Ancient building multi-channel fire and security optical fiber monitoring device

Abstract

The application belongs to the technical field of distributed optical fiber sensing, and solves the problem that electromagnetic monitoring equipment cannot be monitored for a long time and in a large range, which easily causes fire. The application provides an ancient building multi-channel fire and security optical fiber monitoring device. Continuous narrow linewidth laser emitted by a narrow linewidth laser is divided into one detection light and N local lights by an optical fiber coupler I. The detection light is divided into N detection lights by an optical fiber coupler II after being modulated, amplified and filtered, and is input into N temperature / perturbation monitoring modules. The detection light output by the temperature / perturbation monitoring modules is input into an optical isolator through a sensing optical fiber, and then the backscattering signal returned from the sensing optical fiber is input into the temperature / perturbation monitoring modules. The N local lights divided by the optical fiber coupler I are input into the N temperature / perturbation monitoring modules. The application combines coherent detection perturbation monitoring technology and Raman optical time domain temperature monitoring technology, and realizes ancient building multi-channel fire and security monitoring.

Description

Technical Field

[0001] This invention belongs to the field of distributed optical fiber sensing technology, specifically relating to a multi-channel fire and security optical fiber monitoring device for ancient buildings. Background Technology

[0002] The threats to ancient buildings mainly stem from pollution and damage to the natural and social environment, as well as fire. The initial methods of protecting ancient buildings relied primarily on manual measurement, with information about the buildings recorded in written documents. This method of recording suffered from problems such as inaccurate data, large space requirements, easy loss of documents, and cumbersome access, and required a significant amount of manpower and resources.

[0003] Existing electromagnetic monitoring equipment can record information about ancient buildings through photos and videos, but the monitoring system cannot provide real-time monitoring over a large area for extended periods. Furthermore, due to its inherent characteristics, it is prone to causing devastating damage such as fires in extreme weather conditions or when power lines are damaged. Summary of the Invention

[0004] In order to solve at least one of the above-mentioned technical problems in the prior art, the present invention provides a multi-channel fire and security fiber optic monitoring device for ancient buildings.

[0005] This invention employs the following technical solution: a multi-channel fire and security fiber optic monitoring device for ancient buildings, comprising a narrow-linewidth laser, fiber optic couplers, temperature / disturbance monitoring modules, sensing fibers, and optical isolators; the narrow-linewidth laser emits a continuous narrow-linewidth laser with a center wavelength of the second wavelength, which is split into one probe light and N local light by fiber optic coupler I. The probe light is modulated, amplified, and filtered, and then split into N probe light by fiber optic coupler II, which are correspondingly input to N temperature / disturbance monitoring modules. Each temperature / disturbance monitoring module is connected to a sensing fiber deployed around the ancient building. The probe light output from the temperature / disturbance monitoring module is input to the optical isolator via the sensing fiber. Subsequently, the backscattered signal carrying temperature and disturbance information around the ancient building, which returns from the sensing fiber, is input to the temperature / disturbance monitoring module. In the process, the temperature / disturbance monitoring module divides the backscattered light into two different wavelengths and transmits them to the data acquisition card. Based on Raman time-domain temperature monitoring technology, the backscattered light signal of the second wavelength is used as a reference signal to demodulate the backscattered light signal of the first wavelength to obtain temperature information around the optical fiber, thereby achieving the purpose of monitoring fires in ancient buildings. The N local light signals split off by the optical fiber coupler I are correspondingly input to the N temperature / disturbance monitoring modules. The local light in the temperature / disturbance monitoring modules beats the backscattered light of the second wavelength to generate beat frequency signal light, which is then converted and filtered before being input to the data acquisition card. Based on coherent detection disturbance monitoring technology, the optical signal acquired by the data acquisition card is demodulated, processed, and analyzed to obtain disturbance information around the optical fiber, thereby achieving the purpose of security monitoring of ancient buildings.

[0006] Preferably, the temperature / disturbance monitoring module includes a data acquisition card, a circulator, fiber optic coupler III, fiber optic coupler IV, fiber optic coupler V, fiber optic coupler VI, an avalanche photodetector, a polarization controller, a polarization combiner, a balanced photodetector, and an electrical filter.

