13CH4 isotope detection device and method based on optical feedback cavity enhancement

Abstract

The invention belongs to the technical field of laser spectrum, and particularly relates to a 13CH4 isotope detection device and method based on optical feedback cavity enhancement. In order to improve the concentration precision of C13 and C12 in methane gas and the measurement precision of carbon isotopes, a laser controller controls a laser device to emit light beams to pass through a polaroid, a first lens, a quarter-wave plate, a second lens and a reflector to enter an optical cavity, laser is reflected back and forth in the optical cavity, laser in four directions can be emitted, and the concentration precision of C13 and C12 in the methane gas is improved. Wherein one beam returns to the DFB laser in the same path, a signal emitted from the right end of the optical cavity is received by the photoelectric detector, and a cavity mode signal with optical feedback is seen on the oscilloscope. After an acquisition program is started, the high-voltage amplifier is controlled to adjust the PZT, the feedback phase is controlled, and the frequency of the laser is locked to the optical cavity. When the whole device operates normally, only methane gas with different concentrations needs to be replaced, and the corresponding C13 and C12 measurement precision can be obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of laser spectroscopy, and in particular relates to a 13CH4 isotope detection device and method based on optical feedback cavity enhancement. Background Art

[0002] With the continuous development of science and technology and the increasing market demand, trace gas detection technology has gradually shown extremely important application value in the fields of environmental testing, scientific and technological research, industrial production monitoring and disease diagnosis. It is of great significance to study gas detection technology methods and equipment that can achieve high sensitivity and real-time monitoring. However, traditional gas detection technology is far from achieving this goal. Optical feedback cavity enhancement (OF-CEAS) technology based on semiconductor lasers has the advantages of good selectivity, high sensitivity and high resolution, and has great application potential in the field of trace gas detection.

[0003] OF-CEAS technology combines optical feedback (OF) technology with cavity enhanced spectroscopy (CEAS) technology. It solves the problems of low efficiency and noise cavity injection of CEAS by tightly locking the laser in the resonant cavity and significantly reducing the laser linewidth.

[0004] In the past, CEAS technology was mainly used to utilize the FP cavity composed of highly reflective mirrors. Its working principle is to couple the laser into the optical cavity, so that the laser reflects back and forth between the cavity mirrors, which increases the action length of the light and the gas medium in the cavity in disguised form. Its equivalent optical path can reach several thousand meters, thereby greatly enhancing the absorption signal. However, this method can only be used for cavity mirrors with low reflectivity, and the resulting measurement accuracy is low. When optical feedback technology is used on the basis of this technology, a semiconductor laser with a line width of the MHZ order can be injected into a cavity longitudinal mode with a line width of only the KHZ order, and frequency locking can be achieved. When using this experimental technology to detect methane gas, theoretically, when using a cavity mirror with a high reflectivity (more than 99.99%), its measurement accuracy can reach tens of ppb. Summary of the invention

[0005] In view of the above problems, the present invention provides a 13CH4 isotope detection device and method based on optical feedback cavity enhancement, which can use low-cost semiconductor lasers to improve the concentration accuracy of C13 and C12 in methane gas and the measurement accuracy of carbon isotopes.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a 13CH4 isotope detection device based on optical feedback cavity enhancement, comprising: a function generator, a laser controller, a DFB laser, a photodetector, an optical cavity and an oscilloscope;

[0008] The output end of the function generator is connected to the input end of the laser controller, the output end of the laser controller is connected to the input end of the DFB laser, the DFB laser outputs laser light, the laser light passes through the isolator, the polarizer, the first lens, the quarter wave plate, the second lens, the first reflector, and the second reflector in sequence to enter the optical cavity, the cavity mode signal of the optical cavity passes through the focusing lens to enter the photodetector, and the output end of the photodetector is connected to the input end of the oscilloscope.

[0009] Furthermore, the optical cavity is a V-shaped cavity, including a cavity body and three cavity mirrors with high reflectivity adhered to the cavity body.

[0010] Furthermore, the three cavity mirrors with high reflectivity are adjusted in position using three three-dimensional adjustment frames through the frames, and the following conditions must be met: 1) the centers of the three cavity mirrors with high reflectivity are aligned with the centers of the holes left on the cavity; 2) after the laser is incident on the optical cavity, a cavity mode signal with no feedback is displayed on the oscilloscope, and the cavity mode signal is not affected by high-order stray modes.

