1. Introduction

Optical microring resonators are manifested as fundamental building blocks of photonic integrated circuits. They provide an opportunity to realize narrow-band optical filters, switches, conduct sensing with low detection limit and high throughput [1], and perform ultra-compact integrated nonlinear processes [2]. In general, transmission characteristics of ring resonators are strongly polarization dependent, which implies the use of polarization control schemes associated with additional complexity and noise. Moreover, in many applications, such as telecommunications systems, the signals typically have arbitrary polarization. Both of the two orthogonal polarizations have their own distinct advantages. Quasi-TE polarized light is widely used in standard silicon photonics for ultimate miniaturization due to its strongly confined mode profile and low losses at sharp bending. On the other hand, quasi-TM polarization is preferable for number of specific applications such as sensing, certain types of laser sources, and functionalized waveguides. In the context of ring resonators, it experiences less scattering losses on the sidewalls, giving an opportunity to achieve extremely high quality factors for standard nanostrip waveguides [3]. Consequently, there is a strong interest in ring resonators capable of similarly handling both polarizations for high performance cross polarization filters, evanescent field sensing arrays based on polarization multiplexing [4], and microring resonator filters with doubled free spectral range [5].

There are number of works proposing solutions for the elimination of the polarization dependence in ring resonators and generally they can be classified into two groups. The first one involves a direct modification of the design of a ring resonator waveguide and a coupler geometry. For example, it can employ a polarization-independent etching depth in case of rib waveguides [6], and an implementation of a multi-mode interference coupler to access polarization-insensitive operation [7]. The approach can alternatively include the use of two level structures, such as vertically coupled sandwich-like ring resonators [8]. The fundamental issue of the proposed designs is that they provide a polarization-independent operation only within a narrow bandwidth of few nanometers. The problem originates from the dispersion properties of quasi-TE and quasi-TM modes that are remarkably different. As a result, the spacing between the neighboring resonance peaks known as the free spectral range (FSR) exhibits different wavelength dependencies for the two orthogonal polarizations. Consequently, neither fabrication of the waveguides with polarization-independent dimensions [9] nor further post-processing by expanding or shrinking their cross section [10] can guarantee the polarization insensitivity for a sufficient wavelength range unless some extra adjustment schemes are involved [11, 12]. The second group of polarization-independent solutions is based on polarization diversity schemes. For example, in the excellent work of [13], a comprehensive polarization-transparent add-drop filter was demonstrated to eliminate polarization sensitivity for over 60 nm bandwidth. Being a high-grade method that shows almost complete elimination of polarization dependence, the approach, however, requires an additional fabrication of multiple integrated elements, increasing the number of fabrication processes and processing stages.

In this work, we demonstrate a new method for tuning the ring resonators based on the standard nanostrip waveguides towards polarization-independent operation. In order to match the FSR of the two orthogonal polarizations, we selectively apply highly uniform TiO₂ dielectric films formed by atomic layer deposition (ALD) [14]. The overlayers modify the waveguiding properties only at certain locations where the previously fabricated ring resonators are exposed to ambient. The technique relies on the specific properties of amorphous TiO₂, which has been recently shown to be very effective for the engineering of the waveguide dispersion properties [15]. The utilized ALD-based growing of TiO₂ is an inherently low-temperature process and, as we have shown earlier [16], it provides conformal high optical quality overlayers with an excellent control over the film thickness for silicon nanostrip and slot waveguides.

In our study, we trace the impact of the TiO₂ overlayer on the FSR for both polarizations and different overlayer thicknesses, representing the cases from a strongly coupled to a quasi-isolated ring. We show that even the relatively thick TiO₂ overlayers do not promote the beating of the coupling coefficient and the associated distortions of the resonance strength [17]. Finally, we demonstrate an example of the TiO₂-coated ring resonator that retains a good alignment of the resonant peak positions for the two orthogonal polarizations within the broad bandwidth.

2. Theory

Transmission characteristic of a ring resonator can be generally described by the following equation [18]:

3. Methods

The ring resonators with the radius of 50 μm and the waveguide width of 460 nm were patterned by 248 nm deep ultraviolet lithography onto 8-inch silicon-on-insulator wafers with a 220 nm thick device layer and 2 μm thick buried oxide. The processing of the Si device layer was done by the reactive ion etching with Cl₂/HBr chemistry. A set of inverse tapers was fabricated at the chip facets to couple the light into a chip with minimized wavelength dependence. The whole chip area was first covered with 2 μm of PECVD SiO₂, and then the specific locations of ring resonators were completely exposed to ambient by a highly selective SiO₂ etching.

