JPH01165928A - Method and apparatus for measuring normalized double refraction of single polarizing fiber - Google Patents

Abstract

PURPOSE:To facilitate measurement and to enhance measuring accuracy, by detecting the variation width of a beam receiving level based on rotation by sweeping the wavelength of a wavelength variable beam source and measuring the interval of a waveform cyclically becoming max. or min. on a wavelength axis in the variation width. CONSTITUTION:The emitted light of a light source 11 such as a semiconductive laser is swept in its wavelength within an oscillation spectrum range by a spectroscope 12 to be incident on a photodetector 19 through a single mode fiber 13, a polarizer 14, a 1/4 wavelength plate 15, a fiber 16 to be measured, a 1/2 wavelength plate 17 rotated by a 1/2 wavelength plate rotating mechanism 22 and a fixed detector 18. Next, the output signal of the photoreceptor 19 as an input signal and the rotary synchronous signal of the mechanism 22 as a reference signal are inputted to a lock-in amplifier 20, and the output signal of the amplifier 20 and the wavelength sweep signal of the spectroscope 12 are inputted to a recorder 21. Herein, since the variation width of the amplitude of the transmitted light of the detector 18 changes corresponding to the polarizing state of the emitted light of the fiber 16, this variation is detected as the variation of the light receiving level of the photodetector 19 to make it possible to specify the polarizing state of the emitted light of the fiber 16.

Description

[Detailed description of the invention] Table of contents Overview ・ ・ ・ ・ ・ ・ ・ ・ ・
・ ・ ・ 3 pages Industrial application fields ・・・・・・・
・Page 4: Prior art ・・・・・・・・・・・・・・・・・・・・・ 6. Problems to be solved by the invention ・Page 9: Means for solving the problems ・Page 1: Effects ・・
・ ・ ・ ・ ・ ・ ・ ・ ・ ・ 12 truths
Example ・ ・ ・ ・ ・ ・ ・ ・ ・
・ ・ Page 13 Effects of the invention ・・・・・・・・・・・・2
1 page summary Concerning a normalized birefringence measurement method and measurement device for a single polarization fiber that reduces coupling between modes by increasing the difference in propagation constant between polarization modes, this paper aims to facilitate measurement. For the purpose of analysis, the light from the wavelength tunable light source is circularly polarized and input into the fiber to be measured, and the transmission polarization plane rotates relative to the emitted light around the propagation direction of the emitted light from the fiber to be measured. receiving the emitted light with a light receiver via a means;
The wavelength tunable light source is configured to sweep the wavelength of the wavelength tunable light source, detect a fluctuation range of the received light level based on the rotation, and measure a wavelength interval at which this fluctuation range is periodically maximum or minimum on the wavelength axis. do.

INDUSTRIAL APPLICATION FIELD The present invention relates to a method and apparatus for measuring the normalized birefringence of a single polarization fiber, which reduces coupling between modes by increasing the difference in propagation constants of polarization modes.

In a normal axisymmetric single mode optical fiber, two mutually orthogonal independent fundamental modes (I]E11 mode) can propagate. Among these, the linearly polarized wave component that has an electric field in the X direction in the plane perpendicular to the axis of the optical fiber is
Let the X mode be the linearly polarized wave component that has an electric field in the X direction perpendicular to the It is degenerate. However, since actual optical fibers have some degree of unavoidable non-axial symmetry and bending, the above-mentioned degeneracy is resolved and coupling occurs between modes. As a result, (a) when coupling occurs between polarization modes, the polarization state of the 1-IE11 mode observed at the receiving end fluctuates drastically due to disturbances such as temperature changes in the optical fiber, for example, reception in a heterodyne reception method. A fatal fluctuation occurs in the level.

(b) When degeneracy is resolved, polarization dispersion occurs, which limits the information transmission speed of single-mode optical fiber communications.

However, in order to solve these problems, it is necessary to use a single polarization fiber that transmits only a single polarized wave. One of the main fiber parameters of a single polarization fiber is the normalized birefringence, and there is a need for a simple method and apparatus for measuring it.

