JPH01305333A - Measuring method for wavelength dispersion of optical fiber - Google Patents

Abstract

PURPOSE:To detect precisely a minute value of negative wavelength dispersion by using a method of calculating the value of the negative wavelength dispersion from a value of modulation frequency of self-amplitude modulation. CONSTITUTION:An oscillation wavelength of an F center laser 1 is adjusted to be a wavelength lambda1 and an emission light thereof is made to enter an optical fiber 3 to be measured through an objective lens 2. An emission light from this fiber 3 is made to be a parallel beam by an objective lens 4 and reflected by a mirror 5, and a photoelectric power P thereof is measured by an optical power meter 7. The mirror 5 being removed subsequently, the emission light from the fiber 3 is passed through the lens 4 and the spectrum thereof is mea sured by a light spectrum analyzer 6. Using wavelengths lambda1 and lambda2 on the spec trum obtained, the frequency (f) of self-amplitude modulation occurring in the fiber 3 is calculated by f=c/lambda1-c/lambda2. From this frequency (f) and the power P, a dispersion value D is determined by D=K.P/f<2>. In the equations, K denotes a constant and (c) a light flux.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for measuring chromatic dispersion of an optical fiber with high precision.

<Conventional techniques and their problems> Conventional chromatic dispersion measurement methods include (1) a method in which optical pulses with different wavelengths are propagated through a fiber under test and the wavelength characteristics of their delay times are measured; A method has been used in which a two-path optical interferometer is configured with one optical path of 1, and the wavelength characteristics of the relative delay time of light propagating through an optical fiber are measured by sweeping the wavelength of light incident on the interferometer. In dispersion measurements using these methods, the dispersion value is directly determined by differentiating the delay time once with respect to the wavelength. Since it is difficult to accurately measure the difference in delay time between wavelengths, the method has the drawback that the differentiation becomes inaccurate and the chromatic dispersion value cannot be measured accurately.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method for measuring group velocity dispersion that can solve these conventional drawbacks and accurately measure the minute negative group velocity dispersion of an optical fiber.

<Means for Solving the Problems> The present invention achieves the above-mentioned object by making a continuous wave or pseudo-continuous wave light beam of a single wavelength enter an optical fiber to be measured, and measuring the output light of the optical fiber to be measured. Measure the power P and the optical spectrum, and the wavelength λ1λ2 on the obtained optical spectrum
is a self-range generated in the optical fiber to be measured, and from this modulation frequency f and the above-mentioned power P, a dispersion value is obtained by D=-P/f2.

Effect> The value of negative chromatic dispersion is calculated from the modulation frequency f of the self-width modulation of the optical beam, which is caused by the interaction between the self-phase modulation and negative chromatic dispersion of the optical beam propagating in the optical fiber under test. In this case, the dispersion value is inversely proportional to the square of the frequency f, so the smaller the negative chromatic dispersion, the larger the frequency f becomes, and as a result, the interval between λl and λ2 becomes wider. Therefore, measurement becomes easier and measurement accuracy improves.

In this case, when a light beam with power above a certain level propagates in an optical fiber, self-amplitude modulation occurs in the light beam due to the interaction between self-phase modulation and negative wavelength dispersion, which are optical nonlinear effects. Physical Review Letters (Physical Revie+v L
etters) Vol. 50. 1986, pp. 135-138, presenters K, Ta1, A, Hasega 7 (A, I (asegawa), and A,
Published in a paper by Tomital A.

Modulation frequency f(s-') of self-amplitude modulation and negative chromatic dispersion D
(5-m-2) has the following relationship.

Linear refractive index, no is the refractive index, P (wl is the power, human eff
(m') is the effective core cross-sectional area, and C is the speed of light. and,
Outside C2^, n2. No, if the person eft is known, the above equation can be expressed as -P/f2 in D=.

Therefore, by measuring the power P and the modulation frequency f, the value of D with respect to λ can be uniquely determined. As is clear from the formula, D is inversely proportional to f2, so the smaller D is, the more f
is measured as a large value, the dispersion value is small (and sufficient measurement accuracy can be obtained).

Embodiment Example 1 FIGS. 1 and 2 are diagrams for explaining the first embodiment of the method of the present invention. In FIG. 1, 1 is a wavelength-tunable F center laser, 2 is an objective lens, and 3 is a Measured light fu? 4 is an objective lens, 5 is a removable total reflection mirror, 6 is an optical spectrum analyzer, 7 is an optical power meter, and arrows indicate the path of light. The oscillation wavelength of the 10F center laser is λ
and make the emitted light enter the optical fiber 3 to be measured through the objective lens 2. Next, the light emitted from the optical fiber 3 is made into a parallel beam through an objective lens 4, reflected by a mirror 5, and input into an optical power meter 7, and its optical power P is measured. Next, the mirror 5 is removed, and the light emitted from the optical fiber 3 to be measured is made to enter the optical spectrum analyzer 6 through the objective lens 4, and its spectrum is measured. The light emitted from the optical fiber 3 to be measured undergoes self-amplitude modulation in the optical fiber due to the interaction of the above-mentioned self-phase modulation and negative group velocity dispersion, and a modulation sideband is generated on its spectrum. FIG. 2 shows an example of the spectrum of the light emitted from the optical fiber measured by the optical spectrum analyzer 6. On the spectrum, modulation sidebands are observed at both ends of the center wavelength λ1.

