Experimental study on long-distance transmission of high-power laser over single-mode fiber

This thesis studies optical fiber taper coupled dielectric microsphere resonators and their applications. Fundamental properties including ideal coupling and critical coupling in an optical fiber taper to fused silica glass microsphere coupling system is investigated both theoretically and experimentally. A symmetrical dual-taper coupling configuration is proposed to obtain highly efficient power transfer from the taper coupler to the microsphere resonator. Applications as channel add/drop filters and microsphere lasers are also demonstrated. The physical essence of the fiber taper to silica microsphere is analyzed using a two-dimensional model. The relationship between the coupling strength and the cavity loss is unveiled. Adiabatic tapers and high-quality microspheres are fabricated and used to demonstrate actual coupling systems. Perfect agreement between the experimental results and the theoretical prediction is presented. Power transfer from the taper to a microsphere resonator has been significantly improved by employing a dual-taper symmetrically coupling configuration. Up to -28 dB extinction at the central resonant wavelength has been measured. We then propose a device application of the taper-sphere-taper structure as a channel add/drop filter in the wavelength division multiplexing systems. For a filter with a bandwidth of 3.8 GHz and a dropping channel isolation of 26 dB, the bit-error-rate measurement shows no power penalty at 2.5 Gbit/s. A 1.5 µm wavelength single-frequency fiber laser is also demonstrated using a single tapered fiber coupling to a highly doped erbium:ytterbium phosphate glass microsphere. The fiber taper serves the dual purpose of transporting optical pump power into the sphere and extracting the resulting laser emission. As low as 60 µW pump threshold and fiber-coupled output power as high as 3 µW with single mode operation are obtained. Imaging of photoluminescence from the sphere at visible wavelengths reveals the pump power is resonantly coupled into semiclassical orbits due to the strong absorption damping in the phosphate glass. A bi-sphere laser system consisting of two microspheres attached to a single fiber taper is also demonstrated. Finally, a novel hybrid fiber taper, made from a combination of a 980 nm single mode fiber and a 1550 nm single mode fiber, is proposed and demonstrated as the micros ph ere laser coupler. Both the pump wave and laser emission are found to be more efficiently coupled to and from, respectively, the sphere modes. As high as 112 µW single-frequency laser output power is measured with a differential quantum efficiency of 12%.