[0007] The probe light, after being split by fiber coupler II, is input to end a of the temperature / disturbance monitoring module and then to end a of the circulator. It then passes through end b of the circulator and end b of the temperature / disturbance monitoring module before entering the sensing fiber and optical isolator. The backscattered signal carrying temperature and disturbance information around the ancient building, returning from the sensing fiber, passes through end b of the temperature / disturbance monitoring module and end b of the circulator, and is output from end c of the circulator to end a of fiber coupler III. Fiber coupler III divides the input backscattered signal into two equal parts, outputting them from ends b and c respectively. The backscattered signal output from end b of fiber coupler III is processed by optical filter I and fiber amplifier I before being input as the first wavelength backscattered light signal. The backscattered signal output from the c end of fiber coupler III is processed by optical filter II and fiber amplifier II and then input as a second wavelength backscattered light signal to the a end of fiber coupler IV. Fiber coupler IV divides the input backscattered signal into two parts and outputs them from its b and c ends respectively. The second wavelength backscattered signal output from the b end of fiber coupler IV is input to the b end of avalanche photodetector. Avalanche photodetector converts the input light signal into an electrical signal and outputs it from the c and d ends to the b and c ends of the data acquisition card. Based on Raman optical time-domain temperature monitoring technology, the temperature information around the optical fiber is obtained, thereby achieving the purpose of monitoring ancient building fires.

[0008] The backscattered signal of the second wavelength output from end c of fiber coupler IV is input to end a of fiber coupler V. Fiber coupler V divides the input backscattered signal into two equal parts and outputs them from ends b and c respectively. These parts are then converged by a polarization controller into a polarization combiner, and from end c of the polarization combiner, they are input to end a of fiber coupler VI. Simultaneously, the local light split from fiber coupler I is input to end b of fiber coupler VI via end c of the temperature / disturbance monitoring module. The two light signals input to fiber coupler VI beat at the same frequency to generate beat frequency signal light, which is then divided into two identical parts. These parts are input to ends a and b of a balanced photodetector from ends c and d of fiber coupler VI, respectively. The balanced photodetector converts the input light signal into an electrical signal, which is then filtered by an electrical filter and input to end a of a data acquisition card. Based on coherent detection disturbance monitoring technology, disturbance information around the optical fiber is obtained, thereby achieving the purpose of security monitoring of ancient buildings.

[0009] Preferably, the first wavelength is 1450nm and the second wavelength is 1550nm.

[0010] Preferably, N is 4, and the proportions of the probe light split by fiber coupler I and the 4 local light are 80%, 5%, 5%, 5%, and 5%, respectively; the proportions of the 4 probe lights split by fiber coupler II are 25%, 25%, 25%, and 25%.

[0011] Preferably, the probe light output from end b of fiber optic coupler I is input to end a of fiber optic coupler II via an acousto-optic modulator, fiber amplifier III, and optical filter III. A signal generator is connected to end c of the acousto-optic modulator. Driven by the signal generator, the acousto-optic modulator modulates the continuous probe light into pulse light and generates a 200MHz frequency shift. The modulated probe pulse light is output from end b of the acousto-optic modulator, amplified and filtered by fiber amplifier III and optical filter III, and retains the effective probe light with a center wavelength of 1550nm.

[0012] Compared with the prior art, the beneficial effects of the present invention are:

[0013] This invention utilizes fiber optic sensing technology to achieve multi-channel simultaneous distributed sensing and monitoring of fire and security in ancient buildings. A distributed fiber optic sensing system structure based on fiber coupling is constructed, splitting the detection pulse light into four paths to achieve simultaneous distributed sensing of the four sensing fibers. A temperature / disturbance monitoring module is designed to split the backscattered signal in each sensing fiber into two light signals with different wavelengths, 1450nm and 1550nm. This combines coherent detection disturbance monitoring technology with Raman time-domain temperature monitoring technology to achieve four-channel fire and security monitoring of ancient buildings based on distributed fiber optic sensing technology.

