Efficacy and Safety of Endovascular Fenestrated and Branched Grafts Versus Open Surgery in Thoracoabdominal Aortic Aneurysm Repair: An Updated Systematic Review, Meta-analysis, and Meta-regression

OBJECTIVE

To provide an updated systematic review and meta-analysis with meta-regression of efficacy and safety of fenestrated/branched endovascular repair of thoracoabdominal aortic aneurysms (TAAAs) compared with open repair.

BACKGROUND

Endovascular repair of TAAAs may be a promising alternative to open surgery by reducing invasiveness and expanding the eligible population, but evidence remains limited.

METHODS

We applied "Prepared Items for Systematic Reviews and Meta-analysis" guidelines to retrieve, quantitatively pool, and critically evaluate the efficacy and safety (including 30-day mortality, reintervention, spinal cord injury [SCI], and renal injury) of both approaches. Original studies were retrieved from PubMed, Embase, and Cochrane Library until April 20, 2022, excluding papers reporting <10 patients. Pooled proportions and means were determined using a random-effect model. Heterogeneity between studies was evaluated with I2 statistics.

RESULTS

Sixty-four studies met the predefined inclusion criteria. Endovascular cohort patients were older and had higher rates of comorbidities. Endovascular repair was associated with similar proportions of mortality (0.07, 95% confidence intervals [CI]: 0.06-0.08) compared with open repair (0.09, 95% CI: 0.08-0.12; P = 0.22), higher proportions of reintervention (0.19, 95% CI: 0.13-0.26 vs 0.06, 95% CI: 0.04-0.10; P < 0.01), similar proportions of transient SCI (0.07, 95% CI: 0.05-0.09 vs 0.06, 95% CI: 0.05-0.08; P = 0.28), lower proportions of permanent SCI (0.04, 95% CI: 0.03-0.05 vs 0.06, 95% CI: 0.05-0.07; P < 0.01), and renal injury (0.08, 95% CI: 0.06-0.10 vs 0.13, 95% CI: 0.09-0.17; P = 0.02). Results were affected by high heterogeneity and potential publication bias.

CONCLUSIONS

Despite these limitations and the lack of randomized trials, this meta-analysis suggests that endovascular TAAA repair could be a safer alternative to the open approach.

Plug-Based Embolization Techniques of Aortic Side Branches during Standard and Complex Endovascular Aortic Repair

Vascular plugs are an evolving family of vessel occluders providing a single-device embolization system for large, high-flow arteries. Nitinol mesh plugs and polytetrafluoroethylene membrane plugs are available in different configurations and sizes to occlude arteries from 3 to 20 mm in diameter. Possible applications during complex endovascular aortic procedures are aortic branch embolization to prevent endoleak or to gain an adequate landing zone, directional branch occlusion, and false lumen embolization in aortic dissection. Plugs are delivered through catheters or introducers, and their technical and clinical results are comparable to those of coil embolization. Plugs are more accurate than coils as repositionable devices, less prone to migration, and have fewer blooming artifacts on postoperative computed tomography imaging. Their main drawback is the need for larger delivery systems. This narrative review describes up-to-date techniques and technology for plug embolization in complex aortic repair.

Outcomes of Percutaneous Access to the First Versus Third Segment of Axillary Artery During Aortic Procedures

PURPOSE

This article aims at investigating the outcomes of percutaneous access via the first versus third axillary artery (AXA) segments with closure devices during aortic procedures.

MATERIALS AND METHODS

All patients receiving percutaneous AXA access closed with Perclose ProGlide device (Abbott, Santa Clara, California) from 2008 to 2021 were included in a retrospective multicenter registry (NCT: 04589962). Efficacy endpoint was the technically successful percutaneous procedure (no open conversion). Safety endpoints were stroke and access complications according to the Valve Academic Research Consortium-3 reporting standards. The first (AXA1) or third (AXA3) axillary puncture sites were compared.

RESULTS

A total of 412 percutaneous AXA accesses were included: 172 (42%) in AXA1 and 240 (58%) in AXA3. Left AXA was catheterized in 363 cases (76% of AXA1 vs 97% of AXA3, p<0.001) and 91% of fenestrated/branched endovascular repair (F/BEVAR) procedures were conducted from the left. A ≥12F internal diameter (ID) sheath was used in 49% of procedures. Open conversion rate was 1%, no major vascular complications occurred, and only one major non-vascular complication was recorded. Primary closure failure occurred in 18 AXA1 (11%) and 32 AXA3 accesses (13%), treated by covered (8.3%) or bare-metal (2.7%) stenting. Bailout stent patency was 100% at median follow-up of 12 months, with 6 of 6 stents still patent after >36 months of follow-up. Stroke rate was 4.4%. An introducer sheath >12F was independently associated with both access complications (p<0.001) and stroke (p=0.005), while a right-side access was associated with stroke only (p=0.034). Even after adjustment for covariates, AXA1 versus AXA3 showed an equal success rate (odds ratio [OR]=0.537, 95% confidence interval [CI]=0.011-1.22 for AXA3, p=0.104). The combination of AXA3 and a >10F introducer sheath provided worse outcomes compared with >10F sheaths through AXA1 (OR for success=0.367, 95% CI=0.176-0.767, p=0.008). This was not confirmed for >12F sheaths, associated with similar outcomes (p=0.31 AXA 1 vs AXA 3).

CONCLUSION

Major local complications with the percutaneous axillary approach and ≤12F sheaths are infrequent and solvable by complementary endovascular interventions. Stroke risk remains an issue. First and third AXA segments are both amenable for access with good results, but larger sheaths (12F) perform better in AXA1.

CLINICAL IMPACT

Percutaneous access with vascular closure devices at the first or third axillary artery (AXA) segments during aortic procedures is burdened by a negligible risk of open conversion. Local complications with the percutaneous axillary approach are infrequent and solvable by complementary endovascular interventions. First and third AXA segments are both amenable to access with excellent results, but larger sheaths (12F) perform better in the wider first AXA segment. In this setting, bailout stenting does not appear to be associated with mid-term stent occlusion.

