Etalon-enhanced grating-based tunable filter

Abstract

The present invention discloses an etalon-enhanced grating-based tunable filter, comprising: a circulator, a steering mirror, a relay lens, a series structure and a reflector connected in sequence according to an optical path; the pivot point of the relay lens is imaged on the grating in the series structure, the FP etalon is arranged on the optical path between the relay lens and the grating, and the series structure comprises the FP etalon and the grating connected in series in sequence according to the optical path; wherein the FP etalon is placed vertically to the grating and the grating is arranged on the output optical path of the FP etalon, the grating spacing of the grating is equal to one of the resonant wavelengths of the FP etalon, and the diffraction angle of the grating is zero. Through the above technical solution, the present invention can provide a tunable filter that can obtain high resolution at low finesse and relatively small beam size.

Description

Technical Field

[0001] The invention belongs to the technical field of optical filters, and in particular relates to an etalon-enhanced grating-based tunable filter. Background Art

[0002] Optically tunable filters are an important component of many optical instruments. Manufacturing high-resolution optical tunable filters has always been a challenge. Both diffraction filters and scanning filters based on Fabry-Perot (FP) etalons have ways to obtain corresponding high resolutions, but the high resolution requires a large beam size and high fineness at the expense of each. For diffraction filters, the large beam size means that the corresponding instrument cost and volume need to be increased; scanning filters based on high-fineness FP-etalons are relatively difficult to manufacture due to technical limitations. The present invention provides a high optical resolution tunable optical filter that takes into account both cost and instrument volume. Summary of the invention

[0003] To solve the above technical problems, the present invention proposes an etalon-enhanced grating-based tunable filter, which provides a tunable filter that can obtain high resolution with a low-fineness etalon and a relatively small beam size to solve the problems existing in the above prior art.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] A series structure of an FP etalon and a grating, comprising:

[0006] An FP etalon and a grating are connected in series in sequence according to an optical path; wherein the FP etalon is placed vertically to the grating and the grating is arranged on an output optical path of the FP etalon, the grating spacing of the grating is equal to one of the resonant wavelengths of the FP etalon, the diffraction angle of the grating is zero, the incident light is subjected to narrowband comb filtering tuning through the FP etalon, and the light after the comb filtering tuning is subjected to single-peak filtering tuning through the grating to obtain filtered-tuned light.

[0007] Optionally, the FP etalon includes a first reflecting surface and a second reflecting surface which are arranged on a transparent material and are parallel to each other, wherein the reflectivity of the first reflecting surface is the same or nearly the same as that of the second reflecting surface.

[0008] Optionally, the grating is a diffraction grating.

[0009] Based on the same concept, the present invention also provides an etalon-enhanced grating-based tunable filter, comprising:

[0010] A circulator (or a dual-fiber collimator), a steering mirror, a relay lens group, the above-mentioned series structure and a reflector are connected in sequence according to the optical path; the relay lens group images the rotation axis of the steering mirror to the position of the grating, so that the size of the grating can be reduced, and the FP standard tool is arranged on the optical path between the relay lens group and the grating;

[0011] The input light is transmitted to the steering mirror through the circulator, and reflected by the steering mirror to the relay lens group. The rotation axis of the steering mirror is imaged to the position of the grating through the relay lens group. The relay lens group amplifies the collimated light reflected by the steering mirror, and then enters the FP etalon for the first stage of filtering. The angle at which the light enters the etalon is determined by the rotation angle of the steering mirror. After the first stage of filtering, the light beam enters the grating for the second stage of filtering after leaving the FP etalon. The second stage of filtering is completed by the grating and the subsequent reflector. The grating disperses light of different wavelengths to different angles, and only the light with an angle perpendicular to the rear reflector can be returned by the original route. The light beam passes through the grating and the FP etalon for the second time on the return journey, returns to the circulator by the original route, and outputs the light through the third port of the circulator.

[0012] Optionally, the steering mirror is a MEMS steering mirror.

[0013] Optionally, the relay lens group includes a first relay lens and a second relay lens. The first relay lens and the second relay lens are confocal to form a telescope structure, and the collimated light beam is focused on the rear focal plane of the first relay lens after passing through the first relay lens. The rear focal plane of the first relay lens is also the front focal plane of the second relay lens. The light entering the first relay lens forms a collimated light beam after passing through the second relay lens.

[0014] Optionally, the circulator includes a first port for receiving input light, a second port for transmitting the input light to the redirector, and a third port for outputting output light.