[0014] This invention uses a 1550nm wavelength backscattered light signal instead of the 1660nm wavelength backscattered light signal in traditional Raman time-domain temperature monitoring technology as a reference signal for temperature demodulation. This offers advantages such as good stability and high sensitivity, improving the system's temperature measurement accuracy. The passive sensor facilitates deployment and avoids the limitations of traditional electromagnetic monitoring technologies, such as high cost and susceptibility to fires in extreme weather. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of the overall device connection of the present invention;

[0017] Figure 2 This is a schematic diagram of the temperature / disturbance monitoring module of the present invention;

[0018] Figure 3 This is a schematic diagram of the fiber optic sensing arrangement of the present invention.

[0019] In the diagram: 1-Narrow linewidth laser; 201-Fiber optic coupler I; 202-Fiber optic coupler II; 301-Data acquisition card; 302-Circulator; 303-Fiber optic coupler III; 304-Fiber optic coupler IV; 305-Fiber optic coupler V; 306-Fiber optic coupler VI; 307-Avalanche photodetector; 308-Polarization controller; 309-Polarization combiner; 310-Balanced photodetector; 311-Electrical filter; 312-Optical filter I; 313-Fiber optic amplifier I; 314-Optical filter II; 315-Fiber optic amplifier II; 4-Sensing fiber; 5-Optical isolator; 6-Acousto-optic modulator; 7-Fiber optic amplifier III; 8-Optical filter III; 9-Signal generator; 10-Ancient building beam; 11-Ancient building column; 12-Fiber optic fixing point. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] It should be noted that the structures, proportions, sizes, etc., shown in the accompanying drawings of this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed in the specification, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportional relationships, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should fall within the scope of the technical content disclosed in the present invention. It should be noted that in this specification, relational terms such as "first" and "second" are only used to distinguish one entity from several other entities, and do not necessarily require or imply any actual relationship or order between these entities.

[0022] This invention provides an embodiment:

[0023] like Figures 1 to 3 As shown, a multi-channel fire and security fiber optic monitoring device for ancient buildings is constructed. A distributed fiber optic sensing system structure based on fiber optic coupling is established, splitting the detection pulse light into four paths to achieve simultaneous distributed sensing of the four sensing fibers. A temperature / disturbance monitoring module is designed to split the backscattered signal in each sensing fiber into two light signals with different wavelengths, 1450nm and 1550nm. This combines coherent detection disturbance monitoring technology with Raman time-domain temperature monitoring technology to achieve four-channel fire and security monitoring of ancient buildings based on distributed fiber optic sensing technology.

[0024] The system includes a narrow-linewidth laser 1, an optical fiber coupler, a temperature / disturbance monitoring module, a sensing fiber 4, and an optical isolator 5. The narrow-linewidth laser 1 emits a continuous narrow-linewidth laser with a center wavelength of 1550nm, which is split into one probe beam and four local beams via optical fiber coupler I201. The probe beam and the four local beams split by optical fiber coupler I201 account for 80%, 5%, 5%, 5%, and 5% of the total, respectively. The probe beam output from the b-end of optical fiber coupler I201 is then processed by an acousto-optic modulator 6, an optical fiber amplifier III 7, and an optical isolator 5. Filter III8 is input to end a of fiber optic coupler II202; signal generator 9 is connected to end c of acousto-optic modulator 6. Signal generator 9 provides drive pulse signal to acousto-optic modulator 6. Under the drive of signal generator 9, acousto-optic modulator 6 modulates continuous probe light into pulse light and generates a frequency shift of 200MHz. The modulated probe pulse light is output from end b of acousto-optic modulator 6, amplified and filtered by fiber optic amplifier III7 and optical filter III8, and retains effective probe light with a center wavelength of 1550nm.

