WO2002037141A2

WO2002037141A2 - Optical interleaver filter

Info

Publication number: WO2002037141A2
Application number: PCT/CN2001/001534
Authority: WO
Inventors: Xiangfei Chen; Shizhong Xie; Chongcheng Fan; Xuhui Li
Original assignee: Tsinghua University; Nuovo Technology Ltd.
Priority date: 2000-11-06
Filing date: 2001-11-05
Publication date: 2002-05-10
Also published as: CN1100274C; CN1246714C; WO2002037141A3; AU2002221466A1; CN1484775A; CN1289055A

Abstract

An optical chirped SBG-based interleaver filter comprising a Sample Bragg grating (SBG), an interface coupled to the SBG for interfacing with a optical fiber system; wherein the chirp of the SBG is strong enough to cause a stop band or pass band overlapping with at least one neighbor stop band or pass band so that the multiple resonant reflection and transmission peaks are produced.

Description

Optical interleaver filter

TECHNOLOGICAL AREA

This invention relates to an optical filter, and more particularly to an optical interleaver filter employing sampled Bragg grating (SBG). BACKGROUND TECHNOLOGY

It is known by most technicians engaging in the communications field that the Internet has created an explosion in the amount of information transmitted over telecommunications networks. According to the research firm RHK Inc. (San Francisco), roughly 1 million terabits/month are currently transmitted over telecommunication networks, with a forecasted growth to >15 million terabits/month in 2003.

While this explosion has created many new opportunities for service providers, it has created a challenge for system integrators and component manufacturers to offer higher-bandwidth systems. Both higher bit rates and higher-channel-count DWDM systems are being pursued aggressively to meet these bandwidth requirements. While existing technology and products can often be stretched to meet next-generation requirements, innovative new components are needed, as well. That's particularly true in the highest-bandwidth applications, where both high bit rates (10 Gbits/sec or even 40 Gbits/sec) and dense channel spacings are being pursued simultaneously due to the extremely stringent dispersion requirements in these systems. A component, named the interleaver, can enable very-high-channel-count DWDM systems.

An interleaver is essential for an optical router that allows existing DWDM filters designed for operation at wide channel spacing to be extended to system designs with narrow channel spacings in the range of 50 GHz or even less. As shown in Fig. 1, it showsoperating principle of an optical multiplier/de-multiplier with three 1 to 2 interleavers and application can be illustrated in the following. In the simplest case, the interleaver(de-interleaver) 112 combines two sets of channels into one densely packed set with half the channel spacing. In reverse, the de-interleaver 112 routes the single input set of channels into two output streams with twice the channel spacing 121 and 122. This single-stage interleaver can be cascaded in a binary fashion to create additional devices. For example as shown in Fig.l, a 50GHz 1x2 de-interleaver 112 allows a set of channels 111 with 50-GHz spacing to be routed to two output fibers with 100-GHz channel spacing 121 and 122. Then twolOOGhz 1x2 de-interleavers 123 and 124 allow two sets of channels with 100- GHz spacing to be routed to four output fibers with 200-GHz channel spacing 131, 132, 133 andl34 respectively. In this way, an interleaver can be combined with existing DWDM filters to immediately extend its application to narrower channel spacings or take advantage of specific performance characteristics. This capability is particularly attractive for long-haul system designs that require the highest-bandwidth solution. For this application, the design cycle and cost of new higher-bandwidth systems can be reduced by combining existing thin- film filters or array waveguide gratings with an interleaver to create a system with twice or four times the channel count of systems using just the filter or gratings themselves. The interleaver has obvious applications for long-haul DWDM systems, where the highest channel counts are being deployed. However, the modularity of the component is attractive for regional metro applications where first-installed costs are critical. For the de-interleaver shown in Fig. 1, not all of the four outputs 131, 132, 133 and 134 have to be used initially. For example, a 1x4 de-interleaver designed for a 32-channel DWDM system will have eight channels on each output fiber of the device. The initial system could be deployed using one output fiber with just eight channels, and additional sets of transmitters and DWDM filters could be added to the unused interleaver/de-interleaver fibers when the bandwidth is required. Of course, the initial transmitters installed would need to have the stability required for the full-channel-count systems, but that does create an intelligent way to provide scalability in a system

