Standard Telecom Fiber Research Articles

A Fiber-Endface, Fabry–Perot Vapor Microsensor Fabricated by Multiphoton Polymerization

A Fabry-Perot micro-optical sensing resonator fabricated by direct laser writing on the endface of a standard telecom fiber is demonstrated. This endface optical fiber sensing probe has been tested for tracing the vapors of common organic solvents. The device operates in the spectral region lying between 1440 and 1660 nm, while the spectra recorded in reflection mode correlate to the refractive index changes due to the different vapors trapped inside the microcavity. The performance of the device operation with respect to the reflecting surface porosity of the fabricated microstructure is presented, while the sensing of ethanol traces down to 4 ppm concentrations was achieved.

Continuous-variable quantum key distribution with 1 Mbps secure key rate.

We report the first continuous-variable quantum key distribution (CVQKD) experiment to enable the creation of 1 Mbps secure key rate over 25 km standard telecom fiber in a coarse wavelength division multiplexers (CWDM) environment. The result is achieved with two major technological advances: the use of a 1 GHz shot-noise-limited homodyne detector and the implementation of a 50 MHz clock system. The excess noise due to noise photons from local oscillator and classical data channels in CWDM is controlled effectively. We note that the experimental verification of high-bit-rate CVQKD in the multiplexing environment is a significant step closer toward large-scale deployment in fiber networks.

Characterization of refractive index change and fabrication of long period gratings in pure silica fiber by femtosecond laser radiation

Abstract Ultrafast laser induced refractive index (RI) change in the core of a standard telecommunication fiber is quantified using the spectral shift of an in-fiber Bragg grating (FBG) based Fabry–Perot cavity. Measured RI change is used to design and then fabricate long period grating (LPG) in pure silica core single mode fiber (SMF) employing identical laser irradiation conditions used in core index characterization. A core length of 100 µm within the 10 mm long cavity structure is scanned with ultrafast laser pulses, and the corresponding spectral shift is used to calculate index modification. The index change of 0.000449 found in characterization process is used to simulate the LPG in pure silica fiber. Identical index modulation written in pure silica fiber by femtosecond laser radiation provides a rejection band that is in good agreement with the simulation results. The fabricated LPG sensors are also characterized for ambient temperature and RI.

Numerical and Experimental Analysis of Sensitivity-Enhanced RI Sensor Based on Ex-TFG in Thin Cladding Fiber

We report a highly sensitive refractive index (RI) sensor in the aqueous solution, which is based on an 81°-tilted fiber grating structure inscribed into a thin cladding fiber with 40 μm cladding radius. The numerical analysis has indicated that the RI sensitivity of cladding resonance mode of the grating can be significantly enhanced with reducing cladding size. This has been proved by the experimental results as the RI sensitivities of TM and TE resonance peaks in the index region of 1.345 have been increased to 1180 nm/RIU and 1150 nm/RIU, respectively, from only 200 and 170 nm/RIU for the same grating structure inscribed in standard telecom fiber with 62.5-μm cladding radius. Although the temperature sensitivity has also increased, the change in temperature sensitivity is still insignificant in comparison with RI sensitivity enhancement.

Influence of photo-inscription conditions on the radiation-response of fiber Bragg gratings.

We compared the sensitivity to X-rays of several fiber Bragg gratings (FBGs) written in the standard telecommunication fiber Corning SMF28 with different techniques. Standard gratings were manufactured with phase-mask and UV lasers, continuum wave (cw) at 244 nm or pulsed in the nanosecond domain at 248 nm, in a pre-hydrogenated fiber. Others gratings were written by exposures to a femtosecond IR-laser (800 nm), with both phase-mask and point by point techniques. The response of these FBGs was studied under X-rays at room temperature and 100°C, by highlighting their similarities and differences. Independently of the inscription technique, the two types of fs-FBGs have showed no big difference up to 1 MGy(SiO(2)) dose. A discussion on the causes of the radiation-induced peak change is also reported.

Measurement-device-independent quantum key distribution: from idea towards application

We assess the overall performance of our quantum key distribution (QKD) system implementing the measurement-device-independent (MDI) protocol using components with varying capabilities such as different single-photon detectors and qubit preparation hardware. We experimentally show that superconducting nanowire single-photon detectors allow QKD over a channel featuring 60 dB loss, and QKD with more than 600 bits of secret key per second (not considering finite key effects) over a 16 dB loss channel. This corresponds to 300 and 80 km of standard telecommunication fiber, respectively. We also demonstrate that the integration of our QKD system into FPGA-based hardware (instead of state-of-the-art arbitrary waveform generators) does not impact on its performance. Our investigation allows us to acquire an improved understanding of the trade-offs between complexity, cost and system performance, which is required for future customization of MDI-QKD. Given that our system can be operated outside the laboratory over deployed fiber, we conclude that MDI-QKD is a promising approach to information-theoretic secure key distribution.

