Optical Delay Line Research Articles

A Compact Upconversion Single-Photon Imager for Full-Range and Accurate 3-D Imaging

The rapid development of single photon detection technology offers the possibility for an imaging system to go beyond the traditional limits. Recently, single-photon imagers based on upconversion technique have achieved unprecedentedly high resolution and noise tolerance in 3D imaging. Nevertheless, the limited imaging range, complicated optical path and long data acquisition time impose remaining challenges to practical applications. In this work, we build an upconversion single-photon imager for full-range and accurate 3D imaging. A compact all-fiber design allows our system to work robustly in an open environment. Compared to the previous imagers, the imaging speed is increased by three orders of magnitude due to the high upconversion efficiency and photon-efficient algorithm. With the application of a high-repetition laser and a nanosecond optical delay line, we achieve full-range imaging under the upconversion scheme against high solar background noise. For close-up scenes, our imager achieves ultrahigh axial resolution up to 20 μm, which is not affected by the time jitter or pileup effect of the single photon detector. Our proposal can stimulate practical applications in sophisticated scenarios.

A Photonics Approach for Microwave Waveform Generation Based on PM-IM Conversions Using Two Dual-Output Sagnac Interferometers

A photonics-assisted scheme for generating various microwave waveforms has been investigated in this paper. In the proposed scheme, two phase modulation to intensity modulation (PM-IM) conversions are realized using two cascaded Sagnac interferometers (SIs) to manipulate light in the optical domain. From the first SI, by adjusting the amplitude of the microwave signal and time delay of the tunable optical delay line (TODL), both half duty cycle square and full duty cycle triangular waveforms can be generated from the photodetector (PD) and balanced photodetection (BPD). Using the second SI and applying microwave signals with appropriate amplitudes and phases, as well as the BPD process, results in full duty cycle sawtooth and reversed sawtooth waveforms. Detailed theoretical analyses are presented and results are verified by simulations. In the simulations, square-, triangular- and sawtooth-shaped (or reversed sawtooth-shaped) waveforms are obtained successfully. To complete the simulation results, the effects of non-ideal microwave hybrid coupler and optical couplers on the performances of the proposed scheme have been studied and no destructive effects were observed. The proposed structure does not include any optical or electrical filters which result in a wide operational bandwidth.

Low-loss chip-scale programmable silicon photonic processor

Chip-scale programmable optical signal processors are often used to flexibly manipulate the optical signals for satisfying the demands in various applications, such as lidar, radar, and artificial intelligence. Silicon photonics has unique advantages of ultra-high integration density as well as CMOS compatibility, and thus makes it possible to develop large-scale programmable optical signal processors. The challenge is the high silicon waveguides propagation losses and the high calibration complexity for all tuning elements due to the random phase errors. In this paper, we propose and demonstrate a programmable silicon photonic processor for the first time by introducing low-loss multimode photonic waveguide spirals and low-random-phase-error Mach-Zehnder switches. The present chip-scale programmable silicon photonic processor comprises a 1×4 variable power splitter based on cascaded Mach-Zehnder couplers (MZCs), four Ge/Si photodetectors, four channels of thermally-tunable optical delaylines. Each channel consists of a continuously-tuning phase shifter based on a waveguide spiral with a micro-heater and a digitally-tuning delayline realized with cascaded waveguide-spiral delaylines and MZSs for 5.68 ps time-delay step. Particularly, these waveguide spirals used here are designed to be as wide as 2 µm, enabling an ultralow propagation loss of 0.28 dB/cm. Meanwhile, these MZCs and MZSs are designed with 2-µm-wide arm waveguides, and thus the random phase errors in the MZC/MZS arms are negligible, in which case the calibration for these MZSs/MZCs becomes easy and furthermore the power consumption for compensating the phase errors can be reduced greatly. Finally, this programmable silicon photonic processor is demonstrated successfully to verify a number of distinctively different functionalities, including tunable time-delay, microwave photonic beamforming, arbitrary optical signal filtering, and arbitrary waveform generation.

Pulse Burst Generation and Diffraction with Spatial Light Modulators for Dynamic Ultrafast Laser Materials Processing.