Another application of an interleaver in the metro environment is to add/drop a set of channels at a node. In this case, the set of wavelengths on one de-interleaver output fiber can be designated as the drop traffic and de-multiplexed while the other sets of wavelengths are express traffic and travel through to the next node. Although the majority of the traffic is express, the transmission through multiple interleaver/de-interleaver pairs places demands on their performance to minimize channel mixing and crosstalk. However, a key economic benefit of an interleaver design is that it allows lower-cost, older- generation filters to be used in a higher-channel-count DWDM system. Clearly, the economic advantage of interleavers only increases for higher-channel-count systems. This advantage will continue into the future, since even though the price of 100-GHz filters will drop, filters with wider channel spacings will always be further down the cost curve than newer, denser filters and will have better availability.

As is common for a new class of devices, there are a number of different technical approaches to interleavers, with no clear technical winner as yet. The general principal behind interleavers is an interferometric overlap of two beams. The interference creates a periodic repeating output, as different integral multiples of wavelengths pass through the device. The desired channel spacing of the device is set by controlling the fringe pattern. Manufacturers today use fused-fiber interferometers, liquid crystals, birefringent crystals, and other more exotic technical approaches to build interleavers. Probably the simplest design in terms of raw material and technologies is a fused-fiber Mach-Zehnder interferometer. (see Fig. 2). In this design, the interference is created by using an unequal fiber-path length between two 3-dB couplers 22 and 24. By carefully controlling the path-length difference between 23 and 27, the channel spacing can be set to the desired value and matched very well to the ITU grid. Port 21 or 26 is an input port and 25 and 28 are output ports. As Fig. 3 shows, the disadvantages of the conventional interleaver based on interferometer are Lorenzian with sharp peaks 31 and broad bottom areas 32. To obtain a flattop spectrum, the insertion loss was reduced by apodizing the output with either an additional fiber or thin-film filter, etc. This increases the difficulty of fabrication and cost of the interleaver.

Bragg Grating:

Bragg gratings are based on the principle of Bragg reflection. When light propagates through periodically alternating regions of higher and lower refractive index, it is partially reflected at each interface between those regions (see Fig. 4). If the spacing between those regions is such that all the partial reflections add up in phase — when the round trip of the light between two reflections is an integral number of wavelengths — the total reflection can grow to nearly 100%, even if the individual reflections are very small. Of course, that condition will only hold for specific wavelengths. For all other wavelengths, the out-of-phase reflections end up canceling each other, resulting in high transmission.

The condition for high reflection, known as the Bragg condition, relates the reflected wavelength, or Bragg wavelength, λ _Bragg to the grating period Λ41 and the average refractive index n via

λ _Bragg = 2 «Λ . (1) ^*

Fiber Bragg grating (FBG) is a Bragg grating which is formatted in the core of the fiber. In a typical FBG, the grating period is about 535 nm for a Bragg wavelength at 1550 nm. The refractive index modulation is relatively small — on the order of 10^"4 to 10^"3 — so it takes a large number of periods to achieve reflections in excess of 99%.

FBGs are typically between 1 mm and 25 mm long. The high reflection condition will hold over a band of wavelengths around λ _Bragg. The 3 dB bandwidth is inversely proportional to the grating length when the grating is relatively weak, but it increases as a function of grating strength. It is also possible to make FBGs with non-uniform periods, for example, linearly varying along the length of the grating. Such 'chirped' gratings can have much broader bandwidths, up to tens of nanometers. The chirped FBG can be expressed as

