Tail-free operation of an overdriven gain-switched DFB laser by spectral filtering

The tail-free operation of an overdriven gain-switched distributed feedback (DFB) laser by spectral filtering was demonstrated. The filtering was realized using a mechanically tunable fiber Bragg grating (FBG). The unfiltered and filtered signals were traced by corresponding oscilloscope. The spectral filtering removed the nonlinearly chirped components resulting in the pulse shortening. The results showed unwanted relaxation in the overdriven DFB laser were supressed by using a steep-edge notch filter.

Spectrally-controlled, gain-switched operation of InGaN diode laser

Self-seeded, gain-switched operation of an InGaN multi-quantum-well diode laser is reported for the first time. Narrow-line, wavelength-tunable, picosecond pulses have been generated from a standard, uncoated diode laser with an external feedback.

Gain switching of an external cavity grating-coupled surface emitting laser with wide tunability

The gain-switched, single frequency operation of an external cavity grating-coupled surface emitting laser with a wavelength tuning range of 100 nm was presented. The light in the grating section was coupled out of the laser at a specific angle to the surface of the device. Analysis showed that within the driving current range, lasing in the device only occurred when the external cavity was properly aligned.

Estimating ECAP threshold from the variability of the response

Electrical compound action potentials (ECAPs) of the cochlear nerve are used clinically for quick and efficient cochlear implant parameter setting. The ECAP is the aggregate response of nerve fibres at various distances from the recording electrode, and the magnitude of the ECAP is therefore related to the number of fibres excited by a particular stimulus. Current methods, such as the masker-probe or alternating polarity methods, use the ECAP magnitude at various stimulus levels to estimate the neural threshold, from which the parameters are calculated. However, the correlation between ECAP threshold and perceptual threshold is not always good, with ECAP threshold typically being much higher than perceptual threshold. The lower correlation is partly due to the very different pulse rates used for ECAPs (below 100 Hz) and clinical programs (hundreds of Hz up to several kHz). Here we introduce a new method of estimating ECAP threshold for cochlear implants based upon the variability of the response. At neural threshold, where some but not all fibers respond, there is a different response each trial. This inter-trial variability can be detected overlaying the constant variability of the system noise. The large stimulus artefact, which requires additional trials for artefact rejection in the standard ECAP magnitude methods, is not consequential, as it has little variability. The variability method therefore consists of simply presenting a pulse and recording the ECAP, and as such is quicker than other methods. It also has the potential to be run at high rates like clinical programs, potentially improving the correlation with behavioural threshold. Preliminary data is presented that shows a detectable variability increase shortly after probe offset, at probe levels much lower than those producing a detectable ECAP magnitude. Care must be taken, however, to avoid saturation of the recording amplifier saturation; in our experiments we found a gain of 300 to be optimal.

Characteristics of Brillouin gain based distributed temperature sensors

A Brillouin-gain based distributed temperature sensor has been investigated experimentally and theoretically. The relation between Brillouin gain, input probe power and sensing length have been studied. The study shows that there is an optimum probe power providing a maximum Brillouin gain signal for a given sensing length.

22-km distributed temperature sensor using Brillouin gain in an optical fiber

We describe an experimental distributed temperature sensor that uses the temperature dependence of the Brillouin frequency shift. When a 22.2-km sensing length is used, we have observed a temperature resolution of 1°C and have obtained a spatial resolution of 10 m.

