Frequency Drift Compensation Research Articles

Golay Coding Φ-OTDR With Distributed Frequency-Drift Compensation

The performance of optical-pulse-coding (OPC) phase-sensitive optical time-domain reflectometry ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\Phi }$ </tex-math></inline-formula> -OTDR) has been improved significantly in recent years. However, laser frequency drift is still a crucial issue, as it reduces the peak to side-lobe ratio (PSR) in Golay coding <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\Phi }$ </tex-math></inline-formula> -OTDR. In this paper, a novel and simple distributed frequency-drift compensation method is proposed. As the phase accumulations of the reference fiber and the sensing fiber are consistent through theoretical analysis and experimental verification, the frequency drift can be compensated by injecting the modulated Golay coding pulses into two fibers at the same time, and subtracting the phase drift of the sensing fiber and the reference fiber. In order to verify the distributed compensation effect, a two-point perturbation measurement experiment is carried out in 9.7 km sensing range. As a result, the far-end signal-to-noise ratio (SNR) is enhanced and the demodulation perturbation SNRs are increased by 2.8 dB and 18.9 dB separately compared with the case without frequency drift compensation. Furthermore, this scheme can also be used in other complex coding systems, providing more possibilities for system designs.

Laser source frequency drift compensation in Φ-OTDR systems using multiple probe frequencies.

Fully distributed fiber sensors, such as phase sensitive optical time domain reflectometry (Φ-OTDR) systems, have drawn significant attention from researchers, especially for use in geophysical applications. Distributed sensing, cost efficiency, wide dynamic range, good spatial resolution, and high accuracy make these sensors ideal for industrial use and for replacing traditional geophones. However, inevitable drifts in the central frequency of laser sources always cause low frequency noise in the output, which could easily be mistaken with real sub-Hertz environmental vibrations. This deteriorates the data accuracy, especially when dealing with low frequency seismic waves. In this study, we propose a method in which adding an extra probe frequency to a Φ-OTDR setup provides a reference frequency. This reference frequency provides information regarding changes in the laser source and other environmental noises, such as humidity and temperature, helping to refine extracted results from low frequency noise. This feature is also very useful for frequency domain analysis, where we may lose the near DC band information during mathematical measurements. Regarding the adjustable properties of this reference frequency, it can be implemented in various Φ-OTDR applications and commercial devices.

A Phase Noise Measurement System with Frequency Drift Compensation

Commercial equipment provides phase noise measurement solution for voltage-controlled oscillator (VCO), but it also needs to connect additional tuning voltage for VCO frequency stabilization. If the frequency-drifting VCO cannot be tuned directly, the measurement would be difficult. In this article, we present a prototype of a phase noise measurement system, without any connection for frequency stabilization. The innovation of this work is to compensate the frequency drift by introducing a delayed path. The frequency drift of the signal under test is calculated in advance, and then accurately compensates the delayed signal. The results are compared with commercial equipment, and it shows better performance when measuring frequency-drifting signal directly.

A MEMS Real-Time Clock With Single-Temperature Calibration and Deterministic Jitter Cancellation.

This article presents a real-time clock (RTC) system based on a microelectromechanical system (MEMS) resonator coupled to an integrated circuit (IC) that implements a frequency-compensating machine. The MEMS resonator is built with a standard, industrial-grade polysilicon process characterized by a -30-ppm/K linear temperature coefficient of frequency ( TCf ) and the frequency-drift compensation is entirely carried out within the IC using a fractional frequency division. The large, but deterministic, output jitter (≈1 μsrms ) is then suppressed down to less than 40 nsrms with a low-power digital-to-time converter (DTC), whose usefulness in this kind of application is then analyzed. With a single-point temperature calibration, a ±8-ppm output frequency stability is demonstrated at ≈800-nA current consumption from a 1.2-V supply.

Bilateral gradient-echo spectroscopic imaging with correction of frequency variations for measurement of fatty acid composition in mammary adipose tissue.

