Self-mixing feedback in a laser diode for intra-arterial optical blood velocimetry

Intra-arterial measurements of the velocity and the average flow of red-blood cells were investigated by means of a fiber-coupled laser Doppler velocimeter based on the self-mixing effect. The velocity of the red cells was calculated from the frequency of the signal that occurs when light, scattered back from a moving object in front of a fiber into a laser-diode cavity, interferes with the laser cavity's proper mode. These fluctuations occur at the Doppler frequency. The signal was obtained from the photodiode that is present in the laser diode's housing. Temperature control and stabilization of the diode cavity were introduced to reduce the light-intensity fluctuation that is due to mode hopping of the diode. The velocimeter was calibrated with a rotating disk covered with white paper (nonlinearity of 2.6% for velocities up to 0.4 m/s) and tested in vitro as a fluid velocimeter. The velocimeter was used in in vivo tests on the iliac artery of a 35-kg pig and on the arteria pulmonaris of a healthy calf. The optical fiber was placed in the iliac artery by a basket catheter 4 cm proximal to the bifurcation of the femoral artery. The average arterial blood flow velocity of the red cells were measured upstream and downstream. A special cleaving procedure for the fiber tip in downstream measurement is reported. Blood-velocity measurement is compared with values generated by an ultrasound flowmeter, and a difference of less than 9% is found.     

Correlation-based speckle velocimeter with self-mixing interference in a semiconductor laser diode

We propose a novel self-mixing laser diode speckle velocimeter based on autocorrelation. The self-mixing laser diode launches and receives reflected light from a moving surface that has a random reflection profile. Some portion of the scattered light backcouples into the laser cavity and causes random intensity variations in the form of speckle signals. These speckle signals obtained from a self-mixing laser diode are processed by use of autocorrelation. The linear relation between the velocity and the reciprocal of the correlation time of the speckle intensity fluctuations allows us to determine the velocity easily if proper calibration is performed. The range of the investigated velocity is 20-450 mm/s. For an aluminum target that moves at a velocity of 350 mm/s, the measurement error is less than 2%.     

Self-mixing laser speckle velocimeter for blood flow measurement

A velocimeter using speckle phenomena in self-mixing laser diodes (SM-LDs) is used to evaluate the blood flow noninvasively. The mean frequency of the speckle signal obtained from the self-mixing laser diode reflects the activity of the blood flow in a certain probing area. The experimental results show that this new type of speckle velocimeter can be useful for the relative evaluation of blood flow in human tissues     

Fiber-coupled self-mixing diode-laser Doppler velocimeter: technical aspects and flow velocity profile disturbances in water and blood flows

The effects of disturbances of the flow pattern in front of the fiber facet of a fiber-coupled self-mixing diode-laser Doppler velocimeter system are investigated. This was done by comparing measurements and calculations of the Doppler frequency spectrum with the expected values. The calculated Doppler spectrum was obtained from the calculation of light scattered (with or without Doppler shift) by the moving particles in front of the fiber facet. The velocity profile of the particles was calculated with a finite-element method. Measurements were done with water (with polystyrene spheres) and whole blood as the samples. Good agreement between measurements and calculations were obtained. The velocimeter was modeled as a five-mirror setup. The reflectivity of the fiber facet closest to the laser turns out to have the most influence on the sensitivity and stability of the laser. Direct reflection of unwanted light back into the laser cavity was avoided by placing a glass plate in front of the fiber. Design consi ations are presented.     

A Comparative Study for the Assessment on Blood Flow Measurement Using Self-Mixing Laser Speckle Interferometer

We study a self-mixing laser diode (SM-LD) as a low-cost compact optical sensor for noninvasive blood flow measurement over the surface of the skin. We compare the SM-LD system with a commercially available Doppler flowmeter to assess the accuracy and feasibility of the SM-LD sensors for such applications. For the SM-LD flowmeter, we apply two different signal processing methods: (1) the counting method, i.e., counting the intensity fluctuations of the signal to obtain a frequency value, and (2) the autocorrelation method, i.e., measuring the autocorrelation time of the signal. In vitro measurements show good agreement with the commercially available flowmeter. In vivo measurements performed on test subjects revealed that the autocorrelation technique shows much better results. The results of in vitro and in vivo studies and the comparison with the commercial flowmeter confirm the applicability of the SM-LD flowmeter.     

