FM eliminated CH4locked frequency stabilization of 3.39 µm He-Ne laser in dual feedback control

The frequency of the 3.39 μm He-Ne laser was locked to the CH4saturated absorption line by means of integral-proportional feedback control, i.e., dual feedback control. The frequency modulation was applied by a vibrating mirror placed outside a laser cavity, obtaining a modulation-free laser beam with a stabilized frequency. The long-term stability achieved under integral feedback control was aboutpm1.1 times 10^{-11}, which was further improved topm1.35 times 10^{-12}under dual feedback control. The Allan variance measured by the photomixing technique was1.77 times 10^{-12}at an averaging time 100 s.     

FM eliminated CH4locked frequency stabilization of 3.39 µm He-Ne laser in dual feedback control

The frequency of the 3.39 μm He-Ne laser was locked to the CH4saturated absorption line by means of integral-proportional feedback control, i.e., dual feedback control. The frequency modulation was applied by a vibrating mirror placed outside a laser cavity, obtaining a modulation-free laser beam with a stabilized frequency. The long-term stability achieved under integral feedback control was aboutpm1.1 times 10^{-11}, which was further improved topm 1.35 times 10^{-12}under dual feedback control. The Allan variance measured by the photomixing technique was1.77 times 10^{-12}at an average time 100 s.     

Performance analysis and laser linewidth requirements for optical PSK heterodyne communications systems

An optical PSK heterodyne communications receiver is investigated. The receiver is based on the decision-directed phase-locked loop. The performance of the phase-locked loop subsystem is analyzed taking into account both shot noise and laser phase noise. It is shown that for reliable phase locking (rms phase error less than 10°), heterodyne second-order loops require at least 6771 electrons/s per volt every hertz of the laser linewidth. This number corresponds to the limit when the loop dumping factor η is infinitely large; ifeta = 0.7, then the loop needs 10 157 electrons/(s . Hz). If the detector has a unity quantum efficiency andlambda = 1.5 mum, the above quoted numberers give 0.9 pW/ kHz foreta rightarrow inftyand 1.35 pW/kHz fornu = 0.7. The loop bandwidth required is also evaluated and found to be155 Deltanu, whereDeltanuis the laser linewidth. Finally, the linewidth permitted for PSK heterodyne recievers is evaluated and found to be2.26 cdot 10^{-3} R_{b}where Rbis the system bit rate. ForR_{b}=100Mbit/s, this leads toDeltanu < 226kHz. Such and better linewidths have been demonstrated with laboratory external cavity lasers. For comparison, ASK and FSK heterodyne receivers are much more tolerant to phase noise,-they can tolerateDeltanuup to 0.09 Rb. At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth (Deltanu < 3 cdot 10^{4} R_{b}).     

Performances of a frequency offset locked He-Xe laser system at 3.51 µm

A highly stabilized frequency offset locked He-Xe laser system was constructed for high resolution laser spectroscopy of H2CO [5_{1,5}(upsilon = 0) rightarrow 6_{0,6}(upsilon_{5}= 1)] at 3.51μm. It is composed of three He-Xe lasers. The first laser is H2CO-stabilized and is used as a frequency reference in the system. The second laser is frequency offset locked to the first laser by using the beat frequency between these lasers, and is used as a local oscillator. The third laser is frequency offset locked to the second laser, and is used to observe the H2CO spectrum by slowly varying the beat frequency between these lasers. The frequency stability of the first laser, measured against a similarly stabilized and synchronously modulated laser, was1.0times10^{-14}attau = 100s, where τ represents the integration time. The frequency traceability of the second laser to the first laser was expressed as8.0times10^{-13} cdot tau^{-1}for 10 msleq tau leq 100s. It was found that this value of the traceability was independent of the frequency modulation of the first and second lasers. The frequency traceability of the third laser to the second laser was nearly equal to that of the second laser described previously. The variable range of the frequency of the third laser was 19 MHz. In this range, the frequency traceability of the third laser to the second laser was independent of the beat frequency between these two lasers. From these results, it was concluded that this system can be used for the observation of the H2CO spectrum.     

