CN106772751B - Quasi-comb filter with gradually changed wavelength interval - Google Patents
Quasi-comb filter with gradually changed wavelength interval Download PDFInfo
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- CN106772751B CN106772751B CN201710003231.1A CN201710003231A CN106772751B CN 106772751 B CN106772751 B CN 106772751B CN 201710003231 A CN201710003231 A CN 201710003231A CN 106772751 B CN106772751 B CN 106772751B
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/20—Filters
- G02B5/28—Interference filters
- G02B5/285—Interference filters comprising deposited thin solid films
- G02B5/286—Interference filters comprising deposited thin solid films having four or fewer layers, e.g. for achieving a colour effect
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
- G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters
Abstract
The invention discloses a quasi-comb filter with gradually changed wavelength intervals, which consists of a substrate and a quadruple-period multilayer film { [ (HL) arranged on a first surface of the substrate p 1 (xHyL) p 2 ] q } s And H and L on the second surface of the substrate alternately, p 1 、p 2 Q and s are the number of cycles of the quadruple cycle, (HL) p 1 And (xHyL) p 2 Two one-dimensional photonic crystals with different thicknesses are used for generating two photonic forbidden bands, x and y are the thickness coefficients of the quarter-wavelength film layers, H and L are a high-refractive-index film layer and a low-refractive-index film layer with the thickness of the quarter-wavelength film respectively, and the high-refractive-index material is Ti 3 O 5 The low refractive index material is SiO 2 The substrate is a glass substrate. The quasi-comb filter can be used for changing the bandwidth of DWDM in a nonlinear optical communication wavelength division multiplexing system and improving the multiplexing degree of the wavelength division multiplexing system.
Description
Technical Field
The invention relates to a quasi-comb filter with gradually changed wavelength intervals, which is used for changing the bandwidth of DWDM in a nonlinear wavelength division multiplexing system such as optical communication and improving the multiplexing degree.
Background
In the optical communication Wavelength Division Multiplexing system, a comb filter is connected with a DWDM (Dense Wavelength Division Multiplexing) filter in series, so that the Multiplexing degree can be improved.
The existing comb-shaped filter adopts a Fabry-Perot (Fabry-Perot) etalon of which a spacing layer is a quartz crystal as a high-order spacing layer, and then high-reflection films are respectively plated on two sides of the quartz crystal to form a high-order single-cavity interference filter, such as: a | (HL) p Quartz crystal H (LH) p | A or A | (HL) p Quartz crystal (LH) p A where p is a positive integer representing the number of periods, H and L are high and low refractive index quarter wave film layers, respectively, ARepresenting air. There are problems in that: first, the comb filter of the fabry-perot interference filter type is very difficult to control the position of the filter wavelength; secondly, it is difficult to control the phase compensation between the quartz crystal and the film layer during the actual manufacturing; thirdly, the filter cannot be made into multiple cavities, so that the rectangular coefficient of a transmission curve is poor; fourth, and even more, the wavelength and waveform of the transmission band of the comb filter are highly non-uniform because the optical thickness uniformity of the etalon is far from the requirement of the spacer layer of the interference filter.
Therefore, engineering technicians propose that a mica sheet with a cleavage surface structure is used for replacing a quartz crystal etalon to serve as a high-order spacing layer, so that the problems of uneven transmission band wavelength and waveform of the comb filter are relieved to a certain extent, but the other three problems still cannot be solved. In fact, engineers have long proposed an all-film comb filter design, with an example of G if a transmission band wavelength spacing of 1nm is desired (HLHLHLH 3060 LHLHL) 2 L | a, where G denotes the substrate and a denotes air, this design can solve the four problems presented above simultaneously, but the new one is: such thick thin film spacer layers have hitherto not been possible to plate. This is not to be expected, if a denser transmission band wavelength spacing of 0.5nm is required, the film spacer thickness would be as high as 6120L, i.e. G | (HLHLHLH 6120 LHLHLHLHL) 2 L | a, such a film thickness is simply not conceivable for manufacturing. Because of this, what can really be implemented now is: and (3) forming the Fabry-Perot single-cavity optical filter by using a mica sheet as a high-order spacing layer to obtain the comb-shaped optical filter.
