JP5227152B2 - Method for confirming single mode transmission of optical fiber, method and apparatus for measuring cut-off wavelength - Google Patents

Description

  The present invention relates to a technique for determining whether or not light of a certain wavelength or more can be transmitted in a single mode in various optical fibers constituting an optical fiber communication network, or measuring and evaluating a cutoff wavelength.

  In an optical fiber, a wavelength region in which single mode transmission is possible (single mode wavelength region) is generally determined by a core diameter and a relative refractive index difference between a core and a clad. In the single-mode wavelength region, higher-order modes do not propagate through the fiber, so that signal degradation due to multimode dispersion does not occur and signal transmission at high speed is possible.

  Therefore, the cutoff wavelength as a parameter for defining the single mode wavelength region and its measurement evaluation method are important. There are two cutoff wavelengths, a theoretical cutoff wavelength λc and an effective cutoff wavelength λce, and only a fundamental propagation mode LP01 exists at wavelengths longer than the theoretical cutoff wavelength λc. On the other hand, even if the wavelength is longer than the effective cutoff wavelength λce, the first-order higher-order mode LP11 may exist in the optical fiber as long as the wavelength is shorter than the theoretical cutoff wavelength λc. However, since the bending loss of the LP11 is large in the above wavelength range, the optical power of the LP11 mode hardly reaches the end of the optical fiber, so that single mode transmission of only LP01 can be realized effectively. Therefore, as shown in FIG. 1, the effective cutoff wavelength λce is dependent on the optical fiber length, and it is considered that the limit value of λce when the fiber length is extrapolated to 0 corresponds to the theoretical cutoff wavelength λc.

  Conventionally, as a method for measuring the effective cutoff wavelength λce, a bending loss method and a multimode excitation method are known (Non-Patent Document 1).

  As shown in FIG. 2, in the measurement system of the bending loss method, the light from the white light source 20 is dispersed by the spectroscope 21, the dispersed light is incident on the measured fiber 23, and the transmitted light (wavelength characteristic of transmitted power). ) Is measured by the light receiver 24. In this method, the wavelength dependence of the transmitted power is measured with and without the small bend 23a applied to the single-mode measured fiber 23, and the change is analyzed by the computer 25. The cutoff wavelength is obtained. As described above, the bending loss method uses the difference in bending loss between the fundamental mode LP01 and the first-order higher-order mode LP11 of the single mode fiber.

  As shown in FIG. 3, in the multi-mode excitation method, the light from the white light source 20 is dispersed by the spectroscope 21, the dispersed light is incident on the multi-mode optical fiber 30, and the transmitted light (wavelength characteristics of the transmitted power). ) Is measured by the light receiver 24. Next, the measured fiber 23 is connected to the multimode optical fiber 30, and the transmitted light (wavelength characteristic of the transmitted power) is measured by the light receiver 24. Then, the calculator 25 obtains the wavelength characteristic of the ratio of the transmission power in the two acquired measurements, and obtains the wavelength at which the wavelength characteristic has changed greatly as the cutoff wavelength of the measured fiber 23. As described above, the multimode excitation method utilizes the fact that the propagation light intensity of the measured fiber changes greatly in the wavelength region where the multimode changes to the single mode.

Nao Uesugi, Masaharu Ohashi, “Measurement of Optical Fiber Parameters”, Guide to Optical Measuring Instruments: Full Revised Edition, Optronics, 2004, p. 176-177

  In recent years, research and development of optical fibers with extremely low bending loss have been made by devising dopant distribution and adding holes. In such an optical fiber, since the bending loss of the first-order higher-order mode LP11 is small, it is very difficult to measure the cutoff wavelength by the bending loss method. Further, the multimode excitation method also has a problem that errors are likely to occur depending on the mode excitation state of the multimode fiber and the connection state of the multimode fiber and the measured fiber.

  Further, as shown in FIGS. 2 and 3, since it is desirable to bend the measured fiber with a constant diameter (diameter 280 mm) in the conventional method, a very short optical cord fiber used as a connection part or the like It is not suitable as a method of directly measuring

  On the other hand, as a practical countermeasure, even if the specific value of the cutoff wavelength is not determined by various methods, it is confirmed that the target optical fiber is operating in a single mode at a certain wavelength λa. If possible, it can be considered that single-mode transmission is possible in the optical fiber at a wavelength of λa or more, and the problem can be avoided by setting the shortest wavelength of the transmission signal wavelength to λa or more. However, there has been no suitable method for confirming that single mode transmission is performed at a certain wavelength λa using a simple measurement system.

  Therefore, the present invention has been proposed in view of the above-described problems, and can be applied to an optical fiber having extremely small (or large) bending loss, and can be shortened without adjusting the optical fiber length. It is an object of the present invention to provide a method for accurately evaluating a cutoff wavelength in a simple state or a method for simply confirming that single mode transmission is performed at a specific wavelength.

