Ring resonators serve as essential components in integrated optics, offering resonant optical cavities with compact dimensions and exceptional wavelength selectivity. These structures have caught significant attention due to their utility in a broad area of applications, including signal filtering, optical switching, and modulation. Additionally, ring resonators constitute a fundamental element in micro-optical gyroscopes (MOGs), that measure angular velocity. Within the domain of optical gyroscopes, MOGs present a promising avenue for advancing the performance of compact navigation systems. In this scenario, the present work aims to characterise ring resonators fabricated for the first time using femtosecond laser micromachining (FLM), with the objective of estimating the key parameters that determine their performance in optical filtering and gyroscope applications. In conventional silicon-based platforms, various ring resonator layouts have been developed and fabricated using lithographic techniques. In any case, the fundamental configuration consists of a bus waveguide coupled with a ring. This basic architecture was adopted in the present study to fabricate ring resonators via femtosecond laser micromachining; coupling of the ring resonator to the bus waveguide was implemented by a Mach-Zehnder interferometer (MZI). Owing to the three-dimensional structuring capabilities of this technique, it was possible to produce a compact device composed of 20 large-area ring resonators arranged concentrically, with radii up to 21 mm. Different scan speeds of 5 and 7 mm/s were employed to inscribe the waveguides with the femtosecond laser, while maintaining a constant pulse energy of 500 nJ. Additionally, all resonators were equipped with thermo-optic phase shifters (TOPS) to enable active control of the bus-ring coupling. An initial characterisation of the transmission spectra was conducted to evaluate losses, free spectral range (FSR), and quality factor (Q-factor) as a function of the radius. The resonator with a radius of 17.4 mm, fabricated at 7 mm/s, exhibited the lowest losses and highest Q-factor, and was therefore selected for further analysis. A more detailed characterisation of its transmission spectrum was performed to obtain precise values for the relevant figures of merit. The stability of the transmission spectrum was finally assessed by monitoring the optical intensity at the half-transmission point (i.e., the point of maximum sensitivity), under critical coupling conditions. Finally, we demonstrated the ability to actively and controlledly shift the dips of the transmission spectrum by varying the common phase term of the MZI.
I risonatori ad anello costituiscono componenti essenziali nell’ottica integrata, offrendo cavità ottiche risonanti con dimensioni compatte e un’eccezionale selettività in lunghezza d’onda. Queste strutture hanno attirato notevole attenzione grazie alla loro utilità in un’ampia gamma di applicazioni, tra cui il filtraggio di segnali, la commutazione ottica e la modulazione. Inoltre, i risonatori ad anello rappresentano un elemento fondamentale nei giroscopi micro-ottici (MOG), dispositivi che misurano la velocità angolare. Nell’ambito dei giroscopi ottici, i MOG costituiscono una promettente via per il miglioramento delle prestazioni dei sistemi di navigazione compatti. In questo contesto, il presente lavoro si propone di caratterizzare risonatori ad anello realizzati per la prima volta mediante microlavorazione laser a femtosecondi (FLM), con l’obiettivo di stimare i parametri chiave che determinano le loro prestazioni nelle applicazioni di filtraggio ottico e nei giroscopi. Su piattaforme convenzionali a base di silicio, sono stati sviluppati e realizzati vari layout di risonatori ad anello utilizzando tecniche litografiche. In ogni caso, la configurazione fondamentale consiste in una guida d’onda bus accoppiata a un anello. Questa architettura di base è stata adottata nello studio presente per la fabbricazione di risonatori ad anello tramite microlavorazione laser a femtosecondi; l’accoppiamento del risonatore ad anello con la guida d’onda bus è stato realizzato mediante un interferometro di Mach-Zehnder (MZI). Grazie alle capacità di strutturazione tridimensionale di questa tecnica, è stato possibile produrre un dispositivo compatto composto da 20 risonatori ad anello di grande area disposti concentricamente, con raggi fino a 21 mm. Diverse velocità di scansione, pari a 5 e 7 mm/s, sono state impiegate per scrivere le guide d’onda con il laser a femtosecondi, mantenendo costante l’energia di impulso a 500 nJ. Inoltre, tutti i risonatori sono stati dotati di modulatore di fase termo-ottico (TOPS) per consentire un controllo attivo dell’accoppiamento tra bus e anello. È stata condotta una caratterizzazione iniziale degli spettri di trasmissione per valutare le perdite, l’intervallo spettrale libero (FSR) e il fattore di qualità (Q-factor) in funzione del raggio. Il risonatore con raggio di 17.4 mm, fabbricato a 7 mm/s, ha mostrato le perdite minori e il più alto Q-factor, ed è stato pertanto selezionato per un’analisi più approfondita. Una caratterizzazione più dettagliata del suo spettro di trasmissione è stata effettuata per ottenere valori precisi delle figure di merito rilevanti. La stabilità dello spettro di trasmissione è stata infine valutata monitorando l’intensità ottica nel punto di mezza trasmissione (cioè il punto di massima sensibilità), in condizioni di accoppiamento critico. Infine, è stata dimostrata la capacità di spostare attivamente e in modo controllato i picchi dello spettro di trasmissione, variando il termine di fase comune del MZI.
Wide-area ring resonators fabricated by femtoscecond laser micromachining
SARETTO, FABIO
2024/2025
Abstract
Ring resonators serve as essential components in integrated optics, offering resonant optical cavities with compact dimensions and exceptional wavelength selectivity. These structures have caught significant attention due to their utility in a broad area of applications, including signal filtering, optical switching, and modulation. Additionally, ring resonators constitute a fundamental element in micro-optical gyroscopes (MOGs), that measure angular velocity. Within the domain of optical gyroscopes, MOGs present a promising avenue for advancing the performance of compact navigation systems. In this scenario, the present work aims to characterise ring resonators fabricated for the first time using femtosecond laser micromachining (FLM), with the objective of estimating the key parameters that determine their performance in optical filtering and gyroscope applications. In conventional silicon-based platforms, various ring resonator layouts have been developed and fabricated using lithographic techniques. In any case, the fundamental configuration consists of a bus waveguide coupled with a ring. This basic architecture was adopted in the present study to fabricate ring resonators via femtosecond laser micromachining; coupling of the ring resonator to the bus waveguide was implemented by a Mach-Zehnder interferometer (MZI). Owing to the three-dimensional structuring capabilities of this technique, it was possible to produce a compact device composed of 20 large-area ring resonators arranged concentrically, with radii up to 21 mm. Different scan speeds of 5 and 7 mm/s were employed to inscribe the waveguides with the femtosecond laser, while maintaining a constant pulse energy of 500 nJ. Additionally, all resonators were equipped with thermo-optic phase shifters (TOPS) to enable active control of the bus-ring coupling. An initial characterisation of the transmission spectra was conducted to evaluate losses, free spectral range (FSR), and quality factor (Q-factor) as a function of the radius. The resonator with a radius of 17.4 mm, fabricated at 7 mm/s, exhibited the lowest losses and highest Q-factor, and was therefore selected for further analysis. A more detailed characterisation of its transmission spectrum was performed to obtain precise values for the relevant figures of merit. The stability of the transmission spectrum was finally assessed by monitoring the optical intensity at the half-transmission point (i.e., the point of maximum sensitivity), under critical coupling conditions. Finally, we demonstrated the ability to actively and controlledly shift the dips of the transmission spectrum by varying the common phase term of the MZI.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/239855