Charlotte R. Bennett

Characteristics of a Q-switched multicore photonic crystal fiber laser with a very large mode field area

We model and characterize the behavior of a Q-switched fiber laser. The fiber is a doped multicore photonic crystal fiber having six cores in a ring-type geometry. The fiber laser is Q-switched using an intracavity acousto-optic modulator. Using a mode filtering technique in the far field, a mode very close to the fundamental in-phase supermode is obtained with a mode field area of 4200 microm(2) and a divergence of 9 mrad. Pulses with energies of up to 2.2 mJ and durations of 26 ns (limited by end facet damage) at a repetition rate of 10 kHz are obtained.

Multicore Photonic Crystal Fiber Lasers for High Power/Energy Applications

In this paper, the authors discuss the modal and lasing properties of multicore photonic crystal fiber lasers in the context of high power/energy production from fiber cores with very large mode area. Supermode selection methods like Talbot imaging or far-field aperturing are tested using 6-, 7-, and 18-core fibers. It is shown that in-phase mode selection is achieved efficiently by using either method. The fibers have been tested in continuous-wave (CW) and Q-switched laser operation. The mode field area is as large as 4240 mum2 for one of the fibers, providing up to 2 mJ of pulse energy in Q-switched operations with 30 ns pulse duration.

Damage threshold and bending properties of photonic crystal and photonic band-gap optical fibers

Laser damage thresholds of 8μm- and 22μm-core diameter solid-core photonic crystal fibres (PCF) and hollow-core photonic band gap (PBG) fibres have been measured. The studies were carried out using a 1.06μm Nd:Yag laser (30nsec pulses at 10Hz), which is optimally coupled into these fibres by careful mode matching, providing a coupling efficiency greater than 90%. It has been shown that the damage threshold of the 8µm core PBG fiber occurs at pulse energies close to 1 mJ, equivalent to a fluence well in excess of 1kJ/cm2 propagating down the fibre. This is a factor of 4 larger than the damage threshold of a solid-core PCF of similar core diameter. In comparison, the damage threshold of the large-core PBG is smaller than that of the equivalent PCF. Theoretical modelling based only on the optical modal properties of the single mode PBG fibre shows that an enhancement of a factor of 25 should be obtainable. Thus there are different damage mechanisms potentially responsible for the fragility of larger core PBG fibres. In an experimental study of bend losses it has been found that it is possible to bend the 8μm PBG fibre up to the breaking point bend radius (<1mm). The critical bend radius for the 22μm core PBG is close to 2 mm, which is 50 times smaller than the critical bend radius of a 20μm core PCF.

Multi-core photonic crystal fibers for high-power laser and amplifiers

In this paper, we show that it is possible to arrange for an 18-core photonic crystal fibre (PCF) laser to operate in the fundamental in-phase supermode. The mode divergence is as small as 12.5 mrad. The equivalent mode field diameter is about 52 μm. Mode filtering is provided by a pinhole in the far field. The laser is Q-switched using an Acousto-Optic Modulator (AOM). An output power up to 65 W at a repetition rate of 50 kHz (corresponding to 1.3 mJ per pulse), with 22 ns short pulses, has been obtained with a slope efficiency of 46%. Ongoing amplification experiments are briefly described. Limiting factors (end facet damage threshold and thermal dissipation) are discussed for further scaling of this laser concept.

Sub-pixel super-resolution by decoding frames from a reconfigurable coded-aperture camera: theory and experimental verification

In a previous paper we presented initial results for sub-detector-pixel imaging in the mid-wave infra-red (MWIR) using an imager equipped with a coded-aperture based on a re-configurable MOEMS micro-shutter. It was shown in laboratory experiments that sub-pixel resolution is achievable via this route. The purpose of the current paper is to provide detail on the reconstruction method and to discuss some challenges which arise when imaging real-world scenes. The number of different mask patterns required to achieve a certain degree of super-resolution is also discussed. New results are presented to support the theory.

