Huw E. Major

Demonstration of 100 GHz electrically tunable liquid-crystal Bragg gratings for application in dynamic optical networks

We demonstrate liquid-crystal-based integrated optical devices with >140 GHz electrical tuning for potential applications in dynamic optical networks. Bragg wavelength tuning covering five 25 GHz wavelength-division multiplexing channel spacing has been achieved with 170 V (peak-to-peak) sinusoidal voltages applied across electropatterned indium tin oxide-covered glass electrodes placed 60 microm apart. This tunability range was limited only by the initial grating strength and supply voltage level. We also observed two distinct threshold behaviors that manifest during increase of supply voltage, resulting in a hysteresis in the tuning curve for both TE and TM input light.

Green-pumped, picosecond MgO:PPLN optical parametric oscillator

We investigate the performance of a magnesium-oxide-doped periodically poled lithium niobate crystal (MgO:PPLN) in an optical parametric oscillator (OPO) synchronously-pumped by 530nm, 20ps, 230MHz pulses with an average power of up to 2W from a frequency-doubled, gain-switched laser diode seed and a multi-stage Yb:fiber amplifier system. The OPO produces ~165mW (signal, 845nm) and ~107mW (idler, 1421nm) of average power for ~1W of pump power and can be tuned from ~800nm to 900nm (signal) and 1.28µm to 1.54µm (idler). Observations of photo-refraction and green-induced infrared absorption (GRIIRA) in different operational regimes of the MgO:PPLN OPO are described and the role of peak intensity and average power are investigated, both with the aim to find the optimal operating regime for pulsed systems.

Liquid crystal based tunable WDM planar Bragg grating devices based on precision sawn groove substrates

Current optical telecommunication systems employ dense Wavelength Division Multiplexing (WDM) techniques to increase the data carrying capacity of fiber networks. Dynamic add/drop and filtering processes are crucial for the precise control of individual channels on these networks. Reconfigurable integrated optical devices, such as planar Bragg gratings, can tune the reflection wavelength over several standard channel spacings, providing the possibility for all-optical dynamic networks. Planar devices have the potential to address and tune several channels simultaneously, and have greater potential for integration than fiber equivalents.

268 nm period bragg gratings and integrated circuits produced by direct UV writing

We demonstrate 268 nm period planar Bragg gratings and Mach-Zehnder interferometers fabricated by direct UV-writing. Grating reflectivities of ~30 dB and FWHM of ~0.16 nm were measured at operational wavelengths around 800 nm.

Direct UV-Written Near-Visible and Visible Planar Bragg Gratings and their Application in Sensors

We demonstrate sensor devices based on planar Bragg gratings operated at near-visible and visible wavelengths fabricated by direct UV-writing. Latest nano-scale period Bragg gratings offer new advantages in highly integrated sensors applications.

UV written waveguide devices - Bragg gratings and applications in sensors

Ultra‐violet laser direct writing provides a powerful way of creating integrated optical devices. The work reported here describes developments in this field, and particularly the use of two‐beam writing to create Bragg gratings in planar integrated format. The work shows how these structures can be used in a wide range of applications, ranging from creation of wide‐band couplers, through to extremely sensitive sensor devices. UV direct writing removes many of the constraints normally associated with planar integrated optics. It is a mask‐free process that uses direct computer control and it dispenses with the etching steps normally required to make low‐loss waveguide devices. The work reported will show how this flexibility of format may be combined with novel substrate structures to allow new device types.

Structured nonlinear crystals for visible light generation in displays and biosciences

Nonlinear conversion of diode based sources using MgO doped PPLN provides a viable route to watt level power visible laser wavelength systems with high electrical efficiency and offer a low cost solution for applications in laser projection and bio-optics. Recent developments of superior nonlinear materials together with brighter and more powerful diode sources provide an excellent opportunity for optical technology over the coming decade.

Improved SHG phase matching response for focused Gaussian beams in Gouy compensated quasi-phase-matched structures

Nonlinear interactions such as second harmonic generation (SHG) [1] and sum and difference frequency generation are routinely used for the generation of laser wavelengths. With high power lasers it is necessary to use bulk crystals and tight beam focusing to achieve the maximum conversion efficiency. The use of Quasi-phase-matched (QPM) materials such as PPLN and PPKTP are now routinely reported; however, as we will show in this paper, a simple linear QPM crystal does not fully optimize the conversion response. We will report theoretical and experimental results showing that by careful control of the crystal design it is possible to fully compensate for the phase errors associated with the focusing induced Gouy shift.

Direct optical observation of walls and disclination effects in active photonic devices

Liquid crystal tunable Bragg Gratings defined in planar substrates via a laser patterning technique exhibit complex wavelength tuning. This tuning displays threshold points and hysteresis. These tuning features are shown to be a manifestation of physical processes occurring in the confined geometry of our tunable devices. Such physical processes include the formation and removal of line disclinations and an associated wall. We discuss the effect of walls in the liquid crystal with regards to voltage tuning characteristics and whether they may allow faster wavelength tuning.

Gouy phase compensated quasi phase matching (GQPM)

Using an appropriately designed quasi-phase matched structure it is theoretically possible to compensate for the deleterious effects of Gouy phase shift allowing a symmetric frequency response and tighter focussing for higher conversion efficiencies.