[0015] Optionally, the circulator is replaced with a dual-fiber collimator, the first optical fiber in the dual-line collimator replaces the circulator, receives the input light, and transmits the input light to the steering mirror, and the second optical fiber in the dual-line collimator replaces the second port and the third port of the circulator to output the light reflected from the last reflector of the filter.

[0016] Compared with the prior art, the present invention has the following advantages and technical effects:

[0017] The present invention proposes an etalon-enhanced grating-based tunable filter, which uses a series structure of an FP etalon and a diffraction grating to increase the wavelength resolution of the grating-based tunable filter by more than an order of magnitude, thereby improving the optical resolution and reducing the cost and instrument size of the grating-based tunable filter to improve the optical resolution, and has strong practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 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:

[0019] Figure 1 Schematic diagram of the structure of a diffraction filter based on a grating according to an embodiment of the present invention; the reference numerals are: 32, grating; 33, optical receiving device; 34, input light;

[0020] Figure 2 Schematic diagram of the filter structure based on the FP etalon according to an embodiment of the present invention; the reference numerals are: 31, FP etalon; 33, optical receiving device; 34, input light;

[0021] Figure 3 Schematic diagram of single filtering of a grating filter series structure enhanced by an FP etalon according to an embodiment of the present invention; the accompanying drawings are numerals: 31, FP etalon; 32, grating; 33, optical receiving device; 34, input light;

[0022] Figure 4 Schematic diagram of double filtering of the etalon-enhanced grating-based tunable filter according to an embodiment of the present invention; the figure numbers are: 31, FP etalon; 32, grating; 41, circulator; 411, first port; 412, second port; 413, third port; 42, steering mirror; 431, first relay lens; 432, second relay lens; 44, reflector.

[0023] Figure 5 For the embodiment of the present invention Figure 4 Schematic diagram of double filtering by the etalon-enhanced grating-based tunable filter after the steering mirror is rotated; compared to Figure 4 , the steering mirror rotates counterclockwise, making the angle entering the etalon larger and the angle entering the grating smaller. This change causes the wavelength of the filter to move toward a shorter wavelength.

[0024] Figure 6 Schematic diagram of the filtering results, i.e., transmission spectra, of the grating-based filter and the FP etalon-based filter according to the embodiment of the present invention.

[0025] Figure 7It is a schematic diagram of the filtering result, ie, the transmission spectrum, of the effective filter of the series-connected diffraction grating filter and the FP filter according to an embodiment of the present invention. DETAILED DESCRIPTION

[0026] 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.

[0027] 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.

[0028] Optically tunable filters are an important component of many optical instruments. Fabricating high-resolution optical tunable filters has always been a challenge. Both diffraction-type grating filters and scanning Fabry-Perot (FP) etalon filters can achieve high resolution at the expense of large beam size and high finesse, respectively. In the present invention, a method combining gratings and FP-etalons is proposed to obtain high-resolution tunable filters at low finesse and relatively small beam size.

[0029] Grating-based diffractive filters:

[0030] Figure 1 The tunable filter based on the grating 32 is shown, which includes the grating 32 and the optical receiving device 33 in the order of the optical path. In the tunable filter based on the grating 32, the grating 32 is mainly used for the relevant wavelength tuning. The grating 32 mainly adopts a diffraction grating, and the central wavelength λ of the first-order diffraction of the input light 34 is g The following equation is satisfied:

[0031] λ g =P(sinθ+sinφ) (1)

[0032] Wherein, θ is the incident angle of the grating 32, φ is the diffraction angle, and P is the period (grating fringe spacing) of the grating 32. As a tunable filter, by fixing the diffraction angle φ and changing the incident angle θ, the relevant central wavelength can be tuned, wherein the central wavelength of the first-order diffraction is proportional to Psinθ.

[0033] The most critical parameter of the filter is its wavelength resolution, which is equivalent to the filter bandwidth. The smaller the bandwidth, the higher the wavelength resolution. The bandwidth (BW) of the tunable filter based on the grating 32 grating ) is equal to the central wavelength divided by the resolving power of the grating 32.

[0034]

[0035] The wavelength resolution of the grating 32 is approximately equal to the projection length D of the light spot in the direction perpendicular to the stripes of the grating 32 divided by the grating pitch P. In order to have a higher resolution, it is necessary to increase the projection length D of the light spot in the direction perpendicular to the stripes of the grating 32, that is, the spot size must be increased. In the manufacture of the instrument, using a large-sized grating 32 and enlarging the spot size by optical methods will lead to an increase in cost.