An experimental infrared sensor using adaptive coded apertures for enhanced resolution

Adaptive coded aperture imaging (ACAI) has the potential to enhance greatly the performance of sensing systems by allowing sub detector pixel image and tracking resolution. A small experimental system has been set up to allow the practical demonstration of these benefits in the mid infrared, as well as investigating the calibration and stability of the system. The system can also be used to test modeling of similar ACAI systems in the infrared. The demonstrator can use either a set of fixed masks or a novel MOEMS adaptive transmissive spatial light modulator. This paper discusses the design and testing of the system including the development of novel decoding algorithms and some initial imaging results are presented.

Optical design of a coded aperture infrared imaging system with resolution below the pixel limit

Adaptive coded aperture imaging systems can resolve objects that are smaller than the pixel-limited resolution of the detector focal plane array. This is done by combining multiple frames of data, where different frames are taken with different coding patterns on the coded-aperture mask. In the mid-wave infrared the required signal to noise ratio necessitates some form of light concentration. Optical design software has been used to model candidate optical systems with the aim of achieving up to four times resolution enhancement along each linear dimension. As in some other computational imaging systems, the requirements on the optical system are found to be different to those that are normally used in more classical optical designs. The basic needs are a point-spread function of suitable extent that changes gradually with angle and does not vary significantly with the expected changes in input spectra or system temperature. Novel metrics have been derived and used to inform the optical design. The modeling and design trade-offs and resulting performance are discussed.

A new numerical model of optical communications in the maritime environment

QinetiQ, in association with Frazer-Nash Consultancy and Dstl, have developed a new numerical model of optical communications where part of the transmission is through water. The model (called Optical Communications Underwater Model or OCUM) finds the signal from air platform to sea platform (and vice versa) or between underwater platforms including the effects of scattering within the water, the refraction at the sea surface and transmission through cloud. The effects of scattering are found through Monte Carlo simulation before parameterizing the results to be used in subsequent calculations. The background light is also included from the sun to obtain the signal to noise ratio which is then used to find the analytical and numerical (via a message transmission simulation) bit error rate. This paper shows some of the details of the model and the approaches taken to obtain the transmission efficiency and performance. Some basic results will be presented to demonstrate the utility of the model.

Characterization of photonic bandgap fiber for high-power narrow-linewidth optical transport

An investigation of the use of hollow-core photonic bandgap (PBG) fiber to transport high-power narrow-linewidth light is performed. In conventional fiber the main limitation in this case is stimulated Brillouin scattering (SBS) but in PBG fiber the overlap between the optical intensity and the silica that hosts the acoustic phonons is reduced. In this paper we show this should increase the SBS threshold to the multi-kW level even when including the non-linear interaction with the air in the core. A full model and experimental measurement of the SBS spectra is presented, including back-scatter into other optical modes besides the fundamental, and some of the issues of coupling high power into hollow-core fibers are discussed.

An adaptive coded aperture imager: building, testing and trialing a super-resolving terrestrial demonstrator

There is an increasingly important requirement for day and night, wide field of view imaging and tracking for both imaging and sensing applications. Applications include military, security and remote sensing. We describe the development of a proof of concept demonstrator of an adaptive coded-aperture imager operating in the mid-wave infrared to address these requirements. This consists of a coded-aperture mask, a set of optics and a 4k x 4k focal plane array (FPA). This system can produce images with a resolution better than that achieved by the detector pixel itself (i.e. superresolution) by combining multiple frames of data recorded with different coded-aperture mask patterns. This superresolution capability has been demonstrated both in the laboratory and in imaging of real-world scenes, the highest resolution achieved being ½ the FPA pixel pitch. The resolution for this configuration is currently limited by vibration and theoretically ¼ pixel pitch should be possible. Comparisons have been made between conventional and ACAI solutions to these requirements and show significant advantages in size, weight and cost for the ACAI approach.