System and method for locking laser frequency based on hollow-core optical fiber FP resonant cavity

Abstract

A system and method for locking laser frequency based on a hollow fiber FP resonant cavity are characterized in that the output end of a laser is connected with the first port of an optical fiber circulator, the second port of the optical fiber circulator is connected with the input end of an optical fiber coupler, and the first output end of the optical fiber coupler is connected with the input end of a polarization controller; a second output end of the optical fiber coupler is connected with an input end of the optical trap, an output end of the polarization controller is connected with an input end of the hollow-core optical fiber Fabry-Perot resonant cavity, and an output end of the hollow-core optical fiber Fabry-Perot resonant cavity is connected with a third port of the optical fiber circulator; the second output end of the optical fiber coupler is connected with the input end of the optical trap, the second output end of the optical fiber coupler is aligned with the input end of the optical trap, and optical signals output by the second output end of the optical fiber coupler directly enter the optical trap. The system is simple in structure, low in implementation cost, wide in locking range, high in anti-interference capability and high in integration degree.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronic devices, and more specifically, relates to a system and method for locking the frequency of a laser based on a hollow-core fiber FP resonator. Background Art

[0002] The stabilization of the laser frequency is of extremely important significance in scientific research and industrial applications, mainly reflected in precision measurement, quantum technologies (such as quantum computing, quantum communication, and quantum sensing), optical communication, industrial processing (such as laser cutting, welding, and microfabrication), scientific research (such as cold atom physics, nonlinear optics, and quantum optics), and medical applications (such as laser surgery and laser therapy).

[0003] CN119542891A discloses a narrow-linewidth fiber laser system based on a self-injection locking feedback mechanism, belonging to the field of fiber optic communication. The system includes a main cavity, a self-injection locking feedback module, and an erbium-doped fiber amplifier. Among them, the inside of the main cavity includes a composite ring cavity filter for selecting longitudinal modes. The composite ring cavity filter is sequentially connected to a polarization controller, a first erbium-doped fiber, a first wavelength division multiplexer, a second erbium-doped fiber, and a first uniform fiber grating through a first circulator. The output end of the first uniform fiber grating is connected to the input end of a first fiber coupler. One output end of the first fiber coupler is sequentially connected to a fiber delay line and a Faraday rotator in the self-injection locking feedback module, and the rotation angle of the Faraday rotator is 90°. The other output end of the first fiber coupler is connected to the input end of the erbium-doped fiber amplifier. However, this system uses a large number of optical components, with a high system structure complexity and implementation cost, a narrow frequency locking range, and is not suitable for miniaturized integration.

[0004] CN105356294A discloses a tunable narrow-linewidth semiconductor laser, including a distributed feedback semiconductor laser, a current driver, a first temperature controller, a polarization-maintaining fiber circulator, a polarization-maintaining fiber coupler, a polarization-maintaining fiber grating Fabry-Perot cavity, a second temperature controller, a polarization-maintaining fiber variable optical attenuator, and a thermal and vibration isolation housing. This device uses the transmission peak of the fiber grating Fabry-Perot cavity to perform optical self-injection locking on the semiconductor laser to achieve noise suppression of the semiconductor laser, and realizes narrow-linewidth quasi-continuous tunable laser output by adjusting the temperature and current of the semiconductor laser. However, this system uses multiple polarization-maintaining fiber components, with a high overall cost, and will cause an increase in fiber coupling loss, reducing the output efficiency of the laser. The specific tuning range of this system is limited by the fiber grating Fabry-Perot cavity and cannot achieve a wide range of wavelength tuning. And due to its frequency locking principle, it is not applicable to lasers other than semiconductor types or more extensive application scenarios. Summary of the Invention

[0005] In view of the deficiencies of traditional laser frequency stabilization technology, the present invention provides a system and method for locking the frequency of a laser based on a hollow-core fiber Fabry-Perot resonator, thereby solving many problems existing in the prior art, such as complex system structure, high implementation cost, narrow locking range, weak anti-interference ability, low integration level, and long length of HCF used.

[0006] The present invention adopts the following technical solutions.

