Experimental study of MMI structures in a switchable continuous-wave thulium-doped all-fiber laser

Switchable multi-wavelength laser emission from a thulium-doped all-fiber laser is reported by implementing a tapered and a non-tapered multi-modal interference (MMI) filters. The MMI structure relies on a coreless optical fiber spliced in between two single-mode optical fibers. For the non-tapered case, a minimum insertion loss of 12.60 dB is achieved around the 2-μm region, from which stable generation of commutable dual-wavelength emission at 1986.34 nm and 2017.38 nm is obtained. On the other hand, the tapered MMI structure performs a minimum insertion loss of 8.74 dB at the 2-μm region, allowing a stable triple-wavelength emission at 1995.4 nm, 2013.3 nm, and 2038.3 nm. In addition, commutable dual-wavelength emission was also obtained at 1997.9 nm and 2032.1 nm. The generated laser lines perform bandwidths of around 50 pm, low peak spectral power fluctuations and signal-to-noise ratio of 50 dB

Multiwavelength Er-doped fiber laser using an all-fiber Lyot filter

We experimentally demonstrated a multiwavelength Er-doped fiber ring laser system by employing an all-fiber Lyot filter (AFLF) and a highly nonlinear fiber (HNLF). The AFLF was employed as a polarizing filter to generate a nonlinear polarization rotation effect and the highly dense and narrow bandwidth comb-like channels. A 1-km-long HNLF was used to enhance the nonlinearity of the laser cavity and suppress the mode competition for multiwavelength operation. In the experiment, 97 laser output channels within a 3 dB bandwidth simultaneously were excited under 224 mW pump power. The power fluctuation of lasing channels was less than 0.182 dB, and the wavelength shift was less than 0.04 nm in 100 min, after treating the AFLF in a thermostatic ice bath. Meanwhile, the output laser was highly polarized with a degree of polarization up to 99.9%

Near- and mid-IR fibre grating devices and applications

This thesis presents a detailed research work on the fabrication, characterisation and applications of optical fibre grating devices with operation wavelengths cover from near- to mid- infrared (IR) range. One of the major contributions described in this thesis is the systematic investigation on the structures, fabrication methods and spectral, thermal, strain and surrounding refractive index (SRI) characteristics of near-IR fibre gratings including fibre Bragg gratings (FBGs), chirped fibre Bragg grating (CFBGs) and long period grating (LPGs). For some applications, such as special engineering sensors and high power fibre laser, the fibre gratings have been fabricated on different novel fibres (metal coated fibre and large mode field fibre), respectively. Another important contribution from the studies is experimental investigation on 45º tilted fibre gratings (45°-TFGs) and excessively tilted fibre gratings (ex-TFGs), and their applications. 45º-TFGs with high polarisation dependent loss (PDL) in single mode and polarisation maintaining (PM) fibres have been fabricated. The 45°-TFG has been employed as in fibre polariser to obtain the single polarised laser, which has been further developed as transverse loading sensor achieving high sensitivities. Furthermore, all fibre Lyot filter with narrow bandwidth (26 pm), constructed by two 45º-TFGs with 100m long cavity in PM fibre has been demonstrated. For ex-TFGs, SRI sensor based on a surface modified 81°-TFG, showing capability to detect glucose concentration with relatively high RI sensitivity (~168nm/RIU). Finally, an all-fibre loading sensor based on a hybrid 45° and 81° TFG structure has been demonstrated. Finally, I have fabricated fibre gratings into mid-IR 2μm range. The mid-IR FBGs have been evaluated for thermal and strain response, revealing higher temperature sensitivities than that in near-IR range. The mid-IR LPGs have been investigated for the thermal and refractive index sensitivities, also showing significant enhancement. The 45°-TFGs in mid-IR have been investigated for their PDL characteristics. The mid-IR FBGs and 45°-TFGs have been employed in Tm-doped fibre laser cavity to realize multi-wavelength continued wave (CW) and single polarisation operation

Fabrication and characterisation of 45º and Ex 45º:tilted fibre gratings and their applications in fibre lasers and sensors

