JP2007121232A - Wavelength monitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce multiple beam interference noise in a wavelength monitor, to enhance the precision of wavelength measurement; and to reduce the size and the cost of the device. <P>SOLUTION: This wavelength monitor is provided with a first optical path for transmitting first branch light; a second optical path for transmitting second branch light, a light branching means having a first outgoing end and a second outgoing end; a first lens which converts the first branch light and the second branch light into beams of parallel lights and emit them, respectively; a second lens which selectively condenses each beam of parallel lights in the orthogonal direction and emits it; a photoelectric conversion means which has at least four photodetectors; arranged respectively so as to receive each beam of the parallel lights and receive light by dividing equally into four; a portion corresponding to one period of an interference pattern, produced by the interference of the two beams of parallel light; and a signal processing means for calculating the wavelength of light to be measured, by applying predetermined signal processing to the electrical signals output from the photodetectors. The photoelectric conversion means is so arranged that the light-receiving surface of each photodetector has a predetermined inclination with respect to the rectangular direction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

 The present invention relates to a wavelength monitor that measures the wavelength of light to be measured, for example, the wavelength of a laser light source that oscillates in a single mode.

  There are various types of laser light sources used in the fields of optical communication and optical measurement, such as DFB-LD (Distributed Feedback-Laser Diode) light sources, DBR-LD (Distributed Bragg Reflector-Laser Diode) light sources, and diffraction gratings. There is an external resonator type tunable light source using The DFB-LD light source and the DBR-LD light source have a problem that the oscillation wavelength drifts in the long term, and the external resonator type wavelength variable light source has a problem that the wavelength changes due to an external influence (for example, temperature change). . Therefore, when using a laser light source that oscillates in a single mode in the fields of optical communication and optical measurement, it is necessary to measure and monitor the wavelength of the laser light source with high accuracy and high accuracy.

As an apparatus for measuring the wavelength, there are a wavelength monitor using a diffraction grating, a wavelength monitor for causing interference with measured light, and the like. Among these, in the case of a wavelength monitor using an interference signal of the light to be measured, there are those using an interference light filter and those measuring two interference signals (so-called A phase and B phase) whose phases are shifted by 90 °. Yes (for example, see Patent Documents 1 to 4).

FIG. 8 is a diagram showing a configuration of a conventional wavelength monitor (see, for example, Patent Document 1). In FIG. 8, the light to be measured that has entered the wavelength monitor enters the cut filter 50. The cut filter 50 transmits only measured light in a predetermined wavelength range.

  The light to be measured that has passed through the cut filter 50 is incident on the interference light filter 51, and the transmitted light is continuously changed according to the incident position. The slide adjusting mechanism 52 mechanically slides the interference light filter 51 in the X-axis direction by a minute distance, and continuously changes the wavelength of light transmitted through the interference light filter 51.

  The photodiode 53 receives the light to be measured that has passed through the interference light filter 51. The photodiode 54 receives the light reflected by the interference light filter 51. The output ratio calculation means 55 includes IV conversion circuits 55a and 55b, a subtractor 55c, an adder 55d, and a divider 55e. From the signals output from the photodiodes 53 and 54, the output ratio calculation means 55 Calculate the output ratio.

  Each of the IV conversion circuits 55a and 55b converts the output of the photodiodes 53 and 54 into a voltage signal. The subtractor 55c subtracts the voltage signal from the IV conversion circuits 55a and 55b. The adder 55d adds the voltage signals of the IV conversion circuits 55a and 55b. The divider 55e normalizes the output ratio by dividing the calculation results of the subtractor 55c and the adder 55d. The signal processing unit 56 calculates the wavelength of the light to be measured from the output ratio obtained by the divider 55e of the output ratio calculating unit 55. In such a wavelength monitor shown in FIG. 8, the measurement wavelength range and wavelength accuracy are determined by the wavelength characteristics of the interference light filter 51.

FIG. 9 is a diagram showing another configuration of a conventional wavelength monitor (for example, Patent Documents 2 and 4).
2) measuring two interference signals (A phase and B phase) whose phases are shifted by 90 ° using an interferometer (for example, Michelson type), and obtaining the wavelength of the light to be measured.

In FIG. 9, the input optical fiber 60 transmits the light to be measured and emits it to the space. Lens 61
Converts the measured light emitted from the input optical fiber 60 into parallel light. The half mirror (first branching means) 62 branches and multiplexes the light to be measured. The first reflector 63 reflects one branched light to the half mirror 62. The second reflector 64, the reflective surface has a step d = λ _0/8, reflects the other branched light to the half mirror 62. The first reflector 63 and the second reflector 64 are arranged perpendicular to the optical path of each branched parallel light branched by the half mirror 62, and the branched parallel light is reflected again to the half mirror 62 by the same optical path. The optical axis is adjusted.

The reflecting prism (second branching means) 65 receives the combined light (interference light) from the half mirror 62 as 2
Branch. The reflection prism 65, the optical axis plane of the second reflector 64 to the optical path difference of lambda _0/4 is generated and the edge tip surface of the reflecting prism 65 are arranged to coincide. The first photodiode (PD) 66 receives one branched light from the reflecting prism 65. The second photodiode 67 receives the other branched light from the reflecting prism 65. The signal processing unit 68 obtains the wavelength of the light to be measured from the outputs of the first photodiodes 66 and 67.

The operation of such an apparatus will be described.
The light to be measured, which is emitted into the space from the output end of the input optical fiber 60 and converted into parallel light by the lens 61, enters the half mirror 62 disposed on the emission optical axis, and the first reflector 63 and the second reflector 63 Branches to the reflector 64.

