CN113376730B - Cascaded long period grating filter with adjustable wavelength and bandwidth and method of making the same - Google Patents

Abstract

The invention discloses a cascade long-period grating filter with adjustable wavelength and bandwidth and a manufacturing method. The filter comprises 8 long-period gratings with different periods in cascade, of which 4 are high-frequency filtering long-period gratings, and the other 4 are long-period gratings. One is the low-frequency filter long-period grating, which is used to fix the support frame of the cascaded filter grating group, and two precisely controllable graphite heating platforms. The fabrication method includes the steps of fiber preprocessing, drawing long period grating, etching long period grating and assembling filter. By adjusting the temperature of two precisely controllable graphite heating platforms, the invention can realize the widening or reducing of the filtering bandwidth of the filter. If the temperature of the two heating platforms is adjusted at the same time, the position of the center wavelength of the filter can be changed. The filter is an all-fiber structure, which is highly suitable for fiber sensing and fiber lasers, etc. The filtering parameters can be adjusted for different use environments. The filter is convenient to manufacture, low in cost, flexible in adjustment, and has strong stability.

Description

Cascaded long period grating filter with adjustable wavelength and bandwidth and method of making the same

technical field

The invention relates to a cascaded long-period grating filter and a manufacturing method, in particular to a cascaded long-period grating filter with adjustable wavelength and bandwidth and a manufacturing method, belonging to the technical field of fiber grating filters.

Background technique

Optical fiber filter is an optical filter element with wavelength selective reflection or transmission for input optical signal. The quality of the optical fiber filter directly determines the quality of the optical index of the optical fiber system. The sensitivity, resolution and measurement range of the fiber optic sensor are limited by the influence of parameters such as the bandwidth, insertion loss, and extinction ratio of the fiber optic filter. The extinction ratio, wavelength and power stability, linewidth and slope efficiency of the laser light source output are closely related to the fiber filter and optical signal processing capabilities. For example, the differential effect of the optical time domain differentiator and the shaping effect of pulse shaping all depend on the filtering The extinction ratio of the device, etc. For existing optical filters and different filter systems, after production and molding, the filter center wavelength and filter bandwidth are also fixed and difficult to change. When dealing with different situations or improving experimental conditions, the center wavelength and filter bandwidth of the filter are adjusted. Filter bandwidth will produce different requirements, which can only be solved by purchasing or making corresponding filters, which is inconvenient and increases research costs. As a new type of passive optical device, long-period fiber grating has the advantages of low insertion loss, compact size and no retroreflection. It is easy to connect with fiber-optic optical devices to form an all-fiber structure. The all-fiber structure has the advantages of simple structure, small size, light weight, etc. It is more flexible and convenient in practical applications, easy to achieve high conversion efficiency and high power output, and has excellent heat dissipation performance. It provides a new idea for designing and producing filters that meet the requirements of different filter center wavelengths and filter bandwidths.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide a cascaded long-period grating filter with adjustable wavelength and bandwidth and a manufacturing method, so as to solve the problem that the filter wavelength and filter bandwidth cannot be changed in the filter in the prior art.

In order to solve the above technical problems, the first technical solution adopted by the present invention is as follows.

A cascaded long-period grating filter with adjustable wavelength and bandwidth, comprising a high-frequency filter group and a low-frequency filter group with the same structure; a support frame and a first temperature control platform; the low frequency filter group includes more than one group of low frequency long period grating groups, a second support frame, and a second temperature control platform; each high frequency long period grating group and each low frequency long period grating group The grating groups are cascaded and etched on the same fiber; the high-frequency long-period grating groups and the low-frequency long-period grating groups are composed of closely connected first to Nth long-period gratings, N≥1; the The upper surface of the first temperature control platform is provided with a thermal insulation device, and the first support frame is a stainless steel hollow annular frame, which is fixedly installed around the first temperature control platform; the high-frequency long-period grating groups are cured in the place. inside the first support frame and closely fit with the heating platform of the first temperature control platform; the spacing between the long period gratings in each high frequency long period grating group and each low frequency long period grating group is greater than or equal to The period of the previous long-period grating; the period of each long-period grating in the low-frequency filter group is greater than the period of each long-period grating in the high-frequency filter groups.

