CN103438916B - Based on the optical fiber grating wavelength demodulating equipment of saturable absorption optical fiber - Google Patents

Abstract

The invention discloses a fiber grating wavelength demodulation device based on a saturable absorbing fiber, including a pump source, a gain fiber, a saturable absorbing fiber, a circulator, a ring cavity or a linear cavity fiber laser composed of a fiber grating to be measured, and A photoelectric detection system for measuring the laser repetition frequency; the wavelength change of the fiber grating to be measured is obtained by calculating the repetition frequency of the measuring laser, and the wavelength demodulation is realized; the invention avoids the use of spectral analysis equipment and reduces the cost of wavelength demodulation , and at the same time realize the full fiber optic demodulation system, which makes the demodulation system small in size, easy to maintain, and simple and convenient to use.

Description

Fiber Bragg Grating Wavelength Demodulation Device Based on Saturable Absorbing Fiber

technical field

The invention relates to a fiber grating wavelength demodulation device, in particular to a novel fiber grating demodulation method and device based on a saturable absorbing optical fiber.

Background technique

Fiber Bragg grating sensing can realize the measurement of various physical quantities, and has the advantages of compact structure, easy maintenance, and immunity to electromagnetic interference, and has been widely used in engineering.

The wavelength demodulation of FBG is one of the key technologies in FBG sensing applications. The commonly used demodulation methods include wavelength scanning method, F-P cavity filter method and dispersion demodulation method based on volume grating.

The wavelength scanning method uses a wavelength-tunable laser as the light source. During the wavelength scanning process, when the wavelength is consistent with the central wavelength of the fiber Bragg grating to be demodulated, the reflected light of the fiber Bragg grating is the strongest, thus obtaining the wavelength information of the fiber Bragg grating. This method has a high signal-to-noise ratio, but the cost of the wavelength-tunable laser is relatively high.

The F-P cavity filtering method uses a broadband light source. The fiber grating reflects light of a specific wavelength, and the length of the F-P cavity changes under the drive of piezoelectric ceramics, so that the transmission wavelength also changes. When the transmission wavelength of the F-P cavity is consistent with the reflection wavelength of the fiber Bragg grating, the transmission light intensity of the F-P cavity is the largest, thereby obtaining the wavelength information of the fiber Bragg grating. This method has high demodulation speed and fast response, but the signal-to-noise ratio is poor and the cost is high.

In the patented portable fiber grating wavelength demodulator (application number 201220387720.4) and the fiber grating demodulation device based on area array CCD (application number 201210085512.3), the author proposed to use the volume grating to spread the broadband light source in space, and then The light at different positions after unfolding is detected by the detector, so as to obtain the intensity of light signals of different wavelengths. It acts like a small spectrometer. However, since this type of demodulation device needs to use a large number of crystal optical devices, its structure is complicated, its cost is high, and it cannot be used in a vibration environment.

Contents of the invention

The technical problem to be solved by the present invention is to provide a wavelength demodulation method and device for a single fiber grating, which can realize real-time demodulation of the fiber grating wavelength without using spectrum analysis equipment.

The basic idea of the invention is to use the pulse repetition frequency information of the laser to realize the wavelength demodulation of the fiber grating. Since thulium-doped fiber or thulium-holmium co-doped fiber has strong absorption near 1550nm, it can be used as saturable absorption fiber to realize Q-switching of erbium-doped fiber laser. In addition, in the range from 1500nm to 1585nm, the absorption coefficients of thulium-doped fiber and thulium-holmium co-doped fiber all have a monotonous increasing trend. In this way, when the laser oscillates at different wavelengths, the losses introduced by the saturable absorbing fiber in the cavity are different, so that the output pulse repetition frequency of the laser also changes accordingly. Using the corresponding relationship between the pulse repetition frequency and the laser wavelength, the wavelength demodulation of the fiber grating can be realized.