Then, each of the first and second reflectors 63 and 64 reflects the branched parallel light branched by the half mirror 62 to the half mirror 62 again. Here, the second reflector 64 has a step d.
= Λ _0/8 is so flat reflectors of, half of the light beam plane generates an optical path difference of lambda _0/4 when reflected by one reciprocation with the second reflector 64. Note that λ ₀ is a wavelength, and may be set to the center wavelength of the measurement wavelength range, for example, and may be a value such as 1550 [nm] when used in optical communication.

The parallel light reflected by the first reflector 63 and the second reflector 64 and incident again on the half mirror 62 is combined and applied to the reflecting prism 65. Then, the parallel light that is combined and applied to the reflecting prism 65 is bifurcated into light that is 90 ° out of phase at the edge tip surface of the reflecting prism 65, and the first photo arranged on the branching optical axis. The light enters the diode 66 and the second photodiode 67. Furthermore, the light incident on both photodiodes 66 and 67 is output to the signal processing means 68 as a current corresponding to the light intensity (also called optical power).

Then, the signal processing means 68 performs a comparison calculation process on the light intensities from both the photodiodes 66 and 67, and outputs wavelength data. The change in light intensity with respect to the wavelength obtained by a normal Michelson interferometer is represented by the following relational expression (1).

  In the above relational expression (1), I is the normalized light intensity received by each photodiode 66, 67, λ is the wavelength of the light to be measured, and ΔL is the optical path length difference of the Michelson interferometer. One cycle of this light intensity change is called a free spectral region (FSR), and if the optical path length difference is large, the FSR becomes short.

In the structure of this wavelength monitor, because it uses a planar reflector having a step d = λ _0/8 to a second reflector 64, half of the light beam plane when one reciprocation is reflected by the second reflector 64 It generates an optical path difference of λ _0/4, periodic interference signal (a-phase, B-phase) with a [pi / 2 phase difference is obtained. Based on the two interference signals having the π / 2 phase difference, the signal processing means 68 obtains the wavelength change amount and change direction of the light to be measured.

  FIG. 10 is a diagram showing another configuration of a conventional wavelength monitor (see, for example, Patent Document 3). In FIG. 10, the laser light to be measured is emitted from the input optical fiber 70, converted into parallel light by the lens 71, and passes through the polarizer 72. The parallel light that has passed through the polarizer 72 is branched by the half mirror 73, and one branched light is received by the photodiode (PD) 74.

The other branched light branched by the half mirror 73 enters the birefringence retardation plate 75. The birefringent retardation plate 75 includes a “fast axis” and a “slow axis” that cause a delay of λ / 8 corresponding to the phase shift of π / 4 of the light having the first and second polarizations. The s-polarized light causes a phase shift with respect to the p-polarized light. Then, the polarization beam splitter 76 splits the light from the birefringence delay plate 75 into p-polarized light and s-polarized light, the p-polarized light is received by the photodiode 77, and the s-polarized light is received by the photodiode 78.

The output of each photodiode 74, 77, 78 is input to the signal processing means 79, and the wavelength of the light under measurement is calculated. In addition, since the optical power itself of the light to be measured from the input optical fiber 70 varies with time, the offset due to the variation is corrected by the output of the photodiode 74.
By correcting and normalizing the offsets of the photodiodes 77 and 78 in this way, periodic interference signals (A phase and B phase) having a phase shift of 90 ° as shown in FIG. 11 are obtained. In FIG. 11, the horizontal axis represents the wavelength, and the vertical axis represents the normalized optical power.
Japanese Patent Laid-Open No. 10-253452 JP 2000-234959 A JP-A-10-339668 JP 2002-214049 A

  However, the conventional wavelength monitor uses various optical elements (for example, the cut filter 50, the interference light filter 51, the half mirrors 62 and 73, the first reflector 63, the second reflector 64, and the reflecting prism using spatial light. 65, the polarizer 72, the birefringence delay plate 75, the polarization beam splitter 76, and the like), multiple interference is likely to occur on the surface of the optical element, and unnecessary multiple interference noise is generated by each photodiode 53. , 54, 66, 67, 74, 77, and 78, there is a problem that the wavelength measurement accuracy decreases.

In addition, each optical element is an independent optical component. By using many optical components, the optical axis adjustment increases, the manufacturing process increases, and it is difficult to reduce the size and cost of the device. There was a problem that the property was also lowered.

  The present invention has been made in view of such circumstances, and aims to reduce the multiple interference noise in the wavelength monitor, improve the wavelength measurement accuracy, and realize downsizing and cost reduction of the apparatus. And

 In order to solve the above-described problem, in the present invention, as a first solution, a wavelength monitor that measures the wavelength of the light to be measured, the first optical path for transmitting the first branched light of the light to be measured A second optical path having a predetermined optical path length difference with respect to the first optical path and transmitting the second branched light of the measured light, and the first optical path provided at one end of the first optical path. An optical branching means having a first exit end for emitting the branched light and a second exit end provided at one end of the second optical path for emitting the second branched light, and the first exit. The first branched light incident from the end and the second branched light incident from the second emission end are converted into parallel light, respectively, and emitted from the first lens. The parallel light beams are divided into the output optical axes of the first branched light and the second branched light, and the arrangement of the first outgoing end and the second outgoing end. A second lens that selectively collects and emits light in a direction orthogonal to each other, and receives each parallel light incident from the second lens, and one cycle of interference fringes generated by interference of the two parallel lights At least four light receiving elements that are arranged in the same direction as the arrangement direction of the first and second light emitting ends so as to receive light by dividing the light into four equal parts and output an electrical signal corresponding to the light receiving intensity Photoelectric conversion means provided, and signal processing means for calculating the wavelength of the light to be measured by performing predetermined signal processing on the electrical signal input from the light receiving element, and the photoelectric conversion means includes each light receiving element. The light receiving surface has a predetermined inclination with respect to a direction orthogonal to the emission optical axes of the first branched light and the second branched light and the arrangement direction of the first outgoing end and the second outgoing end. It is characterized by being arranged.

