In this paper we present an efficient method for designing discrete, nearly-uniform Bragg gratings in generic planar waveguides. Various schemes have already been proposed to design continuous Bragg gratings in optical fibers, but a general scheme for creating their discrete counterpart is still lacking. Taking a continuous Bragg grating as our starting point, we show that the same grating functionalities can also be realized in any planar waveguide by discretizing it into a series of air holes. The relationship between the two gratings is established in terms of grating strength and local grating period.
© 2005 Optical Society of America
Optical filter design based on nearly-uniform Fiber Bragg gratings (FBGs) have recently attract much attention as it provides convenient, cost-effective, and reliable solutions to some of the important problems in the high bit-rate wavelength-division multiplexed systems –. All the schemes proposed so far, however, involve continuous 1-D gratings in optical fibers. It would be interesting to see how we can carry over the solution into waveguides of other geometries, e.g., is it possible to realize the same grating functionalities in a planar waveguide? In particular, given the recent progress in light confinement in 2-D photonic crystal slab waveguides –, we would like to take the next logic step and explore the possibility of etching a discrete, nearly-periodic grating in such a structure.
As a historic note, Laso et al. first proposed a simple discretization procedure in the microwave regime, in a similar effort to set up an equivalence relation between an FBG and a discrete air-hole grating in a microstrip waveguide . This scheme, while simple to implement, works only for purely periodic structures and hence is limited in its applications. In the following we propose a discretization procedure that is much broader in scope and applies to both periodic and nearly-periodic gratings.
An 1-D FBG is characterized by the evolution of its effective refractive index n(z) along its length:
where n 0 is the average effective refractive index and K 0=(=2π/Λ), Δn(z), and θ(z) specify the grating parameters: Λ is the reference period, Δn(z) accounts for the local grating strength (apodization), and θ(z) determines its phase variation and local period.
Various schemes have been proposed to design continuous FBG’s that can accommodate almost any filter responses. But to the best knowledge of the authors a general scheme for designing discrete gratings is still lacking. In this paper we show that the same grating functionalities can also be realized in any planar waveguide by discretizing the continuous fiber grating into a series of air holes, and the connection between the two is established in terms of grating strength and local grating period.
3. Grating design
We draw our intuition from the discretization procedure outlined in a scheme called “layer peeling algorithm” , which, among others, provides a generic formalism for designing continuous, nearly-uniform gratings. Suppose we first design a continuous grating in the form of Eq. (1) (using the “layer peeling algorithm”, say) and then try to create its discrete counterpart by transforming it into a series of discrete air holes, distributed nearly uniformly along the center of a planar waveguide:
The refractive index profile should then be rewritten as:
ñ 0=the effective refractive index of the unperturbed waveguide,
am=(area of the mth air hole)×(1-nsubstrate),
bm=deviation of the mth air hole from z=mΛ,
(“t” is the thickness of the planar waveguide)
i.e., W(y) is a windowing function that is nonzero only within the thickness of the waveguide. Also embedded in Eqs. (3-a) and (3-b) is the implicit assumption that the areas of the air holes, hence the am’s, are small, for otherwise we would not have been able to approximate the index perturbation by δ functions. Additionally the deviation of the air holes from their respective reference locations, i.e., the bm’s, are also taken to be small since the grating is assumed to be “nearly uniform”.
Comparing the functional forms of n(z) in (1) and ñ(x,y,z) in Eq. (3) we arrive at the following intuition:
where θ′(z) is the first order derivative of θ(z).
To show that n(z) and ñ(x,y,z) give the same filter response we go back to the original wave equation. Assuming linearly polarized electric field, say E⃗=Ex, the perturbed wave equation can be written :
is the electric polarization due to the perturbation. Since Ppert(r⃗,t) is the only term in the wave equation that provides coupling between the forward and backward propagating waves, n(z) and ñ(x,y,z) are equivalent as long as they give equivalent perturbative coupling term Ppert(r,t).
For the 1-D continuous grating, assuming Δn(z)≪n 0, Ppert(r⃗,t) is given by (1):
For sufficiently slowly-varying Δn(z) and θ(z), Δ(n 2) consists of two distinct Fourier components centered respectively at ±K 0 (see Fig. 2). Without loss of generality we filter out the positive frequency component and apply the sampling theorem on its slowly-varying envelope (including the phase factor eiθ(z). The end result is an alternative but equivalent expression for the perturbative component at +K 0 :
where Λ is the sampling period and Γ is one half of the bandwidth of (see Fig. 2).