The invention provides a design for exploring a comb-shaped optical filter based on a semiconductor superlattice theory, and the comb-shaped optical filter is called a quasi-comb-shaped optical filter because the wavelength interval of the designed comb-shaped optical filter is gradually changed, and can be suitable for a nonlinear-change wavelength division multiplexing system. The quasi-comb filter is also a filter of a full-film system, but the total thickness of the film is greatly reduced, so that the quasi-comb filter can be really manufactured by the existing equipment and the existing technology. For the quasi-comb filter with the transmission band wavelength interval of 0.5nm density, the total layer number of the film is countedThe number of transmission bands is 336-560 film layers with quarter wavelength thickness, the total film thickness of the quasi-comb filters is only about the whole-film comb filter G | (HLHLHLH 6120 LHLHL) 2 2.7% -4.6% of the total thickness of the L | a, so that the actual manufacturing can be performed using existing equipment and techniques.
Disclosure of Invention
The invention aims to provide a quasi-comb filter with gradually changed wavelength intervals, which is used for changing the bandwidth of DWDM in a nonlinear wavelength division multiplexing system such as optical communication and improving the multiplexing degree of the wavelength division multiplexing system.
The concept of the invention is as follows: since the design of new comb filters or quasi-comb filters currently lacks design ideas and methods, the present invention seeks to explore the design methods by means of the semiconductor superlattice theory. It is known that the behavior of photonic crystals is very similar to that of semiconductor crystals, thin-film one-dimensional photonic crystals are similar to the one-dimensional periodic potential structure of semiconductors, and the transmission behavior of photons is also similar to that of carriers, so that constructing a new comb filter or quasi-comb filter by using the semiconductor superlattice theory is probably a good design idea and design method.
The superlattice concept is a new concept originated from semiconductor materials, a one-dimensional periodic potential structure can be realized by using ultrathin layers which are formed by alternating two materials with different electronic band gaps, and the carrier transmission behavior of the superlattice structure is completely different from that of a three-dimensional semiconductor bulk material because the lattice potential field can be changed by the ultrathin layers with different periods. When the barrier width of the crystal lattice is narrow, the wave functions of adjacent potential wells are overlapped, so that carriers can pass through the potential barrier through a tunnel effect, and the structure is called a superlattice; on the contrary, when the barrier width is wide, the wave functions of the adjacent potential wells are independent, so that the carriers are completely confined in the independent potential wells, and the structure is called a quantum well.
In the case of thin film one-dimensional photonic crystals, the only alternative dielectric materials for photonic crystals are different dielectric constants rather than different band widths for semiconductor crystals, and thus the superlattice concept of semiconductors can be introduced into thin film one-dimensional photonic crystals. Like a superlattice formed by semiconductor crystals with different bandwidths, photonic crystal superlattices can be formed by different optical films, and the frequency of a plurality of pass bands formed by passing through forbidden bands is obtained by using the concept of one-dimensional photonic crystal superlattices, namely, a multi-transmission-band comb filter in photonic forbidden bands is obtained. The design steps of the method are that firstly, the energy band structure of the one-dimensional photonic crystal is calculated, then the information of photon forbidden bands and pass bands is obtained, and finally the spectral characteristics of the comb-shaped optical filter are calculated. In calculating the spectral characteristics of the photon forbidden band (cut-off band) and pass band (transmission band), the most intuitive method is the transmission matrix method, so the calculation can be directly performed by design software (such as TFCal) in thin film optics.
