JP5191784B2 - Mode coupling evaluation apparatus and mode coupling evaluation method - Google Patents

Description

  The present invention relates to a mode coupling evaluation apparatus and a mode coupling evaluation method that use a mode coupling coefficient of an optical cable including an optical fiber therein, and evaluates the manufacturing process of the optical cable and evaluates the manufactured optical cable based on the mode coupling coefficient. The present invention relates to a mode coupling evaluation apparatus and a mode coupling evaluation method.

  At present, optical cables equipped with optical fibers are indispensable in various scenes of our lives without being conscious. Not only a communication network represented by the Internet but also various types of sensors are applied. In such a communication network or sensor, it is necessary to appropriately grasp the characteristics and quality of the optical cable.

  A method for evaluating the optical loss of an optical fiber in an optical cable or its change is used for evaluating the manufacturing process of an optical cable and evaluating the characteristics and quality of a product. However, the change in the optical loss can be detected when the optical cable is subjected to a large distortion or a change in temperature or the like, but there is a problem that the distortion cannot be detected when a slight distortion is applied during manufacturing. Further, there is a problem that a slight change in characteristics in the longitudinal direction of the optical cable is further difficult to detect. For this reason, a technique capable of evaluating the cable characteristics when a slight distortion is applied at the time of manufacture is required. Conventionally, the following technique has been used.

  For example, there is a technique for evaluating the strain distribution of an optical cable by evaluating the polarization state of an optical fiber (see, for example, Patent Document 1). FIG. 9 shows a schematic device configuration in the technique disclosed in Patent Document 1. In this apparatus, first, an optical pulse repeatedly sent from the light source 811 is incident on an optical fiber 802 in the optical cable 801 to be evaluated after the polarization state is adjusted. Then, one polarization component of the back scattered light from the optical fiber 802 is received, and the received light intensity is averaged as a function of the reception time and displayed. When a side pressure or strain is applied to the optical fiber 802 in the optical cable 801, the polarized light propagating through that portion is modulated. The apparatus detects at least one linearly polarized light out of the backscattered light from the optical fiber 802. Therefore, the intensity of the backscattered light in the portion to which the side pressure or distortion is applied varies in level due to the influence of polarization modulation. By displaying the fluctuating light intensity as a function of the reception time, in other words, the distance in the longitudinal direction of the optical fiber 802, the position and degree of the applied distortion is estimated.

In addition, there is a technique for evaluating the strength of mode coupling, which is closely related to distortion of an optical fiber, as a distribution in the longitudinal direction of the cable (optical fiber) (for example, see Patent Document 2). FIG. 10 shows a schematic device configuration in the technique disclosed in Patent Document 2. In this apparatus, light emitted from the light source 901 is excited as one propagation mode in the measured optical fiber 906 having two propagation modes. Part of the excited propagation mode undergoes mode conversion in the optical fiber 906 to be measured, and a light wave having two propagation modes is emitted. The two propagation modes are separated into separate optical paths by the prism 907, and the optical path length difference between the separated lights is adjusted by the optical path length variable device 903. Next, the light in the propagation mode is combined by the semi-transparent mirror 910, and the amplitude of the interference signal of both propagation lights is detected by the analyzer 911 and the photodetector 904. Further, the amplitude of the interference light is displayed as a relationship of the optical path length differences by the waveform storage device 914 or the like. Since the amplitude of the interference light represents the intensity of mode coupling and the optical path length difference is related to the length of the measured optical fiber 906, the above apparatus can evaluate the longitudinal distribution of the intensity of mode coupling.
Japanese Patent Laid-Open No. 04-259813 JP 63-234130 A

  By the way, with the technique disclosed in Patent Document 1, the strain position and degree of the optical fiber can be estimated, but there is a problem that quantitative evaluation cannot be performed. Furthermore, the spatial resolution of the strain position of the optical fiber depends on the pulse width of the light source of the optical pulse. For example, when the pulse width is 10 ns, the spatial resolution is about 1 m, and a higher resolution is required.

  In the technique disclosed in Patent Document 2, the strength of mode coupling closely related to the distortion of the optical fiber is evaluated as a distribution in the longitudinal direction of the optical fiber, but the mode coupling coefficient indicating the size of the mode coupling is evaluated. There is a problem that the spatial frequency dependence of the system cannot be evaluated.

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a technique for solving the above-described problems and improving the accuracy of evaluation of the manufacturing process and characteristics of an optical cable.

