Digital positron lifetime spectroscopy at EPOS

As the digital equipment to measure positron lifetimes gets cheaper and more widely used, it is decided that EPOS, the ELBE positron source will sample the signals from the photomultipliers directly and evaluate it online or offline by digital means.Still using isotope sources, the EPOS lifetime spectrometer results in a timing resolution of around 170 ps (with ⁶⁰Co), which compares good to analog equipment. A distinct improvement is expected when a coincidence setup will be used at ELBE. However, also the software needs further improvement: while one of the goals is of course to achieve the best time resolution, there is also the aspect of runtime and expandability. Results of evaluations will be presented and compared with results from other groups.     

Optical data timing skews in on-chip optical WDM interconnects

Micro-cavity induced optical data timing skews (ODTS) in wavelength division multiplexing (WDM) on-chip optical interconnect are investigated. In a 10 Gb/s non-return-to-zero (NRZ) WDM system, potentially 28 ps timing skews can be induced by the linewidth non-uniformities among the ring-resonator-based devices, which leads to 3.75 dB signal eye opening penalty. It is shown the coupled type filter structure can help reduce optical data timing skews.     

Saturable absorber using single wall carbon nanotube-poly (vinylalcohol) deposited by the vertical evaporation technique

We employed a vertical evaporation technique to fabricate saturable absorbers by embedding single wall carbon nanotubes (SWCNT) in polymer (vinylalcohol). Two fast recovery time constants, 250 fs and 1.13 ps, respectively, were measured for the absorbers using a transient absorption experimental setup. The saturation intensity of the absorber was found to be 300-400 μJ/cm² at 1060 nm, and a modulation depth as high as 3.5% was achieved.     

Experimental investigation of the impact of optical injection on vital parameters of a gain-switched pulse source

An analysis of optical injection on a gain-switched distributed feedback (DFB) laser and its impact on pulse parameters that influence the performance of the pulse source in high-speed optical communication systems is presented in this paper. A range of 10 GHz in detuning and 5 dB in injected power has been experimentally identified to attain pulses, from an optically injected gain-switched DFB laser, with durations below 10 ps and pedestal suppression higher than 35 dB. These pulse features are associated with a side mode suppression ratio of about 30 dB and a timing jitter of less than 1 ps. This demonstrates the feasibility of using optical injection in conjunction with appropriate pulse compression schemes for developing an optimized and cost-efficient pulse source, based on a gain-switched DFB laser, for high-speed photonic systems.     

Investigations on order and width of RZ super Gaussian pulse in pre-, post- and symmetrical-dispersion compensated 10 Gb/s optical communication system using standard and dispersion compensating fibers

We show the effect of varied order and width of super Gaussian pulse at 10 Gb/s in dispersion compensated optical communication system. The optical communication system consists of standard single-mode fiber of 16 ps/nm/km of a certain length, whose dispersion is compensated using pre-, post- and symmetrical-dispersion compensation schemes with proportionate length dispersion compensating fiber of −80 ps/nm/km. Performance of these three compensation schemes is compared at 14 dBm values of Er-doped fiber amplifiers (EDFA) power at 1st, 2nd and 3rd order RZ super Gaussian optical pulse. The pulse width, full width at half maximum (FWHM) is also varied from 5 to 30 ps to highlight the optimum performance. The graphical results obtained show a relationship among the attributes pulse width, order of RZ super Gaussian optical pulse and dispersion compensation scheme implemented. It shows that to decrease BER and timing jitter in the system, smaller width and 3rd order super Gaussian pulse should be used. It is recommended that to decrease dependency of BER and timing jitter in the communication system on the pulse width i.e. FWHM, the symmetrical compensation scheme should be implemented.     

Electrically ultra-fast tunable optical filter employing HiBi fiber and phase modulator

An electrically tunable optical filter with ultra-fast tuning time of a few picoseconds is proposed and demonstrated. The filter consists of a phase modulator with HiBi pigtails sandwiched between two polarizers. Tuning of 6.62 nm for a 16.06 nm FSR filter is obtained with an applied voltage of 0.9 V. Tuning speed at 10 Gb/s and risetime of 40 ps is demonstrated with a frequency shift keying modulation experimental setup. Faster tuning speed is potential since the scheme is based on the electro-optic effect of the LiNO₃ crystal with sub-picosecond response time.     

