JP5416175B2 - Manufacturing method of etalon filter - Google Patents

Description

  The present invention relates to an etalon filter used in a laser system or an optical communication system and a method for manufacturing the etalon filter.

The solid etalon filter is composed of a substrate and a reflection film provided on the incident / exit surface of the substrate, exhibits a periodic transmission waveform, and emits light at an m-order peak wavelength λ _m (peak frequency ν _m ). This is a filter whose transmittance increases sharply. FSR (Free Spectral Range) that is the frequency interval of the transmission peak is generally determined by the following equation (1).

ただし、ｃは光速、ｎは基板の屈折率、ｄは基板の厚み、θは基板内の屈折角度である。

Where c is the speed of light, n is the refractive index of the substrate, d is the thickness of the substrate, and θ is the angle of refraction within the substrate.

  According to Patent Document 1, it is important to accurately adjust the thickness d of the substrate in order to set the wavelength of the transmission waveform position. On the other hand, it is difficult to precisely adjust the thickness d of the substrate by optical polishing. To mention. Patent Document 1 proposes that the thickness d of the substrate is substantially adjusted with high accuracy by depositing a material having the same refractive index as that of the substrate on the entrance surface or the exit surface of the substrate. .

  In Patent Document 2, in order to satisfy the specification of the MD characteristic, the parallelism is polished with an accuracy within 10 ″ to 30 ″, and the FSR characteristic has an error of the plate thickness with an accuracy of about ± 1 to 3 μm. It mentions that it needs to be processed. In addition, in order to satisfy the specifications as described above, it is mentioned that an extremely excellent polishing technique is necessary, and by forming a thin film having a refractive index equivalent to that of the substrate on a substrate polished by machining. Proposes improvement of MD characteristics and fine adjustment of FSR characteristics.

Japanese Patent Laid-Open No. 6-208022 JP 2006-292911 A

  However, the FSR adjustment methods of Patent Documents 1 and 2 have various disadvantages. For example, since a material having the same refractive index as that of the substrate is formed, the number of steps and the material are increased by the amount of film formation. Further, as a result of intensive studies by the inventors of the present application, it has been found that even if the thickness d of the substrate can be accurately adjusted to a thickness determined based on the equation (1), the FSR does not become a desired value. That is, as in Patent Documents 1 and 2, even if the thickness d of the substrate is substantially adjusted with high accuracy so that the thickness d matches the designed thickness, a desired FSR cannot be obtained.

  An object of the present invention is to provide an etalon filter manufacturing method and an etalon filter capable of suitably adjusting the FSR.

The manufacturing method of the etalon filter which concerns on 1 aspect of this invention has a board | substrate and the 1st reflective film and 2nd reflective film which are arrange | positioned at the entrance / exit surface of the said board | substrate, The said 1st reflective film and the said 2nd reflective film Each of the reflective films is a method of manufacturing an etalon in which a plurality of thin films are stacked, the step of determining the design thickness _dt of the substrate, and the second reflective film the combination of the plurality of thin films, measuring the steps of providing a plurality of candidates ΔFSR are different from each other to be calculated by the following formula (I), and forming the substrate, the actual thickness d _r of the substrate A step of forming the first reflective film, a step of selecting one candidate from the plurality of candidates based on a comparison between the thickness d _t and the thickness _dr, and the plurality of thin films of the selected candidate Combination And forming the second reflective film.
ΔFSR = FSR ₁ −FSR ₀ (I)
However, FSR ₀ is the FSR in a case where the substrate thickness _{d t} of the first reflecting film and the second reflective film is not provided, FSR ₁ includes a plurality on the substrate thickness _{d t} of the thin film This is an FSR when a combination is provided as the first reflective film and the second reflective film.