A pulse burst optical system has been developed, able to alter an energetic, ultrafast 10 ps, 5 kHz output pulse train to 323 MHz intra-burst frequency at the fundamental 5 kHz repetition rate. An optical delay line consisting of a beam-splitting polariser cube, mirrors, and waveplates transforms a high-energy pulse into a pulse burst, circulating around the delay line. Interestingly, the reflected first pulse and subsequent pulses from the delay line have orthogonal linear polarisations. This fact allows independent modulation of these pulses using two-phase-only Spatial Light Modulators (SLM) when their directors are also aligned orthogonally. With hybrid Computer Generated Holograms (CGH) addressed to the SLMs, we demonstrate simultaneous multi-spot periodic surface micro-structuring on stainless steel with orthogonal linear polarisations and cylindrical vector (CV) beams with Radial and Azimuthal polarisations. Burst processing produces a major change in resulting surface texture due to plasma absorption on the nanosecond time scale; hence the ablation rates on stainless steel with pulse bursts are always lower than 5 kHz processing. By synchronising the scan motion and CGH application, we show simultaneous independent multi-beam real-time processing with pulse bursts having orthogonal linear polarisations. This novel technique extends the flexibility of parallel beam surface micro-structuring with adaptive optics.

High accurate self-calibration of photonic integrated circuit using lossless thermo-optic phase shifters.

The accurate calibration of large-scale switch networks is critical for integrated photonics, in which the integrated optical true time delay chip is typical. In this work, a novel self-calibration method without extra testing ports is proposed by introducing lossless thermo-optic phase shifters instead to calibrate the network. As a demonstration, a 5-bit delay line based on silicon nitride is fabricated and calibrated. The extinction ratio of all the switches is greater than 30.9 dB at the cross and bar states. Using this method, the 5-bit optical delay line which can be tuned in a range of 118.53 ps and reach a low delay time deviation less than ±0.4 ps.

Performance comparison of integrated optical switching delay lines on three typical photonic integration pilot lines

Integrated optical switching delay line (OSDL) chip, which is composed of optical switches cascaded with optical waveguides of different lengths, has the merits of ultra-wide delay bandwidth, very high delay accuracy and easy controlling. As one of the key components in microwave photonic beamforming system, OSDLs have drawn great attentions and various integrated OSDL chips have been demonstrated. Among them, the SOI (silicon-on-insulator) and Si3N4 (silicon nitride) are the most popular platforms to fabricate OSDL chips due to their CMOS compatibilities. Nevertheless, the performances and characteristics of the OSDL chips fabricated on the three mainstream platforms have not been compared, which make it difficult to fair evaluate different technologies, which can provide useful guidance for real OSDL applications. To this end, we have designed, fabricated and packaged three OSDL chips based on SOI (The waveguide core thickness is 220 nm and 3 µm respectively) and Si3N4 (double strip silicon nitride waveguide) on three photonic integration pilot lines successively. The performances of the fabricated OSDL chips were investigated and compared comprehensively, including the power consumption, switching time and fiber to fiber insertion loss. Then, the delay times of the three OSDLs were measured, which agree with our design very well. Finally, some discussions and future performance improvement methods of these OSDL chips are given.

Enhancement of calculation accuracy of the integrated photonic tensor flow processer by global optical power allocation.

We present a global optical power allocation architecture, which can enhance the calculation accuracy of the integrated photonic tensor flow processor (PTFP). By adjusting the optical power splitting ratio according to the weight value and loss of each calculating unit, this architecture can efficiently use optical power so that the signal-to-noise ratio of the PTFP is enhanced. In the case of considering the on-chip optical delay line and spectral loss, the calculation accuracy measured in the experiment is enhanced by more than 1 bit compared with the fixed optical power allocation architecture.

Optical delay lines in topological microring resonator array

We use a periodic microring resonator array (MRRA) to realize photonic topological insulator. By tuning the coupling of resonators, two topological edge states can be observed in different bands in the dispersive band, normalized transmission spectrum and light field distribution. From the dispersion band structure, we calculate the group velocity, group velocity dispersion, normalized delay bandwidth product and other delay properties. It is demonstrated that optical delay lines (ODLs) in topological MRRA not only can obtain a comparable delay times, but also show a flatter transmission spectrum and more stable delay time, compared with the traditional coupled resonator optical waveguide. Our work may provide a theoretical reference for the topological ODLs with robust transmission.

Simulations about Stability, Damage, and Heating Impacts for an X-ray Optical Delay Line at SASE3

European XFEL has initiated a project to facilitate two-color pump-probe spectroscopy with X-rays at the SASE3 soft X-ray undulator. To this end, the controllable temporal delay between the two X-ray pulses is obtained by a magnetic chicane and an optical delay line (ODL) that delay the electron and X-ray beams, respectively. This article focuses on describing the alignment process of the ODL and its related specification. The distance between the experimental station and the ODL is nearly 500 m, thus if one of the mirrors in the ODL is misaligned, the focus spot experiences a noticeable shift. Moreover, a pulse train can include up to 2700 X-ray pulses with a minimum spacing of 222 ns. An increase in temperature per pulse would just add up if there was no heat removal during that time and the material could be damaged during a pulse train. The question arises whether the X-ray optics can survive the high X-ray peak power during a single pulse. Finally, the present study also investigates the damage issue.

