Albert L. Kellner

High-performance autofocus circuit for biological microscopy

An autofocus circuit, based on measurement of the high spatial frequency image components, was designed for automated microscopic scanning of biological specimens. By careful consideration of the system transfer function, elimination of video end-of-line filter artifacts, correction for illumination instability, and incorporation of autogain, the focus measurement circuit attained the sensitivity and dynamic range necessary for robust operation even at the extremes of biological specimen detail encountered in exhaustive raster scans of thousands of fields. This new circuit exhibited a 25-fold improvement in dynamic range over a previous analog implementation, matched real-time digital performance at an order of magnitude lower cost, resulted in autofocus precision of 56 nm (or 10-fold better than the depth of field of the objective) in scanning experiments comprising over 10 000 microscope fields, and tracked focus at scanning speeds of up to 3.45 fields/s. Focus was correctly maintained in these scanning experiments without additional compensation for low-detail images. This circuit makes possible the widespread inclusion of high-performance autofocus as a low cost option in video microscopy systems.

Magnification corrected optical image splitting for simultaneous multiplanar acquisition

Simultaneous multi planar microscope imaging enables parallel computation of autofocus for high-sped image cytometry. Although image cytometry exhibits many potential advantages over flow cytometry, substantially slower speed has limited use to fewer applications. In commercial image cytometry instruments, long scanning times have typically been circumvented by identification of small areas of interest during high speed, low resolution scans for subsequent analysis at high resolution. This two-pass strategy of analyzing only a few cells at high resolution is a disadvantage and often cannot be used at all where dim fluorescence demands higher numerical aperture (NA) objectives. Continuous stage motion synchronized with line array or time-delay-and-integrate (TDI) CCD image acquisition is capable of increasing scan speed by an order of magnitude or more, but until recently lacked the autofocus required for higher resolution objectives where depth of field is about the thickness of a cell monolayer. Here we describe an improved design for simultaneous multi planar acquisition and on-the-fly autofocus. This new system replaces more complicated and less light efficient fiberoptic imaging bundles with beamsplitters and mirrors. This new image-splitting design also enables addition of magnification correction optics not easily added to the earlier fiberoptic version. The result is a simplified, high sensitivity, magnification-matched, parallel multi planar acquisition module containing an array of CCD sensors for high-speed focus tracking and 3D imaging.

High-saturation-power waveguide photodetectors for analog fiber optic links

A four-layer asymmetric waveguide structure using a nonabsorbing 1.08 eV bandgap InGaAsP waveguiding layer has been studied for high saturation power, high speed waveguide photodiode operating at 1.32 micrometers wavelength. A peak photocurrent of 32 mA corresponding to an optical power of 76 mW has been obtained for 40 GHz waveguide photodiode.

Observation of chaotic instability in the active mode locking of a semiconductor laser

We examine experimentally the consequence of frequency detuning an actively mode-locked external-cavity semiconductor laser from resonance. We observe a transition of the laser system from a periodic oscillation to a nonperiodic state with broadened spectral tones. By estimating the fractal dimension of the corresponding phase-space attractors, we show the presence of low-dimensional chaos. The route to chaos is a well-defined regime of three-frequency quasi-periodicity preceded by a two-frequency quasi-periodicity.

Microwave characteristics of coplanar waveguides on helium‐implanted epitaxial p‐InP

The electrodes of a microwave coplanar waveguide (CPW) on planar p‐InP epitaxial layers are electrically isolated by helium implantation. Impedance, attenuation, and relative effective dielectric permittivity of implanted and unimplanted samples are compared over a frequency range of 0.13–20 GHz. The results show that coplanar waveguides on 4He+ implanted materials exhibit very low loss (<1.4 dB/cm) and low dispersion comparable to those fabricated directly on semi‐insulating InP. CPWs made on uniformly implanted substrates show no significant change in dielectric permittivity after annealing at temperatures up to 310 °C.

Semiconductor electroabsorption waveguide modulator for shipboard analog link applications

A fully packaged and connectorized 1.32 micrometers InGaAsP electroabsorption modulator for analog fiber optic link applications is described. This Franz-Keldysh effect modulator has a 3-dBe bandwidth exceeding 20 GHz, a fiber-to-fiber optical insertion loss of less than 9 dB, and a high modulation efficiency (equivalent switching voltage less than 10 V). A fiber link spurious free dynamic range of greater than 100 dB in a 1 Hz bandwidth is achievable using this modulator, which makes it attractive for Navy shipboard applications.

Performance of the COD: A transparent optical TDM statistical packet multiplexor for scalable terabit networks

We have constructed an optically transparent, memoryless 2:1 statistical TDM packet multiplexor for constant packet-length input. This multiplexor is the fundamental building block in a class of Cascaded Optical Delay (COD) multiplexors intended for wide-area terabit networks. The multiplexor does not require slot-aligned traffic. This multiplexor can be cascaded to build multiplexors with improved packet loss rate. In this article we present the experimental packet loss rate of the memoryless multiplexor as well as the calculated packet loss rate of larger, buffered COD multiplexors. Furthermore, we outline an optical loss compensation scheme for buffered COD multiplexors.

Electroabsorption effects in InxGa1−xAs/GaAs strained‐layer superlattices

Electroabsorption of strained‐layer Inx Ga1−x As/GaAs superlattice structures grown by molecular beam epitaxy on GaAs substrates was experimentally investigated. Its spectral characteristics were found to be similar to those of Franz–Keldysh electroabsorption of bulk semiconductor materials, and suggest that the widths of ground‐state electron and hole minibands might be larger than the maximum tilt of the potential well caused by an applied voltage. We attribute the electroabsorption of such superlattices to photon‐assisted tunneling between ground‐state electron and heavy hole minibands.

Chaotic instabilities in modulated external-cavity semiconductor lasers

Chaotic stability of external cavity semiconductor laser under modulation is examined both theoretically and experimentally. When the modulation frequency is detuned below the cavity resonant frequency, the simulations show a two-frequency to three-frequency route to chaos, as depicted by the power spectrum and time series of the laser emission at different stages of detuning. This agrees with experimental observations of 1.3 micrometer wavelength distributed feedback (DFB) laser and ridge waveguide (RW) InGaAsP laser. The phase-space attractor of both DFB and RW lasers have well-defined structures at broadband chaotic state, which signifies the presence of dynamical determinism in this state.

High-saturation-intensity InGaAs/InP PIN waveguide photodetector

High-frequency and high-responsivity photodetectors which possess a high saturation intensity are needed for high performance analog fiber optic links. We report the results of a high saturation intensity InGaAs/InP PIN waveguide photodetector. At an optical wavelength of 1.3 micrometers , the detector has a responsivity of 0.5 A/W and a flat frequency response from 800 MHz to 20 GHz. With a bias of -4 V, the normalized frequency response of the detector remains unchanged when optical powers up to 10 mW are incident on the detector. An initial linearity measurement is made by measuring the linearity of a fiber optic link which uses the detector with a DC photocurrent of 140 (mu) A as the receiver. The linearity of the link is limited by the modulator of the link and there is no detectable nonlinearity introduced by the detector.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.