Method for assessing optical characteristics of optical coupler

Abstract

This method is for assessing optical characteristics of an optical coupler (3) provided with: an optical fiber (1) that is set in a communicable state and is side-polished; and an optical waveguide (2) that is side-polished and is to be optically coupled to the optical fiber (1). The method comprises: connecting one end of the optical waveguide (2) to either a nonreflective termination (8) or a light cutoff filter having a known reflectance with respect to first test light; acquiring an OTDR waveform by inputting the first test light to the other end of the optical waveguide (2); and calculating optical characteristics of the optical coupler (3) from a change in the amount of reflection of the first test light indicated by a level difference appearing in the OTDR waveform.

Description

Optical characteristics evaluation method for optical couplers

[0001] The present disclosure relates to a method for evaluating the optical characteristics of an optical coupler that uses a side-polished optical fiber.

[0002] The Passive Optical Network (PON) system is known as a wiring system for optical communication networks that provides optical services at high speed and low cost (see Non-Patent Document 1). Meanwhile, a method for manufacturing an optical coupler using an optical fiber side polishing method has been studied as an optical multiplexing / demultiplexing technology that can demultiplex light from an optical fiber and multiplex light into an optical fiber without cutting the optical fiber during communication (see Non-Patent Document 2).

[0003] “Technology Basic Course GE-PON Technology”, NTT Technology Journal, August 2005 issue, pp. 71-74. T. Uematsu et al., “Optical coupling technique based on fiber side-polishing without service interruption”, IEEE Photonics Technology Letters, vol. 34, no. 19, pp. 1042-1045 (1 Oct. 2022).

[0004] In the optical multiplexing / demultiplexing technology described above, it is important to evaluate the optical characteristics of optical couplers, such as excess loss, branching ratio, and insertion loss. Measuring or adjusting these optical characteristics requires the introduction of a light source and an optical power meter. However, in order to install these devices on an optical fiber in communication, the optical fiber must be cut temporarily to interrupt communication.

[0005] The present disclosure aims to provide a method for evaluating optical characteristics such as excess loss, branching ratio, and insertion loss without interrupting communication when fabricating an optical coupler using an optical fiber side polishing method on an optical fiber in communication.

[0006] One aspect of the present disclosure is a method for evaluating the optical characteristics of an optical coupler that includes an optical fiber that is placed in a state where communication is possible and that has been side-polished, and an optical waveguide that is also side-polished and optically coupled to the optical fiber, wherein a reflection-free termination or a light-blocking filter with a known reflectance to a first test light is connected to one end of the optical waveguide, the first test light is input to the other end of the optical waveguide to obtain an OTDR waveform, and the optical characteristics of the optical coupler are calculated from the change in the amount of reflection of the first test light that is indicated by a step that appears in the OTDR waveform.

[0007] According to the present disclosure, it is possible to provide a method capable of evaluating optical characteristics such as excess loss, branching ratio, and insertion loss without interrupting communication.

[0008] FIG. 1A shows a first fabrication procedure for an optical fiber side-polished optical coupler. FIG. 1B shows a second fabrication procedure for an optical fiber side-polished optical coupler. FIG. 1C shows a third fabrication procedure for an optical fiber side-polished optical coupler. FIG. 2 shows an example of a configuration for evaluating the optical characteristics of an optical coupler. FIG. 3 shows an example of measurement results when the test optical time domain reflectometer is an OTDR. FIG. 4 shows a configuration in which an optical path length adjuster is installed. FIG. 5 shows an example of a configuration for measuring loss from port g2 to port g1. FIG. 6 shows a first example of a configuration for measuring loss from port f1 to port f2. FIG. 7 shows a second example of a configuration for measuring loss from port f1 to port f2. FIG. 8 shows a first example of a configuration for measuring loss from port f2 to port f1. FIG. 9 shows a second example of a configuration for measuring loss from port f2 to port f1. FIG. 10 shows an example of a configuration for adjusting the optical characteristics of an optical coupler. FIG. 11 shows an example of a configuration of a computer system for executing an optical characteristic evaluation method.

