A non-contact ultrafast sensing system

Abstract

The present invention is applicable to the field of sensing technology, and provides a non-contact ultrafast sensing system, the system comprising: a pulse laser generation module, a dispersion module, a circulator, a first tilted fiber grating, a second tilted fiber grating, a signal detection module, and an optical fiber sensor and other units, wherein the first tilted fiber grating and the second tilted fiber grating can perform non-contact signal transmission. By setting a first tilted fiber grating and a second tilted fiber grating, and coordinating the dispersion module, the circulator module, and the signal detection and analysis unit, the present application can realize non-contact signal transmission of optical fiber sensing signals, so that the existing optical fiber sensing system can obtain high-precision and high-speed signal transmission capabilities in harsh environments, significantly improving the applicable fields and scenarios of the existing optical fiber sensing system, and having high application value and market prospects.

Description

Technical Field

[0001] The invention belongs to the field of sensor technology, and in particular relates to a non-contact ultrafast sensor system. Background Art

[0002] Fiber optic sensing technology is a technology that uses optical fiber as a sensing element to detect and measure various physical or chemical quantities in the environment. It is based on the propagation characteristics of light and monitors the target parameters by measuring the changes in optical signals.

[0003] The principle of fiber optic sensing technology is to use the changes caused by the influence of the external environment when light propagates in the optical fiber. By introducing some specific structures or materials into the optical fiber, such as gratings, Bragg fibers, microstructured fibers, etc., the optical fiber can be made sensitive to parameters such as temperature, pressure, strain, and chemical composition. When these parameters change, the characteristics of the optical signal in the optical fiber will change. By detecting these changes, the target parameters can be measured. Fiber optic sensing technology has the advantages of high sensitivity, strong anti-interference ability, and easy remote monitoring. Therefore, it is widely used in various fields, such as industrial monitoring, environmental monitoring, medical diagnosis, security monitoring, etc.

[0004] The current fiber optic sensing system relies on optical fiber as the transmission medium, that is, it is a wired transmission. The arrangement and distribution of cables limit the application scope of sensing. For example, it is difficult to use wired fiber optic sensing in some high temperature and highly corrosive environments. Therefore, it can be seen that the existing fiber optic signal transmission device is significantly restricted by the application environment and needs to be improved. Summary of the invention

[0005] The purpose of the embodiments of the present application is to provide a non-contact ultrafast sensing system, aiming to solve the problem that the existing optical fiber signal transmission devices are significantly restricted by the application environment.

[0006] The embodiment of the present application is implemented by providing a non-contact ultrafast sensing system, the system comprising:

[0007] A pulse laser generating module, a dispersion module, a circulator, a first tilted fiber Bragg grating, a second tilted fiber Bragg grating, a signal detecting module, and a fiber sensor;

[0008] The pulse laser generating module is used to generate pulse laser;

[0009] The dispersion module is used to perform pulse broadening on the pulse laser, and set the time sequence signal of the broadened pulse laser and the frequency domain wavelength intensity to a linear corresponding relationship to generate a linear corresponding signal;

[0010] The circulator has three ports, the first port of the circulator is used to receive the linear correspondence signal, the second port of the circulator is used to output the linear correspondence signal to the first tilted fiber Bragg grating, the second port of the circulator is also used to receive a return signal transmitted by the first tilted fiber Bragg grating, the circulator is also used to modulate the linear correspondence signal based on the return signal and generate a modulated optical signal, and the third port of the circulator is used to output the modulated optical signal to the signal detection module;

[0011] The first tilted fiber Bragg grating has two ports, the first tilted fiber Bragg grating first port is used to receive the linear corresponding signal transmitted from the second port of the circulator, and transmit the linear corresponding signal to the space through the first tilted fiber Bragg grating second port, and the first tilted fiber Bragg grating second port is also used to obtain the return signal emitted into the space by the second tilted fiber Bragg grating second port, and transmit the return signal back to the second port of the circulator through the first tilted fiber Bragg grating first port;

[0012] The second tilted fiber Bragg grating has two ports, the second tilted fiber Bragg grating second port is used to obtain the linear corresponding signal emitted into space by the first tilted fiber Bragg grating second port, and output it to the light sensor via the second tilted fiber Bragg grating first port;

[0013] The optical fiber sensor is used to modulate the linear corresponding signal based on an external signal to generate a return signal, and output the return signal to the first port of the second tilted optical fiber grating.