Emergent endovascular treatment options for thoracoabdominal aortic aneurysm

For a long time, parallel grafting, physician-modified endografts, and, more recently, in situ fenestration were the only go-to endovascular options for ruptured thoracoabdominal aortic aneurysm, offered mixed results, and depended mainly on the operator's and center's experience. As custom-made devices have become an established endovascular treatment option for elective thoracoabdominal aortic aneurysm, they are not a viable option in the emergency setting, as endograft production can take up to 4 months. The development of off-the-shelf (OTS) multibranched devices with a standardized configuration has allowed the treatment of ruptured thoracoabdominal aortic aneurysm with emergent branched endovascular procedures. The Zenith t-Branch device (Cook Medical) was the first readily available graft outside the United States to receive the CE mark (in 2012) and is currently the most studied device for those indications. A new device, the E-nside thoracoabdominal branch endoprosthesis OTS multibranched endograft (Artivion), has been made commercially available, and the GORE EXCLUDER thoracoabdominal branch endoprosthesis OTS multibranched endograft (W. L. Gore and Associates) is expected to be released in 2023. Due to the lack of guidelines on ruptured thoracoabdominal aortic aneurysm, this review summarizes the available treatment options (ie, parallel grafts, physician-modified endografts, in situ fenestrations, and OTS multibranched devices), compares the indications and contraindications, and points out the evidence gaps that should be filled in the next decade.

Multicentre International Registry of Open Surgical Versus Percutaneous Upper Extremity Access During Endovascular Aortic Procedures

OBJECTIVE

To investigate access failure (AF) and stroke rates of aortic procedures performed with upper extremity access (UEA), and compare results of open surgical vs. percutaneous UEA techniques with closure devices.

METHODS

A physician initiated, multicentre, ambispective, observational registry (SUPERAXA - NCT04589962) was carried out of patients undergoing aortic procedures requiring UEA, including transcatheter aortic valve replacement, aortic arch, and thoraco-abdominal aortic endovascular repair, pararenal parallel grafts, renovisceral and iliac vessel repair. Only vascular procedures performed with an open surgical or percutaneous (with a suture mediated vessel closure device) UEA were analysed. Risk factors and endpoints were classified according to the Society for Vascular Surgery and VARC-3 (Valve Academic Research Consortium) reporting standards. A logistic regression model was used to identify AF and stroke risk predictors, and propensity matching was employed to compare the UEA closure techniques.

RESULTS

Sixteen centres registered 1 098 patients (806 men [73.4%]; median age 74 years, interquartile range 69 - 79 years) undergoing vascular procedures using open surgical (76%) or percutaneous (24%) UEA. Overall AF and stroke rates were 6.8% and 3.0%, respectively. Independent predictors of AF by multivariable analysis included pacemaker ipsilateral to the access (odds ratio [OR] 3.8, 95% confidence interval [CI] 1.2 - 12.1; p = .026), branched and fenestrated procedure (OR 3.4, 95% CI 1.2 - 9.6; p = .019) and introducer internal diameter ≥ 14 F (OR 6.6, 95% CI 2.1 - 20.7; p = .001). Stroke was associated with female sex (OR 3.4, 95% CI 1.3 - 9.0; p = .013), vessel diameter > 7 mm (OR 3.9, 95% CI 1.1 - 13.8; p = .037), and aortic arch procedure (OR 7.3, 95% CI 1.7 - 31.1; p = .007). After 1:1 propensity matching, there was no difference between open surgical and percutaneous cohorts. However, a statistically significantly higher number of adjunctive endovascular procedures was recorded in the percutaneous cohort (p < .001).

CONCLUSION

AF and stroke rates during complex aortic procedures employing UEA are non-negligible. Therefore, selective use of UEA is warranted. Percutaneous access with vessel closure devices is associated with similar complication rates, but more adjunctive endovascular procedures are required to avoid surgical exposure.

Experimentally realized in situ backpropagation for deep learning in photonic neural networks

Integrated photonic neural networks provide a promising platform for energy-efficient, high-throughput machine learning with extensive scientific and commercial applications. Photonic neural networks efficiently transform optically encoded inputs using Mach-Zehnder interferometer mesh networks interleaved with nonlinearities. We experimentally trained a three-layer, four-port silicon photonic neural network with programmable phase shifters and optical power monitoring to solve classification tasks using "in situ backpropagation," a photonic analog of the most popular method to train conventional neural networks. We measured backpropagated gradients for phase-shifter voltages by interfering forward- and backward-propagating light and simulated in situ backpropagation for 64-port photonic neural networks trained on MNIST image recognition given errors. All experiments performed comparably to digital simulations ([Formula: see text]94% test accuracy), and energy scaling analysis indicated a route to scalable machine learning.

In-hospital cost-effectiveness analysis of open versus staged fenestrated/branched endovascular elective repair of thoracoabdominal aneurysms

OBJECTIVE

To compare costs and effectiveness of elective open (OR) versus fenestrated/branched endovascular (ER) repair of thoracoabdominal aneurysms (TAAA) in a high-volume center.

METHODS

This single-center retrospective observational study (PRO-ENDO TAAA Study, NCT05266781) was designed as part of a larger Health Technology Assessment analysis. All electively treated TAAAs between 2013 and 2021 were analyzed and propensity-matched. Endpoints were clinical success, major adverse events (MAE), hospital direct costs, and freedom from all causes and aneurysm-related mortality and reinterventions. Risk factors and outcomes were homogeneously classified according to the Society of Vascular Surgery reporting standards. Cost-effectiveness value (CEV) and Incremental Cost-Effectiveness Ratio (ICER) were calculated, considering the absence of MAEs as a measure of effectiveness.

RESULTS

Propensity matching identified 102 pairs of patients out of 789 TAAAs. Mortality, MAE, permanent spinal cord ischemia rates, respiratory complications, cardiac complications, and renal injury were higher for OR (13% vs 5%, p=.048; 60% vs 17%, p<.001; 10% vs 3%, p=.045; 91% vs 18%, p<.001; 16% vs 6%, p=.024; 27% vs 6%, p<.001; respectively). Access complication rate (6% vs 27%; p<.001) was higher in the ER group. Intensive Care Unit stay was longer (p<.001) for OR and ER patients were discharged home more frequently (3% vs 94%; p<.001). No differences in mid-term endpoints were observed at 2 years. Despite ER reducing all the hospital cost items (-42% to -88%, p<.001), the higher expenses (p<.001) of the endovascular devices increased the overall cost of ER by 80%. CEV for ER was favorable to OR (56 365 vs 64 903 €/patient) with an ICER of 48 409 € per MAE saved.

CONCLUSIONS

ER of TAAA reduces perioperative mortality and morbidity compared to OR, with no differences in reinterventions and survival rates at midterm follow-up. Despite the expenses for endovascular grafts, ER resulted more cost-effective in preventing MAEs.