Technical solution of the present invention is:

Fiber Bragg grating wavelength demodulation device based on saturable absorbing fiber, including pump source, wavelength division multiplexer, gain fiber, isolator, saturable absorbing fiber, output coupler, circulator, fiber grating to be measured and photodetection system ; The wavelength division multiplexer, gain fiber, isolator, saturable absorption fiber, output coupler and circulator are connected by a fiber fusion splicer to form a fiber loop; the pumping source is passed through the wavelength division multiplexer Connected on the fiber loop; the fiber grating to be measured is connected to the fiber loop through a circulator; the input and output ends of the circulator are respectively connected to the fiber loop; the middle end of the circulator is connected to the fiber loop to be measured fiber grating connection; the photoelectric detection system is connected to the fiber loop through the output coupler, and is used for the measurement of the light pulse repetition frequency.

In the fiber grating wavelength demodulation device based on saturable absorbing fiber, the pump source is a 980nm semiconductor laser, the wavelength division multiplexer is a 980nm/1550nm wavelength division multiplexer, and the gain fiber is a single Erbium-doped optical fiber, the operating wavelength of the isolator is 1550nm, and the saturable absorption optical fiber is a thulium-doped optical fiber or a thulium-holmium co-doped optical fiber.

In the aforementioned fiber grating wavelength demodulation device based on a saturable absorbing fiber, the central wavelength of the fiber grating to be measured is 1550 nm, the 3 dB bandwidth is less than 0.3 nm, and the reflectivity is 30% to 100%.

In the fiber grating wavelength demodulation device based on saturable absorbing fiber, the photodetection system includes a photodiode, an amplification processing circuit, a counting unit or an oscilloscope, and the photodiode converts the pulsed optical signal into an electrical signal, which is amplified by the amplification processing circuit After that, use a counting unit or an oscilloscope to record the repetition frequency of the output electrical signal.

The above-mentioned fiber grating wavelength demodulation device based on saturable absorbing fiber includes a pump source, a wavelength division multiplexer, a gain fiber, a saturable absorbing fiber, a fiber grating to be measured, and a high-reflection coating fiber; the high-reflection coating The optical fiber, the wavelength division multiplexer, the gain optical fiber, the saturable absorption optical fiber and the fiber grating to be measured are fused by a fiber fusion splicer to form a laser resonator; the high-reflective coated optical fiber is a single-mode optical fiber with a high-reflective coating on the reflective end; the said The pumping source is connected in the laser resonant cavity through a wavelength division multiplexer; the photoelectric detection system is connected with the output end of the fiber grating to be measured for the measurement of the optical pulse repetition frequency.

In the fiber grating wavelength demodulation device based on saturable absorbing fiber, the pump source is a 980nm semiconductor laser, the wavelength division multiplexer is a 980nm/1550nm wavelength division multiplexer, and the gain fiber is a single Erbium-doped optical fiber, the saturable absorption optical fiber is a thulium-doped optical fiber or a thulium-holmium co-doped optical fiber.

The above fiber grating wavelength demodulation device based on saturable absorbing fiber is characterized in that: the central wavelength of the fiber grating to be measured is 1550 nm, the 3dB bandwidth is less than 0.3 nm, and the reflectivity is 10% to 90%.

In the fiber grating wavelength demodulation device based on saturable absorbing fiber, the photodetection system includes a photodiode, an amplification processing circuit, a counting unit or an oscilloscope, and the photodiode converts the pulsed optical signal into an electrical signal, which is amplified by the amplification processing circuit After that, use a counting unit or an oscilloscope to record the repetition frequency of the output electrical signal.

The above fiber grating wavelength demodulation device based on saturable absorbing optical fiber is characterized in that: the reflection rate of the reflection cavity mirror is greater than 80%, the center wavelength of the reflection band is 1550nm, and the 3dB bandwidth is greater than 80nm.

The present invention has following beneficial effect:

1. The demodulation method of the present invention is based on a Q-switched fiber laser, which has a very high signal-to-noise ratio.

2. Since the photoelectric detection system has a faster time response, the present invention can realize the capture and measurement of fast signals and meet the measurement of high repetition frequency signals.

3. The present invention avoids the use of spectral analysis equipment, reduces the cost of wavelength demodulation, and is simple and convenient to use.

4. The present invention avoids the use of various non-fiber optical components, and realizes the full fiber optic demodulation system, making the demodulation system small in size and easy to maintain.

Description of drawings

Fig. 1 is a schematic diagram of the structural principle of a novel fiber grating wavelength demodulation device based on a saturable absorbing fiber according to the present invention, in which the pulsed fiber laser has a ring cavity structure.

