Design of narrowband Bragg spectral filters in subwavelength grating metamaterial waveguides

Jiří Čtyroký; Juan Gonzalo Wangüemert-Pérez; Pavel Kwiecien; Ivan Richter; Ján Litvik; Jens H. Schmid; Íñigo Molina-Fernández; Alejandro Ortega-Moñux; Milan Dado; Pavel Cheben

doi:10.1364/OE.26.000179

1. Introduction

Since the first demonstrations [1, 2], subwavelength grating waveguides (SWG) based on silicon-on-insulator (SOI) platform have become important building blocks in silicon photonics devices. SWG refractive index engineering brings unprecedented flexibility to the design of SOI waveguide components without excessive technological demands. Not only that most of such devices can be fabricated in single lithographic step but clever utilization of specific dispersion properties of SWG waveguides also allows for designing ultra-broadband devices. Among numerous applications of SWG technique, let us mention highly efficient fiber-chip couplers [1–6], broadband directional couplers [7] and multimode interference (MMI) couplers [8], polarization mode splitters [9–11], wavelength-division [2] and mode-division [12] multiplexers, optical delay lines [13], evanescent field sensors [14, 15] and suspended membrane waveguides for mid-infrared applications [16]. Principles of operation and further applications of SWG devices have been recently reviewed in [17, 18].

In this paper, we present the first systematic study of narrowband reflection and transmission spectral filters in SOI SWG waveguides comprising Bragg gratings with lateral loading segments. First reports on Bragg gratings in silicon waveguides were published in early 2000s [19–21]. Since then, several papers were published on Bragg gratings based on longitudinally uniform rib or channel SOI waveguides [22–45], for applications including spectral filtering [23, 25, 33, 35, 46], sensing [24, 40] and optical signal processing [37, 43]. Design of Bragg grating in SWG waveguides was first reported only recently [47]. By adjusting the effective waveguide core index, the mode size and profile can be tailored to optimize overlap with the Bragg grating structure. For example, by geometrically separating the Bragg grating from the waveguide core (so that it acts on the mode evanescent field) while at the same time delocalizing the waveguide mode by the SWG effect, the resulting interaction of the Bragg grating with the mode can be controlled more accurately compared to the conventional Bragg grating geometry. Furthermore, dispersion in SWG waveguides can be engineered [17], offering an additional degree of freedom in controlling the impulse response of the Bragg grating. In principle, Bragg grating in a SWG waveguide comprises two kinds of gratings, both of which are subwavelength, but with significantly different periods. While the SWG forming the waveguide is designed to operate outside its bandgap to ensure lossless propagation, the operation of the Bragg grating relies on its band gap. In [47], this condition was satisfied by choosing the period of the Bragg grating 2 times larger than that of the SWG waveguide. To this end, the authors proposed to change the length of each second Si segment of the original SWG grating. The structure is schematically shown in Fig. 1.

Fig. 1 Bragg grating in a SWG waveguide proposed in [47]. Λ is the period of the SWG input and output waveguides, Λ_B = 2Λ is the period of the Bragg grating. The lengths of even and odd segments, L₁ and L₂, are different.

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In [47], it was claimed that the bandwidth of the Bragg gratings with difference in lengths of about 10% between alternate silicon segments and with about 2000 grating periods is of the order of 1 nm. Our rigorous full-vector 3D simulations of the same structures predicted several times larger bandwidths [48]. A detailed numerical analysis showed that even a small change of the position or width of Si segments (of the order of a few nanometers) induces a comparatively strong perturbation of the periodic structure, resulting in broadening of the Bragg resonance. This observation was also recently confirmed experimentally in our study of the effect of jitter in SWG waveguides [49].

These considerations initiated our systematic analysis of Bragg grating structures in SWG waveguides, as presented in this paper. We first consider several configurations of Bragg gratings, and from different possibilities we select two most promising implementations yielding high spectral selectivity. We also demonstrate that Bragg gratings in SWG waveguides can be described with good accuracy using the coupled mode theory (CMT), and for a particular geometry of the SWG waveguide we determine the necessary CMT parameters from results of rigorous 3D simulations. The CMT is then used for the design of apodized Bragg gratings with suppressed sidelobes. Finally, we propose and analyze narrowband and comb-like transmission filters based on Bragg gratings in SWG waveguides.

