Photodetector Signal Research Articles

Down-conversion en-face optical coherence tomography.

We present an optical coherence tomography (OCT) method that can deliver an en-face OCT image from a sample in real-time, irrespective of the tuning speed of the swept source. The method, based on the master slave interferometry technique, implements a coherence gate principle by requiring that the optical path difference (OPD) between the arms of an imaging interferometer is the same with the OPD in an interrogating interferometer. In this way, a real-time en-face OCT image can originate from a depth in the sample placed in the imaging interferometer, selected by actuating on the OPD in the interrogating interferometer, while laterally scanning the incident beam over the sample. The generation of the en-face image resembles time domain OCT, with the difference that here the signal is processed based on spectral domain OCT. The optoelectronic processor operates down-conversion of the chirped radio frequency signal delivered by the photo-detector. The down-conversion factor is equal to the ratio of the maximum frequency of the photo-detected signal due to an OPD value matching the coherence length of the swept source, to the sweeping rate. This factor can exceed 106 for long coherence swept sources.

Beam offset detection in laser stake welding of tee joints based on photodetector sensing

This paper presents an experimental study where a photodetector is used in a laser beam welding tool to monitor beam deviations (beam offsets) in stake welding of tee joints. The aim is to obtain an early detection of deviations from the joint centerline in this type of welding where the joint is not visible from the top side. The photodetector used is a GaP diode sensitive in the spectral range 150-550 nm corresponding to the spectral emissions form the plasma plume during keyhole welding. The photodetector signal has been evaluated by change point detection methods with respect to their detection performance. Both an off-line and an on-line method have been evaluated. The off-line method can be used to guide post weld inspection and the on-line method has the potential to enable on-line adaptive position control and/or the possibility to stop the process for repair. The results shows that the proposed method can be used as a go/no go system and to guide post weld inspection.

Optical vector analyzer for characterization of Fano resonance structures based on unbalanced double-sideband modulation

We report an optical vector analyzer (OVA) for characterization of Fano resonance structures based on unbalanced optical double-sideband (UB-ODSB) carrier modulation, with capability to investigate the optical structures with asymmetric and ultra-narrowband spectral responses. The ratio of UB-ODSB sidebands amplitudes is not equal to one and constant throughout the measurement process. Transmitted through the optical resonant structure UB-ODSB-radiation is sent to photodetector, that is used to convert the optical signal to electrical one. Output photodetector signal contains the beat components of the UB- ODSB-radiation sidebands and optical carrier. These components splits up by bandpass filters (BPF) for post-processing of measurement results in accordance with new mathematical model that is also presented in this paper. The proposed approach allows to measure phase and magnitude responses of the optical device under test (ODUT) with high resolution; to formulate a criterion for signal falling within the limits of the investigated spectral response; to take one-step measurements.

小波阈值降噪算法在光电探测器信号处理中的应用

小波阈值降噪算法在光电探测器信号处理中的应用

Development of a Micro/Nano Probing System Using Double Elastic Mechanisms.

To meet the requirement of high precision measurement of coordinate measurement machine system, a compact microprobe has been designed for 3D measurement in this paper. Aiming to reduce the influences of signal coupling during the probing process, the probe has been designed by adopting two elastic mechanisms, in which the horizontal and vertical motions of the probe tip can be separated by differential signals of quadrant photodetectors in each elastic mechanism. A connecting rod has been designed to transfer the displacement of the probe tip in vertical direction from lower to upper elastic mechanisms. The sensitivity models in horizontal and vertical directions have been established, and the sensor sensitivity has been verified through experiments. Furthermore, the signal coupling of three axes has been analyzed, and mathematical models have been proposed for decoupling. The probing performance has been verified experimentally.

A novel approach to reduce the influence of external conditions on the laser Doppler vibrometer signal acting in the nanometer range

In this work, the operation of laser Doppler vibrometer (LDV) studies in the presence of a third coherent light wave that interferes in detector in addition to the two waves of classical LDV. If amplitudes of vibration are small, presence of the third wave may give a practical advantage. Specifically, such “tree-wave interference” (TWI) vibrometer with the probing wave and additional waves propagating along the same path through the media with unstable refractive index (such as fiber or turbulent air) would have a higher resistance to the external influence on the propagating light in comparison to the classical LDV. The challenge here is to provide the optimal relations between phases and amplitudes of the interfering waves. We propose a simple approach to check optimal phase difference (Δφ = π ) by monitoring the high frequency component of LDV photo-detector signal, particularly spectrum of its amplitude. Our experiments have demonstrated that LDV modified by the additional coherent wave have increased signal-to-noise ratio (SNR) from ldB to 18 dB in the presence of wideband noise. Therefore, TWI vibrometry can be used in noisy environmental conditions where classical LDV shows poor SNR.