In this thesis, I present the studies on fabrication, spectral and polarisation characterisation of fibre gratings with tilted structures at 45º and > 45º (namely 45º- TFGs and ex 45º-TFGs throughout this thesis) and a range of novel applications with these two types of grating. One of the major contributions made in this thesis is the systematic investigation of the grating structures, inscription analysis and spectral and polarisation properties of both types of TFGs. I have inscribed 45º-TFGs in standard telecom and polarisation maintaining (PM) fibres. Two wavelength regions of interest have been explored including 1.55 µm and 1.06 µm. Detailed analysis on fabrication and characterisation of 45º-TFGs on PM fibres have also been carried out for the first time. For ex 45º- TFGs, fabrication has been investigated only on low-cost standard telecom fibre. Furthermore, thermal responses have been measured and analysed showing that both types of TFG have low responsivity to temperature change. More importantly, their refractive index (RI) responses have been characterised to verify the high responsivity to surrounding medium. Based on the unique polarisation properties, both types of TFG have been applied in fibre laser systems to improve the laser performance, which forms another major contribution of the research presented in this thesis. The integration of a 45º-TFG to the Erbium doped fibre laser (EDFL) enables single polarisation laser output at a single wavelength. When combing with ex 45º-TFGs, the EDFL can be transformed to a multi-wavelength switchable laser with single polarisation output. Furthermore, by utilising the polarisation property of the TFGs, a 45º-TFG based mode locked fibre laser is implemented. This laser can produce laser pulses at femtosecond scale and is the first application of TFG in the field of nonlinear optics. Another important contribution from the studies is the development of TFG based passive and active optical sensor systems. An ex 45º-TFG has been successfully developed into a liquid level sensor showing high sensitivity to water based solvents. Strain and twist sensors have been demonstrated via a fibre laser system using both 45°- and ex 45º-TFG with capability identifying not just the twist rate but also the direction. The sensor systems have shown the added advantage of low cost signal demodulation. In addition, load sensor applications have been demonstrated using the 45º-TFG based single polarisation EDFL and the experimental results show good agreement with the theoretical simulation

Highly Efficient Side-Coupled Acousto-Optic Modulation of a Suspended Core Fiber Bragg Grating

We demonstrate acousto-optic modulation of a fiber Bragg grating in a birefringent suspended core fiber up to frequencies of 5.2 MHz for the first time. At acoustic resonances lower than 820 kHz, the reflectivity of the orthogonal polarization modes is electrically switched to a superposed reflection band with a maximum modulation depth of 51% at 10 V. The wavelength peak of the polarization modes is dynamically shifted by tuning the driven electrical signal from 1.92 to 5.2 MHz. A novel side-coupled based acousto-optic device is demonstrated, indicating new possibilities for ultracompact, fast and efficient all-fiber integrated devices

Fabrication and characterisation of 45º and Ex 45º : tilted fibre gratings and their applications in fibre lasers and sensors

In this thesis, I present the studies on fabrication, spectral and polarisation characterisation of fibre gratings with tilted structures at 45º and > 45º (namely 45º- TFGs and ex 45º-TFGs throughout this thesis) and a range of novel applications with these two types of grating. One of the major contributions made in this thesis is the systematic investigation of the grating structures, inscription analysis and spectral and polarisation properties of both types of TFGs. I have inscribed 45º-TFGs in standard telecom and polarisation maintaining (PM) fibres. Two wavelength regions of interest have been explored including 1.55 µm and 1.06 µm. Detailed analysis on fabrication and characterisation of 45º-TFGs on PM fibres have also been carried out for the first time. For ex 45º- TFGs, fabrication has been investigated only on low-cost standard telecom fibre. Furthermore, thermal responses have been measured and analysed showing that both types of TFG have low responsivity to temperature change. More importantly, their refractive index (RI) responses have been characterised to verify the high responsivity to surrounding medium. Based on the unique polarisation properties, both types of TFG have been applied in fibre laser systems to improve the laser performance, which forms another major contribution of the research presented in this thesis. The integration of a 45º-TFG to the Erbium doped fibre laser (EDFL) enables single polarisation laser output at a single wavelength. When combing with ex 45º-TFGs, the EDFL can be transformed to a multi-wavelength switchable laser with single polarisation output. Furthermore, by utilising the polarisation property of the TFGs, a 45º-TFG based mode locked fibre laser is implemented. This laser can produce laser pulses at femtosecond scale and is the first application of TFG in the field of nonlinear optics. Another important contribution from the studies is the development of TFG based passive and active optical sensor systems. An ex 45º-TFG has been successfully developed into a liquid level sensor showing high sensitivity to water based solvents. Strain and twist sensors have been demonstrated via a fibre laser system using both 45°- and ex 45º-TFG with capability identifying not just the twist rate but also the direction. The sensor systems have shown the added advantage of low cost signal demodulation. In addition, load sensor applications have been demonstrated using the 45º-TFG based single polarisation EDFL and the experimental results show good agreement with the theoretical simulation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo