Frequency stabilization system of microcavity optical comb based on saturated absorption spectrum and CPT effect

Abstract

The application relates to a microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect, and belongs to the microcavity optical comb and wavelength frequency stabilization field.The application comprises a microcavity optical comb module, a pump light source module, a saturated absorption spectrum frequency stabilization module and a CPT frequency stabilization module.The pump light module comprises a pump laser and an optical amplifier.The saturated absorption spectrum frequency stabilization module comprises an atomic cell physical system and an optical path for realizing saturated absorption effect.The microcavity optical comb excitation is realized by a thermal tuning method, sidebands are generated by modulating the pump light, the decoupling of the microcavity optical comb repetition frequency and the microcavity optical comb pump light is realized based on the principle of microcavity injection locking, and the microcavity optical comb repetition frequency is equal to the alkali metal atom ground state hyperfine level frequency difference.The CPT frequency stabilization module comprises an atomic cell physical system, the CPT effect is excited by the microcavity optical comb pump light and the sideband light, and the microcavity optical comb repetition frequency is referenced to the atomic transition frequency.The two degrees of freedom of the microcavity optical comb are stabilized based on the saturated absorption spectrum and the CPT effect, so that the frequency of the microcavity optical comb is stabilized.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of microcavity optical comb based frequency stabilization system, especially a kind of microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect, belong to microcavity optical comb, laser frequency stabilization field. BACKGROUND

[0002] Optical frequency comb is periodic pulse sequence in time domain, is equal frequency interval comb in frequency domain, comb (each frequency component) has coherence, has stable phase relationship.Based on optical frequency comb can realize from optical frequency to microwave frequency link, thus be regarded as the most effective optical frequency measurement tool, has wide application in spectroscopy, laser precision measurement field.Especially the invention of optical frequency comb f-2f self-reference locking technology, greatly improves the frequency stability of optical frequency comb, makes each frequency component of optical frequency comb have traceability, makes optical frequency comb can play a huge role in precision measurement and metrology field.In recent years, with the development of new generation high-speed optical communication and microwave photonics technology, need higher optical comb repetition frequency to meet system demand.Based on the microcavity optical comb generated by continuous light pumped high quality factor microcavity provides new technical possibility.When pump laser is in red detuning, microcavity kerr soliton optical frequency comb can be realized based on microcavity, with the coherence of optical frequency comb comb, due to the filtering effect of microcavity, comb has the characteristic of narrow linewidth.And due to the small size of microcavity, large free spectral range, can produce very high repetition frequency microcavity optical comb, greatly expands the application range of optical frequency comb.But, it is extremely difficult to obtain octave frequency of microcavity optical comb, it is difficult to realize optical frequency comb self-reference locking based on f-2f method.The existing technical scheme is based on the octave frequency microcavity optical comb of terahertz repetition frequency and the double optical comb interlocking of microwave repetition frequency microcavity optical comb, realizes the full frequency domain frequency stabilization of optical frequency comb, system is complex, and it is difficult.The other hand, since microcavity optical comb belongs to complex dynamic system, microcavity optical comb repetition frequency and pump light coupling together, cannot be independently tuned.Further increase the difficulty of microcavity optical comb frequency stabilization. SUMMARY

[0003] The purpose of the present application is to provide a kind of microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect, based on the principle of heat-assisted laser, using the method of microcavity thermal tuning to excite microcavity optical comb, utilizes microcavity injection locking effect to realize the decoupling of microcavity optical comb repetition frequency and pump light.Based on saturated absorption spectrum and CPT effect, pump light and microcavity optical comb repetition frequency are locked respectively.Based on the characteristics that the spacing of microcavity optical comb comb is equal to the repetition frequency of microcavity optical comb, by stabilizing comb spacing and pump comb, each comb of microcavity optical comb is referenced to atomic transition frequency, so as to realize the frequency stabilization of microcavity optical comb.The present application can reduce the loop noise of frequency stabilization system, improve the frequency stability of frequency stabilization system.

