JP2012163534A - Optical spectrum analyzer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost and compact optical spectrum analyzer capable of measuring, with high resolution, the intensity of each set of orthogonal polarized lights of incident light.SOLUTION: In an optical spectrum analyzer, light Px to be measured is make into parallel light by a first lens 22 and directed to a polarized light separating part 23 to separate orthogonal polarized light elements. Then, the polarization direction of one of the separated light elements is turned 90 degrees by a half wavelength plate 24 so as to be matched with that of the other separated light element, and directed to a diffraction grating 30 of a wavelength selecting part 29. A single second lens 40 is used both as a lens for focusing each of the polarized light elements each and directing the elements to polarized light preserving optical fibers 41 and 42 and as a lens for changing each of the polarized light elements returned by the polarized light preserving optical fibers 41 and 42 into parallel beams and directing the beams to the diffraction grating 30 again. Further, selected lights of final order (the fourth-order selected lights in this example) are focused by third and fourth lenses 45 and 46, directed to pass through slits 47 and 48, and received by optical receivers 50 and 51.

Description

  The present invention relates to a technique for realizing an optical spectrum analyzer that can measure the intensity of each orthogonal polarization of incident light and that has high resolution, low cost, and a small size.

  With the recent increase in optical communication capacity due to the rapid advancement of the Internet, the wavelength density of WDM (Wavelength Division Multiplexing) has progressed, and higher resolution is required as the performance of optical spectrum analyzers that evaluate these spectra. I came.

ROADM (Reconfigurable Optical
In a network based on Add / Drop Multiplexer (OSNR), it is assumed that the noise level between adjacent peaks, which is a conventional signal quality evaluation method, is flat.
In addition to the Out Band OSNR method for measuring a Signal-to-Noise Ratio (optical signal-to-noise ratio), an In Band OSNR method within a signal band has been required.

  In the measurement of In Band OSNR, it is necessary to measure the signal intensity of each orthogonal polarization with respect to the wavelength of the WDM signal light. As a configuration for this, the input light is polarized and separated into polarization components orthogonal to each other, incident on a Litman-type spectroscopic unit composed of a diffraction grating and a mirror, and subjected to wavelength selection processing twice for a specific wavelength band by the diffraction grating. The light of each polarization component is extracted by a polarization preserving optical fiber (PMF), is incident again on the spectroscopic unit, is subjected to wavelength selection processing twice more, is extracted by the output optical fiber, and is incident on the light receiver.

  An example of the optical spectrum analyzer having the above configuration is disclosed in the following Patent Document 1.

US patent US6930776 B2

  However, the conventional optical spectrum analyzer disclosed in Patent Document 1 requires four polarization-preserving optical fibers, two outgoing optical fibers, and six lenses, which makes it large and expensive. There was a problem.

  An object of the present invention is to solve this problem and to provide a small optical spectrum analyzer that can measure the intensity of each polarization of incident light orthogonal to each other and that has high resolution, low cost, and small size.

In order to achieve the object, an optical spectrum analyzer according to claim 1 of the present invention comprises:
A light incident terminal (20a) for making the light to be measured incident;
A first lens (22) that collimates the light to be measured incident on the light incident terminal;
A polarization separation unit (23) that receives the parallel light emitted from the first lens and separates the parallel light components into orthogonal polarization components;
A half-wave plate (24) that receives one of the polarization components emitted from the polarization separation unit, rotates the polarization direction by 90 degrees, and matches the polarization direction of the other polarization component;
It has at least one diffraction grating (30, 31) in which engraving lines are provided in parallel on one surface side, and each polarization component whose polarization direction is matched is incident on the position shifted in the length direction of the engraving line. A wavelength selection unit (29) that receives light at a corner and selectively emits light in a specific wavelength band in a specific direction;
It has two reflection surfaces (35a, 35b) orthogonal to each other on a plane orthogonal to the score line of the diffraction grating of the wavelength selection unit, and the specific wavelength band emitted from the wavelength selection unit in the specific direction. Orthogonal mirror (35) that receives the selective light on one reflecting surface, reflects it to the other reflecting surface, reflects it further on the other reflecting surface, and folds it along the optical axis that is parallel to the incident and shifted in the direction of the engraving When,
The orthogonal mirror or the diffraction grating of the wavelength selection unit is rotated by a predetermined angle range about an axis parallel to the score line, and the center wavelength of the light in the specific wavelength band that is folded back from the orthogonal mirror to the wavelength selection unit is swept. A rotating device (36) for causing;
Second, third, and fourth condensing lenses (40, 45, 46) respectively disposed on optical axes that are parallel to the incident optical axis with respect to the wavelength selection unit and shifted in the score direction;
First and second polarization-preserving optical fibers that receive light collected by the second lens on one end side, propagate a predetermined distance, and return to the second lens in the same polarization state as incident from the other end side. (41, 42),
First and second slits (47, 48) for increasing the wavelength selectivity by limiting the width of the light collected by the third and fourth lenses in the direction perpendicular to the score line, respectively. ,
First and second light receivers (50, 51) that receive light that has passed through the first and second slits and detect the intensity thereof,
For each polarization component with the polarization direction matched, the wavelength selection unit folds the primary selection light emitted in the specific direction to the wavelength selection unit with an optical axis shifted in the engraving direction by the orthogonal mirror,
The second selective light of the specific wavelength band selected by the wavelength selective action that is reversible with the time indicated by the wavelength selection unit with respect to the folded light is incident on the second lens, and each polarization component is Incident first and second polarization preserving optical fibers,
Third-order selection of the specific wavelength band that the wavelength selection unit emits in the specific direction with respect to the light that is turned back by the first and second polarization preserving optical fibers and enters through the second lens The light is turned back again by the orthogonal mirror,
The first and fourth lenses condense each polarization component of the fourth-order selection light in the specific wavelength band emitted by the wavelength selection unit with respect to the light refolded by the orthogonal mirror, and the first lens Then, the light passes through the second slit, is incident on the first and second light receivers, and based on the outputs of the first and second light receivers, the spectral characteristics of the orthogonal polarization components of the measured light It is characterized by calculating | requiring.