[0036] Figure 6 The broken line trace in is a typical transmission spectrum of a tunable filter based on grating 32, with a half-width (FWHM) of about 60 GHz (1.55 microns at 480 pm). Assuming P = 1 um, the spot diameter is about 3 mm. This is a fairly small spot for a high-resolution filter.

[0037] FP etalon based filters:

[0038] Figure 2 The figure shows a filter based on the FP etalon 31, which includes the FP etalon 31 and the optical receiving device 33 in the order of the optical path. The relevant tuning is mainly performed through the FP etalon 31. The filter is composed of two parallel surfaces with the same reflectivity, namely the first reflection surface and the second reflection surface. The above two surfaces constitute the structure of the FP etalon 31, and the distance between the two surfaces determines the free spectral range (FSR) of the filter. Its transmission spectrum is a periodic function with the frequency of light. The frequency that can pass through the FP etalon 31 is called the resonant frequency, and the frequency difference between two adjacent resonant frequencies is the FSR of the FP etalon 31.

[0039] The bandwidth of the filter based on the FP etalon 31 is determined by the FSR of the FP etalon 31 and the reflectivity of the two surfaces. Figure 6 In FIG. 1 , the periodic continuous line trace is the transmission spectrum of the comb filter based on the FP etalon 31 with 80% reflectivity and a FSR of 100 GHz. In this setting, the 3 dB bandwidth of each peak is 3.6 GHz (about 29 pm at a wavelength of 1.55 um).

[0040] The comb filter based on the FP etalon 31 has a frequency v corresponding to the mth peak of the penetration spectrum when the input light 34 is incident vertically. N,m and the filter bandwidth BW FP They can be expressed as:

[0041]

[0042] Where C is the speed of light, L is the cavity length, m is the mode number, and R is the reflectivity.

[0043] It can be seen from equation (3) that in order to make the filter based on the FP etalon 31 have a narrower bandwidth, the FSR must be reduced or the reflectivity must be increased.

[0044] Based on the above resonant frequency v N,m , resonant wavelength λ N,m Can be:

[0045]

[0046] When the incident angle of the input light 34 beam into the etalon is not normal incidence, but deviates from the normal line, equation (4) becomes:

[0047] mλ α,m =2Lcos(α) (5)

[0048] where λ α,m is the resonance wavelength when the incident angle of the input light 34 is α.

[0049] Combining equation (4) and equation (5), we obtain:

[0050] λ α,m =λ N,m cos(α) (6)

[0051] Equation (6) shows the relationship between the resonant wavelength at different incident angles of the input light 34 to the FP etalon 31. By changing the incident angle from 0 to α, the wavelength can be changed from λ to N,m “Tuning” to Lambda α,m For simplicity, equation (6) is rewritten as E =λ0cos(α),λ0 and λ E is the wavelength corresponding to the comb peak of the FP etalon 31 when the incident angles are 0 and α.

[0052] Equation (1) and equation (6) show the relationship between the penetration wavelength and the incident angle of the diffraction filter based on the grating 32 and the filter based on the FP etalon 31. They are sine and cosine functions respectively. Since the function shape of the transmission wavelength of these two filters with the angle is different, these two filters cannot be cascaded to improve the filter resolution.

[0053] Etalon-enhanced Grating Diffraction Tunable Filter:

[0054] In the present invention, an improved tunable filter is proposed: an etalon-enhanced grating tunable filter, which is output from an optical receiving device 33 by cascading an FP etalon 31 and a diffraction grating 32. Thus, when the incident angle of the input light 34 changes, their wavelengths can be tuned simultaneously.

[0055] To achieve this, three conditions must be met:

[0056] 1) The FP etalon 31 and the grating 32 are placed vertically (see Figure 3 ), so that the sum of the incident angle θ of the grating 32 and the incident angle α of the FP etalon 31 is That is to say The angle θ or α is determined by the MEMS-controlled steering mirror 42. The above two incident angles will change during the wavelength tuning process. Since the grating 32 and the FP etalon 31 are perpendicular at any time, the incident angle can satisfy

[0057] 2) The fringe pitch P of the grating 32 must be equal to one of the resonant wavelengths λ0 of the FP etalon 31 when the incident angle is 0.

[0058] 3) The incident angle of the grating 32 is determined by the turning mirror 42, but the diffraction angle is zero degrees.

[0059] Under the above three conditions,

[0060]

[0061] Equation (7) shows that when the incident angle θ of the input light 34 changes, the wavelengths of the grating 32 and the FP etalon 31 are tuned simultaneously. Both follow the sine rule.

