JP6586186B1 - Wavelength sweep light source, OFDR apparatus using the same, and measurement method - Google Patents

Abstract

A wavelength swept light source capable of accurately realizing polarization compensation at low cost in an OFDR apparatus, an OFDR apparatus using the same, and a measurement method are provided. In a wavelength swept light source that repeatedly changes the wavelength of light emitted from a semiconductor laser at a predetermined sweep cycle, the light emitted from the semiconductor laser at an optical rotation angle corresponding to the intensity of the applied magnetic field. A Faraday element 18 that rotates the light, an output side optical fiber 21 that transmits the light rotated by the Faraday element 18 to the outside as output light, an electromagnet 19 that applies a magnetic field to the Faraday element 18, and the Faraday element 18 by the electromagnet 19. An optical rotation angle control unit 20 configured to change the intensity of the applied magnetic field and switch the optical rotation angle alternately between the first angle and the second angle in synchronization with a predetermined sweep cycle, and the first angle And the second angle is 90 degrees. [Selection] Figure 1

Description

  The present invention relates to a wavelength swept light source, an OFDR apparatus using the same, and a measurement method.

  Conventionally, an OFDR apparatus that measures a strain distribution or a temperature distribution of a sensor fiber using an optical frequency domain reflectometry (OFDR) is known (for example, see Patent Document 1). In the OFDR apparatus, the reference light and the reflected light from the sensor fiber are caused to interfere using an optical fiber or a fiber coupler.

  As shown in FIG. 11, the conventional OFDR apparatus includes a fiber coupler 51 and a circulator 52 that branch the output light from the wavelength swept light source 50 into measurement light and reference light and input the measurement light to the sensor fiber 100. A measurement interferometer 54 including a fiber coupler 53 that outputs interference light by combining reflected light from the sensor fiber 100 to which measurement light is input and reference light, and output from the measurement interferometer 54 And a light receiver 55 that converts the interference light into an interference signal as an electrical signal.

  In order to acquire the above interference signal with high S / N, polarization compensation is essential. As a polarization compensation method, there is a method in which a polarization maintaining fiber is used for all optical fibers constituting the measurement interferometer 54 including the sensor fiber 100 in the configuration of FIG. The influence on the sensitivity of the measurement interferometer 54 is large. Since the polarization extinction ratio of a commercially available polarization maintaining fiber coupler is about 20 dB, even if only the light passing through the fast axis of the polarization maintaining fiber is intended to interfere, the interference signal has a fast axis. Interference signals due to light propagating along the slow axis are also superimposed. Therefore, a sideband wave having an intensity of about −20 dB is generated on the side of the interference signal between the fast axes. Further, polarization maintaining fibers and polarization maintaining fiber couplers are very expensive.

  In order to avoid this sideband, when a single mode fiber is used as the optical fiber constituting the sensor fiber 100 and the measurement interferometer 54, for example, two methods shown in FIGS. 12 and 13 can be used. it can. In the method shown in FIG. 12, the reflected light from the sensor fiber 100 is divided into two orthogonal polarizations by the polarization separator 56, and the reference lights whose polarization states are adjusted by the polarization controllers 57a and 57b, respectively. And compensating for polarization by synthesizing respective interference signals. However, since the configuration of FIG. 12 requires two light receivers 55a and 55b, there is a problem that costs are increased.

On the other hand, in the method shown in FIG. 13, an optical rotator 58 is inserted between the wavelength swept light source 50 and the sensor fiber 100, and the polarization state of the light incident on the sensor fiber 100 is switched on the time axis. This is a method of compensating for polarization by synthesizing interference signals on the time axis. The rotator 58 is for switching to either of two polarization states in the polarization state of the incident light into a positional relationship rotated 180 degrees about the S ₃ axis (90 ° optical rotation) on the Poincare sphere .

  For example, as shown in FIG. 14A, if the light incident on the optical rotator 58 is linearly polarized light, the two polarization states corresponding to the positions of the counter electrodes on the Poincare sphere, that is, the polarization states are orthogonal to each other. The two linearly polarized lights can be switched from the time axis and emitted from the optical rotator 58. As the optical rotator 58, an inexpensive one using a Faraday element can be used. The method of FIG. 13 can be realized at low cost because only one light receiver 55 is required.

Japanese Patent No. 4122291

  However, in the configuration of FIG. 13, the emission fiber of the wavelength swept light source 50, the incident fiber of the optical rotator 58, the fiber couplers 51, 53, etc. are each several tens of centimeters or more. The polarization is rotated by such as. For this reason, even if the wavelength swept light source 50 emits linearly polarized light, the light may be elliptically polarized before reaching the optical rotator 58. As shown in FIG. 14B, the polarization state cannot be changed to the position of the counter electrode on the Poincare sphere even if the elliptically polarized light is rotated by 90 degrees. It cannot be switched on the axis and emitted from the optical rotator 58. For this reason, in the configuration in which the optical rotator 58 is inserted between the wavelength swept light source 50 and the sensor fiber 100 shown in FIG.

