CN112230322A

CN112230322A - Preparation method of bandpass filter with insertion loss linearly changing

Info

Publication number: CN112230322A
Application number: CN202011049798.0A
Authority: CN
Inventors: 苏炎; 李昱; 陈居凯
Original assignee: Suzhou Zhongwei Photoelectric Co ltd
Current assignee: Suzhou Zhongwei Photoelectric Co ltd
Priority date: 2020-09-29
Filing date: 2020-09-29
Publication date: 2021-01-15
Anticipated expiration: 2040-09-29
Also published as: CN112230322B

Abstract

The invention provides a preparation method of a band-pass filter with linearly-changed insertion loss, which comprises the following steps: cutting the base layer and forming; carrying out ultrasonic cleaning on the base layer before plating; performing membrane system design; the film system structure of the film system stacked on the base layer in the film system design comprises a plurality of Fabry-Perot cavities and connecting layers, each Fabry-Perot cavity is connected with the next Fabry-Perot cavity in a cascade mode through the connecting layers, and the connecting layers are low-refractive-index layers which are not quarter-wavelength and have optical thickness; controlling the thickness of each film layer according to the design of a film system and coating the film; and testing the plated band-pass filter to obtain the qualified band-pass filter. The band-pass filter with linearly-changed insertion loss can be prepared, the band-pass filter has the conventional filtering function, the insertion loss or transmittance of the pass band of the band-pass filter has linear change, the function that the insertion loss is linearly changed along with the wavelength in a specific wavelength range is realized in the pass band, and the function of wavelength identification is realized in the specific wavelength range.

Description

Preparation method of bandpass filter with insertion loss linearly changing

Technical Field

The invention relates to the technical field of optics, in particular to a preparation method of a band-pass filter with linearly-changed insertion loss.

Background

Bandpass Filters (Bandpass Filters) only pass light of a particular wavelength or narrow band, and do not pass light outside the passband. The optical indexes of the band-pass filter are mainly as follows: center Wavelength (CWL), half bandwidth (FWHM). The method is divided into the following steps according to the bandwidth size: a narrow-band filter with the bandwidth less than 30 nm; the bandwidth is more than 60nm and is a broadband filter.

Bandpass filters are commonly used in interferometers, imaging instruments, inspection instruments, data centers, optical communications, industrial lasers, and wavelength identification.

The devices currently used for wavelength identification include grating devices, bandpass filter combination devices, and the like. The bandpass filter is generally applied to the use directions of light splitting and the like, the passband insertion loss or transmittance of the bandpass filter is generally required to be as flat as possible, the ripple wave is small, the bandpass filter is difficult to realize the linear change of the insertion loss along with the wavelength due to the characteristics of the bandpass filter, and the bandpass filter in the prior art can not realize the linear change of the insertion loss along with the wavelength in the passband.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide a preparation method of a band-pass filter with linearly-changed insertion loss.

The technical scheme of the invention is summarized as follows:

the invention provides a preparation method of a band-pass filter with linearly-changed insertion loss, wherein a base layer is cut and formed; wherein the base layer is made of material with refractive index ranging from 1304.5 nm to 1317nm and ranging from 1.45 nm to 3.5 nm;

carrying out ultrasonic cleaning on the base layer before plating;

designing a film system according to the target performance of the band-pass filter; the film system structure of the film system stacked on the base layer in the film system design comprises a plurality of Fabry-Perot cavities and connecting layers, and each Fabry-Perot cavity comprises high-refractive-index film layers and low-refractive-index film layers which are alternately stacked; each Fabry-Perot cavity is cascaded with the next Fabry-Perot cavity through a connecting layer, and the connecting layer is a low-refractive-index layer with optical thickness not equal to one quarter wavelength;

controlling the thickness of each film layer according to the design of a film system and coating the film;

and testing the plated band-pass filter to obtain the qualified band-pass filter.

