Low-noise phase-sensitive optical parametric amplifier with lossless local pump generation using a digital dither optical phase-locked loop.

The low noise figure of phase-sensitive amplifiers (PSAs) is attractive for optically pre-amplified measurement and communication systems. However, a major practical implementation difficulty pertains to the requirement of phase-locked signal, idler, and pump waves. Previously, injection locking to a co-propagating weak pump pilot or tapping portions of the received signal (lossy) for carrier re-generation have been used. Here we present a novel, lossless approach without any pump pilot, that generates a phase-locked receiver-local pump within the PSA using a digital dither-based optical phase-locked loop. We experimentally demonstrate a 2 dB noise figure with a low 0.3 dB penalty due to imperfect locking. By comparing the phase-locking performance in a PSA to that in a 50/50-coupler, we discuss and predict potential performance improvements connected to loop delay and laser phase characteristics.

Surpassing the nonlinear conversion efficiency of soliton microcombs.

Laser frequency combs are enabling some of the most exciting scientific endeavours in the twenty-first century, ranging from the development of optical clocks to the calibration of the astronomical spectrographs used for discovering Earth-like exoplanets. Dissipative Kerr solitons generated in microresonators currently offer the prospect of attaining frequency combs in miniaturized systems by capitalizing on advances in photonic integration. Most of the applications based on soliton microcombs rely on tuning a continuous-wave laser into a longitudinal mode of a microresonator engineered to display anomalous dispersion. In this configuration, however, nonlinear physics precludes one from attaining dissipative Kerr solitons with high power conversion efficiency, with typical comb powers amounting to ~1% of the available laser power. Here we demonstrate that this fundamental limitation can be overcome by inducing a controllable frequency shift to a selected cavity resonance. Experimentally, we realize this shift using two linearly coupled anomalous-dispersion microresonators, resulting in a coherent dissipative Kerr soliton with a conversion efficiency exceeding 50% and excellent line spacing stability. We describe the soliton dynamics in this configuration and find vastly modified characteristics. By optimizing the microcomb power available on-chip, these results facilitate the practical implementation of a scalable integrated photonic architecture for energy-efficient applications.

Thermorefractive noise reduction of photonic molecule frequency combs using an all-optical servo loop.

Phase and frequency noise originating from thermal fluctuations is commonly a limiting factor in integrated photonic cavities. To reduce this noise, one may drive a secondary "servo/cooling" laser into the blue side of a cavity resonance. Temperature fluctuations which shift the resonance will then change the amount of servo/cooling laser power absorbed by the device as the laser moves relatively out of or into the resonance, and thereby effectively compensate for the fluctuation. In this paper, we use a low noise laser to demonstrate this principle for the first time in a frequency comb generated from a normal dispersion photonic molecule micro-resonator. Significantly, this configuration can be used with the servo/cooling laser power above the usual nonlinearity threshold since resonances with normal dispersion are available. We report a 50 % reduction in frequency noise of the comb lines in the frequency range of 10 kHz to 1 MHz and investigate the effect of the secondary servo/cooling noise on the comb.

Compact lithium niobate microring resonators in the ultrahigh Q/V regime.

Lithium niobate (LN) is a promising material for future complex photonic-electronic circuits, with wide applications in such fields as communications, sensing, quantum optics, and computation. LN took a great stride toward compact photonic integrated circuits (PICs) with the development of partially etched LN on insulator (LNOI) waveguides. However, integration density is still limited for future highly compact PICs, owing to the partial etching nature of their waveguides. Here, we demonstrate a fully etched LN PIC platform, which, for the first time to our knowledge, simultaneously achieves ultralow propagation loss and compact circuit size. The tightly confined fully etched LN waveguides with smooth sidewalls allow us to bring the bending radius down to 20 µm (corresponding to 1 THz free spectral range). We have achieved compact high Q microring resonators with Q/V of 8.7 × 104 µm-3, almost one order of magnitude larger than previous demonstrations. The statistical mean propagation losses of our LN waveguides is 8.5 dB/m (corresponding to a mean Q factor of 4.9 × 106), even with a small bending radius of 40 µm. Our compact and ultralow-loss LN platform shows great potential in future miniaturized multifunctional integration systems. As complementary evidence to show the utility of our platform, we demonstrate soliton microcombs with an ultrahigh repetition rate of 500 GHz in LN.

Widely tunable narrow linewidth laser source based on photonic molecule microcombs and optical injection locking.