[0011] Furthermore, the optical path distance from the DFB laser to the leftmost end of the optical cavity is an odd multiple of the cavity length of the optical cavity.

[0012] Furthermore, a PZT is provided on the second reflector, and an input end of the PZT is connected to an output end of the high voltage amplifier.

[0013] Furthermore, it also includes a precision translation stage, and the DFB laser is arranged on the precision translation stage.

[0014] The present invention also provides a 13CH4 isotope detection method based on optical feedback cavity enhancement, comprising the following steps:

[0015] Step 1, assembling the 13CH4 isotope detection device based on optical feedback cavity enhancement;

[0016] Step 2, remove the isolator, adjust the quarter-wave plate, and obtain the cavity mode signal with feedback on the oscilloscope. During operation, the PZT change is controlled by the high-voltage amplifier to accurately control the phase;

[0017] Step 3, introduce the methane to be measured into the cavity, obtain the absorption of C13 and C12 on the oscilloscope, collect data through the data acquisition card, and use MATLAB program to analyze, so as to obtain the concentration accuracy of C13 and C12 in the methane gas and the measurement accuracy of carbon isotopes.

[0018] Further, the assembly of the 13CH4 isotope detection device based on optical feedback cavity enhancement in step 1 is specifically as follows:

[0019] Step 1.1, the laser controller drives the DFB laser to emit laser light, which passes through the isolator, polarizer, first lens, quarter wave plate, second lens, first reflector, second reflector, and focusing lens in sequence and enters the photodetector;

[0020] Step 1.2, using a function generator to output a triangle wave scanning signal and a square wave trigger signal, providing them to a laser controller and an oscilloscope respectively, and obtaining a laser signal of the DFB laser in the oscilloscope;

[0021] Step 1.3, determine the position of the optical cavity, adjust the positions of three cavity mirrors with high reflectivity of the optical cavity, and stick them on the cavity of the optical cavity;

[0022] Furthermore, the position of the optical cavity in step 1.3 is as follows: the optical path distance from the DFB laser to the leftmost end of the optical cavity should be an odd multiple of the cavity length of the optical cavity.

[0023] The positions of the three high-reflectivity cavity mirrors of the optical cavity are adjusted by using three three-dimensional adjustment frames through the mirror frames. The positions must meet the following requirements: 1) the centers of the three high-reflectivity cavity mirrors are aligned with the centers of the holes left on the cavity; 2) after the laser is incident on the optical cavity, there is no feedback cavity mode signal displayed on the oscilloscope, and the cavity mode signal is not affected by high-order stray modes.

[0024] Furthermore, the positions of the three cavity mirrors with high reflectivity of the optical cavity are adjusted as follows:

[0025] A red light source is introduced. After the red light passes through the reflector and the folding reflector, the direction of the red light is consistent with the direction of the laser emitted by the DFB laser. The folding reflector is controlled to make the red light emit to the right or upward. When the red light emits upward, the laser will not be blocked and will reach the photodetector through the focusing lens. Two small holes are placed on the optical path reaching the photodetector, and the laser is allowed to pass through the small holes, thereby determining the specific emission route of the laser. When the red light emits to the right, the laser is blocked by the folding reflector, and the red light is allowed to pass through the two small holes by adjusting the reflector and the folding reflector, thereby replacing the laser with red light on this optical path.

[0026] Place the right cavity mirror so that the red light passes through the center of the cavity mirror and the cavity mirror surface is perpendicular to the entire optical path. Adjust the mirror holder knob of the right cavity mirror so that the reflected light of the cavity mirror to the red light also passes through the two small holes and returns along the original path.

[0027] Place the left cavity mirror so that the red light passes through the center of the cavity mirror and the mirror surface is perpendicular to the optical path. At this time, remove the two small holes and place the optical cavity on the optical path. Align the two cavity mirrors with the cavity position, and then adjust the knob of the left cavity mirror so that the red light is emitted from the center of the other port of the cavity.

