SMF-28 Optical Fiber Research Articles

Grating Coupler Design for Low-Cost Fabrication in Amorphous Silicon Photonic Integrated Circuits

Photonic circuits find applications in biomedicine, manufacturing, quantum computing and communications. Photonic waveguides are crucial components, typically having cross-section orders of magnitude inferior when compared with other photonic components (e.g., optical fibers, light sources and photodetectors). Several light-coupling methods exist, consisting of either on-plane (e.g., adiabatic and end-fire coupling) or off-plane methods (e.g., grating and vertical couplers). The grating coupler is a versatile light-transference technique which can be tested at wafer level, not requiring specific fiber terminations or additional optical components, like lenses, polarizers or prisms. This study focuses on fully-etched grating couplers without a bottom reflector, made from hydrogenated amorphous silicon (a-Si:H), deposited over a silica substrate. Different coupler designs were tested, and of these we highlight two: the superimposition of two lithographic masks with different periods and an offset between them to create a random distribution and a technique based on the quadratic refractive-index variation along the device’s length. Results were obtained by 2D-FDTD simulation. The designed grating couplers achieve coupling efficiencies for the TE-like mode over −8 dB (mask overlap) and −3 dB (quadratic variation), at a wavelength of 1550 nm. The coupling scheme considers a 220 nm a-Si:H waveguide and an SMF-28 optical fiber.

High-efficiency self-focusing metamaterial grating coupler in silicon nitride with amorphous silicon overlay

Efficient fiber-chip coupling interfaces are critically important for integrated photonics. Since surface gratings diffract optical signals vertically out of the chip, these couplers can be placed anywhere in the circuit allowing for wafer-scale testing. While state-of-the-art grating couplers have been developed for silicon-on-insulator (SOI) waveguides, the moderate index contrast of silicon nitride (SiN) presents an outstanding challenge for implementing efficient surface grating couplers on this platform. Due to the reduced grating strength, a longer structure is required to radiate the light from the chip which produces a diffracted field that is too wide to couple into the fiber. In this work, we present a novel grating coupler architecture for silicon nitride photonic integrated circuits that utilizes an amorphous silicon (α-Si) overlay. The high refractive index of the α-Si overlay breaks the coupler’s vertical symmetry which increases the directionality. We implement subwavelength metamaterial apodization to optimize the overlap of the diffracted field with the optical fiber Gaussian mode profile. Furthermore, the phase of the diffracted beam is engineered to focalize the field into an SMF-28 optical fiber placed 55 µm above the surface of the chip. The coupler was designed using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations supported by genetic algorithm optimization. Our grating coupler has a footprint of 26.8 × 32.7 µm2 and operates in the O-band centered at 1.31 μm. It achieves a high directionality of 85% and a field overlap of 90% with a target fiber mode size of 9.2 µm at the focal plane. Our simulations predict a peak coupling efficiency of − 1.3 dB with a 1-dB bandwidth of 31 nm. The α-Si/SiN grating architecture presented in this work enables the development of compact and efficient optical interfaces for SiN integrated photonics circuits with applications including optical communications, sensing, and quantum photonics.

Machine learning assisted BOFDA for simultaneous temperature and strain sensing in a standard optical fiber.

We report, to our knowledge for the first time on simultaneous distributed temperature and strain sensing in a standard telecom optical fiber using a machine learning assisted Brillouin frequency domain analysis (BOFDA) system. The well-known temperature and strain cross-sensitivity problem is addressed by developing a BOFDA system with a high signal-to-noise ratio and applying machine learning. The spectrum consists of four highly resolved peaks, whose Brillouin frequency shifts are extracted and serve as features for the machine learning algorithms. The spectra result from a 450-m standard SMF-28 optical fiber, and particularly from a segment of 30 m. This fiber segment is coiled around a stretcher and placed in a climate chamber. The applied temperature and strain values range from 20 °C to 40 °C and from 0 µɛ to 1380 µɛ, respectively. The total measurement time to achieve a high SNR and resolve four peaks with a spatial resolution of 6 m is 16 min. To discriminate temperature and strain effects, simple frequentist and more sophisticated Bayesian-based algorithms are employed with the powerful Gaussian process regression (GPR) delivering the best performance in terms of temperature and strain errors, which are found to be 2 °C and 45 µɛ, respectively. These errors are calculated using leave-one-out cross-validation, so that an unbiased estimation of the sensor's performance is provided.

