Single-wavelength Signal Research Articles

Polarization-independent wavelength demultiplexer based on a single etched diffraction grating device

Polarization-compatible receivers are indispensable in transceivers used for wavelength division multiplexing optical communications, as light polarization is unpredictable after transmission through network fibers. However, the strong waveguide birefringence makes it difficult to realize a polarization-independent wavelength demultiplexer in a silicon photonic receiver. Here, we utilized the birefringence effect for simultaneously demultiplexing wavelengths and polarizations, and experimentally demonstrated a polarization-independent wavelength demultiplexer with a single device on a SiPh platform. The principle was validated on an etched diffraction grating, which successfully split the arbitrarily polarized light containing four wavelengths into eight channels with single-polarization and single-wavelength signals. Polarization-dependent losses of 0.5–1.8 dB, minimum insertion loss of 0.5 dB, and crosstalks lower than −30 dB were experimentally measured for the fabricated etched diffraction grating. Thus, a promising general solution was developed for implementing polarization-independent wavelength division multiplexing receivers and other polarization-independent devices on photonics platforms with birefringent waveguide devices.

Swept-Source Optical Coherence Vibrometer: Principle and Applications

Optical coherence vibrometer (OCV) is an optical vibration measurement technology which presents the advantages of noncontact, high precision, and large dynamic measurement range. The OCV equipment and the related theories are proposed to avoid excessive demands in monochromaticity and stability of the laser source. The performance of laser Doppler vibrometer (LDV) is complemented by OCV with low cost. In order to obtain large dynamic measurement range (i.e., from nanometer scale to centimeter scale), the differential effect from multiwavelength components is used to break the phase limit of laser interference. However, due to the interference blurry effect, its high-velocity measurement performance is limited. To solve this problem, a novel optical vibrometer with a high-speed measuring capability called swept-source OCV (SSOCV) was proposed. In SSOCV, both a swept-laser source and single-point photodetector were employed to replace the conventional superluminescent diode and spectrometer. These two units help the SSOCV to divide the full bandwidth interference signal into single-wavelength signals, which are subsequently collected by the photodetector. Therefore, the exposure time of interferometric signal was reduced and the measuring speed was highly improved. Taking advantage of these features, our SSOCV can be applied in many different fields particularly in highly accurate measurements applications such as the displacement, mechanical vibration, as well as acoustic and ultrasonic vibration among others.

Wavelength-Division Demultiplexing Enhanced by Silicon-Photonic Tunable Filters in Ultra-Wideband Optical-Path Networks

With the recent growth of network traffic demands, a high-capacity optical-path network based on reconfigurable optical add-drop multiplexers (ROADMs) is highly desirable. Among the various technologies available, extending the useable frequency band is one of the most promising solutions since it can enlarge the network capacity with relatively small signal-quality degradation. However, increasing the number of wavelength signals detracts flexible signal-drop capabilities of the ROADM because the effective dynamic range of the digital coherent receiver is restricted by the number of wavelength signals input to the receiver. To overcome this difficulty, we adopt the ROADM structure that sets an optical wavelength-tunable filter in front of each receiver. This scheme retains the effective receiver dynamic range even if a large number of wavelength signals are multiplexed. In this paper, we demonstrate flexible signal drop in a C+L-band optical-path network by using an optical wavelength-tunable filter implemented on a single silicon-photonic chip. The prototype filter is monolithically implemented in conjunction with an input-port selector so as to reduce the total system loss. The filter comprises multiple asymmetric Mach-Zehnder interferometers for wavelength tuning and symmetric Mach-Zehnder interferometers for input-port selection. Its effectiveness is experimentally confirmed by measuring the bit-error ratios of 32-Gbaud/100-Gbps dual-polarization quadrature-phase-shift-keying signals and 32-Gbaud/200-Gbps dual-polarization 16-ary quadrature-amplitude-modulation signals. A single wavelength signal is extracted out of 285 wavelength signals multiplexed over the C+L band and successfully demodulated with negligible penalty in terms of the optical signal-to-noise ratio. The net fiber capacity reaches 57 Tbps.

Well-Optimized Conjugated GO-DNA Nanosystem for Sensitive Ratiometric pH Detection in Live Cells.