  The present invention has been made to solve such a conventional problem, and provides a wavelength swept light source capable of accurately realizing polarization compensation at low cost in an OFDR apparatus, an OFDR apparatus using the same, and a measurement method. The purpose is to provide.

In order to solve the above problems, a wavelength swept light source according to the present invention comprises a semiconductor laser and wavelength swept means that repeatedly changes the wavelength of light emitted from the semiconductor laser at a predetermined sweep period. in the transmission, before SL receives the light air propagated emitted from the semiconductor laser, a Faraday element which optical rotation in angle of rotation corresponding to the intensity of the applied magnetic field, the external optical rotation has been light by the Faraday element as the output light Output side optical fiber, magnetic field applying means for applying a magnetic field to the Faraday element, and changing the intensity of the magnetic field applied to the Faraday element by the magnetic field applying means to synchronize with the predetermined sweep period. An optical rotation angle control unit that performs control to alternately switch the optical rotation angle between the first angle and the second angle, and the difference between the first angle and the second angle is 90 degrees It is a certain configuration.

  With this configuration, the wavelength swept light source according to the present invention can switch the angle of the linearly polarized light emitted from the semiconductor laser by 90 degrees before being affected by the stress of the optical fiber. Can be used to realize accurate polarization compensation at low cost in the OFDR apparatus.

  Moreover, in the wavelength swept light source according to the present invention, the magnetic field applying means is an electromagnet, and the optical rotation angle control unit is a drive for generating a magnetic field that makes the optical rotation angle the first angle or the second angle. A configuration including a variable current source that supplies current to the magnetic field applying unit may be employed.

  With this configuration, the wavelength swept light source according to the present invention can switch the optical rotation angle of the Faraday element to the first angle or the second angle by controlling the driving current of the electromagnet with the optical rotation angle control unit.

  In the wavelength-swept light source according to the present invention, the Faraday element has a first angle of 45 degrees when a magnetic field in a first direction having a larger absolute value than a saturation magnetic field is applied from the magnetic field applying unit. In addition, the second angle is −45 degrees by applying a magnetic field in the second direction opposite to the first direction having an absolute value larger than the saturation magnetic field from the magnetic field applying unit. Also good.

  With this configuration, the wavelength swept light source according to the present invention can rotate the light emitted from the semiconductor laser at 45 degrees or −45 degrees.

  In the wavelength sweep light source according to the present invention, the normal line of the Faraday element may form an angle other than 0 degrees with respect to the optical path of the light emitted from the semiconductor laser.

  With this configuration, the wavelength swept light source according to the present invention can suppress the light from the semiconductor laser reflected by the surface of the Faraday element from returning to the semiconductor laser side.

  In the wavelength-swept light source according to the present invention, the Faraday element maintains the first angle of 45 degrees or the second angle of −45 degrees with a magnetic field having an absolute value smaller than the coercive force. It may be a configuration.

  With this configuration, the wavelength swept light source according to the present invention can shorten the supply time of the drive current supplied to the electromagnet, and thus can suppress the heat generation of the electromagnet and suppress the fluctuation of the optical rotation angle due to the temperature.

  In the wavelength swept light source according to the present invention, the semiconductor laser has one end surface of two light emitting end surfaces having a low reflectivity surface compared to the other end surface, and the wavelength sweeping means includes the semiconductor laser A diffraction grating that diffracts light emitted from the one end face of the laser at an angle corresponding to the wavelength; a rotating mirror that reflects the light diffracted by the diffraction grating and returns the light to the diffraction grating; and the rotation The angle of the rotating mirror with respect to the diffraction grating is set at a predetermined sweep period so that the light returned from the mirror to the diffraction grating is again diffracted by the diffraction grating and incident on the one end face of the semiconductor laser. And a resonator length from the other end face of the semiconductor laser to the rotating mirror via the diffraction grating changes according to the change in the angle of the rotating mirror. Accordingly, the wavelength of light emitted from the end surface of the other of the Faraday element may be configured to repeatedly changed in the predetermined sweep period.

  With this configuration, the wavelength swept light source according to the present invention can repeatedly change the wavelength of light emitted from the semiconductor laser to the Faraday element at a predetermined sweep period.

  Also, an OFDR apparatus according to the present invention includes the wavelength swept light source according to any one of the above, and the output light from the wavelength swept light source branched into measurement light and reference light, and the measurement light is supplied to the measured optical fiber. An optical branching means for inputting the reference light; a polarization controller for changing the polarization state of the reference light branched by the optical branching means; and the predetermined polarization of the reference light whose polarization state has been changed by the polarization controller An optical signal that outputs interference light by combining a polarizer that transmits a directional component, the reference light that has passed through the polarizer, and reflected light from the optical fiber to be measured to which the measurement light is input. Wave means; a light receiver that converts the interference light into an electrical signal; an A / D converter that converts the electrical signal into a digital signal; and the digital signal based on the digital signal at each position of the optical fiber to be measured. A reflection measuring unit that measures the intensity of incident light, and the polarization controller transmits the reference light through the polarizer when the optical rotation angle is the first angle and when the optical rotation angle is the second angle. The polarization state of the reference light is changed so that the amount of light becomes the same, and the reflection measurement unit performs Fourier transform on the digital signal to calculate a spectrum of the interference light. And the interference light spectrum calculated by the Fourier transform unit when the optical rotation angle is the first angle, and the interference calculated by the Fourier transform unit when the optical rotation angle is the second angle. And an addition processing unit for adding the light spectrum.

