M-ary pulse-position modulation and frequency-shift keying with additional polarization/phase modulation for high-sensitivity optical transmission.

We present a new class of optical modulation formats based on the combination of m-ary pulse-position modulation (m-PPM) or m-ary frequency-shift keying (FSK) with additional polarization and/or phase modulation, which is applied on the information carrying pulses in the case of m-PPM or on the information carrying frequency carriers in the case of m-FSK. We describe the principle and implementation of this class of optical modulation formats, and formulate their theoretical receiver sensitivities in optically pre-amplified receivers. Pilot-assisted frequency-domain equalization, similar to that used in coherent optical orthogonal frequency-division multiplexing (CO-OFDM), is used for reliable channel estimation and compensation. CO-OFDM also allows m-FSK to be implemented with high spectral efficiency. As a particular format in this class, m-PPM in combination with polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK), termed as PQ-mPPM, offers superior receiver sensitivity in optically pre-amplified receivers at bit error ratios (BERs) around the thresholds of common forward-error correction codes. Record receiver sensitivities of 3.5 photons per bit (ppb) at BER = 10(-3) and 2.7 ppb at BER = 1.5 × 10(-2) are experimentally demonstrated at 2.5 Gb/s and 6.23 Gb/s using PQ-16PPM and PQ-4PPM, respectively. We further demonstrate the transmission of a 6.23-Gb/s PQ-4PPM signal over a 370-km unrepeatered ultra-large-area-fiber span with 71.7-dB total loss budget.

Actual modal power distributions in multimode optical fibers and their effect on modal noise.

The modal power distribution, which specifies the amount of power flowing in each mode of a multimode fiber, influences the level of modal noise in optical-fiber systems. Generally, previous work has assumed that power in multimode fibers was distributed uniformly among the fiber modes. By considering the coupling of a semiconductor laser to a lensed fiber, we show that this assumption is grossly incorrect for practical excitations. Typically, half of the power is in just 3 modes, even though the fiber supports 49 modes. This leads to much lower modal noise than would be expected if the power were uniformly distributed. The dependence of the modal noise level on mode excitation is found to be much more complex than previously suggested.

Optical-level shifter and self-linearized optical modulator using a quantum-well self-electro-optic effect device.

We demonstrate an optical-level shifter and a modulator whose transmission varies linearly with drive current, both based on a new, negative-feedback mode of operation of the recently discovered quantum-well self-electro-optic effect device. The system is compatible with both laser diodes and low-power semiconductor electronics and is applicable in both analog and digital optical processing. An extension of the system gives inverted, linear modulation of a coherent beam by an incoherent light source.

Microcomputer stabilization of a Fabry-Perot interferometer.

Fabry-Perot interferometers typically require some form of active stabilization system for long-term operation. A stabilization system based on a microcomputer is described which is able to maintain the finesse of a plane parallel Fabry-Perot interferometer for long periods of time. The microcomputer takes the place of the multichannel analyzer, ramp generator, and stabilizer of a conventionally stabilized system. Sample data illustrating the operation of the system on the Brillouin spectrum of fused silica, with and without computer corrections, are included.

Kinetics of genetic recombination in Escheria coli K-12: competition between genetic integration and degradation.

The effect of shifting zygotes from a rich medium into a poor medium (shift-down) on the frequency of recombination has been examined; our results suggest that protein syntheses other than those required for growth may be involved. Experiments with chloramphenicol support this notion, and further suggest that there is a competition between genetic integration and degradation in the recipient cells. By inhibiting protein synthesis one can shift the competition in favor of either integration or segregation, depending on the physiological state of the zygotes. By calculating the time required for segregation as one generation, we conclude that the recombination chromosome is homozygous at the time of integration.

Genetic mapping in Escherichia coli K-12 by radiation-induced crossing-over.

Conventional methods for chromosomal mapping in Escherichia coli are (i) interruption of matings to obtain minimum marker entry times, (ii) linkage analysis of recombinants, and (iii) cotransduction. Method (i) has a resolution of about 0.5 min (5 x 10(4) nucleotides) and is not useful for distances less than about 1 min; methods (ii) and (iii) are capable of better resolution but are generally not very reproducible and no general theory is available for translating crossing-over and cotransduction frequencies into physical chromosomal distances. We found that when merozygotes are irradiated (X rays or ultraviolet light) soon after marker transfer, high linkage values (0.8 to 1.0) between nearby marker pairs decrease with radiation dose to 0.5. Our results are quantitatively consistent with the idea that radiations induce crossing-over lesions proportional to dose, and the number of such lesions between two markers is proportional to the physical separation of the markers in the range that can also be measured by interruption of mating (0.5 to 4.0 min). Additivity relations among markers are also satisfied. We used this technique to measure the distances (0.1 to 1.0 min) between several pairs of closely linked markers.