An optical spectrum analyzer according to claim 2 of the present invention is the optical spectrum analyzer according to claim 1,
The wavelength selection unit is constituted by a single diffraction grating.

The optical spectrum analyzer according to claim 2 of the present invention is the optical spectrum analyzer according to claim 1,
The wavelength selector is composed of two diffraction gratings facing each other;
The light incident on the wavelength selection unit is emitted after receiving the wavelength selection action twice by the two diffraction gratings for the specific wavelength band.

An optical spectrum analyzer according to claim 3 of the present invention is the optical spectrum analyzer according to claim 1 or 2,
A polarization controller (70) for controlling the polarization of the light to be measured incident on the polarization separation unit is disposed between the light incident terminal and the polarization separation unit.

  As described above, the optical spectrum analyzer of the present invention converts the light to be measured into parallel light by the first lens and enters the polarization separation unit to separate the orthogonal polarization component, and the polarization direction of one of the light is analyzed by the half-wave plate. It is rotated by 90 degrees and is incident on the diffraction grating of the wavelength selection unit in accordance with the other polarization direction. In addition, a lens for condensing and injecting each polarization component into the first and second polarization preserving optical fibers and each polarization component folded back by the first and second polarization preserving optical fibers are made into parallel beams. Thus, a single second lens is used in common with the lens for re-incident on the wavelength selection section. Further, the final selection light is condensed by the third and fourth lenses, passed through the first and second slits, and received by the light receiver.

  For this reason, as compared with the conventional apparatus, only two polarization-preserving fibers are required, no outgoing optical fiber is required, and four lenses can be formed, and the size and cost can be significantly reduced.

Configuration diagram of the first embodiment of the present invention Schematic diagram representing the optical path of the first embodiment in a plane The figure which shows the structural example of a polarization separation part The figure which shows the structural example of a beam expander The figure showing the positional relationship of a 2nd lens and a polarization preserving optical fiber Configuration diagram of second embodiment of the present invention Schematic diagram representing the optical path of the second embodiment in a plane Illustration of the arrangement of two diffraction gratings The figure which shows the example which provided the polarization controller in the light entrance part

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows the configuration of an optical spectrum analyzer 20 to which the present invention is applied, and FIG. 2 is a schematic diagram showing the optical system in a plan view.

  In these figures, the optical spectrum analyzer 20 causes the measured light Px (polarized multiplexed signal light) incident from the light incident terminal 20 a (optical connector) to be incident on the first lens (collimator) 22 via the fiber 21. The light is converted into parallel light, enters the polarization separation unit 23, is separated into two orthogonal polarization components Pp and Ps, and is emitted along parallel optical axes.

  Here, for the optical system of the embodiment, a three-dimensional orthogonal coordinate space of xyz is defined, and the direction of the optical axis of the first lens 22 (incident optical axis of the polarization separation unit 23) is the x-axis direction. The polarization direction of one polarization component Ps of the measurement light Px is in the z-axis direction (direction perpendicular to the optical axis and along the vertical direction), and the polarization of the other polarization component Pp is in the y-axis direction (perpendicular to the optical axis and horizontal). Direction along the direction).

