Mode-programmable laser generator based on degenerate cavity

Abstract

The present invention relates to the field of laser technology, and in particular to a mode-programmable laser generator based on a degenerate cavity. The generator includes a resonant cavity, a 4‑f self-imaging system, a gain medium, a pinhole aperture, a linear polarizer, an F‑P standard tool, a wave plate group and a pump source. The resonant cavity includes a liquid crystal layout and an output coupling mirror of a liquid crystal spatial light modulator arranged in parallel; the 4‑f self-imaging system is arranged in the resonant cavity, and includes a first thin lens and a second thin lens arranged coaxially; the gain medium is arranged between the liquid crystal spatial light modulator and the first thin lens; the pinhole aperture is arranged between the first thin lens and the second thin lens; the linear polarizer is arranged between the gain medium and the pinhole aperture; the F‑P standard tool is arranged obliquely between the pinhole aperture and the second thin lens; the wave plate group is arranged at one end of the outer side of the resonant cavity close to the output coupling mirror; and the pump source is arranged on the side of the gain medium. The present invention can regulate the transverse mode, longitudinal mode and polarization mode of the resonant cavity, thereby realizing the programming of the laser mode.

Description

Technical Field

[0001] The invention relates to the field of laser technology, in particular to a mode-programmable laser generator based on a degenerate cavity. Background Art

[0002] Light field control refers to the control of light field parameters such as space, frequency, and polarization through various optical elements or technical means, and has great application prospects in the fields of optical imaging, laser processing, optical communication, and optical sensing. However, due to the limitations of the geometric aperture of optical elements and the end-face pumping method, traditional lasers can generally only produce low-order cavity eigenmodes, with disadvantages such as low mode purity and poor beam quality.

[0003] For the control of the extra-cavity light field, the use of astigmatism elements, spiral phase plates, liquid crystal spatial light modulators and other devices can easily achieve the control of the extra-cavity beam mode, but there are problems such as low conversion efficiency and inflexible control. In addition, although methods for controlling degrees of freedom have also been proposed in the laser field, most of them are limited to the control of a single degree of freedom.

[0004] Therefore, for application fields such as optical manipulation, laser communication, and material processing, there is an urgent need to develop a mode-programmable laser generator based on a degenerate cavity. Summary of the invention

[0005] The purpose of the present invention is to address the above-mentioned problems and shortcomings and to provide a mode-programmable laser generator based on a degenerate cavity, which can effectively regulate the transverse mode, longitudinal mode and polarization mode of the resonant cavity, thereby achieving the effect of laser mode programming.

[0006] To achieve the above purpose, the technical solution adopted is:

[0007] The present invention provides a mode-programmable laser generator based on a degenerate cavity, comprising:

[0008] A resonant cavity comprising a liquid crystal panel and an output coupling mirror of a liquid crystal spatial light modulator arranged in parallel;

[0009] A 4-f self-imaging system, which is arranged in the resonant cavity, and the 4-f self-imaging system includes a first thin lens and a second thin lens arranged coaxially;

[0010] A gain medium, which is disposed between the liquid crystal spatial light modulator and the first thin lens;

[0011] A pinhole stop is disposed between the first thin lens and the second thin lens, and the resonant cavity, the 4-f self-imaging system, the gain medium and the pinhole stop are coaxial;

[0012] A linear polarizer, which is disposed between the gain medium and the pinhole aperture;

[0013] The FP etalon is tilted and arranged between the pinhole stop and the second thin lens;

[0014] A wave plate group, which is arranged outside the resonant cavity and close to one end of the output coupling mirror, and the wave plate group is used to control the polarization mode of the output light field of the resonant cavity;

[0015] and a pump source, which is arranged on the side of the gain medium, and is used to provide energy for the generation of laser.

[0016] According to the mode-programmable laser generator based on the degenerate cavity of the present invention, further, the first thin lens and the second thin lens are both circular and are convex lenses.

[0017] According to the mode-programmable laser generator based on the degenerate cavity of the present invention, further, the back focal plane of the first thin lens and the front focal plane of the second thin lens coincide with each other to form a common focal plane.

[0018] According to the mode-programmable laser generator based on a degenerate cavity of the present invention, further, the pinhole aperture is arranged at the common focal plane.