[0025] After modulation, amplification, and filtering, the probe light is split into four probe lights by fiber optic coupler II202 and input to four temperature / disturbance monitoring modules respectively. The proportions of the four probe lights split by fiber optic coupler II202 are 25%, 25%, 25%, and 25%. Each temperature / disturbance monitoring module is connected to a sensing fiber 4 deployed around the ancient building. The detection light output from the temperature / disturbance monitoring module is input into the optical isolator 5 via the sensing fiber 4. Subsequently, the backscattered signal carrying the temperature and disturbance information around the ancient building, which returns from the sensing fiber 4, is input into the temperature / disturbance monitoring module. The optical isolator 5 is used to prevent the reflected laser signal from affecting the backscattered signal. The temperature / disturbance monitoring module divides the backscattered light into two different wavelength bands and transmits them to the data acquisition card 301. Based on Raman optical time-domain temperature monitoring technology, the backscattered light signal with a wavelength of 1550nm is used as the reference signal, and the backscattered light signal with a wavelength of 1450nm is used for temperature demodulation to obtain the temperature information around the optical fiber, thereby achieving the purpose of monitoring fires in the ancient building.

[0026] The four local light beams split off by the fiber optic coupler I201 are input to four temperature / disturbance monitoring modules. The local light in the temperature / disturbance monitoring modules beats the backscattered light of the second wavelength to generate a beat frequency signal light. After conversion and filtering, the signal is input to the data acquisition card 301. Based on coherent detection disturbance monitoring technology, the optical signal acquired by the data acquisition card 301 is demodulated, processed, and analyzed to obtain disturbance information around the optical fiber, thereby achieving the purpose of security monitoring of ancient buildings.

[0027] In this embodiment, the temperature / disturbance monitoring module includes a data acquisition card 301, a circulator 302, an optical fiber coupler III 303, an optical fiber coupler IV 304, an optical fiber coupler V 305, an optical fiber coupler VI 306, an avalanche photodetector 307, a polarization controller 308, a polarization combiner 309, a balanced photodetector 310, and an electrical filter 311.

[0028] The probe light, after being split by fiber coupler II202, is input to end a of the temperature / disturbance monitoring module and then to end a of the circulator 302. It then passes through end b of the circulator 302 and the temperature / disturbance monitoring module, before entering the sensing fiber 4 and optical isolator 5. The backscattered signal carrying temperature and disturbance information around the ancient building, returning from sensing fiber 4, passes through end b of the temperature / disturbance monitoring module and the circulator 302, and is output from end c of the circulator 302 to end a of fiber coupler III303. Fiber coupler III303 divides the input backscattered signal into two equal parts, outputting them from ends b and c respectively. The backscattered signal output from terminal b of coupler III303 is processed by optical filter I312 and fiber amplifier I313, and then input as an effective wavelength band backscattered light signal (1450nm) to terminal a of avalanche photodetector 307. Fiber amplifier I313 is used to compensate for optical signal loss caused by the fiber coupling process. The backscattered signal output from terminal c of fiber coupler III303 is processed by optical filter II314 and fiber amplifier II315, and then input as an effective wavelength band backscattered light signal (1550nm) to terminal a of fiber coupler IV304. Fiber coupler IV304 converts the input backscattered light signal... The backscattered signal is divided into two parts and output from its b and c ends respectively. The 1550nm backscattered signal output from the b end of the fiber coupler IV304 is input to the b end of the avalanche photodetector 307. The avalanche photodetector 307 converts the input optical signal into an electrical signal and outputs it from its c and d ends to the b and c ends of the data acquisition card 301. Based on Raman optical time-domain temperature monitoring technology, temperature information around the optical fiber is obtained, thereby achieving the purpose of monitoring fires in ancient buildings. The electrical signal converted from the 1450nm backscattered signal is input to the data acquisition card 301 from its c end. At end b of 01, the electrical signal converted from the backscattered signal with a wavelength of 1550nm is input from end d of the avalanche photodetector 307 to end c of the data acquisition card 301. Based on the advantages of the backscattered light signal with a wavelength of 1550nm, such as higher stability and lower temperature insensitivity, the backscattered light signal with a wavelength of 1550nm is used to replace the backscattered light signal with a wavelength of 1660nm in the traditional Raman optical time-domain temperature monitoring technology as the reference signal. The backscattered light signal with a wavelength of 1450nm is used for temperature demodulation, so as to obtain the temperature information around the optical fiber and thus achieve the purpose of monitoring fires in ancient buildings.

