JPS63218838A - Method for checking cut-off wavelength of optical fiber - Google Patents

Abstract

PURPOSE:To rapidly detect an optical fiber cut-off wavelength, by investigating the magnitude relation between the light output change quantities after and before bending is imparted to an optical fiber, and a predetermined threshold value and judging the magnitude relation between the cut-off wavelength of the optical fiber and the wavelength of a light source. CONSTITUTION:At first, an optical switch 7 couples an optical cord 6A and an optical fiber 3 to be measured, and the light of a light source 5A having a wavelength lambda1 is incident to the fiber 3 in such a state that the fiber 3 is straight and the light output P1 thereof is measured by a light detector 4. Next, light output P2 is measured by the detector 4 in such a state 3A that the fiber 3 is bent by a bending imparting part 8. Next, the switch 7 is changed over to successively couple the fiber 3 with optical cords 6B, 6C. The light outputs P3-P6 are measured with respect to respective couplings in the straight and bent states of the fiber 3. From the light outputs P1-P6 thus calculated, the magnitude relation between the cut-off wavelength of the fiber 3 and the wavelengths lambda1, lambda2, lambda3 of the light source is judged. By this method, the cut-off wavelength of the optical fiber can be rapidly checked.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for rapidly testing whether the cutoff wavelength of a single mode optical fiber is within a predetermined standard range.

[Prior Art] The number of propagation modes in an optical fiber changes depending on the wavelength. It is important to define a single mode operating wavelength range that provides broadband transmission characteristics as a standard according to the wavelength of the light source used. The shortest wavelength for single mode operation is called the cutoff wavelength. An international discussion on its definition is being held at the CCITY, and although agreement in the strict sense has not yet been reached, the general discussion is proceeding on the following points. In other words, when light is input into an optical fiber, the upper limit of the wavelength at which the optical power at the output end changes significantly when a bend with a constant bending radius is applied at a position approximately 2 m from the input end is defined as the cutoff wavelength. ing.

Figure 2 shows the conventional method of measuring the cutoff wavelength according to the above definition (Y, Katsuya Ila et al, ``N
ew Method for Measuring
V-Value of a Single -
ModeOptlcal Fiber, ”,
Electron, Lett, 12. 25゜p
p668-670.1976,), 1
2 is a white light source, 2 is a spectroscope, 3 is an optical fiber to be measured, and 4 is a photodetector. The measurement procedure begins with Figure 2 (A
), the spectrometer 2 is driven without bending the optical fiber to be measured, and the optical output PA (λ) in a predetermined wavelength range is continuously transmitted to the photodetector 4 for each wavelength λ. Take the course and record it. Next, as shown in FIG. 2(B), the optical fiber to be measured is bent and similar measurements are performed to determine the wavelength dependence pe(λ) of the optical output. Next, the wavelength dependence α(λ) of the bending loss is calculated in terms of age.

α(λ) −10108to [PA(λ)/Pa(λ
)] Figure 3 is an example of experimental data obtained by such a method.

Cutoff wavelength λ. is determined as the longest wavelength such that α (λ)〉0, and in the example in Figure 3, regardless of the value of the radius of curvature R,
λ=1.24 μm.

[Problem to be solved by the invention 1 As can be seen from the above explanation, measurement of the cutoff wavelength requires two spectroscopic measurements, one with bending and one without bending.
It usually takes about 10 minutes. For this reason, for example, in the final inspection of a 600-fiber cable, there was a drawback that it took a total of 100 hours just to inspect the cutoff wavelength.

An object of the present invention is to quickly verify the cutoff wavelength, shorten the time required for the test, and provide a low-cost optical fiber cable.

[Means for Solving the Problems] In order to achieve such an object, the present invention involves injecting light of a specific wavelength into an optical fiber to be measured in an unbent state and in a bent state, The output light of each optical fiber to be measured is measured, and the magnitude relationship between the intensity change of the output light due to the presence or absence of bending and a predetermined threshold value is investigated, and the magnitude relationship between the cutoff wavelength and the wavelength of the incident light is determined from the magnitude relationship. It is characterized by making a judgment.

[Function] According to the present invention, the magnitude relationship between the cutoff wavelength of the optical fiber and the light source wavelength is determined based on the magnitude relationship between the amount of change in optical output before and after bending the optical fiber and a predetermined threshold value. As a result, the cutoff wavelength of an optical fiber can be verified more than 100 times faster than conventional methods.

[Examples] Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram illustrating the present invention in detail, 5A, 5
8.5G has wavelengths of λ, λ2, and λ2, respectively. λ3 light source,
6A, 8B, and 6G are optical cords, 7 is an optical switch, and 8 is a bending section. The cutoff wavelength verification operation is carried out as follows.

■Introduction The optical switch 7 connects the optical cord 6A and the fiber under test 3, and when the optical fiber 3 under test is in a straight state, the light from the light source 5A with the wavelength λ is input, and the optical output P is transmitted to the photodetector 4. It is measured in

(2) Next, the optical fiber to be measured is bent by the bending unit 8 as shown in 3A, and the optical output P2 is measured by the photodetector 4.

(2) Next, the optical switch 7 is switched to couple the optical cord 6B and the optical fiber 3 to be measured, and the light from the light source 5B having the wavelength λ is guided to the bent optical fiber 3B to be measured. After measuring the optical output P at this time, (1) Release the bending and measure the optical output P4. '■Next, the optical switch 7 is switched to couple the optical cord 6C and the optical fiber 3 to be measured, and the light from the light source 5C with wavelength λ3 is guided to the optical fiber 3 to be measured in a straight state, and the optical output P at this time is After measuring , (2) Apply bending and measure the optical output P6.