An estimate of the variation of a band-limited process

Upper and lower bounds are established for the mean-square variation of a stationary processX(t)whose power spectrum is bounded byomega_{c}, in terms of its average powerP_{0}and the average powerP_{1}of its derivative. It is shown thatleft( frac{2}{pi} right)^{2} P_{1} tau^{2} leq E {|X(t+tau )-X(t)|^{2}} leq P_{1} tau^{2} leq omega_{c}^{2}P_{0}tau^{2}where the upper bounds are valid for anytauand the lower bound fortau < pi / omega_{c}. These estimates are applied to the mean-square variation of the envelope of a quasi-monochromatic process.     

Five color CO2laser stabilization and switching

A CO2laser is described which may be actively stabilized in any of four differentR/Pline pairs in the 10.4 μ band. The laser may be caused to rapidly switch between line pairs. The degree of frequency stability achieved issim1:10^{10}, 5 Hz and above, andsim1:10^{8}, 0-5 Hz. The laser may be switched through the four line pairs in ∼100 ms.     

High efficient UV generation at 3472 Å in RDA

The high efficient generation of the second harmonic of ruby laser in rubidium dihydrogen arsenate (RDA) is reported. A peak harmonic power of 52 MW was obtained with a fundamental power of 110 MW. In addition, the temperature variation of the birefringence has been measured to bed(n_{2}^{e} - n_{1}^{o})/dT = (9.3 pm 0.4) times 10^{-6}degC^{-1}.     

Analyses of mode-hopping phenomena in an AlGaAs laser

Intensity fluctuations of the longitudinal modes of a 0.8 μm AlGaAs laser were precisely measured during the occurrence of hopping between two modes. It was found from this result that mode hopping follows the stochastics of a Poisson process. The frequency of mode hopping was measured asf_{c} = [exp [-95(I/I_{th} - 1)]] times 10^{7}(Hz). whereI/I_{th}is the injection current normalized to its threshold value. Results of analog computer simulations showed that spontaneous emission worked as a triggering force for mode hopping. Results of the analysis based on the Fokker-Planck equation were compared to the experimental results, from which the root-mean-square value of the fluctuating electric field of spontaneous emission was estimated as2.3 times 10^{2)(V/m)leqlanglesim{E}_{N} leq 3.2 times 10^{2}(V/m). It is concluded that an effective reduction of mode hopping is achieved if the laser is operated at a higher bias or if the coupling constant between the two modes is increased.     

A 63 mA 112/94 dB DR IF Bandpass $DeltaSigma$ Modulator With Direct Feed-Forward Compensation and Double Sampling

We developed a 10.7 MHz intermediate frequency bandpass discrete-time 4th-order 4-bit $DeltaSigma$ modulator for AM/FM car radio tuners. Using direct feed-forward compensation and double sampling, we have achieved a dynamic range (DR) of 112 dB in the 3 kHz AM bandwidth (BW) and a DR of 94 dB in the 200 kHz FM BW. The modulator occupies 3 ${hbox {mm}}^{2}$ , in 0.15 $mu{hbox {m}}$ CMOS technology, and draws 63 mA of current.      

Lateral analysis of quasi-index-guided injection lasers: Transition from gain to index guiding

In double-heterostructure stripe-geometry semiconductor lasers an effective lateral index stepDelta n_{L}over the stripe region can be induced through evanescent-field coupling. Such a quasi-index-guided device exhibits a transition from the gain-guided to the index-guided regime whenDelta n_{L}is progressively increased. Using parameters appropriate to a 1.3-μm InGaAsP laser, the transition is shown to occur aroundDelta n_{L} sim 5 times 10^{-3}. The exact value ofDelta n_{L}depends on the extent of carrier-induced antiguiding. In the transition region the threshold current decreases rapidly, the lateral mode contracts, and the far field changes from a twin-lobe to a single-lobe pattern. Our analysis suggests that a quasi-index-guided device operates most efficiently for values ofDelta n_{L}at which the index-guided regime is just approached. With a further increase ofDelta n_{L}, the mismatch between the gain and mode profiles leads to lower differential quantum efficiencies. Among other structures, the analysis is applicable to a ridge waveguide laser. For a 1.3-μm laser the optimumDelta n_{L}can be obtained using 0.2-μm-thick cladding layers for a 0.2-μm thick active layer.     

