An 18-channel multifrequency laser

A multifrequency laser (MFL) is demonstrated that runs simultaneously CW on 18 channels spaced by 103 GHz. The laser emits -14.6-dBm power per wavelength channel into single-mode fiber. Each wavelength channel can be modulated at 1.24 Gb/s. The MFL exhibits a stable and reproducible optical channel spacing owing to the reproducibility of the waveguide grating router that serves as the intracavity filter element.     

A multifrequency waveguide grating laser by selective area epitaxy

We demonstrate a 6 frequency digitally tunable laser based on a waveguide grating router fabricated using selective area epitaxy (SAE). The laser wavelengths span from 1.527 to 1.557 μm with 6.4 nm spacings. The bandgap variation in the active layer plane is from 1.352 μm for waveguide ribs to 1.605 μm for the amplifiers. ASE noise is suppressed by 35 db. These results demonstrate the flexibility of SAE technology for building complete photonic circuits     

Wavelength conversion in a 1550-nm multifrequency laser

Wavelength conversion of input signals at data rates of 622 and to 1250 Mb/s was as demonstrated using an integrated multifrequency laser having 8 channels with 1.6-nm channel spacing.     

Accurate simulation of multifrequency semiconductor laser dynamicsunder gigabits-per-second modulation

A multifrequency laser diode model which contains Langevin operators is used, in conjunction with models for the pattern generator and optical detector, to illustrate how the averaged response and stochastic response of the laser diode to gigabits-per-second modulation can be simulated accurately. Large-signal, stochastic noise models which retain both the magnitude and phase information are described. This allows the simulated results to be displayed in the form of eye diagrams. Experimental and simulated results for the laser's response to 1.5 Gb/s modulation, over the bias range from 4 mA above threshold to 8 mA below threshold, are presented. Good quantitative agreement was obtained between theory and experiment for all major characteristics of the laser's response over the full bias range investigated. No adjustments to the original set of extracted model parameters were necessary in order to maintain good correlation for the different operating conditions     

Chirped waveguide grating router multifrequency laser with absolute wavelength control

We apply chirping of the waveguide grating router to the multifrequency laser (MFL). We present a 10/spl times/100 GHz chirped MFL that has precise absolute oscillation wavelength control and guaranteed single router free-spectral-range mode oscillation for all 10 channels and, additionally, guaranteed single cavity mode oscillation for 4 of the channels.     

Low threshold 3 μm interband cascade “W” laser

We have demonstrated the operation at λ ≈ 3.0 μm of a 22-stage interband cascade laser with a “W” active region for enhanced gain. The threshold current density for a ridge structure is 170 A/cm² at 80K, and it remains lower than the best reported intersubband quantum cascade laser results at all T up to the maximum lasing temperature of 225K. At T = 100K, peak output powers up to 532 mW are observed, and the slope of 342 m W/A per facet for high injection levels corresponds to a differential quantum efficiency of 1.6 photons emitted for every injected electron.     

A monolithic dual tone multifrequency receiver

A fully integrated DTMF receiver has been fabricated on a single CMOS chip requiring only three external passive components. The circuit combines precision filters, zero crossing detectors and amplitude detectors with digital logic. Switched capacitor techniques are used to implement high-pass, bandpass, and bandstop filters. The receiver architecture and circuit implementation are described.     

Polarization scrambling using a short piece of high-birefringenceoptical fiber and a multifrequency laser diode

Describes a technique for avoiding system performance degradation and outages in an optical fiber system due to state of polarization mismatch between signal and polarization-sensitive system components. The technique uses a short piece of high-birefringence fiber and a multifrequency laser to create a depolarized source that can be used to transmit data without fading     

Chirping of the waveguide grating router for free-spectral-range mode selection in the multifrequency laser

Because the transmission through each input-output combination of a conventional waveguide grating router consists of a periodic sequence of equal-height passbands spaced by the router free spectral range, the absolute laser oscillation wavelength and single-passband oscillation can be difficult to control for all the channels in the multifrequency waveguide grating router laser. However, the router can be designed to have one dominant passband, with the neighboring passbands experiencing lower transmission coefficients for all the input output combinations by means of chirping of the waveguide grating. Such chirping is discussed theoretically and is demonstrated experimentally.     

A multifrequency gyrotron for plasma heating and diagnostics

Gyrotrons are regarded as a necessary component of any modern fusion machine. They are presently used primarily to heat the electron component of the plasma. However, gyrotrons can be utilized also to drive the current, to stabilize MHD modes, to measure the ion temperature by means of the collective Thomson scattering, to mention just a view plasma diagnostic purposes. In this work we present a design of a multifrequency gyrotron, i.e., a gyrotron which generates RF power simultaneously at several frequencies and discuss possible applications of such tubes.     

Sb-heterostructure interband backward diodes

Backward diodes are a version of Esaki tunnel diodes that are useful for mixing and detection. Ge backward diodes in particular have been used as temperature insensitive, zero bias square law detectors, capable of translating low level RF power into DC voltage or current with extreme linearity and low noise. However, Ge diodes are difficult to reproducibly manufacture and are physically fragile. Here we demonstrate specially designed Sb-heterostructure-based backward diodes grown by molecular beam epitaxy. These diodes have superior figures of merit compared to Ge diodes, especially the current density and junction resistance, and are reproducible and physically rugged. In addition, the flexibility of MBE growth allows easy tailoring of the layer structure to maximize the desired figure of merit for a given application     