  The etalon filter which concerns on 1 aspect of this invention has a board | substrate and the 1st reflective film and 2nd reflective film which are arrange | positioned at the entrance / exit surface of the said board | substrate, The said 1st reflective film and the said 2nd reflective film Each of them is formed by laminating a plurality of thin films, and the configuration of the first reflective film and the configuration of the second reflective film are different from each other.

  According to said procedure or structure, adjustment of FSR can be performed suitably.

FIG. 1A is a schematic side view of an etalon filter according to an embodiment of the present invention, and FIG. 1B is a diagram schematically showing transmission characteristics of the etalon filter. It is a figure which shows the some structural example of the reflecting film of an etalon filter. FIG. 3A and FIG. 3B are diagrams showing FSR and ΔFSR calculated by the FSR calculation method of the embodiment. The flowchart which shows the procedure of the manufacturing method of the etalon filter which concerns on embodiment. FIG. 5A and FIG. 5B are diagrams showing various configuration examples of the substrate and the reflective film and modifications thereof. The figure which shows FSR of the structural example of Fig.5 (a) and FIG.5 (b). FIG. 7A and FIG. 7B are diagrams showing other various structural examples and modified examples of the substrate and the reflective film. The figure which shows FSR of the structural example of Fig.7 (a) and FIG.7 (b). FIG. 9A and FIG. 9B are diagrams showing an operation when reflecting films having different configurations are combined. The block diagram which shows the application example of the etalon filter of embodiment.

(Outline of etalon filter)
Fig.1 (a) is a side view which shows typically the etalon filter 1 which concerns on embodiment of this invention. In the following description, for the sake of convenience of explanation, the reference to the etalon filter 1 may be attached to the related art or the comparative example.

  The etalon filter 1 includes a substrate 3, a first reflective film 5A provided on the first main surface 3a of the substrate 3, and a second reflective film 5B provided on the second main surface 3b of the substrate 3 (hereinafter simply referred to as “ It is referred to as a “reflective film 5”, and the two may not be distinguished from each other.)

  The substrate 3 is made of, for example, a glass material such as quartz or a crystal material such as quartz, and the first main surface 3a and the second main surface 3b are formed in a flat plate shape parallel to each other. The thickness d may be appropriately set according to desired optical characteristics such as FSR. The surface roughness and parallelism of the first main surface 3a and the second main surface 3b may also be appropriately set according to the desired optical characteristics and the accuracy thereof. For example, the surface roughness is less than 1 nm and parallel. The degree is less than 1 minute. Such a fine surface roughness and high parallelism can be obtained by optical polishing of the first main surface 3a and the second main surface 3b, for example.

The reflective film 5 is configured by laminating a plurality of thin films 7. The plurality of thin films 7 are made of, for example, a dielectric. Examples of the dielectric include silicon dioxide (SiO ₂ , refractive index = 1.445), titanium dioxide (TiO ₂ , refractive index = 2.3), tantalum pentoxide (Ta ₂ O ₅ , refractive index = 2.2). It is. The plurality of thin films 7 are arranged so that the thin films 7 having different refractive indexes are alternately stacked. The thin film 7 superimposed on the first main surface 3a and the second main surface 3b of the substrate 3 is a thin film 7 having a refractive index different from that of the substrate 3 (in the above example, other than SiO ₂ ). Further, the material, the number of layers, and the thickness of the plurality of thin films 7 are designed so that desired optical characteristics (for example, reflectance) can be obtained in the reflective film 5. As an example, the reflective film 5 is configured by laminating 10 or less thin films 7 having a thickness of about submicron. The thin film 7 is provided on the substrate 3 by vapor deposition, for example.

  The light Lt is incident on the etalon filter 1 in parallel or inclined with respect to the optical axis Ax orthogonal to the first main surface 3a and the second main surface 3b of the substrate 3. 1 illustrates the case where the light Lt is inclined and enters the etalon filter 1 and is transmitted through the substrate 3 at a refraction angle θ. The etalon filter 1 allows transmission of light having a desired wavelength and restricts transmission of other light. Any of the first main surface 3a and the second main surface 3b may be an incident surface or an output surface. In FIG. 1, the first main surface 3a is an incident surface, and the second main surface 3b is an output surface. The case where it is said is illustrated.