  As shown in FIG. 3, the polarization separation unit 23 emits the polarization component Ps of the light to be measured Px along the optical axis obtained by extending the incident optical axis, and the other polarization component Pp in a direction orthogonal to the incident optical axis. The outgoing polarization beam splitter 23a and the other polarization component Pp emitted from the polarization beam splitter 23a have an inclination of 45 degrees with respect to the optical axis of the polarization component Pp, and the polarization component Pp is emitted with an optical axis parallel to the one polarization component Ps. (The polarization direction shown in FIG. 3 is a view seen from the light traveling direction x-axis side).

  Of the polarization components Pp and Ps separated by the polarization separation unit 23, the component Pp whose polarization direction is horizontal is incident on the half-wave plate 24, and the polarization direction is rotated by 90 degrees to be the same as the polarization component Ps. In this way, it is aligned in the direction along the z-axis and emitted as a polarization component Pp ′.

  These two polarization components Pp ′ and Ps are incident on the beam expander 25 along parallel optical axes that are separated in the z-axis direction, and in the state where the respective beam widths are expanded, the wavelength selection unit 29 of this embodiment is used. The light is incident on the diffraction grating 30 constituting the structure.

  For example, as shown in FIG. 4, the beam expander 25 uses the two triangular prism-like lenses 25a and 25b extending in the z-axis direction to change the beam width Wa in the y-axis direction of the incident polarization components Pp ′ and Ps to Wb. Expand to. Here, the beam expander 25 is used, and the spot width of the incident light with respect to the engraving line (diffraction groove) provided in parallel at fine intervals on one surface side of the diffraction grating 30 extending in the z-axis direction with a narrow interval is widened. In this case, the diffraction efficiency is increased. However, the beam expander 25 may be omitted and the two polarization components Pp ′ and Ps may be directly incident on the diffraction grating 30.

  The polarization components Pp (0) (= Pp ′) and Ps (0) (= Ps) emitted from the beam expander 25 along the parallel optical axes separated in the z-axis direction are the lengths of the engraving lines of the diffraction grating 30. The light beams are incident on the positions shifted in the direction (z-axis direction) at the same incident angle, and the wavelength components included in the light are emitted at an angle (on the xy plane) according to the wavelength by the diffraction action of the diffraction grating 30.

  In this embodiment, an example in which the wavelength selection unit 29 is configured by a single diffraction grating 30 is shown, but the wavelength selection unit 29 can also be configured by two diffraction gratings as will be described later.

  An orthogonal mirror 35 is disposed on one side of the diffraction grating 30. The orthogonal mirror 35 has two reflection surfaces 35 a and 35 b that are orthogonal to each other on a plane orthogonal to the engraving line of the diffraction grating 30, and the reflection surfaces 35 a and 35 b are 45 to the engraving line of the diffraction grating 30. It is arranged toward the surface on which the score line is provided in a tilted state. The angle formed by the two reflecting surfaces 35a and 35b is 90 degrees, but the angles of the reflecting surfaces 35a and 35b with respect to the score line need not be equal (45 degrees), and if the sum is 90 degrees, for example, 30 It may be different such as 60 degrees and 40 degrees and 50 degrees.

  The orthogonal mirror 35 is driven to rotate within a predetermined angle range with an axis parallel to the score line of the diffraction grating 30 (an axis parallel to the z axis) by the rotation device 36.

  Here, out of the light diffracted by the diffraction grating 30 with respect to the respective polarization components Pp (0) and Ps (0), in a direction (specific direction) orthogonal to the boundary line 35c of the reflecting surfaces 35a and 35b of the orthogonal mirror 35. , Light in a specific wavelength band (having a center wavelength λ1) is emitted as primary selection light Pp (1), Ps (1), and one of the reflecting surfaces of the orthogonal mirror 35 (here, the lower reflecting surface 35b) ) At an angle of 45 degrees, reflected upward along the z-axis, incident on the upper reflecting surface 35a at an angle of 45 degrees, and shifted parallel to the diffraction grating 30 in the z-axis direction. It is folded at the optical axis.

  The folded lights Pp (1) ′ and Ps (1) ′ are reincident on the diffraction grating 30 at the same incident angle as the outgoing angles of the primary selection lights Pp (1) and Ps (1).

  Accordingly, the incident light of the first incident light Pp (0) and Ps (0) among the diffracted light with respect to the re-incident light Pp (1) ′ and Ps (1) ′ due to reversibility of the diffraction characteristics of the diffraction grating 30. In the direction parallel to the axis and shifted upward along the z-axis, the light of the specific wavelength band is selectively emitted as the second-order selection light Pp (2) and Ps (2), and the second-order selection. Lights Pp (2) and Ps (2) are incident on the beam expander 25, the beam width is reduced to the original width, and incident on the second lens 40.