Further, the high-frequency filter group includes first to second high-frequency long-period grating groups, the first high-frequency long-period grating group includes two closely cascaded long-period gratings, and the first high-frequency long-period grating group The periods of the two long-period gratings in the grating group are the same, and the interval is one period; the second high-frequency long-period grating group includes two closely cascaded long-period gratings, and the second high-frequency long-period grating group The periods of the two long-period gratings are the same, and the interval is one period; the low-frequency filter group includes first to second low-frequency long-period grating groups, and the first low-frequency long-period grating group includes two closely cascaded long-period gratings grating, the period of the two long period gratings in the first low frequency long period grating group is the same, and the interval is 1 period; the second high and low frequency long period grating group includes two closely cascaded long period gratings, the The periods of the two long period gratings in the second low frequency long period grating group are the same, and the interval is one period.

Furthermore, the first to second temperature control platforms use graphite temperature control platforms with a heating upper limit of 400°C, and the drift temperature after thermal stabilization is less than 1°C; the period of the long period gratings in the first high frequency long period grating group is The range is 180 μm to 200 μm; the period range of the long period grating in the second high frequency long period grating group is 220 μm to 240 μm; the period range of the long period grating in the first low frequency long period grating group is 270 μm to 290 μm; The period range of the long period gratings in the second low frequency long period grating group is 310 μm to 330 μm; the high frequency long period grating groups and the low frequency long period grating groups are respectively cured on the first and second support frames with UV curing glue The top of the UV-curable adhesive is resistant to high temperature of 300°C.

The second technical solution adopted in the present invention:

A method for manufacturing a cascaded long-period grating filter with adjustable wavelength and bandwidth, comprising the following steps:

Step 1: Fiber pretreatment: Strip the coating on the surface of the single-mode fiber before etching, reserve more than 20cm for the left and right pigtails, tighten and fix the single-mode fiber to be etched with a fiber clamp, and apply a shaft to the fiber. pulling force;

Step 2: Draw a long period grating: draw a single period long period grating, and draw it according to the required period length;

Step 3: Etch long period grating: including the following sub-steps:

Step 3-1: Use online monitoring equipment to monitor, use a CO ₂ infrared laser workstation with a working wavelength of 1064 μm to etch the center of the long-period grating, and stop when the subsidence reaches 15dB;

Step 3-2: Keep the pre-tensioned state of the optical fiber, and continue to draw another long-period grating closely cascaded with the distance of one period of the previous long-period grating, and follow the step 3 until the etching is completed;

Step 3-3: judge whether the fabrication of all long-period gratings is completed, if so, go to step 6, otherwise go to step 4;

Step 4: Filter assembly: The long period gratings in the first to second high frequency long period grating groups are closely attached to the heating plate of the first temperature control platform, and then cured and packaged on the first temperature control platform by UV curing glue. In the support frame; the first to second low frequency long period grating groups are closely attached to the heating plate of the second temperature control platform, and are cured and packaged in the second support frame by UV curing glue.

Further, the period range of the two long-period gratings in the first high-frequency long-period grating group is 180 μm to 200 μm, 120 periods are written, and the interval is one grating period; The period range of the two long-period gratings is 220 μm to 240 μm, the writing period is 120, and the interval is one grating period; 120 periods, with a spacing of one grating period; the period range of the two long-period gratings in the second low-frequency long-period grating group is 310 μm to 330 μm, with 120 periods of writing and one grating period.

Further, the etching speed of the CO ₂ laser workstation is 8 mm/s, the laser frequency is 7 kHz, the model of the single-mode fiber is HI1060, and the working band is 980 nm-1600 nm.

The technical effects obtained by adopting the above technical solutions are as follows:

1. The core components used for filtering in the present invention are all fiber-optic structures, and the filtering parameters are adjusted by temperature changes. The all-fiber structure has extremely strong anti-electromagnetic interference characteristics, and has extremely high adaptability with other fiber-optic devices.

2. The present invention uses two groups of 8 long-period gratings in cascade for filtering, and makes full use of its filtering characteristics and temperature variation characteristics to realize the adjustment of the filtering center wavelength and bandwidth.

