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Microresonator frequency comb-based high-speed data transmission provides a pathway towards augmented data capacity without increasing the number of laser sources. Their use with intensity-modulated direct detection modulation (IMDD) formats is especially pertinent in data center communications where minimizing cost, latency and complexity is paramount. This however implies that the same extent of digital signal processing techniques commonly used in coherent detection for the management of fiber impairments including chromatic dispersion are not available. With the proliferation of silicon photonics technologies in data center transceivers integrated dispersion compensation which can overcome fiber impairments would be of great merit. We demonstrate low power generation of the primary comb state in a silicon nitride microresonator and transmission of 25 Gb/s NRZ and 50 Gb/s PAM4 data over 20 km of single mode fiber. This represents the longest fiber reach demonstrated to date for the transmission of IMDD data using an integrated, microresonator frequency comb. An integrated, tunable grating device for dispersion compensation that reduces dispersion impairments after several fiber lengths generates significant improvements in the eye diagram, six orders of magnitude improvement in the bit-error rate and 14 dB improvement in the transmitter and dispersion eye closure quaternary values. Concurrently, doubling data transmission is demonstrated via polarization multiplexing a comb line and successful dispersion compensation of up to 20 km.

Silicon nitride (SiN) has surged into prominence as a material for photonic-integrated circuits (PICs) in the past decade, well regarded for its broadband transparency, compatibility with complementary metal oxide semiconductor (CMOS) fabrication processes and high optical bandgap that avoids two-photon absorption. However, current fabrication methods result in users having to choose between low thermal budgets and low losses, which are suboptimal given that both are necessary to facilitate a wide range of applications. Recently, works have emerged featuring PICs fabricated using deuterated silicon nitride (SiNx:D) – SiNx films grown using deuterated precursors instead of conventional hydrogenated ones. This decreases material absorption near the telecommunications bands at 1.55 µm previously present due to parasitic silicon–hydrogen and nitrogen–hydrogen bonds, attaining low-loss PICs realised using a low temperature, back-end-of-line CMOS-compatible fabrication plasma-enhanced chemical vapour deposition process. These devices have shown promise for both linear and nonlinear applications and the platform has the potential to be instrumental in realising highly efficient chips with co-packaged electronics and photonics devices. This paper reviews recent developments on the SiNx:D platform and provides a glance at future advancements for this highly promising material.

Nonlinear propagation in periodic media has been studied for decades, yielding demonstrations of numerous phenomena including strong temporal compression and slow light generation. Gap solitons, that propagate at frequencies inside the stopband, have been observed in optical fibres but have been elusive in photonic chips. In this manuscript, we investigate nonlinear pulse propagation in a chip-based nonlinear Bragg grating at frequencies inside the stopband and observe clear, unequivocal signatures of gap soliton propagation, including slow light, intensity-dependent transmission, intensity-dependent temporal delay and gap soliton compression. Our experiments which are performed in an on-chip ultra-silicon-rich nitride (USRN) Bragg grating with picosecond time scales, reveal slow light group velocity reduction to 35%–40% of the speed of light in vacuum, change in the temporal delay of 7 ps at low peak powers between 15.7 W–36.6 W, which is accompanied by up to 2.7× temporal compression of input pulses. Theoretical calculations using the nonlinear coupled mode equations confirm the observations of intensity-dependent temporal delay. Of fundamental importance, this demonstration opens up on-chip platforms for novel experimental studies of gap solitons as the basis of all-optical buffers, delay lines and optical storage.

Globally, the long-haul transmission of ultra-high bandwidth data is enabled through coherent communications. Driven by the rapid pace of growth in interconnectivity over the last decade, long-haul data transmission has reached capacities on the order of tens to hundreds of terabits per second, over fiber reaches which may span thousands of kilometers. Data center communications operate in regimes featuring shorter reaches and higher cost sensitivity. While integrated microresonator frequency combs are poised to revolutionize light sources used for high-speed data transmission over fiber, recent progress has focused largely on coherent detection schemes. Furthermore, though state-of-the-art intensity modulators are advancing in speed, it has not been demonstrated in the literature if microresonator-based comb lines can accommodate higher intensity modulated direction data (IMDD) line rates in tandem with these advancements. In this manuscript, we demonstrate the use of microresonator frequency combs pumped with a single laser for the transmission of high-speed IMDD data. We demonstrate error-free transmission of 30 Gbs−1 per comb non-return-to-zero data over fiber lengths of 6 km, as well as bit error rates under the forward error correction limit for propagation through 20 km of optical fiber. 60 Gbs−1 and 42 Gbs−1 pulse modulation amplitude 4 (PAM4) data modulated on each frequency comb line is further quantified to have a bit error rate under the forward error correction limit for fiber reaches of up to 6 km and 20 km respectively. The results showcase CMOS-compatible microresonator frequency comb modulated using IMDD formats as a promising technology for high-speed transmission in the data center transceiver industry.