A(z) = A₀ (! - cz) (-//2 < z < //2) (2) where c is the chirping coefficient and it denotes the change of the grating period along the grating, z is the ordinate along the Bragg grating and / is the length of the Bragg grating. Sampled Brgg grating (SBG) is a Bragg grating whose refractive index is modulated by another sampling periodic function F(z). Namely, beside Bragg periodic modulation (order of micrometer) 51, there is another periodic modulation whose period 52 is in order of millimeter. The sampling length 53 is the part of the grating in a sampling period 52 (=E). The sampling ratio is defined by sample length 53 divided by sampling period 52. A usual FBG has a reflection peak. Unlike usual FBG, SBG has multiplex reflection in its spectrum. Conventional SBG is unchirped or small-chirped SBG. There exist multiple stop bands and pass bands. The neighboring stop bands or allowed bands are not overlapped In a SBG- structure, there are multiple stop bands. When a Ught whose frequency falling within the stop band, the Ught will be blocked and portion of it can transmit without encumbrance. When the peak index modulation is larger, only little of the Ught can transmitted with out encumbrance, and most of the Ught will be reflected. The smaller the sampling ratio, the more balanceable channel to channel. When the application of the grating faU in the 1550nm window of the optical communication, sampling periods are usually near 0.5mm, 1mm and 2mm, which the corresponding channel spacing (the spacing between neighboring stop bands) are near l.βnm, 0.8nm, 0.4nm respectively, or 200Ghz, lOOGhz and 50GHz respectively. Therefore, when a light whose frequency falls within these stop bands, the Ught will be reflected. On the contrary, when the Ught falls out of the top bands, the Ught will be transmitted without encumbrance. From the spectrum of conventional SBG, the SBG can be regarded as an interleaver filter. Such type of "interleaver filter" has some ultra disadvantages: the spectrum response is not equaUzed channel to channel, which becomes worse with the increase of the deviation from the center Bragg wavelength. At the same time, the phase response of the conventional SBG is nonlinear within the filtering band, the signal pulse passing through such a SBG, will be distorted seriously. Therefore, conventional SBG can't be used in actual fiber system as an interleaver.

LA. Everall, K. Sugden, J.A.R. Williams, I. Behnion, X. Liu, J.S Aitchison, S. Thomas and K.M DelaRue disclosed a Moire resonators in "Fabrication of multipassband moire resonators in fibers by the dual-phase-mask exposure method", Optics Lett., vol. 22, pp. 1473-1475, 1997. A Moire grating is a special grating with two superimposed gratings at different wavelengths. Usually, when a large chirp in the grating period is included, multiple pass bands may result in such a chirped Moire grating, which can be used as an interleaver with poor performance (see Fig. 7). However, the transmission of this structure is Lorenzian with sharp peaks 72 and broad bottom areas 74.

It is an object of this invention to provide an optical interleaver filter to overcome the disadvantages of the interleavers above mentioned. It is another object of this invention to provide an optical interleaver having a flattening filtering response.

It is further another object of this invention to provide an optical filter having following advantages: compact, low-cost, filtering with good performance, low insertion loss and simple arts and crafts.

It is still another object of this invention to provide an optical filter being low-dispersion and has clear spectrum. The proposed filter can be used as 12.5GHz, 25GHz, 50GHz and 100GHz spacing interleavers.

DISCLOSURE OF THE INVENTION

The above described objects of this invention are achieved by an optical chirped SBG-based interleaver filter, comprising a strong chirped SBG and an interface to couple the SBG to optical fiber system, wherein the chirp of the SBG is strong enough to lead the overlapping of the neighboring at least one stop band or pass band and the multiple resonant reflection and transmission peaks are produced. There are two output streams in the SBG. One is related to the pass bands and the other is related to stop the bands. Such a SBG can be used as an interleaver filter. The operation frequency of the interleaver filter is the frequency spacing between neighboring pass band and stop band.

Optionally, the chirp coefficient c of the SBG is larger than 5.7xl0^"5/mm, and the

2δf corresponding sampling period P is equal to ^■•— ^* — , which lead to expanding the stop bands or l^cl^vι aUowed bands so as to make the bands overlapped, producing resonant multichannel, wherein

δfis the operation frequency spacing of the SBG-based interleaver filter, v, is the Ught velocity m vacuum.

Alternatively, the chirped SBG-based interleaver filter is further characterized in that the structure parameters of the SBG substantiaUy approaching the foUowing relations:

and

wherein c is the chirping coefficient, / is the length of the Bragg grating, λ _Bragg is the center

reflected wavelength, i.e. Bragg wavelength, and n is the average refractive index of the SBG,

P is the sampling period of the SBG, v, is the Ught velocity in vacuum, m is the integer ( m=±!,

±2, ...) and the δfis the operation frequency spacing of the interleaver filter.

For an operation frequency spacing of the interleaver filter δf, 3%-5% departure of the chirping coefficient c and sampling period E will only affect the frequency response a Uttle and the response being acceptable in an interleaver, i.e. the degrade of the performance of the interleaver filtering being allowable.