3. The present invention forms a cascaded long-period grating group through the band-stop characteristic of the long-period grating, so that adjustable filtering is performed in a wide range near the 1 μm band, the width of the adjustable range of the central wavelength reaches 19.5 nm, and the filtering bandwidth can be adjusted. The tuning range width reaches 13.4nm.

4. The present invention adopts an accurate and controllable graphite heating platform to control the temperature of the cascaded long-period grating group, correspondingly changing the filter center wavelength and filter bandwidth, and the change accuracy can reach 0.07nm.

Description of drawings

FIG. 1 is a schematic structural diagram of the present invention.

FIG. 2 is a spectral diagram of the narrowest filtering bandwidth of the filter in Embodiment 1 of the present invention.

FIG. 3 is a spectral diagram of the widest filtering bandwidth of the filter in Embodiment 1 of the present invention.

FIG. 4 is a graph showing the relationship between filter bandwidth and temperature in Embodiment 1 of the present invention.

FIG. 5 is a graph showing the relationship between the filter center wavelength and temperature in Embodiment 1 of the present invention.

FIG. 6 is a graph showing the relationship between wavelength and temperature linearity in Example 1 of the present invention.

The meaning of the arrangement is: 1-first support frame, 2-second support frame, 3-high frequency filter group, 4-low frequency filter group, 5-first temperature control platform, 6-second temperature control platform, 7 -UV curing glue, 8-single-mode fiber, 301-first high frequency long period grating group, 302-second high frequency long period grating group, 401-first low frequency long period grating group, 402-second low frequency long period grating group Raster group.

Detailed ways

The following examples serve to illustrate the invention.

Example 1

A cascaded long-period grating filter with adjustable wavelength and bandwidth, comprising a high-frequency filter group 3 and a low-frequency filter group 4 with the same structure; the high-frequency filter group 3 includes more than one group of cascaded high-frequency long-period gratings group, a first support frame 1 and a first temperature control platform 5; the low frequency filter group 4 includes more than one group of low frequency long period grating groups, a second support frame 2, and a second temperature control platform 6; The periodic grating group and each low-frequency long-period grating group are cascaded and etched on the same single-mode fiber 8; It is composed of long period grating, N≥1; the upper surface of the first temperature control platform 5 is provided with a thermal insulation device, and the first support frame 1 is a stainless steel hollow ring frame, which is fixed around the first temperature control platform 5 installation; each of the high-frequency long-period grating groups is inside the first support frame 1 and is closely attached to the heating platform of the first temperature control platform 5; the high-frequency long-period grating groups and the low-frequency The spacing between the long-period gratings in the long-period grating group is greater than or equal to the period of the previous long-period grating; cycle.

The high-frequency filter group 3 includes first to second high-frequency long-period grating groups 301, 302, the first high-frequency long-period grating group 301 includes two closely cascaded long-period gratings, the first high-frequency long-period grating The period of the two long period gratings in the frequency long period grating group 301 is the same, and the interval is 1 period; the second high frequency long period grating group 302 includes two closely cascaded long period gratings, the second high frequency long period grating The period of the two long period gratings in the long period grating group 302 is the same, and the interval is 1 period; the low frequency filter group 4 includes first to second low frequency long period grating groups 401-402, the first low frequency long period grating The group 401 includes two closely cascaded long-period gratings. The periods of the two long-period gratings in the first low-frequency long-period grating group 401 are the same, and the interval is one period; the second high- and low-frequency long-period grating group 402 It includes two closely cascaded long period gratings, and the periods of the two long period gratings in the second low frequency long period grating group 402 are the same, and the interval is one period.

The first to second temperature control platforms 5, 6 use graphite temperature control platforms with a heating upper limit of 400°C, and the drift temperature after thermal stabilization is less than 1°C; The period range is 180 μm to 200 μm; the period range of the long period gratings in the second high frequency long period grating group 302 is 220 μm to 240 μm; the period range of the long period gratings in the first low frequency long period grating group 401 is 270 μm to 270 μm 290 μm; the period range of the long period gratings in the second low frequency long period grating group 402 is 310 μm to 330 μm; the high frequency long period grating groups and the low frequency long period The tops of the second support frames 1 and 2; the UV-curable adhesive is resistant to high temperature of 300°C.

