T. Bhutta

Spatial dopant profiles for transverse-mode selection in multi-mode waveguides

We theoretically investigated the effect of the spatial distribution of the active-ion concentration in multimode step-index waveguides on transverse-mode selection for continuous-wave laser operation. We found that uniform doping of a central portion of as much as 60% of the full waveguide core width is highly effective for the selection of fundamental-mode operation, even under highly saturated, high-power conditions. Profiling the dopant distribution to match that of the particular mode desired was also found to be effective, especially if it is the saturated inversion profile that is matched to the shape of the mode.

Double-clad structures and proximity coupling for diode-bar-pumped planar waveguide lasers

We report, for the first time, fabrication of double-clad planar waveguide structures and their use for multiwatt, diode-bar-pumped, planar waveguide lasers based on Nd/sup 3+/ and Yb/sup 3+/-doped YAG. The direct-bonded, five-layer structures of sapphire, YAG, and rare-earth-doped YAG have sufficient numerical aperture to capture the fast-axis divergence of a diode bar by proximity coupling with no intervening optics, leading to very simple and compact devices. The restriction of the doped region to the central core leads to diffraction-limited laser output in the guided direction. We also show that the direct-bonding fabrication process can lead to a linearly polarized output.

The effect of particulate density on performance of Nd:Gd3Ga5O12 waveguide lasers grown by pulsed laser deposition

We have successfully grown a range of waveguiding layers of Nd:Gd3Ga5O12 (Nd:GGG) on Y3Al5O12 by pulsed laser deposition for purposes of studying the effects of particulates on waveguiding and lasing performance. We have found that particulates have a detrimental effect on lasing threshold for the range of particulate densities studied, and can increase lasing thresholds from as low as 2.5 up to 167 mW. We have also shown that the detrimental effect of particulates in waveguides becomes less significant with increasing waveguide thickness.

Continuous-wave broadband emitter based on a transition-metal-ion-doped waveguide.

We demonstrate the suitability of a simple continuous-wave-pumped transition-metal-ion-doped waveguide as a broadband light source in the wavelength region 600-1000 nm for interferometric applications. Spatially coherent (single mode in the confined direction), spectrally broadband (~130-nm FWHM) luminescence with output powers of several hundreds of microwatts is obtained from a Ti:sapphire planar waveguide with incident pump powers from an Ar-ion laser of up to 1 W. This result represents an increase in power by several orders of magnitude from previously reported simple broadband light sources in this wavelength range.

Direct-UV-written buried channel waveguide lasers in direct-bonded intersubstrate ion-exchanged neodymium-doped germano-borosilicate glass

We report a technique for producing single-mode buried channel waveguide lasers in neodymium-doped SiO2:GeO2:B2O3:Na2O (SGBN) glass. Direct bonding forms the basis of this process, providing a buried waveguide layer in the photosensitive SGBN material into which channel confinement can be directly written with a focused UV beam. Characterization of a 7.5-mm-long device was performed using a Ti:Sapphire laser operating at 808 nm and the resultant 1059 nm channel waveguide laser output exhibited single-mode operation, milliwatt-order lasing thresholds, and propagation losses of <0.3 dB cm–1.

Low phonon energy, Nd:LaF/sub 3/ channel waveguide lasers fabricated by molecular beam epitaxy

We report the first fabrication and laser operation of channel waveguides based on LaF/sub 3/ planar thin films grown by molecular beam epitaxy. To our knowledge, this is the lowest phonon energy dielectric material to have shown guided-wave laser operation to date. A full characterization, in terms of spectroscopy, laser results, and propagation losses, is given for the planar thin films upon which the channel waveguides are based. Two channel-fabrication methods are then described, the first involves ion milling and the second takes the novel approach of using a photo-definable polymer overlay. Laser operation in Nd-doped samples is demonstrated at 1.06, 1.05, and 1.3 /spl mu/m, and the potential for mid-infrared laser sources based on such guides is discussed.

Buried laser waveguides in neodymium-doped BK-7 by K+–Na+ ion-exchange across a direct-bonded interface

We report a technique for producing single-step buried K+-Na+ ion-exchanged waveguide lasers in neodymium doped BK-7. Direct bonding is the basis for this process, providing atomic contact between two chemically modified BK-7-type substrates followed by a 350°C treatment suitable for simultaneous annealing and intersubstrate ion-exchange. Characterization of a 6-mm long device was performed using a Ti:Sapphire laser operating at 808 nm. The resultant laser output exhibited TE polarized single-spatial-mode operation with losses of < 0.4 dB /cm and a maximum output power of 8.5 mW for 249 mW of absorbed pump power.