2. Design of Bragg gratings in SWG waveguides

Design of a Bragg grating typically starts from its required spectral bandwidth. Since the relative bandwidth of any grating device is ultimately determined by the number of “grooves” irradiated by the incident light, narrowband Bragg filter requires weak grating with large number of periods. The question of the “strength” of the Bragg grating in a SWG waveguide will be discussed in more detail in the next section. Generally, a weak grating implies that the Bloch mode propagating in the SWG waveguide is only weakly affected by the Bragg grating, irrespectively of the groove geometry. Bragg gratings in silicon waveguides are typically formed by surface [21, 26] or sidewall modulation [34, 36, 44–46]. Periodic arrays of cylinders placed aside the waveguide have also been used [33]. The advantage of this technique is that locating these loading elements sufficiently far away from the waveguide core represents only weak modulation and thus enables to obtain reduced bandwidth without the need of shallow corrugation of the waveguide sidewalls. An interesting and simple method of fine tuning the coupling strength of the grating by relative shifting of periodic modulation at the left and right waveguide sidewalls has been presented in [45].

In this paper, we propose a new approach which, building upon these techniques, exploits the unique advantage of dispersion properties of SWG waveguides. For the first time, a Bragg grating comprising SWG metamaterial waveguide core loaded with lateral segments is proposed an analyzed. The structures are designed for SOI waveguides with silicon thickness of 220 nm and 3 micron buried oxide (BOX) layer. For simplicity, we follow the idea of [47, 48] that the Bragg grating period is 2 × the period of the SWG waveguide, albeit other schemes are also possible. This choice ensures that the input and output SWG waveguides operate reasonably far from their bandgap while the Bragg grating exhibits bandgap in the desired wavelength range, while the effective refractive indices of Bloch modes of both sections are above the limit required for negligible leakage to silicon substrate [50].

Based on these considerations, we choose the SWG metamaterial waveguide consisting of 400-nm-wide and 145-nm-long silicon segments with the period $Λ = 242 nm,$ embedded in silica cladding. The structural dimensions are compatible with the 193 nm deep UV fabrication process. The SWG waveguide and calculated distributions of dominant field components of the fundamental Bloch mode at the wavelength of 1550 nm are shown in Fig. 2.

Fig. 2 SWG metamaterial waveguide considered in this paper: (a) top view of the structure, (b) and (c) dominant magnetic and electric field components of the fundamental TE Bloch mode, respectively, (d) detail view of four periods of electric field distribution.

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Eight different configurations of Bragg gratings shown in Fig. 3 were analyzed. In design 1, each second Si segment of the SWG waveguide is shifted by the same amount along the waveguide axis. In designs 2 and 3, the longitudinal and transversal dimensions of each second Si segment are modified, respectively. In designs 4 to 6, small Si blocks–loading segments–are inserted at different locations of the SWG waveguide. Finally, in design 7, larger loading segments are positioned along the SWG waveguide, while in design 8, positions of the segments placed at opposite sides of the waveguide are mutually shifted in the longitudinal direction. Our general design strategy is that by controlling the positions of loading segments within the local field of the Bloch mode, we can optimize the spectral properties of the Bragg gratings.

Fig. 3 Considered designs of Bragg gratings in SWG waveguides. Two periods of Bragg gratings Λ_B are shown. Central rectangles (blue) form the SWG waveguide, loading segments (beige) form the Bragg grating.

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Spectral properties of all these configurations were numerically investigated using our proprietary 3D Fourier modal method (FMM) simulators [51–53]. As it follows from our previous discussion, designs 1-3 lead to rather strong modification of the original SWG waveguide, so that dimensional changes of the order of a few nanometers are required to get narrow (~1 nm) reflection spectrum. The situation is more relaxed for designs 4 to 6 due to smaller sizes of loading silicon segments: for narrowband reflection, the dimensions t and w of these segments are in the range of a few tens of nanometers. From the fabrication point of view, the most promising are designs 7 and 8 since even for narrowband operation, the minimum feature sizes can still be larger than 100 nm, i.e. compatible with 193 nm deep-UV lithography. For this reason, we choose design 7 as our nominal design and design 8 as its modification. In the next sections, spectral properties of these two configurations are analyzed in detail.

