Tunable large free spectral range microring resonators in lithium niobate on insulator

Microring resonators are critical photonic components used in filtering, sensing and nonlinear applications. To date, the development of high performance microring resonators in LNOI has been limited by the sidewall angle, roughness and etch depth of fabricated rib waveguides. We present large free spectral range microring resonators patterned via electron beam lithography in high-index contrast Z-cut LNOI. Our microring resonators achieve an FSR greater than 5 nm for ring radius of 30 μm and a large 3 dB resonance bandwidth. We demonstrate 3 pm/V electro-optic tuning of a 70 μm-radius ring. This work will enable efficient on-chip filtering in LNOI and precede future, more complex, microring resonator networks and nonlinear field enhancement applications.

Microring resonators are challenging photonic components to fabricate, as any losses incurred in the cavity are greatly amplified.To achieve a large FSR for telecommunication applications and sensing, small, single-mode high-index contrast waveguides are required.Microring resonators can also be cascaded to increase the spectral enhancement, or create various types of filters [17] and this requires very precise and careful control of the 3 dB resonance bandwidth and FSR.In general, a ring performance is limited either by the material properties, such as 2-photon absorption in the C-band of Si, or the ability to nanostructure the material to produce small waveguides with smooth sidewalls.A further challenge that plagues ring resonators is their stability as many platforms, such as Si and SiN, are highly sensitive to temperature.
Although Lithium Niobate (LN) could greatly benefit from microring resonators to enhance its nonlinear and electro-optic properties, as well as prepare it for telecommunication use, compact rings in LN are yet to be seen.Traditionally, LN has only supported prohibitive low-index contrast waveguides made from Ti:LN [18] and (soft/reverse) PE:LN [19].With the commercialization of Lithium Niobate on insulator wafers (LNOI) [20], highindex contrast waveguides in LN can now be achieved, enabling the fabrication of microring resonators [21].Due to the complex nature of processing LNOI, compact rings in TFLN with a high FSR and large 3 dB bandwidth are yet to be seen.To date, the sidewall angle of most processes is limited to ∼ 50 • , as the etching process utilizes predominantly Ar, leading to anisotropic etching, this hampers the ability to produce small gaps [22,23].Furthermore, the waveguides demonstrated to date have a limited bend radius because is challenging to etch LNOI deeply enough-to obtain a high-index contrast-due to mask depletion [24].
In this work, we present all-pass microring resonators in LNOI fabricated in Z-cut LNOI from small, low-loss, highindex contrast waveguides.We analyze the performance of multiple rings with varying radii from 30 µm to 90 µm.The demonstrated microring resonators have a maximum FSR of 5.7 nm, and a Q-factor of 10000 in overcoupled regime.We expect the microring resonators in this work to pave the way towards on-chip filtering in LNOI with ring networks, as well as field enhancement applications such as switching and nonlinear photon generation.

DESIGN AND FABRICATION
In order to achieve high FSR and wide resonance bandwidth rings in LNOI, low-loss waveguides with a small bend radius and a small gap were required.Microring resonators with radii 30-90 µm were designed to obtain an FSR from 1.5 to 5.7 nm and were simulated using the commercially available software, Lumerical.Rings of varying radii were fabricated to analyze the FSR and performance for the TE and TM modes.The small gap of 300 nm was chosen to overcouple the rings and obtain wider bandwidth resonances, rather than extremely narrow resonances that require very precise wavelength tuning to access.The small bending loss needed for good operation of a 30 µm microring resonator required high-index contrast single mode waveguides at 1550 nm.A mode solver was used to determine the dimensions required to ensure a sufficiently small TM polarization bend radius.The design of the waveguide includes the following parameters: rib height, top width, sidewall angle, refractive indices of the waveguide and claddings, and film thickness.The cross-section of a Z-cut rib waveguide cladded with SiO 2 is shown in Figure reffig1(a).
The photonic components were fabricated by the process developed and described in our previous work [25].The process starts with 500 nm thick LN film, which is fabricated using the smart-cut technique on 2 µm of SiO 2 layer and supported by a 500 µm LN substrate.The next fabrication steps rely on electron beam lithography and lift-off of the e-beam evaporated metal layer to obtain a hard mask defining the photonic components.The scanning electron microscopy image (SEM) of a waveguide to a ring coupling region just after the metal lift-off process, is shown in Fig. 1(d).The components were then dry etched in a reactive ion etcher Fig. 1(c).Following etching, the waveguides were cladded with 3 µm thick plasma-enhanced chemical vapor deposition (PECVD) SiO 2 .The presented structures were etched deeper than in our previous work to achieve the necessary index contrast, reducing the waveguide bending radius.The rib waveguide cross-section, obtained via focused ion beam (FIB) slicing and scanning electron microscopy (SEM), shows a sidewall angle of 75 • and an etch depth of 350 nm Fig. 1(b).Finally, the waveguide facets were diced using optical grade dicing to facilitate butt-coupling.The length of the chip, after all processing steps were completed, is 6 mm.