  With this configuration, the OFDR apparatus according to the present invention can perform accurate polarization compensation at low cost.

  Further, the measurement method according to the present invention includes a light branching unit that branches the output light from the wavelength swept light source described above into measurement light and reference light, and places the measurement light in the optical fiber to be measured. A step of inputting, a polarization state changing step of changing a polarization state of the reference light branched by the optical branching means by a polarization controller, and the reference in which the polarization state is changed by the polarization state changing step Transmitting a component of a predetermined polarization direction of light with a polarizer, combining the reference light transmitted through the polarizer, and reflected light from the optical fiber to be measured to which the measurement light is input. Outputting the interference light in step, converting the interference light into an electrical signal, converting the electrical signal into a digital signal, and measuring the measured signal based on the digital signal. A reflection measurement step of measuring the intensity of the reflected light at each position of the optical fiber, and the polarization state changing step includes a case where the optical rotation angle is the first angle and a case where the optical rotation angle is the second angle Then, the polarization state of the reference light is changed so that the amount of the reference light transmitted through the polarizer becomes the same amount, and the reflection measurement step Fourier transforms the digital signal, A Fourier transform step for calculating a spectrum of the interference light, a spectrum of the interference light calculated by the Fourier transform step when the optical rotation angle is the first angle, and a case where the optical rotation angle is the second angle Adding the spectrum of the interference light calculated by the Fourier transform step.

  With this configuration, the measurement method according to the present invention can perform accurate polarization compensation at low cost.

  The present invention provides a wavelength swept light source capable of accurately realizing polarization compensation at low cost in an OFDR apparatus, an OFDR apparatus using the same, and a measurement method.

It is a block diagram of the wavelength sweep light source which concerns on embodiment of this invention. It is a graph which shows the relationship between the magnetic field applied to the Faraday element in the wavelength sweep light source which concerns on embodiment of this invention, and an optical rotation angle. It is a figure which shows the Poincare sphere for demonstrating the change of the polarization state of the output light of the wavelength sweep light source which concerns on embodiment of this invention. It is a graph which shows the relationship between the timing of the sweep period of the wavelength sweep light source which concerns on embodiment of this invention, and the switching timing of the optical rotation angle of a Faraday element. It is a block diagram of an OFDR apparatus provided with the wavelength sweep light source which concerns on embodiment of this invention. It is a graph for demonstrating adjustment of the polarization controller of an OFDR apparatus provided with the wavelength sweep light source which concerns on embodiment of this invention. It is a graph for demonstrating the sampling timing in an OFDR apparatus provided with the wavelength sweep light source which concerns on embodiment of this invention. It is a graph which shows the power of the interference light before polarization compensation obtained by the OFDR apparatus provided with the wavelength sweep light source which concerns on embodiment of this invention. It is a graph which shows the power of the interference light after the polarization compensation obtained by the OFDR apparatus provided with the wavelength sweep light source which concerns on embodiment of this invention. It is a flowchart which shows the process of the measuring method using the wavelength swept light source which concerns on embodiment of this invention. It is a block diagram (the 1) of the conventional OFDR apparatus. It is a block diagram (the 2) of the conventional OFDR apparatus. It is a block diagram (the 3) of the conventional OFDR apparatus. It is a figure which shows the Poincare sphere for demonstrating the change of the polarization state of the output light of the wavelength sweep light source in the conventional OFDR apparatus.

  Hereinafter, embodiments of a wavelength swept light source, an OFDR apparatus using the same, and a measurement method according to the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the wavelength swept light source 10 according to the present embodiment includes a configuration of, for example, a Litman-type external resonator light source, and includes a semiconductor laser 11, a collimator lens 12, a diffraction grating 13, and a circuit. A moving mirror 14, a sweep drive unit 15, a condenser lens 16, an isolator 17, a Faraday element 18, an electromagnet 19 as a magnetic field application unit, an optical rotation angle control unit 20, and an output side optical fiber 21. Prepare mainly. The wavelength swept light source 10 uses a wavelength sweep unit including a collimator lens 12, a diffraction grating 13, a rotating mirror 14, and a sweep drive unit 15 to change the wavelength of linearly polarized light emitted from the semiconductor laser 11 at a predetermined sweep period. It is designed to change repeatedly.

  The semiconductor laser 11 emits linearly polarized laser light from the two light emitting end faces 11a and 11b, and one end face 11a of the two light emitting end faces 11a and 11b is lower than the other end face 11b. It is a reflectance surface. One end surface 11a is a low reflectance surface by AR coating, for example.

  The collimating lens 12 is a lens that collimates light emitted from one end face 11 a of the semiconductor laser 11. The diffraction grating 13 diffracts the light from the semiconductor laser 11 collimated by the collimating lens 12 at an angle corresponding to the wavelength. The rotating mirror 14 reflects the light diffracted by the diffraction grating 13 and returns it to the diffraction grating 13.