In order to achieve the above object, the present invention provides a method for confirming single mode transmission, wherein a gap or an axis deviation amount is connected to a lower optical fiber to be confirmed. connect the optical fiber of the upper side through the section, by the incidence of light on the upper side optical fiber, a single mode transmission confirmation method for the optical fiber to verify single mode transmission of the lower-side optical fiber, the The light P1 of light leaking in the vicinity of the connection portion of the lower optical fiber is measured while the light of the optical power P0 is incident on the lower optical fiber and the gap amount or the axial deviation amount of the connection portion is changed. it and, measuring the optical power P3 of the transmitted light at the end of the lower-side optical fiber and, of transmitted light optical power P0 and the measurement of the light incident on the lower side optical fiber optical power And calculating the coupling loss L on the basis of 3, if they meet the following relationship (Equation 1), the lower-side optical fiber, characterized in that it comprises a can be determined that the single-mode transmission.

  In (Expression 1), k is a constant representing the light receiving efficiency.

According to a second aspect of the invention, in the confirmation process of single-mode transmission, to connect the upper side optical fiber gap amount or axial misalignment in the lower-side optical fiber is a confirmation target via the variable connection portion, the upper side by applying a light to the optical fiber, a single mode transmission confirmation method for the optical fiber to verify single mode transmission of the lower-side optical fiber, the lower side while changing the gap amount or axial misalignment of the connecting section When the optical power P1 of the leaked light leaking in the vicinity of the connection portion of the optical fiber is measured and the relationship between the optical power P1 with respect to the changed gap amount X or the axis deviation amount, an extreme point does not occur. And determining that it is mode transmission.

According to a third aspect of the invention, the verification device of the single-mode transmission, to connect the upper side optical fiber gap amount or axial misalignment in the lower-side optical fiber is a confirmation target via the variable connection portion, the upper side An apparatus for confirming single mode transmission of an optical fiber that makes light incident on the optical fiber and confirms single mode transmission of the lower optical fiber, and means for injecting light of optical power P0 into the lower optical fiber; The means for changing the gap amount or the axial deviation amount of the connecting portion and the macro bend that is an arc-shaped bend or the micro bend that is a slight bend in the vicinity of the connecting portion between the lower optical fiber and the upper optical fiber means for measuring the optical power P1 of the leaked light which has leaked from the lower side optical fiber in the vicinity of the connecting portion by means for applying to the lower-side optical fiber, the lower side It means for measuring the optical power P3 of the transmitted light at the end of the fiber, and means for calculating the coupling loss L based on the optical power P3 of the transmitted light optical power P0 and the measurement of the light incident on the lower side optical fiber When the relationship of the following (formula 1) is satisfy | filled, the means which judges that a lower side optical fiber is single mode transmission is provided, It is characterized by the above-mentioned.

  In (Expression 1), k is a constant representing the light receiving efficiency.

According to a fourth aspect of the invention, the verification device of the single-mode transmission, to connect the upper side optical fiber gap amount or axial misalignment in the lower-side optical fiber is a confirmation target via the variable connection portion, the upper side An optical fiber single-mode transmission confirmation device for making light incident on an optical fiber and confirming the single-mode transmission of the lower-side optical fiber, the means for changing the gap amount or the axial deviation amount of the connection part, In the vicinity of the connecting portion between the lower optical fiber and the upper optical fiber, the lower side near the connecting portion is provided by means for giving the lower optical fiber a macrobend that is an arc-shaped bend or a microbend that is a slight bend. The relationship between the means for measuring the optical power P1 of the leaked light leaked from the optical fiber and the optical power P1 with respect to the changed gap amount X or axial deviation amount. Te, if the extreme point is not generated, characterized in that it comprises a means for determining that the single-mode transmission.

  As described above, according to the first aspect of the present invention, the leakage that leaks in the vicinity of the connection portion of the first optical fiber while changing the gap amount or the axial deviation amount between the first optical fiber and the second optical fiber. When the optical power P1 of light is measured, the connection loss L is measured at the end of the first optical fiber 3, and the relationship of (Expression 1) is satisfied, the first optical fiber 3 is in single mode transmission. Since the determination is made, it can be easily confirmed that single mode transmission is performed even for an optical fiber having extremely small (or large) bending loss.

  As described above, according to the second aspect of the present invention, the leaked light leaks in the vicinity of the connection portion of the first optical fiber while changing the gap amount or the axial deviation amount between the first optical fiber and the second optical fiber. In the relationship of the optical power P1 with respect to the changed gap amount X or the amount of axial deviation, it is determined that the single-mode transmission is performed when an extreme point does not occur. It is possible to more easily confirm that single mode transmission is performed even for an optical fiber having a small (or large) bending loss and without adjusting the length of the optical fiber.