[0004] The purpose of the present application is achieved by the following technical solutions:

[0005] The microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect disclosed by the present application comprises a microcavity optical comb module, a pump light source module, a saturated absorption spectrum frequency stabilization module and a CPT frequency stabilization module. The pump light module comprises a pump laser and an optical amplifier. The saturated absorption module comprises a first atomic cell physical system and an optical path for realizing saturated absorption effect. The microcavity optical comb module comprises microcavity optical comb pump light and auxiliary laser. The microcavity optical comb is excited by a thermal tuning method. Sidebands are generated by modulating the pump light. The microcavity optical comb repetition frequency and the microcavity optical comb pump light are decoupled based on the principle of microcavity injection locking. The microcavity optical comb repetition frequency is equal to the alkali metal atomic ground state hyperfine level frequency difference, which is used to excite the CPT effect. The CPT frequency stabilization module comprises a second atomic cell physical system. The CPT effect is excited by the microcavity optical comb pump light and its sideband light, so as to refer the microcavity optical comb repetition frequency to the atomic transition frequency. The two degrees of freedom of the microcavity optical comb are stabilized based on the saturated absorption spectrum and the CPT effect, so as to realize the frequency stabilization of the microcavity optical comb.

[0006] The pump light source module emits pump light. The pump light is split into first pump light and second pump light. The first pump light enters the saturated absorption spectrum frequency stabilization module after frequency doubling. The saturated absorption spectrum frequency stabilization module uses the method of saturated absorption spectrum to frequency discriminate the first pump light. The frequency of the pump light source module is controlled by PID feedback, so as to realize the frequency stabilization of the pump light source.

[0007] The microcavity optical comb module uses the microcavity optical comb pump light and auxiliary laser split by the second pump light as input. The microcavity optical comb is excited by the method of thermally tuning the microcavity based on the principle of thermal auxiliary laser. The microcavity optical comb is detected by a photodetector, and a microwave equal to the microcavity optical comb repetition frequency is obtained. The microcavity optical comb pump light is modulated by the microwave to generate sideband light. The two form coherent dual-color light. The coherent dual-color light is split to generate first coherent dual-color light and second coherent dual-color light. The first coherent dual-color light is injected into the microcavity. Based on the principle of microcavity injection locking, the microcavity optical comb repetition frequency and the microcavity optical comb pump light are decoupled. The second coherent dual-color light is frequency-doubled and injected into the CPT frequency stabilization module to realize the frequency discrimination of the microcavity optical comb repetition frequency. The frequency of the auxiliary laser is controlled by the method of PID feedback to control the repetition frequency of the microcavity optical comb, so as to realize the frequency stabilization of the microcavity optical comb repetition frequency.

[0008] The first atomic cell physical system and the second atomic cell physical system use the same atom and the same fine level transition line.

[0009] The free spectral range value of the microcavity is equal to the microcavity optical comb repetition frequency. The free spectral range value of the microcavity is designed to make the microcavity optical comb repetition frequency equal to half of the ground state hyperfine level frequency difference of the used atom.

[0010] In the microcavity optical comb module, the auxiliary laser is shifted to the blue end by an acousto-optic frequency shifter for compensating the thermal effect of the microcavity. The microcavity optical comb pump light is first modulated by a phase modulator to generate sidebands, and then split to generate first and second coherent dual-color lights. The first coherent dual-color light provides a gain effect to compensate the dissipation of the microcavity.

[0011] The spectrum output by the microcavity optical comb module is a soliton optical frequency comb. On the one hand, the comb tooth spacing of the soliton optical frequency comb is equal to the repetition frequency of the microcavity optical comb, and the repetition frequency of the microcavity optical comb is referenced to the atomic transition frequency by the CPT frequency stabilization module; on the other hand, the pump light corresponds to a comb tooth of the soliton optical frequency comb, and the pump light is referenced to the saturated absorption spectrum frequency stabilization module. Based on the characteristics of the soliton optical frequency comb, the frequency stability of the pump light is transmitted by the stable comb tooth spacing, and the frequency stabilization of the microcavity optical frequency comb is realized.

