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We describe a full field heterodyne interferometry system where the object beam is shifted by a high frequency with respect to the reference beam. The optical path difference between the object and reference beam is encoded in the phase of the envelope of the interference signal which is at the difference frequency. Conventionally, high frequency heterodyne interferometry is restricted to measurement of a single (or a few) point(s) as is the case in displacement measuring interferometers for precision machine tool axis feedback. This is because of the problem of demodulating the signal phase simultaneously for many pixels comprising a full-field image. This problem is overcome here by exploiting the capability of the special pixel structure employed in a Time of Flight (ToF) camera. This structure enables the measurement of the envelope phase of an optical signal at every pixel with respect to an electronic reference at the same frequency. ToF cameras are designed to measure the distance to an object in its field of view by detecting the phase delay, due to time of flight, of reflected light from a modulated source synchronized to the camera. In the described experimental interferometer, a Twyman-Green architecture is used with an acousto-optic modulator to produce interfering beams with a difference frequency of 20MHz. The image detector is a modified ToF camera based on a Texas Instruments OPT8241 sensor. The interferometer directly outputs a wrapped optical phase map with 12-bit resolution (equivalent OPD resolution 0.15 nm) at greater than 50 frames per second with no post-processing. The phase reconstruction is highly insensitive to the reference/object beam intensity ratio or to environmental noise.

Labeling nonquiescent cells with carboxyfluorescein succinimidyl ester (CFSE)-like dyes gives rise to a population width exceeding the threshold for resolving division peaks by flow cytometry. Width is a function of biological heterogeneity plus extrinsic and intrinsic error sources associated with the measurement process. Optimal cytometer performance minimizes extrinsic error, but reducing intrinsic error to the point of facilitating peak resolution requires careful fluorochrome selection and fluorescent cell sorting. In this study, we labeled the Jurkat and A549 cell lines with CFSE, CellTraceViolet (CTV), and eFluor 670 proliferation dye (EPD) to test if we could resolve division peaks in culture after reducing the labeled input widths by cell sorting. Reanalysis of the sorted populations to ascertain the level of reduction achieved always led to widths exceeding the gated limits due to the contribution of errors. Measuring detector-specific extrinsic error by sorting uniform fluorescent particles with similar spectral properties to the tracking dyes allowed us to determine the intrinsic error for each dye and cell type using a simple mathematical approach. We found that cell intrinsic error ultimately dictated whether we could resolve division peaks, and that as this increased, the required sort gate width to resolve any division peaks decreased to the point whereby issues with yield made A549 unsuitable for this approach. Finally, attempts to improve yields by setting two concurrent sort gates on the fluorescence distribution enriched for cells in different stages of the cell cycle that had nonequivalent proliferative properties in culture and thus should be practiced with caution. © 2013 International Society for Advancement of Cytometry

We have calculated the optical structure of wurtzite ZnO doped with Silicon (Si). The calculations are based on the density functional theory with the generalized gradient approximation (GGA) and the projector augmented wave pseudopotentials. The GGA with the Perdew Burke Ernzerhof exchange-correlation functional are employed in the simulations. Ultrasoft pseudopotentials are utilized for the geometry optimization to render the computations tractable as well as to enhance the efficiency. We investigate two kinds of defects in ZnO, namely the substitution of Zn by Si and O by Si. The optical properties, including dielectric function, reflectivity and absorption coefficient of wurtzite ZnO are calculated. The variation in the band gap and energetics have been validated against published results. The dielectric properties follow a steep decreasing trend for the low energy level. In addition, the reflectivity and absorption coefficients reduce abruptly due to the doping.

Cross-sectional scanning tunneling microscopy is used to study defects on the surface of semiconductor laser devices. Step defects across the active region caused by the cleave process are identified. Curved blocking layers used in buried heterostructure lasers are shown to induce strain in the layers above them. Devices are also studied whilst powered to look at how the devices change during operation, with a numerical model that confirms the observed behavior. Whilst powered, low-doped blocking layers adjacent to the active region are found to change in real time, with dopant diffusion and the formation of surface states. A tunneling model which allows the inclusion of surface states and tip-induced band bending is applied to analyze the effects on the tunneling current, confirming that the doping concentration is reducing and defect surface states are being formed.

