Qin Zou

An electro-optic modulation technique for direct and accurate measurement of birefringence

We report a convenient and accurate technique for measuring the optical path difference induced by a birefringent crystal. The method is based on the transmission of a low-frequency signal by the electro-optic amplitude modulation of a monochromatic light source. The linear superposition principle of the induced birefringence and the electro-optic effect are employed in the analysis of the optical parameters. Birefringence measurements at the wavelength 0.6328 lm for some common nematic liquid crystal cells are presented. The measurement accuracy is close to 10 � 3 . In addition, the phase shift of a twisted-nematic liquid crystal spatial light modulator, which is now widely used in optical processing systems, is determined as a function of the gray-scale level. 2003 Elsevier Science B.V. All rights reserved.

Coherence collapse and low-frequency fluctuations in quantum-dash based lasers emitting at 1.57 μm

Optical feedback tolerance is experimentally investigated on a 600-mum-long quantum-dash based Fabry-Pérot laser emitting at 1.57mum. While quantum-dashes are structurally intermediate to quantum-wells and quantum-dots, the observed behaviour is distinctly like that of a quantum-well based laser but with greater stability. Coherence collapse and low-frequency fluctuation regimes are observed and are reported here. The onset of the coherence collapse regime is experimentally determined and is found to vary from -29 dB to -21 dB external feedback level when increasing the current from twice to nine times the threshold current.

Optical Feedback Tolerance of Quantum-Dot- and Quantum-Dash-Based Semiconductor Lasers Operating at 1.55

This paper reports on the tolerance of low-dimensional InAs/InP quantum-dash- and quantum-dot-based semiconductor lasers to optical feedback in the 1.55 mum window. For this purpose, the onset of coherence collapse (CC) is experimentally determined and systematically investigated as a function of different laser parameters, such as the injection current, differential gain, temperature, and photon lifetime. It is in particular found that for both material systems the onset of CC increases with the injection current in a similar way to bulk or quantum-well-based devices. Of most importance, we experimentally show that the differential gain plays a key role in the optical feedback tolerance. It is indeed shown to determine not only the range of the onset of CC but also the dependence of this threshold both on the temperature and laser cavity length. Increasing the operating temperature from 25degC to 85degC leads to a decrease of the onset of CC by a factor of only ~3 dB, well accounted for by the variation of the differential gain in this temperature range. We find no difference in the tolerance to external reflections of a truly 3-D confined quantum-dot-based laser and a quantum dash device of the same cavity length, which have similar differential gains. A tentative analysis of our data is finally carried out, based on existing models.

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Tolerance to optical feedback is investigated on quantum-dash-based lasers emitting at 1.51 mum. The onset of coherence collapse regime is experimentally determined using three criteria: optical spectrum broadening, relative intensity noise increase, and bit-error-rate degradation. Measurements were first performed in static operation at different current values, using the first and second criteria. The onset of coherence collapse was found to increase from ~-41 to -27 dB with the bias current. Then tolerance to optical feedback was assessed in dynamic operation at 10 Gb/s, using the third criterion. In spite of a relatively high linewidth enhancement factor , (alphaH ~4.5) a -32-dB onset of coherence collapse corresponding to -24-dB maximum optical return loss tolerance was achieved at 10-Gb/s rate.

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We describe the performance characteristics of a continuously tunable hybrid liquid-crystal filter over extended spectral ranges for implementation of multiwavelengths systems. This device is based on the linear electrooptic (Pockels) effect where tuning is achieved via externally applied voltage. A folded ten-stage S/spl caron/olc configuration is adopted which demonstrates a tuning range of 34 nm around 1550 nm. This opens the way to the achievement of various wavelength-division-multiplexing functions based on filtering [(de)multiplexers, add and drop devices, tunable emitters, etc.].

Tolerance to Optical Feedback of 10-Gb/s Quantum-Dash-Based Lasers Emitting at 1.51

The tolerance to optical feedback of InAs∕InP quantum dash-based lasers is reported for several structures exhibiting differing values of the linewidth enhancement factor and damping factor. An analysis of the onsets of coherence collapse is carried out based on the experimental dynamic parameters extracted for each structure. It is shown that the relevant significant parameter to explain the optical feedback tolerance for this low dimensional material system is the differential gain.

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This chapter investigates a preliminary interpretation of the experimental results recently obtained with InAs/InP quantum-dash Fabry-Perot lasers, by using the formalism developed from the so-called asymptotic method.

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A novel 1×2 wavelength routing switch is introduced and analyzed. It is based on a tunable hybrid liquid-crystal Solc filter and a polarization beamsplitter. The device can be set into the bar state for an arbitrary subset of WDM channels while remaining in the cross state for the rest of channels. This is achieved via a tuning voltage applied to liquid-crystal cells. Interchannel crosstalk and its reduction are discussed. Application of the switch includes coarse and dense WDM.

Low-voltage continuous tunable hybrid filter for tailored optical-bandwidth operation

In this paper, we present the feasibility of wavelength filtering by a hybrid liquid crystal Solc/Fabry-Perot filter device. The influence of the structure opto-geometric defects, such as the variation of the thickness, on the spectral response of the filter have been reported. Frequency and temporal characteristics are presented and discussed.

Systematic investigation of InAs∕InP quantum-dash based lasers under external optical feedback

In this paper, we report for the first time the feasibility of coding by coherence modulation in 2D optical correlator using a spatially and temporally incoherent light source. This technique allows to carry out simultaneously several correlation products. The reported results have been obtained with a broadband source, such as a white-light source.