[0007] A first aspect of the present invention proposes a system for locking the frequency of a laser based on a hollow-core fiber Fabry-Perot resonator, including: laser 1, optical fiber circulator 2, optical fiber coupler 3, polarization controller 4, hollow-core fiber Fabry-Perot resonator 5, first outer shell 6, second outer shell 7, optical trap 8, characterized in that:

[0008] The output end of the laser is connected to the first port of the optical fiber circulator, the second port of the optical fiber circulator is connected to the input end of the optical fiber coupler, the first output end of the optical fiber coupler is connected to the input end of the polarization controller, the second output end of the optical fiber coupler is connected to the input end of the optical trap, the output end of the polarization controller is connected to the input end of the hollow-core fiber Fabry-Perot resonator, and the output end of the hollow-core fiber Fabry-Perot resonator is connected to the third port of the optical fiber circulator;

[0009] The hollow-core fiber Fabry-Perot resonator is located in the first outer shell, and the first outer shell is used to support and fix the hollow-core fiber Fabry-Perot resonator; the optical fiber circulator, optical fiber coupler, polarization controller, hollow-core fiber Fabry-Perot resonator, and first outer shell are all located in the second outer shell;

[0010] The connection between the second output end of the optical fiber coupler and the input end of the optical trap is the alignment of the second output end of the optical fiber coupler with the input end of the optical trap, and the optical signal output from the second output end of the optical fiber coupler directly enters the input end of the optical trap. All other connections in this system are connected through single-mode optical fibers.

[0011] Preferably, the optical fiber circulator has the functions of an optical coupler and an optical isolator. The function of the optical coupler is to couple the input from one port to the output of another port; the function of the optical isolator is to only allow the unidirectional transmission of light; the optical fiber circulator transmits the optical signal received at the first port to the second port and transmits the optical signal received at the third port to the first port.

[0012] Preferably, the optical fiber coupler used is a 1×2 optical fiber coupler, and the distribution ratio of the output of the second output end and the first output end of the optical fiber coupler is 1:9.

[0013] Preferably, the hollow-core fiber Fabry-Perot resonator includes: a first single-mode fiber 9, a second single-mode fiber 10, a first graded-index fiber 11, a second graded-index fiber 12, a first fiber sleeve 13, a second fiber sleeve 14, and a hollow-core fiber 15; one end of the first single-mode fiber 9 serves as the input end of the hollow-core fiber Fabry-Perot resonator, the other end of the first single-mode fiber 9 is connected to one end of the first graded-index fiber 11, and the other end of the first graded-index fiber 11 is connected to one end of the hollow-core fiber 15 through the first fiber sleeve 13; the other end of the hollow-core fiber 15 is connected to one end of the second graded-index fiber 12 through the second fiber sleeve 14, the other end of the second graded-index fiber 12 is connected to one end of the second single-mode fiber 10, and the other end of the second single-mode fiber 10 serves as the output end of the hollow-core fiber Fabry-Perot resonator; the fiber sleeves 13 and 14 are hollow transparent glass tubes, with one end adapted to the outer diameter of the graded-index fiber and the other end adapted to the outer diameter of the hollow-core fiber.

[0014] Preferably, for the first single-mode fiber 9 and the second single-mode fiber 10, their transmission bandwidths are both adapted to the output wavelength of the laser, and the transmission loss is not higher than 10 dB / km; and the end faces on the side close to the hollow-core fiber are flat-cut.

[0015] Preferably, one ends of the first graded-index fiber 11 and the second graded-index fiber 12 are flat-cut and then respectively fusion-spliced to the first single-mode fiber 9 and the second single-mode fiber 10, and the loss after fusion splicing is not higher than 0.05 dB; the other ends are flat-cut, polished, and then coated with a dielectric film, and the reflectivity of the dielectric film to the laser is not lower than 0.999.

[0016] Preferably, the first single-mode fiber 9, the second single-mode fiber 10, and the hollow-core fiber of the hollow-core fiber Fabry-Perot resonator are all fixed to the bottom plate of the first housing by fiber tape.

[0017] Preferably, the used hollow-core fiber includes a metal-coated hollow optical waveguide, a hollow-core photonic bandgap fiber, and a hollow-core anti-resonant fiber. Its transmission bandwidth is adapted to the output wavelength of the laser, and the transmission loss is not higher than 1 dB / km. The end faces of the two ports of the hollow-core fiber are flat-cut.

