JP2008268094A - Apparatus for measuring light propagation characteristics - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein an apparatus for measuring light propagation characteristics becomes large, a coupling optical system that includes lenses has susceptibility to vibrations, and it is difficult to implement stable measurements, since a coupling optical system which includes lenses needs to be used, when a light enters and exits from an object to be measured 1 in the apparatus. <P>SOLUTION: The light propagation characteristic measuring apparatus comprises a wavelength-variable light source 2; a light modulator 3 for modulating and outputting the output light from the light source; an oscillator 4 for inputting a signal from the light modulator 3 and outputting a modulation signal 31 at a predetermined frequency; a polarization-retaining optical fiber 5 connected to an output end of the light modulator 3 at one end and connected to a first optical fiber 21 at the other end; a third optical fiber 7 connected to a second optical fiber 22 at one end and connected to a photoelectric conversion element 6 at the other end; a phase comparator 8 for inputting a modulation signal 33, in synchronization with the modulation signal 31 and an output signal 32 from the photoelectric conversion element 6 and measuring the phase difference between them; and an output display 16 for displaying the output from the phase comparator 8. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a device for measuring the light propagation characteristics of an object to be measured in a small and vibration-resistant system.

  In an optical communication system, optical propagation characteristics such as group delay characteristics, chromatic dispersion characteristics, and polarization mode dispersion characteristics of optical elements and optical fibers are important for speeding up transmission signals and wavelength multiplexing transmission. Therefore, techniques for accurately measuring these characteristics are important for the construction of an optical communication system.

  Measurement of light propagation characteristics such as group delay characteristics of a device under test has been conventionally performed. For example, Patent Document 1 also describes a device for measuring a light propagation characteristic of an object to be measured. FIG. 8 shows a polarization mode dispersion characteristic measuring apparatus disclosed in Patent Document 1. In FIG. With reference to FIG. 8, a method for measuring the group delay characteristic, chromatic dispersion characteristic, and polarization mode dispersion characteristic of the device under test will be described.

  First, incident light is generated by the wavelength variable light source 2. The generated incident light propagates in the space, enters the optical modulator 3, is modulated by the modulation signal 31 having a predetermined frequency fm input from the oscillator 4, and is emitted from the optical modulation 3 into the free space. . The modulated light propagates in the space, propagates in the space via the polarization controller 14 and enters the device under test 1. The light transmitted through the DUT 1 propagates through the space, enters the photoelectric conversion element 6, and is converted into an electrical signal.

  Next, the phase comparator 8 measures the phase difference φ between the phase φr of the modulation signal 31 input to the optical modulator 3 and the phase φs of the electrical signal obtained by photoelectrically converting the transmitted light. That is, the phase difference φ = φs−φr. The group delay amount τ is obtained from the phase difference φ and the modulation frequency fm by the following equation: τ = φ / 2πfm.

  The group delay characteristic can be obtained by sweeping the wavelength of the light wave with the wavelength tunable light source 2 and measuring the group delay amount in association with the wavelength. Further, the chromatic dispersion characteristic D is obtained from the group delay characteristic τ by the equation D = ∂τ / ∂λ. Further, the polarization direction is controlled by using the polarization controller 14, the phase difference φ is measured, the maximum value φmax and the minimum value φmin of the phase difference are obtained, and the polarization mode dispersion characteristic PMD is obtained from these and the modulation frequency fm. Desired. Note that PMD = (φmax−φmin) / 2πfm. This may be recorded in association with the wavelength of the light source.

The light propagation characteristics of the object to be measured can be measured by the above method.
JP 2001-333708 A JP 2002-070512 A Japanese Patent Laid-Open No. 06-229870 JP-A-10-332537 Optical Network Analyzer Q7760 Instruction Manual (Advantest)

  In the method of Patent Document 1, the outgoing light from the optical modulator 3 propagates through the space and enters the device under test 1, and the outgoing light also propagates through the space and enters the photoelectric conversion element 6. For this reason, it is necessary to use a coupling optical system using a lens for entering and exiting modulated light to and from the device under test 1, and there is a problem that the apparatus becomes large. In addition, there is a problem that a coupling optical system using a lens is vulnerable to vibration and stable measurement is difficult.

