A RF spectrum analyzer with high performance and unique capabilities that traditional all-electronic spectrum analyzers
do not exhibit is demonstrated. The system is based on photonic signal processing techniques that have enabled us to
demonstrate the spectral analysis of a 1.5 GHz bandwidth with a 1.4 ms update time and a resolution bandwidth of 31
kHz. We observed a 100% probability of intercept for all signals, including short pulses, during the measurement
window. The spectrum analyzer operated over the 0.5 to 2.0 GHz range and exhibited a spur-free dynamic range of 42
dB. The potential applications of such a system are extensive and include: detection and location of transient electromagnetic
signals, spectrum monitoring for adaptive communications such as spectrum-sensing cognitive radio, and
battlefield spectrum management.
Experimental and analytical results for semiconductor electro-refractive modulators will be presented. Modulation structures investigated include quantum wells, coupled quantum wells and quantum dots.
This paper describes a concept to generate coherent THz radiation in a semiconductor diode device using phonon generation via high-mobility electrons in semiconductor quantum well heterostructures. The theoretical basis for pumping both acoustic and optical phonons by high-mobility, two-dimensionally confined electrons has been established over the past decade. The electrons drift parallel to the quantum well heterojunction, and because their drift velocity
exceeds either the local velocity of sound or the phase velocity of optical phonons in the crystal, energy is transferred from the electrons to the phonons (Cherenkov radiation). Strong confinement of both electrons and optical phonons in the quantum well leads to highly efficient energy transfer from high mobility electrons to coherent phonon waves. Plasmon oscillations created by coherent phonons in a polar material (such as GaAs or InP) create propagating THz electromagnetic fields. This process is analogous to the physical process that is the basis of a laser: multi-level pumping,
stimulated emission, and a selection of one mode at the expense of the other modes. This paper describes a design approach to design structures that will produce required electron velocities and bias fields, for phonon generation through electrical pumping. This paper will also discuss the applicability of incorporating acoustic mirrors for a high finesse phonon cavity, and approaches for outcoupling the THz radiation.
Preliminary analysis has shown that quantum dots enable tens of millivolt-range operation of phase-shifters in a semiconductor Mach-Zehnder interferometer modulator. Our methodology based upon the quantum dot experimental work of Hse et al, makes use of his measured exciton line shapes to estimate refractive index changes in a PIN structure in which the intrinsic laser is loaded with self-organizing quantum dots and their associated wetting layers. We consider both forward and reversed bias cases; in the former, the interferometer phase shift sections become DFB lasers, and in the latter, the phase shift is caused by the quantum-confined Stark effect (QCSE). With the latter, we found a trade-off between low operating voltage and modulating bandwidth. For a phase shifter insertion loss of 5 dB, a 250-micron long phase section will yield a pi/2 control voltage of 50 mV at a bandwidth of around 18 GHz. Ifi 90 mV control voltage swing can be tolerated, the modulator bandwidth increases to 30 GHz. If a resonant tunneling diode (RTD) is made part of the assembly, the local E-field is enhanced by a factor of 5 to 10, thereby reducing the drive requirements even further. Similar, though narrower bandwidth results were noted for the DFB laser phase modulator concept.
Semiconductor based optical modulators offer flexibility in providing engineerable optical transfer characteristics that can target specific applications. Use of quantum well active regions provides the capability of efficient and linearized transfer characteristics that can benefit analog RF systems in terms of link gain, noise figure and spur free dynamic range. We present experimental results demonstrating the potential for improvements in modulator linearity and efficiency using quantum well based Mach-Zehnder modulators.
Efficient, linear, broadband optical modulators are an essential component in wide bandwidth optical links for high fidelity distribution and processing of wideband signals. We discuss the packaging issues and remedies for the packaging of electroabsorption modulators for greater than 20 GHz link applications. The EAM is a waveguide semiconductor quantum well device which we have demonstrated to have a excellent linearity and electrical to optical conversion efficiency. Several important issues require resolution in order to produce a high bandwidth, packaged product. These include temperature control, EAM back-side metallization, attachment of EAM to submount, Product hermeticity, alignment and attachment of input and output fibers to the device at sub- micron tolerances, retaining this alignment over operational temperatures and anticipated vibration environments, and elimination of outgassing materials that may serve as failure initiators to optical facets. This paper addresses these issues.
Semiconductor optical amplifiers are investigated for use in large optical signal distributions systems requiring high dynamic range. The impact of amplifier length on the gain and noise figure of the microwave signal is illustrated experimentally. The performance of a device which simultaneously splits and amplifies the optical signal using the principle of multimode interference will be discussed, and it will be shown that this device has potentially higher performance that the previous generation Y-branch/amplifier combination.
2D monolithic grating spectrometers for dense wavelength division multiplexing (WDM) show considerable promise to extend the usable bandwidth of optical fibers. Their performance is fundamentally dictated by the grating which is used. First order gratings will theoretically improve the performance of monolithic WDM devices, since WDM devices based on first order gratings do not suffer from an inherent tradeoff between efficiency and broadband operation.
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