[0014] Preferably, the first tilted fiber grating and the second tilted fiber grating are a grating pair.

[0015] Preferably, the tilt angle θ of the first tilted fiber grating and the second tilted fiber grating are both 45 degrees.

[0016] Preferably, the pulse laser generating module is a femtosecond laser, which is used to generate femtosecond pulse laser.

[0017] Preferably, the signal detection module is used to demodulate the modulated optical signal based on a synchronization signal to obtain the external signal, and the synchronization signal is a synchronization signal of the pulse laser generation signal.

[0018] Preferably, the synchronization signal is generated by the pulse laser generating module.

[0019] Preferably, the pulse laser generated by the pulse laser generating module includes a laser light signal in the near-infrared band.

[0020] Preferably, the dispersion module is a chirped fiber grating.

[0021] A non-contact ultrafast sensing system provided in an embodiment of the present application can realize non-contact signal transmission of optical fiber sensing signals by setting a first tilted optical fiber Bragg grating and a second tilted optical fiber Bragg grating, and coordinating a dispersion module, a circulator module, and a signal detection and analysis unit, so as to enable the existing optical fiber sensing system to obtain high-precision and high-speed signal transmission capabilities in harsh environments, significantly improving the applicable fields and scenarios of the existing optical fiber sensing system, and having high application value and market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A module structure diagram of a non-contact ultrafast sensing system provided in an embodiment of the present application;

[0023] Figure 2 Another non-contact ultrafast sensing system module structure diagram provided in an embodiment of the present application;

[0024] Figure 3 A schematic diagram of a demodulation result of an optical signal provided in an embodiment of the present application;

[0025] Figure 4 A simplified roadmap for different wavelength laser signals entering a chirped fiber Bragg grating;

[0026] Figure 5 This is a diagram of the optical signal propagation inside a tilted fiber Bragg grating and a side structure diagram. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0028] It is understood that the terms "first", "second", etc. used in this application may be used herein to describe various elements, but unless otherwise specified, these elements are not limited by these terms. These terms are only used to distinguish a first module from another module. For example, a first port may be referred to as a second port, and similarly, a second port may be referred to as a first port without departing from the scope of this application.

[0029] like Figure 1 As shown, in one embodiment, a non-contact ultrafast sensing system is proposed, which may specifically include:

[0030] A pulse laser generating module, a dispersion module, a circulator, a first tilted fiber Bragg grating, a second tilted fiber Bragg grating, a signal detecting module, and a fiber sensor;

[0031] The pulse laser generating module is used to generate pulse laser;

[0032] The dispersion module is used to perform pulse broadening on the pulse laser, and set the time sequence signal of the broadened pulse laser and the frequency domain wavelength intensity to a linear corresponding relationship to generate a linear corresponding signal;

[0033] The circulator has three ports, the first port of the circulator is used to receive the linear correspondence signal, the second port of the circulator is used to output the linear correspondence signal to the first tilted fiber Bragg grating, the second port of the circulator is also used to receive a return signal transmitted by the first tilted fiber Bragg grating, the circulator is also used to modulate the linear correspondence signal based on the return signal and generate a modulated optical signal, and the third port of the circulator is used to output the modulated optical signal to the signal detection module;

[0034] The first tilted fiber Bragg grating has two ports, the first tilted fiber Bragg grating first port is used to receive the linear corresponding signal transmitted from the second port of the circulator, and transmit the linear corresponding signal to the space through the first tilted fiber Bragg grating second port, and the first tilted fiber Bragg grating second port is also used to obtain the return signal emitted into the space by the second tilted fiber Bragg grating second port, and transmit the return signal back to the second port of the circulator through the first tilted fiber Bragg grating first port;

[0035] The second tilted fiber Bragg grating has two ports, the second tilted fiber Bragg grating second port is used to obtain the linear corresponding signal emitted into space by the first tilted fiber Bragg grating second port, and output it to the light sensor via the second tilted fiber Bragg grating first port;

[0036] The optical fiber sensor is used to modulate the linear corresponding signal based on an external signal to generate a return signal, and output the return signal to the first port of the second tilted optical fiber grating.

