Coherent O-band WDM transmission of DP-16QAM over a 50-km BDFA-amplified link.

We present wavelength-division multiplexed coherent transmission in an O-band amplified link enabled by bismuth-doped fiber amplifiers (BDFAs). Transmission of 4 × 25 GBd DP-16QAM (4 × 200 Gb/s) is demonstrated over a single span of 50-km length, occupying a bandwidth of 4.7 THz across the wavelengths 1323 nm to 1351 nm.

Transient gas-induced differential refractive index effects in as-drawn hollow core optical fibers.

When a hollow core fiber is drawn, the core and cladding holes within the internal cane geometry are pressurized with an inert gas to enable precise control over the internal microstructure of the fiber and counteract surface tension forces. Primarily by considering the temperature drop as the fiber passes through the furnace and the geometrical transformation of the internal microstructure from preform-to-fiber, we recently established that the gas pressure within the final 'as-drawn' fiber is substantially below atmospheric pressure. We have also established that slight changes in the gas refractive index within the core and surrounding cladding holes induced by changes in gas pressure are sufficient to significantly affect both the modality and loss of the fiber. Here we demonstrate, through both simulations and experimental measurements, that the combination of these effects leads to transient changes in the fiber's attenuation when the fibers are opened to atmosphere post-fabrication. It is important to account for this phenomenon for accurate fiber characterization, particularly when long lengths of fiber are drawn where it could take many weeks for every part of the internal microstructure to reach atmospheric pressure.

Double-Clad Antiresonant Hollow-Core Fiber and Its Comparison with Other Fibers for Multiphoton Micro-Endoscopy.

Label-free and multiphoton micro-endoscopy can transform clinical histopathology by providing an in situ tool for diagnostic imaging and surgical treatment in diseases such as cancer. Key to a multiphoton imaging-based micro-endoscopic device is the optical fiber, for distortion-free and efficient delivery of ultra-short laser pulses to the sample and effective signal collection. In this work, we study a new hollow-core (air-filled) double-clad anti-resonant fiber (DC-ARF) as a high-performance candidate for multiphoton micro-endoscopy. We compare the fiber characteristics of the DC-ARF with a single-clad anti-resonant fiber (SC-ARF) and a solid core fiber (SCF). In this work, while the DC-ARF and the SC-ARF enable low-loss (<0.2 dBm-1), close to dispersion-free excitation pulse delivery (<10% pulse width increase at 900 nm per 1 m fiber) without any induced non-linearities, the SCF resulted in spectral broadening and pulse-stretching (>2000% of pulse width increase at 900 nm per 1 m fiber). An ideal optical fiber endoscope needs to be several meters long and should enable both excitation and collection through the fiber. Therefore, we performed multiphoton imaging on endoscopy-compatible 1 m and 3 m lengths of fiber in the back-scattered geometry, wherein the signals were collected either directly (non-descanned detection) or through the fiber (descanned detection). Second harmonic images were collected from barium titanate crystals as well as from biological samples (mouse tail tendon). In non-descanned detection conditions, the ARFs outperformed the SCF by up to 10 times in terms of signal-to-noise ratio of images. Significantly, only the DC-ARF, due to its high numerical aperture (NA) of 0.45 and wide-collection bandwidth (>1 µm), could provide images in the de-scanned detection configuration desirable for endoscopy. Thus, our systematic characterization and comparison of different optical fibers under different image collection configurations, confirms and establishes the utility of DC-ARFs for high-performing label-free multiphoton imaging-based micro-endoscopy.

High power Raman second stokes generation in a methane filled hollow core fiber.

We demonstrate a multi-watt, picosecond pulse duration laser source by exploiting a cascaded Raman process to the second Stokes signal at a wavelength of 2.58 µm in a methane-filled Nested Anti-Resonant Nodeless fiber from a 1 µm disk laser source. A maximum average power of 2.89 W (14.45 µJ) is produced in a 160 cm length of custom-designed and in-house fabricated fiber filled with methane at a pressure of 2 bar. The impact of gas pressure and propagation distance on the second Stokes signal power are investigated experimentally. The experimental results are simulated by solving the Generalized Nonlinear Schrodinger Equation with the experiment carefully modelled by accounting for the impacts of pressure dependent gas-light interactions along the pressure gradient of the fiber. This work offers a laser source for a variety of applications as well as expanding the modelling space to methane filled fibers including pressure gradients, and nonlinear optical activity in the presence of infrared gas absorption.

