High spatial-density, cladding-pumped 6-mode 7-core fiber amplifier for C-band operation.

We present a C-band 6-mode 7-core fiber amplifier in an all-fiberized cladding-pumped configuration for space division multiplexed transmission supporting a record 42 spatial channels. With optimized fiber components (e.g. passively cooled pump laser diode, pump coupler, pump stripper), high power multimode pump light is coupled to the active fiber without any noticeable thermal degradation and an average gain of 18 dB and noise figure of 5.4 dB are obtained with an average differential modal gain of 3.4 dB.

Study on the temperature dependent characteristics of O-band bismuth-doped fiber amplifier.

We report the temperature dependent performance of an O-band bismuth (Bi)-doped fiber amplifier (BDFA) in the temperature range from -60 to +80°C. At room temperature, maximum gains of 27 and 40 dB with noise figures (NFs) of 4.3 and 4.8 dB are measured for -23 dBm signal power in the single and double pass BDFA, respectively. An increment in gain and reduction in NF is observed as the ambient temperature of the BDFA is reduced. In the double pass BDFA, the temperature dependent gain coefficient from -60 to +80°C is found to be around -0.02 and -0.03 dB/°C across the wavelength band of 1300-1360 nm for -10 and -23 dBm signal powers, respectively. We also study the gain and NF characteristics with pump power and signal power at different temperatures, and a maximum gain of 45 dB is obtained at -60°C for -30 dBm signal power.

Demonstration of opposing thermal sensitivities in hollow-core fibers with open and sealed ends.

The output phase and propagation time of an optical signal propagating through a hollow-core optical fiber (HCF) drift with changes in environmental temperature significantly less than in conventional optical fibers. In all earlier experimental studies, however, the simplifying assumption was made that the thermo-optic effect of air was negligible. In this Letter, we present, to the best of our knowledge, the first experimental demonstration that the air inside a HCF core can make an appreciable contribution to the fiber's thermal sensitivity with the performance depending on whether the fiber is open to the atmosphere or sealed at both ends (e.g., spliced to solid fiber pigtails). We measure both the sensitivity of the accumulated phase as well as the signal propagation time for both open and sealed HCF and show that these are opposite in sign. Most importantly, we show that the thermal sensitivity contribution from the air inside an open HCF has the sign opposite to the effect of fiber elongation (which is otherwise the dominant effect responsible for the overall thermal sensitivity of HCF). We then go on to show that these two effects can be used to balance each other out in order to achieve zero thermal sensitivity for both accumulated phase and propagation time. We demonstrate this property experimentally over a large spectral range.

40 dB gain all fiber bismuth-doped amplifier operating in the O-band.

In this Letter, we investigate and compare the gain and noise figure characteristics of bismuth (Bi)-doped fiber amplifiers configured in both single and double signal pass implementations. A maximum gain of 25 dB and a noise figure of 4 dB is measured at 1360 nm in the single pass configuration for -23 dBm input signal power, whereas in the double pass configuration the gain of the amplifier is improved significantly by 14 dB allowing us to achieve a gain of 39 dB with a noise figure of 5 dB. To the best of our knowledge, this is the highest gain reported to date using Bi-doped fiber as a gain medium. Furthermore, we also study the gain and noise figure dependency on pump power, signal power, and pump wavelength for the double pass amplifier configuration. We observed similar gain and noise figure performance in the double pass configuration to that of the single pass configuration but with the benefit of less pump power and a shorter length of the Bi-doped fiber.

Hollow-core fibres for temperature-insensitive fibre optics and its demonstration in an Optoelectronic oscillator.

Many scientific and practical applications require the propagation time through cables to be well defined and known, e.g., an error in the evaluation of signal propagation time in the OPERA experiment in 2011 initially erroneously concluded that Neutrinos are faster than light. In fact, there are many other physical infrastructures such as synchrotrons, particle accelerators, telescope arrays and phase arrayed antennae that also rely on precise time synchronization. Time synchronization is also of importance in new practical applications like autonomous manufacturing (e.g., synchronization of assembly line robots) and upcoming 5G networks. Even when the propagation time through a coaxial cable or optical fibre is carefully calibrated, it is affected by changes in the ambient temperature, posing a serious technological challenge. We show how hollow-core optical fibres can address this issue.

Point-by-point femtosecond laser micro-processing of independent core-specific fiber Bragg gratings in a multi-core fiber.

