Introduction from Professor Sir David Payne, Director of the Optoelectronics Research Centre at the Univ. of Southampton UK, to the SPIE Photonex Industry Talks session on Advances in Resilient Photonics Manufacturing: The Future Photonics Hub.
Test points are essential in allowing optical circuits on a wafer to be autonomously tested after selected manufacturing steps, hence allowing poor performance or device failures to be detected early and to be either repaired using direct write methods, or a cessation of further processing to reduce fabrication costs. Grating couplers are a commonly used method for efficiently coupling light from an optical fibre to a silicon waveguide. They are relatively easy to fabricate and they allow light to be coupled into/out from any location on the device without the need for polishing, making them good candidates for an optical test point. A fixed test point can be added for this purpose, although traditionally these grating devices are fabricated by etching the silicon waveguide, and hence this permanently adds loss and leads to a poor performing device when placed into use after testing. We demonstrate a similar device utilising a refractive index change induced by lattice disorder. Raman data collected suggests this lattice damage is reversible, allowing a laser to subsequently erase the grating coupler.
This paper reviews the progress in active fibers suitable for power scaling, highlighting the advances in fiber design that
will enable the control of nonlinearities such as SRS and SBS in high power fiber lasers, as well as making feasible a
practical high power three-level system.
We report the generation of white light comprising red, green, and blue spectral bands from a frequency-doubled
fiber laser in submicron-sized cores of microstructured holey fibers. Picosecond pulses of green light are launched
into a single suspended core of a silica holey fiber where energy is transferred by an efficient four-wave mixing
process into a red and blue sideband whose wavelengths are fixed by birefringent phase matching due to a slight
asymmetry of the structure arising during the fiber fabrication. Numerical models of the fiber structure and
of the nonlinear processes confirm our interpretation. Finally, we discuss power scaling and limitations of this
white light source.
Fiber lasers and amplifiers offer unique characteristics that are derived from the use of a waveguide and the properties
of rare-earth doped silica glass. Their capability for high output power, with high efficiency, has been demonstrated
both in CW and pulsed regimes. Cladding-pumped Yb-doped fiber lasers have now reached beyond kW levels with
good beam quality. Advances in both fiber technology and high-power multimode diode pump sources, and the
inherent power scalability of cladding-pumped fibers, lie behind this power surge. However, there are still many
challenges to overcome in the high-power fiber laser area. These include, for example, single-mode output at higher
powers and power scaling of a three-level laser. This paper reviews novel W-type fiber and depressed clad hollow
optical fiber waveguide structures designed with distributed wavelength filter characteristics to achieve an efficient and
high power cladding-pumped three-level lasers such as Nd-doped fiber laser operating at 930 nm and Yb-doped fiber
laser at 980 nm. Moreover, such fiber geometries enable to scale up the output power in a small and single-mode core
for generating a single-mode output beam in a robust and reliable manner.
We discuss the dramatic development of high-power fiber laser technology in recent years and the prospects of kilowattclass
single-frequency fiber sources. We describe experimental results from an ytterbium-doped fiber-based multihundred-watt single-frequency, single-mode, plane-polarized master-oscillator power amplifier (MOPA) operating at 1060 nm and a similar source with 0.5 kW of output power, albeit with a degraded beam quality (M2 = 1.6) and not linearly polarized. Experiments and simulations aimed at predicting the Brillouin limit of single-frequency system with a
thermally broadened Brillouin gain are presented. These suggest that single-frequency MOPAs with over 1 kW of output power are possible. In addition, the power scalability of a simple single-strand fiber laser to 10 kW is discussed.
We are currently investigating two infrared glasses for active applications. Gallium lanthanum sulphide (GLS) glass is investigated as a potential host material for rare-earth doped mid-infrared fiber lasers. We have fabricated gallium lanthanum sulphide glass by melt quenching and drawn it into fibers using the rod-in-tube technique. Fluoroaluminate glasses (ALF) are being prepared in planar form by spin coating and clad waveguides have been achieved. The quality of waveguides from both these materials is gradually being improved as methods to eliminate transition metals and other impurities, understand crystallization and reduce the imperfections at the core/clad interface are developed. Although initially motivated by the demand for a practical 1310 nm amplifier, interest has now extended further into the infrared. We describe recent progress in these glasses, their properties and applications.
