Andrea Annoni

Non-invasive monitoring and control in silicon photonics using CMOS integrated electronics

As photonics moves from the single-device level toward large-scale, integrated, and complex systems on a chip, monitoring, control, and stabilization of the components become critical. We need to monitor a circuit non-invasively and apply a simple, fast, and robust feedback control. Here, we show non-invasive monitoring and feedback control of high-quality-factor silicon (Si) photonic resonators assisted by a transparent detector that is directly integrated inside the cavity. Control operations are entirely managed by a CMOS microelectronic circuit that is bridged to the Si photonic chip and hosts many parallel electronic readout channels. Advanced functionalities, such as wavelength tuning, locking, labeling, and swapping, are demonstrated. The non-invasive nature of the transparent monitor and the scalability of the CMOS readout system offer a viable solution for the control of arbitrarily reconfigurable photonic integrated circuits aggregating many components on a single chip.

Automated Routing and Control of Silicon Photonic Switch Fabrics

Automatic reconfiguration and feedback controlled routing is demonstrated in an 8 × 8 silicon photonic switch fabric based on Mach-Zehnder interferometers. The use of noninvasive contactless integrated photonic probes (CLIPPs) enables real-time monitoring of the state of each switching element individually. Local monitoring provides direct information on the routing path, allowing an easy sequential tuning and feedback controlled stabilization of the individual switching elements, thus making the switch fabric robust against thermal crosstalk, even in the absence of a cooling system for the silicon chip. Up to 24 CLIPPs are interrogated by a multichannel integrated ASIC wire bonded to the photonic chip. Optical routing is demonstrated on simultaneous WDM input signals that are labeled directly on-chip by suitable pilot tones without affecting the quality of the signals. Neither preliminary circuit calibration nor lookup tables are required, being the proposed control scheme inherently insensible to channels power fluctuations.

BER Evaluation of a Passive SOI WDM Router

The performance of a ring-resonator based passive wavelength router, suitable for optical networking at chip level, is evaluated through bit error rate measurements in single channel 10 Gb/s and 3-channel 10 Gb/s WDM configurations. As the routing path involves a different number of routing elements, depending on the wavelength used for the propagating signal, the performance of three output ports with respect to a specified input one are experimentally evaluated and discussed. Measurements show that low rejection on the filter elements due to fabrication issues mostly affect the channel on the through path with respect to the dropped ones.

High-Sensitivity In-Band OSNR Monitoring System Integrated on a Silicon Photonics Chip

We report on a compact ( footprint) silicon photonics integrated system performing high-sensitivity monitoring of in-band optical signal-to-noise ratio (OSNR). The system, including a thermally tunable racetrack resonator filter and an unbalanced Mach-Zehnder interferometer, performs the autocorrelation measurement of a filtered fraction of the noisy signal spectrum. Monitor performance is evaluated on a 10 Gb/s ON/OFF keying nonreturn to zero signal, demonstrating an accuracy of 0.4 dB over a wide OSNR range from 8 to 28 dB. We also demonstrate that the proposed system induces a tiny distortion of the optical signal, making it suitable for in-line monitoring of signal quality.

Non-Invasive Monitoring of Mode-Division Multiplexed Channels on a Silicon Photonic Chip

We demonstrate on-chip non-invasive monitoring of orthogonal modes transmitted in a silicon photonic waveguide. The proposed technique exploits a recently developed ContactLess Integrated Photonic Probe (CLIPP) realizing a fully transparent integrated light detector. The optical intensity of the modes propagating in the waveguide is tracked in time by the CLIPP, with no signal quality degradation induced by monitoring operations. We exploit this concept for the simultaneous monitoring of two intensity modulated 10 Gbit/s data channels transmitted at the same wavelength and multiplexed on the fundamental transverse electric and magnetic modes of the silicon waveguide. By labeling each signal with a weakly modulated pilot tone, the CLIPP can discriminate at the same time the two channels, the monitoring of one signal not affecting the readout of the other one. The scalability of the presented technique to several modes on arbitrary polarization states, along with the fabrication simplicity and CMOS compatibility of the CLIPP detector, makes this approach promising for the monitoring and control of integrated components for mode-division multiplexing systems.

Enhancing the Sensitivity of Interferometer Based In-Band OSNR Monitoring by Narrow Band Filtering

We present a method to enhance the sensitivity of interferometer based techniques employed for the in-band measurement of optical signal-to-noise ratio (OSNR) in wavelength division multiplexed (WDM) optical transmission systems. With respect to conventional schemes, a narrow band filter (NBF) is added before the variable delay interferometer performing the autocorrelation measurement. The NBF is used to select within the signal spectrum only those frequencies where the signal power spectral density (PSD) is locally lower and more sensitive to noise effects. Performance monitoring of a 10 Gb/s on-off keying (OOK) non-return to zero (NRZ) signal is numerically and experimentally investigated for an OSNR level ranging from 5 dB to 25 dB. Design criteria are pointed out for the bandwidth and the detuning of the NBF with respect to the signal carrier wavelength, as well as for the delay of the interferometer, in order to maximize the system sensitivity to OSNR variations. Experimental results, achieved by using an integrated ring-resonator NBF, demonstrate that presented technique enables to effectively detect small noise variations at high OSNR levels (> 15 dB), where conventional interferometer based techniques exhibit a poor sensitivity. Moreover we demonstrate that the NBF can operate directly on the transmitted optical signal without introducing any significant perturbation, this making the presented technique suitable for in-line OSNR measurements.