Further, in the present invention, as the second solving means, in the first solving means, the inclination φ of the photoelectric conversion means has the following relationship consisting of the beam radius r of parallel light and the focal length f ₂ of the second lens. It is a value satisfying Equation (4).

 In the present invention, as a third solution, a wavelength monitor that measures the wavelength of the light to be measured, the first optical path for transmitting the first branched light of the light to be measured, and the first A second optical path having a predetermined optical path length difference with respect to the optical path and transmitting the second branched light of the light to be measured; and provided at one end of the first optical path to emit the first branched light A light branching means having a first emission end and a second emission end provided at one end of the second optical path for emitting the second branched light; and a first light incident from the first emission end. A first lens that converts the first branched light and the second branched light incident from the second exit end into parallel light and emits the parallel light; and each parallel light incident from the first lens, A direction orthogonal to the outgoing optical axes of the first branched light and the second branched light and the arrangement direction of the first outgoing end and the second outgoing end A second lens that selectively collects and emits light, receives each parallel light incident from the second lens, and divides one period of interference fringes generated by interference of the two parallel lights into four equal parts. Photoelectric conversion means comprising at least four light receiving elements that are arranged in the same direction as the arrangement direction of the first emission end and the second emission end so as to receive light and output an electrical signal corresponding to the received light intensity. And signal processing means for calculating the wavelength of the light to be measured by performing predetermined signal processing on the electrical signal input from the light receiving element.

 In the present invention, as the fourth solving means, in any one of the first to third solving means, the first lens and the second lens are integrally formed. .

 In the present invention, as a fifth solving means, in any one of the first to fourth solving means, the photoelectric conversion means has a light receiving surface of each light receiving element, the first lens and the second lens. It arrange | positions so that it may correspond with the focus position of this.

 In the present invention, as a sixth solution, a wavelength monitor that measures the wavelength of the light to be measured, the first optical path that transmits the first branched light of the light to be measured, and the first optical path A second optical path having a predetermined optical path length difference with respect to the optical path and transmitting the second branched light of the light to be measured; and provided at one end of the first optical path to emit the first branched light A light branching means having a first emission end and a second emission end provided at one end of the second optical path for emitting the second branched light; and a first light incident from the first emission end. A first lens that converts the first branched light and the second branched light incident from the second emission end into parallel light and emits the parallel light; and receives each parallel light incident from the first lens; In addition, the first emission end is received so that one period of interference fringes generated by interference of two parallel lights is divided into four equal parts. And photoelectric conversion means provided with at least four light receiving elements that are arranged in the same direction as the arrangement direction of the second emission ends and output an electric signal corresponding to the received light intensity, and an electric signal input from the light receiving element Signal processing means for calculating the wavelength of the light to be measured by performing predetermined signal processing, and the photoelectric conversion means has a light receiving surface of each light receiving element for the first branched light and the second branched light. It is characterized by being arranged so as to have a predetermined inclination with respect to a direction orthogonal to the outgoing optical axis and the arrangement direction of the first outgoing end and the second outgoing end.

 In the present invention, as a seventh solving means, in any one of the first to sixth solving means, the light branching means is constituted by a planar optical circuit board.

 In the present invention, as an eighth solving means, in any one of the first to sixth solving means, the optical branching means is constituted by an optical coupler made of an optical fiber.

 Further, in the present invention, as a ninth solving means, in the eighth solving means, a third optical path provided at a subsequent stage of the optical coupler and connected to a first emission end of the optical coupler, A fourth optical path connected to the second emission end of the optical coupler and having the same optical path length as the third optical path, and provided at one end of the third optical path, emits the first branched light. A third emission end, and a fourth emission end provided at one end of the fourth optical path for emitting the second branched light, wherein the third emission end and the fourth emission end It is characterized by comprising pitch conversion means in which the respective emission ends are arranged so that the distance is equal to or smaller than the optical fiber diameter.

 In the present invention, as the tenth solving means, in the ninth solving means, the third optical path and the fourth optical path of the pitch converting means are each constituted by melt drawing of an optical fiber. And

 According to the present invention, the first and second branched lights emitted from the light branching means are converted into parallel lights by the first lens, and these parallel lights are converted by the second lens into the first and second lights. Photoelectric conversion selectively condensed in a direction orthogonal to the emission optical axis of the branched light and the arrangement direction of the first emission end and the second emission end, and arranged with a predetermined inclination in the orthogonal direction Incident on the light receiving surface of each light receiving element of the means. Therefore, since it differs from the conventional configuration in which parallel light is incident on various optical elements, it is possible to reduce the multiple interference generated by the residual reflectance on the surface of the optical element and improve the wavelength measurement accuracy. Further, since the number of parts of the optical element can be reduced as compared with the conventional case, the number of manufacturing steps for adjusting the optical axis can be reduced. As a result, the apparatus can be reduced in size and cost.

[First embodiment]
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1A is a top view of the wavelength monitor in the first embodiment, and FIG. 1B is a side view of the wavelength monitor. As shown in this figure, the wavelength monitor of the first embodiment includes an input optical fiber 10, a planar lightwave circuit (PLC) substrate 11, a lens 12, a condenser lens 17, and a PDA (Photo Diode Array) 13. , The first differential amplifier 14, the second differential amplifier 15, and the signal processing unit 16. In FIG. 1B, illustration of the first differential amplifier 14, the second differential amplifier 15, and the signal processing unit 16 is omitted.