Fiber Bragg Gratings (FBGs) are high-performance versatile devices that have had a major impact on different ares of optical fiber technology. Their unique filtering and dispersion properties allow them to be used in DFB and DBR fiber laser, wavelength-stabilized semiconductor lasers, gain-flattened EDFAs, Raman amplifiers and lasers, dispersion-compensators and add/drop multiplexers. We report some of the latest advances in the area of grating fabrication, fiber grating dispersion-compensators and all- fiber DFB lasers and address how our improved dispersion- compensating gratings have a performance close to that theoretically predicted.
Nd3+-doped 1.3micrometers fiber amplifier has many inherent advantages, but two major problems must be overcome to achieve a device: signal excited-state absorption (ESA), and amplified spontaneous emission (ASE). Signal ESA around 1300 nm prevents gain in that region, and so red-shifts and gain out of the second telecom window. The ASE at 1050 nm has a branching ratio 5 time larger than the 1.3 micrometers transition; as a result it clamps signal gain at low values. Moreover, in many Nd3+-doped glass hosts the peak of emission lies at wavelengths above 1320 nm. In this paper we present new highly ionic fluoroaluminate glasses developed as hosts for the 1.3 micrometers -doped fiber amplifier and discuss aspects of glass design. (Formula available in paper) emission peaks between 1310 and 1317 nm were demonstrated in bulk glasses, and gain in the 1310-1320 nm region was measured in fiber. The gain and emission spectra show evidence of significantly reduced ESA. ASE filtering is discussed, focusing on Bragg grating filters and on absorbing co-dopants, such as (formula available in paper) The paper also assesses thermal and viscous properties of the core and cladding glasses for fiber drawing.
In this paper we describe a multiwatt Nd3+ fiber laser pumped via a second cladding by the DIOMED 25 laser diode unit. This multiple diode array source is designed for coupling up to 25 Watts of diode power into a plastic-clad silica fiber of 400 micrometers diameter. The double-clad laser fiber is interchangeable with the normal PCS delivery fiber. The device operates at 1.058 micrometers with a slope efficiency > 50% and a 150 times brightness enhancement. This laser though useful in itself is also a key intermediate laser for generation of high powers at other wavelengths. Tandem pumping of Tm3+ and Er3+/Yb3+ fiber lasers at 1.058 micrometers enables efficient generation of 2.0 micrometers and 1.55 micrometers radiation respectively. In addition the Nd3+ laser can be operated close to 1.3 micrometers and there are prospects for in-fiber frequency doubling of the 1.06 micrometers line to generate a high power source in the green.
A passive WDM channel equalizer using a twin-core EDFA is studied in detail. Gain saturation limits the range of input signal powers from approximately -20 dBm to approximately 0 dBm. Channel equalization rates as high as 0.35 dB/dB are predicted. When used in cascade, twin-core EDFAs are shown to provide channel power stabilization and self- healing against additional channel losses.
The optical properties of praseodymium-doped glasses have attracted considerable attention recently for their potential application as a 1.3 micron optical amplifier. We report here on our spectroscopic evaluation of a series of low-phonon-energy glasses based on halides and sulphides. These results, though driven by the desire for a practical amplifier, provide insight into the application of these glasses not only for telecommunications applications, but also an understanding of the overall optical properties of a low-phonon-energy glass. Using Raman spectroscopy, the vibrational characteristics of the glass host are determined. Absorption measurements across the visible and infrared allow evaluation of the intrinsic loss of these glasses when in fiber form, as well as providing an indication of glass purity. Fluorescence of Pr3+-doped glasses, through excitation of the 3P0, 1D2 and 1G4 levels, is measured along with the fluorescence lifetimes. These radiative properties are compared to those predicted by a Judd-Ofelt analysis, which has been performed on all glasses. In this way, this work provides an overall spectroscopic evaluation of the optical properties of low-phonon-energy glasses, leading the way towards a practical device.