Long-period fibre grating writing with a slit-apertured femtosecond laser beam ∗

We report on long-period grating (LPG) writing in a standard telecom fibre, , via refractive index modification by femtosecond pulses. A method is proposed for grating writing with a slit-apertured beam, which enables one to produce LPGs with reduced background losses and a resonance peak markedly stronger than that in the case of grating writing with a Gaussian beam. The method can be used to fabricate LPGs for use as spectral filters of fibre lasers and sensing elements of sensor systems.

Quantum storage of entangled telecom-wavelength photons in an erbium-doped optical fibre

The realization of a future quantum Internet requires processing and storing quantum information at local nodes, and interconnecting distant nodes using free-space and fibre-optic links. Quantum memories for light are key elements of such quantum networks. However, to date, neither an atomic quantum memory for non-classical states of light operating at a wavelength compatible with standard telecom fibre infrastructure, nor a fibre-based implementation of a quantum memory has been reported. Here we demonstrate the storage and faithful recall of the state of a 1532 nm wavelength photon, entangled with a 795 nm photon, in an ensemble of cryogenically cooled erbium ions doped into a 20 meter-long silicate fibre using a photon-echo quantum memory protocol. Despite its currently limited efficiency and storage time, our broadband light-matter interface brings fibre-based quantum networks one step closer to reality. Furthermore, it facilitates novel tests of light-matter interaction and collective atomic effects in unconventional materials.

ALD thin ZnO layer as an active medium in a fiber-optic Fabry–Perot interferometer

A novel optical fiber sensor of temperature using a thin ZnO layer fabricated by atomic layer deposition (ALD) is demonstrated for the first time. The thin ZnO layer was grown on the face of a standard optical telecommunication fiber SMF-28 and operates as a Fabry–Perot interferometer sensitive to temperature. The interferometer characterization was made in the temperature range extending from 50 to 300°C with resolution equal to 1°C. The output signal was analyzed by measurement of the shift of the maxima in spectral pattern. The sensitivity of temperature measurement is about 0.05nm/°C. Furthermore, very good linearity of the sensor was achieved with correlation coefficient R2=0.9984.

Implementation of multiplexing in a subcarrier-wave quantum cryptography system

Quantum cryptography allows distributing secure keys in a way that any eavesdropping in the channel is inevitably detected. This work is dedicated to introducing wavelength division multiplexing in a subcarrier-wave quantum cryptography system. Compared to other existing schemes, the resulting device is able to achieve higher bitrates (up to 2.26 Mbit/s at 20 km), is robust against external conditions and compatible with standard telecommunication fibres in multi-user environment.

Fiber-optic flow sensors for high-temperature environment operation up to 800°C.

This Letter presents an all-optical high-temperature flow sensor based on hot-wire anemometry. High-attenuation fibers (HAFs) were used as the heating elements. High-temperature-stable regenerated fiber Bragg gratings were inscribed in HAFs and in standard telecom fibers as temperature sensors. Using in-fiber light as both the heating power source and the interrogation light source, regenerative fiber Bragg grating sensors were used to gauge the heat transfer from an optically powered heating element induced by the gas flow. Reliable gas flow measurements were demonstrated between 0.066 m/s and 0.66 m/s from the room temperature to 800°C. This Letter presents a compact, low-cost, and multiflexible approach to measure gas flow for high-temperature harsh environments.

Femtosecond laser processing of fiber Bragg gratings with photo-induced gradient-index assisted focusing

Gradient-index modulation prefabrication in the cladding of standard telecom fibers was proposed for femtosecond laser processing of Type II-IR fiber Bragg gratings. The refractive index of the prefabricated region in the cladding had a large gradient across the laser beam, which could act as a cylindrical lens to enhance the focusing of laser beam when writing Type II-IR gratings. With the help of prefabrication, the threshold pulse energy for processing Type II-IR FBGs was lowered from 750 μJ to 520 μJ. The fabricated FBGs showed good thermal stability at temperatures over 900 °C.

Fiber Bragg grating inscription based on optical filamentation of UV femtosecond laser pulses

We present the fabrication and characterization of strong first-order Bragg gratings operating around 1550nm in standard telecom fibers (SMF-28) by use of 160fs UV laser pulses at 266nm and a Talbot interferometer. Bragg reflectivities exceeding 50dB are achieved in both hydrogen-free and hydrogen-loaded fibers. In addition, Bragg wavelength tuning over the telecom C-band using a single phase mask is demonstrated. We also show that the formation of Bragg gratings, relying on a two-photon absorption process, is intimately accompanied by the optical filamentation process of UV fs laser pulses during nonlinear propagation in the fiber.

All-Fiber Optofluidic Component to Combine Light and Fluid

A component is presented that combines light and fluid in an optical fiber arrangement for optofluidics. The component couples light from a standard telecom fiber (STF) to the solid-core of a microstructured fiber, and delivers fluid from a capillary to the holes of the same microstructured fiber. The light is then made to interact longitudinally with the delivered fluid in a hollow-core fiber or capillary. The component is all-spliced, hermetic and allows for fluid flow without interrupting the optical beam. Light is brought from the STF to the solid-core/fluid interface with a loss <;0.1 dB.