Further, the structure of the fabry-perot cavity is as follows: ahhlhl 4HLHLHLH, bhhlhl 4HLHL3HLHLH, chhlhl 4 hlhlhlhlh, dhhl 4HLHLA, wherein dhhl 4HLHLA is the last layer fabry-perot cavity;

h is a high refractive index layer of quarter-center wavelength optical thickness, L is a low refractive index layer of quarter-center wavelength optical thickness; 4H is the four quarter-center wavelength optical thicknesses, a is a high refractive index layer of non-quarter optical thickness; a. b, c, d are coefficients of the optical thickness of the quarter-center wavelength of the first high refractive index layer of each of said fabry-perot cavities.

Further, the optical thickness of the connecting layer is a non-integer number of quarter-wave optical thicknesses in the range of 0.675-2.451.

Further, the coefficient of the optical thickness of the quarter center wavelength of the first high refractive index layer of the fabry-perot cavity is obtained by software optimization from the optical thickness of the last layer of the last fabry-perot cavity.

Furthermore, a, b, c and d are 0.974-1.015, and the optical thickness of the quarter-wave center of the first high refractive index layer of the fabry-perot cavity is 0.974-1.015, so that the transmittance at the end of the layer reaches the extreme value.

Further, the number of the fabry-perot cavities is 9.

Further, the controlling the thickness of each film layer and coating the film according to the design of the film system comprises: the first layer of each Fabry-Perot cavity is monitored by an extremum method in coating monitoring, so that the next film layer with the optical thickness of one quarter wavelength of each layer can be monitored by the extremum method.

Further, the controlling the thickness of each film layer and coating the film according to the design of the film system further comprises: the connection layer is monitored by adopting a crystal control/time control method.

Further, the material of the high-refractive-index layer is at least one of Ta2O5, Nb2O5 and TiO2, and the refractive index of the high-refractive-index layer ranges from 1.85 to 2.5 in the range of 1304.5-1317 nm; the material of the low-refractive-index layer is at least one of SiO2, Al2O3 and MgF2, and the refractive index of the low-refractive-index layer ranges from 1.38 to 1.6 in the range of 1304.5-1317 nm.

Further, the plated band-pass filter is tested to obtain a qualified band-pass filter, and the insertion loss of the wavelength of the qualified band-pass filter is linearly increased or decreased along with the wavelength under the condition that the incident angle is 0-13.5 degrees, or the insertion loss in the wave band range of 1304.5-1317nm is increased from-5 dB to-0.2 dB according to the slope 0.3846 dB/nm.

Compared with the prior art, the invention has the beneficial effects that:

the preparation method of the band-pass filter with the linearly-changed insertion loss, provided by the invention, can be used for preparing the band-pass filter with the linearly-changed insertion loss, the band-pass filter has the conventional filtering function, the insertion loss or the transmittance of the band-pass filter has the linear change, the linear change of the insertion loss along with the wavelength in a specific wavelength range is realized in a pass band, and the function of wavelength identification is realized in the specific wavelength range. The preparation method of the invention improves the film system structure, the film thickness and the monitoring mode, selects a proper monitoring method according to the film system structure through multiple experiments, and can prepare the high-precision bandpass filter with linearly-changed insertion loss. The band-pass filter can also be applied to the fields of interferometers, imaging instruments, detecting instruments, data centers, optical communication, industrial laser, wavelength identification and the like.

The foregoing description is only an overview of the technical solutions of the present invention, and in order to make the technical solutions of the present invention more clearly understood and to implement them in accordance with the contents of the description, the following detailed description is given with reference to the preferred embodiments of the present invention and the accompanying drawings. The detailed description of the present invention is given in detail by the following examples and the accompanying drawings.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention. In the drawings:

FIG. 1 is a schematic diagram of a bandpass filter with linearly varying insertion loss according to the present invention;

FIG. 2 is a graph illustrating a target slope according to the present invention;

FIG. 3 is a graph of the wavelength at 8 degrees incidence versus insertion loss, and target slope, for an embodiment of a bandpass filter with linearly varying insertion loss in the range of 0-30dB in accordance with the present invention;

