C. Ferrari

Tunable Delay Lines in Silicon Photonics: Coupled Resonators and Photonic Crystals, a Comparison

In this paper, we report a direct comparison between coupled resonator optical waveguides (CROWs) and photonic crystal waveguides (PhCWs), which have both been exploited as tunable delay lines. The two structures were fabricated on the same silicon-on-insulator (SOI) technological platform, with the same fabrication facilities and evaluated under the same signal bit-rate conditions. We compare the frequency- and time-domain response of the two structures; the physical mechanism underlying the tuning of the delay; the main limits induced by loss, dispersion, and structural disorder; and the impact of CROW and PhCW tunable delay lines on the transmission of data stream intensity and phase modulated up to 100 Gb/s. The main result of this study is that, in the considered domain of applications, CROWs and PhCWs behave much more similarly than one would expect. At data rates around 100 Gb/s, CROWs and PhCWs can be placed in competition. Lower data rates, where longer absolute delays are required and propagation loss becomes a critical issue, are the preferred domain of CROWs fabricated with large ring resonators, while at data rates in the terabit range, PhCWs remain the leading technology.

Continuously tunable 1 byte delay in coupled-resonator optical waveguides

A reconfigurable coupled-resonator optical waveguide made of a few directly coupled ring resonators is employed to control the delay of data streams modulated at tens of gigabits per second. A delay of 8 bit lengths (1 optical byte) with a small pulse broadening and 1 dB/bit fractional loss is achieved by using only eight rings. The limiting role of waveguide loss and spurious backreflections is experimentally investigated. The high storage efficiency (1 bit/ring) of the device enables an easy, reliable, hitless, and relatively low-power-consuming management of the delay. A higher storage efficiency is demonstrated to be associated to an unavoidable higher pulse distortion.

A reconfigurable architecture for continuously variable optical slow-wave delay lines

A novel reconfigurable architecture based on slow-wave propagation in integrated optical ring resonators is proposed for the realization of variable optical delay lines. A continuously variable delay is achieved by combining a coarse discrete (digital) delay, provided by a coupled resonator slow-wave structure, with a fine continuous (analog) delay given by a cascaded ring- resonator phase-shifter. The reflective configuration of the structure enables a simple, accurate and robust tuning of the delay and provides a footprint reduction by a factor 2 with respect to conventional coupled resonator optical waveguides. Proof-of-concept devices realized in 4.4% silicon oxynitride waveguides and activated by a thermal control are discussed. Experimental results demonstrate, in both spectral and time domain, a continuously variable delay, from zero to 800 ps (2 bit fractional delay), on a 2.5 Gbit/s NRZ signal, with less than 8 dB insertion loss and less than 5 mm2 device footprint.

Travelling-wave resonant four-wave mixing breaks the limits of cavity-enhanced all-optical wavelength conversion

Wave mixing inside optical resonators, while experiencing a large enhancement of the nonlinear interaction efficiency, suffers from strong bandwidth constraints, preventing its practical exploitation for processing broad-band signals. Here we show that such limits are overcome by the new concept of travelling-wave resonant four-wave mixing (FWM). This approach combines the efficiency enhancement provided by resonant propagation with a wide-band conversion process. Compared with conventional FWM in bare waveguides, it exhibits higher robustness against chromatic dispersion and propagation loss, while preserving transparency to modulation formats. Travelling-wave resonant FWM has been demonstrated in silicon-coupled ring resonators and was exploited to realize a 630-μm-long wavelength converter operating over a wavelength range wider than 60 nm and with 28-dB gain with respect to a bare waveguide of the same physical length. Full compatibility of the travelling-wave resonant FWM with optical signal processing applications has been demonstrated through signal retiming and reshaping at 10 Gb s−1

Error-free continuously-tunable delay at 10 Gbit/s in a reconfigurable on-chip delay-line