 The input optical fiber 10 transmits to-be-measured light w emitted from a laser light source (not shown) to a planar optical circuit board (hereinafter referred to as PLC board) 11. This PLC substrate (optical branching means) 11 is an optical waveguide type optical branching element, and has a first optical path 11a and a second optical path 11b whose optical path length is longer than the first optical path 11a by ΔL. The measured light w transmitted from the input optical fiber 10 is branched and transmitted to the first optical path 11a and the second optical path 11b, and the optical axes of the branched lights are parallel and arranged in parallel. Branched light is emitted toward the lens 12 from the first emission end 11c and the second emission end 11d. Further, as shown in FIG. 1B, the emission end side of the PLC substrate 11 has a shape having a predetermined inclination. The branched light emitted from the first emission end 11c is referred to as first branched light w1, and the branched light emitted from the second emission end 11d is referred to as second branched light w2.

The lens (first lens) 12 is an optical element such as a collimator lens, for example, is disposed on the outgoing optical axis of the PLC substrate 11, and the first branched light w1 emitted from the first outgoing end 11c; The second branched light w2 emitted from the second emission end 11d is converted into parallel light and emitted to a condenser lens 17 disposed on the lens emission optical axis. Incidentally, the focal length of the lens 12 and _{f 1,} the distance between the first exit end 11c and a second exit end 11d and the lens 12 and _{f 1.}

 The condensing lens (second lens) 17 is, for example, a cylindrical lens, and is a lens that selectively condenses and emits incident light only in one axial direction. In the present embodiment, as shown in FIG. 1B, the outgoing optical axes of the first branched light w1 and the second branched light w2 and the arrangement direction of the first outgoing end 11c and the second outgoing end 11d A cylindrical lens that selectively collects light in a direction perpendicular to the direction is used. As described above, the condensing lens 17 condenses the two parallel lights incident from the lens 12 only in the orthogonal direction and emits the light to the PDA 13 disposed on the lens emission optical axis. The PDA 13 is disposed at the focal position of the lens 12 and the condenser lens 17.

 In the PDA (photoelectric conversion means) 13, photodiodes (first PD 13 a to fourth PD 13 d) as four light receiving elements face the light receiving surface toward the condenser lens 17, and the first emission end 11 c and the second light emitting element 11 c are arranged. Are arranged in the same direction as the arrangement direction of the emission ends 11d. These photodiodes output an electrical signal corresponding to the received light intensity as is well known. As will be described below, the PDA 13 is arranged such that interference fringes generated by two parallel lights emitted from the condenser lens 17 are in one cycle in the first PD 13a to the fourth PD 13d. In other words, the first PD 13a to the fourth PD 13d are arranged so as to output electrical signals (hereinafter referred to as interference signals) whose phases are different by 90 ° when receiving interference fringes.

  FIG. 2 schematically shows the relationship between the first PD 13 a to the fourth PD 13 d arranged as described above and the light intensity distribution 18 of interference fringes generated by two parallel lights incident from the condenser lens 17. It is shown in. As shown in FIG. 2, the reason why interference fringes are generated in the light intensity distribution 18 by two parallel lights will be described later.

 In FIG. 2, 18a represents an interference fringe region received by the first PD 13a, 18b represents an interference fringe region received by the second PD 13b, and 18c represents an interference fringe region received by the third PD 13c. Reference numeral 18d denotes an interference fringe area received by the fourth PD 13d. Thus, the width of each light receiving surface of the first PD 13a to the fourth PD 13d is adjusted so that each light receiving surface receives light by dividing one spatial period of interference fringes into four equal parts. Further, since the period of the interference fringes varies depending on the wavelength of the light to be measured, for example, it is desirable to adjust so that the width of the entire four photodiodes matches the period of the interference fringes at the center wavelength of the wavelength measurement range.

 By disposing the first PD 13a to the fourth PD 13d as described above, the first PD 13a outputs the first interference signal having a phase of 0 ° to the positive phase input terminal of the first differential amplifier 14. . The second PD 13 b outputs a second interference signal having a phase of 90 ° to the positive phase input terminal of the second differential amplifier 15. The third PD 13 c outputs a third interference signal having a phase of 180 ° to the negative phase input terminal of the first differential amplifier 14. The fourth PD 13 d outputs a fourth interference signal having a phase of 270 ° to the negative phase input terminal of the second differential amplifier 15.

 FIG. 3 is a side view showing the detailed structure of the PDA 13. As shown in this figure, a light incident window 13e having a thickness of ΔL1 is provided on the surface of the PDA 13 on the condenser lens 17 side. Here, if the distance from the PD-side end face 13f of the light incident window 13e to the light-receiving faces of the first PD 13a to the fourth PD 13d is ΔL2, the first PD 13a to fourth PD 13a to the fourth PD 13a to the fourth PD 13d are assumed. The distance to the light receiving surface of the PD 13d is ΔL1 + ΔL2.

 In the PDA 13, as shown in FIG. 1B, the light receiving surfaces of the first PD 13a to the fourth PD 13d are arranged such that the first branched light w1 and the outgoing optical axes of the second branched light w2 The light emitting end 11c and the second light emitting end 11d are arranged so as to have an inclination φ with respect to a direction orthogonal to the arrangement direction of the second emitting end 11d.