We present a description of our method to process a set of autocollimator-based deflectometer 1-dimensional line-scans taken over a large optical surface and reconstruct them to a best-fit conic-section surface. The challenge with our task is that each line-scan is in a different (unknown) coordinate reference frame with respect to the other line-scans in the set. This problem arises due to the limited angular measurement range of the autocollimator used in the deflectometer and the need to measure over a greater range; this results in the optic under measurement being rotated (in pitch and roll) between each scan to bring the autocollimator back into measurement range and therefore each scan is taken in a different coordinate frame. We describe an approach using a 6N+2 dimension optimisation (where N is the number of scan lines taken across the mirror) that uses a gradient-based non-linear least squares fitting combined with a multi-start global search strategy to find the best-fit surface. Careful formulation of the problem is required to reduce numerical noise and allow the routine to converge on a solution of the required accuracy., Comment: 12 pages, 11 figures, Published in Applied Optics

The use of autocollimator-based profilometers of the Nanometer Optical measuring Machine (NOM) design has been reported for the evaluation of X-ray optics for some time. We report a related development in the use of a non-contact NOM profilometer for the in situ measurement of base radius of curvature and conic constant for E-ELT primary mirror segments during fabrication. The instrument is unusual in NOM design in that it is deployable onto a CNC polishing machine in an industrial fabrication environment. Whilst the measurement of radius of curvature of spherical surfaces over a single scan has been reported previously, here we report on the use of this instrument to measure optical surfaces with an aspheric departure of 180 micrometers using a grid of multiple scans and bespoke surface fitting software. The repeatability of the measurement has been found to be approximately 1 mm in a measured radius of curvature of approximately 90 m. The absolute accuracy is limited by the accuracy of the calibration of the autocollimator and the in situ calibration of the instrument during operation.

The manufacture of the next generation of large astronomical telescopes, the extremely large telescopes (ELT), requires the rapid manufacture of greater than 500 1.44m hexagonal segments for the primary mirror of each telescope. Both leading projects, the Thirty Meter Telescope (TMT) and the European Extremely Large Telescope (E-ELT), have set highly demanding technical requirements for each fabricated segment. These technical requirements, when combined with the anticipated construction schedule for each telescope, suggest that more than one optical fabricator will be involved in the delivery of the primary mirror segments in order to meet the project schedule. For one supplier, the technical specification is challenging and requires highly consistent control of metrology in close coordination with the polishing technologies used in order to optimize production rates. For production using multiple suppliers, however the supply chain is structured, consistent control of metrology along the supply chain will be required. This requires a broader pattern of independent verification than is the case of a single supplier. \ud This paper outlines the metrology requirements for a single supplier throughout all stages of the fabrication process. We identify and outline those areas where metrology accuracy and duration have a significant impact on production efficiency. We use the challenging ESO E-ELT technical specification as an example of our treatment, including actual process data. We further develop this model for the case of a supply chain consisting of multiple suppliers. Here, we emphasize the need to control metrology throughout the supply chain in order to optimize nett production efficiency.

Self-pulsating laser diodes operating at 420 nm are required in high-density optical storage devices. By growing saturable absorbing quantum wells in the p-doped cladding layer, of a semiconductor laser, it is possible to obtain the interplay between gain and absorption, which is required for pulsation. The dynamics of self-pulsating InGaN laser diodes are investigated to gain an insight into how the quantum-well configuration in the absorber and the cavity length affect the laser output. In particular, the work shows how a carefully designed structure gives rise to stable self-pulsation up to a temperature of 100degC while offering high powered emission at low operating currents. As a result, such blue-emitting laser diodes are highly desirable for use within next-generation optical storage devices

