R. D. Feldman

An evaluation of architectures incorporating wavelength division multiplexing for broad-band fiber access

Wavelength division multiplexing (WDM) offers the potential to transport very high bit rates in access systems serving residential and small business customers. We describe the options for WDM access systems, and evaluate them on the basis of an international standard that is emerging for passive optical networks (PONs) that use wavelength-independent power splitting. The greatest impediment to meeting the high-bandwidth potential of WDM for downstream (i.e., from the central office to the home) transmission is the need for a moderately priced multiple-wavelength light source, implementing WDM in the upstream (home to central office) direction presents greater hurdles than in the downstream. The technical aspects of overcoming these hurdles can all be met. The real challenge will be in finding technical solutions that can meet the very stringent cost requirements of access systems. Systems that use WDM in the downstream direction only, while using wavelength-independent power-combining in a PON configuration for upstream, may prove economical before full WDM systems. WDM may also prove valuable if upgrades are needed in the future for power-splitting PONs.

Photon assisted OMVPE growth of CdTe

Abstract We have investigated the use of ultraviolet light for lowering the temperature of Organometallic Vapor Phase Epitaxial (OMVPE) CdTe growth on GaAs. The effects of growth temperature and stoichiometry on the crystallinity are reported. In addition, we have observed what appears to be anomolously high quantum efficiency for this process, which suggests a more complex phenomena than simply direct photolysis.

Influence of Ga‐As‐Te interfacial phases on the orientation of epitaxial CdTe on GaAs

When CdTe is grown by molecular beam or organometallic vapor phase epitaxy on (100) GaAs, the layer can grow with either a (100) or (111) orientation. Reflection high‐energy electron diffraction and Auger studies are presented here which show that adsorption of different submonolayer amounts of Te on a GaAs surface can change the surface symmetry and the resulting CdTe orientation. A precursor surface to (111) growth results from the formation of a relatively Te‐poor Ga‐As‐Te surface phase. A relatively Te‐rich structure yields a surface with (100) symmetry and lead to (100) growth.

High sensitivity optical image processing device based on CdZnTe/ZnTe multiple quantum well structures

We present results on the operation of a high sensitivity semi‐insulating multiple quantum well device for optical image processing. This device operates in the visible spectrum using II‐VI CdZnTe/ZnTe multiple quantum well structures. Incident light creates charge carriers that screen an applied ac electric field modulating the absorption and refractive index of the structure through the quantum confined Stark effect. In this way, an incident intensity pattern is recorded as an absorption and refractive index variation. The semi‐insulating nature of the material eliminates the need for pixelation. In a wave‐mixing experiment, a peak diffraction efficiency of 0.25% was observed from 2.25 μm active layer of the device. Two‐beam‐coupling gain coefficients of ∼500 cm−1 at wavelengths longer than the exciton absorption peak should be possible.

Effects of Zn to Te ratio on the molecular‐beam epitaxial growth of ZnTe on GaAs

ZnTe films have been grown with Zn:Te flux ratios ranging from 1 to 3.2. The highest quality films have been grown with flux ratios between 2 and 3, substrate temperatures between 300 and 325 °C, and a surface reconstruction that is a combination of c(2×2) and (2×1). Films grown under these conditions have x‐ray rocking curve half‐widths between 125 and 225 arcsec. Photoluminescence spectra show that the relative intensity of emission related to Zn vacancies decreases with increasing Zn:Te ratio. Picosecond photoconductivity measurements show an initial decay rate for photoexcited carriers that correlates well with other material parameters. After several hundred picoseconds, the decay rates for different samples show exponential behavior with a lifetime of approximately 675 ps.

Growth of Cd1−xZnxTe by molecular beam epitaxy

Cd1−xZnxTe has been grown on GaAs substrates for compositions from x=0 to x=1. Binaries are shown to be of high quality, but x‐ray rocking curve half‐widths are extremely broad for most ternary compositions. Attempts to modify the interface yield only modest and uneven improvement in rocking curve half‐widths. The poor quality appears to be due to a phase separation.

Access PON using downstream 1550-nm WDM routing and upstream 1300-nm SCMA combining through a fiber-grating router

We describe an 8-channel access passive optical network using a fiber grating router at the remote node, 50 Mb/s 1550-nm WDM downstream transmission from an integrated multifrequency laser, and 3 Mb/s 1300 nm SCMA upstream transmission. This approach combines the advantages of a downstream WDM PON and an upstream low-cost power-combining PON.

Reduction of optical beat interference in a subcarrier multiple-access passive optical network through the use of an amplified light-emitting diode

We show that a transmitter consisting of a light-emitting diode (LED) followed by a semiconductor amplifier can be used to transmit data in a subcarrier multiple access system for fiber-in-the-loop. Optical beat interference penalties, which have been observed when conventional laser transmitters are used, were eliminated with the amplified LED. Penalty-free system operation is observed when a signal is transmitted with 17 dB less optical power than each of two interfering transmitters.

Room‐temperature optically pumped Cd0.25Zn0.75Te/ZnTe quantum well lasers grown on GaAs substrates

Optically pumped lasers emitting near 600 nm at room temperature have been fabricated for the first time in Cd0.25Zn0.75Te/ZnTe superlattices grown on GaAs substrates. The threshold pump intensity using pulsed 0.53 μm radiation increased from ∼7 kW/cm at 10 K to ∼60 kW/cm2 at room temperature with a threshold temperature dependence described by T0∼111 K.

Demonstration of a cost-effective, broadband passive optical network system

We demonstrate that a combination of coarse wavelength division multiplexing, time division multiplexing and subcarrier multiple access can be used to provide broadband service over a 16-way split, single-fiber passive optical network. We provide an aggregate bit rate of 622 Mb/s outbound and 48 Mb/s return in our system, with power margins of at least 9.4 dB in excess of specifications.<<ETX>>