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We analyze the ultimate timing error that can be achieved in the operation of a LiDAR based on the time-of-flight (ToF) measurement of distance using a pulsed light source and two possible detectors in the optic receiver: (i) an avalanche photodiode APD in linear mode, and (ii) a SPAD single photon detector. We analyze both the random and systematic contributions to the total error and find that the latter becomes dominant at large ( $> 10^{2}$ ) number of detected photons $\text{N}_{\text {ph}}$ . However, the systematic error can be cancelled by a separate measurement of $\text{N}_{\text {ph}}$ . As a conclusion, it is found that, aside from a multiplicative factor of the order of unity, all the schemes supply a timing error given by $\tau /\surd N_{\text {ph}}$ , where $\tau $ is the characteristic time describing the illumination waveform. The theory we have developed provides a theoretical framework for the evaluation of the precision of time-of-flight measurement, and the results are applicable as a benchmark of the timing performance obtained by practical instruments.

This chapter begins with the analysis of thermal and quantum regimes and point out the need for an internal gain in photodetectors. It introduces the figures of merit, which are frequently used in thermal IR detectors and allow to compare completely different detectors. The problem of how to terminate the photodetector on a suitable load resistor, and trade the performances between bandwidth and noise, is common to any quantum detector yielding emitted electrons (or internal electron‐hole pairs) as a response to incoming photons. In single‐photon detection or counting mode and for those detectors that inherently work as photon counters like the semiconductor photodiode avalanche photodiod, the photodetector is cleared of the analogue noise contribution and the figures of merit are improved.

This chapter begins with the description of the photosensitive counterpart of some electronic devices like the bipolar phototransistor, the unipolar phototransistor, and the photo‐SCR. It introduces the photoconductor, the photosensitive counterpart of a resistance, and shows how, surprisingly, it supplies an internal gain. The chapter also introduces the superconducting nanowire photodetector, which has also an internal gain mechanism capable of detecting single photons, a feature that makes the device interesting for applications like quantum cryptography despite the need of working at liquid He temperature. The superconducting detector (SNSPD) is able to detect single photons but one at the time, and like the SPAD, it has a dead time during which it ignores any new incoming photon. Also, the response pulse is just the same for a single photon and multiple simultaneous photons, or there is no photon number resolution.

Multiplication by impact ionization is a very welcome mechanism in photodetectors. Avalanche multiplication by impact ionization is the gain mechanism used for amplification of the photogenerated charge internal to a photodiode junction. The avalanche photodiode (APD) has been the first device to take advantage of the gain of impact ionization. Following the development and the technical maturity of APD, the Single‐Photon Avalanche Detector (SPAD) has been invented, the first semiconductor device able to detect single photons but with a kind of Geiger‐mode limitation. From it, recently the Silicon Photomultiplier (SiPM) emerged as a truly solid‐state counterpart of the vacuum‐tube technology PMT. The SiPM is an example of SPAD array, as it incorporates a large number of individual sensors each with its own resistance for passive quenching.

We show that the speckle phase error incurred when the laser spot is scanned over the diffusing surface can be made as low as 10– $20~\mu \text{m}$ for object sizes in the range 3–10 cm. This result is new, to the best of our knowledge, because it is usually assumed that when the spot moves of its size, the phase becomes uncorrelated and the measurement cannot be done anymore. We carry out numerical simulations of the process and derive an analytical model to evaluate the phase error and the noise equivalent displacement of the profile measurement by a self-mixing interferometer.

The LiDAR-embedded smart laser headlight module employing a single digital micromirror device is demonstrated. This novel scheme enables reduction in optical components and module space requirements for use in the next-generation high-performance autonomous vehicles.

We show that, by adding a photodiode and a plain glass beam splitter to a commercial green laser pointer, we can obtain a very sensitive self-mixing interferometer capable of detecting minute vibrations down to submicrometer amplitude, typically ≈ 13 n m resolution, or make displacement measurements with half-wavelength ( λ / 2 = 266 n m ) count increments.

A headlight module embedding smart laser headlights and a LiDAR is presented. The headlights include blue lasers for the high beam, and blue LEDs for the low beam, each with a high efficiency glass phosphor-converter we have fabricated. The LiDAR is an indirect-mode time-of-flight 905-nm rangefinder. We used two Nichia GaN lasers emitting a total 9.5-W optical power at 445-nm for the high-beam headlight, and five OSRAM GaN LED emitting a total 12-W at 445-nm for the low-beam headlight. The yellow-converter phosphor is a glass-based Ce3+: YAG slab, 100-mm dia. and 0.2-mm thick, that we have fabricated at 750°C temperature, obtaining a high thermal stability, a high conversion efficiency, and a good color rendering index. A digital micromirror device (DMD) placed in the focus of the parabolic reflector is used to switch off sectors of the high-beam when crossing other vehicles. The 2D image of a CCD camera and the LiDAR data are fed to an image processing unit based on CNN that classifies the targets. The headlight module has a >85% detection accuracy of a pedestrian up to 20 m distance.