Phase-shifting digital holographic microscopy for microstructure measurement by sweeping the repetition rate of femtosecond laser

This paper proposes a phase-shifting digital holographic microscopy (PSDHM) for microstructure measurement by sweeping the repetition rate of femtosecond laser, and a multiple reflection arrangement between two quasi-parallel mirrors is constructed for optical multiplication. High precision phase-shifting can be achieved by sweeping the repetition rate of the femtosecond laser referenced to a Rb atomic clock without any mechanical sweeping. Optical multiplication can shorten the spatial distance of the optical delay line used for pulses alignment, make the PSDHM structure compact and stable, and avoid certain environmental disturbances. In the experiments, a ten-step phase-shifting test was first carried out for evaluating the phase-shifting accuracy, and the phase-shifting error was calculated to be in the range of −1° to 0.25°. Then, a USAF 1951 resolution target and a microstructure standard target were measured using a four-step PSDHM, and the measurement results were compared with those from a stylus profiler and a white light interferometer, respectively. The lateral resolution of PSDHM was tested to be about 2.1 μm, and the maximum error of the longitudinal measurement was within 6 nm. Experiments verify that the PSDHM system has good performance in terms of phase-shifting accuracy, surface topography measurement and coherent noise suppression.

All-Optical Format Conversion of 2-Dimensional MQAM to MPSK Based on Nonlinear Mach-Zehnder Interferometer With Wavelength Preservation

An all-optical format conversion scheme of 2-dimensional multiple-quadrature amplitude modulation (MQAM) to multiple-phase shift keying (MPSK) signals is proposed and numerically simulated by using the nonlinear Mach-Zehnder interferometer (MZI) configuration with wavelength preservation. The input octal-quadrature amplitude modulation (8QAM) signal is generated through the phase modulator (PM) and amplitude modulator (AM) and injected into the nonlinear MZI configuration. The designed nonlinear MZI configuration includes two 3 dB optical couplers (OCs), a piece of high nonlinear fiber (HNLF) and a variable optical attenuator (VOA) in the upper interfering arm and a piece of standard single mode fiber (SSMF) or tunable optical delay line (TODL) and a variable phase shifter (VPS) in the lower interfering arm. The converted octal-phase shift keying (8PSK) signal can be obtained from the nonlinear MZI configuration through vector superposition between the input optical signal and itself with some nonlinear phase shift. The needed asymmetric nonlinear phase shift is mainly induced by the big nonlinear index difference between HNLF and SSMF. The error vector magnitude (EVM) and bit error rate (BER) performance of the scheme are calculated before and after format conversion with the different input and receiver optical-signal-noise-ratios (OSNRs). The scheme can be used in the intermediate node to connect the different networks which adopt 8QAM and 8PSK signals, respectively.

Digitally tunable optical delay line based on thin-film lithium niobate featuring high switching speed and low optical loss

A tunable optical delay line (ODL) featuring high switching speed and low optical loss is highly desirable in many fields. Here, based on the thin-film lithium niobate platform, we demonstrate a digitally tunable on-chip ODL that includes five Mach–Zehnder interferometer optical switches, four flip-chip photodetectors, and four delay-line waveguides. The proposed optical switches can achieve a switching speed of 13 ns and an extinction ratio of 34.9 dB. Using a modified Euler-bend-based spiral structure, the proposed delay-line waveguide can simultaneously achieve a small footprint and low optical propagation loss. The proposed ODL can provide a maximum delay time of 150 ps with a resolution of 10 ps and feature a maximum insertion loss of 3.4 dB.

Synaptic delay plasticity based on frequency-switched VCSELs for optical delay-weight spiking neural networks.

In this Letter, we propose an optical delay-weight spiking neural network (SNN) architecture constructed by cascaded frequency and intensity-switched vertical-cavity surface emitting lasers (VCSELs). The synaptic delay plasticity of frequency-switched VCSELs is deeply studied by numerical analysis and simulations. The principal factors related to the delay manipulation are investigated with the tunable spiking delay up to 60 ns. Moreover, a two-layer spiking neural network based on the delay-weight supervised learning algorithm is applied to a spiking sequence pattern training task and then a classification task of the Iris dataset. The proposed optical SNN provides a compact and cost-efficient solution for delay weighted computing architecture without considerations of extra programmable optical delay lines.

