JP5032437B2 - Heterodyne optical spectrum analyzer - Google Patents

Description

  The present invention relates to a heterodyne optical spectrum analyzer, and more particularly to a polarization independence technique.

Conventional heterodyne optical spectrum analyzers divide the swept light into vertically polarized linearly polarized light and horizontally polarized linearly polarized light, perform heterodyne detection, and add the signals to make them polarization independent. (For example, refer to Patent Document 1).
Japanese Patent Laid-Open No. 7-11378

  Conventionally, since each was heterodyne detection, two light receiving systems were required. Therefore, an object of the present invention is to reduce the number of light receiving systems to one in a heterodyne optical spectrum analyzer.

  In order to solve the above-described problems, the heterodyne optical spectrum analyzer according to the present invention is characterized in that a time difference is given between orthogonal polarizations and detection is performed simultaneously.

  Specifically, the heterodyne optical spectrum analyzer according to the present invention includes a local light source that emits, as local light, a pair of two linearly polarized light beams that are orthogonal to each other and have a wavelength difference, measured light, and the local light. And a light receiving unit that receives the combined light from the combining unit and outputs a heterodyne detection signal of the light to be measured and the local light, and the wavelength difference is a heterodyne in the light receiving unit. It is characterized by being narrower than the detection band of detection.

  Since heterodyne detection is performed using a pair of two linearly polarized lights orthogonal to each other and having a wavelength difference as local light, both coherent lights of the two linearly polarized lights orthogonal to each other can be detected simultaneously. At this time, if the wavelength difference of the linearly polarized light is sufficiently narrower than the detection band of the heterodyne detection, by sweeping the wavelengths of the two linearly polarized lights, the measured light in all polarization states in the detection wavelength band Detection can be performed by any one of a pair of linearly polarized light having a wavelength difference. Therefore, the heterodyne optical spectrum analyzer can have one light receiving system.

The heterodyne optical spectrum analyzer according to the present invention may further include a signal processing unit that analyzes a spectrum of the measured light based on a heterodyne detection signal from the light receiving unit.
By further including a signal processing unit, the spectrum of the light to be measured can be acquired.

  In the heterodyne optical spectrum analyzer according to the present invention, the local light source separates a wavelength swept light source that continuously outputs a wavelength and outputs light from the wavelength swept light source into two linearly polarized lights orthogonal to each other. A polarization delay device that emits a delay difference between the two orthogonally polarized light beams orthogonal to each other may be provided.

  In the heterodyne optical spectrum analyzer according to the present invention, the wavelength-swept light source outputs linearly polarized light, and the polarization delay unit is a first polarization that branches the linearly-polarized light from the wavelength-swept light source into two. A holding optical fiber coupler, a second polarization maintaining optical fiber coupler that combines the polarization planes of the two linearly polarized lights from the first polarization maintaining optical fiber coupler to be perpendicular to each other, and the first polarization maintaining optical fiber coupler. An optical path difference is given to two optical paths between the polarization maintaining optical fiber coupler and the second polarization maintaining optical fiber coupler, and the two linearly polarized lights incident on the second polarization maintaining optical fiber coupler are subjected to the wavelength. And a delay device that generates a difference.

In the heterodyne optical spectrum analyzer according to the present invention, the polarization delay device is a polarization maintaining optical fiber, and a fast axis is inclined by approximately 45 ° with respect to a polarization plane of output light from the wavelength swept light source. It may be.
By making the output light from the wavelength swept light source enter into the polarization maintaining optical fiber by 45 °, the output light can be separated into two linearly polarized lights orthogonal to each other. In the polarization maintaining optical fiber, the propagation delay differs between the orthogonal polarizations, so that a wavelength difference occurs in the light output from the polarization maintaining optical fiber. As a result, the local light can be emitted.