[0019] According to the mode-programmable laser generator based on a degenerate cavity of the present invention, further, the liquid crystal spatial light modulator is located at the front focal plane of the thin lens.

[0020] According to the mode-programmable laser generator based on the degenerate cavity of the present invention, further, the output coupling mirror is arranged at the back focal plane of the second thin lens.

[0021] According to the mode-programmable laser generator based on a degenerate cavity of the present invention, further, the wave plate group includes a half-wave plate and a quarter-wave plate arranged in parallel, and the half-wave plate is arranged between the output coupling mirror and the quarter-wave plate.

[0022] According to the mode-programmable laser generator based on the degenerate cavity of the present invention, further, the calculation equation of the resonant frequency of the eigenmode of the resonant cavity is shown in Formula 1,

[0023]

[0024] The v qmn is the resonant frequency of the eigenmode of the resonant cavity, q is the longitudinal mode number of the eigenmode of the resonant cavity, m is the transverse mode number of the eigenmode of the resonant cavity, n is the other transverse mode number of the eigenmode of the resonant cavity, c is the speed of light in vacuum, L is the optical length of the resonant cavity, A is the position transmission factor of the eigenmode of the resonant cavity, and D is the angle holding factor of the eigenmode of the resonant cavity, wherein A=D=1.

[0025] According to the mode-programmable laser generator based on the degenerate cavity of the present invention, further, the calculation equation of the half-width at half maximum of the spectrum line of the FP etalon is shown in Formula 2,

[0026]

[0027] The Δv is the half-height full width of the spectrum line of the FP etalon reflection surface, the c is the speed of light in a vacuum, the d is the thickness of the FP etalon, the θ is the angle between the side normal of the FP etalon and the axis of the resonant cavity, the R is the reflectivity of the FP etalon reflection surface, and the μ is the refractive index of the FP etalon.

[0028] The beneficial effects achieved by adopting the above technical solution are:

[0029] The present invention is ingeniously designed and reasonably structured. The present invention adopts the side pumping technology to realize uniform pumping of the laser working material while reducing the complexity of the device. Starting from the three degrees of freedom of the transverse mode, longitudinal mode and polarization mode of the light field, the transverse mode is controlled through the multi-transverse mode degeneracy characteristics of the self-imaging system, the liquid crystal spatial light modulator and the small aperture aperture mode limiting structure, the longitudinal mode is controlled through the FP standard tool, and the polarization mode is controlled through the combined wave plate, so as to realize efficient real-time regulation of lasers in any mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings of the embodiments of the present invention, wherein the drawings are only used to illustrate some embodiments of the present invention, but not to limit all embodiments of the present invention thereto.

[0031] Figure 1 is a schematic structural diagram of a mode-programmable laser generator based on a degenerate cavity according to an embodiment of the present invention;

[0032] Figure 2 is a schematic structural diagram of an FP etalon according to an embodiment of the present invention;

[0033] Figure 3 It is a diagram of experimental results of a mode-programmable laser generator based on a degenerate cavity according to an embodiment of the present invention.

[0034] The meanings of the numbers in the figure are:

[0035] 1. Liquid crystal spatial light modulator, 2. Gain medium, 3. Pump source, 4. First thin lens, 5. Linear polarizer, 6. Pinhole aperture, 7. FP etalon, 8. Second thin lens, 9. Output coupling mirror, 10. Half-wave plate, 11. Quarter-wave plate. DETAILED DESCRIPTION

[0036] The following will be combined with the drawings of specific embodiments of the present invention to clearly and completely describe the exemplary scheme of the embodiment of the present invention. Unless otherwise defined, the technical terms or scientific terms used in the present invention should be the common meanings understood by people with ordinary skills in the field.

