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An exciting recent development for deep-tissue imaging with cellular resolution is three-photon fluorescence microscopy (3PM) with excitation at long wavelengths (1300 and 1700 nm). In the last few years, long-wavelength 3PM has driven rapid progress in deep-tissue imaging beyond the depth limit of two-photon microscopy, with impacts in neuroscience, immunology, and cancer biology. However, wide adoption of 3PM faces challenges. Three-photon excitation (3PE) is naturally weaker than two-photon excitation, which places a premium on ultrashort pulses with high peak power. The inefficiency, complexity, and cost of current sources of these pulses present major barriers to the use of 3PM in typical biomedical research labs. Here, we describe a fiber-based source of femtosecond pulses with multi-megawatt peak power, tunable from 850 nm to 1700 nm. Compressed pulses from a fiber amplifier at 1030~nm are launched into an antiresonant hollow-core fiber filled with argon. By varying only the gas pressure, pulses with hundreds of nanojoules of energy and sub-100 fs duration are obtained at wavelengths between 850 and 1700 nm. This approach is a new route to an efficient, robust, and potentially low-cost source for multiphoton deep-tissue imaging. In particular, 960-nJ and 50-fs pulses are generated at 1300 nm with a conversion efficiency of 10\%. The nearly 20-MW peak power is an order of magnitude higher than the previous best from femtosecond fiber source at 1300~nm. As an example of the capabilities of the source, these pulses are used to image structure and neuronal activity in mouse brain as deep as 1.1 mm below the dura., Comment: Supplement appended at end

Photonic lanterns are waveguide devices enabling high throughput single mode spectroscopy and high angular resolution. We aim to present the first on-sky demonstration of a photonic lantern (PL) operating in visible light, to measure its throughput and assess its potential for high-resolution spectroscopy of compact objects. We used the SCExAO instrument (a double stage extreme AO system installed at the Subaru telescope) and FIRST mid-resolution spectrograph (R 3000) to test the visible capabilities of the PL on internal source and on-sky observations. The best averaged coupling efficiency over the PL field of view was measured at 51% +/- 10% with a peak at 80%. We also investigate the relationship between coupling efficiency and the Strehl ratio for a PL, comparing them with those of a single-mode fiber (SMF). Findings show that in the AO regime, a PL offers better coupling efficiency performance than a SMF, especially in the presence of low spatial frequency aberrations. We observed Ikiiki (alpha Leo - mR = 1.37) and `Aua (alpha Ori - mR = -1.17) at a frame rate of 200 Hz. Under median seeing conditions (about 1 arcsec measured in H band) and large tip/tilt residuals (over 20 mas), we estimated an average light coupling efficiency of 14.5% +/- 7.4%, with a maximum of 42.8% at 680 nm. We were able to reconstruct both star's spectra, containing various absorption lines. The successful demonstration of this device opens new possibilities in terms of high throughput single-mode fiber-fed spectroscopy in the Visible. The demonstrated on-sky coupling efficiency performance would not have been achievable with a single SMF injection setup under similar conditions, partly because the residual tip/tilt alone exceeded the field of view of a visible SMF (18 mas at 700 nm). Thus emphasizing the enhanced resilience of PL technology to such atmospheric disturbances. The additional, Comment: Accepted in Astronomy and Astrophysics journal on 9/11/2024

A Photonic Lantern (PL) is a novel device that efficiently converts a multi-mode fiber into several single-mode fibers. When coupled with an extreme adaptive optics (ExAO) system and a spectrograph, PLs enable high throughput spectroscopy at high angular resolution. The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system of the Subaru Telescope recently acquired a PL that converts its multi-mode input into 19 single-mode outputs. The single mode outputs feed a R~4,000 spectrograph optimized for the 600 to 760 nm wavelength range. We present here the integration of the PL on SCExAO, and study the device performance in terms of throughput, field of view, and spectral reconstruction. We also present the first on-sky demonstration of a Visible PL coupled with an ExAO system, showing a significant improvement of x12 in throughput compared to the use of a sole single-mode fiber. This work paves the way towards future high throughput photonics instrumentation at small angular resolution., Comment: Proceeding published in SPIE Astronomical Telescopes + Instrumentation (2024) - Paper number 13096-25

