Tunable integrated ring resonators by femtosecond laser micromachining

TL;DR

This paper demonstrates tunable integrated ring resonators fabricated by femtosecond laser micromachining in glass, achieving a high Q factor ($Q \approx 8\times10^5$ at critical coupling) and dynamic resonance tuning using two thermo-optic phase shifters embedded in a reconfigurable Mach-Zehnder interferometer. The authors design a monolithic, open-waveguide layout that enables a self-coupled racetrack resonator with 3D FLM routing, including 20 concentric rings and low propagation losses ($\sim$0.17–0.23 dB/cm). Theoretical modeling links the MZI phases and TOPS powers to the ring’s effective optical length and coupling, predicting tunable spectral shifts and Q-factor modulation; experimental results confirm high-Q operation, a fit to an unbalanced coupler model (64:36), and resonance shifts over more than one FSR via coordinated TOPS actuation. The work highlights a versatile platform for dynamic photonic spectral control, with potential impact on tunable filters, gyroscopes, and integrated sensors, and suggests competitive thermal stability advantages over silicon platforms due to the silica host and structured heating.

Abstract

Femtosecond Laser Micromachining (FLM) is a powerful technology for the fabrication of photonic devices. In this context, the integration of resonant elements within the platform represents a key advancement, enhancing both its versatility and its compatibility with a wide range of optical and fluidic components specifically enabled by this technique. Here, we report the realization of a tunable racetrack resonator fabricated by FLM and operating at telecom wavelengths. Leveraging low-loss waveguides, we obtained a Q factor of the resonator as high as 8 x 10^5 at critical coupling. Moreover, by integrating two thermo-optic phase shifters, we achieved both resonance tuning and dynamic control of the Q factor. This capability makes the device highly versatile for applications requiring dynamic spectral control, such as tunable filters, gyroscopes, and sensors.

Figures (8)

• Figure 1: Layout of the resonator. The waveguide, starting from the input port (IN) first forms a ring with a racetrack geometry, then couples back to itself through a MZI, and is finally brought to the output (OUT) port, passing underneath the ring structure. The MZI is equipped with two TOPS, one per each arm, consisting in metal resistors deposited and patterned on the substrate surface. Different electrical powers $P_1$ and $P_2$ can be dissipated on the resistors, enabling independent control on the MZI phases. To enhance thermal efficiency, U-trenches were ablated around the waveguide segments Ceccarelli2020. Total footprint of the device is $5 \times 6~\mathrm{cm}^2$.
• Figure 2: Characterization of the spectral response of the resonator. (a) Output power of the resonator as a function of wavelength under critical coupling conditions. Multiple resonance dips are visible over a span of 160 pm. (b) Zoom-in of the spectral region highlighted by the red rectangle in (a), showing the shape and depth of individual resonance fringes, with the FWHM and FSR indicated by red and orange arrows, respectively. (c) Maximum (red dot) and minimum (blue dot) transmission values extracted from the spectra as $P_1$ is varied, thereby tuning the coupling coefficient $t$. Solid lines, represent a theoretical fit, taking into account unbalanced directional couplers, see text. The dotted line labeled A (black), B (brown), C (grey) correspond to $|t|=\alpha$ (critical coupling condition), strong undercoupling ($|t| \sim 1$) and strong overcoupling ($|t| \sim 0$), respectively. The purple arrow indicates the internal losses of the ring, while the green arrow marks the extinction ratio achieved experimentally at critical coupling.
• Figure 3: Characterization of all resonators as a function of the ring radius, where blue dotted markers represent the values obtained with a writing speed of 5 mm s$^{-1}$, while red diamonds correspond to those fabricated at 7 mm s$^{-1}$. (a) Measured round-trip losses of the ring resonator. (b) Free Spectral Range (FSR), where the green line represents the theoretical prediction based on the relation $\text{FSR} = \frac{\bar{\lambda}^2}{L_\mathrm{tot}}$, and the data points indicate measured values, calculated by averaging the separation between resonance dips detected over the scanned wavelength range. (c) Quality factor of each resonator, extracted from the FWHM under critical coupling conditions.
• Figure 4: Q factor as a function of dissipated power $P_1$, illustrating the transition between coupling regimes, from strong overcoupling ($|t| \sim 0$) to strong undercoupling ($|t| \sim 1$), passing from critical coupling ($|t|=\alpha$). The highest Q factor is observed in undercoupling regime. No data points are shown at the minima, as the transmission spectrum becomes flat in those regions, preventing the identification of a resonance dip and, consequently, the measurement of a meaningful FWHM.
• Figure 5: Diagram showing the resonance shift as a function of the power dissipated $P_2$. The data show a clear linear trend, highlighted by the black linear fit.