[0018] Preferably, the first housing is a metal aluminum box, the second housing is a polystyrene box, and the inside of the second housing is filled with gel packs whose heat capacity exceeds the set heat capacity threshold.

[0019] Preferably, the used laser is a semiconductor laser, a fiber laser, or a solid-state laser, and the laser is equipped with a built-in temperature and current control system.

[0020] The second aspect of the present invention proposes a method for locking the frequency of a laser based on a hollow-core fiber Fabry-Perot resonator using the system described in the first aspect of the present invention, which is characterized by including:

[0021] The laser emits an optical signal; the optical signal is coupled into the first port of the fiber optic circulator through a single-mode fiber and exits from the second port; the exiting optical signal is coupled into the fiber optic coupler 3 through a single-mode fiber and is distributed inside it in a ratio of 1:9; thereafter, 90% of the optical signal is modulated in polarization state by the polarization controller 4 and then coupled into the hollow-core fiber Fabry-Perot resonator 5 through a single-mode fiber; the remaining 10% of the optical signal is directly emitted and collected by the optical trap 8; the optical signal transmitted by the hollow-core fiber Fabry-Perot resonator 5 is coupled through a single-mode fiber, enters the third port of the fiber optic circulator 2, and exits from the first port; the exiting optical signal is coupled back to the laser through a single-mode fiber, realizing the frequency locking of the laser on the resonance frequency of the hollow-core fiber Fabry-Perot resonator.

[0022] The beneficial effects of the present invention are as follows. Compared with the prior art, the optical main part of the system of the present invention only includes a fiber optic circulator, a fiber optic coupler, a polarization controller, and a hollow-core fiber Fabry-Perot resonator, with a simple structure, low complexity, and low implementation cost, and is particularly suitable for miniaturized integration. The present invention uses a polarization controller to adjust the polarization state of the input light to the desired output polarization state, optimizing the polarization characteristics of the optical signal. The hollow-core fiber Fabry-Perot resonator provides a resonance frequency for the frequency locking of the laser, with a wide locking range, and uses a graded-index fiber with good coupling effect; and due to the excellent polarization purity of the hollow-core fiber, the adjusted polarization state can be stably maintained for several days. A polystyrene box is used to isolate heat fluctuations, thereby reducing energy exchange; and a gel pack with a large heat capacity is used to passively reduce temperature fluctuations. The system has strong anti-interference ability. Description of the Drawings

[0023] Figure 1 It is a schematic structural diagram of a system for locking the frequency of a laser based on a hollow-core fiber FP resonator;

[0024] Figure 2 It is a schematic structural diagram of a hollow-core fiber Fabry-Perot resonator. Detailed Embodiments

[0025] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

[0026] As shown Figure 1 in FIG. 1, Embodiment 1 of the present invention provides a system for locking the frequency of a laser based on a hollow fiber FP resonator, including: a laser 1, an optical fiber circulator 2, an optical fiber coupler 3, a polarization controller 4, a hollow fiber Fabry-Perot resonator 5, a first housing 6, a second housing 7, and an optical trap 8, characterized in that:

[0027] The output end of the laser is connected to the first port of the optical fiber circulator, the second port of the optical fiber circulator is connected to the input end of the optical fiber coupler, the first output end of the optical fiber coupler is connected to the input end of the polarization controller, the second output end of the optical fiber coupler is connected to the input end of the optical trap, the output end of the polarization controller is connected to the input end of the hollow fiber Fabry-Perot resonator, and the output end of the hollow fiber Fabry-Perot resonator is connected to the third port of the optical fiber circulator;

[0028] The hollow fiber Fabry-Perot resonator is located in the first housing, and the first housing is used to support and fix the hollow fiber Fabry-Perot resonator; the optical fiber circulator, the optical fiber coupler, the polarization controller, the hollow fiber Fabry-Perot resonator, and the first housing are all located in the second housing;

[0029] The connection between the second output end of the optical fiber coupler and the input end of the optical trap is to align the second output end of the optical fiber coupler with the input end of the optical trap, and the optical signal output from the second output end of the optical fiber coupler directly enters the input end of the optical trap. All other connections in this system are through single-mode optical fibers.