Starlight suppression techniques for High-Contrast Imaging (HCI) are crucial to achieving the demanding contrast ratios and inner working angles required for detecting and characterizing exoplanets with a wide range of masses and separations. The advent of photonic technologies provides new opportunities to control the amplitude and phase characteristics of light, with the potential to enhance and control starlight suppression. Here, we present a focal plane optical-fiber-based nulling interferometer working with commercially available components for amplitude and phase modulation. The instrument implements single-mode fiber-coupled elements: a MEMS variable optical attenuator (VOA) matches the on-axis and off-axis starlight amplitude, while a piezoelectric-driven fiber stretcher modifies the optical path difference between the channels to achieve the $\pi$ phase shift condition for destructive interference. We show preliminary lab results using a narrowband light source working at 632 nm and discuss future opportunities for testing on-sky with the Astrophotonics Advancement Platform at Lick Observatory (APALO) at the Shane 3-m Telescope., Comment: submitted to SPIE Astronomical Telescopes and Instrumentation 2024, paper number 13095-95

Photoacoustic spectroscopys (PAS)-based methane (CH4) detectors have garnered significant attention with various developed systems using near-infrared (NIR) laser sources, which requires high-energy and narrow-linewidth laser sources to achieve high-sensitivity and low-concentration gas detection. The anti-resonant hollow-core fiber (ARHCF) lasers in the NIR and mid-infrared (MIR) spectral domain show a great potential for spectroscopy and high-resolution gas detection. In this work, we demonstrate the generation of a frequency-comb-like Raman laser with high pulse energy spanning from ultraviolet (UV) (328 nm) to NIR (2065 nm wavelength) based on a hydrogen (H2)-filled 7-ring ARHCF. The gas-filled ARHCF fiber is pumped with a custom-laser at 1044 nm with ~100 {\mu}J pulse energy and a few nanoseconds duration. Through stimulated Raman scattering process, we employ the sixth-order Stokes as case example located at ~1650 nm to demonstrate how the developed high-energy and narrow-linewidth laser source can effectively be used to detect CH4 in the NIR-II region using the photoacoustic modality. We report the efficient detection of CH4 with sensitivity as low as ~550 ppb with an integration time of ~40 s. In conclusion, the main goal of this work is to demonstrate and emphasize the potential of the gas-filled ARHCF laser technology for compact next-generation spectroscopy across different spectral regions., Comment: 18 pages, 7 figures

Photonic lantern nulling (PLN) is a method for enabling the detection and characterization of close-in exoplanets by exploiting the symmetries of the ports of a mode-selective photonic lantern (MSPL) to cancel out starlight. A six-port MSPL provides four ports where on-axis starlight is suppressed, while off-axis planet light is coupled with efficiencies that vary as a function of the planet's spatial position. We characterize the properties of a six-port MSPL in the laboratory and perform the first testbed demonstration of the PLN in monochromatic light (1569 nm) and in broadband light (1450 nm to 1625 nm), each using two orthogonal polarizations. We compare the measured spatial throughput maps with those predicted by simulations using the lantern's modes. We find that the morphologies of the measured throughput maps are reproduced by the simulations, though the real lantern is lossy and has lower throughputs overall. The measured ratios of on-axis stellar leakage to peak off-axis throughput are around 10^(-2), likely limited by testbed wavefront errors. These null-depths are already sufficient for observing young gas giants at the diffraction limit using ground-based observatories. Future work includes using wavefront control to further improve the nulls, as well as testing and validating the PLN on-sky., Comment: 30 pages, 12 figures

Multimode optical fibers represent the ideal platform for transferring multidimensional light states. However, dispersion degrades the correlations between the light's degrees of freedom, thus limiting the effective transport of ultrashort pulses between distant nodes of optical networks. Here, we demonstrate that tailoring the spatiotemporal structure of ultrashort light pulses can overcome the physical limitations imposed by both chromatic and modal dispersion in multimode optical fibers. We synthesize these light states with predefined spatial and chromatic dynamics through applying a sequence of transformations to shape the optical field in all its dimensions. Similar methods can also be used to overcome dispersion processes in other physical settings like acoustics and electron optics. Our results will enable advancements in laser-based technologies, including multimode optical communications, imaging, ultrafast light-matter interactions, and high brightness fiber sources., Comment: 15 pages, 4 figures

We propose and demonstrate a compact and simple vector bending sensor capable of distinguishing any direction and amplitude with high accuracy. The sensor consists of a short segment of asymmetric multicore fiber (MCF) fusion spliced to a standard single mode fiber. The reflection spectrum of such a structure shifts and shrinks in specific manners depending on the direction in which the MCF is bent. By monitoring simultaneously wavelength shift and light power variations, the amplitude and bend direction of the MCF can be unmistakably measured in any orientation, from 0{\deg} to 360{\deg}. The bending sensor proposed here is highly sensitive even for small bending angles (below 1{\deg}).