  An optical network analyzer is commercially available as an apparatus for measuring the light propagation characteristics of an object to be measured having fibers at both ends. However, the optical network analyzer has a problem that the polarization of the optical output from the optical connector of the output unit is not stable. In addition, a method using a polarization controller is disclosed in Non-Patent Document 1 in order to control the polarization direction of the optical output, but there is a problem that the apparatus becomes large because the polarization controller is provided.

  The present invention includes a first optical connector 21 and a second optical connector 22, and the first light of the DUT 1 having a first optical fiber 9 and a second optical fiber 10 at both ends. A third optical connector 23 is connected to the end of the fiber 9 that is not connected to the device under test 1 and a third optical connector 23 is connected to the end of the second optical fiber 10 that is not connected to the device under test 1. 4 optical connectors 24, the first optical connector 21 and the third optical connector 23 are connected by connectors, and the second optical connector 22 and the fourth optical connectors 24 are connected by connectors. A light propagation characteristic measuring apparatus for measuring the light propagation characteristic of the device under test 1 having the configuration as described above, comprising: a tunable light source 2; and an optical modulation that outputs modulated light with the output light of the tunable light source 2 as input 3 and a predetermined frequency as an input signal of the optical modulator 3 A polarization-maintaining optical fiber 5 having one end connected to the output end of the optical modulator 3 and the other end connected to the first optical connector 21; A third optical fiber 7 connected to the second optical connector 22 and having the other end connected to the photoelectric conversion element 6; a modulation signal 33 synchronized with the modulation signal 31; and an output signal 32 of the photoelectric conversion element 6 And a phase comparator 8 that measures each phase difference, and an output display unit 16 that displays the output of the phase comparator 8.

  It is preferable that the first optical connector 21 and the third optical connector 23 include a rotation mechanism that relatively rotates.

  The data displayed on the output device 16 is preferably group delay characteristics and chromatic dispersion characteristics of the DUT 1.

  It is preferable that the data displayed on the output device 16 is the polarization mode dispersion characteristic of the device under test 1.

  According to the present invention, the third optical connector 23 connected to the device under test 1 via the first optical fiber 9 is connected to the first optical connector 21 and the device under test is measured. The fourth optical connector 24 connected to the first optical fiber 10 via the second optical fiber 10 is connected to the second optical connector 22 so that light is incident on and emitted from the object 1 to be measured. There is no need to use a coupling optical system, and the apparatus can be miniaturized. In addition, since a stable polarized output light can be obtained from the first optical connector 21 by the polarization-controlled optical fiber without using a coupling optical system by a lens, it is possible to perform a stable measurement that is resistant to vibration. It becomes.

  When the first optical connector 21 includes a rotation mechanism, it is possible to arbitrarily control the polarization direction of the modulated light incident on the DUT 1 by rotating the first optical connector 21. The polarization characteristic of the DUT 1 can be easily measured. At this time, since the polarization controller is not required for controlling the polarization direction, the apparatus can be miniaturized.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows the configuration of a light propagation characteristic measuring apparatus according to an embodiment of the present invention. An optical propagation characteristic measuring apparatus according to an embodiment of the present invention measures a group delay characteristic, a wavelength dispersion characteristic, and a polarization mode dispersion characteristic of an object to be measured. An optical propagation characteristic measuring apparatus according to an embodiment of the present invention includes a wavelength tunable light source 2, an optical modulator 3, an oscillator 4, a polarization maintaining optical fiber 5, an optical connector 21, an optical connector 22, a third optical fiber 7, and a photoelectric. It comprises a conversion element 6, a phase comparator 8, and an output display unit 16.

  A wavelength variable light source 2 is used as the light source. Incident light is generated by a light source capable of sweeping the wavelength λ, and the generated light is incident on the optical modulator 3.