[0012] Advantages:

[0013] 1. The microcavity optical comb frequency stabilization system based on the saturated absorption spectrum and the CPT effect disclosed in the application realizes the microcavity optical comb by using the method of microcavity thermal tuning based on the principle of auxiliary laser, and realizes the decoupling of the repetition frequency of the microcavity optical comb and the wavelength of the pump light by using the microcavity injection locking effect. It makes it possible to stabilize the frequency of the microcavity optical comb by using the stable pump light and the repetition frequency of the microcavity optical comb. Thus, it is not necessary to expand the microcavity optical comb to an octave, avoiding the shortcomings of the f-2f optical comb frequency stabilization, and reducing the difficulty and complexity of the microcavity optical comb frequency stabilization.

[0014] 2. The existing laser frequency stabilization is mostly realized based on an atomic clock, and needs to pass through part of the microwave link to realize the frequency stabilization. The microcavity optical comb frequency stabilization system based on the saturated absorption spectrum and the CPT effect disclosed in the application stabilizes the frequency of the pump light by using the saturated absorption spectrum, and stabilizes the repetition frequency of the microcavity optical comb by using the CPT frequency stabilization, realizing the all-optical link frequency stabilization, and having low system noise and strong anti-interference ability. At the same time, the microcavity optical comb and the CPT frequency stabilization system have the potential of miniaturization and chipization, and have advantages in small volume and low power consumption application scenarios.

[0015] 3. The microcavity optical comb frequency stabilization system based on the saturated absorption spectrum and the CPT effect disclosed in the application adopts the mode of stabilizing the repetition frequency of the microcavity optical comb and the comb tooth of the pump light, transmits the frequency stability of the pump light source to other optical comb teeth through the stable repetition frequency, and realizes the frequency stabilization of the full spectrum. It can realize multi-wavelength standard application. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The microcavity optical comb frequency stabilization system based on the saturated absorption spectrum and the CPT effect of the application;

[0017] It is divided into four modules, a-saturation absorption spectrum frequency stabilization module; b-microcavity optical comb module; c-CPT frequency stabilization module; d-pump light source module.

[0018] Wherein, a-saturation absorption spectrum frequency stabilization module comprises: 1-first photodetector, 2-first beam splitter, 3-first atomic gas chamber, 4-first magnetic shielding cylinder, 5-second beam splitter, 6-first lens, 7-first frequency doubling crystal. a-31-first mirror, a-32-second mirror, a-33-third mirror.

[0019] b-microcavity optical comb module comprises: 9-third beam splitter, 10-suppressed carrier single sideband modulator, 11-acoustooptic modulator, 12-fourth beam splitter, 13-first EDFA, 14-second EDFA, 15-first polarization controller, 16-second polarization controller, 17-first circulator, 18-second circulator, 19-second photodetector, 20-third photodetector, 21-fifth beam splitter, b-34-fourth mirror, 39-second PID, 41-signal source, 42-phase locked amplifier.

[0020] c-CPT frequency stabilization module comprises: 22-second frequency doubling crystal, 23-second lens, 24-adjustable optical attenuator, 25-1 / 4 wave plate, 26-sixth beam splitter, 27-second atomic gas chamber, 28-fourth photodetector, 29-second magnetic shielding cylinder, 30-fifth photodetector, c-35-fifth mirror, c-36-sixth mirror, 40-third PID.

[0021] d-pump light source module comprises: 8-seventh beam splitter, 37-narrow linewidth laser, 38-first PID. DETAILED DESCRIPTION

[0022] In order to better illustrate the purpose and advantages of the present application, the content of the invention is further illustrated below in combination with the drawings and examples.

[0023] Example 1:

