CN100405104C - Tunable Bandpass Filter Based on Spatial Birefringent Elements - Google Patents

Abstract

A tunable bandpass filter based on a spatial birefringence element is characterized in that an input optical fiber, a collimating lens, a polarizer, K (1 < K ≤ 3) cascaded wedge-shaped medium pairs, an analyzer, a focusing lens and an output optical fiber are sequentially arranged along the forward direction of the input light beam, wherein the cascaded wedge-shaped medium pairs are packaged as a whole and connected to a stepping motor. The invention has a large flat width passband and stopband and high isolation, and also has the tunable characteristics of the central wavelength and bandwidth.

Description

Tunable Bandpass Filter Based on Spatial Birefringent Elements

Technical field:

The present invention relates to a tunable bandpass filter applied in an optical system, especially a tunable bandpass filter based on a spatial birefringence element. It is mainly used in an optical wavelength division multiplexing (WDM) system. In addition, it can also be used For all tuned filters that require flattened passband and stopband.

Background technique:

Optical bandpass filters are key devices in optical wavelength division multiplexing (WDM) systems, optical communication-related measurements, and other optical system applications. In WDM systems, each receiver at the end must select the required channel. Tunable A filter is a wavelength (or frequency) selective device whose function is to select an optical signal of a specific frequency from multiple input optical signals of different frequencies, which is very important in WDM systems. In order to maximize performance Optimizing, optical bandpass filters should have large flat passband and stopband widths, high isolation, tunable characteristics of central wavelength (or frequency) and bandwidth.

Described in the prior art [1] (see M.Oguma, K.Jinguji, T.Kitoh, T.Shibataand A.Himeno, Electron.Lett.36(15), 2000, 1299-1300) is a plane-based Optical interleaving filter with waveguide lattice structure. It adopts a two-stage lattice structure and uses planar waveguide circuit technology to realize the filtering characteristics of flattened passband and stopband, but it does not have tunable characteristics of wavelength or bandwidth.

The prior art [2] (see W.Warzanskyj, F.Heismann and R.C.Alferness, Appl.Phys.Lett, 1988, 53(1), 13-15) describes a polarization-independent electro-optic using a single-mode waveguide Tune narrowband filters. The bandwidth of this filter is very narrow, and it only has the tunable characteristic of the center wavelength.

The prior art [3] (see W. J. Carlsen and C. F. Buhrer, Eltctron. Lett. 23(3), 1987, 106-107) describes a cascaded birefringent polarization interferometry technique that produces a flattened spectral transmittance function. It uses three birefringent crystal wave plates with a thickness ratio of 1:2:2. By controlling the azimuth angle of the crystal wave plate, the spectral transmittance with flattened passband and stop band is realized, but the isolation and flatness of the transmittance The passband width is not very good, and it does not have the tunable characteristics of center wavelength or bandwidth.

Invention content:

The technical problem to be solved by the present invention is to overcome the difficulties of the above-mentioned prior art and provide a tunable bandpass filter based on spatial birefringence elements, which has a large flat width in the passband and stopband and high isolation At the same time, it also has tunable characteristics of central wavelength and bandwidth.

The idea of the present invention is to adopt the structure of cascaded wedge-shaped medium pairs, and to achieve a Large flat passband and stopband width and high isolation filtering characteristics. Among them, the tunable characteristics of central wavelength and bandwidth are realized by changing the optical path difference of light in each wedge-shaped medium pair.

Concrete technical solution of the present invention is as follows:

A tunable bandpass filter based on a spatial birefringence element, characterized in that an input optical fiber, a collimating lens, a polarizer, and K (1<K≤3) cascaded Wedge-shaped dielectric pair, analyzer, focusing lens and output fiber, in which the cascaded wedge-shaped dielectric pair is packaged as a whole and connected with a stepping motor.

The K cascaded wedge-shaped dielectric pairs are sequentially cascaded, and the wedge-shaped dielectric pairs are composed of two wedge-shaped dielectric sheets, wherein the material of the first dielectric sheet is a crystal, and the optical axis is parallel to the crystal surface, and the light beam The incident direction is vertical, and the material of the second dielectric sheet can be glass or crystal. If the material of the second dielectric sheet is crystal, its optical axis is the same as the incident direction of the light beam; when the material of the second dielectric sheet in the wedge medium pair When it is a glass material, the refractive index of the selected glass is close to the two intrinsic refractive indices of the crystal; when the two dielectric sheets in the wedge-shaped medium pair are both crystal materials, the ordinary light refractive index of the selected second dielectric sheet is the same as The two intrinsic refractive indices of the first dielectric sheet are close to each other.

