JPH0564289B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to the measurement of properties of optical fibers.
In particular, it relates to an apparatus for measuring chromatic dispersion of optical fibers.

[Conventional technology]

Conventionally, the chromatic dispersion of an optical fiber has been generally measured by interference of two light beams. In other words, the light beam output from one light source is spatially separated into two, one light beam is propagated into the optical fiber to be measured, the other light beam is propagated in space, and these two light beams are The position where the visibility of the interference fringes is maximum is detected by an imaging device such as a vidicon. This is done for light sources of different wavelengths, and the optical path length difference is measured so that the visibility of the interference fringes is maximum at the same position. Through this measurement, the optical path length difference with respect to wavelength is measured, and chromatic dispersion is determined by calculation from the measured value.

[Problem that the invention seeks to solve]

However, such a conventional optical fiber wavelength dispersion measurement method has the disadvantage that the signal-to-noise ratio of the interference fringes is low and there is a width at the position where the visibility is maximum, making it difficult to measure with high precision.

SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus capable of measuring the optical path length at which interference is maximum with high precision and measuring optical fiber wavelength dispersion with high precision.

[Means for solving problems]

The optical fiber wavelength dispersion device of the present invention includes an optical means for separating a light beam emitted from a light source that outputs a coherent light beam into a signal light and a reference light, and an optical means for making the signal light enter an optical fiber to be measured. an optical means, an optical means for causing interference between a signal light propagated through an optical fiber to be measured and a reference light propagated through a path other than the optical fiber to be measured, a means for detecting interference light obtained by the means, and a light source. In an optical fiber chromatic dispersion measuring device, the optical fiber chromatic dispersion measuring device is equipped with means for variably setting the optical path length difference between the signal light and the reference light so that the interference intensity of each output light with different wavelengths from the signal light is maximized. The present invention is characterized by comprising means for applying intensity modulation or phase modulation to at least one of the reference light and the reference light, and means for extracting information on the intensity of the interfered light at a narrow band frequency including the modulation frequency of the means.

Although it is possible to replace and use light sources that generate output light of different wavelengths, it is preferable to use the same light source that outputs two or more different wavelengths.

[Effect]

The optical fiber wavelength dispersion measuring device of the present invention modulates a light beam with an alternating current signal to give a change in the interference intensity of the two light beams to the alternating current signal component, and detects this at a narrow frequency band to calculate the signal-to-noise ratio. Improve.

〔Example〕

FIG. 1 is a block diagram of an optical fiber wavelength dispersion measurement apparatus according to a first embodiment of the present invention.

A light source 1 outputs a coherent light beam. An example is a semiconductor laser. This light flux is subjected to intensity modulation by an intensity modulator 2. The intensity-modulated light beam is separated into two light beams by a semi-transparent mirror 3. The light beam transmitted through the semi-transparent mirror 3 propagates through the optical fiber 4 to be measured as a signal. The light beam reflected by the semi-transparent mirror 3 propagates in space as a reference light, is reflected by the reflecting mirrors 5 and 6, passes through the semi-transparent mirror 7, and is moved to a movable mirror 8 fixed to a movable pedestal by a pulse motor or the like. reflected. The signal light outputted from the optical fiber 4 to be measured is transmitted through the semi-transparent mirror 7, the reference light reflected by the movable mirror 8 is reflected by the semi-transparent mirror 7, and the combined light beam obtained by combining these light beams is as follows. , linear analyzer 9
, and enters a photodetector 10 constructed of a silicon avalanche photodiode, a germanium avalanche photodiode, or the like.

Photodetector 10 is connected to bandpass filter 12 .
Bandpass filter 12 is connected to recording device 13 . Further, an oscillation circuit 11 is connected to the intensity modulator 2 and the bandpass filter 12.

The optical signal incident on the photodetector 10 is frequency-modulated by the intensity modulator 2. Therefore, the bandpass filter 12 extracts the frequency component from the output signal of the photodetector 10 using the frequency reference signal from the oscillation circuit 11. Bandpass filter 1
The output of No. 2 is recorded by the recording device 13.

A comparison will be made in the case where a light source that outputs light beams of wavelengths 〓 ₁ and 〓 ₂ that are close to each other is used as the light source 1. In either case, the optical path lengths of the signal light and reference light in free space remain unchanged. However, the optical path length of the signal light propagating through the optical fiber 4 to be measured changes depending on the wavelength dispersion of the optical fiber 4 to be measured. Therefore, in order to measure the change in chromatic dispersion, the position of the movable mirror 8 is adjusted so that the optical path lengths of the signal light and the reference light are equal. Due to the wavelength dispersion of the optical fiber, the position of the movable mirror 8 differs for the wavelengths ₁ and ₂ , and the chromatic dispersion can be obtained from the amount of displacement at that position.