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the description of the drawings, the same parts are designated by the same reference numerals and the description thereof will be omitted.

[0010] The optical coupler 3 according to this embodiment includes an optical fiber 1 that is placed in a state where communication is possible and has its side surface polished, and an optical waveguide 2 that is also side surface polished and optically coupled to the optical fiber 1. The optical coupler 3 is fabricated by polishing the side surface of the optical fiber 1 that is placed in a state where communication is possible.

[0011] First, we will explain the method for manufacturing this optical coupler 3. Figures 1A to 1C are diagrams showing steps 1 to 3 for manufacturing the optical coupler 3, respectively.

[0012] (Step 1) As shown in Fig. 1A, transmission devices 4A and 4B are connected to both ends of an optical fiber 1, and the two devices communicate with each other using communication light 20 and communication light 21 propagating through the optical fiber 1. In this state, a portion of the optical fiber 1 in the longitudinal direction is bent to form a bent portion M1. The communication light 20 leaks from the bent portion M1 as leakage light 20a. Note that leakage light (not shown) from the communication light 21 also leaks from the bent portion M1, but in the configuration shown in Fig. 1A, the monitor device 24 observes the leakage light 20a of the communication light 20.

[0013] (Step 2) As shown in FIG. 1B , a bent portion M2 is formed by bending another portion of the optical fiber 1 in the longitudinal direction in order to polish the optical fiber 1. The bent portion M2 is located upstream of the bent portion M1 in the propagation direction of the communication light 20. Next, the side surface of the bent portion M2 (i.e., the coating portion 1c and the cladding portion 1b) is polished to the vicinity of the core 1a (see FIG. 1C ). As the side surface polishing progresses closer to the core 1a, a portion of the communication light 20 begins to leak from the polished surface of the bent portion M2, and its intensity gradually increases. Meanwhile, the communication light 20 leaks from the polished surface of the bent portion M2 before it leaks from the polished surface of the bent portion M1. Therefore, the intensity of the leaked light 20a from the bent portion M1, as observed by the monitor device 24, gradually decreases as polishing progresses.

[0014] The measurement value of the monitor device 24 changes with the polishing time of the bent portion M2. In other words, by observing the intensity of the leaked light 20a at the bent portion M1, the state of polishing at the bent portion M2 can be indirectly determined. When the measurement value of the monitor device 24 reaches a predetermined value, polishing of the bent portion M2 is completed. Also, the bent portion M1 is returned to its original state.

[0015] (Step 3) As shown in Figure 1C, an optical waveguide 2 is prepared in advance, in which the coating portion 2c and cladding portion 2b have been polished to within a few microns from the core 2a. Then, the polished surface D1 of the bent portion M2 of the optical fiber 1 and the polished surface D2 of the bent portion M3 of the optical waveguide 2 are surface-bonded to each other. Next, one or both polished surfaces are slid to change the distance d between the cores 1a and 2a, thereby moving the cores 1a and 2a relative to each other, and holding them at a position where the desired branching ratio is obtained. In other words, the cores 1a and 2a are aligned with each other. This completes the optical coupler 3 with side-polished optical fiber.

[0016] As described above, during communication, a portion of the longitudinal side surface of the optical fiber 1 is polished to the vicinity of the core 1a. Even when polished to the vicinity of the core 1a, propagation of the communication light 20 within the core 1a is maintained, and communication is also maintained. Meanwhile, the optical waveguide 2 to which the optical fiber 1 is joined is also polished in advance to the vicinity of the core 1a. The polished surfaces of the optical fiber 1 and the optical waveguide 2 are then surface-joined, and so-called alignment is performed to adjust the positions so that the cores 1a and 2a approach each other. This alignment optically couples the cores 1a and 2a, allowing a portion of the communication light to be moved to the adjacent core. In other words, light input to the optical fiber 1 or the optical waveguide 2 can be multiplexed or demultiplexed.