FIG. 1B schematically shows the transmission characteristics of the etalon filter 1. In FIG. 1B, the horizontal axis indicates the wavelength λ, and the vertical axis indicates the transmission coefficient T. The transmission coefficient T is a ratio I _out / I _in between the intensity I _in of the light Lt before being incident on the etalon filter 1 and the intensity I _out after being emitted from the etalon filter 1.

As is generally known, in the etalon filter 1, the transmission coefficient T periodically increases at the m-th order peak wavelength λ _m (peak frequency ν _m ). Incidentally, FSR is the spacing of the peak vibration frequency _{ν m (ν m -ν m +} 1), in FIG. 1 (b), shows between the peak wavelength of the FSR for help understanding.

(FSR calculation method)
Here, the inventor of the present application has found from the measurement results of FSR in the prototype of the etalon filter 1 that the FSR changes when the configuration (material, number of layers, thickness, etc.) of the reflective film 5 changes. On the other hand, as shown in the equation (1), in general, it is not recognized that the configuration of the reflective film 5 affects the FSR. Naturally, the configuration of the reflective film 5 is taken into consideration in the etalon filter 1. A method for calculating (predicting) the FSR based on various design values is not known.

  Therefore, the inventor of the present application has devised an FSR calculation method that takes into consideration the influence of the reflective film 5. The calculation method is based on Florin Abeles' matrix method. Specifically, it is as follows.

  The entire substrate 3 and the plurality of thin films 7 are considered as a multilayer structure composed of m layers of medium. At this time, the characteristic matrix Mj of the j-th (1 ≦ j ≦ m) medium is expressed by the following equations (2) and (3).

ただし、λは波長、ｎ_ｊはｊ番目の媒質の屈折率、ｄ_ｊはｊ番目の媒質の厚み、θ_ｊはｊ番目の媒質中の屈折角、δ_ｊはｊ番目の媒質の位相である。

Where λ is the wavelength, n _j is the refractive index of the j-th medium, d _j is the thickness of the j-th medium, θ _j is the refraction angle in the j-th medium, and δ _j is the phase of the j-th medium. .

  The characteristic matrix M of the multilayer structure is represented by the product of the matrix of each layer as shown in the following equation (4).

  The Fresnel reflection coefficient ρ and Fresnel transmission coefficient τ of this multilayer structure are expressed by the following equations (5) and (6).

ただし、ｎ_０及びｎ_ｍはそれぞれ、ｍ層の媒質のうち入射媒質及び出射媒質となる媒質の屈折率である。

Here, n ₀ and n _m are the refractive indexes of the medium serving as the incident medium and the output medium, respectively, among the m layers of medium.

  From the equations (5) and (6), the reflection coefficient R and the transmission coefficient T are expressed by the following equations (7) and (8).

  Then, in the above equation (8), the transmission coefficient T is calculated with the wavelength λ as a variable to obtain the maximum value, and the FSR is calculated from the wavelength interval between the maximum values.

  For the refractive index n, chromatic dispersion (wavelength dependence) must be considered. For example, the refractive index n is preferably calculated based on the wavelength λ according to the following equation (9).

ただし、Ａ_０〜Ａ_６は分散係数である。なお、基板あるいは薄膜に吸収がある場合は、屈折率のみならず消衰係数とその波長依存性も考慮する必要がある。

_However, A 0 to A ₆ is a dispersion coefficient. If the substrate or thin film has absorption, it is necessary to consider not only the refractive index but also the extinction coefficient and its wavelength dependency.

