利用全光纤自外差系统研究光纤光栅半导体激光器的线宽

以带光纤尾纤引出式微体声光移频器（ＡＯＦＳ）为基础,用全光纤结构的延迟自外差激光器线宽测量系统（分辨率为５０ｋＨｚ）,其中声光移频器插入损耗仅为２．９ｄＢ,大大提高了信噪比。采用该系统对窄线宽１．３μｍ光纤光栅半导体激光器（ＦＢＧ－ＬＤ）进行了测量,测得激光器线宽的典型值为１．５ＭＨｚ,同时分析了测量系统近端反射光对线宽测量精度的影响以及ＦＢＧ－ＬＧ的线宽压窄特性,最后验证了ＦＢＧ－ＬＤ 输出线宽     

PMD measurement based on delayed self-heterodyne OFDR andexperimental comparison with ITU-T round robin measurements

A new measurement method of polarisation mode dispersion (PMD) in optical fibres based on delayed self-heterodyne optical frequency domain reflectometry (OFDR) technology is proposed and demonstrated on an automatic measurement system. The results of intercomparison of the ITU-T international PMD round robin measurements on two polarisation-maintaining fibre specimens are presented     

Linewidth measurements of MEMS-based tunable lasers for phase-locking applications

We have used two techniques - the beat spectrum method and the frequency noise spectrum method - to measure linewidths of microelectromechanical systems (MEMS)-based tunable external cavity lasers (ECLs) in the C-band (1527-1567 nm). The two techniques yield similar results, but the latter is able to measure narrow linewidths more accurately in the presence of frequency jitter. The MEMS-based ECL linewidths are found to be inversely proportional to output powers over a wide range of powers. At output powers of 12 dBm, the beat linewidth is no more than 30 kHz, corresponding to per-laser linewidths of about 15 kHz. We show that these lasers are suitable for coherent detection of quadrature phase-shift keying.     

Linewidth requirements for optical synchronous detection systemswith nonnegligible loop delay time

The spectral spreads of quadrature-phase-shift-keying (QPSK) modulation and minimum shift keying (MSK) modulation are narrower than those of other kinds of modulation with the same bit rate. Therefore, they are attractive for systems limited by electrical bandwidth and available optical frequency. The synchronous demodulation offers better sensitivity than differential demodulation. However, the propagation delay time of the phase locked loop affects the performance of synchronous detection systems. Therefore, linewidth requirements while considering the loop delay time must be considered in designing these systems. The linewidth requirements for BPSK, QPSK, and MSK homodyne/heterodyne detection systems are obtained by applying the Pade approximation in evaluating the receivers' performance. A change in the power penalty with respect to the change in these values is also obtained     

Linewidth measurements and phase locking of Josephson oscillatorsusing RSFQ circuits

We present a technique for linewidth measurement and phase-locking of Josephson oscillators using digital rapid single-flux-quantum (RSFQ) circuits. The oscillator consists of a resistively shunted 6 μm×6 μm Nb/AlO_x/Nb Josephson tunnel junction that is integrated with RSFQ input and output circuits. A cascade of RSFQ T flip-flops is used to directly monitor the output of the Josephson oscillator. Spectral characteristics have been measured directly for oscillator frequencies ranging from 10-50 GHz. The linewidth can be reduced by over 100 times by phase-locking the oscillator to an RSFQ pulse train generated by an external sinusoidal signal. These Josephson oscillators can be used as on-chip stable high frequency clocks for RSFQ circuits     

Linewidth measurements of a 1.5 ?m range DFB laser

The linewidth of a 1.5 ?m range DFB laser with a window region has been measured using Fabry-Perot interferometers. The linewidth was observed to increase linearly with inverse output power with a slope which approximately agreed with the theoretically calculated value. The full width at half maximum was estimated at 50 MHz at an output power of 1 mW.     