  The oscillator 4 generates a modulation signal 31 having a predetermined frequency fm. The generated modulation signal 31 is input to the optical modulator 3. The phase of the modulation signal 31 at this time is φr.

  In the optical modulator 3, the incident light is modulated by the modulation signal 31 having the frequency fm. The light modulator 3 has lithium niobate (LN).

  Since the modulation degree of the LN optical modulator varies depending on the polarization of the incident light, the optical modulator 3 and the wavelength tunable light source 2 are connected by a polarization maintaining optical fiber to stabilize the polarization, and the polarization direction is further changed. It is necessary to match the direction of the Z axis of the LN crystal or the direction perpendicular to the Z axis.

  Instead of connecting the optical modulator 3 and the variable wavelength light source 2 with a polarization maintaining optical fiber, the polarization may be controlled using a polarization controller.

  The optical modulator 3 does not necessarily have LN, and may be, for example, an EA (Electro Absorption) modulator.

  Further, the wavelength variable light source 2 and the optical modulator 3 are not necessarily prepared separately, and a direct modulation type optical modulator that oscillates by applying a modulated electric signal directly to the laser light source may be used.

  A polarization maintaining optical fiber 5 is connected to the output end of the optical modulator 3, and light modulated by the optical modulator 3 propagates in the polarization maintaining optical fiber 5. The other end of the polarization maintaining optical fiber 5 is connected to the first optical connector 21, and the modulated light is output from the first optical connector 21.

  At this time, since the polarization maintaining optical fiber 5 is used, the polarization direction of the modulated light output from the optical modulator 3 is maintained even if the fiber is twisted or bent.

  Next, the first optical connector 21 and the third optical connector 23 of the DUT 1 are connected by a connector. Further, the fourth optical connector 24 is connected to the second optical connector 22 through a connector. In the present specification, the connector connection means that the two optical connectors are fitted and connected.

  The modulated light output from the first optical connector 21 is input to the third optical connector 23. The third optical connector 23 is connected to the device under test 1 via the first optical fiber 9, and the modulated light is input to the device under test 1. The second optical fiber 10 is connected to the output end of the device under test 1, and the modulated light transmitted through the device under test 1 propagates through the second optical fiber 10. A fourth optical connector 24 is connected to the other end of the second optical fiber 10, and modulated light propagating through the second optical fiber 10 is output from the fourth optical connector 24. The modulated light output from the fourth optical connector 24 is input to the second optical connector 22 and is incident on the third optical fiber 7 connected to the second optical connector 22.

  At this time, since the coupling optical system using the lens is not used for light incident / exit at the DUT 1, the apparatus can be miniaturized. In addition, since the output light having a stable polarization can be obtained from the first optical connector 21 by the polarization-maintained optical fiber without using the coupling optical system by the lens, it is possible to perform a stable measurement strong against vibration. .

  At this time, the polarization direction of the modulated light output from the first optical connector 21 is adjusted by changing the angle of the first optical connector 21 relative to the third optical connector 23. it can.

  If the first optical connector 21 is provided with a rotation mechanism and the first optical connector 21 is rotated in a state where the third optical connector 23 is fixed, the modulation incident on the DUT 1 as the rotation occurs. It is also possible to control the polarization direction of light. Thereby, modulated light having an arbitrary polarization direction can be incident on the device under test 1, and the polarization characteristics of the device under test 1 can be measured. At this time, since it is not necessary to use a polarization controller, the apparatus can be miniaturized.

  At this time, the first optical connector 21 is not necessarily provided with a rotation mechanism, and the first optical connector 21 may be fixed and the third optical connector 23 may be provided with a rotation mechanism. Further, both the first optical connector 21 and the third optical connector 23 may be provided with a rotation mechanism, and both the connectors may be rotated relatively.

  The third optical fiber 7 is connected to the photoelectric conversion element 6, and the modulated light transmitted through the DUT 1 is converted into an output signal 32 in the photoelectric conversion element 6. The phase of the output signal 32 at this time is φs.

  At this time, it is not always necessary to maintain the polarization direction, and the polarization maintaining optical fiber is not necessarily used for the third optical fiber 7.