[0017] By forming an optical coupler 3 using such side-polished optical fiber 1, it is possible to continue providing predetermined services to users without interrupting communications prior to the establishment, even when establishing a new route by connecting to an existing route. Furthermore, unlike a communication network in which existing optical splitters are installed, there are no restrictions on where a new optical fiber can be inserted.

[0018] Next, a method for evaluating the optical characteristics of the optical coupler 3 according to this embodiment will be described. Fig. 2 is a diagram showing an example of a configuration for evaluating the optical characteristics of the optical coupler 3. The optical fiber 1 is connected to a communication network installed between PON transmission devices. In the PON system, time-division multiplexed communication and time-division multiple access communication are implemented. Note that the optical fiber 1 is not limited to application to star-type networks constructed using the PON system, but can also be applied to bus-type networks.

[0019] As described above, the optical coupler 3 is composed of an optical fiber 1 and an optical waveguide 2, each of which has a polished side surface. As shown in Fig. 2, the optical coupler 3 has four ports f1, f2, g1, and g2. Of these ports, ports f1 and f2 are composed of an optical fiber 1, and ports g1 and g2 are composed of an optical waveguide 2. With respect to the joint surface (optical coupling portion) between the optical fiber 1 and the optical waveguide 2, ports f1 and g1 are located on one side of the optical coupler 3, and ports f2 and g2 are located on the other side of the optical coupler 3.

[0020] A transmission device 4 is connected to the optical fiber 1. Communication light 20 output from the optical fiber 1 is input to port f1 of the optical coupler 3. A test light blocking filter 7 is inserted between the transmission device 4 and the optical coupler 3. The distance from the optical coupler 3 to the test light blocking filter 7 is longer than the distance from the optical coupler 3 to the reflection-free termination 8.

[0021] A reflection-free termination 8 is connected to one end of the optical waveguide 2 extending from port g1. Meanwhile, a test optical time domain reflectometer 5 is connected to the other end of the optical waveguide 2 extending from port g2 via a bandpass filter 6. The test optical time domain reflectometer 5 is, for example, an OTDR (Optical Time Domain Reflectometer), and outputs pulsed light 22 as a first test light while continuously measuring the intensities of its Fresnel reflected light and back-scattered light. The wavelength of the pulsed light 22 is set to be different from the wavelength of the communication light 20. The bandpass filter 6 blocks the communication light 20 from the transmission device 4 while passing the pulsed light 22, its reflected light, and back-scattered light.

[0022] Communication light 20 output from transmission device 4 propagates through optical fiber 1, passes through optical coupler 3, and is blocked by bandpass filter 6. Pulse light 22 output from test optical time domain reflectometer 5 passes through bandpass filter 6 and is split into pulse light 22a and pulse light 22b by optical coupler 3. Pulse light 22a is input to optical fiber 1 and reflected by test optical blocking filter 7. The reflected pulse light 22a propagates in the opposite direction along the original propagation path and is input to test optical time domain reflectometer 5. Pulse light 22b is input to optical waveguide 2 and is blocked by non-reflection termination 8. The test optical time domain reflectometer 5 samples the intensity of the light input to it over time.

[0023] FIG. 3 is a diagram showing an example of an OTDR waveform acquired by the test optical time domain reflectometer 5. As shown in FIG. 3, it can be seen that loss occurs at the optical coupler 3 and the reflectionless termination 8. Between the optical coupler 3 and the reflectionless termination 8, the test optical time domain reflectometer 5 measures the back-scattered Rayleigh light from the optical fiber 1 and the optical waveguide 2. Further from the reflectionless termination 8 (i.e., to the right of the figure), the test optical time domain reflectometer 5 measures the back-scattered Rayleigh light from the optical fiber 1. Generally, the propagation loss of an optical fiber is constant over distance. It is also known that a sloping line in an OTDR waveform represents the propagation loss of the optical fiber. Therefore, the slope of the line far from the reflectionless termination 8 is the propagation loss of the optical fiber 1. Therefore, by extending the line far from the reflectionless termination 8 to the loss point of the optical coupler 3 and estimating the step with the line before the optical coupler 3 at the location of the optical coupler 3, the loss L from port g2 to port f1 of the optical coupler 3 can be calculated. ｇ２ｆ１ can be calculated (estimated).