For several decades silica-based optical fibres have been used for telecommunication and sensor purposes. The single-mode fibre is frequently employed in long-distance networks, whereas the multi-mode fibre is the preferred means of signal transport in campus and in-building networks. Because of the huge bandwidth of optical fibres in comparison to its electrical wireless and copper-based counterparts, the demand for optical fibres keeps increasing. In a competitive market, fibre manufacturers aim to produce ever better fibres that are as cheap and easy to employ as possible. As fibre research, development and manufacturing is a mature discipline, improvements in fibre design can only be achieved through the construction of robust, accurate and efficient numerical fibre models for the computation of those quantities that determine the behaviour of the fibre. We have developed a modular software code, based on Maxwell’s equations, to compute these quantities in a vectorial full-wave way for both single-mode and multi-mode optical fibres. Key is the refractive-index profile, or, more specifically, the dopant profile, as it defines the propagation, splicing and bending-loss characteristics of the fibre. For the single-mode fibre, the fibre quantities that we have concentrated on are dispersion, dispersion slope, mode-field diameter, effective area, bending loss, effective and theoretical cut-off wavelength and MAC-value. We highlight one fibre quantity in particular, viz. the computation of the bending loss in a single-mode fibre. Many approximate modelling techniques have been developed to estimate this loss in a fast way. Our numerical scheme, however, is the first rigorous one, as we have performed a vectorial full-wave analysis of the bent optical fibre. In this context, triple integrals involving products of Bessel functions with large, complex order and argument appear. Due to cancellations in the pertaining computation, a high relative accuracy is needed for the computation of each product. As a result, it takes weeks on a contemporary computer to compute the bending loss as a function of the radius of curvature. We have used the vectorial full-wave bending-loss results to determine the most appropriate approximate method. Subsequently, we have extended that approximate method to compute the bending losses of higher-order modes, since the required effective cut-off wavelength depends on the bending loss of the first higher-order mode. The selected approximate method has been used in the ensuing bending-loss calculations. Since the fibre properties are often conflicting, it is a challenging task to adapt the radial dopant profile to meet a set of predefined design goals. A design goal is a combination of desired values for (some of) the aforementioned fibre quantities, and can mathematically be translated into a cost function. The minimisation of this cost function provides us with the optimal dopant profile for that specific set. For the single-mode fibre, we have performed this minimisation for piecewise-linear profiles, by employing various global and gradient-based local optimisation strategies to speed up the design step considerably. Frequently,these optimisation strategies lead to counter-intuitive dopant profile designs that could not have been contrived otherwise. We have selected a deliberate mix of several optimisation routines and have compared their performances. Perhaps the most important conclusion is that there still appears to be room for improvement in the design of the radial dopant profile of commercially available fibres. For the multi-mode fibre, vectorial full-wave optimisation is not feasible yet because of the long computation times for the large number of propagating modes. Still, our numerical scheme allows for a manual fine-tuning of the popular power-law profile to minimise differential mode delay. Further, we have included mode coupling and differential mode attenuation in our model to obtain intensity patterns that match closely with measurements. We have also analysed the influence of profile variations, e.g. on-axis dips and kinks, on the intensity pattern. A selective excitation of different mode groups in a multi-mode fibre, offers the possibility to create several independent transmission channels, and thus a higher information capacity. Recently, the feasibility of this so-called mode group diversity multiplexing technique has been demonstrated. Simulations provide us with a means to better understand its operation and possibly increase its efficiency. The channel separation may be enhanced by employing a lens between the fibre and the detector, which is called mode-selective spatial filtering. Our numerical simulations of a mode group diversity multiplexing link, with and without mode-selective spatial filtering, are in agreement with the measurements. The above discussion makes clear that the developed software code has a wide range of applicability. Moreover, it is built in a modular way and thus extensions, like the inclusion of more fibre quantities or different profile dopants, are straightforward.

We have successfully compressed the pulse duration of an all-normal-dispersion (ANDi) Yb-fiber laser to sub-100 fs by a single-mode nonlinear fiber amplifier. Through optimizing the input power, prechirp, and the pump power of the amplifier, the pulse duration of the ANDi Yb-fiber laser is compressed to 68 fs when a 2.5-m active fiber is used, although for a 3.5-m active fiber, the pulse duration of the laser can be compressed to 58 fs. We have also shown that a low relative intensity noise is maintained within the nonlinear amplification process. The experimental results prove this ultrashort-pulse-duration and low-noise laser source with moderate output power can be a good candidate for frequency comb, multiphoton imaging, coherent Raman spectroscopy, and other applications.