Further more, the structure parameters of the SBG using Blackman, Hamming, Gauss and Tanh, Sine, Cauchy, or Super-Gauss apodization in every sample.

Alternatively, a strong-chirped SBG-based 50GHz interleaver filter with structure parameters: λ _Bragg=1545nm, w=1.448, =0.259mm to 1.035mm, c=4.978xl0^"4 to 1.991xl0-³/mm.

Further, a strong- chirped SBG-based 25GHz interleaver filter with structure parameters: λ _Bra =1545nm , «=1.448, E=0.259 to 2.07mm, c=1.245xl0-⁴ to 9.956xlONmm; Further, a strong-chirped SBG-based 12.5GHz interleaver filter with structure parameters: λ _Bragg=1545nm , «=1.448, E=0.518 to 2.07mm, c=6.23xl0-⁵ to 2.489xlONmm;

Further, a strong-chirped SBG-based 100GHz interleaver filter with structure parameters: λ =1545nm , «=1.448, E=0.518, c=1.991xl0Nmm.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is schematic diagram of an optical interleaver filter in a de-interleaver;

Fig. 2 is schematic diagram of a Mach-Zehnder based interleaver;

Fig. 3 shows the response of a Mach-Zehnder based interleaver according to Fig. 2;

Fig. 4a and 4b are schematic diagrams of a Bragg grating which showing its operating principle and the optical response thereof;

Fig. 5 is a schematic diagram showing the structure of a sampled Bragg Grating as in this invention;

Fig. 6 is a schematic diagram of the transmission and reflection spectrum of a sampled Bragg grating according to Fig. 5;

Fig. 7 is a schematic diagram of the optical response of a Moire grating.

Fig. 8 shows a filtering spectrum response of a 50GHz interleaver filter according to first preferred embodiment of this invention

Fig. 9 is a schematic diagram of filtering spectrum of a 50 GHz interleaver filter based on our sampling function. The sampling function is characterized by the sample 52 and the sampling period 53. In terms of Fourier analysis, it can be treated as a special FBG with multiple superimposed ghost gratings. There are multiple stop bands corresponding to superimposed ghost gratings, and also multiple reflection peaks. One of stop bands 61 and one of the reflection peaks 62 is showing in Fig. 6a and 6b, respectively. When a SBG is chirped in grating period that is showing by Bragg grating 51, all ghost gratings in the structure are chirped with identical chirping coefficient.

Although the SBG described referred to the Fig.5 shows that sample 52 only in part of the period, the person with the ordinary skill in the art knows that sample 52 of the SBG described in this example can cover full sampling period 53 if the sample 52 is not uniformity in full sampUng period 53.

In order to compare with a Moire gratin, Fig 7 shows the optical response of a Moire grating with a chirp in the gratmg period ( chirped Moire grating ). There are multiple identical transmission peaks and reflection peaks in the chirped Moire grating. One of the transmission peaks is labeled with 71 and one of the reflection peaks is labeled with 72, respectively. The disadvantages of the multichannel filleting of such a grating is the sharp top 73 and broad bottom 74. The filtering performance, i.e. the spectrum response, is poor for optical fiber communications.

In terms of Fourier analysis, a SBG can be considered as a special Bragg grating with multiple superimposed "ghost" gratings. In a chirped SBG, when the chirp is weak, the stop bands are not overlapped, such as shown in Fig. 6. The Ught incident to the SBG is affected only by one of the "ghost" gratings. In such a situation, there is no resonance transmission in the structure, as shown in Fig 6. However, when the chirp is large enough, the stop bands are widely broadened and several neighboring stop bands related to the corresponding ghost gratings may be overlapped. Incident Ught with wavelength located within the overlapped region will be affected by several ghost gratings, and may transmit through the chirped SBG without

- 10 - invention, wherein no-apodization is used in each sample.

Figs. 10-11 are schematic diagrams of filtering spectrum of 50GHz interleaver filter according to other embodiments of this invention.

Figs. 12-15 are schematic diagrams of filtering spectrum of 25GHz interleaver filter according to stiU other embodiments of this invention.

Figs. 16-18 are schematic diagrams of filtering spectrum of 12.5GHz interleaver filter according to further other embodiments of this invention.