Summary Buried heterostructure multiquantum well laser devices are investigated utilizing a near-field scanning optical microscope to characterize and correlate the surface topography, optical output and electronic properties of the device. Near-field photocurrent imaging has been used to accurately measure the unbiased buried heterostructure multiquantum well device in cross-section, successfully revealing the distribution of pn-junctions and their associated fields. Moreover, this has been accurately correlated with the physical structure of the device determined by simultaneous shear-force imaging of the surface. Topographic structure is manifested as a result of strain relaxation (∼10−10 m) of the cleaved cross-section. These imaging modes are similarly correlated with the optical output of the operational device mapped with 50 nm lateral resolution. The collection-mode measurements detected electroluminescence external to the active region, highlighting the existence of carrier recombination away from the multiquantum well device region. The combination and correlation of different near-field scanning optical microscope imaging modes proved powerful in the analysis of the buried heterostructure multiquantum well device, and was shown to assist in the identification of current leakage pathways within the structure.

Two-mode vertical cavity surface-emitting lasers (VCSELs) subject to external optical injection are investigated experimentally. The spectrum of a VCSEL with two parallel-polarized modes demonstrates significantly different behavior compared to that of a VCSEL with two orthogonally-polarized transverse modes. In the latter, external optical injection can be used to achieve single-mode operation. However, in the case of two parallel-polarized transverse modes, VCSEL single-mode operation cannot be obtained via optical injection. In this case, when the injection frequency is close to one of the modes, the coexistence of the injected beam and that mode gives rise to very rich dynamical behavior; however, the other mode is unperturbed by the optical injection.

This paper describes the application of loose-abrasive processes to the manufacture of 1.4 meter, off-axis aspheric, hexagonal mirror-segments. These are prototypes for the 39 m European Extremely Large Telescope (E-ELT). The application of active forces to correct the overall form of segments in the telescope, means that the overall form-accuracy achieved in polishing can be less critical than for a non-active mirror. However, it is a requirement that the base-radii and conic constants of mirror-segments are very closely matched, so that the combined image is not degraded. This means that abrasive processes have to operate with respect to anabsoluterather thanrelativedatum. Furthermore, there are stringent requirements on mid-spatial frequency defects on segment surfaces, and on edge-roll. These control stray-light, and ultimately detectability of faint astronomical targets. We describe the CNC abrasive techniques we have developed in response. We then demonstrate the success of the approach, which represents the first time ever that segments have been processed entirely in the hexagonal shape:- a milestone in loose-abrasive processing. Finally, we address up-scale for the unprecedented number of segments required for the E-ELT build-phase.

Self-pulsating laser diodes operating at 650 nm are required in high-density optical storage devices. By growing saturable absorbing quantum wells in the p-doped cladding layer of a laser it is possible to obtain the interplay between gain and absorption that is required for pulsation. A self-pulsating AlGaInP laser is modelled in order to gain an insight into how the distance between the active and absorber layers affects the laser output. Results indicate that the device performance can be altered significantly as the distance changes, showing that this design feature is a key parameter for optimum performance.

A simple method is presented for determining the minimum feedback level required for the onset of instability and chaos in semiconductor laser diodes. Using this method, the positions of the pair of dominant system poles in the s-plane are found for varying levels of feedback. This analysis determines the feedback level required to send the system unstable. The results are applicable for both long and short external cavities. The method is ideally suited to the prediction of system stability from measurements of microwave modulation response or relative intensity noise (RIN) spectrum, since it only requires knowledge of the natural frequency and damping frequency of the dominant s-plane poles of the laser diode without optical feedback. Copyright © 2001 John Wiley & Sons, Ltd.

Self-pulsating laser diodes operating at a wavelength of 650 nm are required in optical storage devices. Pulsation can be achieved by including saturable absorbing quantum wells in the p-doped cladding layer of an AlGaInP laser. A rate equation model of this kind of device is used to analyze the power output of the pulses by varying the cavity length and the absorber quantum-well configuration. Results indicate that certain performance tradeoffs occur between the power output and other characteristics that are subject to optical storage device constraints.