FIG. 4 is a graph of the wavelength at 8 degrees incidence versus insertion loss, and target slope for an embodiment of a bandpass filter with linearly varying insertion loss in the 0-6dB range in accordance with the present invention;

FIG. 5 is a graph of the wavelength of 0 degree incidence versus the insertion loss and the target slope for a bandpass filter with linearly varying insertion loss in the range of 0-30dB in accordance with the present invention;

FIG. 6 is a graph of the relationship between the wavelength of 13.5 degree incidence and the insertion loss and the target slope for a bandpass filter with linearly varying insertion loss in the range of 0-30dB in accordance with the present invention;

fig. 7 is a flowchart of a method for manufacturing a bandpass filter with linearly changing insertion loss according to the present invention.

Reference numerals: 1. a base layer; 2. a high refractive index layer; 3. a low refractive index layer; 4. a connecting layer; 5. fabry-perot cavity.

Detailed Description

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings, which will enable those skilled in the art to practice the present invention with reference to the accompanying specification. In the drawings, the shape and size may be exaggerated for clarity, and the same reference numerals will be used throughout the drawings to designate the same or similar components. In the following description, terms such as center, thickness, height, length, front, back, rear, left, right, top, bottom, upper, lower, and the like are used based on the orientation or positional relationship shown in the drawings. In particular, "height" corresponds to the dimension from top to bottom, "width" corresponds to the dimension from left to right, and "depth" corresponds to the dimension from front to back. These relative terms are for convenience of description and are not generally intended to require a particular orientation. Terms concerning attachments, coupling and the like (e.g., "connected" and "attached") refer to a relationship wherein structures are secured or attached, either directly or indirectly, to one another through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.

The present invention will be further described with reference to the accompanying drawings and the detailed description, and it should be noted that any combination of the embodiments or technical features described below can be used to form a new embodiment without conflict. It will be understood that terms such as "having," "including," and "comprising," as used herein, do not preclude the presence or addition of one or more other elements or groups thereof.

Filters are optical devices used to select a desired wavelength band of radiation. The optical filter is formed by stacking a base layer and a plurality of film layers on the base layer, and the thickness of the film layers can be divided into two description modes of physical thickness and optical thickness. Physical thickness refers to thickness on a physical scale, such as 100nm or the like; the optical thickness refers to the path traveled by the light, which relates to the refractive index of the material through which the light travels and the wavelength of the light QW ═ n x d)/λ, where n is the refractive index of the material through which the light travels, d is the physical thickness, and λ is the wavelength of the light.

The devices currently used for wavelength identification include grating devices, bandpass filter combination devices, and the like. The bandpass filter is generally applied to the use directions of light splitting and the like, the passband insertion loss or transmittance of the bandpass filter is generally required to be as flat as possible, the ripple wave is small, the bandpass filter is difficult to realize the linear change of the insertion loss along with the wavelength due to the characteristics of the bandpass filter, and the bandpass filter in the prior art can not realize the linear change of the insertion loss along with the wavelength in the passband. Therefore, an object of the present invention is to provide a bandpass filter with linearly changing insertion loss, which realizes the function of linearly changing insertion loss or transmittance with wavelength in a specific wavelength range in a passband, and realizing the function of wavelength discrimination in the specific wavelength range, and can obtain the wavelength of the measuring light by measuring the size of the insertion loss value. Meanwhile, the characteristic of the band-pass filter can cut off the wavelength outside the band-pass with high isolation, and the influence of stray light on the test result is reduced.

Referring to fig. 7, the method for manufacturing a bandpass filter with linearly changing insertion loss according to the present invention includes:

s1, cutting the base layer for forming; wherein the base layer is made of material with refractive index ranging from 1304.5 nm to 1317nm and ranging from 1.45 nm to 3.5 nm;

s2, carrying out ultrasonic cleaning on the base layer before plating;

s3, designing a film system according to the target performance of the band-pass filter;

s4, controlling the thickness of each film layer according to the design of the film system and coating the film;

and S5, testing the plated band-pass filter to obtain the qualified band-pass filter.