[0028] Then place the second right cavity mirror, and make the red light pass through the center and the mirror surface perpendicular to the optical path. Adjust the knob of the second right cavity mirror so that the red light can be reflected by the second right cavity mirror and return to the first left cavity mirror, and then can be reflected to the first right cavity mirror, so that the red light is emitted back and forth on the three reflectors, and the rough adjustment of the positions of the three cavity mirrors is completed;

[0029] Flip the folding mirror again, introduce the laser emitted by the DFB laser, display the cavity mode signal on the oscilloscope, adjust the knobs of the right first cavity mirror and the right second cavity mirror to adjust the cavity mode signal without feedback, and the cavity mode signal is not affected by high-order stray modes. At this time, stick the three cavity mirrors on the cavity.

[0030] The laser reflects back and forth in the optical cavity, and there will be four directions of laser emission, one of which will return to the DFB laser along the original path, reducing the laser line width. The signal emitted from the right end of the optical cavity can be received by the photodetector, and the cavity mode signal with optical feedback can be seen on the oscilloscope. After opening the acquisition program, control the high-voltage amplifier to adjust the PZT, control the distance between the laser output surface and the front mirror of the cavity to control the feedback phase, and achieve the locking of the laser frequency to the optical cavity. When the entire device is operating normally, you only need to replace the methane gas of different concentrations to obtain the corresponding C13 and C12 measurement accuracy.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] The optical feedback of the present invention provides a locking of a laser with an external optical component. In contrast to PDH locking, when operating in a weak feedback state, the optical feedback is a combination of optical injection and electronic locking. Optical feedback can provide a more robust locking mechanism that is more sensitive to mechanical vibrations, simplifies the experimental setup, improves system compatibility, reduces costs, and is conducive to the industrialization of optical feedback cavity enhancement technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the 13CH4 isotope detection device based on optical feedback cavity enhancement of the present invention.

[0034] Figure 2 These are the corresponding values ​​of the laser used in the present invention. DETAILED DESCRIPTION

[0035] In order to further illustrate the technical solution of the present invention, the present invention is further described below through embodiments.

[0036] like Figure 1 As shown, a 13CH4 isotope detection device based on optical feedback cavity enhancement in this embodiment includes: a function generator 1, a laser controller 2, a DFB laser 3, a photodetector 15, an optical cavity 19 and an oscilloscope 20;

[0037] The output end of the function generator 1 is connected to the input end of the laser controller 2, the output end of the laser controller 2 is connected to the input end of the DFB laser 3, the DFB laser 3 outputs laser light, and the laser light passes through the isolator 5, the polarizer 6, the first lens 7, the quarter-wave plate 8, the second lens 9, the first reflector 10, and the second reflector 11 in sequence to enter the optical cavity 19, and the cavity mode signal of the optical cavity 19 passes through the focusing lens 14 to enter the photodetector 15, and the output end of the photodetector 15 is connected to the input end of the oscilloscope 20.

[0038] The optical cavity 19 of this embodiment is a V-shaped cavity, including a cavity and three cavity mirrors with high reflectivity glued to the cavity. The three cavity mirrors with high reflectivity are adjusted in position by three three-dimensional adjustment frames through the frames, and the following conditions must be met: 1) the centers of the three cavity mirrors with high reflectivity are aligned with the center of the hole left on the cavity; 2) after the laser is incident on the optical cavity 19, a cavity mode signal with no feedback is displayed on the oscilloscope 20, and the cavity mode signal is not affected by high-order stray modes.

[0039] Since the optical cavity used is a V-shaped cavity and optical feedback is used for gas measurement, the optical path distance from the DFB laser 3 to the leftmost end of the cavity 19 of the optical cavity 19 of this embodiment is an odd multiple of the cavity length of the optical cavity (19) (referring to a single arm), thereby determining the position of the optical cavity.

[0040] In this embodiment, a PZT 12 is disposed on the second reflector 11 , and the input end of the PZT 12 is connected to the output end of the high-voltage amplifier 13 . The high-voltage amplifier 13 will also control the change of the PZT and accurately control the phase under the control of a corresponding program.

[0041] The device further comprises a precision displacement stage 4 , on which the DFB laser 3 is arranged, for precisely adjusting the position of the DFB laser 3 .

[0042] The corresponding values ​​of the laser used in this embodiment are as follows Figure 2 shown.