This paper examines the effect of carrier heating due to hot carrier injection in directly modulated semiconductor laser diodes. Using a single-mode rate equation model including an equation for the energy stored in the conduction band carriers, the changes in carrier temperature due to hot carrier injection from the barrier layer in quantum-well laser diodes are seen to cause a degradation in the device's sinusoidal modulation response. This degradation is coupled with an increase in the wavelength chirp of the light output. It is concluded that in order to minimize the detrimental effect of carrier heating on directly modulated quantum well laser diodes the barrier height should be kept as small as practical. Additionally, the appreciable change in device performance brought about by carrier heating should be accounted for when modelling such devices.

Numerical simulations are undertaken to optimise the contrast ratios of terahertz optical asymmetric demultiplexer (TOAD) devices subject to frequency-detuned signal and control picosecond optical pulses. Optimum control pulse widths and pulse energies as well as optimum small-signal gains of semiconductor optical amplifiers (SOAs) are identified for achieving maximum contrast ratios in TOAD devices.

A numerical investigation has been undertaken of the operating characteristics of Terahertz Optical Asymmetric Demultiplexer (TOAD) devices subject to frequency-detuned signal and control picosecond optical pulses, taking into account the wavelength dependence of the differential gain coefficient and carrier density linewidth enhancement factor. It is shown that, by shifting the signal pulses toward lower frequencies compared with that of control pulses, the contrast ratio can be significantly enhanced and the amplitude modulation can simultaneously be considerably decreased. The switching energy demonstrates small changes over a broad frequency detuning range. The simulations show excellent agreement with published experimental measurements. Furthermore, two operational modes for the TOAD configuration are identified.

A detailed numerical treatment is undertaken to investigate the operating characteristics of a self-synchronization scheme based on a modified terahertz optical asymmetric demultiplexer (TOAD) device for ultrafast packet switching. The simulations agree with published experimental measurements. It is shown that input picosecond optical pulse energies as low as /spl sim/100 fJ can be utilized to achieve a larger than 10 dB contrast ratio of the first to the remaining pulses of a transmitted signal packet. The utilization of low input pulse energies in the scheme also provides significant advantages including insensitivity of contrast ratio to pulse energies and pulse widths as well as distortionless extracted pulse shapes. Means for achieving a maximum contrast ratio are identified.

PUBLISHED, The potential for using inexpensive compact disc laser diodes as optical clock extraction elements in transparent networks has led to an increase in research into the dynamics of self-pulsating laser diodes. We use a rate-equation model to simulate the synchronization of the self-pulsating laser output pulses to a periodic optical signal, In particular, we investigate the time it takes for the laser to synchronize to the input signal and also, the time taken for the laser to unlock when the signal is removed. The effect of varying the power of the optical signal and the detuning of the input signal frequency relative to the laser's self-pulsation frequency are determined. Our results enable us to identify important issues which need to be addressed when a self-pulsating laser diode is used in a clock extraction subsystem, In particular, we find that the signal frequency and laser free-running frequency must be as close as possibility to minimize errors. Also, the higher the signal power the quicker the laser synchronizes to the signal, although we find that if the power becomes too large the laser can no longer lock, which would cause a significant increase in detection errors.

Self-pulsating laser diodes operating at a wavelength of 650 nm are attractive for high-density optical storage. The main candidate for such a device is an AlGaInP laser diode including an epitaxially integrated saturable absorber. The characteristic self-pulsation occurs due to the interplay between gain in the active region and the absorption within the structure. In the paper, we calculate the dynamics of self-pulsation in this type of AlGaInP laser diode, including a detailed description of gain and absorption within the relative sections. In particular, we identify how, by modifying the structure of the epitaxial absorber layers, we can alter the operating characteristics of these laser diodes.

We report on the first-ever demonstration of grinding and polishing full-size, off-axis aspheric, mirror segments as prototypes for an extremely large telescope, processed entirely in the final hexagonal shape. We first describe the overall strategy for controlling form and mid spatial frequencies, at levels in the vicinity of

We have carried out measurements of the internal losses in a multiple quantum well laser of Cd 0.28 Zn 0.72 Se ZnSe . From the dependence of the slope efficiency as a function of cavity length the loss value obtained for the 40 A well width with 5 wells in the sample is 50 cm−1. This very large value is attributed mainly to absorption in the GaAs substrate on which the quantum wells were grown. This measurement shows the importance of using waveguide structures in order to obtain low threshold conditions in II–VI lasers.