 The first differential amplifier 14 performs differential amplification of the first interference signal having a phase of 0 ° and the third interference signal having a phase of 180 °, and a signal processing unit 16 as an A-phase signal (shown in FIG. 2). Output to. The second differential amplifier 15 performs differential amplification of the second interference signal having a phase of 90 ° and the fourth interference signal having a phase of 270 °, and the signal processing unit 16 as a B-phase signal (shown in FIG. 2). Output to. The A-phase signal and the B-phase signal obtained in this way are signals whose phases around the zero point are shifted by 90 °. The signal processing unit (signal processing means) 16 performs predetermined signal processing based on the A-phase signal and the B-phase signal whose phases are shifted by 90 ° in this way, and calculates the wavelength change amount of the measured light w.

Next, the operation of the wavelength monitor according to the first embodiment configured as described above will be described.
The measured light w transmitted to the PLC board 11 via the input optical fiber 10 is branched and transmitted to the first optical path 11a and the second optical path 11b having the optical path length difference ΔL. Then, the light is emitted from the first emission end 11c to the lens 12 as the first branched light w1 and from the second emission end 11d as the second branched light w2.

The lens 12 converts the first branched light w <b> 1 and the second branched light w <b> 2 into parallel light and outputs the parallel light to the condenser lens 17 disposed on the lens output optical axis. Here, since the distance D between the first emission end 11c and the second emission end 11d is set to several tens of μm, the emission direction of the two parallel lights is relative to the lens emission optical axis. It is in a state of being inclined by an inclination θ shown by the following relational expression (2).
Accordingly, since the two parallel lights exit and intersect with an inclination twice as large as the inclination θ, they are emitted to the PDA 13 in a state where interference fringes are generated spatially. The period of the interference fringes (FSR: free spectral region) is expressed by the following relational expression (3), where λ is the wavelength of the light to be measured w.

 The two parallel lights emitted from the lens 12 are collected only in the orthogonal direction by the condenser lens 17 and emitted to the PDA 13. That is, in the condenser lens 17, the two parallel lights are not changed in the horizontal direction and are emitted to the PDA 13 while maintaining a state in which interference fringes are generated spatially.

 FIG. 4A is a side view showing in detail how parallel light is emitted from the condenser lens 17 to the PDA 13 as described above. As shown in this figure, the PDA 13 is arranged with an inclination φ with respect to the orthogonal direction. On the other hand, FIG. 4B shows a detailed state where the condenser lens 17 is not installed in FIG. 4A, that is, the parallel light is emitted to the PDA 13 without being condensed in the orthogonal direction. FIG.

  The parallel light incident on the light incident window 13e of the PDA 13 is repeatedly reflected after being reflected between the lens side end surface 13g and the PD side end surface 13f, between the PD side end surface 13f and the light receiving surface, and between the lens side end surface 13g and the light receiving surface. It will be incident on the surface. That is, between the lens side end surface 13g and the light receiving surface, the optical path length ΔL1 (between the lens side end surface 13g and the PD side end surface 13f), the optical path length ΔL2 (between the PD side end surface 13f and the light receiving surface), and the optical path length ΔL1 + ΔL2 (lens side end surface). 13g-between the light receiving surfaces), and these resonators cause multiple interference in parallel light. Therefore, noise due to multiple interference as described above is superimposed on the interference signals output from the first PD 13a to the fourth PD 13d, and the wavelength measurement accuracy is lowered.

 In the case of FIG. 4B, the parallel light incident on the light incident window 13e of the PDA 13 is repeatedly reflected between the lens side end surface 13g and the PD side end surface 13f, and then received by the first PD 13a to the fourth PD 13d. Incident on the surface. That is, multiple interference occurs in the shaded portion between the lens side end surface 13g and the PD side end surface 13f. Although not shown, such multiple interference also occurs between the PD side end surface 13f and the light receiving surface and between the lens side end surface 13g and the light receiving surface.

 On the other hand, as shown in FIG. 4A, when the condensing lens 17 is provided between the lens 12 and the PDA 13 and the parallel light condensed in the orthogonal direction is incident on the light incident window 13e, as described above. It can be seen that the multiple interference is greatly reduced.

Further, as the slope φ of PDA 13, the beam radius r of the collimated light, it is preferable to set the value that satisfies the following equation (4) consisting of the focal length f ₂ of the condenser lens 17. Thus, by setting the inclination φ of the PDA 13 (light receiving surface), it is possible to further reduce the multiple interference. Of course, it is not possible to set the slope φ such that a sufficient output signal (first to fourth interference signals) cannot be obtained in the first PD 13a to the fourth PD 13d.

 As described above, two parallel lights that maintain the state in which interference fringes are generated spatially and that significantly reduce multiple interference are incident on the light receiving surfaces of the first PD 13a to the fourth PD 13d. Become. Then, the first PD 13 a outputs the first interference signal having a phase of 0 ° to the positive phase input terminal of the first differential amplifier 14. The second PD 13 b outputs a second interference signal having a phase of 90 ° to the positive phase input terminal of the second differential amplifier 15. The third PD 13 c outputs a third interference signal having a phase of 180 ° to the negative phase input terminal of the first differential amplifier 14. Further, the fourth PD 13 d outputs a fourth interference signal having a phase of 270 ° to the negative phase input terminal of the second differential amplifier 15.

 The first differential amplifier 14 performs differential amplification of the first interference signal having a phase of 0 ° and the third interference signal having a phase of 180 °, and outputs the amplified signal to the signal processing unit 16 as an A-phase signal. The second differential amplifier 15 performs differential amplification of the second interference signal having a phase of 90 ° and the fourth interference signal having a phase of 270 °, and outputs the differential signal to the signal processing unit 16 as a B-phase signal. These A-phase signal and B-phase signal have a period (FSR) represented by the relational expression (3). The signal processing unit 16 counts the A phase signal and the B phase signal whose phases are shifted by 90 ° in this way to obtain the phase, and calculates the wavelength λ of the light to be measured from the phase.