[0037] In the embodiment of the present application, the first tilted fiber grating can mainly have three functions: a, as a spatial light diffraction device, it can realize the diffraction and wavelength separation of light; b, as a spatial light receiving device, it can realize the coupling of light signals from the external space into the optical fiber core; c, as a side light emitting device, it can output the modulated light signal modulated by the external sensing signal to the external space.

[0038] In the embodiment of the present application, the main functions of the second tilted fiber grating can be similar to those of the first tilted fiber grating: a, as a spatial light diffraction device, it can realize the diffraction and wavelength separation of light wavelengths; b, as a spatial light receiving device, it can realize the coupling of light signals from the external space into the optical fiber core; c, as a side light emitting device, it can output the modulated light signal modulated by the external sensing signal to the external space.

[0039] In an embodiment of the present application, the first end of the circulator can be used to receive an optical signal after time domain broadening and linear correspondence of a dispersion module signal, the second end can be used to output the broadened optical signal, and receive a returned external sensor signal, modulate the optical signal after broadening and linear correspondence processing, and the third end can be used to output a modulated optical signal modulated by the external sensor signal.

[0040] In the embodiments of the present application, Figure 5 The figure shows the optical signal propagation inside the tilted fiber Bragg grating. Figure 5 (a), side structure Figure 5 (b) The first end of the first tilted fiber Bragg grating can receive the light signal after widening from the third end of the circulator, and output the returned modulated light signal to the third end of the circulator; the second end receives the modulated light signal from the first end of the second tilted fiber Bragg grating, and outputs the widened light signal to the first end of the second tilted fiber Bragg grating.

[0041] In the embodiment of the present application, the optical fiber sensor can adopt an optical fiber Bragg grating, which can sense external signals and convert the external intensity signal into a change in wavelength. The external signal intensity can be obtained by detecting the change in wavelength. The optical fiber sensor receives the broadened optical signal and returns the optical signal modulated by the external signal through reflection, and returns the modulated optical signal to the second tilted optical fiber Bragg grating. Sense the external signal E (A2, f), and convert the external intensity signal into a change in wavelength. The external signal intensity can be obtained by detecting the change in wavelength. The optical fiber sensor receives the broadened optical signal and returns the optical signal L2 (A2, λ1) modulated by the external signal through reflection, and returns the modulated optical signal L2 (A2, λ1) to the second tilted optical fiber Bragg grating.

[0042] In an embodiment of the present application, the signal detection module can use a high-speed photodetector. While receiving the synchronization signal of the femtosecond laser at the first end, the second end receives the modulated light signal from the circulator. By analyzing the time domain position of the modulated light signal, the size of the reflected light wavelength can be obtained, and the size of the external signal can be demodulated.

[0043] It is known to those skilled in the art that the fiber optic sensing technology on which the traditional fiber optic sensing system relies often relies on optical fiber as the transmission medium and on the sensor as the sensing unit. Therefore, there are two problems with the traditional fiber optic sensing system: First, the traditional fiber optic sensing system relies on optical fiber as the transmission medium, that is, it is a wired transmission, and the arrangement and distribution of the cable limit the application scope of the sensing, such as some high temperature and highly corrosive environments are difficult to use a wired method for fiber optic sensing. Secondly, the response speed of the sensor is generally much higher than the demodulation speed of the signal, that is, there are certain constraints on the detection and demodulation of the signal, which affects the ultrafast response of the signal. A non-contact ultrafast sensing system provided in an embodiment of the present application, by setting a first tilted fiber grating and a second tilted fiber grating, and coordinating a dispersion module, a circulator module, and a signal detection and analysis unit, etc., can realize the non-contact signal transmission of the fiber optic sensing signal, so that the existing fiber optic sensing system obtains high-precision and high-speed signal transmission capabilities in harsh environments, significantly improving the applicable fields and scenarios of the existing fiber optic sensing system, and has high application value and market prospects. The above-mentioned non-contact ultrafast sensing system based on tilted fiber Bragg grating can solve the problem that the wired transmission of the traditional fiber optic sensing system leads to a narrow application range and cannot realize ultrafast signal detection and signal demodulation.