[0043] Based on the above device, a 13CH4 isotope detection method based on optical feedback cavity enhancement in this embodiment includes the following steps:

[0044] Step 1, assembling the 13CH4 isotope detection device based on optical feedback cavity enhancement;

[0045] Step 2, remove the isolator 5, adjust the quarter wave plate 8, obtain the cavity mode signal with feedback on the oscilloscope 20, and during operation, control the change of PZT12 through the high voltage amplifier 13 to accurately control the phase;

[0046] Step 3, introduce the methane to be measured into the cavity, obtain the absorption of C13 and C12 on the oscilloscope 20, collect data through the data acquisition card, and use MATLAB program to analyze to obtain the concentration accuracy of C13 and C12 in the methane gas and the carbon isotope measurement accuracy.

[0047] The method of assembling the device in step 1 is as follows:

[0048] Step 1.1, the laser controller 2 drives the DFB laser 3 to emit laser light, which passes through the isolator 5, the polarizer 6, the first lens 7, the quarter wave plate 8, the second lens 9, the first reflector 10, the second reflector 11, and the focusing lens 14 in sequence and enters the photodetector 15;

[0049] Step 1.2, using the function generator 1 to output a triangle wave scanning signal and a square wave trigger signal, and providing them to the laser controller 2 and the oscilloscope 20 respectively, and obtaining the laser signal of the DFB laser 3 in the oscilloscope 20;

[0050] Step 1.3, determine the position of the optical cavity 19, and adjust the positions of three cavity mirrors with high reflectivity of the optical cavity 19, and stick them on the cavity body of the optical cavity (19).

[0051] Determine the position of the optical cavity 19. Since the optical cavity 19 used is a V-shaped cavity and uses optical feedback to measure gas, the position of the optical cavity 19 is: the optical path distance from the DFB laser (3) to the leftmost end of the optical cavity 19 should be an odd multiple of the cavity length of the optical cavity 19 (referring to a single arm).

[0052] Since the optical cavity 19 is a V-shaped cavity, the positions of the three cavity mirrors with high reflectivity of the optical cavity (19) are adjusted by using three three-dimensional adjustment frames through the mirror frames, and their positions must meet the following requirements: 1) the centers of the three cavity mirrors with high reflectivity are aligned with the centers of the holes left on the cavity; and 2) after the laser is incident on the optical cavity (19), a cavity mode signal without feedback is displayed on the oscilloscope (20), and the cavity mode signal is not affected by high-order heterogeneous modes.

[0053] Due to the use of infrared laser, it is impossible to determine the route of the laser propagation in the three cavity mirrors, so that the position of the cavity mirror cannot be finely adjusted through the mirror frame to obtain the cavity mode signal. Therefore, in order to better adjust the position of the cavity mirror and quickly obtain the cavity mode signal without feedback, a red light source 16 can be introduced from the outside. After the red light passes through the reflector 17 and the folding reflector 18, the direction is consistent with the direction of the laser emitted by the DFB laser 3. The red light is emitted to the right or upward by controlling the folding reflector 18; when the red light is emitted upward, the laser will not be blocked by the folding reflector 18, and will reach the photodetector 15 through the focusing lens 14. Two small holes are placed on the optical path reaching the photodetector 15, and the laser passes through the small holes, thereby determining the specific emission route of the laser; when the red light is emitted to the right, the laser is blocked by the folding reflector 18, and the red light is passed through the two small holes by adjusting the reflector 17 and the folding reflector 18, so that the red light replaces the laser on this optical path. (Note: At this time, the small hole is still on the optical path and does not need to be removed.)

[0054] There is a certain order for adjusting and placing the cavity mirrors. First, the cavity mirror on the lower right side of the cavity (called right one), then the cavity mirror on the left side of the cavity (called left one), and finally the cavity mirror on the upper right side of the cavity (called right two). It is worth noting that the three cavity mirrors should be on the three-position adjustment frame at this time, so the placement of the cavity mirrors described below is to place the three-position adjustment frame. After obtaining the position of the optical cavity, the approximate positions of the three cavity mirrors can be determined.

