Introduction

The rapid growth of Si-photonic industry over the past two decades, driven by the huge investments in the CMOS electronics industry, has made this new technology a potential cost effective solution for many new applications such as LIDAR systems1, bio-sensing2,3, hybrid photonic RF-ICs4, high speed and microwave signal processing5,6, etc. Silicon on Insulator (SOI) is a widespread silicon photonics technology which supports submicron optical waveguides due to its high refractive index contrast. This platform supports many other optical components and devices such as Y-junctions, couplers, interferometers, gratings and resonators7,8,9,10,11 which form the building blocks of many photonic circuits and systems12,13.

One of the most widely used optical devices is the Mach–Zehnder Interferometer (MZI). The simplest MZI configuration consists of two Y-junctions, one acts as a beam splitter and the other as a beam combiner and two waveguide arms. MZI is used in wide range of applications from wavelength division multiplexers14, optical switches15,16 and electro-optical modulators17,18,19 to biosensors20,21. One of the advantages of MZI is its high immunity to temperature fluctuations when compared to resonators22. However, they suffer from large footprint. This large footprint become critical when many MZIs are required on the same chip and when very large length is necessary for achieving certain performance.

In this paper, we propose loop-terminated asymmetric MZI (LT-aMZI) design based on the widespread SOI technology. The LT-MZI is simply constructed from the conventional MZI with directional coupler (DC) splitter and combiner and a loop reflector23,24. Our structure is the integrated version of the fiber reflection MZI (FRMZI) proposed by CA Millar et al.25. This configuration allows the propagating light to travel back and forth in the interferometer arms rather than just once; resulting in smaller footprint and boost the performance of the conventional MZI in different applications. This paper is an exhaustive extension to our previously published work26.

By integrating the structure presented in25, several important advantages are added. Mass production using widespread CMOS process will result in significant reduction in the cost of such devices. In addition, easily on-chip integration of a whole system with source, detector and electronic parts allowing for compact handheld devices. Hence, integrated high performance optical sensors, electro-optical modulators and even gyroscopes using the loop reflector23,24 can be implemented using our structure. Similar structures to our proposed one were published previously. In27,28, the same LT-MZI structure was presented but in the symmetric configuration and was used as a mirror with controllable reflectivity which then used by27 to construct a Fabry–Perot cavity. While in29 Michelson interferometer was proposed employing two loop mirrors compared to just one used in LT-MZI, i.e. more compact. Also, in30 authors use asymmetric LT-MZI together with ring resonator to construct a Vernier sensor. Their work is based on numerical design and optimization using the transfer matrix method. Here we have derived a simple and compact analytical model that accurately describes the LT-aMZI structure rather than the complex transfer matrix method used in30. Using our structure, we have designed optical filters and compared these designs with the common MZI showing our designs to be more compact. Simulation of the optical filter designs have been performed using Lumerical Interconnect31 which is fast circuit simulation software. These LT-aMZI designs have also been fabricated and characterized for the first time, with measurements showing good matching with Interconnect simulations as well as the analytical model. In addition, unlike conventional MZIs and Michelson interferometer in29 the transmission spectrum of our proposed LT-MZI can be engineered using the DCs coupling coefficients and not just by the interferometer arms phase difference, which can be used as spectral tunable filter. Finally, using our compact model we show that our proposed LT-MZI structure enhance significantly the performance of optical sensors and electro-optical modulators that were using the widespread conventional MZI. Analysis also shows that the asymmetric MZI (aMZI) configuration offers an advantage over the symmetric MZI (sMZI) in sensing as both sensitivity and FOM can be engineered independently in aMZI, which is not applicable in the case of sMZI and also ring resonators.

Results

Structure and modeling

Our proposed loop terminated asymmetric Mach–Zehnder Interferometer (LT-aMZI) is shown in Fig. 1a, which is constructed from input directional coupler (DC) that split the input wave to the interferometer arms. The two interferometer arms experience different phase shift φ1 and φ2 and then another DC is used to combine the two arms waves. The outputs of the second DC are connected through a bend waveguide utilizing loop reflector as the one in23,24. This loop reflect the waves back to the interferometer and the output is taken from the first DC as shown in Fig. 1a. Thus, our structure is constructed of a conventional MZI (C-MZI) using DCs, denoted as C-MZIDC, and loop reflector. Hence, the transmission of our proposed LT-MZI can be derived using the C-MZIDC forward tfrw and cross tcr transmission coefficients shown in Fig. 1b.