Fig. 2 is a schematic structural schematic diagram of another simplified linear cavity structure demodulation device according to the present invention.

Fig. 3 is a typical corresponding relationship between wavelength and repetition frequency obtained based on the verification system built in Fig. 1.

In the figure 1—pump source; 2—wavelength division multiplexer; 3—gain fiber; 4—isolator; 5—saturable absorption fiber; 6—output coupler; 7—circulator; 7-a—circulator Input end; 7-b—middle end of circulator; 7-c—output end of circulator; 8-a, 8-b—fiber grating to be tested; 9—photoelectric detection system; 10—reflective cavity mirror; 11—Q switching Erbium-doped fiber lasers.

detailed description

Specific implementations of the present invention will be described below according to the annular cavity structure and the linear cavity structure respectively.

Fig. 1 is a schematic structural diagram of the device of the present invention under an annular cavity structure. As shown in Figure 1, the novel fiber grating wavelength demodulation device based on the saturable absorbing fiber 5 of the present invention consists of a pump source 1, a wavelength division multiplexer 2, a gain fiber 3, an isolator 4, a saturable absorbing fiber 5, an output It consists of a coupler 6, a circulator 7, a fiber grating to be tested 8-a and a photoelectric detection system 9, etc.

First, the wavelength division multiplexer 2, the gain fiber 3, the isolator 4, the saturable absorption fiber 5, the output coupler 6 and the circulator 7 are connected by a fiber fusion splicer to form a fiber loop; the pump source 1 Connected on the optical fiber loop through the wavelength division multiplexer 2, the fiber grating 8-a to be measured is connected on the optical fiber loop through the circulator 7; the optical fiber circulator 7 is a common three-terminal optical fiber passive device, as shown in Fig. 1 Arrow tail to head are respectively input end 7-a, intermediate end 7-b and output end 7-c; Wherein the input end 7-a and output end 7-c of circulator 7 are respectively connected in the optical fiber ring, ring The middle end 7-b of the device 7 is connected to the fiber grating to be measured 8-a; the photodetection system 9 is connected to the fiber loop through the output coupler 6 for the measurement of the optical pulse repetition frequency.

In this way, the pump source 1, the wavelength division multiplexer 2, the gain fiber 3, the isolator 4, the saturable absorbing fiber 5, the output coupler 6, the circulator 7 and the fiber grating to be tested 8-a constitute a Q-switched erbium-doped fiber laser 11. The oscillation wavelength of the laser is determined by the fiber Bragg grating to be tested 8-a; experiments show that the laser oscillation output can be realized if the reflectivity of the fiber Bragg grating is greater than 30%.

In the Q-switched erbium-doped fiber laser 11, the gain fiber 3 is used for generating laser signals; the saturable absorbing fiber 5 is used for pulse modulation of the laser signals. Under the pumping of the pumping source 1, the gain fiber 3 generates a laser signal in the laser cavity. The ground-state dopant ions in the saturable absorption fiber 5 absorb the laser signal generated by the gain fiber 3, so that the resonator is in a state of low Q value. With the continuous absorption of signal light, the saturable absorbing fiber 5 gradually reaches saturation, and the absorption ability of signal light also decreases, so that the resonant cavity is in a state of high Q value, thus generating a laser pulse output. At the same time, the dopant ions in the excited state in the saturable absorption fiber 5 decay to the ground state through processes such as non-radiative transition, and then start the process of absorbing the signal light again. In this way, the pulse modulation of the signal light is realized.

In the above device, the pump source 1 is a 980nm semiconductor laser, the wavelength division multiplexer 2 is a 980nm/1550nm wavelength division multiplexer, the gain fiber 3 is a single-mode erbium-doped fiber, and the working wavelength of the isolator 4 is 1550nm, which can absorb The optical fiber 5 is a thulium-doped optical fiber or a thulium-holmium co-doped optical fiber; the output coupler 6 has an operating wavelength of 1550nm, and the output coupling ratio is between 10% and 90%; the optical fiber circulator 7 has an operating wavelength of 1550nm. The heads are the input end (7-a), the intermediate end (7-b) and the output end (7-c); the center wavelength of the fiber grating 8-a to be measured is 1550nm, and the 3dB bandwidth is less than 0.3nm; the photoelectric detection system 9 includes a photodiode, an amplification processing circuit, a counting unit or an oscilloscope. The photodiode converts the pulsed light signal into an electrical signal. The corresponding relationship between the obtained pulse repetition frequency and the laser wavelength is calculated to obtain the wavelength information of the laser.