  Here, as shown in FIG. 5, the optical axes of the secondary selection lights Pp (2) and Ps (2) incident on the second lens 40 are in the z-axis direction (here, from the central axis of the second lens 40). In this case, the light is condensed at a position shifted in the z-axis direction by an amount corresponding to the shift of the optical axis in the vicinity of the position of the focal length of the second lens 40, and the polarized light arranged at that position. The light is incident on one end 41 a and 42 a of the storage type optical fiber (PMF) 41 and 42.

  The other ends 41b and 42b of the polarization preserving optical fibers 41 and 42 are positioned at the focal length of the second lens 40 and shifted upward from the lens center axis in the same z-axis direction as the one ends 41a and 42a. The light received at one end 41a, 42a is propagated for a predetermined distance, and is folded back to the second lens 40 from the other end 41b, 42b side in the same polarization state as that upon incidence.

  The folded lights Pp (2) ′ and Ps (2) ′ from the polarization preserving optical fibers 41 and 42 are converted into parallel light by the second lens 40, and their beam width is expanded by the beam expander 25. Incident on the diffraction grating 30.

  Here, the optical axes of the folded lights Pp (2) ′ and Ps (2) ′ re-entering the diffraction grating 30 are parallel to the optical axes of the first incident lights Pp (0) and Ps (0) and are z-axis. It is also shifted in the z-axis direction with respect to the optical axes of the secondary selection light Pp (2) and Ps (2).

  Therefore, the same operation as the two wavelength selection operations for the first incident light Pp (0) and Ps (0) hinders the mutual operations for the return lights Pp (2) ′ and Ps (2) ′. Done without.

  That is, the folded lights Pp (2) ′ and Ps (2) ′ are incident on the diffraction grating 30. Among the diffracted lights, the polarization components Pp (3) and Ps (3) in a specific wavelength band are the third-order selection lights. , And the reflected light Pp (3) ′, Ps (3) ′ is re-incident on the diffraction grating 30 and subjected to the fourth selection process for the specific wavelength band. (4), Ps (4) is incident on the beam expander 25 with an optical axis shifted parallel to the z-axis direction with respect to the optical axes of the folded lights Pp (2) ′ and Ps (2) ′. The beam is narrowed and emitted.

  The fourth selection light Pp (4) and Ps (4) are condensed by the third lens 45 and the fourth lens 46, respectively, and the first slit 47 and the second slit 48 extending in the z-axis direction at the focal positions. Therefore, the width direction (direction perpendicular to the score line) of the light passing therethrough is limited, and excess wavelength components are further removed and incident on the first light receiver 50 and the second light receiver 51, respectively. .

  Accordingly, the first light receiver 50 and the second light receiver 51 have a light incident angle on the diffraction grating 30 and an angle of the orthogonal mirror 35 with respect to the diffraction grating 30 out of two orthogonal polarization components of the incident light Px. Components of the determined specific wavelength band are incident upon receiving the wavelength selection process four times, and the intensity is detected. As the orthogonal mirror 35 rotates, the selected specific wavelength band is swept, and signals PDp (λ) and PDs (λ) representing the intensity of the polarization component for each wavelength of the light to be measured are output.

  These signals PDp (λ) and PDs (λ) are converted into digital values by the A / D converters 52 and 53 and input to the arithmetic processing unit 60.

  The arithmetic processing unit 60 obtains wavelength-to-intensity information (spectrum information) for two polarization components, performs various arithmetic processes for evaluating the characteristics and quality of the incident light Px, and displays the results. In this example, a process for obtaining the In Band OSNR of the WDM signal light will be described.

With respect to spectrum data PDp (λ1) to PDp (λN) and PDs (λ1) to PDs (λN) obtained by sweeping in a predetermined wavelength region, WDM spectrum data, that is, spectrum data of the sum of the intensities of both polarization components,
PD (λ1) = PDp (λ1) + PDs (λ1)
PD (λ2) = PDp (λ2) + PDs (λ2)
......
PD (λN) = PDp (λN) + PDs (λN)
Ask for.

  Next, the center wavelength (or peak wavelength) of the signal light and the signal light level Psignal are obtained from the WDM spectrum. The signal light level Psignal is a level obtained by integrating the peak level of the spectrum or the intensity of signal light in the wavelength range including the spectrum peak level.

  Next, for each signal light of each channel of WDM, a plurality of data groups are created using data in an arbitrary wavelength range from the center wavelength.

