Abstract:
A method for manufacturing a multi-dimensional target waveguide grating and volume grating with micro-structure quasi-phase-matching. An ordinary waveguide grating is used as a seed grating, and on this basis, a two-dimensional or three-dimensional sampling structure modulated with a refractive index, that is, a sampling grating, is formed. The sampling grating comprises multiple shadow gratings, and one of the shadow gratings is selected as a target equivalent grating. A sampled grating comprises Fourier components in many orders, that is, shadow gratings, a corresponding grating wave vector is [Formula 1], and the grating profile of all the shadow gratings changes with the sampling structure [Formula 2]. In a case where a seed grating wave vector [Formula 3] and a required two-dimensional or three-dimensional grating wave vector do not match, a certain Fourier periodic structure component of the Fourier components of the sampling structure is used to compensate for the wave vector mismatch. The manufacturing method may be applied to design and manufacture a multi-dimensional target waveguide grating and volume grating for any grating profile, and may simplify the grating manufacturing process and also make possible a variety of grating-based photon devices.

Description:
TECHNICAL FIELD 
       [0001]    This invention belongs to the field of optoelectronic technology involving fiber-optic communications, photonic integration, photoelectric sensing as well as other optoelectronic information processing technologies. The main idea of the invention is to propose the micro-structure of quasi-phase matching technology (i.e. MS-QPM) with which we can equivalently realize any target grating structure for the two-dimensional planar waveguide Bragg grating or three-dimensional volume grating using the sampling structure. Based on this design idea, the applicants also propose a variety of new photonic devices such as new wavelength division multiplexer or filter without retro-reflection, tilted waveguide grating, DFB semiconductor lasers with suppressed 0 th  channel based on the reconstruction equivalent chirp technology as well as directional coupler and power splitters. 
       BACKGROUND 
       [0002]    With the advancement of computer network technology, the demand for information is increasing exponentially. Over the past decade, the fast development of Internet and the accompanying huge requirement for information transmission not only call for a more advanced fiber communication system, but also demands the development of advanced fiber communication technology. This is especially true for optical communication devices. This new demand requires new theories and cost-effective devices to support the further development of optical networks. Fortunately, the photonic integration circuits (PICs) technology developed in recent years conforms to the development of times and is opening a new era of optical networks. PICs technology is considered to be the cutting-edge and the most promising technology of optical communications. In the Silicon Valley of the United States of America, Infinera Corporation has realized the integration of a large number of complex optoelectronic integrated devices with indium phosphide and other materials, lowering the cost of optical communication while increasing its capacity. In the field of passive optical devices, Bragg grating waveguide shows an excellent property of wavelength selectivity and has been used in a variety of optical communication devices and photonic integrated devices, such as planar integrated Bragg grating waveguide filter [1] with which multiplexer/demultiplexer or filter of light signal with different wavelengths waveguide grating assisted components (OADM) [2-3], tilted waveguide grating mode converter, etc. [4] are achieved. In actual design, it is necessary to achieve different functions of the waveguide grating in a single photonic chip, which means different grating profiles have to be written individually for different functions. In particular, in order to achieve different grating directions, cycles, phase shifts, chirps and even arbitrary structures in the same chip, the traditional low-cost holographic exposure is almost impossible to actualize. Therefore, more advanced nano-fabrication technology, such as electron beam (E-Beam) lithography, is often utilized. However, the high cost and time-consuming properties of E-Beam lithography have increased the difficulty and cost of fabrication and limited its large-scale implementation. 
         [0003]    In order to solve this practical problem, Chen Xiangfei et al first proposed an effective solution to simplify the fabrication of fiber gratings, and they called it “Reconstruction-Equivalent-Chirp (REC) technology” [5-6]. With this technology, we can fabricate a nano-scale grating structure with micron-scale precision. This method has also been successfully applied to the design and fabrication of the distributed feedback (DFB) semiconductor laser and the DFB laser array [7-9], which provides an effective solution for high performance of semiconductor laser array in photonic integration. In order to further solve the monolithic integration problem of different waveguide gratings with complex structure in the planar photonic integration and to lower the fabrication costs, based on the previous research of the applicants of this invention, the applicants propose a micro-structure quasi-phase matching (MS-QPM) technology. This technology not only provides a new method of design and fabrication of the waveguide grating with two or three dimensions, but also gives some novel grating structure and corresponding optical properties of waveguide gratings or volume gratings. For instance, we can change the grating period and even rotate the grating directions equivalently and simultaneously by sampling in the same seed grating. The REC technology is a special one-dimensional case of the micro-structure of quasi-phase matching (MS-QPM) [5]. The mathematical expression of this technology shares some similarities with the famous quasi-phase matching (QPM) described in non-linear optical materials [10,11], and therefore it can be considered as a new discovery and development of quasi-phase matching technology. In summary, this technology can achieve arbitrary shape of two or three-dimensional gratings by changing the large-scale sampling structure while keeping the seed grating period unchanged. Any of the physical achievable of two or three-dimensional grating structure can be achieved as long as the two or three dimensional gratings design is used by sampling structure with micrometer scale and uniform seed grating. With this structure we can achieve a variety of optical properties of the waveguide grating or volume grating with the fine grating structures. We only need to change the sampling structure while keep the seed grating uniform. The sampling structure size is normally a few micrometers, the implementation of this method only requires a standard holographic exposure technique with conventional photolithography technique. This greatly eases fabrication process and substantially improves productivity and product quality. The idea of two or three-dimensional sampled grating structures can be used to design new photonic devices, such as wavelength division multiplexer, which until now are array waveguide grating (AWG) and multimode interference (MMI) in the mainstream market. These existing devices have high requirement for waveguide accuracy, aside from relatively large size. Based on this two-dimensional sampling structure, combined with Bragg grating reflection principle, we can make a new compact wavelength division multiplexer. In addition, some other photonic devices such as filter without retroreflection, DFB semiconductor lasers with suppressed 0 th  channel resonance based on the Reconstruction-Equivalent-Chirp technology, directional coupler and power splitters of any angle, optical waveguide mode converter, any other photonic devices based on waveguide grating and the volume grating, can be achieved. We believe this method can open a new avenue and bring a new dawn to the design of planar photonic integration and other relative photonic devices. 
         [0004]    The main idea of this invention is to propose the micro-structure of quasi-phase matching technology. Based on this technology, the target waveguide grating or volume waveguide grating with any grating shape and the corresponding photonic devices can be achieved by two or three-dimensional sampling structures using a uniform grating. 
       REFERENCES 
       [0000]    
       