Variable optical true-time delay line breaking bandwidth-delay constraints

Continuously variable true-time optical delay lines are typically subject to a constraint of the bandwidth-delay product, limiting their use in several applications. In this Letter, we propose an integrated topology that breaks the bandwidth-delay product limit. The device is based on multiple Mach-Zehnder Interferometers (MZIs) arranged in parallel, providing easier control and a larger bandwidth compared to ring resonator-based solutions. The functionality of this architecture is demonstrated with a 4-stage delay line by performing measurements in both the time and frequency domains. The delay line introduces a delay of 90 ps over a bandwidth of more than 22 GHz with a negligible group delay distortion, operates on a wavelength range of about 60 nm, and is scalable to a higher number of MZI stages.

Efficacy of a Drug-Eluting Stent Versus Bare Metal Stents for Symptomatic Femoropopliteal Peripheral Artery Disease: Primary Results of the EMINENT Randomized Trial

BACKGROUND

A clear patency benefit of a drug-eluting stent (DES) over bare metal stents (BMSs) for treating peripheral artery disease of the femoropopliteal segment has not been definitively demonstrated. The EMINENT study (Trial Comparing Eluvia Versus Bare Metal Stent in Treatment of Superficial Femoral and/or Proximal Popliteal Artery) was designed to evaluate the patency of the Eluvia DES (Boston Scientific, Marlborough, MA), a polymer-coated paclitaxel-eluting stent, compared with BMSs for the treatment of femoropopliteal artery lesions.

METHODS

EMINENT is a prospective, randomized, controlled, multicenter European study with blinded participants and outcome assessment. Patients with symptomatic peripheral artery disease (Rutherford category 2, 3, or 4) of the native superficial femoral artery or proximal popliteal artery with stenosis ≥70%, vessel diameter of 4 to 6 mm, and total lesion length of 30 to 210 mm were randomly assigned 2:1 to treatment with DES or BMS. The primary effectiveness outcome was primary patency at 12 months, defined as independent core laboratory-assessed duplex ultrasound peak systolic velocity ratio ≤2.4 in the absence of clinically driven target lesion revascularization or surgical bypass of the target lesion. Primary sustained clinical improvement was a secondary outcome defined as a decrease in Rutherford classification of ≥1 categories compared with baseline without a repeat target lesion revascularization. Health-related quality of life and walking function were assessed.

RESULTS

A total of 775 patients were randomly assigned to treatment with DES (n=508) or commercially available BMSs (n=267). Baseline clinical, demographic, and lesion characteristics were similar between the study groups. Mean lesion length was 75.6±50.3 and 72.2±47.0 mm in the DES and BMS groups, respectively. The 12-month incidence of primary patency for DES treatment (83.2% [337 of 405]) was significantly greater than for BMS (74.3% [165 of 222]; P<0.01). Incidence of primary sustained clinical improvement was greater among patients treated with the DES than among those who received a BMS (83.0% versus 76.6%; P=0.045). The health-related quality of life dimensions of mobility and pain/discomfort improved for the majority of patients in both groups (for 66.4% and 53.6% of DES-treated and for 64.2% and 58.1% of BMS-treated patients, respectively) but did not differ significantly. At 12 months, no statistical difference was observed in all-cause mortality between patients treated with the DES or BMS (2.7% [13 of 474] versus 1.1% [3 of 263]; relative risk, 2.4 [95% CI, 0.69-8.36]; P=0.15).

CONCLUSIONS

By demonstrating superior 1-year primary patency, the results of the EMINENT randomized study support the benefit of using a polymer-based paclitaxel-eluting stent as a first-line stent-based intervention for patients with symptomatic peripheral artery disease attributable to femoropopliteal lesions.

REGISTRATION

URL: https://www.

CLINICALTRIALS

gov; Unique identifier: NCT02921230.

A scoping review on the approaches for cannulation of reno-visceral target vessels during complex endovascular aortic repair

OBJECTIVE

To assess the approaches to reno-visceral target vessels (TV) cannulation during complex endovascular aortic repair (B/FEVAR), determine the evidence base that links these approaches to clinical outcomes, and identify literature gaps.

METHODS

A scoping review following the PRISMA Protocols Extension for Scoping Reviews was performed. Available full-text studies published in English (PubMed, Cochrane and EMBASE databases; last queried, 31 June 2022) were systematically reviewed and analyzed. Data were reported as descriptive narrative or tables, without any statistical analysis nor quality assessment.

RESULTS

Fourteen retrospective articles were included. Seven articles studied the use of upper extremity access (UEA) during B/FEVAR, three the use of steerable sheaths, and four included both approaches. A left UEA was used in 757 patients (technical success: 99%, stroke rate: 1-3%) and a right UEA in 215 patients (technical success: 92-98%, stroke rate: 0-13%). Seven studies (1066 patients) described a surgical access only (technical success: 80-99%, stroke rate: 0-13%), while three studies (146 patients) described a percutaneous access only (technical success: 83-90%, stroke rate: 3%), and lastly four studies compared UEA versus use of steerable sheaths from the transfemoral approach (TFA) (UEA: 563 patients, technical success: 95-98%, stroke rate: 1-8%; TFA: 209 patients, technical success: 98-100%, stroke rate: 0-1%).

CONCLUSIONS

Both UEA and TFA as cannulation approaches were associated with high technical success and low peri-operative complications. Currently, there is a paucity of high-quality data to provide definitive indication. Optimal UEA in terms of side (left vs right) and approach (surgical vs percutaneous) needs further study.

Open Thoracoabdominal Aortic Procedures following Endovascular Intervention

Open conversion of thoracoabdominal aortic (TAA) disease after failed attempts of endovascular treatment is increasingly required. The main causes are endoleak, endograft failure, infection, disease progression, or persistent false lumen perfusion in dissected aortas. Mortality and morbidity rates are high, higher than after standard TAA open repair. Therefore, this surgery should be performed only in dedicated centers by experienced teams. Specific perioperative organ protection protocols, as well as surgical techniques, are crucial to guarantee acceptable results.

Temperature and wavelength drift tolerant WDM transmission and routing in on-chip silicon photonic interconnects

We demonstrate a temperature and wavelength shift resilient silicon transmission and routing interconnect system suitable for multi-socket interconnects, utilizing a dual-strategy CLIPP feedback circuitry that safeguards the operating point of the constituent photonic building blocks along the entire on-chip transmission-multiplexing-routing chain. The control circuit leverages a novel control power-independent and calibration-free locking strategy that exploits the 2^nd derivative of ring resonator modulators (RMs) transfer function to lock them close to the point of minimum transmission penalty. The system performance was evaluated on an integrated Silicon Photonics 2-socket demonstrator, enforcing control over a chain of RM-MUX-AWGR resonant structures and stressed against thermal and wavelength shift perturbations. The thermal and wavelength stress tests ranged from 27°C to 36°C and 1309.90 nm to 1310.85 nm and revealed average eye diagrams Q-factor values of 5.8 and 5.9 respectively, validating the system robustness to unstable environments and fabrication variations.

