N. R. Das

Study of Component Crosstalk and Obtaining Optimum Detection Threshold for Minimum Bit-Error-Rate in a WDM Receiver

In this paper, bit error rate and power penalty in a WDM receiver system in the presence of component crosstalk with finite interferers are studied using a simplified analysis and some optimum detection thresholds are suggested for minimum bit error rate. Error probabilities are calculated considering a unipolar bit stream of optical signals at the receiver input. Probability density function in the presence of crosstalk deviates from conventional Gaussian function. Effects of receiver noise, number of interfering channels and crosstalk levels on the receiver performance are shown. Optimum detection thresholds for minimum bit error rates in the WDM receiver in presence of component crosstalk are investigated and summarized in tabular form.

Calculating the responsivity of a resonant-cavity-enhanced Si1−xGex/Si multiple quantum well photodetector

In this paper, the responsivity of a resonant-cavity-enhanced SiGe/Si multiple quantum well photodetector has been theoretically investigated. The present model considers the effects of material parameters of Si1−xGex and recombination of carriers confined by the potential barriers at the heterointerfaces of Si1−xGex/Si quantum wells on the detector responsivity for different Ge-contents (x). The effect of electric field due to applied bias on the emission of confined carriers from quantized subbands has also been included in the analysis. Results show that though high Ge-content is required to enhance the responsivity of the SiGe/Si photodetector for long-haul communication (at 1.3 and 1.55 μm), the confinement of holes at the heterointerfaces may be significant to affect the responsivity adversely. Studies at 1.3 and 1.55 μm also show that the responsivity of the photodetector for a particular Ge-content in the active layer can be increased by suitable choice of well parameters and applied bias.

Tunneling current calculations for nonuniform and asymmetric multiple quantum well structures

In this paper, we present our studies on current-voltage characteristics due to tunneling in nonuniform and asymmetric multiple quantum well (MQW) structures. First, the transmission coefficient is calculated by solving the Schrodinger equation with the piecewise-constant potential approximation and by considering the effects of nonuniformity and the asymmetry of layer dimensions and band-offsets. Then the tunneling current through the structure is calculated as a function of bias for different structural combinations of the MQW structure. The configurations suitable for some applications are indicated in the results.

Effect of Ge-composition on the frequency response of a Si/Si 1-y Ge y P-i-N photodetector

The effect of Ge-composition on the transit-time limited frequency response of a vertical Si/Si1?yGey P-i-N photodetector has been investigated. The change in Ge-content (y) causes the changes in properties of the SiGe layer and the Si/SiGe interfaces and, hence, affects the transit time of carriers in the Si/SiGe photodetector. The results obtained from the analysis show that at low bias, the bandwidth of the photodetector initially increases with increase in Ge-content, but after an optimum value of Ge-content, the bandwidth starts decreasing. This optimum value increases with increase in applied bias.

A finite-difference technique for computation of electron states in core–shell quantum wires of different configurations

In this paper, electron energies in core–shell quantum wires (CSQW) of rectangular, triangular, T-shaped, H-shaped and circular geometries are numerically computed by solving a time-independent Schrodinger equation using the finite difference technique. Computation is performed for both normal and inverted structures of CSQW, taking into account Kane-type nonparabolicity, conduction band discontinuity, and effective mass mismatch at the hetero-interface. Sparse, structured Hamiltonian matrices are produced for the calculation of energy eigenvalues and intersubband transition energies. Comparative study reveals that for a given core width of normal CSQW, the eigenenergy is the highest for the triangular geometry and the lowest for the rectangular geometry. For inverted CSQW, the ground-state energy of the triangular wire decreases with increasing core width, unlike other geometries. Studies on the intersubband transition energy show that for triangular and H-shaped inverted CSQW, it varies in a direction opposite to that of other inverted structures. Suitable tailoring of wire dimensions indicates the possibility of tuning the transition energy for photonic applications.

Gain and bandwidth analysis and comparison for gallium arsenide and silicon avalanche photodiodes with very thin multiplication layer

Abstract. We describe the calculation of gain and bandwidth of an n+-i-p+ avalanche photodiode (APD) for a multiplication layer down to tens of nanometers. The computed results are used to make a comparative study of gallium arsenide (GaAs) and silicon (Si) APDs. In the analysis, the depletion region is discretized into equal energy segments to take into account the discontinuous nature of impact ionization in the multiplication layer due to dead-space effect. Also, the carrier diffusion from undepleted regions is considered to study the effect of low bias on the frequency response. Carrier distribution within the structure is obtained by a numerical solution of coupled equations and recurrence relations. The model is verified with some experimental data taken from literature. Results show that gain increases with bias more rapidly for thinner multiplication layer. The Si APD is thinner than GaAs APD for the same gain at a given bias. Diffusion causes significant reduction of bandwidth at a low gain, with the change being sharper for GaAs APD than for Si APD.

Probability of Power Depletion in SRS Cross-Talk and Optimum Detection Threshold for Minimum BER in a WDM Receiver

In this paper, the probability of power depletion due to stimulated Raman scattering (SRS) is derived for an arbitrary number of interferers to estimate bit error rate and power penalty of a wavelength division multiplexed (WDM) receiver. Gaussian function approximation can be used only when the numbers of interferers is very large. The effects of number of interfering channels, channel separation and receiver noise on the bit error rate (BER) are studied. Minimum BER varies non-monotonically with channel separation. Optimum detection thresholds for minimum BER in the WDM receiver in presence of SRS crosstalk are investigated and summarized in tabular form.

Investigating the effects of hetero-interface trapping on the performance of a Ge-based RCE Schottky photodiode at 1.55 µm

In this paper, the performance of a Ge-on-Si resonant-cavity-encapsulated (RCE) Schottky photodetector at 1.55 µm has been investigated considering the effects of trapping of carriers at the Si/Ge hetero-interface. The transit-time delay and loss of carriers by recombination have been included in the model to calculate the photocurrent, bandwidth, quantum efficiency and bandwidth–quantum-efficiency product of the photodiode. Results show that the effect of trapping plays a significant role in the bandwidth and quantum efficiency of the photodetector at low bias. Possible optimum designs of the photodetector at different biases have been suggested in this paper.

On the Frequency Response and Optimum Designs for Maximum Bandwidth of a Lateral Silicon Photodetector

In this paper, the photocurrent in a CMOS compatible Si photodetector is calculated considering the effect of diffusion of photogenerated carriers from the substrate region. The parasitic effects due to the multiple diodes in lateral configuration have also been considered in the analysis. Normalized frequency response is computed for a square-area photodetector. The results on the 3 dB bandwidth are shown as a function of number of diodes, finger width, finger spacing, and the photodetector length. It is shown that some optimum choices of the device parameters exist to obtain the bandwidth maxima of the lateral Si photodetector.

A comparative study on determination of optimum detection threshold for minimum BER in a WDM receiver with component crosstalk

Component crosstalk causes severe system performance degradation in an optical network. In this paper the influence of the component crosstalk originated from signal-crosstalk beat noise, on the performance of a preamplified WDM receiver is investigated assuming both Gaussian and non-Gaussian model. From the analysis it is evident that for the finite interfering channels the results obtained using non-Gaussian model varies significantly from Gaussian model in terms of BER and the optimum detection threshold for obtaining minimum BER, but as the number of interfering channels approaches large value the variations are minimized. Effects on BER and optimum detection thresholds for minimum bit error rates are compared, summarized and analyzed in both graphical and tabular form for the two different model.