CWDM filter with four channels
Technical Field
The invention relates to an optical filter, in particular to a CWDM optical filter which is suitable for a Coarse Wavelength Division Multiplexing (CWDM) system (Coarse Wavelength Division multiplexing 1ex) and simultaneously has four channels.
Background
Optical filters, or optical filters, are wavelength selective devices that have important applications in fiber optic communication systems and optical sensor systems. The optical filter is divided into a passive type and an active type, and the basis of the passive optical filter is a prism, a diffraction grating and a spectrum (frequency) filter; while an active optical filter is a combination of passive devices and tunable detectors, each tuned to a particular frequency.
One of the passive optical filters is an interference film type filter, which uses a material with a high refractive index and a low refractive index to be deposited on a substrate made of a material such as glass in a predetermined thickness (usually λ/4) so as to achieve a desired wavelength response characteristic. In general, a dielectric film interference filter is made by overlapping thin films of high refractive index and low refractive index each having a thickness of λ/4, the phase of light reflected in the high refractive index layer is not shifted, and the phase of light reflected in the low refractive index layer is shifted by 180 °. Due to the difference in light travel (multiples of 2 x λ/4), successive reflected light rays recombine overlapping in front, producing a high intensity reflected light beam in a narrow wavelength range, while output wavelengths outside this wavelength range are abruptly reduced. Such filters may be used as high pass filters, low pass filters or highly reflective layers. Moreover, since the optical characteristics of the optical film depend on the reflection and transmission characteristics of the optical film, the optical film is generally plated to be a band-pass filter (band-pass filter), a low-pass or high-pass filter (low pass or high pass filter), or a band-stop filter (band reject filter).
Fig. 2 shows a four-channel CWDM filter of the prior art. The whole film layer structure of the four-channel CWDM optical filter is as follows: n is a radical of0/L(HLH)4L(HLH)L(HLH)2L(HLH)L(HLHLH)2L(HLHLH)L(HLH)4L(HLH)L(HLHLH)2L(HLHLH)L(HLH)4L(HLH)L(HLHLH)2L(HLHLH)L(HLH)4L(HLH)L(HLHLH)2L(HLHLH)L(HLH)4L(HLH)L(HLHLH)2L(HLHLH)L(HLH)4L(HLH)L(HLHLH)2L(HLHLH)L(HLH)2L(HLH)L(HLH)4L(HLH)/NS(ii) a Wherein N isSDenotes a substrate refractive index of 1.658, N0Represents air, L represents lambda0A film layer of low refractive index,/4, H represents lambda0High refractive index film layer of/4, lambda0Representing the center wavelength of the channels. As shown in fig. 2, the 67 th layer and the 87 th layer are coupling layers from the left, and are low-refractive-index film layers with one quarter of the center-wavelength film thickness respectively.
The corresponding spectral characteristic diagram is shown in FIG. 3. In the four-channel CWDM filter of the prior art, the film thickness of the coupling layers of the 67 th layer and the 87 th layer is set to be only one quarter of a center wavelength. However, the CWDM filter has a large light transmittance of noise in the two bands of 1500-1505nm and 1575-1580nm, thereby generating a large interference ripple (ripple), which reduces the light transmittance. With a typical design, so-called optimization would be performed with a film thickness other than a quarter of the center wavelength in the last two layers of the overall film structure. However, stacking non-quarter-wave film thicknesses is a more complex process.
Disclosure of Invention
The present invention is directed to overcome the above-mentioned deficiencies of the prior art, and provides a CWDM filter having four channels and being suitable for a Coarse Wavelength Division Multiplexing (CWDM) system, which can reduce the overall loss of the CWDM assembly.
The technical points of the invention are as follows: a multi-channel CWDM filter is provided, which comprises a substrate and N stacked structures, wherein N is an odd number not less than 2, and each stacked structure comprises a Fabry-Perot resonant cavity structure and a coupling layer from the substrate side.