Separating arbitrary free-space beams with an integrated photonic processor

Free-space optics naturally offers multiple-channel communications and sensing exploitable in many applications. The different optical beams will, however, generally be overlapping at the receiver, and, especially with atmospheric turbulence or other scattering or aberrations, the arriving beam shapes may not even be known in advance. We show that such beams can be still separated in the optical domain, and simultaneously detected with negligible cross-talk, even if they share the same wavelength and polarization, and even with unknown arriving beam shapes. The kernel of the adaptive multibeam receiver presented in this work is a programmable integrated photonic processor that is coupled to free-space beams through a two-dimensional array of optical antennas. We demonstrate separation of beam pairs arriving from different directions, with overlapping spatial modes in the same direction, and even with mixing between the beams deliberately added in the path. With the circuit's optical bandwidth of more than 40 nm, this approach offers an enabling technology for the evolution of FSO from single-beam to multibeam space-division multiplexed systems in a perturbed environment, which has been a game-changing transition in fiber-optic systems.

Fenestrated-branched endovascular repair for distal thoraco-abdominal aortic pathology after total aortic arch replacement with frozen elephant trunk

OBJECTIVE

To report the outcomes of fenestrated-branched endovascular repair (FBEVAR) for thoracoabdominal aortic pathology after total aortic arch replacement with frozen elephant trunk (TAR+FET).

METHODS

Interrogation of prospectively-maintained databases from four high volume aortic centres identified consecutive patients treated with distal FBEVAR after prior TAR+FET between August 2013 and September 2020. Primary endpoint was 30-day/in-hospital mortality. Secondary end points were technical success, early clinical success, mid-term survival and freedom from re-intervention. Data are presented as median (IQR).

RESULTS

39 patients [21 men; median age, 73 years (67-75)] with degenerative (n=22) and post-dissection TAAAs (n=17) [median diameter 71 mm (61-78)] were identified. Distal FBEVAR was intended in 27 patients [median interval 9.8 months (6.2-16.6)], anticipated in seven and unexpected in five. 31 patients had a two (n=24) or three (n=7) stage distal FBEVAR. Reno-visceral target vessel preservation was 99.3% (145 of 146). Early primary and secondary technical success was 92% and 97%, respectively. 30-day mortality was 2.6% [n=1; respiratory failure and spinal cord ischaemia (SCI)]. Six survivors also developed SCI which was associated with complete (n=4), or partial recovery (n=2) at hospital discharge. No patients required renal replacement therapy or suffered a stroke. Early clinical success was 95%. Median follow-up was 30.5 months (23.7-49.7). Eleven patients required 16 late re-interventions. Estimated 3-year survival and freedom from re-intervention were 84±6% and 63±10%, respectively.

CONCLUSIONS

Distal FBEVAR after prior TAR+FET is associated with high technical success and low early mortality. The risk of SCI is significant although the majority of patients demonstrate full or partial recovery before hospital discharge. Mid-term patient survival is favourable but there remains a high requirement for late re-intervention. FBEVAR represents an acceptable alternative to distal open TAAA repair.

Amorphous-silicon visible-light detector integrated on silicon nitride waveguides

Visible-light integrated photonics is emerging as a promising technology for the realization of optical devices for applications in sensing, quantum information and communications, imaging, and displays. Among the existing photonic platforms, high-index-contrast silicon nitride (Si₃N₄) waveguides offer broadband transparency in the visible spectral range and a high scale of integration. As the complexity of photonic integrated circuits (PICs) increases, on-chip detectors are required to monitor their working point for reconfiguration and stabilization operations. In this Letter, we present a semi-transparent in-line power monitor integrated on Si₃N₄ waveguides that operates in the red-light wavelength range (660 nm). The proposed device exploits the photoconductivity of a hydrogenated amorphous-silicon (a-Si:H) film that is evanescently coupled to an optical waveguide. Experimental results show a responsivity of 30 mA/W, a sensitivity of -45 dBm, and a sub-µs time response. These features enable the use of the proposed photoconductor for high-sensitivity monitoring and control of visible-light Si₃N₄ PICs.

Active Opto-Magnetic Biosensing with Silicon Microring Resonators

Integrated optical biosensors are gaining increasing attention for their exploitation in lab-on-chip platforms. The standard detection method is based on the measurement of the shift of some optical quantity induced by the immobilization of target molecules at the surface of an integrated optical element upon biomolecular recognition. However, this requires the acquisition of said quantity over the whole hybridization process, which can take hours, during which any external perturbation (e.g., temperature and mechanical instability) can seriously affect the measurement and contribute to a sizeable percentage of invalid tests. Here, we present a different assay concept, named Opto-Magnetic biosensing, allowing us to optically measure off-line (i.e., post hybridization) tiny variations of the effective refractive index seen by microring resonators upon immobilization of magnetic nanoparticles labelling target molecules. Bound magnetic nanoparticles are driven in oscillation by an external AC magnetic field and the corresponding modulation of the microring transfer function, due to the effective refractive index dependence on the position of the particles above the ring, is recorded using a lock-in technique. For a model system of DNA biomolecular recognition we reached a lowest detected concentration on the order of 10 pm, and data analysis shows an expected effective refractive index variation limit of detection of 7.5×10-9 RIU, in a measurement time of just a few seconds.

High-sensitivity transparent photoconductors in voltage-controlled silicon waveguides

On-chip optical power monitors are essential elements to calibrate, stabilize, and reconfigure photonic integrated circuits. Many applications require in-line waveguide detectors, where a trade-off has to be found between large sensitivity and high transparency to the guided light. In this work, we demonstrate a transparent photoconductor integrated on standard low-doped silicon-on-insulator waveguides that reaches a photoconductive gain of more than 10⁶ and an in-line sensitivity as high as -60 dBm. This performance is achieved by compensating the effect of electric charges in the cladding oxide through a bias voltage applied to the chip substrate or locally through a gate electrode on top of the waveguide, allowing one to tune on demand the conductivity of the core to the optimum level.