A fiber-ring laser incorporating dual mode-locking mechanism

A fiber-ring laser incorporating dual mode-locking mechanisms is presented and demonstrated. A multi-quantum well (MQW) semiconductor optical amplifier (SOA) acts as active loss modulator, because the external optical signal injection results in gain depletion. In addition, a reflective SOA (RSOA) and a polarization beam splitter (PBS) introduce nonlinear polarization rotation (NPR), and also function as a saturable absorber. With this dual mode-locking mechanism, 10 GHz repetition-rate stable pulses, with pulse width of 16 ps and a root-mean-square timing jitter of 1 ps, was obtained without the use of external compression approaches.     

Progress of the intense positron beam project EPOS

EPOS (the ELBE POsitron Source) is a running project to build an intense, bunched positron beam for materials research. It makes use of the bunched electron beam of the ELBE radiation source (Electron Linac with high Brilliance and low Emittance) at the Research Centre Dresden-Rossendorf (40 MeV, 1 mA). ELBE has unique timing properties, the bunch length is <5 ps and the repetition time is 77 ns. In contrast to other Linacs made for Free Electron Lasers (e.g., TTF at DESY, Hamburg), ELBE can be operated in full cw-mode, i.e., with an uninterrupted sequence of bunches. The article continues an earlier publication. It concentrates on details of the timing system and describes issues of radiation protection.     

Reduction in timing jitter by chirp selection for externally modulated return to zero optical soliton pulse at 10 Gb/s

We investigate the chirp selection of externally modulated RZ soliton pulse at 10 Gb/s for fiber optical communication systems for the reduction in timing jitter. We have chosen single arm Mach-Zehnder amplitude modulator with sin² electrical shaped input-output (P-V) characteristic and its chirp range has been varied in the range of −5 to 5. The timing jitter, Q factor and bit error rate (BER) generated for the chirp range has been studied for various fiber lengths and post compensation has been demonstrated to reduce the timing jitter. The number of fixed output amplifiers after every 60 km span is varied from 2 to 10 and corresponding accumulated ASE noise has been studied to manage timing jitter and BER in permissible range, i.e. 5 ps and 10⁻⁹, respectively. It is observed that when two fiber spans are taken then the compensating fiber length for the system is less than 20 km for each case of the chirp considered. For 10 fiber spans, the compensating fiber length increases in the range 60-90 km depending upon the value of chirp taken. Finally it is shown that the chirp value of external modulator should be set to either 0 or −1 for externally modulated RZ soliton pulse in 10 Gb/s optical communication system which makes the system more insensitive to the timing jitter and the selection of dispersion compensating fiber length.     

Realization of a multipath ultrasonic gas flowmeter based on transit-time technique

A microcomputer-based ultrasonic gas flowmeter with transit-time method is presented. Modules of the flowmeter are designed systematically, including the acoustic path arrangement, ultrasound emission and reception module, transit-time measurement module, the software and so on. Four 200 kHz transducers forming two acoustic paths are used to send and receive ultrasound simultaneously. The synchronization of the transducers can eliminate the influence caused by the inherent switch time in simple chord flowmeter. The distribution of the acoustic paths on the mechanical apparatus follows the Tailored integration, which could reduce the inherent error by 2–3% compared with the Gaussian integration commonly used in the ultrasonic flowmeter now. This work also develops timing modules to determine the flight time of the acoustic signal. The timing mechanism is different from the traditional method. The timing circuit here adopts high capability chip TDC-GP2, with the typical resolution of 50 ps. The software of Labview is used to receive data from the circuit and calculate the gas flow value. Finally, the two paths flowmeter has been calibrated and validated on the test facilities for air flow in Shaanxi Institute of Measurement & Testing.     

Optimizing amplifier spacing to improve performance in RZ-rectangular pulse based 10 Gb/s single channel dispersion managed optical communication system

In this paper, we optimize the inter-amplifier spacing in combination with duty cycle of RZ data format and EDFAs power so that link length of system can be maximized. The results for EDFA amplifier placement in 10 Gbps single channel dispersion managed optical communication system have been presented. By increasing the length of standard single mode fiber of dispersion 16 ps/nm/km in proportion to the increase in length of compensating fiber of dispersion −80 ps/nm/km, the pre-, post- and symmetrical-dispersion compensation schemes of the system have been compared. Further, schemes are observed at 8, 10 and 12 dBm values of EDFA power in the link with different duty cycle values of RZ optical pulse in the range of 0.2-0.8 with step size of 0.2 in relation to amplifier spacing to get lower value of bit error rate and timing jitter. The graphical results obtained show strong relationship among duty cycle of RZ optical pulse, EDFA power and, dispersion compensation scheme.     