In the all-fiber filter based on long period grating, after the core LP01 mode enters the cladding, it couples with the LP0m mode of the cladding to form the notch loss of the transmission spectrum at different wavelengths. A cascaded long-period fiber grating can be regarded as a combination of two long-period fiber gratings with different periods separated by a cascaded fiber of length d. When the structural parameters of the cascaded long period gratings are the same, after the light passes through the first long period grating, the core mode is partially coupled to the cladding mode, and the cladding mode and the remaining core modes are transmitted along the cladding and the core respectively. , at the cascaded long-period grating, the cladding mode is coupled back to the core to become the core mode, and interferes with the uncoupled core mode of the previous long-period grating.

Phase matching conditions of long period grating:

为纤芯基模的有效折射率；

为m阶包层模的有效折射率；Λ为相应长周期光栅的周期。用r_j和t_j分别来表示均匀长周期光纤光栅的包层传输模振幅和基模传输振幅：where: λ ^m is the center wavelength of the loss peak after the m-order cladding mode is coupled with the core fundamental mode;

is the effective refractive index of the fundamental mode of the core;

is the effective refractive index of the m-order cladding mode; Λ is the period of the corresponding long period grating. Let r _j and t _j denote the cladding transmission mode amplitude and fundamental mode transmission amplitude of the uniform long-period fiber grating, respectively:

Where: L _j is the length of the j-th grating; δ _m is the coupling coefficient, which is defined as follows:

The transmission amplitude r of the cladding mode and the transmission amplitude of the core fundamental mode of the cascaded long period grating group can be expressed as:

The transmission rate T of the cascaded long period grating is:

In this embodiment, the model of the single-mode fiber 8 is HI1060, and the working wavelength range is from 980 nm to 1600 nm. A total of 8 tightly cascaded long-period gratings are etched on the single-mode fiber 8 . The temperature control range of the first to second temperature control platforms 5 and 6 is from room temperature to 400°C. When only the filter wavelength is changed, the two temperature control platforms are set to the same temperature. When the filter bandwidth needs to be changed, the temperature of the two platforms needs to be set separately. .

First, set the temperature of the heating platform to 20 degrees. After the temperature is stabilized, the central wavelength of the entire filter is stabilized at 1046.08 nm, and the filter bandwidth is 10.25 nm, which is 3dB of the central wavelength. When the center wavelength of the filter needs to be changed, and the temperature of the temperature control platform is changed at the same time, the filter frequency of the high-frequency grating group and the low-frequency grating group will decrease by 20GHz for each degree of increase, and the corresponding wavelength will increase by about 0.07nm. By precisely controlling the temperature, the precise control of the center wavelength of the filter can be achieved.

Determine the required filter center wavelength and bandwidth by adjusting the filter bandwidth. The first to second graphite temperature control platforms 5 and 6 are set at different temperatures, so that the filter wavelengths of the high-frequency filter group 3 and the low-frequency filter group 4 are shifted relative to each other. Under isothermal conditions, the 3dB bandwidth is 10.25nm. 8nm bandwidth, the temperature difference between the first graphite temperature control platform 5 and the second graphite temperature control platform 6 should be kept at about 32°C. To obtain a bandwidth of 10nm, the first graphite temperature control platform 5 and the second graphite temperature control platform The temperature difference of 6 should be kept around 25℃.

At the same time, the first to second graphite temperature control platforms 5 and 6 are heated, and the filter center wavelength will increase with the increase of temperature. After the temperature is completely stabilized, the overall filter wavelength will shift accordingly, and only the first graphite The temperature of the temperature control platform 5, or only lowering the temperature of the second graphite temperature control platform 6, or increasing the temperature of the first graphite temperature control platform 5 while reducing the temperature of the second graphite temperature control platform 6, can reduce the filter bandwidth accordingly. Small; only reduce the temperature of the first graphite temperature control platform 5, or only increase the temperature of the second graphite temperature control platform 6, or increase the temperature of the second graphite temperature control platform 6 while reducing the temperature of the first graphite temperature control platform 5 temperature, the filter bandwidth can be increased accordingly.

