JP2002031714A - Optical filter and tunable optical filter device - Google Patents

Abstract

(57) [Problem] To provide a wavelength tunable optical filter that enables high-speed and high-accuracy transmission center wavelength sweeping without receiving mechanical disturbance and realizes high-accuracy and high-resolution optical wavelength separation, and Provided is a wavelength tunable optical filter device used. This optical filter is a wavelength-variable optical filter having a disk-shaped outer shape, and has a disk-shaped transparent parallel flat substrate (4) and a film thickness that changes with respect to an expected angle around a rotation axis of the disk. A cavity (resonator portion) is formed integrally with a spacer layer 2 made of a dielectric film having the same refractive index as that of the substrate, and a high-reflectance mirror 3 made of a multilayer coating is arranged on both sides of the cavity. Thus, an etalon-type resonator is formed. This eliminates the need to move the mirror directly to control the resonator length, and can eliminate mechanical disturbance. It is preferable to provide a rotary encoder mark for detecting an absolute position on the outer edge of the optical filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an etalon-type optical filter having a variable transmission center wavelength, and a wavelength-variable optical filter device which uses the optical filter to sweep the transmission center wavelength at high speed.

[0002]

2. Description of the Related Art An etalon-type optical filter has been used as a narrow-band optical filter required for optical spectrum analysis. An etalon-type optical filter refers to an optical resonator formed between two mirrors arranged to face each other. Light that can pass through such an optical resonator is limited to light having a specific wavelength. This wavelength is equal to the resonance wavelength of the optical resonator, and the higher the Q value of the resonator, the narrower the transmission wavelength bandwidth.

In general, a transmittance T when a mirror having a reflectance R is opposed to a mirror is represented by a plane wave approximation.

[0004]

(Equation 1)

[0005] Here, λ is a transmission wavelength, L is a resonator length (distance between mirrors), and n is a refractive index of a substance filled between the mirrors.

The equation (1) is obtained by calculating the optical frequency v (= c / λ)
In conversion,

[0007]

(Equation 2)

[0008] At the same time, the transmission bandwidth (full width at half maximum) w is

[0009]

(Equation 3)

[0010] It is also shown that

Therefore, an extremely narrow transmission bandwidth can be realized by increasing the length L of the resonator at the same time as increasing the reflectivity of the mirror (making R close to 1). For example, even if R = 0.99, L = 2 mm, n = 1.5
The glass (cavity part) is used as the cavity (resonator part), so that the bandwidth (full width at half maximum) is w = 16 MHz (0.001
25 nm). This bandwidth is a conventional bandpass filter in which a half-wavelength spacer layer is sandwiched between high-reflection mirrors of a multilayer coating in which two types of quarter-wave films having different refractive indexes are alternately laminated. Is significantly smaller than the bandwidth (about 0.3 nm).

Optical spectrum measurement is realized by utilizing such a steep transmission wavelength characteristic of an etalon type optical filter having a narrow bandwidth. FIG. 7 is a diagram for explaining the principle of this etalon-type optical filter. The periodicity of the transmission wavelength characteristic shown by the above equation (2) is shown. One of the peaks has a wavelength selection function with the transmission bandwidth as the resolution.
The center wavelength of this peak can be controlled by moving the mirror constituting the resonator and changing the resonator length L as shown by the broken line in FIG. In addition, the amount of movement of the mirror required for sweeping one cycle of the peak wavelength is a half wavelength.

[0013]

As described above, since the transmission center wavelength of the conventional etalon type optical filter is determined by the distance between the mirrors, when the center wavelength is to be set precisely, this distance is precisely controlled. There is a need to. But,
Since the distance between mirrors mainly fluctuates due to mechanical and thermal disturbances, it has been difficult to precisely set the transmission center wavelength. Here, the mechanical disturbance refers to irregular resonator length fluctuations caused by natural vibration of a mirror constituting the etalon and a mount supporting the mirror, and has a band covering about several tens of kilohertz. On the other hand, thermal disturbance refers to a resonator length variation caused by thermal expansion of the support mount, the mirror substrate, and the mirror itself, and the bandwidth of the variation is limited to about several hertz. Since it also depends on the structure of the mechanical components constituting the resonator, drift fluctuations that are difficult to predict are caused.