Regional differences in oxygen saturation in retinal arterioles and venules

BACKGROUND: Retinal vessel oxygenation saturation measurements have been the focus of much attention in recent years as a potential diagnostic parameter in a number of ocular and systemic pathologies. This interest has been heightened by the ability to measure oxygen saturation in vivo using a photographic technique. METHODS: Retinal vessel oxygenation in venules and arterioles of 279 retinal vessels of 12 healthy Caucasian participants (mean age: 30 SD (+/- 6) years) were measured consecutively three times to evaluate short-term variation in oxygen saturation and regional variability of retinal vessel oxygen saturation using dual-wavelength technology (Oxymetry Modul, Imedos, Germany). All subjects underwent standard optometric assessment including non-contact intra-ocular pressure assessment as well as having their systemic blood pressure measured. RESULTS: Vessels were grouped as either near-macula or peripheral, depending on their location. Peripheral arterioles and venules exhibited significantly lower oxygen saturation compared to their near-macula counterparts (arterioles: 94.7% (SD 3.9) vs. 99.7% (SD 3.2); venules: 65.1% (SD 7.2) vs. 90.3% (SD 6.7)). Both arterioles and venules, main branches, and those feeding and draining the retina near the macula and periphery showed low short-term variability of oxygen saturation (arterioles: COV 1.2-1.8%; venules: COV 2.9-4.9%). CONCLUSIONS: Retinal arterioles and venules exhibit low short-term variation of oxygen saturation in healthy subjects. Regional differences in oxygen saturation could be a potential useful marker for risk stratification and diagnostic purposes of area-specific retinal pathology such as age-related macula degeneration and diabetic maculopathy.

Multi-wavelength erbium/Raman gain based random distributed feedback fiber laser

A multiwavelength generation in a random distributed feedback fiber laser based on hybrid Raman and erbium gain and a Lyot all-fiber spectral filter is demonstrated for the first time. The use of erbium-doped fiber allows a multi-wavelength generation to be achieved at lower pump powers in comparison with random fiber lasers based on Raman gain only. The operating bandwidth and flatness of power distribution between different lines in generation are also improved in the hybrid gain configuration.

Analysis of nocturnal oxygen saturation recordings using kernel entropy to assist in sleep apnea-hypopnea diagnosis

In this study, a new entropy measure known as kernel entropy (KerEnt), which quantifies the irregularity in a series, was applied to nocturnal oxygen saturation (SaO 2) recordings. A total of 96 subjects suspected of suffering from sleep apnea-hypopnea syndrome (SAHS) took part in the study: 32 SAHS-negative and 64 SAHS-positive subjects. Their SaO 2 signals were separately processed by means of KerEnt. Our results show that a higher degree of irregularity is associated to SAHS-positive subjects. Statistical analysis revealed significant differences between the KerEnt values of SAHS-negative and SAHS-positive groups. The diagnostic utility of this parameter was studied by means of receiver operating characteristic (ROC) analysis. A classification accuracy of 81.25% (81.25% sensitivity and 81.25% specificity) was achieved. Repeated apneas during sleep increase irregularity in SaO 2 data. This effect can be measured by KerEnt in order to detect SAHS. This non-linear measure can provide useful information for the development of alternative diagnostic techniques in order to reduce the demand for conventional polysomnography (PSG). © 2011 IEEE.

Phase shift keyed systems based on a gain switched laser transmitter

Return-to-Zero (RZ) and Non-Return-to-Zero (NRZ) Differential Phase Shift Keyed (DPSK) systems require cheap and optimal transmitters for widespread implementation. The authors report on a gain switched Discrete Mode (DM) laser that can be employed as a cost efficient transmitter in a 10.7 Gb/s RZ DPSK system and compare its performance to that of a gain switched Distributed Feed-Back (DFB) laser. Experimental results show that the gain switched DM laser readily provides error free performance and a receiver sensitivity of -33.1 dBm in the 10.7 Gbit/s RZ DPSK system. The standard DFB laser on the other hand displays an error floor at 10(-1) in the same RZ DPSK system. The difference in performance, between the two types of gain switched transmitters, is analysed by investigating their linewidths. We also demonstrate, for the first time, the generation of a highly coherent gain switched pulse train which displays a spectral comb of approximately 13 sidebands spaced by the 10.7 GHz modulation frequency. The filtered side-bands are then employed as narrow linewidth Continuous Wave (CW) sources in a 10.7 Gb/s NRZ DPSK system.