Axial Calibration of QPD Signal based on Stuck Bead Method for Optical Trapping Applications

Calibration of axial quadrant photodetector (QPD) signal to the trapped bead position in an optical tweezer is important to measure the quantitative mechanical parameter in axial (laser propagation) direction. An alternative calibration based on the Stuck Bead Method (SBM) was proposed in this study. 3 µm polystyrene beads were stuck at the surface of glass coverslip and moved axially around the laser focus. QPD was used to obtain the position dependent intensity profile at three different laser powers (19.8 mW, 34.1 mW, 48.5 mW). The QPD signal-to-distance calibration value was consistent at 26 mV/µm for the used bead at the three laser powers. It was found that the calibration values are independent of laser powers and limited by the resolution of distance adjustment.

On the Indoor VLC Link Evaluation Based on the Rician <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math> </inline-formula>-Factor

In visible light communication (VLC) technology, the channel is generally modeled as a multi-path propagation environment. This is due to the presence, on the photo-detector, of specular and non-specular signal components. The Rician $K $ -factor is exploited in such environments to measure the dominance of the line-of-sight (LoS) signal power over that of the non-LoS link. In this letter, $K $ is exploited in the evaluation of the VLC link. Examples of the use of $K $ is underlined in the computation of few link evaluation metrics, including the signal-to-noise ratio, $\gamma $ , the channel capacity, $C $ , and the outage probability, $p_{\text {out}} $ .

Optimal Data Acquisition Methods for Single-Pixel Imaging

Single-pixel imaging is an image reconstruction technique, which uses spatially modulated illumination of an object and a single-pixel detector collecting the light scattered by the object. This technique is complementary to the conventional imaging based on multipixel matrices and becomes especially interesting when one needs to go beyond the capabilities of the standard silicon-based sensors, e.g., to perform imaging in the infrared range. One of the factors that limit the resolution of the reconstructed images is the signal-to-noise ratio during the data acquisition. We propose several methods for processing the photodetector signal and develop a universal method to determine the optimal pixel number in light patterns with respect to illuminance of the object. We use these methods to capture the standard test images and discuss their advantages.

Detecting beam offsets in laser welding of closed-square-butt joints by wavelet analysis of an optical process signal

Robotized laser beam welding of closed-square-butt joints is sensitive to the positioning of the laser beam with respect to the joint since even a small offset may result in a detrimental lack of sidewall fusion. An evaluation of a system using a photodiode aligned coaxial to the processing laser beam confirms the ability to detect variations of the process conditions, such as when there is an evolution of an offset between the laser beam and the joint. Welding with different robot trajectories and with the processing laser operating in both continuous and pulsed mode provided data for this evaluation. The detection method uses wavelet analysis of the photodetector signal that carries information of the process condition revealed by the plasma plume optical emissions during welding. This experimental data have been evaluated offline. The results show the potential of this detection method that is clearly beneficial for the development of a system for welding joint tracking.

Single-Cell Force Spectroscopy of Interaction of Lipopolysaccharides from Yersinia pseudotuberculosis and Yersinia pestis with J774 Macrophage Membrane Using Optical Tweezers

In order to investigate quantitatively the role of lipopolysaccharides (LPS) from outer bacterial membrane at the initial state of bacterium adhesion to a host cell membrane, a model system for single cell force spectroscopy was developed and used. The system comprised of an LPS-coated microsphere placed into optical trap and a J774 macrophage being approached the microsphere to initiate their binding and then moved back to rupture the bond. An “object shadow” phenomenon was discovered, manifested as large-scale variations of the signal of photodetector registering the trapped microsphere displacement, such variations emerging long before the actual interaction between the macrophage and microsphere. The theory and the measurements technique were developed for registration of the force required for detachment of bounded microsphere from the object investigated by means of optical tweezers under the “object shadow” conditions. Characteristic spectra of binding force between J774 macrophage and microspheres functionalized with various LPS, as well as LPS plus complementary antibodies preparations were obtained at the rate of detachment force application of 3–6 pN/s. Force spectrum characteristic of Yersinia pseudotuberculosis LPS possessing O-antigen had a maximum at ~14 pN with half-width of ~23 pN. The treatment of O-antigen with complementary antibodies resulted in transformation of this spectrum into a spectrum with maximum at ~10 pN and half-width of ~14 pN, being almost identical to the spectrum of Y. pestis LPS devoid of O-antigen, with a maximum at ~9 pN and half-width of ~13 pN. A possible mechanism of force spectra formation has been proposed under assumptions of nonspecific binding of O-antigen and probable receptor-type binding of LPS core region to the macrophage surface. The elastic modulus of macrophage envelope, as estimated using analysis of displacement of the contacting microsphere as an indenter, was ≈0.17 pN/nm.