[0024] Generally, a step in the waveform measured by an OTDR for one optical path (path) represents the loss of light traveling back and forth through the point that causes the step. In other words, the step that appears in the OTDR waveform represents twice the loss that occurs when the test light passes through the point that causes the step. Considering this tendency in the configuration according to this embodiment, the step between the line S1 before the optical coupler 3 and the line S3 after the non-reflective termination 8 corresponds to the square of the branching ratio of the optical coupler 3. On the other hand, the step between the line S1 before the optical coupler 3 and the line S2 after the optical coupler 3 and before the non-reflective termination 8 corresponds to the loss L of the optical coupler 3. ｃ The loss of the optical coupler 3, L ｃ is the loss L from port g2 to port f1 ｇ２ｆ１ and the loss L from port g2 to port g1 ｇ２ｇ１ Therefore, the loss L ｇ２ｆ１ and loss L ｇ２ｇ１ If one of these is known, it is possible to estimate the loss of the other.

[0025] For example, as shown in FIG. 3, the loss L ｇ２ｆ１ (branching ratio) is A [dB], and the loss before and after the position of the optical coupler 3 is L ｃ If the loss is B [dB], then the loss L ｇ２ｆ１ The step caused by this is 2A [dB], and the loss L ｃ In this case, the loss L from port g2 to port g1 is 2B [dB]. ｇ２ｇ１ The value C [dB] of the insertion loss is expressed by Equation 1. C = -10 log(10 －２Ｂ / １０ -10 －２Ａ / １０ ) ... (Equation 1) Furthermore, the loss L from port g2 to port f1 ｇ２ｆ１ is also the branching ratio. This branching ratio (A [dB]) and the insertion loss L ｇ２ｇ１ (C [dB]) is known, the excess loss L of the optical coupler 3 ｅｘ The value D [dB] is expressed by Equation 2. D = -10 log(1-10 －Ｃ / １０ -10 －２Ａ / １０ ) = -10log(1-(10 －２Ｂ / １０ -10 －２Ａ / １０ )-10 －２Ａ / １０) = -10log(1-10 －２Ｂ / １０ ) (Equation 2) where B [dB] is the loss L before and after the position of the optical coupler 3 ｃ is.

[0026] In this way, by connecting the test optical time domain reflectometer 5 to one end of the optical coupler 3, at least one of the branching ratio, insertion loss, and excess loss of the optical coupler 3 can be calculated from the change in the reflection amount of the pulsed light 22 indicated by the step that appears in the OTDR waveform. In other words, by analyzing the measurement results obtained by the test optical time domain reflectometer 5, the optical characteristics of the optical coupler 3 can be calculated (estimated).

[0027] The test optical time domain reflectometer 5 may be connected to port g1 instead of port g2. Using the same procedure as when the test optical time domain reflectometer 5 is installed at port g2, the loss (branching ratio) from port g1 to port f2 of the optical coupler 3 and the loss (insertion loss) from port g1 to port g2 of the optical coupler 3 can be calculated as estimated values, and the excess loss can be calculated from these results as an estimated value. Furthermore, by performing measurements when the test optical time domain reflectometer 5 is installed at port g2 of the optical coupler 3, it is also possible to estimate the optical characteristics of the optical coupler 3 in both directions of light propagation.

[0028] 4 is a diagram showing a configuration in which an optical path length adjuster 12 is installed. As shown in Fig. 4, the optical path length adjuster 12 may be installed between the optical coupler 3 and the reflectionless termination 8, in other words, between one end of the optical waveguide 2 functioning as port g1 and the reflectionless termination 8. The optical path length adjuster 12 adjusts the length of the optical fiber between the optical coupler 3 and the reflectionless termination 8. In this case, the test light time domain reflectometer 5 may change the pulse width of the pulsed light 22 in accordance with the branching ratio of the optical coupler 3.