Single-mode UV-written buried channel waveguide lasers in direct-bonded neodymium-doped SGBN

Summary form only given. We present the first demonstration of single-mode buried channel waveguide lasers in neodymium-doped SGBN glass by a combination of direct bonding and direct UV writing techniques. Based on intersubstrate ion-exchange between specifically designed glass substrate materials we have used direct bonding to provide a region of atomic contact between Nd:SGBN and a potassium-rich borosilicate cladding substrate, between which K/sup +/-Na/sup +/ ion-exchange can occur. By taking this approach we have achieved a low-loss buried planar waveguide layer in the Nd:SGBN glass, which retains the photosensitive characteristics of the bulk material and into which single-mode channel waveguide structures can be directly written using a focussed UV beam. For this initial demonstration, a Nd:SGBN substrate containing SiO/sub 2/ (60 wt.%), GeO/sub 2/ (10 wt.%), B/sub 2/O/sub 3/ (10 wt.%), Na/sub 2/O (19 wt.%), and Nd/sub 2/O/sub 3/ (1 wt.%), was prepared and direct-bonded to a potassium-rich glass of similar composition to BK-7. Characterization of the 7.5-mm-long buried channel laser waveguide device was performed using a Ti:sapphire laser operating at 808 nm. The resultant 1059 nm laser output exhibited milliwatt-order laser thresholds, single-mode operation, propagation losses of <0.3 dB cm/sup -1/, and a maximum output power of 2 mW for 32 mW of absorbed pump power. These initial results demonstrate that optimisation of glass composition and direct UV writing parameters could lead to efficient low-loss buried waveguide devices in versatile bulk multicomponent oxide glasses for use with integrated optics.

Ti:sapphire planar waveguide coherent broadband emitter

Summary form only given. In recent years, broadband fiber interferometers have become very popular as basic instruments used in optical coherence tomography (OCT) for imaging applications in the biomedical field. A major challenge in the further development and applicability of OCT has been the improvement of both its spatial resolution and dynamic range. The longitudinal resolution is inversely proportional to the optical bandwidth of the light source. Broadband luminescence from transition-metal-ion doped materials, (e.g., Ti:sapphire) can significantly improve the longitudinal resolution compared to superluminescent diodes (/spl sim/30 nm FWHM), but the low brightness of its luminescence is insufficient for achieving a useful dynamic range in OCT. Femtosecond lasers have, therefore, been used as large-bandwidth high-brightness light sources, and subcellular imaging has recently been demonstrated in this way. However, current femtosecond light sources do not necessarily meet the requirements of compactness, ease of use, and low cost. We present a simple broadband light source based on a Ti:sapphire planar waveguide. It operates in a wavelength region near 800 nm, applicable to the investigation of biotissue and detectable with simple silicon diodes, with a bandwidth comparable to that of a femtosecond light source.

Nd:LaF/sub 3/ channel waveguide lasers fabricated by molecular beam epitaxy

Summary form only given. Recently we demonstrated the first laser action in a neodymium-doped lanthanum fluoride planar waveguide. This was the first laser action achieved in a RE-doped fluoride thin-film fabricated by MBE. The fabrication technique allows very accurate control of the thickness and doping level of the thin film, and potentially also over the refractive index profiles. Here we report on investigations into producing slab-loaded channel waveguide lasers based on these MBE planar thin films, to further lower the laser threshold and give a more circular spatial output. The first fabrication technique reported, employs an organic photodefinable polymer (benzocyclobutene) that can be processed using standard photolithographic techniques to produce the type of structures where various width polymer strips are laid over the Nd:LaF/sub 3/ thin film. Although benzocyclobutenes potential for producing integrated optical devices is well recognised we believe that this is the first demonstration of active laser channels that incorporate this polymer. We report laser operation at both 1.06 and 1.3 /spl mu/m, with thresholds as low as 15 mW (incident power) for these structures. We also report the fabrication and laser operation of channels produced by etching via ion beam milling through a CaF/sub 2/ upper cladding. The prospects for the future development of mid-IR laser sources based on this technology are discussed.