Michelson-GT tunable filter

Abstract

The invention discloses a Michelson-GT adjustable filter, which comprises a first reflecting surface, a GT etalon and a beam splitter, and is characterized in that a first reflecting mirror and the GT etalon are respectively arranged on a reflecting path and a transmitting path of the beam splitter for input light, the input light is reflected and transmitted on the beam splitter to form a reflected light beam and a transmitted light beam, and the reflected light beam and the transmitted light beam are reflected by the first reflecting surface and transmitted by the GT etalon. The first reflector reflects the reflected light beam, the GT etalon reflects the transmitted light beam, the light beam reflected by the first reflector and the light beam reflected by the GT etalon intersect on the beam splitter, and output light is formed through the reflection and transmission effects of the beam splitter after the light beam reflected by the first reflector and the light beam reflected by the GT etalon intersect.

Description

Technical Field

[0001] The invention belongs to the technical field of filters, and in particular relates to a Michelson-GT tunable filter. Background Art

[0002] Existing tunable filters using optical resonant cavities (Fabry-Perot Cavity) are based on changing the optical path length of light in the cavity to adjust the wavelength. However, existing tunable filters using optical resonant cavities cannot take into account both resolution and cost, and require MEMS with special processes, which is relatively expensive. Summary of the invention

[0003] In order to solve the above technical problems, the present invention proposes a Michelson-GT tunable filter to manufacture a low-cost, high-efficiency wavelength-tunable optical filter, which can be used as a core component of a high-speed scanning light source and a high-speed spectrometer in medical equipment.

[0004] To achieve the above object, the present invention provides a Michelson-GT tunable filter, comprising:

[0005] A first reflector, a GT etalon and a beam splitter, wherein the first reflector and the GT etalon are respectively arranged on the reflection and transmission paths of the beam splitter for input light, the input light is reflected and transmitted on the beam splitter to form a reflected light beam and a transmitted light beam, the first reflector reflects the reflected light beam, the GT etalon reflects the transmitted light beam, the light beam reflected by the first reflector and the light beam reflected by the GT etalon meet on the beam splitter, and after meeting, the output light is formed through the reflection and transmission action of the beam splitter.

[0006] Optionally, the GT standard device includes a second reflection surface and a third reflection surface, wherein the second reflection surface is parallel to the third reflection surface and is spaced apart from each other, i.e., separated by a certain distance, and the optical path difference between the third reflection surface and the first reflection surface is half a wavelength; the optical path difference here includes the optical path resulting from the phase conversion generated by each coating surface.

[0007] Optionally, the position of the third reflecting surface is fixed, and the position of the second reflecting surface is not fixed and moves closer or farther along a direction perpendicular to the third reflecting surface.

[0008] Optionally, the second reflecting surface is moved by half the wavelength of the input light toward a direction close to the third reflecting surface, and the frequency of the tunable filter is tuned toward high frequency by 1 FSR.

[0009] Optionally, a first anti-reflection film and a second anti-reflection film are arranged between the first reflective surface and the beam splitter, and the first anti-reflection film and the second anti-reflection film are approximately parallel to the first reflective surface and are arranged at intervals.

[0010] Optionally, a third anti-reflection film and a fourth anti-reflection film are arranged between the GT etalon and the beam splitter, wherein the third anti-reflection film and the fourth anti-reflection film are approximately parallel to the GT etalon and are arranged at intervals.

[0011] Optionally, the second reflecting surface is moved by a mechanical device.

[0012] Optionally, the mechanical device adopts a mechanically driven moving mirror, wherein the second reflecting surface is arranged on one side of the mechanically driven moving mirror, and the mechanically driven moving mirror includes a MEMS piston moving mirror, a PZT driven mirror, and an electric motor driven mirror.

[0013] Optionally, a solid material is further arranged between the second reflecting surface and the third reflecting surface, and the second reflecting surface and the third reflecting surface are respectively provided with solid materials on two sides corresponding to each other, wherein the solid material changes the optical path length between the two interfaces through temperature or electric field.

[0014] Optionally, the tunable filter is used to construct a scanning spectrometer or as a wavelength tuning driver for a tunable laser source.

[0015] Compared with the prior art, the present invention has the following advantages and technical effects:

[0016] The tunable filter invented by the present invention utilizes Michaelson interferometer-like technology to adjust the length of the GT-cavity to achieve the purpose of wavelength tuning.

[0017] The present invention has the following advantages:

[0018] From an optical point of view, to achieve filters with the same resolution, the coating cost of the method adopted by the present invention is lower.