Among all non-linear optical effects observed in single-mode optical fibres, stimulated Brillouin scattering (SBS) is of particular importance since it has numerous practical implications. SBS occurs when laser waves are scattered through the interaction of light with hypersonic acoustic waves in the medium. It manifests through the generation of a backscattered Stokes wave that carries most of the input optical power once a definite intensity level is reached within the fibre core. It limits the maximum optical power that can be transmitted through an optical fibre and therefore can cause a severe penality for fibre optics telecommunications. The backscattered Stokes wave is down shifted in frequency with respect to the incident lightwave frequency. This frequency shift – often called the Brillouin frequency shift – depends on the fibre parameters and on the wavelength of the incident light. It is directly proportional to the acoustic velocity and ranges from 12 to 13 GHz for silica fibres at a 1.3 µm wavelength. The Brillouin frequency shift is also very sensitive to environmental quantities changing the acoustic velocity such as temperature and strain. This feature makes SBS very interesting for temperature and strain monitoring in optical fibres and has been used in the design of fibre optics sensors. In the present work, a novel method to measure SBS characteristics in single-mode optical fibres has been developed. It is based on the so-called pump and probe technique using two counterpropagating optical waves within the test fibre. It basically presents the originality to require only one laser source and relies on an electro-optic modulator to generate the probe signal by microwave modulation of the pump light. The inherent high stability of the experimental set-up has made possible the characterisation of different fibres in terms of SBS parameters with a higher accuracy than the traditional two lasers set-up. The influence of dopant concentration in silica fibres on the SBS properties has been fully characterised. Furthermore it has been determined that the presence of core dopants decreases the acoustic velocity resulting in a smaller Brillouin frequency shift. The temperature and strain effects on the SBS characteristics have been extensively investigated. The problem of the evolution of the polarisation of the interacting lightwaves has been studied and a model clarified the so far unexplained behaviour in low birefringence fibres. A new measurement procedure has been defined to achieve polarisation-independent Brillouin gain determination. The second facet of the present thesis deals with the concept of distributed fibre optics sensors based on SBS. The determination of the Brillouin frequency shift gives access to the temperature or strain experienced by the fibre, while a modified OTDR technique provides the information on the position. Here again the use of a single laser source together with an electro-optic modulator brings several advantages. Besides the convenient flexibility in the generation of the probe signal, it makes possible to pulse the optical signals to localise the interaction within the test fibre. The overall temperature or strain distribution of the test fibre can be carried out by measuring the Brillouin frequency shift at any locations throughout the test fibre. A sensor has been experimentally achieved and its performances can be summarised as follow: spatial resolution less than 10 meters over more than 10 kilometres, with a resolution on the Brillouin frequency determination of 1 MHz, that corresponds to a ±1°C temperature resolution or to a 2x10-5 strain resolution. The dynamic range can be increased up to 30 kilometres to the detriment of the spatial resolution. The ultimate performances of the SBS distributed fibre optics sensors have been investigated in terms of spatial resolution and dynamic range. It turns out that these sensors are eventually dedicated to distributed measurements over several tens of kilometres with a spatial resolution limited to the meter range. Finally two practical applications of such sensors are described: the measurement of the strain distribution in installed fibre optics cables for telecommunications and the temperature monitoring of electrical energy distribution cables. On site measurements using a Brillouin sensor have been performed for the first time thanks to the high stability and reliability of the sensor.

As the need for higher information throughput increases, standard solutions such as copper lines and radio links seem to approach their limits. Therefore, optical solutions, after having conquered the long and medium-range networks, are nowadays also migrating into short-range data communication scenarios, offering the possibility of high capacity information transfer for both professional as well as consumer applications. The challenge is to offer cost-effective and robust optical solutions at relatively short (? 1 km) transmission distances, where traditional single-mode fiber for long-haul transmission systems are unsuitable. Solutions such as multimode glass fibers (MMF), plastic optical fibers (POF), using light-emitting diodes (LED) or low-cost vertical cavity surface emitting laser diodes (VCSEL), and optical wireless links (based on LEDs) are therefore being proposed and seem to be promising candidates. These solutions feature low costs, easy handling and installation, flexibility, and robustness, which are all very suitable characteristics for consumer needs. However, this comes at the expense of less bandwidth when compared to single-mode fiber systems. This thesis investigates the use of digital signal processing in order to overcome the bandwidth limitations in short-range optical communication systems, ensuring that such solutions are future-proof. In particular, discrete multitone (DMT) modulation is proposed and investigated in order to increase the capacity of such systems. Derived from the more general orthogonal frequency division multiplexing (OFDM), DMT is a baseband multicarrier modulation technique that is already widely employed in copper-based digital subscriber lines (DSL) systems such as asymmetrical DSL (ADSL) and very high data rate DSL (VDSL). By dividing a high-speed serial data stream into multiple parallel low-speed sub-streams and transmitting them simultaneously using different frequencies, DMT can be used to efficiently combat various signal impairments such as dispersion and narrowband interference. Due to the use of intensity-modulation and direct-detection (IM/DD) in low-cost optical systems, where only the intensity of light is modulated and not the phase, the application of DMT is different from standard electrical systems. Characteristics such as high crest factor, which is the ratio of the peak to root-mean-square amplitude value of the DMT signal, and clipping have different consequences and are studied in this thesis. After an introduction to the principles of DMT and rate-adaptive bit-loading, an analytical model of the optical IM/DD channel for short-range optical communications is presented. Making use of this model, the theoretical capacity of such a channel is derived for both a Gaussian and a first-order low-pass electrical-to-electrical channel response by means of the water-filling method. It is found that the crest factor of the modulation signal plays a dominant role in defining the capacity of the optical IM/DD channel. Furthermore, by including characteristics of DMT modulation such as clipping and quantization, it is shown that the calculated capacity values can be refined and optimum parameters for DMT transmission over an optical IM/DD channel exist. Following this, the optimum clipping values and number of subcarriers for maximizing DMT transmission performance over an optical IM/DD channel are investigated. It is shown that the optimum clipping value, which depends on various system parameters such as receiver noise power and modulation order, can be determined by using an analytical expression. In the case of the number of subcarriers, larger values generally lead to better performance when DMT with bit-loading is used. Additionally, various experiments to explore the system limits of DMT techniques have been performed and the results for POF, MMF, and optical wireless are presented. It is shown that record bit-rates of up to 47 Gbit/s can be achieved using DMT. Finally, an efficient way to implement DMT is presented, together with results regarding the implementation of a real-time DMT transmission system operating at 1.25 Gbit/s. System complexity issues of real-time hardware implementation are also discussed, showing that pipelining and parallelization are essential in high-speed designs, adding to the need of extra hardware resources. Moreover, it is verified that for DMT, the Fast Fourier Transform (FFT) operations require most hardware resources. After the presentation of some alternative modulation techniques such as pulse-amplitude-modulated DMT (PAM-DMT), which also were investigated by the author, this thesis ends with the conclusions and some recommendations for further research work.