3. Theory of Bragg gratings in SWG metamaterial waveguides

Operation of Bragg gratings in optical waveguides can be described using the coupled-mode theory (CMT). Let us remind that properly formulated CMT, which takes into account all waveguide modes (including radiation modes), is rigorous [54, 55]. For weak gratings, it is often possible to use only the fundamental forward and backward propagating modes and neglect all other modes. Complex amplitudes $a (z)$ and $b (z)$ of the fundamental modes satisfy the set of coupled first-order differential equations

\frac{d}{d z} (\begin{matrix} a (z) \\ b (z) \end{matrix}) = - j (\begin{matrix} Δ β & κ \\ - κ & - Δ β \end{matrix}) \cdot (\begin{matrix} a (z) \\ b (z) \end{matrix}) .

Here, the time harmonic dependence of the fields in the complex representation is supposed to be of the form $\exp (j ω t)$ (suppressed in the equations), $Δ β = β_{u} + κ^{'} - K / 2$ is the phase detuning factor, $β_{u}$ is the propagation constant of the unperturbed waveguide without grating (here the SWG metamaterial waveguide), $κ^{'}$ is the self-coupling coefficient representing the change of the propagation constant due to the presence of the grating, $κ$ is the coupling coefficient which determines the strength of the grating, and $K = {2 π / Λ}_{B}$ is the grating constant. The coupling coefficients are usually calculated by overlap integrals of the mode field of the unperturbed waveguide with the perturbation of the permittivity distribution due to the presence of the grating. In the following, without loss of generality, we consider both $κ$ and $κ^{'}$ to be real and positive. As it is known in the coupled-mode theory, the solution of the set of Eqs. (1) with boundary conditions $a (0) \neq 0, b (L) = 0$ , where L is the length of the grating, gives the (power) modal reflectance of the grating in the form

R = {| \frac{b (0)}{a (0)} |}^{2} = \frac{κ^{2} \sin h^{2} (σ L)}{σ^{2} + κ^{2} \sin h^{2} (σ L)},

where

σ = \sqrt{κ^{2} - Δ β^{2}} .

The reflectance reaches its maximum for

Δ β = 0

. In this case,

σ = κ

and Eq. (2) reduces to

R_{\max} = \tan h^{2} (κ L) .

It is also useful to determine the bandwidth between the first nulls, BWFN [56], which is determined according to (2) by condition $σ L = \pm i π$ . For small bandwidths it can be approximately expressed as

B W F N = \frac{λ_{\max}^{2}}{N_{g} L} \sqrt{1 + {(\frac{κ L}{π})}^{2}},

where

N_{g}

is the group index of the fundamental mode of the unperturbed waveguide at the central wavelength

λ_{\max} .

Note that for long gratings,

B W F N = λ_{\max}^{2} κ / (N_{g} π) .

This bandwidth exactly corresponds to the wavelength interval in which

σ

is real, i.e., to the stopband of the Bragg grating.

Although the CMT was originally derived for gratings on conventional (longitudinally homogeneous) waveguides, it can be equally well applied to gratings in SWG waveguides, providing we substitute the forward and backward propagating modes of the conventional waveguide with the Bloch modes of the SWG waveguide. Since the transverse mode field distribution of Bloch modes varies periodically along the SWG waveguide, the determination of the coupling constants from overlap integrals would be quite complex. Here we propose and demonstrate that this problem can be circumvented by determining the coupling constant directly from reflectance spectra of the gratings calculated by another, rigorous numerical method. Once the coupling constants are determined, the design of the Bragg grating can be optimized for required specifications using the simpler and more flexible CMT. This approach has been carefully tested on a 2D (planar) model of the SWG waveguide with Bragg gratings similar to our nominal design using the proprietary software FEXEN based on the Fourier modal method (FMM) [57]. The coupling coefficient was determined from the reflectance maxima using Eq. (3) for different lengths (or number of periods) of the grating. Exponential dependence of the coupling coefficient on the separation $s$ of loading segments from the waveguide was found, as expected from basic physical considerations (evanescent field decay).