[0019] The tunable filter provided by the present invention and the traditional tunable filter both need to change the length of the resonant cavity to adjust the wavelength. The present invention can use mature MEMS technology to achieve the purpose of adjustment. This MEMS is a reflector that can be translated; while the traditional design requires a MEMS with a special process. This MEMS has two reflective surfaces, one movable and the other fixed. The reflectivity of the two reflective surfaces is the same, and the manufacturing cost of this MEMS is very high. The present invention can further reduce costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0021] Figure 1 Schematic diagram of the structure of the Michelson-GT tunable filter according to an embodiment of the present invention; the accompanying drawings are marked as follows: A, first reflection surface, B, first anti-reflection film, C, second anti-reflection film, D, beam splitter, E, third anti-reflection film, F, fourth anti-reflection film, G, third reflection surface, H, second reflection surface, P, input point, Q, meeting point, L, input light, R, second output light, T, first output light, W, fourth light beam, Z, third light beam, X, second light beam, Y, first light beam;

[0022] Figure 2 The output power of the embodiment of the present invention is a schematic diagram showing the function of the normalized frequency in a linear scale when the reflectivity is 90%; a change of 1 unit on the horizontal axis is equivalent to a change of 1 FSR in frequency;

[0023] Figure 3 is a graph showing a relationship between a phase difference and a normalized frequency between a first beam and a second beam according to an embodiment of the present invention;

[0024] Figure 4 It is a relationship diagram of IL loss (dB) and normalized frequency when the reflectivity is 90% coated according to an embodiment of the present invention; wherein the broken line corresponds to the Michelson-GT tunable filter MGT, and the continuous line corresponds to the tunable filter FP of the existing optical resonant cavity.

[0025] Figure 5 is a relationship diagram between the phase difference and the normalized frequency of the first beam and the second beam after the second reflecting surface moves in an embodiment of the present invention;

[0026] Figure 6 It is a relationship diagram of IL loss (dB) and normalized frequency at 90% reflectivity after the second reflection surface is moved in an embodiment of the present invention; wherein, the broken line corresponds to the Michelson-GT tunable filter MGT, and the continuous line corresponds to the tunable filter FP of the existing optical resonant cavity. DETAILED DESCRIPTION

[0027] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0028] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0029] like Figure 1 As shown, this embodiment provides a Michelson-GT tunable filter, which mainly utilizes the technology of Michelson-GT interferometer.

[0030] Michelson-GT tunable filter:

[0031] Figure 1 The structure of the Michelson-GT tunable filter is shown. The top is the first reflection surface A, and the right side is the GT-etalon. The positions of the first reflection surface A and the GT etalon can be interchanged. The GT etalon consists of two reflectors, namely the third reflection surface G and the second reflection surface H, where the third reflection surface G is usually a high reflection surface and the second reflection surface H is a reflector (about 100% reflectivity). The medium between the third reflection surface G and the second reflection surface H can be air or vacuum. For a solid etalon, the third reflection surface G and the second reflection surface H can be two surfaces of a solid material, such as glass, silicon, etc.

[0032] Reference Figure 1 , input light L enters the interferometer from the left hand side, hits the beam splitter D at the input point P, and then splits into two paths, the reflected beam and the transmitted beam on the top and right. Both paths are reflected and return to the beam splitter D again, intersecting at the meeting point Q. The light in the top path PAQ is divided into the first beam Y (bottom) and the third beam Z (left); the beam of the right path PHQ is divided into the second beam X (bottom) and the fourth beam W (left). The first beam Y and the second beam X interfere at the bottom of the beam splitter D to form the first output light T. The third beam Z and the fourth beam W interfere on the left side of the beam splitter D to form the second output light R.

[0033] A second anti-reflection film C and a first anti-reflection film B are arranged in sequence between the beam splitter D and the first reflection surface A, and a third anti-reflection film E and a fourth anti-reflection film F are arranged in sequence between the beam splitter D and the GT-etalon, wherein the second anti-reflection film C and the third anti-reflection film E are arranged on one side of the prism corresponding to the beam splitter D, the second anti-reflection film C and the first anti-reflection film B are fixed by a solid material such as glass or silicon, the first anti-reflection film B and the first reflection surface A are arranged on both sides of a material such as glass or silicon and are parallel in sequence, the third anti-reflection film E and the fourth anti-reflection film F are fixed by a material such as glass or silicon, the fourth anti-reflection film F and the third reflection surface G are arranged on both sides of a solid material such as glass or silicon and are parallel in sequence, and the second reflection surface H is arranged on a translatable firmware.

