JPS63144231A - Incidence device for fourier transformation type optical fiber loss measuring instrument - Google Patents

Abstract

PURPOSE:To measure the loss spectrum of an optical fiber with high accuracy by providing an aperture behind a concave mirror which converges luminous flux projected by a Michelson interferometer and reducing the diameter of converged light incident on the optical fiber. CONSTITUTION:The Michelson interferometer consists of a light source 1, the concave mirror 2, fixed mirrors 3 and 3', a movable mirror 4, and a beam splitter 5. Composite wave luminous flux from the interferometer is converged by an elliptic mirror 6, and made incident on a convex lens 8 after being transmitted through the aperture 7 to obtain parallel luminous flux. Then part of the luminous flux is branched by an optical demultiplexer and this luminous flux is guided to a photodetector 14' through an optical attenuator 10 and the convex lens 8. The remaining luminous flux demultiplexed by the optical demultiplexer 9, on the other hand, is converged by the convex lens 9 and incident on an optical fiber 13 arranged by an optical fiber holder 12 so that its incidence end surface is on the focus, and its projection light is detected by a photodetector 14. The outputs of photodetectors 14 and 14' are guided to a computer through a preamplifier and an A/D converter and processed by Fourier transformation to obtain a projection light spectram.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an input device for a Fourier transform type optical fiber loss measuring instrument, and more particularly, to an input device for measuring the transmission loss spectrum of an optical fiber in a short time and with high precision. The present invention relates to a light input device used in a Fourier transform type optical fiber loss measuring instrument required for.

[Prior art to the invention]

Conventionally, a cutback method has been used to measure the transmission loss spectrum of an optical fiber. In the measurement procedure using this cutback method, first, a long optical fiber is
(Km) ), and then, while keeping the input condition of the optical fiber constant tη, move the fiber a short distance from the input point (this length is expressed as L2 (Km). For example, 0 Cut at .001~0.002 Km),
Measure the emitted light intensity again. When the output light intensities of these long fibers and short fibers are rx and rt, respectively, the loss (α) of the optical fiber is given by the following equation (1).

In conventional transmission loss spectrum measurements, a light source such as a tungsten lamp is used as a light source, and the light is made monochromatic through a dispersive spectrometer (a spectrometer that uses a diffraction grating to separate white light), and the equation ( 1) was performed to obtain the transmission loss spectrum. Therefore, for example, when determining the transmission loss spectrum between wavelength λ1 and wavelength λ1) (λ2>λ1), first measure I1 from λ1 to λ2, and then sequentially measure I2 from λ1 to λ2. Intensity must be measured. For this reason, it takes a long time to measure, and
In a dispersive spectrometer, the separated light becomes significantly weaker as it deviates from the blaze wavelength of the diffraction grating used.
In addition, because filters are used to remove high-order diffracted light and slits are used to obtain a certain wavelength resolution, the final monochromatic light intensity is weak. . Therefore, transmission loss spectrum measurement using a dispersive spectrometer has the disadvantage that it is time-consuming and difficult to measure with a high S/N ratio over a wide wavelength range.

As a measurement method that solves this drawback and enables highly accurate transmission loss spectrum measurement with a high S/N ratio in a short time,
There is a measurement method using a Fourier transform spectrometer. This Fourier transform spectrometer uses a Michelson interference needle to modulate the light emitted from the light source, enters the sample, and spectra the transmitted light signal received by the detector using Fourier transform. In addition to being able to significantly shorten measurement time compared to dispersive spectrometers that monochromate one by one,
Even if the optical signal transmitted through the sample is weak, integrating the signal has the advantage of improving the S/N ratio of the signal and enabling highly accurate measurement.