The modulation frequency f of self-amplitude modulation can also be obtained as the difference frequency f = C/λ, -C/^2 between the center wavelengths λ2 and ^ of the modulation sideband. Here C represents the speed of light. λ, ^
The value of the optical power P measured by the optical power meter 7 and f obtained by the measurement in step 2 is calculated using the relational expression ~ [C・■n2・P/^
By substituting 3moAeffl·f−2, negative dispersion 4rliD at wavelength λ1 is calculated.

Next, the same procedure for measuring the oscillation wavelength of laser No. 1 is repeated by an eccentric person, and finally the wavelength characteristic of negative wavelength dispersion is measured.

Practical example 2 FIG. 3 is a diagram illustrating a second embodiment of the method of the present invention.

If the configuration in which the polarizers 8 and 9 and the optical fiber support jigs 10 and 11 with rotating mechanisms are rounded in Example 1 is rounded, and the optical fiber 3 to be measured is a polarization-maintaining optical fiber, the X of the optical fiber is rounded.
It has the effect of being able to measure the wavelength dispersion of the polarization axis and the X polarization axis.

That is, the light beam from the laser 1 is linearly polarized by the polarizer 8, and the jig 10 is rotated so that the linear polarization is incident on the X polarization axis of the optical fiber 3. Also at the output end of the optical fiber 3, the jig 11 is rotated to align the X polarization axis with the polarization plane of the polarizer 9, and the light propagating along the X polarization axis of the optical fiber 3 is extracted as output. The power P and spectrum λ1λ2 of the extracted optical output were measured according to the procedure of Example 1,
Calculate the value of negative chromatic dispersion on the X polarization axis.

Next, rotate the jigs 10 and 11 to align the polarization plane of the light with the Y polarization axis of the optical fiber 3, and use the same procedure as when aligning it with the X polarization axis to make the Y polarization axis of the optical fiber negative. Calculate the value of chromatic dispersion.

This measurement procedure is repeated by changing the wavelength of the laser 1, and finally the wavelength characteristics of the negative chromatic dispersion of the X polarization axis and the Y polarization axis of the edge wave maintaining optical fiber are determined.

The wavelength dispersion measuring method has been explained above in Examples 1 and 2. This method is used to measure the characteristics of the base material after the drawing process in optical fiber manufacturing, and the results are fed back to the previous process. It is possible to obtain a single mode fiber with extremely precisely controlled dispersion.

<Effects of the Invention> As explained above, the chromatic dispersion measuring method according to the present invention uses a method of calculating the value of negative chromatic dispersion from the value of the modulation frequency of self-amplitude modulation, and the value of minute negative chromatic dispersion is It also has the advantage that it can be determined with high precision. Also,
By applying the optical fiber chromatic dispersion measurement method of the present invention to the production of optical fibers, it is possible to produce single mode fibers with very precisely controlled dispersion. Furthermore, by applying the obtained single mode fiber to so-called coherent communication or sonoton communication, there is an advantage that optical communication with a very large transmission capacity can be realized.

[Brief explanation of the drawing]

1 to 3 are for explaining an embodiment of the method of the present invention, FIG. 1 is a block diagram for the first embodiment, FIG. 2 is a graph showing a spectrum due to self-amplitude modulation, and FIG. The figure is a configuration diagram for the second embodiment. In the figure, 1 is a wavelength tunable F center laser, 2.4 is an objective lens, 3 is an optical fiber to be measured, 5 is a total reflection mirror, 6 is an optical spectrum analyzer, 7 is an optical power meter, 8.9 is a polarizer, 10.11 is an optical fiber support jig with a rotating mechanism. Patent applicant: Agent of Nippon Telegraph Tiwa Co., Ltd.

Claims (1)

[Claims] A continuous wave or pseudo-continuous wave light beam of a single wavelength is incident on an optical fiber to be measured, and the power P and optical spectrum of the light emitted from the optical fiber to be measured are measured. The modulation frequency f of self-amplitude modulation occurring in the optical fiber under test at wavelength λ_1λ_2 on the optical spectrum is f=c/λ_
1-c/λ_2, and from this modulation frequency f and the above-mentioned power P, a dispersion value D is obtained as D=K·P/f^2. However, K is a constant and C is the speed of light.