The comb filter designed by utilizing the superlattice concept of the thin film one-dimensional photonic crystal is different from a Fabry-Perot filter, and according to the semiconductor superlattice theory, the comb filter can be simply expressed into a multilayer film with a quadruple periodic structure by means of two one-dimensional photonic crystals: { [ (HL) p 1 (xHyL) p 2 ] q } s Wherein p is 1 、p 2 Q and s are the number of cycles of the quadruple cycle, respectively, and are positive integers, the basic cycle (HL) p 1 And (xHyL) p 2 The reflector is a one-dimensional photonic crystal reflector with different wavelengths, x and y are quarter-wavelength film thickness coefficients, and H and L respectively represent a quarter-wavelength film thick high-refractive-index film and a quarter-wavelength film thick low-refractive-index film. It is clear that this structure is completely different from a Fabry-Perot interference filter, in which the basic periodic structure (HL) p 1 (xHyL) p 2 The photonic crystal is a photonic crystal with two different thicknesses and can generate two photon forbidden bands, but according to the photonic crystal superlattice theory, photons with certain frequencies can obtain different multiple narrow transmission bands through a tunneling effect. The simplest example of the basic periodic structure of a fabry-perot interference filter is: (HL) p HLLH(LH) p Or (HL) p HH(LH) p Where LL and HH are spacer layers, the spacer layers must satisfy integer multiples of half-wavelength. It is clear that the center wavelength of such a fabry-perot structure is itself a photon passband, not a photon forbidden band, because of all LL or HH halvesThe wave layers are all dummy layers, so that LL or HH are eliminated one by one from the middle of the basic periodic structure of the Fabry-Perot interference filter, and finally the whole basic periodic structure can be eliminated completely, so that one transmission band is generated, which is an explanation of the reason why the Fabry-Perot interference filter only generates one transmission band at the central wavelength.
The comb filters designed in the above-described manner are generally quasi-comb filters, and the transmission bands at both ends of the quasi-comb filters become very narrow, and the passband waveforms on the short-wave side and the long-wave side, which do not meet the use requirements, need to be cut off by means of a high-steepness bandpass filter provided on the second surface of the substrate.
In order to achieve the purpose, the invention adopts the following specific technical scheme:
a quasi-comb filter with gradually changed wavelength interval comprises a substrate and a quadruple-period multilayer film { [ (HL) arranged on a first surface of the substrate p 1 (xHyL) p 2 ] q } s And a bandpass filter on the second side of the substrate;
wherein p is 1 、p 2 Q and s are the number of cycles of the quadruple-cycle multilayer film respectively and are positive integers;
h and L respectively represent a high refractive index film layer and a low refractive index film layer with quarter-wavelength film thickness;
x and y are quarter-wavelength film thickness coefficients;
the high-refractive-index film layer is titanium oxide (Ti) 3 O 5 ) The low refractive index film layer is silicon dioxide (SiO) 2 )；
The substrate is optical glass.
In the invention, the first surface and the second surface of the substrate are two opposite surfaces on the substrate, the substrate is optical glass, and one surface of the optical glass is provided with a quadruple period multilayer film { [ (HL) p 1 (xHyL) p 2 ] q } s And a band-pass filter is arranged on the other surface of the optical glass. Basic cycle (HL) p 1 And (xHyL) p 2 Is one-dimensional light of different thicknessAnd x and y are quarter-wavelength film thickness coefficients, and H and L respectively represent a high refractive index film and a low refractive index film which are quarter-wavelength film thick. The high refractive index film layer is titanium oxide (Ti) 3 O 5 ) The low refractive index film layer is silicon dioxide (SiO) 2 ). The band-pass filter consists of H and L alternate band-pass filters.
The quadruple period multilayer film { [ (HL) p 1 (xHyL) p 2 ] q } s Triple-period multilayer film consisting of s repeating units [ (HL) p 1 (xHyL) p 2 ] q The triple-period multilayer film [ (HL) p 1 (xHyL) p 2 ] q Double-period multilayer film (HL) consisting of q repeating units p 1 (xHyL) p 2 Composition, double period multilayer film (HL) p 1 (xHyL) p 2 From p 1 A HL alternating layer and p 2 And a plurality of xHyL alternating layers. The HL alternating layers are composed of high refractive index film layers with the film thickness of a quarter wavelength and low refractive index film layers with the film thickness of a quarter wavelength. The xHyL alternating layers are composed of high refractive index film layers with thickness coefficient x multiplied by the thickness of a quarter-wave film and low refractive index film layers with thickness coefficient y multiplied by the thickness of a quarter-wave film.