In this thesis I will describe the basic concepts necessary for acquiring functional data from nerve tissue by application of two-photon laser scanning microscopy and its miniaturization for in vivo imaging in freely moving animals. The construction of a laser with enhanced repetition rate and the design of a novel non-resonant fiber scanning technique is emphasized. The increased repetition rate of the femtosecond laser source should decrease undesirable non-linear effects when transmitting ultra-short pulses with a single mode fiber. By implementing the prototype of a head-mounted microscope involving the novel fiber scanner and testing with fluorescent probes I can demonstrate its feasibility - not only for imaging in freely moving animals but for mobile laser scanning microscopy in general.

Single-Mode Fibers for High Speed and Long-Haul Transmission Multimode Fibers for Data Centers Multi-core Fibers for Space Division Multiplexing Optical Coherent Detection and Digital Signal Processing of Channel Impairments A Brief History of Fiber-Optic Soliton Transmission Perturbations of Solitons in Optical Fibers Emission of Dispersive Waves from Solitons in Axially Varying Optical Fibers Nonlinear Waves in Multimode Fibers Shock Waves A Variety of Dynamical Settings in Dual-Core Nonlinear Fibers Advanced Nano-engineered Glass-Based Optical Fibers for Photonics Applications Fabrication of Negative Curvature Hollow Core Fiber Optimized Fabrication of Thulium Doped Silica Optical Fiber Using MCVD Microfiber: Physics and Fabrication Flat Fibers: Fabrication and Modal Characterization 3D Silica Lithography for Future Optical Fiber Fabrication Rare-Earth-Doped Laser Fiber Fabrication Using Vapor Deposition Technique Powder Process for Fabrication of Rare Earth-Doped Fibers for Lasers and Amplifiers Progress in Mid-infrared Fiber Source Development Crystalline Fibers for Fiber Lasers and Amplifiers Cladding-Pumped Multicore Fiber Amplifier for Space Division Multiplexing Optical Amplifiers for Mode Division Multiplexing Optical Fibers for High-Power Lasers Multicore Fibers Polymer Optical Fibers Optical Fibers in Terahertz Domain Optical Fibers for Biomedical Applications Basics of Optical Fiber Measurements Measurement of Active Optical Fibers Characterization of Specialty Fibers Characterization of Distributed Birefringence in Optical Fibers Characterization of Distributed Polarization-Mode Coupling for Fiber Coils Materials Development for Advanced Optical Fiber Sensors and Lasers Optoelectronic Fibers Fiber Grating Devices CO2-laser-inscribed long period fiber gratings: from fabrication to applications Micro-/Nano-optical Fiber Devices Measurement of Optical Fiber Grating Measurement of Optical Fibre Amplifier Measurement of Optical Fiber Laser Distributed Rayleigh Sensing Distributed Raman Sensing Distributed Brillouin Sensing: Time-Domain Techniques Distributed Brillouin Sensing: Frequency-Domain Techniques Distributed Brillouin Sensing: Correlation-Domain Techniques Optical Fibre Sensors for Remote Condition Monitoring of Industrial Structures Optical Fiber Sensor Network and Industrial Applications Fibre Optic Sensors for Coal Mine Hazard Detection Optical Fiber Sensors in Ionizing Radiation Environments Polymer Optical Fiber Sensors and Devices Solid Core Single-Mode Polymer Fiber Gratings and Sensors Microstructured Polymer Optical Fiber Gratings and Sensors Polymer Fiber Sensors for Structural and Civil Engineering Applications Photonic Microcells for Sensing Applications Filling Technologies of Photonic Crystal Fibers and Their Applications Photonic Crystal Fiber-Based Grating Sensors Photonic Crystal Fiber-Based Interferometer Sensors Optical Fiber Microfluidic Sensors Based on Opto-physical Effects Micro-/Nano-Optical Fiber Microfluidic Sensors All Optical Fiber Optofluidic or Ferrofluidic Microsensors Fabricated by Femtosecond Laser Micromachining