[Problem that the invention seeks to solve]

However, the Fourier transform spectrometers that have been developed so far have optical systems designed for bulk-shaped samples, and cannot be used to measure the transmission loss of optical fibers, which requires highly accurate transmission spectrum measurements. That is, in order to fully utilize the advantages of the Fourier transform spectrometer described above, it is necessary to construct an optical system for optical fiber. This is because with a Fourier transform spectrometer, white light is incident on the measurement sample, so the amount of light incident on the sample is significantly larger than when using a dispersive spectrometer, and the incident light flux is not rushed into the optical fiber. When this occurs, excess light that is not incident on the optical fiber hits the optical system holding the optical fiber, causing a temperature rise in that part, and causing a slight optical axis misalignment between the incident light beam and the optical fiber during measurement. This is because high-precision measurements cannot be made. Furthermore, for highly accurate loss spectrum measurement, it is necessary to eliminate measurement errors due to time fluctuations in the light source output.

The present invention has been made in view of the above points, and it provides an optical system for inputting optical fiber light to a Fourier transform spectrometer configured for bulk sample measurement, and uses the Fourier transform spectrometer to The purpose of this invention is to enable highly accurate loss spectrum measurement of optical fibers.

[Means for solving problems]

In order to solve the above-mentioned problems, the Fourier transform type optical fiber loss measurement injector according to the present invention focuses only a part of the image formed after the concave mirror that converges the interference light beam emitted from the Michelson interference needle. an aperture that transmits light; a convex lens that converts the light beam transmitted through the aperture into a parallel light beam;
an optical demultiplexer for changing the propagation direction of a part of the light that has passed through the convex lens and become a parallel beam; and a photodetector for measuring the intensity of one of the lights split by the optical demultiplexer. and a convex lens for condensing the other beam split by the optical demultiplexer and inputting it into the optical fiber.

According to the present invention, in a Fourier transform spectrometer designed for measuring bulk samples, an aperture is provided after an ellipsoidal mirror to remove the light beam emitted from a Michelson interferometer that is not focused to a sufficiently small diameter. It becomes possible to condense the light in a sufficiently small amount on the end face of the optical fiber, and it is possible to suppress the extra light flux that does not enter the optical fiber from hitting the optical fiber entrance part. Therefore, it is possible to suppress deviation of the optical axis due to temperature rise during measurement of the optical fiber entrance part. In addition, by guiding a part of the light flux incident on the optical fiber to a photodetector and measuring its intensity, the time-varying error in the light source output that occurs during the measurement of the light output from the optical fiber is measured. This has the advantage of being able to provide a Fourier transform type optical fiber loss measuring instrument that can withstand minute loss measurements without being included in the strength.

〔Example〕

FIG. 1 is a schematic diagram showing an embodiment according to the present invention,
In the figure, 1 is a light source (such as a halogen lamp), and 2 is a concave mirror (
ellipsoidal mirror), 3.3' is a fixed mirror, 4 is a movable mirror, and 5 is a beam splitter, making the whole a Michelson interference needle. Further, 6 is a concave mirror (ellipsoidal mirror), 7 is an aperture, 8 is a convex lens, 9 is an optical demultiplexer, 10 is an optical attenuator, 1) is an aperture, 12 is an optical fiber holder, 1
3 indicates an optical fiber, and 14.14゛ indicates a photodetector.

As is clear from FIG. 1, according to one embodiment of the inlet device for an optical fiber transmission loss measuring instrument according to the present invention, a light source 1 such as a halogen lamp is provided.
It has a concave mirror 2 that converts the twisted light into parallel light; and the light made parallel by the ellipsoidal mirror 2 is transmitted to a plane mirror 3.
3 and guided to the beam splitter 5, where it is split into two. That is, part of the light passes through the beam splitter 5 and is guided toward the fixed mirror 3'', and the other part of the light is reflected by the beam splitter 5 and guided toward the movable mirror 4. In this way, the fixed mirror 3 The light guided to the fixed mirror 3° and the beam splitter 5
and reaches the plane mirror 3 and the ellipsoidal mirror 6. On the other hand, the light guided by the movable mirror 4 is reflected by the movable mirror 4, passes through the beam splitter 5, passes through the plane mirror 3, and is guided to the concave mirror 6. At this time,
The light emitted from light source 1 is transmitted through beam splitter 5 and fixed mirror 3°.
When there is a difference of χ/2 in the distance from the fixed mirror 3' to the movable mirror 4, an optical path difference of χ occurs between the light reflected by the fixed mirror 3' and the light reflected by the movable mirror 4. Waves cause interference, either canceling each other out or reinforcing each other.