[0055] At this time, there should be only two small holes and the red light reflected by the folding reflector 18 on the optical path where the optical cavity is placed. Place the right cavity mirror so that the red light passes through the center of the cavity mirror and the cavity mirror surface is perpendicular to the entire optical path. Adjust the mirror holder knob of the right cavity mirror so that the reflected light of the cavity mirror to the red light also passes through the two small holes and returns along the original path; place the left cavity mirror so that the red light also passes through the center of the cavity mirror and the mirror surface is perpendicular to the optical path. At this time, remove the two small holes and place the optical cavity 19 on the optical path. Align the two cavity mirrors with the cavity position, and then adjust the knob of the left cavity mirror so that the red light is emitted and then emitted from the center of the other port of the cavity; then place the right second cavity mirror so that the red light also passes through the center and the mirror surface is perpendicular to the optical path. Adjust the knob of the right second cavity mirror so that the red light can be reflected by the right second cavity mirror and return to the left first cavity mirror, and can also be reflected to the right first cavity mirror, so that the red light is emitted back and forth on the three reflectors, and the rough adjustment of the positions of the three cavity mirrors is completed; flip the folding reflector 18 again, and after the laser emitted by the DFB laser 3 is introduced, the cavity mode signal is displayed on the oscilloscope 20. At this time, the signal may be messy, but only by adjusting the knobs of the right first cavity mirror and the right second cavity mirror, the cavity mode signal without feedback can be adjusted, and the cavity mode signal is not affected by high-order stray modes. At this time, the three cavity mirrors are glued to the cavity.

Claims

1. A 13CH4 isotope detection device based on optical feedback cavity enhancement, characterized in that: include: Function generator (1), laser controller (2), DFB laser (3), photodetector (15), optical cavity (19) and oscilloscope (20); The output end of the function generator (1) is connected to the input end of the laser controller (2), the output end of the laser controller (2) is connected to the input end of the DFB laser (3), the DFB laser (3) outputs laser light, the laser light passes through an isolator (5), a polarizing plate (6), a first lens (7), a quarter wave plate (8), a second lens (9), a first reflector (10), and a second reflector (11) in sequence and enters an optical cavity (19), the cavity mode signal of the optical cavity (19) passes through a focusing lens (14) and enters a photodetector (15), the output end of the photodetector (15) is connected to the input end of an oscilloscope (20).

2. The 13CH4 isotope detection device based on optical feedback cavity enhancement according to claim 1, characterized in that: The optical cavity (19) is a V-shaped cavity, comprising a cavity body and three cavity mirrors with high reflectivity adhered to the cavity body.

3. The 13CH4 isotope detection device based on optical feedback cavity enhancement according to claim 2, characterized in that: The three cavity mirrors with high reflectivity are adjusted in position by using three three-dimensional adjustment frames through the mirror frames, and the following conditions must be met: 1) the centers of the three cavity mirrors with high reflectivity are aligned with the centers of the holes left on the cavity; 2) After the laser is incident on the optical cavity (19), a cavity mode signal without feedback is displayed on the oscilloscope (20), and the cavity mode signal is not affected by high-order heterogeneous modes.

4. The 13CH4 isotope detection device based on optical feedback cavity enhancement according to claim 3, characterized in that: The optical path distance from the DFB laser (3) to the leftmost end of the optical cavity (19) is an odd multiple of the cavity length of the optical cavity (19).

5. The 13CH4 isotope detection device based on optical feedback cavity enhancement according to claim 4, characterized in that: A PZT (12) is provided on the second reflector (11), and an input end of the PZT (12) is connected to an output end of a high-voltage amplifier (13).

6. The 13CH4 isotope detection device based on optical feedback cavity enhancement according to claim 5, characterized in that: It also comprises a precision displacement stage (4), on which the DFB laser (3) is arranged.

7. A 13CH4 isotope detection method based on optical feedback cavity enhancement, characterized in that: The following steps are involved: Step 1, assembling the 13CH4 isotope detection device based on optical feedback cavity enhancement as described in any one of claims 5 or 6; Step 2, remove the isolator (5), adjust the quarter wave plate (8), obtain the cavity mode signal with feedback on the oscilloscope (20), and during operation, control the change of PZT (12) through the high voltage amplifier (13) to accurately control the phase; Step 3, introducing the methane to be measured into the cavity, obtaining the absorption of C13 and C12 on the oscilloscope (20), and then collecting data through a data acquisition card, and analyzing it using a MATLAB program to obtain the concentration accuracy of C13 and C12 in the methane gas and the carbon isotope measurement accuracy.