One embodiment of the present invention relates to a mode evaluation apparatus. This apparatus is a mode coupling evaluation apparatus that evaluates mode coupling of an optical cable including an optical fiber having two propagation modes whose polarization directions are orthogonal to each other. A first light wave is excited in the optical fiber from the light source. Linearly polarized light and second linearly polarized light generated by mode coupling in a propagation mode different from the first linearly polarized light in the optical fiber are obtained from the optical cable, and the first linearly polarized light and the second linearly polarized light are obtained. Interfering means that generates an optical path length difference and interfering with linearly polarized light to generate interference light, a light receiving means that receives the interference light, and a path from the light source to the light receiving means, the light receiving means receiving light A power spectrum of a longitudinal intensity distribution of the optical fiber is obtained with respect to the interference light received by the light receiving means and the modulation means for causing the interference light to be modulated. And calculating means for calculating a spatial frequency dependence of a mode coupling coefficient for the mode coupling, wherein the interference means calculates the optical path length of either the first linearly polarized light or the second linearly polarized light. Optical path length control means for variably controlling is provided.
The modulating means may phase-modulate the first and / or second linearly polarized light in the interference means.
The modulation means may amplitude-modulate the light wave before incidence on the interference means.
The modulating means modulates at a predetermined frequency, and the calculating means refers to the signal of the predetermined frequency when obtaining the intensity distribution in the longitudinal direction of the optical fiber with respect to the signal of the predetermined frequency. It may be used as a signal.
The computing means may obtain the length of the optical fiber and calculate the extinction ratio of the optical fiber based on the length and the mode coupling coefficient.
Further, the calculation means acquires the polarization dispersion of the optical fiber, and based on the optical path length difference between the first linear polarization and the second linear polarization and the polarization dispersion, the optical fiber. The position where the mode coupling occurs may be calculated.
Further, the computing means excites either the slow axis or the fast axis of the optical fiber based on the control amount of the optical path length control means when the interference occurs. You may specify whether or not it was done.
Another embodiment of the present invention relates to a mode coupling evaluation method. This method is a mode coupling evaluation method for evaluating an optical cable including an optical fiber having two propagation modes whose polarization directions are orthogonal to each other. The optical wave excitation excites the first linearly polarized light in the optical cable to be evaluated. And when the second linearly polarized light is generated in a propagation mode different from the first linearly polarized light by mode coupling in the optical cable, the two polarized lights of the first and second linearly polarized light are acquired from the optical cable. A two-polarization acquisition step, a separation step of separating the two polarizations of the first linear polarization and the second linear polarization into different optical paths, and the optical paths of the first linear polarization and the second linear polarization An optical path length difference control step for changing the length, a modulation step for causing the first linearly polarized light to be modulated at a predetermined frequency with respect to the light wave of the light source, and the first linearly polarized light separated in the separation step And before A combining step of combining the second linearly polarized light, an interference step of generating interference light by causing the first linearly polarized light combined in the combining step to interfere with the two polarized light of the second linearly polarized light, and A signal processing step for processing the interference light signal obtained by acquiring the interference light based on the predetermined frequency in the modulation step, and the optical cable of the interference light signal processed in the signal processing step. And a mode coupling coefficient calculating step of calculating a power spectrum based on the intensity distribution in the longitudinal direction and calculating a mode coupling coefficient.
The method may further include an extinction ratio calculating step of acquiring a length of the optical fiber and calculating an extinction ratio of the optical fiber based on the length and the mode coupling coefficient.
In addition, the polarization dispersion of the optical fiber is acquired, and the mode coupling in the optical fiber is determined based on the optical path length difference between the first linear polarization and the second linear polarization and the polarization dispersion. A mode coupling position specifying step of calculating the generated position may be included.

  ADVANTAGE OF THE INVENTION According to this invention, the technique which improves the precision of evaluation, such as a manufacturing process and a characteristic of an optical cable, can be provided.

  The best mode for carrying out the invention (hereinafter referred to as “embodiment”) will be described below with reference to the drawings. First, the basic technique of the present embodiment will be described, and then more specific application examples (specific examples 1 to 3) will be described.

<Basic technology>
FIG. 1 is a functional block diagram showing a schematic configuration of a mode coupling evaluation apparatus 10 according to the basic technique of the present embodiment. The object of evaluation of the mode coupling evaluation apparatus 10 is an optical fiber (Birefringent Fiber; hereinafter referred to as “BF”) 13 having birefringence, and two orthogonal polarization modes can propagate. Then, the light source 12 of the light wave incident on the BF 13 emits a linearly polarized light parallel to the birefringence axis of the BF 13 as a light wave having a short coherence distance that excites the BF 13.

  The mode coupling evaluation device 10 includes a light source 12, an interferometer 14, an optical path length control device 15, a light receiver 16, a light modulation device 17, a signal processing / recording device 18, and a mode coupling coefficient calculation device 19. I have. The light source 12 may be provided separately.

  The interferometer 14 is of an amplitude division type, and causes the linearly polarized light excited by the BF 13 and the mode-coupled linearly polarized light to interfere with each other. Specific configuration examples of the interferometer 14 will be described later in specific examples 1 to 3.

  The optical path length control device 15 adjusts the optical path length of at least one of excited linearly polarized light or mode-coupled linearly polarized light in the interferometer 14. In general, only one optical path is adjusted.

  The light receiver 16 detects light (interference light) in which interference has occurred in the interferometer 14 and performs O / E (photoelectric) conversion.

  The light modulation device 17 acts on the interference signal measurement system 11 to modulate the interference light. Specific modulation will be described later in a specific example. In addition, the signal processing / recording device 18 processes and records the detected output of the interference light in the light receiver 16 including the cable longitudinal direction information, and outputs it to the mode coupling coefficient computing device 19.

  Based on the signal output from the signal processing / recording device 18, the mode coupling coefficient computing unit 19 BF longitudinal distribution (hereinafter simply referred to as “longitudinal”) of the interference light signal between the excited linearly polarized light and the mode-coupled linearly polarized light. The autocorrelation function for the direction) is obtained, and Fourier transform is further performed to calculate the spatial frequency dependence of the mode coupling coefficient of BF13. The interferometer 14 has an optical system configured such that only the linearly polarized light excited by the BF 13 and the linearly polarized light that is mode-coupled interfere with each other.

  Next, a method for calculating the mode coupling coefficient from the longitudinal distribution of the interference light signal, which is characteristic of the present embodiment, will be described.