Further, the number p of cycles of the quadruple-cycle multilayer film 1 、p 2 The method mainly represents the transmission wavelength position of the quasi-comb filter, the cut-off degree of a cut-off area and the half width of a transmission band, and the number q and s of cycles represent the number of the transmission bands of the quasi-comb filter.
Further, the number of cycles p 1 、p 2 And taking the equivalent value and taking the value of an integer from 6 to 8, taking the value of the period number q as an integer from 2 to 6, and taking the value of s as an integer from 2 to 6. Specifically, such as p 1 、p 2 The value 7, q 5, s 4.
Furthermore, the film thickness coefficient x, y is equivalent and the value is 0.5-0.9 or 1.1-3. Specifically, x and y both take the value of 0.8.
Further, the band-pass filter is composed of three cyclesThe four-cavity Fabry-Perot filter consists of a substrate and a three-period four-cavity Fabry-Perot filter [ (HL) 4 HLLHHH(LH) 4 L(HL) 5 HLLHHH(LH) 5 L(HL) 5 HLLHHH(LH) 5 L(HL) 4 HLLHHH(LH) 4 L] 3 。
In order to improve the pass band transmittance, the outermost two-layer film thickness on the air side was corrected from H and L to 1.243H and 1.326L, respectively. I.e. from the substrate outwards, the three-period four-cavity Fabry-Perot filter is [ (HL) 4 HLLHHH(LH) 4 L(HL) 5 HLLHHH(LH) 5 L(HL) 5 HLLHHH(LH) 5 L(HL) 4 HLLHHH(LH) 4 L] 2 [(HL) 4 HLLHHH(LH) 4 L(HL) 5 HLLHHH(LH) 5 L(HL) 5 HLLHHH(LH) 5 L(HL) 4 HLLHHH(LH) 3 L1.243H1.326L]。
Further, the high-refractive-index film layer is titanium oxide (Ti) 3 O 5 ) The refractive index at the wavelength of 1550nm is 2.27, and the low-refractive-index film layer is made of silicon dioxide (SiO) 2 ) A refractive index at 1550nm of 1.443; the refractive index of the substrate at 1550nm is 1.52, and a substrate WMS-13 can be selected specifically.
Furthermore, the substrate is optical glass with high thermal expansion coefficient, and the linear expansion coefficient of the optical glass is 90-130 multiplied by 10 -7 A specific example of the linear expansion coefficient of WMS-13 produced by OHARA, japan is 110X 10/. Degree.C -7 A linear expansion coefficient of 20 to 30 x 10 at/° C relative to the film material -7 W/C, WMS-13 may be referred to as a high CTE material. This high thermal expansion coefficient characteristic of the substrate WMS-13 can compensate for the shift in the wavelength of the filter transmission band due to the change in ambient temperature. The principle is that when the temperature of the wavelength division multiplexing system rises, on one hand, the thickness of each film layer is thickened due to the expansion and contraction effect of the film, so that the wavelength of each transmission band of the comb-shaped optical filter drifts to long wave, on the other hand, the thickness of each film layer is stretched and thinned when the substrate expands due to the fact that the thermal expansion coefficient of the substrate is much higher than that of the film, so that the wavelength of each transmission band drifts to short wave, and the two effects are combined to achieve the purpose that the wavelength driftsThe purpose of compensation is achieved, so that the sparse filter is stable within a certain temperature variation range; conversely, the temperature decreases similarly.