The use of detailed balance relationships between absorption and emission to obtain a gain spectrum from a spontaneous emission spectrum is a relatively easy method of measuring the gain from a semiconductor laser. It has been shown that the usual theoretical gain and emission spectra do not satisfy these relationships casting doubt on the validity of the procedure. We show that this arises from the incorrect implementation of spectral broadening that leads to a situation where the carriers and photons are no longer in thermal equilibrium and the relationship between absorption and gain in the calculations breaks down. We show that with a more exact implementation of spectral broadening thermal equilibrium is preserved and that the relationship between absorption and emission remains valid. We discuss the accuracy of the measurements required to obtain a correct gain spectra. >

We describe progress on a novel process-chain being used to produce eight 1.4m hexagonal segments as prototypes for the European Extremely Large Telescope - a Master Spherical Segment as a reference, and seven aspheric segments. A new pilot plant integrates a bespoke full-aperture test-tower designed and built by OpTIC Glyndwr, with a Zeeko 1.6m polishing machine. The process chain starts with aspherising hexagonal segments on the Cranfield BoX™ grinder, followed by smoothing, corrective-polishing and edge-rectification using the Zeeko CNC platform. The paper describes the technology and progress, and anticipates how the process-chain is expected to evolve through the seven segments to increase both process-speed and surface-quality.

Following a 'Call for Tenders' by the European Southern Observatory (ESO) a collaboration headed by OpTIC Glyndwr Ltd is producing seven prototype segments for the European Extremely Large Telescope (E-ELT). Each hexagonal segment is 1.4 m corner-to-corner with a base radius of curvature (ROC) of 84 m and the combination of 984 segments will lead to a primary mirror with a diameter of 42 m. The polishing of the prototype segments occurs in-house at OpTIC Glyndwr using a Zeeko polishing machine, while in situ interferometry is undertaken using a specifically designed optical test tower built above the polishing machine. To confirm the base radius of curvature of the prototype segments an on-machine non-contact profiler is used. The profiler attaches to the bridge of the polishing machine and is removed during polishing. The design of the profiler is based upon a nanometre optical component measuring machine (NOM) system, originating from the sychrotron optics community. This instrument determines the height at a series of points across the surface through the integration of measured slope data. This paper presents the operation of the OpTIC-NOM as both an on-machine and off-machine profiler. The accuracy of the profiler both on- and off-machine and its accuracy is compared against similar profilers through its participation in a round robin study. Radius of curvature results are presented for the master spherical segment (MSS), which is a reference optic for the E-ELT segments and highlights a standard deviation on the mean radius of curvature of 2.5 mm. Finally, a summary of future work is presented regarding the polishing of the prototype segments and the development of a stitching algorithm to produce 3D surface maps from profilometry data.

The development of an experimental setup capable of contra sting fluorescent materials by their recombinative lifetimes in an imaging mode is discussed. Such materials might include molecular dyes and QDs. The system is comprised of a standard upright microscope fitted with an imaging CCD, and a white light laser that illuminates a circular region within the field of view with variable period excitation pulse trains. Different fluorescent species within this region absorb the laser light and fluoresce with a recombination lifetime dependent on material composition and local environment. Species with differing fluorescent lifetimes can be distinguished in an imaging mode by their contrasting intensity response to the pulse train at the range of different pulse frequencies. The technique is discussed and applied to samples containing both CdTe (705 nm) and CdSe (611 nm) QDs, showing contrast between long (70-100 ns) and (relatively) short (25-35 ns) lifetime within an image. Keywords: Fluorescence, lifetim e, FLIM, imaging, quantum dot s, CdTe, CdSe, fibre laser.