Calibration of Distributed Temperature Sensors Using Commercially Available SMF-28 Optical Fiber From 22 °C to 1000 °C

Optical frequency domain reflectometry (OFDR) is a family of optical techniques which can be used to produce distributed temperature measurements from the spectral shift of an interference pattern based on the Rayleigh backscatter signature of an optical fiber. Adaptive signal processing techniques have recently been used with OFDR to record meaningful spectral shift data from commercially available SMF-28 optical fibers heated beyond 950 °C. However, a correlation between the measured spectral shift and temperature has not yet been developed at these high temperatures. To extend the measurable temperature range of OFDR in SMF-28, this work describes the development of such a correlation from room temperature (22 °C) to 1000 °C. The relationship between spectral shift and temperature change over this range was found to be best characterized by the fourth-order polynomial <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta {T}=(-4.241\ast 10^{-11})\text {S}^{4} +(-2.017\ast 10^{-7})\text {S}^{3} +(-3.677\ast 10^{-4})\text {S}^{2}+(-0.8057)\text{S}$ </tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta {T}$ </tex-math></inline-formula> represents the temperature difference compared to the reference temperature, and S represents the spectral shift measured by the fibers. The calibration developed in this work assumes that the fiber has been fully annealed by heating the fiber to 1000 °C for a few hours. This paper is the first to demonstrate the calibration and use of SMF-28 distributed optical fiber sensors up to 1000 °C, enabled using adaptive OFDR-based signal processing.

Experimental observation of self-imaging in SMF-28 optical fibers.

Spatial self-imaging, consisting of the periodic replication of the optical transverse beam profile along the propagation direction, can be achieved in guided wave systems when all excited modes interfere in phase. We exploited material defects photoluminescence for directly visualizing self-imaging in a few-mode, nominal singlemode SMF-28 optical fiber. Visible luminescence was excited by intense femtosecond infrared pulses via multiphoton absorption processes. Our method permits us to determine the mode propagation constants and the cutoff wavelength of transverse fiber modes.

Correlation technique using of-the-shelf CW DFB laser for interrogation of fiber optic sensors

The paper presents a simple fiber-optic sensor technique based on methods of correlation optical time domain reflectometry. A correlation reflectometry technique can measure distribution of reflection coefficient along the optical fiber by calculating the correlation function between a probe optical signal (reference) and the signal returned back due to reflections or/and back-scattering from the fiber under the test. To obtain the best sensor performance, the probe signal power should be a truly random function of time. As an optical source we use a free-running DFB laser diode operating in a continuous wave regime without any external modulation. To generate the probe test signal, laser light is passed through an interferometer with an optical path difference much longer than the coherence length of the laser light. The light intensity at the interferometer output has a truly random fluctuations and its auto correlation function is suitable for correlation optical reflectometry. We present results of experimental verification of the techniques in different sensor configurations. Multipoint sensor using very low reflective fiber Bragg gratings with reflectivity of 0.01% printed in a long SMF-28 optical fiber was demonstrated.

Thermal Regeneration of Tilted Bragg Gratings UV Photo-Inscribed in Hydrogen-Loaded Standard Optical Fibers

We report on the thermal regeneration of tilted Bragg gratings (TFBGs) produced in hydrogen loaded SMF-28 optical fiber using two types of UV lasers: 266 nm femtosecond pulses and 193 nm nanosecond pulses. Different isochronal and isothermal annealing profiles are used to improve the regeneration of the cladding modes. Experimental results are in good agreement with simulation results. We show that TFBGs regenerate efficiently in the temperature range 850 °C - 900 °C when isothermal stepwise annealing profile is used. The regeneration efficiency of the cladding modes does not exceed 4.35%. The sensitivity to the surrounding refractive index is comparable to that of non-regenerated tilted Bragg gratings. These regenerated tilted Bragg gratings can be applied in several applications, like for example the follow up of high-temperature fabrication processes.