Compared with the prior art, the invention has the beneficial effects that:
1) In the prior art, a high-order spacing layer is formed by a quartz crystal or a mica sheet with the thickness of several millimeters commonly used according to the wavelength spacing requirement, and two reflectors are respectively plated on two surfaces of the spacing layer to manufacture the comb-shaped optical filter. The invention is based on the semiconductor crystal superlattice theory, two adjacent photonic forbidden bands are mutually overlapped by means of two optical thin film photonic crystals with different thicknesses so as to form a photonic crystal superlattice structure, a one-dimensional photonic crystal superlattice theory is used for obtaining a passband which can tunnel the forbidden bands to form a plurality of similar frequencies, and a quasi-comb-shaped optical filter of a plurality of transmission bands in the photonic forbidden bands is obtained.
2) In the prior art, the distances between transmission peaks of the comb filter formed by a quartz crystal or mica sheet high-order spacing layer or the comb filter of a designed full-film system are equal, but nonlinear conditions are often encountered in reality, the passband wavelength distance formed by the photonic crystal superlattice structure can be gradually increased or decreased, and even can be centrosymmetric, and the number of transmission bands and the wavelength distance can be properly adjusted and changed through a basic periodic structure and each period number thereof, so that the method is very suitable for manufacturing the quasi-comb filter used in a nonlinear wavelength division multiplexing system.
3) The quasi-comb filter of the present invention is also a comb filter of a full-film system, but the total thickness of the film is greatly reduced, so that the quasi-comb filter can be really manufactured by the existing equipment and the existing technology. For comb filters with a transmission band having a wavelength spacing of 0.5nm density, the typical design of the prior art full-film is G | (HLHLHLH 6120 LHLHL) 2 L | A, the total optical thickness of which is 12263 quarter-wavelengths (with the wavelength of 1550 nm), and the total optical thickness of the all-film quasi-comb filter of the present invention is 336-560 quarter-wavelengths depending on the number of transmission bandsIn between long, that is, the total thickness of the full-film quasi-comb filter of the present invention is only 2.7% to 4.6% of the total thickness of the prior art full-film comb filter.
Drawings
FIG. 1 is a graph of the transmittance of a prior art comb filter;
wherein, fig. 1 (a) is a transmittance curve of a mica sheet | HLHLH | a with a quarter wavelength thickness of a comb filter a | HLHLH |6200 in the prior art; FIG. 1 (b) shows a comb filter full film G | of the prior art (HLHLHLH 6120 LHLHLHL) 2 The transmission curve of | a;
FIG. 2 is a one-dimensional photonic crystal (HL) 6 And (0.8H0.8L) 6 Wherein (a) in FIG. 2 is (HL) 6 (ii) the transmittance curve of (2) (b) is (0.8H0.8L) 6 Fig. 2 (c) is a synthesized transmittance curve;
FIG. 3 is a graph of the invention G | { [ (HL) 7 (0.8H0.8L) 7 ] 5 } 4 The transmission curve of the | A quasi-comb filter;
FIG. 4 is a three cycle four cavity bandpass filter [ (HL) of the invention for clipping unwanted transmission peaks 4 HLLHHH(LH) 4 L(HL) 5 HLLHHH(LH) 5 L(HL) 5 HLLHHH(LH) 5 L(HL) 4 HLLHHH(LH) 4 L] 3 The transmittance curve of (a);
FIG. 5 is a graph of the transmittance of a quasi-comb filter of the present invention with gradually decreasing wavelength spacing;
FIG. 6 is a graph of the transmittance of quasi-comb filters of the present invention with gradually increasing wavelength spacing;
FIG. 7 is a graph of the transmittance of a quasi-comb filter of the present invention with a centrosymmetric wavelength spacing;
fig. 8 is a schematic structural diagram of a quasi-comb filter with gradually changing wavelength intervals according to the present invention.