  The sweep drive unit 15 is configured so that the light returned from the rotating mirror 14 to the diffraction grating 13 is again diffracted by the diffraction grating 13 and incident on one end face 11 a of the semiconductor laser 11 through the collimator lens 12. The angle of the rotating mirror 14 with respect to the diffraction grating 13 is changed at a predetermined sweep cycle. In addition, the sweep drive unit 15 outputs a sweep signal a including information on the timing of the sweep cycle to the optical rotation angle control unit 20 and a reflection measurement unit 34 described later.

  As the resonator length from the other end face 11b of the semiconductor laser 11 through the diffraction grating 13 to the turning mirror 14 changes according to the change in the angle of the turning mirror 14, as shown in the upper part of FIG. The wavelength of light emitted from the end face 11b to the Faraday element 18 repeatedly changes sinusoidally at a predetermined sweep period T.

  The wavelength swept light source 10 is not limited to the above-described Littman type, and may be an arbitrary configuration such as a Littrow type, a Fabry-Perot type, a ring laser type, or a DBR (Distributed Bragg Reflector) type.

  The Faraday element 18 is made of a magneto-optical crystal such as a bismuth-substituted rare earth iron garnet single crystal, and rotates the light emitted from the other end face 11b of the semiconductor laser 11 at an optical rotation angle corresponding to the intensity of the applied magnetic field. It has become.

  The electromagnet 19 applies a magnetic field to the Faraday element 18, and is disposed so as to apply a magnetic field having a component parallel to the traveling direction of light from the semiconductor laser 11 that passes through the Faraday element 18, for example. Note that when the direction of the magnetic field applied to the Faraday element 18 changes in the opposite direction, the direction of change in the optical rotation angle also changes. Hereinafter, the magnetic field in the direction from the semiconductor laser 11 toward the Faraday element 18 is also referred to as “first direction magnetic field”, and the magnetic field in the direction from the Faraday element 18 toward the semiconductor laser 11 is also referred to as “second direction magnetic field”.

  The optical rotation angle control unit 20 changes the intensity of the magnetic field applied to the Faraday element 18 by the electromagnet 19 according to the sweep signal a, and sets the optical rotation angle to the first angle and the second angle in synchronization with a predetermined sweep cycle. Control to switch alternately is performed. The optical rotation angle control unit 20 includes a variable current source 20a that supplies the electromagnet 19 with a drive current for generating a magnetic field that makes the optical rotation angle the first angle or the second angle. Here, the difference between the first angle and the second angle is 90 degrees. In addition, the optical rotation angle control unit 20 outputs a control signal including information on the timing of switching the optical rotation angle to the reflection measurement unit 34 described later.

  As the Faraday element 18, for example, when a magnetic field in the first direction having a larger absolute value than the saturation magnetic field is applied from the electromagnet 19, the first angle becomes 45 degrees and the first magnetic field has a larger absolute value than the saturation magnetic field. It is convenient to use a magnetic field in which the second angle is −45 degrees by applying a magnetic field in the second direction opposite to the one direction from the electromagnet 19.

  Further, as the Faraday element 18, a magnetic field having an absolute value smaller than its coercive force and maintaining a first angle of 45 degrees or a second angle of −45 degrees can be used. For example, a pulsed magnetic field having a duty ratio of less than 50% (for example, about 5%) as shown in FIG. 2 may be applied to such a Faraday element 18, and the drive current is supplied to the electromagnet 19. Since the time can be shortened, the heat generation of the electromagnet 19 can be suppressed and fluctuations in the angle of rotation can be suppressed.

  A condensing lens 16 that condenses the light emitted from the other end surface 11b of the semiconductor laser 11 toward the output-side optical fiber 21 between the other end surface 11b of the semiconductor laser 11 and the Faraday element 18; An isolator 17 that prevents light from the semiconductor laser 11 that has passed through the condenser lens 16 from returning to the semiconductor laser 11 side is disposed.

  According to the above configuration, since the light from the semiconductor laser 11 is rotated while propagating in the air, the polarization state of the light from the semiconductor laser 11 is changed to the Poincare sphere before being affected by the stress of the optical fiber. It becomes possible to switch to one of two polarization states corresponding to the position of the upper counter electrode. For example, the polarization state of the light from the semiconductor laser 11 corresponding to point A on the equator of the Poincare sphere in FIG. 3 (rotation angle 0 degree) corresponds to point B (rotation angle 45 degrees) or point C on the equator. The state is switched to one of the polarization states (the angle of rotation -45 degrees).

  FIG. 4 is a graph showing an example of the timing of the sweep cycle and the timing of switching the optical rotation angle. In this example, the optical rotation angle is switched every sweep cycle T, the first angle is 45 degrees, and the second angle is −45 degrees. Note that the timing of switching the optical rotation angle is not limited to the above, and may be any timing such as every half of the sweep cycle T, for example. Further, hereinafter, the polarization state of the light rotated by the Faraday element 18 at an angle of rotation of 45 degrees is “P-polarized light”, and the polarization state of the light rotated by the Faraday element 18 at an angle of rotation of −45 degrees is “ S-polarized light.