 As described above, according to the wavelength monitor of the first embodiment, the number of parts of the optical element is reduced instead of the conventional structure in which parallel light is incident on various optical elements. As a result, it is possible to reduce the multiple interference that occurs due to the residual reflectance on the surface of the optical element, and in particular, it is possible to greatly reduce the multiple interference of parallel light that occurs in the PDA 13. As a result, it is possible to suppress the noise caused by multiple interference as in the past from being superimposed on the interference signals output from the first PD 13a to the fourth PD 13d, and to improve the wavelength measurement accuracy. Further, by condensing light by the condensing lens 17, the intensity of the parallel light incident on the PDA 13 can be increased. That is, a large output signal (first to fourth interference signals) that is resistant to noise can be obtained from the first PD 13a to the fourth PD 13d, which can contribute to improvement of wavelength measurement accuracy.

 In addition, by making the output end side of the PLC substrate 11 into an inclined shape, it is possible to reduce multiple interference generated at the output end, that is, the first output end 11c and the second output end 11d. Further, in the exit end of the PLC substrate 11, the lens 12, the condenser lens 17, and the PDA 13, a non-reflective process may be performed on a place where multiple interference may occur. Thereby, it can contribute to reduction of the multiple interference which generate | occur | produces in each optical element.

 Furthermore, according to the present wavelength monitor, the number of parts of the optical element can be reduced as compared with the conventional wavelength monitor, so that the manufacturing process for adjusting the optical axis can be reduced. Can be realized.

[Second Embodiment]
Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 5 is a top view of the wavelength monitor in the second embodiment. In FIG. 5, the same components as those in FIG. As shown in FIG. 5, the wavelength monitor of the second embodiment is different from the wavelength monitor of the first embodiment in that a two-input two-output optical coupler 20 made of an optical fiber instead of the PLC substrate 11 and V And a groove substrate 21. In FIG. 5, the first differential amplifier 14, the second differential amplifier 15, and the signal processing unit 16 are not shown.

 The optical coupler 20 is an optical waveguide type optical branching element, and includes a first optical path 20c having a first input end 20a and a first output end 20b, a second input end 20d, and a second output end 20e. And a second optical path 20f whose optical path length is longer by ΔL than the first optical path 20c. The first input end 20a is connected to an input optical fiber 10 (not shown), and the second input end 20d is subjected to antireflection processing.

 The optical coupler 20 also splits and transmits the light to be measured w transmitted from the input optical fiber 10 to the first optical path 20c and the second optical path 20f, and makes the optical axes of the branched lights parallel to each other. Branched light is emitted toward the lens 12 from the first emission end 20b and the second emission end 20e arranged in parallel. The first emission end 20 b and the second emission end 20 e are mechanically fixed by a V-groove substrate 21.

 The V-groove substrate 21 has two V-shaped grooves on the substrate parallel to the output optical axis of the optical coupler 20, and as described above, the first output end 20b of the optical coupler 20 and the first output end 20b. The two exit ends 20e are mechanically fixed by two V-shaped grooves.

Next, the operation of the wavelength monitor of the second embodiment configured as described above will be described.
The measured light w transmitted to the optical coupler 20 through the input optical fiber 10 is branched and transmitted by the optical coupler 20 to the first optical path 20c and the second optical path 20f having the optical path length difference ΔL. Then, the light is emitted from the first emission end 20b to the lens 12 as the first branched light w1 and from the second emission end 20e as the second branched light w2. In addition, the 1st output end 20b and the 2nd output end 20e are arrange | positioned in parallel with the distance D similarly to 1st Embodiment.

 As described above, the first branched light w1 and the second branched light w2 emitted to the lens 12 arranged on the outgoing optical axis of the optical coupler 20 held by the V-groove substrate 21 are collimated by the lens 12. And output to the condenser lens 17. Since the operation after the condenser lens 17 is the same as that of the first embodiment, the description thereof is omitted.

 As described above, according to the wavelength monitor of the second embodiment, since the optical coupler 20 made of an optical fiber is used as the optical branching element, the optical path length difference ΔL can be made very long as compared with the PLC substrate 11. It becomes possible. In this way, a wavelength monitor having high wavelength resolution can be easily obtained by increasing the optical path length difference ΔL.

[Third embodiment]
Next, a third embodiment of the present invention will be described with reference to FIG. FIG. 6 is a top view of the wavelength monitor in the third embodiment. In FIG. 6, the same components as those in FIG. As shown in FIG. 6, the wavelength monitor of the third embodiment includes a pitch conversion element 30 instead of the V-groove substrate 21 as compared with the wavelength monitor of the second embodiment. In FIG. 6, the first differential amplifier 14, the second differential amplifier 15, and the signal processing unit 16 are not shown.

 The pitch conversion element 30 includes a first optical path 30c having a first input end 30a and a first output end 30b, a second input end 30d and a second output end 30e, and the first optical path 30c. And a second optical path 30f having the same optical path length. The first input end 30 a is connected to the first output end 20 b of the optical coupler 20, and the second input end 30 d is connected to the second output end 20 e of the optical coupler 20.

 Further, the pitch conversion element 30 transmits the first branched light w1 input to the first input end 30a via the optical coupler 20 through the first optical path 30c, and the lens 12 from the first output end 30b. And the second branched light w2 input to the second input end 30d via the optical coupler 20 is transmitted by the second optical path 30f, and is directed from the second output end 30e to the lens 12. And exit. Note that the first emission end 30 b and the second emission end 30 e are arranged in parallel to the emission optical axis of the optical coupler 20.