We report on recent progress towards the application of both mixed cadmium halides and sulphide-based glasses as host materials for a Pr3+-doped 1.3 micron optical fiber amplifier. Both of these materials offer the potential for higher gains than can be currently achieved in a Pr3+-doped ZBLAN fiber. Fundamental glass properties, including optical and thermal characteristics, are compared. Losses of these glasses in fiber form have been estimated and spectroscopic measurements are summarized. From these studies we show that quantum efficiencies over an order of magnitude higher than those demonstrated with Pr3+-doped ZBLAN amplifiers are in principle obtainable. Measured efficiencies of 11% and 52% for the Cd halide and Ga sulphide, respectively, are achieved, while 25% and 80% are predicted. Numerical modelling allows comparison of the expected amplifier performance. Thermal analysis has identified the challenges which remain for the drawing of single mode fibers and the results of preliminary fiber drawing trials are described. The relative merits of each of the various glasses are considered and the challenges before a practical amplifier is achieved are outlined.
A composite-EDFA configuration which incorporates an optical isolator has been investigated theoretically and experimentally. The isolator prevents the build-up of the backward-ASE and results in an amplifier with high gain and near-quantum-limited noise figure (NF). The optimum position of the isolator has been calculated as a function of the pump power so that minimum NF and maximum gain are achieved simultaneously. It is shown that under practical pump powers, the optimized composite EDFA exhibits a gain improvement of about 5 dB and a NF reduction in excess of 1.5 dB when compared with an optimized conventional EDFA. Finally, a high-gain composite EDFA has been experimentally demonstrated which exhibits a gain of 51 dB and NF of 3.1 dB for only 45 mW of pump power.
The gain efficiency of a fully optimized EDFA is calculated as a function of the fiber NA and dopant confinement in the core and is shown to agree well with experimental data. A gain efficiency of 8.9 dB/mW is demonstrated which is the best reported value to date for MCVD fibers. In addition, the detrimental effect of pump and signal background losses on the optimum gain efficiency is considered in detail.
Optimisation of optical fibre design to fully realise the potential of optical sensing systems is discussed. In particular, waveguide geometry and host glass composition are discussed with reference to specific sensor applications.
Ultrafast all-optical, fibre optic switches are of great interest as all-optical signal processing elements in telecommunication systems and as passive mode-lockers for all-fibre mode-lock lasers. We describe the operation and characteristics of two such switches, the conventional and amplifying Sagnac switch, and describe the use of these components in a recently developed, passive, self-starting, mode-locked erbium doped fibre laser capable of the generation of solitons with durations as short as 320 femtoseconds.
The paper describes how active fiber devices and special fibers can enhance the performance of multiplexed and distributed sensing systems. The use of active fibers, such as rare-earth-doped types, can act as powerful CW or pulsed sources, as wavelength-tunable sources, high-power amplifiers and as low-noise detector preamplifiers. Thus many arrangements for sensor-multiplexing, or distributed sensing, which may appear unattractively lossy on first consideration, may become perfectly viable with the insertion of active devices. The paper also reviews possible applications of special fibers in multiplexed and distributed sensors. The use of such fibers can greatly enhance the performance of sensor systems and even allow the construction of new types of optical sensor which were not previously possible with conventional fibers.
Laser sources based on rare-earth-doped single-mode optical fibres offer considerable potential as narrow-linewidth sources. Fibre laser sources have the capacity to produce highly-coherent low-noise output pumped by laser diodes. Fibre lasers are inherently compatible with optical fibres for transmission and sensing applications. Techniques for producing narrow-linewidth and single-longitudinal-mode operation in fibre lasers are reviewed.
The A. M. and F. M. noise mechanisms in a number of rare-earth doped fiber devices are reviewed. Spontaneous emission noise presents an ultimate limit to the performance of a number of fiber laser and amplifier devices and the paper concentrates primarily on this area. Important concepts are reviewed and experimental and theoretical data are presented.
Over the last few years the erbium-doped fibre amplifier has emerged as the preferred amplifier for use in transmission systems operating around 1.55 μm. This is due to its high fibre-to-fibre gain, polarisation-insensitivity, quantum-limited 3dB noise figure, low crosstalk and high operating bandwidth. This paper will discuss the characteristics of erbium-doped fibre amplifiers, highlight their advantages and review current applications.