A narrow-spectrum long-wavelength VCSEL using a fiber Bragg grating

Market forecasts and trends for the usage of fiber optical sensors confirm that demand for them will continue to increase in the near future. This article focuses on the research of fiber Bragg grating (FBG) sensor network, their applications in IoT and structural health monitoring (SHM), and especially their coexistence with existing fiber optical communication system infrastructure. Firstly, the spectrum of available commercial optical FBG temperature sensor was experimentally measured and amplitude-frequency response data was acquired to further develop the simulation model in the environment of RSoft OptSim software. The simulation model included optical sensor network, which is combined with 8-channel intensity-modulated wavelength division multiplexed (WDM) fiber optical data transmission system, where one shared 20 km long ITU-TG.652 single-mode optical fiber was used for transmission of both sensor and data signals. Secondly, research on a minimal allowable channel spacing between sensors’ channels was investigated by using MathWorks MATLAB software, and a new effective and more precise determination algorithm of the exact center of the sensor signal’s peak was proposed. Finally, we experimentally show successfully operating coexistence concept of the spectrum-sliced fiber optical transmission system with embedded scalable FBG sensor network over one shared optical fiber, where the whole system is feed by only one broadband light source.

The interrogation of fiber-Bragg-grating (FBG) sensors using a vertical-cavity surface-emitting laser (VCSEL) is discussed. A long-wavelength (1.54 μm) VCSEL was used as a wavelength-tunable source by variation in the current. Temperature stabilization was performed with a thermoelectric device. Characteristics of temperature and strain sensing were investigated. FBGs with different reflectivities were compared. For temperature sensing, the root-mean-square error in the measurement was reduced to 1/3 that without temperature stabilization. The dependence of the measurement error on the reflectivities of the FBGs was investigated. The measurement error was larger for FBGs with lower reflectivities in both temperature and strain sensing. Improvement on the sensing with low-reflectivity FBGs is discussed.

The rapid growth of internet and cloud computing applications drives a huge demand for bandwidth capacity in communication networks, while power consumption, cost, and space density must scale down. This growth leads to an increase in the size of data centers (longer optical links), and of the fibers’ channel data rate, rooted in Moore’s Law. Until now, multi-mode fibers (MMF) have been largely employed in datacom applications due to the large coupling tolerance. However, the data-carrying capability of MMF decreases with the transmission distance due to pulse broadening resulting from modal and chromatic dispersion. In order to overcome those limits, transceivers based on single mode fiber (SMF) are under development and the first systems are on the market. Vertical-cavity surface-emitting lasers (VCSELs) are the transmitters of choice for short-reach applications due to their low cost, energy efficiency , and small footprint. InP-based VCSELs emitting at long wavelengths (i.e. 1.3 and 1.55 µm) have gained large interest due to their intrinsic lower power consumption (lower band gap) and low losses in silicon waveguides and silica-based optical fibers, which allows longer transmission distances. While short-wavelength GaAs-based VCSELs have achieved small-signal modulation bandwidths up to 30 GHz [1], InP-based VCSELs show inferior modulation capabilities [2, 3]. Up to date, the highest small-signal bandwidth demonstrated on InP-based devices is 22 GHz [3]. The distributed Bragg reflectors (DBRs) commonly used for GaAs-based VCSELs are made of binary and ternary semiconductor compounds, which offer several advantages such as high refractive-index contrast between the layers, good electrical conductivity and low thermal resistivity. The inferiority of semiconductor DBRs lattice matched to InP challenges the modulation bandwidth enhancement of InP-based devices which suffer of poor thermal conductivity, and high lateral spreading resistance. A further challenge is the single-mode laser operation that has motivated the transition from MMF to SMF in datacom systems. In this chapter, the challenges related to InP-based VCSELs are discussed with focus on active region design, cavity engineering, and current and optical confinement. These arguments apply to all InP-based VCSELs with emission wavelength between 1.3 and 2.0 µm. Stationary and dynamic characteristics are presented for a 1.55 µm VCSEL. Finally, datacom and telecom transmission experiments are presented.