Real-time tracking of time-varying velocity using a self-mixing laser diode

A new method is proposed for estimating the time-varying velocity of a moving target with a low-cost laser sensor using optical feedback interferometry. A new algorithm is proposed to track velocity variations from real-time analysis of the output signal of a self-mixing laser diode. This signal is strongly corrupted by a multiplicative noise caused by the speckle effect, which occurs very often with noncooperative targets used in many industrial applications. The proposed signal processing method is based on a second order adaptive linear predictor filter, which enables us to track the digital instantaneous Doppler frequency, which is proportional to the velocity. A model of the laser diode output signal is proposed, and it is shown that the sensor and its associated algorithm have a global first-order lowpass transfer function with a cutoff frequency expressed as a function of the speckle perturbations, the signal to noise ratio and the mean Doppler frequency. Numerical as well as experimental results illustrate the properties of this sensor.     

Shadow Doppler technique for sizing particles of arbitrary shape

The output from a linear diode array is used in a modified laser Doppler velocimeter to measure the size and shape of irregular particles. The sizing accuracy for transparent and opaque particles between 30 and 140 μm is better than 10%. The inaccuracy caused by trajectories that lay at angles of less than 24° to the axis of the array was less than +5%, and a further inaccuracy of +5% was caused by defocusing of the particle from the center of the velocimeter measuring volume by up to ±500 μm. The advantages of the shadow Doppler technique over other techniques for sizing irregular particles, such as amplitude systems with pointer volumes, are that the shadow Doppler technique records shape, the optical arrangement is more robust, less precise alignment is required, and the equipment can be constructed at low cost.     

Frequency-shifted optical feedback in a pumping laser diode dynamically amplified by a microchip laser

Compared with conventional optical heterodyne detection, laser optical feedback imaging (LOFI) allows for a several orders of magnitude higher intensity modulation contrast. The maximum contrast amplification is typically 10(3) for a diode laser in the gigahertz range and 10(6) for a microchip laser in the megahertz range. To take advantage of the wavelength tunability of a laser diode and of the lower resonant detection frequency of a microchip laser, we used LOFI modulation induced by the frequency-shifted optical feedback in a laser diode as a modulated pumping power for a microchip laser for resonant dynamic amplification. In this way, we were able to transfer the optical feedback sensitivity of the laser diode to the megahertz range. Application to telemetry is also reported.     

Automatic measurement of velocity and length of moving plate usingself-mixing laser diode

We propose and demonstrate to automatically measure both a velocity and a length of a moving plate, employing a novel laser speckle velocimeter using a self-mixing laser diode (SM-LD). We derive two empirical equations including velocity of length of a moving plate made of white paper or plastic. After determining a pair of constants fs(0) and β in advance calibration, we can automatically obtain velocity or length from the measured mean speckle signal frequency and/or the number of speckle pulses counted during the measuring time. The investigated range of velocity and length is 100 mm/s to 950 mm/s, and 10 mm to 100 mm, respectively. The measurement accuracy of the velocity and the length is approximately 1% and a few percent, respectively     

Measurement of absolute displacement by a double-modulation technique based on a Michelson interferometer

A novel method is presented for of measuring absolute displacement with a synthesized wavelength interferometer. The optical phase of the interferometer is simultaneously modulated with a frequency-modulated laser diode and optical path-length difference. The error signal originating from the intensity modulation of the source is eliminated by a signal processing circuit. In addition, a lock-in technique is used to demodulate the envelope of the interferometric signal. The displacement signal is derived by the self-mixing technique.     

Loop mirror laser neural network with a fast liquid-crystal display

In our laser neural network (LNN) all-optical threshold action is obtained by application of controlled optical feedback to a laser diode. Here an extended experimental LNN is presented with as many as 32 neurons and 12 inputs. In the setup we use a fast liquid-crystal display to implement an optical matrix vector multiplier. This display, based on ferroelectric liquid-crystal material, enables us to present 125 training examples/s to the LNN. To maximize the optical feedback efficiency of the setup, a loop mirror is introduced. We use a delta-rule learning algorithm to train the network to perform a number of functions toward the application area of telecommunication data switching.     

Optical time-domain reflectometry in optical fiber with reflection delay time matched to the period of the optical frequency modulation

A method of optical time-domain reflectrometry in optical fiber is described that uses a single distributed feedback diode laser and a reference reflector. When the period of the frequency modulation of the laser matches the time difference between the reference reflection and the reflection of interest an increase in the noise at the detector occurs. The locations of reflections within the fiber are then mapped to frequencies at which the noise at the detector increases. A sinusoidal frequency modulation is analyzed, and an experiment is described in which the system is used to measure the location and the wavelength of two Bragg gratings located 10 cm apart in an optical fiber. Wavelength measurement is accomplished by temperature tuning the diode laser.     

