Self similar solutions of nonlinear partial differential equations are chirped solutions which maintain their mathematical form, whilst being scaled in time and amplitude. Standard techniques for finding such solutions have been developed with applications in nonlinear acoustics, plasma physics and other areas. More recently these techniques have been applied to locate new solutions of the NLSE, which are finding increasing applications in high power amplifiers and mode locked fibre lasers. Several such similariton generation systems have been experimentally realised in the last decade, taking advantage of different fibre architectures. Chirped self similar pulse propagation has now been demonstrated in both normal and anomalous dispersion fibre amplifiers. Self similar pulse amplification systems can also be used to build similariton lasers, and this talk will compare these lasers to other fibre laser systems, such as the all normal dispersion (ANDi) laser and the giant chirp oscillator (GCO) laser systems. The construction of such lasers using all PM fibre has enabled the demonstration of laser systems exhibiting great stability. These newly developed laser systems based on all normal dispersion cavities, are ideally suited to act as reliable seed lasers for high energy femtosecond laser systems as they have a wide bandwidth and strong linear chirp enabling stretched pulse amplification and subsequent compression.

Single-frequency, narrow linewidth and low noise fibre laser sources are intensively desirable for a range of applications in optics including high-resolution interferometry, sensing, coherent communications, dense-wavelength-division-multiplexing (DWDM) network systems, and light detection and ranging (LIDAR). To date, most of these sources have typically been achieved from heavily rare-earth doped fibre lasers with unidirectional ring configurations, short-cavity distributed Bragg reflector (DBR) designs and distributed-feedback (DFB) structures. However, the lasing wavelength region of rare-earth-doped lasers is naturally limited by the specific rare-earth ion used leaving some wavelength regions inaccessible. Furthermore, a high concentration of rare-earth ions in silica glass has been shown to result in problems with output-power stability due to ion-pair quenching causing inefficient pump- to signal-energy transfer and excessive thermal loading. Because of these performance-related issues linked to the rare-earth gain, a Raman gain based single-frequency fibre laser would be of considerable interest to investigate whether some of the above mentioned drawbacks could be mitigated. Moreover, employing Raman gain for such lasers could open up the prospect of generating high-power narrow linewidth low-noise oscillation at, in principle, any wavelength spanning from the visible to the infrared regions depending only on the pump-source used, and the host material into which the laser is formed. Recently, the concept of such a laser was demonstrated based around DFB fibre Bragg gratings written directly into standard, commercially available silica fibres.

This talk will review our recent work on centimetre length Raman DFB fibre lasers UV-written directly into such standard commercially available silica fibres. We will show examples of highly efficient singly-frequency lasers with threshold powers as low as 440mW and output powers of more than 2W, and also discuss design guidelines for high-efficiency operation in non-silica based fibres. We will demonstrate how thermal effects are largely absent in these lasers leading to extraordinarily narrow linewidths typically at the sub-kHz level. We will also demonstrate how these new sources facilitate four-wave mixing with conversion efficiencies up to -24dB and wavelength-conversion in excess of 165nm, and speculate on what the next steps will be for this very exciting fibre laser technology.

Of the parameters which characterise stimulated Brillouin scattering (SBS), perhaps the most popular is its threshold. While the fundamental physical threshold for the SBS effect is very low (on the level few quanta), the “SBS threshold” parameter is introduced for practical convenience as the pump power, Pth, at which the output Stokes power approaches some measurable fraction, r, of Pth; r then serves as a threshold criterion. In this work I show that in optical fibers such a SBS threshold is only fully adequate in a CW SBS interaction regime. Because of the relatively low SBS gain coefficient such a regime is feasible in long enough optical fibers. However, in practice long fibers are often carrying pulsed radiation. Examples include optical communication fibers, pulsed power delivery fiber systems and Brillouin-based fiber sensors, to name but a few. An essential difference between the threshold power and threshold energy in the single and repetitive pulse regimes of the SBS interaction are among the nontrivial peculiarities of the SBS threshold in long optical fibers. The nature and scale of this difference and a theoretical framework for its description will be presented and discussed.

Passively mode-locked fiber laser has been intensively studied due to its advantage in free of alignment, excellent beam profile and without special maintenance. However, passively mode-locked fiber laser with performance of changeable centre wavelength and free of environmental disturbance have potential application in micromachining, optical signal processing, spectroscopy, and biomedical researches.

In this paper, we demonstrate a polarization-maintaining mode-locked fiber laser based on a semiconductor saturable absorber mirror (SESAM). The structure of the proposed fiber laser mainly composed with a nonlinear amplification mirror (NALM), SESAM, and a 2*2, 70:30 optical coupler (OC). Self-started mode-locking can be observed at the pump current of 130 mA (corresponding to the pump power of 53.4 mW). The spectral width is about 2.5 nm with steep edge, which indicates that dissipative soliton can be observed in the cavity. It is demonstrated that the centre wavelength can be tunable only by increasing the pump power. The proposed mode-locked fiber laser operates in an environmental-stable state, which is insensitive to the environments. We attribute the wavelength-tunable characteristic to the birefringence effect and the power dependent filtering effect. Theoretical result agrees well with the experimental result.