  The output side optical fiber 21 is a single mode fiber that transmits light rotated by the Faraday element 18 to the outside as output light of the wavelength swept light source 10. The output side optical fiber 21 is inserted into the fiber hole 22 a of the cylindrical ferrule 22. The output side optical fiber 21 is fixed to the ferrule 22 by, for example, adhesive fixing with an adhesive provided in the fiber hole 22a, fixing by press-fitting into the fiber hole 22a, or the like.

  The end surface 22b of the ferrule 22 is subjected to oblique polishing such as APC (Angled Physical Contact) polishing that forms an inclined surface inclined about 7 to 9 degrees with respect to a virtual vertical plane perpendicular to the axis of the fiber hole 22a. It may be. The tip end surface of the output side optical fiber 21 fixed to the ferrule 22 is aligned with the end surface 22 b of the ferrule 22. By sticking and fixing the Faraday element 18 to the end face 22b thus obliquely polished, the normal line of the Faraday element 18 is 7 with respect to the optical path of the light emitted from the other end face 11b of the semiconductor laser 11. An angle other than 0 degrees of about 9 degrees will be formed. Thereby, it can suppress that the light from the semiconductor laser 11 reflected on the surface of the Faraday element 18 returns to the semiconductor laser 11 side.

  FIG. 5 shows the distortion of the DUT 110 by observing the optical signal from the sensor fiber 100 in a state where the sensor fiber 100 as the optical fiber to be measured is attached to the device under test (DUT) 110. It is a figure which shows the structural example of OFDR apparatus 30 which measures temperature.

  The OFDR device 30 includes the wavelength swept light source 10 according to the present embodiment, a measurement interferometer 31, a light receiver 32, an A / D converter 33, a reflection measurement unit 34, and a display unit 35.

  The measurement interferometer 31 is a fiber coupler as an optical branching unit that branches the output light transmitted from the output side optical fiber 21 of the wavelength swept light source 10 into measurement light and reference light and inputs the measurement light to the sensor fiber 100. 36, a circulator 37, a polarization controller 38 that changes the polarization state of the reference light branched by the fiber coupler 36, and a component in a predetermined polarization direction of the reference light whose polarization state has been changed by the polarization controller 38. And a polarizer 39 to transmit.

  The polarization controller 38 adjusts the reference light so that the amount of the reference light transmitted through the polarizer 39 is the same when the angle of rotation of the Faraday element 18 is the first angle and when the angle is the second angle. It has been adjusted in advance to change the polarization state. For example, this adjustment is performed so that the direction of linearly polarized light output from the polarization controller 38 is inclined by 45 degrees with respect to the polarization axis of the polarizer 39. Therefore, the light emitted from the polarizer 39 is always linearly polarized light that coincides with the polarization axis of the polarizer 39 regardless of whether the optical rotation angle of the Faraday element 18 is the first angle or the second angle. The amount of light is constant.

  For example, in a situation where the P-polarized light and the S-polarized light of the reference light branched by the fiber coupler 36 are switched at the timing shown in FIG. 6A, the output light amount of the polarizer 39 before adjusting the polarization controller 38 is shown. 6 (b), the polarization controller 38 adjusts so that the output light amount of the polarizer 39 is constant regardless of the timing of switching the optical rotation angle, as shown in FIG. 6 (c). Has been.

  Further, the measurement interferometer 31 is an optical multiplexing unit that outputs interference light by combining the reflected light from the sensor fiber 100 to which the measurement light is input and the reference light transmitted through the polarizer 39. A circulator 37 and a fiber coupler 40 are included.

  That is, the output light is split into two beams, the measurement light and the reference light, by the fiber coupler 36, and the measurement light propagates through the sensor fiber 100 after passing through the circulator 37. The measurement light propagating through the sensor fiber 100 is reflected at each point of the sensor fiber 100. This reflected light propagates again through the sensor fiber 100, passes through the circulator 37, and enters the fiber coupler 40. On the other hand, the reference light branched by the fiber coupler 36 passes through the polarization controller 38 and the polarizer 39 and enters the fiber coupler 40.

  The sensor fiber 100 is a single mode fiber. Each optical component on the optical paths of the measurement light, reflected light, and reference light from the fiber coupler 36 to the fiber coupler 40 is also connected by a single mode fiber. For this reason, when the optical rotation angle of the light emitted from the wavelength swept light source 10 is the first angle and the second angle, the polarization state of the reflected light from the sensor fiber 100 input to the fiber coupler 40 is It corresponds to the position of the counter electrode on the Poincare sphere.

  The light receiver 32 includes a photodiode (PD) that outputs an electric signal proportional to the intensity of the input light, and converts the interference light output from the measurement interferometer 31 into an electric signal (hereinafter also referred to as “interference signal”). It is supposed to be. The frequency f of the interference signal is a difference in optical frequency between the reflected light from the sensor fiber 100 and the reference light transmitted through the polarizer 39.