[0034] It should be noted that if the working wavelength of the tunable filter is around 1.5um, the movement of the second reflection surface H is less than 1um. Since the incident angle of the light entering the first reflection surface A and the second reflection surface H is close to 90 degrees, in actual use, the position of the reflected light spot caused by the translation of the second reflection surface H on the beam splitter D can be approximately fixed. This position is Figure 1 Q point.

[0035] Based on the law of energy conservation, the optical powers in the second output light R and the first output light T are complementary. If absorption and scattering can be ignored, the sum of the first output light R and the second output light T is a constant. In the following discussion, the present invention will only focus on the second output light T and take the second output light T as an example for relevant explanation.

[0036] Assuming the input power is 1, using a 50 / 50 beam splitter D, the power of the first beam Y and the second beam X are both equal to 1 / 4. At the bottom of the beam splitter, the optical power of the first output light T is determined by the phase difference between the first beam Y and the second beam X. In the present invention, all optical phases are relative to the phase of the input light L at point P. The parameters are defined as follows:

[0037] v: optical frequency;

[0038] λ: wavelength;

[0039] C: speed of light in vacuum;

[0040] L0: optical path length of path PAQ;

[0041] L'0: optical path length of path PGQ;

[0042] L g : The optical path length from the third reflecting surface G to the second reflecting surface H;

[0043]

[0044] Round trip phase shift from the third reflection surface G to the second reflection surface H;

[0045] φ0: light wave phase change corresponding to the path PAQ;

[0046] φ'0: light wave phase change corresponding to path PGQ;

[0047] φ g : Phase change of the light wave from the third reflection surface G to the second reflection surface H of the GT etalon path;

[0048] φ1: light wave phase of the first light beam Y;

[0049] φ2: light wave phase of the second light beam X;

[0050] φ diff =(φ2-φ1): The optical phase difference between the first beam Y and the second beam X

[0051] R g : reflectivity of the third reflecting surface G, 0 <R g <1;

[0052] r: reflection coefficient

[0053]

[0054] P T : output power of the first output light T;

[0055] m: integer;

[0056] n: refractive index of glass;

[0057] Based on the above definition,

[0058]

[0059] In the interferometer design, the optical path lengths PAQ and PGQ are slightly offset, so that the optical path difference between the paths PAQ and PGQ is about half a wavelength. The optical path difference includes the optical path converted by the phase generated by each coating surface, so the corresponding light wave phase difference is π (φ'0=φ0+π). Therefore, the phase difference and output power of the two interfering light beams are expressed as follows:

[0060]

[0061] In equation (2), φ diff and P T All are φ RT is a periodic function with a period of 2π. In terms of optical frequency, P T is a periodic function of ν, with period ν g , also known as the free spectral range (FSR) of the etalon. In the present invention, in all figures, the optical frequency is normalized by the FSR of the etalon.

[0062] Figure 2 Display P T The function of normalized frequency is shown on a linear scale. As expected, it is a periodic function with a period equal to 1 (normalized to the FSR). For high-resolution spectrometer applications, it is required that these peaks be as narrow as possible. Usually, the filter resolution is expressed by its 3-dB bandwidth, or equivalently FWHM (full width at half maximum). A key parameter called finesse is used to characterize the resolution:

[0063]

[0064] It is the ratio of period to FWHM. High finesse means high resolution. With some algebra we can show that Where T is the transmittance of the third reflective surface G. For example, if T = 1% (or reflectivity is 99%), then the finesse = 628. For a Fabry-Perot etalon with the same reflectivity, the corresponding finesse is about half (~314). This shows that the Michelson-GT etalon has a higher wavelength resolution than the Fabry-Perot etalon. For example, at an operating wavelength of around 1550 nanometers, with a cavity length of 30 um (FSR ~ 5 THz) and a reflectivity of 99%, the FWHM of the filter is ~ 8 GHz (or 64 pm).

[0065] Figure 3 The phase difference in equation (2) is shown. At normalized frequency = 0, the phase difference is zero; therefore, the interference is constructive. Once the frequency deviates from the center, the phase difference moves away from zero and quickly approaches 180° (π) or 180° (-π) (e.g. Figure 3 As shown), these frequencies of light cannot pass through the bottom of the beam splitter ( Figure 1 The frequency with a phase difference equal to π can be eliminated 100%, which is the mechanism of forming a narrow-band filter. Figure 4 The filtering spectra of the MGT and Fabry-Perot etalon with 90% coating on the G surface are shown. The bandwidth of the Michelson-GT tunable filter MGT (broken line) is about half of the tunable filter FP (continuous line) of the existing optical resonator.

