A micro resonator quantum well intensity modulator for operation in the wavelength band around 1μm is described.
High efficiency 90° bends are used to form the resonator and also provide optimal coupling to the external
waveguide. The benefits are to reduce loss, to relax the lithography requirements and to provide more flexible
contact designs to the modulator. The characteristics of modulator are analyzed using optical simulation tools and
based on measured absorption parameters. The modulator operates with two distinctly different electrode
configurations which are both based on the index change calculated using Kramers-Kronig relations. A model
including parasitic is developed for HSPICE transient simulations and run in the AGILENT ADS environment. The
performance parameters are determined to be an extinction ratio of 10.4dB, a bandwidth of 33GHz, and a dc power
less than 1mW for device dimensions of 16×6μm2.
An optical switching fabric based on resonant optoelectronic switches is presented. The elements are comprised
of two optoelectronic thyristors which have their own switching characteristic. Simultaneous switching of 40Gbps
baseband data along with mm-wave optical signals is shown in a 4x4 fabric. The possible unique capabilities and
performance advantages of implementation with thyristor-based switches in the fabric are discussed.
A technology for monolithic device integration of lasers and transistors is described. It is based on a GaAs double
modulation-doped epitaxial structure creating both n type and p type conduction channels. In the epitaxy, there are total
four levels of contacts including a top level p contact, a bottom level n contact, and two intermediate level channel
contacts. The device operation is determined by the contact utilization. The laser operation is achieved within a vertical
cavity structure. In laser fabrication, ion implantation is used instead of an oxidized AlAs layer to steer the injection
current. The main advantage of ion implantation is accuracy in control of the aperture dimension. For a 12μm diameter
VCSEL, the threshold is about 1.7mA. Complementary HFETs are obtained using the n and p channels for the n-HFET
and p-HFET respectively. The top p layer and the p-channel are used as n-HFET gate contact and collector (back-gate )
contact respectively. The bottom n layer and the n-channel are used as p-HFET gate contact and collector contact
respectively. Complementary HFET operation is demonstrated with balanced threshold voltages of about 0.5V and -0.5V
for n-HFETs and p-HFETs, respectively. For 1μm gate length n-HFETs, gm~170ms/mm at Vg=1.2V and Vds=4V, which
is similar to that of comparable HEMTs. For 1μm gate length p-HFETs, gm~6.5ms/mm at Vg=1.1V and Vds=4V. With
better lithography and shorter gate features, higher gm can be expected. These first results indicate that optoelectronic
device performance has not been sacrificed by the monolithic integration.
Intersubband absorption is reported in a new modulation doped structure using strained InGaAs quantum wells
(QWs) that support transistor operation. Well defined absorption peaks (1000 cm-1 to 1700 cm-1) from 8μm to 11.5μm
have been obtained using either n- or p- type modulation doped wells. The absorption wavelength may be extended to as
low as 3.2μm by using quantum dots placed within the quantum well. Both n and p well responses show strong
polarization dependence with maximum values at incident angles of 65-70° and peak positions which are adjusted by the
quantum well parameters. The p well shows a double peaked response with a peak separation of about 1.5μm which
results from heavy and light hole contributions. A thyristor infrared detector model has been established based upon the
intersubband absorption mechanism and simulation results are shown.
A novel transistor based quantum well modulator structure is presented and analyzed for applications in RF photonic
links. The modulator has been realized in the GaAs epitaxial system using both GaAs/AlGaAs and InGaAs/AlGaAs
modulation-doped quantum wells. The modulator operates on the principle of charge filling of a quantum well to shift
the absorption edge to shorter wavelengths (blue shift). A generalized absorption model is presented for the modulator in
which the relaxed k-selection rule and Lorentzian weighting function are used to represent the absorption coefficient in
terms of the carrier Fermi energy. Then the blue shift of the absorption edge is determined by the channel charge density
in which the Fermi level is controlled by the applied gate-to-source voltage. From this charge control model the
transmission of the modulator is determined to be an increasing function of gate-to-source voltage. Absorption spectra
and relative transmission curve for both devices are then calculated and validated by comparison to measurement data. It
is found that the enhancement interface offers better performance. It is also found that deionization of charge sheet sets
the upper limits on input optical power. The analytical T(V) response enables full distortion analysis. Thus RF link
performance is studied based on calculation results and SFDR of 120 dB·Hz2/3 and 127 dB·Hz2/3 are predicted for the two
modulators respectively.