Specifically, in step S2, the power of the ultrasonic wave for cleaning is 600W-900W, the cleaning chemical is RS-26, and the frequency value is set to 28KHZ-40 KHZ.

In step S3, the film system structure of the film system stacked on the base layer in the film system design includes a plurality of fabry-perot cavities and connecting layers, each fabry-perot cavity includes alternately stacked high refractive index film layers and low refractive index film layers; each fabry-perot cavity cascades the next fabry-perot cavity through a connecting layer, which is a low refractive index layer of optical thickness other than a quarter wavelength.

As shown in fig. 1, in the step of film system design, a base layer 1 and a film system stacked on the base layer are used. The membrane system structure of the membrane system comprises a plurality of Fabry-Perot cavities 5 and connecting layers 4, wherein each Fabry-Perot cavity comprises high-refractive-index membrane layers 2 and low-refractive-index membrane layers 3 which are alternately stacked; each fabry-perot cavity 5 cascades the next fabry-perot cavity 5 through a connecting layer 4, the connecting layer 4 being a low refractive index layer of optical thickness other than a quarter wavelength.

Specifically, the structure of the fabry-perot cavity is as follows: ahhlhl 4HLHLHLH, bhhlhl 4HLHL3HLHLH, chhlhl 4 hlhlhlhlh, dhhl 4HLHLA, wherein dhhl 4HLHLA is the last layer fabry-perot cavity;

The insertion loss of the wavelength of the optical filter is increased or decreased linearly along with the wavelength under the incident angle of 0-13.5 degrees.

The insertion loss of the filter in the 1304.5-1317nm wave band range is increased from-5 dB to-0.2 dB with the slope of 0.3846 dB/nm.

The number of fabry-perot cavities is 9.

The coefficient of the optical thickness of the quarter-center wavelength of the first high-index layer of the fabry-perot cavity is obtained by software optimization from the optical thickness of the last layer of the last of said fabry-perot cavities. a. b, c and d are integers of 1 or approximate 1. The purpose of this design is to better cascade with the connecting layer 4 (i.e. a refractive index layer that is not one-quarter of the optical thickness). And the optical thickness of approximate quarter wavelength is used for compensating the transmittance of the central wavelength at the end of the layer to reach an extreme value, and an extreme value method is adopted for monitoring in coating monitoring, so that the subsequent film layer with the optical thickness of the quarter wavelength of each layer can be monitored by the extreme value method, the optical thickness can be accurately controlled, and the optical filter with the accurate linear transmittance change can be obtained.

Specifically, a, b, c and d are 0.974-1.015, namely the optical thickness of the quarter-center wavelength of the first high-refractive-index layer of the Fabry-Perot cavity is 0.974-1.015 quarter-wavelength optical thickness, so that the transmittance of the layer reaches an extreme value when the layer is finished, an extreme value method is adopted for monitoring the change trend of light intensity in coating monitoring, and the light intensity signal is switched to the next layer after the light intensity signal reaches an extreme value point; this algorithm has the function of compensating for the errors of the front film layer.

The optical thickness of the connecting layer is a non-integer number of quarter-wave optical thicknesses in the range of 0.675-2.451.

Preferably, the material of the high refractive index layer is at least one of Ta2O5, Nb2O5 and TiO2, and the refractive index of the high refractive index layer is 1.85 to 2.5 in the range of 1304.5-1317 nm.

The material of the low-refractive-index layer is at least one of SiO2, Al2O3 and MgF2, and the refractive index of the low-refractive-index layer ranges from 1.38 to 1.6 in the range of 1304.5-1317 nm.

The base layer is made of silicon dioxide material or silicon material, and the refractive index of the base layer is in the range of 1304.5-1317nm and is 1.45-3.5. The base layer is at least one of D263T, WMS-15, BK7, FS and Si. Preferably, the substrate material is ordinary K9 optical glass.