The shape of the medium sheet in the wedge-shaped medium pair is a wedge with a base angle equal to 90°, both ends of which have a certain width, or only one end has a certain width.

When the number K=2 of the selected cascaded wedge-shaped medium pairs, the shape of the two medium pairs satisfies the optical path difference of light propagating in the medium pair is 1:2, and the first dielectric sheet in the two medium pairs The azimuth angles θ ₁ , θ ₂ and the angle θ _P between the analyzer and polarizer polarization direction should meet the following angle combinations:

Combination θ ₁ (°) θ ₂ (°) θ _P (°)

1 45 -79 -4

2 45 -78 -4

3 45 -77 -4

4 45 -76 -4

5 45 -14 4

6 45 -13 4

7 45 -12 4

8 45 -11 4

When the number K=3 of the selected cascaded wedge-shaped medium pairs, the shape of the three medium pairs satisfies the optical path difference of light propagating in the medium pairs is 1:2:2, and the first medium of the three medium pairs The azimuth angles θ ₁ , θ ₂ , θ ₃ of the slice satisfy the following angle combinations in the text:

Combination θ ₁ (°) θ ₂ (°) θ ₃ (°)

1 45 -72 84

2 45 -18 6

3 45 -71 83

4 45 -19 7

5 45 -70 82

6 45 -20 8

7 45 -70 83

8 45 -20 7

9 45 -69 81

10 45 -21 9

11 45 -69 82

12 45 -21 8

Technical effect of the present invention:

The tunable bandpass filter based on the spatial birefringence element of the present invention simultaneously realizes the spectral characteristics of the large flat and high-contrast passband and stopband and the tunable characteristics of the center wavelength and bandwidth. Compared with each prior art, it overcomes It overcomes the tunable characteristics of the prior art [1] that cannot realize the central wavelength and bandwidth; overcomes the tunable characteristics of the prior art [2] that cannot realize the flattened passband, stopband and bandwidth; overcomes the prior art [2]. 3] Insufficient isolation and flattening width, and the tunable characteristics of central wavelength and bandwidth cannot be realized.

Description of drawings:

Fig. 1 is a schematic structural diagram of an embodiment of a tunable bandpass filter based on a spatial birefringent element in the present invention.

Figure 2 shows the variation of the distance that a light beam with a certain width passes when propagating in the dielectric sheet 11 in the wedge-shaped dielectric pair.

Fig. 3 is a tunable characteristic diagram of the central wavelength of the embodiment of the present invention.

Fig. 4 is a characteristic diagram of bandwidth tunability according to an embodiment of the present invention.

Detailed ways:

Please refer to Fig. 1 first, as can be seen from the figure, the tunable bandpass filter based on the spatial birefringence element of the present invention includes an input optical fiber 1, a collimator lens 2, a polarizer 3, and K (1<K≤3) stages Coupled wedge media pairs 4, 5, 6, stepping motor 7, analyzer 8, focusing lens 9 and output fiber 10.

The K cascaded wedge-shaped medium pairs are sequentially formed by cascading wedge-shaped medium pairs 4, 5, and 6. Said above-mentioned wedge-shaped dielectric pairs are all made up of two wedge-shaped dielectric sheets 11 and 12, wherein the material of the first dielectric sheet 11 is crystal, the optical axis is parallel to the crystal surface, and is perpendicular to the incident direction of the light beam, and the second dielectric sheet 12 The material is glass or crystal (the optical axis is the same as the incident direction of the light beam). When the second dielectric sheet 12 in the wedge-shaped medium pair is glass material, the refractive index of the selected glass is close to the two intrinsic refractive indices of the crystal; when the wedge-shaped medium When the second dielectric sheet 12 in the dielectric pair is all crystal materials, the ordinary light refractive index of the selected dielectric sheet 12 is close to the two intrinsic refractive indices of the first dielectric sheet 11. The first dielectric sheet 11 in the wedge-shaped dielectric pair The shape of the second dielectric sheet 12 is a wedge with a base angle equal to 90°, which can be a wedge with a certain width at both ends, or a wedge with a certain width at only one end. When the number of selected cascaded wedge-shaped medium pairs is K=2, the shape of the two medium pairs satisfies the optical path difference of light propagating in the medium pair is 1:2, and the first dielectric sheet in the two medium pairs The azimuth angle of 11, that is, the deflection angles θ ₁ and θ ₂ of the crystal fast axis or slow axis relative to the polarization direction of the input light and the angle θ _P between the analyzer and the polarization direction of the polarizer meet the angles shown in Table 1 in the text Combination. When the number of selected cascaded wedge-shaped medium pairs is K=3, the shape of the three medium pairs satisfies the optical path difference of light propagating in the medium pair is 1:2:2, and the three medium pairs The azimuth angle of the first dielectric sheet 11 satisfies the angle combinations shown in Table 2 in the text, where the polarization directions of the analyzer and the polarizer are parallel.