Fig. 2 shows the structure diagram of the device of the present invention under the linear cavity structure, including pumping source 1, wavelength division multiplexer 2, gain fiber 3, saturable absorption fiber 5, fiber grating to be measured 8-b and reflective cavity mirror 10; the wavelength division multiplexer 2, the gain fiber 3, the saturable absorption fiber 5 and the fiber grating 8-b to be measured are fused by a fiber fusion splicer to form a laser line resonator; the reflective cavity mirror can be an ordinary dielectric substrate Or the reflection mirror plated on the metal substrate can also be directly plated on the end face of the single-mode optical fiber; the described pumping source is connected in the laser resonant cavity through the wavelength division multiplexer 2; the described photoelectric detection system 9 It is connected with the output end of the fiber grating 8-b to be measured, and is used for the measurement of the optical pulse repetition frequency.

Most of the above-mentioned devices are the same as those in Fig. 1, and will not be repeated here, except that the laser resonator in Fig. 2 is composed of a reflective cavity mirror 10 and a fiber grating 8-b to be tested, and the resonant cavity structure is a linear cavity. The reflectivity of the reflective cavity mirror is greater than 80%, the central wavelength of the reflective band is 1550nm, and the 3dB bandwidth is greater than 80nm.

The novel fiber grating wavelength demodulation method based on the saturable absorbing fiber of the present invention needs to calibrate the corresponding relationship between the laser oscillation wavelength and the pulse repetition frequency under a certain pump power before use. During the calibration, the pump power of the pump source 1 is fixed, and fiber gratings of different known wavelengths are connected to the port of the circulator 7 to form Q-switched erbium-doped fiber lasers 11 with different oscillation wavelengths. Then, the photoelectric detection system 9 is used to measure the repetition frequency of the laser pulse at different laser wavelengths to obtain the corresponding relationship between the laser oscillation wavelength and the pulse repetition frequency. When performing wavelength demodulation of the fiber gratings 8-a and 8-b to be tested, the photoelectric detection system 9 is used to measure the repetition frequency of the laser pulses to obtain the center wavelength of the fiber gratings 8-a and 8-b to be tested.

Figure 3 shows the experimental results of the typical correspondence between the laser oscillation wavelength and the pulse repetition frequency at a certain pump power obtained by calibrating the wavelength demodulation system built based on Figure 1 .

The present invention is not limited to the above-mentioned specific embodiment, for example in the linear cavity structure of the present invention, the 1550nm laser pulse can also be drawn by adding an output coupler 6 in the resonant cavity, and the size of the pulse signal intensity can also be obtained by changing the output coupler The output coupling ratio of 6 is adjusted. For the technical field to which the present invention belongs, several simple substitutions and changes can also be made under the premise of the concept of the present invention. These all belong to the protection scope of the present invention.

Claims (10)

1. based on the optical fiber grating wavelength demodulating equipment of saturable absorption optical fiber, it is characterized in that: comprise pumping source (1), wavelength division multiplexer (2), gain fibre (3), isolator (4), saturable absorption optical fiber (5), output coupler (6), circulator (7), fiber grating to be measured (8 ?a) and Photodetection system (9); Described gain fibre (3), isolator (4), saturable absorption optical fiber (5) and circulator (7) are connected by single-mode fiber, and form optical fiber loop; Described pumping source is connected on optical fiber loop by wavelength division multiplexer (2); Described fiber grating to be measured (8 ?a) be connected on optical fiber loop by circulator (7); The input end of described circulator (7) (7 ?a) and output terminal (7 ?c) be connected in optical fiber loop respectively; The intermediate ends of described circulator (7 ?b) with fiber grating to be measured (8 ?a) connect; Described saturable absorption optical fiber (5) monotone increasing in fiber grating demodulation wavelength coverage, make the laser instrument loss that saturable absorption optical fiber (5) is introduced in resonator cavity when different wave length vibrates different, laser instrument is caused to export the change of pulse repetition rate, described Photodetection system (9) is connected on optical fiber loop by output coupler (6), for the measurement of light pulse repetition rate, realize the Wavelength demodulation of fiber grating.