  The A / D converter 33 converts the analog interference signal output from the light receiver 32 into a digital signal in response to an instruction signal c from the reflection measuring unit 34 described later. As shown in FIG. 7, the interference signal is sampled by the A / D converter 33 in synchronization with the sweep cycle. FIG. 7 shows an example in which sampling is performed at the time of sweeping from a short wavelength to a long wavelength, but the present invention is not limited to this, and the timing of sampling is arbitrary. For example, sampling may be performed during a sweep from a long wavelength to a short wavelength, or may be performed during both a sweep from a short wavelength to a long wavelength and a sweep from a long wavelength to a short wavelength. Good.

  The reflection measuring unit 34 measures the intensity of the reflected light at each position of the sensor fiber 100 based on the digital signal output from the A / D converter 33. Part 42 and addition processing part 43. The reflection measuring unit 34 A / D converts the instruction signal c for instructing the sampling timing based on the sweep signal a from the sweep driving unit 15 and the control signal b from the optical rotation angle control unit 20. Is output to the device 33.

  The Fourier transform unit 41 calculates the spectrum of interference light by performing fast Fourier transform (FFT) on the digital signal output from the A / D converter 33.

  For example, the optical path length of the fiber coupler 36 → the circulator 37 → the incident end of the sensor fiber 100 → the circulator 37 → the fiber coupler 40 matches the optical path length of the fiber coupler 36 → the polarization controller 38 → the polarizer 39 → the fiber coupler 40. As described above, if the fiber length of the measurement interferometer 31 is adjusted, the signal intensity at the frequency 0 of the spectrum of the interference light is proportional to the intensity of the reflected light from the incident end of the sensor fiber 100.

In addition, the optical frequency sweep speed of the wavelength swept light source 10 is V [Hz / s], the light velocity is c [m / s], the refractive index of the sensor fiber 100 is n, and the sensor fiber 100 from the incident end of the sensor fiber 100. If the distance to each point is Z [m], the frequency f [Hz] of the interference light spectrum can be expressed by the following equation (1). According to Expression (1), it is understood that the frequency f of the spectrum of the interference light is proportional to the position Z in the sensor fiber 100 if the optical frequency sweep speed V is assumed to be constant.
f (Z) = V × 2nZ / c (1)

  However, the frequency of the light output from the actual wavelength swept light source 10 is swept nonlinearly or modulated. As a result, the spectrum of the interference light is blurred or has sidebands, which hinders highly accurate measurement.

  Therefore, it is desirable to remove the influence of the nonlinearity of the wavelength sweep by the wavelength swept light source 10 by performing a known resampling process on the digital signal output from the A / D converter 33. In this case, the Fourier transform unit 41 performs Fourier transform on the digital signal from the A / D converter 33 that has been subjected to the resampling process, and calculates the spectrum of the interference light.

  FIG. 8 shows the power of the interference light obtained by converting the frequency of the spectrum of the interference light into the position Z in the sensor fiber 100 according to the equation (1). As shown in the upper part of FIG. 8, in the region where the signal intensity is reduced when the light emitted from the wavelength swept light source 10 is P-polarized light (hereinafter also referred to as “P-polarized light”), The polarization state of the reflected light is orthogonal to the polarization state of the reference light transmitted through the polarizer 39. On the other hand, as shown in the lower part of FIG. 8, when the light emitted from the wavelength swept light source 10 is S-polarized light (hereinafter also referred to as “S-polarized”), the polarization of reflected light from the sensor fiber 100. Since the state is not orthogonal to the polarization state of the reference light transmitted through the polarizer 39, the power of the interference light is increased in the same region as the upper stage of FIG.

  The storage unit 42 is configured to store the spectrum data of the interference light calculated by the Fourier transform unit 41 based on the control signal b from the optical rotation angle control unit 20 in association with the optical rotation angle.

  The addition processing unit 43 calculates the interference light spectrum calculated by the Fourier transform unit 41 when the optical rotation angle is the first angle and the interference light spectrum calculated by the Fourier transform unit 41 when the optical rotation angle is the second angle. Are read from the storage unit 42 and added together. FIG. 9 shows an example of a result obtained by performing polarization compensation by adding the power of interference light at the time of P polarization obtained by the Fourier transform unit 41 and the power of interference light at the time of S polarization by the addition processing unit 43. It is a graph to show. As shown in FIG. 9, the power of the interference light after the addition processing by the addition processing unit 43 becomes almost constant without depending on the position Z.

  The display unit 35 is configured by a display device such as an LCD or a CRT, for example, and displays the power of interference light after addition processing obtained by the addition processing unit 43 and the like.

  Hereinafter, an example of the process of the measurement method using the wavelength swept light source 10 according to the present embodiment will be described with reference to the flowchart of FIG.

  First, the optical rotation angle control unit 20 sets the optical rotation angle of the Faraday element 18 to the first angle according to the sweep signal a (step S1).

  Next, the fiber coupler 36 and the circulator 37 as the optical branching means branch the output light from the wavelength swept light source 10 into measurement light and reference light and input the measurement light to the sensor fiber 100 (step S2).

  Next, the polarization controller 38 changes the polarization state of the reference light branched by the fiber coupler 36 (polarization state change step S3).