Four 3rd order fiber Bragg gratings were inscribed into separate cores of a 7 core multi-core fiber using the point-by-point inscription technique. A 1030 nm, 206 ± 5 fs laser was used, operating at a frequency of 1 kHz and pulse energy of 2.1 ± 0.2 µJ. Independent Bragg resonances at λB = 1541.01 ± 0.02, 1547.82 ± 0.02, 1532.66 ± 0.02, and 1537.42 ± 0.02 nm and extinction ratios of 13.97 ± 0.4, 16.02 ± 0.4, 10.08 ± 0.4 and 13.40 ± 0.4 dB were recorded. Our data analysis shows that refractive index changes, Δn, of the order 10-3 were induced. Core-specific inscription of fiber Bragg gratings in a multi-core fiber can provide a flexible and versatile platform to address the needs of recent space division multiplexed transmission and optical sensor networks.

49.6 Gb/s direct detection DMT transmission over 40 km single mode fibre using an electrically packaged silicon photonic modulator.

We present the characterization of a silicon Mach-Zehnder modulator with electrical packaging and show that it exhibits a large third-order intermodulation spurious-free dynamic range (> 100 dB Hz2/3). This characteristic renders the modulator particularly suitable for the generation of high spectral efficiency discrete multi-tone signals and we experimentally demonstrate a single-channel, direct detection transmission system operating at 49.6 Gb/s, exhibiting a baseband spectral efficiency of 5 b/s/Hz. Successful transmission is demonstrated over various lengths of single mode fibre up to 40 km, without the need of any amplification or dispersion compensation.

Thulium-fiber-laser-pumped, high-peak-power, picosecond, mid-infrared orientation-patterned GaAs optical parametric generator and amplifier.

We report a high-peak-power, picosecond, mid-infrared optical parametric generator (OPG) and optical parametric amplifier (OPA) based on orientation-patterned gallium arsenide pumped by a thulium:fiber master-oscillator-power-amplifier employing direct diode-seeded amplification. An OPG tuning range of 2550-2940 nm (signal) and 5800-8300 nm (idler) is demonstrated with peak powers as high as 3 (signal) and 2 kW (idler), and with pulse energies of 0.26 and 0.16 µJ, respectively. When seeded with a 0.6 cm-1 linewidth tunable Cr:ZnSe laser, the OPA idler linewidth is narrowed to 1.4 cm-1 and a small-signal parametric gain of 60 dB is achieved. A maximum peak power of 13.3 (signal) and 3.2 kW (idler) is obtained at an overall conversion efficiency of 36%. The corresponding maximum pulse energies for the signal and idler are 1.07 and 0.26 µJ, respectively.

Low-loss Kagome hollow-core fibers operating from the near- to the mid-IR.

We report the fabrication and characterization of Kagome hollow-core antiresonant fibers, which combine low attenuation (as measured at ∼30 cm bend diameter) with a wide operating bandwidth and high modal purity. Record low attenuation values are reported: 12.3 dB/km, 13.9 dB/km, and 9.6 dB/km in three different fibers optimized for operation at 1 µm, 1.55 µm, and 2.5 µm, respectively. These fibers are excellent candidates for ultra-high power delivery at key laser wavelengths including 1.064 µm and 2.94 µm, as well as for applications in gas-based sensing and nonlinear optics.

Optoelectronic oscillator incorporating hollow-core photonic bandgap fiber.

We demonstrate, to the best of our knowledge, the first optoelectronic oscillator that uses hollow-core photonic bandgap fiber (HC-PBGF) as a delay element of a sufficient length to allow for low-noise operation. We show experimentally that HC-PBGF can improve the temperature stability of the oscillator by a factor of more than 15, as compared to standard optical fiber. We also measured the oscillator's phase noise, allowing evaluation of the suitability of HC-PBGF for this application. Additionally, this Letter also provides, to the best of our knowledge, the first characterization of the temperature stability of a long length (>800 m in our Letter) of low-thermal sensitivity (2 ps/km/K) HC-PBGF wound on a spool.

Radially and azimuthally polarized nanosecond Yb-doped fiber MOPA system incorporating temporal shaping.

We report an Yb-doped fiber master-oscillator power-amplifier (MOPA) system with the capability of selectively generating doughnut-shaped radially and azimuthally polarized beams with user-defined temporal pulse shapes. The desired output polarization was generated with the aid of a nanograting spatially variant half-waveplate (S-waveplate). The latter was used to convert the linearly polarized fundamental (LP01) mode output from the preamplification stages to a doughnut-shaped radially polarized beam prior to the power amplifier stage. A maximum output pulse energy of ∼860 µJ was achieved for ∼100 ns pulses at 25 kHz with user-defined pulse shape for both radial and azimuthal polarization states. The polarization purity and beam propagation factor (M2) were measured to be >12 dB and 2.2, respectively.