In a self-mixing interferometer based on a diode laser, the measurement of the external reflector displacement $s(t)$ is carried out by looking at the optical-phase signal $\cos 2ks(t)$ , a signal readily detected as an amplitude modulation (AM) of emitted power. In contrast, the other available signal, sin $2ks(t)$ , a frequency modulation (FM) of the emitted field at optical frequency, is never used because difficult to recover. Recently, Contreras et al. used a narrow-band acetylene cell to convert the FM into an amplitude signal, finding it is larger and has a better SNR than the AM. In this paper, we analyze the advantages of the new CFM (converted-FM) signal, calculating both amplitude and SNR, and compare theoretical results to published experimental evidence, finding good agreement. We then present options for realizing the selective filter in different configurations and technology. Finally, we evaluate the improvement offered by CFM in a number of measurements, like sub-wavelength vibrations, digital readout displacement, and diode laser alpha factor.

At the mirrors of a laser diode self-mixing interferometer, the output beams carry anti-correlated (i.e., in phase opposition) interferometric signals, whereas the superposed noise fluctuations are (partially) correlated. Therefore, by using an instrumental output of the interferometer as the difference of the two, we double the amplitude of the self-mixing useful signal, while the superposed noise is reduced. To validate the idea, we first calculate the noise reduction by means of a second-quantization model, finding that in a laser diode the signal-to-noise ratio (SNR) can be improved by 8.2 dB, typically. Then, we also carry out an experimental measurement of SNR and find very good agreement with the theoretical result.

We present a new concept to implement phase modulation in DQPSK transmitter: moving the physical phase modulator from outside the laser cavity, where it is usually placed, to inside the cavity. On analyzing the new scheme, we find that the in-cavity process of phase–frequency–phase conversion provides a phase gain, which is larger than one up to bit-frequency of about 100 GHz. We supplement the first-hand calculations by numerical simulations based on a travelling-wave model of the field internal to the cavity and find good agreement with theory.

This study numerically investigates the enhancement of photonic microwave generation using an optically injected semiconductor laser operating at period-one (P1) nonlinear dynamics through ultrashort optical feedback. For the purpose of practical applications where system miniaturization is generally preferred, a feedback delay time that is one to two orders of magnitude shorter than the relaxation resonance period of a typical laser is emphasized. Various dynamical states that are more complicated than the P1 dynamics can be excited under a number of ultrashort optical feedback conditions. Within the range of the P1 dynamics, on one hand, the frequency of the P1 microwave oscillation can be greatly enhanced by up to more than three folds. Generally speaking, the microwave frequency enhances with the optical feedback power and phase, while it varies saw-wise with the optical feedback delay time. On the other hand, the purity of the P1 microwave oscillation can be highly improved by up to more than three orders of magnitude. In general, the microwave purity improves with the optical feedback power and delay time, while it only varies within an order of magnitude with the optical feedback phase. These results suggest that the ultrashort optical feedback provides the optically injected laser system with an extra degree of freedom to manipulate/improve the characteristics of the P1 microwave oscillation without changing the optical injection condition.

We develop a new method to read the frequency modulation (FM) of a laser diode self-mixing interferometer (SMI), based on using a Mach–Zehnder interferometer as an edge filter, and obtain improved performance respect to the normally used amplitude modulation (AM) signal. The converted FM signal is much larger and has a better signal-to-noise ratio than the normally used AM signal. The minimum detectable signal of the SMI is improved by a factor of 150 with our setup that uses an all-fiber technology to realize the compact Mach–Zehnder filter (MZF). The MZF has a record 19 (GHz) $^{-1}$ conversion factor and is tuned at half-fringe on the laser wavelength with a feedback loop acting on the laser bias current. On small amplitude vibrations measured with the converted-FM signal, we attain a displacement (NED) of 1.3-pm/ $\surd $ Hz, about two orders of magnitude better than the AM channel or a normal SMI.

A novel configuration of interferometry, SMI (self-mixing interferometry), is described in this paper. SMI is attractive because it doesn’t require any optical part external to the laser and can be employed in a variety of measurements – indeed it is sometimes indicated as the “interferometer for measuring without an interferometer”. On processing the phase carried by the optical field upon propagation to the target under test, a number of applications have been developed, including traditional measurements related to metrology and mechanical engineering – like displacement, distance, small-amplitude vibrations, attitude angles, velocity, as well as new measurements, like mechanical stress-strain hysterisis and microstructure/MEMS electro-mechanical response. In another field, sensing of motility finds direct application in a variety of biophysical measurements, like blood pulsation, respiratory sounds, chest acoustical impedance, and blood velocity profile. And, we may also look at the amplitude of the returning signal in a SMI, and we can measure weak optical echoes – for return loss and isolation factor measurements, CD readout and scroll sensing, and THz-wave detection. Last, the fine details of the SMI waveform reveal physical parameters of the laser like the laser linewidth, coherence length, and alpha factor. Worth to be noted, SMI is also a coherent detection scheme, and measurement close to the quantum limit of received field with minimum detectable displacements of 100 pm/√Hz are currently achieved upon operation on diffusive targets, whereas in detection mode returning signal can be sensed down to attenuations of -80dB.

