CN106154405B - A New Realization Method of Long Period Fiber Grating - Google Patents

Abstract

The invention provides a method for realizing a novel long-period fiber grating. The fiber adopts a hybrid fiber with a high thermal-optical coefficient in the core and a low thermal-optical coefficient in the cladding, and produces a sufficiently large refractive index change on the core through a relatively low temperature change. , and then realize the refractive index modulation required by the long-period fiber grating; cooperate with the programmable heating array to control the temperature gradient along the waveguide structure, and the controllable temperature gradient then controls the refractive index distribution on the optical waveguide to realize programmable long-period grating. The new realization method of long-period fiber grating provided by the present invention can be digitally programmed, and its characteristics can be changed in real time according to application requirements, and has the characteristics of low insertion loss and low power consumption.

Description

A New Realization Method of Long Period Fiber Grating

technical field

The invention relates to a new realization method of long-period fiber grating, and the long-period fiber grating has the characteristic of being digitally programmable.

Background technique

Searching the prior art from the perspective of the realization method of long-period fiber grating, it is found that there are many related invention patents and papers: using arc and high-power laser to cause periodic deformation of optical fiber to realize long-period grating, or using high-power ultraviolet light or electrons The beam achieves periodic refractive index modulation of the fiber. A number of related patents have also been retrieved from the perspective of tunable long-period fiber gratings. But from the perspective of programmable long-period fiber gratings, similar prior art has not yet been found.

The traditional long-period grating cannot be re-structured after one-time fabrication. As an important photonic device, long period grating plays an important role in various fields of photonics, such as photonic filter, optical pulse signal processing, mode conversion and other devices. In these applications, there are high requirements for the tunable performance of the device. Due to its fixed structure and limited tuning performance, the long-period gratings currently involved are limited in their applications.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a new realization method of long-period fiber grating, which can be digitally programmed, its characteristics can be changed in real time according to application requirements, and has the characteristics of low insertion loss and low power consumption. The invention is based on a hybrid optical fiber with a high thermal-optical coefficient as the optical fiber core, and realizes a rewritable and real-time redefinition long-period fiber grating on the waveguide by controlling the temperature distribution along the optical fiber on the waveguide structure. Combined with digital control circuits, real-time functional repeat definition and digital programming of the device can be achieved.

For achieving the above object, the technical scheme adopted in the present invention is as follows:

A new realization method of long-period fiber grating. The fiber adopts a hybrid fiber with a high thermal-optical coefficient in the core and a low thermal-optical coefficient in the cladding, and produces a sufficiently large refractive index change on the core through a low temperature change, thereby realizing long-term growth. The refractive index modulation required by the periodic fiber grating, combined with the programmable heating array, controls the temperature gradient change along the waveguide structure, and the temperature gradient controls the refractive index structure of the optical waveguide, thereby realizing the long period grating.

The high thermo-optic coefficient is that TOC (thermo-optics coefficient, TOC) is in the order of 10e-4/°C, the low thermo-optic coefficient is that the TOC is in the order of 10e-6/°C to 10e-5/°C, and the lower The temperature is 10 °C and below, which is sufficiently large to be on the order of 10e-3.

The hybrid optical fiber is a liquid core optical fiber or a plastic core optical fiber.

The cladding layer is made of silicon dioxide or other materials with low light guide loss and low thermo-optic coefficient.

The uncoated hybrid fiber is fixed or wrapped on the heating array, assisted by a heat sink to provide a stable packaging environment and heat dissipation, a voltage source provides a programmable voltage for the heating array, and a control unit gives instructions to adjust the heating array And then control the characteristics of the long period fiber grating.

The heating array is composed of micro-heating units arranged longitudinally. The working state of each heating unit can be turned on or off independently. Uniform refractive index modulation enables apodization of long-period fiber gratings to provide filtering properties with fewer filtering side lobes.

The geometric size of each heating unit along the waveguide direction is less than 200 microns, and the smaller the size, the better the heating accuracy and control accuracy can be.