Further, when trying to quickly and precisely sweep the transmission center wavelength required for optical spectrum measurement, the limitation of speeding up due to the movement form and the mechanical disturbance essentially generated by the movable mechanism are caused by the high speed and precision. It was a major impediment to the purpose of the sweep. Here, the movement form refers to a reciprocating linear movement accompanied by an acceleration movement of the mirror along the optical axis of the resonator for the wavelength sweep, and the sweep speed is limited by the driving force of the actuator.

The present invention has been made in view of the above points, and has as its object to provide an etalon-type optical filter which is not affected by mechanical disturbance while its transmission center wavelength is variable, and a transmission filter using this optical filter. An object of the present invention is to provide a wavelength tunable optical filter device capable of quickly and precisely sweeping a center wavelength.

[0016]

In order to achieve the above object, an optical filter according to the present invention is a wavelength-variable optical filter having a disk-shaped outer shape, and comprises a disk-shaped transparent parallel filter. A planar substrate and a spacer layer made of a dielectric film whose film thickness changes with respect to the expected angle around the rotation axis of the disk and having the same refractive index as the substrate constitute a cavity (resonator portion). In addition, a high-reflectance mirror made of a multilayer coating is arranged on both sides of the mirror to form an etalon-type resonator.

With this configuration, if the optical filter of the present invention is used, it is not necessary to move the mirror directly to control the resonator length, and it is possible to eliminate mechanical disturbance.

Here, a rotary encoder mark for detecting an absolute position is provided on either the outer edge of the disk-shaped wavelength tunable optical filter (hereinafter referred to as a disk) or the disk surface. it can. With this configuration, it is possible to predict the wavelength characteristics of the optical filter only from the material properties without depending on the structure of the mechanical components of the resonator.

According to a third aspect of the present invention, there is provided a wavelength tunable optical filter device comprising: an optical system for connecting a pair of optical fiber ends facing each other with a collimated beam; A wavelength-variable optical filter according to claim 1 or 2, wherein the wavelength-variable optical filter is disposed so as to sandwich the wavelength-variable optical filter so as to be vertically transmitted through the substrate, and a disk (disk-shaped wavelength-variable optical filter) is rotated to perform collimation The transmission wavelength characteristic is controlled by changing the transmission position of the beam.

With such a configuration that limits the incident angle,
According to the tunable optical filter device of the present invention, the transmission center wavelength can be swept with high accuracy and high speed without depending on the beam alignment.

Here, the drive mechanism for rotating the disk may have a control means for rotating at a constant speed in synchronization with an electric signal. With this configuration, it is possible to rotate the disk at a constant speed with electrical accuracy, and as a result, it becomes possible to sweep the transmission center wavelength stably at high speed without mechanical disturbance.

According to a fifth aspect of the present invention, there is provided a wavelength tunable optical filter according to the second aspect, wherein the detecting means detects a rotary encoder mark and counts the output of the detecting means. Counter means for providing the count value as position information of the wavelength tunable optical filter; and using the position information provided by the counter means as a parameter, a center wavelength of the transmitted light spectrum and a rate of change of the center wavelength are obtained. Calculating the optimum instantaneous rotation speed such that the time change rate of the center wavelength of the transmitted light spectrum becomes constant, and the optimum instantaneous rotation using the position information calculated by the first operation means as a parameter The instantaneous rotation speed is calculated from the position information provided by the storage means for storing the speed, and the position information provided from the counter means. A second calculating means for calculating a deviation from the instantaneous rotational speed with reference to the instantaneous rotational speed; and a rotation of a motor for rotating the tunable optical filter using the deviation calculated by the second calculating means as a rotational speed correction value. A rotation number control means for finely adjusting the number. With this configuration, the center wavelength of the transmitted light spectrum can be swept linearly.