Unscrambling light—automatically undoing strong mixing between modes

Propagation of light beams through scattering or multimode systems may lead to randomization of the spatial coherence of the light. Although information is not lost, its recovery requires a coherent interferometric reconstruction of the original signals, which have been scrambled into the modes of the scattering system. Here, we show that we can automatically unscramble four optical beams that have been arbitrarily mixed in a multimode waveguide, undoing the scattering and mixing between the spatial modes through a mesh of silicon photonics Mach-Zehnder interferometers. Using embedded transparent detectors and a progressive tuning algorithm, the mesh self-configures automatically and reset itself after significantly perturbing the mixing, without turning off the beams. We demonstrate the recovery of four separate 10 Gbits/s information channels, with residual cross-talk between beams of -20dB. This principle of self-configuring and self-resetting in optical systems should be applicable in a wide range of optical applications.Propagation of light beams through scattering or multimode systems may lead to the randomization of the spatial coherence of the light. Although information is not lost, its recovery requires a coherent interferometric reconstruction of the original signals, which have been scrambled into the modes of the scattering system. Here we show that we can automatically unscramble optical beams that have been arbitrarily mixed in a multimode waveguide, undoing the scattering and mixing between the spatial modes through a mesh of silicon photonics tuneable beam splitters. Transparent light detectors integrated in a photonic chip are used to directly monitor the evolution of each mode along the mesh, allowing sequential tuning and adaptive individual feedback control of each beam splitter. The entire mesh self-configures automatically through a progressive tuning algorithm and resets itself after significantly perturbing the mixing, without turning off the beams. We demonstrate information recovery by the simultaneous unscrambling, sorting and tracking of four mixed modes, with residual cross-talk of −20 dB between the beams. Circuit partitioning assisted by transparent detectors enables scalability to meshes with a higher port count and to a higher number of modes without a proportionate increase in the control complexity. The principle of self-configuring and self-resetting in optical systems should be applicable in a wide range of optical applications.

Integrated all-optical MIMO demultiplexer for mode- and wavelength-division-multiplexed transmission

A photonic integrated circuit performing simultaneous mode and wavelength demultiplexing for few-mode-fiber transmission is demonstrated for the first time. The circuit is realized on an InP-based technological platform; it can handle up to eight mode- and wavelength-division-multiplexed (MDM/WDM) channels and allows all-optical multiple-input-multiple-output processing to unscramble mode mixing generated by fiber propagation. A single arrayed waveguide grating is used to demultiplex the WDM channels carried by all the propagating modes, optimizing circuit complexity, chip area, and operational stability. Combined with an integrated wideband mode multiplexer the circuit is successfully exploited for the transmission of 10  Gbit/s on-off-keying non-return-to-zero channels with a residual cross talk of about -15  dB.

Wavelength Locking of Silicon Photonics Multiplexer for DML-Based WDM Transmitter

We present a wavelength locking platform enabling the feedback control of silicon (Si) microring resonators (MRRs) for the realization of a 4 × 10 Gb/s wavelength-division-multiplexing (WDM) transmitter. Four thermally tunable Si MRRs are employed to multiplex the signals generated by four directly modulated lasers (DMLs) operating in the L-band, as well as to improve the quality of the DMLs signals. Feedback control is achieved through a field-programmable gate array controller by monitoring the working point of each MRR through a transparent detector integrated inside the resonator. The feedback system provides an MRR wavelength stability of about 4 pm (0.5 GHz) with a time response of 60 ms. Bit error rate (BER) measurements confirm the effectiveness and the robustness of the locking system to counteract sensitivity degradations due to thermal drifts, even under uncooled operation conditions for the Si chip.

Design Guidelines for Contactless Integrated Photonic Probes in Dense Photonic Circuits

This paper provides detailed guidelines for the optimal design of contactless integrated photonic probes suitable to track and control the local optical power in photonic circuits. With reference to current technology platforms, this paper provides a guide to extract the electrical parameters of the probe and to highlight their role in defining the achievable resolution. Crucial technological and geometrical choices are discussed, together with layout and interconnection solutions oriented to a highly dense integration of the probes. Finally, the criteria for the optimal coupling of the probes to the most suitable readout electronics providing the maximization of the SNR are presented. With these guidelines in mind, transparent in-line local power monitors featuring –35 dBm sensitivity, 40 dB of dynamic range, broadband response from 1.3 to 1.6 μm, a speed down to tens of microsecond and a minimum size of tens of micrometer can be effectively designed for high performance reconfiguration and closed-loop control of complex photonic circuits.