Cross-sectional scanning tunneling microscopy is performed on operating semiconductor quantum well laser devices to reveal real time changes in device structure. Low and nominally doped capping regions adjacent to the quantum well active region are found to heat under normal operating conditions. The increase in anion-vacancy defect formation and the generation of surface states pins the Fermi level at the surface and begins the process of catastrophic optical degradation which eventually destroys the device. The technique has implications for the study of defect generation and in-operation changes in all nanostructures.

Low-coherence self-pulsating laser diodes operating at a wavelength of 650 nm and at temperatures in excess of 70/spl deg/C are required for high density optical storage systems. We report on AlGaInP lasers operating at this wavelength which exhibit stable self-pulsation up to a temperature of 100/spl deg/C. The lasers are 50-/spl mu/m-wide oxide-isolated stripe devices in which the saturable absorption necessary for pulsation is provided by multiple-quantum wells placed within the p-doped cladding layer. The pulsation frequency of the devices increases linearly with increasing drive current and is present up to 1.5 times, lasing threshold.

Self-pulsation within 650 nm, AlGaInP laser diodes can be achieved via the use of a saturable absorbing, epitaxial layer within the structure. A rate equation model of this type of device is presented, which includes the process of thermally activated charge transfer from the quantum well and gain region to the absorbing layer. The results indicate that this thermal leakage mechanism saturates the absorber and hence destroys the self-pulsation at high temperatures. Comparison of the model with recently published experimental data shows good agreement and provides a consistent explanation for the loss of pulsation at temperatures in excess of 60 °C.

The Lyapunov exponents and plotted bifurcation diagrams for a self-pulsating laser diode under periodic current modulation have been calculated. Different dynamics are obtained dependent on the modulation depth and frequency of the drive current. It is shown that it is very difficult to distinguish between regimes of high periodicity and true chaos. However by introducing a return map relating the pulse peak height and the inter pulse interval clear differences are observed between these regimes. This has consequences for the use of chaotic self-pulsating laser diodes for data encryption applications.

We investigate both experimentally and theoretically the behavior of pulse jitter in compact disk (CD) self-pulsating lasers that have been synchronized to an applied periodic electrical signal. Our investigations reveal that pulse-to-pulse jitter in a synchronized laser is almost entirely unchanged from that in an unsynchronized laser under the same bias conditions. The major effect of synchronization is the suppression of long term jitter.

Experimental data regarding the temperature dependence of compact disc lasers are presented. The authors demonstrate, for the first time, a temperature-dependent model which has been adapted from a well established compact disc laser model. By comparing the experimental trends and calculation, they attempt to highlight the role of certain phenomena such as non-radiative recombination and charge carrier diffusion on the device behaviour. From these results they come to a number of important conclusions regarding the origin of self-pulsation in these types of laser diodes.

Semiconductor lasers subject to optical feedback are of both scientific and technological importance. The laser and external mirror form a compound cavity which can be modeled using a coupled cavity approach. Using this approach we have calculated the longitudinal mode frequencies and the associated mode lifetimes of all the modes in the system. We have found that for a narrow range of external cavity reflectivities, the mode lifetimes become almost degenerate. In this regime the mode lifetimes become highly sensitive to variations in the refractive index of the laser and external cavity reflectivities. Close inspection of this regime has revealed ever finer detail, suggesting a fractal structure to the mode solutions. The calculations have been undertaken in the 'cold cavity' limit and indicate a catastrophic loss of mode discrimination, even in a system with no inherent material gain. In the short cavity limit the external reflectivity required to induce the loss of mode discrimination varies as a function of external cavity round-trip time.

We have studied self-pulsation in InGaAs quantum dot lasers with an emission wavelength in the 1 μm band. The use of saturable absorption to produce internal optical feedback in semiconductor lasers is well established and leads to the phenomenon of self-pulsation. The characteristics of this self-sustaining oscillation in the optical intensity are determined by the optical characteristics of the amplifying and absorbing media. These experiments therefore provide a direct measure of the intrinsic dynamics of the dot laser system free of any external parasitics. At room temperature, pulsation is observed up to a drive current of 1.5 Ith with a maximum pulsation frequency of 700 MHz. The self-pulsation is strongly temperature dependent, and cannot be maintained below a temperature of 150 K. Studies of the optical gain and carrier lifetime within the lasers indicate that the dynamic characteristics are controlled by the interaction of the quantum dots with the two-dimensional wetting layer surrounding them. ...