Acknowledgments: This work was supported by the CAS Special Grant for Postgraduate Research, Innovation and Practice, the CAS/SAFEA International Partnership Program for Creative Research Teams. It is also partially funded by A*STAR SERC grant (Grant Number 1122904018).

Infrared applications have been limited by the lake of high quality infrared optical fibers and assemblies. During the eighties, fluoride glass fibers have been intensively developed for ultra long haul telecommunication application, due to their theoretical ultra low loss (10-3db/km at 2,6 µm). Unfortunately after two decades of intensive development, Fiber loss is still two orders of magnitude far from the theoretical value, and one order of magnitude higher than attenuation of silica fibers. Consequently, in the end of the nineties this intensive development slowed down significantly. Only few groups are still developing fluoride glass fibers for short and medium application, such as fiber lasers and amplifiers, spectroscopy, medical, military and astronomy applications.

The first generation of fluoride fibers has suffered from high attenuation and manly from their poor mechanical performances. Significant progress has been made during the last few years. The technology is now very mature to provide high quality fluoride fibers, including rare-earth doped and un-doped single and multimode fibers.  More recently we have reported high optical and mechanical quality indium fluoride fibers. These new fibers have transmission widow from 0.3 up to 5.5 microns, which is 1 micron wider than ZBLAN fiber’s transmission window.

Current commercial fluoride fiber losses are ranging from 30 to 150 dB/ km and mechanical strength is ranging from 75 to 135 kpsi depending on fiber geometry and fiber optical parameters. As far as fiber handling is concerned, fluoride glass fibers can be cleaved, spliced and tapered as silica fibers using standard equipments. Significant progress has been made also in fiber cables assemblies too; we have developed and reported hermitic cables for very demanding environment. The presentation will report the latest development of fluoride fiber technology.

A diversity of application areas for the phenomenon of stimulated Brillouin scattering (SBS), together with a growing variety of SBS interaction regimes and media suitable for its realization, stimulate continuing interest in the phenomenon. These studies obviously require an adequate theoretical framework.

Physically, SBS is the reflection of incident (pump) radiation by the acoustic wave that is excited in an optically transparent medium through electrostriction. The acoustic wave in SBS serves as the Bragg grating travelling with the velocity of sound, and resulting in a Doppler shift of the reflected (scattered) light. It was noted in [1] that, since the period of such a grating is roughly half of the pump radiation wavelength, excitation of an acoustic wave in a medium makes it macroscopically inhomogeneous. In spite of this, the currently prevalent theoretical formalism of SBS uses the optical wave equation (OWE) for the scattered field obtained in [2] in the approximation of a macroscopically homogeneous medium; equations for the pump field and the material response are respectively fitted.

I shall present (i) a revised formalism of SBS which is based on an OWE which explicitly accounts for the medium’s macroscopic inhomogeneity, (ii) a corrected OWE for the pump field and (iii) a consequently re-examined description of the material response. The new formalism opens up, firstly, opportunities for a more adequate description of already observed features of SBS and predictions of new effects and, secondly, gives grounds for revealing and rejecting certain speculative claims. 

References:

[1]. V.I.Kovalev, R.G.Harrison. Phys. Lett. A 374, 2297 (2010).

[2]. J.A.Armstrong, N.Bloembergen, et al. Phys. Rev. 127 1918 (1962).

Fiber lasers are widely used to perform precise mechanical operations used for industrial applications, such as cutting, sintering, engraving and welding. The precision of the mechanical process is largely dependent on the beam quality of the fiber laser, as determined by their M² parameter. A smaller M² value indicates better beam quality, which could be used to improve performance in industrial applications. To improve the precision of mechanical process, we have designed dielectric-pillars based lenses to convert the measured laser beam profile into a Gaussian beam and improve M² of the laser beam.

The in-house code based on diffractive optics has been used to calculate the required lens structure to transform the measured laser beam profile into a Gaussian shape with near unity M². The procedure is based on the generalized coordinate transformation of the measured beam profile to that of a Gaussian shape. Lens structure is calculated using diffractive optics based calculation, by solving the general form of diffraction equation. This converts the measured beam profile into Gaussian shape. The built-in lens has a dimension less than 1mm and is thus extremely difficult to fabricate by conventional methods using either grinding and polishing, or diamond turning. The proposed lens curvature is then converted to an array of dielectric pillars with sub-wavelength dimensions using effective medium and filling fraction theory. The dielectric nanopillar array is being fabricated on a quartz substrate using E-beam lithography and plasma etching. Experiment has shown that the nanopillar array can withstand fibre laser power.

