Signal-to-noise ratio analysis of all-fiber common-path optical coherence tomography

We present theoretical analysis and experimental verification of the signal to noise ratio (SNR) of a common-path interferometer-based optical coherence tomography (OCT) system. Based on fully integrated all-fiber implementation of a common-path time-domain OCT system, we derived the SNR of the system including the effect of beat noise, which turns out to be twice as large as the excess noise term. We verified the theoretical SNR through a series of experiments, utilizing both controlled phantom and biological samples such as a rat brain with tumor and a frog retina. The results showed that the source power and the reference reflectivity can be easily controlled to optimize the SNR of OCT imaging. We have also analyzed the effect of the fiber delays and the offset in the fiber autocorrelator of the common-path OCT system on the overall SNR.     

Media limitations on achievement of maximum detection window

The recording performance of a digital recording system is primarily dependent upon the isolated half pulse width (T50), signal amplitude, intersymbol interference (ISI), signal to noise ratio (SNR) and the horizontal detection window margin (Tm). These characteristics are related to the thickness and magnetic properties of the media, as well as the head/media interface. This paper addresses the relationship of the media characteristics in longitudinal digital magnetic recording for both particulate and thin film media to the overall recording system performance. The results are shown to proceed from further development of the quasi-optimum time containment filter analysis for media noise dominant and electronic (white) noise dominant recording channels [1], [2].     

Noise properties of a quadrature phase tracker for interferometricoptical fiber sensors

The noise performance of an electronic quadrature phase-detection system for interferometric optical fiber sensors is presented. Three noise sources are discussed in this work, namely, synchronous detection-circuit noise, phase-perturbation noise; and additive amplitude noise. We determined the output signal-to-noise ratio (SNR) experimentally as a function of input phase power for each of the three noise sources. For uncorrelated synchronous detection-circuit noise the output SNR increases monotonically with input phase power. For correlated noise the output SNR has distinct peaks due to noise cancellation. System performance is limited by uncorrelated detection-circuit noise which exhibits a threshold behavior in output SNR at a phase shift of 25 mrad/Hz^½. The phase noise has a more conventional behavior in the sense that SNR gain occurs only at the expense of dynamic performance. Uncorrelated amplitude noise also displays noise cancellation at certain discrete values of input phase, as is the case for correlated synchronous detection-circuit noise. System insensitivity to correlated light-source amplitude noise is evident from the fact that the output SNR is more than 30 dB higher than the input SNR     

Fiber Bragg Grating Sensor System With Two-Level Ring Architecture

This investigation proposes a fiber Bragg grating (FBG) sensor system with a two-level ring architecture. The survivability and capacity of a FBG for a multipoint sensor system are enhanced by adding remote nodes and optical switches in the two-level ring architecture. Additionally, to enhance the signal-to-noise ratio (SNR) of the sensor system, a fiber ring laser approach is utilized to construct the proposed two-level ring architecture. The fiber ring laser adopted herein yields the high SNR of the sensor system. The proposed system can increase the reliability of FBG sensor systems for multipoint smart structures.      

Direct frequency counting with enhanced beat signal-to-noise ratio for absolute frequency measurement of a He-Ne/I2 laser at 633 nm

We obtain a heterodyne beat signal between an iodine-stabilized He-Ne laser at 633 nm and a low power comb mode of an optical frequency synthesizer with an enhanced signal-to-noise ratio (SNR) up to approximately 35 dB. This is accomplished by the adoption of a fiber coupler and the reduction of the shot noise induced by adjacent comb modes. SNR is improved more than 15 dB compared with that of conventional methods. We measure the absolute frequency of the He-Ne laser mentioned above directly utilizing the enhanced SNR. A quantitative analysis of SNR and some information on experimental criteria for the correct frequency counting are given.     

Noise properties of a bidirectional optical fiber transmission system using Raman amplifers

Abstract

Noise properties of a bidirectional optical fiber transmission system using Raman amplification are analyzed. Analytical expressions for both forward and backward amplified spontaneous emission (ASE) noise are established in terms of system parameters. The signal to ASE noise ratio (SNR) and the achievable repeater length are discussed. Based on the results, we find that there exist optimum pump and initial power levels for the maximum SNR and repeater spacing. The maximum system transmission length and accumulated ASE noise for a multiple‐amplifier system are calculated. A maximum system transmission length up to 5,000 km with 12 dB SNR is then predicted.     