The feature of this embodiment is that the signal light and the reference light are intensity modulated. As a result, when the optical path lengths are equal, the frequency component of the interference intensity of the light beams combined by the semi-transparent mirror 7 becomes maximum. This change in frequency component is large, and it can be easily seen that the optical path lengths are equal. Therefore, the optical path length difference can be measured with high precision, and accurate wavelength dispersion can be obtained.

The present example will be described quantitatively below using equations.

_The frequency component of the light intensity detected by the photodetector 10, that is, the interference intensity I, is: I=[I _{1 +} I ₂ +2√ _{1 2} |
〕×cos2〓ft……(1). Here, I ₁ : signal light intensity, I ₂ : reference light intensity, ｜〓(〓 ₁₂ )｜: degree of coherence (〓 is a function of 〓 ₁₂ ), 〓 ₁₂ : two points from the semi-transparent mirror 3 to the photodetector 10. Group delay time difference between the two light beams, 〓 ₁₂ : Phase difference between the two light beams reaching from the semi-transparent mirror 3 to the photodetector 10. When the movable mirror 8 is swept, at the position 〓 ₁₂ =0, |〓(〓 ₁₂ )|=1, cos〓 ₁₂ =1, and the amplitude of the interference intensity I becomes maximum.

The position of the movable mirror 8 where the interference intensity I becomes maximum is determined using light sources of different wavelengths. This relative positional shift of the movable mirror 8 becomes an optical path length difference due to wavelength dispersion of the optical fiber 4 to be measured. By dividing the optical path length difference by the speed of light c, the group delay time difference 〓 can be obtained. This group delay time difference 〓 is the optical fiber 4 to be measured with length L
There is a relationship between the wavelength dispersion D and D=(1/L)(d〓/d〓)...(2). Here, 〓 is the wavelength. The chromatic dispersion of the optical fiber 4 to be measured can be obtained by determining the group delay time difference 〓 for different wavelengths λ using equation (2).

In this embodiment, the light beam output from the light source 1 is intensity-modulated and then separated into the signal light and the reference light. The present invention can be carried out in the same manner.Also, the present invention can be carried out even if only one of the signal light and the reference light is subjected to intensity modulation, although the interference intensity is reduced.

FIG. 2 is a block diagram of an optical fiber wavelength dispersion measurement apparatus according to a second embodiment of the present invention.

The light beam output from the light source 1 is separated into two light beams by the semi-transparent mirror 3. The light beam transmitted through the semi-transparent mirror 3 propagates through the optical fiber 4 to be measured as signal light. The light beam reflected by the semi-transparent mirror 3 propagates in space as a reference light, is reflected by the reflecting mirror 5, and is transmitted to the phase modulating element 1.
4, the signal is phase-modulated, reflected by a reflecting mirror 6, transmitted through a semi-transparent mirror 7, and reflected by a movable mirror 8.
The signal light output from the optical fiber 4 to be measured is transmitted through the semi-transparent mirror 7, the reference light reflected by the movable mirror 8 is reflected by the semi-transparent mirror 7, and these light beams are combined.
The combined light flux passes through the linear analyzer 9 and enters the photodetector 10.

Photodetector 10 is connected to bandpass filter 12 .
Bandpass filter 12 is connected to recording device 13 . Further, an oscillation circuit 11 is connected to a phase modulation element 14 and a bandpass filter 12.

The feature of this embodiment is that the phase modulation element 14 applies phase modulation to the reference light instead of intensity modulating the light beam output from the light source 1. This results in
The interference intensity between the signal light and the reference light changes over time with the modulation frequency, and reaches its maximum when the optical path lengths of the signal light and the interference light match.

The effect of this example will be further explained quantitatively.

The phase modulation element 14 generates a rectangular wave of frequency rect
Assume that the reference light is phase modulated at (t). At this time, the interference intensity I detected by the photodetector 10 is expressed as I=I ₁ +I ₂ +2√ _{1 2} |〓(〓 ₂ )|cos(rect(t
)+〓 ₁₂ )……(3) is given.