The generation of optical pulse trains from continuous-wave light has attracted growing attention in recent years because it provides a simple way to obtain high repetition rate ultrashort pulses. While pulse generation has been extensively demonstrated in optical fibers, pulse train generation from weak, continuous wave light in photonic chips has posed significant challenges because of the short interaction length and therefore difficulty in acquiring sufficient new frequency content, and/or absence of the appropriate dispersion environment. In this manuscript, we report the pulse train generation of a low continuous-wave signal to 18 ps, by leveraging cross-phase modulation induced by co-propagating pump pulses with a peak power of 3.7 W in an ultra-silicon-rich nitride grating. The pulse train generation dynamics are documented both experimentally and theoretically to arise from cross-phase modulation-induced generation of new spectral content, and dispersive re-phasing. This is a new approach in which picosecond pulse generation may be achieved from low power, continuous-wave light.

Background: The "gold standard" marker for freezing of gait severity is percentage of time spent with freezing observed through video analysis. Objective: This study examined inter- and intra-rater reliability and variability of physiotherapists rating freezing of gait severity through video analysis and explored the effects of experience. Methods: Thirty physiotherapists rated 14 videos of Timed Up and Go performance by people with Parkinson's and gait freezing. Ten videos were unique, while four were repeated. Freezing frequency, total duration, and percentage of time spent with freezing were computed. Reliability and variability were estimated using ICC (2,1) and mean absolute differences. Between-group differences were calculated with the one-way ANOVA. Results: Inter- and intra-rater reliability ranged from moderate to good (ICC: inter-rater frequency = 0.63, duration = 0.78, percentage = 0.50; intra-rater frequency = 0.84, duration = 0.89, percentage = 0.50). Variability for freezing frequency was two episodes. Inter- and intra-rater variability for total freezing duration was 18.8 and 12.3 seconds, respectively. For percentage of time spent with freezing, this was 15.2% and 13.5%. Physiotherapy experience had no effect. Conclusion: Physiotherapists demonstrated sufficient reliability, but variability was large enough to cause changes in severity classifications on existing rating scales. Percentage of time spent with freezing was the least reliable marker, supporting the use of freezing frequency or total duration instead. [ABSTRACT FROM AUTHOR]

Nonlinear phenomena are investigated in a topological photonic valley Hall insulator (VHI) using a Kagome system designed to have localized topological boundary state (TBS) at telecommunications wavelengths. Valley Hall topologically protected nonlinear phenomena are demonstrated. Similar transmission properties and parametric wavelength conversion of up to −12 dB are experimentally achieved using sub‐milliwatt average powers in a zigzag interface without back‐scattering, compared with a straight interface. Furthermore, a 7 dB slow light enhancement is observed in the conversion efficiency near the red edge in the transmission band of the boundary state, demonstrating how slow light‐induced backscattering may be circumvented by leveraging the topologically protected boundary state. A Kerr‐induced delocalization of the valley Hall topological state (VHTS) is further experimentally observed, which can be harnessed to control the properties of topological edge states. [ABSTRACT FROM AUTHOR]

We demonstrate a complementary metal–oxide–semiconductor (CMOS)-compatible optical parametric Bragg amplifier on an ultra-silicon-rich nitride chip. The amplifier design incorporates advantageous group index properties in a nonlinear Bragg grating to circumvent phase matching limitations arising from the bulk material and waveguide dispersion. The grating structure further augments the effective nonlinear parameter of 800 W−1/m, considerably lowering the power required for the observation of strong parametric gain. On/off optical parametric gain of 20 dB is achieved using a low peak power of 1.6 W, in good agreement with numerical calculations. This represents a 7 dB improvement in the parametric gain compared to the absence of grating enhancement which is attributed to the Bragg grating induced superior phase matching.

The Freezing of Gait Severity Tool (FOG Tool) was developed because of limitations in existing assessments. This cross-sectional study investigated its validity and reliability. People with Parkinson's disease (PD) were recruited consecutively from clinics. Those who could not walk eight-metres independently (with or without an assistive device), comprehend instructions, or with co-morbidities affecting walking were excluded. Participants completed a set of assessments including the FOG Tool, Timed Up and Go (TUG), and Freezing of Gait Questionnaire. The FOG Tool was repeated and those reporting no medication state change evaluated for test-retest reliability. Validity and reliability were investigated through Spearman's correlations and ICC (two-way, random). McNemar's test was applied to compare the FOG Tool and TUG on the proportion of people with freezing. Thirty-nine participants were recruited [79.5%(n = 31) male; Median(IQR): age–73.0(9.0) years; disease duration–4.0(5.8) years]. Fifteen (38.5%) contributed to test-retest reliability analyses. The FOG Tool demonstrated strongest associations with the Freezing of Gait Questionnaire (ρ = 0.67, 95%CI 0.43–0.83). Test-retest reliability was excellent (ICC = 0.96, 95%CI 0.88–0.99). The FOG Tool had 6.2 times the odds (95%CI 2.4–20.4, p < 0.001) of triggering freezing compared to the TUG. The FOG Tool appeared adequately valid and reliable in this small sample of people with PD. It was more successful in triggering freezing than the TUG. The Freezing of Gait Severity Tool's assessment course is more effective than the commonly-used Timed Up and Go's assessment course for eliciting freezing of gait for clinical evaluation in people with Parkinson's disease. The Freezing of Gait Severity Tool can be considered for scoring freezing of gait severity in people with Parkinson's disease in the clinical setting. [ABSTRACT FROM AUTHOR]