Fig. 19 is a schematic diagram of filtering spectrum of a 100GHz interleaver filter according to this invention

DETAILED DESCRIPTION OF PREFERED EMBODINEMENTS.

In order to make the technician in the art to practice this invention, more detailed description of the preferred embodiments will be disclosed with the reference to the figures.

Referring to Fig. 4, wherein, Fig. 4a refers to a Bragg grating 41 and Fig. 4b shows the optical response of the Bragg grating 41. There exists stop band 42 in the Bragg grating and leads to a reflection peak 43. When an optical beam 44 is incident to the Bragg grating, the beam may be reflected. When the frequency of the optical beam faUs within the stop band 42 where there is a reflection peak 43, the beam will be blocked. When the peak index modulation is larger, only little of the beam can transmitted with out encumbrance, and most of the Ught wUl be reflected, as shown in Fig 4b.

Fig. 5 shows an SBG according to a performed embodiment of this invention and its optical response is shown in Fig. 6. A SBG is a special Bragg grating 51 modulated by a periodic impediment. Therefore, when the chirp is strong enough, there may be multiple resonant transmission and reflection peaks in such a chirped SBG.

Fig 8 shows the response of first example according to this invention. The SBG according to this invention employs strong chirp. As Fig. 8a shows, a chirped SBG according to this invention has multiple resonant transmission and reflection peaks. Label 81 is one of resonant transmission peaks and label 82 is one of resonant reflection peaks, respectively. The resonance transmission peaks and reflection peaks may be non-Lorenzian because the optical response in a strong-chirped SBG is the interference of the response of multiple "ghost" chirp ed Bragg gratings .

Fig. 8b shows a spectrum response of the SBG filter according to this invention, clearly, the filtering according to this invention has a flat top 83 and a steep edge 84. Fig. 8c shows the dispersion of the SBG according to this invention, the SBG filter shows low dispersion because of small change of group delay 85 and 86.

Now, referring again to Fig. 8, the response of a first example SBG with strong chirp is showing. As you can see, Channel 1 of the interleaver filter is related to the transmission peaks, and Channel 2 of the interleaver filter is related to the reflection peaks. The response of both channel of the SBG according to the invention have below specifications: Channel 1: the IdB filtering bandwidth is about 0.32nm, the steep edge is characterized by 151dB/nm at IdB bandwidth and the maximum group delay change is less than 9ps within the IdB bandwidth. Channel 2: IdB bandwidth is about 0.34nm, the steep edge is characterized by 136dB/nm and the maximum group delay change is less than 19ps within the IdB bandwidth.

The SBG in Fig 8 is a 50GHz spacing interleaver filter. In this example, an interleaver filter is designed pursuant to equations (3) and (4) wherein a Hamming apodization is used in each sample 52 when fabrication. In this example, λ _Bragg=1545nm, «=1.448, E=1.035mm, c=4.978xlONmm, m=!. As above mentioned, in this example, the structure parameters of the SBG substantially approaching the foUowing relations, which may lead to an interleaver filter with good performance

and

wherein c is the chirping coefficient, / is the length of the Bragg grating, λ _Bragg is the center reflected wavelength, i.e. Bragg wavelength, and n is the average refractive index of the SBG,

E is the sampling period of the SBG, v, is the light velocity in vacuum, m is the integer ( m=±l, ±2, ...) and the δfis the operation frequency spacing of the interleaver filter.

The SBG according to this invention is a strong chirped SBG, the chirp of the SBG is strong enough to lead the overlapping of the neighboring at least one stop band or pass band and the multiple resonant reflection and transmission peaks are produced. These two equations show a SB G with such features.

corresponding sampling period E is equal to τ-X—, which lead to expanding the stop bands or

allowed bands so as to make the bands overlapped, producing resonant multichannel, wherein

δfis the operation frequency spacing of the SBG-based interleaver filter, , is the Ught velocity

in vacuum.

Although the equations are described, the person with ordinary skill in this art shaU know the key skirt of this invention is that the structure of the SBG shall make more than one stop band or pass band overlap, and produce more than one resonant reflection and transmission peaks. In Fig. 8, there shows a SBG with Hamming apodization in each sample. It shaU be know that apodization of other types also can be used, such as Blackman, Gauss and Tanh, Sine, Cauchy, or Super-Gauss.