An investigation of high-frequency bias-induced tape noise

Bias-induced tape noise remains a major limitation of the SNR in audio magnetic tape recording systems. Defined as the increment in system noise incurred when the bias oscillator is turned on, the noise can originate from a number of different causes; namely, bias oscillator harmonic distortion, magnetized heads, the earth's magnetic field, and an intrinsic noise source. The latter noise source is our primary concern here. Such record system parameters as head-to-tape spacing, gap length, bias current, and bias frequency were investigated with regard to their influence on this intrinsic bias noise source. Two models of the mechanism of intrinsic bias noise are examined. The first, the "amplitude modulation model." proposes that bias noise is generated by amplitude modulation of the recorded bias signal by the physical and magnetic variations of the head-tape system. In this model, bias noise is merely the lower AM sidebands of the recorded bias signal. The second model relates bias noise to the interaction fields in erased tape. This model proposes that these fields behave similarly to normal recording fields and can be "re-recorded" on the tape at an enhanced level. The two proposed mechanisms are examined in the light of the experimental data. The amplitude modulation model is shown to agree with all the observed data with the exception of the existence of bias noise at bias wavelengths smaller than the particle size. The second model, which does not incorporate a wavelength dependency of bias noise, is in qualitative agreement with the observed data. Methods are discussed for reducing the bias noise without materially affecting the system performance.     

Synthetic ferrimagnetic media: effects of thermally assisted writing

Thermally assisted writing on high-coercivity synthetic ferrimagnetic media (SFM) was demonstrated using a conventional spin stand equipped with an optical head for commercial magnetooptical drives. The laser light (/spl lambda/ = 685 nm) was focused through a glass substrate onto a recording layer. The optical spot size was 1.1 /spl mu/m and a commercial magnetic head had a writer width of /spl sim/0.25 /spl mu/m. The recording properties were measured as a function of the writing current (I/sub w/) and the laser power (P/sub w/). For the thermally stable medium with H/sub c/ = 6 kOe, a laser irradiation with an optimum power significantly improved the overwrite performance and the signal-to-noise ratio (SNR) values. The SNR values were improved by optimizing P/sub w/ over a wide writing current range. The improvements with the assist were found in both the signal and the noise. The media with a large dynamic coercivity value or with thick magnetic layers clearly showed the advantages with thermal assist.     

Advanced noise reduction techniques for ultra-low phase noise optical-to-microwave division with femtosecond fiber combs

We report what we believe to be the lowest phase noise optical-to-microwave frequency division using fiber-based femtosecond optical frequency combs: a residual phase noise of -120 dBc/Hz at 1 Hz offset from an 11.55 GHz carrier frequency. Furthermore, we report a detailed investigation into the fundamental noise sources which affect the division process itself. Two frequency combs with quasi-identical configurations are referenced to a common ultrastable cavity laser source. To identify each of the limiting effects, we implement an ultra-low noise carrier-suppression measurement system, which avoids the detection and amplification noise of more conventional techniques. This technique suppresses these unwanted sources of noise to very low levels. In the Fourier frequency range of ～200 Hz to 100 kHz, a feed-forward technique based on a voltage-controlled phase shifter delivers a further noise reduction of 10 dB. For lower Fourier frequencies, optical power stabilization is implemented to reduce the relative intensity noise which causes unwanted phase noise through power-to-phase conversion in the detector. We implement and compare two possible control schemes based on an acousto-optical modulator and comb pump current. We also present wideband measurements of the relative intensity noise of the fiber comb.     