  Thus, according to the third aspect of the invention, the wavelength λ is continuously changed in a state where the first optical fiber and the second optical fiber are set to have a predetermined gap amount or axial deviation amount. Then, the light is incident on the second optical fiber, and the relationship between the optical power P1 of the leaked light and the optical power P1 of the incident light with respect to the wavelength λ is a predetermined threshold value S. Since the cutoff wavelength is determined as the cutoff wavelength, even for optical fibers with extremely small (or large) bending loss, the cutoff wavelength can be adjusted without adjusting the optical fiber length. Can be measured.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Outline of the embodiment)
In the present embodiment, the lower fiber, which is the fiber to be measured, and the upper fiber to which the measuring instrument (power meter 5) is connected are butt-connected while providing an appropriate amount of axial deviation or gap. At this time, at wavelengths longer than the theoretical cutoff wavelength, only light (cladding mode light) propagating through the cladding or coating of the lower fiber is produced, but at wavelengths below the theoretical cutoff wavelength, in addition to the cladding mode light, A higher-order mode (LP11 or the like) that propagates by being coupled to the core of the side fiber is generated. Using this fact, it is confirmed whether the target optical fiber is operating in a single mode at a certain wavelength λa and the cutoff wavelength is measured.

  The optical power P1 of the clad mode light is selectively changed to the side of the optical fiber at a position very close to the butt connection portion provided with an appropriate amount of axial misalignment or gap, typically at a distance of several centimeters from the connection portion. Measure from the side. In the single mode wavelength region, the experimental relationship between the optical power P1 of the clad mode light and the connection loss L (of the basic propagation mode LP01) is very well expressed by a simple theoretical formula. On the other hand, since a higher order mode (LP11 or the like) is generated in the multimode wavelength region, if the higher order mode is removed before the termination and the connection loss L is correctly measured at the termination, a large value from the above theoretical formula is obtained. Deviation occurs. Therefore, it is possible to confirm whether the optical fiber is operating in the single mode by measuring the optical power P1 of the clad mode light under an appropriate condition.

  On the other hand, when light of a plurality of different wavelengths is sequentially input to the connection portion with an appropriate fixed amount of axial deviation or gap, a higher-order LP11 mode is generated at a wavelength shorter than the cutoff wavelength. As a result, the value of the optical power P1 of the clad mode light decreases. Therefore, the wavelength λm in which the amount of change due to the wavelength of P1 exceeds the set threshold value can be determined as the cutoff wavelength.

  Unlike the fundamental mode LP01 and the first-order higher-order mode LP11 propagating through the core, the clad mode light propagates through the clad, and therefore, microbends, which are minute bends caused by a very slight pressing in any fiber. Alternatively, leakage is caused by a macrobend which is an arc-shaped bend when the optical fiber is bent (particularly, sufficient power is often leaked when an intended bending or the like is not applied). Therefore, the clad mode light can be easily separated from the LP01 mode and the LP11 mode and the optical power thereof can be easily observed. Therefore, this method can be applied to a fiber having a small bending loss.

  FIG. 4 is a diagram schematically showing how the clad mode light is generated. FIG. 4A shows a state of the connection portion when incident light having a wavelength longer than the theoretical cutoff wavelength λc is incident, and FIG. 4B shows incidence of a wavelength shorter than the theoretical cutoff wavelength λc. The state of the connecting portion when light is incident is shown. As shown in FIG. 4A, when incident light in a single mode wavelength region having a wavelength longer than the theoretical cutoff wavelength λc is incident, if there is a loss factor such as an axis deviation or a gap at the connection portion, A part of the light of the fundamental mode LP01 propagating through the light becomes clad mode light in the lower optical fiber, and leaks after propagating through the clad or coating for a very short distance.

  On the other hand, in the case where incident light having a wavelength shorter than the theoretical cutoff wavelength λc is incident as shown in FIG. 4B, under the condition that an axial deviation and a gap equivalent to those in FIG. Since not only the cladding mode light but also the first-order higher-order mode LP11 is generated, the power of the cladding-mode light is reduced by the amount of the generated higher-order mode LP11. In addition, since the higher-order mode LP11 has a bending loss smaller than that of the clad mode light, the microbend provided in the vicinity of the connection portion does not leak, and a strong bend or the like is not applied. Most of it propagates. Therefore, the clad mode light can be separated from the fundamental mode LP01 and the higher order mode LP11.

(First embodiment)
5 and 6 are diagrams showing an example of a configuration for carrying out the method for confirming single mode transmission according to the present invention. In FIG. 5, a light source 1 that emits light incident on an optical fiber is connected to an upper optical fiber 2, and a lower optical fiber 3 is butt-connected to the lower side of the upper optical fiber 2. The V-groove connecting portion 4 butt-connects the upper optical fiber 2 and the lower optical fiber 3 while providing an appropriate amount of axial deviation or gap. The power meter 5 is provided in the vicinity of the connecting portion of the lower optical fiber 3, and the power meter 6 is provided at the end of the lower optical fiber 3. Further, the power meter 5 and the power meter 6 are connected to a computer 7. In this configuration, the optical fiber to be measured is the lower optical fiber 3 connected to the rear stage of the V-groove connecting portion 4. The lower optical fiber 3 can be sufficiently shorter than the upper optical fiber 2.

  The light source 1 is connected to the upper optical fiber 2 and makes light of a predetermined wavelength enter the upper optical fiber 2 with a predetermined power. The upper-side optical fiber 2 is butt-connected to the lower-side optical fiber 3 by a V-groove connecting portion 4 at the end portion.