Spinal Cord Ischemia After Thoracoabdominal Aortic Aneurysms Endovascular Repair: From the Italian Multicenter Fenestrated/Branched Endovascular Aneurysm Repair Registry

PURPOSE

The aim of this study is to report an Italian multicenter experience analyzing the incidence and the risk factors associated with spinal cord ischemia (SCI) in a large cohort of thoracoabdominal aortic aneurysms (TAAAs) treated by fenestrated-branched endovascular aneurysm repair (F-/B-EVAR).

MATERIALS AND METHODS

All consecutive patients undergoing F-/B-EVAR in 4 Italian university centers between 2008 and 2019 were prospectively recorded and retrospectively analyzed. Spinal cord ischemia, 30 day/in-hospital adverse events, and mortality were assessed as early outcomes. Risk factors for SCI were determined by multivariable analysis.

RESULTS

A total of 351 patients received F-/B-EVAR for a TAAA. Twenty-eight (8.0%) patients died within 30 postoperative days or during the hospitalization. Regarding SCI, 47 patients (13.4%) developed neurological symptoms related to spinal cord impaired perfusion. Among them, 17 (4.8%) had a major permanent impairment. The multivariable analysis identified that SCI was associated with Crawford extent I to III (odds ratio [OR]: 20.90, p=0.004, 95% confidence interval [CI]=2.69-162.57), and with endovascular procedures performed for ruptured TAAA (OR: 5.74, p=0.010, 95% CI=1.53-21.57). Spinal cord ischemia was also significantly associated with a grade 3 bleeding during the visceral stage (OR: 4.34, p=0.005, 95% CI=1.55-12.16) and a grade 2 renal insufficiency at 30 days (OR: 7.45, p=0.002, 95% CI=2.12-26.18).

CONCLUSION

The present study indicates that SCI is still an open issue after extent I to III TAAA endovascular repair, while its incidence in extent IV TAAA and pararenal/juxtarenal aneurysms is rare. Thoracoabdominal aortic aneurysms extension, urgent TAAA repair for rupture, severe bleeding, and 30 day renal insufficiency have been identified as significant risk factors for SCI. In the presence of such factors, adjunctive strategies may be considered to reduce SCI rates, while in low-risk patients invasive or potentially-risky maneuvers might not be justified.

Late surgical conversion of failed Multilayer Flow Modulator stenting in thoraco-abdominal aneurysms

OBJECTIVES

The aim of this study was to report the outcomes of open or hybrid repair of failed thoraco-abdominal aortic aneurysm endovascular treatment with Multilayer Flow Modulator (MFM) stents.

METHODS

All patients who underwent open or hybrid repair of a failed MFM aortic treatment were retrospectively analysed. Perioperative and postoperative data, as well as midterm survival, were assessed.

RESULTS

Between 2013 and 2020, 39 patients received an open or hybrid conversion after endovascular treatment. Five of them [13%; 4 males; median age 68 years (interquartile range 66-76)] were previously treated with aortic MFM stents (Cardiatis, Isnes, Belgium). Among these, the median interval between index repair and conversion was 84 months (interquartile range 75-84). The median aneurysm diameter was 9.6 cm (interquartile range 8-10). Renovisceral vessels steno-occlusion was highly prevalent: 2 renal arteries were occluded; 3 coeliac trunks, 2 renal arteries and 1 superior mesenteric artery had a >70% ostial stenosis. Open standard thoraco-abdominal aneurysm conversion was performed in 3 fit patients, while a hybrid approach with visceral debranching and tube endografting was performed in 2 high-risk patients. Two patients (2 open repairs) died intraoperatively, and 1 (hybrid repair) postoperatively. The 2 successfully treated patients are alive at 4- and 34-month follow-up, respectively, with patent visceral branches.

CONCLUSIONS

Open or hybrid thoraco-abdominal aortic aneurysm treatment after failed endovascular aortic repair with MFM stents might be the only surgical option to address sac enlargements and ruptures or branch-related failures. However, both procedures had a poor prognosis due to both the impaired preoperative patient's status and the surgical complexity in the presented series.

Early and mid-term outcomes of open popliteal artery aneurysm repair with prosthetic grafts

OBJECTIVES

The aim of this study is to assess the early and mid-term outcomes of open surgical repair (OR) for popliteal artery aneurysm (PAA) with prosthetic grafts.

MATERIALS AND METHODS

The pre-, intra-, and postoperative data of all patients who underwent PAA OR with prosthetic grafts at our Institution between January 2009 and July 2019 were included in a prospectively maintained database which was retrospectively analysed. Primary patency was defined as uninterrupted flow (<50% stenosis) in the graft with no additional procedures performed. Secondary patency was defined as the restoration of graft patency.

RESULTS

Eighty-two patients underwent OR for 104 PAA (age: 71, 67-78; 82 males) with prosthetic grafts. Seventy-two aneurysms (68%) were asymptomatic. The median diameter was 30mm (24-37). A medial approach was used in 35 aneurysms (34%) while a posterior approach (PA) in 69 (65%). Repairs either consisted of aneurysmectomy or aneurysm ligation without removal with an interposition graft with end-to-end anastomoses. Median operative time was 120 (103-142) minutes. The estimated blood loss (EBL) was 281 (150-281) ml. Only one patient treated by PA sustained a permanent peroneal nerve lesion, and a second patient treated via the same approach needed a surgical revision due to bleeding on postoperative day two. No temporary lesions were recorded. There were no early amputations. No perioperative deaths occurred. The median length of stay (LOS) was 3 (3-4) days. An expanded poly-tetra-flour-ethylene graft was used in 102 cases (98%) and a Dacron graft in the remain two cases (2%). As for the caliber, the 8mm graft was used in 64 cases (62%). Median follow-up was 34.6 (8.5-62.7) months. There was no related mortality. Nineteen PAA underwent reintervention with primary and secondary rate patency of 78% and 88% at three years. The median time to reintervention was 28.3 months.

CONCLUSIONS

Popliteal artery aneurysms open repair with prosthetic grafts are safe and feasible, with good mid-term results and satisfactory primary and secondary patency at three years.

Dynamic mitigation of nonlinear effects in a silicon photonic add-drop filter

Nonlinear effects limit the maximum amount of optical power that can be handled by silicon photonic integrated circuits (PICs). This limitation is particularly tight in resonant devices such as microring resonator (MRR) filters, suffering from a power-dependent resonance spread due to intracavity power enhancement. In this Letter, we present an automatic control system that can dynamically mitigate the nonlinear spectral distortion of silicon MRR filters by thermally controlling each MRR. The benefit of the proposed scheme is demonstrated on the spectral response of a polarization-transparent coupled-MRR filter operating on a 200 Gbit/s signal. The proposed technique, which does not require a priori information on the PIC topology and functionality, is scalable to more complex architectures and can be employed to compensate for generic nonlinear effects in different photonic platforms.