Figure 1
figure 1

(a) Schematic of our proposed LT-aMZI, (b) schematic of C-aMZIDC c output transmission versus interferometer arms phase difference of both LT-MZI and C-MZI with 3-dB directional couplers.

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The intensity transmission of our LT-MZI can be expressed as:

$$ T_{LT {\text{-}} MZI} = \left| {t_{LT {\text{-}} MZI} } \right|^{2} = A\left[ {\frac{H}{4} + \cos \left( {\Delta \phi } \right)} \right]^{2} $$
(1)
$$ \text{where} \quad A = 16a_{1}^{2} a_{2}^{2} b_{1}^{2} b_{2}^{2} $$
(2a)
$$ \text{and} \quad H = - \frac{{a_{1} a_{2} }}{{b_{1} b_{2} }} - \frac{{b_{1} b_{2} }}{{a_{1} a_{2} }} + \frac{{a_{1} b_{2} }}{{a_{2} b_{1} }} + \frac{{a_{2} b_{1} }}{{a_{1} b_{2} }} $$
(2b)

where ɑ1, b1 are the forward and cross coupling coefficients of the first DC and ɑ2, b2 are the forward and cross coupling coefficients of the second DC. Also, φ1 and φ2 are the phase shifts of the upper and lower arms of the interferometer, respectively. We here focus on asymmetric (or unbalanced) MZI, i.e. arms with different lengths L1 and L2 and with identical waveguides at both arms, i.e. β1 = β2 = β. Hence, φ1 = βL1, φ2 = βL2 and Δφ = φ2 − φ1 = βΔL with ΔL = L2 − L1 is the interferometer path length difference.

From Eq. (1) we can simply derive the main parameters of the LT-aMZI which are the resonance wavelength (λres), free spectral range (FSR) and full width half maximum (FWHM) as:

$$ \lambda_{res} = \frac{{2n_{eff} \Delta L}}{q} $$
(3a)
$$ FSR_{LT {\text{-}} aMZI} = \frac{{\lambda^{2} }}{{2n_{g} \Delta L}} $$
(3b)

For \(a_{1} = a_{2} = b_{1} = b_{2} = \frac{1}{\sqrt 2 }\), H = 0:

$$ FWHM_{LT {\text{-}} aMZI} \approx \frac{{FSR_{LT {\text{-}} aMZI} }}{\pi } $$
(3c)

with q an integer number, neff and ng are the waveguide mode effective index and group index, respectively. Note that, FWHM is the spectral width Δλ with transmission greater than or equal to Tmax/2, which can be derived from Eq. (1) using \(T(\lambda_{res} + \frac{\Delta \lambda }{2}) = \frac{{T_{\max } }}{2}\).

Figure 1c shows the change in the output transmission due to change in the phase difference of both C-MZI and LT-MZI with ɑ1 = b1 = ɑ2 = b2 = 1/\(\surd 2\), i.e. 3-dB directional couplers. It can be seen that, LT-MZI have higher rate of change compared to C-MZI, specifically two times higher. Due to this characteristic LT-aMZI can achieve the same FSR of the C-aMZI using half its path length difference (ΔL). Also, the FWHM of the LT-aMZI is half that of C-aMZI with the same path length difference, because the FSR of the LT-aMZI is half that of C-aMZI. Reducing the FWHM of the interferometer is useful in many applications such as sensors and modulators.

Designs and simulations

We use the quasi-TE mode of the standard SOI strip waveguide with width w = 500 nm and thickness h = 220 nm and silicon dioxide cladding to construct our interferometer designs. We have performed modal analysis for this waveguide’s TE mode using finite difference eigenmode solver32 to get its effective and group index over the wavelength, see Fig. 2a. This waveguide width, w = 500 nm, is used as it exhibits the least bend and roughness loss while still supporting a single mode.

Figure 2
figure 2

(a) Effective index and group index versus wavelength for the fundamental TE like mode of the (500nmx220nm) strip waveguide obtained from mode solver and (b) output transmission spectra versus wavelength of both LT-aMZI and C-aMZIY for path length differences ΔL = 44.7 µm.