By using, for example, an arithmetic processing processor and a random access memory as the arithmetic means, the center wavelength of the transmitted light spectrum and the rate of change of this central wavelength can be determined using the position information provided by the counter circuit as a parameter (probable angle). Then, from this change rate, the optimum instantaneous rotation speed can be calculated such that the time change rate of the center wavelength of the transmitted light spectrum becomes constant. That is, the relationship between the center wavelength λc of the transmitted light spectrum and the expected angle θ is

[0024]

(Equation 4)

It is assumed that Here, Π (θ) represents a deviation from the linearity of the rate of change of the transmission center wavelength with respect to the expected angle. From this equation (4), the time change rate of the transmission center wavelength is

[0026]

(Equation 5)

Can be written as Here, the time derivative of the angle is the instantaneous rotational angular velocity ω of the disk. Separating the instantaneous angular velocity into a constant rotation speed and a deviation component

[0028]

(Equation 6)

[0029]

[0030]

(Equation 7)

## EQU1 ## Here, the second minute amount was ignored.
It is assumed that the time change rate of the transmission center wavelength is always constant as follows.

[0032]

(Equation 8)

Applying this to the above equation (7),

[0034]

(Equation 9)

## EQU1 ## This δω is a correction value of the rotational angular velocity of the disk to satisfy Expression (8). Therefore, the instantaneous angular velocity

[0036]

(Equation 10)

By controlling the rotation of the disk as follows, the transmission center wavelength is swept linearly.

By utilizing the arithmetic processing processor and the random access memory, the instantaneous rotation speed is actually measured from the position information of the disk provided from the counter circuit, and at the same time, the expected angle stored in the non-volatile memory by the position information of the disk. The deviation from the actually measured instantaneous rotation speed can be calculated with reference to the optimum instantaneous rotation speed. If this deviation is input as a rotation speed correction value (error signal) to a circuit for finely adjusting the rotation speed of the motor, the above-described linear sweep of the center wavelength of the transmitted light spectrum can be realized.

The invention described in claim 6 is the same as that in claim 5
A detecting means for detecting a rotary encoder mark provided on the tunable optical filter according to claim 2; and counting the output of the detecting means to change the count value to the position of the tunable optical filter. Counter means for providing the information, position information provided by the counter means as a parameter, storage means for storing the center wavelength of the transmitted light spectrum and the rate of change of the center wavelength, and instantaneous information from the position information provided by the counter means. The rotational speed is calculated, and the rate of change of the center wavelength of the transmitted light spectrum of the storage means is referred to from the position information. Is calculated, and the deviation between the optimum instantaneous rotational speed and the calculated instantaneous rotational speed is calculated. It characterized by having a calculating means for generating a driving signal which gives the optimum rotation to the motor to rotate the motor.

[0040]

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

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
1 shows an outer shape of a wavelength tunable optical filter according to the embodiment. This tunable optical filter 1 has a disk shape. The inner circumference and the outer circumference of the disk (disk-shaped tunable optical filter) 1 are substantially concentric. The transmission wavelength characteristic of the wavelength tunable optical filter 1 changes linearly (at a constant rate) with respect to an expected angle around the rotation axis of the disk.

FIG. 2 shows a sectional structure of the tunable optical filter 1 described above. On a disk-shaped parallel flat substrate 4, a spacer layer 2 of a dielectric multi-layered film having a film thickness which changes in proportion to an estimated angle is deposited. By matching the refractive index of the substrate 4 with the refractive index of the dielectric film of the spacer layer 2, they function as an integral optical medium. In order to make the refractive indices coincide, the materials of these optical media may be made the same. For example, when fused quartz is used for the substrate 1, a dense SiO ₂ film having substantially the same refractive index as a bulk deposited by a sputtering method can be used as the spacer layer 2. A high reflectivity mirror 3 made of a multilayer high reflectivity coating is provided on the front and back of the substrate with the film.

The multilayer high reflectivity coating is obtained by alternately laminating two kinds of quarter-wave films having different refractive indexes. It is desirable that the higher refractive index of the quarter-wave film is higher than the refractive index of the substrate 4. H,
When L (H: high refractive index, L: low refractive index), the layer structure of the high-reflectance mirror 3 is as follows.