[0029] A 1550nm backscattered signal output from the c-end of fiber coupler IV304 is input to the a-end of fiber coupler V305. Fiber coupler V305 divides the input backscattered signal into two equal parts and outputs them from its b-end and c-end respectively. The parts are then converged by polarization controller 308 into polarization combiner 309, and then input from the c-end of polarization combiner 309 to the a-end of fiber coupler VI306. The two polarization controllers 308 are tuned to orthogonal polarization states to suppress polarization fading noise of the 1550nm wavelength backscattered light signal, thereby improving the sensitivity of the system.

[0030] Simultaneously, 5% of the local light split from fiber optic coupler I201 is input to the b end of fiber optic coupler VI306 via the c end of the temperature / disturbance monitoring module. The two light signals input to fiber optic coupler VI306 are beat-frequency generated and split into two identical parts, which are input to the a and b ends of balanced photodetector 310 from the c and d ends of fiber optic coupler VI306, respectively. Balanced photodetector 310 converts the input light signal into an electrical signal and inputs it to electrical filter 311. Electrical filter 311 is used to filter out electrical noise of other frequencies, retaining only the effective electrical signal with a frequency of 200MHz. The filtered electrical signal is input from electrical filter 311 to the a end of data acquisition card 301. Based on coherent detection disturbance monitoring technology, the light signal acquired by data acquisition card 301 is demodulated, processed, and analyzed to obtain disturbance information around the optical fiber, thereby achieving the purpose of security monitoring of ancient buildings.

[0031] The sensing fiber optic cable 4 is deployed on the beams 10 and columns 11 of the ancient building through fiber optic fixing points 12. When a fire or abnormal disturbance / intrusion event occurs around the ancient building, the sensing fiber optic cable 4 can detect the abnormal information and feed it back to the designed four-channel fire and security fiber optic monitoring device for demodulation, processing, and analysis, so as to take timely protective measures. The four sensing fiber optic cables can monitor different buildings separately, realizing the purpose of simultaneous distributed real-time online monitoring of fire and security in the ancient building across four channels.