Further, the FSR is generally obtained without considering the order m as exemplified in the equation (1). However, if the FSR is obtained in consideration of the variation according to the order m, the FSR includes the number of peak frequency intervals, the maximum peak frequency included in the frequency range in which the etalon filter 1 is used, and Let the minimum peak frequency be L, ν _m , ν _{m + L} respectively.
FSR = (ν _m −ν _{m + L} ) / L
May be required.

  It is clear from the theory that the FSR calculation method described above can calculate the FSR with high accuracy unless the refractive index is nonuniform in each medium.

(Conditions of Examples and Comparative Examples)
In the following, with respect to examples and comparative examples in which specific materials and dimensions are set, the effects of the reflective film on the FSR will be described with reference to the results of calculating FSR calculated values and actually measured values. In this case, unless otherwise specified, it is assumed that the incident angle is 0 °, the wavelength range is 1530 to 1610 nm, and the substrate 3 is composed of a quartz Z-cut plate.

(Investigation of the effect of reflective film on FSR)
FIG. 2 is a diagram showing a plurality of configuration examples of the reflection film 5 (a plurality of examples of combinations of a plurality of thin films 7). In FIG. 2, each column (vertical column) corresponds to each configuration example, and each row indicates a feature of each configuration example. Specifically, it is as follows.

  The first line “No.” in FIG. 2 indicates a number given to the configuration example for convenience. In FIG. 1-No. 15 types of configuration examples up to 15 are shown.

  The second row “R” in FIG. 2 indicates the reflectance (%) of the reflective film 5 in each configuration example. In FIG. 2, the material, the number of layers, and the thickness are set so that the reflection film 5 has a reflectance of 20% in any configuration example.

  The row “d” near the center of FIG. 2 indicates the thickness d (unit: nm) of the substrate 3. In FIG. 2, all the configuration examples have the same thickness d.

In FIG. 2, “SiO 2” and “Ta 2 O 5” above and below “d” indicate the thickness (unit: nm) for each material of the thin film 7 in the order of stacking with respect to the substrate 3. That is, in the configuration example of FIG. 2, Ta ₂ O ₅ , SiO ₂ , Ta ₂ O ₅ , SiO ₂ , Ta ₂ O ₅ and SiO _{2 with} respect to one main surface of the substrate 3 in that order from the main surface side. 6 layers of Ta ₂ O ₅ , SiO ₂ , Ta ₂ O ₅ , SiO ₂ , Ta ₂ O ₅ and SiO ₂ are sequentially formed from the main surface side in the same manner on the other main surface. Layers are stacked. In the plurality of configuration examples, the thickness of the thin film 7 is different from each other.

  In addition, No. The thickness of some of the thin films 7 in 11 to 15 means that the thin film of the material was not provided. No. 1-No. Up to 13, the first reflective film 5A and the second reflective film 5B have the same configuration. 14 and no. Up to 15, the first reflective film 5A and the second reflective film 5B have different configurations.

  FIG. 1-No. 15 is a diagram illustrating a result of calculating the FSR in the configuration example 15 by the FSR calculation method of the present embodiment described above, and FIG. 1-No. It is a figure which shows (DELTA) FSR in the example of a structure of 15. 3A and 3B, the horizontal axis indicates the thickness D of the etalon filter (FIG. 1A). In FIG. 3A, the vertical axis indicates FSR, and in FIG. 3B, the vertical axis indicates ΔFSR.

Here, ΔFSR is obtained by the following equation (10).
ΔFSR = FSR ₁ −FSR ₀ (10)
However, FSR ₀ is FSR in the case where the substrate 3 having a thickness of d first reflective layer 5A and the second reflective film 5B is not provided, FSR _1, the first reflective layer 5A and the second substrate of the thickness d This is an FSR in the case where two reflective films 5B are provided.

