Sideband Filtering Research Articles

Bidirectional wavelength-division-multiplexing fibre-free-space optical communications using polarisation multiplexing technique and tunable optical vestigial sideband filter

To address the growing demand from emerging applications, high transmission capacity is essential for both fibre backbones and last-mile communications. This can be achieved by integrating optical fibre with optical wireless technologies, facilitating the development of fibre-free-space optical communications. Here we report a bidirectional wavelength-division-multiplexing fibre-free-space optical communication employing polarisation multiplexing technique and tunable optical vestigial sideband filter. The transmission capacity is considerably increased by integrating the polarisation multiplexing technique with the wavelength-division-multiplexing scheme. The transmission performance is extensively enhanced by using a tunable optical vestigial sideband filter and vestigial sideband-four-level pulse amplitude modulation. Moreover, the optical wireless link is substantially extended through the operation of triplet lenses. Low bit error rates and clear vestigial sideband-four-level pulse amplitude modulation eye diagrams are attained with a high aggregate transmission capacity of 480 Gb/s for downstream/upstream transmission. This capability of bidirectional fibre-free-space optical communications holds substantial potential for enhancing advanced wired-wireless communications.

A spatial phase-shifting method for real-space wave reconstruction of off-axis electron holograms

The Fourier transform with a side-band filter is the well-established method for reconstructing off-axis fringe-type holograms due to its ease of implementation and fast processing. However, this method works in reciprocal space and requires inversion of a side-band sub-region, which can degrade the spatial resolution of reconstructed wave compared to the original hologram. We present a new method, the spatial phase-shifting (SPS) method, for real-space wave reconstruction of off-axis electron holograms. We describe the working principles of the SPS method in analogy to the temporal phase-shifting method. We conducted both hologram simulations and experiments to evaluate its applicability and effectiveness. We compared the wave reconstruction results of the SPS and the conventional Fourier transform method, highlighting the advantages of the newly proposed SPS method. Our results demonstrate that the proposed SPS method is particularly effective for real-space wave reconstruction of small-sized hologram, providing an alternative approach to off-axis type holography wave reconstruction.

Integrated photonic RF self-interference cancellation on a silicon platform for full-duplex communication

In-band full-duplex (IBFD) technology can double the spectrum utilization efficiency for wireless communications, and increase the data transmission rate of B5G and 6G networks and satellite communications. RF self-interference is the major challenge for the application of IBFD technology, which must be resolved. Compared with the conventional electronic method, the photonic self-interference cancellation (PSIC) technique has the advantages of wide bandwidth, high amplitude and time delay tuning precision, and immunity to electromagnetic interference. Integrating the PSIC system on chip can effectively reduce the size, weight, and power consumption and meet the application requirement, especially for mobile terminals and small satellite payloads. In this paper, the silicon integrated PSIC chip is presented first and demonstrated for IBFD communication. The integrated PSIC chip comprises function units including phase modulation, time delay and amplitude tuning, sideband filtering, and photodetection, which complete the matching conditions for RF self-interference cancellation. Over the wide frequency range of C, X, Ku, and K bands, from 5 GHz to 25 GHz, a cancellation depth of more than 20 dB is achieved with the narrowest bandwidth of 140 MHz. A maximum bandwidth of 630 MHz is obtained at a center frequency of 10 GHz. The full-duplex communication experiment at Ku-band by using the PSIC chip is carried out. Cancellation depths of 24.9 dB and 26.6 dB are measured for a bandwidth of 100 MHz at central frequencies of 12.4 GHz and 14.2 GHz, respectively, and the signal of interest (SOI) with 16-quadrature amplitude modulation is recovered successfully. The factors affecting the cancellation depth and maximum interference to the SOI ratio are investigated in detail. The performances of the integrated PSIC system including link gain, noise figure, receiving sensitivity, and spurious free dynamic range are characterized.

