TW387055B - Distributed fiber grating sensing systems using birefringence fiber interferometers for detecting wavelength shifts - Google Patents

Abstract

A distributed fiber-grating-based sensing system, the system comprising one or more scanned birefringence fiber interferometers to detect the small wavelength shift of the light reflicted from the fiber grating, a wavelength demultiplexer to separate the lights from each fiber grating, a reference fiber grating for determining the absolute optical wavelengths, compared to the prior art, the system has advantages of easier fabrication, better stabilizatin, and can be customized for different applications.

Description

V. Description of the Invention u) 'I. Field of the Invention The present invention relates to the architecture of a distributed fiber grating sensing system, and particularly to a method for accurately detecting the wavelength of light in such a system. Related Patents 2 3 4 5 6 7

US Patent 53 6 1 1 30. Nov. 1, 1994 US Patent 58 1 8 1 85. Oct., 6, 1998 US Patent 574831 2. May 5, 1998 US Patent 54 1 0 40 4. Apr. 2 5, 1 9 9 5 US Patent 5798891. Mar. 14, 1995 US Patent 5401956. Mar. 28, 1995 US Patent 5838437. Nov. 17, 1998 Background of the Invention Fiber Bragg Gratings can be used to detect the temperature, temperature, or location of the fiber grating. The change of physical variables such as strain is because the change of the environmental physical quantity will cause the change of the reflection center wavelength of the fiber Bragg grating. By detecting the change of the reflection center wavelength, the change of the environmental physical quantity can be deduced. To detect the change in the center wavelength of the fiber grating reflection, the most commonly used method is to irradiate the fiber grating with a broadband light source or adjustable frequency laser, and then detect the center skin length of the narrow-frequency reflected light reflected by the fiber grating, Optical properties such as intensity and time delay are as described in the aforementioned Patent No. 丄 I'6. If multiple wells with different reflection wavelengths are used at the same time ^ " ^ + 疋 fiber first grid, you can also detect the reflection of different fiber gratings first to reach eight ~ 々 ~ distributed multi-point detection

Page 7 V. Purpose of Invention (2). Among them, if it is to detect the central wavelength of light transmitted by optical fiber light, the narrowest anti-unbalanced Mach-Zehnder interferometer emitted by the most commonly used method at present, if you want to use (1)) use adjustable frequency F abry- Per Ot filters, such as ^, the first piece (2 3 pieces; (3) the use of fiber grating elements, as mentioned in the patent 2 and the month | J mentioned in the patent 4 and the first two. In the first two In this method, due to the adjustable frequency, the detection value will be drifted due to the influence of the sadness, wave, or Mach-Zehnder interferometer. Therefore, a reference light source is needed to determine the absolute wavelength of the light. This reference light source can be a temperature-compensated fiber grating or a fixed Fabry-per〇t ^ wave device 'as in the aforementioned patent 7. The combination of these technologies can be obtained as shown in Figure 1 or Figure 2. Typical fiber grating sensing system. These current systems have some of the following. Disadvantages: (1) adjustable frequency Fabry-Pert filter is too expensive, sweep, speed is not fast; (2 interferometer price Too expensive, the optical path length of the two paths is not different The random drift of the difference is very large;) Integrated optical unbalanced Mach-Zehnder unbalanced Mach-Z ehnder interferometer is easy to control and accurate, and it is not easy to make, and the phase (3) is detected by Mach-Zehnder interferometer Multiple fiber gratings have the problem of ambiguous wavelength. The purpose of the present invention is to propose a new architecture of a distributed fiber grating sensing system to solve or alleviate the above problems. BRIEF SUMMARY OF THE INVENTION The object of the present invention is to propose a Easy-to-manufacture, stable, and high-sensitivity distributed fiber grating sensing system. This system includes a broadband light source, multiple fiber optic thumb sensors, and one or more concealed birefringent light