Such a composite wave beam is condensed by an ellipsoidal mirror 6, and the ellipsoidal mirror 6 and a convex lens [for example, ZrF 4 (60)
-BaF e (32) -LaF 3 (4)
After passing through an aperture installed between AIF 3 (4) compositions (all fluoride glass made of fluoride glass in mol %) 8, the light is incident on a convex lens 8 to form a parallel beam of light.

A part of the light beam is divided by an optical demultiplexer 9 having a support made of fluoride glass having the same composition as the convex lens 8, and the demultiplexed light beam is sent to an optical attenuator! 0 and a convex lens 8, for example, an InSb photodetector 1.
It led to 4.

- The other light beam split by the tip splitter 9 is sent to the convex lens 8
The light is focused by the optical fiber holder 12 and enters the optical fiber 13 installed so that the incident end face is on the focal point. At this time, the condensing diameter of the incident light beam on the focal plane is 1
It was 00 μ... The light emitted from the optical fiber 13 was received by the photodetector 14. Light equipment - Exhibits 14 and 14
The output (interferogram) of 0.2° was led to a computer through a preamplifier and an A/D converter (not shown), and was Fourier-transformed to obtain an output light spectrum (or reference light spectrum).

The detailed principle of such an interferometric spectroscopic needle using a so-called Michelson interferometer is described in, for example, New Experimental Chemistry Course Basic Technology 3 Light (1), page 210 (Maruzen Co., Ltd.). A detailed explanation will be omitted and a brief explanation will be provided.

As mentioned above, when there is a difference of χ/2 in the distance between the beam splitter 5, the fixed mirror 3'', and the movable mirror 4, the light from the light source 1 is reflected by the light wave reflected by the fixed mirror 3'' and the light wave reflected by the movable mirror 4. An optical path difference of χ occurs between the two light waves, and as a result, the combined waves of both light waves cancel each other out or strengthen each other.

Therefore, the outputs from the photodetectors 14 and 14' described above become a function of χ [this is called an interferogram,
It is called (χ)].

Now, consider monochromatic light with a wave number of 7 as the incident light, its amplitude is U, and the amplitude reflectance and amplitude transmittance of the beam subrifter are 7. t, the composite wave is γtU (e-upper πP engineering 1 +0-element π-nominal,), where χ1/
2 and χ2/2 are from the beam substitute 75 to the fixed mirror 3'
and the respective distances to the movable mirror 4.

Since the intensity of light received by the photodetector is the square of the absolute value of the composite wave, ■ (χ, V) = 2 r T” t t @00” (1+cos2y
cfz) −(21, where χmiχl−χ
! represents the optical path difference, and 0 represents the complex conjugate.

In the case of general incident light containing various wave numbers, the above (2
) is integrated by V, and the result is the energy received by the photodetector 14.14. That is,
This is an interferogram. Here, η”rγ@1
) @ is the efficiency of the beam subrifter, and B(v) is the spectral distribution of the incident light (here, the light that has undergone fiber loss).

In order to know the spectral distribution B<v> from equation (3) above, it is sufficient to perform the Fourier transform of (η is omitted as it is included in B(9')).

From the above measurement system, the output light spectrum (I3) of the optical fiber and the reference light spectrum (1
'3) and the output light spectrum of the optical fiber ([4
), the reference light spectrum (I'm) is used, and the transmittance obtained by multiplying by 100, that is, the 100% transmittance line is shown in FIG. If the above value is 100%, 1oΦ
This is a case where there is no measurement error due to time variation of the light source at time and tl.

In this measurement, a stone/silica-based multimode optical fiber was used as the optical fiber 13.