  As described above, no. 1-No. In the configuration example 15, the thickness d of the substrate 3 is the same. Therefore, it was confirmed from FIGS. 3A and 3B that the FSRs are different from each other even if the thickness d of the substrate 3 is the same. In other words, no matter how accurately the thickness d of the substrate 3 is adjusted, as long as the etalon filter 1 is designed so that the FSR is determined only by the thickness d of the substrate 3, the FSR is adjusted with high accuracy. It was confirmed that it was not possible.

  Further, from FIGS. 3A and 3B, it was confirmed that the FSR is different when the configuration of the reflective film 5 is different. This shows that the FSR can be adjusted by adjusting the configuration of the reflective film 5.

  Further, from the dispersion of the plots of FIGS. 3A and 3B, when the thickness D of the etalon filter or the thickness of the reflective film (Dd) increases macroscopically, the FSR tends to decrease. However, it can be seen that increasing the thickness D or thickness (D-d) does not necessarily reduce the FSR. Note that the square of the correlation coefficient in FIG. 3A is 0.28 (the same applies to FIG. 3B).

  In general, as exemplified in the equation (1), the FSR is determined only by the thickness d of the substrate 3, and the configuration of the reflective film 5 affects the FSR, and the thickness D is It is not recognized that the FSR cannot be adjusted with high precision only by adjustment. In other words, the above knowledge has been obtained as a result of intensive studies by the present inventors.

(Etalon filter manufacturing method)
Based on the above knowledge, in the present embodiment, the etalon filter 1 is manufactured as follows, so that highly accurate FSR adjustment is suitably performed.

  FIG. 4 is a flowchart showing the procedure of the manufacturing method of the etalon filter 1. Steps ST1 and ST2 are steps in the design stage, and are performed once for one design. Steps ST3 to ST9 are processes in a narrowly-defined manufacturing stage, and are generally performed repeatedly, and a large number of etalon filters 1 are manufactured in steps ST3 to ST9.

In step ST1, the design thickness _dt of the substrate 3 is determined. The thickness _dt is determined so as to obtain desired optical characteristics (for example, FSR). The thickness _dt may be determined by a conventionally known method, or may be determined by a method including the above-described FSR calculation method of the present embodiment.

In step ST2, a plurality of candidates for designing the reflective film 5 are prepared. For example, in FIG. 1-No. Thirteen configuration examples are prepared as a plurality of design candidates. As for the plurality of candidates, ΔFSR (FSR) when it is provided on the substrate 3 having the thickness _dt is known. Note that the FSR and ΔFSR may be obtained based on actual measurement of a prototype in which the thickness d of the substrate 3 can be controlled to the thickness _dt with high accuracy, or calculated based on the FSR calculation method of the present embodiment described above. May be. Also, those having different ΔFSR are selected as the plurality of candidates. It should be noted that it is preferable that a plurality of candidates are selected such that the difference between the FSR and the desired FSR _t is within a certain range.

In step ST3, the substrate 3 is formed. The substrate 3 is formed such that its thickness is the designed thickness _dt . In addition, the formation method of the board | substrate 3 may be the same as a well-known method.

In step ST4, measuring the actual thickness _{d r} of the substrate 3 formed in step ST3. Measurement of thickness d _r, such as a micrometer or a laser length measuring meter, may be carried out by known techniques. The measurement of thickness _{d r} is preferably is preferably carried out in the following accuracy 0.1 [mu] m.

In step ST5, the first reflective film 5A is formed. The configuration of the first reflective film 5A is, for example, that the magnitude of ΔFSR is intermediate among the plurality of candidates prepared in step ST2. Further, for example, in the process of calculating ΔFSR, FSR (FSR ₁ ) is obtained, but the FSR is close to a desired FSR _t . In addition, the formation method of 5 A of 1st reflective films may be the same as a well-known method.

In step ST6, comparing the actual thickness _{d r} of the thickness _{d t} and the substrate 3 on the design the substrate 3. Here, as understood from equation (1), in the case actual thickness d _r is smaller than the thickness d _t of the design, the actual FSR is larger than the desired FSR _t, actual thickness d _{In the case where r} is larger than the designed thickness d _t , the actual FSR is smaller than the desired FSR _t .

