An investigation in to a novel pivot point free external cavity tunable laser (PPI ECTL). Existing external cavity tunable lasers are constructed using tunable optics, such as Bragg gratings to manipulate the cavity length. These elements although functional provide instability with prolonged use as they are defined by a centre of rotation or pivot point. The aim of this research was to provide a lab demonstrator of a new concept tunable laser based on conventional designs. The novel approach to the system design was in the removal of a defined pivot point, whilst still maintaining a moving optical component. The demonstration system will investigate the improvements upon traditional methods of tuning whilst providing greater tuning range and stability. The main defect of conventional systems was the reliance on the defined pivot point of the movable optics. By changing the position of the pivot point, the cavity length will change and the laser mode will change or hop to a side mode. This was known as a mode hop. The objective of the research was to design and implement a durable ECTL with a large tuning range (>200GHz), mode hop free tuning, fibre coupled, circular output beam and a wavelength in the visible. Design and production of a prototype PPI ECT laser that demonstrate the process of mode hop free tuning without the need of a defined pivot point. The impact of the research may lead in to the integration in to lower cost and more reliable commercial applications.

A new scheme of fabricating the tapered hyperbolic-end fibers (THEFs)microlenses using unique etching and fusion techniques is proposed. TheTHEFs were fabricated by symmetrically tapering the fiber during theetching process and hyperbolically lensing the tip during the fusing process.The tapered hyperbolic microlenses have demonstrated up to 82% couplingefficiency for a laser with an aspect ratio of 1:1.5. The influence of the tapering asymmetry on the coupling has also been investigated experimentally and theoretically. The axially symmetrical taperedmicrolenses of the THEFs showed that far-field profiles were well approximated to a Gaussian profile, while the asymmetric taper had deviated significantly from a Gaussian profile. A theoretical analysis illuminated a larger wavefront transformation of the hemispherical microlenses. A lesser phase aberration of the normalized optical path difference (OPD) was found in the hyperbolic-end lens, and that resulted in more than 2 dB improvement in the coupling efficiency when compared to the currently available hemispherical microlenses. The high-coupling performance of the hyperbolic microlens was due to an improved wavefront matching between the laser and the fiber, which was one of the most important contributions in this study.The 1.55 µm fiber grating external cavity lasers (FGECLs), packaged with THEF microlens for coupling the fiber grating external cavity, have been investigated for different combinations of coupling efficiency (£b) and Bragg reflectivity (Rg). Various tapered hyperbolic-end fiber microlenses with different coupling efficiency have been fabricated for this study. The FGL of higher £b = 72% and Rg = 0.52 has a stronger resonant feedback as the spectral output showed a single longitudinal mode with the side-mode-suppression-ratio (SMSR) greater than 45dB, a high output power of greater than 5mW, and a lower threshold current. However, for the case of £b = 68% and Rg = 0.35, the FGL exhibited a more stable SMSR against the variation of pumping current and temperature. Numerical simulations have also been performed on the SMSR at different coupling efficiencies and Bragg reflectivity for the FGLs. The high performance of the FGLs can be achieved through a higher coupling efficiency between a laser diode and a single-mode fiber. The calculated SMSR showed an excellent agreement with the measured data.

The coupling of Fabry-Perot laser and fiber-grating for single frequency operation was studied experimentally and theoretically. A 1.55

ABSTRACT The spectral characteristics for non-AR coated fiber grating lasers were studied theoretically and experimentally. The lensed fiber was used to improve coupling efficiency between laser and fiber. The tapered fibers were fabricated by using the mixture of HF and oil with different density to increase etched taper angle. The coupling efficiency could reach more than 60%. A single-mode operation for a fiber grating external cavity laser (FGECL) was simulated. The results showed that the SMSR, emitted power, and wavelength drift were dependent on the related device parameters. Our calculations showed that the strong current-dependent SMSR oscillation was from the mode selection by the fiber grating external cavity and the heating effect in the Fabry-Perot (FP) laser. A 1.55mm FP laser chip that one facet was coated a high reflectivity (HR) of 90% and another facet was uncoated. In our experiment and simulation of FGECL, the reflectivity of fiber gratings were 50% and 70% and 86%, and the length of external cavity was about 0.9cm. The measured result of FGECL showed that the side-mode suppression ratio (SMSR) was more than 35dB and the output power was larger than 1.5mW at the injected current 2 to 3 times of threshold current. Furthermore, the spectrums of fiber grating external cavity lasers were studied in order to understand the external laser characteristics.