         [1] Xu Mai, Research Progreess on Waveguide Gratings for integrated Optics “Chinese Journal of Luminescence”, 2005, 26 (4); 415-425. 
         [2] Jose M. Castro, David F. Geraghty, Seppo Honkanen, Christoph M. Greiner, Dmitri Iazikov, and Thomas W. Mossberg, Optical add-drop multiplexers based on the antisymmetric waveguide Bragg grating, Applied Optics, 2006, 45(6); 1236-1243. 
         [3] Ming Li, Yaming Wu, Jiangyi Yang, and Hongchang Qu, Return loss reduction of integrated grating-assisted optical Add/Drop multiplexer by control the reflective spectrum, Journal of lightwave technology, 2005, 23(3): 1403-1409. 
         [4] Jose M. Castro, David F. Geraghty, Demonstration of mode conversion using anti-symmetric waveguide Bragg gratings, Optics Express, 2005, 13(11): 4180-4184. 
         [5] Yitang Dai and Xiangfei Chen Yi, etc, CN200410007530.5. 
         [6] Yitang Dai, Xiangfei Chen, Li Xia, Yejin Zhang, and Shizhong Xie, Sampled Bragg grating with desired response in one channel by use of reconstruction algorithm and equivalent chirp, Optics Letters, 2004, 29(12): 1333-1335. 
         [7] Yitang Dai and Xiangfei Chen, DFB semiconductor lasers based on reconstruction-equivalent-chirp technologyOptics Express, 2007, 15(5): 2348-2353. 
         [8] Jingsi Li, Huan Wang, Xiangfei Chen, Zuowei Yin, Yuechun Shi, Yanqing Lu, Yitang Dai and Hongliang Zhu, Experimental demonstration of distributed feedback semiconductor lasers based on reconstruction-equivalent-chirp technology. Optics Express, 2009, 17(7): 5240-5245. 
         [9] Jingsi Li, Xiangfei Chen, Ningzhou, etc, Monolithically integrated 30-wavelength DFB laser array, Proc. of SPIE-OSA-IEEE, 2009, SPIE 7631, 763104. 
         [10] J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan. Interactions between light waves in a nonlinear dielectric, Physical review, 1962, 127(6): 1918-1939. 
         [11] Shi-ning Zhu, Yong-yuan Zhu, Nai-ben Ming, Quasi-Phase-Matched third-harmonic generation in a quasi-periodic optical superlattice, Science, 1997, 278 (843). 
       
     
       DESCRIPTION OF THE INVENTION 
       [0016]    This invention proposes a technology named the micro-structure quasi-phase-matching. This technology is used to fabricate the two dimensional (2-D) planar waveguide grating and the three dimensional (3-D) volume grating with complex grating corrugation profile. The arbitrary grating corrugation profile can also be equivalently achieved by designing the sampling structure and the seed grating with relatively small grating period. This technology simplifies the fabrication process and lowers the cost of production for the various photonic devices. 
       THE DETAILED DESCRIPTIONS OF THE INVENTION ARE AS FOLLOWS 
       [0017]    The main characteristic of the invention is to use uniform grating period and uniform grating index modulation strength as the seed grating with grating period between 50 nm and 1000 nm. Then the sampling structure further modulates the seed grating&#39;s index corrugation to form the sampled grating structure. This sampled grating with specific sampling structure can equivalently realize the arbitrary 2-D or 3-D grating structure. Based on the Fourier series analysis, such a sampled grating contains a series of Fourier components. Each of these Fourier components can also be considered as a grating which is called ghost grating or sub-grating. If one of these ghost gratings is selected to perform a specific function, this ghost grating is called the target equivalent grating. When the wave-vector of the seed grating is described as {right arrow over (K)} 0 ({right arrow over (r)}), the wave-vector difference Δ{right arrow over (k)} between the light {right arrow over (k)} and the seed grating can be expressed as, 
         [0000]      Δ {right arrow over (k)}={right arrow over (k)}−{right arrow over (K)}   0 ( {right arrow over (r)} )  (1)
 
         [0018]    When a light beam hit on a grating based device (either two or three dimensional), strong coupling will happen if some certain condition is satisfied. This condition is usually called the phase matching condition. Specifically, it can be written as Δ{right arrow over (k)}={right arrow over (k)}−{right arrow over (K)} 0 ({right arrow over (r)})=0. If it is satisfied, the light will have the largest diffractive efficiency. This phenomenon is also called the Bragg diffraction. 
         [0019]    Usually, the index corrugation of the seed grating can be expressed as Δn({right arrow over (r)})=½Δn exp(j{right arrow over (K)} 0 ·{right arrow over (r)})+c.c. The corresponding sampled grating with arbitrary sampling structure can be expressed as, 
         [0000]      Δ n   s ( {right arrow over (r)} )= S[{right arrow over (r)}]·Δn ( {right arrow over (r)} )  (2)
 
         [0020]    Here, S[{right arrow over (r)}] describes the sampling structure, Δn({right arrow over (r)}) is the amplitude of the index modulation of the seed grating, {right arrow over (r)} denotes the spatial vector. If the Fourier analysis is applied here, the Eq. (2) can be further expressed as 
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         [0021]    Here j denotes imaginary unit, {right arrow over (G)} sn ({right arrow over (r)}) are the wave-vectors of the n th  Fourier components of the sampling structure. These Fourier components are called sampling structure&#39;s Fourier sub-grating. {right arrow over (K)} 0 ({right arrow over (r)}) is the grating wave-vector of the seed grating and can be written as 
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         [0000]    is the period of the seed grating. {right arrow over (e)} 0  denotes the direction of the seed grating. C n  is the Fourier coefficients of each Fourier component. n denotes the order of the Fourier components. Therefore, n th  order of the ghost gratings is characterized by wave-vector {right arrow over (K)} n ({right arrow over (r)}). Obviously, all of these ghost gratings can be manipulated by changing the sampling structure S[{right arrow over (r)}]. This is because {right arrow over (K)} n ({right arrow over (r)}) is only composed by seed grating&#39;s wave-vector {right arrow over (K)} 0 ({right arrow over (r)}) and the wave-vector of sampling structure&#39;s Fourier sub-grating {right arrow over (G)} sn ({right arrow over (r)}). For a certain order of the Fourier components of the sampled grating, i.e. n=N, the wave-vector transformation relation according to Eq. (3) can be expressed as 
         [0000]        {right arrow over (K)}   N ( {right arrow over (r)} )= {right arrow over (G)}   sN ( {right arrow over (r)} )+ {right arrow over (K)}   0 ( {right arrow over (r)} )  (4)
 
