CN102359943B - Photonic crystal fibre-optical air chamber active cavity absorption-type gas detection device - Google Patents

Abstract

The invention relates to the fields of optical fiber sensing and optical fiber gas detection. In order to solve the problem that the fiber optic gas detection technology has a small absorption coefficient of ammonia gas in the detection of gas pollutants, it is difficult to detect and the detection sensitivity is low; to solve the problem of single-pass gas chamber Due to the limitation of the length of the gas pool, the absorption distance is short, the detection sensitivity is low, and the long-distance gas chamber has the disadvantages of complex optical structure and poor stability in the gas chamber. The technical solution adopted in the present invention is: photonic crystal fiber gas chamber active cavity absorption type The gas detection device includes: 980nm semiconductor laser 1, 980/1550nm wavelength division multiplexer 2, erbium-doped fiber 3, hollow photonic crystal fiber 4, fiber optic mirror 5, tunable fiber grating 6, fiber optic power meter 7. The invention is mainly applied to optical fiber gas detection.

Description

Photonic crystal fiber gas cell active cavity absorption type gas detection device

technical field

The invention relates to the fields of optical fiber sensing and optical fiber gas detection, in particular to a photonic crystal optical fiber gas chamber active inner cavity absorption type gas detection device.

Background technique

In the process of industrial production, timely and accurate monitoring, forecasting and automatic control of flammable, explosive, toxic and harmful gases have become one of the important problems to be solved in the current coal, petroleum, chemical, electric power and other industries. Based on the gas spectral absorption method, the detection of gas concentration using the low loss window (1-2 μm) of silica optical fiber has gained widespread attention. In this band, it is found that many polluting gases or toxic gases, such as acetylene (C ₂ H ₂ ), methane (CH ₄ ), carbon monoxide (CO), ammonia (NH ₃ ) and other gases, have absorption lines. Among them, ammonia gas is one of the common air pollutants in industrial production waste gas. Since the absorption cross section of ammonia gas at the near-infrared absorption peak is relatively small, it is difficult to detect ammonia gas with infrared direct absorption optical fiber, and the sensitivity is low. Usually, photoacoustic ammonia fiber optic detection technology and optical-microwave double resonance ammonia fiber optic detection technology are used instead. Since VM Baev and others successfully used diodes as light sources to realize active cavity gas detection in 1992, active cavity absorption gas detection technology has attracted people's attention and developed rapidly. In recent years, thanks to fiber laser technology and fiber optic sensing With the development of technology, the optical fiber active cavity gas detection technology has also developed rapidly, and has become one of the research hotspots of gas detection technology.

Active intracavity gas detection technology refers to the detection technology that places the sample in the laser resonator, that is to say, in the laser resonator there is a sample absorption medium in addition to the laser gain medium, and the intracavity beam is both the laser oscillation line and the sample. Absorption line. The active ammonia detection of optical fiber usually uses erbium-doped fiber as the gain medium. peak absorption (Figure 2); second, the power output of the erbium-doped fiber laser is stable.

The active intracavity gas detection based on erbium-doped fiber laser is to insert a sample chamber in the fiber laser cavity. Figure 1 is a typical active intracavity gas detection system based on erbium-doped fiber laser. The erbium-doped fiber laser is pumped by a 980nm or 1480nm semiconductor laser Pu. Intracavity detection is achieved by implanting a gas cell between the erbium fiber and the fiber optic mirror. Traditional air chambers are generally divided into one-way air chambers and long-distance air chambers. The single-pass gas cell is usually composed of a pair of fiber optic collimators. Since the working distance of the fiber optic collimator is generally not more than 50cm, the effective absorption distance of the gas is very short, which greatly limits the system test sensitivity. On the other hand, the one-way air chamber has a large volume due to the increased absorption distance, and is not flexible enough to use, which is not conducive to the use of on-site activities in the industry. The long-range gas chamber mainly relies on the special optical path design to make the light reflect back and forth in the gas chamber multiple times, thereby increasing the effective optical path of the gas. Compared with the single-pass air chamber, the long-range air chamber has a smaller volume and a longer effective absorption distance, but its structure is more complicated, difficult to debug, and poor in stability. In addition, due to multiple reflections of light in the gas chamber, a large transmission loss will be caused. Hollow-core photonic crystal fiber is a new type of transmission fiber, and its light guiding mechanism is different from ordinary refractive index-guided and total internal reflection fibers. The core of the hollow-core photonic crystal fiber is an air hole structure, and the cladding is a two-dimensional photonic crystal structure composed of periodically arranged air. The structure can support the mode of a certain wavelength in the photon band gap of the cladding to propagate in the air hole core region, and can realize that more than 95% of the light is trapped in the air core. Fig. 3 is a cross-sectional view of two kinds of hollow-core photonic crystal fibers in the 1.5 micron band commercially produced by NKT Company of Denmark. Among them, 7 thin-walled capillaries are removed from the central area, and the diameter of the air hole in the core is 10 microns, and the transmission loss can be lower than 0.03db/m.b at a central wavelength of 1.55 microns. , the diameter of the central air hole is 10 microns, and the loss at 1.57 microns can be reduced to 0.02dB/m. The all-fiber optical path using the hollow-core photonic crystal fiber as the gas chamber can effectively solve the traditional one-way gas chamber. Due to the limitation of the length of the gas pool, the absorption distance is short, the detection sensitivity is low, and the optical structure in the long-distance gas chamber is complex and the stability is poor. and other shortcomings.