[0030] Among them, the optical fiber circulator has the functions of an optical coupler and an optical isolator. The function of the optical coupler is to couple the input from one port to the output of another port; the function of the optical isolator is to only allow the unidirectional transmission of light; the optical fiber circulator transmits the optical signal received at the first port to the second port and transmits the optical signal received at the third port to the first port.

[0031] The optical fiber coupler used is a 1×2 optical fiber coupler, and the distribution ratio of the output of the second output end and the first output end of the optical fiber coupler is 1:9.

[0032] As shown in the appendix Figure 2As shown in the figure, the hollow-core fiber Fabry-Perot resonator includes: a first single-mode fiber 9, a second single-mode fiber 10, a first graded-index fiber 11, a second graded-index fiber 12, a first fiber sleeve 13, a second fiber sleeve 14, and a hollow-core fiber 15; one end of the first single-mode fiber 9 serves as the input end of the hollow-core fiber Fabry-Perot resonator, the other end of the first single-mode fiber 9 is connected to one end of the first graded-index fiber 11, and the other end of the first graded-index fiber 11 is connected to one end of the hollow-core fiber 15 through the first fiber sleeve 13; the other end of the hollow-core fiber 15 is connected to one end of the second graded-index fiber 12 through the second fiber sleeve 14, the other end of the second graded-index fiber 12 is connected to one end of the second single-mode fiber 10, and the other end of the second single-mode fiber 10 serves as the output end of the hollow-core fiber Fabry-Perot resonator; the fiber sleeves 13 and 14 are hollow transparent glass tubes, with one end adapted to the outer diameter of the graded-index fiber and the other end adapted to the outer diameter of the hollow-core fiber.

[0033] The first single-mode fiber 9 and the second single-mode fiber 10 have a transmission bandwidth that is adapted to the output wavelength of the laser, and the transmission loss is not higher than 10 dB / km; and the end faces on the side close to the hollow-core fiber are flat-cut.

[0034] Specifically, the core diameters of the first single-mode fiber 9 and the second single-mode fiber 10 in this embodiment are 10.4 μm; the core diameter of the hollow-core fiber 15 is 24 μm, and the length of use is 30 m.

[0035] One ends of the first graded-index fiber 11 and the second graded-index fiber 12 are flat-cut and then respectively fused with the first single-mode fiber 9 and the second single-mode fiber 10, and the loss after fusion is not higher than 0.05 dB; the other ends are flat-cut, polished, and then coated with a dielectric film, and the reflectivity of the dielectric film to the laser is not lower than 0.999.

[0036] The first single-mode fiber 9, the second single-mode fiber 10, and the hollow-core fiber of the hollow-core fiber Fabry-Perot resonator are all fixed to the bottom plate of the first housing by fiber tape.

[0037] The hollow-core fiber used includes a metal-coated hollow optical waveguide, a hollow-core photonic bandgap fiber, and a hollow-core anti-resonant fiber. Its transmission bandwidth is adapted to the output wavelength of the laser, and the transmission loss is not higher than 1 dB / km. The end faces of the two ports of the hollow-core fiber are flat-cut.

[0038] It should be noted that the steps for constructing a hollow-core fiber Fabry-Perot resonator are as follows: ① First, sleeve the first and second fiber sleeves on both ends of the hollow-core fiber respectively; ② Then insert the graded-index fiber into the other end of the fiber sleeve; ③ Under a microscope, finely adjust the alignment of the first graded-index fiber 11 and the second graded-index fiber 12 with the hollow-core fiber; ④ When the optical field coupling is completed and the insertion loss of the coupling is not higher than 0.02 dB, quickly cure the joints of the fiber sleeve, the two graded-index fibers and the hollow-core fiber with ultraviolet glue.

[0039] The first outer shell is a metal aluminum box, the second outer shell is a polystyrene box, and the inside of the second outer shell is filled with gel packs whose heat capacity exceeds the set heat capacity threshold.

[0040] The laser used is a semiconductor laser, a fiber laser or a solid-state laser, and the laser is equipped with a temperature and current control system.