  The BF 13 has birefringence as described above, and can propagate two orthogonal polarization modes. In the present embodiment, for convenience, these propagation modes (polarization modes) are referred to as a first polarization mode Px and a second polarization mode Py. In particular, linearly polarized light excited by the BF 13 is referred to as the first polarization mode. The linearly polarized light that is P1 and mode-coupled is defined as a second polarization mode P2. The orthogonal first and second polarization modes P1 and P2 have different propagation speeds. In general, a fast propagation axis is a fast axis, and a slow propagation axis is a slow axis (Slow). Axis). Hereinafter, the fast axis is the x axis, the slow axis is the y axis, and the longitudinal direction is the z axis. When the propagation axis of the first polarization mode P1 is the fast axis (x axis), “first polarization mode P1x”, and when the propagation axis is the slow axis (y axis), the “first polarization mode” P1y ". Similarly, when the propagation axis of the second polarization mode Py is the fast axis (x axis), “second polarization mode P2x” and when the propagation axis is the slow axis (y axis), “second polarization mode”. Mode P2y ”.

Here, a case where linearly polarized light is excited parallel to the x-axis will be described. If the optical power of the first polarization mode P1x at the z-axis (longitudinal) position z = z ₁ is I ₀ (z ₁ ), and the mode coupling coefficient from the x-axis to the y-axis is h _xy (z ₁ ). , Z = z ₁ , the optical power I ₁ (z ₁ ) of the second polarization mode P2y that is mode-coupled is expressed by the following equation (1).

ここで、Ｊ_ｍ：ｍ次のベッセル函数、λ_０：測定波長、Ω：光変調装置１７における変調周波数、ａ：光変調装置１７における周波数変調の振幅である。
参考文献（１）"Interference Between Two Orthogonally Polarized Modes Traversing a Highly Birefringent Air-Silica Microstructure Fiber", N.Shibata, A.Nakazono, and Y.Inoue, Journal of Lightwave Technology, vol. 23, pp.1244-1252,2005 As described above, the interferometer 14 causes the first polarization mode P1x and the second polarization mode P2y to interfere with each other. The interference signal intensity I (z ₁ ) output from the light receiver 16 is as follows. When derived with reference to the reference (1), it is expressed by the following equation (2).

Here, J _m : m-order Bessel function, λ ₀ : measurement wavelength, Ω: modulation frequency in the light modulation device 17, a: amplitude of frequency modulation in the light modulation device 17.
Reference (1) "Interference Between Two Orthogonally Polarized Modes Traversing a Highly Birefringent Air-Silica Microstructure Fiber", N. Shibata, A. Nakazono, and Y. Inoue, Journal of Lightwave Technology, vol. 23, pp.1244-1252 , 2005

Next, the signal processing / recording device 18 detects the output of only the last term of the equation (2). In general, the mode coupling coefficient h _xy (z ₁ ) is a small value, and when the signal of the equation (2) is detected as it is, the mode coupling coefficient h _xy (z ₁ ) is buried in noise. Therefore, information including the mode coupling coefficient h _xy (z ₁ ) can be obtained by detecting only the frequency component indicated by cos (2Ωt) of the final term by the lock-in amplifier 162. When this output is P (z ₁ ) and the square process is performed, a signal output expressed by the following equation (3) and proportional to the mode coupling coefficient h _xy (z ₁ ) is obtained.

ここで、ｋは波数、ｉは虚数（−１）^１／２である。
参考文献（２）Ivan. P.Kaminow,"Polarization in Optical Fibers", IEEE Journal of Quantum Electronics, Vol.QE-17, No.1, pp.15-22, Jan.1981 Assuming that the longitudinal fluctuation of the mode coupling coefficient is γ _xy (z), from the following reference (2), the z-axis direction distribution h _xy (z) of the mode coupling coefficient is expressed by the following equation (4). It is done.

Here, k is the wave number, and i is the imaginary number (−1) ^1/2 .
Reference (2) Ivan. P. Kaminow, "Polarization in Optical Fibers", IEEE Journal of Quantum Electronics, Vol.QE-17, No.1, pp.15-22, Jan.1981

さらに、その自己相関関数のパワースペクトラムは、次の（６）式で表される。

すなわち、モード結合係数のパワースペクトラムｈ（Ｋ）は、（４）式〜（６）式により、次の（７）式で表される。

ここで、Ｋは空間周波数であり、波数ｋ＝２π／λとモード複屈折Ｂとから、Ｋ＝Ｂｋで表される。以下、モード結合係数のパワースペクトラムｈ（Ｋ）を、「モード結合係数ｈ（Ｋ）」と称する。 The autocorrelation function of γ _xy (z) is expressed by the following equation (5).

Further, the power spectrum of the autocorrelation function is expressed by the following equation (6).

That is, the power spectrum h (K) of the mode coupling coefficient is expressed by the following equation (7) by equations (4) to (6).

Here, K is a spatial frequency, and is represented by K = Bk from the wave number k = 2π / λ and the mode birefringence B. Hereinafter, the power spectrum h (K) of the mode coupling coefficient is referred to as “mode coupling coefficient h (K)”.

  That is, the mode coupling coefficient h (K) represents the spatial frequency dependence, and the z-axis direction (longitudinal direction) of the interference signal in the two orthogonal polarization modes (first and second polarization modes P1x and P2y). The distribution is signal-processed, the fluctuation of the mode coupling coefficient is calculated, the autocorrelation coefficient is calculated, and the power spectrum is calculated.

Also, if the eta extinction ratio BF13 much smaller than 1, when the length of the BF13 and _{L f,} is represented by _{η = h (K) × L} f. Therefore, the extinction ratio of BF13 can be calculated from the mode coupling coefficient h (K).