[0024] As Figure 1As shown, the embodiment discloses a microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect, which comprises a microcavity optical comb module b, a pump light source module d, a saturated absorption spectrum frequency stabilization module a, and a CPT frequency stabilization module c. The pump light module d comprises a narrow linewidth laser 37 with a wavelength of 1550 nm as a pump laser, and the output power is greater than 100 mW. The saturated absorption module a comprises a first beam splitter 2, a first atomic cell 3, a first magnetic shielding cylinder 4, a second beam splitter 5, a first lens 6, and a first frequency doubling crystal 7. The first mirror a-31, the second mirror a-32, and the third mirror a-33 constitute a first atomic cell physical system. The first photodetector 1 is used to detect the signal light of the saturated absorption spectrum, and is used to realize saturated absorption spectrum frequency discrimination. The microcavity optical comb module b comprises a light beam 2-2 as a microcavity optical comb pump light, and a light beam 2-1 as auxiliary laser. The microcavity optical comb is excited by a thermal tuning method. The light beam 2-2 is modulated by a single sideband modulator 10 to generate a sideband, which is used as coherent double-color light. Based on the principle of microcavity injection locking, the repetition frequency of the microcavity optical comb and the pump light of the microcavity optical comb are decoupled, and the repetition frequency of the microcavity optical comb is equal to 87 The Rb atom ground state hyperfine level frequency difference is used to excite the CPT effect. The CPT frequency stabilization module c comprises a second lens 23, an adjustable optical attenuator 24, a 1 / 4 wave plate 25, a sixth beam splitter 26, a second atomic cell 27, a second magnetic shielding cylinder 29, a fifth mirror c-35, and a sixth mirror c-36, which constitute a second atomic cell physical system. The coherent double-color light formed by the microcavity optical comb pump light and its sideband light passes through the fourth beam splitter 12 to generate a light beam 2-2-2. After frequency doubling by the second frequency doubling crystal 22, the CPT effect is excited, so that the microcavity optical comb repetition frequency is referenced to the atomic transition frequency. Based on the saturated absorption spectrum and the CPT effect, the two degrees of freedom of the microcavity optical comb are stabilized, so that the frequency of the microcavity optical comb is stabilized.

[0025] The working method of the microcavity optical comb frequency stabilization system based on the saturated absorption spectrum and the CPT effect disclosed in the embodiment is as follows:

[0026] First, the pump light source module d emits pump light. The pump light passes through the seventh beam splitter 8 and is split into light beam 1 as the first pump light and light beam 2 as the second pump light. The light beam 1 passes through the first frequency doubling crystal 7, is reflected by the first mirror a-31, is focused by the first lens 6, is split by the second beam splitter 5 into light beam 1-1 as alkali metal atom pumping light, and enters the first atomic cell 3 after being reflected by the second mirror a-32, the third mirror a-33, and the first beam splitter 2. The light beam 1-2 directly enters the first atomic cell 3 as a probe light, and the saturated absorption signal is detected by the first photodetector 1 to realize the frequency discrimination function of the saturated absorption spectrum frequency stabilization module. The frequency of the pump light source module d is stabilized by controlling the narrow linewidth laser 37 by the first PID 38.