[0044] In a preferred embodiment, the first tilted fiber grating and the second tilted fiber grating are a grating pair.

[0045] In the embodiment of the present application, the meaning of being a grating pair is that the length, period, and material of the first tilted fiber grating and the second tilted fiber grating can be the same, so as to facilitate wireless communication and signal processing. It is known to those skilled in the art that communication can be achieved when the above lengths, periods, and materials are not exactly the same, but the data needs to be additionally processed to make them match each other, so preferably, the first tilted fiber grating and the second tilted fiber grating can be exactly the same.

[0046] In one embodiment, the tilt angle θ of the first tilted fiber grating and the second tilted fiber grating are both 45 degrees.

[0047] In an embodiment of the present application, the first end of the first tilted fiber grating can receive the broadened optical signal L0 (A0, λ(t)) from the third end of the circulator, and output the returned modulated optical signal to the third end of the circulator; the second end receives the modulated optical signal from the first end of the second tilted fiber grating, and outputs the broadened optical signal L1 (A1, λ(x, t)) to the first end of the second tilted fiber grating, where A1 is the amplitude value of the laser pulse signal, and λ is the wavelength of the optical signal.

[0048] After the fiber grating is tilted, since the tilted fiber grating is a spatial light diffraction device, a one-to-one correspondence between wavelength λ and spatial position x is achieved. The spatial light diffraction angle α of the tilted fiber grating is:

[0049]

[0050] Where n is the refractive index of the tilted fiber Bragg grating core, θ is the tilt angle of the tilted fiber Bragg grating, Λ is the period of the tilted fiber Bragg grating, and λ is the wavelength of the optical signal transmitted in the tilted fiber Bragg grating. When θ = 45°, the angular dispersion D (which measures the divergence of the tilt angle of the tilted fiber Bragg grating with wavelength) is:

[0051]

[0052] At this time, the angular dispersion value of the tilted fiber Bragg grating is the largest.

[0053] In one embodiment, the first end of the second tilted fiber Bragg grating receives the light signal L1(A1, λ(x, t)) after being widened from the second end of the first tilted fiber Bragg grating, and outputs the returned modulated light signal to the second end of the first tilted fiber Bragg grating; the second end receives the modulated light signal from the optical fiber sensor, and outputs the widened light signal to the optical fiber sensor. The first end of the second tilted fiber Bragg grating receives the light signal L1(A1, λ(x, t)) after being widened from the second end of the first tilted fiber Bragg grating, and outputs the returned modulated light signal to the second end of the first tilted fiber Bragg grating; the second end receives the modulated light signal from the optical fiber sensor, and outputs the widened light signal L1(A1, λ(x, t)) to the optical fiber sensor. The second tilted fiber Bragg grating and the first tilted fiber Bragg grating can be a grating pair.

[0054] In one embodiment, the pulse laser generating module is a femtosecond laser, which is used to generate femtosecond pulse laser.

[0055] In the embodiment of the present application, the pulse laser generation module may use a femtosecond laser to generate high-speed femtosecond laser pulses.

[0056] like Figure 2 As shown, in one embodiment, the signal detection module is used to demodulate the modulated optical signal based on a synchronization signal to obtain the external signal, and the synchronization signal is a synchronization signal of the pulse laser generation signal.

[0057] As an embodiment of the present application, Figure 3 The figure shows an example of a local area detection method provided by the present application. Figure 3 (a) represents the time domain signal returned by the fiber Bragg grating sensor when there is no external signal; Figure 3 (b) represents the frequency domain signal returned by the fiber Bragg grating sensor when there is no external signal; Figure 3(c) shows the time domain signal returned by the fiber Bragg grating sensor when there is an external signal after 200 μs.