This thesis describes the development of a technique for measuring frequency noise of semiconductor lasers. Equivalent laser linewidths were calculated from frequency noise measurements on several InGaAsP lasers with short external cavities to give single mode operation. Conventional 250 um lasers demonstrated linewidths of about 125 MHz-mW, compressively strained quantum well lasers of commensurate length had linewidth of 37 MHz-mW, and 500 um strained quantum well lasers had linewidth of 18 to 28 MHz-mW with an apparent strain dependence. The short external configuration allowed selection of a number of laser modes. Measurement of linewidth variation with laser mode showed a 20% to 40% change over six to eight modes. The system was adapted to make measurements of the optical frequency tuning with fine external cavity length change. This measurement provided a novel means to estimate the linewidth enhancement factor and the reflectivity of the external cavity element. The estimated values of the linewidth enhancement factor for 250 um conventional and quantum well lasers were found to be in the correct ratio to account for the measured difference in linewidth. / Thesis / Master of Engineering (ME)

This thesis describes the use of short external cavity (SXC) semiconductor diode lasers as single longitudinal mode (SM) tunable sources. A SXC forces a multimode diode laser to lase on a single longitudinal mode. Various laser types were investigated in SXC configurations using both planar and spherical external mirrors. The side mode suppression ratio (SMSR) and the SM tuning range were measured with respect to the positioning of the external cavity element. With a planar mirror as the SXC element, SMSR of —33 dB and SM tuning ranges of 1 nm (110% of a mode spacing) were obtained with inverted rib waveguide (IRW) lasers. For external cavity lengths of ~ 60 um the total continuous SM tuning range summed over all modes was found to be 72 cm^-1 or 12 nm. The use of a spherical mirror improved the results. A SXC laser consisting of a spherical mirror and an IRW laser had SMSR values of —37 dB and SM tuning ranges of 1.10 nm. Power and voltage characteristics of SM SXC lasers were also examined. It was found possible to use the laser voltage and electronic feedback to control the external cavity length for optimum SM output. The external differential quantum efficiency (DQE) was found to be wavelength dependent and may be explained by the wavelength dependence of the scattering/absorption loss. One aspect of the characteristic trend of the DQE with respect to wavelength is that it offers the possibility of determining the lasing wavelength of the SM without the use of a monochromator. / Thesis / Master of Engineering (ME)

In this thesis, with external cavity semiconductor laser, a high lateral resolution thickness measurement is proposed and demonstrated. The approach is typical an intra-cavity measurement of focused cell thickness by wavelength tuning of an external cavity laser diode. In addition, using blue light of 406nm as laser diode, higher lateral resolution is also observed. Using the proposed thickness method, the lateral resolution and longitudinal resolution have been demonstrated with 20£gm and 0.15£gm, respectively. We also discuss the feasibility of £gm scaled lateral resolution through improvement of laser diode, such as M^2~1.

The width of an ultra high-power laser diode is greater than 50 £gm and more than 20 times of low-power laser diode.The core diameter of Erbium Doped Fiber Amplifier fiber (EDFA) is 4~6 £gm and it is a single-mode fiber (SMF).However¡Athe ultra high-power laser diode is multi-mode laser. Therefore¡Athe mismatch between high-power laser and SMF resulting in low coupling efficiency. We improve coupling efficiency by using a wedge-shaped graded-index fiber (GIF) tip spliced to a SMF then fused a fiber bragg grating (FBG) to form an external cavity laser. The GIF is a focusing action like a graded-index fiber. From near-field pattern (NFP)¡Awe find the best GIF length is 400 £gm. The coupling efficiency between ultra high-power laser diode and wedge-shape lensed fiber is only 5% .

This thesis use laser diode, lens, grating, and mirrors to composite the external cavity system to retrieve the thickness of the liquid crystal and its characteristics. This way is different to the traditional way of measure. It has good accuracy on the vertical resolution and the vertical resolution can reach to 0.3£gm. At the same time, it can develop the high quality of horizontal resolution. On the basis of the correlation between the cavity length and the wavelength of the semiconductor laser, the system is capable of developing high horizontal resolution of accurate liquid crystal measurements. The horizontal resolution can reach to 40£gm. Furthermore, by adding bias on liquid crystal through this system, it expanded more understandings on the influences of bias and induced electric field of the electrodes to the tilting angle of the liquid crystal.

Thesis (MSc (Physics))--University of Stellenbosch, 2006. / External cavity diode lasers are used increasingly as sources of light in applications ranging from industrial photonic systems to basic laboratory research on the interaction of light and atoms. External cavity diode lasers offer more stable output frequency and narrow spectral bandwidth than the typical free-running diode lasers. These characteristics are achieved by exploiting the sensitivity of diode lasers to external optical feedback. In this study the design and development of an external cavity diode laser system for future applications in spectroscopy and laser cooling of rubidium atoms is presented. The external cavity diode laser including mechanical components and control electronics of the system is developed from basic components. The system uses frequency selective optical feedback from a diffraction grating in a Littrow configuration to provide collimated, narrow-band, frequency tunable light near 780 nm. The external cavity diode laser is designed to increase the mode-hop-free frequency tuning range, and allow accurate frequency tuning and stabilisation. A low-noise current source and a temperature controller for thermal stability were developed as part of the system since the output frequency changes with temperature and current. The temperature controller is optimised experimentally for the thermal characteristics of the external cavity. An electronic sidelock servo circuit for frequency locking of the external cavity diode laser to an external reference for long term frequency stabilisation is proposed and discussed. The servo circuit electrically controls the grating tilt and the current through the diode laser in order to lock the frequency of the diode laser. The external cavity diode laser is optimised and characterised near 780 nm. Results obtained in this study indicate that the external cavity diode laser is suitable for future applications in spectroscopy and laser cooling of neutral rubidium atoms.