Mode attraction, rejection and control in nonlinear multimode optics.

Novel fundamental notions helping in the interpretation of the complex dynamics of nonlinear systems are essential to our understanding and ability to exploit them. In this work we predict and demonstrate experimentally a fundamental property of Kerr-nonlinear media, which we name mode rejection and takes place when two intense counter-propagating beams interact in a multimode waveguide. In stark contrast to mode attraction phenomena, mode rejection leads to the selective suppression of a spatial mode in the forward beam, which is controlled via the counter-propagating backward beam. Starting from this observation we generalise the ideas of attraction and rejection in nonlinear multimode systems of arbitrary dimension, which paves the way towards a more general idea of all-optical mode control. These ideas represent universal tools to explore novel dynamics and applications in a variety of optical and non-optical nonlinear systems. Coherent beam combination in polarisation-maintaining multicore fibres is demonstrated as example.

Fiber-delivered heterodyne spectroscopy with a mid-infrared frequency comb.

By exploiting the excellent short-term phase stability between consecutive pulses from a free-running optical parametric oscillator frequency comb, we report the first example of hollow-core fiber-delivered heterodyne spectroscopy in the 3.1-3.8 µm wavelength range. The technique provides a means of spectroscopically interrogating a sample situated at the distal end of a fiber, with all electronics and light sources situated at the proximal end and with an inherent capability to suppress spectroscopically interfering features present in the free-space and in-fiber delivery path. Using a silica anti-resonant, hollow-core delivery fiber, we demonstrate high quality transmission and attenuated total reflectance spectroscopy of a plastic sample for fiber lengths of up to 40 m, significantly exceeding the few-meter lengths typically possible using solid-core fibers. The technique opens a route to implementing multi-species spectroscopic monitoring in remote and / or hostile industrial environments and medical applications.

All-fiberized 1840-nm femtosecond thulium fiber laser for label-free nonlinear microscopy.

We report an all-fiberized 1840-nm thulium-fiber-laser source, comprising a dissipative-soliton mode-locked seed laser and a chirped-pulse-amplification system for label-free biological imaging through nonlinear microscopy. The mode-locked thulium fiber laser generated dissipative-soliton pulses with a pre-chirped duration of 7 ps and pulse energy of 1 nJ. A chirped-pulse fiber-amplification system employing an in-house-fabricated, short-length, single-mode, high-absorption, thulium fiber delivered pulses with energies up to 105 nJ. The pulses were capable of being compressed to 416 fs by passing through a grating pair. Imaging of mouse tissue and human bone samples was demonstrated using this source via third-harmonic generation microscopy.

The copper-responsive regulator CsoR is indirectly involved in Bradyrhizobium diazoefficiens denitrification.

The soybean endosymbiont Bradyrhizobium diazoefficiens harbours the complete denitrification pathway that is catalysed by a periplasmic nitrate reductase (Nap), a copper (Cu)-containing nitrite reductase (NirK), a c-type nitric oxide reductase (cNor), and a nitrous oxide reductase (Nos), encoded by the napEDABC, nirK, norCBQD, and nosRZDFYLX genes, respectively. Induction of denitrification genes requires low oxygen and nitric oxide, both signals integrated into a complex regulatory network comprised by two interconnected cascades, FixLJ-FixK2-NnrR and RegSR-NifA. Copper is a cofactor of NirK and Nos, but it has also a role in denitrification gene expression and protein synthesis. In fact, Cu limitation triggers a substantial down-regulation of nirK, norCBQD, and nosRZDFYLX gene expression under denitrifying conditions. Bradyrhizobium diazoefficiens genome possesses a gene predicted to encode a Cu-responsive repressor of the CsoR family, which is located adjacent to copA, a gene encoding a putative Cu+-ATPase transporter. To investigate the role of CsoR in the control of denitrification gene expression in response to Cu, a csoR deletion mutant was constructed in this work. Mutation of csoR did not affect the capacity of B. diazoefficiens to grow under denitrifying conditions. However, by using qRT-PCR analyses, we showed that nirK and norCBQD expression was much lower in the csoR mutant compared to wild-type levels under Cu-limiting denitrifying conditions. On the contrary, copA expression was significantly increased in the csoR mutant. The results obtained suggest that CsoR acts as a repressor of copA. Under Cu limitation, CsoR has also an indirect role in the expression of nirK and norCBQD genes.