From wave number 10,000 cm-' to 4,800 cm-',
The slope of the 100% line is less than 0.1%, and 70%
00c1) to 4800 cm-'', very good results were obtained in which the fluctuation of the 100-% line was less than ±0.1%.

In order to compare with the above measurement, the reference light spectrum was not measured, and 13/I a −−−−−−−−−−−−−−・−
・The 100% transmittance line obtained by multiplying (6) by 100 is shown in Figure 3 (the measurement time interval of I x and It is 3
(obtained by integrating interferograms 50 times each), unlike the spectrum shown in FIG. 2, wavelength dependence is observed in the 100% transmittance line.

This is caused by fluctuations in the intensity of the incident light flux due to temporal fluctuations in the light source output. In other words, measure the reference light spectrum,
It can be seen that by determining the transmittance 100% line from equation (5), it was possible to eliminate measurement errors due to fluctuations in the output of the light source.

10 obtained from equation (6) when no aperture is provided
The 0% line is shown in FIG. 4 (the measurement time interval for Ia and 4 was 30 minutes, and was determined by integrating interferograms 50 times each). The slope of the 100% line is larger than when measuring with the aperture shown in Figure 3, and is accurate to 1o.

It can be seen that the % line is not obtained.

In the case of a Fourier transform spectrometer, the intensity of the light beam incident on the material (optical fiber in this example) is extremely large compared to the case of a dispersive spectrometer because the light beam is not separated by a diffraction grating. Therefore, if the light is made to enter the optical fiber without worrying about the focusing diameter, the light that has not entered the optical fiber will hit the optical fiber entrance part, causing a temperature change at the entrance part, and this temperature change will result in a change in the temperature during measurement. This causes optical axis misalignment and impairs high-precision measurements.

Therefore, when measuring the loss of an optical fiber, which requires highly accurate measurement, it is desirable that the diameter of the condensed light at the entrance part be made small to the extent that the S/N of the high intensity output from the optical fiber does not deteriorate significantly. In this example, the converging diameter was 100 μm.

Next, FIG. 5 shows an example of loss measurement of a quartz optical fiber obtained using the input optical system of the present invention (FIG. 1). A long optical fiber with a length of 2 Km and a short optical fiber with a length of 2a+ were used, and the cut-back method was used to determine the value using the following formula.

Here, Ll is the length of the long optical fiber, L2 is the length of the short optical fiber, If is the output light spectrum of the long optical fiber, (2) is the output light spectrum of the short optical fiber, 10.
．． 10. are reference light spectra immediately after measuring the output light spectra of long and short optical fibers (each spectrum was obtained by integrating interferograms 50 times).

When the reference light spectrum is not measured, and when an aperture is not installed to narrow down the condensing diameter of the incident light beam on the input end face of the optical fiber, the time fluctuation of the light source output and the The extra light flux that does not enter the optical fiber and hits the optical fiber entrance part supported by the optical fiber holder 12 heats the entrance part, causing a shift in the optical axis, making it difficult to measure a reproducible loss spectrum. there were.

However, by providing an aperture and measuring the reference light spectrum, even if the measurement interval of the output light spectrum of long and short optical fibers is set arbitrarily, the effects of time fluctuations in the light source output and optical axis deviation can be avoided. This does not appear in the measurement results, improving the reproducibility of measurements and making it possible to measure loss spectra with high reliability.

As a support for the above-mentioned convex lens 8 and optical demultiplexer 9,
For example, group Ia, group Ua, group ma, group IVa, group IIIb
A fluoride glass containing an element selected from the group consisting of the group consisting of the group consisting of the group consisting of the group rVb and the group rVb is used. Such fluoride glass-
is cheaper than single crystals such as KBr that have been conventionally used as supports for optical demultiplexers, does not have significant deliquescence like single crystals such as KBr, and has good weather resistance. It has various advantages over conventional ones, including the fact that it is easier to manufacture large-sized ones.