Excess delay estimation from time-varying mobile radio channelimpulse response measurements

This letter proposes a method for estimating the instantaneous excess delay in time-varying mobile radio channel impulse response measurement data using image processing techniques. The method is shown by example to produce more accurate mean power delay profiles than those produced assuming either constant or linearly varying excess delay     

Propagation delay times of ISL and STL

Propagation delay times of high-speed VLSI candidates ISL and STL are calculated analytically. It is shown by calculations and measurements that STL is marginally faster than ISL in oxide-isolated processes, at the cost of higher process complexity. Both logic forms suffer from speed degradation due to fan-in. Measures to obtain delay times that are independent of fan-in are discussed. Fan-out aspects are also considered. It is shown that ring oscillators exhibit a somewhat better speed than logic gates that start to switch from the DC state. This speed difference is expressed in an empirical formula.     

Bifurcations in electrooptic bistable devices with short delay times in the feedback

We have investigated experimentally the behavior of a hybrid bistable electrooptic device with a delayed feedback when the delay time is comparable to the time constant of the device. The behavior is governed by a nonlinear difference-differential equation. The threshold of instability and the period of the oscillations observed close to threshold agree relatively well with the predictions obtained from a linear stability analysis of this equation. For larger input signals or feedback, bifurcations take place. We then observe period-2, period-4, and chaotic behavior following the period-doubling scheme of Feigenbaum. For delay times shorter than the time constant this bifurcation behavior can still be observed for large input signals. Furthermore, for such short delay times the threshold for onset of instabilities was observed to exhibit hysteresis, so different thresholds were observed when the input signal was increasing and decreasing.     

Modelling of SiGe heterobipolar transistors: 200 GHz frequencies with symmetrical delay times

Our basic approach is to develop a symmetrical design with equal delay times for collector, base and the total load to simulate the high frequency behaviour of SiGe heterobipolar transistors (HBTs). On this base we have investigated the feasibility of SiGe HBTs with transit frequencies f_T above 200 GHz. A symmetrical design reaching f_T=208 GHz is presented. The dependence of the high frequency behaviour on Ge content and vertical transistor design is shown. Critical parameters like the maximum current density and the breakthrough voltage are considered. An analytical model is compared to numerical simulations and experimental data.     

Angles-of-arrival and delay times on terrestrial line-of-sight microwave links

A ray-tracing approach is used to generate quantitative information on the ranges of angles-of-arrival, delay times, and amplitudes associated with individual paths in a multipath propagation situation as expected on typical terrestrial microwave links. Some experimental evidence relating to, and confirming, the predicted values is considered.     

Negative group delay times in frustrated Gires-Tournois andFabry-Perot interferometers

It is demonstrated in this paper that a Gires-Tournois interferometer illuminated with an angle of incidence greater than the critical angle for total internal reflection introduces a negative group delay time, whatever the orientation of the electric field vector of the wave with respect to the plane of incidence, when the evanescent wave in the dielectric layer is reflected by a dielectric substrate whose refractive index is between those of the incident medium and of the dielectric layer. When the evanescent wave in the dielectric layer is reflected by a nonabsorbing metal, the group delay time is negative when the electric field vector is in the plane of incidence and positive when the electric field vector is perpendicular to the plane of incidence. Similarly, a frustrated Fabry-Perot interferometer shows negative group delay times for angles of incidence greater than specific p-wave and s-wave critical angles     

Compact Quantum-Dot Microbeads with Sub-Nanometer Emission Linewidth

Fluorescent microbeads are widely used for applications in life sciences and medical diagnosis. The spectral contrast and sharpness of photoluminescence are critical in the utilities of microbeads for imaging and multiplexing. Here, microbeads capable of generating single-peak laser emission with a sub-nanometer linewidth are demonstrated. The microbeads are made of quantum dots that are tightly packed and crosslinked via ligand exchange for high optical gain and refractive index as well as material stability. Bright single-mode lasing with no photobleaching is achieved with particle diameters as small as 1.5 µm in the air. Sub-nm lasing emission is maintained even inside high-index surroundings, such as organic solvents and biological tissues. Feasibility of intracellular tagging and multi-color imaging in vivo is demonstrated.     

Two-dimensional index distribution determination from near-field measurements in single-mode fibres

The refractive-index distribution of single-mode optical fibres can be obtained from transmitted near-field intensity measurements. The generalisation of the method from conventional fibres to noncircularly symmetric guides (e.g. polarisation-maintaining fibres) is discussed. Results obtained both on circular and on polarisation-maintaining fibres are presented.     