[0037] like Figure 1 As shown, the mode-programmable laser generator based on the degenerate cavity of this embodiment includes a resonant cavity, a 4-f self-imaging system, a gain medium 2, a pinhole aperture 6, a linear polarizer 5, an FP etalon 7 and a wave plate group. The resonant cavity includes a liquid crystal panel of a liquid crystal spatial light modulator 1 and an output coupling mirror 9 arranged in parallel; a 4-f self-imaging system is arranged in the resonant cavity, and the 4-f self-imaging system includes a first thin lens 4 and a second thin lens 8 arranged coaxially; a gain medium 2 is arranged between the liquid crystal spatial light modulator 1 and the first thin lens 4; a pinhole aperture 6 is arranged between the first thin lens 4 and the second thin lens 8, and the resonant cavity, the 4-f self-imaging system, the gain medium 2 and the pinhole aperture 6 are arranged coaxially; a linear polarizer 5 is arranged between the gain medium 2 and the pinhole aperture 6; an FP standard tool is tiltedly arranged between the pinhole aperture 2 and the second thin lens 8; a wave plate group is arranged at one end of the outer side of the resonant cavity close to the output coupling mirror 9, and the wave plate group is used to control the polarization mode of the output light field of the resonant cavity; a pump source 3 is arranged on the side of the gain medium, and the pump source 3 is used to provide energy for the generation of laser.

[0038] The resonant cavity composed of the liquid crystal spatial light modulator 1 and the output coupling mirror 9 realizes the extension of the working length of the gain medium 2 by reflecting the light in the cavity back and forth. In addition, the liquid crystal panel of the spatial light modulator 1 is loaded with a computational hologram to control the loss of different regions of the target mode in the resonant cavity, suppress non-target modes, and realize the control of the laser transverse mode.

[0039] The first thin lens 4 and the second thin lens 8 form a 4-f self-imaging system, so that the light field of the liquid crystal spatial light modulator 1 can achieve self-reproduction on the output coupling mirror 8 after a single-pass transition, reducing the diffraction loss of high-order modes and enabling the cavity to support multi-transverse mode resonance.

[0040] The pinhole aperture 6 acts as a transverse mode selector, which increases the loss of high-order modes in the spectrum, increases the mode loss spacing, reduces the number of oscillation modes for modes with the same quality factor, and controls the number of transverse modes.

[0041] The linear polarizer 5 is used to control the polarization state of the light field.

[0042] The FP etalon 7 performs longitudinal filtering on the light field by adjusting its angle and length, so as to achieve controllable longitudinal mode output.

[0043] The half-wave plate 10 regulates the polarization direction of the light field, and the quarter-wave plate 11 regulates the ellipticity and handedness of the light field output by the resonant cavity. The polarization mode can be controlled by using the wave plate combination.

[0044] Gain medium 2 is used to achieve population inversion and increase light energy density, while determining the wavelength of the laser.

[0045] The pump source 3 serves as an energy source, and its function is to emit photons to pump electrons in the gain medium from the ground state energy level to the high energy level to achieve population inversion.

[0046] Optionally, the liquid crystal spatial light modulator 1 adopts a high-power phase-type liquid crystal spatial light modulator, loads a computational hologram to match the corresponding light field mode, or realizes amplitude modulation of the target light field through a chessboard grid method and a blazed grating method.

[0047] Optionally, the liquid crystal spatial light modulator 1 adopts a high-power amplitude type spatial light modulator.

[0048] Optionally, the gain medium 2 is cylindrical or rectangular.

[0049] Optionally, the gain medium 2 may be made of neodymium-doped yttrium aluminum garnet laser crystal (Nd:YAG), neodymium-doped yttrium lithium fluoride crystal (Nd:YLF), neodymium-doped yttrium vanadate crystal (Nd:YVO4), or other neodymium-doped yttrium aluminum garnet laser crystal (Nd:YAG). 3+ ) solid gain medium, concentration 0.8at%-1.2at%, thickness 1-3mm.

[0050] Optionally, the gain medium 2 may be selected as a quantum well material.

[0051] Optionally, the gain medium 2 and the pump source 3 are arranged as a whole between the liquid crystal spatial light modulator 1 and the thin lens 4 , or between the thin lens 8 and the output coupling mirror 9 .

[0052] Furthermore, the linear polarizer 5 needs to have high transmittance for the laser wavelength. Optionally, the linear polarizer 5 can be a dichroic polarizing lens, a crystal polarizing lens, or a wire grid polarizing lens. In addition, the laser wavelength range here is 800nm ​​to 1600nm.

[0053] Optionally, the laser wavelength is 808 nm, 1064 nm or 1550 nm.

[0054] Optionally, the pump source 3 may be a high optical power device such as a xenon lamp or a semiconductor laser.