  In the phase comparator 8, the phase difference φ between the output signal 32 and the modulation signal 33 synchronized with the modulation signal 31 is measured. That is, φ = φs−φr is measured.

  The optical group delay amount τ is obtained from the measured phase difference φ and the frequency fm of the modulation signal 31. The group delay amount τ can be expressed by the following equation: τ = φ / 2πfm. Thereby, the group delay amount τ can be obtained.

  Next, the operation of the embodiment according to the present invention will be described with reference to the flowchart of FIG.

  First, the wavelength of the wavelength tunable light source 2 is set to the lower limit of the measurement range. Then, the phase difference φ is measured by the above-described method, the group delay amount τ is obtained from the phase difference φ, and recorded in association with the wavelength of the wavelength tunable light source 2. Thereafter, the wavelength of the wavelength tunable light source 2 is increased, and it is checked whether the wavelength has reached the upper limit of the measurement range. If not, the phase difference is measured in the same manner as described above, the group delay amount is obtained and recorded, and the wavelength of the wavelength tunable light source 2 is increased. This is repeated until the wavelength reaches the upper limit of the measurement range, and the measurement is terminated when the upper limit is reached.

  The chromatic dispersion characteristic D can be obtained by differentiating the group delay characteristic τ with respect to the wavelength. That is, it can be obtained from the equation D = ∂τ / ∂λ. At this time, since the measurement value is discrete data, data processing such as difference is required.

  The measurement result is displayed on the output display unit 16. The output display may be a table or a graph. Moreover, the output display of the measured value may be performed sequentially every time the measured value is recorded, or may be displayed collectively after the measurement is completed. FIG. 9 (a) and Table 1 (a) show output display examples of the group delay characteristics, and FIG. 9 (b) and Table 1 (b) show output display examples of measurement results of the chromatic dispersion characteristics.

以上により、被測定物１の群遅延特性および波長分散特性を測定できる。

As described above, the group delay characteristic and the wavelength dispersion characteristic of the DUT 1 can be measured.

  Note that the wavelength of the wavelength tunable light source 2 does not necessarily need to be set to the lower limit of the measurement range at the start and gradually increased, but may be set to the upper limit of the measurement range at the start and sequentially shortened.

  When the first optical connector 21 is provided with a rotation mechanism and the polarization direction is controlled, the group delay characteristic and the chromatic dispersion characteristic can be measured for any polarization.

  When the first optical connector 21 is provided with a rotation mechanism and the polarization direction is controlled, the polarization mode dispersion characteristic can be measured according to the flowchart of FIG. This will be described below.

  First, the wavelength of the wavelength tunable light source 2 is set to the lower limit of the measurement range. Next, the first optical connector 21 is set to a predetermined angle, and the polarization direction of incident light is set to an initial value. Thereafter, the phase difference φ is measured and recorded by the method described above. Subsequently, after the first optical connector 21 is rotated by a predetermined angle with respect to the third optical connector 23 to change the polarization direction of the incident light, the phase difference φ is similarly recorded. The step of rotating the first optical connector 21 by a predetermined angle and recording the phase difference φ is repeated until the first optical connector 21 rotates 360 degrees.

  Next, the maximum value φmax and the minimum value φmin are obtained from the recorded φ. Here, φmax and φmin each correspond to a phase difference in orthogonal polarized waves, and therefore the polarization mode dispersion characteristic PMD is obtained from PMD = (φmax−φmin) / 2πfm. This is recorded in association with the wavelength of the wavelength tunable light source 2.

  Next, the wavelength is increased to check whether the wavelength has reached the upper limit of the measurement range. If the upper limit has not been reached, the polarization mode dispersion characteristic is measured and recorded in the same process as described above, and the measurement and recording are repeated by increasing the wavelength of the light source. If the wavelength of the wavelength tunable light source 2 reaches the upper limit, the measurement is terminated.