2. the optical fiber grating wavelength demodulating equipment based on saturable absorption optical fiber according to claim 1, it is characterized in that: described pumping source (1) is 980nm semiconductor laser, described wavelength division multiplexer (2) is 980nm/1550nm wavelength division multiplexer, described gain fibre (3) is single mode Er-doped fiber, the operation wavelength of described isolator (4) is 1550nm, and described saturable absorption optical fiber (5) is thulium doped fiber or Tm Ho co doped fiber; Described output coupler (6) operation wavelength is 1550nm, exports coupling ratio between 10% to 90%; The operation wavelength of described optical fiber circulator (7) is 1550nm.

3. the optical fiber grating wavelength demodulating equipment based on saturable absorption optical fiber according to claim 1, it is characterized in that: described fiber grating to be measured (8 ?centre wavelength a) be 1550nm, three dB bandwidth is less than 0.3nm, and reflectivity is 30% ~ 100%.

4. the optical fiber grating wavelength demodulating equipment based on saturable absorption optical fiber according to claim 1, is characterized in that: described Photodetection system (9) comprises photodiode, amplification treatment circuit and counting unit; Pulsed optical signals is converted to electric signal by photodiode, amplifies after process through amplification treatment circuit, adopts counting unit record to export the repetition frequency of electric signal.

5. based on the optical fiber grating wavelength demodulating equipment of saturable absorption optical fiber, it is characterized in that: comprise pumping source (1), wavelength division multiplexer (2), gain fibre (3), saturable absorption optical fiber (5), fiber grating to be measured (8 ?b), Photodetection system (9) and reflecting cavity mirror (10); Described gain fibre (3) and saturable absorption optical fiber (5) are connected between reflecting cavity mirror (10) and fiber grating to be measured (8 ?b) by single-mode fiber, and form the linear resonator cavity of laser; Described pumping source (1) is connected in laserresonator by wavelength division multiplexer (2); Described saturable absorption optical fiber (5) monotone increasing in fiber grating demodulation wavelength coverage, make laser instrument when different wave length vibrates, the loss that saturable absorption optical fiber (5) is introduced in resonator cavity is different, laser instrument is caused to export the change of pulse repetition rate, described Photodetection system (9) and fiber grating to be measured (8 ?output terminal b) be connected, for the measurement of light pulse repetition rate, realize the Wavelength demodulation of fiber grating.

6. the optical fiber grating wavelength demodulating equipment based on saturable absorption optical fiber according to claim 5, is characterized in that: described reflecting cavity mirror (10) direct plating is on the end face of single-mode fiber.

7. the optical fiber grating wavelength demodulating equipment based on saturable absorption optical fiber according to claim 5, it is characterized in that: described pumping source (1) is 980nm semiconductor laser, described wavelength division multiplexer (2) is 980nm/1550nm wavelength division multiplexer, described gain fibre (3) is single mode Er-doped fiber, and described saturable absorption optical fiber (5) is thulium doped fiber or Tm Ho co doped fiber.

8. the optical fiber grating wavelength demodulating equipment based on saturable absorption optical fiber according to claim 5, it is characterized in that: described fiber grating to be measured (8 ?centre wavelength b) be 1550nm, three dB bandwidth is less than 0.3nm, and reflectivity is 10% ~ 90%.

9. the optical fiber grating wavelength demodulating equipment based on saturable absorption optical fiber according to claim 5, it is characterized in that: described Photodetection system (9) comprises photodiode, amplification treatment circuit and counting unit, pulsed optical signals is converted to electric signal by photodiode, amplify after process through amplification treatment circuit, adopt counting unit record to export the repetition frequency of electric signal.

10. the optical fiber grating wavelength demodulating equipment based on saturable absorption optical fiber according to claim 5, it is characterized in that: the reflectivity of described reflecting cavity mirror is greater than 80%, zone of reflections centre wavelength is 1550nm, and three dB bandwidth is greater than 80nm.