EXPERIMENTAL RESULTS
In order to confirm that the photonic components are not limited by the propagation loss, loss measurements were performed prior to the characterization of the microrings, using the Fabry-Perot loss measurement technique [26].Laser light at 1550 nm wavelength is coupled into and out of the polished facets of the waveguide using polarization maintaining (PM) lensed fibers with a mode field diameter of 2 µm .A typical optical transmission spectrum for TM (the TE and TM modes have a similar response) is shown on the Fig. 2. Inverse tapers down to 200 nm width, at the waveguide ends, are used to improve the coupling efficiency of the laser light from the optical fiber to the chip, and improve the signal to noise ratio of the Fabry-Perot measurements.As the waveguide narrows, the mode field diameter at the input and output of the waveguide significantly increases, allowing improved mode matching with the mode of the lensed fiber.The total input and output coupling and propagation loss is 8 dB for a 6 mm long chip, compared to the 15 dB loss achieved with the straight waveguide without tapering section.The estimated propagation loss is less than 0.5 dB/cm for both the TE and TM modes, which is in agreement with the results obtained in our previous work [25] The fabricated microring resonators were characterized by sweeping the wavelength of the laser between 1530 to 1610 nm and recording their spectral responses with a commercially available high-speed InGaAs photodiode.The laser light was injected into and out of a 6 mm buswaveguide via PM lensed fibers.To decrease the chance of interference between multiple oscillations inside of the photonic component, the inverse tapering section was not implemented for the microrings-this led to a drop in the achieved mode matching efficiency.We observe that both TE and TM (Fig. 3(b)) modes achieve the largest FSR for the ring with the smallest radius 30 µm ; however, the TE and TM modes show different results in terms of the achievable Q-factor for this geometry.A Q-factor of ∼ 9000 was achieved for the TM mode whilst for the TE mode the Q-factor is significantly smaller ∼ 1200.As the radius of the ring increases, the Q-factor for TM mode preserves almost unchangeable values Fig. 3(a); meanwhile, for the TE mode, it significantly increases Fig. 3(a) and the highest Q-factor has been achieved for the ring with radius of 90 µm Fig. 3(c).This dissimilarity can be attributed to the difference in the bending loss between both modes.By comparing theoretical and experimental results, the effective index for the TE mode is 1.85 and for the TM mode is 1.72 and the TM mode was confined to have an index contrast of ∼ 0.272, whilst the TE mode is lower ∼ 0.247.As the TM mode has a higher index contrast, a smaller bend radius is achieved, enabling smaller microring resonators to be realized.The TE mode bending loss decreased with increasing microring resonator radius, leading to an improvement in the Q-factor.
The group indices for the TE and TM modes respectively, n TE g and n TM g , are deduced from the fully-vectorial mode solver using the Sellmeier equations for lithium niobate: n TM g = 2.33 and n TE g = 2.38.The FSR can be calculated using FSR = λ 2 /(n g L), where L is the circumference of the ring (L = 2πR), R is the radius of the ring.The simulation curve is plotted with the measured FSR for different microring resonator dimensions in Fig. 4(a) and Fig. 4(d).The simulated E-field distributions of the fundamental waveguide modes at a wavelength of 1550 nm (found using an in-house mode solver) are included to the figures as insets: 4(b) for the TE mode, and 4(e) for the TM mode.Also included as insets, Fig. 4(c) and Fig. 4(f), show the measured power distribution at a wavelength of 1550 nm in a 3 × 3 µm window; each cell defined by the white grid lines represents a single pixel (a single power measurement).The measured power distribution is performed by sweeping the fiber over the output facet of the waveguide, resulting in a convolution between the fiber mode and the waveguide mode, smearing and enlarging the appearance of the waveguide mode.