Open or endovascular treatment of downstream thoracic or thoraco-abdominal aortic pathology after frozen elephant trunk: perioperative and mid-term outcomes

OBJECTIVES

The aim of this study was to evaluate the outcomes of open and endovascular treatment of downstream thoracic or thoraco-abdominal aortic pathology in patients who underwent previous frozen elephant trunk (FET).

METHODS

Data were retrieved to evaluate mortality, cardiac, pulmonary, cerebrovascular, renal and spinal cord major adverse events, early- and mid-term reintervention and survival rates. The Society for Vascular Surgery endovascular reporting standards were used.

RESULTS

From 2011 to 2020, 48 patients (36 males, median age 60 years) underwent downstream aortic repair at a median of 18 months (interquartile range: 6-57) after the initial FET. Twenty-eight patients (58.3%) received open and 20 (41.7%) endovascular repair. The overall 30-day mortality was 6.3% and the initial clinical success was 88%, with no inter-group differences (P = 0.22 and 0.66 respectively). Six spinal cord deficits were recorded (13%): 3 (6.3%) were permanent. The major adverse events incidence was lower in the endovascular cohort [4 (20%) vs 14 (50%); P = 0.047], mainly due to a lower rate of grade ≥2 respiratory complications (5% vs 42.9%; P = 0.004). Assisted primary clinical success at 5 years was higher in the endovascular group (95% vs 68%, P = 0.022); freedom from reintervention at competing risk analysis (P = 0.3) and overall survival at Kaplan-Meier curves (log-rank P = 0.29) were similar.

CONCLUSIONS

Downstream aortic repair after FET is feasible with both open and endovascular repair with acceptable mortality and permanent paraplegia rates. The endovascular approach has potential perioperative and mid-term advantages, but long-term durability has to be further investigated in larger cohorts.

Endovascular embolization of a lower limb arteriovenous fistula using a vascular plug deployed with a through-and-through arteriovenous access

A 66-year-old man was referred to our institution for a popliteal arteriovenous fistula (AVF). The patient presented with left lower limb edema associated with a pulsatile popliteal mass with bruit at auscultation, and no sign of lower limb ischemia. Relevant history included penetrating stabbing wound to that leg 50 years prior. A computed tomography scan demonstrated an AVF with a tract 10 mm in length and 6 mm in diameter at the level of the popliteal fossa. An AMPLATZER Vascular Plug III 12 mm (AVP - AGA Medical Corporation, Minneapolis, MN) was initially deployed through a percutaneous femoral arterial access. Initial angiographic check showed inadequate sealing, so the plug was repositioned from the venous side of the AVF with a through-and-through arteriovenous access from the posterior tibial vein, achieving a correct sealing. At 24-month follow-up the patient does not present any complications from the procedure, and the AVF remains occluded.

Thoracic Endovascular Aortic Repair With Additional Distal Bare Stents in Type B Aortic Dissection Does Not Prevent Long-Term Aneurysmal Degeneration

PURPOSE

TEVAR (thoracic endovascular aortic repair) + PETTICOAT (Provisional ExTension to Induce COmplete ATtachment) technique has been selectively employed since 2005 at our institution during endovascular treatment of type B aortic dissection (TBD). The aim of this study is to evaluate the long-term (>5 years) clinical results and the evolution of aortic volume.

MATERIALS AND METHODS

All the patients receiving an endovascular treatment for TBD with the PETTICOAT technique were collected in a prospectively maintained database and follow-up computed tomography scan were retrospectively analyzed. Study endpoints included short- and long-term clinical success (absence of need for reintervention) and any major adverse event. The volumes of thoracic and abdominal aorta at long-term follow-up were also analyzed.

RESULTS

Twenty-eight patients received a TEVAR + PETTICOAT and were followed up (median follow-up 85 months). Primary 30-day clinical success rate was 82% with an adverse event rate of 31%; 4 type I endoleak and 1 retrograde dissection were recorded. Secondary mid-term clinical success was 96% while the long-term clinical success rate was 79%. Six cases (21%) received either an open repair or an endovascular repair for a significant distal aortic enlargement at follow-up. With regards to volumetric analysis, an increase of overall (thoracic and abdominal) aortic volume was observed in 8 cases mainly related to an increase (mean: +31%) of the abdominal volume that was observed in 11 cases.

CONCLUSIONS

PETTICOAT technique does not protect from long-term significant aneurysmal degeneration that may require aortic open or endovascular reinterventions. Aortic growth occurs mainly in the bare-stented aorta and thus, life-long surveillance is advisable in these patients.

Electrical conductance of silicon photonic waveguides

Many optoelectronic devices embedded in a silicon photonic chip, like photodetectors, modulators, and attenuators, rely on waveguide doping for their operation. However, the doping level of a waveguide is not always reflecting in an equal amount of free carriers available for conduction because of the charges and trap energy states inevitably present at the Si/SiO₂ interface. In a silicon-on-insulator technology with 10¹⁵cm^-3p-doped native waveguides, this can lead to a complete depletion of the core from free carriers and to a consequently very high electrical resistance. This Letter experimentally quantifies this effect and shows how the amount of free carriers in a waveguide can be modified and restored to the original doping value with a proper control of the chip substrate potential. A similar capability is also demonstrated by means of a specific metal gate integrated above the waveguide that allows fine control of the conductance with high locality level. This paper highlights the linearity achievable in the conductance modulation that can be exploited in a number of possible applications.

Kidney protection in thoracoabdominal aortic aneurysm surgery

Acute kidney injury (AKI) is a common complication of both open and endovascular repair of thoracoabdominal aortic aneurysms (TAAA). Its definition varies across difference studies, some standardized definitions (RIFLE, AKIN, KDIGO) have been proposed but still not uniformly employed in published papers. Acute kidney injury is multifactorial and is associated with increased in-hospital mortality, long-term mortality and late renal function decline. In addition, AKI is also associated with perioperative spinal cord ischemia. No specific pharmacological strategy has received a strong recommendation with high level of evidence as a protective measure. Fenoldopam, methylprednisolone or mannitol use to prevent AKI is commonly employed, but not supported by literature data. Avoiding nephrotoxic drugs and maintaining an adequate MAP, during and after the procedure plays a key role in preserving kidney function. During open TAAA surgery, renal arteries may be reimplanted using different techniques. The choice of the best option must be tailored to the patient, to reduce ischemic time and guarantee long-term patency. Current experience suggests that cold crystalloid solutions are the best substrates in preventing ischemia-reperfusion injury. Renal perfusion using Custodiol^® (Dr Franz-Kohler Chemie GmbH; Bensheim, Germany) 4 °C, even if currently considered off-label, represents an encouraging organ protection tool. In endovascular TAAA repair, techniques such as fusion imaging, use of diluted contrast, and CO<inf>2</inf> subtraction angiography have the potential to reduce postoperative AKI. Visceral vessels patency is closely related to the anatomy. Therefore, accurate endograft design according to these characteristics is crucial for long-term preservation of renal function.