15-µJ picosecond hollow-core-fiber-feedback optical parametric oscillator.

We report a high-energy, picosecond, mid-infrared (MIR) optical parametric oscillator (OPO), in which a length of hollow-core-fiber (HCF) is employed to enable operation at 1-MHz repetition rate in a compact cavity format. The OPO is synchronously pumped by an ytterbium-doped-fiber (YDF) master-oscillator-power-amplifier (MOPA) system, seeded by a 1040-nm gain-switched laser diode (GSLD). Using periodically poled lithium niobate (PPLN) as the nonlinear crystal, the OPO generates signal and idler beams with tunable wavelengths in the range of 1329-1641 nm and 2841-4790 nm, respectively. The OPO provides 137-ps pulses with a maximum signal energy of 10.05 µJ at 1600 nm and a maximum idler energy of 5.13 µJ at 2967 nm. This, to the best of our knowledge, represents the highest energy MIR pulses, as well as the highest total converted pulse energy (15.18 µJ), ever achieved from a fiber laser pumped picosecond OPO.

The effect of source backing materials and excitation pulse durations on laser-generated ultrasound waveforms.

In this article, it is shown experimentally that a planar laser-generated ultrasound source with a hard reflective backing will generate higher acoustic pressures than a comparable source with an acoustically matched backing when the stress confinement condition is not met. Furthermore, while the source with an acoustically matched backing will have a broader bandwidth when the laser pulse is short enough to ensure stress confinement, the bandwidths of both source types will converge as the laser pulse duration increases beyond stress confinement. The explanation of the results is supported by numerical simulations.

Gap design to enable functionalities into nested antiresonant nodeless fiber based systems.

By modifying the interconnection design between standard single-mode fiber (SSMF) and nested antiresonant nodeless type hollow-core fiber (NANF), we create an air gap between SSMF and NANF. This air gap enables the insertion of optical elements, thus providing additional functions. We show low-loss coupling using various graded-index multimode fibers acting as mode-field adapters resulting in different air-gap distances. Finally, we test the gap functionality by inserting a thin glass sheet in the air gap, which forms a Fabry-Perot interferometer and works as a filter with an overall insertion loss of only 0.31 dB.

Hollow core fiber Fabry-Perot interferometers with reduced sensitivity to temperature: erratum.

In the original publication of our research article "Hollow core fiber Fabry-Perot interferometers with reduced sensitivity to temperature" [Opt. Lett.47, 2510 (2022)10.1364/OL.456589OPLEDP0146-9592], we identified an error that requires correction. The authors sincerely apologize for any confusion that may have arisen from this error. The correction does not affect the overall conclusions of the paper.

Experimental investigation of BDFA-based O-band direct-detection transmission using an optical recirculating loop.

We implemented a bismuth-doped fiber amplifier (BDFA) based optical recirculating loop to investigate the performance of amplified O-band transmission over appreciable distances. Both single-wavelength and wavelength-division multiplexed (WDM) transmission were studied, with a variety of direct-detection modulation formats. We report on (a) transmission over lengths of up to 550 km in a single-channel 50-Gb/s system operating at wavelengths ranging from 1325 nm to 1350 nm, and (b) rate-reach products up to 57.6 Tb/s-km (after accounting for the forward error correction redundancy) in a 3-channel system.

Hollow-core fiber with stable propagation delay between -150°C and +60°C.