Depolarized Interferometric Fiber Optic Gyroscopes (D-IFOGs) that are constructed with inexpensive single mode (SM) fiber have provided an opportunity for developers to meet Army emerging missions goals for affordable, small volume, reliable inertial guidance systems for use in small missiles, munitions, and future micro-unmanned autonomous vehicles. However, there remain several vital issues associated with substantially reducing the diameter of the sensor coil. Optical fiber that is precision-wound onto a micro coil experiences increased stress due to small radius bending, fiber distortions at crossover sites, and increased interlayer pressures as a result of multiple layers of fiber wound under tension. Tension and small radius bending stresses can have a detrimental effect on the performance of D-IFOGs. Therefore, other scenarios for the application of SM fiber to a micro-sensor coil must be considered. One scheme involves taking advantage of the bending-induced birefringence and employing the low cost SM fiber as a polarization-maintaining (PM) fiber. The mechanics of how a substantial reduction in the coil radius produces PM fiber properties in SM fiber is investigated under this research effort. Conventional and specialty SM fibers are characterized to identify optimal fibers for the development of micro-sensor coils. The results from extinction ratio measurements on the SM fibers and micro-sensor coils are presented in this paper. The significant cross coupling suggests that scattering centers are present in very small radius bending. Also, measurements show that optical loss is significant in micro IFOG coils.

The next generation of stellar interferometer arrays will develop methods for observing fainter sources with greater resolution and create synthesized images at optical and IR wavelengths like those obtained from radio telescope arrays. Techniques using single-mode (SM) fiber optics offer significant advantages for future ground and space-based interferometers. SM fibers and integrated optics components can be used for nearly lossless transport and combination of light beams in a stellar interferometer, avoiding the multiple lossy reflective surfaces that must be kept in precise alignment with conventional optics. Furthermore, SM fibers spatially filter light corrupted by atmospheric seeing fluctuations or optical aberrations, increasing the fringe visibility and potentially improving measurement accuracy for bright sources by an order of magnitude. Controlling the dispersion and polarization properties of SM fibers is possible, but the poor coupling efficiency to aberrated light is a major limitation. MC fibers consist of a symmetrical arrangement of SM fiber cores inside a common cladding. One MC fiber is placed at the focus of each telescope, where light couples into the fiber and propagates through the cores for a short distance. Each MC fiber is then drawn apart into individual single-core fibers, and the resulting SM fiber beams are interfered pair-wise with beams from other telescopes. MC fibers are predicted to have an improved coupling efficiency over standard SM fibers, and the MC fiber geometry is well suited for transporting and combining light beams with minimal losses in an interferometer array. Computer simulations of fiber-linked interferometer arrays were performed to evaluate the performance of MC and standard SM fibers with different conditions of atmospheric, photon and detector noise. The effects of waveguide and material dispersion over a broad band at visible wavelengths are also included. The simulations determine the fiber modes, calculate the light coupling, propagate light through the fibers, and measure the beam correlations. Photon and detector noise are added and noisy estimates of the fringe power and bispectrum are found for the interferometer baselines. Statistics are then calculated over large ensembles and the measurements are processed to reconstruct an image of the source object. MC fibers are found to have greatly improved coupling efficiency over conventional SM fibers to aberrated light, and the noise sensitivity of visibility measurements and images also improves under certain conditions. Simulated images are shown and attempts to make MC fibers are discussed.