[0029] Increasing the pulse width of the test light time domain reflectometer 5 increases the measurable dynamic range of the test light time domain reflectometer 5, but decreases the spatial resolution. When the spatial resolution decreases, the optical fiber length may be extended using the optical path length adjuster 12 in order to measure the branching ratio using the test light time domain reflectometer 5. It is desirable to set the optical fiber length using the optical path length adjuster 12 to at least half the pulse width of the test light time domain reflectometer 5. For example, if the pulse width is 100 ns and the refractive index of the optical fiber is 1.46, an optical fiber length of approximately 10 m or more is desirable.

[0030] The length of the optical fiber in optical path length adjuster 12 must be shorter than the length of the optical fiber between optical coupler 3 and a transmission device or the next optical coupler 3. Therefore, it is desirable to adjust the length of the optical fiber in optical path length adjuster 12 and the pulse width of pulsed light 22 output by test light time domain reflectometer 5 according to the length of the optical fiber between optical coupler 3 and a transmission device 4 or an adjacent optical coupler (not shown).

[0031] The optical path length adjuster 12 may be provided with optical fibers of several lengths, any of which may be connected to the optical coupler 3, or two 1xN optical switches (not shown) may be installed, with optical fibers of multiple lengths connected between the optical switches, allowing the appropriate optical fiber length to be selected by the optical switch.

[0032] The loss between port g1 and port g2 can be calculated by measuring the change in intensity of the second test light (output light 25) propagating through the optical waveguide 2 before it is input to the optical coupler 3 and after it is output from the optical coupler 3.

[0033] For example, the loss L from port g2 to port g1 ｇ２ｇ１ may be measured as follows: Figure 5 shows the loss L from port g2 to port g1. ｇ２ｇ１ 5 is a diagram showing an example of a configuration for measuring the second test light. As shown in FIG. 5, a test light source 10 is connected to port g2 instead of the test light pulse tester 5. The test light source 10 outputs output light 25 as the second test light. The output light 25 may be either continuous light or pulsed light. On the other hand, an optical power meter 9 is connected to port g1 via a bandpass filter 6.

[0034] The output light 25 output from the test light source 10 is split by the optical coupler 3, and one of the split light beams is output from port g1. The bandpass filter 6 blocks light of wavelengths other than that of the output light 25, and the optical power meter 9 measures the intensity of the output light 25 that has passed through the bandpass filter 6. The intensity of the output light 25 before it is input to the optical coupler 3 is measured in advance, and by comparing this intensity with the intensity of the output light 25 output from port g1 (i.e., from the change in these two intensities), the loss L from port g2 to port g1 can be calculated. ｇ１ｇ２ Furthermore, the measured loss L ｇ２ｇ１ and the loss L measured by the test optical pulse tester 5 ｇ２ｆ１ Using Equation 2, the excess loss L ｅｘ can also be calculated as an estimate.

[0035] In addition, the loss L ｇ２ｇ１ When measuring the optical power meter 9, the test light source 10 may be connected to the port g1 instead of the port g2. In this case, the optical power meter 9 is connected to the port g2 via the bandpass filter 6.

[0036] The loss between port f1 and port f2 can be calculated by measuring the change in intensity of the second test light (communication light 20, communication light 21 or output light 25) propagating through the optical fiber 1 via the optical coupler 3 while aligning the optical fiber 1 and the optical waveguide 2.

[0037] For example, the loss L from port f1 to port f2 ｆ１ｆ２ may be measured as follows: Figure 6 shows the loss L from port f1 to port f2. ｆ１ｆ２ 6 is a diagram showing a first example of a configuration for measuring the optical power meter 9. As shown in Fig. 6, in this example, an optical power meter 9 is added to the configuration shown in Fig. 2. In addition, a bent portion M4 is formed by partially bending the optical fiber 1 downstream from port f2. From the bent portion M4, the communication light 20 output from the transmission device 4 leaks as leakage light 20a. The optical power meter 9 measures the change in intensity of the leakage light 20a from the bent portion M4 during alignment.