[0041] Specifically, the laser in this embodiment is a fiber laser, its output center wavelength is 1550 nm, and the output laser power is 5 dBm;

[0042] Embodiment 2 of the present invention proposes a method for locking the frequency of a laser based on a hollow-core fiber FP resonator using the system described in the first aspect of the present invention, which is characterized by including:

[0043] The laser emits an optical signal; the optical signal is coupled into the first port of the fiber circulator through a single-mode fiber and exits from the second port; the exiting optical signal is coupled into the fiber coupler 3 through a single-mode fiber and is distributed inside it in a ratio of 1:9; thereafter, 90% of the optical signal is modulated in polarization state by the polarization controller 4 and then coupled into the hollow-core fiber Fabry-Perot resonator 5 through a single-mode fiber; the remaining 10% of the optical signal is directly emitted and collected by the optical trap 8; the optical signal transmitted by the hollow-core fiber Fabry-Perot resonator 5 is coupled through a single-mode fiber, enters the third port of the fiber circulator 2, and exits from the first port; the exiting optical signal is coupled back to the laser through a single-mode fiber to lock the frequency of the laser on the resonance frequency of the hollow-core fiber Fabry-Perot resonator.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: still can modify the specific implementation manners of the present invention or make equivalent replacements, and any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A system for locking the laser frequency based on a hollow-core fiber FP resonant cavity, comprising: A laser (1), an optical fiber circulator (2), an optical fiber coupler (3), a polarization controller (4), a hollow-core optical fiber Fabry-Perot resonant cavity (5), a first housing (6), a second housing (7), and a light trap (8), characterized in that: The output end of the laser (1) is connected to the first port of the optical fiber circulator (2), the second port of the optical fiber circulator (2) is connected to the input end of the optical fiber coupler (3), the first output end of the optical fiber coupler (3) is connected to the input end of the polarization controller (4), the second output end of the optical fiber coupler (3) is connected to the input end of the light trap (8), the output end of the polarization controller (4) is connected to the input end of the hollow core optical fiber Fabry-Perot resonant cavity (5), and the output end of the hollow core optical fiber Fabry-Perot resonant cavity (5) is connected to the third port of the optical fiber circulator (2); The hollow-core optical fiber Fabry-Perot resonant cavity (5) is located in a first housing (6), and the first housing (6) is used to support and fix the hollow-core optical fiber Fabry-Perot resonant cavity (5); the optical fiber circulator (2), the optical fiber coupler (3), the polarization controller (4), the hollow-core optical fiber Fabry-Perot resonant cavity (5), and the first housing (6) are all located in a second housing (7); The connection between the second output end of the optical fiber coupler (3) and the input end of the light trap is that the second output end of the optical fiber coupler is aligned with the input end of the light trap (8), and the optical signal output by the second output end of the optical fiber coupler (3) is directly incident on the input end of the light trap (8). Apart from this, all other connections in the system are made through single-mode optical fibers.

2. A method and system for locking laser frequency based on hollow core fiber FP resonant cavity according to claim 1, characterized in that: The optical fiber circulator (2) couples the optical signal received at the first port and transmits it unidirectionally to the second port, and couples the optical signal received at the third port and transmits it unidirectionally to the first port.

3. The system for locking the laser frequency based on the hollow core fiber FP resonant cavity according to claim 1, characterized in that: The optical fiber coupler (3) used is a 1×2 optical fiber coupler, and the distribution ratio of the outputs of the second output end of the optical fiber coupler (3) and the first output end of the optical fiber coupler (3) is 1:

9.