Step S4 specifically includes: the first layer of each Fabry-Perot cavity is monitored by adopting an extreme method in coating monitoring, so that the next film layer with the optical thickness of one quarter wavelength of each layer can be monitored by adopting the extreme method;

the connection layer is monitored by adopting a crystal control/time control method.

Specifically, the crystal control/time control method is carried out by adopting a crystal oscillator controller, and the principle is as follows: in the coating process, a film layer can be deposited on the surface of a crystal oscillator wafer of the crystal controller; the oscillation frequency of the crystal oscillator wafer can change along with the increase of the thickness of the upper surface film layer of the crystal oscillator wafer; the thickness change of the film layer on the crystal oscillator is obtained by obtaining the change of the oscillation frequency of the crystal oscillator plate on the crystal oscillator controller. The control method has the disadvantages that optical thickness supplement cannot be realized, and the peak transmittance of the pass band cannot be improved. And is often used only to control the deposition rate of the coating process.

For a high-precision optical filter, it is often considered to control the film thickness by using a direct light control method. The reason is that the characteristics of the film layer often deviate from the theoretical characteristics due to the inherent characteristics of the film and the differences of the production process and equipment in the film coating process, for example, the refractive index of the film layer is lower compared with that of the bulk material, and the refractive index of the film layer is different due to different processes. And the final application of the filter is light, it is contemplated that the optical thickness can be controlled by monitoring the light signal during the deposition process.

In the coating process, because the test error and the error between the process and the theory can cause the actual test value and the theoretical test value to come in and go out, the influence caused by the error needs to be reduced or compensated through an optical monitoring algorithm. Conventional algorithms include a ratio method, an extremum method and the like. The ratio method is usually used for monitoring of non-regular (non-integer) 1/4 wavelengths, but has the disadvantage that in the case of a single monitoring chip, errors accumulate, resulting in failure of the final product. Is often used for optical products with lower spectral accuracy requirements. The extreme method is used for monitoring the change trend of the light intensity and switching to the next layer after judging that the light intensity signal reaches an extreme point; this algorithm has the function of compensating for the errors of the front film layer.

The invention improves on the consideration of the thickness of the film layer and the monitoring mode, the first layer of each Fabry-Perot cavity is monitored by adopting an extreme method in the film coating monitoring, and the light intensity signal is switched to the next layer after being judged to reach an extreme point, so that the film layer with the optical thickness of one quarter wavelength of each next layer can be monitored by adopting the extreme method, and the error of the previous film layer can be compensated.

The refractive index layer with non-quarter optical thickness is used as a connecting layer to be monitored by adopting a crystal control/time control method, and the thickness change of the film layer on the crystal oscillator plate is obtained by obtaining the change of the oscillation frequency of the crystal oscillator plate and is used for controlling the deposition rate in the film coating process. The crystal control/time control method can be used for plating a complex film layer with any thickness, and the extreme value method is combined for monitoring to make up for error accumulation caused by single monitoring, so that the failure of the product is avoided; various monitoring methods monitor the process curve to achieve the desired filtering effect without accumulating errors, improving accuracy in the thickness of the film layer in the bandpass filter.

In step S4, the coating includes performing film deposition by a coater, specifically, a hard dielectric coating by sputtering or ion beam assisted deposition.

How to precisely control the film layer in the film deposition process is the core of realizing the high-precision optical filter. In this process, there are two important control factors: firstly, how to keep a stable deposition rate of a film layer in the deposition process; secondly, how to ensure the thickness of the film layer to meet the requirement.

In the ion beam assisted deposition process, an electron gun is used for heating the film material to realize the thermal evaporation of the film material, and the crystal oscillator controller is used for acquiring the physical thickness of the film layer in the process so as to control the deposition rate of the film layer. In the deposition process of magnetron sputtering and ion beam sputtering, the target material is bombarded by plasma, the film material is sputtered by utilizing the principle of elastic collision, and the concentration of the plasma is ensured by controlling the power stability of a power supply in the process, so that the stability of the deposition rate is realized.