Vascular Surgery During COVID-19 Emergency in Hub Hospitals of Lombardy: Experience on 305 Patients

Objective

During the most aggressive phase of the COVID-19 outbreak in Italy, the Regional Authority of Lombardy identified a number of hospitals, named Hubs, chosen to serve the whole region for highly specialised cases, including vascular surgery. This study reports the experience of the four Hubs for Vascular Surgery in Lombardy and provides a comparison of in hospital mortality and major adverse events (MAEs) according to COVID-19 testing.

Methods

Data from all patients who were referred to the Vascular Surgery Department of Hubs from 9 March to 28 April 2020 were collected prospectively and analysed. A positive COVID-19 polymerase chain reaction swab test, or symptoms (fever > 37.5 °C, upper respiratory tract symptoms, chest pain, and contact/travel history) associated with interstitial pneumonia on chest computed tomography scan were considered diagnostic of COVID-19 disease. Patient characteristics, operative variables, and in hospital outcomes were compared according to COVID-19 testing. A multivariable model was used to identify independent predictors of in hospital death and MAEs.

Results

Among 305 included patients, 64 (21%) tested positive for COVID-19 (COVID group) and 241 (79%) did not (non-COVID group). COVID patients presented more frequently with acute limb ischaemia than non-COVID patients (64% vs. 23%; p < .001) and had a significantly higher in hospital mortality (25% vs. 6%; p < .001). Clinical success, MAEs, re-interventions, and pulmonary and renal complications were significantly worse in COVID patients. Independent risk factors for in hospital death were COVID (OR 4.1), medical treatment (OR 7.2), and emergency setting (OR 13.6). COVID (OR 3.4), obesity class V (OR 13.5), and emergency setting (OR 4.0) were independent risk factors for development of MAEs.

Conclusion

During the COVID-19 pandemic in Lombardy, acute limb ischaemia was the most frequent vascular disease requiring surgical treatment. COVID-19 was associated with a fourfold increased risk of death and a threefold increased risk of major adverse events.

Programmable photonic circuits

The growing maturity of integrated photonic technology makes it possible to build increasingly large and complex photonic circuits on the surface of a chip. Today, most of these circuits are designed for a specific application, but the increase in complexity has introduced a generation of photonic circuits that can be programmed using software for a wide variety of functions through a mesh of on-chip waveguides, tunable beam couplers and optical phase shifters. Here we discuss the state of this emerging technology, including recent developments in photonic building blocks and circuit architectures, as well as electronic control and programming strategies. We cover possible applications in linear matrix operations, quantum information processing and microwave photonics, and examine how these generic chips can accelerate the development of future photonic circuits by providing a higher-level platform for prototyping novel optical functionalities without the need for custom chip fabrication.

Outcomes of routine use of percutaneous access with large-bore introducer sheaths (>21F outer diameter) during endovascular aneurysm repair

OBJECTIVE

The objective of this study was to evaluate the performance of percutaneous femoral access with large-bore sheaths (>21F outer diameter) mainly employed for thoracic and thoracoabdominal aortic endovascular treatment and to stratify the outcomes on the basis of the introducer size.

METHODS

Between December 2015 and December 2018, all consecutive patients who received endovascular repair through a percutaneous approach with a suture-mediated vascular closure device (VCD) and the preclose technique were included in a retrospective single-center study called Totally Percutaneous Approach to Endovascular Treatment of Aortic Aneurysms (PEVAR-PRO). The morphologic characteristics of the access vessels and patients' demographics were recorded, and 30-day closure success was defined as the primary end point. Analysis of the closure success comparing large-bore sheaths vs small-bore sheaths (≤21F outer diameter) was performed after 1:1 propensity score matching of preoperative confounding variables.

RESULTS

The closure success rate of the entire study cohort was 94% (622 femoral accesses in 360 patients; median age, 74 years; 84% male). Univariate analysis identified eight different factors associated with failure, but only two remained significant on multivariate analysis: diabetes (odds ratio [OR], 2.8; 95% confidence interval [CI], 1.3-6.2; P = .011) and common femoral artery stenosis >50% (OR, 4.5; 95% CI, 1.3-13.7; P = .019). After propensity score matching (1:1, 172 femoral accesses per group), closure success rate was not significantly different between large-bore and small-bore sheaths (90.7% vs 93.0%; P = .43). Multivariate analysis of the large-sheath group identified two factors associated with failure: small (<9 mm) femoral arteries (OR, 6.9; 95% CI, 1.5-31.6; P = .13) and access vessel calcifications involving more than one-third of the circumference (OR, 7.9; 95% CI, 2.1-29.4; P = .002). Neither previous femoral cutdown (44 accesses [23%]) nor percutaneous closure with VCDs (38 accesses [20%]) affected the closure success rate in the large-sheath group. Closure failure did not significantly increase the need for postoperative blood transfusions or hospital length of stay.

CONCLUSIONS

Off-label use of VCDs and the preclose technique for percutaneous approach with large-bore sheaths needed for complex aortic endovascular procedures is safe and feasible. Closure success rate is not significantly different from that obtained with on-label application of VCDs with smaller sheaths.

Electrically driven source of time-energy entangled photons based on a self-pumped silicon microring resonator

Time-energy entangled photon pairs are fundamental resources for quantum communication protocols since they are robust against environmental fluctuations in optical fiber networks. Pair sources based on spontaneous four-wave mixing in silicon microring resonators usually employ expensive external tunable lasers to compensate for ambient fluctuations; adopting self-pumped configurations, instead, lifts the need for such external source. Here we demonstrate the emission of time-energy entangled photon pairs at telecom wavelengths from a silicon self-pumped ring, obtaining a Franson interference fringe with 93.9%±0.9% visibility.