  The measurement result is displayed on the output display unit 16. The measurement value output display may be sequentially performed every time the measurement value is recorded, or may be collectively displayed after the measurement is completed. FIG. 9 (c) and Table 1 (c) show output display examples of measurement results of polarization mode dispersion characteristics.

  As described above, the wavelength characteristic of the polarization mode dispersion characteristic can be measured.

(Example)
A measurement example will be described in which measurement is performed using a device in which a fiber pigtail is connected to an input / output end of an LN waveguide having a wavelength dependency of the group delay amount as the DUT 1. The LN waveguide as the object to be measured is provided with a periodic change in the effective refractive index by periodically digging the grooves 11 in the upper portion of the optical waveguide 12 to form an uneven structure as shown in the figure.

  The transmission characteristics of the DUT 1 are shown in FIG. The figure shows the transmission characteristics of the DUT 1 from a wavelength of 1540 nm to 1555 nm, and has a light blocking range in the wavelength range of 1551 to 1558 nm, and further a localized mode near the wavelength of 1554.1 nm in the blocking range. The light transmission mode called is confirmed. In the localized mode, it is known that the group velocity of light decreases and the amount of group delay increases.

  Actually, a group delay characteristic is measured in a measurement range of 1540 nm to 1555 nm using a measurement system as shown in FIG.

  The measurement system includes a wavelength tunable light source 2, an optical modulator 3, an optical amplifier 17, a polarization maintaining optical fiber 5, a first optical connector 21, a second optical connector 22, a third optical fiber 7, and a photoelectric conversion element 6. , Network analyzer 18 and output display unit 16.

  First, the variable wavelength light source 2 is connected to the input end of the optical modulator 3 by a polarization maintaining optical fiber. Next, the output end of the optical modulator 3 is connected to the input end of the optical amplifier 17 with a polarization maintaining optical fiber. The output end of the optical amplifier 17 is connected to the polarization maintaining optical fiber 5, and the first optical connector 21 is connected to the other end of the polarization maintaining optical fiber. The second optical connector 22 is connected to the photoelectric conversion element 6 through the third optical fiber 7.

  A modulation signal 31 is input from the port 1 of the network analyzer 18 to the optical modulator 3 via the amplifier 19. The output signal 32 of the photoelectric conversion element 6 is input to the port 2 of the network analyzer 18. The network analyzer 18 measures the phase difference between the modulation signal 31 output from the port 1 signal and the output signal 32 input to the port 2. That is, they serve as the oscillator 4 and the phase comparator 8.

  As the wavelength variable light source 2, a wavelength variable LD light source is used. The light output from the light source is linearly polarized, the light intensity is 5 mW, and is kept constant when the wavelength is swept. The wavelength is first set to 1540 nm, which is the lower limit of the measurement range.

  The light modulator 3 is a 40 Gbit / s LN intensity modulator manufactured by Sumitomo Osaka Cement Co., Ltd. Since the wavelength tunable light source 2 and the optical modulator 3 are connected by a polarization-maintaining optical fiber, light having a stable polarization can be incident on the optical modulator 3 regardless of bending or twisting of the optical fiber. .

  As the modulation signal 31, a signal from the port 1 of the network analyzer 18 is amplified by the amplifier 19, and a signal having a frequency of 3 GHz and a signal intensity of 15 mW is input.

  The light emitted from the optical modulator 3 is amplified to 15 dBm by the optical amplifier 17 and then reaches the first optical connector 21 via the polarization maintaining optical fiber 5. The first optical connector 21 adjusts the angle so as to obtain a predetermined polarization while confirming the polarization direction of the outgoing light with an analyzer.

  Next, the first optical connector 21 and the third optical connector 23 are connected to each other, and modulated light is incident on the DUT 1 via the first optical fiber 9. Further, the fourth optical connector 24 and the second optical connector 22 are connected to each other, and the transmitted light of the DUT 1 is input to the photoelectric conversion element 6 through the third optical fiber 7. A photodiode is used as the photoelectric conversion element 6.