[0027] The light beam 2 is used as the second pump light for pumping the microcavity optical comb module b. The second pump light is split by the third beam splitter 9 to form the light beam 2-2 as the microcavity optical comb pump light and the light beam 2-1 as the auxiliary laser. Both of them pump the microcavity from two ends. The light beam 2-2 passes through the suppressed carrier single sideband modulator 10 to modulate coherent dual-color light, and is split by the fourth beam splitter 12. The light beam 2-2-1 passes through the first EDFA 13 for amplification, and passes through the first polarization controller 15 and the first circulator 17 to be injected into the microcavity. After the light beam 2-1 passes through the acousto-optic frequency shifter 11 to be shifted by 80 MHz, it is amplified by the second EDFA 14, and passes through the second polarization controller 16 and the second circulator 18 to be injected into the microcavity from the other end. The powers of the first EDFA 13 and the second EDFA 14 are adjusted to be matched, and the power monitoring is performed by the second photodetector 19. The method of heating the microcavity is used until the microcavity optical comb at the port of the second circulator 18, and the output light beam is split by the fifth beam splitter 21 into the light beam a and the light beam b. The light beam b is the microcavity optical comb output. The light beam a is detected by the third photodetector 20 to obtain a microwave signal equal to the repetition frequency of the microcavity optical comb. The microwave signal is used to drive the suppressed carrier single sideband modulator 10 to realize the closed-loop modulation of the light beam 2-2 to generate sideband light and form coherent dual-color light. The fourth beam splitter 12 generates the light beam 2-2-1 as the first coherent dual-color light and the light beam 2-2-2 as the second coherent dual-color light. The first coherent dual-color light is injected into the microcavity to realize the repetition frequency of the microcavity optical comb and the decoupling of the microcavity optical comb pump light based on the microcavity injection locking principle. The second coherent dual-color light is frequency-doubled by the second frequency-doubling crystal 22, reflected by the sixth mirror c-36, focused by the second lens 23, adjusted in light power by the adjustable optical attenuator 24, adjusted to circularly polarized light by the 1 / 4 wave plate 25, split by the sixth beam splitter 26, and reflected by the fifth mirror c-35. The light passes through the fifth photodetector 30 and the third PID 40 to control the adjustable optical attenuator 24 to stabilize the light power. The other light is directly injected into the second atomic gas chamber 27 containing Rb atoms, and the CPT transmission signal light is detected by the fourth photodetector 28 to realize frequency discrimination. The phase-sensitive demodulation differentiation is performed by the lock-in amplifier 42, and the output frequency of the 80 MHz signal source is feedback controlled by the second PID 39 to realize the frequency stabilization of the microcavity optical comb repetition frequency based on the CPT frequency stabilization module c. The signal source 41 outputs a signal to modulate the 80 MHz signal source, and provides a reference signal to the lock-in amplifier 42 to support the phase-sensitive demodulation differentiation function. The auxiliary laser of the microcavity optical comb module b is shifted to the blue end by the acousto-optic frequency shifter 11 to compensate for the thermal effect of the microcavity. In addition, the first coherent dual-color light provides a gain effect to compensate for the dissipation of the microcavity. 87 Rb atoms, and the CPT transmission signal light is detected by the fourth photodetector 28 to realize frequency discrimination. The phase-sensitive demodulation differentiation is performed by the lock-in amplifier 42, and the output frequency of the 80 MHz signal source is feedback controlled by the second PID 39 to realize the frequency stabilization of the microcavity optical comb repetition frequency based on the CPT frequency stabilization module c. The signal source 41 outputs a signal to modulate the 80 MHz signal source, and provides a reference signal to the lock-in amplifier 42 to support the phase-sensitive demodulation differentiation function. The auxiliary laser of the microcavity optical comb module b is shifted to the blue end by the acousto-optic frequency shifter 11 to compensate for the thermal effect of the microcavity. In addition, the first coherent dual-color light provides a gain effect to compensate for the dissipation of the microcavity.

[0028] The free spectral range value of the microcavity needs to be designed as 3.417 GHz, so that the repetition frequency of the microcavity optical comb is equal to 87 Half of the ground state hyperfine level frequency difference of Rb atoms.

[0029] Finally, the output spectrum of the microcavity optical comb module b is a soliton optical frequency comb. On the one hand, the comb tooth spacing of the soliton optical frequency comb is equal to the repetition frequency of the microcavity optical comb, and the repetition frequency of the microcavity optical comb is referenced to the atomic transition frequency through the CPT frequency stabilization module c; on the other hand, the pump light corresponds to a comb tooth of the soliton optical frequency comb, and the pump light is referenced to the saturated absorption spectrum frequency stabilization module a. Based on the characteristics of the soliton optical frequency comb, the frequency stability of the pump light is transferred from the stable comb tooth spacing, thereby realizing the frequency stabilization of the microcavity optical comb.

[0030] Due to the fixed correspondence between the comb teeth, the repetition frequency of the microcavity optical comb is referenced to the atomic transition spectrum, and at the same time, one comb tooth is also referenced to the atomic transition, which can realize the frequency stabilization of the microcavity optical comb. The relationship between the mth comb tooth and the nth comb tooth is as follows:

[0031] f m = f n +(m-n)f rep

[0032] Where f m represents the frequency of the mth comb tooth, f n represents the frequency of the nth comb tooth, and f rep represents the repetition frequency of the microcavity optical comb.

[0033] The above specific description further details the purpose, technical scheme and beneficial effects of the application. It should be understood that the above description is only a specific embodiment of the application and is not used to limit the protection scope of the application. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the application should be included in the protection scope of the application.

