CN100424494C - Method for Realizing Uniform Intensity of Circular Halo and Optical Fiber Mode Scrambler - Google Patents

Abstract

The invention discloses a method for realizing the homogenization of circular halo intensity in a differential absorption spectrometer and an optical fiber mode scrambler, which squeezes a section of the optical fiber between the telescope of the differential absorption spectrometer and the spectrometer through a workpiece with a concave-convex surface, The high-order modes in the fiber are mixed, and the light spot with uniform output intensity distribution enters the monochromator. The optical fiber mode scrambler is to clamp a section of optical fiber between a pair of splints with uneven surfaces and matching each other. According to the optical fiber mode scrambling effect, in the optical fiber mode scrambler, the high-order modes in the optical fiber are mixed to output a light spot with uniform intensity distribution. Applied to a differential absorption spectrometer, it can eliminate the influence of the circular halo on the measurement accuracy, reduce the residual noise of the system, increase the detection accuracy, and improve the lower limit of detection.

Description

Method for Realizing Uniform Intensity of Circular Halo and Optical Fiber Mode Scrambler

technical field

The invention relates to a differential absorption spectrometer, in particular to a processing method for homogenizing the circular light ring received by the differential absorption spectrometer.

Background technique

Differential absorption spectroscopy (DOAS) technology is a method to invert the concentration of various gas molecules in the atmosphere by using the characteristic absorption spectra of various gas molecules in the ultraviolet and visible bands when light is transmitted in the atmosphere. The differential absorption spectrometer mainly includes: light source, Telescope, corner reflector, quartz optical fiber, spectrometer, photodiode array (PDA), data acquisition system and computer, etc. Optical signals are transmitted between the telescope and the spectrometer through optical fibers.

The Cassegrain type telescope is used in the system, and the transmitting and receiving are designed as a whole through the corner reflector placed at the other end of the optical path. In order to realize the transceiver function of the telescope, the main mirror of the telescope is divided into two parts. The outer ring is used as a transmitter, and the middle part is used to receive the returning light. Due to the occlusion of the optical elements in the telescope on the optical axis, the exit spot is not a full circle, but a ring. When the light returns through the corner reflector and is received by the telescope, it is also a halo, that is, the optical fiber conducts when measuring the atmosphere. for the halo. When the telescope is measuring the lamp, the diaphragm in Figure 1 rotates to block the light from the lamp to the main mirror of the telescope, and only makes the light enter the optical fiber through the small hole on the diaphragm and the gap between M1. At this time, the optical fiber transmits Circular spot. In this way, the design of the telescope with integrated transceiver causes the light structure received by the spectrometer to be different when measuring the light and measuring the atmosphere.

One of the main problems of using PDA to measure spectra in DOAS system is the principle defect of individual differences among pixels. The differential optical density obtained from the absorption lines produced by atmospheric trace gases by differential absorption spectroscopy is on the order of 0.1%-0.001%, although the photodiode array can record a wide range of spectra at the same time as a readout and is increasingly It is widely used as a DOAS detector, but the sensitivity difference of this type of optical multi-channel analyzer on the unit pixel is in the order of 1%; in addition, water vapor and dust also lead to the fixed structure of PDA. Therefore, how to correctly remove the fixed structure of PDA from the measured atmospheric spectrum is very important. For measurements of smaller optical densities—below 1-3 orders of magnitude of pixel-to-pixel variance, pixel-to-pixel variance must be calibrated. For DOAS instruments, it is difficult to record the sensitivity reference spectra of individual pixels under accurately maintained constant illumination. So far, the following methods have been used to correct the fixed structure of the PDA: first, the reference optical path measures the spectrum of the xenon lamp after each measurement of the atmospheric spectrum; second, the multi-channel scanning technology is used. Combination of road detection and scanning technology.

The first method used in the system here corrects the difference between PDA pixels, that is, divides the atmospheric spectrum by the corresponding xenon lamp spectrum to remove the fixed structure of the PDA pixel. The actual sensitivity of this method depends largely on the illumination of individual pixels, ie, the angle of incidence and the distribution of light intensity over the entire surface of the pixel. However, in the DOAS system, as mentioned above, due to the difference between the reference light path and the atmospheric measurement light path, the structure of the light spectrum and the atmospheric spectrum is not uniform. Limited by the mismatch, the division of the two spectra only removes the fixed structure of the PDA pixel but brings residual noise due to the difference in the measurement light path.