  In the network analyzer 18, the phase difference φ between the modulation signal 31 input to the optical modulator 3 and the photoelectrically converted output signal 32 is measured. The phase difference is multiplied by 1 / 2πfm to obtain the group delay amount, recorded in association with the wavelength of the light source, and displayed on the output display unit 16.

  Next, the wavelength is increased by 0.005 nm, the group delay amount is recorded in the same step, and the output display is updated. This is repeated until the wavelength reaches 1555 nm, and the group delay characteristics from 1540 nm to 1555 nm are measured.

  An example of the actual measurement results is shown in FIG. A group delay of about 15 psec was confirmed at the wavelength 1554.1 nm where the localized mode exists in the DUT 1, and the group delay characteristic could be measured.

  The light propagation characteristic measuring apparatus according to the present invention is useful for measuring light propagation characteristics such as group delay characteristics, wavelength dispersion characteristics, and polarization mode dispersion characteristics of optical elements such as optical fibers and optical waveguide elements.

The figure which shows the structure of the optical propagation characteristic measuring apparatus by this invention Flow chart showing a group delay characteristic measurement procedure according to the present invention. Flow chart showing a procedure for measuring polarization mode dispersion characteristics according to the present invention. Graph showing transmission characteristics of measured object The figure which shows an example of the measurement system of the group delay characteristic by this invention The graph which shows the group delay characteristic of the to-be-measured object measured with the measuring system by this invention The figure which shows the optical waveguide where the group velocity decrease is obtained The figure which shows the structure of the polarization mode dispersion characteristic measuring apparatus by patent document 1 The figure which shows the example of an output display of the light propagation characteristic measuring apparatus by this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Measured object 2 Wavelength variable light source 3 Optical modulator 4 Oscillator 5 Polarization maintaining optical fiber 6 Photoelectric conversion element 7 Third optical fiber 8 Phase comparator 9 First optical fiber 10 Second optical fiber 11 Groove 12 Light Waveguide 13 Intermediate section 14 Polarization controller 15 Polarization mode dispersion measurement section 16 Output display section 17 Optical amplifier 18 Network analyzer 19 Amplifier 21 First optical connector 22 Second optical connector 23 Third optical connector 24 Fourth light Connector 31 Modulation signal 32 Output signal 33 Modulation signal

Claims (4)

The first optical fiber 9 of the DUT 1 having the first optical connector 21 and the second optical connector 22 and having the first optical fiber 9 and the second optical fiber 10 at both ends is provided. A third optical connector 23 is connected to the end not connected to the device under test 1, and a fourth optical connector is connected to the end of the second optical fiber 10 not connected to the device under test 1. 24, the first optical connector 21 and the third optical connector 23 are connected by connectors, and the second optical connector 22 and the fourth optical connectors 24 are connected by connectors. , A light propagation characteristic measuring device for measuring the light propagation characteristic of the device under test 1,
A wavelength tunable light source 2;
An optical modulator 3 that outputs the modulated light using the output light of the wavelength tunable light source 2 as an input;
An oscillator 4 that outputs a modulation signal 31 having a predetermined frequency as an input signal of the optical modulator 3;
A polarization maintaining optical fiber 5 having one end connected to the output end of the optical modulator 3 and the other end connected to the first optical connector 21;
A third optical fiber 7 having one end connected to the second optical connector 22 and the other end connected to the photoelectric conversion element 6;
A phase comparator 8 that inputs a modulation signal 33 synchronized with the modulation signal 31 and an output signal 32 of the photoelectric conversion element 6 and measures the respective phase differences;
An optical propagation characteristic measuring apparatus comprising an output display unit 16 for displaying the output of the phase comparator 8. The optical propagation characteristic measuring apparatus according to claim 1, further comprising a rotation mechanism that relatively rotates the first optical connector (21) and the third optical connector (23). The optical propagation characteristic measuring device according to claim 1 or 2, wherein the data displayed on the output device (16) is a group delay characteristic and a chromatic dispersion characteristic of the device under test (1). The light propagation characteristic measuring device according to claim 1 or 2, wherein the data displayed on the output device (16) is a polarization mode dispersion characteristic of the device under test (1).

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