  In the heterodyne optical spectrum analyzer according to the present invention, the polarization delay unit includes a first polarization beam splitter that separates output light from the wavelength swept light source into two linearly polarized light beams orthogonal to each other, and the first polarization beam. A second polarizing beam splitter for combining the two linearly polarized light beams from the beam splitter so that their polarization planes are perpendicular to each other; and two optical paths between the first polarizing beam splitter and the second polarizing beam splitter An optical path difference, and a delay unit that causes the wavelength difference between the two linearly polarized light incident on the second polarization beam splitter.

  In the heterodyne optical spectrum analyzer according to the present invention, the delay unit may be a polarization maintaining optical fiber that maintains a polarization plane of linearly polarized light from the first polarization maintaining optical fiber coupler.

  In the heterodyne optical spectrum analyzer according to the present invention, the local light source includes a first wavelength swept light source that continuously varies the wavelength and outputs linearly polarized light, and linearly polarized light that is different from the first wavelength swept light source by a certain wavelength. A second wavelength swept light source that outputs light, and a polarization that combines so that the polarization planes of the linearly polarized light from the first wavelength swept light source and the linearly polarized light from the second wavelength swept light source are perpendicular to each other. A wave holding optical fiber coupler.

  According to the present invention, in the heterodyne optical spectrum analyzer, one light receiving system can be provided.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.

  FIG. 1 is a schematic configuration diagram of a heterodyne optical spectrum analyzer according to the present embodiment. The heterodyne optical spectrum analyzer according to the present embodiment includes a local light source 2, a multiplexing unit 3, a light receiving unit 4, a filter 5, a detection unit 21, a signal processing unit 6, a control unit 8, and a display unit. 7. An information processing device may be used for the signal processing unit 6, the control unit 8, and the display unit 7. The local light source 2 to the multiplexing unit 3 are connected by a PMF (Polarization Maintaining Fiber).

Local light source 2 emits two pairs of linearly polarized light having orthogonal and wavelength difference from each other as a local light P _L. For example, vertically polarized linearly polarized light and horizontally polarized linearly polarized light having different wavelengths are emitted. The constant wavelength difference is preferably generated by delaying one of the two linearly polarized light beams by the delay device 13, and heterodyne detection can be performed with one light source. In order to acquire the spectrum of the light P _{0 to} be measured, the local light source 2 is swept in wavelength by a control signal from the control unit 8.

The multiplexing unit 3 multiplexes the measured light P ₀ and the local light P _L. The light receiving unit 4 receives the combined light from the combining unit 3 and outputs a heterodyne detection signal of the measured light P ₀ and the local light. The filter 5 extracts a beat signal from the output signal of the light receiving unit 4. The detector 21 detects the beat signal from the filter 5.

Control unit 8, the wavelength of the local light P _L of the local light source 2 is emitted, and controls the wavelength of the signal processing unit 6 processes. For example, the wavelength sweep light source 9 sweeps the wavelength at the wavelength sweep speed instructed from the control unit 8. The signal processing unit 6 acquires the wavelength sweep speed of the wavelength sweep light source 9 from the control unit 8 and stores the output signal from the detection unit 21 and the wavelength of the wavelength sweep light source 9 in association with each other. Then, the spectrum of the measured light P ₀ is analyzed based on the heterodyne detection signal from the light receiving unit 4. The display unit 7 displays the processing result of the signal processing unit 6.

FIG. 2 shows an example of local light. The vertically polarized linearly polarized light 101 and the horizontally polarized linearly polarized light 102 are swept in frequency while maintaining a certain wavelength difference. The wavelength difference is (delay time δt) × (local light sweep speed V). At this time, the wavelength sweeping speed V of the local light _{P L} in a local light source 2 is preferably 2 THz / ms or more.

  FIG. 3 shows an example of the relationship between the wavelength difference of the local light and the detection band of the light receiving unit. The wavelength difference δλ between the vertically polarized linearly polarized light 101 and the horizontally polarized linearly polarized light 102 is narrower than the detection band 100 of the heterodyne detection in the light receiving unit. When local light having a wavelength difference δλ between the linearly polarized lights 101 and 102 is detected from the measured light and heterodyne, both linearly polarized lights are detected as shown in the figure. At this time, if the detection band 100 is sufficiently wider than the wavelength difference δλ between the linearly polarized lights 101 and 102, the orthogonally polarized light orthogonal to the measured light can be simultaneously received by heterodyne detection. As a result, the same effect as the polarization independence at each swept wavelength can be obtained. Therefore, if this method is used, it is possible to construct a heterodyne optical spectrum analyzer that is independent of polarization with only one light receiving system.