Wavelength conversion of complex modulation formats in a compact SiGe waveguide.

We report a nonlinear signal processing system based on a SiGe waveguide suitable for high spectral efficiency data signals. Four-wave-mixing (FWM)-based wavelength conversion of 10-Gbaud 16-Quadrature amplitude modulated (QAM) and 64-QAM signals is demonstrated with less than -10-dB conversion efficiency (CE), 36-dB idler optical signal-to-noise ratio (OSNR), negligible bit error ratio (BER) penalty and a 3-dB conversion bandwidth exceeding 30nm. The SiGe device was CW-pumped and operated in a passive scheme without giving rise to any two-photon absorption (TPA) effects.

Full quadrature regeneration of QPSK signals using sequential phase sensitive amplification and parametric saturation.

We demonstrate all-optical regeneration of both the phase and the amplitude of a 10 GBaud quadrature phase shift keying (QPSK) signal using two nonlinear stages. First we regenerate the phase using a wavelength converting phase sensitive amplifier and then we regenerate the amplitude using a saturated single-pump parametric amplifier, returning the signal to its original wavelength at the same time. We exploit the conjugating nature of the two processing stages to eliminate the intrinsic SPM distortion of the system, further improving performance.

The metabolic impact of extracellular nitrite on aerobic metabolism of Paracoccus denitrificans.

Nitrite, in equilibrium with free nitrous acid (FNA), can inhibit both aerobic and anaerobic growth of microbial communities through bactericidal activities that have considerable potential for control of microbial growth in a range of water systems. There has been much focus on the effect of nitrite/FNA on anaerobic metabolism and so, to enhance understanding of the metabolic impact of nitrite/FNA on aerobic metabolism, a study was undertaken with a model denitrifying bacterium Paracoccus denitrificans PD1222. Extracellular nitrite inhibits aerobic growth of P. denitrificans in a pH-dependent manner that is likely to be a result of both nitrite and free nitrous acid (pKa = 3.25) and subsequent reactive nitrogen oxides generated from the intracellular passage of FNA into P. denitrificans. Increased expression of a gene encoding a flavohemoglobin protein (Fhp) (Pden_1689) was observed in response to extracellular nitrite. Construction and analysis of a deletion mutant established Fhp to be involved in endowing nitrite/FNA resistance at high extracellular nitrite concentrations. Global transcriptional analysis confirmed nitrite-dependent expression of fhp and indicated that P. denitrificans expressed a number of stress response systems associated with protein, DNA and lipid repair. It is therefore suggested that nitrite causes a pH-dependent stress response that is due to the production of associated reactive nitrogen species, such as nitric oxide from the internalisation of FNA.

Raman-shifted wavelength-selectable pulsed fiber laser with high repetition rate and high pulse energy in the visible.

A high-pulse-energy, diffraction-limited, wavelength-selectable, visible source, based on Raman frequency shifting of a frequency-doubled Yb-doped fiber laser, has been studied. The relative length-scaling laws of Raman gain and self-phase modulation push the design towards short fiber lengths with large core size. It is experimentally demonstrated that the Raman clean-up effect in a graded-index multi-mode fiber is not sufficient to obtain diffraction-limited beam quality in the short fiber length. Thus, a large-core photonic crystal fiber is used to maintain diffraction-limited performance and output pulse energies of ~1 µJ, at a 1-MHz repetition rate and 1.3-ns pulse-width are successfully achieved. This step-tunable visible source should find applications in photoacoustic microscopy.

Exploiting the short wavelength gain of silica-based thulium-doped fiber amplifiers.

Short wavelength operation (1650-1800 nm) of silica-based thulium-doped fiber amplifiers (TDFAs) is investigated. We report the first demonstration of in-band diode-pumped silica-based TDFAs working in the 1700-1800 nm waveband. Up to 29 dB of small-signal gain is achieved in this spectral region, with an operation wavelength accessible by diode pumping as short as 1710 nm. Further gain extension toward shorter wavelengths is realized in a fiber laser pumped configuration. A silica-based TDFA working in the 1650-1700 nm range with up to 29 dB small-signal gain and noise figure as low as 6.5 dB is presented.

Anaerobic Metabolism in Haloferax Genus: Denitrification as Case of Study.