Therefore, in the case actual thickness d _r is smaller than the thickness d _t of the design, of the plurality of candidates prepared in step ST2, the configuration of the first reflective layer 5A (candidate) DerutaFSR is smaller than (DerutaFSR is absolute value since the negative of the large) candidate selected as the configuration of the second reflective layer 5B (step ST7), when the actual thickness d _r is greater than the thickness d _t of the design, of the plurality of candidates, the A candidate having a ΔFSR larger than the configuration (candidate) of the first reflective film 5A (the absolute value is small because ΔFSR is negative) is selected as the configuration of the second reflective film 5B (step ST8). Then, the second reflective film 5B is formed with the selected candidate configuration (step ST9).

Thus, offset by the actual thickness d _r is an error of FSR by that differs from the thickness d _t in design, to increase or reduce the ΔFSR according to the second reflecting film 5B, the configuration of the second reflective layer 5B Compared with the case where is the same as the configuration of the first reflective film 5A, the actual FSR approaches the desired FSR _t .

Incidentally, when the selection of the candidate having the second reflecting film 5B, select the actual thickness d _r and the difference is greater ΔFSR candidates with ΔFSR of the first reflection film 5A larger the difference between the thickness d _t of the design For example, the selection may be made according to the difference between the thickness _dr and the designed thickness _dt .

Although not particularly illustrated, in the case where the actual thickness _dr is equal to the designed thickness _dt , for example, the second reflective film 5B may have the same configuration as the first reflective film 5A.

  The order of steps ST1 and ST2 may be reversed. Further, the order of steps ST5 to ST9 can be changed as appropriate. For example, steps ST6 to ST8 may be performed before step ST5, and step ST5 may be performed after step ST9.

(Relevance of manufacturing method of embodiment)
Below, the validity of the manufacturing method of embodiment is demonstrated.

  FIG. 5A is a diagram showing various configuration examples of the substrate 3 and the reflective film 5 as in FIG. In addition, No. of FIG. 21-A-No. In the configuration examples up to 24-A, the thickness d of the substrate 3 is the same, and the configuration of the reflective film 5 is different. In each configuration example, the configuration of the first reflective film 5A and the configuration of the second reflective film 5B are the same.

A configuration example of FIG. 5A was made as a prototype, and FSR and ΔFSR were measured. Then, in consideration of ΔFSR, a configuration example was designed in which the thickness d of the substrate 3 in FIG. 5A was changed so that a desired FSR _t (100 GHz in this example) was obtained.

  FIG. 5B is a view similar to FIG. 5A showing a configuration example in which the thickness d is changed. No. 21-B-No. 24-B is No. 24, respectively. 21-A-No. It corresponds to 24-A. In the following description, A and B may be omitted to indicate the configuration of the reflective film 5.

  Then, the configuration example of FIG. 5B was made as a prototype, and the FSR was measured in the same manner as the configuration example of FIG.

  FIG. 6 is a diagram showing a comparison between the FSR of the configuration example of FIG. 5A and the FSR of the configuration example of FIG.

  In FIG. 6, each row corresponds to the configuration of the reflective film 5. The first column “No.” indicates the No. of the configuration example of the reflective film 5. Is shown with A and B omitted. Note that the row in which “No.” is “−” corresponds to no reflection film. The columns “A” and “B” (vertical columns) correspond to the configuration examples in FIGS. 5A and 5B, respectively.

As shown in this figure, in the configuration example of FIG. 5B, the FSR is close to the desired FSR _t (100 GHz) compared to the configuration example of FIG. From this, it was confirmed that FSR _t is ideally obtained by appropriately combining the thickness d and the configuration of the reflective film 5.

