Spectral multiplexing with optical-path-tailored interferometric sensing system for real-time differential phase detection

Optical nanofibers (ONF) have emerged as versatile platforms for studying light-gas interactions at the micro/nanoscale, yet existing ONF gas sensors remain limited in detection sensitivity. Here, we report a polarization-mode photothermal interferometry technique that precisely measures the gas absorption-induced phase difference between two polarization states of the symmetric supermode of a single-mode ONF coupler. The high power density and large evanescent field associated with the ONF coupler enhance the efficiency of photothermal phase modulation, while the strong waveguide birefringence and noise-immune differential phase detection confer environmental immunity, jointly yielding an order-of-magnitude enhancement in the signal-to-noise ratio. With a 2 cm-long overcoupled ONF coupler, we achieved an acetylene detection limit of 6 ppb and an instability below ± 1.2% over 30 h. This compact ONF gas sensor, based on standard fused directional coupler technology, provides a promising route toward cost-effective and high-performance solutions for environmental monitoring and industrial applications.

This article presents a 28-GHz, sub-40-fs-jitter single-stage differential sampling phase-locked loop (DSPLL) with accelerated slide locking (ASL) and sampling voltage shift technique. To solve the cycle slips caused by the limited linear detection range (DR) of the differential sampling phase detector (DSPD), the detailed frequency capture behavior is analyzed and the ASL technique is introduced to eliminate cycle slips during the frequency pull-in stage. Additionally, a sampling voltage shift DSPD (SVS-DSPD) is employed to adjust the sampling voltage position, which reduces the variation in the sampling voltage for better in-band phase noise (PN). A dual-core voltage-controlled oscillator (VCO) is utilized to achieve a 3-dB out-of-band PN reduction. Fabricated in 65-nm CMOS process, the proposed DSPLL demonstrates a jitter performance of 36.75-fsrms and a locking time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.66~\mu $ </tex-math></inline-formula>s. Furthermore, the measured power is 32.69 mW at 28-GHz output, resulting in an figure-of-merit jitter (FOMJ) of −253.6 dB and an FOMN of −275 dB.

Abstract Deep learning models trained to estimate the probability of seismic P and S phases are rapidly expanding the scale of local event detections. Here, we evaluate the potential for deep learning model output phase detection probabilities to contribute to event-type classification, particularly discrimination of single-fired borehole explosions and earthquakes at local distances (&amp;lt;300 km). Motivated by the empirical success of P/S amplitude ratios, we consider the difference between P and S pick probability output from previously developed phase detection models, Pprob−Sprob, as a discriminant. Test data include ML∼1–4 earthquakes and explosions observed by common seismographs in ten geologically diverse localities. Depending on the picking model and training data, binary classification using Pprob−Sprob with at least three stations can achieve approximately equivalent classification accuracy as P/S amplitude ratios without requiring any customization. Joint classification with P/S and Pprob−Sprob improves accuracy for most quality control scenarios. Pick probabilities are an efficient attribute to consider in explosion discrimination because they can be automated byproducts of event detection. They avoid the binary choice of picking or not picking weakly visible S waves common to explosions.

We present a stable optical frequency comb (OFC) that utilizes a Fabry-Perot phase modulator. The environmental-induced state variation of the OFC is accurately detected by measuring the relative phase changes of beat signals from its upper and lower sidebands. We then compensate for this variation by controlling OFC bias voltage through a homodyne phase-locked loop. The differential phase detection eliminates the common-mode detection noise, enabling long-term stability of the OFC without requiring any additional reference signal. The relative phase change is only 0.056° over 3800 s. Even under a drastic temperature change, the OFC remains stable, validating the effectiveness of the proposed stabilization method.

To overcome the poor low-frequency response sensitivity of differential phase detecting interferometer, an improved differential phase detecting optical fiber interferometer with a time domain low frequency compensation scheme is designed and implemented, effectively compensating the low frequency loss of the system by means of recursive accumulation method. Theoretical derivation and experimental verification are conducted in this paper. The experimental results indicate that the improved system can significantly enhance the amplitude of 10 Hz signal and signals with high fidelity can be obtained because the compensated system responds equally both to the high and low frequency signals. Further experiments in the buried pipelines monitoring reveal that the signal-to-noise ratio is obviously improved by 22 dB approximately compared with the traditional system. The positioning error of the proposed system is within &#x00B1;20 m along a 40 km sensing fiber. The differential phase detection interferometer compensated by the proposed scheme has potential application in low-frequency fields such as seismic wave detection, hydrophones, sonar, pipeline monitor and geological detection.

A differential phase sensor based on a narrowband perfect absorber uses the phase singularity at the point of darkness (100% absorption). Herein, a structure with a top SiO2 layer is proposed that is index matched with a glass flow cell that shows the point of darkness for both s‐ and p‐polarizations. Detailed numerical results to optimize the structure, study the role of fluctuation in thickness and roughness of analyte layer show that the proposed structure has refractive index sensitivity of 147.3° RIU−1. Experimental results of launch angle independent ≈99.9% absorption in the 45°–73° range are reported, with the highest reported till date quality factor in planar narrowband absorbers of ≈42, and high figure of merit of 556 for p‐polarization. The structure also exhibits the generalized Brewster effect with a perfect absorption (≈99.99%) for s‐polarization at ≈74° angle. Experimental results are presented to demonstrate the sensor operation for two different biomolecules at the p‐polarization resonance. The configuration presented is ideal for reusable and cost‐effective biosensors.