We demonstrate a method to generate a widely and arbitrarily tunable laser source with very narrow linewidth. By seeding a coupled-cavity microcomb with a highly coherent single-frequency laser and using injection locking of a Fabry-Perot laser to select a single output comb tone, a high power, high side mode suppression ratio output wave is obtained. The system is demonstrated across 1530 -1585 nm with a linewidth below 8 kHz, having 5 dBm output power and sidemode suppression of at least 60 dB. Prospects of extending the performance are also discussed.

Coherent combining of low-power optical signals based on optically amplified error feedback.

In free-space optical communication links, the combining of optical signals from multiple apertures is a well-known method to collect more power for improved sensitivity or mitigation of atmospheric disturbances. However, for analog optical combining no detailed analysis has been made in cases when the optical signal power is very low (<-60 dBm) as would be the case in very long-haul free-space links. We present a theoretical and experimental study of analog coherent combining of noise-limited signals from multiple independent apertures by applying low frequency optical phase dithering to actively compensate the relative phases. It is experimentally demonstrated that a 97% combining efficiency of four 10 GBaud QPSK signals is possible with a signal power per aperture exceeding -80 dBm, in fair agreement with theory. We also discuss the scaling aspects to many apertures.

Spectral Interferometry with Frequency Combs.

In this review paper, we provide an overview of the state of the art in linear interferometric techniques using laser frequency comb sources. Diverse techniques including Fourier transform spectroscopy, linear spectral interferometry and swept-wavelength interferometry are covered in detail. The unique features brought by laser frequency comb sources are shown, and specific applications highlighted in molecular spectroscopy, optical coherence tomography and the characterization of photonic integrated devices and components. Finally, the possibilities enabled by advances in chip scale swept sources and frequency combs are discussed.

Coherent supercontinuum generation in all-normal dispersion Si3N4 waveguides.

Spectral broadening of optical frequency combs with high repetition rate is of significant interest in optical communications, radio-frequency photonics and spectroscopy. Silicon nitride waveguides (Si3N4) in the anomalous dispersion region have shown efficient supercontinuum generation spanning an octave-bandwidth. However, the broadening mechanism in this regime is usually attained with femtosecond pulses in order to maintain the coherence. Supercontinuum generation in the normal dispersion regime is more prone to longer (ps) pulses, but the implementation in normal dispersion silicon nitride waveguides is challenging as it possesses strong requirements in propagation length and losses. Here, we experimentally demonstrate the use of a Si3N4 waveguide to perform coherent spectral broadening using pulses in the picosecond regime with high repetition rate. Moreover, our work explores the formation of optical wave breaking using a higher energy pulse which enables the generation of a coherent octave spanning spectrum. These results offer a new prospect for coherent broadening using long duration pulses and replacing bulky optical components.

Influence of inflammation and cardiac hypertrophy on mechanical properties of human pericardium.

Different devices for mechanical circulatory support (MCS) have been developed for the treatment of refractory cardiogenic shock. However, all of them are associated with direct blood contact, the need for anticoagulation and bleeding complications. To overcome these limitations the pericardial sac got into the focus as a promising implantation site for MCS. For this purpose, further knowledge about the mechanical properties of human pericardium is required. In this prospective, monocentric, experimental pilot study 56 samples of human pericardium were extracted postmortem from 13 critically ill patients. After preparation of test specimens uniaxial tensile tests were performed. The primary end points were load at fracture per sample width and strain at fracture. Acute inflammation was assessed by blood levels of C-reactive protein, white blood count and procalcitonin measured at several times during hospital stay. Inflammatory load was estimated by area under the inflammatory curves. Correlation and regression analysis were used to assess the relationship of primary end points to inflammation, comorbidities and postmortem time to preparation. Human pericardium showed a load at fracture per sample width of 1.95 [1.38-2.94] N/mm (median [inter quartile range]) and a strain at fracture of 89.29 [73.84-135.23] %. Markers of acute inflammation and cardiac hypertrophy did not correlate to load or strain at fracture. However, strain at fracture increased with higher body mass index and an increasing number of postmortem days. In contrast, higher patient age was associated with a lower strain at fracture. Inflammation and cardiac hypertrophy did not influence mechanical properties of human pericardium.

Phase-sensitively amplified wavelength-division multiplexed optical transmission systems.

The throughput and reach in fiber-optic communication links are limited by in-line optical amplifier noise and the Kerr nonlinearity in the optical transmission fiber. Phase-sensitive amplifiers (PSAs) are capable of amplifying signals without adding excess noise and mitigating the impairments caused by the Kerr nonlinearity. However, the effectiveness of Kerr nonlinearity mitigation depends on the dispersion pre-compensation in each span. This paper investigates dense wavelength-division multiplexed PSA-amplified links using joint processing with a less complex digital domain Volterra nonlinear equalizer at the receiver. Both numerically and with experiments, it is shown that this significantly reduces the impact of the dispersion pre-compensation in each span. Also, with simulations, a substantial improvement in transmission reach is demonstrated for PSA links.