Transition from purely elastic to viscoelastic behavior of silica optical fibers at high temperatures characterized using regenerated Bragg gratings.

In this study, the response of regenerated fiber Bragg gratings (RFGBs) to axial forces was investigated in a temperature range from room temperature to 900 °C. For the first time, the transition from pure elastic to viscoelastic behavior around 700 °C of a standard SMF28 optical fiber was measured with an inscribed RFBG. An elastic model with linear temperature dependencies of Young's modulus and Poisson's ratio was established, and showed good agreement with the measurements up to temperatures of ∼500 °C. In the temperature range up to 900 °C, the RFBG response could be well described with a simple, single-material approach and a Burgers model that consists of a Kelvin and a Maxwell part. Based on the elastic parameter of the Maxwell part, the temperature-dependent force sensitivity of the RFBG was determined, and it showed a linear decrease in the range from room temperature to ∼500 °C, constant values in the range between ∼500 °C and ∼600 °C, and a strong increase at higher temperatures. While fulfilling the condition to operate in the elastic domain of the silica fiber, the investigations demonstrate that RFBGs can be used as force sensors up to temperatures of ∼600 °C - the range in which temperature-dependent force sensitivities have to be considered. The temperature-dependent parameters of the effective single-material model (elastic and viscoelastic part) are essential to describe the effective mechanical behavior of the optical fiber at high temperatures.

The ROGUE: a novel, noise-generated random grating.

We propose a novel device defined as Random Optical Grating by Ultraviolet or ultrafast laser Exposure (ROGUE), a new type of fiber Bragg grating (FBG), exhibiting a weak reflection over a large bandwidth, which is independent of the length of the grating. This FBG is fabricated simply by dithering the phase randomly during the writing process. This grating has an enhanced backscatter, several orders of magnitude above typical Rayleigh backscatter of standard SMF-28 optical fiber. The grating is used in distributed sensing using optical frequency domain reflectometry (OFDR), allowing a significant increase in signal to noise ratio for strain and temperature measurement. This enhancement results in significantly lower strain or temperature noise level and accuracy error, without sacrificing the spatial resolution. Using this method, we show a sensor with a backscatter level 50 dB higher than standard unexposed SMF-28, which can thus compensate for increased loss in the system.

Passively Q-switched erbium fiber laser with a thin crystalline film of bismuth telluride used as a saturable absorber

Performance of a passively Q-switched erbium fiber laser with a narrow bandgap Bi2Te3 semiconductor thin film used as a saturable absorber was studied experimentally. In order to obtain repetitively-pulsed lasing, a Q-switcher was fabricated as a section of the standard SMF-28 optical fiber with a short (∼4 mm in length) chemically etched out tapered section laterally covered with the bismuth telluride film. This Q-switcher was installed in a specially designed circuit of a ring-type erbium fiber laser pumped by a laser diode at 975 nm wavelength. The fiber diameter in the middle of the tapered section was 10-15 μm. Two types of taper coverage were studied: Bi2Te3 flakes suspended in isopropanol and a ZnTe/Bi2Te3 uniform polycrystalline thin-film heterostructure synthesized on a lateral surface of a tapered fiber section via MOCVD. In the first case, we observed laser oscillation at a wavelength of 1565 nm as a train of 7-26 μs pulses with a 86-50 μs repetition period at pump powers varying from 26 to 75 mW. In the second case, CW oscillation in the circuit appeared at a pump power of 30 mW while conversion to the repetitively-pulsed mode with a kilohertz repetition rate occurred at pump powers within the range of 34-40 mW. The results obtained indicate the feasibility of effective interaction of the evanescent field with a bismuth telluride film used as a saturable absorber. This interaction is sufficient to achieve a stable repetitively-pulsed lasing in the circuits of passively Q-switched fiber lasers.