In this paper, we report on a multicore fiber-based (MCF) temperature sensor that operates in a wide thermal range and that is robustly packaged to withstand harsh environments. To develop the sensor, the fundamentals concerning the effect of temperature on such fibers have been analyzed in detail to predict the most temperature sensitive MCF geometry. Thanks to it, the device, which operates in reflection mode and consists of a short segment of strongly coupled MCF fusion spliced to a standard single mode fiber, shows higher sensitivity than other devices with identical configuration. Regarding its packaging, it consists of an inner ceramic and two outer metallic tubes to provide rigidity and protection against impacts or dirt. The device was calibrated for a thermal range from 25 C to 900 C and a K-type thermocouple was used as reference. Our results suggest that the manufactured optical thermometer is as accurate as the electronic one, reaching a sensitivity up to 29.426 pm/C with the advantage of being passive, compact and easy to fabricate and interrogate. Therefore, we believe this device is appealing for industrial applications that require highly sensitive temperature sensing in very demanding environments, and that the analysis included in this work could be analogously applied to develop sensitivity-optimized devices for other parameters of interest.

The present work deals with a curvature sensor that consists of two segments of asymmetric multicore fiber (MCF) fusion spliced with standard single mode fiber (SMF). The MCF comprises three strongly coupled cores; one of such cores is at the geometrical center of the MCF. The two segments of MCF are short, have different lengths (less than 2 cm each), and are rotated 180 degrees with respect to each other. The fabrication of the sensor was carried out with a fusion splicing machine that has the means for rotating optical fibers. It is demonstrated that the sensor behaves as two SMF-MCF-SMF structures in series, and consequently, it has enhanced sensitivity. The device proposed here can be used to sense the direction and amplitude of curvature by monitoring either wavelength shifts or intensity changes. In the latter case, high curvature sensitivity was observed. The device can also be used for the development of other highly sensitive sensors to monitor, for example, vibrations, force, pressure, or any other parameter that induces periodic or local curvature or bending to the MCF segments.

We report on a compact, highly sensitive all-fiber accelerometer suitable for low frequency and low amplitude vibration sensing. The sensing elements in the device are two short segments of strongly coupled asymmetric multicore fiber (MCF) fusion spliced at 180{\deg} with respect to each other. Such segments of MCF are sandwiched between standard single mode fibers. The reflection spectrum of the device exhibits a narrow spectrum whose height and position in wavelength changes when it is subjected to vibrations. The interrogation of the accelerometer was carried out by a spectrometer and a photodetector to measure simultaneously wavelength shift and light power variations. The device was subjected to a wide range of vibration frequencies, from 1 mHz to 30 Hz, and accelerations from 0.76 mg to 29.64 mg, and performed linearly, with a sensitivity of 2.213 nW/mg. Therefore, we believe the accelerometer reported here may represent an alternative to existing electronic and optical accelerometers, especially for low frequency and amplitude vibrations, thanks to its compactness, simplicity, cost-effectiveness, implementation easiness and high sensitivity.

Quantum communication implementations require efficient and reliable quantum channels. Optical fibers have proven to be an ideal candidate for distributing quantum states. Thus, today's efforts address overcoming issues towards high data transmission and long-distance implementations. Here, we experimentally demonstrate the secret key rate enhancement via space-division multiplexing using a multicore fiber. Our multiplexing technique exploits the momentum correlation of photon pairs generated by spontaneous parametric down-conversion. We distributed polarization-entangled photon pairs into opposite cores within a 19-core multicore fiber. We estimated the secret key rates in a configuration with 6 and 12 cores from the entanglement visibility after transmission through 411 m long multicore fiber.