Encouraged with results of the 2D analysis, we then performed similar but computationally more intensive simulations using our 3D FMM tools. We choose the square loading segments of the size of 130 × 130 nm², for compatibility with 193 nm DUV lithography. For our nominal design we calculated reflectance spectra of gratings for different separations of loading segments from the SWG core, specifically s = 150 nm, 250 nm, 350 nm, 450 nm, 750 nm and 1000 nm. For each value of separation s, the reflectance spectra were calculated for several gratings with different number of Bragg periods, i.e., of different lengths. Correspondingly, the gratings length varied approximately from 8 µm to 2000 µm. From the calculated results, and making use of Eq. (3), the coupling coefficient associated to each separation s was determined. For the CMT to be applicable also for gratings in SWG waveguides (which should not be taken as granted because of a high refractive index contrast of the SWG waveguide and a possible role of evanescent modes not taken into account in the CMT), the reflectance values for the same separation $s$ but different grating lengths must satisfy Eq. (3). As it is evident from a typical example in Fig. 4, this condition is clearly satisfied. For each s, the coupling coefficient is then chosen from the best fit of the corresponding $\tanh^{2} (κ L)$ dependence.

Fig. 4 (a) Spectral dependence of Bragg reflectance for various numbers of periods. Nominal design, separation $s = 250 nm;$ (b) Reflectance maxima for various numbers of periods (i.e., grating lengths); red line – the best fit to Eq. (3) with $κ = 0.0205 {µm}^{- 1} .$

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Our simulations showed that the wavelengths of maximum reflectance depend rather significantly on the separation $s .$ This spectral shift, shown in Fig. 5(a), is a consequence of the self-coupling described by the coefficient $κ^{'}$ in the CMT Eqs. (1). Its value can be obtained from the spectral position of maximum reflectance,

κ^{'} = π (1 / Λ_{B} - 2 N_{e f f} / λ_{\max}),

where

N_{e f f}

is the effective index of the Bloch mode of the SWG waveguide at the wavelength

λ_{\max} .

The dependences of both coupling coefficients

κ

and

κ^{'}

on the separation s are shown in Fig. 5(b). It is observed that, for both coupling coefficients, the dependence on the separation s can be fitted with good accuracy (except for separations

s

smaller than ~200 nm) with exponential functions with decay factors:

κ = 0.1171 \exp (- 6.94 s), κ^{'} = 0.2155 \exp (- 6.95 s),

where separation

s

is given in micrometers and the coupling constants in µm^–1. (Note that unlike in the 2D case, the evanescent decay of a guided mode of a channel waveguide is exponential only at larger distances; for example, in an optical fiber the decay follows the modified Bessel function K which asymptotically approaches exponential function for large arguments).

Fig. 5 (a) Dependence of the wavelength of maximum reflectance and (b) of the coupling coefficients $κ$ and $κ^{'}$ (dots) on the separation s of loading segments from the edge of SWG waveguide (nominal design), with the corresponding exponential fits (dashed lines).

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We also examined the dependence of the coupling constant of the Bragg grating on the relative shift $Δ z$ of the loading segments in the modified nominal design 8 in Fig. 3. In [45] it has been shown that for longitudinally uniform waveguides the coupling constant decays with increasing $Δ z$ according to the cosine law,

κ (Δ z) = κ (0) \cos (π Δ z / Λ_{B}) .

Results obtained from 3D simulations of the spectral reflectance of such Bragg gratings are shown in Fig. 6(a).

Fig. 6 (a) Dependence of the coupling coefficient $κ$ of the Bragg grating on the longitudinal shift $Δ z$ of loading segments in the modified nominal design for three values of separation s: 150, 250 and 350 nm; (b) dependence of the maximum reflectance wavelength on $Δ z$ .