Effective Method for Determining Chromatic Dispersion From a Spectral Interferogram

We present a novel method for determining chromatic dispersion profile from a broadband spectral interferogram obtained by using a white-light interferometry technique. The proposed method is based on direct calculations of a second derivative of the registered spectral intensity at extremal points, which in result gives a spectral phase difference derivative further used for dispersion determining. Although the method is best suited for processing interferograms with modulation around a zero level (zero-mean value interferograms) and slowly changing envelope, the conducted numerical tests show that it is highly tolerable to different types of perturbations of the input interference signal. The proposed method can be applied both to spectral interferograms with monotonically changing phase difference as well as containing stationary phase difference points. We demonstrate the effectiveness and accuracy of the proposed method in measurements of chromatic dispersion of commercially available samples like the BK7 glass plate and the Corning SMF-28 optical fiber.

Comparative study of quantitative phase imaging techniques for refractometry of optical waveguides.

A comparative study of quantitative phase imaging techniques for refractometry of optical waveguides is presented. Three techniques were examined: a method based on the transport-of-intensity equation, quadri-wave lateral shearing interferometry and digital holographic microscopy. The refractive index profile of a SMF-28 optical fiber was thoroughly characterized and served as a gold standard to assess the accuracy and precision of the phase imaging methods. Optical waveguides were inscribed in an Eagle2000 glass chip using a femtosecond laser and used to evaluate the sensitivity limit of these phase imaging approaches. It is shown that all three techniques provide accurate, repeatable and sensitive refractive index measurements. Using these phase imaging methods, we report a comprehensive map of the photosensitivity to femtosecond pulses of Eagle2000 glass. Finally, the reported data suggests that the phase imaging techniques are suited to be used as precise and non-destructive refractive index shift measuring tools to study and control the inscription process of optical waveguides.

Waveguide magnetoplasmonic structure based on ferrite garnet film

The magnetoplasmonic structure, which is a planar multimode magnetooptical waveguide, 10 μm thick with plasmon resonance SiO2 / Cu coating on the surface is experimentally realized. With the total length of the magnetooptical waveguide 4.6 mm and the length of the SiO2 / Cu coating equal to 3.9 mm, the insertion loss of the structure for TM- and TE-polarized light was 22 and 4 dB, respectively, at a wavelength of 1550 nm. Linearly polarized light was launched into the magneto-optical waveguide using a SMF-28 single-mode optical fiber coupled to the polished input edge of the waveguide. The structure is of interest for use as miniature magnetically controlled modulators of light intensity.

An additional channel for FM signal transmission in standard fiber-optic AM communication lines

A paradigm has been developed according to which an additional FM signal transfer channel is formed in a standard optical fiber transmission line without violating the main AM channel operation. Using the proposed approach, an RS-232 interface signal has been experimentally transferred via a standard intraobject 100-Mbit Ethernet line based on an SMF-28 single-mode optical fiber.

Detailed investigation of intermodal four-wave mixing in SMF-28: blue-red generation from green.

A short piece of commercial-grade SMF-28 optical fiber is pumped with a 680 ps high-peak power green laser. Red Stokes and blue anti-Stokes beams are generated spontaneously from vacuum noise in different modes in the fiber via intermodal four-wave mixing. Detailed experimental and theoretical analyses are performed and are in reasonable agreement. The large spectral shifts from the pump protect the Stokes and anti-Stokes from contamination by spontaneous Raman scattering noise. This work highlights the predictive power and limitations of a theoretical model to explain the experimental results for a process that relies on the amplification of quantum vacuum energy over more than 11 orders of magnitude.

Subhertz linewidth laser by locking to a fiber delay line.

An ultralow-noise, subhertz 1.55 μm erbium-doped fiber laser that is locked on an all-fiber-based Michelson interferometer is presented in this paper. The interferometer uses 500 m SMF-28 optical fiber and an acousto-optic modulator to allow heterodyne detection. By comparing two identical laser systems, a 0.67 (0.21) Hz linewidth beat-note signal is achieved and we obtain fractional frequency instability of 7×10(-15) at short timescales (0.1-1 s). The frequency noise power spectral density of two identical lasers is below -1 dB Hz(2)/Hz at 1 Hz and it reaches -18 dB Hz(2)/Hz from 200 Hz to 1 kHz.