Conventional technology Single polarization fiber has characteristics such that transmission loss in one polarization mode is extremely large (absolute single polarization characteristics).
There are two known methods: one in which the propagation constant difference between the two polarization modes is made as large as possible, and the coupling between the two modes is reduced. The latter is common.

FIG. 11 is a cross-sectional view of a typical single-polarization fiber. In the figure, 41 is a cladding, 42 is a core,
Each has a different refractive index.

43 and 44 are stress-applying parts installed in the longitudinal direction of the fiber axially symmetrically, for example, in the Afterwards, the core 42 contracts, causing different stress distributions in the X direction and the y direction (7X).

This causes birefringence ′b<difference in propagation constants in the X and y directions Δβ(-1β −β, 1)× in the core 42, giving it single polarization characteristics. .

The propagation constant difference Δβ is generally the polarization birefringence (A is called the mode birefringence, and is usually the normalized birefringence B B−Δβ/k (k=2π/λ: the polarization birefringence of light in vacuum). wave number).

FIG. 12 is a configuration diagram of a conventional normalized birefringence measuring device. The emitted light from the light source 51 passes through a spectroscope 52 for wavelength sweeping, a single mode optical fiber \53.1/4 wavelength plate 54, a polarizer 55, a core fiber to be measured\56, and an analyzer 57 in this order. The output signal from the optical power meter 58 and the wavelength sweep signal from the spectroscope 52 are recorded by a recorder 59 such as a pen recorder.

The transmission polarization planes of the polarizer 55 and the analyzer 57 (or they are set in the same direction, and this direction is the main axis of the core to be measured < 56 (
It is tilted at 45° with respect to the X-axis or the Y-axis.

FIG. 13 shows an example of recording by the recorder 59.
The vertical axis is the output of the optical power meter 58, and the horizontal axis is the sweep wavelength. Maximum values and minimum values (zero) appear periodically as the wavelength increases.

Now, assuming that the length of the fiber under test 56 is λ and the wavelength of the propagating light is λ, the phase difference φ between the two main axes of the fiber 56 under test is as follows: φ −kBL=2 π BL/λ
It is expressed as...ku1). This phase difference φ is 2
Since wavelengths of πn (n is an integer) exist discretely,
These are λ, λ, ..., λ, λ in order from the short wavelength side, 1. ...12
Let it be k. Consider the adjacent wavelengths λ and λ
k+1 Since the phase difference is 2π, from the relationship of equation (1), 2π−2πBL (1/λ −1/λ )k
k+1 B=, ;l λ /L(λ −λ )...(2
) ka, 1 k k+1 k is obtained. Therefore, adjacent wavelengths λ giving a phase difference of 2π, λ1<+
By actually measuring 1, the birefringence B can be determined for normalization.

In the measurement results shown in FIG. 13, the wavelengths λ and λ
The fact that the output of the optical power meter is at its maximum when
The minimum value (zero) at 14 indicates that the linearly polarized light incident on the fiber 56 to be measured is converted into linearly polarized light with a vertical plane of polarization and is emitted.

Moreover, at wavelengths intermediate between these, the light is emitted as elliptically polarized light or circularly polarized light.

Therefore, when calculating the normalized birefringence B, the adjacent wavelengths λ and λ are set to k of λ and λ13.
h+i 11 pairs or λ and λ14
A set of can be used.

Note that in order to improve the measured viscosity, the wavelength interval that gives the minimum value is usually measured.

Problems to be Solved by the Invention However, in the conventional measurement method described above, the transmission polarization planes of the measurement source fill, polarizer, and analyzer are set in the direction of the main axis at the input end and output end of the fiber under test, respectively. Since it is necessary to incline it by 45 degrees with respect to the measurement, a problem arises in that complicated work is required when arranging each member prior to measurement. On the other hand, in order to improve the measurement accuracy, it is possible to obtain a large number of measurement data by rotating the analyzer by 90 degrees so that the wavelengths λ and λ13 in the measurement example shown in FIG. 13 give the minimum values. Although this method is effective, it also takes time and effort to set the main axis direction again in this case, which is a problem.