Two pulse-shortening schemes based on the nonlinear propagation of picosecond pulses from a Nd: YAG laser in single-mode optical fibres are described. In the first experiment, a spectral-windowing technique was employed to compress self-phase modulated pulses exiting 100 m of optical fibre, and compression factors of 2.5 were recorded in good agreement with theoretical predictions. The technique was adapted to enable the time-dependent frequency shift occurring after pulse propagation through 2 km of fibre to be measured. In the second experiment, cascaded stimulated Raman generation, occurring when a high-power pump pulse propagated in 4 m of fibre, was studied. Lower-order components in the cascade were severely depleted through generation of higher-order Stokes signals, leaving negatively chirped pulse fragments. These were compressed with a dispersion-tunable Gires-Tournois interferometer and compression ratios of times 4 were obtained.

Fiber amplifiers with a robust monolithic seed coupling and very high peak power in a near diffraction-limited beam are increasingly demanded by many industrial applications in laser materials processing. A large mode area fiber is used to scale up the peak power and suppress the nonlinear effects. An approach of local adiabatic taper is proposed to provide a monolithic signal path and selectively excite the fundamental mode in highly multimode fiber. The powder-sintering technology was employed to achieve rod-type fibers with excellent refractive index homogeneity. First experiments were performed with 56m core diameter rod fibers. While non-tapered fiber amplifier achieved a peak power of 544kW, tapered amplifier reached 230kW. For comparable average powers of 10W, the taper improves the beam quality from M2 values of about 10 to 3.5, while the monolithic seed coupling significantly improves the beam stability. It was observed that the dopants diffuse during the tapering process because of high temperature, possibly providing further sources for coupling to higher order modes. Second experiments with improved rod-type fiber amplifiers (reduce the Al3+-content of fiber core and use suitable material of outer clad to mitigate the diffusion problem) delivered 2ns pulses with peak powers of 210kW for the non-tapered rod and 140kW for the tapered rod (limited by facet damage). For the tapered fiber, the beam quality was between 1.3 and 1.7, significantly improved compared to the beam quality of the non-tapered fiber (M2 = 3.3 ~ 4.5). An endcap was adopted for the tapered fiber amplifier and the peak power is scaled up to 375kW in the nearly diffraction limited region. For future work, the confined doped fibers and a picosecond seed laser source are envisioned.

The mode competition mechanism in concentric 4-core and 7-core fiber lasers with large mode area single mode (SM) fiber as in-phase supermode selection component is presented. The coupling coefficient between the fundamental mode in large mode area SM fiber and each supermode in mutlicore fiber is discussed. For individual supermode in multicore fiber, the coupling coefficient is optimized as a function of the core radius of SM fiber as well as the distance between multicore fiber and SM fiber. The optimization results demonstrate that only two supermodes are involved in concentric-type fiber lasing - in-phase and anti-phase supermode, owing to the negligible coupling coefficients of the other supermodes. Furthermore, to achieve the best in-phase supermode selection, the core radius of SM fiber will be optimized for maximum coupling coefficient difference between in-phase supermode and anti-phase supermodes. The numerical results illustrate that in-phase supermode always dominate the output and is the highest when the distance equals zero. Compared to conventional multicore fiber lasers with Talbot cavity, this all-fiber configuration based on large mode area SM fiber has higher-order supermodes more efficiently suppressed and high-brightness output may be achieved.