FIG. 1 shows a first example of a local light source. The first example of the local light source 2 includes a wavelength swept light source 9 and a polarization delay device 10. The wavelength swept light source 9 outputs continuous light having a constant output power while continuously varying the wavelength. The polarization delay unit 10 separates the output light from the wavelength swept light source 9 into two linearly polarized lights orthogonal to each other and emits the two orthogonally polarized lights orthogonal to each other with a delay difference δt. Thus, a local light source 2 can emit two pairs of linearly polarized light having orthogonal and wavelength difference from each other as a local light P _L.

  The wavelength swept light source 9 outputs linearly polarized light. The polarization delay device 10 includes a first polarization maintaining optical fiber coupler 11, a second polarization maintaining optical fiber coupler 12, and a delay device 13. The wavelength sweeping light source 9 to the multiplexing unit 3 are connected by PMF.

  The first polarization maintaining optical fiber coupler 11 branches the linearly polarized light from the wavelength swept light source 9 into two. The second polarization maintaining optical fiber coupler 12 combines the polarization planes of the two linearly polarized lights from the first polarization maintaining optical fiber coupler 11 so that they are perpendicular to each other. For example, the connection of the two optical paths to the second polarization maintaining optical fiber coupler 12 is rotated by 90 °. Thereby, a pair of two linearly polarized lights orthogonal to each other can be generated.

  The delay unit 13 gives an optical path difference to the two optical paths between the first polarization maintaining optical fiber coupler 11 and the second polarization maintaining optical fiber coupler 12, and enters the second polarization maintaining optical fiber coupler 12. A wavelength difference is caused between the two linearly polarized lights. For example, the optical fiber delay device is connected to one of the optical fibers between the first polarization maintaining optical fiber coupler 11 and the second polarization maintaining optical fiber coupler 12.

  The delay device 13 is preferably a polarization maintaining optical fiber that maintains the polarization plane of linearly polarized light from the first polarization maintaining optical fiber coupler 11. By changing the length of the polarization-maintaining optical fiber connecting the two optical paths between the first polarization-maintaining optical fiber coupler 11 and the second polarization-maintaining optical fiber coupler 12, the delay device 13 is configured. Also good.

  The first example of the local light source 2 can emit two linearly polarized light pairs orthogonal to each other and having a wavelength difference from the second polarization maintaining optical fiber coupler 12 by adopting the above configuration. it can.

  FIG. 4 shows a second example of the local light source. In the second example of the local light source 2, a polarization maintaining optical fiber is used for the polarization delay device 10. In this case, the wavelength swept light source 9 outputs linearly polarized light. The polarization maintaining optical fiber is disposed so that the fast axis is inclined by approximately 45 ° with respect to the polarization plane of the output light from the wavelength swept light source 9. When the output light from the wavelength swept light source 9 is incident on the polarization maintaining optical fiber by 45 °, a pair of two linearly polarized lights orthogonal to each other is generated. The polarization maintaining optical fiber only needs to have a different refractive index depending on the plane of polarization, and may be a panda type or an elliptical clad type.

  The polarization maintaining optical fiber has such a length that a wavelength difference is generated when it is output from the local light source 2. In the polarization maintaining optical fiber, since the refractive index is different between the orthogonal polarizations, the propagation speeds of the two linearly polarized lights are different. Therefore, by adjusting the length of the polarization maintaining optical fiber, it is possible to generate a delay difference δt, that is, a wavelength difference between two pairs of linearly polarized light. If the refractive index difference of the polarization maintaining optical fiber is 4/10000, the length of the polarization maintaining optical fiber is preferably 300 m or more, for example.