  Next, the polarizer 39 transmits the component in the predetermined polarization direction of the reference light whose polarization state has been changed in the polarization state changing step S3 (step S4). Note that the polarization state changing step S3 described above is performed so that the amount of reference light transmitted through the polarizer 39 is the same amount when the optical rotation angle is the first angle and when it is the second angle. It changes the polarization state of light.

  Next, the circulator 37 and the fiber coupler 40 as optical multiplexing means combine the reference light transmitted through the polarizer 39 and the reflected light from the sensor fiber 100 to which the measurement light is input, thereby generating interference light. Output (step S5).

  Next, the light receiver 32 converts the interference light into an electrical signal (step S6).

  Next, the A / D converter 33 converts the electrical signal into a digital signal (step S7).

  Next, the Fourier transform unit 41 performs Fourier transform on the digital signal to calculate the spectrum of the interference light (Fourier transform step S8).

  Next, the reflection measuring unit 34 determines whether or not the spectrum of the interference light when the optical rotation angle is the second angle is calculated in the Fourier transform step S8 (step S9). If the determination is negative, the process proceeds to step S10. If the determination is affirmative, the process proceeds to step S11.

  In step S10, the optical rotation angle control unit 20 sets the optical rotation angle of the Faraday element 18 to the second angle according to the sweep signal a, and returns to step S2.

  In step S11, the addition processing unit 43 calculates the interference light spectrum calculated by the Fourier transform step S8 when the optical rotation angle is the first angle and the interference calculated by the Fourier transform step S8 when the optical rotation angle is the second angle. Add the spectrum of light.

  Steps S8 and S11 constitute a reflection measurement step for measuring the intensity of the reflected light at each position of the sensor fiber 100 based on the digital signal.

  As described above, the wavelength swept light source 10 according to the present embodiment can switch the angle of the linearly polarized light emitted from the semiconductor laser 11 by 90 degrees before being affected by the stress of the optical fiber. When used as a wavelength swept light source of the apparatus, accurate polarization compensation can be realized at low cost in the OFDR apparatus.

  Further, the wavelength swept light source 10 according to the present embodiment can switch the optical rotation angle of the Faraday element 18 to the first angle or the second angle by controlling the driving current of the electromagnet 19 by the optical rotation angle control unit 20.

  Further, the wavelength swept light source 10 according to the present embodiment can rotate the light emitted from the semiconductor laser 11 at 45 degrees or −45 degrees.

  Further, in the wavelength swept light source 10 according to this embodiment, the normal line of the Faraday element 18 forms an angle other than 0 degrees with respect to the optical path of the light emitted from the semiconductor laser 11. It can suppress that the light from the semiconductor laser 11 reflected on the surface returns to the semiconductor laser 11 side.

  Further, the wavelength swept light source 10 according to the present embodiment is such that the Faraday element 18 can maintain an optical rotation angle of 45 degrees or an optical rotation angle of −45 degrees with a magnetic field whose absolute value is smaller than its coercive force. In this case, since the supply time of the drive current supplied to the electromagnet 19 can be shortened, the heat generation of the electromagnet 19 can be suppressed and the fluctuation of the optical rotation angle due to the temperature can be suppressed.

  Further, the wavelength sweep light source 10 according to the present embodiment can repeatedly change the wavelength of light emitted from the semiconductor laser 11 to the Faraday element 18 at a predetermined sweep cycle.

  In addition, since the OFDR device 30 according to the present embodiment includes the wavelength swept light source 10 according to the present embodiment, accurate polarization compensation can be performed at low cost.

  In the present embodiment, the configuration example in the case where the wavelength swept light source 10 is used as a wavelength swept light source for OFDR has been mainly described, but the present invention is not limited to this. For example, the wavelength swept light source 10 of this embodiment can be used for any application that switches and emits light in two polarization states corresponding to the positions of the counter electrodes on the Poincare sphere.

DESCRIPTION OF SYMBOLS 10 Wavelength sweep light source 11 Semiconductor laser 11a, 11b Light emission end surface (end surface)
DESCRIPTION OF SYMBOLS 12 Collimating lens 13 Diffraction grating 14 Rotating mirror 15 Sweep drive part 18 Faraday element 19 Electromagnet 20 Optical rotation angle control part 20a Variable current source 21 Output side optical fiber 30 OFDR apparatus 31 Measurement interferometer 32 Light receiver 33 A / D converter 34 Reflection measurement unit 36 Fiber coupler 37 Circulator 38 Polarization controller 39 Polarizer 40 Fiber coupler 41 Fourier transform unit 43 Addition processing unit 100 Fiber 110 for sensor 110 DUT

Claims (8)