Each stack structure comprises a first film stack, a space layer, a second film stack and a coupling layer from the substrate side; the first film stack and the second film stack are symmetrically disposed with respect to the space layer. Wherein the spatial layer is kL, k is a positive integer of 2 or more, preferably a positive even number, and L represents λ0Low refractive index film layer of/4, lambda0Representing the center wavelength of the channels.
And, the [ (N +1)/2-1 ] th]The coupling layer and the [ (N +1)/2+1 ] th layer of the stacked structure]The coupling layers of the stacked structure are all (2n +1) L, n is a positive integer, and L represents lambda0Low refractive index film layer of/4, lambda0Representing the center wavelength of the channels.
Each Fabry-Perot cavity structure is sequentially H (LH) from the substrate sideakL(HL)aH, a is a positive integer, k is an integer of not less than 2, L represents lambda0A film layer of low refractive index,/4, H represents lambda0High refractive index film layer of/4, lambda0Representing the center wavelength of the channels. Wherein,the first membrane stack is H (LH)aAnd the second film stack is (HL)aH。
The coupling layer of the rest stacked structure is L, L represents lambda0Low refractive index film layer of/4, lambda0Representing the center wavelength of the channels.
When the multi-channel CWDM optical filter of the invention provides four channels of 1510nm, 1530nm, 1550nm and 1570nm, the Fabry-Perot resonant cavity structure from the substrate side is HLH4LHLH, HLH2LHLH, H (LH) in sequence22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH2LHLH、HLH4LHLH。
The center wavelength lambda of the multi-channel CWDM optical filter0Is 1540 nm. The coupling layers from the substrate side are L, L, L, L, L, L, 3L, L, and 3L, L, L, L, L, L, L in this order.
The film layer design technique of the CWDM optical filter with four optical channels of the invention is as follows: odd-number times of low refractive index coupling layer (cladding) with quarter of center wavelength is adopted to reduce Ripple (Ripple) generated by passband, and further improve light transmittance of the whole filter, thereby reducing overall loss on the assembled CWDM.
Drawings
FIG. 1 is a schematic diagram of a film structure of a CWDM filter with four channels according to the present invention;
FIG. 2 is a schematic diagram showing the relationship between the overall film structure and the corresponding light transmittance of a CWDM filter of the prior art;
FIG. 3 is a graph of the corresponding spectral characteristics of FIG. 2;
FIG. 4 is a schematic diagram showing the relationship between the overall film structure and the corresponding optical transmittance of a CWDM filter with four channels according to the present invention; and
fig. 5 is a spectral characteristic diagram of the CWDM filter of the present invention.
Detailed Description
The cwdm (coarse wavelength division multiplex) filter 1 having four channels according to the present invention will now be described in further detail with reference to the drawings.
As shown in fig. 1, the CWDM filter 1 having four channels of the present invention includes a substrate 10 and N stacked structures 40; wherein N is an odd number of not less than 2. Each stack structure includes a first film stack 21, a spacer layer 25, a second film stack 22 and a coupling layer 30 from the substrate side. Accordingly, the first film stack 21, the spacer 25 and the second film stack 22 constitute a Fabry-Perot Cavity (Fabry-Perot Cavity) structure 20. And the first film stack 21 and the second film stack 22 are symmetrically arranged with respect to the space layer 25. The spatial layer 25 is kL, and k is a positive integer of 2 or more, preferably a positive even number.
Referring to FIGS. 1 and 4, the [ (N +1)/2-1 ] th]The coupling layer 30' and the [ (N +1)/2+ 1) th layer of the stacked structure 40]The coupling layers 30 of the stacked structure 40 "are all (2n +1) L, n is a positive integer, and L represents λ0Low refractive index film layer of/4, lambda0Representing the center wavelength of the channels. And the coupling layer 30 of the remaining stack is L, L represents lambda0Low refractive index film layer of/4, lambda0Representing the center wavelength of the channels.