Resonant interband tunneling FET

A lateral three-terminal Resonant Interband Tunneling Field Effect Transistor (RITFET) has been fabricated. It consists of a resonant interband tunnel diode (RITD) built using the GaSb-AlSb-InAs material system and a pseudomorphic InGaAs channel FET in which the current is controlled by a Schottky gate. The three terminal device has current-voltage characteristics that exhibit negative differential conductance with a peak-to-valley current ratio of 8 at room temperature     

A novel feed for a multifrequency hybrid resonator antenna

A novel feed technique for a hybrid resonator antenna is proposed and developed. The antenna consists of microstrip-patch radiator and a pair of twin resonator slots. The proposed feed mechanism is based on a new coupling aperture technique, in which a single microstrip feed line is etched on the same side of the substrate as the radiating patches. The patches are coupled to the feed line through a twin slots located in the ground plane. By selecting suitable structure parameters for the slots and the patches, multifrequency operation is obtained. The proposed antenna structure is suitable for the development of large antenna arrays.     

Context-based lossless interband compression-extending CALIC

This paper proposes an interband version of CALIC (context-based, adaptive, lossless image codec) which represents one of the best performing, practical and general purpose lossless image coding techniques known today. Interband coding techniques are needed for effective compression of multispectral images like color images and remotely sensed images. It is demonstrated that CALIC's techniques of context modeling of DPCM errors lend themselves easily to modeling of higher-order interband correlations that cannot be exploited by simple interband linear predictors alone. The proposed interband CALIC exploits both interband and intraband statistical redundancies, and obtains significant compression gains over its intrahand counterpart. On some types of multispectral images, interband CALIC can lead to a reduction in bit rate of more than 20% as compared to intraband CALIC. Interband CALIC only incurs a modest increase in computational cost as compared to intraband CALIC.     

A multichannel continuously selectable multifrequency electrical impedance spectroscopy measurement system

There is increasing evidence that alterations in the electrical property spectrum of tissues below 10 MHz is diagnostic for tissue pathology and/or pathophysiology. Yet, the complexity associated with constructing a high-fidelity multichannel, multifrequency data acquisition instrument has limited widespread development of spectroscopic electrical impedance imaging concepts. To contribute to the relatively sparse experience with multichannel spectroscopy systems this paper reports on the design, realization and evaluation of a prototype 32-channel instrument. The salient features of the system include a continuously selectable driving frequency up to 1 MHz, either voltage or current source modes of operation and simultaneous measurement of both voltage and current on each channel in either of these driving configurations. Comparisons of performance with recently reported fixed-frequency systems is favorable. Volts dc (VDC) signal-to-noise ratios of 75-80 dB are achieved and the noise floor for ac signals is near 100 dB below the signal strength of interest at 10 kHz and 60 dB down at 1 MHz. The added benefit of being able to record multispectral information on source and sense signal amplitudes and phases has also been realized. Phase-sensitive detection schemes and multiperiod undersampling techniques have been deployed to ensure measurement fidelity over the full bandwidth of system operation.     

Mid-infrared type-II interband cascade lasers

Interband cascade (IC) lasers that utilize optical transitions between the conduction and valence bands in a staircase of Sb-based type-II quantum wells (QWs) represent a new class of mid-IR diode lasers. By combining the advantages of quantum cascade lasers and type-II QW interband lasers, type-II IC lasers show promise of operating in continuous-wave (CW) mode up to room temperature with high output powers. Significant advances toward such high performance have been reported in terms of record-high differential external quantum efficiency (DEQE>600%), peak output power (~6 W/facet at 80 K), CW power conversion efficiency (>16% at 80 K), and room-temperature operation under pulsed conditions. Here, we will review the progress made in the past few years and discuss the issues encountered during the development. Also, the current status of type-II IC lasers and the remaining challenges will be discussed     

A three-dimensional multifrequency large signal model for helixtraveling wave tubes

A three-dimensional (3D) multifrequency large signal model of the beam wave interaction in a helix TWT is described. The beam is divided into a set of discrete rays, or “beamlets”, instead of the disks or rings used in one-dimensional (1-D) or two-dimensional (2-D) models. The RF fields supported by the helix are represented by a tape helix model that uses a modal expansion including the full (Bessel function) radial dependence of the fields; both forward and backward synchronous space harmonics are included in the model. RF space charge fields are obtained from solutions of the Helmholtz equations for the RF electric and RF magnetic fields, using the beam current and charge densities as sources. The dc space charge electric field is similarly obtained from a solution of Poisson's equation. This model has been implemented in a code called CHRISTINE 3D, a generalization of the one dimensional CHRISTINE code. The full three dimensional treatment permits the accurate computation of large signal gain and efficiency, taking into account the self-consistent variation of beam radius along the interaction space. The code also computes helix interception current and transverse beam distributions at the entrance to the collector-important design data that are unavailable from a 1D model. Results from the CHRISTINE 3D code are shown to compare very favorably with measurements of output power, efficiency, and interception current vs. drive power. Its predictions for spent beam distributions also compare very well with measurements. Run times for the code are problem dependent, but for a single case of interest are typically 1 to 5 min on a 450 MHz PC, orders of magnitude shorter than that required for a comparable 3D particle-in-cell simulation     

Saturation of interband absorption in semiconductors

The nonlinear absorption factors in a direct-gap semiconductor are calculated both for the pump and probe waves using the density matrix formalism. The key feature of the calculation is the consideration of the carrier dispersion. It is shown for the first time that the dispersion law defines the saturation behavior of the absorption factor. The intensity and frequency dependences of the absorption factors calculated differ significantly from those obtained previously. It is also shown that the conventional theory of nonlinear interband absorption based on the kinetic equations for carriers leads to the correct results only in the limit of small pump intensities.