In a wavelength sweep light source (10) comprising: a semiconductor laser (11); and wavelength sweeping means (12 to 15) that repeatedly change the wavelength of light emitted from the semiconductor laser at a predetermined sweep cycle.
Before and Symbol semiconductor laser receives the light air propagated emitted from the Faraday element to optical rotation in angle of rotation corresponding to the intensity of the applied magnetic field (18),
An output side optical fiber (21) for transmitting the light rotated by the Faraday element to the outside as output light;
Magnetic field applying means (19) for applying a magnetic field to the Faraday element;
An optical rotation angle for controlling the optical rotation angle to be alternately switched between the first angle and the second angle in synchronization with the predetermined sweep period by changing the intensity of the magnetic field applied to the Faraday element by the magnetic field applying means. A control unit (20),
The wavelength swept light source, wherein a difference between the first angle and the second angle is 90 degrees. The magnetic field applying means is an electromagnet (19),
The optical rotation angle control unit includes a variable current source (20a) that supplies a driving current for generating a magnetic field that sets the optical rotation angle to the first angle or the second angle to the magnetic field application unit. The wavelength swept light source according to claim 1.   The Faraday element has a first angle of 45 degrees when a magnetic field in the first direction having a larger absolute value than the saturation magnetic field is applied from the magnetic field applying means, and has a larger absolute value than the saturation magnetic field. The magnetic field in the second direction opposite to the first direction is applied from the magnetic field applying unit, so that the second angle becomes -45 degrees. The wavelength swept light source described.   4. The wavelength swept light source according to claim 1, wherein a normal line of the Faraday element forms an angle other than 0 degrees with respect to an optical path of light emitted from the semiconductor laser.   5. The Faraday element maintains the first angle of 45 degrees or the second angle of −45 degrees with a magnetic field having an absolute value smaller than its coercive force. The wavelength swept light source according to any one of the above. In the semiconductor laser, one end surface (11a) of the two light emitting end surfaces (11a, 11b) is a low reflectance surface compared to the other end surface (11b),
The wavelength sweeping means includes:
A diffraction grating (13) for diffracting light emitted from the one end face of the semiconductor laser at an angle corresponding to the wavelength;
A rotating mirror (14) for reflecting the light diffracted by the diffraction grating and returning it to the diffraction grating;
The angle of the rotating mirror with respect to the diffraction grating is set to a predetermined angle so that the light returned from the rotating mirror to the diffraction grating is again diffracted by the diffraction grating and incident on the one end surface of the semiconductor laser. A sweep drive unit (15) that changes with a sweep cycle,
In accordance with the change in the angle of the rotating mirror, the resonator length from the other end surface of the semiconductor laser to the rotating mirror via the diffraction grating changes, so that the Faraday element from the other end surface changes. The wavelength swept light source according to any one of claims 1 to 5, wherein the wavelength of the light emitted to the light source is repeatedly changed at the predetermined sweep period. A wavelength-swept light source according to any one of claims 1 to 6,
Optical branching means (36, 37) for branching the output light from the wavelength swept light source into measurement light and reference light and inputting the measurement light to the optical fiber to be measured (100);
A polarization controller (38) for changing the polarization state of the reference light branched by the light branching means;
A polarizer (39) that transmits a component in a predetermined polarization direction of the reference light whose polarization state has been changed by the polarization controller;
Optical multiplexing means (37, 40) for outputting interference light by combining the reference light transmitted through the polarizer and the reflected light from the optical fiber to be measured to which the measurement light is input;
A light receiver (32) for converting the interference light into an electrical signal;
An A / D converter (33) for converting the electrical signal into a digital signal;
A reflection measuring unit (34) for measuring the intensity of the reflected light at each position of the optical fiber to be measured based on the digital signal;
The polarization controller controls the reference light so that the amount of the reference light transmitted through the polarizer is the same amount when the optical rotation angle is the first angle and when the rotation angle is the second angle. Which changes the polarization state of
The reflection measuring unit is
Fourier transform of the digital signal to calculate the spectrum of the interference light (41),
The spectrum of the interference light calculated by the Fourier transform unit when the optical rotation angle is the first angle, and the spectrum of the interference light calculated by the Fourier transform unit when the optical rotation angle is the second angle And an addition processing unit (42) for adding together. The output light from the wavelength swept light source according to any one of claims 1 to 6 is branched into measurement light and reference light by an optical branching means, and the measurement light is input to the measured optical fiber (100). Performing step (S2);
A polarization state changing step (S3) in which the polarization state of the reference light branched by the light branching means is changed by a polarization controller (38);
Transmitting a component of a predetermined polarization direction of the reference light whose polarization state has been changed by the polarization state changing step through a polarizer (39) (S4);
Outputting interference light by combining the reference light transmitted through the polarizer and the reflected light from the measured optical fiber to which the measurement light is input (S5);
Converting the interference light into an electrical signal (S6);
Converting the electrical signal into a digital signal (S7);
A reflection measurement step (S8, S11) for measuring the intensity of the reflected light at each position of the optical fiber to be measured based on the digital signal,
In the polarization state changing step, the light amount of the reference light transmitted through the polarizer is the same amount when the optical rotation angle is the first angle and when the optical rotation angle is the second angle. It changes the polarization state of the reference light,
The reflection measurement step includes
Fourier transform step (S8) for Fourier transforming the digital signal to calculate the spectrum of the interference light;
The spectrum of the interference light calculated by the Fourier transform step when the optical rotation angle is the first angle, and the spectrum of the interference light calculated by the Fourier transform step when the optical rotation angle is the second angle. And a step of adding (S11).

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