Each Fabry-Perot resonator structure is sequentially H (LH) from the substrate 10 sideakL(HL)aH, a is a positive integer, k is an integer of not less than 2, L represents lambda0A film layer of low refractive index,/4, H represents lambda0High refractive index film layer of/4, lambda0The center wavelength of each channel of the present invention is shown. Wherein the first membrane stack can be expressed as H (LH)aAnd the second film stack can be expressed as (HL)aH。
When the multi-channel CWDM filter 1 of the invention provides four channels of 1510nm, 1530nm, 1550nm and 1570nm, the Fabry-Perot resonant cavity structure from one side of the substrate 10 can be expressed as HLH4LHLH, HLH2LHLH, H (LH) in sequence22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH4LHLH、H(LH)22L(HL)2H、HLH2LHLH、HLH4LHLH。
The center wavelength lambda of the multi-channel CWDM optical filter0Is 1540 nm. The coupling layers from the substrate side are L, L, L, L, L, L, 3L, L, and 3L, L, L, L, L, L, L in this order.
The CWDM optical filter 1 of the invention is formed by stacking a plurality of (15) Fabry-Perot resonant cavity film stacks 20 with the thickness of one quarter of the center wavelength film, and simultaneously has four optical channels of 1510nm, 1530nm, 1550nm and 1570 nm. The center wavelength of the CWDM optical filter is positioned at the center point lambda of the four optical channels01540 nm; and the film thickness of all the films is one fourth of the central wavelength lambda0。
Wherein each space layer 25 adopts a quarter of the center wavelength λ0A film thickness; each fabry-perot resonator film stack 20 is composed of two single or even number layers of multilayer films (stacks), and the coupling layer 30 of the fabry-perot resonator film stack 20 is a low refractive index film layer with odd number times of quarter of the center wavelength.
And coupling layer 30 at layers 67 and 87, respectively, may be (2n +1) times the quarter-center wavelength lambda0A film thickness of the low refractive index film layer. In the present invention, it is preferable to use a low refractive index film layer three times as thick as the quarter-center wavelength film.
The glass substrate 10 of the present invention had a surface polishing diameter of 90mm and a thickness of 10 mm; the peak transmittance is more than 90%; the band pass is 1502nm-1578 nm. The low refractive index film layer can be made of silicon oxide, the high refractive index film layer is made of tantalum oxide, and the substrate is composed of silicon oxide, barium, lithium, sodium and other elements. Wherein the refractive index of the low refractive index film layer is 1.44, and the refractive index of the high refractive index film layer is 2.1-2.5.
Fig. 4 shows an embodiment of the present invention. In the figure, starting from the left, the first film layer of the present invention is shown, and the 67 th and 87 th layers are also coupling layers and are low-refractive-index film layers with a thickness of three times the center-wavelength film. FIG. 5 is a graph of the spectral characteristics corresponding to the embodiment, and it can be seen from FIG. 5 that the noise ripples in the two bands of 1500-1505nm and 1575-1580nm in the embodiment have been eliminated, so that the overall light transmittance of the filter 1 is improved.
In summary, a key technical point of the present invention is: the coupling layer 30 connecting the fabry-perot resonator film stack and the fabry-perot resonator film stack adopts the film thickness of odd-number times of quarter of the central wavelength to reduce the ripple generated by the passband, thereby improving the overall light transmittance of the optical filter 1. Thus, the whole design does not adopt the film thickness of non-quarter wavelength at all, so that the corresponding production process is simplified, and the corresponding production cost is reduced.
The design technique of the film layer of the CWDM optical filter 1 with four optical channels of the invention is that a coupling layer (cladding) with low refractive index of odd number times of quarter center wavelength is adopted to reduce the Ripple (Ripple) generated by the passband, thereby improving the optical transmittance of the whole optical filter and reducing the integral loss on the assembled CWDM.