Optical fiber sensors are widely applied in modern sensing systems. Taking advantage of the high sensitivity of optical interference, optical coherence fiber sensors, such as fiber Bragg gratings (FBGs) and Fabry-Perot interferometers (FPIs), have been investigated intensively and utilized broadly in optical sensing systems. Multimode fibers (MMFs) offer low coupling loss, high compatibility with various light sources, and insensitivity to ambient fluctuations, which are preferred for reliable and low-cost sensing systems. Therefore, the combination of optical coherence sensors and MMFs have facilitated and will continue to contribute to various optical sensor designs with desirable performances. This dissertation addresses the design and construction of optical coherence sensors in multimode fibers and presents several fully multimode fiber sensing systems with low coherence light source. Moreover, a theoretical analysis of fiber mode excitations, model coupling, and multimodal interference is conducted, and a numerical model is constructed to study the behaviors of optical coherence sensors in MMFs. With the femtosecond laser point-by-point inscription method, parallel fiber Bragg gratings (pFBGs), scattering array interferometers (SAIs), and densely multiplexable scattering array interferometers (DMSAIs) in sapphire fibers are proposed, fabricated and characterized, achieving excellent performance in multiplexed high temperature sensing. The study on SAI signals also revealed the unique coherence properties of MMFs. This work points out that the coherence properties in MMFs play a significant role in affecting the performances of optical coherence sensors, and such properties are closely related to both geometrical and optical properties of the fibers. This work also presents both theoretical and experimental tools to explore such properties and predict and test the performance of optical coherence sensors in MMFs, which is of great significance in the applications of such sensors in the real world. / Doctor of Philosophy / Optical fibers have been considered a powerful media that opened a new era in the field of telecommunication and optical sensing. Owing to their excellent resistance to chemical corrosion, immunity to electromagnetic interference, extremely low loss transmission at long distance, small size, and large aspect ratio, optical fibers are considered an ideal media to construct optical sensors. Optical coherence sensors are a very important type of optical fiber sensors that utilize the optical coherence property, such as interference, for sensing purposes. A lot of such sensors are generally constructed with single mode fibers (SMFs) owing to the high-quality coherence interaction supported by the fundamental-mode-only operation. Multimode fibers (MMFs), however, process high compatibility with various light sources owing to the high power-coupling efficiency because of large core size, which is desirable for industrial applications that requires low-cost and robust sensing systems. Meanwhile, the high modal volume of MMFs causes severe challenges on the design and fabrication of optical coherence sensors in MMFs. In this work, theoretical analysis of the mode excitation and coupling in MMFs is discussed, and a numerical model to simulate the behaviors of optical coherence sensors in MMFs is built. Then, using femtosecond laser point-by-point fabrication technique, parallel fiber Bragg gratings (pFBGs), scattering array interferometers (SAIs), and densely multiplexable scattering array interferometers (DMSAIs) are demonstrated and proven to be effective in multiplexable temperature sensing. Furthermore, using the numerical model and the SAI structures, coherence properties in MMFs are studied. This research may facilitate a deeper understanding of coherence properties in optical waveguides and support the design of novel fiber sensors that can be utilized in the real world.

This dissertation focuses on the use of subcarrier multiplexing (SCM) in multimode fibers, utilizing carrier frequencies above what is generally utilized for multimode fiber transmission, to achieve high bit rates. In the high frequency region (i.e., frequencies larger than the intermodal bandwidth), the magnitude response of multimode fiber does not decrease monotonically as a function of the frequency but is shown to become relatively flat (but with several deep nulls) with an amplitude below that at DC. The statistical properties of this frequency response at high frequencies are analyzed. The probability density function of the magnitude response at high frequencies is found to be a Rayleigh density function. The average amplitude in this high frequency region does not depend on the frequency but depends on the number of modes supported by the fiber. To transmit a high bit rate signal over the multimode fiber, subcarrier multiplexing is adopted. The performance of the SCM multimode fiber system is presented. The performance of the SCM system is significantly degraded if there are some subcarriers located at the deep nulls of the fiber. Equalization and spread spectrum techniques are investigated but are shown to be not effective in combating the effects of these nulls. To cancel the effects of these deep nulls, training process and diversity coding are considered. The basic theory of diversity coding is given. It is found that the performances of the system with training process and the system with diversity coding are almost identical. However, diversity coding is more appropriate since it requires less system complexity. Finally, the practical limits and capacity of the SCM multimode fiber system are investigated. It is shown that a signal with a bit rate of 1.45 Gbps can be transmitted over a distance up to 5 km. / Ph. D.