Photonics offer numerous functionalities that can be used to realize astrophotonic instruments. The most spectacular example to date is the ESO Gravity instrument at the Very Large Telescope in Chile. Integrated astrophotonic devices stand to offer critical advantages for instrument development, including extreme miniaturization, as well as integration, superior thermal and mechanical stabilization owing to the small footprint, and high replicability offering cost savings. Numerous astrophotonic technologies have been developed to address shortcomings of conventional instruments to date, including for example the development of photonic lanterns, complex aperiodic fiber Bragg gratings, complex beam combiners to enable long baseline interferometry, and laser frequency combs for high precision spectral calibration of spectrometers. Despite these successes, the facility implementation of photonic solutions in astronomical instrumentation is currently limited because of (1) low throughputs from coupling to fibers, coupling fibers to chips, propagation and bend losses, device losses, etc, (2) difficulties with scaling to large channel count devices needed for large bandwidths and high resolutions, and (3) efficient integration of photonics with detectors, to name a few. In this roadmap, we identify 24 areas that need further development. We outline the challenges and advances needed across those areas covering design tools, simulation capabilities, fabrication processes, the need for entirely new components, integration and hybridization and the characterization of devices. To realize these advances the astrophotonics community will have to work cooperatively with industrial partners who have more advanced manufacturing capabilities. With the advances described herein, multi-functional instruments will be realized leading to novel observing capabilities for both ground and space platforms., Comment: 191 pages, 47 figures. This is the version of the article before peer review or editing, as submitted by an author to J. Phys. Photonics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://iopscience.iop.org/article/10.1088/2515-7647/ace869/meta

Astrophysical research into exoplanets has delivered thousands of confirmed planets orbiting distant stars. These planets span a wide ranges of size and composition, with diversity also being the hallmark of system configurations, the great majority of which do not resemble our own solar system. Unfortunately, only a handful of the known planets have been characterized spectroscopically thus far, leaving a gaping void in our understanding of planetary formation processes and planetary types. To make progress, astronomers studying exoplanets will need new and innovative technical solutions. Astrophotonics -- an emerging field focused on the application of photonic technologies to observational astronomy -- provides one promising avenue forward. In this paper we discuss various astrophotonic technologies that could aid in the detection and subsequent characterization of planets and in particular themes leading towards the detection of extraterrestrial life., Comment: 9 pages, 2 figures, SPIE Optics and Photonics conference

Off-axis digital holography is used to align a few-mode fiber to a multi-mode fiber in a free-space optical setup. Alignment based on power coupling measurements alone cannot guarantee low mode-dependent loss. The proposed alignment method enables reliable fiber coupling with low mode-dependent loss and crosstalk., Comment: Accepted for presentation at ECOC 2022

Angular resolved digital holography is presented as a technique for real-time characterization of the full optical field (amplitude and phase) of space-division multiplexing components and fibers, here a 6-mode photonic-lantern is characterized., Comment: Demo zone paper to be presented at the 2022 Optical Fiber Communication Conference (OFC), March 7, 2022 from 2:00 PM to 4:15 PM PST

The increase in capacity provided by coupled SDM systems is fundamentally limited by MDG and ASE noise. Therefore, monitoring MDG and optical SNR is essential for accurate performance evaluation and troubleshooting. Recent works show that the conventional MDG estimation method based on the transfer matrix of MIMO equalizers optimizing the MMSE underestimates the actual value at low SNR. Besides, estimating the optical SNR itself is not a trivial task in SDM systems, as MDG strongly influences the electrical SNR after the equalizer. In a recent work we propose an MDG and SNR estimation method using ANN. The proposed ANN-based method processes features extracted at the receiver after DSP. In this paper, we discuss the ANN-based method in detail, and validate it in an experimental 73-km 3-mode transmission link with controlled MDG and SNR. After validation, we apply the method in a case study consisting of an experimental long-haul 6-mode link. The results show that the ANN estimates both MDG and SNR with high accuracy, outperforming conventional methods., Comment: Submitted to JLT, currently under review

We report a study of soliton self-frequency shifting in hydrogen-filled hollow-core fiber. The combination of hydrogen and short 40-fs input pulses underlies clean and efficient generation of Raman solitons between 1080 and 1600 nm. With 240-nJ input pulses, the Raman soliton energy ranges from 110 to 20 nJ over that wavelength range, and the pulse duration is approximately 45 fs. In particular, 70-nJ and 42-fs pulses are generated at 1300 nm. Numerical simulations agree reasonably well with experiments and predict that microjoule-energy tunable pulses should be possible with higher-energy input pulses.