The collimating lens 2 is used to collimate and expand the randomly polarized WDM signal beam x emitted by the input optical fiber 1. The collimating lens 2 outputs a parallel thin beam, and its polarization direction after entering the polarizer 3 is the same as that of the polarizing beam. The polarization direction of the filter is the same, and then the incident cascade medium pair 4, 5, 6. The light beams output from the cascaded medium pair 4, 5, 6 are then incident on the analyzer 8, and finally focused by the focusing lens 9 and coupled into the output optical fiber 10 to finally output the light beam y.

The cascaded wedge-shaped dielectric pair in the polarizer 3 and the polarizer 8 is the core of the whole system. The first dielectric sheet 11 (crystal) in each cascaded dielectric pair is used to generate a certain phase delay; the second dielectric sheet 12 The refractive index of (glass or crystal) is close to the intrinsic refractive index of the first dielectric sheet 11 (crystal), which is used to compensate the refraction of light at the interface of the two dielectric sheets, so that the light beam exits each medium pair along the original incident direction. The light beam incident on the cascaded medium pair first enters the first medium pair, and is decomposed into two polarized lights whose polarization directions are orthogonal to each other in the first medium plate 11. After a certain distance, a certain phase delay is generated, and then enters the The second dielectric sheet 12 in the medium pair, since the refractive index of the dielectric sheet 12 is close to the intrinsic refractive index of the dielectric sheet 11, the refraction of light at its interface can be ignored, and the two polarized lights finally exit the medium along the original direction Right. In the second medium pair, since the azimuth angle of the medium sheet 11 in the medium pair is different from the azimuth angle of the previous medium sheet 11, in the medium sheet 11 of the second medium pair, the two The two polarized lights are respectively decomposed into two polarized lights whose polarization states are orthogonal to each other, and a certain phase delay is produced after passing through the dielectric sheet. The characteristic refractive index is close, and the light beam finally exits the second medium pair along the original direction. After passing through multiple medium pairs with the same structure in this way, a plurality of different beams are generated, and finally these light beams pass through the polarizer 8 Interference occurs in the polarization direction, wherein by controlling the azimuth angle of the dielectric sheet 11 in each medium pair, the angle between the polarizer and the polarizer polarization direction and the optical path difference of the light in it, to produce a flattened passband and stop Finally, the optical signal is focused by the focusing lens 9 and coupled into the output fiber 10 for output. The stepper motor 7 connected to the entire cascaded wedge-shaped medium pair is used to move the entire cascaded wedge-shaped medium pair in the vertical direction , so that the optical path difference of light in each wedge-shaped medium pair can be adjusted to realize the tuning of the central wavelength and bandwidth.

The analysis for above-mentioned invention is as follows:

Flattened Spectral Transmittance

Under the premise that the beam width can be ignored, the cascade behavior of the wedge-shaped cascaded medium pair is the same as that of a common equal-thickness crystal wave plate. Corresponding to a certain wedge-shaped medium pair thickness position, its spectral transmittance can be expressed as:

T(f)＝T ₀ +T ₁ cos(2πγ′ ₁ f)+T ₂ cos(2πγ′ ₂ f)+....+T _n cos(2πγ′ _n f)+.... (1)

Among them, T _n is a coefficient related to the azimuth angle θ ₁ , θ ₂ , ... θ _K of the first dielectric sheet 11 in each wedge-shaped medium pair, and γ′ _n is the medium in each wedge-shaped medium pair at this position The time delay γ ₁ , γ ₂ , ... γ _K of slice 1 is the difference term, sum term or sum difference term of individual or any combination thereof. Wherein, the time delay of the medium slice 11 in each wedge-shaped medium pair at this position The delay γ _k or the frequency interval Δf _k can be expressed as:

${\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;n</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}}{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Here Δn is the birefringence of the dielectric sheet 11, d _k is the thickness of the dielectric sheet 11 at this position, and C is the speed of light.