[0062] Figure 4 A practical MEMS-driven etalon-enhanced diffraction grating tunable filter is presented.

[0063] Specifically, in the order of the optical path, the circulator 41, the MEMS-based steering mirror 42, the relay lens group, the FP etalon 31, the grating 32 and the reflector 44 are sequentially included. The FP etalon 31 and the grating 32 meet the above three conditions; the relay lens group includes the first relay lens 431 and the second relay lens 432 which are sequentially arranged in the order of the optical path. ， The first relay lens 431 and the second relay lens 432 are both positive lenses, and the two lenses have a common focus to form a telescope structure to amplify the collimated light beam reflected from the steering mirror. After passing through the first relay lens 431, the collimated light beam is focused on the rear focal plane of the first relay lens 431. The rear focal plane of the first relay lens 431 is also the front focal plane of the second relay lens 432.

[0064] The light beam is collimated before entering the relay lens group and after leaving the relay lens group. The purpose of the relay lens group is twofold. First, the axis of rotation of the steering mirror 42 is imaged to the position of the grating 32 to reduce the required size of the grating 32. Second, it determines the magnification of the spot size on the grating 32 and the spot size on the steering mirror 42. This magnification is determined by the ratio of the focal lengths of the two lenses.

[0065] The input light beam is input into the tunable filter from the first port 411 of the circulator 41. After the circulator 41 guides the input light beam to the second port 412, it hits the MEMS steering mirror 42, which reflects the light and guides it into the relay lens group. The relay lens group images the rotation axis of the steering mirror 42 to the position of the grating 32. The collimated light spot reflected from the steering mirror 42 is magnified by a certain multiple and roughly imaged on the grating 32. After leaving the relay lens group, the light beam enters the FP etalon 31 at an angle determined by the steering mirror 42, and the light beam is filtered by the FP etalon 31. This is the first stage of filtering. In the first stage of filtering, the frequency of the light that can be filtered through is the resonant frequency of the FP etalon 31. After leaving the FP etalon 31, it immediately enters the grating 32 for the second stage of filtering. In the second stage of filtering, only one resonant frequency can pass through the grating 32. After the light beam leaves the grating 32, it is transmitted to the reflector 44, and the reflector 44 returns the light along the original path, passes through the grating 32 and the FP etalon 31 again, and returns to the second port of the circulator 41 along the original path, and outputs the light through the third port 413 of the circulator 41. Figure 4-Figure 5 In FIG. 4 , the solid line and the dotted line correspond to the incident angles θ and θ′ of the grating 32, respectively. These two rays correspond to the two angles of the turning mirror 42. Figure 5 In, for Figure 4 Except for the counterclockwise rotation of the steering mirror 42, no other settings are made on other devices. The counterclockwise rotation of the steering mirror 42 increases the incident angle entering the etalon, and this angle change causes the wavelength passing through the filter to move toward the short wavelength. Regardless of the incident angle, it complies with the rule embodied in the above equation (7).

[0066] In the above content, a structure of a circulator 41 and a reflector 44 is used, and the reflector 44 returns the light beam to its original path and outputs it through the circulator 41, thereby achieving a double filtering effect. The present invention can also use a collimator device to replace the reflector 44. This collimator receives the light passing through the grating 32 and outputs it directly. This is a single filtering. The advantage of double filtering is that the bandwidth of the tunable filter is relatively narrow, but the difference loss will be relatively large. In some applications, in order to reduce the difference loss, a single filtering solution can be adopted.

[0067] At the same time, as an alternative, the circulator 41 can be replaced with a dual-fiber collimator. The first optical fiber of the dual-fiber collimator receives the input light and converts it into a collimated light beam through the collimating lens. The collimated light beam is then transmitted to the steering mirror 42, and passes through the relay lens group, FP etalon 31, and grating 32 in sequence. After being reflected by the reflector 44, it passes through the grating 32, FP etalon 31, and relay lens group in sequence, and then is reflected by the steering mirror 42 to the collimator (which acts as a focusing lens at this time), enters the second optical fiber, and then is output.

[0068] for Figure 4 For the device shown, since the grating 32 and the FP etalon 31 are connected in series, the transmission spectrum is the product of the spectrum of the grating 32 (broken line trace) and the spectrum of the FP etalon 31 (solid line trace). Figure 7 As shown in Figure 2, it can be seen that the bandwidth of the improved tunable filter is close to that of the comb filter, but there is only one peak. The improved filter can achieve a narrow bandwidth by reducing the FSR and increasing the reflectivity.