To fabricate a SBG with a structure pursuant to any functions is well know in this art, as the text book " Andreas Othonos and Kyriacos KaUi: FiberBragg gratings: fundamentals and applications in telecommunications and sensing, Arteck House Inc, Norwood, MA, United Sates, 1999" describes.

Fig. 9 show the second example, it shows an interleaver filter Uke Fig. 8, but no-apodization in each sample is used in the fabrication. Clearly, there are some ripples 91 to decrease the filtering performance.

Although this invention disclosed a SBG with Hamming apodization, the person with ordinary skill in this art knows that other apodization also can be used to improve the response of the filter, such as, Gauss (as shown in Fig 10). By these apodizations, a better response also can be obtained.

Fig. 10 shows the third example of this invention, it shows a response of a 50Ghz (δf=50GΗz) spacing interleaver filter according to equations (3) and (4). For a SBG-based interleaver filter, the average index n is determined by the material, λ _Bragg is determined by the actual system and v, is the constant. According to equations (3) and (4) and with a selected m, the chirping coefficients c and the sampling period E can be determined. In this example, a fiber with a index of «=1.448 is fabricated to a SBG with λ _Bragg=1545nm, E=0.259mm, c=1.991xl0Nmm, m=4.

Other examples are also showed in other Figs., to make the description concision, no detail descriptions are further described. Fig. 1 1 shows 50Ghz spacing interleaver filter in terms of equations (3) and (4), ^λ _Bragg=1545nm, «=1.448, E=0.518mm, c=9.956xlONmm, m=2.

Fig. 12 shows, the response of the fifth example, a 25Ghz spacing interleaver filter in terms of equations (3) and (4), _Bragg=1545nm, «=1.448, E=0.259mm, c=9.956xlONmm, m=8.

Fig. 13 shows, the response of the sixth example, a 25Ghz spacing interleaver filter in terms of equations (3) and (4), λ _Bragg=1545nm, w=1.448, E=0.518mm, c=4.978xl0-⁴/mm, m=4.

Fig. 14 shows, the response of the seventh example, a 25Ghz spacing interleaver filter in terms of equations (3) and (4), λ _Bragg=1545nm, «=1.448, E=1.035mm, c=2.489xlONmm, m=2.

Fig. 15 shows, the response of the 8^th example, 25Ghz spacing interleaver filter in terms of equations (3) and (4), λ _Bragg=1545nm, «=1.448, E=2.07mm, c=1.245xlONmm, m=l.

Fig. 16 shows, the response of the 9^th example, 12.5Ghz spacing interleaver filter in terms of equations (3) and (4), λ _Bragg=1545nm, Λ=1.448, E=0.518mm, c=2.489xl0^'4/mm, m=%.

Fig. 17 shows, the response of the 10^th example, 12.5Ghz spacing interleaver filter in terms of equations (3) and (4), λ _Bragg=1545nm, w=1.448, E=l.035mm, c=1.245xl0-⁴/mm, m=4.

Fig. 18 shows, the response of the 11^th example, 12.5Ghz spacing interleaver filter in terms of equations (3) and (4), λ _Bragg=1545nm, «=1.448, E=2.07mm, c-=6.23xlONmm, m=2.

Fig. 19 shows , the response of the 12^th example, lOOGhz spacing interleaver filter in terms of equations (3) and (4), λ _Bragg=1545nm, n=1.448, E=0.518mm, c=1.991xl0^'3/mm, m=l.

As described above, the person with ordinary skill in the art know that there shall be infinitude embodiments to practice the SBG-based interleaver filter according to this invention. It shall be known that these examples only show the best modes to practice this invention in particular conditions.

While the present invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention, , that is, use a strong chirp to make the neighboring stop bands of the SBG overlapped.

Claims

What is claimed is

1. An optical chirped SBG-based interleaver filter comprising: a Sample Bragg grating(SBG), an interface coupled to the SBG for interfacing with a optical fiber system; wherein the chirp of the SBG is strong enough to cause a stop band or pass band overlapping with at least one neighbor stop band or pass band so that the multiple resonant reflection and transmission peaks are produced.