4. The system for locking laser frequency based on hollow core fiber FP resonant cavity according to claim 1, characterized in that: The hollow-core fiber Fabry-Perot resonant cavity comprises: a first single-mode fiber (9), a second single-mode fiber (10), a first gradient-refractive-index fiber (11), a second gradient-refractive-index fiber (12), a first fiber ferrule (13), a second fiber ferrule (14), and a hollow-core fiber (15); one end of the first single-mode fiber (9) serves as the input end of the hollow-core fiber Fabry-Perot resonant cavity, the other end of the first single-mode fiber (9) is connected to one end of the first gradient-refractive-index fiber (11), and the other end of the first gradient-refractive-index fiber (11) is connected to the hollow-core fiber (15). 5) is connected to one end of the hollow-core optical fiber (15) through a first optical fiber sleeve (13); the other end of the hollow-core optical fiber (15) is connected to one end of the second graded-refractive-index optical fiber (12) through a second optical fiber sleeve (14); the other end of the second graded-refractive-index optical fiber (12) is connected to one end of the second single-mode optical fiber (10), and the other end of the second single-mode optical fiber (10) serves as the output end of the hollow-core optical fiber Fabry-Perot resonant cavity; the optical fiber sleeves (13) and (14) are hollow transparent glass tubes, and one end is adapted to the outer diameter of the graded-refractive-index optical fiber, and the other end is adapted to the outer diameter of the hollow-core optical fiber.

5. A system for locking laser frequency based on hollow core fiber FP resonant cavity according to claim 4, characterized in that: The transmission bandwidths of the first single-mode optical fiber (9) and the second single-mode optical fiber (10) are adapted to the output wavelength of the laser, and the transmission loss is not higher than 10 dB / km; and the end faces on the side close to the hollow-core optical fiber are cut flat.

6. A system for locking laser frequency based on hollow core fiber FP resonant cavity according to claim 4, characterized in that: One end of the first graded-refractive-index optical fiber (11) and the second graded-refractive-index optical fiber (12) are respectively fused with the first single-mode optical fiber (9) and the second single-mode optical fiber (10) after being cut flat, and the loss after fusion is not higher than 0.05 dB; the other end is successively cut flat, ground and polished, and then coated with a dielectric film, and the reflectivity of the dielectric film to laser is not lower than 0.

999.

7. The system for locking laser frequency based on hollow core fiber FP resonant cavity according to claim 4, characterized in that: The first single-mode optical fiber (9), the second single-mode optical fiber (10) and the hollow-core optical fiber of the hollow-core optical fiber Fabry-Perot resonant cavity are all adhered and fixed to the bottom plate of the first housing by optical fiber adhesive tape.

8. The system for locking laser frequency based on hollow core fiber FP resonant cavity according to claim 1, characterized in that: The hollow-core optical fiber used includes a metal-plated hollow optical waveguide, a hollow-core photonic bandgap optical fiber, and a hollow-core antiresonant optical fiber. Its transmission bandwidth is adapted to the output wavelength of the laser, and the transmission loss is no higher than 1dB / km. The end faces of the two ports of the hollow-core optical fiber are cut flat.

9. The system for locking laser frequency based on hollow core fiber FP resonant cavity according to claim 1, characterized in that: The first shell is a metal aluminum box, the second shell is a polystyrene box, and the interior of the second shell is filled with gel packs whose heat capacity exceeds a set heat capacity threshold.

10. The system for locking laser frequency based on hollow core fiber FP resonant cavity according to claim 1, characterized in that: The laser (1) used is a semiconductor laser, a fiber laser or a solid laser, and the laser is equipped with a temperature and current control system.

11. A method for locking the laser frequency based on a hollow core fiber FP resonant cavity according to any one of claims 2 to 10, characterized in that: include: The laser emits an optical signal; the optical signal is injected from the first port of the optical fiber circulator through single-mode optical fiber coupling and emitted from the second port; The emitted optical signal enters the optical fiber coupler (3) through single-mode optical fiber coupling and is distributed in the optical fiber coupler at a ratio of 1:9; thereafter, 90% of the optical signal is modulated in polarization state by a polarization controller (4) and then coupled into the hollow-core optical fiber Fabry-Perot resonant cavity (5) through single-mode optical fiber; the remaining 10% of the optical signal is directly emitted and collected by a light trap (8); the optical signal transmitted by the hollow-core optical fiber Fabry-Perot resonant cavity (5) is coupled through a single-mode optical fiber, injected from the third port of the optical fiber circulator (2), and emitted from the first port; The emitted optical signal is fed back to the laser (1) via single-mode optical fiber coupling, so that the frequency of the laser is locked to the resonance frequency of the hollow-core optical fiber Fabry-Perot resonant cavity.

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