A coupled-resonator optical waveguide (CROW) consisting of a chain of directly coupled ring-resonators (RRs) fabricated in 4.5%-indexcontrast silicon oxynitride technology is employed to control the delay of optical pulses with continuity and over several bit-slots. The moderate deterioration of the signal quality versus the delay is demonstrated by the observation of error-free transmission (BER < 10(-9)) at 10 Gbit/s for fractional delays of up to 3 bits, with fractional losses below 1 dB per bit-delay. The high storage efficiency of the device, exceeding 0.5 bit/RR, enables an easy management of the delay and the reduction of the footprint down to 7 mm(2). The presented reconfiguration scheme is hitless with respect to data transmission, since the CROW delay can be tuned without halting the data flow, while preserving the signal quality.

Silicon coupled-ring resonator structures for slow light applications: potential, impairments and ultimate limits

Coupled-ring resonator-based slow light structures are reported and discussed. By combining the advantages of high index contrast silicon-on-insulator technology with an efficient thermo-optical activation, they provide an on-chip solution with a bandwidth of up to 100 GHz and a slowdown factor of up to 16, as well as a continuous reconfiguration scheme and a fine tunability. The performance of these devices is investigated in detail for both static and dynamic operation, in order to evaluate their potential in optical signal processing applications at high bit rate. The main impairments imposed by fabrication imperfections are also discussed in relation to the slowdown factor. In particular, the analysis of the impact of backscatter, disorder and two-photon absorption on the device transfer function reveals the ultimate limits of these structures and provides valuable design rules for their optimization.

Strain and surface damage induced by proton exchange in Y‐cut LiNbO3

When Y‐cut LiNbO3 substrates are proton exchanged in pure benzoic acid to fabricate optical waveguides, they suffer surface damage, and a consequent degradation in optical properties. This effect is mainly produced by a remarkably large strain in the exchanged layer in a direction normal to the surface. This strain leads to a large number of cracks and to the peeling off of the exchanged layer itself. This paper presents a probable explanation of the mechanism involved.

Reconfigurable silicon filter with continuous bandwidth tunability

We present the design and the fabrication of compact tunable silicon-on-insulator bandpass filters based on the integration of a Mach-Zehnder interferometer with ring resonators and activated via thermo-optic phase shifters. The proposed architecture provides wide filter bandwidth tunability from 10% to 90% of the free spectral range preserving the filter off-band rejection. Possible applications are channel subset selection in wavelength division multiplexing optical systems, adaptive filtering to signal bandwidth, and reconfigurable filters for gridless networking.

Photo-induced trimming of coupled ring-resonator filters and delay lines in As2S3 chalcogenide glass

Selective exposure to visible light is used to permanently trim the resonant wavelengths of coupled ring-resonator filters and delay-lines realized on a chalcogenide As2S3 platform. Post-fabrication manipulation of the circuit parameters has proved an effective tool to compensate for technological tolerances, targeting demanding specifications in photonic integrated circuits with no need for always-on power-hungry actuators. The same approach opens a way to realize photonic integrated circuits that can be reconfigured after fabrication to fulfill specific applications.

Disorder in coupled-resonator optical waveguides

Disorder in coupled-resonator optical waveguides (CROWs) is modeled by exploiting the concept of the characteristic impedance of a periodic slow-light waveguide. Every imperfection in the CROW structure is modeled as an impedance discontinuity, and the related backreflection is evaluated by using well-known reflection rules. We demonstrate that backreflections induced by disorder scale with the square of the slowing factor and the square of the disorder parameter, both independently of the specific structure. The method is simple and accurate, holds even when the slowing factor of the CROW is modified by disorder, and can be applied to any slow-light structure where the characteristic impedance can be defined. Theoretical and numerical results are supported by an experimental investigation showing the effects of increasing disorder on both frequency and time domain responses of a ring resonator CROW. Pulse envelope distortions due to distributed backreflections along the disordered CROW arise as one of the main limiting factors for applications based on CROWs.