The working state of the heating array is represented by a number sequence, the working state of a single heating unit is represented by a number '1', and the off state is represented by a number '0'; by changing the distance between the heating units in the working state, that is, in the digital sequence' The number of '0's between 1' can realize the adjustment of the grating period; the change of the length of the digital sequence corresponds to the change of the length of the long-period grating, and the length of the grating is an important parameter that determines the coupling efficiency; the heating voltage directly determines its The temperature change brought about by the waveguide structure, and the temperature change directly determines the change of the refractive index of the fiber core, that is, determines the modulation depth of the long-period fiber grating. With the grating length programming, it can be realized from the two parameters of voltage and grating length. Program control of grating coupling efficiency; the number of heating units in continuous working state in each grating period can control the duty cycle of the grating.

The tuning of the resonance wavelength is achieved by controlling the overall ambient temperature.

By introducing a phase shift inside the formed long-period fiber grating, and changing the size of the phase shift and the position of the phase shift in the grating by programming, single or multiple controllable phase shifts can be realized at any position in the grating; position, can realize the cascade operation of a pair of gratings or multiple gratings.

In the hybrid optical fiber used in the present invention, the TOC of the core reaches the order of 10e-4/°C, which is more than an order of magnitude higher than that of traditional optical fiber materials such as silica; the cladding of the hybrid optical fiber adopts low TOC Optical materials such as silicon dioxide. Through a relatively low temperature change (10°C or lower), a sufficiently large (10e-3 order) refractive index change of the core is generated, and at this time, it can be considered that the refractive index of the cladding does not change, thereby realizing a long-period fiber grating desired refractive index modulation. With the designed high-precision programmable heating array, the temperature gradient along the waveguide structure can be controlled with high precision; the programmed temperature gradient directly controls the refractive index structure of the optical waveguide, thereby realizing long period grating; on this basis , more complex structures, such as cascaded long-period gratings, and phase-shifted long-period gratings, can be implemented through programming.

Compared with the prior art, the present invention can greatly expand the application range of the current long-period grating.

Description of drawings

Other features, objects and advantages of the present invention will become more apparent by reading the detailed description of non-limiting embodiments with reference to the following drawings:

1 is a cross-sectional view of a liquid-core optical fiber used in an embodiment of the present invention;

Figure 2 shows the heating array and supporting digital drive;

3 is a schematic diagram of forming a periodic temperature distribution on an optical fiber to form a long-period fiber grating;

Figure 4 shows the change of transmission spectrum under different grating periods, ∧ is the grating period;

Fig. 5 is the change of coupling efficiency at the resonant wavelength by voltage control;

Fig. 6 shows the variation of coupling efficiency under different grating lengths;

Figure 7 is a comparison of the characteristics of the transmission spectrum with and without phase shift.

1. Liquid core of liquid-core fiber; 2. Cladding of liquid-core fiber; 3. Heat sink, 4. Liquid-core fiber (without coating); 5. Heating array; 6. Voltage source; 7. Control unit; 8. Heat sink; 9. Inactive heating unit; 10. Activating heating unit in working state.

Detailed ways

The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the inventive concept. These all belong to the protection scope of the present invention.

In the embodiments of the present invention, a liquid-core fiber is used as an example of such a hybrid fiber, and a high-precision heating array is designed to realize a programmable long-period fiber grating, and its programming characteristics will be described. The cross-sectional structure of the liquid-core fiber is shown in Figure 1:

Under the premise of selecting a reasonable liquid material, the liquid-core optical fiber is an excellent waveguide structure. The material of the hybrid fiber cladding layer 2 is silicon dioxide (SiO2). Both liquid materials have good light-guiding properties, and their absorption is negligible within the transmission length of the long-period fiber grating. At the same time, in order to reduce the interference of multi-mode transmission in the optical fiber, a suitable solution ratio can be used to make the optical fiber in the working wavelength window (in this embodiment, the wavelength range is 1.1-1.6 microns) for single-mode transmission. In order to achieve low-loss connections to conventional fiber systems, we used a fiber geometry with a core diameter of 8.6 microns and a cladding diameter of 125 microns. Similarly, in order to adapt to different access fibers and further customize the corresponding cladding mode of the long-period fiber grating, this parameter can be modified appropriately.