[0038] By measuring the change in the intensity of the leaked light 20a before and after aligning the optical coupler 3, the loss L from port f1 to port f2 can be calculated.ｆ１ｆ２ That is, the communication light 20 has a loss L ｆ１ｆ２ The second test light is measured by the optical power meter 9 to measure the loss L from port g2 to port f1. ｇ２ｆ１ and the loss L from port f1 to port g2 ｆ１ｇ２ are equal, the loss L from port f1 to port f2 measured by the optical power meter 9 ｆ１ｆ２ and the loss L from port g2 to port f1 measured by the test optical time domain reflectometer 5. ｇ２ｆ１ Using Equation 2, the excess loss L ｅｘ can also be calculated as an estimate.

[0039] FIG. 7 shows the loss L from port f1 to port f2. ｆ１ｆ２ 7 is a diagram showing a second example of a configuration for measuring the second test light. As shown in Fig. 7, in this example, a test light source 10 is added to the configuration shown in Fig. 6. In addition, a bent portion M5 is formed by partially bending the optical fiber 1 upstream from port f1. Output light 25 from the test light source 10 is input to the bent portion M5 as second test light toward the optical coupler 3. An optical power meter 9 measures leakage light 25a of the output light 25 leaking from the bent portion M4.

[0040] The optical power meter 9 measures the change in the intensity of the leaked light 25a before and after the alignment of the optical coupler 3, and the loss L from the port f1 to the port f2 is calculated. ｆ１ｆ２ Furthermore, the loss L from port f1 to port f2 measured by the optical power meter 9 can be calculated. ｆ１ｆ２ and the loss L from port g2 to port f1 measured by the test optical time domain reflectometer 5. ｇ２ｆ１ Using Equation 2, the excess loss L ｅｘ can also be calculated as an estimate.

[0041] It is desirable to install a bandpass filter 11 between the bending portion M4 and the optical power meter 9. The bandpass filter 11 blocks the communication light 20 from the transmission device 4, while passing the leaked light 25a leaking from the bending portion M4. The test light source 10 may also be connected to a test optical coupler unit (not shown) installed immediately after the transmission device 4 on the station side. In this case, it is desirable that the test light source 10 be remotely operable.

[0042] FIG. 8 shows the loss L from port f2 to port f1. ｆ２ｆ１ 8 shows a first example of a configuration for measuring the optical time domain difference (f1). As shown in Fig. 8, in this example, the bandpass filter 6, test optical time domain reflectometer 5, optical power meter 9, and bending unit M4 of the example shown in Fig. 6 are arranged on opposite sides of the optical coupler 3. That is, the test optical time domain reflectometer 5 is connected to port g1 via the bandpass filter 6. The bending unit M4 is formed between the transmission device 4 and port f1.

[0043] Leakage light 21a of communication light 21 from another transmission device 4B (see FIG. 1A) propagating toward port f2 leaks from bent portion M4. The optical power meter 9 measures the change in intensity of the leaked light 21a from bent portion M4 during alignment. By measuring the change in intensity of the leaked light 21a before and after aligning the optical coupler 3, the loss L from port f2 to port f1 can be calculated. ｆ２ｆ１ That is, the communication light 21 has a loss L ｆ１ｆ２ The measured loss L from port f2 to port f1 is used as the second test light to be measured by the optical power meter 9. ｆ２ｆ１ may be used instead of the loss (insertion loss) from port g1 to port g2, taking into account the reciprocity of light.

[0044] The transmission device 4B may be directly connected to the optical fiber 1, or may be connected to the optical fiber 1 via another optical coupler, splitter, etc. In either case, however, a test light blocking filter 7 is inserted between the transmission device 4B and the optical coupler 3 and in the vicinity of the transmission device 4B.

