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:YVO<sub>4</sub> 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 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 450μm<sup>2</sup> 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.
Fiber delivery of intense laser radiation is important for a broad range of application sectors, from medicine through to industrial laser processing of materials, and offers many practical system benefits relative to free space solutions. In recent years, photonic crystal fiber technology has revolutionized the dynamic field of optical fibers, bringing with them a wide range of novel optical properties that make them ideally suited to power delivery with unparalleled control over the beam properties. The DTI funded project: Photonic Fibers for Industrial beam DELivery (PFIDEL), aims to develop novel fiber geometries for use as a delivery system for high power industrial lasers and to assess their potential in a range of "real" industrial applications. In this paper we review, from an industrial laser user perspective, the advantages of each of the fibers studied under PFIDEL. We present results of application demonstrations and discuss how these fibers can positively impact the field of industrial laser systems and processes, in particular for direct write and micromachining applications.
Direct Write (DW) is an emerging group of technologies that allow printing of electronic and other functional components out of vacuum, directly onto structural parts and assemblies. With its ability to deposit a wide range of dissimilar materials, and transfer details directly from CAD/CAM, the process is very flexible, enabling rapid progress from design to fabrication. This paper provides an introduction to direct write, and describes the BAE Systems activities in this field. The paper also describes the use of lasers in direct write, and some provisional results on laser curing are presented.
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.
Fiber delivery of intense laser radiation is important for a broad range of application sectors, from medicine through to industrial laser processing of materials, and offers many practical system design and usage benefits relative to free space solutions. Optical fibers for high power transmission applications need to offer low optical nonlinearity and high damage thresholds. Single-mode guidance is also often a fundamental requirement for the many applications in which good beam quality is critical. In recent years, microstructured fiber technology has revolutionized the dynamic field of optical fibers, bringing with them a wide range of novel optical properties. These fibers, in which the cladding region is peppered with many small air holes, are separated into two distinct categories, defined by the way in which they guide light: (1) index-guiding <i>holey fibers</i> (HFs), in which the core is solid and light is guided by a modified form of total internal reflection, and (2) <i>photonic band-gap fibers</i> (PBGFs) in which guidance in a hollow core can be achieved via photonic band-gap effects. Both of these microstructured fiber types offer attractive qualities for beam delivery applications. For example, using HF technology, large-mode-area, pure silica fibers with robust single-mode guidance over broad wavelength ranges can be routinely fabricated. In addition, the ability to guide light in an air-core within PBGFs presents obvious power handling advantages. In this paper we review the fundamentals and current status of high power, high brightness, beam delivery in HFs and PBGFs, and speculate as to future prospects.
This paper presents the design and construction of a photonic fibre pumped OPO. The photonic fibre is used to provide high-energy pump power to an optical parametric oscillator (OPO) from a remotely sited pump laser source. Delivery of the high power radiation required for these systems is not possible using conventional fibre, as the fibres would need to be highly multimode to handle the high intensities without damage. Photonic fibres are a disruptive technology for power transmission and light manipulation/control. The fibres have the potential for supporting high irradiation powers and can operate with robust singlemode guidance. The OPO is to be used to provide a 3-5 μm wavelength source for active sensor applications. The integration of high power lasers into air platforms for remote sensing applications would therefore be facilitated, as the fibre delivery would enable the laser to be sited remotely from the sensor head and open up the possibility of several sensor types sharing the same multi-functional laser. This could reduce the complexity and hence the cost of such sensors systems leading to the potential for an affordable, robust system for military platforms.