A number of species of Haloferax genus (halophilic archaea) are able to grow microaerobically or even anaerobically using different alternative electron acceptors such as fumarate, nitrate, chlorate, dimethyl sulphoxide, sulphide and/or trimethylamine. This metabolic capability is also shown by other species of the Halobacteriaceae and Haloferacaceae families (Archaea domain) and it has been mainly tested by physiological studies where cell growth is observed under anaerobic conditions in the presence of the mentioned compounds. This work summarises the main reported features on anaerobic metabolism in the Haloferax, one of the better described haloarchaeal genus with significant potential uses in biotechnology and bioremediation. Special attention has been paid to denitrification, also called nitrate respiration. This pathway has been studied so far from Haloferax mediterranei and Haloferax denitrificans mainly from biochemical point of view (purification and characterisation of the enzymes catalysing the two first reactions). However, gene expression and gene regulation is far from known at the time of writing this chapter.

Mechanisms of Bacterial Extracellular Electron Exchange.

The biochemical mechanisms by which microbes interact with extracellular soluble metal ions and insoluble redox-active minerals have been the focus of intense research over the last three decades. The process presents two challenges to the microorganism. Firstly, electrons have to be transported at the cell surface, which in Gram-negative bacteria presents an additional problem of electron transfer across the ~6nm of the outer membrane. Secondly, the electrons must be transferred to or from the terminal electron acceptors or donors. This review covers the known mechanisms that bacteria use to transport electrons across the cell envelope to external electron donors/acceptors. In Gram-negative bacteria, electron transfer across the outer membrane involves the use of an outer membrane ß-barrel and cytochrome. These can be in the form of a porin-cytochrome protein, such as Cyc2 of Acidithiobacillus ferrooxidans, or a multiprotein porin-cytochrome complex like MtrCAB of Shewanella oneidensis MR-1. For mineral-respiring organisms, there is the additional challenge of transferring the electrons from the cell to mineral surface. For the strict anaerobe Geobacter sulfurreducens this requires electron transfer through conductive pili to associated cytochrome OmcS that directly reduces Fe(III)oxides, while the facultative anaerobe S. oneidensis MR-1 accomplishes mineral reduction through direct membrane contact, contact through filamentous extensions and soluble flavin shuttles, all of which require the outer membrane cytochromes MtrC and OmcA in addition to secreted flavin.

Nitrous Oxide Metabolism in Nitrate-Reducing Bacteria: Physiology and Regulatory Mechanisms.

Nitrous oxide (N2O) is an important greenhouse gas (GHG) with substantial global warming potential and also contributes to ozone depletion through photochemical nitric oxide (NO) production in the stratosphere. The negative effects of N2O on climate and stratospheric ozone make N2O mitigation an international challenge. More than 60% of global N2O emissions are emitted from agricultural soils mainly due to the application of synthetic nitrogen-containing fertilizers. Thus, mitigation strategies must be developed which increase (or at least do not negatively impact) on agricultural efficiency whilst decrease the levels of N2O released. This aim is particularly important in the context of the ever expanding population and subsequent increased burden on the food chain. More than two-thirds of N2O emissions from soils can be attributed to bacterial and fungal denitrification and nitrification processes. In ammonia-oxidizing bacteria, N2O is formed through the oxidation of hydroxylamine to nitrite. In denitrifiers, nitrate is reduced to N2 via nitrite, NO and N2O production. In addition to denitrification, respiratory nitrate ammonification (also termed dissimilatory nitrate reduction to ammonium) is another important nitrate-reducing mechanism in soil, responsible for the loss of nitrate and production of N2O from reduction of NO that is formed as a by-product of the reduction process. This review will synthesize our current understanding of the environmental, regulatory and biochemical control of N2O emissions by nitrate-reducing bacteria and point to new solutions for agricultural GHG mitigation.

Optimisation of amplitude limiters for phase preservation based on the exact solution to degenerate four-wave mixing.

Adopting an exact solution to four-wave mixing (FWM), wherein harmonic evolution is described by the sum of two Bessel functions, we identify two causes of amplitude to phase noise conversion which impair FWM saturation based amplitude regenerators: self-phase modulation (SPM) and Bessel-order mixing (BOM). By increasing the pump to signal power ratio, we may arbitrarily reduce their impact, realising a phase preserving amplitude regenerator. We demonstrate the technique by applying it to the regeneration of a 10 GBaud QPSK signal, achieving a high level of amplitude squeezing with minimal amplitude to phase noise conversion.