[0055] like Figure 1 As shown, the first thin lens 4 and the second thin lens 8 are both circular and are convex lenses.

[0056] Optionally, the first thin lens 4 and the second thin lens 8 can be double convex lenses or plano-convex lenses with a focal length of 20 cm or 30 cm, and the surfaces of the first thin lens 4 and the second thin lens 8 need to be coated with a high-transmittance film layer for the laser wavelength. In addition, the laser wavelength range here is 800nm ​​to 1600nm.

[0057] Optionally, the laser wavelength is 808 nm, 1064 nm or 1550 nm.

[0058] Optionally, the diameters of the first thin lens 4 and the second thin lens 8 are 25.4 cm or 50.8 cm.

[0059] like Figure 1 As shown, the back focal plane of the first thin lens 4 and the front focal plane of the second thin lens 8 coincide with each other to form a common focal plane.

[0060] like Figure 1 As shown, the pinhole stop 6 is arranged at the common focal plane of the first thin lens 4 and the second thin lens 8 .

[0061] Optionally, the pinhole aperture 6 may adopt a square aperture or a circular aperture structure, and the aperture is continuously adjustable within the range of 1 mm-10 mm.

[0062] like Figure 1 As shown, the liquid crystal spatial light modulator 1 is located at the front focal plane of the first thin lens 4 .

[0063] like Figure 1 As shown, the output coupling mirror 9 is arranged at the back focal plane of the second thin lens 8.

[0064] Optionally, the output coupling mirror 9 can adopt a reflection: transmission splitting ratio of 5:5, 6:4, 7:3, etc., and the surface of the output coupling mirror 9 needs to be plated with a reflection and transmission film layer for the laser wavelength, with a diameter of 25.4 cm and 50.8 cm. In addition, the wavelength range here is 800nm ​​to 1600nm.

[0065] Optionally, the laser wavelength is 808 nm, 1064 nm or 1550 nm.

[0066] Optionally, the FP etalon 7 may be a solid single-cavity etalon or an air-gap etalon.

[0067] like Figure 1 As shown, the wave plate set includes a half-wave plate 10 and a quarter-wave plate 11 which are arranged in parallel, and the half-wave plate 10 is arranged between the output coupling mirror 9 and the quarter-wave plate 11 .

[0068] Optionally, the half-wave plate 10 may be a zero-order wave plate, a true zero-order wave plate, a multi-order wave plate, a quartz wave plate, a polymer wave plate, or an air-gap wave plate.

[0069] Optionally, the quarter wave plate 11 may be a zero-order wave plate, a true zero-order wave plate, a multi-order wave plate, a quartz wave plate, a polymer wave plate, or an air-gap wave plate.

[0070] In this application, the calculation equation of the resonant frequency of the eigenmode of the resonant cavity is shown in Formula 1,

[0071]

[0072] where v qmn is the resonant frequency of the eigenmode of the resonant cavity, q is the longitudinal mode number of the eigenmode of the resonant cavity, m is the transverse mode number of the eigenmode of the resonant cavity, n is the other transverse mode number of the eigenmode of the resonant cavity, c is the speed of light in vacuum, L is the optical length of the resonant cavity, A is the position transmission factor of the eigenmode of the resonant cavity, D is the angle holding factor of the eigenmode of the resonant cavity, and A=D=1.

[0073] In this application, the resonant cavity regulates the transverse mode:

[0074] Substituting A=D=1 into Formula 1, we get: It can be seen from this that the resonant frequency v of the eigenmode of the resonant cavity in this application is qmn It is only related to the speed of light in vacuum c, the optical length L of the resonant cavity and the longitudinal mode number q, but has nothing to do with the two transverse mode numbers m and n. This shows that all transverse mode modes in the resonant cavity are completely degenerate, and the frequency does not depend on the transverse mode number, so the resonant cavity of this device supports resonance of any transverse mode.

[0075] On this basis, the liquid crystal spatial light modulator 1 is used to load the computational hologram of the target transverse mode in the resonant cavity, and the phase of the target transverse mode in the resonant cavity is matched, thereby realizing the lasing of the target transverse mode in the resonant cavity.