The present invention was created in view of these problems.
The purpose is to facilitate measurement and improve measurement accuracy.

A foolproof way to solve problems FIG. 1 is a diagram showing the principle of the present invention.

The method for measuring the normalized birefringence of a single-polarized fiber according to the present invention involves making the light from the wavelength tunable light source 2 circularly polarized and making it enter the fiber 4 to be measured, and changing the propagation direction of the light emitted from the fiber 4 to be measured. The output light is received by a light receiver 6 via an analyzer 5 having a transmission polarization plane that rotates relative to the output light around the center, and the wavelength of the wavelength tunable light source 2 is swept to detect the rotation. The variation width of the received light level based on the wavelength is detected, and the interval between wavelengths at which this variation range is periodically maximum or minimum on the wavelength axis is measured.

In addition, as a device directly used for implementing the above measurement method,
A wavelength tunable light source 2 capable of changing the wavelength of emitted light, a quarter wavelength plate 3 that converts the light from the wavelength tunable light source 2 into circularly polarized light and enters the fiber under test 4, An analyzer 5 having a transmission polarization plane that rotates relative to the output light around the propagation direction of the output light, a light receiver 6 that receives the transmitted light of the analyzer 5, and a wavelength tunable light source 2. Received light level fluctuation range detection means 7 for detecting the fluctuation range of the received light level based on the rotation when the wavelength is swept
Provided is a normalized birefringence measuring device for a single polarization fiber, characterized in that it is comprised of the following.

The reason why the light from the working wavelength tunable light source is made into circularly polarized light and is made incident on the fiber to be measured is to eliminate the need to set the orientation of the principal axis of the fiber to be measured. The circularly polarized light incident on the fiber to be measured is converted into linearly polarized light, elliptically polarized light, or circularly polarized light by the birefringence of the fiber to be measured, and is emitted. The analyzer into which this emitted light is incident has a transmission polarization plane that rotates relative to the fiber under test around the propagation direction of the emitted light, so the polarization state of the emitted light from the fiber under test is The intensity of the light emitted from the analyzer varies accordingly. That is, when the light emitted from the fiber to be measured is linearly polarized light, the variation range is maximum, when it is circularly polarized light it is minimum, and when it is elliptically polarized light it is an intermediate value.

On the other hand, the polarization state of the light emitted from the fiber to be measured changes depending on the wavelength of the light used for measurement. Therefore, by sweeping the relevant wavelength and detecting the change in the fluctuation range of the light reception level of the optical receiver, and measuring the interval of wavelengths that give the maximum value (for linearly polarized light) or the minimum value (for circularly polarized light), the birefringence can be determined.

In the measuring device according to the present invention, since the transmitted polarization plane of the analyzing means rotates relative to the output light of the fiber to be measured, its implementation can be divided into two ways. That is, there are two cases in which the analysis means consists of a means for rotating the plane of polarization of the light emitted from the fiber under test and a fixed analyzer, and another case in which the analysis means consists of a rotating analyzer that itself rotates. .

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 2 is an overall configuration diagram of a normalized birefringence measuring device for a single polarization fiber embodying the first embodiment of the present invention. The output light of light +1iii11 is transmitted through a spectroscope 12, a single mode fiber 13, a polarizer 14, a 1/4 wavelength plate 15,
The measured fiber 16 enters the light receiver 19 via the 1/2 wavelength plate 17 rotated by the 1/2 wavelength plate rotation mechanism 22 and the fixed analyzer 18. 20 is a lock-in amplifier that performs synchronous amplification, and this lock-in amplifier 20 receives the output signal of the light receiver 19 as an input signal and the rotation synchronization signal of the 1/2 wavelength plate rotation mechanism 22 as a reference signal. It has been entered. 21 is a recording device such as a pen recorder for two parameters, and the output signal of the lock-in amplifier 20 and the spectrometer 1.
Two wavelength sweep signals are input.