As such fluoride glass, for example, ZrF
4-BaF 2-LaF 3-AIF 3 series, ZrF
4-BaF II-LaF 3-AIF 3-NaF system, ZrF 4-BaF 2-Y F 3-AIF 3L
iF system, ZrF 4-BaF 2-GdF 3-AIF
3 series, 1lfF a BaF 2 LaF 3
'AIF 3 series, IIfF 4ZrF A
BaF2-LaF3-NaF AIFs
-PbF 2 series, 8 IF 3 Bar
2 CaF 2-Y F 3 type glass can be mentioned, and even when the above-mentioned fluoride glass was used, the same effect as above could be obtained. ' Although the optically polished fluoride glass slope was used in the optical demultiplexer, a metal coating may be applied to the surface of the fluoride glass to change the demultiplexing efficiency as necessary.

〔Effect of the invention〕

As explained above, according to the incidence device for a Fourier transform loss measuring instrument according to the present invention, an aperture is provided after the concave mirror that condenses the light beam emitted from the Michelson interference needle to narrow down the diameter of the incident light to the optical fiber. This allows the light beam that is not incident on the optical fiber to hit the input section, suppressing the shift of the optical axis caused by temperature fluctuations, and splitting a part of the incident light beam using an optical demultiplexer, monitoring the incident spectrum intensity, and separating the optical fiber from the optical fiber. By taking the ratio with the emission spectrum of
By eliminating measurement errors due to time fluctuations in the light source output, it is now possible to measure loss spectra with high precision in a short time, which eliminates the long time required to fabricate optical fibers using a dispersive spectrometer. The time required to test product characteristics can be significantly reduced. Therefore, if a Fourier transform loss measuring device equipped with the injection device according to the present invention is introduced into an optical fiber production line, there is an advantage that it will lead to labor saving on the production line.

Furthermore, when fluoride glass is used as a material for the convex lens used in the input device, the refractive index of the glass is n□-1
, 50 to 1.53, which is higher than CaF 2 (n□-1,43), which is usually used as an infrared transmitting material, can be easily obtained, so the input optical system can be made compact. In addition, since the glass is easy to manufacture in large sizes, if the glass is used as a material for the optical demultiplexer and convex lens, the input device can be constructed at a lower cost than when CaF 9 single crystal is used. This has the advantage of improving economic efficiency.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an embodiment of the incidence device for a Fourier transform type optical fiber loss measuring instrument according to the present invention, and FIG. 2 is a diagram showing the formula (5
), the transmittance 100% line in Figure 3 is calculated using equation (6
), and Figure 4 shows the 100% transmittance line determined from equation (6) without an aperture.
The line in FIG. 5 is a diagram showing a loss spectrum of a quartz optical fiber measured in an example according to the present invention. 1...Light source, 2...Ellipsoidal mirror, 3.3゛...
Fixed mirror, 4, 6... Movable mirror, 5... Beam splitter, 6... Ellipsoidal mirror, 7... Aperture,
8...Convex lens, 9...Optical demultiplexer, 1o...
Optical attenuator, 1)...Aperture, 12...Optical fiber holder, 13...Optical fiber, 14.14°...
Photodetector.

Claims (2)

[Claims] (1) An aperture that transmits only a part of the image formed after the concave mirror that condenses the interference light beam emitted from the Michelson interferometer, a convex lens that converts the light beam that has passed through the aperture into a parallel light beam, and the convex lens. an optical demultiplexer for changing the propagation direction of a portion of the light that has passed through and become a parallel beam;
It has a photodetector for measuring the intensity of one beam split by the optical demultiplexer, and a convex lens for condensing the other beam divided by the optical demultiplexer and inputting it into an optical fiber. An input device for a Fourier transform type optical fiber loss measuring instrument, characterized in that: (2) The material of the convex lens and/or optical demultiplexer is
The Fourier transform type according to claim 1, which is a fluoride glass containing an element selected from the group consisting of Group Ia, Group IIa, Group IIIa, Group IVa, Group IIIb, and Group IVb. Injection device for optical fiber loss measuring instruments.