[0045] FIG. 9 shows the loss L from port f2 to port f1. ｆ２ｆ１9 is a diagram showing a second example of a configuration for measuring the optical fiber 1. As shown in Fig. 9, in this example, the bandpass filters 6 and 11, the test optical time domain reflectometer 5, the optical power meter 9, the test optical light source 10, the bending portion M4, and the bending portion M5 of the example of the configuration shown in Fig. 7 are arranged on opposite sides of the optical coupler 3. That is, the test optical time domain reflectometer 5 is connected to port g1 via the bandpass filter 6. The bending portion M4 is formed between the transmission device 4 and port f1. The bending portion M5 is also formed in the optical fiber 1 downstream from port f2.

[0046] The output light 25 of the test light source 10 is input to the bending portion M5 as second test light toward the optical coupler 3. The optical power meter 9 measures the leaked light 25a of the output light 25 leaked from the bending portion M4. The optical power meter 9 measures the change in the intensity of the leaked light 25a from the bending portion M4 during alignment. By measuring the change in the intensity of the leaked light 25a before and after the optical coupler 3 is aligned by the optical power meter 9, the loss L from port f2 to port f1 can be calculated. ｆ２ｆ１ can be calculated.

[0047] Instead of the reflectionless termination 8, a test light blocking filter (not shown) with a known reflectance for the output light (i.e., pulsed light 22) of the test light time domain reflectometer 5 may be installed. In OTDR measurement using the test light time domain reflectometer 5, the difference between the backscattered light level and the Fresnel reflected light level is calculated from the reflectance of the test light blocking filter and the return attenuation of the backscattered light. If the reflectance of the test light blocking filter is known, and the optical fiber is a single-mode optical fiber, the return attenuation of the backscattered light is known based on the pulse width, so the difference between the backscattered light level and the Fresnel reflected light level can be estimated. Therefore, if the Fresnel reflected light level can be measured by OTDR measurement, the backscattered light level of only the optical fiber connected to port g2 can be estimated. Since the backscattered light level of only the optical fiber connected to port g2 and the backscattered light level of the optical fiber connected to port g1 are also known from the OTDR measurement results, the difference between them is the insertion loss.

[0048] Furthermore, if it is desired to adjust the branching ratio again after adjusting the branching ratio of the optical coupler 3 by the above-described alignment, the amount of fluctuation in the branching ratio may be estimated from the amount of fluctuation in the reflected light power of the test light blocking filter 7 in the configuration shown in Fig. 1. This is because the amount of fluctuation in the reflected light power of the test light blocking filter 7 is equal to the amount of fluctuation in the branching ratio. It is known that the reflected light power of the test light blocking filter 7 can be measured, for example, by a test optical time domain reflectometer 5.

[0049] Next, a method for adjusting the optical characteristics of the optical coupler 3 will be described. Fig. 10 is a diagram showing an example of a configuration for adjusting the optical characteristics of the optical coupler 3. For example, in the configuration shown in Fig. 2, an alignment machine 13 is installed in the optical waveguide 2 of the optical coupler 3, as shown in Fig. 10. By installing the alignment machine 13, the optical characteristics of the optical coupler 3 can be adjusted on-site. This is because the length at which the optical fiber 1 and the optical waveguide 2 are coupled can be changed. By using this method when adjusting the optical characteristics of the optical coupler 3 on-site, it is possible to align the side-polished optical fibers of the optical coupler 3 while measuring the optical characteristics, and align the optical coupler 3 to the desired optical characteristics.

[0050] The aligning device 13 may be a manual moving stage or an electrically operated moving stage. The aligning device 13 may also be a combination of multiple moving stages so that multiple axes can be aligned. It is also desirable to combine a rotary stage with the aligning device 13. The optical characteristics of the optical coupler 3 may be displayed on a display device 14 such as a display, and the optical characteristics may be adjusted by the aligning device 13 while viewing the displayed values. The electrically operated stage or rotary stage may be controlled by a program to measure and estimate the optical characteristics at each coordinate, select the coordinate that is closest to the desired optical characteristics from the optical characteristics at each coordinate, and control the electrically operated stage or rotary stage to that coordinate, thereby automatically adjusting the optical characteristics.