Optical fiber sensors have already been developed from the experimental stage to practical applications in the past 20 years. There is no doubt that this technology can bring a wealth of applications, ranging from sensors in medical industry, aerospace and wind-energy industries, through to distributed sensors in oil and gas industry. Among a large amount of physical and chemical parameters which optical fiber sensors could measure, temperature and strain are the most widely studied. This thesis presents several low-cost optical fiber sensor configurations primarily for temperature and strain measurement. Several basic optical fiber components which are good candidates as optical fiber sensors are used in our experiments, such as fiber Bragg gratings (FBGs), multimode fibers (MMFs), small-core dispersion compensation fibers (SCDCFs), high-birefringence fiber loop mirrors (HBFLMs), and polarization-maintaining photonic crystal fibers (PMPCFs). Temperature and strain cross sensitivity is a crucial issue when designing high performance optical fiber sensors, since most of the sensing components are both sensitive to temperature and strain. This would introduce an error when measuring each of them independently. We developed several schemes to overcome this problem by cascading an FBG and a section of MMF, inserting an FBG into an HBFLM, and space division multiplexing two HBFLMs. By measuring the wavelength shifts of the two independent components' spectra in each scheme, simultaneous measurement of temperature and strain could be achieved. However, all the above schemes need optical spectrum analyzers to monitor the spectral information, which increases the cost of the system and limits the operation speed. In order to avoid using optical spectrum analyzers, we use an intensity-based interrogation method with MMFs and HBFLMs as edge filters. By measuring power ratio changes, instead of monitoring spectra shifts, simultaneous measurement of temperature and strain could be realized with a low cost and high speed. The resolutions of the above five configurations are between 0.26 - 1.2 ^oC in temperature and 9.21 - 29.5 με in strain, which are sufficient for certain applications. We also investigate the sensing applications with the SCDCF. Since the cutoff wavelength of this kind of fiber is around 1663 nm, which makes it naturally an MMF in the wavelength range of 1550 nm. By slightly offsetting the core of the SCDCF with respect to that of the standard single-mode fiber (SMF), a high extinction ratio could be achieved with almost 9 dB. When a lateral force (lateral strain) applied on the SCDCF, extinction ratio will decrease. The change of the extinction ratio is almost independent of temperature variation. The measured extinction ratio change has a good quadratic relationship with respect to applied lateral force. This feature could be used to measure lateral force (lateral strain). In addition, we also use this feature to realize simultaneous measurement of both the longitudinal strain and lateral strain, since the applied longitudinal strain results in the whole spectrum shift. Moreover, a miniature high temperature sensor could also be made using the SCDCF. One end of a 4-mm long SCDCF is spliced directly to SMF with the other end cleaved. By monitoring the reflection spectrum of the SCDCF, temperature information could be obtained. This sensing head is very compact and could realize high temperature measurement up to 600 ^oC. Recently, a kind of PMPCF has been found to have very small responses to temperature change. This offers an opportunity to measure other parameters without considering temperature influence. We construct a compact 7-mm long transmission-type sensor with this kind of PMPCF. The interference spectrum generated by the coupling of cladding modes and core mode is obtained by slightly offsetting the PMPCF core to SMF core. The experiment shows that the interference spectrum is almost unchanged within the temperature range of 25-60 ^oC. The presented sensor has the potential to be used to measure strain and refractive index in the normal environment without temperature discrimination for practical applications.

Optical fiber sensors have already been developed from the experimental stage to practical applications in the past 20 years. There is no doubt that this technology can bring a wealth of applications, ranging from sensors in medical industry, aerospace and wind-energy industries, through to distributed sensors in oil and gas industry. Among a large amount of physical and chemical parameters which optical fiber sensors could measure, temperature and strain are the most widely studied. This thesis presents several low-cost optical fiber sensor configurations primarily for temperature and strain measurement. Several basic optical fiber components which are good candidates as optical fiber sensors are used in our experiments, such as fiber Bragg gratings (FBGs), multimode fibers (MMFs), small-core dispersion compensation fibers (SCDCFs), high-birefringence fiber loop mirrors (HBFLMs), and polarization-maintaining photonic crystal fibers (PMPCFs). Temperature and strain cross sensitivity is a crucial issue when designing high performance optical fiber sensors, since most of the sensing components are both sensitive to temperature and strain. This would introduce an error when measuring each of them independently. We developed several schemes to overcome this problem by cascading an FBG and a section of MMF, inserting an FBG into an HBFLM, and space division multiplexing two HBFLMs. By measuring the wavelength shifts of the two independent components' spectra in each scheme, simultaneous measurement of temperature and strain could be achieved. However, all the above schemes need optical spectrum analyzers to monitor the spectral information, which increases the cost of the system and limits the operation speed. In order to avoid using optical spectrum analyzers, we use an intensity-based interrogation method with MMFs and HBFLMs as edge filters. By measuring power ratio changes, instead of monitoring spectra shifts, simultaneous measurement of temperature and strain could be realized with a low cost and high speed. The resolutions of the above five configurations are between 0.26 - 1.2 ^oC in temperature and 9.21 - 29.5 με in strain, which are sufficient for certain applications. We also investigate the sensing applications with the SCDCF. Since the cutoff wavelength of this kind of fiber is around 1663 nm, which makes it naturally an MMF in the wavelength range of 1550 nm. By slightly offsetting the core of the SCDCF with respect to that of the standard single-mode fiber (SMF), a high extinction ratio could be achieved with almost 9 dB. When a lateral force (lateral strain) applied on the SCDCF, extinction ratio will decrease. The change of the extinction ratio is almost independent of temperature variation. The measured extinction ratio change has a good quadratic relationship with respect to applied lateral force. This feature could be used to measure lateral force (lateral strain). In addition, we also use this feature to realize simultaneous measurement of both the longitudinal strain and lateral strain, since the applied longitudinal strain results in the whole spectrum shift. Moreover, a miniature high temperature sensor could also be made using the SCDCF. One end of a 4-mm long SCDCF is spliced directly to SMF with the other end cleaved. By monitoring the reflection spectrum of the SCDCF, temperature information could be obtained. This sensing head is very compact and could realize high temperature measurement up to 600 ^oC. Recently, a kind of PMPCF has been found to have very small responses to temperature change. This offers an opportunity to measure other parameters without considering temperature influence. We construct a compact 7-mm long transmission-type sensor with this kind of PMPCF. The interference spectrum generated by the coupling of cladding modes and core mode is obtained by slightly offsetting the PMPCF core to SMF core. The experiment shows that the interference spectrum is almost unchanged within the temperature range of 25-60 ^oC. The presented sensor has the potential to be used to measure strain and refractive index in the normal environment without temperature discrimination for practical applications.