The field of communications has been revolutionized by the advent of optical fiber. Optical fiber now connects most of the world carrying a vast amount of information through a very limited physical medium. To the materials engineer, optical fiber technology has stimulated interest in a new type of micro-sensor application, where the size, weight, and the ability to integrate the sensor into a material structure play a major role. Areas of interest currently include materials characterization, cure monitoring, and structural health monitoring. The fiber Bragg grating (FBG) is such a sensor technology, creating an optical strain gauge within the core of an optical fiber through the use of a wavelength specific filter. As the FBG experiences induced strain along its major axis, its light signal indicates the amount of strain with great accuracy and sensitivity. In FBG applications, where loading may occur in all directions, complex changes take place in the FBG signal. As these changes impact the usefulness of the FBG as a strain sensor, this thesis endeavors to study and demonstrate the effect of transverse load on the FBG structure. A review of the basics of optical fiber fabrication, light transmission, and optical fiber sensor technology is presented first, followed by a detailed discussion of the FBG's manufacturing and operation. The crux of the thesis is presented with the results of experiments conducted to characterize the FBG signal in both unloaded and transversely loaded configurations. Polarization and wavelength based experiments and analyses are conducted to detect changes in the FBG signal. The results of the study are used to propose cause and effect relationships between transverse loading, its effect on the material properties of the FBG, and changes in the FBG signal.

Fiber optics are the right transmission medium to monitor the movement of the human body, one of them is knee activity. Single-mode fiber (SMF) and fiber Bragg grating (FBG) as sensing sensors that can monitor damage to bad conditions of the human joint area so can prevent further damage. The purpose of this study was to design fiber optic-based belts due to macrobending with sinusoidal patterns, determine the relationship of knee benders with average power change (P) of SMF and FBG, measure changes in power loss (L) fiber optics as a function of optical fiber diameter (d) and knee angle (k), and determine the optimum sensitivity (S) of SMF and FBG in detecting knee bending. The results showed that L of 0.0751 dB and S of 0.017442 dB are the largest values produced from the SMF belt with values d of 12 mm and k of 180°. The values L of 2.0177 dB and S of 0.591382 dB are the largest values produced from FBG belts with values d of 8 mm and k of 180°. The results of this study explain that FBG is more effective to use because it has a higher S value than SMF.

Optical fibres (OF) with inscribed arrays of fibre Bragg gratings (AFBG) are interesting for different types of fibre devices such as a fibre sensors and fibre lasers and optical signal management. The most advanced method of manufacturing such AFBG is to write them directly in-line during the fibre drawing process. This gives the possibility of making a huge number of fibre Bragg gratings (FBG) in a single OF. The parameters of these AFBG could be constant or variable along the OF length. We present results of an investigation AFBG produced during fibre drawing process. Typical parameters of such AFBG are: working wavelength into 1550 nm band, spectral width 0.2mm (3dB level), reflective contrast (excess to Rayleigh backscattering level) up to 45 dB. We use polymer or aluminum as AFBG coating. AFBG can be written either in single or multi core preforms. OF with fibre Bragg gratings A(FBG) arrays written over a long length and in-line metal coating have been made for the first time. The optical parameters of the FBG arrays are tested by the optical frequency domain reflectometer method in a wide temperature range, demonstrating no degradation in reflection at heating up to 600C for a fibre with Al coating. Moreover, array of metal coated FBG can not be erased thermally. The mechanical strength of the developed fibre is practically the same as for “ordinary” OF with similar coating, showing the absence of the influence of FBG writing process on fibre strength. Some possibilities of the developed AFBG use in advanced sensor systems with special resolution about 10 mm, Brillouin random fibre laser with linewidth less 40 kHz are considered.