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Using our proposed LT-aMZI, we demonstrate four different designs with different path length differences (ΔL) shown in Table 1. These devices are suitable for optical filtering as well as wavelength de-multiplexing (WDM) applications. We compare these designs with C-aMZI of the same path length difference ΔL. When simulated the C-aMZI y-junctions rather than DCs were used to split and combine the power to and from the interferometer arms, denoted C-aMZIY. This C-aMZIY is used when comparing with our LT-aMZI as it is more practical due to the even splitting of the y-junction over broadband. Lumerical Interconnect software tool was mainly used in devices simulations as it is more computationally efficient when compared to 3D FDTD simulations. Interconnect uses scattering matrices of optical components to determine the transfer function of photonic integrated circuits (PICs). In addition, some results have been verified with analytical modeling Eq. (1) showing a very good matching.

Table 1 LT-aMZI designs dimensions and FSR.
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Figure 2b shows the Interconnect simulation results of our proposed LT-aMZI and the corresponding C-aMZIY simulation with ΔL = 44.7 µm. From this result we can see that, as mentioned before, for the same interferometer path length difference ΔL the FSR of the LT-aMZI is half that of the C-aMZI. The DC and y-junction used in the simulations are optimized designs from Lumerical E-Beam Compact Model Library (CML)33 that result in even power splitting at both outputs over 100 nm bandwidth around 1.55 µm wavelength.

Measurements and characterization

The dimensions of the fabricated LT-aMZIs designs are included in Table 1. The designs were fabricated at the UW NNCI Washington Nanofabrication Facility using Electron Beam lithography34 on standard SOI wafers with silicon device layer of 220 nm. We fabricated three copies for each design to account for the fabrication tolerance. Figure 3a,b show SEM photos of the fabricated LT-aMZIs. Note that, the SEM photos are taken before depositing the SiO2 clad. As can be seen, bend waveguide loop is crucial in determining the device footprint thus, LT-MZI structure is favorable over the Michelson interferometer where two loop mirrors are needed and hence will increase the footprint significantly. A tunable laser is used to span the wavelength region from 1500 to 1600 nm. A polarization maintaining fiber array containing 4 fibers separated by 127 µm is used to couple light in and out of the chip through the grating couplers while a detector is used to measure the output power from the fibers. The GC used is also from the library in33. A typical SOI MZI have around 0.5 dB losses (with arms’ length in micrometer dimensions)35,36 and we can expect roughly that our device losses will be twice that value.

Figure 3
figure 3

SEM photos of the fabricated LT-aMZIs: (a) with ΔL = 44.7 µm, (b) with ΔL = 89.4 µm.

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The measurement results of the fabricated designs are compared with different modelling methods to ensure the feasibility of the proposed LT-aMZI structure. Figure 4a shows the device output power spectrum of the fabricated and characterized LT-aMZI Design1 with ΔL = 44.7 µm together with the Interconnect simulation, and analytical model from Eq. (1). Note that we have added the device IL to the Interconnect and analytical model results. The results show very good agreement among the different modelling methods. The FSR of the different modelling methods is listed in Table 2, showing less than 2.2% difference among the different modelling methods.

Figure 4
figure 4

Comparison of the: (a) output power versus wavelength and, (b) resonance wavelengths from measured, Interconnect and analytical modelling of the LT-aMZI with ΔL = 44.7 µm. The measured (solid) and simulated (dotted) output power spectra versus wavelength of the LT-aMZI designs with path length differences: (c) ΔL = 44.7 µm (red) and ΔL = 89.4 µm (blue) and (d) ΔL = 178.8 µm (red) and ΔL = 357.6 µm (blue).

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Table 2 Calculated FSR from measurements, Interconnects, and analytical model of the LT-aMZI with ΔL = 44.7 µm.
Full size table

As mentioned above, the four designs, ΔL = 44.7 µm, 89.4 µm, 178.8 µm and 357.6 µm, were fabricated with three copies for each design to account for the fabrication tolerances. Figure 4b shows the resonance wavelengths of the LT-aMZI as calculated from the different modelling methods for the same design ΔL = 44.7 µm with error-bars showing the variations due to fabrication tolerances. Overall, it can be seen that the different modelling methods have good matching with the experimental results with deviation that is lying within the fabrication tolerances. Exact matching is not satisfied for two main reasons. Firstly, the used refractive indices for the silicon and silicon dioxide in Interconnect simulations are not exactly the same as the real refractive indices used in the fabrication of the devices. Secondly, is the dimension mismatch between the devices dimensions on the layout and the fabricated dimensions. Hence, the effective, group index of the waveguide mode, the lengths of the interferometer components and accordingly the resonant wavelengths and FSR will not be matched. We will stick to Interconnect simulations in the rest of the paper because, as mentioned above, it is computationally efficient, especially as the interferometer length is increased.