Suppression of the Thermal Effects in the Femtosecond Laser Processing of Fiber Bragg Gratings

The thermal effects on the processing of type-I IR fiber Bragg gratings (FBGs) using a femtosecond laser with a phase mask are investigated. Thermal effects are significantly suppressed by using interval exposure mode and reducing the tension on the fiber. FBGs with improved photo-induced refractive index modulation are fabricated in the standard telecom fiber. The index modulation reaches 1.6 × 10−3. The reflectivity and bandwidth are measured to be −0.36 dB and 1.27nm, respectively.

A high-Q low threshold thulium-doped silica microsphere laser in the 2 μm wavelength region designed for gas sensing applications

A high-Q and low threshold laser resonator, operating in the 2 μm wavelength region, has been demonstrated by coupling a thulium-doped silica microsphere to a tapered fibre. Microspheres with diameters ranging from fifty to a few hundred micrometres were carefully fabricated for this purpose by melting an etched-clad thulium-doped silica fibre tip using a focused beam from a CO2 laser, while the tapered fibre with waist diameter in the desired range of 2 μm was fabricated by using heating and stretching of standard single-mode telecommunication fibre. The tapered fibre served the dual purpose of transporting pump power into the sphere and allowing the extraction of the resulting laser emission. Under excitation at a wavelength of ∼1.6 μm, lasing occurred at wavelengths over the range from 1.9 to 2.0 μm. Single-mode laser operation was obtained by exciting the fundamental whispering gallery mode resonance of the microsphere, while multi-mode lasing occurred for non-fundamental mode excitation. The threshold power of the laser was measured to be about 50 μW delivered pump power, and a maximum laser power of 0.8 mW at around 1.94 μm was observed for a 6 mW pump power, operating at wavelengths around 1.6 μm. The laser was designed as a low threshold and compact source for miniaturized gas sensing devices operating over this important wavelength region.

Femtosecond direct-writing of low-loss fiber Bragg gratings using a continuous core-scanning technique

We report the inscription of low-loss fiber Bragg gratings using focused femtosecond (fs) pulses and a continuous core-scanning technique. This direct-write technique produces high-fidelity Type I-IR gratings that share the inherent advantages of other direct-write methods, such as the point-by-point (PbP) method, for which the grating period is a free parameter. However, here we demonstrate an order of magnitude improvement in scattering loss compared to PbP gratings, to a level comparable with that of phase-mask-based fs inscription. A first-order grating was inscribed in standard telecommunications fiber with -49 dB transmission at the Bragg wavelength and 0.1 dB broadband scattering loss. Potential application of these gratings to large-mode-area fibers and chirped grating fabrication are highlighted.

The effect of annealing and temperature on transmission spectra of long period gratings written in hydrogen loaded standard single mode fiber

The influence of annealing and temperature on the transmission spectrum of Long Period Grating (LPG) is investigated by monitoring the wavelength shift and the intensity variation in the loss peaks. The LPGs are inscribed in a hydrogen loaded standard telecommunication fiber and experiments are performed at different annealing temperature and time to observe the spectral changes. The effect of temperature changes on the spectral profile of LPG is also studied.

Signal Processing Enhanced Fiber Vibration Sensors

The paper analyzes the feasibility of cost-effective fiber sensors for the measurement of small vibrations, from low to medium-high frequencies, in which the complexity of the measurement is moved from expensive optics to cheap electronics without losing too much performance thanks to signal processing algorithms. Two optical approaches are considered: Bragg gratings in standard telecom fibers, which represent the most common type of commercial fiber sensors, and specifically developed sensors made with plastic optical fibers. In both cases, to keep the overall cost low, vibrations are converted into variations of the light intensity, although this makes the received signal more sensitive to noise. Then, adaptive filters and advanced spectral estimation techniques are used to mitigate noise and improve the sensitivity. Preliminary results have demonstrated that the combined effect of these techniques can yield to a signal-to-noise improvement of about 30 dB, bringing the proposed approaches to the level of the most performing sensors for the measurement of vibrations.

Modelling the transmission of a NRZ-ADPSK signal in networks

We study numerically the effect of linear and nonlinear distortions on the transmission of a NRZ-ADPSK signal. The results of the numerical simulation coincide within the measurement accuracy with the experimental data, which indicates the adequacy of the mathematical model of the communication line, transmitter and receiver. We have found experimentally and numerically that in communication systems based on a standard telecommunication fibre negative dispersion (approximately ) introduced into the signal being transmitted provides an increase in the maximum admissible signal power coupled into a span and in the value of the admissible attenuation of the spans at . It is established that nonlinear distortions caused by the presence of adjacent channels in a frequency network with a step of and (crosstalks) are insignificant in comparison with the interchannel nonlinear distortions.