Single-mode operation from an index-guided vertical-cavity surface-emitting laser (VCSEL) has been achieved by using a fiber Bragg grating (FBG) as an external selective wavelength reflector. The spectral characteristics of the external cavity controlled VCSEL under static and dynamic conditions have been studied. The FBG provided stable single mode operation under pseudorandom modulation with a sidemode suppression ratio of /spl sim/30 dB from 500 Mb/s up to 4 Gb/s at room temperature.

In this paper, the development of loop mirror multi-wavelength Brillouin fiber laser utilizing semiconductor optical amplifier and fiber Bragg grating is successfully demonstrated. A multi-wavelength BFL structure employs a single mode fiber that acts as the gain medium, while the fiber Bragg grating act to reflect the signals and semiconductor optical amplifier is used to amplify the signal are employed to produce multiple channels. The implementation of difference parameter for semiconductor optical amplifier driven current, Brillouin pump power, Brillouin pump wavelength and length of single mode fiber, plays an important role in producing the multiple channels. The highest number of channels which is 15 achieved once 9 km of SMF length, semiconductor optical amplifier current at 800mA and low Brillouin pump power at -12 dBm are utilized in the laser structure. Furthermore, the implementation of semiconductor optical amplifier and reflectivity of fiber Bragg grating in Brillouin fiber laser structure gives a better performance in producing the multiple channels.

Current spreading and carrier diffusion in vertical cavity surface-emitting lasers (VCSELs) critically affect the threshold current density, transverse gain profile, and optical modal characteristics of the devices. We have numerically analyzed these effects for four popular long-wavelength VCSEL structures with wafer-bonded GaAs-AlGaAs Bragg bottom mirrors. The results show that current confinement for p-mirror VCSELs is approximately twice as effective as the corresponding n-mirror VCSEL's although the confinement factor for all practical VCSEL structures studied here is considerably lower than the ideal structure. Calculations also show a strong dependence of current confinement on device aperture size and a tradeoff between the optical modal property and the current confinement efficiency for different apertured devices.

We introduced the vertical cavity surface emitting laser (VCSEL) as the laser diode in tour external cavity system. Because VCSELs are now commercially available, and the External cavity diode laser (ECDL) systems using them are expected to improve their frequency stability, we have replaced a Fabry-Perot type laser diode with a VCSEL, and examined its oscillation-frequency stability. Therefore we were able to expect that the VCSELs with our double optical feedback system have good oscillation frequency stability. The obtained VCSEL's oscillation-frequency stability, i.e., the square root of Allan variance σ was 4×10^-10, at an averaging time of τ=1 sec.

Improving the performance of a bidirectional radio over fiber (RoF) link by the use of fiber Bragg grating(FBG) in a carrier suppressed double sideband(CS-DSB)modulation based wavelength reuse incorporating coherent detection and signal processing unit like DSP is proposed. A comparison study is used to evaluate the performance improvement by the use of FBG. At the transmitter, a downstream optical signal containing optical single sideband+carrier(OSSB+C)is generated and transmitted to base station via a single mode fiber(SMF). The signal received is reused at the base station, in two ways in order to study the performance improvement of RoF by the use of FBG. The optical single side band and carrier is separated by the help of a FBG. The carrier is further reused for the upstream signal transmission and the downstream signal can be recovered. The upstream signal is recovered using coherent detection along with DSP unit to avoid the noise due to the phase difference from the transmitter laser source and to eliminate the cross talk effects.

The dependence of the mode partition noise (MPN) and the power penalty associated with it can be measured from the source spectral width. Our findings show that there is strong dependence of the carrier lifetime on the bit error rate degradation caused by MPN on the spectral width of the vertical cavity surface emitting laser (VCSEL). VCSELs with smaller spectral width (shorter carrier lifetime) exhibited smaller MPN induced power penalty. We found that the theoretical calculation of the power penalties caused by MPN from the carrier lifetime and the spectral width is in good agreement with the measured system penalties.