 The distance D between the first emission end 30b and the second emission end 30e is narrower than that in the second embodiment, and may be, for example, equal to or smaller than the optical fiber diameter.

Next, the operation of the wavelength monitor of the third embodiment configured as described above will be described.
The first branched light w1 input to the first input end 30a of the pitch conversion element 30 via the optical coupler 20 is transmitted through the first optical path 30c and is output from the first output end 30b to the lens 12. The In addition, the second branched light w2 input to the second input end 30d via the optical coupler 20 is transmitted through the second optical path 30f and output to the lens 12 from the second output end 30e.

 As described above, the first branched light w1 and the second branched light w2 emitted to the lens 12 arranged on the outgoing optical axis of the pitch conversion element 30 are converted into parallel light by the lens 12, and are collected into a condensing lens. 17 is emitted. Since the operation after the condenser lens 17 is the same as that of the first embodiment, the description thereof is omitted.

 As described above, according to the wavelength monitor of the third embodiment, the distance between the first emission end 20b and the second emission end 20e of the optical coupler 20 can be arbitrarily adjusted by the pitch conversion element 30. Is possible. When using two optical fibers like the optical coupler 20, it is physically difficult to make the distance between the emission ends smaller than the optical fiber diameter, but by using the pitch conversion element 30, It becomes possible to make the distance between the emission ends narrower than the optical fiber diameter. Thereby, the focal distance of the lens 12 can be shortened. As a result, the size of the wavelength monitor can be reduced, and the intensity of the parallel light incident on the PDA 13 can be increased. In addition, as another pitch conversion element, you may use the thing comprised from the melt drawing of two optical fibers, for example.

[Fourth embodiment]
Next, a fourth embodiment of the present invention will be described with reference to FIG. FIG. 7A is a top view of the wavelength monitor in the fourth embodiment, and FIG. 7B is a side view of the wavelength monitor. In FIG. 7, the same components as those in FIG. As shown in FIG. 7, the wavelength monitor of the fourth embodiment includes an integrated lens 40 instead of the lens 12 and the condenser lens 17 as compared with the wavelength monitor of the second embodiment. In FIG. 7, the first differential amplifier 14, the second differential amplifier 15, and the signal processing unit 16 are not shown.

 The integrated lens 40 has a structure in which a first lens 40a and a second lens 40b made of, for example, a cylindrical lens are integrally disposed. The first lens 40a transmits the first branched light w1 emitted from the first emission end 20b of the optical coupler 20 and the second branched light w2 emitted from the second emission end 20e in the orthogonal direction. On the other hand, the light is condensed and emitted to the second lens 40b. The second lens 40b converts the two branched lights collected by the first lens 40a into parallel lights and emits them to the PDA 13.

Next, the operation of the wavelength monitor of the fourth embodiment configured as described above will be described.
The light to be measured w transmitted to the optical coupler 20 via the input optical fiber 10 (not shown) is branched into the first optical path 20c and the second optical path 20f having the optical path length difference ΔL in the optical coupler 20. The light is transmitted and emitted from the first emission end 20b to the integrated lens 40 as the first branched light w1 and from the second emission end 20e as the second branched light w2.

 The first branched light w1 and the second branched light w2 incident on the integrated lens 40 are collected in the first lens 40a with respect to the orthogonal direction, and then in the second lens 40b with respect to the orthogonal direction. Then, the light is converted into parallel light that is condensed and emitted to the PDA 13. Since the operation after the PDA 13 is the same as that of the first embodiment, the description thereof is omitted.

As described above, according to the fourth embodiment, as compared with the case where the lens 12 and the condenser lens 17 are used as in the first to third embodiments, the optical axes of these lenses are individually aligned. This is unnecessary, and the optical axis alignment time in the manufacturing process can be shortened. In addition, since the structure of the wavelength monitor is simplified, the apparatus cost can be reduced.
Such an integrated lens 40 can also be used in the first embodiment and the third embodiment. Further, the first lens 40a and the second lens 40b may be integrally disposed with the positions thereof reversed.

 In the first to fourth embodiments, the two parallel lights collected in the orthogonal direction are incident on the PDA 13 having the inclination φ to reduce the multiple interference generated in the PDA 13. The following method may be adopted instead of the method.

(1) The condensing lens 17 is not used, that is, the two parallel lights are incident on the PDA 13 having the inclination φ without condensing in the orthogonal direction.

(2) Two parallel lights condensed with respect to the orthogonal direction using the condenser lens 17 are incident on the PDA 13 having an inclination φ = 0 °.

  The methods (1) and (2) as described above are less effective in reducing multiple interference compared to the first to fourth embodiments, but can contribute to reduction of multiple interference compared to the conventional method. It is.

It is the upper side figure and side view of the wavelength monitor in 1st Embodiment of this invention. It is a schematic diagram which shows the relationship between arrangement | positioning of each photodiode of the wavelength monitor and interference fringe in 1st Embodiment of this invention. It is a side view of PDA13 of the wavelength monitor in 1st Embodiment of this invention. It is explanatory drawing of the reduction effect of the multiple interference in 1st Embodiment of this invention. It is a top view of the wavelength monitor in 2nd Embodiment of this invention. It is a top view of the wavelength monitor in 3rd Embodiment of this invention. It is the upper side figure and side view of the wavelength monitor in 4th Embodiment of this invention. It is a 1st structural example of the conventional wavelength monitor. It is a 2nd structural example of the conventional wavelength monitor. It is a 3rd structural example of the conventional wavelength monitor. It is explanatory drawing which shows the wavelength measurement principle in the 3rd structural example of the conventional wavelength monitor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Input optical fiber is 11 ... Planar light circuit (PLC: Planar Lightwave Circuit) board | substrate, 12 ... Lens, 17 ... Condensing lens, 13 ... PDA (Photo Diode Array), 14 ... 1st differential amplifier, 15 2nd differential amplifier 16 Signal processing unit 20 Optical coupler 30 Pitch conversion element 40 Integrated lens