Hereinafter, a specific configuration will be described.
<Specific example 1>
FIG. 2 is a functional block diagram illustrating a schematic configuration of the mode coupling evaluation apparatus 110 according to the first specific example of the present embodiment. The mode coupling evaluation apparatus 110 includes a light source 112, an interference signal measurement unit 120, a recording processing control unit 160, and a main control unit 111 that comprehensively controls these components. Then, the BF to be measured is installed on the path between the light source 112 and the interference signal measurement unit 120. The light source 112 outputs a light wave with a short coherence distance that excites linearly polarized light (first polarization mode P1x) parallel to the birefringence axis of the PMF 113, similarly to the light source 12 of the basic technology of FIG. .

  Here, as an object to be measured, a polarization maintaining fiber (hereinafter referred to as “PMF”) 113 which is a kind of BF is illustrated.

  The interference signal measurement unit 120 includes an interferometer 140, a light receiver 156, and a movable mirror position control unit 158. The interferometer 140 causes the linearly polarized light (first polarization mode P1x) excited by the PMF 113 to interfere with the mode-coupled linearly polarized light (second polarization mode P2y). The light receiver 156 detects the interference signal and performs O / E conversion. The movable mirror position controller 158 moves the position of the movable mirror 146 of the interferometer 140 to perform optical path length control.

  The interferometer 140 includes a half-wave plate (hereinafter referred to as “WP”) 142, a polarizing beam splitter (hereinafter referred to as “PBS”) 144, a movable mirror 146, a fixed mirror 148, and a beam splitter ( (Hereinafter referred to as “BS”) 150 and an analyzer 152. The WP 142 is provided on the upstream side (the light source 112 side) of the interferometer 140 and adjusts the linear polarization of the light wave. More specifically, the WP 142 causes the polarization directions of the first polarization mode P1x and the second polarization mode P2y to coincide with the main axis of the PBS 144. The PBS 144 demultiplexes the light wave emitted from the PMF 113 into two optical paths (optical path 1 and optical path 2). The movable mirror 146 functions as an optical path length adjuster that is provided in the optical path 1 and reflects a light wave that travels straight through the BS 150 to the BS 150 and gives an optical path length difference between the optical path 1 and the optical path 2. The fixed mirror 148 is provided in the optical path 2 and reflects the light wave from the PBS 144 toward the BS 150. The BS 150 is provided in a path between the PBS 144 and the movable mirror 146, and multiplexes two light waves that have propagated through two different optical paths (optical path 1 and optical path 2). The analyzer 152 causes two orthogonally polarized light beams (first and second polarization modes P1x and P2y) to interfere with each other. In the movable mirror 146, the optical path length difference between the two optical paths is controlled by the movable mirror position control unit 158 as described above. Further, a piezo element 154 is attached to the movable mirror 146, and modulation is performed by a drive signal given by an arbitrary waveform generator 170 described later.

  Here, the optical path 1 is a path that goes straight through the PBS 144, passes through the BS 150, is reflected by the movable mirror 146, and returns to the BS 150. The optical path 2 is a path from the PBS 144 toward the fixed mirror 148 and reflected by the fixed mirror 148 toward the BS 150. Then, the two light waves traveling along the optical path 1 and the optical path 2 are combined by the BS 150 and proceed to the analyzer 152. The optical path 1 will be described later as an optical path of the first polarization mode P1x, and the optical path 2 will be described later as an optical path of the second polarization mode P2y.

  The recording processing control unit 160 includes a lock-in amplifier 162, a signal processing unit 164, a recording unit 166, a mode coupling coefficient calculation unit 168, an arbitrary waveform generator 170, and a frequency changing unit 172.

  The arbitrary waveform generator 170 drives the piezo element 154 attached to the movable mirror 146 with an arbitrary waveform and frequency. As a result, the light wave incident on the movable mirror 146 is modulated. Therefore, the arbitrary waveform generator 170 functions as a light modulation device together with the piezo element 154. The arbitrary waveform generator 170 outputs the same signal as the signal that drives the piezo element 154 to the lock-in amplifier 162 as a reference signal. The reference signal output from the arbitrary waveform generator 170 is doubled in frequency by the frequency changing unit 172 and input to the lock-in amplifier 162.

  The lock-in amplifier 162 receives the detection signal from the light receiver 156 and receives the reference signal of the arbitrary waveform generator 170 that has been doubled by the frequency changing unit 172. The signal processing unit 164 acquires the output of the lock-in amplifier 162, performs predetermined signal processing described later, and outputs the processed signal to the recording unit 166. In addition, the recording unit 166 acquires the position of the movable mirror 146 (movement distance d from the initial position) from the movable mirror position control unit 158.

  The mode coupling coefficient calculation unit 168 calculates the mode coupling coefficient based on the data acquired by the recording unit 166 (output from the signal processing unit 164 and the movable mirror position control unit 158).

The operation of the mode coupling evaluation apparatus 110 having the above configuration will be described.
As shown in the figure, first, the PMF 113 that is the object to be measured is installed between the light source 112 and the interferometer 140.

  Next, the polarization direction of the light wave emitted from the light source 112 is adjusted, and linearly polarized light parallel to the birefringence axis of the PMF 113 is excited in the PMF 113. FIG. 2 illustrates a state in which the first polarization mode P1x, which is linearly polarized light, is excited by the PMF 113 with the x-axis (indicated by an intermediate double circle in the figure) as a fast axis. Further, the y axis (indicated by a bidirectional arrow in the figure) is the slow axis. When an external force such as strain or lateral pressure is applied to the PMF 113, mode coupling occurs, and a second polarization mode P2y orthogonal to the excited first polarization mode P1x is induced. While the first polarization mode P1x and the second polarization mode P2y propagate to the output end of the PMF 113, the propagation between the first polarization mode P1x and the second polarization mode P2y due to the birefringence of the PMF 113. A group delay time difference depending on the distance is generated, and the first polarization mode P1x propagating through the fast axis is emitted first.