Aircraft can be equipped with a number of radios which need to be operated simultaneously and in the full-duplex mode. To prevent interference required a combination of antenna separation, frequency separation, transmitter power control, and specific design considerations. Interference cancellation systems often use a common antenna, a common Low Noise Amplifier, and signal splitting to each receiver. Onboard transmitter signal would be sampled, delayed, adjusted in amplitude equal to the unwanted received signal (the transmitted signal), adjusted to 180 degrees out of phase with the unwanted received signal; and injected prior to the LNA, thus canceling the unwanted transmitter signal. The time delay can be accomplished by using coaxial cable or by digitizing the transmit signal spectrum. An optical alternative would convert the RF spectrum to the optical domain, use fiber or polymeric waveguide for time delay, and then convert back to the RF domain for injection into the receiver system to accomplish the cancellation. The optical system would have to process the RF signal(s) without creating distortion, and provide sufficient flexibility to allow the system to be readily adjusted to optimize performance. The optical system, installed at the receiver, would consist of a transmitter, a programmable delay line and a receiver. An attractive implementation would be a single chip to incorporate the optical generation and switching functions to redirect the optical signal into a selected optical path. The optical paths might be polymer waveguides patterned onto a PCB. With newly developed optoelectronic integration technologies, the combination of sources, detectors, waveguide switches and amplifiers becomes a realistic possibility. The result is a rugged system with low cost and high performance. This paper describes these optical technologies and the optical interference cancellation implementation approach.
This paper describes a novel laser communications transceiver for use in multi-platform satellite networks or clusters that provides internal pointing and tracking technique allowing static mounting of the transceiver subsystems and minimal use of mechanical stabilization techniques. This eliminates the need for the large, power hungry, mechanical gimbals that are required for laser cross-link pointing, acquisition and tracking. The miniature transceiver is designed for pointing accuracies required for satellite cross-link distances of between 500 meters to 5000 meters. Specifically, the designs are targeting Air Force Research Lab's TechSat21 Program, although alternative transceiver configurations can provide for much greater link distances and other satellite systems. The receiver and transmitter are connected via fiber optic cabling from a separate electronics subsystem containing the optoelectronics PCBs, thereby eliminating active optoelectronic elements from the transceiver's mechanical housing. The internal acquisition and tracking capability is provided by an advanced micro-electro-mechanical system (MEMS) and an optical design that provides a specific field-of-view based on the satellite cluster's interface specifications. The acquisition & tracking control electronics will utilize conventional closed loop tracking techniques. The link optical power budget and optoelectronics designs allow use of transmitter sources with output powers of near 100 mW. The transceiver will provide data rates of up to 2.5 Gbps and operate at either 1310 nm or 1550 nm. In addition to space-based satellite to satellite cross-links, we are planning to develop a broad range of applications including air to air communications between highly mobile airborne platforms and terrestrial fixed point to point communications.
A new approach to sensing with intersubband absorption is introduced. In contrast to the conventional Quantum Well Infrared Photodetector (QWIP) which is a multi-quantum well device, our structure has 1 - 3 wells and uses resonant enhancement to achieve nearly complete absorption. In the QWIP the dark current is limited by the quantum well barrier in the range of 0.125 ev and thus cryogenic cooling is required in general to achieve BLIP operation. In the new structure, the dark current is limited by the band gap of GaAs/AlGaAs layers (>= 1.4 eV). This difference implies that BLIP operation may be possible near room temperature. The detecting quantum well is used to form the storage well of an active pixel or a CCD device and the intersubband absorption mechanism removes charge from the quantum well starting from the full well condition. The state of depletion of the well is then clocked to the output amplifier as in a conventional CCD using noise reduction techniques such as correlated double sampling. Therefore, the hybrid bump bonding of the Si ROIC is no longer required. In this paper, we describe the concept and its advantage vis-a-vis the existing approach and a preliminary analysis of the sensitivity of the detection.