Claims (10)

A wavelength monitor for measuring the wavelength of light to be measured,
A first optical path for transmitting the first branched light of the measured light and a second optical path for transmitting the second branched light of the measured light having a predetermined optical path length difference with respect to the first optical path A first emission end provided at one end of the first optical path for emitting the first branched light, and a first emission end provided at one end of the second optical path for emitting the second branched light. An optical branching means having two exit ends;
A first lens that converts the first branched light incident from the first output end and the second branched light incident from the second output end into parallel light and outputs the parallel light;
Each parallel light incident from the first lens is orthogonal to the outgoing optical axes of the first branched light and the second branched light and the arrangement direction of the first outgoing end and the second outgoing end. A second lens that selectively collects and emits light;
Each of the parallel light incident from the second lens is received, and the first emission end and the second light are received so that one period of interference fringes generated by interference of the two parallel lights is equally divided into four. Photoelectric conversion means provided with at least four light receiving elements that are arranged in the same direction as the arrangement direction of the emission ends of each of the light emitting elements and output an electrical signal corresponding to the received light intensity;
Signal processing means for calculating the wavelength of the light to be measured by performing predetermined signal processing on the electrical signal input from the light receiving element;
In the photoelectric conversion means, the light receiving surface of each light receiving element is in a direction orthogonal to the outgoing optical axes of the first branched light and the second branched light and the arrangement direction of the first outgoing end and the second outgoing end. A wavelength monitor, wherein the wavelength monitor is arranged so as to have a predetermined inclination. 2. The wavelength monitor according to claim 1, wherein the inclination φ of the photoelectric conversion means is a value satisfying the following relational expression (4) consisting of a beam radius r of parallel light and a focal length f2 of the _second lens. .

A wavelength monitor for measuring the wavelength of light to be measured,
A first optical path for transmitting the first branched light of the measured light and a second optical path for transmitting the second branched light of the measured light having a predetermined optical path length difference with respect to the first optical path A first emission end provided at one end of the first optical path for emitting the first branched light, and a first emission end provided at one end of the second optical path for emitting the second branched light. An optical branching means having two exit ends;
A first lens that converts the first branched light incident from the first output end and the second branched light incident from the second output end into parallel light and outputs the parallel light;
Each parallel light incident from the first lens is orthogonal to the outgoing optical axes of the first branched light and the second branched light and the arrangement direction of the first outgoing end and the second outgoing end. A second lens that selectively collects and emits light;
Each of the parallel light incident from the second lens is received, and the first emission end and the second light are received so that one period of interference fringes generated by interference of the two parallel lights is equally divided into four. Photoelectric conversion means provided with at least four light receiving elements that are arranged in the same direction as the arrangement direction of the emission ends of each of the light emitting elements and output an electrical signal corresponding to the received light intensity;
A wavelength monitor comprising: signal processing means for calculating a wavelength of light to be measured by performing predetermined signal processing on an electrical signal input from the light receiving element.   The wavelength monitor according to claim 1, wherein the first lens and the second lens are integrally formed.   The photoelectric conversion means is arranged so that a light receiving surface of each light receiving element coincides with a focal position of the first lens and the second lens. Wavelength monitor. A wavelength monitor for measuring the wavelength of light to be measured,
A first optical path for transmitting the first branched light of the measured light and a second optical path for transmitting the second branched light of the measured light having a predetermined optical path length difference with respect to the first optical path A first emission end provided at one end of the first optical path for emitting the first branched light, and a first emission end provided at one end of the second optical path for emitting the second branched light. An optical branching means having two exit ends;
A first lens that converts the first branched light incident from the first output end and the second branched light incident from the second output end into parallel light and outputs the parallel light;
The parallel light incident from the first lens is received, and the first emission end and the second light are received so that one period of interference fringes generated by the interference of the two parallel lights is equally divided into four. Photoelectric conversion means provided with at least four light receiving elements that are arranged in the same direction as the arrangement direction of the emission ends of each of the light emitting elements and output an electrical signal corresponding to the received light intensity;
Signal processing means for calculating the wavelength of the light to be measured by performing predetermined signal processing on the electrical signal input from the light receiving element;
In the photoelectric conversion means, the light receiving surface of each light receiving element is in a direction orthogonal to the outgoing optical axes of the first branched light and the second branched light and the arrangement direction of the first outgoing end and the second outgoing end. A wavelength monitor, wherein the wavelength monitor is arranged so as to have a predetermined inclination.  The wavelength monitor according to any one of claims 1 to 6, wherein the optical branching unit is configured by a planar optical circuit board.  The wavelength monitor according to claim 1, wherein the optical branching unit includes an optical coupler made of an optical fiber. Provided after the optical coupler,
A third optical path connected to the first output end of the optical coupler; a fourth optical path connected to the second output end of the optical coupler and having the same optical path length as the third optical path; A third emission end provided at one end of the third optical path for emitting the first branched light, and a fourth emission provided at one end of the fourth optical path for emitting the second branched light. And a pitch converting means in which each output end is arranged such that a distance between the third output end and the fourth output end is equal to or less than the diameter of the optical fiber. Item 9. The wavelength monitor according to Item 8. The wavelength monitor according to claim 9, wherein the third optical path and the fourth optical path of the pitch converting means are each configured by melt drawing of an optical fiber.

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