High performance GaAs- and InGaAs-based selectively oxidized vertical-cavity surface-emitting laser (VCSEL) arrays emitting in the 780 to 980 nm wavelength regime are investigated as light sources for high bit rate optical interconnects. Solid source molecular beam epitaxy (MBE) with carbon as p-type dopant is used for crystal growth. Single-mode devices show record-high output powers up to 4.8 mW with a butt-coupling efficiency of more than 80% into single-mode fiber. Linear 1/spl times/8 and 1/spl times/16 as well as two-dimensional 4/spl times/8 VCSEL arrays have been employed as transmitters in optical fiber links. Non-return-to-zero (NRZ) pseudo-random bit sequence (PRBS) signals with 2/sup 31/-1 wordlength at total data rates up to 24 Gb/s (8/spl times/3 Gb/s) have been transmitted over 500 m of multimode fiber. Bias-free data transmission of 2.5 Gb/s over 500 m of multimode fiber is demonstrated. Polymer optical fiber (POF) was used for short distance data transmission. A visible, red emitting VCSEL (/spl lambda/=780 nm) is employed to transmit 2.5 Gb/s over 1 m of small core diameter (120 /spl mu/m) POF. For all data transmission experiments bit-error rates (BERs) of better than 10/sup -11/ were observed for back-to-back testing as well as for fiber transmission.

Optical time domain multiplexed (OTDM) technology helps to respond to the need for increased capacity and demand in internet traffic using optical transmissions. High-speed optical networks are highly affected by chromatic dispersion (CD) that causes pulse broadening and intersymbol interference (ISI) at the light detector. This paper presents a comparative analysis of chromatic dispersion compensation on 10 Gbps single mode fibre (SMF) and 40 Gbps OTDM operating at C-band using Gaussian apodized linear chirped fibre Bragg grating, Erbium Doped Fibre Amplifier (EDFA) and uniform fibre Bragg grating (UFBG) hybrid approach in OptiSystem 18. A cascade of four UFBG and chirped Bragg gratings design approach presents minimal improvement in the quality factor and achievable single mode fibre length. Quality factor results at a reference wavelength of 1550 nm were obtained for respective chirped Bragg grating lengths of 8 mm, 20 mm, 50 mm, 87 mm, and 90 mm using linear Gaussian apodization. The Q-factor results obtained using the proposed simulation model for single 10 Gb/s channel were respectively 51.14 dBm over 30 km at a grating length of 20 mm and 15.64 dBm over 150 km at 45 mm of grating length. Different eye-diagrams corresponding to varying grating lengths have also been presented for 10 Gb/s and 40 Gb/s at corresponding SMF lengths.

High-bandwidth single-mode selectively oxidized vertical-cavity surface-emitting laser (VCSEL) arrays operate at 980 nm or 850 nm emission wavelength for substrate or epitaxial side emission. Coplanar feeding lines and polyimide passivation are used to reduce electrical parasitics in top-emitting GaAs and bottom-emitting InGaAs VCSELs. To enhance fundamental single-mode emission for larger devices of reduced series resistance a surface relief transverse mode filter is employed. Fabricated VCSELs are applied in various interconnect schemes. InGaAs quantum-well based VCSELs at 935 nm emission wavelength are investigated for use in perfluorinated graded-index plastic-optical fiber (GI-POF) links. We obtain a 7 Gb/s pseudo random bit sequence (PRBS) nonreturn-to-zero (NRZ) data transmission over 80 m long 155 /spl mu/m diameter GI-POF. We investigate data transmission over standard 1300 nm, 9 /spl mu/m core diameter single-mode fiber with selectively oxidized single-mode GaAs and InGaAs VCSELs. We achieve biased 3 Gb/s and bias-free 1 Gb/s pseudo-random data transmission over 4.3 km at 830 nm emission wavelength where a simple fiber mode filter is used to suppress intermodal dispersion caused by the second order fiber mode. For the first time, we demonstrate 12.5 Gb/s data rate transmission of PRBS signals over 100 m graded-index multimode fiber or 1 km single-mode fiber using high performance single-mode GaAs VCSELs of 12.3 GHz modulation bandwidth emitting at /spl lambda/=850 nm.