Super-broadband on-chip continuous spectral translation unlocking coherent optical communications beyond conventional telecom bands.

Today's optical communication systems are fast approaching their capacity limits in the conventional telecom bands. Opening up new wavelength bands is becoming an appealing solution to the capacity crunch. However, this ordinarily requires the development of optical transceivers for any new wavelength band, which is time-consuming and expensive. Here, we present an on-chip continuous spectral translation method that leverages existing commercial transceivers to unlock the vast and currently unused potential new wavelength bands. The spectral translators are continuous-wave laser pumped aluminum gallium arsenide on insulator (AlGaAsOI) nanowaveguides that provide a continuous conversion bandwidth over an octave. We demonstrate coherent transmission in the 2-µm band using well-developed conventional C-band transmitters and coherent receivers, as an example of the potential of the spectral translators that could also unlock communications at other wavelength bands. We demonstrate 318.25-Gbit s-1 Nyquist wavelength-division multiplexed coherent transmission over a 1.15-km hollow-core fibre using this approach. Our demonstration paves the way for transmitting, detecting, and processing signals at wavelength bands beyond the capability of today's devices.

Laser frequency stabilization and spectroscopy at 2051 nm using a compact CO2-filled Kagome hollow core fiber gas-cell system.

We describe a compact, all fiber, frequency stabilized diode laser system at 2051 nm using CO2 gas-filled Kagome Hollow Core Fiber (HCF), capable of tuning continuously over four transitions in 12C16O2: R(24), R(26), R(28), and R(30). This laser system has been designed for use in future space-based atmospheric monitoring using differential absorption lidar (DIAL). The fully spliced Kagome HCF gas cell is filled to 2 kPa CO2 partial pressure and we compare the observed CO2 lineshape features with those calculated using HITRAN, to quantify the properties of the CO2-filled fiber cell. In this first demonstration of Kagome HCF used in a fully sealed gas cell configuration for spectroscopy at 2 µm, we characterize the frequency stability of the locked system by beat frequency comparison against a reference laser. Results are presented for the laser locked to the center of the 12C16O2 R(30) transition, with frequency stability of ∼40 kHz or better at 1 s, and a frequency reproducibility at the 0.4-MHz level over a period of > 1 month. For DIAL applications, we also demonstrate two methods of stabilizing the laser frequency ~3 GHz from this line. Furthermore, no pressure degradation was observed during the ~15-month period in which frequency stability measurements were acquired.

Anisotropic Superattenuation of Capillary Waves on Driven Glass Interfaces.

Metrological atomic force microscopy measurements are performed on the silica glass interfaces of photonic band-gap fibers and hollow capillaries. The freezing of attenuated out-of-equilibrium capillary waves during the drawing process is shown to result in a reduced surface roughness. The roughness attenuation with respect to the expected thermodynamical limit is determined to vary with the drawing stress following a power law. A striking anisotropic character of the height correlation is observed: glass surfaces thus retain a structural record of the direction of the flow to which the liquid was submitted.

Accurate modelling of fabricated hollow-core photonic bandgap fibers.

We report a novel approach to reconstruct the cross-sectional profile of fabricated hollow-core photonic bandgap fibers from scanning electron microscope images. Finite element simulations on the reconstructed geometries achieve a remarkable match with the measured transmission window, surface mode position and attenuation. The agreement between estimated scattering loss from surface roughness and measured loss values indicates that structural distortions, in particular the uneven distribution of glass across the thin silica struts on the core boundary, have a strong impact on the loss. This provides insight into the differences between idealized models and fabricated fibers, which could be key to further fiber loss reduction.

Low-loss and low-bend-sensitivity mid-infrared guidance in a hollow-core-photonic-bandgap fiber.

Hollow-core-photonic-bandgap fiber, fabricated from high-purity synthetic silica, with a wide operating bandwidth between 3.1 and 3.7 µm, is reported. A minimum attenuation of 0.13 dB/m is achieved through a 19-cell core design with a thin core wall surround. The loss is reduced further to 0.05 dB/m following a purging process to remove hydrogen chloride gas from the fiber-representing more than an order of magnitude loss reduction as compared to previously reported bandgap-guiding fibers operating in the mid-infrared. The fiber also offers a low bend sensitivity of <0.25 dB per 5 cm diameter turn over a 300 nm bandwidth. Simulations are in good agreement with the achieved losses and indicate that a further loss reduction of more than a factor of 2 should be possible by enlarging the core using a 37-cell design.

Fast and broadband fiber dispersion measurement with dense wavelength sampling.

We report on a method to obtain dispersion measurements from spectral-domain low-coherence interferograms which enables high accuracy (≈ ps/(nm · km)), broadband measurements and the determination of very dense (up to 20 points/nm over 500 nm) data sets for both dispersion and dispersion slope. The method exploits a novel phase extraction algorithm which allows the phase associated with each sampling point of the interferogram to be calculated and provides for very accurate results as well as a fast measurement capability, enabling close to real time measurements. The important issue of mitigating the measurement errors due to any residual dispersion of optical elements and to environmental fluctuations was also addressed. We performed systematic measurements on standard fibers which illustrate the accuracy and precision of the technique, and we demonstrated its general applicability to challenging problems by measuring a carefully selected set of microstructured fibers: a lead silicate W-type fiber with a flat, near-zero dispersion profile; a hollow core photonic bandgap fiber with strongly wavelength dependent dispersion and dispersion slope; a small core, highly birefringent index guiding microstructured fiber, for which polarization resolved measurements over an exceptionally wide (≈ 1000 nm) wavelength interval were obtained.