Intracellular pH is a vital parameter which can reflect the physiological process, and the detection of intracellular pH with a high signal-to-noise ratio (SNR) remains a challenge. Compared to pH biosensors based on a single-wavelength signal, it is much easier to obtain better sensitivity and higher SNR from the biosensors by two-wavelength ratiometric signals. In this study, we used DNA-grafted graphene oxide (GO) to ratiometrically detect intracellular pH ranging from basic to acidic. A high SNR with a 35-fold difference in the ratiometric output has been achieved through careful optimization: (1) A high DNA conjugation yield of 45% has been gained through utilizing the partial double-stranded assembly strategy. (2) Herring sperm DNA (HSD) plays an important role in improving the sensitivity of the nanosystem by purifying and passivating the surface of GO; therefore, the concentration of HSD has been optimized to pursue the most sensitive ratiometric response. Apart from the ultrahigh SNR, fabricated GO-AR-Cy5/IFO-Cy3 exhibited excellent stability and biocompatibility in biological environments. Further experiments demonstrated that the nanosystem worked well in live cells in response to pH changes. It is possible to distinguish small pH differences and realize quantitative detection based on ratiometric fluorescence imaging by laser scanning confocal microscope analysis, which makes the nanosystem a promising candidate for further biological study and clinical applications.

Inter‐core crosstalk measurement in multi‐core fibre amplifier using multiple intensity tones

An inter-core crosstalk measurement method in a multi-core erbium-doped fibre amplifier that uses a single-wavelength signal with multiple intensity tones is investigated. Its feasibility is confirmed and the output signal power dependence of the crosstalk values for the same signal gains between cores is successfully characterised.

Stimulated Brillouin scattering limited intensity-dependent four-wave mixing between pump and signal in a non-zero dispersion shifted fiber

The intensity-dependent four-wave mixing (FWM) efficiency is derived by the nonlinear Schrodinger equations including the self phase modulation and cross phase modulation between the two Fabry–Perot modes of a pump and a single-wavelength signal. The maximum FWM for mixing the pump with signal in non-zero dispersion shifted fiber is explained as the minimum phase mismatch for FWM due to equal group delay for the pump and signal. It was also found that the experimental measurements for the generated FWM power are limited by the stimulated Brillouin scattering (SBS) threshold. Under the SBS threshold, the experimental results are still coincided well with the theoretical calculations of the FWM generated power ratio to have the difference of 0.12dB for the input signal power of 12mW.

Non-degenerated Four-Wave Mixing Generated Power Ratio between Co-Propagated Pump Fabry–Perot Modes and Probe Signal in a Non-zero Dispersion Shifted Fiber

We theoretically and experimentally study of the four-wave mixing (FWM) that exists between the co-propagated pump Fabry–Perot (FP) modes of a FP laser diode (FPLD) and the single wavelength probe signal in a 25 km of non-zero dispersion shifted fiber (NZDSF) with zero dispersion wavelength of 1495 nm. The 100-mW co-propagated FP spectra centered at 1440 nm are reproduced via FWM around the 1-mW single wavelength signal around 1552 nm. The maximum FWM generated power ratio is found to have the first condition in matching of the group indices of the pump and signal to obtain the maximum FWM efficiency. And the second condition is mapping the signal wavelength to the center wavelength of the FP pump modes to have the adjacent FP modes with larger powers. The zero-dispersion wavelength of NZDSF can also be determined at the middle of the signal and the center wavelength of pump to have the maximum FWM generated power ratio.

Real-Time End-Point Detection Using Modified Principal Component Analysis for Small Open Area SiO2 Plasma Etching

Principal component analysis (PCA) was modified for real-time applications and applied to the end-point detection of small open area SiO2 plasma etching. Typically, the end point of plasma etching is determined from a few manually selected wavelengths. Determining the end point of the plasma etching using this approach is quite a challenge when the exposed open area is less than several percent. To increase the sensitivity, information was extracted from the entire spectra of 2755 signals in the range of 200−1100 nm, using a PCA algorithm. In this study, the PCA algorithm was modified to allow real-time applications of end-point detection. The loading vector was determined from the model wafer, and the score vector was determined using the real-time data of the target wafer to reduce the processing time. This algorithm was tested for the small open area of SiO2 etching of a 200 ms sampling period, using the entire optical emission spectra, through a comparison with a defined signal-to-noise ratio. The results were compared with the conventional single wavelength signals of SiF (440.2 nm), CO (482.5 nm), and Si (505.6 nm). The end-point detection of 0.4%−0.8% SiO2 open area was achieved using the suggested algorithm, while the single wavelength showed limitations in the open areas above a few percent. The sensitivity was also increased by a factor of 2.15, compared to the signal-to-noise ratio of the single wavelength method.