Long-haul optical fiber communication employing digital signal processing (DSP)-based dispersion compensation can be distorted by the phenomenon of equalization-enhanced phase noise (EEPN), due to the reciprocities between the dispersion compensation unit and the local oscillator (LO) laser phase noise (LPN). The impact of EEPN scales increases with the increase of the fiber dispersion, laser linewidths, symbol rates, signal bandwidths, and the order of modulation formats. In this work, the phase noise cancellation (PNC) employing a radio frequency (RF) pilot tone in coherent optical transmission systems has been investigated. A 28-Gsym/s QPSK optical transmission system with a significant EEPN has been implemented, where the carrier phase recovery (CPR) was realized using the one-tap normalized least-mean-square (NLMS) estimation and the differential phase detection (DPD), respectively. It is shown that the RF pilot tone can entirely eliminate the LPN and efficiently suppress the EEPN when it is applied prior to the CPR.

An improved self-coherent digital-signal-processing-based optical receiver that utilizes polarization diversity is presented and analyzed. It demonstrates that coherent performance can be approached by employing field reconstruction on differential-phase detection and judicious digital signal processing. Performance is improved compared to known techniques by addressing two shortcomings that characterize self-coherent detection: 1. the phase drift caused during field reconstruction; and 2. the loss of synchronization induced by a zero-intensity sample with undefined phase. These will be met with phase estimation and periodic correction, and with polarization-diversity and a non-linear quantization scheme, respectively. It is demonstrated by means of simulations that the improved receiver allows achieving detection of an optical orthogonal frequency division multiplexing at uncoded bit-error-rate of 3*10-5, with 12 bit non-uniform quantization, over an 18 dB signal-to-noise ratio channel.

A real-time and robust phase detection method for the in-line inspection of phase objects is proposed. Time differential phase detection (TDPD) detects phase signals by using four cameras within a short interval of time while moving an object across the beam path. Theory shows that the proposed method is substantially less susceptible to intensity deviation among multiple cameras and to spatial phase variation in both the object beam and reference beam when amplifying a weak signal by optical homodyne amplification. Experimental results show that stable phase retrieval is feasible in the presence of a non-uniform camera gain of up to 20% and a peal-to-valley wave aberration error in the optics of up to four wavelengths.

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We demonstrate a size sensing technique for nano-particles using optical differential phase measurement by a dual fiber interferometer through phase-generated carrier (PGC) demodulation. Nano-particle diameters are obtained from the differential phase shift as a result of adding an optical scattering perturbation into two-beam interference. Polystyrene nano-particles with diameters from 200 to 900 nm in a microfluidic channel are detected using this technique to acquire real-time particle diameters. Compared with amplitude sensing with over 10 mW of laser irradiance, particle sizing by PGC phase sensing can be achieved at a laser power as low as 1.18 mW. We further analyze major sources of noise in order to improve the limits of detection. This sensing technique may find a broad range of applications from the real-time selection of biological cell samples to rare cell detection in blood samples for early cancer screening.

Track error (TE) detection system for readout of a high density super-resolution optical disc beyond Blu-ray Disc unaccompanied by its complication, based on a differential phase detection method is proposed. An evaluation of quality of TE signal with this system is described on a numerical simulation using experimentally acquired readout signal of the super-resolution optical disc.

델타 구동 GMSK는 바이폴라 델타 신호로 가우시안 펄스를 생성하고 <TEX>$+T_b/2$</TEX>와 <TEX>$-T_b/2$</TEX> 사이의 차분 신호로 위상변조하는 방식이다. 이러한 방식에 대한 복조는 기존의 GMSK와는 다르게 설계해야 한다. 본 논문에서는 인접한 비트 간의 위상 변화를 검출하고 위상 영역을 경판정한 다음에 상태 전이로 이진 데이터를 복원하는 방식을 설계하였다. AWGN 채널에 대한 전산모의실험 결과, 이진 데이터의 런 랭스에 의한 비트 오율의 영향이 확인되었으며, 이론적인 코히어런트 복조의 BER과 비교해서 <TEX>$BT_b$</TEX>=0.3에 대하여 2[dB] 정도의 성능 저하가 있는 것으로 분석되었다. The delta driven GMSK is a modulation scheme to generate the gaussian pulse by the bipolar delta signal and to modulate the phase function with the differential gaussian pulse between <TEX>$+T_b/2$</TEX> and <TEX>$-T_b/2$</TEX>. The demodulation of delta driven GMSK differs from conventional GMSK schemes. This paper proposed a demodulation with the differential phase detector at the end of each bit interval. A designed detector then finds the one of the possible region by hard decision, in which the phase difference lies. Finally the binary data can be recovered by state transition process. Through the BER simulation for AWGN channel, it was found that the proposed demodulation effected by the run length of binary data. The simulated BER degradation of about 2[dB] was analyzed, as compared to the theoretical coherent demodulation with <TEX>$BT_b$</TEX>=0.3.