         [0022]    Let {right arrow over (K)} N ({right arrow over (r)}) be equal to K d ({right arrow over (r)}) of our desired grating or target grating, i.e. {right arrow over (K)} N ({right arrow over (r)})={right arrow over (K)} d ({right arrow over (r)}), then {right arrow over (K)} N  ({right arrow over (r)}) can be called as target equivalent grating wave-vector. Then, we call {right arrow over (G)} sN ({right arrow over (r)}) target sampling structure&#39;s Fourier sub-grating wave-vector. {right arrow over (K)} d  is the wave-vector of the target grating corresponding to the specific grating corrugation that we select to perform a specific function. The target equivalent grating is denoted by {right arrow over (K)} N  ({right arrow over (r)}) and can be easily manipulated by changing the sampling structure S[{right arrow over (r)}]. The detailed wave-vector transform procedure can be summarized as follows: 
         [0023]    We first change the sampling pattern or sampling structure S[{right arrow over (r)}] on purpose. Then, the wave-vector {right arrow over (G)} sN ({right arrow over (r)}) can be changed accordingly. According to Equ. (4), the wave-vector of the target equivalent grating {right arrow over (K)} N ({right arrow over (r)}) can also be changed. To carefully pre-design the sampling structure, the target equivalent grating can be same as the target grating. Therefore, the arbitrary grating profile can be realized equivalently only by changing the sampling pattern under a certain seed grating. In other words, we use one of the Fourier components of the sampling structure to compensate the wave-vector difference between the seed grating wave-vector {right arrow over (K)} 0 ({right arrow over (r)}) and the target grating wave-vector {right arrow over (K)} d ({right arrow over (r)}) when {right arrow over (K)} 0 ({right arrow over (r)}) and {right arrow over (K)} d ({right arrow over (r)}) is not equal. In general, this method is similar to the Quasi-phase-matching technology of the nonlinear optics for high order harmonic generation. In the nonlinear medium, an additional periodic structure is provided to compensate the wave-vector mismatch between the incident fundmental and the output high order harmonic light frequency. This transformation can contribute an additional momentum to satisfy the necessary of momentum conservation. By contrast, the Micro-structure Quasi-phase-matching technology has the similar physical picture and mathematical expression that can be expressed as follows: 
         [0000]      Δ {right arrow over (k)}={right arrow over (K)}   d   −{right arrow over (K)}   0 ( {right arrow over (r)} )− {right arrow over (G)}   sN ( {right arrow over (r)} )=0  (5)
 
         [0024]    Here, {right arrow over (K)} d  can be usually expressed as {right arrow over (K)} d ={right arrow over (k)} re −{right arrow over (k)} in . 
         [0025]    Furthermore, according to Eq. (3), the phase of the sampled grating is also a function of the space. For a certain ghost grating, the phase change can be expressed as 
         [0000]      exp[ j{right arrow over (G)}   sn   ·{right arrow over (r)} ]exp( j ( {right arrow over (K)}   0   ·{right arrow over (r)} ))  (6)
 
         [0026]    Assuming there is a phase shift in the sampling structure, and the value of the shift is Δ{right arrow over (r)}′, the phase of the sampled grating will be, 
         [0000]      exp[ j{right arrow over (G)}   sn ·( {right arrow over (r)}+Δ{right arrow over (r)} ′)]exp( j ( {right arrow over (K)}   0   ·{right arrow over (r)} ))=exp[ j{right arrow over (G)}   sn ·( {right arrow over (r)} )]exp( j ( {right arrow over (K)}   0   ·{right arrow over (r)} ))exp( j{right arrow over (G)}   sn   ·Δ{right arrow over (r)} ′)  (7)
 