[0058] The repetition frequency of the femtosecond laser used in this embodiment is 50MHz, that is, the time of each cycle is 20ns. The dispersion amount used in the dispersion module is 1ns / nm. The bandwidth of the high-speed photodetector used is 20GHz, and a high-speed data acquisition card with a rate of 20GS / s is used for data acquisition. In each cycle (20ns), a total of 20ns*20GS / s=400 data are collected. The strain coefficient of the fiber grating sensor used is 1.2pm / με. When there is no external signal, the time domain signal and frequency domain signal returned by the fiber grating sensor are as follows: Figure 3 (a) and Figure 3 As shown in (b), the wavelength returned by the fiber Bragg grating sensor is 1550.16nm. After 10000 cycles (measured by synchronization signal), that is, 10000*20ns=200μs, the time domain signal returned by the fiber Bragg grating sensor when there is an external signal is as follows Figure 3 (c). Through data analysis, we know that Figure 3 (c) Peak data ratio Figure 3 (b) The peak data lags behind 9 data in the time domain. According to the fact that 400 data are collected in each cycle, the lag time of 9 data is: 9 / 400*20ns=0.45ns. According to the dispersion parameter 1ns / nm, it can be analyzed that the wavelength change returned by the fiber Bragg grating sensor is 0.45nm=450pm, that is, the wavelength returned by the fiber Bragg grating sensor at this time is: 1550.16nm+0.45nm=1550.61nm. And because the strain coefficient of the fiber Bragg grating sensor is 1.2pm / με, it can be known that the size of the external strain signal at this time is 450pm / (1.2pm / με)=375με.

[0059] In one embodiment, the synchronization signal is generated by the pulse laser generating module.

[0060] In the embodiment of the present application, the synchronization signal can be generated by an independent signal source or by a pulse laser generation module. Preferably, it is generated by a pulse laser generation module to improve accuracy and reduce cost.

[0061] In one embodiment, the pulse laser generated by the pulse laser generating module includes a laser light signal in a near-infrared band.

[0062] In the embodiment of the present application, the advantage of the laser light signal in the near-infrared band is that communication can be used during the day or night, is not affected by the use period, and the band is relatively cheap.

[0063] In one embodiment, the dispersion module is a chirped fiber grating.

[0064] like Figure 4 As shown, a simplified route map of laser signals of different wavelengths entering a chirped fiber grating is given. In the embodiment of the present application, the dispersion module can use a chirped fiber grating to perform pulse broadening on femtosecond pulses while achieving a one-to-one linear relationship between the intensity value of the pulse time domain signal and the frequency domain wavelength.

[0065] In this embodiment, the dispersion module can be composed of a chirped fiber grating. According to the principle of chirped fiber grating, after laser pulse signals of different wavelengths (or frequencies) pass through the chirped fiber grating, their reflection time is different, that is, the paths of laser pulse signals of different wavelengths are different. Therefore, after the femtosecond laser pulse signal passes through the chirped fiber grating, its pulse time domain signal and the frequency domain wavelength realize a one-to-one linear relationship of intensity value. For example, the laser pulse signal at this time is: L0(A0, λ(t)), A0 is the amplitude value of the laser pulse signal, λ is the wavelength of the optical signal, and after passing through the chirped fiber grating, it is a one-to-one mapping function corresponding to time t.

[0066] Compared with the prior art, the present application also has the following advantages: by adopting a scheme based on chirped fiber grating to adjust dispersion, a linear relationship between the timing information in the pulse time domain and the intensity value of the wavelength in the frequency domain can be achieved, and the dispersion amount of the chirped fiber grating can be adjusted within a certain range. The tilted fiber grating based on the 45° tilted fiber grating adopted is an integrated fiber optic device as a spatial light transmitter, a spatial light diffraction device and a light receiving device, which can significantly reduce the system volume and improve energy efficiency. The scheme of ultrafast wavelength detection performed by the device adopted by the present application can greatly reduce the external signal detection time and improve the time resolution of signal detection.