As the light source 11, one having an oscillation wavelength width wider than the wavelength sweep range by the spectroscope 12 can be used.

An LD (semiconductor laser) having a spectral width of ±10 nm 100 nm with respect to the center wavelength is suitable. In this example, an LD having an oscillation spectrum of (1300±8) nm is used.

A YAG laser with Raman scattering can also be used.

The spectroscope 12 is only for sweeping wavelengths within the oscillation spectrum range of the light source 11, and has a resolution of about 0.5 nm. As such a spectrometer 12, one constructed using a diffraction grating is suitable.

As the polarizer 14 and the fixed analyzer 18, a Glan-Thompson prism having a birefringent crystal attached with an optical adhesive can be used.

The single mode fiber 13 is for making the light emitted from the spectrometer 12 enter the polarizer 14, and depending on the case, it may be omitted and direct optical coupling may be performed.

The length of the fiber under test 16 is within the sweep wavelength range (approximately 1
0 nm), the wavelength characteristics described later can be obtained for one period.

The half-wave plate rotation mechanism 22 rotates the half-wave plate 17 by, for example, 1351-IZ, thereby rotating the polarization plane of the light emitted from the fiber to be measured 16 by 270H2.

FIG. 3 shows a refractive index ellipsoid of a birefringent crystal shown to explain the action of the half-wave plate 17, and for convenience it is assumed that the half-wave plate 17 is made of a positive uniaxial crystal. Now 1/2
The refractive index of the wavelength plate 17 for ordinary rays is n. The maximum value of the refractive index for extraordinary rays is n. (no<no)
. Assume that light is propagating in the direction of arrow S from the origin O of an orthogonal three-dimensional coordinate axis with the optical axis of the half-wave plate 17 as the y-axis, and that the projection of arrow S onto the X7 plane coincides with the Z-axis. Suppose that At this time, the refractive index ellipsoid is x2/n 2+y
2/n 2+Z2/n2-10
Represented by e O. The refractive index n for ordinary rays is always 〇 constant, and the refractive index ellipsoid up to the point P where the circle A cut by the XZ plane and the ellipse B cut by the plane perpendicular to the propagation direction S at the origin O intersect. It is expressed as the distance OP from the origin O. Also, the refractive index n for extraordinary rays. ' is the propagation direction S
and yilIll change according to the angle θ, and are expressed as the distance OQ from the origin O to the point Q where the ellipse B intersects with the yz plane. In other words, the refractive index n for the extraordinary ray
' is from n to n depending on the propagation direction S of the light. It changes continuously until

In this way, the refractive index differs depending on the propagation direction S of light, so for example, set the desired optical axis so that the propagation direction S coincides with 7' tllll (θ-90°), and calculate the phase difference due to birefringence. By setting the thickness of the 1/2 wavelength plate 17 so that π, the polarization plane of the incident light can be converted to a polarization plane that is symmetrical about the optical axis (y-axis).

FIG. 4 is for explaining the effect of rotation of the 1/2 wavelength plate 17, and shows the directions of the polarization planes of the incident light and the output light when linearly polarized light is incident on the 1/2 wavelength plate 17. has been done. In the initial state of rotation, the plane of polarization (A) of the incident light is y
Assuming that the 1/2 wavelength plate 17 is aligned with the axis, the angular velocity ω
When rotated at a constant speed, after t (time unit) has elapsed, the optical axis is on the y' axis rotated by ω from the y axis position in the initial state, so the polarization plane (B) of the output light is the polarization of the input light. Wave front (
This results in a rotation of 2ω with respect to A>.

FIG. 5 shows the relationship between the amplitude of the transmitted light of the fixed analyzer 18 and the rotation angle of the half-wave plate 17 when the half-wave plate 17 is rotated as described above. It is assumed that the transmitted polarization plane of the analyzer 18 coincides with the direction (X-axis) perpendicular to the optical axis in the initial state of rotation. 1/
If the amplitude of the light incident on the two-wave plate 17 is A, and the loss of the propagating light in the half-wave plate 17 is ignored and is 1q, the amplitude of the light emitted from the fixed analyzer 18 is A. It is expressed as I Sin 2ωt l . At this point, that is, when the incident light on the half-wave plate 17 is linearly polarized light, the amplitude fluctuation range of the transmitted light of the fixed analyzer 18 becomes maximum, and correspondingly, the fluctuation range of the light reception level of the light receiver 19 is also maximum.