  Here, the V-groove connecting portion 4 will be described with reference to FIG. As shown in FIG. 6, the V-groove connecting portion 4 includes stages 41 and 43 for supporting the fibers, fixing members 42 and 44 for fixing the fibers to the upper surfaces of the stages 41 and 43, and light on the stage. And transparent lid members 45 and 46 for covering the connecting portions of the fibers. On the upper surfaces of the stages 41 and 43, long grooves 41a and 43a notched in a V-shape are provided. An optical fiber is placed on the grooves 41a and 43a along the longitudinal direction to fix the members. By fixing from above with 42 and 44, the optical fiber can be fixed onto the stages 41 and 43. V-shaped grooves 41a and 43a are installed on the stages 41 and 43 so as to face each other. When the optical fibers are fixed on the stages 41 and 43, respectively, the termination surface and the lower side of the upper optical fiber 2 are placed. The optical fiber 3 is butt-connected so as to face the front end surface.

  The stage 43 has an X-axis drive unit 43b and a Y-axis drive unit 43c. By driving the X-axis drive unit 43b and the Y-axis drive unit 43c, the stage 43 is finely moved in the X direction and the Y direction. be able to. The relative position of the optical fiber fixed to the stage 43 with respect to the optical fiber fixed to the stage 42 can be changed by finely moving the stage in the X and Y directions. In the example shown in the figure, the amount of optical fiber spacing can be adjusted by fine movement in the X direction, and the amount of optical fiber misalignment can be adjusted by fine movement in the Y direction.

  Returning to FIG. 5, the lower optical fiber 3 in the vicinity of the V-groove connecting portion 4 is pressed to generate a minute bend, and the power meter 5 measures the clad mode light. The pressing method will be described later with reference to FIG.

  The lower optical fiber 3 is provided with a bend 3a in the middle thereof. The bend 3a is different from the minute bend provided in the vicinity of the connecting portion in order to measure the clad mode light. By providing the bending 3a, when the higher-order mode LP11 is generated in the lower-side optical fiber 3, it can be removed by radiating the generated higher-order mode LP11 to the outside.

  The power meter 5 measures the optical power (power) P <b> 1 of the leaked light that leaks in the vicinity of the connection portion of the lower-side fiber 3. The leaked light measured by the power meter 5 is clad mode light leaked due to a minute bending of the lower-side fiber 3 in the vicinity of the connecting portion.

  The power meter 6 measures the transmitted light power (optical power of transmitted light) P3 at the end of the lower optical fiber 3. The transmitted light power P3 measured by the power meter 6 is used when calculating the connection loss L of the basic propagation mode LP01 as will be described later.

  The computer 7 connected to the power meter 5 and the power meter 6 acquires the optical power P1 of the leaked light measured by the power meter 5 and the transmitted light power P3 measured by the power meter 6. When the optical power P0 of the incident light is separately input to the computer 7 and the optical power P2 and the transmitted light power P3 of the leaked light are acquired, the connection loss L is calculated and the following method is performed to perform single mode transmission. It can be determined whether or not. Here, the connection loss L is obtained by, for example, subtracting the transmitted light power P3 at the end of the lower optical fiber 3 from the transmitted light power P0 at the end of the upper optical fiber 2 when the output Pout of the light source 1 is the same. It can be obtained as a value. At this time, the transmitted light power P0 can be measured in advance by connecting the power meter 6 at the end of the upper optical fiber 2 without connecting the lower optical fiber 3. Since the lower optical fiber 3 is sufficiently shorter than the upper optical fiber 2, the transmission loss in the lower optical fiber 3 can be ignored.

  Next, a method for confirming single mode transmission in the above configuration will be described. The light source 1 is adjusted in advance so that the power P0 of light incident on the lower optical fiber 3 from the terminal end via the upper optical fiber 2 becomes a predetermined value P0. The adjustment of the power P0 of the incident light may be separately measured and adjusted in advance, or the light source 1 may be adjusted in consideration of the transmission loss of the upper optical fiber 2.

  In the V-groove connecting portion 4, the lower portion in the vicinity of the V-groove connecting portion 4 is continuously changed while the distance amount X or the axial deviation amount Y between the end surface of the upper optical fiber 2 and the distal end surface of the lower optical fiber 3 is continuously changed. The power P1 of the clad mode light leaking from the side optical fiber 3 is measured by the power meter 5, and the optical power P3 of the transmitted light at the end of the lower side optical fiber 3 is measured by the power meter 6. L is calculated.

  Next, the computer 7 determines whether or not the measured result satisfies the following expression (1) when the interval amount X or the axis deviation amount Y is continuously changed. If the following expression (1) is satisfied, it is determined that single mode transmission is being performed. Specifically, when the maximum value of the deviation between the function obtained by fitting the measured value group with a polynomial by the least square method and the equation (1) does not exceed a predetermined threshold, the relationship of the equation (1) Judge that it satisfies. This threshold can typically be between 0.5 and 1 dB.