In this letter, we present a high pulse energy Raman laser at 1946 nm wavelength directly pumped with a 1533 nm custom-made fiber laser. The Raman laser is based on the stimulated Raman scattering (SRS) in an 8-meter carbon dioxide (CO2) filled nested anti-resonant hollow-core fiber (ARHCF). The low energy phonon emission combined with the inherent SRS process along the low-loss fiber allows the generation of high pulse energy up to 15.4 {\mu}J at atmospheric CO2 pressure. The Raman laser exhibits good long-term stability and low relative intensity noise (RIN) of less than 4%. We also investigate the pressure-dependent overlap of the Raman laser line with the absorption band of CO2 at 2 {\mu}m spectral range. Our results constitute a novel and promising technology towards high energy 2 {\mu}m lasers., Comment: I would like to withdrawal this paper because a mistake needs to be corrected. I will upload a new version

In this letter, an energetic and highly efficient dispersive wave (DW) generation at 200 nm has been numerically demonstrated by selectively exciting LP$_{02}$-like mode in a 10 bar Ar-filled hollow-core anti-resonant fiber pumping in the anomalous dispersion regime at 1030 nm with pulses of 30 fs duration and 7 $\mu$J energy. Our calculations indicate high conversion efficiency of $>$35\% (2.5 $\mu$J) after propagating 3.6 cm fiber length which is due to the strong shock effect and plasma induced blue-shifted soliton. It is observed that the efficiency of fundamental LP$_{01}$-mode is about 15\% which is much smaller than LP$_{02}$-like mode and also emitted at longer wavelength of 270 nm.

The PolyOculus technology produces large-area-equivalent telescopes by using fiber optics to link modules of multiple semi-autonomous, small, inexpensive, commercial-off-the-shelf telescopes. Crucially, this scalable design has construction costs which are > 10x lower than equivalent traditional large-area telescopes. We have developed a novel photonic lantern approach for the PolyOculus fiber optic linkages which potentially offers substantial advantages over previously considered free-space optical linkages, including much higher coupling efficiencies. We have carried out the first laboratory tests of a photonic lantern prototype developed for PolyOculus, and demonstrate broadband efficiencies of ~91%, confirming the outstanding performance of this technology., Comment: 13 pages, 8 figures

The development and wide application of quantum technologies highly depend on the capacity of the communication channels distributing entanglement. Space-division multiplexing (SDM) enhanced channel capacities in classical telecommunication and bears the potential to transfer the idea to quantum communication using current infrastructure. Here, we demonstrate an SDM of polarization-entangled photons over a 411m long 19-core multicore fiber distributing polarization-entangled photon pairs through up to 12 channels simultaneously. The quality of the multiplexed transfer is evidenced by high polarization visibility and CHSH Bell inequality violation for each pair of opposite cores. Our distribution scheme shows high stability over 24 hours without any active polarization stabilization and can be effortlessly adapted to a higher number of channels. This technique increases the quantum-channel capacity and allows the reliable implementation of quantum networks of multiple users based on a single entangled-photon pair source.

Understanding the impact of the cladding tube structure on the overall guiding performance is crucial for designing single-mode, wide-band, and ultra low-loss nested hollow-core anti-resonant fiber (HC-ARF). Here we thoroughly investigate on how the propagation loss is affected by the nested elements when their geometry is realistic (i.e., non-ideal). Interestingly, it was found that the size rather than the shape of the nested elements, have a dominant role in the final loss performance of the HC-ARFs. We identify a unique 'V-shape' pattern for suppression of higher-order modes loss by optimizing free design parameters of HC-ARF. We find that a 5-tube nested HC-ARF has wider transmission window and better single-mode operation than 6-tube HC-ARF. We show that the propagation loss can be significantly improved by using anisotropic nested anti-resonant tubes elongated in the radial direction. Our simulations indicate that with this novel fiber design, a propagation loss as low as 0.11 dB/km at 1.55 $\mu$m can be achieved. Our results provide design insights towards fully exploiting single-mode, wide-band, and ultra low-loss HC-ARF. In addition, the extraordinary optical properties of the proposed fiber can be beneficial for several applications such as future optical communication system, high energy light transport, extreme non-nonlinear optics and beyond.