[0032] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A multi-channel fire and security fiber optic monitoring device for ancient buildings, characterized in that: It includes a narrow linewidth laser (1), an optical fiber coupler, a temperature / disturbance monitoring module, a sensing fiber (4), and an optical isolator (5). A continuous narrow-linewidth laser with a center wavelength of the second wavelength emitted by a narrow-linewidth laser (1) is split into one probe light and N local lights by fiber coupler I (201). The probe light is modulated, amplified, and filtered, and then split into N probe lights by fiber coupler II (202) and input to N temperature / disturbance monitoring modules. Each temperature / disturbance monitoring module is connected to a sensing fiber (4) deployed around the ancient building. The probe light output by the temperature / disturbance monitoring module is input to the optical isolator (5) through the sensing fiber (4). Then, the backscattered signal carrying the temperature and disturbance information around the ancient building is input to the temperature / disturbance monitoring module. The temperature / disturbance monitoring module splits the backscattered light into two different bands and transmits them to the data acquisition card (301). Based on Raman optical time-domain temperature monitoring technology, the backscattered light signal of the second wavelength is used as a reference signal to demodulate the backscattered light signal of the first wavelength to obtain the temperature information around the fiber, thereby achieving the purpose of monitoring fires in the ancient building. The N local light streams split off by the fiber optic coupler I (201) are input to the N temperature / disturbance monitoring modules. The local light input to the temperature / disturbance monitoring modules beats the backscattered light of the second wavelength to generate a beat frequency signal light. After conversion and filtering, the signal is input to the data acquisition card (301). Based on coherent detection disturbance monitoring technology, the optical signal acquired by the data acquisition card (301) is demodulated, processed and analyzed to obtain disturbance information around the optical fiber, thereby achieving the purpose of security monitoring of ancient buildings. The temperature / disturbance monitoring module includes a data acquisition card (301), a circulator (302), fiber optic coupler III (303), fiber optic coupler IV (304), fiber optic coupler V (305), fiber optic coupler VI (306), an avalanche photodetector (307), a polarization controller (308), a polarization combiner (309), a balanced photodetector (310), and an electrical filter (311). After being spun by fiber coupler II (202), the probe light is input to end a of the temperature / disturbance monitoring module and then to end a of the circulator (302). After passing through end b of the circulator (302) and end b of the temperature / disturbance monitoring module, it enters the sensing fiber (4) and the optical isolator (5). The backscattered signal carrying the temperature and disturbance information around the ancient building, which returns from the sensing fiber (4), passes through end b of the temperature / disturbance monitoring module and end b of the circulator (302) and is output from end c of the circulator (302) to end a of fiber coupler III (303). Fiber coupler III (303) divides the input backscattered signal into two parts and outputs them from ends b and c, respectively. The backscattered signal output from end b of fiber coupler III (303) is processed by optical filter I (312) and fiber amplifier I (313) and then inputs the first wavelength backscattered signal. The backscattered light signal is sent to end a of the avalanche photodetector (307); the backscattered signal output from end c of fiber coupler III (303) is processed by optical filter II (314) and fiber amplifier II (315) and then input as a second wavelength backscattered light signal to end a of fiber coupler IV (304). Fiber coupler IV (304) divides the input backscattered signal into two parts and outputs them from ends b and c respectively. The second wavelength backscattered signal output from end b of fiber coupler IV (304) is input to end b of the avalanche photodetector (307). The avalanche photodetector (307) converts the input light signal into an electrical signal and outputs it from ends c and d to ends b and c of the data acquisition card (301). Based on Raman time-domain temperature monitoring technology, the temperature information around the optical fiber is obtained, thereby achieving the purpose of monitoring fires in ancient buildings. The backscattered signal of the second wavelength output from the c end of fiber coupler IV (304) is input to the a end of fiber coupler V (305). Fiber coupler V (305) divides the input backscattered signal into two parts and outputs them from its b and c ends respectively. The outputs are then converged into the polarization combiner (309) by the polarization controller (308), and then input from the c end of the polarization combiner (309) to the a end of fiber coupler VI (306). At the same time, the local light split from fiber coupler I (201) is input to fiber coupler VI through the c end of the temperature / disturbance monitoring module. At end b of (306), two optical signals input to the fiber optic coupler VI (306) are beat-frequency generated and divided into two identical parts, which are input from ends c and d of the fiber optic coupler VI (306) to ends a and b of the balanced photodetector (310). The balanced photodetector (310) converts the input optical signal into an electrical signal, which is then filtered by the electrical filter (311) and input to end a of the data acquisition card (301). Based on coherent detection disturbance monitoring technology, disturbance information around the optical fiber is obtained, thereby achieving the purpose of security monitoring of ancient buildings.

2. The multi-channel fire and security fiber optic monitoring device for ancient buildings according to claim 1, characterized in that: The first wavelength is 1450nm, and the second wavelength is 1550nm.

3. The multi-channel fire and security fiber optic monitoring device for ancient buildings according to claim 2, characterized in that: When N is 4, the proportions of the probe light split by fiber coupler I (201) and the 4 local light are 80%, 5%, 5%, 5%, 5%, 5%, respectively; the proportions of the 4 probe lights split by fiber coupler II (202) are 25%, 25%, 25%, 25%.

4. The multi-channel fire and security fiber optic monitoring device for ancient buildings according to claim 3, characterized in that: The probe light output from the b end of the fiber optic coupler I (201) is input to the a end of the fiber optic coupler II (202) via the acousto-optic modulator (6), fiber optic amplifier III (7), and optical filter III (8). The c end of the acousto-optic modulator (6) is connected to a signal generator (9). Under the drive of the signal generator (9), the acousto-optic modulator (6) modulates the continuous probe light into pulse light and generates a frequency shift of 200MHz. The modulated probe pulse light is output from the b end of the acousto-optic modulator (6), amplified and filtered by the fiber optic amplifier III (7) and optical filter III (8), and retains the effective probe light with a center wavelength of 1550nm.

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