  Subsequently, the first and second polarization modes P 1 x and P 2 y that have propagated to the output end of the PMF 113 are incident on the interferometer 140. At this time, the WP 142 is adjusted so that the polarization directions of the first and second polarization modes P1x and P2y coincide with the main axis of the PBS 144. By this adjustment, the first and second polarization modes P1x and P2y are incident on the two optical paths of the interferometer 140 separately from each other.

  The first polarization mode P1x is incident on the optical path 1. Here, the first polarization mode P1x propagating in the optical path 1 is referred to as “optical path 1 polarization mode P1x1”. The second polarization mode P2y is incident on the optical path 2. Similarly, the second polarization mode P2y propagating through the optical path 2 is referred to as “optical path 2 polarization mode P2y2”. The optical path length of the optical path 1 polarization mode P1x1 is adjusted by the movable mirror 146, and is combined with the optical path 2 polarization mode P2y2 by the BS 150. Further, when the piezo element 154 attached to the movable mirror 146 is vibrated at the frequency Ω under the control of the arbitrary waveform generator 170, the optical path 1 polarization mode P1x1 undergoes phase modulation. Therefore, this mode coupling evaluation apparatus 110 is a phase modulation method. The light wave to be modulated is not limited to the first polarization mode P1x, but may be the second polarization mode P2y, or both polarization modes (light waves) may be modulated.

  The combined light wave interferes with the analyzer 152 installed at an inclination of 45 degrees with respect to the main axis of the PBS 144, and the light receiver 156 detects the interference light. According to the above-mentioned reference (1), the frequency component of the interference signal O / E converted by the light receiver 156 becomes maximum at a frequency 2Ω which is twice the modulation frequency. Therefore, the interference signal is amplified by the lock-in amplifier 162 based on the reference signal sin2Ωt, and is output as an output P to the signal processing unit 164.

In the signal processing unit 164, the output P of the lock-in amplifier 162 is squared as described in the above equation (3). Thereby, an interference signal proportional to the mode coupling coefficient h _xy can be obtained. Then, the recording unit 166 records the movement distance d of the movable mirror 146 and the interference signal output (P ² ) proportional to the mode coupling coefficient h _xy in association with each other.

ここで、ｃは光速、τ_ｐはＰＭＦ１１３の偏波分散である。 Here, the relationship between the propagation distance (mode coupling occurrence position) L from the emission end of the PMF 113 to the position where mode coupling occurs and the moving distance d of the movable mirror 146 is expressed by the following equation (8).

Here, c is the speed of light, and τ _p is the polarization dispersion of the PMF 113.

That is, if the polarization dispersion τ _p of the PMF 113 is obtained from this equation (8) by another method, the mode coupling occurrence position L in the PMF 113 is determined from the moving distance d of the movable mirror 146 where mode coupling is observed. Can be sought. In this way, the optical interference signal distributed in the longitudinal direction is measured.

  Next, the mode coupling coefficient calculation unit 168 calculates a mode coupling coefficient h (K) from the longitudinal distribution of the interference signal according to a method based on the above-described equations (4) to (7). This mode coupling coefficient h (K) is spatial frequency dependent.

  FIG. 3 is a graph showing the spatial frequency dependence of the mode coupling coefficient h (K) obtained using the mode coupling evaluation apparatus 110. Here, the object to be measured is a polarization maintaining holey fiber (hereinafter referred to as “PM-HF”), which is a kind of PMF, and has a length of 13 m and is cabled with a plastic resin coating or the like.

In FIG. 3, the vertical axis represents the mode coupling coefficient h (K), and the horizontal axis represents the spatial frequency K. It can be seen that the mode coupling is a large value although there is some variation when it is less than about K = 30 rad / m. Further, it can be confirmed that the high frequency component of K = 30 rad / m or more is small. Now, an extinction ratio η at a wavelength of 972 nm will be obtained. At a wavelength of 972 nm, the mode birefringence B = 1.3 × 10 ⁻⁴ and the spatial frequency K is
K = B × k = B × 2π / λ = 8.46 × 10 ²
It becomes. At this time, the mode coupling coefficient h (K) is 1.5 × 10 ⁻⁴ from the figure. And the extinction ratio η is
η = h (K) × L _f
= 1.5 × 10 ⁻⁴ × 13
= 1.95 × 10 ⁻³
When this value is logarithmically converted, 27.1 dB is obtained. Similarly, 31.1 dB is calculated at a wavelength of 1312 nm.

  And in order to verify the validity of the spatial frequency dependence of the mode coupling coefficient calculated above, it compared with the mode coupling coefficient calculated | required from the extinction ratio which is another calculation method. FIG. 3 also shows the value of the mode coupling coefficient obtained from the extinction ratio. In the figure, circles indicate values for the wavelength 972 nm, and triangles indicate values for the wavelength 1312 nm. As shown in the figure, both agree very well, and it is confirmed that the method for calculating the mode coupling coefficient of this embodiment is appropriate.

  In addition, as a result of obtaining the mode coupling coefficient in a state where the optical fiber strand was stripped by removing the coating of the optical cable, it was confirmed that the spatial frequency dependency was almost the same as the optical cable state. From this, it was confirmed that the measured optical cable was properly manufactured without applying an abnormal external force or the like in the manufacturing process such as resin coating.