Experimental and numerical results of optical preamplification in alaser Doppler anemometer receiving head

The first application of a fiber amplifier in a laser Doppler anemometer (LDA) system is presented. Using an Nd-doped fiber amplifier as optical preamplifier in a conventional 1064-nm LDA, a small signal gain of 30 and 35 dB signal-to-noise (SNR) enhancement of the LDA radiation are achieved. The amplifier model, numerical, and experimental results are described     

2.07μm光纤激光在弱湍流条件下的传输特性研究

本文报道了一种2.07μm波段可调谐主动锁模光纤激光并在室内模拟大气湍流条件下进行传输特性研究。增益介质为1.5 m长的掺钬光纤,主动锁模通过LiNbO3强度调制器在腔内引入周期强度调制实现。腔内引入非线性偏振旋转效应实现波长2058.4nm^2078.6nm可调谐。实验获得了稳定的基频锁模脉冲和10阶,24阶,48阶谐波锁模脉冲,对应频谱信噪比为66.79 dB、61.37 dB、54.82 dB和49.66 dB。锁模脉冲经过数字调制后在实验室内大气湍流模拟池中进行传输,分别获得了?T为70℃,140℃和210℃时三种湍流强度和背对背条件下的眼图;与背对背条件相比,在?T=210℃时光信噪比降低了9.14 dB。     

Linearly polarized er(3+)-doped fiber laser by use of a fiber loop polarizer and a wavelength-selective fiber loop reflector

We demonstrate a novel single-polarization Er(3+)-doped fiber Fabry-Perot laser. The all-fiber Fabry-Perot cavity is simple inasmuch as it is formed by only two fiber-based components, a fiber loop polarizer and a wavelength-selective fiber loop reflector. The threshold power is 6.5 mW, the slope efficiency is 12.2%, and the extinction ratio is approximately 25 dB.     

Magneto-optical disk drive technology using multiple fiber-coupled flying optical heads. Part I. System design and performance

A novel flying-optical-head data storage technology is described. It is based on a micro-optical recording head that contains a silicon micromachined torsional mirror for high-bandwidth track following. Multiple heads and disks are contained in a Winchester-style rotating disk drive. Single-mode optical fibers provide light delivery to and from the heads. Both polarization-maintaining and low-birefringence fiber systems have been implemented for magneto-optical (MO) recording. A fixed optics module containing a laser diode, MO detection optics, and a 1 x N fiber bundle switch has been developed as an integral part of this new recording architecture. A 5.25-in. (13.33-cm), half-height prototype drive design and its performance are presented.     

基于常温干燥法的纤维素基泡沫制备及性能分析

传统的石油基泡沫难以降解,因而带来环境污染和安全问题。纤维素基泡沫借助其可生物降解的天然特性,逐渐成为研究热点。然而,目前的成型技术在很大程度上依赖于干燥条件（如冷冻干燥和超临界干燥）,存在干燥耗时长、成本高的问题,因而难以实现泡沫的规模化生产。为解决此问题,提出一种常温干燥制备可再生纤维素基泡沫的新方法。以纸浆纤维为主料、纳米纤维素为黏结剂、聚乙烯醇作为纤维分散剂和泡沫助剂,经过充分混合、发泡、排水和干燥后,制成纤维素基泡沫。最后,测试泡沫密度、孔隙率,分析导热性能、力学性能。结果表明：制备的纤维素基泡沫具有密度低（(0.015±0.002)~(0.028±0.004) g/cm3）、孔隙率高（>98%）、热导率低（(0.060±0.003)~(0.069±0.003) W/(m·K)）等特点。纤维素基泡沫在80%应变下的最大应力值为59.366 kPa,比其他文献报道的类似纳米纤维素基泡沫高37.1%。未来,纤维素基泡沫有望替代石油基泡沫,在冷链运输过程中对产品进行缓冲保护和隔热保温。     

Characterizing a fiber-based frequency comb with electro-optic modulator

We report on the characterization of a commercial- core fiber-based frequency comb equipped with an intracavity free-space electro-optic modulator (EOM). We investigate the relationship between the noise of the pump diode and the laser relative intensity noise (RIN) and demonstrate the use of a low-noise current supply to substantially reduce the laser RIN. By measuring several critical transfer functions, we evaluate the potential of the EOM for comb repetition rate stabilization. We also evaluate the coupling to other relevant parameters of the comb. From these measurements, we infer the capabilities of the femtosecond laser comb to generate very-low-phase-noise microwave signals when phase-locked to a high-spectral-purity ultra-stable laser.     