DISCUSSION
The Fabry-Perot transmission measurements were conducted on straight waveguides with inverse tapers at either end and indicate low propagation loss for this platform.The overall insertion loss of the waveguides is dominated by mode-mismatch, despite the significant improvement of the inverse tapers.Given that the straight waveguides measured have identical dimensions to the waveguides used in the ring resonators and were fabricated on the same chip, the propagation loss in the rings are concluded to be equally low loss.
The demonstrated ring resonators are designed to be heavily overcoupled, increasing their 3 dB resonance bandwidth (and, conversely, reducing their Q-factor).A 300 nm gap in the bus waveguide to microring coupling region provides strong overcoupling.The potential of the presented nanofabrication process could be further extended to photonic components including grating couplers and compact directional couplers.
The Q-factor measurements show that it is possible to achieve small and high performance microring resonators for the TM mode-critical for electo-optic and nonlinear applications.Meanwhile, the TE mode bending losses significantly limit the Q-factor of the smaller radius microring resonators; however, increasing the ring radius leads to a substantial increase in Q-factor.It was anticipated that the TM mode would have a smaller bend radius than the TM mode, as the index contrast of the TE fundamental mode is less than that of the TM fundamental mode, as verified by both our in-house mode solver, and by the Q-factor simulations conducted in Lumerical Mode for the 30 µm ring-accurate results of microring resonators larger than ∼ 50 µm rings are challenging using Lumerical Mode.
Contrary to the TE mode, the TM mode does not experience such significant bending loss.The Q-factor is a balanced combination of bending loss and overcoupling.As the ring radius increases, the Q-factor drops (the resonance 3 dB bandwidth increases) due to the increase in overcoupling, but the drop in Q-factor is lessened by the decrease in bending losses, leading to a mostly constant Q-factor across all the microring resonators.
It was found that the theoretically predicted results for the microring resonators demonstrated in this paper are in a good agreement with the experimental results.The deviation for n g is less than 2% leading to precise agreement between the designed and measured FSRs for different ring geometries.Using 350 nm deep ribs, a small TM bend radius was achieved to enable 30 µm TM microring resonators with an FSR of 5.5 nm.This result is competitive with other high-index contrast leading platforms, such as SiN and AlN, and will enable future low-loss and tunable filtering in LNOI.

CONCLUSION
We have demonstrated and investigated high FSR rings in LNOI by fabricating rings of varying radii.The waveguides used in this work are etched 350 nm, to enable the creation of small microring resonators.Fabry-perot loss measurements are presented, showing that the waveguides are state-of-the-art with low-loss and a good side-wall profile.We have studied the bandwidth of the resonances and Q-factor of the rings, and showed that they match well with the design and simulation.We anticipate that these rings will precede more complex filtering on LNOI, including tunable rings and multi-stage filters.

FIG. 1 .
FIG. 1.(a) Design of a single mode LNOI rib waveguide at 1550 nm wavelength; the top width is 650 nm, the bottom width is 840 nm and the waveguide height is 350 nm.Scanning electron microscope pictures: (b) cross-section taken by FIB slicing and SEM imaging; (c) an etched 30 µm radius ring with a 300 nm gap between the bus waveguide and the ring prior to PECVD SiO2 cladding; (d) false-color image of the coupling region after the lift-off process and prior to etching; the false-red highlights the metal etch mask.

FIG. 2 .
FIG. 2. (a) Transmission spectrum of the inverse taper with 200 nm width LNOI used for calculating the propagation loss of the TM mode.

FIG. 3 .
FIG. 3. (a) Measured Q-factors of the ring resonators versus their radius for the TE and TM modes.The blue curve corresponds to the TE mode and the red curve corresponds to the TM mode.(b) Spectral response of the ring with radius 30 µm for TM mode.(c) Spectral response of the ring with radius 90 µm for TE mode.

FIG. 4 .
FIG. 4. (a) Measured FSR as the function of a microring resonator radius for the TE mode and (d) for TM mode; the blue circles are measured values, whilst red line is theoretically predicted dependence of FSR on microring resonator radius; (b) the simulated electrical field distribution for the TE mode and (e) for the TM mode; (c) measured optical power distribution at the output of the chip for the TE mode and (f) for the TM mode, where black lines schematically show the actual waveguide dimensions.

FUNDING
Australian Research Council Centre for Quantum Computation and Communication Technology CE170100012; Australian Research Council Discovery Early Career Researcher Award, Project No. DE140101700; RMIT University Vice-Chancellors Senior Research Fellowship.