The Heterostructure Field Effect Optical Modulator (HFEOM) is a waveguide modulator that operates via band filling of quantum wells using charge transfer from an adjacent n+ charge sheet. The control of this charge transfer is with a gate electrode as in a field effect transistor. The band filling of the quantum wells produces a blue-shift of the absorption edge that is used to modulate the incident light. This device is compatible in both growth and processing with the associated in- plane laser and field effect transistor. The initial high speed results of HFEOMs in the InGaAs/GaAs material system are presented using a double quantum well active region. This structure has demonstrated a 35:1 extinction ratio for a 2 volt swing (-1 V to +1 V) on a 300 micrometers long device along with excellent wavelength compatibility with a 400 micrometers in-plane laser fabricated from the same wafer. Capacitance limited modulation bandwidths of 1.2 GHz and 1.6 GHz are measured for 5 micrometers and 2 micrometers rib widths respectively.
In this paper we discuss the effects of incorporating carbon doping in semiconductor lasers. Data is presented that demonstrates that very high quality carbon doped epilayers for the fabrication of AlGaAs-GaAs and AlGaAs-GaAs-InGaAs quantum well lasers can be grown by solid source molecular beam epitaxy using a resistively heated graphite filament as a p-type dopant source. Also results are presented that indicate that the use of carbon instead of beryllium improves the contact resistance for refractory ohmic contacts.
The inversion channel technology is a new approach to monolithic optoelectronic integration that offers the possibility of FET logic, optical detection, and laser emission from a single chip. The detection is performed by the three terminal configuration of the DOES biased in the off state. Incident light switches the DOES into the on state and recovery from the on state is provided by the conduction of electrons from the inversion channel through a FET connected to the third terminal. In this paper we demonstrate the functionality of this operation with an OEIC that integrates the three terminal DOES device with four FETs. The operation is discussed both as an optical clock and as an electrically clocked optical gate. Sensitivity issues are considered.
The operation of the inversion-channel Resonant-Cavity Enhanced (RCE) photodetector is demonstrated in a configuration compatible with the Vertical Cavity Surface Emitting Laser (VCSEL). The phototransistor used 3 strained InGaAs/GaAs quantum well's as the absorbing region and a post-growth dielectric top stack. A quantum efficiency of 41% was obtained at a resonant wavelength of 0.94 micrometers , thereby giving a resonant enhancement factor of 13.5. A bipolar transistor gain of 6.8 at a current density of 10 A/cm2 allowed the phototransistor responsivity to reach 2.1 A/W at the resonant wavelength. We also demonstrate the movement of the resonant peak through the use of Focussed Ion-Beam (FIB) etching which has potential applications in Wavelength Division Multiplexed (WDM) systems.
A new model for the small signal intensity modulation response is described which utilizes a new analytic description of the quantum well laser based on the concept of a stimulated lifetime. The new analysis is able to qualitatively predict the modulation response of quantum well lasers without the concept of non-linear gain. The analysis uses the quasi-Fermi level separation as a parametric variable to couple the electron and photon rate equations. In particular, the Fermi level allows the effects of the diode losses to be an integral part of the small signal transfer function. It is shown that by including all the current components in the electron rate equation, the modulation response of the laser is fundamentally limited by the diode carrier lifetime. This is a consequence of the less than unity value of the electrical confinement factor within the quantum well. The recombination and diffusion current components outside the well are shown to be the cause of the increased damping of the laser resonance as the bias applied to the laser is increased.
A quantum well resonant cavity structure is modelled using energy balance considerations by computing the Poynting vectors across the absorbing region. The quantum efficiency (eta) is shown to yield terms that depend on the Q of the cavity, the position of the quantum wells in the structure, and the relative reflectivities of the two stacks at either end of the cavity. The bandwidth B.W. of the detector on the other hand depends only on the cavity Q. Thus an optimum (eta) X B.W. product can be found. Experimental results corroborate the model.
A new approach to optoelectronic integration is presented in which all optical and electronic devices are derived from a single crystal growth and a single fabrication sequence. The approach uses a self-aligned inversion channel capable of functioning as an FET or bipolar transistor, a detector, a modulator or a laser in either an analog or a digital mode. Topics discussed include a three-terminal switching laser, a bipolar inversion channel field-effect transistor, a three-terminal analogue laser, an HFET detector, and an HFET optical modulator.
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