Performance estimation for maximum-likelihood detection forchannels with transition noise

Transition noise is known to be a major cause of errors for high density magnetic recording. This noise is signal dependent and can be modeled as multiplicative noise in a linear channel model. A maximum-likelihood algorithm was considered for detection of signals in such noise. In this work, the performance of the detector, based on this algorithm, is compared to the traditional Viterbi algorithm (VA) and a modified Viterbi algorithm (MVA) by computer simulations. Results show an improvement of up to 5 dB In signal-to-noise-ratio (SNR) under typical conditions with a reasonable complexity     

Thermal lens micro optical systems

This paper describes two types of miniaturized thermal lens optical systems that use optical fibers, SELFOC microlenses and light sources. The first system consists of a compact diode pumped solid-state laser (532 nm) as an excitation light source, a laser diode (635 nm) as a probe light source, an acoustoptic modulator as an excitation light modulator, fiber-based and conventional optics, and a detection system that combines a pinhole, an interference filter, and a photodiode. The second system consists of two laser diodes as the excitation (658 nm) and probe (780 nm) light sources, fiber-based optics, and the same detection system as the first one. The performance of the two systems was evaluated by the limit of detection (LOD) using standard solutions of sunset yellow (SY) and nickel(II) phthalocyaninetetrasulfonic acid tetrasodium salt (NiP). The LODs of the first system for SY and second system for NiP were calculated to be 3.7 x 10(-8) (1.7 x 10(-6) AU) and 7.7 x 10(-9) M (3.4 x10(-6) AU), respectively. These results were consistent with the expected values obtained from photothermal parameters.     

DFB光纤激光器声致弯曲振动研究

对利用DFB（分布反馈式）光纤激光器进行水声探测时的弯曲振动问题进行了分析与实验研究。总结了采用非平衡干涉仪解调系统解调的DFB光纤激光水听器的声压灵敏度计算公式;基于梁的弯曲理论,通过数值方法计算了两端固定的DFB光纤激光器在50Hz～2000Hz频率范围内的声压灵敏度,绘制了该频率范围内的频响曲线：采用振动液枉法对一支DFB光纤激光器在该频率范围内进行了实验研究,实验数据具有良好的可重复性,实验结果与理论分析吻合。表明了细长型结构的DFB光纤激光器在水声场中很容易由于弯曲振动而引入较大的非声压振动的干扰信号,影响其水声探测性能,有必要在DFB光纤激光水听器探头的设计中考虑这一因素。     

Tunable single longitudinal mode S-band fiber laser using a 3 m length of erbium-doped fiber

In this paper a tunable single-longitudinal mode (SLM), short-wavelength band (S-band) fiber laser using a conventional erbium-doped fiber (EDF) with a length of 3?m and a step index erbium dopant profile as opposed to the commonly used depressed cladding erbium-doped fiber (DC-EDF) is proposed and demonstrated. The proposed SLM fiber laser has a tuning range of 1496 to 1507?nm in a ring configuration using two 0.15?m of EDF which acts as saturable absorbers (SAs). The highest peak power measured is about ?0.6?dBm at a wavelength range of 1502 to 1507?nm. The measured signal-to-noise ratio (SNR) is approximately 74?dB for the same wavelength range. The line-width of the SLM output is measured to be 140?kHz.     

Magnetooptics, lasers, and memory systems

The field of magnetooptics is reviewed and the application of a magnetic memory system as a readout technique is discussed. A review and comparison of the fundamental magneto-optic effects and their utility in a system is presented. It is shown for a longitudinal Kerr readout system that laser and shot noise limit wide-band (1 MHz) signal-to-noise ratios to about 40 dB. Media noise problems are reviewed. The limitations to packing density are discussed, and it is concluded that packing densities greater than 10⁷bit/in²(including suitable guardbands) are practical. The various techniques for optico-thermal recording are surveyed. A discussion of related hardware components (such as optical modulators and lasers) is presented. It is concluded that a viable magnetooptic detection-laser beam memory system is practical. No suitable nonmechanical scanning system has yet been developed.