For example,
PDp (λ1) → PD1 (λa1)
PDs (λ1) → PD2 (λa1)
PDp (λ2) → PD3 (λa2)
PDs (λ2) → PD4 (λa2)
......
......
PDp (λ [N−1]) → PD1 (λm1)
PDs (λ [N−1]) → PD2 (λm1)
PDp (λN) → PD3 (λm2)
PDs (λN) → PD4 (λm2)

Each data group,
{PD1 (λa1), PD2 (λa1), PD3 (λa2), PD4 (λa2)}
......
{PD1 (λm1), PD2 (λm1), PD3 (λm2), PD4 (λm2)}
Pase (λa),..., Pase (λm) is calculated.

  Then, the plurality of Pase (λa),..., Pase (λm) obtained above are subjected to a fitting process using an arbitrary fitting function (for example, first to fifth order functions, Gaussian functions, etc.) Ask.

  The ASE noise level (Amplified Spontaneous Emission Noise) Pase at the center wavelength (or peak wavelength) of the spectrum is calculated from the Pase function, and the OSNR is calculated from the Pase and the signal light level (Psignal).

Here, the calculation of the ASE noise level Pase will be described in detail.
If the data groups of the detection levels of the optical spectra PDs and PDp at different wavelengths are PD1 (λm1), PD2 (λm1), PD3 (λm2), and PD4 (λm2), within the band of the Add / Drop filter of the transmission line However, the signal light level differs depending on the wavelength sweep position of the optical spectrum analyzer, but the separation ratio α of the orthogonal component separated by polarization does not change for the ASE noise level.

  Therefore, the detection data PD1 (λm1), PD2 (λm1), PD3 (λm2), and PD4 (λm2) have ASE noise power Pase, signal light power Ps, OSNR Ps / Pase, and optical bandpass of the optical spectrum analyzer. If the bandwidth of the filter is B0 and the light separation ratio of the polarization component is α, it is expressed as follows.

PD1 (λm1) = Ps (λm1) (1-α) + Pase · B0 / 2 (1)
PD2 (λm1) = Ps (λm1) · α + Pase · B0 / 2 (2)
PD3 (λm2) = Ps (λm2) (1-α) + Pase · B0 / 2 (3)
PD4 (λm2) = Ps (λm2) · α + Pase · B0 / 2 (4)

From formula (1) -formula (3),
PD1 (λm1) -PD3 (λm2)
= (1-α) [Ps (λm1) −Ps (λm2)] (5)

From formula (2) -formula (4),
PD2 (λm1) -PD4 (λm2)
= Α [Ps (λm1) −Ps (λm2)] (6)

By the calculation of Expression (5) / Expression (6),
α = [PD2 (λm1) −PD4 (λm2)]
/ [PD1 (λm1) −PD3 (λm2) + PD2 (λm1) −PD4 (λm2)]
...... (7)
Is obtained.

On the other hand, from the calculation of Formula (1) × α−Formula (2) × (1-α),
Pase = 2 × [α · PD1 (λm1) − (1-α) PD2 (λm1)]
/ [B0 (2α-1)] (8)
Thus, Pase is obtained by the calculation of this equation.

And as mentioned above,
OSNR = (Psignal−Pase) / Pase
In Band OSNR can be obtained by the above calculation. However, Psignal is a power or peak power obtained by integrating the total spectrum (sum of each polarization component) in an arbitrary range. Pase is an ASE noise level obtained by averaging processing or fitting processing.

  As described above, the optical spectrum analyzer 20 according to the first embodiment converts the light to be measured Px into parallel light by the first lens 22 and enters the polarization separation unit 23 to separate the orthogonal polarization component, and one polarization direction thereof. Is rotated 90 degrees with the half-wave plate 24 and is incident on the diffraction grating 30 of the wavelength selection unit 29 in accordance with the other polarization direction. Further, a lens for condensing and injecting each polarization component into the polarization preserving optical fibers 41 and 42 and each polarization component folded back by the polarization preserving optical fibers 41 and 42 are converted into parallel beams and reentered into the diffraction grating 30. A single second lens 40 is used as a common lens. Further, the final order selection light (fourth order selection light in this example) is collected by the third and fourth lenses 45 and 46, passed through the slits 47 and 48, and received by the light receivers 50 and 51.

  For this reason, as compared with the conventional apparatus, only two polarization-preserving optical fibers are required, no outgoing optical fiber is required, and four lenses can be formed, and the size and cost can be significantly reduced.

(Second Embodiment)
The wavelength selection unit 29 of the above embodiment is composed of a single diffraction grating 30 and performs wavelength selection processing four times by the diffraction grating 30 for a specific wavelength band. However, when higher wavelength selectivity is required. 4 and 5, the wavelength selection unit 29 is configured by two opposing diffraction gratings 30 and 31, and the light incident on the wavelength selection unit 29 has a specific wavelength band. By performing the wavelength selection processing by the diffraction gratings 30 and 31 and emitting the light, it is possible to obtain light that has been subjected to the wavelength selection processing eight times in total.

