Presently, there is no truly flexible delivery system for light from Er:Yag medical lasers (λ = 2.94 μm) which allows
surgeons to work unrestricted. Instead, either a relatively inflexible articulated arm or multi-mode fibre, limited to large
bend radii, must be used. One proposed solution is the use of novel types of hollow core - band gap optical fibre rather
than more traditional large area solid core fibres. In these silica based fibres, material absorption and damage limitations
are overcome by using a photonic band gap structure. This confines radiation to lower order modes, that are guided in a
small diameter air core. The overall fibre diameter is also smaller, which allows a smaller mechanical bend radius.
Together with the guidance in air, this improves the laser power damage threshold. However, there are many practical
hurdles that must be overcome to achieve a robust system for use in surgery.
One of the main problems is that the fibre structure is hollow and ingress of dust, vapour, fluids and other contaminants
need to be prevented to ensure safe in-vivo usage. Additionally, any infibre contamination will degrade the laser damage
resistance of the fibre leading to potential catastrophic failure. The development of a robust and hermetically sealed end
cap for the fibre, without adversely affecting beam quality or damage threshold is an essential prerequisite for the safe
and efficient use of such fibres in surgery. In this paper we report on the progress on implementing end caps and describe
novel methods of sealing off these hollow fibres in particular for surgical applications. This work will demonstrate that
the use of these superior fibres with low loss guidance at 2.94 μm in surgery is feasible.
In current laser countermeasure technology concepts where frequency conversion is required, each active component has
its own laser source. During this paper we show that by using microstructured fibre technology as a delivery system,
output in multiple wavebands can be efficiently generated at locations remote from the laser pump source. We
demonstrate that laser radiation (with specifications close to those currently on airframes) can be delivered without
significant spectral, temporal or modal degradation over lengths representative of that in an airframe. This fibre delivered
radiation is used as a pump source for active frequency conversion, generating tuneable laser output in the 2 μm, 3.5 μm
and 0.532 μm regions, i.e. in wavebands of interest to countermeasure applications.
A Nd:YVO4 laser (λ = 1.064 μm) with 16 W of average power in a train of 15 ns pulses acts as the single pump source
for our system. Different types of microstructured fibre are assessed for high power delivery over lengths greater than
6.5 m. Three frequency conversion devices were constructed here to demonstrate the quality of the fibre-delivered
radiation - the devices are all based around periodically poled lithium niobate (PPLN) crystals and consist of two optical
parametric oscillators converting the pump source to wavelengths of ~2 μm and ~3.5 μm and a second harmonic
generator to double the frequency to 0.532 μm. The efficiencies of the frequency conversion sources are comparable
whether radiation is delivered through free space or by microstructured fibre.
In this paper we discuss recent work at the Advanced Technology Centre of BAE Systems on photonic technology, in particular photonic crystal fibres, applied to infra-red and electro-optic countermeasure systems. The use of Photonic Crystal fibres or holey fibres in countermeasure systems could significantly simplify platform integration by enabling remote location of laser sources, the generation of multiple wavelengths or continuum generation from a single pump source .The paper will describe the development of these fibres, drawing examples from recent civil collaborative research projects such as PFIDEL and LAMPS.
In this paper we seek to assess the potential impact of microstructured fibres for security and defence applications. Recent literature has presented results on using microstructured fibre for delivery of high power, high quality radiation and also on the use of microstructured fibre for broadband source generation.
Whilst these two applications may appear contradictory to one another the inherent design flexibility of microstructured fibres allows fibres to be fabricated for the specific application requirements, either minimising (for delivery) or maximising (for broadband source generation) the nonlinear effects.
In platform based laser applications such as infrared counter measures, remote sensing and laser directed-energy weapons, a suitable delivery fibre providing high power, high quality light delivery would allow a laser to be sited remotely from the sensor/device head. This opens up the possibility of several sensor/device types sharing the same multi-functional laser, thus reducing the complexity and hence the cost of such systems.
For applications requiring broadband source characteristics, microstructured fibres can also offer advantages over conventional sources. By exploiting the nonlinear effects it is possible to realise a multifunctional source for applications such as active hyperspectral imaging, countermeasures, and biochemical sensing.
These recent results suggest enormous potential for these novel fibre types to influence the next generation of photonic systems for security and defence applications. However, it is important to establish where the fibres can offer the greatest advantages and what research still needs to be done to drive the technology towards real platform solutions.
In this paper we demonstrate how Holey Fibre (HF) technology can positively impact the field of materials processing and fabrication, specifically Direct Write (DW). DW is the large scale, patterned deposition of functional materials onto both flat and conformal surfaces. Currently, DW techniques involve thermal post-processing whereby the entire structure is enclosed inside an oven, so limiting the DW technique to small, heat resistant surfaces.
Selectively laser curing the ink would allow the ink to be brought up to the required temperature without heating the surrounding substrate material. In addition the ability to trim components would allow miniature circuits to be written and devices to be tuned by changing the capacitance or resistance. HF technology enables in-situ curing and trimming of direct write components using the same rig and length of fibre. HF's with mode areas in excess of 450μm2 can be routinely fabricated allowing high power transmission whilst retaining the high beam quality of the radiation source.
We will present results of curing and trimming trials which demonstrate that HF's provide a distinct advantage over standard multimode fibres by allowing both curing and machining to be achieved through a single delivery fibre.
Microstructured fibers (MOFs) are among the most innovative developments in optical fiber technology in recent years. These fibers contain arrays of tiny air holes that run along their length and define the waveguiding properties. Optical confinement and guidance in MOFs can be obtained either through modified total internal reflection, or photonic bandgap effects; correspondingly, they are classified into index-guiding Holey Fibers (HFs) and Photonic Bandgap Fibers (PBGFs). MOFs offer great flexibility in terms of fiber design and, by virtue of the large refractive index contrast between glass/air and the possibility to make wavelength-scale features, offer a range of unique properties. In this paper we review the current status of air/silica MOF design and fabrication and discuss the attractions of this technology within the field of sensors, including prospects for further development. We focus on two primary areas, which we believe to be of particular significance. Firstly, we discuss the use of fibers offering large evanescent fields, or, alternatively, guidance in an air core, to provide long interaction lengths for detection of trace chemicals in gas or liquid samples; an improved fibre design is presented and prospects for practical implementation in sensor systems are also analysed. Secondly, we discuss the application of photonic bandgap fibre technology for obtaining fibres operating beyond silica's transparency window, and in particular in the 3μm wavelength region.