When the depth of phase modulation by the rectangular wave is set near 〓, Equation (3) becomes: I=I ₁ +I ₂ + _{2√ 1 2} |
...(4) Here, 〓 is a constant. Therefore, the interference intensity I changes over time depending on the frequency, and when 〓 ₁₂ =0, the degree of coherence 〓(〓 ₁₂ ) becomes maximum, and the amplitude of the interference intensity I becomes maximum. Therefore, as in the first embodiment, the movable mirror 8 is swept, the position where the interference intensity I is maximum is measured for light sources of different wavelengths, and the chromatic dispersion of the optical fiber is obtained from the optical path length difference with respect to the wavelength. be able to.

In this embodiment, the phase modulation element 14 is placed between the reflecting mirror 5 and the reflecting mirror 6, but it can be placed at any position as long as it is on the optical path of the reference light.

FIG. 3 is a block diagram of an optical fiber wavelength dispersion measurement apparatus according to a third embodiment of the present invention.

The light beam output from the light source 1 is separated into two light beams by the semi-transparent mirror 3. The light beam reflected by the semi-transparent mirror 3 propagates through the optical fiber 4 to be measured as signal light,
It is reflected by the mercury bath 15 and propagates through the optical fiber 4 to be measured again. The light beam transmitted through the semi-transparent mirror 3 propagates in space as a reference light, undergoes phase modulation by the phase modulation element 14, is reflected by the movable mirror 8, and is phase modulated by the phase modulation element 14 again.
The signal light that has traveled back and forth through the optical fiber 4 to be measured passes through the semi-transparent mirror 3.
The reference light that has been phase-modulated twice is passed through the semi-transparent mirror 3.
and these beams are combined. The combined light beam passes through a linear analyzer 9 and is detected by a photodetector 1.
0.

Photodetector 10 is connected to bandpass filter 12 .
Bandpass filter 12 is connected to recording device 13 . Further, an oscillation circuit 11 is connected to a phase modulation element 14 and a bandpass filter 12.

This embodiment is basically the same as the second embodiment, except that the signal light travels back and forth through the optical fiber 4 to be measured, and the reference light also travels back and forth along one optical path. In this embodiment, the end face of the optical fiber 4 to be measured is immersed in a mercury bath, so that the signal light is totally reflected at this end face. The interference intensity I measured by the photodetector 10 is the same as in the second embodiment, but since the signal light passes through the optical fiber 4 to be measured twice, it is equivalent to an optical fiber twice the length. Furthermore, since the reference light also passes through the phase modulation element 4 twice, the depth of phase modulation is also doubled. Therefore, the phase modulation element 14 is set so that the reference light travels back and forth and has a modulation depth of approximately .

The measurement of the interference intensity between the signal light and the reference light is the same as in the second embodiment.

〔Effect of the invention〕

As explained above, according to the present invention, the chromatic dispersion of an optical fiber can be measured with a good signal-to-noise ratio. Therefore, it is also possible to measure the chromatic dispersion of a relatively short optical fiber.

Since the present invention enables highly accurate measurement with a simple configuration, it is highly effective in quality inspection and quality control of optical fibers.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an optical fiber wavelength dispersion measuring device according to a first embodiment of the present invention. FIG. 2 is a block diagram of an optical fiber wavelength dispersion measurement apparatus according to a second embodiment of the present invention. FIG. 3 is a block diagram of an optical fiber wavelength dispersion measurement apparatus according to a third embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Light source, 2... Intensity modulator, 3, 7... Semi-transparent mirror, 4... Optical fiber to be measured, 5, 6... Reflecting mirror, 8... Movable mirror, 9... Linear analyzer, 10 ……
Photodetector, 11...Oscillation circuit, 12...Band filter, 13...Recording device, 14...Phase modulation element,
15...Mercury tank.

Claims (1)

[Claims] 1. Optical means for separating a light beam emitted from a light source that outputs a coherent light beam into a signal light and a reference light, and an optical means for inputting this signal light into an optical fiber to be measured. an optical means for causing interference between the signal light propagated through the optical fiber to be measured and the reference light propagated through a path other than the optical fiber to be measured; means for detecting the interference light obtained by this means; and a wavelength from the light source. In an optical fiber wavelength dispersion measuring device, the optical fiber wavelength dispersion measuring device is equipped with means for variably setting the optical path length difference between the signal light and the reference light so that the interference intensity of each of the different output lights is maximized. An optical fiber wavelength characterized by comprising means for applying intensity modulation or phase modulation to at least one of the reference light and means for extracting information on the intensity of the interfered light at a narrow band frequency including the modulation frequency of this means. Dispersion measuring device.