Contents of the invention

In order to overcome the deficiencies of the existing technology, in the trace detection of gas pollutants, the fiber optic gas detection technology has a small ammonia absorption coefficient in the ammonia gas detection, which makes the detection difficult and the detection sensitivity is low; solves the difficulty of the traditional absorption type fiber optic ammonia gas In the detection technology, the single-pass gas chamber has short absorption distance and low detection sensitivity due to the limitation of the length of the gas cell, and the long-pass gas chamber has disadvantages such as complex optical structure and poor stability in the gas chamber. In order to achieve the above object, the technical solution adopted in the present invention is: photonic crystal fiber gas chamber active cavity absorption type gas detection device, comprising: 980 nanometer semiconductor laser 1, 980/1550 nanometer wavelength division multiplexer 2, erbium-doped optical fiber 3 , hollow-core photonic crystal fiber 4, fiber mirror 5, tunable fiber grating 6, fiber power meter 7, 980nm semiconductor laser 1 uses fiber coupling to output to 980/1550nm wavelength division multiplexer 2, 980/1550nm wavelength division The other end of the multiplexer 2 is welded to the erbium-doped optical fiber 3, and the other end of the erbium-doped optical fiber 3 and the fiber reflector are movably connected to a 1m long hollow-core photonic crystal fiber, the fiber reflector 5 and the 980/1550 nanometer wavelength division multiplexer 2 The tunable fiber grating fused at the third end constitutes a resonant cavity, the 980nm semiconductor laser 1 tunes the fiber grating output, and uses the fiber power meter 7 to measure the output power and wavelength. For connection, a 100um gap is left between the end faces of the two optical fibers, and the surrounding gas enters the central hole area of the hollow-core photonic crystal fiber 4 through the gap and the end face of the hollow-core photonic crystal fiber 4 .

The hollow-core photonic crystal fiber 4 is wound together to form several circles, so as to increase the optical path of the laser for gas absorption.

The erbium-doped fiber 3 and the 980nm semiconductor laser 1 form an erbium-doped fiber laser. During work, the erbium-doped fiber laser works in a stable state at first, and then gradually reduces the pumping power, so that the erbium-doped fiber laser operates under the condition slightly exceeding the threshold value, with Higher detection sensitivity is obtained by utilizing the nonlinear effect of the erbium-doped fiber laser near the threshold.

The present invention is characterized in that: the hollow-core photonic crystal fiber is used as the gas chamber, which can realize the effective absorption distance of several meters, which is beneficial to improve the detection degree of the fiber optic ammonia gas detection technology; the active inner cavity absorption type gas based on the erbium-doped fiber laser is adopted Detection technology can greatly improve the detection sensitivity of ammonia and other gases.

Description of drawings

Figure 1 is a typical absorption spectrum and gain curve of an erbium-doped fiber. The abscissa is the wavelength, in nanometers, and the ordinate is the absorptivity, in db/m.

Figure 2 is the absorption spectrum of ammonia gas at 1530nm-1542nm. The abscissa is the wavelength, the unit is micrometer, and the ordinate is the intersection section, the unit is cm ² .

Figure 3(a) is the end view of HC-1550-02 hollow core photonic crystal fiber.

(b) is the sensing end face of HC19-1550-01 hollow-core photonic crystal fiber.

Fig. 4 Structural diagram of the photonic crystal fiber gas cell active cavity ammonia gas detection system. Among them: 1 is 980 semiconductor laser, 980/15502 is wavelength division multiplexer, 3 is erbium-doped fiber, 4 is hollow photonic crystal fiber, 5 is fiber mirror, 6 is tunable fiber grating, 7 is fiber power meter.