Femtosecond laser ablation of silver foil with single and double pulses

The average ablation depth per pulse of silver foil by 130 fs laser pulses has been measured in vacuum over a range of three orders of magnitude of pulse fluence up to 900 J cm⁻². In addition, double pulses with separations up to 3.4 ns have been used to probe time scales of relevance for femtosecond ablation. The double pulse ablation depth, when each pulse fluence is 0.7 J cm⁻², falls to that of a single pulse as the pulse separation is increased from 0 ps to 700 ps. This time scale decreases to only 4 ps as the fluence is increased to 11 J cm⁻². It then jumps to 500 ps across a transition fluence where the slope of the ablation depth versus logarithmic fluence characteristic changes abruptly to a higher value. In addition, for pulse separations near 1000 ps, the second pulse can cause re-deposition of ejecta from the first pulse resulting in a double pulse ablation depth only 40% that of the first pulse alone. This has important implications for the interpretation of double pulse femto-LIBS intensities. Our results suggest that the optical properties of nano or mesoparticles play a significant role in double pulse ablation with large pulse separations.     

基于DRS4测试板的数字波形分析方法（英文）

使用不同的方法来确定La Br3晶体信号的到达时间。在文中信号经过光电倍增管的放大之后由DSR4测试板进行数字采集,其中DRS4是由瑞士PSI研究所生产的高带宽、低功耗以及快读出时间的开关电容阵列。这些优势使得DRS4很具有吸引力,很多实验将传统的ADC与TDC替换为DRS4。采集的波形可以通过不同的方法进行后续处理。其中包括:(1)恒分甄别、(2)波形拟合、(3)PMT脉冲模型法以及(4)均值过滤法。文中实现的恒分甄别的时间分辨与使用模拟电路获取的平均时间分辨相比没有提高。高斯波形拟合法虽然与数字CFD的结果相当,但是却更加耗时。均值滤波法虽然容易实现,但是通过这个方法得到的时间分辨与采样时间在一个量级。而PMT脉冲模型法得到的平均时间分辨为195.4 ps,优于模拟信号的恒分甄别的时间分辨254.7 ps。     

Experimental investigation of self-starting operation in a F8L based on a symmetrical NOLM

We experimentally analyze the self-starting operation of a figure-eight mode-locked fiber laser. The design is based on a power-balanced nonlinear optical loop mirror (NOLM) with highly twisted low-birefringence fiber and a quarter-wave (QW) retarder in the loop. The NOLM operates by nonlinear polarization rotation. Self-starting mode-locking requires a careful adjustment of the NOLM low-power transmission, which is easily realized with our setup by adjusting the angle of the QW retarder. The laser is capable of generating ∼20 ps pulses at the fundamental repetition frequency of 0.78 MHz.     

High-speed compact silicon digital optical switch with slot structure

A high-speed compact silicon digital optical switch (DOS) is proposed in this paper. The direct electro-optic effect is applied by filling electro-optic polymer in the void slot of the branches, which compensates the limitation of silicon itself. The crosstalk of about 35 dB and the insertion loss of 0.7 dB is obtained, the switching speed is less than 1 ps, and the whole device length can be shortened to 616 μm even using the basic mode-evolving principle and a simple Y-type structure. Analysis also shows that the device has good fabrication tolerance and wavelength independence over the C-band.     

30-ps impulse response monolithically waveguide coupled photo-detector for high-speed performance monitoring applications

Optical power performance functions monitoring can be demonstrated using a waveguide coupled photo-detector using a reflector and a metal coplanar waveguide. A full-width half-maximum temporal response of 30 ps is illustrated with minimal ringing from a monolithic integrated photo-detector, suitable for 10-Gb/s digital communication applications. The high-speed response of the pin photo-detector is limited by an average saturation drift velocity of 3.5 × 10⁶ cm/s and the impurity concentration of the absorbing In_0.53Ga_0.47As layer of 1.9 × 10¹⁷ cm^− 3 will be demonstrated.     

