JPH037061B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a single mode optical fiber evaluation device.

Traditionally, single mode optical fibers have been used in long distance transmission lines, particularly for communication purposes. This single mode optical fiber has a core having a thickness comparable to the wavelength used and coated with a coating material whose refractive index is slightly smaller than that of the core. The important characteristics of such a single mode optical fiber that affect its communication quality are the diameter of the core, the difference in refractive index between the core and the coating material, and the single mode limit, which is the limit wavelength for use as a single mode optical fiber. can be given. These characteristics also affect the coupling efficiency in coupling between optical fibers, coupling with elements, and the like. Therefore, these properties must be evaluated before use.
Conventionally, the core diameter and refractive index difference have been evaluated by dipping the fiber in a liquid such as glycerin and measuring it from the side using a microscope. However, as shown in Fig. 1, the refractive index distribution in the cross section of an actual optical fiber is curve A at the preform stage, but curve B at the fiber stage. It is difficult to obtain accurate values when measuring with a microscope. To measure the single mode limit, use a monochromator to sequentially change the wavelength of light from one end of the optical fiber to be measured, and observe the single mode pattern that appears at the other end with a television camera. It is measured by changes in the brightness distribution. However, with this method, as shown by curves No. 1 to No. 4 in Figure 2, the measured value (shown on the vertical axis of the figure) changes depending on the length of the optical fiber to be measured (shown on the horizontal axis of the figure). Accurate measurement is difficult. The present invention has been devised to eliminate these drawbacks.

Therefore, in the single-mode optical fiber evaluation apparatus of the present invention, it is possible to divide the optical fiber to be measured into two and connect the two halves by placing one end face of each of the two halves facing each other with a certain distance between them. The device is characterized in that it consists of an optical connector that can be used, two light sources with different emission wavelengths that are switched and connected to one optical fiber to be measured, and an optical power meter that is connected to the other optical fiber to be measured. be.

Embodiments of the present invention will be described in detail below based on the accompanying drawings.

FIG. 3 shows a structural diagram of the embodiment. In the figure, reference numerals 1 and 2 are optical fibers to be measured, 3 is an optical connector, 4 is a light source, and 5 is an optical power meter.
In order to provide a certain distance between the opposing surfaces of the optical fibers 1 and 2 in the optical connector as shown in FIG. 4, a ring-shaped spacer 8 can be inserted between the end surfaces of the cores 6 and 7 that hold each optical fiber. It's becoming like that. The cores 6 and 7 are guided by an adapter 10 in the socket 9 so that the optical axes of the optical fibers coincide with each other, and are connected by cap nuts 11 and 11'. In addition, the light source 4 is 4a as shown in Figure 3.
It has two light sources 4a and 4b, and is configured so that light can be input independently into the optical fiber 1 by a switching mechanism (not shown).
b has a different wavelength of emitted light.

Next, the operation of the apparatus of this embodiment configured as described above will be explained. First, after setting the optical fibers 1 and 2 to be measured in the device, one of the light sources 4a projects light, and the value input to the power meter 5 is calculated.
Read L ₁ . Next, the light source is switched to 4b, and the value _L2 input to the power meter 5 is read.

In general, when single mode optical fibers are connected with a spacing D and light of wavelength λ is input from one optical fiber, the power loss L that appears in the other optical fiber
(λ) is expressed by the following approximate expression, assuming that the light emitted from one optical fiber has a Gaussian distribution.

L(λ)=−10log(4/(4+λD/πW ² (λ)) ² )
...(1) However, W (λ) is the beam waist and the beam waist W (λ) is W (λ) = A (0.65 + 1.619 / V ^1.5 + 2.879 / V ⁶ )
...(2) However, A is the radius of the core and V is according to the following formula: V=2πAn ₀ √2/λ ...(3) However, n ₀ is the refractive index of the core Δ=n ₀ ² −n ² /2n ₀ ² n is the refractive index of the cladding. Inserting equations (2) and (3) into equation (1) and giving the measured values L(λ ₁ ) and L(λ ₂ ) of the power meter at wavelengths λ ₁ and λ ₂ , A and Δ can be calculated.

By inserting A and Δ into the following cutoff condition (4), the single mode limit λ _c is calculated.

λ _c =2πAn√2/2.405≒5.3781A√ ...(4) The optical fiber shown in FIG. 2 was measured using the example device shown in FIG. 3, and calculated using the above equation (4). The results are shown in Figure 2 with a circle. N ₀ corresponds to those measured by the conventional method.
From equation (4), it can be seen that when the device of this embodiment is used, the measured value is independent of the length of the optical fiber.

As explained above, the single mode optical fiber evaluation apparatus of the present invention makes it possible to obtain accurate measured values of the characteristics of a single mode optical fiber.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing the refractive index distribution of the core of a single mode optical fiber, and Fig. 2 shows the measured value of the single mode limit of the single mode optical fiber by the conventional measurement method and the evaluation device according to the present invention. A line diagram showing the measured values used, FIG. 3 is a schematic explanatory diagram of a single mode optical fiber evaluation apparatus according to an embodiment of the present invention, and FIG. 4 is a sectional view of the optical connector. 1, 2...Optical fiber, 3...Optical connector, 4
... light source, 5 ... power meter, 6, 7 ... core, 8 ... spacer.

Claims (1)

[Claims] 1 Divide the optical fiber to be measured into two, and switch to an optical connector that can connect one end face of each of the two halves to face each other with a certain interval, and one optical fiber to be measured. 1. A single mode optical fiber evaluation device comprising two connected light sources having different emission wavelengths and an optical power meter connected to the other optical fiber to be measured.