Tip‐mould microcontact printing for functionalisation of optical microring resonator

We present an approach to functionalise optical microring resonators as hybridisation platforms, using tip‐mould reactive microcontact printing process. Derived from reactive microcontact printing using an ad hoc mould of polydimethylsiloxane (PDMS), the method functionalises single microring resonator with a target‐specific capture agent. The authors report the functionalisation of silicon nitride (SiN) 200 μ m diameter microring resonator with single‐strand DNA and the hybridisation detection of 100 nM target analyte, while concurrently monitoring not‐functionalised microring as a control sensor. Results show that the functionalisation approach permits to address single microring resonators with mutual distance lower than 100 μ m with high precision, enabling a better integration of multiple spotting zones on the chip concerning traditional functionalisation procedures.

Unscrambling light-automatically undoing strong mixing between modes

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.

Gamma radiation effects on silicon photonic waveguides

To support the use of integrated photonics in harsh environments, such as outer space, the hardness threshold to high-energy radiation must be established. Here, we investigate the effects of gamma (γ) rays, with energy in the MeV-range, on silicon photonic waveguides. By irradiation of high-quality factor amorphous silicon core resonators, we measure the impact of γ rays on the materials incorporated in our waveguide system, namely amorphous silicon, silicon dioxide, and polymer. While we show the robustness of amorphous silicon and silicon dioxide up to an absorbed dose of 15 Mrad, more than 100× higher than previous reports on crystalline silicon, polymer materials exhibit changes with doses as low as 1 Mrad.

Reconfigurable photonic integrated mode (de)multiplexer for SDM fiber transmission

Spatial division multiplexing in multi-mode fibers allows to largely enhance transmission capacity compared to single-mode links. Photonic integrated circuits can provide solutions for mode multiplexing at the transmitter and demultiplexing at the receiver but have to generally face high losses and inter-modal cross-talk issues. Here a photonic circuit for efficient mode multiplexing and demultiplexing in few-mode fibers is presented and demonstrated. Two 10 Gbit/s channels at the same wavelength and polarization are simultaneously transmitted over modes LP₀₁ and LP_11a of a few-mode fiber relying only on integrated mode MUX and DEMUX. The proposed Indium-Phosphide-based circuits have a good coupling efficiency with fiber modes and mode-dependant loss smaller than 1 dB. Measured mode excitation cross-talk is as low as -20 dB and a channel cross-talk after propagation and demultiplexing of -15 dB is achieved. An operational bandwidth of the full transmission system of at least 10 nm is demonstrated. Both mode MUX and DEMUX are fully reconfigurable and allow a dynamic switch of channel routing in the transmission system. These results enable fully-integrated fiber mode handling for high-bandwidth flexible optical networks.

Uncertainty quantification of silicon photonic devices with correlated and non-Gaussian random parameters

Process variations can significantly degrade device performance and chip yield in silicon photonics. In order to reduce the design and production costs, it is highly desirable to predict the statistical behavior of a device before the final fabrication. Monte Carlo is the mainstream computational technique used to estimate the uncertainties caused by process variations. However, it is very often too expensive due to its slow convergence rate. Recently, stochastic spectral methods based on polynomial chaos expansions have emerged as a promising alternative, and they have shown significant speedup over Monte Carlo in many engineering problems. The existing literature mostly assumes that the random parameters are mutually independent. However, in practical applications such assumption may not be necessarily accurate. In this paper, we develop an efficient numerical technique based on stochastic collocation to simulate silicon photonics with correlated and non-Gaussian random parameters. The effectiveness of our proposed technique is demonstrated by the simulation results of a silicon-on-insulator based directional coupler example. Since the mathematic formulation in this paper is very generic, our proposed algorithm can be applied to a large class of photonic design cases as well as to many other engineering problems.

Optical radiative crosstalk in integrated photonic waveguides

We report on the comprehensive experimental characterization of optical crosstalk between waveguides caused by scattering. Our results reveal that a strong power exchange between close-placed waveguides due to sidewall roughness exists also for high-quality, low-loss waveguides. We derive a power-law dependence of the coupling on the distance between the waveguides, confirmed by an ad hoc developed electromagnetic model. Further, we demonstrate higher order mode excitation caused by scattered light and the appearance of decorrelation between the guided modes propagating in waveguides coupled via radiative mechanism, providing a full description of this phenomenon.

Post-fabrication trimming of athermal silicon waveguides

We experimentally demonstrate the post-fabrication trimming of polymer-coated athermal silicon waveguides by exploiting the photosensitivity of As(2)S(3) chalcogenide glass to near-bandgap visible light. Our technique enables compensation of fabrication tolerances and modification of specific circuit functionalities after fabrication. Moreover, our athermal and trimmable waveguide technology is highly resilient to high optical power, and thus extremely appealing for nonlinear applications. Finally, it enables to fix the absolute wavelength and spectral response of silicon devices with extremely low dependence from temperature and power.

Tunable silicon photonics directional coupler driven by a transverse temperature gradient

A compact directional coupler fabricated on a silicon photonic platform is presented, with a power-splitting ratio that can be tuned through a transverse temperature gradient induced by a laterally shifted integrated heater. The tuning mechanism exploits the thermally induced phase velocity mismatch between the coupled modes of the silicon waveguides. The positions of the integrated heater and the waveguide design are optimized to maximize the tuning range and to reduce electric power consumption. Asynchronous devices with an intrinsic phase mismatch are demonstrated to be more efficient, providing a tunable coupled power from 0.7 to 0.01 with 36 mW maximum power dissipation.

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 chalcogenide-assisted silicon waveguides

A chalcogenide-assisted silicon waveguide is realized by depositing a thin layer of A(2)S(3) glass onto a conventional silicon on insulator optical waveguide. The photosensitivity of the chalcogenide is exploited to locally change the optical properties of the waveguide through exposure to visible light radiation. Waveguide trimming is experimentally demonstrated by permanently shifting the resonant wavelength of a microring resonator by 6.7 nm, corresponding to an effective index increase of 1.6·10(-2). Saturation effects, trimming range, velocity and temporal stability of the process are discussed in details. Results demonstrate that photo-induced treatments can be exploited for a post-fabrication compensation of fabrication tolerances, as well as to set and reconfigure the circuit response.

Slow pulses in disordered photonic-crystal waveguides

Using a 3D fully-vectorial coupled Bloch-mode method, we present a systematic study of the transport of slow-light pulses in single-mode photonic-crystal waveguides (PhCW) with a realistic disorder model. For the intermediate regime corresponding to waveguide lengths of the order of the mean-free path (3 dB attenuation), we show that the group-velocity has a strong impact on the pulse broadening and distortion, limiting the practical use of PhCW to group indices below ≈50. For smaller group velocities, the pulse experiences an additional delay and the group-velocity is no longer a meaningful quantity.

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.