Single lens logarithmic confocal distance measurement array.

This article presents a method and results based on confocal microscopy for non-contact axial distance measurement from a lens to a partially reflective surface using an array of photodetectors. The ratio of the photodetector signals is independent of the surface reflectivity and light source variations over a range of distance. By using logarithmic photodetector signals the object distance is proportional to the difference between the signals. This technique potentially allows measurement of sound produced by single living cells, or the high velocity surface shape changes in the hohlraum in laser fusion experiments.

Polarimetric Optical High-Voltage Sensor Using Synthetic-Heterodyne Demodulation and Hilbert Transform With Gain Control Feedback

Optical voltage sensors are widely used for high-voltage (HV) sensing, but the measurement accuracy is often compromised by a sensitivity drift due to variations in the ambient conditions. Synthetic-heterodyne demodulation is a useful technique for dynamic displacement measurements using interferometric sensors as it can provide a detected signal that is immune to an interferometric drift. In this paper, a new synthetic-heterodyne technique, employing the Hilbert transform and gain control feedback, is applied to a polarimetric-based optical sensor system and used to measure an HV signal. The system comprises an He–Ne laser source, optical phase modulator, and HV sensor located between two polarizers followed by a photodetector and real-time electrical signal acquisition and processing. The use of a control loop results in significant improvements in the stability and accuracy of the detected HV signal. The sensor system was used to measure a 60-Hz HV signal with an amplitude range of 300 V to 7.5 kV. The sensor was also used to successfully measure the total power line harmonic distortion of a highly distorted 4 kV signal. The experimental results show very good agreement with measurements obtained by using the industry standard signal coincidence method.

Gb/s One-Time-Pad Data Encryption With Synchronized Chaos-Based True Random Bit Generators

Ultrafast physical random bit generators based on broadband optical signals have been presented lately at astounding speeds. Some of the most popular mechanisms to obtain such random sequences are through signals that emerge from the coherence collapse operation of semiconductor lasers or from the photodetection signal beating of amplified spontaneous emission optical noise. Especially in the first case, the potential of chaotic signals to synchronize offers a great potential for secure communications. In this paper, we combine two unique properties of semiconductor lasers that operate at a chaotic regime: Their potential to become highly synchronized when optically coupled through appropriate configurations and their ability to seed ultrafast true random bit generators. The concurrent fulfillment of both conditions is shown in this paper and is used to demonstrate experimentally the one-time-pad encryption communication protocol. We report an error-free operation of such an encryption system, exceeding for the first time the Gb/s rate. Forward-error-correction coding is applied on the encryption scheme, in order to drastically reduce the errors of the synchronized true random bit sequences and optimize the decoding performance of the system, while securing the distribution of the random seed.

Underwater wireless transmission of high-speed QAM-OFDM signals using a compact red-light laser.

We first study the transmission property of red light in water in terms of extinction coefficient and channel bandwidth via Monte Carlo simulation, with an interesting finding that red light outperforms blue-green light in highly turbid water. We further propose and experimentally demonstrate a broadband underwater wireless optical communication system based on a simple and cost-effective TO56 red-light laser diode. We demonstrate a 1.324-Gb/s transmission at a bit error rate (BER) of 2.02 × 10-3 over a 6-m underwater channel, by using 128-QAM OFDM signals and a low-cost 150-MHz positive-intrinsic-negative photodetector, with a record spectral efficiency higher than 7.32 bits/Hz. By using an avalanche photodetector and 32-QAM OFDM signals, we have achieved a record bit rate of 4.883 Gb/s at a BER of 3.20 × 10-3 over a 6-m underwater channel.

Frequency-multiplying microwave photonic phase shifter for independent multichannel phase shifting.

A frequency-multiplying microwave photonic phase shifter with independent multichannel phase shifting capability is proposed and demonstrated using an integrated polarization division multiplexing dual-parallel Mach-Zehnder modulator (PDM-DPMZM) and a polarizer. By building a proper power distribution network to drive the PDM-DPMZM, two sidebands along two orthogonal polarization directions are generated with a spacing of two or four times the frequency of the driving signal. Leading the signal to a polarizer and a photodetector, a frequency-doubled or frequency-quadrupled signal with its phase adjusted by the polarization direction of the polarizer is achieved. The magnitude of the signal remains almost unchanged when the phase is adjusted. The proposed approach features compact configuration, scalable independent phase-shift channels and wide bandwidth, which can find applications in beam forming and analog signal processing for millimeter-wave or terahertz applications.