Semiconductor laser lineshape and parameter determination from fringe visibility measurements

The field autocorrelation function of single-mode 1.3 ?m InGaAsP lasers has been measured using a Michelson interferometer. Comparison with theory for semiconductor laser line broadening yields the relaxation oscillation frequency, its damping rate and the linewidth broadening factor. A comparison with other techniques for lineshape measurement is presented.     

Correction of single frequency altimeter measurements forionosphere delay

This study is a preliminary analysis of the accuracy of various ionosphere models to correct single frequency altimeter height measurements for ionospheric path delay. In particular, research focused on adjusting empirical and parameterized ionosphere models in the parameterized real-time ionospheric specification model (PRISM) 1.2 using total electron content (TEC) data from the Global Positioning System (GPS). The types of GPS data used to adjust PRISM included GPS line-of-sight (LOS) TEC data mapped to the vertical, and a grid of GPS derived TEC data in a Sun-fixed longitude frame. The adjusted PRISM TEC values, as well as predictions by IRI-90, a climatological model, were compared to TOPEX/Poseidon (T/P) TEC measurements from the dual-frequency altimeter for a number of TIP tracks. When adjusted with GPS LOS data, the PRISM empirical model predicted TEC over 24 1 h data sets for a given local time to within a global error of 8.60 TECU rms during a midnight centered ionosphere and 9.74 TECU rms during a noon centered ionosphere. Using GPS derived sun-fixed TEC data, the PRISM parameterized model predicted TEC within an error of 8.47 TECU rms centered at midnight and 12.83 TECU rms centered at noon. From these best results, it is clear that the proposed requirement of 3-4 TECU global rms for TOPEX/Poseidon Follow-On will be very difficult to meet, even with a substantial increase in the number of GPS ground stations, with any realizable combination of the aforementioned models or data assimilation schemes     

千赫兹量级窄线宽半导体激光器研究进展

窄线宽半导体激光器由于其高单色性、低频率噪声、高可调谐性等优点,广泛应用于高速相干光通信、分布式传感、激光雷达等领域.随着高品质因子(Q)光学谐振腔、硅光异构集成芯片等技术的发展,窄线宽半导体激光器近十年经历了革命式发展,线宽压缩至千赫兹(kHz)量级,甚至到亚千赫兹量级.文章阐述了千赫兹量级窄线宽半导体激光器的最新进展,针对不同压缩线宽机制的窄线宽激光器进行了分类介绍,深入讨论了优化耦合系数、减少外腔损耗等对窄线宽激光器性能的影响,并针对未来应用需求展望了千赫兹量级窄线宽激光器在进一步压缩线宽、提升输出光功率方面的发展方向.     

Effect of random secondary delay times and emission velocities inelectron multipactors

Traditionally, the modeling of rf driven electron cascades (multipactors) has employed a constant emission velocity and zero delay time for all secondaries. This paper introduces the effects from the random secondary emission velocities, as well as the random delay time in the secondary emission. By combining the emission related random phase kicks with the focusing rf action it is shown that the electron impact phase, though wandering erratically, nevertheless stays in the vicinity of a stable phase-locked multipactor phase; the rms phase deviation from phase locking is bounded and directly proportional to the rms fluctuation in the secondary emission. The random phase wandering around the locked phase reduces the nominal secondary emission coefficient δ to an effective value δ* between δ>δ*⩾δ/2 and depending not only on the impact energy of the primary but also on the thermal spread of the secondaries. Multipactor quenching from secondary spreads may occur through the reduction in δ*. For sufficiently small fluctuations, the modeling of a “cold” multipactor with zero thermal spreads is sufficient to determine the actual multipactor behavior in terms of only two control parameters; the normalized rf and magnetic field strengths     

Group delay measurements of optical components near 800 nm

Phase-locked interferometry was used to measure the group delay of several optical components in the spectral region 700-850 nm. The dispersive and absorptive properties of several different types of linear filters were measured. Measurements of the absorption and dispersion of several linear, causal optical filters show that a generalized Kramers-Kronig relation does not exist for many commonly used filters. This result emphasizes the importance of making direct measurements of the dispersive properties of filters, since the dispersion of a linear filter cannot always be determined from a measurement of its absorption. Data are presented for a birefringent filter, dielectric mirror, Gires-Tournois interferometer, and titanium-doped sapphire