Long-period fiber grating moisture sensor with nano-structured coatings for structural health monitoring

In this article, we inscribe long-period gratings on an SMF-28 optical fiber by a focused CO2 laser beam to develop a humidity sensor with high sensitivity and selectivity for moisture detection for civil infrastructures health monitoring. We propose two types of nano-structured film to deposit on surface of the grating region by electrostatic self-assembly deposition process. The primary nanofilm coating (PAH+/PAA−) does not affect long-period grating properties such as resonance wavelength and transmission intensity; however, it increases the sensitivity by changing the refractive index of the surrounding analytes. The secondary nanofilm coating (Al2O3+/PSS−), due to its molecule structure compatibility, can selectively adsorb analyte molecules of interest (in this case H2O only). The experimental results reveal that the nanofilm-coated long-period gratings are more sensitive to moisture in terms of both wavelength shift and resonance intensity change as of 0.019%/pm and 0.01%/10−3 dBm, respectively, while the bare LPG coefficients are 0.021%/pm and 0.0603%/10−3 dBm. The proposed sensor shows excellent thermal stability as well.

Growth of a seven pointed star shaped of vertical and uniform ZnO nanostructures on optical fiber via catalyst-free VLS mechanisms

In this paper, the growths of ZnO nano- and microstructures by VLS mechanism have been studied. A piece of silica fiber (SMF-28 optical fiber) was used as a substrate. By controlling the experimental conditions, nano- and microstructures of ZnO with different shapes and sizes were grown around the optical fiber, which can be used as an optical fiber sensor. A star-shaped ZnO microstructure was obtained in this research. Pyramidal structures were grown on each side of vertical hexagonal microrods which formed the seven-pointed star-shaped microstructures. This growth process was carried out in a special quartz container in a tube furnace at temperatures around 400–550 °C without any additional catalyst. XRD, SEM and PL spectroscopy were used for the characterization of the produced nanostructures. The influence of temperature, reactant and carrier gases flow rate on the morphology of ZnO nanostructure was investigated.

Comparative study between LPFG- and FBG-based bending sensors

A comparative study on behavior of grating-based bending sensors, which include long-period fiber grating (LPFG) and fiber Bragg grating (FBG), has been performed in this article. In order to simulate these grating-based bending sensors, a comprehensive simulation model has been presented. According to our simulation results, LPFG-based bending sensor on the standard SMF-28 optical fiber has bending sensitivity of 8.78nm/m−1. The linear operation range for this bending sensor is between K=0m−1 and K=5m−1 (K is the amount of bending curvature). On the other hand, a FBG-based bending sensor on a single-mode 4-core fiber (4CF) has been studied. Our simulation results show that this bending sensor has bending sensitivity of 85.57pm/m−1 and it has linear response over the entire considered range of bending, K=0–6m−1. For full-rotation bend sensing, two adjacent cores of this optical fiber should be utilized.

Ultralow-frequency-noise stabilization of a laser by locking to an optical fiber-delay line

We report the frequency stabilization of an erbium-doped fiber distributed-feedback laser using an all-fiber-based Michelson interferometer of large arm imbalance. The interferometer uses a 1 km SMF-28 optical fiber spool and an acousto-optic modulator allowing heterodyne detection. The frequency-noise power spectral density is reduced by more than 40 dB for Fourier frequencies ranging from 1 Hz to 10 kHz, corresponding to a level well below 1 Hz2/Hz over the entire range; it reaches 10(-2) Hz2/Hz at 1 kHz. Between 40 Hz and 30 kHz, the frequency noise is shown to be comparable to the one obtained by Pound-Drever-Hall locking to a high-finesse Fabry-Perot cavity. Locking to a fiber delay line could consequently represent a reliable, simple, and compact alternative to cavity stabilization for short-term linewidth reduction.