In this work, we present a high pulse energy multi-wavelength Raman laser spanning from 1.53 um up to 2.4 um by employing the cascaded rotational stimulated Raman scattering (SRS) effect in a 5-m hydrogen (H2) -filled nested anti-resonant fiber (NARF), pumped by a linearly polarized Er/Yb fiber laser with a peak power of ~13 kW and pulse duration of ~7 ns in the C-band. The developed Raman laser has distinct lines at 1683 nm, 1868 nm, 2100 nm, and 2400 nm, with pulse energies as high as 18.25 uJ, 14.4 uJ, 14.1 uJ, and 8.2 uJ, respectively. We demonstrate how the energy in the Raman lines can be controlled by tuning the H2 pressure from 1 bar to 20 bar

We demonstrate 1 Tbit/s/$\lambda$ single-span transmission over a heterogeneous link consisting of graded-index 50 $\mu$m core multi-mode fiber and 6LP few-mode fiber using a Kramers-Kronig receiver structure. Furthermore, the link budget increase by transmitting only three modes while employing more than three receivers is investigated., Comment: Accepted for presentation at European Conference on Optical Communications (ECOC) 2020

We experimentally validate a mode-dependent loss (MDL) estimation technique employing acorrection factor to remove the MDL estimation dependence on the SNR when using a minimum meansquare error (MMSE) equalizer. A reduction of the MDL estimation error is observed for both transmitter-side and in-span MDL emulation., Comment: Accepted for presentation at European Conference on Optical Communications (ECOC) 2020

Digital holography measures the complex optical field and transfer matrix of a device, polarization-diversity is often achieved through spatial multiplexing. We introduce angular multiplexing, to increase flexibility in the optical setup. Comparatively, similar values for cross-talk and mode-dependent loss are measured for a photonic lantern., Comment: Accepted for presentation at the European Conference on Optical Communications (ECOC) 2020

Bright, coherent soft X-ray (SXR) radiation is essential to a variety of applications in fundamental research and life sciences. So far, high photon flux in this spectral region can only be delivered by synchrotrons, free electron lasers or high-order harmonic generation (HHG) sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment, in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched HHG in a single, helium-filled antiresonant hollow-core fibre (ARHCF). This enables the first 100 kHz-class repetition rate, table-top SXR source, that delivers an application-relevant flux of 2.8*10^6 Photons/s/eV around 300 eV. The fibre-integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure controlled phase-matching will allow compact, high repetition rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux SXR sources., Comment: 12 pages, 3 figures

The capacity in space division multiplexing (SDM) systems with coupled channels is fundamentally limited by mode-dependent loss (MDL) and mode-dependent gain (MDG) generated in components and amplifiers. In these systems, MDL/MDG must be accurately estimated for performance analysis and troubleshooting. Most recent demonstrations of SDM with coupled channels perform MDL/MDG estimation by digital signal processing (DSP) techniques based on the coefficients of multiple-input multiple-output (MIMO) adaptive equalizers. Although these methods provide a valid indication of the order of magnitude of the accumulated MDL/MDG over the link, MIMO equalizers are usually updated according to the minimum mean square error (MMSE) criterion, which is known to depend on the channel signal-to-noise ratio (SNR). Therefore, MDL/MDG estimation techniques based on the adaptive filter coefficients are also impaired by noise. In this paper, we model analytically the influence of the SNR on DSP-based MDL/MDG estimation, and show that the technique is prone to errors. Based on the transfer function of MIMO MMSE equalizers, and assuming a known SNR, we calculate a correction factor that improves the estimation process in moderate levels of MDL/MDG and SNR. The correction factor is validated by simulation of a 6-mode long-haul transmission link, and experimentally using a 3-mode transmission link. The results confirm the limitations of the standard estimation method in scenarios of high additive noise and MDL/MDG, and indicate the correction factor as a possible solution in practical SDM scenarios.