  Next, the excitation shaft will be described. Although the movable mirror 146 changes the optical path length of the light wave propagating in the optical path 1 as described above, the interference signal distribution (here, the output of the lock-in amplifier 162) when the movable mirror 146 is moved is illustrated in FIG.

  The object to be measured is the aforementioned PM-HF, and the wavelength of the light source 112 is 972 nm. The horizontal axis is the position of the movable mirror 146, where the position where the lengths of the optical path 1 and the optical path 2 coincide with each other is zero, the direction in which the length of the optical path 1 is shortened is shown on the minus side, and the direction in which the length is increased is shown on the plus side. Yes.

  FIG. 4A shows a case where the first polarization mode P1 is excited on the fast axis (x axis) (in the case of P1x). It can be seen that when the movable mirror 146 is moved in the direction (plus side) in which the optical path 1 becomes longer, an interference signal appears up to a distance of about 1.6 mm. That is, it is confirmed that the optical path length is increased and the propagation time of the optical path 1 polarization mode P1x1 is increased to interfere with the optical path 2 polarization mode P2y2. Since nothing is detected on the minus side, it is also confirmed that no interference other than the optical path 1 polarization mode P1x1 and the optical path 2 polarization mode P2y2 occurs.

  Similarly, FIG. 4B shows a case where the first polarization mode P1 is excited on the slow axis (y axis) (in the case of P1y). In the interferometer 140, the WP 142 is adjusted as described above, and the first polarization mode P1y excited on the slow axis (y axis) is incident on the optical path 1. In the case of this slow axis excitation, the first polarization mode P1y reaches the emission end of the PMF 113 later than the second polarization mode P2x that is mode-coupled to the fast axis (x axis). Is moved in the direction (minus side) in which the optical path of the first polarization mode P1y is shortened, an interference signal is obtained. This result is clearly shown in FIG.

Thus, when the first polarization mode P1y is excited on the slow axis (y-axis), the interference signal distribution and mode coupling with the second polarization mode P2x mode-coupled to the fast axis (x-axis) are obtained. The coefficient is obtained based on the mode coupling coefficient h _yx (z ₁ ) from the y axis to the x axis. On the other hand, in the case of fast axis (x axis) excitation, as described above, the interference signal distribution and the mode coupling coefficient based on the mode coupling coefficient h _xy (z ₁ ) from the x axis to the y axis are obtained. In FIGS. 4A and 4B, since the interference signal distribution of the fast axis excitation and the interference signal distribution of the slow axis excitation are clearly distinguished, in this embodiment, h _xy (z ₁ ) And h _yx (z ₁ ) can be detected.

Comparing the waveforms shown in FIGS. 4A and 4B, the waveform is almost symmetrical about the position 0 of the movable mirror 146. That is, it is expected that h _xy (z ₁ ) = h _yx (z ₁ ). When the interference signal distributions in FIGS. 4A and 4B were processed to calculate the mode coupling coefficient, it was confirmed that the two coincided.

  4A and 4B, in the interference signal measurement system (light source 112, PMF 113, interferometer 140) shown in this embodiment, linearly polarized light from the light source 12 is excited by the fast axis of the PMF 113. In this case, the interference signal distribution appears on the plus side of the position of the movable mirror 146, and when excited on the slow axis, the interference signal distribution appears on the minus side of the position of the movable mirror 146. There is an advantage that the axis can be identified.

  Next, the spatial resolution in the longitudinal distribution measurement of the interference light signal will be described. The mode resolution was deliberately generated by applying pressure from the side of the cable at a position about 1 mm wide at a position 1 m from the output end side of the optical cable containing PM-HF, and the spatial resolution was grasped.

  FIG. 5 shows a standardized waveform of an intensity signal due to interference in two orthogonal polarization modes observed at a position 1 m from the cable output end. If the spatial resolution is defined as the full width at half maximum of the interference light intensity (B in the figure), the spatial resolution is 62 mm from FIG. According to the reference (1), it is shown that the full width at half maximum of the interference light intensity decreases as the spectrum width of the light source 112 used increases. Therefore, the spatial resolution can be increased by using a light source showing a full width at half maximum of a spectrum wider than 17 nm of the full width at half maximum of the spectrum of the light source 112 described above.

<Specific example 2>
FIG. 6 is a functional block diagram showing a schematic configuration of the mode coupling evaluation apparatus 210 according to the specific example 2 of the present embodiment, which is a modification of the mode coupling evaluation apparatus 110 of the specific example 1 described above. In the interferometer 240 of the interference signal measuring unit 220. Hereinafter, mainly different configurations and operations will be described, and components having the same function are partially denoted by the same reference numerals and description thereof will be omitted.

  The interferometer 240 of the mode coupling evaluation apparatus 210 shown in FIG. 6 provides a BS 250 as an optical demultiplexer at a location where the light wave emitted from the PMF 113 is demultiplexed into two optical paths, and multiplexes the light waves propagating through the two optical paths. PBS244 is provided as an optical multiplexer at the location. As in the first specific example, a movable mirror 246 that is an optical path length adjuster that gives an optical path length difference between two optical paths, and a WP (1/2 wavelength plate) that is an optical system that adjusts the direction of linear polarization of light waves. ) 242 and an analyzer 252 which is an optical system for interfering two orthogonal linearly polarized lights. The positions of the fixed mirror 248 and the movable mirror 246 are interchanged with the arrangement of the first specific example. In addition, a piezo element 254 is attached to the movable mirror 246 as in the first specific example.