FIGS. 7A, 7B, and 8 are other examples corresponding to FIGS. 5A, 5B, and 6. FIG. In this configuration example, a desired FSR _t is 50 GHz. Also in this example, as in the previous example, it was confirmed that FSR _t can be obtained ideally by appropriately combining the thickness d and the configuration of the reflective film 5.

  FIG. 9A and FIG. 9B are diagrams showing the effects when the reflective films 5 having different ΔFSR are combined as the first reflective film 5A and the second reflective film 5B.

  No. 1 shown in FIG. 21 and no. In the 22 configuration examples, the configurations of the first reflective film 5A and the second reflective film 5B were the same. On the other hand, No. 1 shown in FIG. In the 29 configuration example, no. No. 21 is the first reflective film 5A. The 22 reflective films 5 are used as the second reflective film 5B. And No. 29 configuration examples were prototyped and ΔFSR was measured.

As a result, no. 29 has a ΔFSR of No. 29. 21 and no. The average value of 22 ΔFSR was approximately average. That is, no. 21 and no. No. 22 is assumed to be ΔFSR1 and ΔFSR2, respectively. 29 ΔFSR12 is
ΔFSR12 = (ΔFSR1 + ΔFSR2) / 2
It became.

  FIG. 21, no. 22 and no. Similar to No. 29, no. 23, no. 24 and no. 30 shows the result of trial manufacture and measurement of ΔFSR. In addition, No. 21, no. 22 and no. 29 and No. 23, no. 24 and no. 30 is different from the reflectivity of the reflective film.

  In the example of FIG. The ΔFSR of No. 30 is no. 23 and no. 24 ΔFSR was approximately the average size.

Therefore, as described with reference to FIG. 4, when the second reflective film 5B is configured to have a ΔFSR larger or smaller than the first reflective film 5A, the reflective film having the same configuration as the first reflective film 5A is the second. ΔFSR is larger or smaller than that in the case of being configured as the reflective film 5B. Therefore, it was confirmed that actual thickness d _r of the substrate can be canceled larger or smaller influence than the desired thickness d _t.

(Application example of etalon filter)
FIG. 10 is a block diagram illustrating an application example of the etalon filter 1.

  The etalon filter 1 is incorporated in a wavelength locker 13 for keeping the wavelength of light of the laser system 11 constant. The wavelength locker 13 includes, for example, a beam splitter 15 on which light dropped from the laser system 11 is incident, an etalon filter 1 on which light transmitted through the beam splitter 15 is incident, and a first light on which light transmitted through the etalon filter 1 is incident. It has a photodetector 17A and a second photodetector 17B on which the light reflected from the beam splitter 15 enters. Then, the control device 19 detects the wavelength of the light by comparing the intensity of the light detected by the first photodetector 17A and the intensity of the light detected by the second photodetector 17B, and the detected wavelength is constant. Control of the laser system 11 is executed so as to be maintained at

  The etalon filter 1 in which the FSR of the present embodiment is adjusted with high accuracy is used for such a wavelength locker 13, so that the wavelength of light can be monitored with high accuracy.

  The present invention is not limited to the above embodiment, and may be implemented in various aspects.

  In the embodiment, the substrate is made of a quartz Z-cut plate, but similar results can be obtained even if quartz glass is used. The substrate may be in a state where different materials are bonded. In other words, the etalon filter may be a composite etalon filter.

How to choose the thickness d _t and on the basis of a comparison between the thickness d _r from a plurality of candidates of one of the first and second reflecting films candidates, DerutaFSR larger or smaller than the first reflective layer in accordance with the magnitude relation candidate It is not limited to the method of selecting. For example, it may be larger or smaller candidates ΔFSR is selected for the reference ΔFSR which is set appropriately, and ΔFSR candidate to be selected from the formula that the thickness d _t and the thickness d _r and parameters are calculated Also good.