[0051] 11 is a diagram showing an example of the configuration of a computer system 50 for executing the optical property evaluation method according to this embodiment. The computer system 50 is a computer system having a well-known configuration. For example, as shown in FIG. 11 , the computer system 50 includes a CPU 51, a memory 52, a storage 53, a communication unit 54, an input unit 55, and an output unit 56. The memory 52 and the storage 53 are storage devices. In this computer system, the CPU 51 executes a predetermined program loaded on the memory 52, thereby realizing various functions of the optical property evaluation method, such as light measurement, calculation of each loss, and alignment control.

[0052] The computer system 50 may be implemented by one computer, or by multiple computers, or may be a virtual machine implemented on a computer.

[0053] The program for the computer system 50 can be stored in a computer-readable recording medium such as a HDD, SSD, USB memory, CD, or DVD. The computer-readable recording medium is, for example, a non-transitory recording medium. The program for the computer system 50 can also be distributed via a communication network.

[0054] Generally, in optical multiplexing / demultiplexing technology that multiplexes and demultiplexes light, the optical power of the optical fiber being communicated is attenuated each time an optical coupler is installed by the branching ratio and excess loss of the optical coupler. Therefore, when building an optical communication network using optical multiplexing / demultiplexing technology, the configuration must be designed in advance. The optical characteristics of the optical coupler must then be adjusted to achieve the designed optical characteristics.

[0055] Measuring and adjusting the optical characteristics of optical couplers requires the introduction of a light source and an optical power meter. However, in order to install a light source or an optical power meter in an optical fiber during communication, the optical fiber must be cut, and cutting the optical fiber interrupts communication.

[0056] On the other hand, according to this embodiment, when constructing an optical coupler by polishing the side surface of an optical fiber in communication, the optical characteristics of the optical coupler, such as the branching ratio, insertion loss, and excess loss, can be measured and adjusted without cutting the optical fiber. In other words, it is possible to evaluate the above-mentioned optical characteristics without interrupting communication.

[0057] REFERENCE SIGNS LIST 1 Optical fiber 2 Optical waveguide 3 Optical coupler 4, 4A, 4B Transmission device 5 Test light pulse tester 6 Bandpass filter 7 Test light blocking filter 8 Reflection-free termination 9 Optical power meter 10 Test light source 11 Bandpass filter 12 Optical path length adjuster 13 Alignment device 14 Display 20 Communication light 20a Leakage light 21 Communication light 21a Leakage light 22 Pulse light 22a Pulse light 22b Pulse light 24 Monitor device 25 Output light 25a Leakage light 50 Computer system 52 Memory f1, f2 Ports g1, g2 Ports M1 to M5 Bending section

Claims

1. A method for evaluating the optical characteristics of an optical coupler comprising an optical fiber that is placed in a state where communication is possible and has its side polished, and an optical waveguide that is also side polished and optically coupled to the optical fiber, comprising: connecting a non-reflection termination or a light blocking filter with a known reflectance to a first test light to one end of the optical waveguide; acquiring an OTDR waveform by inputting the first test light to the other end of the optical waveguide; and calculating the optical characteristics of the optical coupler from the change in the amount of reflection of the first test light that is indicated by a step appearing in the OTDR waveform.

2. The optical characteristic evaluation method according to claim 1, further comprising adjusting an optical path length between said one end of said optical waveguide and said non-reflection termination or said light blocking filter.

3. A method for evaluating optical characteristics according to claim 1 or 2, comprising: measuring a change in intensity of a second test light propagating through the optical fiber via the optical coupler while performing alignment between the optical fiber and the optical waveguide; and calculating an excess loss from the change in the amount of reflection of the first test light and the change in the intensity of the second test light.

4. A method for evaluating optical characteristics according to claim 1 or 2, comprising: measuring a change in intensity of the second test light propagating through the optical waveguide before it is input to the optical coupler and after it is output from the optical coupler; and calculating excess loss from the change in the amount of reflection of the first test light and the change in the intensity of the second test light.

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