On the other hand, if the light emitted from the fiber to be measured 16 is elliptically polarized light with a set of electric field vector tips indicated by C as shown in FIG. 6(a), the fixed analyzer 1
As shown in FIG. 8(b), the fluctuation in the amplitude of the transmitted light in No. 8 is smaller than that in the case where the incident light is linearly polarized light. The range of variation at this time is determined according to the ellipticity of the ellipse.

When the light emitted from the fiber to be measured 16 is circularly polarized light with a set of electric field vectors at the tips indicated by D as shown in FIG. 7(a), the transmitted light of the fixed analyzer 18 The amplitude does not change at all, as shown in FIG. 2(b). This can be understood from the properties of circularly polarized light -1,8-.

In this way, since the fluctuation range of the amplitude of the transmitted light of the fixed analyzer 18 changes depending on the polarization state of the light emitted from the fiber 16 to be measured, by detecting this fluctuation as a fluctuation of the light reception level of the optical receiver 6, , the polarization state of the light emitted from the fiber 4 to be measured can be specified.

A lock-in amplifier that uses a synchronous detection method is optimal for detecting such fluctuations in the received light level that are synchronized with the rotation of the 1/2 wavelength plate 17, and by using this, measurement accuracy can be improved. It is.

FIG. 8 shows an example of recording according to the recording rule 21 when the wavelength of light incident on the fiber under test 16 is swept by the spectroscope 12, where the vertical axis shows the fluctuation width of the received light level and the horizontal axis shows the wavelength. ing. Wavelength λ. The fluctuation range is maximum at
The fluctuation width becomes minimum at wavelength λ1, and thereafter this characteristic appears periodically.

Wavelength λ. Regarding odor, the polarization state of the light emitted from the fiber 16 to be measured is shown in FIG. It is linearly polarized light as shown in , and this gives the maximum value of the fluctuation width. As the wavelength is swept, the output of the fiber under test 16!
) The polarization state that appears at the l end passes through elliptically polarized light and becomes circularly polarized light indicated by Pl at wavelength λ1, and further passes through elliptically polarized light and becomes linearly polarized light P2 that gives the maximum fluctuation width at wavelength λ2, and this is repeated thereafter. To calculate the normalized birefringence, Ic requires a wave spacing such that the phase difference is 2π, so this wavelength spacing is λ and 2.
A combination of 4 or a combination of λ1 and 25 can be used. At this time, the wavelength at which the fluctuation width of the received light level is minimum, that is, when the output light from the fiber 16 to be measured becomes circularly polarized light, can be measured with extremely high accuracy, so the normalized birefringence can be measured with high precision. is possible.

FIG. 10 is an overall configuration diagram of a measuring device showing a second embodiment of the present invention, and the same reference numerals as in FIG. 2 refer to the same members. In this embodiment, an analyzer with a rotating mechanism is used in place of the 1/2 wavelength plate on the side of the rotating mechanism and the fixed detector = 20-photons, which constitute the analyzing means.

Reference numeral 31 denotes an analyzer which is supported so that its transmission polarization plane rotates around the propagation direction of the light emitted from the fiber to be measured, and is composed of, for example, a Glan-Thompson prism. This analyzer 31 is rotationally driven by an analyzer rotation mechanism 32, and a signal synchronized with the rotational movement at this time is used as a reference signal for the lock-in amplifier 20. According to this example, the wavelength interval at which the fluctuation width of the received light level is the maximum or minimum can be determined in the same way as the previous example except that the period of fluctuation of the received light level is twice that of the previous example. be.

Effects of the Invention As described in detail above, according to the present invention, it is not necessary to set the principal axis direction of the fiber to be measured during measurement, so that the measurement work is facilitated and the measurement accuracy is improved.