[0044]

[Equation 11]

N is a positive integer.

With such a configuration, it is possible to realize the tunable optical filter 1 whose wavelength characteristic changes along the circumferential direction. The wavelength characteristic does not change in the radial direction of the disk (disk-shaped wavelength-variable optical filter) 1.

(Second Embodiment) FIG. 3 shows a tunable optical filter according to a second embodiment of the present invention. As shown in FIG. 3A, a rotary encoder mark 5 is provided on the outer peripheral portion of the disk (disk-shaped variable optical filter) 1 having the configuration described with reference to FIGS. When the mark 5 is provided on the disk surface, the area near the outer edge of the disk 1 is a mark drawing area in order to maximize the angular resolution.

As shown in FIG. 3B, a general mark that can be used as the rotary encoder mark 5 is:
It comprises a Z-phase mark 6, an A-phase mark 7, and a B-phase mark 8.
The Z-phase mark 6 is a mark that provides a reference for the estimated angle, and is provided only once per revolution. The A-phase mark 7 and the B-phase mark 8 are marks that give relative angles of the expected angles, and the rotation direction can be known from the phase difference shifted by 90 degrees from each other. When such a mark 5 is detected by transmitted light, a metal film pattern such as chromium can be used. Even when the filter area extends over the entire surface of the disk 1, light having a wavelength significantly different from the wavelength to be filtered can pass through the filter, and can be used as mark detection light. For example, if the filter targets a long wavelength band of 1.5 μm, short wavelength light of 0.8 μm can be used.

When the temperature changes, the transmission wavelength characteristic of the optical filter 1 changes due to thermal expansion. However, regardless of the temperature change, the position of the rotary encoder mark 5 does not change in the circumferential direction. For this reason, the temperature characteristic of the transmission wavelength characteristic of the optical filter 1 can be predicted because it substantially depends only on the physical properties (coefficient of thermal expansion) of the material constituting the resonator of the optical filter 1. That is, the correction value of the transmission wavelength characteristic can be easily calculated from the output of the temperature sensor. Furthermore, if SiO ₂ having an extremely low expansion coefficient is used as the material of the resonator,
It is possible to realize a temperature-variable optical filter that has extremely low temperature dependence and is stable in temperature.

(Third Embodiment) FIG. 4 shows a configuration of a tunable optical filter device according to a third embodiment of the present invention. The collimating lens 1 is placed on the exit end face of the optical fiber 9.
A pair of optical systems for obtaining collimated light 11 by disposing 0 are prepared. By opposing the optical systems as shown in FIG. 4, a system for coupling the optical fibers 9 with the collimated light 11 is realized. The disc-shaped wavelength tunable optical filter (disc) 1 shown in FIG. 1 or FIG.
Is disposed so as to transmit light vertically to the substrate 4 of FIG.

With this configuration, an effective optical resonator is formed along the optical axis of the collimated light 11. When the disk 1 is rotated, the transmission position of the collimated light 11 changes,
The length of the effective resonator changes. Therefore, if the disk 1 is precisely rotated, the center wavelength of the transmitted light spectrum can be precisely controlled. The rate of change of the resonator length with respect to the prospective angle along the circumference of the disk 1 is very small, so that even a mechanical positioning control can control a very small length of sub-nanometer or less.

Further, if the disk 1 is mounted on the spindle (not shown) of the motor so as not to be eccentric, high-speed rotation is possible, and therefore, high-speed wavelength sweep is possible.

(Fourth Embodiment) FIG. 5 shows a configuration of a wavelength variable optical filter device according to a fourth embodiment of the present invention, and shows a specific example in which a drive system is added to the configuration of FIG. As a motor for rotating the disk 1, a servo motor 16 whose rotation speed can be controlled by voltage is used. The servo motor 16 includes an encoder (not shown) that generates an instantaneous rotation speed signal 13. The motor drive circuit 14 monitors the rotation speed signal 13 and supplies a drive signal 12 to the servo motor 16 so that the rotation speed of the disk 1 becomes constant, thereby obtaining a stable disk rotation.