Air-Filled SIW Remote Antenna Unit With True Time Delay Optical Beamforming for mmWave-Over-Fiber Systems

A low-complexity and efficient mmWave-over-fiber remote antenna unit (RAU) is proposed for broadband transmission and wide-angle squint-free beam steering in the full [26.5–29.5] GHz n257 5G band. It leverages an optical beamforming network (OBFN), implemented on a silicon photonics integrated circuit, and a broadband optically enabled 1x4 uniform linear array (ULA). The antenna elements (AEs) of the ULA are implemented in air-filled substrate-integrated-waveguide technology. They adopt an improved aperture-coupled feeding scheme to achieve high efficiency, high isolation and minimal back radiation over a broad frequency band. Each AE is compactly integrated and co-optimized with a dedicated opto-electrical transmit chain, maximizing the RAU’s performance, including beamforming flexibility and energy efficiency, while minimizing its size. The separately packaged OBFN implements true-time-delay beamforming by means of four switchable optical delay lines that are capable of discretely tuning the delay difference between AEs with a resolution of 1.6 ps, up to a maximum delay of 49.6 ps to fully exploit the ULA’s full grating-lobe-free scan range. The measured AEs are excellently matched in the [25.1–30.75] GHz band, exhibit high isolation ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &gt;$</tex-math></inline-formula> 15 dB) in the operating band, and feature a stable peak gain of 6.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.72 dBi with a beamwidth of at least 95 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> . Additionally, optical beamforming was successfully demonstrated by steering the RAU’s beam towards angles up to 51.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> without grating lobes. The optically enabled 1 × 4 ULA successfully establishes a 64-QAM wireless communication link at 2.2 Gbaud (13.2 Gbps) while beam steering up to 50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ ^\circ$</tex-math></inline-formula> with an error vector magnitude below 7.6%.

Triggerless data acquisition in asynchronous optical-sampling terahertz time-domain spectroscopy based on a dual-comb system.

By using two mutually phase-locked optical frequency combs with slightly different repetition rates, we demonstrate asynchronous optical-sampling terahertz time-domain spectroscopy (ASOPS THz-TDS) without using any trigger signals or optical delay lines. Due to a tight stabilization of the repetition frequencies, it was possible to accumulate the data over 48 minutes in a triggerless manner without signal degradation. The fractional frequency stability of the measured terahertz signal is evaluated to be ∼8.0 × 10-17 after 730 s. The frequency accuracy of the obtained terahertz spectrum is ensured by phase-locking the two frequency combs to a frequency standard. To clarify the performance of our system, we characterized the absorption line of water vapor around 0.557 THz. The good agreement of the measured center frequency and linewidth with the values predicted from the HITRAN database verifies the suitability of our ASOPS THz-TDS system for precise measurements.

Zero-GVD slow light of coupled topological edge states in a sandwiched photonic crystal waveguide

We propose a new scheme to realize topological photonic states with low group velocity (vg) and zero group velocity dispersion (zero-GVD) based on a sandwiched photonic crystal (PC) waveguide, which are composed of finite sized PCs with different topological phases. In our proposed sandwiched heterostructure, two coupled topological edge states (CTESs) can be found, one of which can be applied to slow light by modifying the radii of the dielectric rods. The slow light characteristics of CTES, including zero-GVD, large average group index and normalized delay-bandwidth product, are discussed in detail based on finite element method (FEM) simulation. Besides, the robustness of CTES with slow light is verified, when introducing random disorders. Time-domain simulation results demonstrate the dispersionless transport of CETS in zero-GVD region. Our findings pave a way of topological slow light, enrich the topological PC research, and have new application in optical buffers and optical delay lines.

Photonic-assisted 2-D terahertz beam steering enabled by a LWA array monolithically integrated with a BFN.