In this embodiment, through precise temperature control, the band-stop frequency of the long-period grating is changed, which is reflected in the change of the overall center wavelength of the filter, and can provide an adjustment range of the filter bandwidth of 13.4nm, and the center wavelength from 1045.45nm to 1064.95nm, The adjustable range can reach 19.5nm, and the accuracy of its wavelength change can reach 0.07nm.

Example 2

A method for manufacturing a cascaded long-period grating filter with adjustable wavelength and bandwidth, comprising the following steps:

Step 1: Optical fiber pretreatment: strip off the coating on the surface of the single-mode fiber 8 before etching, reserve more than 20 cm for each of the left and right pigtails, and fasten and fix the single-mode fiber 8 to be etched with a fiber clamp. Apply axial tension;

Step 2: Draw a long period grating: draw a single period long period grating, and draw it according to the required period length;

Step 3: Etch long period grating: including the following sub-steps:

Step 3-1: Use online monitoring equipment to monitor, use a CO ₂ infrared laser workstation with a working wavelength of 1064 μm to etch the center of the long-period grating, and stop when the subsidence reaches 15dB;

Step 3-2: Keep the pre-tensioned state of the optical fiber, and continue to draw another long-period grating closely cascaded with the distance of one period of the previous long-period grating, and follow the step 3 until the etching is completed;

Step 3-3: judge whether the fabrication of all long-period gratings is completed, if so, go to step 6, otherwise go to step 4;

Step 4: filter assembly: the long period gratings in the first to second high frequency long period grating groups 301, 302 are closely attached to the heating plate of the first temperature control platform 5, and cured by high temperature resistant ultraviolet curing glue , and encapsulated in the first support frame 1; the first to second low frequency long period grating groups 301, 302 are closely attached to the heating plate of the second temperature control platform 6, and cured by high temperature resistant UV curing glue , and packaged in the second support frame 2 .

The period range of the two long-period gratings in the first high-frequency long-period grating group 301 is 180 μm to 200 μm, 120 periods are written, and the interval is one grating period; the second high-frequency long-period grating group 302 The period range of the two long-period gratings is 220 μm to 240 μm, the writing period is 120, and the interval is one grating period; 120 periods are written with one grating period apart; the two long period gratings in the second low-frequency long period grating group 402 have periods ranging from 310 μm to 330 μm, with 120 periods of writing and one grating period.

The etching speed of the _CO2 laser workstation is 8mm/s, the laser frequency is 7kHz, the single-mode fiber model is HI1060, and the working band is from 980nm-1600nm.

Claims (5)

1. A cascade long-period grating filter with adjustable center wavelength and bandwidth is characterized in that: the light transmitted in the fiber core couples partial modes into a cladding layer through a long-period grating to generate a cladding mode, the cladding mode and a fiber core mode are transmitted along the optical fiber together, when the cladding mode returns to the fiber core, the light is coupled with the fiber core mode transmitted in the fiber core, the coupled wavelength components generate loss and are not transmitted continuously, the wavelength components which are not coupled are transmitted continuously along the fiber core, and the non-transmitted light components are eliminated through high-frequency and low-frequency long-period grating groups, so that the function of band-pass filtering is realized, and the high-frequency and low-frequency long-period grating groups comprise a high-frequency filtering group (3) and a low-frequency filtering group (4) which have the same structure; the high-frequency filtering group (3) comprises more than one high-frequency long-period grating group, a first support frame (1) and a first temperature control platform (5) which are cascaded; the low-frequency filtering group (4) comprises more than 1 low-frequency long-period grating group, a second supporting frame (2) and a second temperature control platform (6); each high-frequency long period grating group and each low-frequency long period grating group are cascaded and etchedEtched on the same single mode fiber (8); each high-frequency long period grating group and each low-frequency long period grating group are respectively a first grating group, a second grating group and a third grating group which are closely connectedNLong period grating composition; the first temperature control platform (5) is tightly attached to the heating platform; the distance between the long-period gratings in each high-frequency long-period grating group and each low-frequency long-period grating group is greater than or equal to the period of the previous long-period grating; the period of each long period grating in the low-frequency filtering group is larger than that of each long period grating in each high-frequency filtering group.