Simultaneous Cancellation of Fiber Loss, Dispersion, and Kerr Effect in Ultralong-Haul Optical Fiber Transmission by Midway Optical Phase Conjugation Incorporated With Distributed Raman Amplification

An alternative application of distributed Raman amplification (DRA) for ultralong-haul optical fiber transmission is proposed. In our study, the DRA is employed in a transmission system using midway optical phase conjugation (OPC) for amplifying an optical signal and, at the same time, for constructing signal power evolution, which is symmetrical with respect to the midpoint of the system where the OPC is performed. Then, the nonlinear signal waveform distortions that are caused by the Kerr effect, as well as fiber dispersion, are almost completely compensated by the OPC, whereas the fiber loss is compensated by the DRA. Three possible symmetrical signal power maps - a power map that has a reverse sign of the power map that is caused by lump amplification, a flat signal power map, and an arbitrary symmetrical signal power map - are numerically designed by using appropriate Raman pump powers. We show that the flat power map exhibits smaller difference from the target and a higher optical signal-to-noise ratio and requires lower pump power than the other two power maps. Numerical simulation results demonstrate that, by employing the flat power maps with a span of 40 km, a single-wavelength signal whose data rate is 160 Gb/s can be successfully transmitted over 5000 km, and the Kerr effect is sufficiently suppressed near limitation due to the nonlinear accumulation of noise. Finally, we study the feasibility of expanding our method to wavelength-division-multiplexed signal transmission by designing a DRA gain with multiple-wavelength pumping to simultaneously obtain a flat power map and a wide-and-flat gain bandwidth. By using four-wavelength Raman pumps while carefully choosing pump wavelengths and their powers, we achieve the DRA gain that simultaneously gives a fluctuation of the signal power of only 3.5%, a gain ripple of only 5.3%, and, at the same time, a gain bandwidth of as wide as 46 nm.

Optical packet compressor

A highly flexible optical packet compressor is presented. The compressor is capable of providing various compression ratios using the same hardware. It is composed of three parts: a chirped packet generator, a signal compressor, and a wavelength converter. In the chirped packet generator, the modulator intensity-modulates a series of supercontinuum chirped optical carriers and generates a series of chirped optical packets. The signal compressor compresses the chirped packets. The wavelength converter then transforms the compressed wideband optical packets into single-wavelength signals. We numerically demonstrate that using dispersive devices (chirped Bragg grating array or dispersion compensation fiber), we can compress both the width of pulses and the distance among pulses at the same time. This results in an increase of the bit rate. We also show that during the compression, the optical packet suffers distortion in the time domain, which can be defined as an extinction ratio. The distortion can be minimized by control parameters such as carrier chirping and modulating bandwidth. We present a highly flexible optical packet compressor, which is capable of compressing hundreds of bits packets from low speed (mega- or gigabits per second) to very high speed (up to 40 Gbits/s).

Detection, location, and imaging of fast moving targets using multifrequency antenna array SAR

In this correspondence, we generalize the linear antenna array synthetic aperture radar (SAR) from transmitting single-wavelength signals to transmitting multiple-wavelength signals (called multifrequency antenna array SAR). We show that, using multifrequency antenna array SAR, not only the clutters can be suppressed but also the locations of both slow and fast moving targets can be accurately estimated: A robust Chinese remainder theorem (CRT) is developed and used for the location estimation of fast and slowly moving targets. Simulations of SAR imaging of ground moving targets are presented to show the effectiveness of the multifrequency antenna array SAR imaging algorithm.

An ASE Reduction Filter Using Cascaded Optical Fiber Grating Couplers in EDFA Repeater

A new configuration for an ASE reduction filter in EDFA repeaters for WDM optical transmission systems using optical fiber grating couplers (FGCs) was proposed and first demonstrated. The filtering characteristics of the proposed filter for amplified single wavelength signal and 2-channel WDM signals using EDFA were identified. It was found that the filter was effective to improve SNR in optical domain by reducing ASE noise. The required FGC characteristics for the filter were theoretically predicted under the condition that the total filter loss did not exceed 3 dB. More than 70% of drop efficiency was required in each FGC for the application of the proposed filter to an 8-channel WDM system using EDFA repeaters.

Anomalous signal of solvent bromides used for phasing of lysozyme

The anomalous signal of bromide ions, present in the crystal structure of tetragonal hen egg-white lysozyme through the substitution of NaCl by NaBr in the crystallization medium, was used for phasing of X-ray data collected to 1.7 Å resolution with a wavelength near the absorption edge of bromine. Phasing of a single wavelength data set, based purely on anomalous δf″″ contribution, led to easily interpretable electron density, equivalent to the complete multiwavelength anonalous dispersion phasing based on four-wavelength data. The classic small-structure direct methods program SHELXS run against all anomalous differences gave a successful solution of six highest peaks corresponding to six bromide ions in the structure with data limited up to a resolution of 3.5Å. Interpretable maps were obtained at a resolution up to 3.0Å using programs MLPHARE and DM. Bromide ions occupy well ordered positions at the protein surface. Phasing based on the single wavelength signal of anomalous scatterers introduced into the ordered solvent shell can be proposed as a tool for solving structures of well diffracting crystals.