We realize mode-multiplexed full-field reconstruction over six spatial and polarization modes after 30-km multimode fiber transmission using intensity-only measurements without any optical carrier or local oscillator at the receiver or transmitter. The receiver's capabilities to cope with modal dispersion and mode dependent loss are experimentally demonstrated.

We demonstrate the first transmission of a new twelve-dimensional modulation format over a three-core coupled-core multicore fiber. The format occupies a single time slot spread across all three linearly-coupled spatial modes and shows improved MI and GMI after transmission compared to PDM-QPSK., Comment: 4 pages, 4 figures

Deep-UV (DUV) supercontinuum (SC) sources based on gas-filled hollow-core fibers constitute perhaps the most viable solution towards ultrafast, compact, and tunable lasers in the UV spectral region. Noise and spectral stability of such broadband sources are key parameters that define their true potential and suitability towards real-world applications. In order to investigate the spectral stability and noise levels in these fiber-based DUV sources, we generate an SC spectrum that extends from 180 nm (through phase-matched dispersive waves - DWs) to 4 {\mu}m by pumping an argon-filled hollow-core anti-resonant fiber at a wavelength of 2.45 {\mu}m. We characterize the long-term stability of the source over several days and the pulse-to-pulse relative intensity (RIN) noise of the strongest DW at 275 nm. The results indicate no sign of spectral degradation over 110 hours, but the RIN of the DW pulses at 275 nm is found to be as high as 33.3%. Numerical simulations were carried out to investigate the spectral distribution of the RIN and the results confirm the experimental measurements and that the poor noise performance is due to the RIN of the pump laser, which was hitherto not considered in numerical modelling of these sources. The results presented herein provide an important step towards an understanding of the noise mechanism underlying such complex light-gas nonlinear interactions and demonstrate the need for pump laser stabilization.

Geometric phases appear ubiquitously in many and diverse areas of physical sciences, ranging from classical and molecular dynamics to quantum mechanics and solid-state physics. In the realm of optics, similar phenomena are known to emerge in the form of a Pancharatnam-Berry phase whenever the polarization state traces a closed contour on the Poincare sphere. While this class of geometric phases has been extensively investigated in both free-space and guided wave systems, the observation of similar effects in photon-tunneling arrangements has so far remained largely unexplored. Here, for the first time, we experimentally demonstrate that the tunneling or coupling process in a twisted multi-core fiber system can display a chiral geometric phase accumulation-analogous to that of the Aharonov-Bohm effect resulting from the presence of a nonzero magnetic flux. In our experiments, the tunneling geometric phase is manifested through the interference of the corresponding supermodes. In this system, and for specific values of the twist rate, the tunneling between opposite cores ceases, thus signifying an Aharonov-Bohm suppression of tunneling. Our work provides the first observation of this intriguing effect in an optical setting.

Supercontinuum (SC) generation based on ultrashort pulse compression constitutes one of the most promising technologies towards an ultra-wide bandwidth, high-brightness and spatially coherent light sources for applications such as spectroscopy and microscopy. Here, multi-octave SC generation in a gas-filled hollow-core antiresonant fiber (HC-ARF) is reported spanning from 200 nm in the deep ultraviolet (DUV) to 4000 nm in the mid-infrared (mid-IR). A measured average output power of 5 mW was obtained by pumping at the center wavelength of the first anti-resonance transmission window (2460 nm) with ~100 fs pulses and an injected pulse energy of ~7-8 {\mu}J. The mechanism behind the extreme spectral broadening relies upon intense soliton-plasma nonlinear dynamics which leads to efficient soliton self-compression and phase-matched dispersive wave (DW) emission in the DUV region. The strongest DW is observed at 275 nm having an estimated pulse energy of 1.42 {\mu}J, corresponding to 28.4 % of the total output energy. Furthermore, the effect of changing the pump pulse energy and gas pressure on the nonlinear dynamics and their direct impact on SC generation was investigated. The current work paves a new way towards novel investigations of gas-based ultrafast nonlinear optics in the emerging mid-IR spectral regime.

We use white Gaussian noise as a test signal for single-mode and multimode transmission links and estimate the link capacity based on a calculation of mutual information. We also extract the complex amplitude channel estimations and mode-dependent loss with high accuracy., Comment: submitted to The Optical Networking and Communication Conference (OFC) 2018