2. The cascaded long-period grating filter with adjustable center wavelength and bandwidth of claim 1, wherein: the first temperature control platform, the second temperature control platform (5) and the second temperature control platform (6) use a graphite temperature control platform with the upper heating limit of 400 ℃, and the drift temperature after thermal stabilization is less than 1 ℃; when the parameters of the cascaded long-period grating structures are the same, after light passes through the first long-period grating, the fiber core mold is partially coupled to the cladding mold, the cladding mold and the residual fiber core mold are respectively transmitted along the fiber core of the cladding, and at the second cascaded long-period grating, the cladding mold is coupled back to the fiber core to form the fiber core mold and is interfered with the fiber core mold which is not coupled before; the first high-frequency long-period grating group comprises 2 closely cascaded long-period gratings with the interval of 1 period, and the period range of the long-period gratings is 180-200 mu m; the second high-frequency long-period grating group comprises 2 closely cascaded long-period gratings with the interval of 1 period, and the period range of the long-period gratings is 220-240 mu m; the first low-frequency long-period grating group comprises 2 closely cascaded long-period gratings with the interval of 1 period, and the period range of the long-period gratings is 270-290 mu m; the second low-frequency long-period grating group comprises 2 closely cascaded long-period gratings with the interval of 1 period, and the period range of the long-period gratings is 310-330 μm; the high-frequency long-period grating groups and the low-frequency long-period grating groups are respectively cured on the tops of the first support frame (1) and the second support frame (2) by ultraviolet curing glue; the ultraviolet curing adhesive resists high temperature of 300 ℃.

3. A method for manufacturing a cascaded long period grating filter with adjustable wavelength and bandwidth according to claim 1, wherein: the method comprises the following steps:

step 1: optical fiber pretreatment: before etching, a coating layer on the surface of the single-mode optical fiber (8) is stripped, the left and right tail fibers are respectively reserved for more than 20cm, the single-mode optical fiber (8) to be etched is fastened and fixed by an optical fiber clamp, and axial tension is applied;

step 2: drawing a long-period grating: drawing a long-period grating with a single period, and drawing according to the required period length;

and step 3: etching the long-period grating: the method comprises the following steps:

step 3-1: monitoring with online monitoring equipment, and using CO with working wavelength of 1064 μm ₂ Etching the center of the long-period grating by the infrared laser workstation, and stopping when the sinking reaches 15 dB;

step 3-2: keeping the pre-tensioned state of the optical fiber, keeping a distance of one period of the previous long-period grating at intervals, continuously drawing another long-period grating which is closely cascaded with the previous long-period grating, and finishing etching according to the step 3;

step 3-3: judging whether the manufacture of all long-period gratings is finished, if so, turning to the step 4, and otherwise, re-performing the step 3;

and 4, step 4: assembling the filter: the long-period gratings in the first high-frequency long-period grating group, the second high-frequency long-period grating group and a heating plate of the first temperature control platform (5) are tightly attached, and are cured and packaged in the first supporting frame (1) through ultraviolet curing glue; and (3) closely attaching the first to second low-frequency long-period grating groups to the heating plate of the second temperature control platform (6), and curing and packaging the grating groups in the second supporting frame (2) through ultraviolet curing adhesive.

4. The method of claim 3, wherein the wavelength and bandwidth tunable cascaded long period grating filter is characterized in that: the period ranges of 2 long period gratings in the first high-frequency long period grating group (301) are all 180-200 mu m, 120 periods are written, and the distance between the periods is one grating period; the period ranges of 2 long period gratings in the second high-frequency long period grating group (302) are all 220-240 mu m, 120 periods are written, and the distance between the periods is one grating period; the period ranges of 2 long-period gratings in the first low-frequency long-period grating group (401) are 270-290 mu m, 120 periods are written, and the distance between the periods is one grating period; the period range of 2 long period gratings in the second low-frequency long period grating group (402) is 310-330 μm, 120 periods are written, and the distance between every two grating periods is one grating period.

5. The method of claim 3 or 4, wherein the wavelength and bandwidth of the cascade long period grating filter are adjustable, and the method comprises the following steps: the CO is ₂ The etching speed of the laser workstation is 8mm/s, the laser frequency is 7kHz, the type of the single-mode optical fiber 8 is HI1060, and the working wave band is 980nm-1600 nm.

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