  In the second specific example, the WP 242 and the PBS 244 are adjusted so that the polarization directions of the first polarization mode P1x and the second polarization mode P2y coincide with the main axis of the PBS 244. As a result, only the optical path 1 polarization mode P1x1 of the optical path 1 and the optical path 2 polarization mode P2y2 of the optical path 2 are combined by the PBS 244. Depending on how the main axis is adjusted, only the second polarization mode P2y (referred to as “P2y1”) passing through the optical path 1 and the first polarization mode P1x (referred to as “P1x2”) passing through the optical path 2 are PBS244. It is also possible to combine them with each other. As a result, the same operations and effects as those of the first specific example can be realized, and the degree of freedom in measurement is improved.

  In specific example 1 and specific example 2, the configurations using PBSs 144 and 244, BSs 150 and 250, and WPs 142 and 242 are illustrated. In addition to this, for example, by combining a polarizer or the like, the light wave incident on the analyzers 152 and 252 can be made only to the linearly polarized light that is mode-coupled with the linearly polarized light incident on the PMF 113.

<Specific example 3>
FIG. 7 is a functional block diagram showing a schematic configuration of the mode coupling evaluation apparatus 310 according to the specific example 3 of the present embodiment, which is another modification of the mode coupling evaluation apparatus 110 of the specific example 1 described above, and has different configurations. Are mainly in the light modulation device of the recording processing control unit 360. Hereinafter, mainly different configurations and operations will be described, and components having the same function are partially denoted by the same reference numerals and description thereof will be omitted.

  In the first specific example, the piezo element 154 and the arbitrary waveform generator 170 are provided as the light modulation device. On the other hand, in the third specific example, an optical chopper 372 and an optical chopper controller 370 are provided as optical modulators, and the optical modulator of this configuration is an amplitude modulation method that changes the amplitude of excited linearly polarized light. Yes. Specifically, an optical chopper 372 is disposed in the path between the light source 112 and the PMF 113. Then, the optical chopper controller 370 of the recording processing controller 360 controls the driving of the optical chopper 372 to a predetermined frequency. The frequency (Ω) is output to the lock-in amplifier 162 as a reference signal. In the case of the mode coupling evaluation apparatus 310 having this configuration, the lock-in amplifier 162 uses the frequency of the reference signal as it is, so that the frequency changing unit 172 is omitted.

  In order to confirm the validity of the mode coupling evaluation device 310 that is the amplitude modulation method, a comparison measurement with the mode coupling evaluation device 110 that is the phase modulation method of the first specific example was performed. FIG. 8 shows the spatial frequency dependence of both mode coupling coefficients at a wavelength of 972 nm. As shown in the figure, in the amplitude modulation system, a lot of noise is seen, but the spatial frequency dependence of the mode coupling coefficients of both is very good, confirming the validity of the mode coupling evaluation device 310 of the amplitude modulation system. It was done. The optical chopper 372 may be disposed in the path between the interferometer 140 and the PMF 113, and the second linearly polarized light P2y generated by mode coupling in the PMF 113 may also be modulated.

The basic effects of this embodiment and the general effects of specific examples 1 to 3 are summarized as follows.
1) According to the present embodiment, the spatial frequency dependence of the mode coupling coefficient can be obtained based on the optical fiber longitudinal direction distribution of the orthogonal two-polarization mode interference signals measured with high spatial resolution. As a result, the spatial frequency dependency of the mode coupling coefficient and the change thereof can be detected, so that the manufacturing process evaluation of the optical fiber and the optical cable and the characteristics and quality of the manufactured optical cable and optical cable can be evaluated.
2) The spatial resolution for detecting the mode coupling position of the optical fiber has been about 1 m in the past, but in this embodiment, a very high performance of 10 cm or less can be realized.
3) Since the extinction ratio at an arbitrary wavelength can be obtained from the spatial frequency characteristics of the mode coupling coefficient, it is not necessary to measure the extinction ratio which has been conventionally performed.
4) The mode coupling coefficient from the fast axis to the slow axis and the mode coupling coefficient from the slow axis to the fast axis can be distinguished and evaluated.
5) Identify whether the linearly polarized light excited in the polarization-maintaining fiber is the slow axis or the fast axis depending on whether the interference signal appears on the minus side or the plus side when the movable mirror of the interferometer is moved it can. This identification is important in a polarization maintaining optical fiber or the like whose optical loss characteristics differ depending on the birefringence axis to be excited, and is a great merit obtained by this embodiment.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and it is understood by those skilled in the art that various modifications can be made to each of those components and combinations thereof, and such modifications are also within the scope of the present invention.

It is a functional block diagram which shows schematic structure of the mode coupling | bonding evaluation apparatus based on the basic technique of embodiment. It is a functional block diagram which shows schematic structure of the mode coupling | bonding evaluation apparatus based on the specific example 1 of embodiment. It is the graph which showed the spatial frequency dependence by the relationship between the mode coupling coefficient h (K) calculated | required using the mode coupling evaluation apparatus based on the specific example 1 of embodiment, and a spatial frequency. It is the graph which showed interference signal distribution when moving a movable mirror in the mode coupling evaluation apparatus based on the specific example 1 of embodiment. In the mode coupling evaluation apparatus according to the specific example 1 of the embodiment, it is a diagram showing a standardized waveform of the intensity signal due to the interference of two orthogonal polarization modes observed at a position of 1 m from the cable output end. . It is a functional block diagram which shows schematic structure of the mode coupling | bonding evaluation apparatus based on the specific example 2 of embodiment. It is a functional block diagram which shows schematic structure of the mode coupling | bonding evaluation apparatus based on the specific example 3 of embodiment. It is the graph which showed the spatial frequency dependence of the mode coupling coefficient of the phase modulation system and wavelength modulation system in wavelength 972nm based on the specific example 1 and the specific example 3 of embodiment. It is the figure which showed an example of the schematic apparatus structure in a prior art. It is the figure which showed an example of the schematic apparatus structure in a prior art.