And in the ion beam assisted deposition process, the method is more beneficial to monitoring the film thickness to meet the requirement by adopting a crystal control/time control method.

The qualified band-pass filter in the step S5 has a wavelength insertion loss increasing or decreasing linearly with the wavelength at an incident angle of 0-13.5 degrees, or an insertion loss increasing from-5 dB to-0.2 dB at a slope of 0.3846dB/nm in a wave band range of 1304.5-1317 nm.

The bandpass filter prepared by the method has the following specific examples:

the present embodiment is a bandpass filter with a linearly varying passband insertion loss. FIG. 2 is a graph showing the relationship between the wavelength and the insertion loss of this embodiment at the target slope of the insertion loss in the passband range, wherein the slope is 0.3846dB/nm, and the insertion loss in the 1304.5-1317nm range is increased from-5 dB to-0.2 dB at a slope of 0.3846dB/nm according to the target slope curve of Table 1.

Wavelength of light	0.3846dB/nm slope insertion loss (dB)
		1304.5	-5
1305	-4.807692308
		1305.5	-4.615384615
1306	-4.423076923
		1306.5	-4.230769231
1307	-4.038461538
		1307.5	-3.846153846
1308	-3.653846154
		1308.5	-3.461538462
1309	-3.269230769
		1309.5	-3.076923077
1310	-2.884615385
		1310.5	-2.692307692
1311	-2.5
		1311.5	-2.307692308
1312	-2.115384615
		1312.5	-1.923076923
1313	-1.730769231
		1313.5	-1.538461538
1314	-1.346153846
		1314.5	-1.153846154
1315	-0.961538462
		1315.5	-0.769230769
1316	-0.576923077
		1316.5	-0.384615385
1317	-0.192307692

TABLE 1 target slope Curve

Fig. 3 and 4 are graphs showing the comparison between the actual insertion loss value and the target insertion loss value in the range of 1304.5-1317nm in this example under the condition of the incident angle of 8 degrees. The solid line is a product insertion loss curve actually measured by the product, the black points are insertion loss target values of different wavelengths, and fig. 3 is a relation graph of the wavelength, the insertion loss and the target slope in the range of 0-30dB in the embodiment; FIG. 4 is a graph of wavelength versus insertion loss and target slope for the present example in the 0-6dB range.

The film structure comprises 138 layers of film system formed by stacking two materials. The initialization structure with characteristic wavelength of 1319nm is as follows:

HLHL4HLHLHB

HLHLHL4HLHLHLHB

HLHLHLHL4HLHL3HLHLHB

HLHLHL4HLHLHLHB

HLHL4HLHLAB

the structure of the Fabry-Perot cavity with characteristic wavelength of 1319nm is as follows: ahhlhl 4HLHLH, bhhlhl 4HLHL3HLHLH, chhlhl 4 hlhlhlhlh, dhhl 4HLHLA, wherein dhhl 4HLHLA is the last layer fabry-perot cavity.

In the embodiment, 9 Fabry-Perot cavities are formed by cascading low-refractive-index materials B with optical thicknesses different from 1/4; a is a high refractive index material with optical thickness of not 1/4, A is a high refractive index material with optical thickness of not 1/4, and the thickness of A and B and the thickness of the first H of the next Fabry-Perot cavity are optimized to obtain a corresponding film system. The order of the layers of the stack and the control of the film thickness of each layer are shown in table 2 below:

TABLE 2 film system Structure and control method

In the embodiment, the material of the high-refractive-index layer is Ta2O5, and the refractive index near 1310nm is 2.094; the material of the low-refractive index layer is SiO2, and the refractive index near 1310nm is 1.473; the substrate material is ordinary K9 optical glass with the refractive index of 1.52. As can be seen from fig. 3, 4, 5, and 6, the wavelength varies linearly with the insertion loss.