Effect of laser linewidth on the performance of heterodyne detection

According to the statistical theory, the influence of laser linewidth on the performance of optical heterodyne system is studied. Also, the effect of laser linewidth on the visibility of fringe pattern is discussed. The expressions of self-correlation function power-spectrum and definition of visibility of fringe patterns are obtained in this paper. Based on the analytical expressions, the numerical simulation is performed. The obtained results demonstrate that the laser linewidth influences the visibility of fringe patterns according to the result shown in Fig. 3, and that the intermediate frequency can be still detected by heterodyne detection technique as the laser linewidth increases. For different linewidths, the measurement of intermediate frequency is accurate without the influence of noise as the delay d between the received signal and locally generated signal is less than the coherence time c of laser source. If the delay d is greater than the coherence time c, the full width at half maximum of intermediate frequency in the frequency spectrum of the output signal of photodetector will broaden as the laser linewidth increases. However, for a wide linewidth, the measurement of intermediate frequency is inaccurate due to the influence of noise when the delay d is greater than the coherence time c. The wider the linewidth, the less accurate the measurement of intermediate frequency will be. In order to check the correctness of theoretical results, an experiment is carried out by using a laser with a linewidth of 1 MHz, which has an 8.1 km channel path. In our experimental set-up, a cooperative target is employed to modulate and reflect the transmitted beam. In this way, an echo signal is received. The mixing process of the received signal and local signal on the photodetector surface produces an electrical current known as the photomixing current. A spectrum analyzer is used to observe the output signal of detector. The obtained spectrum shows that intermediate frequency can be checked, which is in agreement with the theoretical result. In this work, the obtained conclusions can be directly used to choose a proper laser for optical heterodyne system. According to the target characteristics and measurement requirements, and by following the conclusions obtained in this paper, the laser linewidth can be evaluated.

Laser-phase-fluctuation-insensitive offset-frequency-spaced two-tone optical coherent detection scheme and its transmission for radio-over-fiber systems

We propose a new optical coherent detection scheme with a "two-tone" local light, which is also in principle insensitive to the laser phase fluctuation and is assisted by the digital signal processing technique for radio-over-fiber (RoF) systems. The main feature of our proposal is that the frequency separation of the two-tone local light is different from that of the RoF signal, which is called "offset-frequency-spaced" in this paper. First, we explain the principle of our new proposal and experimentally demonstrate the data recovery. Then, the influence of the frequency detuning between photo-detected modulated and unmodulated signals is discussed. Moreover, the transmission performance with an error vector magnitude (EVM) is evaluated for the optical coherent detection of a 10-Gbaud quadrature-phase-shift-keying RoF signal after a 20-km-long standard single-mode fiber transmission.

Laser-Frequency Stabilization Based on Steady-State Spectral-Hole Burning inEu3+∶Y2SiO5

We present and analyze a method of laser-frequency stabilization via steady-state patterns of spectral holes in Eu(3+)∶Y(2)SiO(5). Three regions of spectral holes are created, spaced in frequency by the ground-state hyperfine splittings of (151)Eu(3+). The absorption pattern is shown not to degrade after days of laser-frequency stabilization. An optical frequency comparison of a laser locked to such a steady-state spectral-hole pattern with an independent cavity-stabilized laser and a Yb optical lattice clock demonstrates a spectral-hole fractional frequency instability of 1.0×10(-15)τ(-1/2) that averages to 8.5(-1.8)(+4.8)×10(-17) at τ=73 s. Residual amplitude modulation at the frequency of the rf drive applied to the fiber-coupled electro-optic modulator is reduced to less than 1×10(-6) fractional amplitude modulation at τ>1 s by an active servo. The contribution of residual amplitude modulation to the laser-frequency instability is further reduced by digital division of the transmission and incident photodetector signals to less than 1×10(-16) at τ>1 s.

Nanodisplacement Measurements of Piezoelectric Flextensional Actuators Using a New Interferometry Homodyne Method

Piezoelectric flextensional actuator (PFA) devices are increasingly being applied in precision mechanics, such as nanotechnology equipments, electronic microscopy instruments, cell manipulation systems, and microsurgery tools. These PFAs need to be characterized by measuring the produced nanodisplacements, and optical interferometry is well established as an accurate and precise technique for this application. In this paper, a new and efficient method for optical phase detection, here named generalized J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> /J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> method, is presented. As in the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> /J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> conventional method, the new method is based on information contained in the photodetected signal spectrum when the drive voltage is sinusoidal and has a known frequency. The dynamic range of this method is from 0.26 to 100π rad (and higher), and only a limited number of frequencies in the magnitude spectrum of the photodetected signal are used, without the need to know the phase spectrum. The method has the advantages of being simple, passive homodyne, self-consistent, and immune to fading. Using the new method, two novel PFAs prototypes, designed using topology optimization method, are tested in terms of displacement linearity (relative to applied voltage) and frequency response.