Ultrasonic Imaging of Seismic Physical Models Using an Ultra-Thin Open-Cavity Microbubble Sensor

A sensitivity-improved fiber-optic ultrasonic sensor is proposed and employed for ultrasonic imaging of seismic physical models (SPMs). The sensor consists of a single mode fiber (SMF) embedded into the hollow-core-fiber (HCF) bubble. Using gas pressure assisted arc discharge, the diameter of the bubble can be larger than 430 um with a wall thickness of approximately 0.48 um. Besides, by using the coating technique, the optical reflectivity and extinction ratio of the ultra-thin bubble are further improved. Fiber tails of SMF and the HCF are partly bonded to form an open-cavity sensor. When applied with ultrasonic waves (UWs), the compact bubble can balance the difference of air pressure between inside and outside. Eventually, a side-band filter technique is used for UWs interrogation. Compared with enclosed uncoating fiber bubble, the sensor shows an enhanced response to pulsed ultrasound. The SPMs of rugged topography and reservoir type are both designed and the proposed sensor is used for ultrasonic imaging of the two specific models.

WITHDRAWN: An air-coupled ultrasonic sensor based on cascaded fibre Bragg grating and Fabry-Perot interference cavity

Abstract In this paper, we proposed and demonstrated experimentally an air-coupled fiber-optic ultrasonic sensor, based on cascaded fibre Bragg grating (FBG) and Fabry-Perot interference (FPI) cavity. Because of the modulation effect of FPI cavity, the sensor’s responses to ultrasonic wave (UW) have been improved. With side-band filtering technology-based intensity interrogation, the voltage responses of 1.2 V and 1.34 V to the continuous and pulse UW at 1 MHz respectively are obtained in air, and its SNR is about 37 dB. Further results of the directivity and stability of the sensor indicate that the sensor has the potential of imaging in air.

Compact fiber-optic ultrasonic sensor using an encapsulated micro-cantilever interferometer.

A compact optical fiber Fabry-Perot interferometer is proposed and demonstrated for ultrasound wave (UW) measurement. The sensor consists of a suspended cantilever in a sealed hollow-core fiber, while the end-face of the leading-in fiber together with the end-face of the suspended fiber act as two reflection mirrors and form an air cavity. A short section of graded index fiber is also employed as a micro-lens to improve the fringe contrast of the interference spectral pattern. Due to the suspended fiber structure, the sensor presents high sensitivity to UW loading, which allows the freely suspended fiber to be stretched and compressed easily. By spectral side-band filtering technology, a simple intensity interrogation technology is used for UW demonstration. Moreover, the sealed sensor structure avoids disturbance from other environment parameters, thus presenting good stability for UW detection.

Sensitivity-Improved Ultrasonic Sensor for 3D Imaging of Seismic Physical Model Using a Compact Microcavity.

A sensitivity-improved ultrasonic sensor is proposed and demonstrated experimentally in this present study. The device is comprised only a fiber-optic microcavity that is formed by discharging a short section of hollow core fiber (HCF). The key to ensuring the success of the sensor relies on the preprocessing of hydrogen loading for HCF. When discharging the HCF, the hydrogen is heated up during the formation of the air bubble, which enlarges the bubble diameter, smoothens its surfaces simultaneously and decreases Young’s modulus of the material of the bubble. Ultimately, this results in the probe being highly sensitive to ultrasound with a SNR of 69.28 dB. Once the compact air cavity is formed between the end face of the leading-in fiber and the top wall of the bubble, a well-defined interference spectrum is achieved based on the Fabry–Perot interference. By using spectral side-band filtering technology, we detect the ultrasonic waves reflected by the seismic physical model (SMF) and then reconstruct its three-dimensional image.