5. Description of the invention (3) Fiber interferometer, a wavelength demultiplexer, a reference fiber grating, and a signal processor required for system operation. Scanning birefringent fiber interferometer is used to detect the center wave of the reflected light from the fiber grating. Long and wavelength demultiplexers are used to distinguish the reflected signals of different fiber gratings. The reference fiber optic shed is used to determine the light. The absolute value of the wavelength. Brief description of the drawings Figure 1: Architecture of an existing fiber Bragg grating sensing system using an adjustable frequency F a b r y-P er ot filter. Figure 2: Architecture of an existing fiber Bragg grating sensing system using an unbalanced Mach-Zehnder interferometer. Figure 3: The architecture of a distributed fiber grating sensing system using a scanning birefringent fiber interferometer according to the present invention. Figure 4: Architecture of a distributed fiber grating sensing system using two scanning birefringent fiber interferometers according to the present invention. Figure 5. The transmission spectrum of the interferometer. Description of the drawing number 1 • • See frequency light source 2 ·. Fiber optic facet combiner 3.. Reference fiber grating 4 · • Fiber grating sensor 5 · • Reference light source 6 ·. Adjustable frequency Fabry-Per Filter Device

Page 9 V. Description of the invention (4) 7… light detector 8… scanning signal generating circuit 9… signal acquisition and processing circuit 10… system / user interface 11… unbalanced Mac-Zeh nder interferometer 12 … Light detector 13… scanning signal generation circuit 14… signal acquisition and processing circuit 15… system / user interface 16… fiber optics σσ 17 • _.PZT phase modulator 18… fiber coupler 19… light Detecting signal and amplifier circuit 21… scanning birefringent fiber interferometer 22… wavelength demultiplexer 23… light detector array 24… scanning signal generation circuit 25… signal extraction and processing circuit 26… system / use User interface 27… optical polarizer 28… birefringent fiber 29… birefringent down-converter 30… optical polarizer 31… scanning birefringent fiber interferometer

Page 10 V. Description of the invention ⑸ 32 .. • Scanning birefringent fiber interferometer 33 ·. · Wavelength demultiplexer 34 · · · Photodetector array 35 · · • Scanning signal generating circuit 36 · · • Signal Acquisition and Processing Circuit 37 ·· • System / User Interface 38 ·· • Optical Polarizer 39 • Birefringent Fiber 40 ·· • PZT Birefringent Modulator 41 ·· • Optical Polarizer 42 ·· • Birefringent Fiber 43 ·· • PZT birefringent modulator 44 ·· • Detailed explanation of the invention of the optical polarizer Now please refer to the system architecture proposed by the present invention in FIG. 3. Figure 3 shows a distributed fiber grating sensing system. Broadband light source 1 generates a wideband light injected into the fiber grating sensing array. This wideband light source can be a semiconductor light emitting diode or an erbium-doped fiber broadband light source. This wide-band light passes through the fiber coupler 2 and enters the reference fiber grating 3 and the fiber grating sensor 4 to generate a series of narrow-frequency reflected light of different frequencies. Part of these reflected light passes through the same fiber coupler 2 and is incident on Scanning Birefringent Fiber Interferometer 2 1. Scanning birefringent fiber interferometer 21 is composed of a light polarizer 27, a birefringent fiber 28, a birefringence down-converter 29 composed of PZT, and a light polarizer

Page 11 V. Description of Invention (6) 30. The optical axis of the two fibers must be L1, of which the B 2 birefringent modulator 2 9 will oscillate when this part changes. The scanning type double-folding multiplexer 22 is adjusted by different fiber arrays 2 3 and 2 1 respectively. The reflection light component coefficient of the reflection frequency of the fiber grating is determined by the following devices 27 and 3. The polarization direction of the output and the angle of the birefringence, the total length of the birefringent fiber 28 ~ X, the birefringent fiber is tightly wound on ρ ζτ to form two = this is when the PTZ is driven by an external electrical signal, and The birefringence characteristic will also be separated with the light output of the tuned fiber interferometer, and the reflected light incident on the wavelength-resolved fiber grating will be separated and detected by the light detection 4, photometric detection theft array 2 and 3 respectively. The reflected light of $ is scanned by the birefringent optical fiber operator to detect the phase of these output signals respectively, and the wavelengths of different optical f-centers can be determined accordingly. Narrow enough for either. The scanning formula of the scanning type birefringent fiber interferometer 2 丨 is determined by + +)) T 2 cos 2πΑηβ ^

C The speed at which it strikes is 疋, the frequency of light waves, Δ η is the unadjusted mark, which is the amortization :: The refractive index difference between the two polarization directions, and Li is a double fold. Body: Light "

(Upper dagger), the birefringence modulator and the phase difference between different offsets generated by the modulation signal. If the scan signal applied to Pζτ is a sawtooth wave, φ (t) will be a linear function of t during the scan °

