Signal beam

About: Signal beam is a research topic. Over the lifetime, 1881 publications have been published within this topic receiving 20717 citations.

Optical sampling of subnanosecond light pulses

Abstract: In this paper we shall describe a technique for the display of subnanosecond light pulses which is the optical analog of the technique used in electronic sampling oscilloscopes. The optical pulse waveform to be displayed is assumed to repeat itself with period T. A mode-locked laser provides a source of sampling pulses of a few picoseconds duration. The period of this laser is adjusted to be T + δT, where δT s is proportional to ∫ I 1 I 2 dt, where I 1 and I 2 are the intensities of the light signal beam and sampling beam, respectively. If the signal beam is slowly varying compared to the very brief sampling pulse, we have: Y s = constant × I 1 (t n ) where t n is the arrival time of the nth sampling pulse at the nonlinear crystal. Because of the slightly unequal periods, the sampling pulse scans the light signal in steps of δT seconds. Displays of the sum frequency signal Y s on a conventional oscilloscope therefore constitute a sampled display of the light signal. In an experiment to test the method, sampled displays of subnanosecond pulses generated by a mode-locked He-Ne laser (wavelength 0.633µ) were obtained. The source of sampling pulses, ∼4 psec in duration, was a mode-locked Nd:glass laser (wavelength 1.06µ). The two unfocussed beams were mixed in a KDP crystal and the sum frequency signal at 0.397µ was detected by a photomultiplier and displayed on an oscilloscope. The period T of the He-Ne laser pulses was 12.44 nanoseconds, and the sampling step δT was varied between 100 psec and 400 psec. Using this method the He-Ne laser pulse width was observed to vary between 700 psec and 900 psec depending on the He-Ne laser adjustment. These measurements were confirmed with a fast photodiode and an electronic sampling oscilloscope. The average He-Ne laser power was ∼1 mw. In the KDP crystal the sampling pulses converted red light into UV light with an efficiency of about 5%. Since with focussed beams an efficiency of better than 20% can be achieved, it appears possible to optically sample light signals with average powers much less than a milliwatt.

Adaptive postbeamformer interference canceler with improved performance in the presence of broadband directional sources

Abstract: A postbeamformer interference canceler (PIC) is a beam space processor that processes the signals derived from an antenna array by forming two beams using fixed beamforming weights. One beam, referred to as the signal beam, is formed to have a fixed response in the known signal direction. The processed output of the second beam, referred to as the interference beam, is subtracted from the output of the signal beam to form the output of the PIC. In the presence of broadband directional sources, the formation of a beam is usually carried out by a set of steering delays followed by a weight and sum network. For this case, the processing of the interference beam output is carried out by a tapped delay line filter. When the two beams of the PIC are formed using conventional weights and the tapped delay line filter weights are adjusted to minimize the mean output power of the PIC, a substantial amount of the signal gets suppressed. It is due to the presence of signal component in the interference channel. This paper considers a frequency domain structure of the PIC (FDPIC) in the presence of broadband directional sources; derives conditions required for the performance of the FDPIC to be equal to the performance of the time‐domain structure of the PIC (TDPIC); presents an algorithm to calculate the fixed weights of the two beams at each frequency bin such that the FDPIC maintains a specified frequency response in the signal direction and its optimized performance is better than that of the TDPIC using conventional beam weights; and suggests a scheme to select a gradient step size, which allows the misadjustment of the convergence time constant of the iterative gradient algorithm to be specified.

Ultrasound detection on rough surfaces using heterodyne photorefractive interferometer: applications to NDE

Abstract: Heterodyne interferometers using two-wave mixing in photorefractive cubic crystals for ultrasound detection on rough surfaces is demonstrated. The speckled scattered beam from a rough surface sample interferes with a planar coherent pump beam inside a photorefractive crystal. A third frequency-shifted beam is used to read the grating. The diffracted readout beam and the transmitted signal beam are wavefront matched, resulting in an optimal heterodyne interference signal. We show that the interferometer sensitivity is independent of the surface quality and an optical fiber in the path of the probe beam can be implemented. The interferometer is characterized and results are compared with a classical heterodyne interferometer.

Analysis of counting measurements on narrowband frequency up-converted single photons and the influence of heralding detector dead time

Abstract: We present a theoretical model of photon counting measurements on conditionally generated narrowband single-photon states that are produced via cavity-enhanced spontaneous parametric down-conversion, then frequency up-converted from the telecom wavelength of 1550 nm to the visible wavelength of 532 nm. The highly nonclassical character of the up-converted states is certified by a quantum non-Gaussianity witness that is determined from coincidence measurements with single-photon detectors in a Hanbury-Brown--Twiss configuration. We find our model in good agreement with the experimental data, and we investigate a useful effect caused by the dead time of the trigger detector, whose clicks herald conditional preparation of the single-photon state. Due to the dead time, a click of the trigger detector excludes the possibility of a trigger event at a certain preceding time interval, during which the measured idler beam is thus projected onto a vacuum state. Due to quantum correlations between signal and idler beams, this reduces the multiphoton contributions in the conditionally generated state of the signal beam and accordingly increases the value of the quantum non-Gaussianity witness. We also show that spurious heralding detections due to after-pulsing can be suppressed by accepting a click of the trigger detector only if its distance from a previous click of this detector exceeds a certain suitably chosen threshold.

High-power optical fiber laser

Abstract: The invention discloses a high-power optical fiber laser. The laser structurally comprises a residual light collector, a red light/signal light beam combiner, a red light laser, a reverse cladding light mode stripper, a forward pumping pump source module, a forward pumping/signal beam combiner, a high-reflection grating, doped optical fiber, a low-reflection output grating, a reverse pumping/signal beam combiner, a reverse pumping pump source module, a forward cladding light stripper, and output optical fiber with an antireflection film end cap. Based on a linear fabry-perot resonance cavity structure, the forward pumping pump source module and the reverse pumping pump source module, with different wavelengths, provided with narrow-band protection filters are adopted, the forward pumping/signal beam combiner and the reverse pumping/signal beam combiner pump doped optical fiber from the front end and the back end of the doped optical fiber through the high-reflection grating and the low-reflection output grating at the same time, so that multi-kilowatt-class power stable output of the optical fiber laser is achieved, and mutual damage of residual pumping light on two sides to a pumpsource chip is avoided.