To develop a simultaneous dual-slab three-dimensional gradient-echo spectroscopic imaging (GSI) technique with frequency drift compensation for rapid (<6 min) bilateral measurement of fatty acid composition (FAC) in mammary adipose tissue. A bilateral GSI sequence was developed using a simultaneous dual-slab excitation followed by 128 monopolar echoes. A short train of navigator echoes without phase or partition encoding was included at the beginning of each pulse repetition time period to correct for frequency variation caused by respiration and heating of the cryostat. Voxel-wise spectral fitting was applied to measure the areas of the lipid spectral peaks to estimate the number of double-bond (ndb), number of methylene-interrupted double-bond (nmidb), and chain length (cl). The proposed method was tested in an oil phantom and 10 postmenopausal women to assess the influence of the frequency variation on FAC estimation. The frequency drift observed over 5:27 min during the phantom scan was about 10 Hz. Phase correction based on the navigator reduced the median error of ndb, nmidb, and cl from 9.7%, 17.6%, and 3.2% to 2.1%, 9.5%, and 2.8%, respectively. The in vivo data showed a mean ± standard deviation frequency drift of 17.4 ± 2.5 Hz, with ripples at 0.3 ± 0.1 Hz. Our reconstruction algorithm successfully separated signals from the left and right breasts with negligible residual aliasing. Phase correction reduced the interquartile range within each subject's adipose tissue of ndb, nmidb, and cl by 18.4 ± 10.6%, 18.5 ± 13.9%, and 18.4 ± 10.6%, respectively. This study shows the feasibility of obtaining bilateral spectroscopic imaging data in the breast and that incorporation of a frequency navigator improves the estimation of FAC.

Stimulated Brillouin laser-based carrier recovery in a high-Q microcavity for coherent detection.

We demonstrate all-optical carrier recovery exploring a stimulated Brillouin laser (SBL) in a high-Q whispering-gallery-mode microcavity, to achieve coherent data detection without requiring an independent local oscillator laser. An ultra-high optical signal-to-noise ratio better than 70 dB is achieved for the recovered carrier, thanks to the fact that the generated SBL counter-propagates with the incoming data signal and experiences high SBS efficiency. High-frequency stability is obtained between the recovered carrier tone and the original data signal, enabling high-performance coherent detection without the need of electrical frequency drift compensation. This Letter offers a low complexity, high energy efficiency, and high robust carrier recovery solution.

Bipolar-Coding $\Phi$-OTDR with Interference Fading Elimination and Frequency Drift Compensation

Optical-pulse-coding (OPC) technique can break the trade-offs between key parameters of distributed fiber-optic sensing systems, therefore it has received great attention in recent years. OPC Φ-OTDR has achieved significant performance improvements over conventional Φ-OTDR; however, scan-rate, interference fading and frequency drift remain critical issues for this type of sensing systems. In this article, for the first time bipolar Golay coding Φ-OTDR with heterodyne detection is proposed and realized to address the scan-rate issue, the laser frequency drift is compensated by a novel real-time compensation method, and the interference fading is eliminated by incorporating the spectrum extraction and remix method. Theoretical analysis is described in detail, and the benefits provided by bipolar code are experimentally demonstrated in 10 km sensing range with 0.92m spatial resolution, achieving 7.1 dB SNR improvement and halving measurement time compared with unipolar code case. By addressing frequency drift and fading issues, the distributed sensing capability of OPC Φ-OTDR becomes available, at a reasonable expense of enlarged spatial resolution (6 m). It should be noted that, such a bipolar scheme can also be applied in other coding schemes, opening up more possibilities for OPC in Φ-OTDR.

Compensation of Frequency Drift in Frequency-Sweep Polarization-Modulation Ranging System

In frequency-sweep polarization-modulation ranging, distance is determined by the frequency of modulated waves and the corresponding wavelength multiple when emitted and returned waves are in phase. However, measurement of the frequency and the wavelength multiple is affected by thermally induced phase delay of the polarized wave. In this article we systematically discuss the principle of the ranging method and analyze the influences of thermally induced phase delay. New approaches to measurement are proposed to eliminate the impact on frequency and the wavelength multiple. Theoretical analysis and experimental results proved the efficiency and applicability of the methods.

All-in-fiber SESAM based comb oscillator with an intra-cavity electro-optic modulator for coherent high bandwidth stabilization.

We demonstrate the stabilization of an all-in-fiber polarization maintaining semi-conductor saturable absorber mirror (SESAM) mode locked frequency comb oscillator with an intra-cavity waveguide electro-optic phase modulator (EOM) to a narrow linewidth HeNe laser over 46 hours. The high feedback bandwidth of the EOM allows a coherent optical lock with an in-loop integrated phase noise of 1.12 rad (integrated from 10 Hz to 3 MHz) from the carrier signal. No piezo fiber stretcher was required to guarantee long-term stabilization, preventing mechanical degradation of the optical fibers and enabling a long lifetime of the oscillator. As an application a hybrid stabilization scheme is presented, where a comb tooth is phase locked to a longitudinal mode of the large ring laser "G" located at the Geodatic Observatory Wettzell. The hybrid stabilization scheme describes the optical lock of the frequency comb to the G laser and the simultaneous compensation of the ring laser frequency drift by comparing the comb repetition rate against an active H-maser reference. In this context the ring laser reached a fractional Allan deviation of 5 · 10-16 at an integration time of 16384 s.