Real-time prediction of structural and optical properties of hollow-core photonic bandgap fibers during fabrication.

We formulate a simple model based on mass conservation to accurately predict the structural parameters of hollow-core photonic bandgap fibers from knowledge of the second stage preforms from which they are drawn. We show that combining this model with precalculated property maps can allow real-time prediction of the optical properties of manufactured fibers.

Phase regeneration of DPSK signals in a highly nonlinear lead-silicate W-type fiber.

We experimentally demonstrate phase regeneration of a 40-Gb/s differential phase shift keying (DPSK) signal in a 1.7-m long highly nonlinear lead silicate W-type fiber using a degenerate two-pump phase-sensitive amplifier (PSA). Results show an improvement in the Error Vector Magnitude (EVM) and a reduction of almost a factor of 2 in the phase noise of the signal after regeneration for various noise levels at the input.

Phase sensitive amplification in a highly nonlinear lead-silicate fiber.

We experimentally demonstrate phase-sensitive amplification in a highly nonlinear and low-dispersion lead-silicate W-type fiber. A phase-sensitive gain variation of 6 dB was observed in a 1.56-m sample of the fiber for a total input pump power of 27.7 dBm.

All-solid highly nonlinear singlemode fibers with a tailored dispersion profile.

We investigate a novel approach to obtain highly nonlinear fibers with a tailored group velocity dispersion around a desired wavelength region of interest. Rather than exploiting longitudinal holes to control the average refractive index of the cladding and hence the fiber's waveguide dispersion, as in holey fibers, we propose using an all-solid cladding with a suitably chosen refractive index difference relative to the core. We demonstrate numerically that this solution allows a large freedom in the manipulation of the overall fiber dispersive properties, while enabling, in practice, a much more accurate control of the fiber's structural properties during fabrication. Effectively single mode guidance over a broad wavelength range can be achieved through the use of a second outer cladding forming a W-type index profile. We derive simple design rules for dispersion controlled fibers, based on which an algorithm for the automatic dispersion optimization is proposed, implemented and used to design various nonlinear fibers for all-optical processing and supercontinuum generation. Fabrication of a lead silicate fiber with flattened dispersion at telecoms wavelengths confirms the potential of these new fibers.

Near-zero dispersion, highly nonlinear lead-silicate W-type fiber for applications at 1.55 microm.

We report the design, fabrication and characterization of a lead-silicate glass highly nonlinear W-type fiber with a flattened and near-zero dispersion profile in the 1.55 microm region. The fiber was composed of three types of commercial lead silicate glasses. Effectively single-mode guidance was observed in the fiber at 1550 nm. The nonlinear coefficient and the propagation loss at this wavelength were measured to be 820 W(-1)km(-1) and 2.1 dB/m, respectively. Investigations of the Brillouin threshold revealed no evidence of stimulated Brillouin scattering for continuous wave signal powers up to 29 dBm in a 2m sample of the fiber. A broadband dispersion measurement confirmed the near-zero dispersion values and the flat dispersion profile around 1550 nm, in good agreement with our simulations. Efficient four-wave-mixing, tunable across the whole C-band, was demonstrated in a 2.2m length of the fiber.

Multi-Line Fit Model for the Detection of Methane at ν(2) + 2ν(3) Band using Hollow-Core Photonic Bandgap Fibres.

Hollow-core photonic bandgap fibres (HC-PBFs) have emerged as a novel technology in the field of gas sensing. The long interaction pathlengths achievable with these fibres are especially advantageous for the detection of weakly absorbing gases. In this work, we demonstrate the good performance of a HC-PBF in the detection of the ν(2) + 2ν(3) band of methane, at 1.3 µm. The Q-branch manifold, at 1331.55 nm, is targeted for concentration monitoring purposes. A computationally optimized multi-line model is used to fit the Q-branch. Using this model, a detection limit of 98 ppmv (parts per million by volume) is estimated.

Gas Sensor Based on Photonic Crystal Fibres in the 2ν(3) and ν(2) + 2ν(3) Vibrational Bands of Methane.

In this work, methane detection is performed on the 2ν(3) and ν(2) + 2ν(3) absorption bands in the Near-Infrared (NIR) wavelength region using an all-fibre optical sensor. Hollow-core photonic bandgap fibres (HC-PBFs) are employed as gas cells due to their compactness, good integrability in optical systems and feasibility of long interaction lengths with gases. Sensing in the 2ν(3) band of methane is demonstrated to achieve a detection limit one order of magnitude better than that of the ν(2) + 2ν(3) band. Finally, the filling time of a HC-PBF is demonstrated to be dependent on the fibre length and geometry.

Micro-channels machined in microstructured optical fibers by femtosecond laser.

Micro-channels were fabricated in hollow-core photonic bandgap fiber (HC-PBGF) and suspended-core holey fiber (SC-HF) by femtosecond Ti:sapphire laser irradiation. Gaseous access was demonstrated via these engineered ports to the core of HC-PBGF and the hollow cladding of SC-HF. Femtosecond laser micro-machining caused no additional transmission loss in HC-PBGFs. This allowed a novel gas cell to be produced, in which gaseous access was provided solely through two micro-channels. Acetylene diffusion was also confirmed through a micro-channel leading to a single cladding airhole in SC-HF. This further highlighted the fabrication technique's precision, selectivity, and potential for developing fiber-based micro-fluidic devices.