Backcoupling manipulation in silicon ring resonators

In this paper, we theoretically propose and experimentally demonstrate the manipulation of a novel degree of freedom in ring resonators, which is the coupling from the clockwise input to the counterclockwise propagating mode (and vice versa). We name this mechanism backcoupling, in contrast with the normal forward-coupling of a directional coupler. It is well known that internal reflections will cause peak splitting in a ring resonator. Our previous research demonstrated that the peak asymmetry will be strongly influenced by the backcoupling. Thus, it is worth manipulating the backcoupling in order to gain full control of a split resonance for the benefit of various resonance-splitting-based applications. While it is difficult to directly manipulate the backcoupling of a conventional directional coupler, here we design a circuit explicitly for manipulating the backcoupling. It can be potentially developed for applications such as single sideband filter, resonance splitting elimination, Fano resonance, and ultrahigh-Q and finesse.

Ultrasonic imaging of seismic physical models using a fringe visibility enhanced fiber-optic Fabry-Perot interferometric sensor.

A fringe visibility enhanced fiber-optic Fabry-Perot interferometer based ultrasonic sensor is proposed and experimentally demonstrated for seismic physical model imaging. The sensor consists of a graded index multimode fiber collimator and a PTFE (polytetrafluoroethylene) diaphragm to form a Fabry-Perot interferometer. Owing to the increase of the sensor's spectral sideband slope and the smaller Young's modulus of the PTFE diaphragm, a high response to both continuous and pulsed ultrasound with a high SNR of 42.92 dB in 300 kHz is achieved when the spectral sideband filter technique is used to interrogate the sensor. The ultrasonic reconstructed images can clearly differentiate the shape of models with a high resolution.

Comparison of VSB PAM4 and OOK Signal in an EML-Based 80-km Transmission System

A comparison between 51.84-Gb/s vestigial sideband 4-level pulse amplitude modulation (PAM4) and ON–OFF keying (OOK) signals at a transmission distance of 80 km is experimentally demonstrated for the first time. An electro-absorption modulated laser-based intensity-modulation and direct-detection 80-km transmission is achieved using both modulation formats without dispersion compensating fiber. The relative frequency between single sideband filter and signal is adjusted to find an optimized condition for both carrier to signal power ratio and sideband suppression ratio. The experimental results show that with linear equalization and without bandwidth limitation, the OOK signal has a 4-dB better sensitivity compared with PAM4 after 80-km standard single-mode fiber transmission.

An optical fiber Fabry-Perot interferometric sensor based on functionalized diaphragm for ultrasound detection and imaging

A miniature optical fiber ultrasonic sensor based on the Fabry-Perot interferometer (FPI) is proposed and experimentally demonstrated. The cavity of the interferometer is based on a section of hollow core fiber coated with a 353ND diaphragm. A well-define interference pattern is achieved, and the spectral sideband filter technique is used to interrogate the sensor. Because the thin reflective diaphragm is extremely sensitive to sound pressure, the signal-to-noise ratio of the sensor response reaches to 31.22 dB with the pulse ultrasonic excitation of 300 kHz. A clear interface information can be distinguished in physical model imaging, and the longitudinal spatial resolution is 5 mm. This proposed sensor has the advantages of being simple to fabricate, ultracompact in size, cost effective, corrosion resistance, and being highly stable.

High-Frequency Fiber-Optic Ultrasonic Sensor Using Air Micro-Bubble for Imaging of Seismic Physical Models.

A micro-fiber-optic Fabry-Perot interferometer (FPI) is proposed and demonstrated experimentally for ultrasonic imaging of seismic physical models. The device consists of a micro-bubble followed by the end of a single-mode fiber (SMF). The micro-structure is formed by the discharging operation on a short segment of hollow-core fiber (HCF) that is spliced to the SMF. This micro FPI is sensitive to ultrasonic waves (UWs), especially to the high-frequency (up to 10 MHz) UW, thanks to its ultra-thin cavity wall and micro-diameter. A side-band filter technology is employed for the UW interrogation, and then the high signal-to-noise ratio (SNR) UW signal is achieved. Eventually the sensor is used for lateral imaging of the physical model by scanning UW detection and two-dimensional signal reconstruction.