  In the following equation (1), k is a constant representing the light receiving efficiency. The light receiving efficiency k is a value determined by the measurement position of the leaked light P2, the measurement conditions such as the pressing force that generates the leaked light P2, and the state of the core and coating of each fiber. This k is determined by substituting the measured values P0, P1, and L obtained by changing the interval amount X or the axis deviation amount Y a few times every time the determination is performed, into the following equation (1). Can do.

  In addition, it is confirmed by the following experiment that Formula (1) is established when single mode transmission is performed.

  Here, the experiment confirmed about the said Formula (1) is demonstrated. In the experimental system, normal single mode optical fibers (SMF) 2 and 3 were used as the upper optical fiber 2 and the lower optical fiber 3 in the configurations shown in FIGS. The fiber to be evaluated is the SMF 3 connected to the rear stage of the V-groove connecting portion 4. The input optical power P0 to the connection part was measured separately, and the value was about 0 dBm. In this experiment, a light source having a wavelength λa = 1550 nm was used, and the value of the connection loss L was adjusted by changing the gap amount. In order to prevent the generation of reflected light, a refractive index matching material was used for the connection portion. Since the effective cutoff wavelength of the SMF used for the experiment is about 1200 nm (value at 2 m), 1550 nm is considered to be completely a single mode wavelength region (theoretical cutoff wavelength λc or more). In addition, even if the LP11 mode is generated, the lower SMF 3 is bent to have a radius of about 5 cm in order to remove (radiate outside) the LP11 mode.

  On the other hand, in the theoretical calculation, a single mode condition of λc or more is assumed, and it is assumed that the fundamental propagation mode LP01 lost at the connection portion is all converted into clad mode light. Then, from the definition of the connection loss L, the relationship between P1-P0 (dB) and the connection loss L (dB) is given by the equation (1) using the light receiving efficiency k of the clad mode light.

  FIG. 7 shows the measurement result (point) of the connection loss L (dB) with respect to P1-P0 (dB) measured in the experimental system, together with the theoretical calculation value (solid line). As shown in FIG. 7, the experimental value measured on the connection portion and the calculated value obtained by fitting the experimental value with Equation (1) show good agreement. In particular, in a region where the connection loss L is 3 dB or less, the relative error is about 0.2 dB or less. This indicates that the equation (1) is satisfactorily established in the single mode wavelength region, as predicted. That is, using the experimental system of FIGS. 5 and 6, it can be proved that the correspondence between the experimental result and the formula (1) is well established, so that single mode transmission is possible at the used light source wavelength λa. I can confirm.

  On the other hand, at a wavelength shorter than λc, the lost fundamental mode LP01 is converted into both the cladding mode light and the higher-order LP11 mode, resulting in a deviation from Equation (1). The characteristics in this case differ depending on the fiber core, but the LP11 mode does not leak due to the minute bending right at the connection part, and propagates through the core to the vicinity of the fiber end, so that the leaked light power P1 correspondingly. Decrease. As the tendency, the characteristic as schematically shown by the thick line in FIG. 8 is shown. In FIG. 8, the relationship of the difference power (P1−P0) with respect to the connection loss L is indicated by a thick line for a wavelength λa (λa> λce: single mode wavelength) longer than the effective cutoff wavelength λce, and from the effective cutoff wavelength λce. A short wavelength λa (λa <λce: multimode wavelength) is indicated by a thin line. When judging whether or not there is a deviation from the equation (1), the inflection point in FIG. 8, that is, the difference power (P1−P0) at the single mode wavelength and the difference power at the multimode wavelength (predetermined connection loss L) ( It may be determined as a threshold value that the maximum difference value a with respect to (P1-P0) is typically about 0.5 to 1 dB.

  As described above, in the first embodiment, the amount of leakage light that leaks in the vicinity of the connection portion of the lower optical fiber 3 while changing the gap amount or the axial deviation between the upper optical fiber 2 and the lower optical fiber 3 is changed. The optical power P1 is measured, the connection loss L is measured at the end of the lower optical fiber 3, and when the relationship of (Equation 1) is satisfied, it is determined that the lower optical fiber 3 is in single mode transmission. Therefore, it can be easily confirmed that single mode transmission is performed even for an optical fiber having extremely small (or large) bending loss. That is, the clad mode light leaks out of the optical fiber very easily as compared with the light propagating through the core, so that it can be applied to a fiber with a small bending loss, and an accurate evaluation is possible.

(Second Embodiment)
As a simpler method, the relationship between the gap amount X or the axis deviation amount Y and the leakage light power P1 is measured, and it is determined whether single mode transmission is performed from the function shape of P1 (X) or P1 (Y). You can also As schematically shown in FIG. 10B, the leakage light power P1 monotonously increases with respect to X or Y in the single mode wavelength region. On the other hand, in the multimode wavelength region where the higher-order LP11 mode is generated, as shown in FIG. 10A, an extreme point is generated in the leakage light power P1. This difference in characteristics makes it possible to confirm whether or not the optical fiber is operating in a single mode at the wavelength λa of the light source used for the measurement.