  In this case, the two diffraction gratings 30 and 31 have the same characteristics (same engraving interval), and the engraving lines are parallel to the z axis, and are opposed to each other while satisfying a predetermined positional relationship described later.

  Here, the diffraction grating 30 receives the respective polarization components Pp (0) and Ps (0), whose polarization directions are matched, at the same incident angle at positions shifted in the direction of the score line (z-axis direction), and receives the diffracted light. The light is emitted to the second diffraction grating 31.

  The diffraction grating 31 receives diffracted lights Pp (1) and Ps (1) in a specific wavelength band among the diffracted lights emitted from the diffraction grating 30 at a predetermined incident angle, and the reflecting surfaces 35a and 35b of the orthogonal mirror 35. Light in a specific wavelength band is emitted as primary selection light Pp (2) and Ps (2) in a direction (specific direction) orthogonal to the boundary line 35c. In the following description, in order to correspond to the first embodiment, light emitted from the wavelength selection unit 29 is referred to as n-th order selection light, and light between the diffraction gratings 30 and 31 is simply referred to as diffracted light. Call it.

  The orthogonal mirror 35 has two reflecting surfaces 35a and 35b orthogonal to each other on a plane orthogonal to the engraving line of the diffraction grating 31, and the first-order selection light Pp (2) and Ps ( 2) is folded back to the diffraction grating 31 with an optical axis that is parallel to the incident time and shifted in the direction of the marking, as described above. The rotation device 36 sweeps a specific wavelength by changing the angle of one of the diffraction gratings 30 and 31 (here, the diffraction grating 30) within a predetermined range with respect to the other.

  The diffracted lights Pp (3) and Ps (3) in the specific wavelength band are shifted in the z-axis direction with respect to the optical axes of the diffracted lights Pp (1) and Ps (1) from the diffraction grating 31 that has received the folded light. The diffraction grating 30 is re-incident on the diffraction grating 30 in the state, and the diffraction grating 30 has an optical axis parallel to the optical axis of the incident light Pp (0) and Ps (0) with respect to the diffracted lights Pp (3) and Ps (3). Secondary selection light Pp (4), Ps (4) in a specific wavelength band is emitted.

  The secondary selection lights Pp (4) and Ps (4) are incident on the second lens 40 via the beam expander 25 and are on the one end 41a and 42a side of the polarization preserving optical fibers 41 and 42, as described above. The light is condensed and folded, and emitted from the other end 41b, 42b side to the second lens 40 in the same polarization state.

  The folded lights Pp (4) ′ and Ps (4) ′ are incident on the diffraction grating 30 via the beam expander 25 at the same incident angle as the first incident lights Pp (0) and Ps (0). Diffraction light Pp (5), Ps (5) in a specific wavelength band is obtained by the wavelength selection processing of the diffraction grating 30, and the specific wavelength is obtained by the wavelength selection processing of the diffraction grating 31 for the diffraction light Pp (5), Ps (5). Band third-order selection lights Pp (6) and Ps (6) are obtained, which are folded back by the orthogonal mirror 35, and the wavelength of the diffraction grating 31 with respect to the folded lights Pp (6) ′ and Ps (6) ′. The diffracted light Pp (7) and Ps (7) in the specific wavelength band is obtained by the selection process, and the fourth order of the specific wavelength band is obtained by the wavelength selection process of the diffraction grating 30 for the diffracted light Pp (7) and Ps (7). Selection lights Pp (8) and Ps (8) are obtained.

  The fourth-order selection lights Pp (8) and Ps (8) finally obtained in this way are parallel to the optical axes of the incident lights Pp (0) and Ps (0) and in the z-axis direction, as described above. Are incident on the third lens 45 and the fourth lens 46, respectively, along the optical axis deviated from each other, pass through the first slit 47 and the second slit 48, and the first light receiver with higher wavelength selectivity. 50 is incident on the second light receiver 51.

  Accordingly, the first light receiver 50 and the second light receiver 51 include the light incident angle on the diffraction grating 30 and the angles of the diffraction gratings 30 and 31 and the orthogonal mirror 35 among the two orthogonal polarization components of the incident light Px. The components in the specific wavelength band determined by the position are incident upon receiving eight wavelength selection processes, and the intensity thereof is detected. As the one diffraction grating 30 rotates, the selected specific wavelength band is swept, and signals PDp (λ) and PDs (λ) representing the intensity of the polarization component for each wavelength of the light to be measured are output. The

  Then, the processing for the signals PDp (λ) and PDs (λ) is performed in the same manner as described above, and the spectrum characteristics and In Band OSNR for each polarization component are obtained.