Measurement of the characteristics of the optical-microwavedouble-resonance effect of the 87Rb D1line for application as an atomic frequency standard

The OMDR (optical-microwave double resonance) spectrum of ⁸⁷ Rb with the aim of using a frequency-stabilized GaAs semiconductor laser instead of an Rb lamp as a pumping source in a gas-cell-type Rb frequency standard. Natural isotope ⁸⁷Rb was sealed in a glass cell with buffer gases (Ar/N₂=1.2, total pressure=39 torr). The double resonance signal in the 5P_1/2(F=2)←5S_1/2( F=1) transition appearing at the resonance to the F=2←1 hyperfine transition of the 5S_1/2 state was detected. The optimum operational cell temperature was 56°C. The peak-to-peak frequency width of the atomic hyperfine resonance discriminator used to stabilize the microwave frequency shifts induced by detuning of the laser frequency, changes in the laser and microwave powers, and temperature drift of the cell were investigated     

A Fully Integrated Variable-Gain Multi-tanh Low-Noise Amplifier for Tunable FM Radio Receiver Front-End

In this paper, we present a two-stage variable-gain low-noise amplifier (LNA) used in a frequency modulation (FM) radio receiver front-end for a variety of portable audio devices. We also describe a tunable antenna interface and a voltage regulator used in the front-end. The front-end can be tuned to resonate at FM frequencies from 63 to 129 MHz. The two-stage LNA achieves a programmable small-signal voltage gain from 0.5 to 32.5 dB in around 1-dB steps across the FM band from 76 to 108 MHz. Its input-referred noise voltage is less than 1.5 ${hbox {nV}}/sqrt{hbox {Hz}}$ in the high gain mode. Furthermore, the linearity of the LNA and front-end is improved by employing the multi-tanh design principle. In the low and high gain modes of the LNA, the measured IIP3 are $-$5.2 and $-$ 16.5 dBm, respectively, compared with $-$10.5 and $-$ 25.5 dBm without using the multi-tanh doublets. The whole LNA is fabricated in a TSMC 0.18-$mu{hbox {m}}$ CMOS technology. It draws maximum 4.5 mA current (5.5 mA including the regulator) from a 1.35-V supply and occupies 0.15 ${hbox {mm}}^{2}$ of chip area. Experimental and simulation results are both provided to demonstrate the performance of the LNA and front-end.      

A sensitive fiber-optic Fabry-Perot interferometer

A Fabry-Perot interferometric sensor utilizing a single-mode optical fiber has been demonstrated, exhibiting a finesse of 18 and a sensitivity to phase modulation of7.9 times 10^{-7}rad/sqrt{Hz}at 1 kHz.     

Low-frequency noise in permeable base transistors

The predominant noise is1/fnoise and consists of two parts: a) Noise varying asImin{C}max{2}, generated mostly with conducting channel and predominating for normal values of the collector voltage VCE. b) Noise at low VCEand practically independent of VCE; it is generated chiefly in the space charge region around the base grating and gives collector1/fnoise atV_{CE} = 0. The turnover frequency of the first noise source lies at about 20 MHz forV_{CE} = 0.30V,V_{BE} = 0.20V. At sufficiently high frequencies the PBT shows thermal noise of the output conductanceg_{c0}at zero bias. Generation-recombination noise is observed at large VBEand low VCEand comes mostly from the space charge region around the base grating.     

VLSI Design of Diminished-One Modulo $2^{n}+1$ Adder Using Circular Carry Selection

The diminished-one modulo $2^{n}+1$ addition is an important arithmetic operation for a high-performance residue number system. In this paper, we propose a new circular-carry-selection (CCS) technique for modulo $2^{n}+1$ addition in the diminished-one number domain. The architecture design of CCS modular adder is simple and regular for various bit-width inputs. For actual VLSI implementation, the proposed modular adder can demonstrate its superiority of savings up to 39.5% in AreaxTime and 46.3% in TimexPower performances over those of the previous existing solutions under 180-nm CMOS technology. Finally, the chip area and the clock rate of CCS diminished-one modulo $2^{16}+1$ adder are 26746 $mu{hbox{m}}^{2}$ and 476 MHz, respectively.      