  The configuration for carrying out this method is that the power meter 6 for measuring the transmitted light at the end of the lower optical fiber 3 is unnecessary, and the lower optical fiber 3 does not need to bend 3a. Except for this, the same configuration as that of the first embodiment can be used, and the description thereof is omitted. However, in this method, the gap amount X or the axis deviation amount Y is input to the computer 7. When the gap amount X or the axis deviation amount Y and the corresponding optical power P1 of the leaked light are input to the computer 7, the computer 7 executes the following procedure to perform single mode transmission at the used light source wavelength. Confirm that it is possible.

The process flow of the second embodiment will be described with reference to FIG. First, the computer 7 has a plurality of gap amounts X _N (or axial deviation amounts Y _N ) and leakage light optical power P1 measured in the state of the gap amounts X _N (or axial deviation amounts Y _N ). The input _N (N = 0, 1,..., N) is accepted (step S1).

  Upon receipt of the above input, the computer 7 measures the measurement point group from N = 0 to m and the measurement point from N = m to n for each of m = 1, 2,..., N−1. Each group is fitted with a polynomial or the like by the least square method (step S2).

  For each of m = 1, 2,..., n−1, the differential coefficients at the N = m measurement points of the two fitting functions obtained are obtained, and the difference between the differential coefficients is equal to or greater than a predetermined threshold value. If there is, it is determined that the measurement point of N = m is an extreme point (step S3).

  If all the measurement points are not extreme points in the determination in step S3, it is determined that the measurement wavelength is longer than the cutoff wavelength, and the propagation light of the optical fiber to be measured is single mode, while one of the measurement points is If it is an extreme point, the measurement wavelength is shorter than the cutoff wavelength, and the propagation light of the optical fiber to be measured is determined to be multimode (step S4).

  As described above, in the second embodiment, the light of the leaked light that leaks in the vicinity of the connection portion of the lower optical fiber 3 while changing the gap amount or the axial deviation amount between the upper optical fiber 2 and the lower optical fiber 3. When the power P1 is measured and the extreme value point does not occur in the relationship of the optical power P1 with respect to the changed gap amount X or the amount of axial deviation, it is determined that the single mode transmission is performed, so that bending loss is extremely large. It is possible to more easily confirm that single mode transmission is performed even for a small (or large) optical fiber. That is, the clad mode light leaks out of the optical fiber very easily as compared with the light propagating through the core, so that it can be applied to a fiber with a small bending loss, and an accurate evaluation is possible.

(Third embodiment)
In the third embodiment of the present invention, when the leakage light power P1 and the power P0 of the incident light are measured while changing the wavelength λ of the incident light, the power difference P1−P0 is abrupt before and after the cutoff wavelength. The present invention relates to a method for measuring a cut-off wavelength that is performed by utilizing the fact that the change is made to the above. The configuration for implementing the third embodiment includes a light source 1 capable of changing the wavelength, an upper optical fiber 2, a lower optical fiber 3 as an optical fiber to be measured, an upper optical fiber 2 and a lower optical fiber. A V-groove connecting portion 4 that butt-connects the optical fiber 3, a power meter 5 that measures leak light in the vicinity of the connecting portion, and a computer 7 are used.

  As shown in FIG. 10 above, if an appropriate amount of interval X or axis deviation Y is set (X1, Y1 in FIG. 10A), the leakage light power P1 before and after the cutoff wavelength. The value changes rapidly. FIG. 10 shows the behavior of P1 when the input light power P0 is assumed to be substantially constant.

  Considering the case where P0 varies greatly depending on the wavelength, the input optical power P0 to the connection unit is measured for each measurement wavelength, and when X1 or Y1 is set and the measured value of P1 is input to the computer 7, The computer 7 plots the difference optical power (P1−P0) instead of P1 as the vertical axis, and obtains two functions by fitting the plot groups with polynomials by the least square method, and cuts off by the following method. The wavelength λce can be obtained. As P0, it is also possible to use output light power of each wavelength in the light source.

  In the above formula (1), k representing the light receiving efficiency and the connection loss L each have some wavelength λ dependency. However, changes in k and L with respect to the wavelength λ are more gradual than changes in (P1−P0) with respect to the wavelength λ. Therefore, the cutoff wavelength λce can be determined based on the change of (P1-P0) with respect to the wavelength λ. Specifically, for example, when n measurement points are obtained, the power difference (P1−P0) (1, 2, 3,...) With respect to λ (1, 2, 3,... K). The function f1 obtained by fitting the measurement point of k) with a polynomial or the like by the least square method and the difference power (P1−P0) (k, k + 1, k + 2) with respect to λ (k, k + 1, k + 2,..., n) ,..., N) and a function f2 obtained by fitting the measurement points with a polynomial or the like by the least square method. Further, as shown in FIG. 11, the wavelength λm (k) when the value of the power difference (P1−P0) at the intersection of the fitting curves f1 and f2 indicated by both functions f1 and f2 becomes the minimum value is defined as a cutoff wavelength λm. To do. When there is no intersection of both functions (fitting curves), the measurement wavelength range can be expanded or the number of measurement points can be increased.