As shown in the Fig. 1, such a design enables local deposition, leaving the waveguiding properties of the rest of the structure intact. Throughout this work, the ALD-TiO₂ films were fabricated with the titanium tetrachloride (TiCl₄)/water (H₂O) process at 120°C using Beneq TFS 500 reactor. The process parameters were found to be optimal as they provide an amorphous uniform coating that has low losses [19] and retains its profile even in narrow gaps [20, 21], as in proximity coupling regions. Before and during each deposition run, a test sample was used to verify the film uniformity to be better than 1%.

 

Fig. 1 (a) Scanning electron micrograph of the fabricated sample taken with an angle of 52 degrees with respect to the normal of the surface, and (b) schematic drawing of the ALD deposition stage.

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The characterization of the samples was performed using a superluminescent LED source with a center wavelength of 1570 nm and a bandwidth of 100 nm. The input polarization was controlled with switchable fiber polarizers and polarization maintaining (PM) fibers. The light was coupled into the chip using a tapered PM fiber with a spot size of 2.5 μm, which provides an efficient coupling to the inverse tapers on the chip. The optical transmission spectra shown in this work were collected with an optical spectrum analyzer ANDO AQ-6315A without output polarization control. In order to verify that the samples do not exhibit any observable polarization rotation [22], the obtained resonant features were confirmed with an output polarizer similar as in the input.

4. Absolute magnitude and beating period of the coupling coefficient

In this section, we show the examples of how the deposition of a TiO₂ overlayer can manipulate the resonance characteristics of the samples. For that, a set of ring resonators was covered with TiO₂ films of different thicknesses from 0 to 150 nm, and then characterized as described in section 3. We utilized ring resonators with a 300 nm gap between the ring and the bus waveguide, as they featured detectable resonant peaks for both polarizations.

In order to maximize the resonance strength, the portion of light coupled to the ring should be equal to the amount of power lost in the resonator per one round trip. This operation regime is known as the critical coupling condition or the steady state of a resonator. Analytically, it corresponds to the situation when α = t in Eq. (1), giving zero transmission at resonance due to the complete destructive interference in the coupler region.

A control on the overall resonance performance implies preserving a balance between the internal losses and the coupling coefficient within the required spectral range. The application of an ALD overlayer simultaneously changes number of parameters affecting this balance, however, the strongest effect is caused on the resonator coupling coefficient. This is of great significance, since while the absolute magnitude of the coupling coefficient with respect to the total round trip loss solely defines the extinction ratio at resonance, its oscillation period affects the bandwidth where this ratio holds [23].

Figure 2 presents the evolution of the resonator transmission spectra for the two polarizations and the scanning electron micrographs showing the corresponding modification of the resonator coupling regions. The uncoated samples first exhibit poor TE-like mode resonances [Fig. 2(b)] due to the large 300 nm gap and initially weak coupling. At the same time, the TM-like signal is low and essentially noisy [Fig. 2(c)], which originates from the significant mode mismatch at the interfaces where SiO₂ cladding was removed. The behavior of the quasi-TM mode is governed by its mode profile that expands to the material above the core more than that of the TE-like mode. For the same reason, even with a thin 50-nm-thick ALD layer, the quasi-TM polarized mode experiences less losses and the resonances become more pronounced [Fig. 2(f)]. When the TiO₂ thickness approaches 100 nm, it already makes the ring resonator operate near the critical coupling regime for the quasi-TM mode, exhibiting strong transmission dips [Fig. 2(i)], while the quasi-TE mode is still undercoupled [Fig. 2(h)]. Further increase in TiO₂ thickness results in the TM-like polarized mode being overcoupled and driven away from the point of strongest resonances [Fig. 2(l)]. Simultaneously, the quasi-TE mode starts to approach the critical coupling condition [Fig. 2(k)].

 

Fig. 2 (a, d, g, j) Scanning electron micrographs of the coupling region covered by the different TiO₂ overlayer thicknesses. The corresponding resonator transmission spectra for (b, e, h, k) quasi-TE and (c, f, i, l) quasi-TM polarizations.

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One of the key points shown in the spectra is that even the thick TiO₂ overlayers do not boost the distortions caused by the periodic beating of the coupling coefficient, which can essentially limit the device performance [17, 23]. We believe that in our case the TiO₂ overlayer simultaneously modifies the physical length of the coupling region (L_c) and the effective distance over which light is coupled (L_π), which, as discussed in [24], both affect the phase shift and therefore the resonator transmission function. Consequently, we can conclude that although the distributed coupling is present, as seen in the cases in Figs. 2(f), 2(i), and 2(l), the beating is fundamentally small in comparison with the considered wavelength range.