Detailed ways

An active cavity absorption type ammonia gas detection technology based on an erbium-doped fiber laser using a hollow-core photonic crystal fiber as a gas chamber. It includes: 980 semiconductor laser (1), 980/1550 wavelength division multiplexer 2, erbium-doped fiber 3, hollow photonic crystal fiber 4, fiber optic mirror 5, tunable fiber grating 6, fiber optic power meter 7, photonic crystal fiber The characteristics of the gas chamber active inner cavity ammonia gas detection technology are: all-fiber optical path design, simple debugging and high sensitivity. There are several reasons for the increase in sensitivity: the first is the laser multiple-pass effect. The laser cavity is a resonant cavity, and the laser oscillates repeatedly in the cavity, as if the light passes through the sample to be tested multiple times. The absorption of monochromatic light by matter follows the Beer-Lambert law. The number of reflections of the beam in the resonant cavity mainly depends on the reflectivity of the output mirror. If the reflectivity of the fiber optic loop mirror is 100%, the transmittance of the output grating is 2%, so the power density in the cavity is 50 times of the output power of the laser. If the power density in the cavity does not saturate the corresponding absorption, the variation of the laser intensity absorbed in the cavity increases by 50 times compared with the absorption outside the cavity, that is, the minimum detectable absorption value of the absorption in the cavity is 50 times smaller than that outside the cavity, In other words, the detection sensitivity is increased by a factor of 50. The second is the nonlinear effect of the laser gain near the threshold. According to laser physics, when the pumping power is constant, in the working area slightly beyond the threshold, the small change in the cavity due to the absorption loss of ammonia will cause a drastic change in the laser power. The third is the model competition effect. For a uniform gain dielectric laser, there is a mode competition effect, that is, although the initial gain curve contains multiple longitudinal modes, the intensity growth of the mode with high gain consumes the intensity of other modes, leaving only one oscillation mode in the end. In fact, there are many laser modes of erbium-doped fiber lasers due to spatial hole burning and uneven doping of erbium fiber. The absorption of ammonia gas in the near-infrared is a narrow-band absorption. When a broadband laser is used to irradiate a narrow-band absorption of ammonia gas, there is mode competition in the absorption of different spectral lines of the laser by ammonia gas. The result of the mode competition greatly enhances the absorption intensity of the absorption center, thereby improving Ammonia detection sensitivity. Fourth, the hollow-core photonic crystal fiber is used as the gas chamber. Since the fiber can be wound, the effective absorption path length of ammonia gas is increased, thereby greatly improving the detection sensitivity of ammonia gas.

The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

Fig. 4 is a schematic structural diagram of a photonic crystal fiber gas cell active inner cavity absorption type ammonia gas detection system. The whole system is realized by placing the hollow-core photonic crystal gas chamber into the resonant cavity of the fiber erbium-doped fiber laser. Erbium-doped fiber lasers are pumped by 980 diode lasers. The pump 980 semiconductor laser adopts fiber coupling output, and the output fiber is fused with 980/1550 wavelength division multiplexer. The other end of the wavelength division multiplexer is fused with a 4m-long erbium-doped optical fiber. The other end of the erbium-doped optical fiber and the optical fiber reflector are flexibly connected to a 1m long hollow-core photonic crystal optical fiber. The resonant cavity is composed of a light reflector and a tunable fiber grating welded at the third end of the wavelength division multiplexer. The laser can tune the output of the fiber grating, and use the fiber wavelength power to measure the output power and wavelength. The connection between the hollow-core photonic crystal fiber and the erbium-doped fiber adopts an active connection. Leave a 100um gap between the end faces of the two optical fibers. The surrounding gas enters the central air through the end face of the hollow photonic crystal fiber. Hollow-core photonic crystal fibers are wound together into several circles to increase the absorption path of ammonia gas by laser. Under the condition of certain pumping power, the fiber tunable grating is used to tune the output wavelength of the erbium-doped fiber laser, and the output power corresponding to each wavelength is recorded, so as to obtain the near-infrared absorption line of ammonia gas. Finally, according to the absorbance of a certain absorption peak and the absorption cross section of ammonia gas at the absorption peak, the concentration of ammonia gas in the gas sample to be measured is reversed.

Claims (3)

1. photonic crystal fibre-optical air chamber active cavity absorption-type gas detection device, it is characterized in that, comprise: 980 Nano semiconductor laser instruments (1), 980/1550 nanometer wave division multiplexer (2), Er-doped fiber (3), hollow-core photonic crystal fiber (4), fiber reflector (5), tunable fiber grating (6), Fiber Dynamometer (7), 980 Nano semiconductor laser instruments (1) adopt coupling fiber to output to 980/1550 nanometer wave division multiplexer (2), 980/1550 nanometer wave division multiplexer (2) other end and Er-doped fiber (3) welding, movable access 1m vast sky core photonic crystal fiber between Er-doped fiber (3) other end and fiber reflector, the tunable fiber grating of fiber reflector (5) and 980/1550 nanometer wave division multiplexer (2) the 3rd end welding consists of resonator cavity, 980 Nano semiconductor laser instrument (1) tunable fiber grating outputs, and utilize Fiber Dynamometer (7) to measure tunable fiber grating output power and wavelength, be connected between hollow-core photonic crystal fiber (4) and Er-doped fiber (3) to adopt and be flexibly connected, reserve 100 μ m spaces between two fiber end faces, it is regional that ambient gas this space of process and hollow-core photonic crystal fiber end face enter the hollow-core photonic crystal fiber center pit.

2. device as claimed in claim 1, is characterized in that, hollow-core photonic crystal fiber is intertwined into some circles, to increase laser to the absorption light path of gas.

3. device as claimed in claim 1, it is characterized in that, Er-doped fiber (3) and 980 Nano semiconductor laser instruments (1) consist of erbium doped fiber laser, during work, at first erbium doped fiber laser is operated in steady state (SS), then reduce gradually pump power, make erbium doped fiber laser operate at and slightly surpass under the condition of threshold value, obtain higher detection sensitivity to utilize erbium doped fiber laser in the nonlinear effect of Near Threshold.

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