         [0027]    Then, a phase shift is easily introduced into the ghost grating. The corresponding phase shift is exp(j{right arrow over (G)} sn ·Δ{right arrow over (r)}′). For example, if the seed grating is uniform and {right arrow over (G)} sn  Δ{right arrow over (r)}′=φ, a phase shift value φ is then introduced in the n th  order ghost grating. 
         [0028]    The index modulation strength of the target equivalent grating can be changed by changing the sampling structure shape in one sampling period such as the duty cycle. In the case of 2-D, we can continuously change the duty cycle of sampling structure along the grating, the equivalent apodization can be realized. The relation between the index modulation of a certain sub-grating and the duty cycle can be determined by the Fourier analysis. 
         [0029]    When an optical signal is actually transmitted and processed, this optical signal has a certain bandwidth instead of a single frequency. That means this optical signal has a certain wavelength within a certain bandwidth. So the corresponding grating structures should be designed according to the process of the realistic optical signal to obtain the largest efficiency. For an actual optical signal, the specific grating is called as the target grating if we can get the best efficiency of single process. The wave-vector of the target grating is called as the wave-vector of the target grating {right arrow over (K)} d ({right arrow over (r)}). 
         [0030]    In most of the cases, the target grating is not a uniform grating. The period of the target grating is usually in the magnitude of a few hundreds of nanometers. Nanotechnology is needed to fabricate this grating. Meanwhile, the wavelength space of two channels of signals is usually smaller than 1 nm. This requires very high precision fabrication method and brings great difficulties to the massive fabrication for commercial application. 
         [0031]    Because there is a one-to-one correspondence between the grating structure and its wave-vector, the key idea to obtain the target grating is how to design the corresponding target grating wave-vector {right arrow over (K)} d ({right arrow over (r)}). This invention provides a method to achieve the target grating structure or the required wave-vector of the target grating {right arrow over (K)} d  ({right arrow over (r)}). Because the sampled grating still maintains the form of periodic structure, it is also a superposition of the Fourier components according to the Fourier analysis. Each Fourier component is called a ghost grating. Each wave-vector of these ghost gratings is a composite of the wave-vector of the seed grating and that of the sampling structure&#39;s Fourier sub-grating. Therefore, we can equivalently realize the target grating by designing the sampling structure to obtain a suitable wave-vector of the sampling structure&#39;s Fourier sub-grating. Here, the selected sampling structure&#39;s Fourier sub-grating is called the target sampling structure&#39;s Fourier sub-grating wave-vector. And the corresponding ghost grating with the same Fourier order of the sampled grating is called the target equivalent grating. In other words, the wave-vector of the target grating is equal to the vector composed by the wave-vectors of the target sampling structure&#39;s Fourier sub-grating and that of the seed grating. The seed grating can be fabricated by low cost traditional technologies, such as, holography exposure and near-field holography exposure to get the large area massive fabrication. The sampling structure can be achieved by using conventional photo-lithography. 
         [0032]    Let us take two dimensional uniform planar waveguide grating as seed grating (any other grating structures can be used as the seed grating, we choose the uniform grating for the convenience of fabrication). The grating period usually ranges from 50 nm to 1000 nm. Base on such a seed grating, the two dimensional larger scale periodic structure (which is called the sampling structure) is further fabricated on it to form the sampled grating. If a certain order of ghost grating is selected as the target equivalent grating, the target waveguide grating or target volume grating can be equivalently achieved. The seed grating is common two dimensional or three dimensional Bragg grating, whose period is relatively small and usually a few hundred nanometers. In this invention, this kind of the basic gratings is called seed grating, because it only provides basic feedback of light instead of complex optical response. In practice, the seed grating can be of any shape and we conveniently fabricate the seed grating in uniform period by using holographic exposure. As a result, we always choose the uniform grating as the seed grating, then we sample the seed grating to form sampled grating and the sampling pattern can also be any shape principally. Because the sampling structure is also a kind of periodic structure, according to Eq. (3), there are many Fourier components. And all of their periods are equal or less than the period of the sampling structure. These Fourier components are also a series of gratings. If the seed grating is further modulated by the sampling structure, it can form the refractive index modulation with the sampling pattern. Such periodic refractive index modulation structure is the sampled grating, which is familiar to us. 
         [0033]    When the wave-vector of the seed grating and the wave-vector of the target grating are not equal, we can change the sampling structure while keep seed grating unchanged. The mismatch between them can be compensated by using the wave-vector of the target sampling structure&#39;s Fourier sub-grating. This is similar to the quasi-phase matching in the nonlinear optics, which makes the phase difference in the transmission zero by an additional periodic modulation structure. This process is described in Eq. (5) for this invention. The mathematical expression and physical meaning of Eq. (5) is similar to that of the QPM in non-linear materials. Both of them use an additional periodic modulation structure to compensate their mismatch. In this invention, we use the sampling structure of large period to compensate the difference between the seed grating and the target grating. As a result, we match the wave-vector of the seed grating and that of the target grating, which is called the micro-structure quasi-phase match. Consequently, this invention is named Micro-Structure of Quasi-Phase Matching technology (MS-QPM technology). 
         [0034]    Under physically realizable conditions, we can achieve the target equivalent grating of any grating structure by using the MS-QPM technology, especially the tilted/arc grating, chirp/phase shifted grating. The sampled grating contains a series of Fourier components. And the wave-vector of the ghost grating is {right arrow over (K)} n ({right arrow over (r)}). According to Eq. (4) and (5), if we want to obtain a particular target equivalent grating, that is the ghost grating of a certain Fourier component, it can be composed by the wave-vectors of the seed grating and the sampling structure&#39;s Fourier sub-grating through corresponding design of the sampling structure. Specifically, in order to change the direction of grating, the target equivalent grating with the tilt/arc grating structure can be equivalently achieved by changing the direction of the wave-vector {right arrow over (G)} sN ({right arrow over (r)}) of the sampling structure according to Eq. (4). To achieve the multi-dimensional phase shifted grating, the equivalent phase shift can also be realized by inserting the phase in the sampling structure according to Eq. (6) and (7). The chirped grating can also be equivalently realized according to Eq. (3) and (4) by changing the period and the direction of the sampling structure. Since the period of the uniform seed grating is usually several hundred nanometers, the seed grating is very easy to fabricate by the traditional holographic exposure with two beam interference or the near-field holographic exposure. The period of the sampling structure is usually several micrometers, so the sampling structure is also easy to fabricate by traditional photo-lithography. The whole fabrication process of a sampled grating with a specific sampling pattern is easily implemented with the MS-QPM technology. 
