ultrasonic deflector 2. The angle ζ formed by the scattered light L _b is ζ=2nβ. However, n
is the refractive index of the ultrasonic deflector element. Now, the core diameter of the light-receiving optical fiber 10 is assumed to be d, and the backscattered light from the optical fiber 4 to be measured is incident on the core center of the light-receiving optical fiber 10, and its spot size is less than or equal to the core diameter d. If fζ≧d...Equation (2), then the Fresnel reflected light B from the incident end surface 2b of the ultrasonic deflector 2 will not enter the receiving optical fiber 10, and will therefore not be guided to the detector 7. do not have. For example, if d=50 μm, f=3.6 mm, and n=2.3, then β≧0.17 degrees is sufficient. In particular, in the case of Figure 5 a, β = θ/n (where θ is half the deflection angle 2θ of the ultrasonic deflector 2 used to explain equation (1). See Figure 2). , light L _f from light source 1
is incident perpendicularly to the end face of the ultrasonic deflector, so the insertion loss of the optical branching circuit is reduced.

As explained above with reference to the embodiments, according to the present invention, the same lens is used for the input and output of light of the optical deflector, and optical fibers are used for the input and output ends of the light respectively. An optical branch circuit that is compact and has stable characteristics can be obtained. As a result, the optical branching circuit can be easily incorporated into an instrument for measuring backscattered light of an optical fiber.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a conventional optical branching circuit, FIGS. 2, 3, and 4 are diagrams showing respective embodiments of the present invention, and FIGS. 5a and 5 are implementations of FIG. 4. FIG. 3 is a diagram showing, by way of example, the trajectory of light passage and reflection of an ultrasonic deflector; In the drawing, 1 is a light source, 2 is an ultrasonic deflector, 3 is a second lens, 4 is an optical fiber to be measured, 5 is an oscillator, 7 is a detector, 8 is an optical fiber for transmitting light, and 9 is a first optical fiber. The lens 10 is an optical fiber for receiving light.

Claims (1)

[Claims] 1. An optical fiber for transmitting light, a first lens that collimates the light emitted from the optical fiber, an optical deflector that can change the traveling direction of the parallel light by an electric signal, and a second lens for making the light that has passed through the optical deflector enter the optical fiber to be measured;
light that has passed through the lens, the optical deflector, and the first lens in this order and is deflected by the optical deflector;
Among the light that is not deflected by the optical deflector and goes straight, the first
An optical branching circuit comprising: a light-receiving optical fiber that receives one of the lights separated by a lens. 2. The optical deflector does not operate when the light emitted from the transmitting optical fiber enters the optical fiber to be measured and allows the light to travel straight, and when the light from the optical fiber to be measured enters the receiving optical fiber. 2. The optical branching circuit according to claim 1, wherein the optical branching circuit is an optical deflector that operates to change the traveling direction of light. 3. The optical branch circuit according to claim 1, wherein the light transmitting optical fiber is a single mode optical fiber, and the light receiving optical fiber is a multimode optical fiber. 4 When the core diameter of the light-receiving optical fiber is d and the focal length of the first lens is f, the light from the light-transmitting optical fiber that enters the optical deflector and is reflected by the incident surface of the optical deflector and the angle ζ [radian] formed between the optical deflector and the first lens by the light from the optical fiber to be measured that passes through the optical deflector and enters the receiving optical fiber satisfies ζ≧d/f. An optical branch circuit according to claim 1.