Explanation of symbols

10, 110, 210, 310 Mode coupling evaluation apparatus 11 Interference signal measurement system 12, 112 Light source 13 BF
14, 140, 240 Interferometer 15 Optical path length control device 16, 156, 256 Light receiver 17 Optical modulation device 18 Signal processing / recording device 19 Mode coupling coefficient calculation device 113 PMF
120, 220 Interference signal measuring unit 142, 242 WP
144, 244 PBS
146, 246 Movable mirror 148, 248 Fixed mirror 150, 250 BS
152, 252 Analyzer 154, 254 Piezo element 158 Movable mirror position control unit 160, 360 Recording processing control unit 162 Lock-in amplifier 164 Signal processing unit 166 Recording unit 168 Mode coupling coefficient calculation unit 170 Arbitrary waveform generator 172 Frequency changing unit 370 Light chopper controller 372 Light chopper

Claims (10)

A mode coupling evaluation apparatus for evaluating mode coupling of an optical cable including an optical fiber having two propagation modes having orthogonal polarization directions,
The optical cable includes a first linearly polarized light in which a light wave from a light source is excited in the optical fiber, and a second linearly polarized light generated by mode coupling in a propagation mode different from the first linearly polarized light in the optical fiber. Interfering means that generates an interference light by causing an optical path length difference between the first linearly polarized light and the second linearly polarized light to interfere with each other, and
A light receiving means for receiving the interference light;
A modulating means that acts in a path from the light source to the light receiving means, and causes the interference light received by the light receiving means to be in a modulated state;
With respect to the interference light received by the light receiving means, comprising calculating means for calculating the spatial frequency dependence of the mode coupling coefficient for the mode coupling by obtaining the power spectrum of the intensity distribution in the longitudinal direction of the optical fiber,
The mode coupling evaluation apparatus, wherein the interference unit includes an optical path length control unit that variably controls an optical path length of either the first linearly polarized light or the second linearly polarized light.   The mode coupling evaluation apparatus according to claim 1, wherein the modulation unit causes the interference unit to phase-modulate the first and / or second linearly polarized light.   The mode coupling evaluation apparatus according to claim 1, wherein the modulation unit amplitude-modulates the light wave before incident on the interference unit. The modulation means performs modulation at a predetermined frequency,
The mode coupling according to any one of claims 1 to 3, wherein the calculation means uses the signal of the predetermined frequency as a reference signal when obtaining the intensity distribution in the longitudinal direction of the optical fiber. Evaluation device.   5. The calculation unit according to claim 1, wherein the calculating unit obtains a length of the optical fiber and calculates an extinction ratio of the optical fiber based on the length and the mode coupling coefficient. The mode coupling evaluation apparatus according to 1.   The arithmetic means obtains the polarization dispersion of the optical fiber, and based on the optical path length difference between the first linear polarization and the second linear polarization and the polarization dispersion, 6. The mode coupling evaluation apparatus according to claim 1, wherein a position where mode coupling occurs is calculated.   The calculating means is based on a control amount of the optical path length control means when the interference occurs, and whether the first linearly polarized light is excited to the slow axis or the fast axis of the optical fiber. The mode coupling evaluation apparatus according to claim 1, wherein the mode coupling evaluation apparatus is specified. A mode coupling evaluation method for evaluating an optical cable provided with an optical fiber having two propagation modes having orthogonal polarization directions,
A light wave excitation step of exciting the first linearly polarized light in the optical cable to be evaluated;
Two-polarization acquisition for acquiring two polarizations of the first and second linear polarizations from the optical cable when a second linear polarization occurs in a propagation mode different from the first linear polarization by mode coupling in the optical cable Process,
A separation step of separating the two polarizations of the first linear polarization and the second linear polarization into different optical paths;
An optical path length difference control step for making variable the respective optical path lengths of the first linearly polarized light and the second linearly polarized light;
A modulation step of causing the first linearly polarized light to be modulated at a predetermined frequency with respect to the light wave of the light source;
A combining step of combining the first linearly polarized light and the second linearly polarized light separated in the separating step;
An interference step of generating interference light by causing interference between the first linearly polarized light and the second linearly polarized light combined in the combining step;
A signal processing step of processing an interference light signal obtained by acquiring the interference light based on the predetermined frequency of the modulation step;
Based on the intensity distribution in the longitudinal direction of the optical cable of the interference light signal processed in the signal processing step, a mode coupling coefficient calculation step for calculating a power spectrum and calculating a mode coupling coefficient;
A mode coupling evaluation method comprising:   The mode coupling according to claim 8, further comprising: an extinction ratio calculating step of obtaining a length of the optical fiber and calculating an extinction ratio of the optical fiber based on the length and the mode coupling coefficient. Evaluation method.   The polarization coupling of the optical fiber is obtained, and the mode coupling occurs in the optical fiber based on the optical path length difference between the first linear polarization and the second linear polarization and the polarization dispersion. The mode coupling evaluation method according to claim 8, further comprising a mode coupling position specifying step of calculating a position.

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