[0066] Michelson-GT tunable filter

[0067] exist Figure 1 In the figure, when the second reflective surface H on the right side is translated toward the beam splitter D, the phase curve ( Figure 5 Therefore, the peak frequency moves to high frequencies accordingly. Figure 6 The optical frequency and power spectrum of the bottom output light is shown when the second reflective surface H is translated 0.1λ toward the beam splitter. This translation shifts the comb frequency by 0.2FSR. To tune 1FSR, the mirror needs to be moved by half a wavelength.

[0068] Mechanical devices that can be used to translate the mirror include: MEMS piston-driven mirrors, PZT-driven mirrors, and electric motor-driven mirrors.

[0069] In addition to mechanical actuators, solid materials can be used to make GH cavities and the wavelength can be tuned by changing the optical path length of the GH through temperature or electric fields. Among these methods, MEMS (micro-electromechanical systems) and PZT (piezoelectric ceramics) are the best choices for driving miniaturized and high-speed tuning devices.

[0070] This embodiment introduces a new design for making a fixed wavelength or tunable wavelength optical comb filter. The conventional GT etalon consists of two reflective surfaces, and the reflectivity of the second reflective surface H is close to 100%. With this arrangement, all light entering the etalon is reflected, but the phase is modulated. In the present invention, the reflected light beam from the GT etalon interferes with another beam of light reflected by a mirror to form a periodic power output, such as Figure 4 As shown. The bandwidth of the filter is only half of that of a conventional FP etalon with the same reflectivity. When the second reflection surface H of the GT etalon is translated, the peak wavelength of the tunable filter shifts accordingly, which makes the device wavelength tunable. This high-speed tunable filter can be used to build a scanning spectrometer, or as a wavelength tuning driver for a tunable laser source. It has a wide range of applications, such as a scanning source for OCT (optical coherence topography) or an optical sensor.

[0071] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. Michelson-GT tunable filter, characterized in that, include: A first reflecting surface, a GT etalon and a beam splitter, wherein the first reflecting surface and the GT etalon are respectively arranged on the reflection and transmission paths of the beam splitter for the input light, the input light is reflected and transmitted on the beam splitter to form a reflected light beam and a transmitted light beam, the first reflecting mirror reflects the reflected light beam, the GT etalon reflects the transmitted light beam, the light beam reflected by the first reflecting mirror and the light beam reflected by the GT etalon meet on the beam splitter, and after meeting, the output light is formed through the reflection and transmission action of the beam splitter.

2. The Michelson-GT tunable filter according to claim 1, characterized in that: The GT etalon comprises a second reflection surface and a third reflection surface, wherein the second reflection surface is parallel to and spaced from the third reflection surface, and the optical path difference between the third reflection surface and the first reflection surface is half a wavelength; the optical path difference here includes the optical path resulting from phase conversion generated by each coating surface.

3. The Michelson-GT tunable filter according to claim 2, characterized in that: The position of the third reflecting surface is fixed, and the position of the second reflecting surface is not fixed and moves closer or farther along a direction perpendicular to the third reflecting surface.

4. The Michelson-GT tunable filter according to claim 3, characterized in that: The second reflecting surface moves by half the wavelength of the input light toward the direction close to the third reflecting surface, and the frequency of the tunable filter is tuned toward high frequency by 1 FSR.

5. The Michelson-GT tunable filter according to claim 1, characterized in that: A first anti-reflection film and a second anti-reflection film are disposed between the first reflective surface and the beam splitter. The first anti-reflection film and the second anti-reflection film are approximately parallel to the first reflective surface and are spaced apart from each other.

6. The Michelson-GT tunable filter according to claim 1, characterized in that: A third anti-reflection film and a fourth anti-reflection film are disposed between the GT etalon and the beam splitter, wherein the third anti-reflection film and the fourth anti-reflection film are substantially parallel to the GT etalon and are spaced apart from each other.

7. The Michelson-GT tunable filter according to claim 3, characterized in that: The second reflecting surface is moved by a mechanical device.

8. The Michelson-GT tunable filter according to claim 7, characterized in that: The mechanical device adopts a mechanically driven moving mirror, wherein the second reflecting surface is arranged on one side of the mechanically driven moving mirror, and the mechanically driven moving mirror includes a MEMS piston-type moving mirror, a PZT-driven mirror, and an electric motor-driven mirror.

9. The Michelson-GT tunable filter according to claim 3, characterized in that: There is a solid material between the second reflecting surface and the third reflecting surface. The second reflecting surface and the third reflecting surface are respectively provided with solid materials on two sides corresponding to each other. The solid material changes the optical path length between the two interfaces through temperature or electric field.

10. The Michelson-GT tunable filter according to claim 1, characterized in that: The tunable filter is used to construct a scanning spectrometer or as a wavelength tuning driver of a tunable laser source.

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