2. An optical chirped SBG-based interleaver filter as in claim 1, wherein the chirp coefficient

c of the SBG is larger than 5.7xl0^"5/ m, and the corresponding sampling period P is

lead to expanding the stop bands or aUowed bands so as to make the bands

overlapped, producing resonant multichannel, wherein δf is the operation frequency spacing

of the SBG-based interleaver filter, , is the Ught velocity in vacuum

3. An optical chirped SBG-based interleaver filter as in claim 1, wherein the chirped SBG- based interleaver filter is further characterized in that the structure parameters of the SBG substantially approaching the foUowing relations:

c = ^8m77λ;^ragg δf (m=±l, +2,...) (3)

and

wherein c is the chirping coefficient, / is the length of the Bragg grating, λ _Braggis the center reflected wavelength, i.e. Bragg wavelength, and n is the average refractive index of the SBG, P is the sampling period of the SBG, v, is the Ught velocity in vacuum, m is the integer ( m=±!, ±2, ...) and the <^is the operation frequency spacing of the interleaver filter.

4. An optical chirped SBG-based interleaver filter as in claims l,2or3, wherein the chirped SBG-based interleaver filter further characterized in that the structure parameters of the SBG using Blackman Hamming, Gauss and Tanh, Sine, Cauchy, or Super-Gauss apodization in every sample.

5. An optical chirped SBG-based interleaver filter in claims 1 , 2, or3 configured to operate as a multiplexer.

6. An optical chirped SBG-based interleaver filter in claim 1 ,2, or3 configured to operate as a router.

7. An optical chirped SBG-based interleaver filter as in claim 3, wherein the chirped SBG- based interleaver filter further characterized in that the structure parameters of the SBG substantially approaching the foUowing relations: λ _Bragg=1545nm, «=1.448, E=0.259mm to 1.035mm, c=4.978xl0^"4 to 1.991 xl0-³/mm.

8. An optical chirped SBG-based interleaver filter as in claim 3, wherein the chirped SBG- based interleaver filter further characterized in that the structure parameters of the SBG substantially approaching the following relations: λ _Bragg ^*=1545nm , w=1.448, E=0.259 to 2,07mm, c=1.245xl0-⁴ to

9. An optical chirped SBG-based interleaver filter as in claim 3, wherein the chirped SBG- based interleaver filter further characterized in that the structure parameters of the SBG in a 50 GHz interleaver filter substantially approaching the foUowing relations: λ _Bragg=1545nm , w=1.448, E=1.035mm, c=4.978xlONmm, m=l; or λ _Bragg=1545nrn , «=1.448, E=0.259rnrn, c=1.991xlONrnrn, m=4; or ^λ _Bragg ⁼1545nm , «=1.448, E=0.518mm, c=9.956xl0-⁴/mm, m=2.

10. An optical chirped SBG-based interleaver filter as in claim 3, wherein the chirped SBG- based interleaver filter further characterized in that the structure parameters of the SBG in a 25 GHz interleaver filter substantially approaching the following relations: ^λ _Bragg ^:=1545nm , «=1.448, E=0.259mm, c=9.956xl0Nmm, m=8; or ^λ _Bragg ⁼¹545nm , «=1.448, E=0.518mm, c=4.978xlONmm, m=4; or λ _Bragg=1545nm , «=1.448, E=1.035mm, c=2.489xl0^"4/mm, m=2; or λ _Bragg=1545nm , «=1.448, E^*=2.07mm, c=1.245xl0-⁵/mm, m=\.

10

11. An optical chirped SBG-based interleaver filter as in claim 3, wherein the chirped SBG- based interleaver filter further characterized in that the structure parameters of the SBG in a 12.5 GHz interleaver filter substantially approaching the foUowing relations: λ _Bragg=1545nm , n=1.448, E=0.518mm, c=2.489 lONmm, m=8 ; or I^{5 λ} _Bragg=1545nm , n=1.448, E=1.035mm, c=1.245xlONmm, m=4; or λ _Bragg=1545nm , «=1.448, E=2.07mm, c^*=6.23xlONmm, m=2.

12. An optical chirped SBG-based interleaver filter as in claim 3, wherein the chirped SBG- based interleaver filter further characterized in that the structure parameters of the SBG in a

20 100 GHz interleaver filter substantially approaching the following relations: «=1.448, E^*=0.518mm, c=9.956xl0Nmm.