In–Sb-based super-resolution near-field structure read-only-memory discs with radial density increased by introducing a narrow track pitch corresponding to the diffraction limit of an optical system were developed. Using an optical system with a laser diode with a wavelength of 405 nm and an objective lens with a numerical aperture of 0.85, we confirmed that differential phase detection (DPD) could detect track errors from disc samples recorded random data including a minimum pit length of 75 nm in a 240 nm track period. It has higher capability of track error detection than push–pull detection at a narrowed track pitch. Moreover, bit error rates satisfying the criterion of 3.0×10-4 were experimentally obtained for 66.7-GB-capacity disc samples with a 240 nm track pitch through signal processing with the partial response maximum likelihood of the (1,2,2,1)-type, by applying DPD to tracking servo control. The feasibility of increasing the track density of the Blu-ray DiscTM physical format by 1.33 times was indicated.

High performance and reliability are important aspects of space-based systems. In many cases correct system functionality must be continuously monitored to ensure the validity of collected data. In this research we develop a new digital circuit which can detect minute phase differences in time-varying analog signals. These phase shifts can be determined for both a single channel input versus a known model of the signal and across two channels with simultaneously-sampled data. Phase shift data can be used in spaceborne systems to either confirm correct system operation or identify potential problems. Current systems primarily transfer data to the Earth for error checking rather than performing error analysis in real-time. The benefits of our field-programmable gate array (FPGA)-based approach are evaluated using a 3 gigasamples per second (GSamp/s) data acquisition system developed as part of the NASA Surface Water Ocean Topography (SWOT) initiative. Phase calculations with an error of less than 0.021 deg (0.006% of 360 deg) are determined using our adaptive approach. The high accuracy of differential phase detection allows for the real-time monitoring of environmental metrics, such as temperature fluctuations, which affect signal phase.

A system to interrogate optical fiber interferometric sensors with digital control is presented. The system is based on a receiving white light Mach-Zehnder interferometer and is capable of operating with four distinct synthetic and pseudo-heterodyne signal detection schemes. A differential phase detection scheme was implemented and system performance with the different processing schemes was compared using fiber Bragg grating based Fabry-Perot cavity strain sensors. With a lock-in time constant of 1 s, most digital techniques were able to nearly match the performance of a standard hardware system, demonstrating the feasibility of low-cost high-resolution interferometric systems operated with virtual instrumentation.

The sub-land/sub-pit affects the characteristic of the tracking error signal which is generated by the conventional differential phase detection (DPD) method in the signal waveform modulation multi-level (SWML) read-only disc. To solve this problem, this paper proposes a new tracking error detection method using amplitude difference. Based on the diffraction theory, the amplitude difference is proportional to the tracking error and is feasible to be used for obtaining the off-track information. The experimental system of the amplitude difference detection method is developed. The experimental results show that the tracking error signal derived from the new method has better performance in uniformity and signal-to-noise ratio than that derived from the conventional DPD method in the SWML read-only disc.

Signal waveform modulation multi-level (SWML) read-only discs employ sub-lands/sub-pits to achieve multi levels. However, the tracking error signal derived from the conventional differential phase detection (DPD) method deteriorates in the SWML read-only disc owing to sub-lands/sub-pits inserted in the conventional pits/lands. To solve this problem, a new DPD method, which employs a signal adjuster and a phase regenerator, is proposed. The experimental results show that the tracking error signal derived from the new DPD method has better characteristics of uniformity and signal-to-noise ratio than that derived from the conventional DPD method in the SWML read-only disc.

The Bit-Error-Ratio (BER) floor caused by the laser phase noise in the optical fiber communication system with differential quadrature phase shift keying (DQPSK) and coherent detection followed by digital signal processing (DSP) is analytically evaluated. An in-phase and quadrature (I&Q) receiver with a carrier phase recovery using DSP is considered. The carrier phase recovery is based on a phase estimation of a finite sum (block) of the signal samples raised to the power of four and the phase unwrapping at transitions between blocks. It is demonstrated that errors generated at block transitions cause the dominating contribution to the system BER floor when the impact of the additive noise is negligibly small in comparison with the effect of the laser phase noise. Even the BER floor in the case when the phase unwrapping is omitted is analytically derived and applied to emphasize the crucial importance of this signal processing operation. The analytical results are verified by full Monte Carlo simulations. The BER for another type of DQPSK receiver operation, which is based on differential phase detection, is also obtained in the analytical form using the principle of conditional probability. The principle of conditional probability is justified in the case of differential phase detection due to statistical independency of the laser phase noise induced signal phase error and the additive noise contributions. Based on the achieved analytical results the laser linewidth tolerance is calculated for different system cases.