Modulation instability (MI) as the main limit to the sensing distance of distributed fiber sensors is thoroughly investigated in this thesis in order to obtain a model for predicting its characteristics and alleviating its effects. Starting from Maxwell's equations in optical fibers, the nonlinear Schrödinger equation (NLSE) describing the propagation of wave envelope in nonlinear dispersive media is derived. As the main tool for analyzing modulation instability, the NLSE is numerically evaluated using the split-step Fourier method and its analytical closed-form solutions such as solitons are utilized to validate the numerical algorithms. As the direct consequence of the NLSE, self-phase modulation is utilized to measure the nonlinear coefficient of optical fibers via a self-aligned interferometer. The modulation instability gain is obtained by applying a linear stability analysis to the NLSE assuming a white background noise as the seeding for the nonlinear interaction. The MI gain spectrum is expressed by hyperbolic functions for lossless fibers and by Bessel functions with complex orders for fibers with attenuation. An approximate gain spectrum is presented for lossy fibers based on the gain in lossless optical fibers. The accuracy of the analytical results and approximate formulas is evaluated by performing Monte Carlo simulations on the NLSE. The impact of background noise on the onset and evolution of modulation instability is analytically investigated and experimentally demonstrated. Power depletion due to the nonlinear process of modulation instability is modeled by integrating its gain spectrum using Laplace's method. Based on that, a critical power for MI is proposed by introducing the notion of depletion ratio. The model is verified by numerical simulation and experimental measurement. An optimal input power for distributed fiber sensors is proposed to maximize the output optical power and thus, the far end signal-to-noise ratio. Furthermore, the recurrence phenomenon of Fermi-Pasta-Ulam is experimentally observed and numerically simulated, validating the utilized numerical techniques. A standard Brillouin optical time-domain analyser serves as the experimental test bench for the proposed model. As the physical phenomenon behind the experiment, stimulated Brillouin scattering is described based on a pump-probe interaction mechanism through an acoustic wave. A 25 km single-mode fiber is employed as a nonlinear medium with anomalous dispersion at the pump wavelength 1550 nm. The evolution of pump power propagating along the fiber is mapped using the Brillouin interaction with the probe lightwave. The measured longitudinal power traces are processed to extract the impact of MI on the pump power. It is experimentally demonstrated that distributed fiber sensors with orthogonally-polarized pumps suffer less from modulation instability. As the scalar modulation instability of each pump reduces, vector modulation instability occurs because of interaction between the pumps; however, the overall performance improves. A version of the coupled nonlinear Schrödinger equations known as the Manakov system is shown to describe the behavior of two-pump distributed fiber sensors employing optical fibers with random birefringence. The excellent agreement between the experimental and numerical results indicates that the performance limit of two-pump distributed fiber sensors is determined by polarization modulation instability.

A simple fiber temperature sensor based on Michelson interferometer is investigated experimentally. The sensor is formed successive splicing of a single mode fiber (SMF) spliced to a short section of multimode fiber (MMF) followed by another SMF, which also known as single mode-multimode-single mode (SMS) structure. Temperature response of three different sensor lengths of 10 mm, 20 mm and 30 mm are experimented with increasing and decreasing temperature. The sensor exhibits good linearity, stability and repeatability for the test range from room temperature to 180 °C. The highest sensitivity is attained by the 10 mm sensor with response ~0.108 nm/°C. Factors that affect sensitivity are discussed and related issues are addressed. This sensor is most suitable for low to intermediate temperature applications.

Optical feedback can reduce the linewidth of a semiconductor laser by several orders of magnitude, but it can also cause line broadening. Although these effects on the temporal coherence of the laser are well known, a good understanding of the effects of feedback on the spatial coherence is still lacking. Here we present an experimental technique that allows discriminating the effects of feedback on temporal and spatial coherence of the laser beam. We analyze the output of a commercial edge-emitting laser diode, comparing the contrast of speckle images recorded using a multimode (MM) or single mode (SM) fiber and an optical diffuser, and also, comparing the optical spectra at the end of the MM or SM fiber. Optical spectra reveal feedback-induced line broadening, while speckle analyses reveal reduced spatial coherence due to feedback-excited spatial modes. These modes reduce the speckle contrast (SC) up to 50% when speckle images are recorded using the MM fiber, but do not affect the SC when the images are recorded using the SM fiber and diffuser, because the spatial modes that are excited by the feedback are filtered out by the SM fiber. This technique is generic and can be used to discriminate spatial and temporal coherence of other types of lasers and under other operating conditions that can induce a chaotic output.