As a method of performing such drive control, for example, a phase is compared between a stable electric signal serving as a reference (reference signal) and the rotational speed signal 13 and an error signal component thereof is converted into an appropriate loop filter ( A well-known phase-locked loop circuit that obtains rotation synchronized with an electric signal by performing phase compensation by a not-shown) can be used.

(Fifth Embodiment) FIG. 6 shows the configuration of a tunable optical filter device according to a fifth embodiment of the present invention. The configuration shown in FIG. 5 linearly sweeps the center wavelength of the transmitted light spectrum. A specific example of adding a control system for this will be shown. A rotary encoder mark 5 is provided on the outer edge of the disc-shaped variable optical filter 1, and a Z-phase signal, an A-phase signal, and a B-phase signal are obtained by the encoder sensor 15. These signals are input to the counter circuit 18 and the position information of the disk 1 based on the Z phase is obtained from the count value.

As described in detail in the section of the means for solving the problems, if this position information is processed by the arithmetic processor 19, an instantaneous rotation speed ω ₀ can be obtained and further stored in the nonvolatile memory 21 in advance. By referring to the obtained optimum instantaneous rotation speed, a rotation speed correction value δω for obtaining a constant sweep transmission wavelength can be obtained.
4 is given to the rotation speed fine adjustment circuit 25 via the input / output interface 22. At the same time, the rotation speed signal 13 is input to the motor drive circuit 14 for stably rotating the servo motor 16 itself via the input / output interface 22 as described above.

The rotation speed fine adjustment circuit 25 obtains a drive signal 12 for giving an optimum rotation from these signals (the fine adjustment amount 24 and the rotation speed signal 13), and controls the servo motor 16 by the drive signal 12. As a result, the sweep speed of the transmission center wavelength (the center wavelength of the transmission light spectrum) at a constant rate is realized.

Such a servo control system includes a data bus 2
By transferring the data more quickly in 3, a loop including arithmetic processing is established.

(Other Embodiments) The drive signal 12 for giving the above-mentioned optimum rotation can also be generated by calculation processing by the arithmetic processor 19, and in this case, this optimum rotation is given. Even if the drive signal 12 is directly input to the motor drive circuit 14 via the input / output interface 22 to control the rotation, a linear transmission center wavelength sweep speed can be realized.

In the fifth embodiment, the optimum instantaneous rotational speed is stored in the nonvolatile memory 21. However, the present invention is not limited to this.
1 stores, in advance, the center wavelength of the transmitted light spectrum and the rate of change of this center wavelength using the position information provided from the counter means as a parameter, and a rotational speed correction value δω for obtaining the above-mentioned constant swept transmission wavelength. May be configured such that the arithmetic processor 19 calculates the optimum instantaneous rotation speed based on the rate of change of the center wavelength.

The above-mentioned optimum instantaneous rotational speed and instantaneous rotational speed may be optimum instantaneous angular speed and instantaneous angular speed, respectively.

[0062]

As described above, according to the present invention,
Without receiving mechanical disturbance, it is possible to perform high-speed wavelength separation having high wavelength accuracy as well as wavelength resolution.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating an outer shape of an optical filter according to a first embodiment of the present invention.

FIG. 2 is a sectional view showing a resonator structure of the optical filter according to the first embodiment of the present invention.

FIG. 3A is a perspective view showing an outer shape of an optical filter according to a second embodiment of the present invention, and FIG. 3B is an enlarged view of a rotary encoder mark provided on an outer peripheral edge of the optical filter. .

FIG. 4 is a perspective view illustrating a configuration of a wavelength tunable optical filter device according to a third embodiment of the present invention.

FIG. 5 is a perspective view illustrating a configuration of a wavelength tunable optical filter device according to a fourth embodiment of the present invention.

FIG. 6 is a block diagram illustrating a configuration of a tunable optical filter device according to a fifth embodiment of the present invention.