         [0035]    There are two methods to design the WDM multiplexer/demultiplexer based on the MS-QPM technology. The first method is the cascade sampling structure which consists of several sections in the two dimensional waveguide grating. Each section of the particular sampling structure includes a particular sampling period and a particular direction of the corresponding wave-vector as shown in  FIG. 3 , while the seed grating is kept uniform. A light beam with certain frequency (Bragg wavelength) is diffracted from a certain section of the sampled grating. The second method is the sampled grating with chirped seed gating. As shown in  FIG. 4 , the seed grating is chirp and the sampling structure keeps uniform but there is a particular angle between them. Therefore, the light with different wavelength will be reflected in different directions by the different target equivalent gratings in the different sections of the sampled grating. Hence, the demultiplexer can be achieved. According to the reversibility of optical path, the lights with different wavelengths propagate in particular direction will be reflected in the same direction. So the multiplexer can be realized. The cavity length of the device is determined by the number of the channels. Usually, the sampling period is from 0.5 to 20 μm. The sampling period of the volume sampled grating based multiplexer/demultiplexer will be longer. 
         [0036]    When fabricating the Bragg grating filter, two equivalent it phase shifts are inserted at ¼ and ¾ of the length of the total cavity, this establishes a narrow passband in middle of the stopband. The equivalent it phase shift can be realized by Eq. (6) and (7). The cavity length is between 50.0 μm to 5000.0 μm. The index modulation is 0.2 or larger. The stopband is up to 40 nm or even wider which can cover one whole optical communication window. 
         [0037]    When we fabricate the DFB semiconductor laser based on the Reconstruction-Equivalent-Chirp (REC) technology with the 0 th  order channel suppressed, the period of the seed grating is kept uniform but tilted with an include angle between the direction of the wave-vector {right arrow over (K)} 0 ({right arrow over (r)}) and the axial direction of the waveguide working as resonant cavity. The titled angle is from 2° to 15°. If the designed sampling structure is also tilted with a certain tilted angle, the direction of the target equivalent grating&#39;s wave-vector {right arrow over (K)} N ({right arrow over (r)}) will be rotated to be parallel to the axial direction of the resonant cavity. Here, N is usually equal to ±1. The specific tilted angle can be determined according to the effect on the suppression of 0 th  order channel&#39;s resonance. Usually the tilted angle is from 3° to 15°, which will have a good effect on the suppression. The detailed parameters of the seed grating and the sampling structure can be determined according to Eq. 4. The period of the sampling structure is usually from 0.5 to 20 μm. The larger of the title angle will lead to a better suppression effect. When the tilted angle is larger than 10°, the light resonance of the 0 th  order channel can be fully suppressed. 
         [0038]    When we fabricate the tilted waveguide grating and the tilted waveguide grating based photonic devices, the uniform seed grating is used and the direction of the wave-vector {right arrow over (K)} 0 ({right arrow over (r)}) is designed according to the actual requirement. The sampling period is also uniform but there is a tilted angle between the sampling structure and the seed grating. Hence, there will be a tilted angle between the target equivalent grating and the axial direction of the waveguide according to Eq. (4). This tilted angle is usually designed to be from 2° to 15°. 
         [0039]    When we fabricate the power divider with arbitrary power division ratio and directional coupler with arbitrary coupling direction, the seed grating on the 2-D planar waveguide keeps uniform, and the sampling structures are different in different sections of the waveguide. The corresponding target equivalent gratings with different directions in different sections of the waveguide will reflect incident lights with specific wavelength on different directions. Each of the reflective directions can be designed on purpose. The detailed parameters can be calculated according to Eq. 4. Then, the directional coupler can be realized. If the index modulation is designed correctly, the sampled grating only reflects a portion of the incident light power. Therefore, the power divider can be obtained. The index modulation is usually designed to be from 0.001 to 0.2. The larger index modulation of the seed grating will lead to a high diffractive efficiency. 
         [0040]    When we fabricate the arbitrary volume grating filter and volume grating based photonic device, the seed grating keeps uniform and the sampling structure can be realized by common photo-mask using the common photo-lithography. The sampling structure can be designed according to the equations from Eq. 3 to Eq. 7. Therefore, the required target grating can be realized by the target equivalent grating. 
         [0041]    When we fabricate the waveguide grating based photonic device with fine grating structure, the seed grating can be fabricated by holographic exposure. The required target equivalent grating can be obtained by designing the sampling structure according to the equations from Eq. 3 to Eq. 7. The sampled grating can be realized by the common photo-lithography. Therefore, the various waveguide grating based photonic devices can be easily realized. 
         [0042]    When we fabricate the monolithic photonic integrated circuits with DFB semiconductor laser array based on REC technology and the waveguide grating filters on the same chip, the DFB laser array and the waveguide grating can share the same seed grating and the sampling structure can be designed respectively according to the equations from Eq. (3) to Eq. (7). Consequently, the whole sampling structure can be fabricated on the same mask. Therefore, the whole sampled grating on that chip can be realized at the same time to achieve the different photonic elements integrated monolithically. 
         [0043]    This invention benefits the fabrication of the various grating based photonic devices. The invention proposes a method named the Micro-Structure Quasi-Phase-Matching (MS-QPM) technology to fabricate the arbitrary required target multi-dimensional waveguide grating and volume grating with complex grating structure. A detailed description of the MS-QPM technology is presented in this invention. This technology requires only the uniform seed grating and the designed large scale sampling structure in order to greatly reduce the cost of fabrication. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0044]      FIG. 1 , the wave-vector transform diagram to form the target equivalent grating. The seed grating is uniform as an example. 
           [0045]      FIG. 2 , the diagram of the interaction between the incident light and the target equivalent grating. 
           [0046]      FIG. 3 , the diagram of the cascaded sampled grating used as WDM multiplexer/demultiplexer. The seed grating is uniform. 
           [0047]      FIG. 4 , the diagram of the sampled grating used as WDM multiplexer/demultiplexer with chirped seed grating. 
           [0048]      FIG. 5 , the diagram of the filter with no retraced light. 
           [0049]      FIG. 6 , the diagram of DFB sampled grating with suppressed 0 th  order channel resonance. 
           [0050]      FIG. 7 , the design of the tilted waveguide grating. 
           [0051]      FIG. 8 , the diagram of the power divider and directional coupler 
           [0052]      FIG. 9 , (a, b) the diagram of fabricating the sampled grating, (c, d) one example of designing a equivalent tilted grating based on the Micro-structure Quasi-phase-matching technology and the composition of the wave-vectors of the sampled grating. The strong diffraction happens when the incident light satisfy the Bragg condition. 
           [0053]      FIG. 10 , the diagram of the fabrication method for sampled grating using the phase mask with metal coating. 
           [0054]      FIG. 11 , the facility for fabricating the volume grating. 
       