Figure 4c,d show the measured output power of the fabricated LT-aMZI designs together with the simulation results of Interconnect after adding to this simulation results the insertion loss from the measured data. The measured data are from the samples that best fit the Interconnect simulations. Table 3 shows the FSR determined from simulations and measurements showing less than 2.6% difference among them at 1.55 µm wavelength for the four designs.

Table 3 Calculated FSR from simulations and measurements near 1.55 µm of the four different LT-aMZI designs and the difference percentage.
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Discussion

Spectral tunable filter

Another unique characteristic of our proposed LT-MZI is that its transmission spectrum can be tuned using the DCs coupling coefficients, working as spectral tunable filter. While this effect is mentioned in30 however, they just investigated three sets <ɑ1 − ɑ2> of DCs coefficients and they studied the effect of fabrication tolerances for this specific sets. Here, we discuss the different possible trends of the LT-MZI transmission spectrum which occur due to specific relation between DC1 and DC2 coupling coefficients and which are of interest for spectral filter applications.

In the previous sections we focused on LT-MZI with 3-dB DCs (ɑ12 = ɑ22 = 0.5). In this case, H = 0 and the transmission spectrum reduce to cos2 function, see Eqs. (1) and (2). In addition, the FSR and FWHM is half that obtained using a C-MZI with the same path length difference, see Fig. 1c. However, changing the DCs coefficient from this condition will change the H parameter to non-zero values and accordingly the transmission spectrum changes. Figure 5a shows H values at different a1 = ɑ12 and a2 = ɑ22 with b12 = 1 − ɑ12 and b22 = 1 − ɑ22, i.e. lossless DCs. H is positive when a1 > 0.5 and a2 < 0.5 or vice versa and negative if both a1,a2 > 0.5 or a1,a2 < 0.5.

Figure 5
figure 5

(a) H parameter versus a1 coefficient at different a2. Transmission spectrum versus wavelength of SOI waveguide TE mode at w = 500 nm and ΔL = 25.4 µm with: (b) a2 = 0.8 at different a1, (c) at different a1 with a2 = a1 and (d) at different a1 with a2 = 1 − a1 where a1 increase from 0.15 to 0.5 with 0.05 step as indicated by the arrow. (e) Percentage change of the main loop FWHM versus a1 for part c and d.

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There are mainly three special cases of interest resulting in different trends of the LT-MZI output spectrum. First, changing a1 while a2 is fixed at 0.8, see Fig. 5b. It can be seen that, by sweeping DC1 coupling coefficients the transmission intensity of even and odd resonances are changed inversely and we can flip from a spectrum having only the even resonances to a spectrum having only the odd resonances, thus controlling the relative intensity of the even and odd resonances. Second, changing the DCs coefficients with a1 = a2, see Fig. 5c, in this case even resonances are always maximum with T = 1 while odd resonances intensity can be controlled from 0 to 1 by changing a1 from 0.15 to 0.5. Finally, changing DCs coefficients such that a1 + a2 = 1, see Fig. 5d, in this case odd resonances have T = 1 while even resonances intensity can be controlled. Note that, in this cases the FSR can be doubled from the case when a1 = a2 = 0.5. All the transmission spectra of Fig. 5 are obtained using SOI waveguide with w = 500 nm and ΔL = 25.4 µm.

As can be seen from the figures, in the two later cases the main loop FWHM changes while changing the DCs coefficients. FWHM is minimum when a1 = a2 = 0.5 and as a1 moves far from 0.5 the FWHM increase. Figure 5e shows the FWHM percentage change from the minimum value at a1 = a2 = 0.5 as a function of a1. We can see that for a1 = 0.15 the FWHM increase by 44%. Also note that, while figures shows only a1 varying from 0.15–0.45 the response is symmetric for a1 values from 0.5–0.85, i.e. a1 = 0.45 and a1 = 0.55 give the same response as they result in the same H value, see Fig. 5a.

Unlike the transfer matrix method used in30, this behavior can be easily interpreted using our derived closed form expression of Eq. (1). Simply, as DCs coefficients change as H parameter which is a constant shift to the cosine function changes. Accordingly, the part of the cosine below (or above) the zero changes and hence when squaring it the relative intensities of the even and odd resonances changes. Such phenomena does not exist in C-MZI or the Michelson interferometer proposed in29, changing the DCs coefficients in these structures will only change the transmission intensity of the whole spectrum evenly.