Summary form only given. Recently we reported an experimental demonstration of this effect using semiconductor lasers. Numerical studies of chaos synchronisation in semiconductor lasers with optical feedback have been reported recently. In this paper we study, experimentally and numerically a) the conditions under which the anticipating synchronisation becomes lag synchronisation and b) the effect of external cavity parameters on the anticipating synchronisation. We have used two single-mode, Fabry-Perot diode lasers emitting at 830 nm.

Summary form only given. We investigate the dynamical behaviour of a VCSEL with single transverse mode subject to optical injection from a master laser. We show optical spectra of a VCSEL subjected to optical injection with about 19 /spl mu/W injection power.

A nearly single mode vertical cavity surface emitting laser (VCSEL) subject to optical injection have been investigated experimentally. It is found that regions of polarization-resolved chaotic behavior exist for both positive and negative detuning from the stable injection locking regime. Outside the chaos regimes, several nonlinear dynamical phenomena including frequency pushing, nearly degenerate four-wave mixing, injection locking, limit cycle and period doubling, were also observed.

Summary form only given. Repetitively Q-switched or self-pulsating semiconductor lasers are key components in high density optical storage systems such as digital versatile disc (DVD) where low intensity-noise sources are required. The saturable absorption required to produce the Q-switched action can be provided by an epitaxial layer placed within the cladding regions of the laser. This provides a mechanism by which the absorption characteristics can be engineered via the position, thickness and material composition of the layer. Thus the gain and dispersion characteristics can be controlled to optimise the self pulsation characteristics. We have fabricated laser diodes with such epitaxial absorbers operating at 650 nm which exhibit self-pulsating behaviour over a wide range of injection currents and temperatures.

The paper presents spectrally resolved measurements and theoretical predictions of self-pulsating output in multiple longitudinal mode laser diodes. The variation in behaviour between the peak-gain mode and its satellite modes is seen to be due to the interaction of the optical field with the carrier reservoir. A measure of the carrier dynamics in the gain section is obtained by measuring the optical power between a pair of optical modes, an approach that is verified by comparison with results of the theoretical model.

Synchronisation of two chaotic semiconductor lasers, viz. transmitter and receiver laser has been achieved such that the receiver laser dynamics neither lags nor leads the transmitter laser dynamics i.e., the receiver and transmitter lasers evolve together.

Instabilities in the polarisation-resolved output of a vertical cavity surface emitting laser (VCSEL) subject to optical injection have been investigated experimentally. It is found that regions of chaotic behaviour exist for both positive and negative detuning from the stable injection locking regime. Outside the chaos regimes, several nonlinear dynamical phenomena including frequency pushing, nearly degenerate four-wave mixing (NDFWM), injection locking, limit cycle and period doubling, were also observed.

We analyse the applicability of the two-flat-test calibration method across wide spatial frequency range based on its calculated accuracy. A number of simulations have been performed to determine the accuracy of the absolute calibration. The simulation results show that the two-flat-test calibration method is applicable to surfaces within the spatial frequency range from 1.67×10−1 to 1.67×10−3 mm−1.

The model presented in this paper views the system as a coupled-cavity problem, and self-consistently calculates the longitudinal modes of the cavity and the optical susceptibility in semiconductor lasers. The optical susceptibility is calculated over the full frequency range of expected optical cavity modes and includes many-body effects. In this way the evolution of the mode spectrum from spontaneous emission can be obtained. The model is applied to a specific topical phenomenon-the observation of low frequency fluctuations, and the results are compared with those of single mode models, in order to assess the importance of multi-mode effects on low frequency fluctuation (LFF) dynamics.