Escherichia coli K-12 mutants resistant to nalidixic acid: genetic mapping and dominance studies.

Escherichia coli K-12 strains tested so far (approximately 20) can be separated into three groups on the basis of their abilities to form colonies on nutrient agar supplemented with nalidixic acid (NAL): (i) Nal(s) or wild type (no growth at 1 to 2 mug/ml); (ii) NalA(r) (growth at 40 mug/ml or higher); and (iii) NalB(r) (growth at 4 mug/ml, but no growth at 10 mug/ml). The NalA(r) group has a spectrum of sensitivity ranging from 60 to over 100 mug/ml. All Hfr strains of the NalA(r) and NalB(r) groups transfer NAL resistance to recipient cells at genetic loci which are at 42.5 +/- 0.5 and 51 +/- 1 min, respectively, on the Taylor-Trotter map. Some members of the NalA(r) group also have the genetic locus for NalB(r). The nalA(s) allele is completely dominant to nalA(r) in a partial diploid configuration. In haploids, nalA(r)-nalB(r) is phenotypically NalA(r); nalA(r)-nalB(s) is NalA(r); and nalA(s)-nalB(r) is NalB(r). The map location of nalA and the easy differentiation between NalA(r) and NalA(s) allow this marker to be used as a counterselector in bacterial conjugation experiments.

Conjugation in Escherichia coli K-12 and its modification by irradiation.

The steps of normal bacterial conjugation (union, transfer, integration and segregation) are described in analytical terms. Only two parameters are utilized: nu(mt) (0), the probability of interruption of transfer of the male chromosome per unit chromosomal distance; and nu(r) (0), the probability per unit chromosomal distance of a recombinational event. Experimentally these two parameters have the same value (0.06 min(-1) or 10(-6) per nucleotide pair). Irradiation of the donor parent prior to mating increases the transfer parameter (nu(mt) = nu(mt) (0) + sigma(mt)D) and a complete description of the radiation response of recombinant production is obtained by a consideration of the single parameter sigma(mt). Irradiation of the recipient parent prior to mating increases the recombination parameter (nu(r) = nu(r) (0) + sigma(r)D) and a complete description of the radiation response of recombinant production is obtained by the addition of the parameter sigma(r). Experimentally sigma(mt) and sigma(r) are found to have the same value, approximately 0.004 krad(-1) min(-1) for X-irradiation. It is thus possible to describe mathematically the behavior of the unperturbed mating system by a single parameter nu(0); a single additional parameter sigma is adequate to describe the behavior of the system when either parental type is irradiated prior to mating. The unexpected observation that nu(mt) and nu(r) have the same value suggests that common molecular mechanisms are involved in the transfer and integration steps.

Effects of temperature, agitation, and donor strain on chromosome transfer in Escherichia coli K-12.

Recombinant production in Escherichia coli K-12 can be described by three parameters: (i) the distance x of a selected male marker from the donor origin; (ii) the gradient constant k (the probability of interruption of the donor chromosome per unit distance during transfer into a recipient cell); and (iii) the extrapolate number A (the probability that a donor cell will produce a recombinant inheriting the donor marker contiguous with the origin). It is usually assumed that chromosomal distances can be measured by marker entry times, i.e., that the velocity of chromosome transfer v is constant along the chromosome. The dependencies of k, A, x, and v on temperature, agitation during mating, and donor strain were studied. The transfer velocity of the HfrH chromosomal region from the origin to his increases 15-fold between 16 and 43 C, and the chromosomal regions studied have the same temperature dependence that was found for the separate transfer velocities of the O-trp and trp-his regions. These data and radiation studies on chromosome transfer indicate that, at a given temperature, chromosomal transfer velocity varies by less than 10% as the distance of any given region from the origin increases. The gradient constant k is temperature-independent between 20 and 45 C if mating times at different temperatures are inversely proportional to the chromosome velocities; also, k is insensitive to agitation during mating and is not decreased by mating on membrane filters. However, the extrapolate number A is highly temperature-dependent, having its maximum value between 30 and 38 C. These results suggest that the spontaneous interruption of transfer which produces the gradient of transfer is a property of the chromosome itself and not of the fragility of the connection between mating cells.

Genetic recombination in Escherichia coli: clone heterogeneity and the kinetics of segregation.

Several recombinant types may be produced from a single Escherichia coli zygote when the donor parent is Hfr-Hayes and segregation is delayed; usually only one type is produced with Hfr-Cavalli. Statistical experiments confirm well-known pedigree analyses by micromanipulation and suggest that DNA synthesis is required before final genetic integration.