Detailed Description
FIG. 1 is a graph of transmittance of a prior art comb filter, wherein (a) the high order spacer layer is mica sheets with an optical thickness of 6200 quartersOne wavelength, the optical thickness can also be noted as 2402.5 microns because the reference wavelength is 1550nm, and the geometric thickness of the mica is 1580.6 microns because the refractive index of the mica at the wavelength of 1550nm is 1.52. The two sides of the mica sheet are respectively coated with 5 layers of reflectors, namely A | HLHLHLH |6200 quarter-wavelength mica sheet | HLHLHLH | A, and the transmissivity curve of the system is shown in figure 1 (a). The disadvantages of such comb filters are: 1. because the geometric thickness 1580.6 microns of the mica sheet is difficult to control, the wavelength position error of the comb filter is large; 2. after the HLHLHLH of the first surface of the mica is plated in actual manufacturing, when the HLHLHLH of the second surface is plated, due to the thickness deviation of the mica sheet, a low-refractive-index material is needed to compensate the phase to an extreme value; 3. the filter cannot be made into multiple cavities, so that the rectangular coefficient of a transmission curve is poor; 4. although the mica sheet has a cleavage plane structure, wavelength and waveform non-uniformity of the transmission band of the comb filter still exist at different positions. In order to solve the above problems, an all-thin-film comb filter G | (HLHLHLH 6120 LHLHLHL) shown in FIG. 1 (b) was designed 2 The transmission curve of L | a, which overcomes all the problems with mica spacer comb filters, unfortunately, has not been possible to manufacture to date due to the thick spacer film thickness.
As shown in FIG. 8, the quasi-comb filter with gradually changed wavelength intervals of the present invention comprises a substrate 1 and a quadruple period multi-layer film { [ (HL) disposed on a first surface of the substrate 1 p 1 (xHyL) p 2 ] q } s 2 and a bandpass filter 3 on a second side of the substrate 1.
The invention designs the comb filter by means of the superlattice concept of the thin film one-dimensional photonic crystal, and according to the semiconductor superlattice theory, the comb filter consists of a periodic structure of two one-dimensional photonic crystals and can be simply expressed as a multilayer film with a quadruple periodic structure: { [ (HL) p 1 (xHyL) p 2 ] q } s Wherein p is 1 、p 2 Q and s are the number of cycles of the quadruple cycle, the basic cycle (HL), respectively p 1 And (xHyL) p 2 Two one-dimensional photonic crystal mirrors of different wavelengthsTwo photon forbidden bands are respectively generated, and by the theory of photonic crystal superlattice, photons with certain frequencies can obtain a string of transmission bands with gradually changed wavelength intervals through a tunneling effect. FIG. 2 is a one-dimensional photonic crystal (HL) 6 And (0.8H0.8L) 6 And a resultant transmittance curve thereof, wherein (a) is (HL) 6 (ii) the transmittance curve of (b) is (0.8H0.8L) 6 The transmittance curve of (c) is [ (HL) 6 (0.8H0.8L) 6 ] 4 The resultant transmittance curve of (2). As can be seen from fig. 2, the characteristic of such multiple transmission bands is actually generated by one-dimensional photonic crystals of two different frequencies, and if x = y =1 in fig. 2 (b), the characteristic of multiple transmission bands is no longer generated because it becomes a photonic crystal. As long as x = y = 0.5-0.9 or 1.1-3, that is, when two photonic crystals exist, multiple sets of multiple transmission bands as shown in fig. 2 (c) appear on the overlapped forbidden bands, and the wavelength subdivision and multiple transmission band curve is developed, and one set with better characteristics is selected to be used as the quasi-comb filter. The number of transmission bands is mainly determined by the film thickness coefficient x, y and the period number p 1 ，p 2 And q. When x = y = 0.5-0.9, with x, y or p 1 ，p 2 Increasing, the passband group is increased; when x and y are reduced to completely separate the forbidden bands of the two photonic crystals, the positions of the overlapped pass bands have irregular spectral curves. When x = y =1.1 to 3, the reflection band is narrowed due to the high order, so that the overlap of two photon forbidden bands can be maintained even if 3 is equal to, but the use is avoided as much as possible because the film layer is too thick for the order, so that the case where the order is greater than 3 is not listed here. The principle of this multi-set multi-transmission band can be further explained by using a tight-binding (TB) method, which is a method of binding (xHyL) p 2 The photonic crystal is considered to have defects.