8. The 13CH4 isotope detection method based on optical feedback cavity enhancement according to claim 7, characterized in that: The assembling of the 13CH4 isotope detection device based on optical feedback cavity enhancement according to any one of claims 5 or 6 in step 1 is specifically: Step 1.1, the laser controller (2) drives the DFB laser (3) to emit laser light, which passes through the isolator (5), the polarizer (6), the first lens (7), the quarter wave plate (8), the second lens (9), the first reflector (10), the second reflector (11), and the focusing lens (14) in sequence and enters the photodetector (15); Step 1.2, using a function generator (1) to output a triangular wave scanning signal and a square wave triggering signal, and providing them to a laser controller (2) and an oscilloscope (20) respectively, and obtaining a laser signal of a DFB laser (3) in the oscilloscope (20); Step 1.3, determine the position of the optical cavity (19), adjust the positions of three cavity mirrors with high reflectivity of the optical cavity (19), and stick them to the cavity body of the optical cavity (19).

9. The 13CH4 isotope detection method based on optical feedback cavity enhancement according to claim 8, characterized in that: The position of the optical cavity (19) in step 1.3 is as follows: the optical path distance from the DFB laser (3) to the leftmost end of the optical cavity (19) should be an odd multiple of the cavity length of the optical cavity (19). The positions of the three cavity mirrors with high reflectivity of the optical cavity (19) are adjusted by using three three-dimensional adjustment frames through the mirror frames, and the positions thereof must meet the following requirements: 1) the centers of the three cavity mirrors with high reflectivity are aligned with the centers of the holes left on the cavity; 2) After the laser is incident on the optical cavity (19), a cavity mode signal without feedback is displayed on the oscilloscope (20), and the cavity mode signal is not affected by high-order heterogeneous modes.

10. The 13CH4 isotope detection method based on optical feedback cavity enhancement according to claim 9, characterized in that: The position of the three cavity mirrors with high reflectivity of the optical cavity (19) is adjusted as follows: A red light source (16) is introduced. After the red light passes through a reflector (17) and a folding reflector (18), the direction of the red light is consistent with the direction of the laser light emitted by the DFB laser (3). The folding reflector (18) is controlled to make the red light emit to the right or upward. When the red light emits upward, the laser light is not blocked by the folding reflector (18) and reaches the photodetector (15) through a focusing lens (14). Two small holes are placed on the light path that reaches the photodetector (15), and the laser light is allowed to pass through the small holes, thereby determining the specific emission route of the laser light. When the red light is emitted to the right, the laser light is blocked by the folding reflector (18), and the red light is allowed to pass through the two small holes by adjusting the reflector (17) and the folding reflector (18), thereby replacing the laser light with the red light on this optical path; Place the right cavity mirror so that the red light passes through the center of the cavity mirror and the cavity mirror surface is perpendicular to the entire optical path. Adjust the mirror holder knob of the right cavity mirror so that the reflected light of the cavity mirror to the red light also passes through the two small holes and returns along the original path. Place the left cavity mirror so that the red light passes through the center of the cavity mirror and the mirror surface is perpendicular to the optical path. At this time, remove the two small holes and place the optical cavity (19) on the optical path. Align the two cavity mirrors with the cavity position, and then adjust the knob of the left cavity mirror so that the red light is emitted from the center of the other port of the cavity. Then place the second right cavity mirror, and make the red light pass through the center and the mirror surface perpendicular to the optical path. Adjust the knob of the second right cavity mirror so that the red light can be reflected by the second right cavity mirror and return to the first left cavity mirror, and then can be reflected to the first right cavity mirror, so that the red light is emitted back and forth on the three reflectors, and the rough adjustment of the positions of the three cavity mirrors is completed; The folding reflector (18) is flipped again, and after the laser light emitted by the DFB laser (3) is introduced, the cavity mode signal is displayed on the oscilloscope (20). By adjusting the knobs of the first right cavity mirror and the second right cavity mirror, a cavity mode signal without feedback can be adjusted, and the cavity mode signal is not affected by high-order heterogeneous modes. At this time, the three cavity mirrors are glued to the cavity.

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