[0067] It is understandable that dispersion compensating fiber or ordinary single-mode fiber can also achieve a one-to-one correspondence between the pulse time domain signal and the frequency domain wavelength intensity value, and the reason why chirped fiber grating is preferred is that the chirped fiber grating can fine-tune the correspondence between the intensity and the pulse time domain signal.

[0068] Those skilled in the art will understand that Figure 1 and Figure 2 The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0069] It should be understood that, although each step in the flow chart of each embodiment of the present application is shown in sequence according to the indication of the arrow, these steps are not necessarily performed in sequence according to the order indicated by the arrow. Unless there is clear explanation in this article, the execution of these steps does not have strict order restriction, and these steps can be performed in other orders. Moreover, at least a portion of the steps in each embodiment may include a plurality of sub-steps or a plurality of stages, and these sub-steps or stages are not necessarily performed at the same time, but can be performed at different times, and the execution order of these sub-steps or stages is not necessarily performed in sequence, but can be performed in turn or alternately with at least a portion of other steps or sub-steps or stages of other steps.

[0070] A person of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be implemented by instructing related hardware or several hardware devices themselves through a computer program. The program can be stored in a non-volatile computer-readable storage medium. When the program is executed, it can include the processes of the embodiments of the above-mentioned methods.

[0071] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0072] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A non-contact ultrafast sensing system, characterized in that: The system comprises: A pulse laser generating module, a dispersion module, a circulator, a first tilted fiber Bragg grating, a second tilted fiber Bragg grating, a signal detecting module, and a fiber sensor; The pulse laser generating module is used to generate pulse laser; The dispersion module is used to perform pulse broadening on the pulse laser, and set the time sequence signal of the broadened pulse laser and the frequency domain wavelength intensity to a linear corresponding relationship to generate a linear corresponding signal; The circulator has three ports, the first port of the circulator is used to receive the linear correspondence signal, the second port of the circulator is used to output the linear correspondence signal to the first tilted fiber Bragg grating, the second port of the circulator is also used to receive a return signal transmitted by the first tilted fiber Bragg grating, the circulator is also used to modulate the linear correspondence signal based on the return signal and generate a modulated optical signal, and the third port of the circulator is used to output the modulated optical signal to the signal detection module; The first tilted fiber Bragg grating has two ports, the first tilted fiber Bragg grating first port is used to receive the linear corresponding signal transmitted from the second port of the circulator, and transmit the linear corresponding signal to the space through the first tilted fiber Bragg grating second port, and the first tilted fiber Bragg grating second port is also used to obtain the return signal emitted into the space by the second tilted fiber Bragg grating second port, and transmit the return signal back to the second port of the circulator through the first tilted fiber Bragg grating first port; The second tilted fiber Bragg grating has two ports, the second tilted fiber Bragg grating second port is used to obtain the linear corresponding signal emitted into space by the first tilted fiber Bragg grating second port, and output it to the optical fiber sensor via the second tilted fiber Bragg grating first port; The optical fiber sensor is used to modulate the linear corresponding signal based on an external signal to generate a return signal, and output the return signal to the first port of the second tilted optical fiber grating; The first tilted fiber grating and the second tilted fiber grating are a grating pair; The signal detection module is used to demodulate the modulated optical signal based on a synchronization signal to obtain the external signal, and the synchronization signal is a synchronization signal of the pulse laser generation signal.

2. A non-contact ultrafast sensing system according to claim 1, characterized in that: The tilt angles θ of the first tilted fiber grating and the second tilted fiber grating are both 45°.

3. A non-contact ultrafast sensing system according to claim 1, characterized in that: The pulse laser generating module is a femtosecond laser, which is used to generate femtosecond pulse laser.

4. A non-contact ultrafast sensing system according to claim 3, characterized in that: The synchronization signal is generated by the pulse laser generating module.

5. The non-contact ultrafast sensing system according to claim 1, characterized in that: The pulse laser generated by the pulse laser generating module includes a laser light signal in the near-infrared band.

6. A non-contact ultrafast sensing system according to claim 1, characterized in that: The dispersion module is a chirped fiber grating.

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