  In the SMF, when only the gap amount is given, X1 may be set, for example, in the vicinity of 150 μm, and when only the axial deviation amount is given, Y1, for example, may be set near 3 μm. In addition, it is desirable in terms of measurement accuracy that the measurement wavelength interval is as fine as possible, the number of measurement wavelengths is large, and the measurement wavelength range is wide. It ’s fine.

  As described above, in the third embodiment, the wavelength λ is continuously set in a state in which the V-groove connecting portion 4 is set so that the upper optical fiber 2 and the lower optical fiber 3 have a predetermined gap amount or axial deviation amount. The optical power P1 of the leaked light and the optical power P1 of the incident light with respect to the wavelength λ has a predetermined optical power relationship (P1−P0). Since the wavelength when changing beyond the threshold value S is determined as the cut-off wavelength, even for an optical fiber having extremely small (or large) bending loss, and without adjusting the optical fiber length, Cut-off wavelength can be measured. That is, the clad mode light leaks out of the optical fiber very easily as compared with the light propagating through the core, so that it can be applied to a fiber with a small bending loss, and an accurate evaluation is possible. In addition, since the leakage power is measured near the connection, it is not necessary to strictly adjust the length of the fiber to be measured, so it is difficult to directly measure the cutoff wavelength when the optical fiber is short. It is possible to measure a value close to the theoretical cutoff wavelength.

(Fourth embodiment)
In the fourth embodiment of the present invention, when the optical power P1 of the leaked light is measured by the power meter 5 in the first to third embodiments, the clad mode light is used. The light receiving efficiency coefficient k, that is, the value of the leakage power P1 of the clad mode light to be detected is increased, and the method and apparatus for improving the measurement accuracy.

  As a result of investigations under various conditions, the leakage of clad mode light may be observed even when no microbend or macrobend is intentionally applied. In the first place, this is caused by minute irregularities such as the outer diameter of the coating that are inevitable in the manufacturing technology, and is considered to be due to the microbend in a broad sense, but in order to improve the measurement accuracy, the following means can be used. desirable.

  FIG. 12 is a diagram illustrating a configuration example of a light receiving portion of clad mode light. In FIG. 12, 10 is a support base, and 20 is a pressing member. Further, 2 is an upper optical fiber, 3 is a lower optical fiber as a fiber to be measured, 4 is a V-groove connecting portion detailed in FIG. 6, and 5 is a power meter.

  The pressing member 20 is a member having a refractive index higher than that of the clad of the lower optical fiber 3, and is in the range in which the higher-order LP11 mode is not leaked in contact with the lower optical fiber 3 in the vicinity of the V groove connecting portion 4. A minute bending is imparted by adjusting the pressing force so as to achieve the optimum pressing in the direction indicated by the arrow pointing from the middle to the top. In order not to attenuate the power of the clad mode light, it is desirable that the pressing member 20 has a high transmittance and a small thickness such as glass or resin.

  The support base 10 is for condensing the leaked clad mode light to the power meter 5, and is preferably a member having high reflectivity. Similarly, the material of the head of the power meter 5 is desirably a highly reflective member. Similarly, by enlarging the head area of the power meter 5, the effect that the leaked clad mode light can be condensed with high accuracy can be obtained.

  Although FIG. 12 shows a case where the support base 10 is installed below the optical fiber (core wire) for simplicity, the configuration in which the entire power meter 5 is completely shielded from the periphery of the core wire has a higher light collection effect. Can be expected. Thereby, the effect which shields the light used as noise components other than clad mode light can also be acquired. If noise light is shielded, highly accurate measurement can be realized even if continuous light (CW light) is used as incident light for measurement.

  In order to increase the leakage power of the detected clad mode light, a plurality of heads (light receiving portions) of the power meter 5 may be installed. In the measurement using the power meter 5 having one head, the optical power P1 of the leaked light is dependent on the measurement position, which may cause errors and measurement variations. In that case, it is preferable to use the heads of a plurality of power meters 5 arranged at appropriate intervals and calculate the sum of the measured values at the heads of each power meter 5 to obtain the value of P1. In that case, since the loss in the longitudinal direction of the clad mode light is about 1 dB / cm, the effect cannot be obtained if the distance from the connection portion to the measurement position is too large. Therefore, it is desirable to install a plurality of heads of the power meter 5 within a range of 10 cm or less from the connection portion in the longitudinal direction of the optical fiber.

  In the above description, the means for applying the microbend as an example of the means for leaking and measuring the clad mode light has been described as an example, but instead of the microbend, the macrobend is an arc-shaped bend as shown below. It is also possible to use means for imparting.

  FIG. 13 is a diagram showing a means for applying a macro bend, which is an arc-shaped bend, which can be used instead of a micro bend. In FIG. 13, 10 is a support base, and 20 is a pressing member. Further, 2 is an upper optical fiber, 3 is a lower optical fiber as a fiber to be measured, 4 is a V-groove connecting portion detailed in FIG. 6, and 5 is a power meter.

  In the configuration shown in FIG. 13, the support base 10 and the pressing member 20 have curved surfaces, and the pressing member 20 presses the optical fiber 3 in the direction of the support base 10 along the curved surface. Is granted. The curvature of this bending can select the curvature of the range which does not leak high order LP11 mode.