    
    
     PROCESS OF IMPLEMENTATION 
       [0055]    The strength of the index modulation of the target equivalent grating can be varied by changing the duty cycle of the sampling structure. In the case of the one-dimensional sampled grating, the relationship between the strength of the index modulation of the target equivalent grating and the duty cycle of the sampling structure is described in the patent titled “Equivalent Apodized Planar Waveguide Grating and DFB Semiconductor Laser Based on the Reconstruction-equivalent-chirp Technology” (China Patent Application No. 200910264486.9). In the case of the two-dimensional sampled grating, the maximum strength of the index modulation of the target equivalent grating can also be obtained by using proper sampling structure. The specific structure can be calculated by the Fourier analysis. 
         [0056]    As described in Eq. (3) to Eq. (5) that the wave-vector of the Fourier components {right arrow over (K)} n ({right arrow over (r)}) of the sampled grating are composed by the corresponding order Fourier components {right arrow over (G)} sn ({right arrow over (r)}) of the sampling structure and the seed grating&#39;s wave-vector {right arrow over (K)} 0 . As shown in  FIG. 1 , the Fourier components of the sampled grating can be changed with the sampling structure which can be varied with space. Therefore, the arbitrary profile of the target equivalent grating can be equivalently realized by changing the sampling structure. The wave-vector {right arrow over (K)} d ({right arrow over (r)}) of the target grating which includes grating direction and period can be arbitrarily changed. 
         [0057]    If the sampling structure is designed to be uniform, i.e. the wave-vector of the sampling structure is a constant, while there is an included angle between the wave-vector of the sampling structure and that of the seed grating, all of the ghost gratings are also uniform but rotating an angle according to Eq. (3) and Eq. (4). In this condition, a light beam which meets the Bragg condition with the target equivalent grating will diffract significantly in one certain direction as shown in  FIG. 2 . 
         [0058]    When the required target grating is not uniform, i.e. changing with space, such as phase shift grating, chirped grating or curved grating, the wave-vector {right arrow over (G)} sN ({right arrow over (r)}) of the target sampling structure&#39;s Fourier sub-grating is also required to change with space. Therefore, the target sampling structure&#39;s Fourier sub-grating or the sampling structure should be designed suitably to obtain the corresponding target equivalent grating which has the same grating profile with the target grating. As shown in Eq. (3), the phase of the sampled grating is also a function regarding space. According to Eq. (6) and Eq. (7), a phase shift in the sampling structure can also lead to a phase shift in the ghost grating. For example, if the relative shift is Δ{right arrow over (r)}′ in some place of the sampling structure, a phase shift of φ={right arrow over (G)} sN ·Δ{right arrow over (r)}′ will be introduced in the N th  order ghost grating. 
         [0059]    All the above conclusions from two dimensional situation can be applied to the three dimensional cases as well. For example, the fabrication of the volume grating with complex grating structure will be greatly simplified with this design. Moreover, from Eq. (4), it also can be seen that the Reconstruction-Equivalent-Chirp technology is only the one dimensional case of this invention. For the other kinds of patterns of the seed grating and sampling structure such as the lattice structure which usually appears in photonic crystals or even the seed grating with arbitrary shapes, where {right arrow over (K)} 0 ={right arrow over (K)} 0 ({right arrow over (r)}) is a function regarding space, the above conclusions are also true. The invention can be applied to any kinds of materials such as silicon-based materials and III-V group materials. 
         [0060]    The above gives the basic method to equivalently obtain the target grating using Micro-structure quasi-phase matching technology. In the following, some detailed photonic devices based on Micro-structure quasi-phase matching technology are illustrated. 
         [0061]    Grating structure used as WDM Multiplexer/Demultiplexer. 
         [0062]    (1) The Grating Structure of Multiple Layers Sampling ( FIG. 3 ) 
         [0063]    The two dimensional planar waveguide is divided into several sections or layers with different sampling structure in each section. All of these sections share the same uniform seed grating. But with the different sampling structures in each section, the target equivalent grating is also different. So, each section corresponds to a certain Bragg wavelength. The light with specific wavelength is reflected in different section of the device and propagates in the given direction. Therefore, the demultiplex will be realized. Based on the reversibility of optical path, if the light with different wavelength incident onto the device in the given direction, all of the light will be reflected in the same direction and coupled into the single waveguide though the diffraction for each light happens in different section of planar waveguide. So the Multiplex can be realized. This principle of the Multiplex/Demultiplex is also true for the volume grating. 
         [0064]    (2) The Grating Structure of Chirped Seed Grating with Uniform Tilted Sampling ( FIG. 4 ) 
         [0065]    If the sampling structure is designed to be chirped, the corresponding target sampling structure&#39;s Fourier sub-grating also appears to be chirped. Therefore, the designed target equivalent grating is also of a special chirped profile with the direction and grating period varying with space. An alternative sample grating structure is that the seed grating being designed to be chirped but with uniform sampling structure. The resulting wave-vector {right arrow over (K)} N ({right arrow over (r)}) of the target equivalent grating can also be chirped with varying direction. The incident lights with different frequencies can be diffracted on the different directions. Each light with specific frequency meets the Bragg condition in a certain region of the sample grating. So, the lights with different frequencies will be separated in respect of space and the demultiplex can be realized. Accordingly, the multiplex can be realized if the lights with different frequencies incident in the different direction at different place. These lights will prorogate at the same direction and couple into the single waveguide. 
         [0066]    This two dimensional principle of the Multiplex/Demultiplex applies to the volume grating as well. 
         [0067]    The following is one design example of the 8 channels WDM Multiplexer/Demultiplexer based on the multiple layers sampling grating structure. 
         [0068]    The core material of this two-dimensional planar waveguide is Ge:SiO 2  with an effective refractive index of 1.455 and an index modulation of the seed grating of 0.006.  FIG. 3  is the schematic diagram of the sampled grating. The width of the device is 40 μm along X direction. The length of each section of the unique sampling pattern is 100 μm. The total length of device along Z direction is 1200 μm. The direction of the wave-vector {right arrow over (K)} 0 ({right arrow over (r)}) of the seed grating is parallel to the +Z direction with a grating period equal to 500 nm. The channel spacing is 2 nm and the operating wavelength ranges from 1544 nm to 1558 nm. The incident polychromatic light is parallel to the +Z direction. The detailed design parameters are listed in the following Table.1. 
         [0069]    Table.1. the channel wavelength/sampling period/The included angle between wave-vector {right arrow over (G)} sN ({right arrow over (r)}) of the sampling structure and the wave-vector of the seed grating/the Corresponding reflective angle between the −Z direction. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Included 
               
               
                   
                   
                   
                 angle 
               
               
                   
                   
                   
                 between the 
               
               
                   
                   
                   
                 Reflective 
               
               
                 Channel 
                 Sampling 
                 Included angle 
                 Light and -Z 
               
               
                 wavelength 
                 period 
                 between {right arrow over (G)} sN  ({right arrow over (r)}) and {right arrow over (K)} 0  ({right arrow over (r)}) 
                 direction 
               
               
                   
               
             
             
               
                 1544 nm 
                 4.20 μm 
                 55° 
                 11.95° 
               
               
                 1546 nm 
                 4.00 μm 
                 56° 
                 12.71° 
               
               
                 1548 nm 
                 3.85 μm 
                 56° 
                 13.42° 
               
               
                 1550 nm 
                 3.60 μm 
                 57° 
                 14.36° 
               
               
                 1552 nm 
                 3.30 μm 
                 58° 
                 15.90° 
               
               
                 1554 nm 
                 3.20 μm 
                 58° 
                 16.44° 
               
               
                 1556 nm 
                 3.10 μm 
                 58° 
                 17.00° 
               
               
                 1558 nm 
                 3.00 μm 
                 58° 
                 17.62° 
               
               
                   
               
             
          
         
       
     