[0076] In addition, in order to avoid the lasing of other high-order modes, a small aperture 6 is used to control the transverse mode loss in the resonant cavity. The principle is as follows: TEMmn The total loss is shown in Formula 2,

[0077]

[0078] Among them, γ mn Represents the laser mode in the resonant cavity TEMmn The total loss, represents the additional loss of each additional component in the resonant cavity, Represents the laser mode in the resonant cavity TEMmn The diffraction loss is It represents the sum of transmission loss, absorption, scattering and other losses of each optical element. In different laser modes of 4-f self-imaging system, and are basically the same, and It can be adjusted according to the light intensity distribution of the target mode. Therefore, the pinhole aperture 6 is introduced into the resonant cavity as a high-order mode loss, so that it cannot oscillate due to the loss and does not meet the laser oscillation conditions.

[0079] Through the coordination between the multi-transverse mode degeneracy characteristics of the 4-f self-imaging system, the phase control capability of the liquid crystal spatial light modulator 1 and the additional loss of the pinhole aperture 6, the control of any order of transverse mode in the resonant cavity can be achieved.

[0080] In this application, the calculation equation of the half-width at half maximum of the spectral line of the FP etalon is shown in Formula 3,

[0081]

[0082] △v is the half-height full width of the spectrum line of the FP etalon reflection surface, c is the speed of light in a vacuum, d is the thickness of the FP etalon, θ is the angle between the normal line of the side surface of the FP etalon and the axis of the resonant cavity, R is the reflectivity of the FP etalon reflection surface, and μ is the refractive index of the FP etalon.

[0083] In this application, the resonant cavity regulates the longitudinal mode:

[0084] The FP etalon 7 is arranged obliquely between the pinhole aperture 6 and the second thin lens 8 to prevent the FP etalon 7 from forming parasitic oscillations in the resonant cavity.

[0085] In addition, the FP standard has 7 pairs of frequencies The longitudinal mode of the resonant cavity has extremely high transmittance, θ is the angle between the normal line of the side of the FP etalon and the axis of the resonant cavity, c is the speed of light in vacuum, d is the thickness of the FP etalon, and q is the longitudinal mode number.

[0086] In the resonant cavity, by adjusting the thickness and tilt angle of the FP etalon 7, the light that does not meet the maximum transmission condition of the etalon is attenuated to achieve the regulation of the longitudinal mode frequency, free spectral range and half-maximum full width in the resonant cavity.

[0087] In this application, the resonant cavity regulates the polarization mode:

[0088] The linear polarizer 5 is arranged between the first thin lens 4 and the pinhole aperture 6. At this time, the light emitted from the output coupling 9 of the resonant cavity is linearly polarized light. When the linearly polarized light passes through the half-wave plate 10, the polarization direction of the linearly polarized light is deflected, that is, the half-wave plate 10 is used to adjust the polarization direction of the linearly polarized light.

[0089] Then, the light passes through the quarter wave plate 11. If the fast axis of the quarter wave plate 11 is consistent with the polarization direction of the incident light, the outgoing light passing through the quarter wave plate 11 is still linearly polarized light. At this time, the quarter wave plate 11 needs to be adjusted so that the fast axis of the quarter wave plate 11 is inconsistent with the polarization direction of the incident light, so that the outgoing light passing through the quarter wave plate 11 becomes elliptically polarized light or circularly polarized light.

[0090] By using the half-wave plate 10 and the quarter-wave plate 11 in coordination, the regulation of any polarization mode in the resonant cavity can be achieved.

[0091] like Figure 3 As shown, the normalized grayscale value in the figure represents the light intensity, each light spot represents a single independent mode, and the number of modes is 12, indicating that the laser mode can be manipulated by using a spatial light modulator;

[0092] The technical solution of the present invention is described above in combination with the accompanying drawings. It is obvious that the specific implementation of the present invention is not limited to the above-mentioned method. As long as various non-substantial improvements are made using the method concept and technical solution of the present invention, or the concept and technical solution of the invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

[0093] In the description of the present invention, it should be understood that the expressions “first” and “second” are used to describe the various elements of the present invention and do not represent any limitation of order, quantity or importance, but are only used to distinguish one component from another.

[0094] It should be noted that when one element is “connected”, “coupled” or “connected” to another element, it may mean that they are directly connected, coupled or connected, but it should be understood that there may be intermediate elements between the two; that is, the positional relationships of direct connection and indirect connection are covered.