Spectral transmittance with flattened passband and stopband can be obtained by controlling the azimuth and thickness of each wedge-shaped dielectric centering dielectric sheet 11 and the included angle between the polarizer and polarizer polarization directions. The following table 1 shows that when two wedge-shaped dielectric pairs with a thickness ratio of 1:2 are cascaded, the flat width of the passband and stop band is greater than 2/11 period, and the isolation is not less than -22dB, each wedge-shaped dielectric pair The azimuth angles θ ₁ , θ ₂ of the dielectric sheet 1, and the angle θ _p between the polarizer and the polarization direction of the polarizer should satisfy the angle values. The following table 2 shows when the thickness ratio of the three pieces is 1:2: In the cascaded structure of 2 wedge-shaped dielectric pairs, when the flat width of the passband and stopband is greater than 2/11 period, and the isolation degree is not less than -30dB, the azimuth angles θ ₁ , θ ₂ and Angle value that _θ3 should satisfy (at this time, the polarization direction of the analyzer and the polarizer is parallel, that is, θ _p =0°).

Table 1 Table 2

tuning characteristics

By adjusting the optical path difference of the light in each wedge-shaped medium pair, while keeping the ratio of the optical path difference at a certain value, the tuning characteristics of the central wavelength and bandwidth can be realized without affecting the shape of the spectral transmittance.

If the entire cascaded wedge-shaped medium pair moves along the vertical direction, the optical path difference of light in each wedge-shaped medium pair can be adjusted, and the ratio of the optical path difference can be kept unchanged, so that the tuning characteristic can be realized.

When just starting to move, the distance that the light passes in the dielectric sheet 11 changes very little, and the change of the bandwidth is negligible at this time, and only the central wavelength shifts; with the continuous movement of the whole cascaded wedge-shaped medium pair, The distance passing through the dielectric sheet 11 changes more and more, at this time, the change of the bandwidth cannot be ignored, and the change of the central wavelength and the bandwidth exist at the same time.

Beam Width Limitations

The above characteristics are all obtained under the condition that the beam width can be ignored. If the beam width cannot be ignored, the corresponding thickness of the light propagating in the wedge-shaped medium pair is not unique, that is, there is a certain deviation, which will cause certain blurring of the spectral characteristics. Therefore, the beam width must be limited.

In Fig. 2, let the beam width be D, the shortest distance that light of this width passes in the dielectric sheet 11 is d _i , the longest distance is d _i + δd _i , and the wedge angle is α. From the above analysis, the beam When the distance passed in the dielectric sheet 11 changes from d _i to d _i + δd _i , the corresponding center wavelengths are shifted respectively. In order not to affect the crosstalk (ie, isolation) of the device, the maximum translation amount must be required not exceed the half-width of the passband.

The frequency intervals corresponding to the shortest and longest distances that the light beam of this width passes through in the dielectric sheet 11 are respectively:

${\mathrm{<span\; class="notranslate">\; \&Delta;f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; C</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;n</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}<span\; class="notranslate">\; ;</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; \&Delta;f</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; C</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;\; n</span>}<span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Then the translation amount of the center frequency is

$\mathrm{<span\; class="notranslate">\; \&delta;\; f</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&Delta;\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; ctg\&alpha;</span>}}{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; ctg\&alpha;</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

If the frequency width of the passband is L, then the restriction on the beam width D is

$\frac{\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; ctg\&alpha;</span>}}{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; ctg\&alpha;</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

When the entire wedge-shaped medium pair in cascade moves along the vertical direction, d _i changes between the short side and the long side of the wedge-shaped medium sheet 11, and d _i should be calculated by taking the length of the short side of the medium sheet 11. Different wedge-shaped media The length of the short side of the dielectric sheet 11 is different, and the size of the wedge angle is also different. Finally, the limit of the beam width is calculated according to the requirements according to (6) and the smallest value is taken.