Summary form only given. In this paper, the characteristics of pump power dependent transparency carrier density of semiconductor optical amplifiers (SOAs) are simulated numerically, based on a comprehensive pulse model for describing the propagation and the amplification of strong optical pulses in SOAs. A detailed material gain model is also used for calculating optical gains and refractive index. The numerical simulations show good agreement with published experimental measurements.

Summary form only given. Among the all-optical switches currently being researched and implemented, the Terahertz Optical Asymmetric Demultiplexer (TOAD) topology, is one of the most attractive because of its unique advantages including high operating speed, low control pulse energy and compactness suitable for easy manufacture. In this paper, the dependence of the operating characteristics of TOADs upon control pulse energies and frequency detunings between the signal and control pulses are investigated numerically, using a comprehensive model for describing propagation of intense picosecond pulses in SOAs and a detailed gain model. The material gain model prescribes, in particular, the wavelength dependence of the linewidth enhancement factor and the differential gain coefficient. These are shown to be crucial to analysis of the device performance. The control pulse frequency is assumed to be located at the peak of the gain spectrum of the SOA.

Summary form only given. Examines the variation in the optical output of different longitudinal modes of a self-pulsating, multiple-quantum well AlGaInP/GaInP laser diode that incorporates a number of epitaxially integrated absorbing quantum wells that act as a saturable absorber. Spectrally-resolved time-domain measurements using a monochromator and photon-counting detector are compared with a multi-mode rate-equation model. Time-resolved spontaneous emission measurements are also taken.

Summary form only given. Considerable attention has been given to using chaos to implement secure optical communication systems. A message is encoded onto a chaotic carrier generated in a transmitter laser. Decoding of the message can be effected using a chaotic receiver if it is synchronised to the transmitter. In this work we show theoretically that synchronisation of two chaotic single mode self-pulsating laser diodes can be achieved. We consider both the multimode nature of these lasers and the effect of random spontaneous emission noise on the previously described synchronisation. The analysis is performed using a rate equation mode describing the dynamics of the carriers and the E-field of the optical mode.

Nonlinear optical loop mirrors based on transparent SOAs (TSOA-NOLMs) exhibit fast switching with a recovery time of the order of a few picoseconds, cascadable switching functionalities, low crosstalk between adjacent channels and low noise figures. In a TSOA-NOLM, following the passage of the pump pulse, the SOA may be considered to be transparent to the probe pulse if the carrier density is immediately restored to the value existing prior to the arrival of the pump pulse. The transparency carrier density is, however, significantly dependent upon the pump pulse power. Furthermore, due to the small net optical gain in a TSOA-NOLM, a high pump pulse power is usually required to achieve a specific switching output energy. Therefore, both the strong pump power dependence of the transparency carrier density of SOAs and the requirement of high switching powers would seriously limit the utilisation of TSOA-NOLMs in practice. In this paper, calculations predict that long SOAs (>500 /spl mu/m) can be employed in TSOA-NOLMs to not only considerably reduce the pump power dependence of the transparency carrier density of the SOAs but also to significantly increase the transmission of the switches.

An approach of utilising long SOAs in transparent SOA nonlinear optical loop mirrors, has been shown to reduce the pump power dependence of the transparency carrier density of the SOA, and simultaneously enhance the probe pulse transmission of the device.

Two-transverse-mode vertical cavity surface emitting lasers (VCSELs) subject to external optical injection is experimentally investigated. The spectra of the VCSEL with two parallel-polarized modes, under optical injection, demonstrate significantly different behaviors, compared to the VCSEL with two orthogonally-polarized transverse modes. In a VCSEL with two orthogonally-polarized transverse modes, the experimental results are good qualitative agreement with the theoretical prediction that the external optical injection can be used to achieve single-mode operation. However, in the case of a two parallel-polarized transverse modes VCSEL, single-mode operation cannot be obtained via optical injection. When the injection frequency is close to one of the modes, the coexistence of the injected beam and the mode gives rise to very rich dynamical behavior, however, the other mode is unperturbed by the optical injection.