Few-mode fibers (FMFs) and multimode fibers (MMFs) can provide much higher data-carrying capacities compared with single-mode fibers. But in order to achieve this goal, one must address the challenge of intermodal coupling and dispersion. Therefore the ability to accurately control the optical signal propagation in FMFs/MMFs can play a pivotal role in FMF/MMF applications. This thesis demonstrates the ability to excite, in FMFs and MMFs, the desired linearly polarized (LP) modes as well as their superpositions through adaptive optics (AO). Specifically, in the case of step-index FMFs, a phase-only spatial light modulator (SLM) is employed to manipulate the light at the fiber input end, driven by the feedback signal provided by the correlation between the charge coupled device (CCD) camera captured images at the fiber output end and the target light intensity profile. Through such an adaptive optical system, any arbitrarily selected LP modes can be excited at the distal end of the four-mode and seventeen-mode fibers, respectively. For a graded-index MMF with a uniform Bragg grating, we use a deformable mirror (DM) to perform the wavefront modulation at the fiber input end, where the feedback is based on the ratio of the grating-reflected signal power to the transmitted signal power. At the Bragg grating position of this highly multimode fiber, any desired principal mode groups can be successfully chosen. These experimental results suggest that adaptive control of optical wavefront in FMFs/MMFs is indeed feasible. / Master of Science / Optical fibers, in terms of the number of modes they support, can be generally divided into single-mode fibers (SMFs), and few-mode fibers/multimode fibers (FMFs/MMFs). FMFs/MMFs can provide much higher data-carrying capacities than SMFs. For example, an FMF/MMF that supports M modes can ideally increase the data transmission rate by a factor of M, where each mode can serve as a distinct communication channel. However, in order to achieve good performance, one must accurately control signal propagation in FMFs/MMFs, which are often degraded due to the multiple-mode nature. This thesis demonstrates the ability, using adaptive optics (AO), to control signal propagation in FMFs and a highly MMF, respectively. Specifically, in the case of FMFs, a phase-only spatial light modulator (SLM) is employed to manipulate the light at the fiber input, driven by AO feedback signal provided by the similarity between the real-time fiber output image and the target mode profile. Through such an adaptive optical system, any desired linearly-polarized (LP) modes can be excited at the output of the four-mode and seventeen-mode fibers, respectively. For the highly MMF with uniform Bragg grating, we use a deformable mirror (DM) to perform the wavefront modulation at the fiber input, where AO feedback is provided by the fiber Bragg grating (FBG) reflectivity. At the FBG position, any desired principal mode groups can be successfully chosen. These experimental results suggest that adaptive control of optical wavefront in FMFs/MMFs is indeed feasible, and may find a large number of applications in optical communication, sensing, and imaging.

A complex environment causes strong distortion of the field, inhibiting tasks such as imaging and communications in both the optical and radio-frequency (RF) region. In the optical regime, strong modal dispersion in highly multimode fiber (MMF) results in a scrambled output field in both space (intensity speckles) and time (spectral and temporal speckles). Taking advantage of the pulse shaping technique, spatial and temporal focusing has been achieved in this thesis, offering potential opportunities for nonlinear microscopy and imaging or space-division multiplexed optical communication through MMF. In the RF regime, multipath effect in wireless RF channel gives multiple echoes with random delay and amplitude attenuation at the receiver end. Static channel sounding and compensation with ultra-broadband spread spectrum technique resolves the issue by generating a peaking signal at the receiver, significantly improving the signal-to-noise/interference performance. However, the limited communication speed in the static approach makes it challenging for sounding and compensation in a dynamic channel. Here, we achieve real-time channel sounding and compensation for dynamic wireless multipath channel with 40 micro-seconds refresh rate by using a fast processing field programmable gate array (FPGA) unit, providing potential opportunities for mobile communications in indoor, urban, and other complex environments. Furthermore, by combining broadband photonics and RF radar technologies, a high depth and transverse resolution wide bandwidth (15 GHz) W-band (75 - 110 GHz) photonic monopulse-like radar system for remote target sensing is demonstrated, offering prospects for millimeter wave 3-D sensing and imaging.