Stabilized Optoelectronic Oscillator With Enlarged Frequency-Drift Compensation Range

To improve the frequency-drift compensation range for the stabilized optoelectronic oscillator, a novel stabilization scheme based on the frequency conversion pair has been proposed and experimentally demonstrated. The cavity length is adjusted by controlling the phase shift of oscillation signal at relatively low frequency via frequency division and frequency multiplication. In the proof-of-concept experiment, a 10-GHz signal has been successfully generated with the phase noise about −123 dBc/Hz at 10-kHz offset frequency assisted by the external triggering. The optoelectronic resonant cavity is tuned at 5 GHz via a divide-by-2 prescaler and a frequency doubler, and the frequency compensation range can be enlarged more than three times compared with a conventional phase-locked-loop-based stabilization method. Finally, the stability of the locked optoelectronic oscillator is improved from $4.1\times 10^{{-7}}$ to $1.1\times 10^{{-10}}$ at 1000-s averaging time.

MEMS Seismic Sensor with FPAA Based Interface Circuit for Frequency-Drift Compensation using ANN

&lt;p&gt;Electrochemical MEMS seismic sensor is limited by its non-ideality of frequency dependent characteristics hence interface circuits for compensation is necessary. The conventional compensation circuits are limited by high power consumption, bulky external hardware circuitry. In these methods digital circuits are also limited by inherent analog to digital conversion and vice versa which consumes significant power, acquires more size and limits speed. A Field programmable analog array (FPAA) overcomes these limitations and gives fast, simple and user friendly development platform with less development speed comparable to ASIC. Recently FPAA becoming popular for rapid prototyping. The proposed system presents FPAA (Anadigm AN231E04) based hardware implementation of ANN model. Using this FPAA based compensation circuit, the error in frequency drift have been minimized in the range of 3.68% to about 0.64% as compared to ANN simulated results in the range of 23.07% to 0.99 %. This single neuron consumes of power of 206.62 mW. and has minimum block wise resource utilization. The proposed hardware uses all analog blocks which remove the requirement of ADC and DAC reducing significant power and size of interface circuit. This work gives the SMART MEMS seismic sensor with reliable output and ANN based intelligent interface circuit implemented in FPAA hardware.&lt;strong&gt;&lt;/strong&gt;&lt;/p&gt;

MEMS Seismic Sensor with FPAA-Based Interface Circuit for Frequency-Drift Compensation Using ANN

Abstract Electrochemical micro-electro mechanical systems (MEMS) seismic sensor is limited by its nonideality of frequency dependent characteristics hence interface circuits for compensation is necessary. The conventional compensation circuits are limited by high power consumption, bulky external hardware. These digital circuits are limited by inherent analog to digital conversions which consumes significant power, acquires more size and limits processing speed. This system presents field programmable analog array (FPAA) (Anadigm AN231E04) based hardware implementation of artificial neural network (ANN) model with minimized error in frequency drift in the range of 3.68% to about 0.64% as compared to ANN simulated results in the range of 23.07% to 0.99%. Single neuron consumes power of 206.62 mW with minimum block wise resource utilization. The proposed hardware uses all analog blocks removing the requirement of analog to digital converter and digital to analog converter, reducing significant power and size of interface circuit and enhancing processing speed. It gives the reliable, SMART MEMS seismic sensor with ANN-based intelligent interface circuit implemented in FPAA hardware.

The SwissFEL Experimental Laser facility

The hard X-ray laser SwissFEL at the Paul Scherrer Institute is currently being commissioned and will soon become available for users. In the current article the laser facility is presented, an integral part of the user facility, as most time-resolved experiments will require a versatile optical laser infrastructure and precise information about the relative delay between the X-ray and optical pulse. The important key parameters are a high availability and long-term stability while providing advanced laser performance in the wavelength range from ultraviolet to terahertz. The concept of integrating a Ti:sapphire laser amplifier system with subsequent frequency conversion stages and drift compensation into the SwissFEL facility environment for successful 24 h/7 d user operation isdescribed.

A temperature stabilized CMOS VCO

The established method of frequency drift compensation in voltage controlled oscillators (VCOs) resulting from temperature variance involves modulation of control voltage using a non-linear voltage...