The performance of optical phase-locked loops in the presence of nonnegligible loop propagation delay

The optical phase-locked loop is analyzed taking into account shot noise, phase noise, and loop propagation delay. The degradation of loop phase error due to propagation delay is evaluated in terms of the delay bandwidth productomega_{n} cdot tau_{D}. This product was found to have a maximum value of 0.736 for absolute loop stability. The resulting effect on a Costas loop system optimized for zero time delay is discussed. It is found that in order to maintain a 10^-9BER system performance withxi = 1/2^{0.5}, R = 0.85A/W,P_{DATA} = -59.2dBm, and a 1-MHz beat linewidth, the delay time must be kept below 1.8 ns. If the beat linewidth increases to 15 MHz this figure drops to 0.12 ns.     

Modes in lossy stratified media with application to underground propagation of radio waves

The problem considered is that of determining the complex propagation constant of a plane wave on a flat layer of lossy dielectric with a loss-tangent of less than one, and of uniform thickness, surrounded on both sides by identical layers having a loss-tangent greater than one, and thicknesses much larger than the skin depth. A formula given, relates this propagation constant to the path loss between two antennas provided their excitation efficiencies are known. As an example, curves showing attenuation and wavelength are given for the following properties of the layers: Middle layer =sigma = 10^{-6}(ohm meter)^{-1},epsilon_{r} = 10, mu_{r} = 1. Surrounding layers =simga = 10^{-2}(ohm meter)^{-1},epsilon_{r} = 10,mu_{r} = 1. Frequencies between 1 kc and 10 Mc are covered, as well as values on thickness of the meddle layers ranging from 10 m-200 m. (The computations were performed under the direction of John Nihen).     

The effect of distributed series resistance on the dark and illuminated current—Voltage characteristics of solar cells

Distributed series resistance effects in solar cells are analyzed and the correctness of representing these by a lumped parameter is discussed for any conditions of bias and illumination. In addition to a general mathematical methodology, analytical expressions are derived to simplify the estimation of series resistance effects on the dark and illuminatedJ-Vcharacteristics of the cell. The equivalent series resistance (rs) in the dark is found to decrease with current densityJfromr_{b} + r_{e}/3at smallJto (r_{e} r_{b})^{1/2}at very highJ, where reand rbare the emitter layer and base region resistances, respectively. For illuminated conditions rsdepends onJas well, being maximum near short-circuit and minimum near open-circuit; however, rsfurther depends on the photogenerated current JL: its short-circuit value increases with JLfromr_{b} + r_{e}/3tor_{b} + r_{e}/2and the open-circuit value decreases with JLfromr_{b} + r_{e}/3to(r_{e}r_{b})^{1/2}. The variability of rsis therefore related to the relative importance of rbandr_{e};r_{b}plays the role of attenuating this variability, a situation not well recognized previously. Previous theoretical and experimental work is critically reviewed throughout this paper.     

A 500 MHz tunable CO2waveguide laser for optical pumping

A long-pulse CO2waveguide laser is described, which can be single frequency tuned over 500 MHz in about 80 lines. A peak power reaching 125 W at line center and 85 W at the tuning edge in the strongest lines makes the laser ideally suited for optically pumped FIR lasers. New FIR lines have been observed by pumping CH3OH in absorptions located up to 268 MHz from the CO2line center. FIR wavelengths have been measured, identified, and compared to calculations at a level of relative accuracy of5 cdot 10^{-5}.     

An All-Digital Fast-Locking Programmable DLL-Based Clock Generator

An all-digital fast-locking programmable DLL-based clock generator is presented. By resetting the output clock every two input clock periods, the initial minimal delay constraint in the conventional architecture is eliminated. Compared with the previous work, the short locking time is also achieved. The proposed circuit has been fabricated in 0.35-$mu$ m CMOS process and occupies the active area of 0.216 ${hbox {mm}}^{2}$. The clock multiplication ratio is programmed from 2 to 15. The frequency ranges of the input and output clocks are 4 $sim$ 200 MHz and 60 $sim$ 450 MHz, respectively. It dissipates less than 17 mW at all operating frequencies from a 3.3-V supply.