Electro-optical modulator

Our proposed LT-MZI design is very promising to enhance the performance of electro-optical modulators that use C-MZI, similar to the Michelson interferometer modulator proposed in29. One of the main electro-optical modulator parameters is the term Vπ × L which defines the voltage Vπ needed to change the output power of the interferometer with length L from High to Low. Other important parameters are the energy consumption of the modulator and modulation speed (bandwidth). Modulation speed is mainly limited by the RF losses, walk-off between the electrical and optical signal and RC time constant. On the other hand, energy consumption are determined by the RF losses, capacitive loading and the 50 Ω termination resistor. For state-of-the-art modulators such as the ones based on silicon organic hybrid (SOH) platform37,38,39,40, the modulator’s length can be as small as 500 µm. At such compact length RF losses are reduced37,38 and the modulator behave as a lumped element that can be derived without the 50-Ω termination38,39,40. Accordingly, walk-off is negligible and RC time constant is the main bandwidth limitation, f3dB = 1/2πRC37,38. Also, the energy consumption is reduced and determined mainly by the capacitive load38,39,40, which is proportional to CV2. Hence, decreasing the capacitance of such modulators will further enhance their performance.

Now assume a symmetric (balanced) interferometer, L1 = L2 = L, where the applied voltage V changes the interferometer arms effective index and hence switching the output power from high to low. For LT-aMZI and C-aMZI with \(a_{1} = a_{2} = b_{1} = b_{2} = 1/\sqrt 2\) we have:

$$ T_{LT {\text{-}} aMZI} = \cos^{2} \left( {\Delta \phi } \right) $$
(4a)
$$ T_{C {\text{-}} aMZI} = \cos^{2} \left( {\frac{\Delta \phi }{2}} \right) $$
(4b)

Hence to switch from ON to OFF we need:

$$ \Delta \phi = \pi \Rightarrow \frac{{2\pi \Delta n_{eff} L_{C - MZI} }}{\lambda } = \pi \Rightarrow L_{C {\text{-}} MZI} = \frac{\lambda }{{2\Delta n_{eff} }} $$
(5a)
$$ \Delta \phi = \frac{\pi }{2} \Rightarrow \frac{{2\pi \Delta n_{eff} L_{LT - MZI} }}{\lambda } = \frac{\pi }{2} \Rightarrow L_{LT {\text{-}} MZI} = \frac{\lambda }{{4\Delta n_{eff} }} $$
(5b)

Δneff is proportional to the applied ΔV. For the same waveguide configuration and same applied voltage V, both C-MZI and LT-MZI will have the same Δneff, Hence, Eq. (5) shows that the Vπ × L term will be two times smaller for our proposed LT-MZI. This shows that LT-MZI design is more compact (LLT-MZI = LC-MZI/2). On the other hand, the capacitance of the modulator expressed by Eq. (6) will also decrease to half the value of the C-MZI (CLT-MZI = CC-MZI/2). Accordingly, for compact state-of-the-art modulators this will effectively reduce the energy consumption and increase the modulation speed for the LT-MZI design.

$$ C = \varepsilon_{0} \varepsilon_{r} \frac{h \times L}{d} $$
(6)

where εr is the permittivity of the capacitor dielectric material, d is the conducting plate separation, h is the waveguide thickness and L is the interferometer arm’s length.

Optical sensor

The main performance parameter of optical sensors is the figure of merit (FOM) which is defined as41:

$$ FOM = \frac{{d\lambda /dn_{med} }}{FWHM} = \frac{S}{FWHM} $$
(7)

where S is the wavelength sensitivity and nmed is the refractive index of the sensed medium. The resonance wavelengths for LT-aMZI and C-aMZI are:

$$ \Delta \phi = q\pi \Rightarrow \lambda_{res,LT {\text{-}} aMZI} = \frac{2}{q}(n_{eff,sens} L_{sens} - n_{eff,ref} L_{ref} ) $$
(8a)
$$ \frac{\Delta \phi }{2} = q\pi \Rightarrow \lambda_{res,C {\text{-}} aMZI} = \frac{1}{q}(n_{eff,sens} L_{sens} - n_{eff,ref} L_{ref} ) $$
(8b)

where neff,sens, Lsens and neff,ref, Lref are the effective index, arm length of the sensing and reference arm, respectively. Hence, the wavelength sensitivity of both interferometers can be derived to be:

$$ S_{LT {\text{-}} aMZI} = \frac{{\lambda_{res} }}{{(n_{eff,sens} L_{sens} - n_{eff,ref} L_{ref} )}}L_{sens} S_{wg} $$
(9a)
$$ S_{C {\text{-}} aMZI} = \frac{{\lambda_{res} }}{{(n_{eff,sens} L_{sens} - n_{eff,ref} L_{ref} )}}L_{sens} S_{wg} $$
(9b)

where Swg = dneff/dnmed is the sensing arm waveguide sensitivity. So, both LT-aMZI and C-aMZI configuration have the same wavelength sensitivity S for the same waveguide structure and same dimensions. This sensitivity increase as Lsens → (nref/nsens) × Lref. On the other hand, FWHM is expressed as:

$$ FWHM = \frac{FSR}{\pi } $$
(10)
$$ \text{with} \quad FSR_{LT {\text{-}} aMZI} = \frac{{\lambda^{2} }}{{2(n_{eff,sens} L_{sens} - n_{eff,ref} L_{ref} )}} $$
(11a)
$$ \text{and} \quad FSR_{C {\text{-}} aMZI} = \frac{{\lambda^{2} }}{{(n_{eff,sens} L_{sens} - n_{eff,ref} L_{ref} )}} $$
(11b)

The FWHM of LT-aMZI is half that of C-aMZI, while both increase as Lsens → (nref/nsens) × Lref. Accordingly:

$$ FOM_{LT {\text{-}} aMZI} = \frac{S}{FWHM} = \frac{{2\pi S_{wg} L_{sens} }}{\lambda } = 2FOM_{C {\text{-}} aMZI} $$
(12)

FOM of LT-aMZI is double that of C-aMZI with both proportional to sensing arm length Lsens and its waveguide sensitivity Swg. In general, aMZI (or unbalanced MZI) sensor offers advantage over the sMZI. For the symmetric LT-MZI (LT-sMZI) where Lsens = Lref = L we have:

$$ S_{LT {\text{-}} sMZI} = \frac{{\lambda_{res} }}{{\Delta n_{eff} }}S_{wg} $$
(13a)
$$ FSR_{LT {\text{-}} sMZI} = \frac{{\lambda^{2} }}{{2\Delta n_{eff} L}} $$
(13b)
$$ FOM_{LT {\text{-}} sMZI} = \frac{{2\pi S_{wg} L}}{\lambda } $$
(13c)

Hence, for the LT-sMZI the sensitivity is independent of L while FWHM decrease as L increase and FOM expression is the same as in the case of LT-aMZI. However, in the asymmetric MZI sensor you can engineer both S and FOM independently using Lref and Lsens, Eq. (9) and Eq. (12), and independent of the used waveguide structures (i.e. Swg and neff), while in the case of symmetric MZI the sensor sensitivity is determined only by the waveguide structure, Eq. (13a). This is also an advantage over ring resonator sensors.

Conclusion

In conclusion a LT-aMZI structure using the popular SOI technology have been proposed, this design is a compact version of the widespread C-aMZI. Four designs for optical filters and WDM applications with different FSR were implemented and characterized both numerically and experimental as well as analytically using our derived compact model. The different modelling techniques show good matching with the experimental results of the fabricated devices. It was also shown that, our LT-MZI spectrum can be tuned by changing its DCs coefficients for spectral tunable filter application. Finally, using our compact model the interferometer proved to have better performance when compared with the widespread C-MZI in sensing and modulation applications, and we also demonstrated that LT-aMZI sensor configuration is preferable over the LT-sMZI.

Materials and methods

The designs were fabricated at the UW NNCI Washington Nanofabrication Facility using Electron Beam lithography34 on standard SOI wafers with silicon device layer of 220 nm. We fabricated three copies for each design to account for the fabrication tolerance.

The device measurements were performed by the team of Lukas Chrostowski at The University of British Columbia and by Maple Leaf Photonics42,43. A tunable laser is used to span the wavelength region from 1500 to 1600 nm. A polarization maintaining fiber array containing 4 fibers separated by 127 µm is used to couple light in and out of the chip through the grating couplers while a detector is used to measure the output power from the fibers. Also note, that the SEM photos are taken before depositing the SiO2 clad.