[0095] It should be noted that the use of "a" or "an" and similar words does not necessarily indicate a quantitative limitation. "Include" or "comprising" and similar words mean that the elements or objects preceding the word include the elements or objects listed after the word and their equivalents, but do not exclude other elements or objects.

[0096] It should be noted that terms such as "up", "down", "left" and "right" indicating orientation or positional relationship are only used to indicate relative positional relationship. This is for the convenience of describing the present invention and does not mean that the device or element must have a specific orientation, be constructed and operated in a specific orientation. When the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0097] The preferred embodiments for implementing the present invention have been described in detail above, but it should be understood that the role of these embodiments is only to exemplify, and is not intended to limit the scope, applicability or configuration of the present invention in any way. The protection scope of the present invention is defined by the attached claims and their equivalents. A person of ordinary skill in the art can make many changes to the above embodiments under the guidance of the present invention, and these changes all fall within the protection scope of the present invention.

Claims

1. A mode-programmable laser generator based on a degenerate cavity, characterized in that: include: A resonant cavity comprising a liquid crystal panel and an output coupling mirror of a liquid crystal spatial light modulator arranged in parallel; A 4-f self-imaging system, which is arranged in the resonant cavity, and the 4-f self-imaging system includes a first thin lens and a second thin lens arranged coaxially; A gain medium, which is disposed between the liquid crystal spatial light modulator and the first thin lens; A pinhole stop is disposed between the first thin lens and the second thin lens, and the resonant cavity, the 4-f self-imaging system, the gain medium and the pinhole stop are coaxial; A linear polarizer, which is disposed between the gain medium and the pinhole aperture; The FP etalon is tilted and arranged between the pinhole stop and the second thin lens; A wave plate group, which is arranged outside the resonant cavity and close to one end of the output coupling mirror, and the wave plate group is used to control the polarization mode of the output light field of the resonant cavity; and a pump source, which is arranged on the side of the gain medium, and is used to provide energy for the generation of laser.

2. The mode-programmable laser generator based on a degenerate cavity according to claim 1, characterized in that: The first thin lens and the second thin lens are both circular and convex lenses.

3. The mode-programmable laser generator based on a degenerate cavity according to claim 2, characterized in that: The back focal plane of the first thin lens and the front focal plane of the second thin lens coincide with each other to form a common focal plane.

4. The mode-programmable laser generator based on a degenerate cavity according to claim 3, characterized in that: The pinhole stop is arranged at the common focal plane.

5. The mode-programmable laser generator based on a degenerate cavity according to claim 2, characterized in that: The liquid crystal spatial light modulator is located at the front focal plane of the first thin lens.

6. The mode-programmable laser generator based on a degenerate cavity according to claim 5, characterized in that: The output coupling mirror is arranged at the back focal plane of the second thin lens.

7. The mode-programmable laser generator based on a degenerate cavity according to claim 1, characterized in that: The wave plate group includes a half-wave plate and a quarter-wave plate which are arranged in parallel, and the half-wave plate is arranged between the output coupling mirror and the quarter-wave plate.

8. The mode-programmable laser generator based on a degenerate cavity according to claim 1, characterized in that: The calculation equation of the resonant frequency of the eigenmode of the resonant cavity is shown in Formula 1, The v qmn is the resonant frequency of the eigenmode of the resonant cavity, q is the longitudinal mode number of the eigenmode of the resonant cavity, m is the transverse mode number of the eigenmode of the resonant cavity, n is the other transverse mode number of the eigenmode of the resonant cavity, c is the speed of light in vacuum, L is the optical length of the resonant cavity, A is the position transmission factor of the eigenmode of the resonant cavity, and D is the angle holding factor of the eigenmode of the resonant cavity, wherein A=D=1.

9. The mode-programmable laser generator based on a degenerate cavity according to claim 8, characterized in that: The calculation equation of the full width at half maximum of the spectrum line of the FP etalon is shown in Formula 2. The Δv is the half-height full width of the spectrum line of the FP etalon reflection surface, the c is the speed of light in a vacuum, the d is the thickness of the FP etalon, the θ is the angle between the side normal of the FP etalon and the axis of the resonant cavity, the R is the reflectivity of the FP etalon reflection surface, and the μ is the refractive index of the FP etalon.

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