As an example of implementation, FIG. 3 shows WMS-13| { [ (HL) of the present invention 7 (0.8H0.8L) 7 ] 5 } 4 The transmission curve of the | a quasi-comb filter. The curve is of a quadruple cycle structure { [ (HL) p 1 (xHyL) p 2 ] q } s Middle value p 1 ＝p 2 =7, q =5 and s =4, andcase of x = y =0.8, number of cycles p 1 、p 2 The method mainly represents the transmission wavelength position of the quasi-comb filter, the cut-off degree of a cut-off area and the half width of a transmission band, the number q and the number s of cycles represent the number of the transmission bands of the quasi-comb filter, if the number q of cycles is only used, the number of the transmission bands is far away from each other and is dispersed too much, and the number s of cycles is added, so that the continuity of the number of the transmission bands is greatly increased, and the number of the transmission bands is convenient to select practically. The high-refractive-index film layer (H) in the structure is titanium oxide (Ti) 3 O 5 ) A refractive index at a wavelength of 1550nm of 2.27; the low refractive index film layer (L) is silicon dioxide (SiO) 2 ) A refractive index at 1550nm of 1.443; the substrate 1 is WMS-13 with a refractive index of 1.52 at 1550 nm; the refractive index of air a is 1.0. Due to the second photonic crystal (xHyL) p 2 Corresponding to x = y of 0.8, shifts the overall synthetic curve to a shorter wavelength, so (HL) p 1 The reference wavelength of (2) needs to be shifted from 1550nm to 1582.72nm. 560 layers in total, starting from the substrate 1 (WMS-13), are alternately films with high and low refractive indexes, i.e. the odd layers are high refractive index film layers, namely, titanium pentoxide (Ti) 3 O 5 ) The even number layer is low refractive index film layer silicon dioxide (SiO) 2 ). As can be seen from fig. 3, the quasi-comb filter generates 19 transmission bands in total, which are symmetric about 1550nm, the wavelength interval and half width of the transmission band become smaller and smaller as the wavelength is farther from the center wavelength, and the peak value of the transmission band decreases at the edges of the two ends because the half width of the transmission band is too narrow and the calculated wavelength interval is wider.
In order to intercept the multiple transmission bands of fig. 3 as quasi-comb filters, a high steepness bandpass filter 3 needs to be designed. FIG. 4 is a three cycle four cavity bandpass filter WMS-13| [ (HL) for cutting off unwanted transmission bands in accordance with the invention 4 HLLHHH(LH) 4 L(HL) 5 HLLHHH(LH) 5 L(HL) 5 HLLHHH(LH) 5 L(HL) 4 HLLHHH(LH) 4 L] 3 The transmission curve of | a. In order to obtain a sufficiently high pass-band steepness, a three-cycle design must be used, but in this case sharp secondary peaks will be generated on both sides of the main transmission band, and it should be noted that: these sharp secondary peaks must be such that they fall on the graph3, the cut-off region of the quasi-comb filter. In order to improve the transmittance in the pass band of the bandpass filter 3, a four-cavity structure is first designed to match the substrate 1 and the air admittance, and two cavities (HL) are arranged in the middle of the four cavities 5 HLLHHH(LH) 5 With higher admittance, two chambers on both sides (HL) 4 HLLHHH(LH) 4 The lower admittance is arranged, so that two rabbit ears on two sides of the passband can be greatly reduced, further, the spacing layer is synthesized by two materials of LLHH, the stress accumulation of the thin film can be reduced, and the decrease of the transmissivity of the passband caused by an angle effect is reduced; next, the outermost two film thicknesses on the air-side thereof were corrected from H and L to 1.243H and 1.326L, respectively, to reduce residual reflection in the pass band and increase the transmittance, and in fact, these two films were the antireflection films in the pass band of the bandpass filter 3. On the basis that the curve shown in fig. 3 has been plated on the first surface of the substrate 1 (WMS-13), the bandpass filter 3 is plated on the second surface of the substrate 1 (WMS-13), so as to obtain the quasi-comb filter transmission curve shown in fig. 5. As can be seen from fig. 5, the wavelength spacing of the quasi-comb filter is gradually decreased as the wavelength increases.