  Next, the arrangement of the two diffraction gratings 30 and 31 for realizing the above operation will be described. As shown in FIG. 8, the incident angle to the diffraction grating 30 at the center wavelength in the measurement wavelength range is i1, the diffraction angle is β1, the angle formed by the incident angle and the diffraction angle is 2 · γ1, and the incident angle of the diffraction grating 31 is i2, the diffraction angle is β2, and the angle between the incident angle and the diffraction angle is 2 · γ2.

  Here, as shown in FIG. 8, the diffraction grating 30 and the imaginary diffraction grating 31 ′ are arranged symmetrically at the vertices B and F of the isosceles triangle EBF in which two equal angles are (90−β1). . Further, it is assumed that the optical axis of the incident light of the diffraction grating 30 coincides with the line segment AB, and the optical axis of the diffracted light coincides with the line segment BF. However, the point A is a point on the bisector drawn from the vertex E to the side BF.

  Considering the incident angle and the diffraction angle of the diffraction grating 30 and the imaginary diffraction grating 31 ', as is clear from the symmetry of the figure, the diffraction grating 30 is incident on the light incident at an incident angle i1 from A to B. Emits diffracted light from B → F at a diffraction angle β1 and makes it incident on the diffraction grating 31 ′ at an incident angle i2 equal to β1, and the diffraction grating 31 ′ is equal to i1 with respect to the incident light. Diffracted light is emitted from F → A at the bending angle β2.

  Subsequently, the diffracted light emitted from F → A is folded back along the optical axis that is parallel to the optical axis and shifted in the engraving direction, and enters A → F at the diffraction grating 31 ′ at an incident angle i3 equal to β2. Incident light is emitted from F → B at a diffraction angle β3 equal to i2.

  The diffracted light is incident on the diffraction grating 30 at an incident angle i4 equal to β1, and is emitted in the B → A direction at a diffraction angle β4 equal to i1.

  Accordingly, the imaginary diffraction grating 31 ′ is translated along the line segment BF to a mountable position closer to the diffraction grating 30 as shown in the figure, and the line segment from the diffraction surface of the diffraction grating 31 is translated. If the orthogonal mirror 35 is arranged on the optical axis parallel to the AF, the wavelength selection and folding of the specific wavelength band by 2 degrees can be realized in a compact state while maintaining the angular relationship of the optical axis.

  Then, the selected wavelength can be swept by rotating the diffraction grating 30 with respect to the diffraction grating 31 about an axis parallel to the score line.

  At this time, in order to reflect (fold back) the fixed diffraction grating 31 by 180 degrees with the orthogonal mirror 35, the diffraction angle β2 of the diffraction grating 31 needs to be constant (the optical axis moves in parallel), For the fixed diffraction angle β2, it is necessary to change the wavelength λ and the incident angle i2 so as to satisfy the following conditions.

sin (i2) = m · λ / d−sin (β2)
Here, sin (β2) is a constant determined by the arrangement angle of the orthogonal mirror 35 and the diffraction grating 31, m is the diffraction order, and d is the groove spacing of the diffraction grating.

  The wavelength λ can be swept by rotating the diffraction grating 30 while satisfying the above conditions.

  As described above, the optical spectrum analyzer 20 ′ of the second embodiment has one more diffraction grating of the wavelength selection unit 29 than the optical spectrum analyzer 20 of the first embodiment, but the measured light Px is the first. The light is converted into parallel light by the lens 22 and is incident on the polarization separation unit 23 to separate the orthogonal polarization components. One of the polarization directions is rotated by 90 degrees by the half-wave plate 24 to match the other polarization direction. The point where the light is incident on the selection unit 29, the lens for converging each polarization component to the polarization preserving optical fibers 41 and 42, and each polarization component folded back by the polarization preserving optical fibers 41 and 42 The single second lens 40 shares a lens for making a parallel beam and re-entering the wavelength selection unit 29, and the final selection light (fourth selection light) is the third, 4 lenses 45 and 46 A common point which is received by the photodetector 50 and 51 light is passed through a slit 47.

  For this reason, compared with the conventional device, only two polarization-preserving fibers are required, no outgoing optical fiber is required, and four lenses can be formed, which is extremely small and low in cost and has high wavelength selectivity. A spectrum analyzer can be realized.

  In the embodiment, two orthogonal polarization components of the light to be measured Px are incident on the polarization separation unit 23 in the best state along the x-axis and the y-axis, respectively, so that the two polarization components are correctly separated. However, in the case of a polarization multiplexed optical signal, if there is a deviation in the polarization plane at the time of incidence, the polarization components of each other are not correctly separated, and highly accurate measurement cannot be performed.