Absolute distance measurement with an optical feedback interferometer

An important use of the self-mixing effect inside a frequency-modulated single-mode laser diode is in laser velocimetry and range-finding applications. The optical beam reflected by a target and injected into the laser diode cavity modulated by a reshaped current is mixed with the light inside the cavity, causing variations of the optical output power. A theoretical analysis of this effect is proposed, based on the determination of the beat frequencies of the optical power variations, to improve the accuracy of laser distance measurement. A resolution of ?1.5 mm from 50 cm to 2 m is obtained when thermal effects are taken into account.     

激光自混合变速测量的多项式相位参数提取

激光自混合能有效用于匀速运动物体的速度测量,为了加速激光自混合用于物体速度测量的实用化进程,对激光自混合用于变速运动物体的速度测量进行了探索.引入了基于多项式相位参数提取的信号处理算法;基于激光自混合的测速原理,推导了激光自混合用于变速测量时输出信号的多项式相位表示式,也即建立了激光自混合用于变速测量的仿真模型.通过仿真分析证明:在激光自混合变速测量中采用基于相位多项式参数提取的方法,可以获得物体运动的速度和加速度信息;该方法在较低信噪比的情况下仍然适用.     

Wavelength switching of picosecond pulses generated from a self-seeded Fabry-Perot laser diode with a tilted fiber Bragg grating formed in a graded-index multimode fiber

We demonstrated the generation of wavelength-switchable picosecond pulses from a self-seeded Fabry-Perot laser diode that used a tilted fiber Bragg grating (FBG) formed in a graded-index multimode fiber as an external optical feedback element, where wavelength switching was achieved by controlling the modal distribution in the FBG. We measured the reflection spectra of multimode FBGs fabricated with different tilt angles and discussed the effects of the tilt angle on wavelength selection. By using a 20?mm long 1.65° tilted FBG and a fiber deformer to control the modal distribution in the FBG, we generated 2?GHz pulses with a wavelength switchable over 14 wavelengths at a spacing of ～0.8?nm.     

Laser vibrometer based on optical-feedback-induced frequency modulation of a single-mode laser diode

We describe a sensitive and inexpensive vibrometer based on optical feedback by diffuse scattering to a single-mode diode laser. Fluctuations in the diode laser's operating frequency that are due to scattered light from a vibrating surface are used to detect the amplitude and frequency of surface vibrations. An additional physical vibration of the laser provides an absolute amplitude calibration. The fundamental bandwidth is determined by the laser response time of roughly 10(-9)s. A noise floor of 0.23 nm/Hz(1/2) at 30 kHz with 5 × 10(-5) of the incident light returning is demonstrated. This instrument provides an inexpensive and sensitive method of noncontact measurement in solid materials with low or uneven reflectivity. It can be used as a vibration or velocity sensor.     

High-contrast saturated absorption spectrometer with a grating reflector as an optical frequency locker

By optical feedback of signals obtained with a high-contrast saturated absorption spectrometer as a reference frequency selector, diode laser frequency has been locked to the hyperfine transition line of a Cs atom. We used a grating instead of a mirror for optical feedback to avoid feedback of unwanted modes, because such modes cause mode hopping or generate another frequency component, whose intensity does not decrease even after it passes through an atomic cell twice. Long-term frequency stability has been improved by compensating the cavity length for phase fluctuations.     

Amplitude-stabilized frequency-modulated laser diode and its interferometric sensing applications

A direct frequency-modulated (FM) laser diode light source without light power variation is developed. The amplitude variation of the FM laser diode is compensated by means of a feedback system with use of a superluminescent diode as an external light power controller. Output power greater than 1 mW is obtained at the modulation frequency to 5 kHz with a >10 stabilization factor. By use of the amplitude-stabilized FM laser diode, we measured subfringes with high accuracy in FM continuous wave interferometry, increased the dynamic range of the displacement measurement, and improved the stabilization factor in the laser diode feedback interferometer.     

Laser diode facet modal reflectivity measurements

A simple and accurate method for measuring the front facet modal reflectivity of a Fabry-Perot laser diode is presented. In this method, optical feedback from an external mirror of known reflectivity, R(ext), is used to alter the laser diode threshold current. The effect of the external mirror and front facet reflectivities on the threshold current then allows for a measurement of the front facet modal reflectivity of the laser diode and is theoretically and experimentally studied. This method was used to measure a facet reflectivity of R(2) = 0.0151(+0.0018/-0.0032) [R(2) = 0.00592(+0.00085/-0.00123)] for a commercially antireflection-coated facet of a laser diode with a center wavelength of 795 nm (935 nm). The results of the reflectivity measurements based on the threshold current as a function of the external mirror reflectivity are compared with the results of the reflectivity measurements based on modulation depth of the optical spectrum [IEEE J. Quantum Electron. QE-19, 493 (1983)].