Similarly, as shown in fig. 6, a quasi-comb filter in which the wavelength spacing gradually increases as the wavelength increases can also be obtained. At this time, there are two selection methods according to the different used wave bands: the first method is to shift the reference wavelength of the quasi-comb filter shown in fig. 3 to a long wavelength, for example, from 1582.72nm to 1584.35nm to prepare a quasi-comb filter, and then plate the bandpass filter 3 shown in fig. 4 on the second surface of the substrate 1 (WMS-13), to obtain the quasi-comb filter shown in fig. 6. The second method is to shift the reference wavelength of the bandpass filter 3 shown in fig. 4 to a short wavelength, and intercept multiple transmission bands in the short wavelength region in fig. 3 to form a quasi-comb filter in the shorter wavelength region.
According to the above concept, if the usage requirement is met, a quasi-comb filter with a symmetric wavelength interval around the center wavelength can be obtained, and fig. 7 is a transmittance curve of the quasi-comb filter with a symmetric wavelength interval around the center according to the present invention, which is obtained by intercepting multiple transmission bands on both sides of 1550nm of the center wavelength in fig. 3 with a band-pass filter with a double bandwidth similar to that shown in fig. 4. It is apparent that, as shown in FIG. 7, on the short wavelength side of the center wavelength of 1550nm, the wavelength interval of the transmission band increases as the wavelength increases, and on the long wavelength side of the center wavelength of 1550nm, the wavelength interval of the transmission band decreases as the wavelength increases.
The quasi-comb filter with gradually changed wavelength intervals can be used for changing the bandwidth of DWDM in an optical communication nonlinear wavelength division multiplexing system so as to improve the multiplexing degree.
Claims (7)
1. A quasi-comb filter with gradually changed wavelength interval is characterized by comprising a substrate and a quadruple-period multilayer film { [ (HL) arranged on a first surface of the substrate p 1 (xHyL) p 2 ] q } s And a bandpass filter on the second side of the substrate;
wherein p is 1 、p 2 Q and s are the number of cycles of the quadruple-cycle multilayer film respectively and are positive integers;
h and L respectively represent a high refractive index film layer and a low refractive index film layer with quarter-wavelength film thickness;
x and y are quarter-wavelength film thickness coefficients;
the high-refractive-index film layer is titanium pentoxide, and the low-refractive-index film layer is silicon dioxide;
the substrate is optical glass;
the number of cycles p 1 、p 2 Taking the equivalent value, taking the value of an integer from 6 to 8, taking the value of the period number q as an integer from 2 to 6, and taking the value of s as an integer from 2 to 6;
the film thickness coefficient x, y is equivalent and the value is 0.5-0.9 or 1.1-3.
2. The quasi-comb filter with gradually changed wavelength intervals of claim 1, wherein p is 1 、p 2 The value 7, q 5, s 4.
3. The quasi-comb filter with gradually changed wavelength intervals of claim 1, wherein x and y are both 0.8.
4. The quasi-comb filter according to claim 1, wherein the bandpass filter comprises a three-cycle four-cavity Fabry-Perot filter.
5. The quasi-comb filter with gradually changing wavelength intervals as claimed in claim 4, wherein the three-period four-cavity Fabry-Perot filter is [ (HL) from the substrate to the outside 4 HLLHHH(LH) 4 L(HL) 5 HLLHHH(LH) 5 L(HL) 5 HLLHHH(LH) 5 L(HL) 4 HLLHHH(LH) 4 L] 3 。
6. The quasi-comb filter with gradually changed wavelength intervals of claim 4 or 5, wherein the outermost two-layer film thickness of the band-pass filter on the air side is corrected from H and L to 1.243H and 1.326L respectively.
7. The quasi-comb filter with gradually changed wavelength intervals as claimed in claim 1, wherein the substrate is optical glass with high thermal expansion coefficient, and the linear expansion coefficient is 90-130 x 10 -7 /℃。
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