V. Description of the invention (7) Φ (ι) = κ ^ 2 The proportionality coefficient κ, which compares the pong to the fiber length L M: and the size of the modulation signal. In this case, in principle, the magnitude of the light wave frequency f can be inferred only by detecting the phase of the 2K, frequency component. From the formula (1), it can be known that the interference frequency interval of the birefringent interferometer when no modulation signal is added is: Δ / 'iSnL · So the interference frequency interval of the interferometer can be set by adjusting the size of L t. Generally, the A η value of a high birefringence fiber can reach about 0.05, so if the fiber length is 6 meters, the frequency interval at this time is 1 0 0 G Ιί ζ (about equal to 0.8 nm (Wavelength interval). For the unbalanced Mach-Zehnder interferometer 11 in Figure 2, the formula for calculating the interference frequency interval is: △ /. 4 β

Page 13 V. Description of the invention (8) where η is the equivalent refractive index of the optical fiber (η = 1 · 5). It can be known from formula (4) that if the unbalanced Mach-Zehnder interferometer 11 is used to obtain the same 200 GHz frequency interval, the difference in fiber length between the two paths | L2 — L | | This makes the scanning birefringent fiber interferometer 21 of the present invention much easier to control than the asymmetric fiber Mach-Zehnder interferometer in terms of manufacturing, which is a great advantage of the present invention. In addition, because the two optical beams in the scanning birefringent fiber interferometer 21 have the same fiber path, this makes the scanning birefringent fiber interferometer 21 more stable than the unbalanced Mach-Zehnder interferometer 11 Many, this is another advantage because the penetration spectrum of the interferometer is a periodic function like Figure 5, which makes the so-called ambiguous problem in determining the size of the light wavelength. In order to get rid of this ambiguous wavelength, we use a wavelength demultiplexer 22 to make a rough determination of the wavelength, and then use the modulation signal of the interferometer to accurately determine the exact wavelength of each reflected light component. Generally speaking, as long as the frequency interval of the wavelength demultiplexer 2 2 is smaller than the frequency interval of the interferometer, there will be no problem with frequency. At present, the frequency separation of the wavelength demultiplexer 22 in optical fiber communication applications can be as small as 100 GHz, so it is sufficient to use with the above interferometer. Because the penetration spectrum of the interferometer is a sine wave function like Figure 5, this makes the measurement sensitivity smaller when the frequency interval is larger. Therefore, when high sensitivity is required, the frequency interval cannot be too large, which makes the ambiguous operation range

Page 14 V. Description of Invention (9)

The circumference becomes smaller. Although the problem of ambiguousness can be overcome by using a wavelength demultiplexer, a wavelength demultiplexer with a small frequency interval is more expensive, and for the application of a fiber grating strain sensing system, an ambiguous operation is expected. The range is 5ηηι, but it must be able to maintain the high sensitivity of the measurement. In order to meet this demand, we propose a distributed fiber grating sensing system architecture as shown in Figure 4. By concatenating two cat-type bi-fold fiber interferometers 3 1 and 3 2 with different fiber lengths, we can expand Ambiguous operating range and maintain high sensitivity of measurement. This design is based on the following working principle: assuming that the input scanning signal is a sawtooth wave, the penetration coefficient of the first birefringent fiber interferometer 31 during the scanning process is as follows: Γ 丨 = cos— the second The transmission spectrum of the birefringent fiber interferometer 3 2 is as follows

T2 = cos total penetration coefficient is the product of the above two formulas

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V. Description of the invention (10) 7 '= Γ, Γ: 1 + cos

+ COS

J + I

COS +. 丨-[> +2 (

+ COS where 2 and 2 can be nf L ″ c, so by detecting the phase of (K,-K2) frequency components, we can expand K 1 + K 2) frequency range without ambiguous operation range: by simultaneously detecting the amount Phase, we have better measurement sensitivity. In an actual system, we can not know the values of parameters such as Li, L2, △ η very accurately, so we must use a reference light source for calibration to get the absolute The wavelength of light. In Figure 3 and Figure 4, we use a temperature-compensated fiber grating to generate the reference light source. At present, the temperature-compensated fiber grating has a center wavelength temperature coefficient close to 1 pm / degree, which can meet the general purpose. Demand. In practice, multiple references can also be used for calibration using fiber gratings or fixed Fabry-Perot resonators.

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Claims (1)