  The second example of the local light source 2 can emit two pairs of linearly polarized light that are orthogonal to each other and have a wavelength difference from the polarization maintaining optical fiber by adopting the above configuration. In the second example of the local light source 2, the configuration for separating and coupling the output light from the wavelength swept light source 9 can be omitted.

  FIG. 5 shows a third example of the local light source. In the third example of the local light source 2, the polarization delay device 10 includes a first polarization beam splitter 14, a delay device 15, and a second polarization beam splitter 16.

  The wavelength swept light source 9 outputs linearly polarized light. The linearly polarized light from the wavelength swept light source 9 is incident so that the polarization direction of the linearly polarized light is inclined by 45 ° with respect to the reflecting surface of the first polarizing beam splitter 14. Thereby, linearly polarized light of vertical polarization and horizontal polarization is generated. The first polarization beam splitter 14 splits the output light from the wavelength swept light source 9 into two linearly polarized lights orthogonal to each other. The delay unit 15 gives an optical path difference to the optical paths of the two linearly polarized lights from the polarizing beam splitter 14, and causes a wavelength difference between the two linearly polarized lights incident on the second polarizing beam splitter 16. As shown in FIG. 5, the delay unit 15 includes, for example, two reflection mirrors 15a and a corner cube 15b, and arbitrarily adjusts the optical path length L between the reflection mirror 15a and the corner cube 15b to obtain a desired value. An optical path difference of 2L corresponding to the wavelength difference δt of is given. The second polarization beam splitter 16 combines the two linearly polarized light beams from the polarization beam splitter 14 so that the polarization planes are perpendicular to each other.

  The third example of the local light source 2 can emit two linearly polarized light pairs orthogonal to each other and having a wavelength difference from the second polarizing beam splitter 16 by adopting the above configuration.

  FIG. 6 shows a fourth example of the local light source. The fourth example of the local light source 2 includes a first wavelength swept light source 17, a second wavelength swept light source 18, and a polarization maintaining optical fiber coupler 19. The optical path between the first wavelength swept light source 17 and the second wavelength swept light source 18 and the polarization maintaining optical fiber coupler 19 and between the polarization maintaining optical fiber coupler 19 and the multiplexing unit 3 is PMF. Connected with. The first wavelength swept light source 17 and the second wavelength swept light source 18 are connected to a common control unit 20.

  The first wavelength swept light source 17 continuously varies the wavelength and outputs linearly polarized light. The second wavelength swept light source 18 outputs linearly polarized light different from the first wavelength swept light source 17 by a certain wavelength. The first wavelength swept light source 17 and the second wavelength swept light source 18 sweep the wavelength based on a control signal from the control unit 20 provided outside the local light source 2. Thereby, the 1st wavelength sweep light source 17 and the 2nd wavelength sweep light source 18 can output the linearly polarized light which has a fixed wavelength difference.

  The polarization maintaining optical fiber coupler 19 combines the linearly polarized light from the first wavelength swept light source 17 and the linearly polarized light from the second wavelength swept light source 18 so that the polarization planes are perpendicular to each other. For example, when the optical output fiber 17a of the first wavelength swept light source 17 and the polarization maintaining optical fiber coupler 19 are connected, they are connected so that their fast axes are parallel to each other. When connecting the output fiber 18a and the polarization maintaining optical fiber coupler 19, they are connected so that their fast axes are perpendicular to each other. As a result, two pairs of linearly polarized light that are orthogonal to each other and have a wavelength difference can be emitted from the polarization maintaining optical fiber coupler 19.

  The fourth example of the local light source 2 can emit two pairs of linearly polarized light that are orthogonal to each other and have a wavelength difference from the polarization maintaining optical fiber coupler 19 by adopting the above configuration. In the fourth example of the local light source 2, since the two wavelength swept light sources 17 and 18 are used, the configuration for separating the output light from the wavelength swept light source 9 can be omitted.

  The present invention can measure the spectrum of an LD or the like used in a coherent optical communication system.

