Spectral modification of type IA fibre Bragg gratings by high-power near-infrared lasers

We report the first experimental measurements on the spectral modification of type IA fibre Bragg gratings, incorporated in an optical network, which result from the use of high-power, near-infrared lasers. The fibre grating properties are modified in a controlled manner by exploiting the characteristics of the inherent 1400 nm absorption band of the optical fibre, which grows in strength during the type IA grating inscription. If the fibre network is illuminated with a high-power laser, having an emission wavelength coincident with the absorption band, the type IA centre wavelength and chirp can be modified. Furthermore, partial grating erasure is demonstrated. This has serious implications when using type IA gratings in an optical network, as their spectrum can be modified using purely optical methods (no external heating source acts on the fibre), and to their long-term stability as the grating is shown to decay. Conversely, suitably stabilized gratings can be spectrally tailored, for tuning fibre lasers or edge filter modification in sensing applications, by purely optical means. © 2006 IOP Publishing Ltd.

Advanced fibre gratings fabricated in standard and infrared glass fibres by ultraviolet and near-infrared-femtosecond lasers

In this thesis, I describe studies on fabrication, spectral characteristics and applications of tilted fibre gratings (TFGs) with small, large and 45° tilted structures and novel developments in fabrication of fibre Bragg gratings (FBGs) and long period gratings (LPGs) in normal silica and mid-infrared (mid-IR) glass fibres using near-IR femtosecond laser. One of the major contributions presented in this thesis is the systematic investigation of structures, inscription methods and spectral, polarisation dependent loss (PDL) and thermal characteristics of TFGs with small (<45°), large (>45°) and 45° tilted structures. I have experimentally characterised TFGs, obtaining relationships between the radiation angle, central wavelength of the radiation profile, Bragg resonance and the tilt angle, which are consistent with theoretical simulation based on the mode-coupling theory. Furthermore, thermal responses have been measured for these three types of TFGs, showing the transmission spectra of large and 45° TFGs are insensitive to the temperature change, unlike the normal and small angle tilted FBGs. Based on the distinctive optical properties, TFGs have been developed into interrogation system and sensors, which form the other significant contributions of the work presented in this thesis. The 10°-TFG based 800nm WDM interrogation system can function not just as an in-fibre spectrum analyser but also possess refractive index sensing capability. By utilising the unique polarisation properties, the 81 °-TFG based sensors are capable of sensing the transverse loading and twisting with sensitivities of 2.04pW/(kg/m) and 145.90pW/rad, repectively. The final but the most important contribution from the research work presented in this thesis is the development of novel grating inscription techniques using near-IR femtosecond laser. A number of LPGs and FBGs were successfully fabricated in normal silica and mid-IR glass fibres using point-by-point and phase-mask techniques. LPGs and 1st and 2nd order FBGs have been fabricated in these mid-IR glass fibres showing resonances covering the wavelength range from 1200 to 1700nm with the strengths up to 13dB. In addition, the thermal and strain sensitivities of these gratings have been systematically investigated. All the results from these initial but systematic works will provide useful function characteristics information for future fibre grating based devices and applications in mid-IR range.

Thermal properties of fibre Bragg gratings inscribed point-by-point by infrared femtosecond laser

Direct, point-by-point inscription of fibre Bragg gratings by an infrared femtosecond laser has been reported recently. Response of these gratings to annealing at temperatures in the range 500 to 1050°C is studied for the first time. Gratings inscribed by infrared femtosecond lasers were thermally stable at temperatures up to 900°C, representing a significant improvement in comparison with the 'common', UV-inscribed, gratings. Annealing at temperatures up to 700°C increased grating reflectivity. © IEE 2005.

Inscription of in-fibre photonic devices by an infrared femtosecond laser

In this work, a point by point method for the inscription of fibre Bragg gratings using a tightly focused infrared femtosecond laser is implemented for the first time. Fibre Bragg gratings are wavelength-selective, retro-reflectors which have become a key component in optical communications as well as offering great potential as a sensing tool. Standard methods of fabrication are based on UV inscription in fibre with a photosensitive core. Despite the high quality of the gratings, a number of disadvantages are associated with UV inscription, in particular, the requirements of a photosensitive fibre, the low thermal stability and the need to remove the protective coating prior to inscription. By combining the great flexibility offered by the point by point method with the advantages inherent to inscription by an infrared femtosecond laser, the previous disadvantages are overcome. The method here introduced, allows a fast inscription process at a rate of ~1mm/s, gratings of lengths between 1cm and 2cm exhibiting reflections in excess of 99%. Physical dimensions of these gratings differ significantly from those inscribed by other methods, in this case the grating is confined to a fraction of the cross section of the core, leading to strong and controllable birefringence and polarisation dependent loss. Finally, an investigation of the potential for their exploitation towards novel applications is carried out, devices such as directional bend sensors inscribed in single-mode fibre, superimposed but non-overlapping gratings, and single-mode, single-polarisation fibre lasers, were designed, fabricated and characterised based on point by point femtosecond inscription.

Structure of fiber gratings directly written by infrared femtosecond laser

Structural modification m gratings inscribed point-by-point by a femtosecond laser is investigated using quantitative phase microscopy. The gratings present a central region with a depressed refractive index surrounded by an outer corona with increased index. © 2006 Optical Society of America.