Optical fibers with a low thermal coefficient of delay (TCD) have been developed for frequency and timing transmission/distribution. However, their temperature sensitivity changes as a function of temperature and, to date, no study of such fibers has demonstrated improved performance over extended temperature ranges, especially at sub-zero temperatures. Here, we show that a hollow core fiber (HCF) with a thin acrylate coating can have a TCD within ±2.0 ps/km/°C over a broad temperature range from -150°C to +60°C. In addition, this thinly coated HCF can be fully insensitive to temperature around -134°C, making it of interest, e.g., for laser stabilization close to cryogenic temperatures.

A Cysteine Pair Controls Flavin Reduction by Extracellular Cytochromes during Anoxic/Oxic Environmental Transitions.

Many bacteria of the genus Shewanella are facultative anaerobes able to reduce a broad range of soluble and insoluble substrates, including Fe(III) mineral oxides. Under anoxic conditions, the bacterium Shewanella oneidensis MR-1 uses a porin-cytochrome complex (Mtr) to mediate extracellular electron transfer (EET) across the outer membrane to extracellular substrates. However, it is unclear how EET prevents generating harmful reactive oxygen species (ROS) when exposed to oxic environments. The Mtr complex is expressed under anoxic and oxygen-limited conditions and contains an extracellular MtrC subunit. This has a conserved CX8C motif that inhibits aerobic growth when removed. This inhibition is caused by an increase in ROS that kills the majority of S. oneidensis cells in culture. To better understand this effect, soluble MtrC isoforms with modified CX8C were isolated. These isoforms produced increased concentrations of H2O2 in the presence of flavin mononucleotide (FMN) and greatly increased the affinity between MtrC and FMN. X-ray crystallography revealed that the molecular structure of MtrC isoforms was largely unchanged, while small-angle X-ray scattering suggested that a change in flexibility was responsible for controlling FMN binding. Together, these results reveal that FMN reduction in S. oneidensis MR-1 is controlled by the redox-active disulfide on the cytochrome surface. In the presence of oxygen, the disulfide forms, lowering the affinity for FMN and decreasing the rate of peroxide formation. This cysteine pair consequently allows the cell to respond to changes in oxygen level and survive in a rapidly transitioning environment. IMPORTANCE Bacteria that live at the oxic/anoxic interface have to rapidly adapt to changes in oxygen levels within their environment. The facultative anaerobe Shewanella oneidensis MR-1 can use EET to respire in the absence of oxygen, but on exposure to oxygen, EET could directly reduce extracellular oxygen and generate harmful reactive oxygen species that damage the bacterium. By modifying an extracellular cytochrome called MtrC, we show how preventing a redox-active disulfide from forming causes the production of cytotoxic concentrations of peroxide. The disulfide affects the affinity of MtrC for the redox-active flavin mononucleotide, which is part of the EET pathway. Our results demonstrate how a cysteine pair exposed on the surface controls the path of electron transfer, allowing facultative anaerobic bacteria to rapidly adapt to changes in oxygen concentration at the oxic/anoxic interface.

Sub-ppm gas phase Raman spectroscopy in an anti-resonant hollow core fiber.

We demonstrate recent progress in the development of a Raman gas sensor using a single cladding ring anti-resonant hollow core micro-structured optical fiber (HC-ARF) and a low power pump source. The HC-ARF was designed specifically for low attenuation and wide bandwidth in the visible spectral region and provided low loss at both the pump wavelength (532 nm) and Stokes wavelengths up to a Raman shift of 5000 cm-1. A novel selective core pressurization scheme was also implemented to further reduce the confinement loss, improving the Raman signal enhancement by a factor of 1.9 compared to a standard fiber filling scheme. By exploiting longer lengths of fiber, direct detection of both methane and hydrogen at concentrations of 5 and 10 ppm respectively is demonstrated and a noise equivalent limit-of-detection of 0.15 ppm is calculated for methane.

Hundred-meter-scale, kilowatt peak-power, near-diffraction-limited, mid-infrared pulse delivery via the low-loss hollow-core fiber.

We report a high-power single-mode mid-infrared (MIR) pulse delivery system via anti-resonant hollow-core fiber (HCF) with a record delivery distance of 108 m. Near-diffraction-limited MIR light was transmitted by HCFs at wavelengths of 3.12-3.58 µm using a tunable optical parametric oscillator (OPO) as the light source. The HCFs were purged beforehand with argon in order to remove or reduce loss due to parasitic gas absorption (HCl, CO2, etc.). The minimum fiber loss values were 0.05 and 0.24 dB/m at 3.4-3.6 µm and 4.5-4.6 µm, respectively, with the 4.5-4.6 µm loss figure representing, to the best of our knowledge, a new low loss record for a HCF in this spectral region. At a coupling efficiency of ∼70%, average powers of 592 mW and 133 mW were delivered through 5 m and 108 m of HCF, respectively. Assuming the 120-ps duration of the MIR pulses remained constant over the low-dispersion HCF (theoretical maximum: 0.4 ps/nm/km), the corresponding calculated peak powers were 4.9 kW and 1.1 kW.

Low loss and broadband low back-reflection interconnection between a hollow-core and standard single-mode fiber.

We report simultaneous low coupling loss (below 0.2 dB at 1550 nm) and low back-reflection (below -60 dB in the 1200-1600 nm range) between a hollow core fiber and standard single mode optical fiber obtained through the combination of an angled interface and an anti-reflective coating. We perform experimental optimization of the interface angle to achieve the best combination of performance in terms of the coupling loss and back-reflection suppression. Furthermore, we examine parasitic cross-coupling to the higher-order modes and show that it does not degrade compared to the case of a flat interface, keeping it below -30 dB and below -20 dB for LP11 and LP02 modes, respectively.

Optical time domain backscattering of antiresonant hollow core fibers.

Today's lowest-loss hollow core fibers are based on antiresonance guidance. They have been shown both theoretically and experimentally to have very low levels of backscattering arising from the fiber structure - 45 dB below that of traditional optical fibers with a solid silica glass core. This makes their longitudinal characterization using conventional reflectometric techniques very challenging. However, it was recently estimated that when filled with air, their backscattering coefficient increases to about 30 dB below that of standard solid core fibers. This level should be measurable with commercially available high performance optical time domain reflectometers (OTDR). Here we demonstrate - for the first time to the best of our knowledge - the measurement of backscattering from the air inside a hollow core fiber. We show that the characterization of multi-km long hollow core fibers with 15 m spatial resolution is possible using a commercial OTDR instrument. To benefit from its full dynamic range, we strongly suppress the 4% back-reflections that ordinarily occur at the OTDR's standard fiber output when directly-connected to a hollow core fiber. Furthermore, low coupling loss into the hollow core fiber (0.3 dB in our experiment) also helps to maximize the achievable OTDR signal-to-noise ratio. This approach enables distributed characterization and fault-finding in low-loss hollow core fibers, a topic of increasing importance as these fibers are now starting to be installed in commercial optical communication networks.

Experimental demonstration of single-span 100-km O-band 4×50-Gb/s CWDM direct-detection transmission.

We report on what is to the best of our knowledge the longest 50-Gb/s/λ O-band wavelength-division multiplexed (WDM) transmission. A pair of in-house built bismuth-doped fiber amplifiers (BDFAs) and the use of Kramers-Kronig detection-assisted single-sideband transmission are adopted to overcome the fiber loss and chromatic dispersion, respectively, in a reach-extended O-band coarse WDM (CWDM) system with a channel spacing of ∼10 nm. Through experiments on an amplified 4×50-Gb/s/λ direct-detection system based on booster and pre-amp BDFAs, we show the superior performance of single-sideband transmission in terms of both optical signal-to-noise ratio sensitivity and uniformity in performance amongst CWDM channels relative to double-sideband transmission after both 75-km and 100-km lengths of single-mode fiber. As a result, up to 100-km reach with comparable performance at all 50-Gb/s channels was achieved without the need for in-line optical amplification.