1 is a schematic configuration diagram of a heterodyne optical spectrum analyzer according to the present embodiment. An example of local light is shown. An example of the relationship between the wavelength difference of local light and the detection zone | band of a light-receiving part is shown. The 2nd example of a local light source is shown. The 3rd example of a local light source is shown. The 4th example of a local light source is shown.

Explanation of symbols

2 local light source 3 multiplexing unit 4 light receiving unit 5 filter 6 signal processing unit 7 display unit 8 control unit 9 wavelength swept light source 10 polarization delay device 11 first polarization maintaining optical fiber coupler 12 second polarization maintaining light Fiber coupler 13 Delay device 14 First polarization beam splitter 15 Delay device 16 Second polarization beam splitter 17 First wavelength swept light source 17a Optical output fiber 18 Second wavelength swept light source 18a Optical output fiber 19 Polarization maintaining optical fiber Coupler 20 Control unit 21 Detection unit

Claims (8)

A local light source that emits, as local light, a pair of two linearly polarized light beams that are orthogonal to each other and have a wavelength difference;
A multiplexing unit that combines the light to be measured and the local light;
A light receiving unit that receives the combined light from the combining unit and outputs a heterodyne detection signal of the light to be measured and the local light; and
The heterodyne optical spectrum analyzer, wherein the wavelength difference is narrower than a detection band of heterodyne detection in the light receiving unit.   The heterodyne optical spectrum analyzer according to claim 1, further comprising a signal processing unit that analyzes a spectrum of the light to be measured based on a heterodyne detection signal from the light receiving unit. The local light source is
A wavelength swept light source that outputs continuously variable wavelengths;
A polarization delay device is provided that separates the output light from the wavelength swept light source into two linearly polarized light beams orthogonal to each other, and emits the two orthogonally polarized light beams that are orthogonal to each other with a delay difference. Item 3. The heterodyne optical spectrum analyzer according to Item 1 or 2. The wavelength swept light source outputs linearly polarized light,
The polarization delay is
A first polarization maintaining optical fiber coupler for branching the linearly polarized light from the wavelength swept light source into two;
A second polarization-maintaining optical fiber coupler that synthesizes so that the polarization planes of the two linearly-polarized light from the first polarization-maintaining optical fiber coupler are perpendicular to each other;
Two linearly polarized light beams that give an optical path difference to two optical paths between the first polarization maintaining optical fiber coupler and the second polarization maintaining optical fiber coupler and are incident on the second polarization maintaining optical fiber coupler The heterodyne optical spectrum analyzer according to claim 3, further comprising a delay device that causes the wavelength difference in light.   4. The polarization delay device is a polarization maintaining optical fiber, and a fast axis is inclined by approximately 45 ° with respect to a polarization plane of output light from the wavelength swept light source. The heterodyne optical spectrum analyzer described in 1. The polarization delay is
A first polarization beam splitter that separates output light from the wavelength swept light source into two linearly polarized light beams orthogonal to each other;
A second polarization beam splitter that combines the polarization planes of the two linearly polarized light beams from the first polarization beam splitter so that they are perpendicular to each other;
A delay that gives an optical path difference to two optical paths between the first polarizing beam splitter and the second polarizing beam splitter, and causes the wavelength difference in two linearly polarized lights incident on the second polarizing beam splitter. The heterodyne optical spectrum analyzer according to claim 3, further comprising:   The heterodyne optical spectrum analyzer according to claim 4, wherein the delay unit is a polarization maintaining optical fiber that maintains a polarization plane of linearly polarized light from the first polarization maintaining optical fiber coupler. The local light source is
A first wavelength swept light source that continuously varies the wavelength and outputs linearly polarized light;
A second wavelength swept light source that outputs linearly polarized light different from the first wavelength swept light source by a certain wavelength;
A polarization maintaining optical fiber coupler that combines linearly polarized light from the first wavelength swept light source and linearly polarized light from the second wavelength swept light source so that the planes of polarization are perpendicular to each other;
The heterodyne optical spectrum analyzer according to claim 1 or 2, further comprising:

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