FIG. 7 is a graph illustrating the principle of the wavelength sorting function of an etalon-type tunable optical filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Disc-shaped wavelength tunable optical filter (disk) 2 Spacer layer 3 High reflectivity mirror (multilayer high reflectivity coating) 4 Parallel plane substrate 5 Rotary encoder mark 6 Z phase mark 7 A phase mark 8 B phase mark 9 Optical fiber 10 Collimating lens 11 Collimated light 12 Drive signal 13 Rotation speed signal 14 Motor drive circuit 15 Encoder sensor 16 Servo motor 18 Counter circuit 19 Arithmetic processor 20 Random access memory 21 Nonvolatile memory 22 Input / output interface 23 Data bus 24 Fine adjustment amount

Continued on the front page F term (reference) 2H041 AA21 AB10 AC01 AZ03 AZ06 AZ08 2H048 GA09 GA13 GA22 GA35 GA48 GA51 GA62

Claims (6)

[Claims] 1. A disk-shaped wavelength tunable optical filter, comprising: a disk-shaped transparent parallel flat substrate; and a layer laminated on one surface of the substrate and proportional to an expected angle along a disk rotation axis. A spacer layer of a dielectric film having the same refractive index as the substrate having a variable thickness, and a high-reflectance mirror made of a multilayer coating laminated on the upper surface of the spacer layer and the other surface of the substrate, An optical filter, wherein the spacer layer and the substrate are integrally formed to form a resonator portion. 2. A rotary encoder mark for detecting an absolute position is provided at one of an outer edge portion of the disc-shaped tunable optical filter and a disc surface of the tunable optical filter. An optical filter according to claim 1. 3. An optical system for connecting a pair of optical fiber ends facing each other by a collimated beam so that the collimated beam vertically passes through the substrate of the tunable optical filter according to claim 1 or 2. A wavelength tunable optical filter device, wherein the wavelength tunable optical filter is interposed, and the transmission wavelength characteristic is controlled by rotating the disk-shaped wavelength tunable optical filter to change the transmission position of the collimated beam. 4. The tunable optical filter device according to claim 3, wherein the drive mechanism for rotating the disk-shaped tunable optical filter has control means for performing constant-speed rotation in synchronization with an electric signal. 5. A detecting means provided on the tunable optical filter according to claim 2, for detecting the rotary encoder mark, and an output of the detecting means is counted, and the count value is used as positional information of the tunable optical filter. And a center wavelength of the transmitted light spectrum and a rate of change of the center wavelength, using the position information provided by the counter means as a parameter, and calculating the time of the center wavelength of the transmitted light spectrum from the change rate. First calculating means for calculating an optimum instantaneous rotational speed such that the rate of change is constant; storage means for storing the optimum instantaneous rotational speed using the position information calculated by the first calculating means as a parameter; Calculating an instantaneous rotation speed from the position information provided by the counter means, and calculating the optimum instantaneous rotation of the storage means from the position information. A second calculating means for calculating a deviation from the instantaneous rotational speed with reference to a speed; and a motor for rotating the wavelength variable optical filter using the deviation calculated by the second calculating means as a rotation speed correction value. 5. The tunable optical filter device according to claim 3, further comprising a rotation speed control unit for finely adjusting the rotation speed. 6. A detecting means for detecting the rotary encoder mark provided on the tunable optical filter according to claim 2, and counting the output of said detecting means and using the count value as the position information of the tunable optical filter. Counter means for providing as a parameter, storage means for storing a center wavelength of a transmitted light spectrum and a rate of change of the center wavelength using the position information provided from the counter means as a parameter, and the position provided from the counter means. The instantaneous rotation speed is calculated from the information, the change rate of the center wavelength of the transmitted light spectrum in the storage means is referred to from the position information, and the time change rate of the center wavelength of the transmitted light spectrum is constant from the change rate. Calculate the optimal instantaneous rotational speed such that the deviation between the optimal instantaneous rotational speed and the calculated instantaneous rotational speed is calculated. 5. The tunable optical filter device according to claim 3, further comprising: a calculating unit configured to generate a drive signal that gives optimal rotation to a motor that rotates the tunable optical filter as a rotation speed correction value. 6.