Widely tunable, low linewidth, and high power laser source using an electro-optic comb and injection-locked slave laser array.

We propose and implement a tunable, high power and narrow linewidth laser source based on a series of highly coherent tones from an electro-optic frequency comb and a set of 3 DFB slave lasers. We experimentally demonstrate approximately 1.25 THz (10 nm) of tuning within the C-Band centered at 192.9 THz (1555 nm). The output power is approximately 100 mW (20 dBm), with a side band suppression ratio greater than 55 dB and a linewidth below 400 Hz across the full range of tunability. This approach is scalable and may be extended to cover a significantly broader optical spectral range.

One photon-per-bit receiver using near-noiseless phase-sensitive amplification.

Space communication for deep-space missions, inter-satellite data transfer and Earth monitoring requires high-speed data connectivity. The reach is fundamentally dictated by the available transmission power, the aperture size, and the receiver sensitivity. A transition from radio-frequency links to optical links is now seriously being considered, as this greatly reduces the channel loss caused by diffraction. A widely studied approach uses power-efficient formats along with nanowire-based photon-counting receivers cooled to a few Kelvins operating at speeds below 1 Gb/s. However, to achieve the multi-Gb/s data rates that will be required in the future, systems relying on pre-amplified receivers together with advanced signal generation and processing techniques from fibre communications are also considered. The sensitivity of such systems is largely determined by the noise figure (NF) of the pre-amplifier, which is theoretically 3 dB for almost all amplifiers. Phase-sensitive optical amplifiers (PSAs) with their uniquely low NF of 0 dB promise to provide the best possible sensitivity for Gb/s-rate long-haul free-space links. Here, we demonstrate a novel approach using a PSA-based receiver in a free-space transmission experiment with an unprecedented bit-error-free, black-box sensitivity of 1 photon-per-information-bit (PPB) at an information rate of 10.5 Gb/s. The system adopts a simple modulation format (quadrature-phase-shift keying, QPSK), standard digital signal processing for signal recovery and forward-error correction and is straightforwardly scalable to higher data rates.

Waveguide tapering for improved parametric amplification in integrated nonlinear Si3N4 waveguides.

In this paper, we propose and numerically investigate waveguide tapering to improve optical parametric amplification in integrated nonlinear Si3N4 circuits. The phase matching condition of parametric amplification changes along the length of uniform Si3N4 waveguides, due to the non-negligible propagation loss, potentially causing peak-gain wavelength shifts of more than 20 nm. By tapering the waveguide width along propagation, we can achieve a 2.5 dB higher maximum parametric gain thanks to the improved phase matching, which can also broaden the amplification bandwidth. Therefore, the length of an optimally tapered Si3N4 waveguide can be 23% shorter than a uniform one in the case of a 3.0 dB/m propagation loss and a single continuous-wavelength pump. Quasi-continuous tapers are efficient to approximate continuous ones and might simplify the fabrication of long tapered nonlinear Si3N4 waveguides, which are promising for optical signal processing and optical communications.

Phase-coherent lightwave communications with frequency combs.

Fiber-optical networks are a crucial telecommunication infrastructure in society. Wavelength division multiplexing allows for transmitting parallel data streams over the fiber bandwidth, and coherent detection enables the use of sophisticated modulation formats and electronic compensation of signal impairments. Optical frequency combs can replace the multiple lasers used for the different wavelength channels. Beyond multiplexing, it has been suggested that the broadband phase coherence of frequency combs could simplify the receiver scheme by performing joint reception and processing of several wavelength channels, but an experimental validation in a fiber transmission experiment remains elusive. Here we demonstrate and quantify joint reception and processing of several wavelength channels in a full transmission system. We demonstrate two joint processing schemes; one that reduces the phase-tracking complexity and one that increases the transmission performance.

Overhead-optimization of pilot-based digital signal processing for flexible high spectral efficiency transmission.