         [0070]    The +1 st  ghost grating works as the target equivalent grating. Because the wave-vector of the seed grating and the incident light are all parallel to the +Z direction, all of these included angles can be either clockwise or counterclockwise. The corresponding sampling structures of these two different included angles are mirror symmetrical on Z axis. 
         [0071]    2. Grating Filter with No Retraced Light. ( FIG. 5 ) 
         [0072]    If there are two equivalent it phase shifts inserted at ¼ and ¾ of the length of the total cavity, a narrow passband will be established in middle of the stopband. The equivalent π phase shift can be realized by Eq. (6) and (7). If the index modulation is increased to 0.2 or larger, the stopband could be increased to 40 nm or larger, which can cover the whole optical communication window. Thanks to the characteristic of the Bragg reflection, all of the lights outside of the passband will be reflected back in a certain direction instead of retracing. So the external feedback to the light source can be suppressed without using the optical circulator which is usually difficult to realize in photonic integrated circuits (PICs). 
         [0073]    3. DFB Semiconductor Laser Based on Reconstruction-Equivalent-Chirp (REC) Technology with the 0 th  Order Channel Suppressed ( FIG. 6 ) 
         [0074]    The +1 st  order or the −1 st  order ghost grating is generally used as the resonant cavity in the DFB semiconductor laser design based on the Reconstruction-Equivalent-Chirp (REC) technology. Because of the largest index modulation of the 0 th  order ghost grating among all of these ghost gratings, the 0 th  order ghost grating is easy to form the light resonance and the single longitudinal mode operation property may deteriorate. So the 0 th  order ghost must be removed outside of the gain region to suppress the potential lasing. The index modulation of the ±2st ghost grating is so weak that the potential resonance is negligible. In order to further suppress the 0 th  order potential lasing, the tilted seed grating can be used together with tilted uniform sampling pattern so that the 0 th  order radiation loss will be greatly increased. If designed appropriately, the wave-vector of the resulting −1 st  order or +1 st  order ghost grating will be parallel to the waveguide direction. This results in dominant light resonance of the −1 st  order or +1 st  order ghost grating with the other ghost gratings suppressed. This process is illustrated in  FIG. 6 . This sampled grating can improve the single mode property greatly. The equivalent phase shift or the equivalent chirp can be realized simultaneously according to Eq. (6-7) and (3-4). 
         [0075]    4. Tilted Waveguide Grating ( FIG. 7 ) 
         [0076]    Similar to the design of the sampled grating with tilted seed grating to suppress the 0 th  order ghost grating as described above, the equivalent tilted waveguide grating can be realized by tilting sampling structure. Because there exist an included angle between the direction of the waveguide axis and that of the sampling structure, the target equivalent grating can be rotated by a certain angle. So the equivalent tilted grating can be realized. The detailed design parameters can be calculated according to Eq. (4). 
         [0077]    The following is one design example for an equivalent tilted single mode waveguide grating. The material of the core layer is Ge:SiO 2  and the width of the waveguide is 6 μm. The thickness of the core is 2 μm. The effective refractive index n eff  is 1.455. The direction of the wave-vector of the seed grating is parallel to the direction of the waveguide axis. The period of the seed grating is 500 nm. If the required tilted target grating with the grating period is equal to 532.6 nm and tilted angle is equal to 6°, i.e. the included angle between the wave-vector of the equivalent tilted grating and the waveguide axis, the designed sampling period should be 4.211 μm and the tilted angle regarding to the waveguide axis should be 55.93°. 
         [0078]    For instance, to design the Optical Add-Drop Multiplexer (OADM) which is usually based on the tilted waveguide grating, the structure parameter is shown in [3].  FIG. 1  in Ref [3] is the diagram of Optical Add-Drop Multiplexer based on null coupler, in which the tilted waveguide grating can be equivalently realized according to Eq. (4-5). 
         [0079]    5. Power Splitter and Directional Coupler ( FIG. 8 ) 
         [0080]    If the period and direction of the sampling structure are designed appropriately, the target equivalent grating can be rotated by a certain angle. This will diffract the light which meets the Bragg condition on the specific direction. So the required diffractive angle can be designed to make the light propagate in a specific direction and the directional coupler can be realized. If the index modulation of the seed grating is appropriately weak, part of the light which meets the Bragg condition will be reflected, and the remaining continues to prorogate in the original direction. Therefore a power splitter is realized only to properly design the index modulation. The certain magnitude of the index modulation leads to a certain diffractive efficiency. A combined device of the power splitter and the directional coupler can be realized as schematically illustrated in  FIG. 8 . 
         [0081]    A design example is given below. The directional coupler with a power division ratio of 0.5 is carried out. The width of the device is 30 μm along the X axis and the length of the device is 50 μm along the Z axis. The material of the core layer is Ge:SiO2 with the effective refractive index of 1.455. The thickness of the core layer is 2 μm. The wave-vector of the seed grating is parallel to the +Z direction with the period of 485 nm. The incident light with the wavelength of 1545.5 nm and beam width of 30 μm propagate along the +Z direction. There is a diffractive beam with diffractive efficiency of about 50% propagating along the designed direction with an angle of 15.74° between the direction of reflective direction and the Z axial direction if the index modulation is about 0.001. The sampling pattern is square wave with duty cycle of 0.5. The included angle is 50° between the wave-vector of the sampling structure and that of the seed grating. 
         [0082]    The Detailed Fabrication Process 
         [0000]    1. In this Invention, the Key of Fabricating the Two or Three Dimensional Target Equivalent Grating is to Fabricate the Sampled Grating Structure. Detailed Fabrication Processes are as Follows:
       (1) The first step is to fabricate the sampling pattern on the photo-mask. The sampling pattern is structured according to the design principle of the proposed technology. The stripes with metal in the mask correspond to sampling structure in wafer.   (2) The fabrication process of the sampled grating structure has two steps. First, a uniform seed grating with a period of several hundreds nanometers (for example 500 nm) is fabricated through conventional holographic exposure on the photo-resist. The sampling structure is then transferred to the photo-resist by conventional photolithography using the photo-mask as described in step (1). The sampling period is several micrometers (for example 2 μm). The corresponding wave-vector of sampling structure is at an angle of 60 degree to that of the seed grating, which means that the angle between {right arrow over (G)} sN  and {right arrow over (K)} 0  is 60 degree, as the schematic of wave-vectors shown in  FIG. 9(   c ). Then the sampled grating pattern is transferred from the photo-resist to the wafer by dry or wet etching. The order of the two exposures can be changed.  FIG. 9  (a/b) shows the fabrication of the sampled grating structure based on 2D MS-QPM technology. The effective refractive index is 1.06 and the wavelength of incident light, whose propagation direction is parallel to the wave-vector of seed grating, is 1.142 um. As a result, intensive reflection will occur when the angle between the light and the wave-vector of seeding grating is 28 degree (see  FIG. 9   c/d ). The device can control the propagation direction of light.   (3) Target equivalent grating can also be fabricated by phase mask. The sampling structure pattern is first fabricated on phase mask with metal stripes and the lithography is carried out on the Ge-doped silica material or some light-sensitive materials. The phase mask should be close to the light-sensitive material when exposure happens. Then the sampled grating structure designed can be obtained after annealing. The sampling structure pattern on the phase mask has a complementary relationship with the sampling structure pattern on the photo-mask if the positive photo-resist is used in (2), which means that the metal stripes on mask such as Chrome on the phase mask corresponds to the region without grating. The schematic of the fabrication is shown in  FIG. 10 .       
 