Since the loss due to bending behaves as shown in Figure 3, the six optical outputs PI obtained above are P2゜Ps+P4
．． The cutoff wavelength λ of the optical fiber 3 is determined from Ps and Pa. and the light source wavelength λ1. λ2. The magnitude relationship with λ3 can be determined as follows.

First, loss values α(λ1) and α(λ2) due to bending.

α(λ3) is calculated using the following formula.

, (λI)=101ag (p+/p2) (da)
D) α(λ2) = 10 log
(P4/P3) (dB) (2) α
(λ3) = 10 log (Ps/Pa) (
dB) (3) Next, set the judgment threshold to ΔP (d
As B), if α(λ1)>ΔP, then λ. 〉λ+ (i
-1, 2, 3) (4) If αζλ+)<ΔP then λ.

It is determined that <λ+ (i-1, 2, 3) (5). Δ
How to choose the value of P affects the accuracy of determining the magnitude relationship between the cutoff wavelength and the test wavelength, but as seen in the CCITT discussion, by setting ΔP = about 0.1 dB,
The cutoff wavelength verification accuracy can be set to about 0.01 μm, which is sufficient for practical use.

For example, the cutoff wavelength standard for a single mode optical fiber used at 1.3 μm is Q, 10 < λc < 1.29 μm, so for example, the light source 5A is a laser diode with a wavelength of 1.10 μm, and the light source 5B is a wavelength of 1.29 μm. By setting the laser diode to , and the bending radius R = 10 am,
α(1,10) > 0.1dB and α(1,29)
When < 0.1 dB, 1.10 < λc < 1.29 μm
And passed.

If a (1, 10) < O, ldB then λ, < 1.10
If a (1, 28) > 0.1 dB, then λc) 1.29 μm, the test will fail. Furthermore, if the wavelength λ3 of the light source 5C is, for example, 1.20 μm, then the magnitude relationship between the value of a (1, 20) and ΔP is 1.10 for a certified optical fiber (1, 10 < λc < 1.29 μm).
<λc<1.20μm, or 1.20<λ. <1.2
It is also possible to determine which one is 9 μm. (1),
(2) (As can be understood from Equation 31 and the configuration in Figure 1, the light source 5A was used in this measurement.

It has the advantage that it is completely unaffected by the coupling loss between 5B, 5C and the optical cords 6A, 6B, 6C and the insertion loss of the optical switch 7. In addition, as a light source, a laser diode with an optical output 1000 times larger than that of a conventional spectrometer can be used, eliminating the need for high-precision axis alignment for highly efficient light input, and enabling highly accurate verification. becomes possible.

A combination of light emitting diodes and narrow band filters can also be used as the light sources 5A, 5B, 5C. In other words, a normal light emitting diode has an emission spectrum width of 0.1 μm.
Due to the above, it is not possible to perform high-accuracy verification as is, but for example, a wavelength filter made of a dielectric multilayer film can be used to reduce the spectral width of transmitted light to 0. By setting the diameter to about 1 μm, highly accurate verification becomes possible. At this time, the optical output of the transmitted light decreases to about 1/10 of the light emitting diode output, but compared to the conventional method in which the light from a halogen lamp is separated using a spectrometer, it is possible to receive a sufficiently large amount of light and emit light. Diodes have the advantage of being very inexpensive and having excellent output stability without the need for a temperature control device.

In the embodiment described here, the number of light sources is three, but
By increasing the number of light sources, the range in which the cutoff wavelength exists can be known in more detail. Further, when the purpose is limited to standard verification, the verification work can be speeded up by using a light source with two wavelengths, the standard upper limit wavelength and the standard lower limit wavelength.

Since the cutoff wavelength can be verified through the steps described above, the required time is determined by the stabilization time of the photodetector, and can be done within 5 seconds, for example.

In other words, it is possible to perform verification 100 times faster than the conventional method, which required about IO minutes.

[Effects of the Invention] As explained above, depending on the magnitude relationship between the amount of change in optical output before and after bending the optical fiber and a predetermined threshold value,
By determining the magnitude relationship between the cutoff wavelength of the optical fiber and the light source wavelength, the cutoff wavelength of the optical fiber can be verified more than 100 times faster than conventional methods.

Furthermore, since a laser diode with a large output or a light emitting diode with low cost and excellent stability can be used as a light source, there is an advantage that highly accurate verification is possible.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a diagram illustrating a conventional method for measuring a cutoff wavelength, and FIG. 3 is a diagram of wavelength dependence characteristics of bending loss. DESCRIPTION OF SYMBOLS 1...White light source, 2...Spectroscope, 3.3A...Optical fiber to be measured, 4...Photodetector, 5A, 5B, 5C...Light source, 6A, 6B, 6C... - Optical cord, 7... Optical switch, 8... Bending section.

Claims (1)

[Claims] 1) Light of a specific wavelength is incident on the optical fiber to be measured in an unbent state and in a bent state, and the output light of the optical fiber to be measured is measured, and the presence or absence of bending is determined. A method for testing an optical fiber cutoff wavelength, characterized in that the magnitude relationship between the amount of change in the intensity of the emitted light and a predetermined threshold is determined, and the magnitude relationship between the cutoff wavelength and the wavelength of the incident light is determined from the magnitude relationship. . 2) The optical fiber cutoff wavelength testing method according to claim 1, wherein the wavelengths of the incident light are two wavelengths, an upper limit and a lower limit of a cutoff wavelength range to be tested.