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It is apparent that the coupling constant of the SWG Bragg grating with smaller separations s decays slightly faster with the shift $Δ z$ than according to the cosine function, and approaches the cosine function for larger separations $s .$ In Fig. 6(b) it is observed that for separations larger than ~200 nm, the wavelength of maximum reflectance is independent of the shift $Δ z .$ This is because, for larger separations, the self-coupling coefficient $κ^{'}$ depends only on the separation $s$ and not on $Δ z .$ This behavior is a consequence of the transversal field distribution of the Bloch mode of the SWG waveguide shown in Fig. 2: at larger separations, the longitudinal variations of the Bloch mode field are comparatively weak. From Fig. 5 and Fig. 6 it is apparent that by judicious choice of the lateral and longitudinal positions of loading segments the coupling coefficients can be controlled in a wide range (several orders of magnitude). This needs to be carefully considered in the design of apodized Bragg gratings, as will be shown in Section 4.

Rather weak longitudinal variations of the Bloch mode field at larger separations invokes the idea that both coupling constants $κ$ and $κ^{'}$ might be rather insensitive to simultaneous longitudinal shifting of loading segments at both sides along the waveguide in the same direction. Our numerical simulations (not explicitly shown here for brevity) clearly confirmed this expectation. This is an important observation since it opens the possibility of chirping the Bragg grating on the SWG waveguide without changing its strength.

Figure 7(a) shows the reflectance bandwidth of the nominal Bragg grating as the function of the separation s and the grating length, determined as follows: We first calculated the wavelength dependence of the effective index $N_{e f f}$ of the Bloch mode of a SWG waveguide using the Fourier modal method and calculated the group effective index $N_{g} .$ Then, for each value of separation s, the central wavelength $λ_{\max}$ was determined from the curve shown in Fig. 5(a). Finally, the BWFN bandwidth was calculated from Eq. (4) in which the value of the coupling constant $κ$ was determined from its exponential dependence on s, see Fig. 5(b).

Fig. 7 (a) BWFN dependence on the length of the Bragg grating on SWG waveguide (nominal design) calculated for various separations s of loading segments; the inset shows the same dependence for larger grating lengths; (b) spectral reflectance of the Bragg grating with separation s = 1000 nm and 65536 Bragg periods.

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It is apparent that nominal SWG Bragg gratings longer than ~1 mm, created by loading segments separated by more than 500 nm from the SWG waveguide, yield sub-nanometer reflection bands. This is remarkable given all structural dimensions of the gratings are larger than 100 nm. This case is illustrated in Fig. 7(b), showing the reflectance spectrum of a very weakly modulated $(s = 1000 nm)$ grating with 65536 Bragg periods $(L = 31.8 mm),$ calculated with the 3D FMM. Note that this value of separation s is just at the edge of simulation window shown in Fig. 2(c), where the mode field of the SWG waveguide is very weak. The resulting BWFN bandwidth is ~50 pm, which agrees well with that calculated from the CMT theory, see inset in Fig. 6. Similar bandwidth can also be obtained using modified grating with shifted loading segments. However, for the separation of $s = 450 nm,$ the required shift is $Δ z = 238 nm,$ which is only by 4 nm smaller than $Λ_{B} / 2 .$

4. Apodization of SWG Bragg filters

The CMT approach is especially efficient for analysis and design of apodized Bragg filters. Already in the early paper [58] it was demonstrated that a quadratic apodization of the coupling coefficient results in a strong suppression of sidelobes. As an example we consider here a Bragg grating with a linear profile of the longitudinal shift $Δ z$ (modified nominal design). According to Fig. 6, for the separations $s$ of loading segments larger than 350 nm, the coupling coefficient closely follows the cosine dependence on $Δ z .$ To be on the safe side, we choose the separation $s = 450 nm .$ To get the maximum reflectance close to 100%, the grating was designed with 2500 periods (grating length 1210 µm). Longitudinal apodization profile of the shift $Δ z$ and the corresponding profiles of the coupling coefficients $κ$ and $κ^{'}$ are shown in Fig. 8(a) and Fig. 8(b), respectively. The reflectance spectrum of the apodized grating was calculated by numerical integration of the Runge-Kutta equation [58], and it is shown in Fig. 8(c). Non-apodized (uniform) grating spectrum is also shown for the reference $(Δ z = 0) .$ The separation $s$ of the uniform grating was adjusted so that its coupling coefficient equals the average value of the coupling coefficient of the apodized grating; this is obtained for $s = 514 nm .$ As expected, the cosine (i.e., close to parabolic) apodization of the coupling coefficient results in strong suppression of sidelobes of the reflectance spectrum, as shown in Fig. 8(c). Spectral shift between the reflectance spectra of gratings with and without apodization arises as a consequence of the difference in the self-coupling coefficients $κ^{'}$ for separations $s = 450 nm$ and $514 nm .$