Therefore, the CWDM with linearly changing passband insertion loss in this embodiment can satisfy the linear change of the insertion loss of the P-polarized light with a certain slope at the commonly used 8-degree angle; the hard medium film coating of sputtering or ion beam assisted deposition is adopted, and the requirements of communication products and automobile products on friction resistance, high temperature resistance and high humidity resistance can be met.

It should be noted that: the precedence order of the above embodiments of the present invention is only for description, and does not represent the merits of the embodiments. And specific embodiments thereof have been described above. Other embodiments are within the scope of the following claims. In some cases, the actions or steps recited in the claims may be performed in a different order than in the embodiments and still achieve desirable results.

The embodiments in the present specification are described in a progressive manner, and the same and similar parts among the embodiments are referred to each other, and each embodiment focuses on the differences from the other embodiments.

The foregoing description has disclosed fully preferred embodiments of the present invention. It should be noted that those skilled in the art can make modifications to the embodiments of the present invention without departing from the scope of the appended claims. Accordingly, the scope of the appended claims is not to be limited to the specific embodiments described above.

Claims

1. A method for preparing a bandpass filter with linearly-changed insertion loss is characterized by comprising the following steps:

cutting the base layer and forming; wherein the base layer is made of material with refractive index ranging from 1304.5 nm to 1317nm and ranging from 1.45 nm to 3.5 nm;

carrying out ultrasonic cleaning on the base layer before plating;

2. The method of manufacturing a bandpass filter with linearly varying insertion loss according to claim 1, wherein the structure of the fabry-perot cavity is: ahhlhl 4HLHLHLH, bhhlhl 4HLHL3HLHLH, chhlhl 4 hlhlhlhlh, dhhl 4HLHLA, wherein dhhl 4HLHLA is the last layer fabry-perot cavity;

3. The method of claim 1, wherein the optical thickness of the connecting layer is a non-integer number of quarter-wavelengths from 0.675 to 2.451.

4. A method of manufacturing a bandpass filter with linearly varying insertion loss according to claim 1, wherein the coefficient of the optical thickness of the quarter-center wavelength of the first high refractive index layer of the fabry-perot cavity is optimized by software from the optical thickness of the last layer of the last fabry-perot cavity.

5. A method of manufacturing a bandpass filter with linearly varying insertion loss according to claim 2, wherein a, b, c, d are 0.974-1.015, and the optical thickness of the quarter-wave center of the first high refractive index layer of the fabry-perot cavity is 0.974-1.015 quarter-wave optical thickness, so that the transmission at the end of this layer reaches an extreme value.

6. The method of manufacturing a bandpass filter with linearly varying insertion loss according to claim 1, wherein the number of the fabry-perot cavities is 9.

7. The method of manufacturing a bandpass filter with linearly changing insertion loss according to claim 1, wherein the controlling of the film thickness and the coating of each film layer according to the film system design comprises: the first layer of each Fabry-Perot cavity is monitored by an extremum method in coating monitoring, so that the next film layer with the optical thickness of one quarter wavelength of each layer can be monitored by the extremum method.

8. The method of manufacturing an optical filter having a linear transmittance change according to claim 1, wherein the controlling of the thickness of each film layer and the coating are performed according to a film system design, further comprising: the connection layer is monitored by adopting a crystal control/time control method.

9. The method of manufacturing a bandpass filter with linearly changing insertion loss according to claim 1, wherein the material of the high refractive index layer is at least one of Ta2O5, Nb2O5, and TiO2, and the refractive index of the high refractive index layer is in the range of 1304.5 to 1317nm from 1.85 to 2.5; the material of the low-refractive-index layer is at least one of SiO2, Al2O3 and MgF2, and the refractive index of the low-refractive-index layer ranges from 1.38 to 1.6 in the range of 1304.5-1317 nm.

10. The method of claim 1, wherein the step of testing the coated bandpass filter results in a qualified bandpass filter having a wavelength insertion loss that increases or decreases linearly with wavelength at an incident angle of 0-13.5 degrees, or an insertion loss that increases from-5 dB to-0.2 dB with a slope of 0.3846dB/nm in the wavelength range of 1304.5-1317 nm.