       2. The Fabrication of the Planar Waveguide Bragg Grating Devices Based on MS-QPM Technology 
       [0086]    All of the conventional materials for fabricating planar waveguide Bragg Gratings, such as silica-on-silicon, polymer, and other III-V compound materials, can be used to realize the photonic devices based on Micro-structure Quasi-phase-matching technology. The key of fabricating such kinds of filters is to fabricate the sampled grating structure. Specific implementing method has been described in part  1  above. 
         [0087]    Let&#39;s take the Bragg grating filter fabricated by Silicon On Insulator (SOI) ridge waveguide for example.
       (1) Two photo-masks are needed before fabrication. One is used to fabricate the ridge waveguide on the SOI. This step is the same as the conventional method. The other mask with the sampling structures, such as equivalent phase shift, equivalent chirp, equivalent tilted structure, is designed.   (2) After cleaning the SOI wafer and coating with photo-resist, the SOI wafer is exposed for the first time by the photo-mask, then developed, fixed and cleaned by plasma stripper for 30 s. The ICP etching is taken after coating adhesive at 180° C. for 30 minute. The etching depth is determined by etching time. The residual photo-resist is cleaned out by plasma stripper at 150 w for 3 minutes. Thus the ridge waveguide fabrication is completed.   (3) After cleaning the wafer again and taking another exposure, the transfer of the sampled grating pattern of the second photo-mask to the photo-resist proceeds as illustrated in  FIG. 9 . After the process of developing, fixing, cleaning, coating adhesive, ICP etching and cleaning of residual photo-resist, the fabrication of sampled grating is finished. Finally, a 1 μm layer of SiO 2  is deposited on the device surface with PECVD. The two ends are polished to eliminate facet reflection.
 
3. The DFB Semiconductor Laser Based on REC Technology with Suppressed 0 th  Channel Lasing
       
 
         [0091]    The DFB semiconductor laser is composed of n-type substrate, n-type InP buffer, undoped lattice-matched InGaAsP lower optical confinement layer, strained-layer InGaAsP multiple-quantum-well (MQW) active structure, InGaAsP grating layer, InGaAsP waveguide layer, InP optical confinement layer and InGaAs ohm contact layer. The sampled gratings including equivalent phase shift or other structures are on the InGaAsP grating layer. The surface of the sampled grating is covered by 200-400 nm thick SiO 2  insulating layer. 
         [0092]    The following is the concrete steps of fabricating the DFB semiconductor laser with work wavelength of 1550 nm based on REC and MS-QPM technology with the 0 th  channel lasing suppressed. 
         [0093]    The photo-mask with the tilted sampling structure or other special sampling structure patterns is fabricated by conventional micro-electronic fabrication processes. The sampling structure patterns are determined by the equations (3)-(7). 
         [0094]    The device is mainly fabricated by metal organic vapor phase epitaxy (MOVPE). A 200 nm thick n-type InP buffer layer with doping concentration about 1.1×10 18  cm −2 , a 100 nm thick undoped lattice-matched InGaAsP lower optical confinement layer, a strained-layer InGaAsP multiple-quantum-well (MQW) active structure. The MQW structure contains five-layer undoped 6 nm-thick 1.2% compressive strain AlGaInAs wells separated by six-layer 9 nm-thick-0.45% tensile-strain AlGaInAs barriers. Then the upper separate-confinement-heterostructure (SCH) layer is grown. The sampled grating is then formed on the upper SCH layer by a conventional holographic exposure combined with conventional photolithography. After the fabrication of the sampled grating, a p-InP cladding layer and a p+-InGaAs contact layer are successively grown over the entire structure in the second epitaxial growth. Then a conventional ridge waveguide processing is performed. Ti—Au p-contacts and AuGeNi n-contacts are formed on the p-side and the n-side, respectively. 
         [0095]    The two ends of the device are coated with antireflection coating (AR) and antireflection coating (AR) or AR and high reflective coating (HR). The typical threshold current and SMSR of the device are 14 mA and above 40 dB, respectively. 
       4. The Fabrication of the Volume Grating Based on the MS-QPM Technology (FIG. 11) 
       [0096]    The detailed optical system of the fabrication is described as follows: 
         [0097]    The Verdi-5 laser is used as the light source. The narrow light emitted from the laser is expanded by the beam expander system. 
         [0000]    The expanded light is then spitted into two light beams with two orthogonal polarization stations through polarization beam splitter. The reflection light beam perpendicular to the experimental platform is used as the reference light. The transmission light with horizontal polarization is used as the object light. The intensity ratio of the two lights can be adjusted by the half-wave plate before the polarization beam splitter.
 
The horizontal polarization is transformed into vertical polarization when the object light passes though the half-wave plate. After passing through shutter  1  and reflected by a mirror, the object light reaches the crystal. The reference light also reaches on the crystal after it passes though shutter  2  and reflected by the other mirror.
 
When we write the holographic grating, shutters  1  and  2  open simultaneously. The object light and the reference light are interferential in the overlapped area where the volume grating can be formed. The whole system including shutter, power meter are all controlled by the computer.
 
         [0098]    The above procedure is the traditional method used to fabricate the volume grating. We use same procedure to fabricate the seed grating in this invention. 
         [0099]    Because a second exposure is required to fabricate the sampling structure, the detailed procedure is described as follows: 
         [0100]    According to Eq. (3) through (7) and the target grating, the sampling structure is determined and fabricated by the photo-mask. 
         [0101]    The volume grating is exposed again by using the mask with sampling structure to fabricate the sampled grating. Because the index modulation strength of the grating is related to the exposure time and light intensity, a correct exposure time is selected suitably to obtain the correct index modulation. 
         [0102]    The second method to fabricate the volume sampled grating is to use the phase mask with sampling structure, which is the same as fabrication of the waveguide mentioned above. The phase mask with sampling structure is placed before the crystal. The two light beams are interfered and projected on the mask and the crystal. The sampled grating can be fabricated all at once. 
         [0103]    An example to realize the narrow bandwidth optical filter using equivalent it phase shift is given in the following. The included angle between the propagation direction of the incident light and the normal direction of the surface of the volume grating is 5°. The normal direction is defined as the +Z direction. The direction of the wave-vector of the seed grating is parallel to the +Z direction. The record material of the volume grating is the 0.05% Fe:LiNbO3 crystal with the size of 5 mm×5 mm×5 mm. The refractive index n 0  is 2.287. The seed grating period is 500 nm. 
         [0104]    The direction of {right arrow over (G)} sN ({right arrow over (r)}) is the same as {right arrow over (K)} 0 ({right arrow over (r)}). Using Eq. (7), the equivalent π phase shift can be inserted in the sampled grating. The period of the target equivalent grating is 543.88 nm and the corresponding sampling period is 6.197 μm. The light with wavelength of 1550 nm can transmits through the volume grating. All of the other lights can be reflected with the reflection angle of 5°. The index modulation of the seed grating is usually 0.0015. The bandwidth of the reflection light is wider if the index modulation is stronger.