[Brief explanation of the drawing]

Fig. 1 is a diagram of the principle of the present invention, Fig. 2 is an overall configuration diagram of a normalized birefringence measuring device for a single polarization fiber showing a first embodiment of the present invention, and Fig. 3 is a diagram of the same embodiment. Figure 4 is a diagram for explaining the effect of rotation of the 1/2 wavelength plate, and Figure 5 is a diagram for explaining the effect of rotation of the 1/2 wavelength plate. In the example, the output light of the fiber to be measured is linearly polarized light, and Figure 6 is a diagram for explaining the fluctuation in the amplitude of the transmitted light of the fixed analyzer. Figure 7 is a diagram for explaining the variation in the amplitude of the transmitted light of the fixed analyzer at a certain time. In the same example, Fig. 7 shows the amplitude of the transmitted light of the fixed analyzer when the output light of the fiber under test is circularly polarized light. Figure 8 is a diagram to explain that there is no fluctuation. Figure 8 is a diagram showing an example of recording by the recorder in the same example. Figure 9 is a diagram showing how the polarization state at the output end of the fiber to be measured changes according to the sweep wavelength in the same example. Figure 10 is an overall configuration diagram of a birefringence measuring device for a single polarized fiber showing the second embodiment of the present invention, and Figure 11 is a diagram for explaining the birefringence of a general single polarized fiber. A cross-sectional view of a polarized fiber, Fig. 12 is an overall configuration diagram of a conventional normalized birefringence measurement device for a single polarized fiber, and Fig. 13 is a diagram showing an example of recording by a recorder in a conventional device. 2... wavelength tunable light source, 3.15... 1/4 wavelength plate, 4.16... fiber to be measured, 5... analysis means, 6, 19... light receiver, 7.
...Received light level fluctuation detection means, 11...Light source, 12...Spectrometer, 14...
・Polarizer, 1A...1/2 wavelength plate, 18...
Fixed analyzer 20...Lock-in amplifier, 22...
- 1/2 wavelength plate rotation mechanism, 31... Analyzer, 32... Analyzer rotation mechanism. Kai Gin◇)

Claims (4)

[Claims] (1) The light from the wavelength tunable light source (2) is made circularly polarized and input into the fiber to be measured (4), and the light is polarized relative to the emitted light from the fiber to be measured (4) centered on the propagation direction of the emitted light. The emitted light is received by a light receiver (6) through an analysis means (5) having a rotating transmitted polarization plane, and the wavelength of the wavelength variable light source (2) is swept and the wavelength is detected based on the rotation. Normalized birefringence measurement of a single polarization fiber, characterized by detecting the fluctuation range of the received light level and measuring the interval between wavelengths at which this fluctuation range is periodically maximum or minimum on the wavelength axis. Method. (2) A wavelength tunable light source (2) that can change the wavelength of emitted light, and a quarter-wave plate that converts the light from the wavelength tunable light source (2) into circularly polarized light and enters the fiber under test (4). (3); and an analyzer (5) having a transmission polarization plane that rotates relative to the output light around the propagation direction of the output light of the fiber to be measured (4); and the analyzer (5). a light receiver (6) that receives transmitted light; and a received light level fluctuation range detection means (7) that detects a fluctuation range of the received light level based on the rotation when the wavelength of the variable wavelength light source (2) is swept. A normalized birefringence measurement device for a single polarization fiber, characterized by comprising: (3) The analysis means (5) includes the fiber to be measured (4).
) and a fixed analyzer.Single polarization according to claim 2, characterized in that it is comprised of a 1/2 wavelength plate whose optical axis is rotated around the propagation direction of the emitted light of ) and a fixed analyzer. Fiber normalized birefringence measuring device. (4) The analysis means (5) includes the fiber to be measured (4).
3. The normalized birefringence measuring device for a single polarization fiber according to claim 2, wherein the device is a rotating analyzer whose transmitted polarization plane is rotated around the propagation direction of the emitted light.