Investigation of EDFA power transients in circuit-switched and packet-switched optical networks

Erbium-doped fibre amplifiers (EDFA’s) are a key technology for the design of all optical communication systems and networks. The superiority of EDFAs lies in their negligible intermodulation distortion across high speed multichannel signals, low intrinsic losses, slow gain dynamics, and gain in a wide range of optical wavelengths. Due to long lifetime in excited states, EDFAs do not oppose the effect of cross-gain saturation. The time characteristics of the gain saturation and recovery effects are between a few hundred microseconds and 10 milliseconds. However, in wavelength division multiplexed (WDM) optical networks with EDFAs, the number of channels traversing an EDFA can change due to the faulty link of the network or the system reconfiguration. It has been found that, due to the variation in channel number in the EDFAs chain, the output system powers of surviving channels can change in a very short time. Thus, the power transient is one of the problems deteriorating system performance. In this thesis, the transient phenomenon in wavelength routed WDM optical networks with EDFA chains was investigated. The task was performed using different input signal powers for circuit switched networks. A simulator for the EDFA gain dynamicmodel was developed to compute the magnitude and speed of the power transients in the non-self-saturated EDFA both single and chained. The dynamic model of the self-saturated EDFAs chain and its simulator were also developed to compute the magnitude and speed of the power transients and the Optical signal-to-noise ratio (OSNR). We found that the OSNR transient magnitude and speed are a function of both the output power transient and the number of EDFAs in the chain. The OSNR value predicts the level of the quality of service in the related network. It was found that the power transients for both self-saturated and non-self-saturated EDFAs are close in magnitude in the case of gain saturated EDFAs networks. Moreover, the cross-gain saturation also degrades the performance of the packet switching networks due to varying traffic characteristics. The magnitude and the speed of output power transients increase along the EDFAs chain. An investigation was done on the asynchronous transfer mode (ATM) or the WDM Internet protocol (WDM-IP) traffic networks using different traffic patterns based on the Pareto and Poisson distribution. The simulator is used to examine the amount and speed of the power transients in Pareto and Poisson distributed traffic at different bit rates, with specific focus on 2.5 Gb/s. It was found from numerical and statistical analysis that the power swing increases if the time interval of theburst-ON/burst-OFF is long in the packet bursts. This is because the gain dynamics is fast during strong signal pulse or with long duration pulses, which is due to the stimulatedemission avalanche depletion of the excited ions. Thus, an increase in output power levelcould lead to error burst which affects the system performance.

Widely tunable gain-switched operation of external cavity grating-coupled surface emitting laser

Widely tunable gain switching of a grating-coupled surface-emitting laser (GCSEL) has been demonstrated in a simple external cavity configuration for the first time. Pulse duration in range of 40-100ps and wavelength tuning over 100nm have been achieved. High power, tail-free optical pulses have been observed at 980nm.

Self-seeded gain-switched operation of an InGaN MQW laser diode

Self-seeded, gain-switched operation of an InGaN multi-quantum-well laser diode has been demonstrated for the first time. An external cavity comprising Littrow geometry was implemented for spectral control of pulsed operation. The feedback was optimized by adjusting the external cavity length and the driving frequency of the laser. The generated pulses had a peak power in excess of 400mW, a pulse duration of 60ps, a spectral linewidth of 0.14nm and maximum side band suppression ratio of 20dB. It was tunable within the range of 3.6nm centered at a wavelength of 403nm.

Q-switched induced gain switching of a twotransition cascade laser

A gain-switched laser transition, of a two-laser-transition cascade laser, that is driven by the adjacent laser transition which is Q-switched is demonstrated using a Ho3+ -doped fluoride fiber laser. Q-switching the 5|6 ? 5|7 transition at 3.002 µm produces stable gain-switched pulses from the 5|7 ? 5|8 transition at 2.074 µm; however, Q-switching the 5|7 ? 5|8 transition produced multiple gain switched pulses from the 5|6 ? 5|7 transition. The gain-switched pulses were measured to be of a similar duration to the Q-switched pulses suggesting that much shorter pulses of closer duration could be generated at pump power higher levels.