Tunable slow and fast light with large bandwidth in the band-edge of gain spectrum of the wide band fiber optical parametric amplifier

We propose a novel method theoretically to generate the slow and fast light with large bandwidth and low gains, which is based on the parametric process in fiber. In our scheme, the wide band fiber optical parametric amplifier is employed and the whole signal bandwidth should be located at a certain frequency range of the band-edge of gain spectrum, and then signal waves will be delayed or advanced with low signal gains because of the peculiar feature of signal gain and phase shift. By changing the pump power, the delay time is continuously-tunable optically. The ultimate delay bandwidth and the delay bandwidth product are constrained by the shape of time delay spectrum. Our simulation verifies that 22.4 ps delay or advanced time for the bandwidth of 10 GHz with little distortion can be obtained at certain wavelengths in the optical communication waveband, and their gains are nearly zero. The tunable range is from 0 ps to 22.4 ps for the signal bandwidth of 10 GHz, and it is from 0 ps to 15.6 ps for the bandwidth of 15 GHz. This type of slow and fast light in wide band FOPA has the potential capability to produce the tunable slow and fast light for large bandwidth with low signal gains in future.     

All-optical clock extraction from 10-Gbit/s NRZ-DPSK data using modal interference in a two-mode fiber

All-optical clock extraction from a 10-Gbit/s NRZ-DPSK input signal is demonstrated using modal interference in a two-mode fiber (TMF) and a mode-locked fiber ring laser. The TMF has a Mach-Zehnder configuration with two arms along the core and cladding regions. Using the difference in propagation delay between two arms, the non-return-to-zero differential phase shift keying (NRZ-DPSK) signal is converted to the return-to-zero on-off keying (RZ-OOK) signal. To obtain repetitive pulses as a clock signal from the RZ-OOK signal, a ring laser with a semiconductor optical amplifier (SOA) is used. Subsequently, the carrier-to-noise ratio (CNR) of the RZ-OOK and clock signals are enhanced up to 30 dB and 40 dB, respectively, compared to that of the original NRZ-DPSK signal. Also, the clock signal centered at 10 GHz has a low timing jitter of <1.6 ps. It is expected that this method can be applied to high speed fiber-optic systems of >10 Gbit/s due to its small time delay between the core and cladding regions.     

Impact of extinction ratio of single arm sin LiNbO3 Mach-Zehnder modulator on the performance of 10 and 20 Gb/s NRZ optical communication system

We investigate the impact of extinction ratio of single arm sin² LiNbO₃ Mach-Zehnder (MZ) amplitude modulator on the performance of 10 and 20 Gb/s single-channel optical communication system. For different fiber lengths, the system performance has been analyzed with the increase in the extinction ratio. The effect of variation in dispersion parameter has also been illustrated. The impact of extinction ratio (ζ), dispersion parameter and length of the fiber has been further optimized with minimum bit error rate (BER) at optimal decision threshold (10⁻⁹) for 10 and 20 Gb/s bit rate. It is found that the system gives optimum performance at extinction ratio (ζ) value 20 dB. The increase in the transmission distance from 468 km for 10 Gb/s to 532 km for 20 Gb/s has been reported, and 8 dB improvement in the Q value has been observed as the value of ζ is increased from 10 to 20 dB. At 20 Gb/s, the system gives optimum performance for dispersion parameter value only up to 4 ps/nm km; however, at 10 Gb/s the system can operate for dispersion values up to 14.3 ps/nm km. Further we investigate the self-phase modulation (SPM) effect for the increase in the input power. It is observed that the SPM effect is negligible below 3 dB m input power and it increases at higher power levels.     

Theoretical analysis of the all-fiberized, dispersion-managed regenerator for simultaneous processing of WDM channels

The concept of the all-fiberized multi-wavelength regenerator is analyzed, and the design methodology for operation at 40 Gb/s is presented. The specific methodology has been applied in the past for the experimental proof-of-principle of the technique, but it has never been reported in detail. The regenerator is based on a strong dispersion map that is implemented using alternating dispersion compensating fibers (DCF) and single-mode fibers (SMF), and minimizes the nonlinear interaction between the wavelength-division multiplexing (WDM) channels. The optimized regenerator design with + 0.86 ps/nm/km average dispersion of the nonlinear fiber section is further investigated. The specific design is capable of simultaneously processing five WDM channels with 800 GHz channel spacing and providing Q-factor improvement higher than 1 dB for each channel. The cascadeability of the regenerator is also indicated using a 6-node metropolitan network simulation model.