  In such a case, as shown in FIG. 9, a polarization controller 70 is inserted between the light incident terminal 20a and the polarization separation unit 23, and the polarization plane of the measured light Px is adjusted by the polarization controller 70. What is necessary is just to set it as the state (For example, the state where the sum of the level of both polarization components becomes the maximum) which can correctly isolate | separate the orthogonal | polarized-light component of the to-be-measured light Px by the polarization separation part 23.

  20, 20 '... Optical spectrum analyzer, 20a ... Light incident terminal, 22 ... First lens, 23 ... Polarization separator, 23a ... Polarizing beam splitter, 23b ... Planar mirror, 24 ... 1/2 Wave plate 25 ...... Beam key spander 29 ...... Wavelength selection unit 30, 31 ...... Diffraction grating 35, Orthogonal mirror 36, Rotating device 40, Second lens 41, 42, ... Polarization-preserving optical fiber, 45 ... 3rd lens, 46 ... 4th lens, 47 ... 1st slit, 48 ... 2nd slit, 50 ... 1st light receiver, 51 ... 1st 2 light receivers, 52, 53... A / D converter, 60... Arithmetic processing unit, 61... Display unit, and 70.

Claims (4)

A light incident terminal (20a) for making the light to be measured incident;
A first lens (22) that collimates the light to be measured incident on the light incident terminal;
A polarization separation unit (23) that receives the parallel light emitted from the first lens and separates the parallel light components into orthogonal polarization components;
A half-wave plate (24) that receives one of the polarization components emitted from the polarization separation unit, rotates the polarization direction by 90 degrees, and matches the polarization direction of the other polarization component;
It has at least one diffraction grating (30, 31) in which engraving lines are provided in parallel on one surface side, and each polarization component whose polarization direction is matched is incident on the position shifted in the length direction of the engraving line. A wavelength selection unit (29) that receives light at a corner and selectively emits light in a specific wavelength band in a specific direction;
It has two reflection surfaces (35a, 35b) orthogonal to each other on a plane orthogonal to the score line of the diffraction grating of the wavelength selection unit, and the specific wavelength band emitted from the wavelength selection unit in the specific direction. Orthogonal mirror (35) that receives the selective light on one reflecting surface, reflects it to the other reflecting surface, reflects it further on the other reflecting surface, and folds it along the optical axis that is parallel to the incident and shifted in the direction of the engraving When,
The orthogonal mirror or the diffraction grating of the wavelength selection unit is rotated by a predetermined angle range about an axis parallel to the score line, and the center wavelength of the light in the specific wavelength band that is folded back from the orthogonal mirror to the wavelength selection unit is swept. A rotating device (36) for causing;
Second, third, and fourth condensing lenses (40, 45, 46) respectively disposed on optical axes that are parallel to the incident optical axis with respect to the wavelength selection unit and shifted in the score direction;
First and second polarization-preserving optical fibers that receive light collected by the second lens on one end side, propagate a predetermined distance, and return to the second lens in the same polarization state as incident from the other end side. (41, 42),
First and second slits (47, 48) for increasing the wavelength selectivity by limiting the width of the light collected by the third and fourth lenses in the direction perpendicular to the score line, respectively. ,
First and second light receivers (50, 51) that receive light that has passed through the first and second slits and detect the intensity thereof,
For each polarization component with the polarization direction matched, the wavelength selection unit folds the primary selection light emitted in the specific direction to the wavelength selection unit with an optical axis shifted in the engraving direction by the orthogonal mirror,
The second selective light of the specific wavelength band selected by the wavelength selective action that is reversible with the time indicated by the wavelength selection unit with respect to the folded light is incident on the second lens, and each polarization component is Incident first and second polarization preserving optical fibers,
Third-order selection of the specific wavelength band that the wavelength selection unit emits in the specific direction with respect to the light that is turned back by the first and second polarization preserving optical fibers and enters through the second lens The light is turned back again by the orthogonal mirror,
The first and fourth lenses condense each polarization component of the fourth-order selection light in the specific wavelength band emitted by the wavelength selection unit with respect to the light refolded by the orthogonal mirror, and the first lens Then, the light passes through the second slit, is incident on the first and second light receivers, and based on the outputs of the first and second light receivers, the spectral characteristics of the orthogonal polarization components of the measured light An optical spectrum analyzer characterized by   2. The optical spectrum analyzer according to claim 1, wherein the wavelength selection unit is constituted by a single diffraction grating. The wavelength selector is composed of two diffraction gratings facing each other;
2. The optical spectrum analyzer according to claim 1, wherein the light incident on the wavelength selection unit is emitted after receiving a wavelength selection action of two times by the two diffraction gratings for the specific wavelength band.   The polarization controller (70) for controlling the polarization of the light to be measured incident on the polarization separation unit is disposed between the light incident terminal and the polarization separation unit. 2. An optical spectrum analyzer according to 2.

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