Fiber-optic Michelson interferometer fixed in a tilted tube for direction-dependent ultrasonic detection

A fiber-optic interferometer is proposed and demonstrated experimentally for ultrasonic detection. The sensor consists of a compact Michelson interferometer (MI), which is fixed in a tilted-tube end-face (45°). Thin gold films are used for the reflective coatings of two arms and one of the interference arms is etched serving as the sensing arm. The spectral sideband filter technique is used to interrogate the continuous and pulse ultrasonic signals (with frequency of 300KHz). Furthermore, because of the asymmetrical structure of the sensor, it presents strong direction-dependent ultrasonic sensitivity, such that the sensor can be considered a vector detector. The experimental results show that the sensor is highly sensitive to ultrasonic signals, and thus it can be a candidate for ultrasonic imaging of seismic physical models.

Performance analysis of long reach colorless wavelength division multiplexed-orthogonal frequency division multiplexing-passive optical network with broadcast capability

A colorless wavelength division multiplexed-orthogonal frequency division multiplexing-passive optical network (WDM-OFDM-PON) is presented, which is capable of supporting symmetric 10 Gbps downlink direct detection (DD) OFDM unicast signal, broadcast signal, and uplink on–off keying (OOK) signal up to 60 km that includes both single mode and dispersion compensation fiber. At each optical network unit (ONU), DD has been used to receive downlink unicast and broadcast OFDM data. A delay interferometer (DI) has been used at a central office to achieve 10 Gbps uplink signal transmission over 60 km distance utilizing a reflective semiconductor optical amplifier (bandwidth=1.5 GHz) at the ONU because DI works as a vestigial sideband filter and an optical equalizer. Broadcast channel does not affect the system performance because it generates a limited interference of the order of 0.1 to 0.28 dB to downlink and uplink channels, and this interference is distributed to every ONU. For bit error rate of 10−9, the receiver sensitivity of −24, −23.1, and −20.14 dBm is achieved by simulating downlink OFDM unicast channels, OFDM broadcast channel, and uplink OOK unicast channels, respectively, for a symmetric data rate of 10 Gbps over 60 km.

A Tunable Si3N4 Integrated True Time Delay Circuit for Optically-Controlled K-Band Radio Beamformer in Satellite Communication

In this paper we present the design, realization, and experimental characterization of a photonic integrated true time delay circuit on a CMOS-compatible Si3N4 platform. The true time delay circuit consists of an optical side band filter for single side band modulation and an optical ring resonator for broadband time delay. Two methods of optical delay tuning are investigated: 1) optical wavelength and 2) thermo-optic delay tuning. The wavelength controlled tuning enables a large delay tuning range and can be done remotely from a distant location. The close to a linear phase measurements can be used for full beam-scanning of radio signals with frequencies in the 20 GHz band. The thermal control results in a 5 GHz RF delay bandwidth. A proof-of-concept 2 × 1 beamforming is demonstrated in the 20 GHz band. The design presented here can be employed to realize multi-beams for multi-users serviced by multiple satellites.

Four-Channel 100-Gb/s Per Channel Discrete Multitone Modulation Using Silicon Photonic Integrated Circuits

The emerging applications for inter and intradata center communications demand low-cost and small-from-factor optical transceivers with 100G/400G capacity. Various techniques, such as wavelength-division multiplexing (WDM), multilevel pulse-amplitude modulation, and discrete multitone (DMT), are heavily investigated in this field. Among these techniques, DMT can offer 100G capacity with 10–20G optical devices using direct detection, which provides a very promising low-cost 100G/400G solution. In this paper, we report silicon photonic four-channel DMT integrated circuits, which demonstrate net channel rates of 70 and 100 Gb/s detected by integrated germanium receivers and commercial receivers, respectively. The silicon photonic chip integrates four silicon Mach–Zehnder modulators and WDM multiplexers using thermally tuned second-order microring filters. Besides the wavelength multiplexing functionality, we also demonstrate that these microrings can serve as vestigial sideband filters to enhance the transmission performance in 20–40 km ranges. These demonstrations indicate the promise of using low-cost and high-integrated silicon photonic circuits in high capacity 100G/400G applications.