This paper presents simulation results on propagation characteristics of all possible excited modes in a non-adiabatic tapered step index single-mode fiber with 8 µm core diameter. The power evolution of each mode along the propagation direction was simulated using the Beam Propagation Method. Four modes were observed to be supported in the tapered SMF with symmetrical down-taper and up-taper length of 15mm and waist length of 10mm. The waist diameters used were 15 µm and 5 µm. The power of mode LP₀₂ is seen to be the most dominant mode at the waist region. As the waist diameter was reduced, the power of all modes experience larger oscillation with higher oscillation frequency at the waist region.

The highly versatile class of microstructured optical fiber (MOF) can provide a broad range of unique and tailorable optical properties, made possible by the presence of the characteristic longitudinal air holes. Whatever material is used or configuration of holes desired, the final fabricated MOF geometry is affected by a large set of conditions, including material viscosity, the effects of surface tension, and the pressure difference of the holes and atmosphere. The efficient and accurate fabrication of MOFs requires a practical understanding of the `draw process’. Our results, when applied to suspended-core and exposed-core MOF fabrication, experimentally validate the Fitt, et al. fluid-mechanics model for describing the draw process.

In recent work by Kostecki, et al., the silica `exposed-core’ MOF, in which the central light-guiding core is directly accessible to the outside environment, was published for the first time. The fabrication of exposed-core fibers is challenging due to their asymmetric cross section, where the method involved modification of established procedures and the practical application of the Fitt model. By understanding the draw process, both in terms of the draw tower temperature profile and establishing a method to define the geometry, it was found that the Fitt model could be used to predict the parameters necessary for the chosen structure. This model provided a practical framework that contributed to the efficient and accurate fabrication of the desired MOF geometry by predicting the fiber draw conditions.

Time-domain spectroscopy (TDS) is widely used at analysis of data of emission and absorption measurements on optical fibers due to its higher sensitivity and accuracy as compared to usual frequency domain approach. Traditional modeling approach for such analysis is based on decomposition of the measured TDS signal into sum of Fourier components (harmonics), dumped due to different kinds of the energy loss [1]. Analytical Fourier transform in complex frequency domain provides spectral representation of the measured signal in such a case.
Generic problem of the harmonic decomposition approach consists in the fact that there is no unique combination of magnitudes and phases of those harmonics, as well as of corresponding dumping factors, which are known a priory. Moreover, possible effects of the measurements errors (‘noise’) are ignored at standard harmonic decomposition. Therefore, separation of physically meaningful information (i.e. parameters of factual harmonic oscillators in the studied system) from the artifacts (noise) becomes one of the key problems at TDS data analysis and interpretation. For that reason, ‘sensitivity’ of the modeling approach and its general applicability for TDS data analysis are obviously limited.

Appropriate approach to numerical analysis of TDS data could be based on so-called ‘regularization’ procedures [2]. Within framework of this approach, ‘common sense’ criteria are routinely implemented in order to separate physically meaningful and stochastic (noise) contributions to the system response based on its spectral representation. Practical example of Fourier-transform-based ‘regularization’ algorithm is described in Ref. [3]. Implication of similar ‘regularization’ approach to TDS data analysis is discussed in the presentation.

1. M.C. Nuss, J. Orenstein, In Millimeter and Submillimeter Wave Spectroscopy of Solids; Ed. G. Gruner, Springer-Verlag: Berlin, 1998; Vol. 74.
2. A.N. Tikhonov and V.A. Arsenin, Solution of Ill-posed Problems, Winston & Sons, Washington, 1977.
3. V. Ligatchev, S.K. Chin, ECS Transactions 25, 19 (2009).

A novel method based on the optical spectrum stitching technique incorporating with optical channel estimation is proposed to realize the high-resolution and large measurement range optical component characterization, including amplitude, phase responses and polarization sensitivity property. Two kinds of fiber Bragg grating based Fabry-Perot cavity with ultra-fine structures have been characterized. By using 1024-point fast Fourier transform and a narrow line-width wavelength-tunable laser source, the achieved frequency resolution and measurement range of this method are ~ 10 MHz and 250 GHz respectively.

A class of diverse mathematical complexity problems such as the travelling salesman problem of looking for a shortest possible route on a map are often referred as NP-complete problems. No efficient algorithms have been identified to tackle these interlinked problems on a conventional computer. Unconventional approaches such as quantum and DNA computing are seen as a way to attack them. We demonstrate an all-fiber optical computer for solving one of the most difficult complexity problems, the Hamiltonian path challenge of finding if a map can be traveled in a unidirectional way that each town is visited exactly once. To solve it we implement the map as a network of optical fiber (roads) of non-commensurate length that connect nodes (towns) and probe the network with a short optical pulse. Each node (town) introduces a unique delay. The decision on the existence of a Hamiltonian path can be made by monitoring the delay of the returning pulse.:A positive answer is given if a pulse with delay equal to the sum of each town’s delay, meaning that this pulse has visited all the towns exactly once. With single-mode fiber networks representing a map of five towns and operating at a telecom wavelength we performed a proof-of-principle Hamiltonian path tests that took only a few tens of nanoseconds. We argue that current fiber technology shall allow interrogating graphs of hundreds of nodes and providing a simple-to-implement alternative to a quantum computer approach.