Coarse wavelength division multiplexing (CWDM) system is generally considered to be a low-cost substitute of DWDM system. Fig. 1 is the optical system structure of the embodiment used in the coarse wavelength division multiplexing system of the present invention K=3 Schematic diagram. Among them, the material of the dielectric sheet 11 in each wedge-shaped dielectric pair is quartz crystal, and the material of the dielectric sheet 12 is the glass of the Soviet brand FK13. The ratio of the light-passing distance of the light in the dielectric sheet 11 of the three wedge-shaped dielectric pairs is 1:2:2, and the azimuth angles are 45°, -70° and 82° respectively. In the 1.55μm band, the intrinsic refractive index of quartz is: n _o = 1.528, _ne = 1.536, and the refractive index of FK13 glass is n = 1.531. It can be seen that the refractive index of the selected glass and the intrinsic double of quartz The refraction is very close, and the light travels in each medium pair with almost no direction deflection. The frequency interval corresponding to the long side of the crystal plate in the first dielectric pair is 2500GHz, that is, the wavelength interval is 20nm, and the length of the short side is one-third of the long side. The length of the long side and short side of the first wedge-shaped crystal plate can be calculated About 14.1mm and 4.7mm, the lengths of the long side and short side of the other two wedge-shaped crystal plates are twice the length of the first wedge-shaped crystal plate respectively. The wedge angle of the selected first wedge-shaped crystal is α ₁ =60°, Then the wedge angles of the second and third wedge-shaped crystals are α ₂ ≈41°, and the heights of the three wedge-shaped crystal plates are all h=16.3mm. When the azimuth angles of the three crystal plates are 45°, -70° and 82° respectively, the ratio of the passband width to the period can be calculated to be 0.202, and the width of the incident beam can be limited to D ≤0.9mm. Figure 3 shows the tunable characteristics of the center wavelength of the filter, where the solid line, dotted line and dotted line correspond to the cases where the distances of the first crystal passed are 14.03mm, 14.05mm and 14.07mm, respectively. Figure 4 shows the tunable characteristics of the bandwidth of the filter, where the solid line, dotted line and dotted line correspond to the cases where the distances of the first crystal passed are 4.7mm, 5.4mm and 14.1mm respectively.

Claims (6)

1. tunable bandpass filters based on the Space Double refracting element, it is characterized in that being provided with the wedge-shaped medium of input optical fibre (1), collimation lens (2), the polarizer (3), a K cascade successively to (4 along the working direction of input beam, 5,6), analyzer (8), condenser lens (9) and output optical fibre (10), this K cascade wedge-shaped medium is to (4,5,6) encapsulation is as a whole and link to each other with a step motor (7), wherein K=2 or 3.

2. tunable bandpass filters according to claim 1, it is characterized in that described K cascade wedge-shaped medium is to (4,5,6) cascade constitutes successively, described wedge-shaped medium is to (4,5,6) form by two wedge-shaped medium sheets (11,1 2), wherein the material of first dieelctric sheet (11) is a crystal, and optical axis is parallel to plane of crystal, vertical with the light beam incident direction, the material of second dieelctric sheet (12) is a glass, and two intrinsic refractive indexes of the refractive index of selected glass and first dieelctric sheet (11) crystal are approaching.

3. tunable bandpass filters according to claim 1, it is characterized in that described K cascade wedge-shaped medium is to (4,5,6) cascade constitutes successively, described wedge-shaped medium is to (4,5,6) form by two wedge-shaped medium sheets (11,12), wherein the material of first dieelctric sheet (11) is a crystal, and optical axis is parallel to plane of crystal, and is vertical with the light beam incident direction, and the material of second dieelctric sheet (12) is a crystal, the optical axis of this crystal is identical with the light beam incident direction, and two intrinsic refractive indexes of the ordinary refraction index of this crystal and first dieelctric sheet (11) are approaching.

4. tunable bandpass filters according to claim 2, it is characterized in that described wedge-shaped medium centering each piece dieelctric sheet (11,12) be shaped as the wedge shape that a base angle equals 90 °, its two all has the wedge shape of certain width, or has only a wedge shape with certain width.

5. according to claim 2 or 3 described tunable bandpass filters, it is characterized in that when the right number K of selected cascade wedge-shaped medium=2, two media are to being 1: 2 satisfying the optical path difference that light propagates medium centering in shape, the azimuth angle theta of first dieelctric sheet (11) that two each media of medium centering are right ₁, θ ₂Angle theta with analyzer and polarizer polarization direction _PShould satisfy any of following 1 to 8 kind combination of angles:

Combination θ ₁(°) θ ₂(°) θ _P(°)

1 45 -79 -4

2 45 -78 -4

3 45 -77 -4

4 45 -76 -4

5 45 -14 4

6 45 -13 4

7 45 -12 4

8 45 -11 4

6. according to claim 2 or 3 described tunable bandpass filters, it is characterized in that when the right number K of selected cascade wedge-shaped medium=3, three media are to (4,5,6) be 1: 2: 2 satisfying the optical path difference that light propagates medium centering in shape, the azimuth angle theta of first dieelctric sheet (11) that three each media of medium centering are right ₁, θ ₂, θ ₃Satisfy any of following 1 to 12 kind of combination of angles in the literary composition:

Combination θ ₁(°) θ ₂(°) θ ₃(°)

1 45 -72 84

2 45 -18 6

3 45 -71 83

4 45 -19 7

5 45 -70 82

6 45 -20 8

7 45 -70 83

8 45 -20 7

9 45 -69 81

10 45 -21 9

11 45 -69 82

12 45 -21 8

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