We present a new distributed time domain model (DTDM) using Maxwell's wave equations with a time dependent polarization in the form of classical electron oscillators (CEO)s with randomly excited spontaneous emission using a virtual field. The model is based upon the neoclassical rate equations of A.E. Siegman and includes effects such as chromatic dispersion, line-width enhancement, gain suppression, optically induced gratings, and excess noise. Although our equations were independently derived we have found that they do resemble the Maxwell-Bloch equations. However, most authors appear to favor the Ginzburg-Landau equations for their DTDM models. We demonstrate that the model can reproduce results comparable with those of others, as well as new results.
In this paper we demonstrate a prototype pump-probe system comprising of a pair of gain-switched semiconductor lasers and present results of a study of the influence of the dc and rf components on pulse shape and position. We demonstrate the system with and without a pulse compression stage consisting of a length of dispersion compensating fiber (DCF).
We have studied the potential profile of gateless MESFET devices using electro-optic probing. We have used smooth samples as previous work has shown how rough devices produce excessive noise generated from the Fabry-Perot effect. The profiles measured show non-linear behavior at low fields but high duty cycles. These non-linearities were more noticeable at the edges of the devices and we believe they are associated with device heating which would be prominent at the edges due to 'edge effect.' To remove this effect we have used very low duty cycles and the resulting potential profiles are as expected. Using low duty cycles and applying high electric fields allows us to study non-linear transport behavior in these devices. The samples were designed to exhibit non-linear behavior due to the Gunn Effect. At high applied electric fields the current saturates and becomes noisy, indicative of non-linear behavior. We show the first reported device field profiles under these conditions measured using electro-optic probing. The observed non-linear behavior can be explained in terms of the Gunn Effect.
Further investigations on a hot electron barrier light emitter (HEBLE), which has a potential for use in the area of wavelength domain multiplexing (WDM), have been undertaken. These investigations follow the works, which were presented over the last two years in Photonics West 98 and 99. The structure of the device is based on Al<SUB>x</SUB>Ga<SUB>1-x</SUB>As-GaAs system with a single quantum well of GaAs. The novelty of the device is how it is operated. Unlike a normal light emitter, HEBLE has a barrier between the n-doped region and the quantum well, which prevents the electrons flooding into the quantum well, hence no light output, when it is forward biased. To obtain the light output, not only the device must be forward biased, but also the n-doped region must be electrically heated, so the electrons will have enough energy to overcome the barrier and move into the quantum well. The devices have been fabricated following the results of the computer simulation, which was presented last year in Photonics West 99. The results of these devices, which have been studied in many aspects including spectral analysis of the light output at different temperatures, will be presented.
In this paper we outline work we have recently completed on the novel high speed photodetector which has come to be known as the `back-gated metal-semiconductor-metal' photodetector or BG-MSM for short. This device was first realized by E. Greger et. al. and us. Recent theoretical work by Hurd et. al. provided a sound basis by which to test the device against a model. The work presented here clearly indicates that the main conclusion of Hurd et. al. is sound. That conclusion was that the main response speed gain obtained by the back-gating is obtained by simply earthing the back contact. As well as this confirmation we have found strong dependence of the peak height of the response on the back gating voltage and a very strong position dependence of the excitation spot on the response particularly at low applied fields. We conclude that the back gating is an advantage over the conventional metal-semiconductor-metal photodetector and needs to be studied more extensively both theoretically and experimentally using devices capable of operation at above 100 GHz.
We present a structure which is capable of being fabricated into two distinct devices, both with considerable potential in the field of optical communications in particular with reference to wavelength domain multiplexing. The structure is based on two back to back p-i-n Ga<SUB>x</SUB>Al<SUB>1-x</SUB>As structures with a single quantum well of GaAs in each intrinsic region. The light emitter device operates by forward biasing either of the p-i-n elements. In forward bias holes flood into the quantum well in the intrinsic region. Electrons are prevented from doing so by a potential barrier. A longitudinal electric field applied along the central n-doped region heats the electrons in this region and gives them sufficient energy to overcome the barrier and flood into the quantum well and hence recombine with holes which are already present. The wavelength converter device operates with one p-i-n structure forward biased and one reverse biased. The forward biased element has a quantum well positioned near the p-doped region. Light of the appropriate wavelength is absorbed in this quantum well. The holes scatter out of the quantum well and drift into the p- doped region. The electrons are scattered out of the quantum well and drift towards the n-doped region, creating additional carriers through impact ionization, thereby creating gain. The electrons flooding over the n-doped region, must overcome a potential barrier to enter the forward biased element, therefore cold electrons are prevented from entering this region. Electrons which are able to overcome the barrier fall into a quantum well positioned near the barrier, where holes are already waiting, as in the light emitting device.
We used electro-optic voltage probing to map the potential distributions along a degenerate single quantum well that exhibited current oscillations when subjected to high fields at low temperatures. The results given here were obtained over a wide range of field strengths at room temperature. The potential profiles were found to contain some very unusual features which might be indicative of the poor quality of the barrier material in this sample.