Fig. 8 Performance of the Bragg grating with linear apodization obtained by varying the longitudinal shift $Δ z$ with $s = 450 nm .$ (a) Longitudinal profile of the shift $Δ z$ ; (b) longitudinal dependence of coupling coefficients $κ$ (blue line) and $κ^{'}$ (red line); (c) spectral dependence of reflectance of the apodized Bragg grating (solid red line) and of the uniform (non-apodized) Bragg grating with $s = 514 nm$ (dashed blue line).

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Essentially the same apodization profile of the coupling coefficient $κ$ as in Fig. 8(b) can be obtained by longitudinal variation of the separation $s$ according to

s (z) \approx - \ln [(κ_{0} / 0.1171) \cos (π Δ z (z) / Λ_{B})] / 6.94,

where

κ_{0}

is the coupling coefficient for

s = 450 nm

and

Δ z = 0,

Δ z (z)

is the

Δ z

profile shown in Fig. 8(a). The resulting

s (z)

profile is plotted in Fig. 9(a).

Fig. 9 Calculated performance of the apodized SWG Bragg filters. (a) longitudinal profile of s for the s-apodized grating; (b) longitudinal dependence of coupling coefficients $κ$ (blue line) and $κ^{'}$ (red line); (c) spectral dependence of reflectance of the Bragg gratings apodized by varying s (dashed blue line) and by varying $Δ z$ (solid red line).

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For this type of apodization, the variation of $s$ results in corresponding variation of the self-coupling coefficient $κ^{'}$ , as shown in Fig. 9(b). The calculated reflectance spectrum is shown in Fig. 9(c), together with the spectrum of a grating apodized by varying $Δ z$ . While the sidelobes of the reflectance spectrum at longer wavelengths are strongly suppressed for the s-apodized grating, the sidelobes at the short-wavelength side are actually increased. This is a consequence of the apodization-induced variations of the self-coupling coefficient which effectively manifests itself as a grating chirp [59–61].

5. Narrow-band transmission SWG Bragg filters

Bragg gratings act essentially as narrow-band reflection filters. However, narrow-band transmission filters can also be implemented with Bragg gratings as Fabry-Perot (FP) filters schematically shown in Fig. 10.

Fig. 10 Schematic configuration of a FP resonator with DBR mirrors in a SWG waveguide.

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From a large number of possible implementations we select here one typical example. In Fig. 11(a) we show transmittance and reflectance spectra of the distributed Bragg reflector (DBR) formed by the SWG Bragg grating with separation $s = 250 nm$ and 256 Bragg periods. In Fig. 11(b) it is shown the transmittance spectrum of a FP resonator created by two DBRs separated by 16 periods of a SWG waveguide. The calculated spectral width of the FP resonance is about 90 pm, which corresponds to the resonator Q-factor of 17500.

Fig. 11 (a) Reflectance and transmittance of SWG DBR; (b) transmittance of the FP resonator with DBR mirrors and SWG waveguide.

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Very high refractive index contrast of SOI waveguides can also be advantageously used to make broadband Bragg mirrors with only a few grating periods. Such mirrors can be readily implemented in SWG waveguides even without silicon loading segments, e.g., by doubling the SWG period and the length of the SWG waveguide core segments. An example of a FP resonator with such mirrors is schematically shown in Fig. 12.

Fig. 12 Schematic of a FP resonator with broadband DBR mirrors in a SWG waveguide.

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The reflectance spectra of Bragg mirrors created by doubling the period of the SWG waveguide are shown in Fig. 13(a) for various numbers of periods. The calculated reflectance spectrum of the semi-infinite grating is also shown for reference.

Fig. 13 (a) Reflectance spectra of broadband DBR mirrors with different number of periods, ∞ sign denotes reflectance from the semi-infinite grating; (b) transmittance spectrum of the FP resonator with 7 Bragg mirror periods and 2048 SWG waveguide periods.