A novel photonic-assisted 2-D Terahertz beam steering chip using only two tuning elements is presented. The chip is based on an array of three leaky wave antennas (LWAs) with a monolithically integrated beamforming network (BFN) on a 50 µm-thick indium phosphide substrate. The THz beam angle in elevation (E-plane) is controlled via optical frequency tuning using a tunable dual-wavelength laser. An optical delay line is used for azimuth (H-plane) beam control. The simulated beam scanning range is 92° in elevation for a frequency sweep from 0.23 THz to 0.33 THz and 69.18° in azimuth for a time delay of 3.6 ps. For the frequency range from 0.26 THz to 0.32 THz, it is confirmed experimentally that the THz beam scans from -12° to +33°, which is in good agreement with the numerical simulations. The beam direction in azimuth scans with a total angle of 39° when applying a delay difference of 1.68 ps. A good agreement is found between theoretically predicted and experimentally determined THz beam angles with a maximum angle deviation below 5°. The experimental scanning angles are limited due to the mechanical constraints of the on-wafer probes, the on-chip integrated transition and the bandwidth of the THz receiver LNA. The mechanical limitation will be overcome when using a packaged chip.

Design and Validation of a Multilink Phase- Compensated Long-Range Ultrawideband VNA-Based Channel Sounder

This article presents a novel and cost-effective vector network analyzer (VNA)-based phase-compensated channel sounder operating in the frequency range of 10–50 GHz using radio-over-fiber (RoF) techniques, which can support multilink/channel long-range phase-coherent measurements. The optical cable enables long-range channel measurements with a dynamic range of 115.7 dB at 30 GHz (for the back-to-back connection). The phase compensation scheme is utilized for stabilizing the inherent phase variations introduced by the optical fiber of the channel sounder to enable its application in multichannel/antenna measurements. A novel optical delay line and combiner scheme is proposed and implemented to separate the signals, thereby saving the port resource on the VNA for multilink/channel measurements. The proposed channel sounder is validated in back-to-back measurements under two optical cable conditions, i.e., with the presence of thermal changes and mechanical stress. The phase change could be maintained within 3° at 10–30 GHz and 7° in 30–50 GHz compared to the over 80° phase variation introduced by the cable effects at 10–50 GHz, demonstrating the robustness and effectiveness of the developed channel sounder in practice. With the proposed optical delay line and combiner scheme, multiple channels can be measured simultaneously with minimal VNA ports, significantly reducing the measurement cost and time for multilink channel measurements. Two multilink indoor channel measurements are conducted and analyzed using a virtual uniform rectangular array (URA) at 28-GHz bands. The results demonstrate the capability of the proposed channel sounder to perform high-fidelity long-range ultrawideband multilink channel measurements.

Optical Generation and Transmission of mmWave Signals in 5G ERA: Experimental Evaluation Paradigm

We demonstrate the generation, of a mmWave signal via the injection of an optical frequency comb (OFC) into an integrated tunable dual distributed Bragg reflector (DBR) laser as well as the fiber transmission and the processing of this signal by an optical beamforming network (OBFN). The dual DBR laser is based on a hybrid indium phosphide (InP) – polymer photonic integrated circuit (PIC). Two different cases have been examined in which the microwave signal is centered around 39 GHz and 60 GHz respectively, carrying quadrature amplitude modulation (QAM) formats at 0.5 Gbaud. In this proof-of-concept scenario, the OBFN consists of two optical paths, where the relative true time delay is induced by an optical delay line (ODL). Extensive comparison between the back-to-back (B2B) case and scenarios with transmission over 25 km of standard single-mode fiber (SSMF) has been made using the error-vector magnitude (EVM) and the bit-error ratio (BER) as evaluation criteria. In all cases, error-free transmission was suggested for all QPSK signals, whereas a worst-case EVM of 11.8% was observed for 16-QAM transmission, successfully showcasing the concept’s potential. The generated microwave signal’s frequency can be set arbitrarily high, provided that high-speed photodetection equipment is available for the detection and down-conversion of the signal. Extension to higher antenna elements (AEs) numbers is straight-forward, relying only on the number of available photodetectors.

Arbitrary distance measurement without dead zone by chirped pulse spectrally interferometry using a femtosecond optical frequency comb.

We demonstrate an arbitrary distance measurement method by chirped pulse spectrally interferometry (CPSI) using femtosecond optical frequency comb (OFC). In this paper, the chirped fiber Bragg grating (CFBG) is used to investigate the mapping relationship between displacement and the center frequency of the chirped spectral interferogram. We overcome the direction ambiguity of dispersive interferometry (DPI) ranging and expand the range of distance measurement to 18 cm. Besides, we achieve a full range of dead-zone free ranging by introducing a variable optical delay line (VODL). And through principles simulation and experiment, it is demonstrated that the measurement accuracy is 12 µm in comparison with an incremental He-Ne laser interferometer and the minimum Allen deviation is 52 nm at an average time of 1.76 ms. Similarly, in the experiment with long-distance of ∼30m, the accuracy reaches 20 µm, and 2.51 µm repeatability is achieved under harsh environment.