[0069] For a 100G comb filter, with a reflectivity of 80%, the bandwidth can be easily less than 30pm. However, the multi-peak nature makes it impossible to use it as a tunable filter over a wide wavelength range. By connecting a grating 32 in series to screen one of the resonance peaks, a single-peak narrowband filter is effectively formed, thereby improving the optical resolution.

[0070] In view of the above, if the tunable filter is made of only the grating 32, in order to achieve a bandwidth of 30 pm and a tuning range of 40 nm, the spot diameter must be greater than 5 cm, which will greatly increase the size and cost of the instrument. If the tunable filter is made of only the FP etalon 31, the FSR must be greater than 40 nm (about 5 Hz in the C band), and the fineness of the FP etalon 31 must be greater than 1300. Such an FP etalon 31 is extremely difficult to manufacture. However, using the improved method provided by the present invention, the FP etalon 31 and the grating 32 are directly cascaded, and a bandwidth of 30 pm and a tuning range of 40 nm can be achieved when the spot size is about 3 mm. In addition to reducing the size and production cost of the instrument, the optical system is also simplified.

[0071] The present invention proposes an improved tunable filter, which improves the wavelength resolution of the tunable filter based on the grating 32 by one order of magnitude through the series structure of the FP etalon 31 and the grating 32, while maintaining the advantages of low cost and small size, and has strong practicality.

[0072] 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. A series structure of a FP etalon and a grating, characterized in that: include: FP etalon and grating connected in series in the order of optical path; The FP etalon is placed vertically to the grating and the grating is arranged on the output light path of the FP etalon, the grating spacing of the grating is equal to one of the resonant wavelengths of the FP etalon, the diffraction angle of the grating is zero, the incident light is subjected to narrowband comb filtering tuning through the FP etalon, and the light after the comb filtering tuning is subjected to single-peak filtering tuning through the grating to obtain filtered and tuned light.

2. The series connection structure according to claim 1, characterized in that: include: The FP etalon includes a first reflecting surface and a second reflecting surface which are disposed on a transparent material and are parallel to each other, wherein the first reflecting surface and the second reflecting surface have the same reflectivity.

3. The series connection structure according to claim 1, characterized in that: The grating is a diffraction grating.

4. An etalon-enhanced grating-based tunable filter, characterized in that: include: A circulator, a steering mirror, a relay lens group, a series structure and a reflector as claimed in any one of claims 1 to 3, which are connected in sequence according to the optical path; The relay lens group images the rotation axis of the steering mirror to the position of the grating, and the FP etalon is arranged on the optical path between the relay lens group and the grating; The input light is transmitted to the steering mirror through the circulator, and reflected to the relay lens group through the steering mirror. The rotation axis of the steering mirror is imaged to the position of the grating through the relay lens group. The relay lens group amplifies the collimated light reflected by the steering mirror, and then enters the FP etalon for first-stage filtering. The angle at which the light enters the etalon is determined by the rotation angle of the steering mirror. After the first-stage filtering, the light beam enters the grating after leaving the FP etalon for second-stage filtering. The second-stage filtering is completed by the grating and the subsequent reflector. The grating disperses light of different wavelengths to different angles, and only the light with an angle perpendicular to the rear reflector can be returned by the original route. The light beam passes through the grating and the FP etalon for the second time on the return journey, returns to the circulator by the original route, and outputs the light through the third port of the circulator.

5. The tunable filter according to claim 4, characterized in that: The steering mirror is a MEMS steering mirror.

6. The tunable filter according to claim 4, characterized in that: The relay lens group includes a first relay lens and a second relay lens; the first relay lens and the second relay lens have a common focal point to form a telescope structure, and the collimated light beam is focused on the rear focal plane of the first relay lens after passing through the first relay lens. The rear focal plane of the first relay lens is also the front focal plane of the second relay lens. The collimated light entering the first relay lens is still collimated after passing through the second relay lens.

7. The tunable filter according to claim 4, characterized in that: The circulator includes a first port for receiving input light, a second port for transmitting the input light to the redirector, and a third port for outputting output light.

8. The tunable filter according to claim 4, characterized in that: The circulator is replaced by a dual-fiber collimator, the first optical fiber in the dual-line collimator replaces the first port and the second port of the circulator to receive input light and transmit the input light to the steering mirror, and the second optical fiber in the dual-line collimator replaces the second port and the third port of the circulator to output the light reflected from the last reflector of the filter.

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