The structure of the stripped liquid-core fiber fixed on the programmable long-period grating is shown in Figures 2 and 3, and the heating array and its design with the liquid-core fiber are shown in the figure. In FIG. 2 , the heat sink 3 and the heating array 5 fix the uncoated liquid-core optical fiber 4 in the middle, so as to provide a stable packaging environment and heat dissipation. The voltage source 6 provides a programmed voltage for the heating array; the control core 7 of the whole device gives instructions to adjust the heating array and then control the characteristics of the long-period fiber grating.

The heating array is composed of micro-heating units arranged longitudinally, and the working state of each heating unit can be turned on or off independently; Heating accuracy and control accuracy; each heating unit can be individually controlled by the magnitude of the current, thereby controlling longitudinally uniform or non-uniform refractive index modulation, and non-uniform refractive index modulation can realize apodization of long-period fiber gratings to provide less filtering lobe filtering characteristics.

Figure 3 shows the principle of the thermally induced long-period fiber grating in the working state. The program-controlled thermal distribution determines the grating period, length and other characteristics of the grating.

The size of a single heating element in this example is 125 microns x 125 microns. The working state of the array is controlled by the micro-control unit 7 . The working state of a single heating unit can be represented by a number sequence, '0' means it is turned off, and the working state is represented by a number '1', corresponding to 9 and 10 in Figure 3. In the sequence: '...000100010001...', the grating period is the distance between the heating units '1' in the working state, that is, 125 μm×4=500 μm. The number of the '0001' sequences in the fiber grating determines the length of the entire long period grating. It can be seen that if the size of the heating unit can be further reduced, the control accuracy of the device can be further improved. The heating current directly determines the temperature change it brings on the waveguide structure. The temperature change directly determines the change of the refractive index of the liquid core, that is, the modulation depth of the long-period fiber grating. The number of heating units in continuous working state in each grating period can control the duty ratio of the grating. For example, the grating period corresponding to the sequence '...000100010001...' and the sequence '...001100110011...' is both 500 microns, but the grating duty cycle is increased from 1/4 to 1/2. The parameters that can be adjusted by the long period grating include: grating period, grating length, modulation depth, duty cycle of the grating, and apodization of the grating. The adjustment of the resonant wavelength is achieved by controlling the overall ambient temperature, such as controlling the temperature of the overall packaged device. By controlling these parameters, the resonant wavelength and the corresponding coupling efficiency can be digitally programmed within the operating wavelength. On this basis, the controllable phase shift within the grating and the cascade operation of multiple gratings can be realized through programming.

The implementation of these functions is explained below, and the corresponding tuning results are given in part:

A-Gating period adjustment: By changing the distance between the heating units in the working state, such as changing the sequence '...000100010001...' to '...000010000100001...', the period can be changed from 125 microns × 4 = 500 microns to 125 μm×5=625 μm; therefore, the grating period can be changed in steps of 125 μm. Figure 4 shows the change of the transmission spectrum under different grating periods. The adjustment resolution of this grating period is determined by the size of the individual heating elements. In this example the radial size of the grating is 125 microns. However, by improving the process, it can be further reduced to the order of tens of microns or even lower, thereby improving the control accuracy of the grating period. Figure 4 shows the tuning of the grating transmission spectrum by changing the grating period:

B-Gating modulation depth adjustment: The modulation depth of the grating is determined by the change rate of the core refractive index, which is determined by the temperature change, that is, the applied voltage. Thus by varying the applied voltage, the modulation depth of the grating can be controlled. The change effect is shown in Figure 5;

C-Gating length adjustment: The length of the grating is another important parameter that determines the coupling efficiency in addition to the voltage control. The length of the grating can directly change the length of the control sequence, such as '000100010001' is changed to '00010001000100010001', then the grating length is changed from 500 microns × 3 = 1.5 mm to 500 microns × 5 = 2.5 mm. Therefore, in the long-period fiber grating, its length adjustment accuracy can be as fine as one grating period, and the control effect of the grating length (number of periods) on the transmission spectrum is shown in Figure 6;

D – Insert digitally controlled phase shift into the grating: by inserting a digitally controllable phase shift into the grating: the digital sequence corresponding to the original grating is '000001000001000001', then the sequence '000' corresponds to half the grating period, which corresponds to a π phase shift . A π-phase shift at the center of the grating can form another transmission peak within the grating's transmission spectrum. More complex filter designs can be realized by manipulating the phase shift (programming to change the magnitude of the phase shift and the position of the phase shift in the grating) and cascading fiber gratings. Figure 7 shows the effect of the presence or absence of this π phase shift on the transmission spectrum.

Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above-mentioned specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essential content of the present invention. The embodiments of the present application and features in the embodiments may be combined with each other arbitrarily, provided that there is no conflict.

Claims (9)

1. A method for realizing a novel long-period fiber grating, characterized in that the optical fiber adopts a hybrid optical fiber whose core is a high thermo-optic coefficient and a cladding is a low thermo-optic coefficient, and a sufficiently large refraction is produced to the core through a relatively low temperature change. In order to realize the refractive index modulation required by the long-period fiber grating, the temperature gradient along the fiber can be controlled with the programmable heating array, and the refractive index structure of the fiber core can be controlled by the controllable temperature gradient, so as to realize the programmable length change. Periodic fiber grating, the high thermo-optic coefficient is in the order of 10e-4/°C, the low thermo-optic coefficient is in the order of 10e-6/°C to 10e-5/°C, and the lower temperature is 10°C and below, The sufficiently large refractive index is of the order of 10e-3. 2 . The realization method of the novel long-period fiber grating according to claim 1 , wherein the hybrid optical fiber is a liquid core optical fiber or a plastic core optical fiber. 3 . 3 . The realization method of the novel long-period fiber grating according to claim 1 , wherein the cladding layer is silicon dioxide. 4 . 4. The realization method of the novel long-period fiber grating according to claim 1 is characterized in that, the hybrid fiber without the coating layer is fixed or surrounded by the heating array, and at the same time passes through the heat sink to provide a stable packaging environment and heat dissipation, a The voltage source provides a programmable voltage for the heating array, and a control unit gives instructions to adjust the heating array and then control the characteristics of the long-period fiber grating. 5 . The method for realizing the novel long-period fiber grating according to claim 1 , wherein the heating array is formed by longitudinally arranging micro heating units, and the working state of each heating unit can be turned on or off independently, and each heating unit can be turned on or off independently. 6 . The heating unit can be individually controlled by the magnitude of the current, thereby controlling the longitudinal uniform or non-uniform refractive index modulation of the fiber, and realizing the apodization of the long-period fiber grating to provide filtering characteristics with less filtering side lobes. 6. The realization method of the novel long-period fiber grating according to claim 5, wherein the geometric size of each heating unit along the fiber direction is less than 200 microns, and the smaller the size is, the better the heating accuracy and control accuracy are improved. . 7. The method for realizing the novel long-period fiber grating according to claim 6, wherein the working state of the heating array is represented by a number sequence, the working state of a single heating unit is represented by a number '1', and the off state is represented by a number '0' ' means; by changing the distance between the heating units in the working state, that is, the number of '0' between '1' in the digital sequence, the grating period can be adjusted; and the change in the length of the digital sequence corresponds to a long The length of the periodic grating changes, and the length of the grating is an important parameter to determine the coupling efficiency; the heating voltage directly determines the temperature change on the fiber, and the temperature change directly determines the change in the refractive index of the fiber core, that is, determines the long period The modulation depth of the fiber grating, combined with the grating length programming, can realize the program control of the grating coupling efficiency from the two parameters of voltage and grating length; the number of heating units in continuous working state in each grating period can control the duty of the grating Compare. 8 . The realization method of the novel long-period fiber grating according to claim 7 , wherein the adjustment of the resonance wavelength can also be realized by controlling the overall ambient temperature. 9 . 9 . The method for realizing the novel long-period fiber grating according to claim 7 , wherein the phase shift can be introduced into the formed long-period fiber grating, and the magnitude of the phase shift and the amount of the phase shift in the grating can be changed by programming. 10 . position, to achieve single or multiple controllable phase shifts at any position in the grating; and control the position of the grating on the fiber to realize the cascade operation of multiple gratings.