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The reflectance bandwidth exceeds 120 nm. A part of the transmittance spectrum of a FP resonator with 2048 SWG periods (i.e., about 0.5 millimeter long) is shown in Fig. 13(b). The structure behaves as a comb filter with free spectral range ~1 nm. The apparent unevenness of the transmittance maxima is a consequence of a limited computational wavelength grid.

6. Influence of jitter

In our recent study [49] we theoretically and experimentally analyzed the influence of disorder (“jitter”) in the size and position of silicon segments due to fabrication imperfections on the functionality of SWG metamaterial waveguide devices. We have found that random variations in positions and dimensions of silicon segments larger than several nanometers can substantially deteriorate device performance. Since Bragg gratings rely on interference effect, they may be particularly sensitive to such defects. In order to get some insight into this problem we simulated the effect of random fluctuations of lengths of silicon segments in the nominal design of the narrowband Bragg grating with 2048 Bragg periods, with loading segments separated by $s = 450 nm$ , see Fig. 14(a). We also studied an example of a FP resonator with DBR mirrors shown on Fig. 14(b).

Fig. 14 (a) Effect of jitter in silicon segment length on the reflectance of the narrowband SWG Bragg grating; (b) jitter influence on the transmittance of the FP resonator with DBR mirrors. Solid blue lines–ideal devices, dashed red lines–devices with jitter. $σ_{L} = 4 nm$ is the standard deviation of segment length fluctuations.

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The lengths of all silicon segments (both of the SWG waveguide core and of the loading segments) were jittered using pseudorandom numbers with normal distribution and standard deviation $σ = 4 nm .$ Simulation results are shown in Fig. 14. Interestingly, it is observed that effect of jitter manifests itself mainly outside the stopbands of the Bragg gratings. As a result, although the characteristics of both devices are affected, their basic functionality–the existence of reflection and transmission peaks–remains preserved. This finding is encouraging for practical implementations of these devices.

7. Summary

We reported results of numerical simulations of reflection and transmission spectral filters based on Bragg gratings in subwavelength grating metamaterial waveguides. We demonstrated that filters with spectral bandwidths as small as a few tens of picometers can be implemented in silicon waveguides while keeping minimum structural dimensions compatible with deep-UV lithography (>100 nm). This is achieved by using a fundamental Bloch mode in the SWG metamaterial waveguide with delocalized electric field, weakly coupled with the lateral loading segments forming the Bragg grating. The Bloch field delocalization allows to substantially increase the distance between the loading segments and the SWG core, therefore relaxing requirement on the positioning accuracy. Furthermore, by judiciously shifting the positions of loading segments at both sides of the waveguide, the grating strength as well as its chirp is controlled. This allows to engineer the filter bandwidth and to shape its spectral and impulse response. We also showed that high refractive index contrast of silicon waveguides can be exploited to design comb-like transmission filters simply by doubling the period of the SWG waveguide, to form the Fabry-Perot resonator. An additional practical advantage of our design strategy is that only a single lithographic step and full etch of silicon waveguide layer is required. We have also shown, for the first time, that the efficient and widely used CMT formalism can be advantageously applied to Bragg gratings in SWG metamaterial waveguides, providing the coupling coefficients are determined by rigorous 3D tools such as FMM. Our results open exciting prospects for development of new types of spectral filters in silicon nanophotonic waveguides building upon the fundamental principle of metamaterial waveguide engineering.

Funding

Czech Science Foundation (GACR) (16-00329S); Ministerio de Economía y Competitividad, Programa Estatal de Investigación, Desarrollo e Innovación Orientada a los Retos de la Sociedad (cofinanciado FEDER), (TEC2016-80718-R); Universidad de Málaga.

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Design of narrowband Bragg spectral filters in subwavelength grating metamaterial waveguides

Abstract

1. Introduction

2. Design of Bragg gratings in SWG waveguides

3. Theory of Bragg gratings in SWG metamaterial waveguides

4. Apodization of SWG Bragg filters

5. Narrow-band transmission SWG Bragg filters

6. Influence of jitter

7. Summary

Funding

References and links

Cited By

Figures (14)

Equations (8)

Optics Express