A temperature stabilized CMOS VCO

The established method of frequency drift compensation in voltage controlled oscillators (VCOs) resulting from temperature variance involves modulation of control voltage using a non-linear voltage internally generated. An innovative frequency drift compensation scheme for a VCO, based on amplitude control, is described in this paper. Two peak detectors are used to generate voltages representing positive and negative peaks of the sinusoidal driving an error amplifier. The amplifier output controls the delivery of transconductance accessible to the oscillator, thereby keeping the oscillation amplitude steady. Frequency stability has improved to 16 ppm/°C from an uncompensated value of 189 ppm/°C and is applicable where frequency stability requirements are not stringent, such as HS-USB and S-ATA. The temperature stabilized VCO at 2.4 GHz center frequency is prototyped using CMOS technology from ams AG (formerly austriamicrosystems AG). The result obtained from this study indicates that better frequency stability may be achievable if the traditional compensation scheme is preceded by amplitude control.

Integration of p-n Junction Diode to Cantilever Mass Sensor for Frequency Drift Compensation due to Temperature Fluctuation

Integration of p-n Junction Diode to Cantilever Mass Sensor for Frequency Drift Compensation due to Temperature Fluctuation

Data-aided phase tracking detection of frequency-shiftedDPSK signalswith baseband frequency-drift compensation

A new data-aided phase tracking detector for frequency-shifted DPSK signals is proposed in which the baseband frequency shift compensation technique of Choi and Lee (1997) is incorporated into the structure of data-aided phase tracking detection. Simulations show that the new detector outperforms the differential detector with perfect frequency-shift compensation for a wide range of frequency shifts, and even approaches the coherent detector in terms of its performance.

Adaptive crosstalk cancellation and laser frequency drift compensation in dense WDM networks

Two variations of the LMS algorithm are proposed to cancel received linear crosstalk in dense WDM networks. Analysis and simulations show that with the addition of a few photodetectors, channel spacing requirements can be reduced by over 50 percent. Simulations using a demultiplexer with Gaussian bandpass characteristics show that a 2.5 dB signal-to-crosstalk-plus-noise ratio can be increased to over 35 dB. The decision-directed algorithm will work with OOK data or any intensity modulation scheme which uses the absence of light as one symbol. The decision-directed algorithm makes assumptions on the desired laser frequency so it can accommodate only limited laser drift. A second cancellation algorithm uses pilot tones added to each laser signal in order to cancel the crosstalk. It will work with analog or digital intensity-modulated data and will automatically configure itself to account for laser drift. Both algorithms are blind in that they do not require training sequences for initialization. Analysis shows that weights derived from pilot tones are nearly optimum for canceling crosstalk for the data. Simulations of both algorithms are presented. Finally both algorithms are shown to be capable of canceling nonlinear beating terms.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of the I-V characteristics of superconducting dipoles : automatic compensation of low frequency drift

Superconducting microbridges and Josephson junctions showing RSJ-like I-V characteristics have potential applications through their current I dependences, at fixed bias voltage V, versus various parameters such as temperature (T), magnetic field (B) or incident optical power (P)... The main problem associated with both low values of voltage (μV-mV range) and dynamic resistance Rd (0.1-10Ω range) was solved earlier; however, the measurement system still suffers from excessive I/f noise due to static biasing conditions if neither the incoming signals nor the preamplifier can be chopped. We have overcome this difficulty through the use of a periodic sampling of four points of the I-V characteristic taking advantage of the odd symmetry of this characteristic. In each period, two of the samples occur in the superconducting state and give the zero voltage reference of the measurement system. This allows us to automatically compensate for the low frequency drifts occuring at preamplifier level. The two other periodic samples, opposite in sign, must appear symmetric with respect to the zero voltage reference at the preamplifier output. This constraint is also used to automatically compensate for the low frequency drifts of the ac square signal witch controls the I-V operating point. For example, a high Tc superconducting microbridge, used as a temperature sensor, has an equivalent low frequency drift of 0.4 mK pp and 6 mK pp respectively with and without the automatic control

Baseband-processing feedforward carrier frequency drift compensation for DPSK mobile radio

The Letter proposes combining baseband-processing feedforward carrier frequency drift compensation (baseband feedforward FDC) with synchronisation word (SW) detection to realise burst DPSK signal reception with differential detection. Crosscorrelation between the differential detector output and the known SW pattern, both in complex representation, is taken both for SW detection and for estimating the phase rotation which appears on the detector output due to carrier frequency drift. The estimate is used to cancel the phase rotation before symbol decision. Computer simulation results of the bit error rate performance of the proposed baseband feedforward FDC are presented for burst π/4-shift QDPSK signal reception in a Rayleigh fading channel.