  In the present embodiment, the means for applying the microbend or the macrobend to the optical fiber 3 has been described by taking the configuration shown in FIGS. 12 and 13 as an example, but other known means as means for leaking the clad mode light. Can be used.

  The present invention can be used for structural design and selection of optical fibers used for high-speed transmission.

It is a schematic diagram of the optical fiber length dependence of the effective cutoff wavelength λce. It is a figure which shows an example of the measuring system of the bending loss method which is a prior art. It is a figure which shows an example of the measuring system of the bending loss method which is a prior art. It is the figure which showed typically the mode of generation | occurrence | production of the clad mode light utilized by this invention. It is a figure which shows an example of the structure used for the confirmation method of the single mode transmission of this invention. It is a figure which shows the structure of a V-groove connection part. It is a figure which shows an example of the relationship between measured P1-P0 and the connection loss L. FIG. It is a schematic diagram of the relationship between P1-P0 and connection loss L. It is a flowchart which shows the confirmation method of the single mode transmission by 2nd Embodiment. It is a schematic diagram which shows the relationship between P1 and X or Y. It is a schematic diagram which shows the relationship between P1-P0 and wavelength (lambda). It is a figure which shows the structural example of the light-receiving part of clad mode light. It is a figure which shows the other structural example of the light-receiving part of clad mode light.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Upper side optical fiber 3 Lower side optical fiber 4 V groove connection part 5, 6 Power meter 7 Computer 41, 43 Stage 41a, 43a Groove 43b X-axis drive part 43c Y-axis drive part 42, 44 Fixing member 45, 46 Lid member 10 Support base 20 Pressing member

Claims (4)

The upper optical fiber is connected to the lower optical fiber to be checked through a connection with variable gap or axial deviation, and light is incident on the upper optical fiber to transmit the lower optical fiber in single mode. A method for confirming single mode transmission of an optical fiber,
Entering light of optical power P0 into the lower optical fiber;
Measuring the optical power P1 of the leaked light leaking in the vicinity of the connecting portion of the lower optical fiber while changing the gap amount or the axial deviation amount of the connecting portion;
Measuring the optical power P3 of the transmitted light at the end of the lower optical fiber;
Calculating a connection loss L based on the optical power P0 of the light incident on the lower optical fiber and the measured optical power P3 of the transmitted light;
A method for confirming single mode transmission, comprising: determining that the lower optical fiber is in single mode transmission when the relationship of the following (formula 1) is satisfied.
In (Expression 1), k is a constant representing the light receiving efficiency.

The upper optical fiber is connected to the lower optical fiber to be checked through a connection with variable gap or axial deviation, and light is incident on the upper optical fiber to transmit the lower optical fiber in single mode. A method for confirming single mode transmission of an optical fiber,
Measuring the optical power P1 of the leaked light leaking in the vicinity of the connecting portion of the lower optical fiber while changing the gap amount or the axial deviation amount of the connecting portion;
In the relationship of the optical power P1 with respect to the changed gap amount X or the amount of axis deviation, it is determined that single mode transmission is performed when no extreme point is generated. Confirmation method. The upper optical fiber is connected to the lower optical fiber to be checked through a connection with variable gap or axial deviation, and light is incident on the upper optical fiber to transmit the lower optical fiber in single mode. An optical fiber single mode transmission confirmation device for confirming
Means for injecting light of optical power P0 into the lower optical fiber;
Means for changing the gap amount or the axial deviation amount of the connecting portion;
In the vicinity of the connection portion between the lower optical fiber and the upper optical fiber, the lower side in the vicinity of the connection portion is provided by means for giving the lower optical fiber a macrobend that is an arc-shaped bend or a microbend that is a slight bend. Means for measuring the optical power P1 of the leaked light leaked from the optical fiber;
Means for measuring the optical power P3 of the transmitted light at the end of the lower optical fiber;
Means for calculating a connection loss L based on the optical power P0 of the light incident on the lower optical fiber and the measured optical power P3 of the transmitted light;
An apparatus for confirming single-mode transmission of an optical fiber, comprising: a unit that determines that the lower-side optical fiber is single-mode transmission when the following relationship (Equation 1) is satisfied.
In (Expression 1), k is a constant representing the light receiving efficiency.

The upper optical fiber is connected to the lower optical fiber to be checked through a connection with variable gap or axial deviation, and light is incident on the upper optical fiber to transmit the lower optical fiber in single mode. An optical fiber single mode transmission confirmation device for confirming
Means for changing the gap amount or the axial deviation amount of the connecting portion;
In the vicinity of the connection portion between the lower optical fiber and the upper optical fiber, the lower side in the vicinity of the connection portion is provided by means for giving the lower optical fiber a macrobend that is an arc-shaped bend or a microbend that is a slight bend. Means for measuring the optical power P1 of the leaked light leaked from the optical fiber;
Means for determining single-mode transmission when an extreme point does not occur in the relationship of the optical power P1 with respect to the changed gap amount X or axial displacement amount. Confirmation device.

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