  The etalon filter in which the configuration of the first reflective film and the configuration of the second reflective film are different is not limited to the one formed by the manufacturing method in which the configuration of the second reflective film is selected according to the measurement result of the substrate thickness. In the initial design stage, the configuration of the first reflective film and the configuration of the second reflective film may be designed differently. Even in this case, for example, when the FSR is adjusted by changing the thickness of one reflective film, the FSR is adjusted more delicately than when the FSR is adjusted by changing the thickness of both reflective films. It is expected to be possible.

  DESCRIPTION OF SYMBOLS 1 ... Etalon filter, 3 ... Substrate, 5 ... Reflective film, 7 ... Thin film.

Claims (5)

A first reflective film and a second reflective film disposed on an incident / exit surface of the substrate, each of the first reflective film and the second reflective film being configured by laminating a plurality of thin films; An etalon manufacturing method comprising:
Determining a design thickness d _t of the substrate;
Preparing a plurality of candidates having different ΔFSR calculated by the following equation (I) for the combination of the plurality of thin films on the design that constitute the second reflective film;
Forming the substrate;
Measuring an actual thickness d _r of the substrate,
Forming the first reflective film;
ΔFSR candidate ΔFSR candidate thickness _{d r} on the basis of a comparison between the thickness _{d t} and the thickness _{d r} is selected is smaller than the thickness _{d t} is selected when the thickness _{d r} is greater than the thickness _{d t} Selecting one candidate from the plurality of candidates to be smaller than ,
Forming the second reflective film with a combination of the plurality of selected candidate thin films;
The manufacturing method of the etalon filter which has this.
ΔFSR = FSR ₁ −FSR ₀ (I)
However, FSR ₀ is the FSR in a case where the substrate thickness _{d t} of the first reflecting film and the second reflective film is not provided, FSR ₁ includes a plurality on the substrate thickness _{d t} of the thin film This is an FSR when a combination is provided as the first reflective film and the second reflective film. The plurality of candidates are one or more candidates having a ΔFSR larger than a combination of a plurality of thin films constituting the first reflective film and one or more candidates having a ΔFSR smaller than a combination of the plurality of thin films constituting the first reflective film. Including
In the step of selecting the one candidate, when the measured thickness d _r is smaller than the thickness d _t of the design, rather than a combination of a plurality of thin film constituting the first reflective layer of the plurality of candidate DerutaFSR select small candidate, when the measured thickness d _r is greater than the thickness d _t of the design, DerutaFSR than the combination of a plurality of thin film constituting the first reflective layer of the plurality of candidate The method for producing an etalon filter according to claim 1, wherein a large candidate is selected. The method for manufacturing an etalon filter according to claim _{1, wherein} FSR ₀ and FSR ₁ are obtained by actual measurement of a prototype. 3. The method for manufacturing an etalon filter according to claim 1, wherein FSR ₀ and FSR ₁ are calculated by the following FSR calculation method based on a combination of a design thickness d _t and a plurality of design thin films.
FSR calculation method:
The substrate and the plurality of thin films are regarded as a multilayer structure composed of m layers of medium, and the characteristic matrix Mj of the jth (1 ≦ j ≦ m) medium is calculated by the following equations (II) and (III): To do.

Where λ is the wavelength, n _j is the refractive index of the j-th medium, d _j is the thickness of the j-th medium, θ _j is the refraction angle in the j-th medium, and δ _j is the phase of the j-th medium. .
The characteristic matrix M of the multilayer structure is calculated by the following equation (IV).

Then, the transmission coefficient T of the multilayer structure is calculated by the following equation (V).

Here, n ₀ and n _m are the refractive indexes of the medium serving as the incident medium and the output medium, respectively, among the m layers of medium.
Then, the FSR is calculated from the wavelength interval between the maximum values of the transmission coefficient T calculated using the wavelength λ as a variable in the equations (II) to (V). The method for manufacturing an etalon filter according to claim 1, wherein the plurality of candidates have the same reflectance.

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