We present a low-complexity fully pilot-based digital signal processing (DSP) chain designed for high spectral efficiency optical transmission systems. We study the performance of the individual pilot algorithms in simulations before demonstrating transmission of a 51×24 Gbaud PM-64QAM superchannel over distances reaching 1000 km. We present an overhead optimization technique using the system achievable information rate to find the optimal balance between increased performance and throughput reduction from adding additional DSP pilots. Using the optimal overhead of 2.4%, we report 9.3 (8.3) bits/s/Hz spectral efficiency, or equivalently 11.9 (10.6) Tb/s superchannel throughput, after 480 (960) km of transmission over 80 km spans with EDFA-only amplification. Moreover, we show that the optimum overhead depends only weakly on transmission distance, concluding that back-to-back optimization is sufficient for all studied distances. Our results show that pilot-based DSP combined with overhead optimization can increase the robustness and performance of systems using advanced modulation formats while still maintaining state-of-the-art spectral efficiency and multi-Tb/s throughput.

Optical injection locking at sub nano-watt powers.

We demonstrate optical injection locking (OIL) at record low injection power of -65 dBm using EDFA-based pre-amplification and an electrical phase locked loop (PLL). Investigating the phase noise characteristics of OIL, we find that at low injection powers the slave laser linewidth and injection ratio strongly influence the phase noise of the locked slave output. By introducing an EDFA pre-amplifier, the minimum locking power for OIL is reduced. Moreover, using this pre-amplifier we find that there exists an optimum injection power into the slave where the output phase noise is minimized and is below the phase noise without EDFA. We evaluate an OIL-based pump recovery in a phase sensitive amplifier (PSA) receiver system aimed at free-space communications.

Reducing Elevated Heart Rates in Patients with Multiple Organ Dysfunction Syndrome with The If (Funny Channel Current) Inhibitor Ivabradine.

INTRODUCTION: A heart rate higher than 90 beats/min indicates an unfavorable prognosis for patients with multiple organ dysfunction syndrome (MODS). We sought to investigate the effect of the pacemaker current (If) inhibitor ivabradine on heart rate, hemodynamics, and disease severity among patients with MODS. PATIENTS AND METHODS: In this prospective, controlled, randomized, open-label, two-arm phase II trial, 70 patients with MODS, a sinus rhythm of at least 90 beats/min, and contraindications to ß-blocker therapy were randomly assigned to receive the standard treatment ± ivabradine (5 mg twice daily) for 96 h via the enteral route. The primary outcome was the percentage of patients with a heart rate reduction of at least 10 beats/min after 96 h. Secondary outcomes included the effect of ivabradine on hemodynamics, disease severity, vasopressor use, mortality, and adverse events. RESULTS: There were no significant differences in the primary outcome between the ivabradine and control groups (P = 0.147). After 96 h, the daily median heart rate was reduced by 7 beats/min in the control group and by 16 beats/min in the ivabradine group (P = 0.014). No differences in secondary outcomes were observed. CONCLUSIONS: The number of critically ill patients with MODS and a sinus rhythm of at least 90 beats/min that experienced a heart rate reduction of at least 10 beats/min after oral ivabradine treatment did not differ significantly between groups. The moderate but significant reduction of heart rate by 7 beats/min did not affect hemodynamics or disease severity.

Polarization independent optical injection locking for carrier recovery in optical communication systems.

An optical injection locking (IL) system that is independent of the incoming signal's polarization is demonstrated for carrier recovery in coherent optical communication systems. A sub-system that enables polarization independence is discussed and experimentally verified. The system is tested over a 20-km test field link using a broad-linewidth laser (40 MHz), and shows the suppression of phase noise when using the carrier recovered by injection locking as the local oscillator.

Complete spatiotemporal characterization and optical transfer matrix inversion of a 420 mode fiber.

The ability to measure a scattering medium's optical transfer matrix, the mapping between any spatial input and output, has enabled applications such as imaging to be performed through media which would otherwise be opaque due to scattering. However, the scattering of light occurs not just in space, but also in time. We complete the characterization of scatter by extending optical transfer matrix methods into the time domain, allowing any spatiotemporal input state at one end to be mapped directly to its corresponding spatiotemporal output state. We have measured the optical transfer function of a multimode fiber in its entirety; it consists of 420 modes in/out at 32768 wavelengths, the most detailed complete characterization of multimode waveguide light propagation to date, to the best of our knowledge. We then demonstrate the ability to generate any spatial/polarization state at the output of the fiber at any wavelength, as well as predict the temporal response of any spatial/polarization input state.

Polarization-resolved cross-correlated (C2) imaging of a photonic bandgap fiber.

We demonstrate polarization-resolved frequency domain cross-correlated (C2) imaging to characterize a 5m length of hollow-core photonic bandgap fiber. We produce a spectrogram of the fiber response to investigate the spatial, polarization, spectral, and temporal behavior. We then show how this temporally-resolved technique can be used to characterize multiple fiber launch conditions simultaneously by assigning each a unique time delay.