Claims

1. A microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect, characterized in that, The system includes a microcavity optical comb module, a pump light source module, a saturable absorption spectrum frequency stabilization module, and a CPT frequency stabilization module. The pump light module includes a pump laser and an optical amplifier. The saturable absorption module includes a first atomic gas cell physical system and an optical path for realizing the saturable absorption effect. The microcavity optical comb module includes a microcavity optical comb pump light and an auxiliary laser. The microcavity optical comb is excited by a thermal tuning method, and sidebands are generated by modulating the pump light. Based on the principle of microcavity injection locking, the repetition frequency of the microcavity optical comb and the pump light are decoupled. The repetition frequency of the microcavity optical comb is equal to the frequency difference of the hyperfine energy level of the ground state of the alkali metal atom, which is used to excite the CPT effect. The CPT frequency stabilization module includes a second atomic gas cell physical system. The CPT effect is excited by the microcavity optical comb pump light and its sideband light, thereby referencing the repetition frequency of the microcavity optical comb to the atomic transition frequency. The spectrum output by the microcavity optical comb module is a soliton optical frequency comb. On the one hand, the soliton optical frequency comb tooth spacing is equal to the microcavity optical comb repetition frequency, and the microcavity optical frequency comb repetition frequency is referenced to the atomic transition frequency through the CPT frequency stabilization module. On the other hand, the pump light corresponds to one tooth of the soliton optical frequency comb, and the pump light is referenced to the saturated absorption spectrum frequency stabilization module. Based on the characteristics of the soliton optical frequency comb, the frequency stability of the pump light is transmitted by the stable tooth spacing, thereby realizing the frequency stabilization of the microcavity optical frequency comb.

2. The microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect as described in claim 1, characterized in that, The pump light source module emits pump light, which is split into a first pump light and a second pump light. The first pump light is frequency-doubled and then enters the saturated absorption spectrum frequency stabilization module. The saturated absorption spectrum frequency stabilization module uses the saturated absorption spectrum method to discriminate the first pump light and controls the output frequency of the pump light source module through PID feedback, thereby achieving frequency stabilization of the pump light source.

3. The microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect as described in claim 1, characterized in that, The microcavity optical comb module uses the pump light of the microcavity optical comb, which is split by the second pump light, and the auxiliary laser as inputs; based on the principle of thermally assisted laser, the microcavity optical comb is excited by thermally tuned microcavity; a photodetector detects the microcavity optical comb to obtain microwaves with the same repetition frequency as the microcavity optical comb; Microwave modulation of the microcavity optical comb pump light generates sideband light, which together form coherent dichromatic light. The coherent dichromatic light is then split to generate a first coherent dichromatic light and a second coherent dichromatic light. The first coherent dichromatic light is injected into the microcavity, and based on the microcavity injection locking principle, the repetition frequency of the microcavity optical comb is decoupled from the pump light of the microcavity optical comb. The second coherent two-color light is frequency-doubled and injected into the CPT frequency stabilization module to achieve frequency discrimination of the microcavity optical comb repetition frequency. By using PID feedback control of the auxiliary laser frequency, the repetition frequency of the microcavity optical comb is adjusted, thereby achieving frequency stabilization of the microcavity optical speed repetition frequency.

4. The microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect as described in claim 1, characterized in that, The first atomic gas chamber physics system uses the same atoms as the second atomic gas chamber physics system, and uses the same fine level transition lines.

5. The microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect as described in claim 4, characterized in that, In the microcavity optical comb module, since the free spectral range of the microcavity is equal to the repetition frequency of the microcavity optical comb, the repetition frequency of the microcavity optical comb is made equal to half of the hyperfine energy level difference of the ground state of the atoms used by designing the free spectral range of the microcavity.

6. The microcavity optical comb frequency stabilization system based on saturated absorption spectrum and CPT effect as described in claim 4, characterized in that, In the microcavity optical comb module, the auxiliary laser is shifted to the blue end via an acousto-optic frequency shifter to compensate for the microcavity thermal effect. The pump light of the microcavity optical comb is first modulated by a phase modulator to generate sidebands, and then split to generate a first coherent two-color light and a second coherent two-color light. The first coherent two-color light provides a gain effect to compensate for the dissipation of the microcavity.

Images