Middle-atmospheric zonal and meridional wind profiles from polar, tropical and midlatitudes with the ground-based microwave Doppler wind radiometer WIRA

Abstract. WIRA is a ground-based microwave Doppler spectroradiometer specifically designed for the measurement of profiles of horizontal wind in the upper stratosphere and lower mesosphere region where no other continuously running measurement technique exists. A proof of principle has been delivered in a previous publication. A technical upgrade including a new high-frequency amplifier and sideband filter has improved the signal to noise ratio by a factor of 2.4. Since this upgrade the full horizontal wind field comprising zonal and meridional wind profiles is continuously measured. A completely new retrieval based on optimal estimation has been set up. Its characteristics are detailed in the present paper. Since the start of the routine operation of the first prototype in September 2010, WIRA has been measuring at four different locations at polar, mid- and tropical latitudes (67°22' N/26°38' E, 46°57' N/7°26' E, 43°56' N/5°43' E and 21°04' S/55°23' E) for time periods between 5.5 and 11 months. The data presented in this paper are daily average wind profiles with typical uncertainties and resolutions of 10 to 20 m s−1 and 10 to 16 km, respectively. A comparison between the data series from WIRA and European Centre for Medium-Range Weather Forecasts (ECMWF) model data revealed agreement within 10% in the stratospheric zonal wind. The meridional wind profiles agree within their error bars over the entire sensitive altitude range of WIRA. However, significant differences in the mesospheric zonal wind speed of up to 50% have been found.

Iterative intersymbol interference cancellation in vestigial sideband Nyquist–subcarrier modulation system

The intersymbol interference caused by dispersion, chirp, and a vestigial sideband filter in intensity modulation and a direct detection single carrier system is analyzed theoretically and numerically. An iterative nonlinear intersymbol interference cancellation technique is proposed and experimentally demonstrated in a 40-Gbps 16-QAM Mach-Zehnder modulator-based vestigial sideband intensity modulation and direct detection half-cycle Nyquist–subcarrier modulation system over a 100-km uncompensated standard single-mode fiber transmission for the first time. The experimental results show that 2.2-dB receiver sensitivity improvement is obtained at the forward error correction limit by using the iterative technique.

Simulation of FBG Wavelength Signal Demodulation Based on Sideband Filter

The wavelength of Fiber Bragg grating (FBG) sensor signal demodulation simulation system based on the quasi-linear relationship between the transmission side band of optical filters and wavelength was established by the simulation software of MABTLAB. The simple system only consists of broadband source, sideband optical filters, couplers, a FBG, data acquisition and processing system. The relationship between the information of FBG central wavelength and the transmitted light intensity of sideband filter were obtained when the alternating and gradually varying signal applied to the FBG sensors, respectively. The FBG wavelength demodulation can be achieved by sideband filtering, and results are consistent with the theory.

Long reach colorless WDM OFDM-PON using direct detection OFDM transmission for downstream and OOK for upstream

Here we have proposed a novel architecture of wavelength division multiplexed orthogonal frequency division multiplexing (OFDM)-passive optical network (WDM OFDM-PON) which is capable of transmitting 10 Gbps data both in downstream and upstream directions up to 50 km. Here direct detection OFDM is used for downstream unicast transmission and simple on–off keying data is used for upstream unicast transmission. OFDM eliminates inter-symbol interference through use of a cyclic prefix also provides good protection against co-channel interference and impulsive parasitic noise. Since delay interferometer (DI) acts as a optical equalizer and vestigial sideband filter hence a DI is used at central office for successful transmission of 10 Gbps upstream data up to 50 km using reflective semiconductor optical amplifier at optical network unit (ONU) having bandwidth of 1.5 GHz. Since no extra laser source is used at ONU and downstream data is further used for upstream data modulation so colorless scheme is implemented. Simulation is performed for all downstream and upstream channels having data rate of 10 Gbps with a transmission distance of 50 km having acceptable performance.