Ignoring δ(x) and W(y) for now and once again assuming am ’s and bm ’s are sufficiently slowly varying, we plot the Fourier spectrum of Δ(ñ 2) in Fig. 3. Note only the two components located at K 0 and -K 0 can provide coupling between the forward and backward propagating waves and hence are relevant in our analysis.
Once again we filter out the component at +K 0 with a pass-band bandwidth of 2Γ :
Equating with and assuming bm ’s are small, we obtain the following relations:
If we define the local period at the mth air hole as the average of Λ1 and Λ2, and recall K 0=2π/Λ, we get:
where we have substituted in (11-b) for bm+1 and b m-1 and made the slowly-varying assumption on θ(z). (12) indeed confirms our second intuition in Eq. (4-b).
One last thing that needs to be addressed is the δ(x) and W(y) functions that we have deliberately left out of Δ(ñ 2) in our derivation so far. We show in the following that the inclusion of the two functions requires the introduction of an extra scaling factor γ into Δñ(x, y, z), i.e.,
where γ is presumably highly dependent on the geometry of the waveguide.
We compute γ by matching coupled mode equations. In the Appendix we derive the coupled mode equations for wave propagation in FBG’s (i.e., the continuous grating), and so long as the goal is to generate the same filter response via a discrete grating in a planar waveguide, we need to reproduce the same set of coupled mode equations for the new waveguide as well. Following through the derivation in the Appendix, we realize that we need to make the following substitution for the discrete grating:
Since we are considering a TE wave (i.e., E⃗=Ex)), the orthonormality relation is given by :
where the integral is taken over the entire transverse plane and km is the propagation constant of the mth mode.
As in the Appendix we replace the ∂2/∂t2 operator in Eq. (15) by -ω 2, multiply both sides by E*B(x,y)=E*F(x,y), integrate over the entire transverse plane, and then apply the orthonormality relation (16). Collecting resonant terms in the resultant equation such as the ones that vary as and comparing them with Eq. (A-12), which is the target mode equation we are hoping to reproduce, we realize the following equality needs to be satisfied:
where t is the thickness of the planar waveguide and Em=EB=EF. Or in other words,
where we have approximated ω by ω 0, the center frequency, and km by K 0/2. c is the speed of light in vacuum and ñ is the effective modal index given by Eq. (3-a).
We presented in this paper an efficient scheme for designing discrete and nearly-uniform Bragg gratings in planar waveguides, which in principle are capable of performing any functionalities that can be achieved by continuous FBG’s. One limitation of our design, which we did not explore in this paper, is the potentially significant scattering losses that could be associated with the air holes. More theoretical and experimental work is needed on this subject.
In this appendix we derive the coupled mode equations for wave propagation in FBG’s. The coupled mode equations provide a mathematical description for the coupling between two counter-propagating waves in the fiber: bB(z,β) (backward propagating) and bF(z,β) (forward propagating) 
where β is the detuning parameter
and k is the wavevector in vacuum (=ω/c). q(z) (and its complex conjugate q*(z)) gives the coupling strength and is related to the grating parameters as:
where Δn(z) and θ(z) have been defined in (1).
To simplify the derivation we first assume the electric field is linearly polarized, say E⃗=Ex, and then expand the total field as:
Plugging the above expression into the perturbed wave equation
where prime denotes derivative with respect to z. Note in the derivation above we have also made the following slowly-varying assumptions on wave amplitudes bB and bF :
So all terms containing bB″ and b″F have been omitted from (A-7).
From the unperturbed wave equation we know
where k=ω/c and n 0 is the effective modal refractive index of the unperturbed waveguide.
We then expand (kn 0)2 as
since β≪K 0. Plugging (A-9) and (A-10) into (A-7), we find the first term in the square brackets becomes
Similarly the second term in the square brackets becomes
Putting them together, we get
For a lossless fiber we have EB(x,y)=EF(x,y). So we can cross them out on both sides of (A-11). Also if we replace the ∂2/∂t 2 operator by -ω 2 and collect the resonant terms, say the ones that vary as
, we get the following equation:
But we also know
Plugging into (A-12), we have
where q(z) has been defined in (A-4). Thus (A-14) gives back the original mode equation (A-1). In a similar fashion, by collecting resonant terms with
dependence we could also derive (A-2) from (A-11).
The authors acknowledge with thanks the support of this work by the Office of Naval Research (Y. Park), the Air Force Office of Scientific Research (H. Schlossberg), and by DARPA (R. Athale). George Ouyang would also like to thank Dr. Yanbei Chen and Dr. Song Wang for sound suggestions and fruitful discussions.
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