Contents of the invention

The purpose of the present invention is to solve the influence of the circular halo on the measurement accuracy, and propose a method for realizing the homogenization of the circular halo intensity in the differential absorption spectrometer and an optical fiber mode scrambler to mix the high-order modes of the light transmitted in the optical fiber and make the output The circular halo becomes a circular spot of uniform intensity. In the differential absorption spectroscopy system, the residual noise of the system can be reduced, the detection accuracy can be increased, and the detection lower limit can be improved.

Technical scheme of the present invention is as follows:

Realize the method for the homogenization of circular halo intensity in the differential absorption spectrometer, the differential absorption spectrometer comprises a light source, a telescope integrating sending and receiving, a corner reflector, an optical fiber, a spectrometer, a PDA, a data acquisition system and a computer, and is characterized in that a section in the optical fiber Squeeze through a workpiece with a concave-convex surface, so that the high-order modes in the fiber are mixed, and the light spot with uniform output intensity distribution enters the monochromator.

The optical fiber mode scrambler includes an optical fiber, and is characterized in that a section of optical fiber is sandwiched between a pair of splints that are uneven and coincident with each other.

The surface of the splint attached to the surface of the optical fiber has a plastic or rubber protective layer.

The splints are connected and fixed by adjusting bolts, and a spring is arranged between the adjusting bolts and the splints.

The optical fiber receiving the circular light ring enters the optical fiber mode scrambler. According to the optical fiber mode scrambler effect, in the mode mixer, the high-order modes in the optical fiber are mixed to output a light spot with uniform intensity distribution.

Main features of the present invention are:

1. The length of the optical fiber in the splint can be adjusted according to the actual situation;

2. In application, it is only necessary to add a fiber optic mode scrambler to solve the problem, and there is no need to change or modify other parts of the system.

3. Realize the homogenization of the intensity of the circular halo, and apply it to the differential absorption spectrometer, which can eliminate the influence of the circular halo on the measurement accuracy, reduce the residual noise of the system, increase the detection accuracy, and improve the lower limit of detection.

Description of drawings

Fig. 1 is a schematic diagram of optical fiber transmission applied to the optical fiber mode scrambler of the present invention in a differential absorption spectrometer.

Fig. 2 is a schematic diagram of the structure of the optical fiber mode scrambler of the present invention.

Detailed ways

See Figure 1.

A method for uniformizing the intensity of a circular halo in a differential absorption spectrometer. The differential absorption spectrometer includes a light source, a telescope integrated with sending and receiving, a corner reflector, an optical fiber, a monochromator, a photomultiplier tube detector, a data acquisition system, and a computer. A section of the fiber is extruded by a workpiece with a concave-convex surface, so that the high-order modes in the fiber are mixed, and the light spot with uniform output intensity distribution enters the monochromator.

See Figure 2.

The present invention has upper and lower metal splints 4 and 6 with concavo-convex structures on the surface, the surface of the metal splint 4 is covered with a layer of plastic protective layer 5, the surface of the metal splint 6 is covered with a layer of plastic protective layer 7, and the metal splint 4 and 6 pass through The four adjusting bolts 2 and the adjusting spring 3 are connected and fixed. The optical fiber 1 is sandwiched between the plastic protective layers 5 and 7. The concave-convex structure of the metal splint squeezes the optical fiber 1 to cause the mode-scrambling effect; the plastic protective layers 5 and 7 protect the optical fiber from breaking; The pressure on the plate makes the scrambler work in the best state; and the adjustment spring 3 is for protection.

Claims (1)

1. realize the method for circular ring of light intensity homogenising in the difference absorption spectrum instrument, the difference absorption spectrum instrument comprises telescope, corner reflector, optical fiber, spectrometer, photoelectron diode array detector, data acquisition system (DAS) and the computing machine of light source, sending and receiving one, it is characterized in that one section in the optical fiber is pushed by the workpiece with male and fomale(M﹠F), make that the high-order mode in the transmission signals is mixed in the optical fiber, the circular light spot that output intensity is evenly distributed enters in the monochromator.

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