Vector bending sensors based on fiber bragg gratings inscribed by an infrared femtosecond laser

A direction-sensitive bend sensor in standard single-mode fiber is demonstrated for the first time based on an axially-offset fiber Bragg grating, directly written by an infrared femtosecond laser.

Optimized design of high power mid-infrared Er3+, Pr3+ -codoped ZBLAN fiber laser

Based on the rate equations describing the operation of the Er3+, Pr3+ -codoped ZBLAN fiber lasers with different pump configurations, theoretical calculations that relate to the population characteristics and optimization of CW operation of high power Er3+, Pr3+ :ZBLAN double-clad fiber lasers are presented. Using the measured ET (energy-transfer), ETU (energy-transfer-upconversion) and CR (cross-relaxation) parameters relevant to Er3+, Pr3+ -codoped ZBLAN, a good agreement between the theoretical results from the model and recently reported experimental measurements is obtained. The effects on the slope efficiency of a number of laser parameters including fiber length, reflectance of the output mirror and pumping configuration are quantitatively analyzed and used for the design and optimization of high power Er3+, Pr3+ -codoped ZBLAN fiber lasers.

Optimized design of high power mid-infrared Er3+, Pr3+ -codoped ZBLAN fiber laser

Based on the rate equations describing the operation of the Er3+, Pr3+ -codoped ZBLAN fiber lasers with different pump configurations, theoretical calculations that relate to the population characteristics and optimization of CW operation of high power Er3+, Pr3+ :ZBLAN double-clad fiber lasers are presented. Using the measured ET (energy-transfer), ETU (energy-transfer-upconversion) and CR (cross-relaxation) parameters relevant to Er3+, Pr3+ -codoped ZBLAN, a good agreement between the theoretical results from the model and recently reported experimental measurements is obtained. The effects on the slope efficiency of a number of laser parameters including fiber length, reflectance of the output mirror and pumping configuration are quantitatively analyzed and used for the design and optimization of high power Er3+, Pr3+ -codoped ZBLAN fiber lasers.

Implications of high power losses in IR femtosecond laser inscribed fibre Bragg gratings

We report on high power issues related to the reliability of fibre Bragg gratings inscribed with an infrared femtosecond laser using the point-by-point writing method. Conventionally, fibre Bragg gratings have usually been written in fibres using ultraviolet light, either holographically or using a phase mask. Since the coating is highly absorbing in the UV, this process normally requires that the protective polymer coating is stripped prior to inscription, with the fibre then being recoated. This results in a time consuming fabrication process that, unless great care is taken, can lead to fibre strength degradation, due to the presence of surface damage. The recent development of FBG inscription using NIR femtosecond lasers has eliminated the requirement for the stripping of the coating. At the same time the ability to write gratings point-by-point offers the potential for great flexibility in the grating design. There is, however, a requirement for reliability testing of these gratings, particularly for use in telecommunications systems where high powers are increasingly being used in long-haul transmission systems making use of Raman amplification. We report on a study of such gratings which has revealed the presence of broad spectrum power losses. When high powers are used, even at wavelengths far removed from the Bragg condition, these losses produce an increase in the fibre temperature due to absorption in the coating. We have monitored this temperature rise using the wavelength shift in the grating itself. At power levels of a few watts, various temperature increases were experienced ranging from a few degrees up to the point where the buffer completely melts off the fibre at the grating site. Further investigations are currently under way to study the optical loss mechanisms in order to optimise the inscription mechanism and minimise such losses.

Mid-infrared passively switched pulsed dual wavelength Ho3+ -doped fluoride fiber laser at 3 μm and 2 μm

Cascade transitions of rare earth ions involved in infrared host fiber provide the potential to generate dual or multiple wavelength lasing at mid-infrared region. In addition, the fast development of saturable absorber (SA) towards the long wavelengths motivates the realization of passively switched mid-infrared pulsed lasers. In this work, by combing the above two techniques, a new phenomenon of passively Q-switched ~3 μm and gain-switched ~2 μm pulses in a shared cavity was demonstrated with a Ho3+-doped fluoride fiber and a specifically designed semiconductor saturable absorber (SESAM) as the SA. The repetition rate of ~2 μm pulses can be tuned between half and same as that of ~3 μm pulses by changing the pump power. The proposed method here will add new capabilities and more flexibility for generating mid-infrared multiple wavelength pulses simultaneously that has important potential applications for laser surgery, material processing, laser radar, and free-space communications, and other areas.

Extreme value statistics in Raman fiber lasers

We present the numerical study of the statistical properties of the partially coherent quasi-CW high-Q cavity Raman fiber laser. The statistical properties are different for the radiation generated at the spectrum center or spectral wings. It is found that rare extreme events are generated at the far spectral wings at one pass only. The mechanism of the extreme events generation is a turbulent-like four-wave mixing of numerous longitudinal generation modes. The similar mechanism of extreme waves appearance during the laser generation could be important in other types of fiber lasers. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

Extreme value statistics in Raman fiber lasers

We present the numerical study of the statistical properties of the partially coherent quasi-CW high-Q cavity Raman fiber laser. The statistical properties are different for the radiation generated at one or many cavity passes. It is found that rare extreme events are generated at the far spectral wings of the spectrum. The mechanism of the extreme events generation is a turbulent-like four-wave mixing of numerous longitudinal generation modes. © 2011 Optical Society of America.