5. Polarization-independent free spectral range

In this chapter, we demonstrate the potential of the tuning by a TiO₂ overlayer to control the FSR of the two orthogonal polarizations. We first acquired the transmission spectra of the ring resonators with various TiO₂ overlayer thicknesses. Then, the positions of resonant peaks were identified with a software and, in order to keep the experimental data clear, the obtained wavelength dependencies of the peak-to-peak spacing were subjected to a fitting algorithm. We verified that the oscillating modes are fundamental by the numerical estimation of their FSR values using 2D finite element method simulations.

Figure 3 demonstrates how drastically the wavelength dependence of the FSR is modified with the increase of the TiO₂ overlayer thickness. The effect is particularly intense for the quasi-TM polarized mode since its profile, as described earlier, experiences strong influence from the change in surrounding material. Consequently, the dispersion properties of the deposited TiO₂ govern the wavelength dependence of n_eff and, as given by the Eq. (3), promote the corresponding FSR shift. Thus, when the TiO₂ thickness equals to 100 nm [Fig. 3(c)], both the absolute values and the slopes of the FSRs for quasi-TE and quasi-TM fundamental modes start to remarkably coincide. Finally, as the thickness achieves 150 nm [Fig. 3(d)], the peak-to-peak spacings of the two polarizations exhibit a good alignment with each other across the broad bandwidth.

 

Fig. 3 Wavelength dependence of the free spectral range for the two input polarizations: blue - TE, red - TM. Experimental data (dots) is approximated by the fit curves (solid lines). The plots correspond to the following thicknesses of TiO₂ overlayer: (a) - 0 nm, (b) - 50 nm, (c) - 100 nm, and (d) - 150 nm.

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In order to illustrate the agreement of the peak-to-peak spacing for the TE and the TM input polarizations, we present the transmission spectra of the ring resonator with 150-nm-thick TiO₂ overlayer taken with a tunable external cavity laser with a 20 pm resolution and a manual polarization controller in Fig. 4. As seen from the spectra, the coupling conditions for TM-like polarized mode are not optimal, which results in a gradual reduction of the resonance peak strength while moving towards the longer wavelengths. Nevertheless, as we discussed earlier, the proposed ALD overlayer does not enhance the distributed coupling effects, and thus the coupling regime for both polarizations can be further optimized. It can be implemented at the fabrication stage by selecting the particular waveguide cross section and the gap between the bus and the ring waveguide, when the TiO₂ overlayer thickness that results in polarization-independent operation is known. Such a fine adjustment, however, lies beyond the scope of this work, as our goal was to demonstrate the principles of making the FSRs equal.

 

Fig. 4 Transmission spectra of the ring resonator with the 300 nm gap and the 150-nm-thick TiO₂ overlayer. Input polarizations are: blue - TE, red - TM.

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6. Conclusion

We have demonstrated the feasibility of a new method to tune microring resonators towards polarization-independent operation, which relies on the specific dispersion properties of ALD TiO₂ thin films. The advantages of the approach are that it does not require extra polarization-independent couplers and is applicable for standard nanostrip waveguides. In addition, it allows for partial compensation of fabrication imperfections and releases manufacturing tolerances imposed by the proximity effects and the control of the device layer etching.

Our results show that the technique offers a vast tuning potential that is sufficient to match the absolute values and the slopes of the FSR wavelength dependencies of the two polarizations. At the same time, the application of the TiO₂ overlayer does not promote polarization conversion effects, as it preserves the initially high birefringence of ring resonator waveguides, and alleviates their surface roughness with associated backscattering. The shown fine control on the peak-to-peak spacing with the flexible adjustment of the coupling strength for both polarizations can benefit numbers of applications, including polarization multiplexing, filtering, switching, and sensing. For example, the use of cross-polarized light in a microring resonator filter covered by a TiO₂ overlayer can double its free spectral range, which is crucial in the conditions when a ring resonator with a small radius is required. Moreover, as the thickness of the fabricated film can be controlled with the accuracy of up to a few molecular layers, the method simultaneously enables a precise regulation of the coupling coefficients. Specifically, it can be utilized for the control of intercoupling coefficients of apodized multiorder microring resonator arrays [25], nonlinearities in ring resonators cascades [2], and microcavity-resonator-based active devices [26]. Finally, the proposed ALD based growing of TiO₂ is an inherently low temperature process, which means that there are no issues with its integration into the existing silicon ring resonator designs. From the practical point of view, the advantage is that the ring resonators initially fabricated in one cycle can be later customized for various specifications.