We demonstrate a flow-meter sensor using a bare laser diode (LD) as a self-mixing interferometer and no additional lens nor optical component. The diode is placed directly on the edge of the duct, exploiting the natural laser divergence. The flow measurement is based on the Doppler shift induced by the scattering particles inside the fluid and read by the self-mixing effect inside the LD. The proposed approach has been modeled by a numerical simulator in order to find the best signal processing algorithm. A set of measurements on blood proves good performances as a flow meter, while simulations confirm the system robustness to mechanical misalignments and changes in liquid properties.

We analyze the regime of strong perturbation in a laser diode subjected to delayed optical feedback (DOF) from an external reflector, and study the intermediate region between short and long cavities, using Lang and Kobayashi equations to follow the dynamic regimes of the DOF system. We find that well-known regimes of unperturbed oscillations, period one, multi-periodic, and chaos are dictated by interplay of coupling factor K, distance L, and phase $\Phi ({\rm mod}.2\pi)$ of the external reflector, and linewidth enhancement factor $\alpha$ . We plot the boundaries of different regimes in the ${\rm K}\hbox{-}\Phi$ plane for several values of L and $\alpha$ , and characterize them in the transition from very short cavity $({\rm L} with negligible high-level effects, to long cavity $({\rm L}=0.5~{\rm L}_{{\rm fr}})$ where the ${\rm K}\hbox{-}\Phi$ plane is almost completely filled with chaos. We show that chaos and periodicity regimes are only found at ${\rm C}>1$ , though for ${\rm C} the DOF system is also subject to self-mixing perturbations. Self-mixing induced FM and AM of the optical signal are found in all regions of stable oscillations, and for ${\rm C}>1$ frequency switching occurs at a certain $\Phi$ for any K and L. Chaos develops at increased K and L in correspondence to loci of frequency switching.

Responsivity and noise properties of the self-mixing (SM) detection process are analyzed. Starting with the photodiode and SM laser scheme as a detector of weak optical echoes from remote targets, we find equivalence to a coherent homodyne detection of the returning signal. In particular, in the VIS-NIR wavelength range, echoes can be detected down to about -90 dB of relative amplitude, typically. Then we consider the laser-diode voltage self-mixing (LV-SM) detection and find it is also a coherent homodyne scheme, though noise performance is limited by shot-noise of the bias current and by the relatively small signal supplied as an output. Theoretical results are finally compared to recent experimental data, finding a substantial agreement and confirming that the LV-SM is an attractive alternative to conventional photo-detection, especially for THz waves and other spectral ranges where good low-noise detectors may be difficult to employ.

In this review, self-mixing interferometry (SMI), a new configuration of interferometry, is discussed. SMI has practical advantages compared to standard interferometry, for example SMI does not require any optical part external to the laser chip and can be employed in a variety of measurements. Applications range from the traditional measurements related to optical pathlength – like displacement, small-amplitude vibrations, velocity – to sensing of weak optical echoes – for return loss and isolation factor measurements, CD readout and scroll sensing – and also, a special feature because of the interaction with the medium, measurements of physical parameters, like the laser linewidth, coherence length, and the alfa factor. Because it is also a coherent detection scheme, the SMI works close to the quantum limit of the received field, typically -90 dBm, so that minimum detectable amplitudes of 100 pm/ √Hz are currently achieved upon operation on diffusive targets, whereas a corner cube allows half-wavelength counting mode – or 0.5 μm resolution – on a dynamic range up to 2 m and more. With its compact setup, the SMI is easy to deploy in the field and can interface a variety of experiments – from MEMS testing to rotating machines vibration testing to pickup of biological motility. The illustration shows a double-channel, differential SMI incorporated in a thermomechanical test equipment to trace the mechanical hysteresis cycle of the beads of a motor-engine brake.

We analyze the performance of position-sensing devices and of distance- or displacement-measuring instruments, and we find that the ultimate uncertainty at the quantum limit of detected signal is given by the same expression in all cases, namely, a characteristic length Lc divided by radicNph, the square root of the number of photons detected in the time interval of the measurement. We derive the expression of the length Lc for well-known position-sensing devices (the quadrant photodiode and the position-sensing detector) and for several measuring instruments (pulsed and sine-wave-modulated rangefinders, triangulation telemeter, laser interferometer, and the optical rule). We also extend the analysis of the uncertainty results to the thermal regime case of detection, i.e., when the detector dark current and preamplifier noises are dominant with respect to quantum noise.

While solitary single-mode semiconductor lasers normally emit continuous-wave radiation, their rich nonlinear dynamics, such as periodic oscillations and chaos, can be excited through various perturbation schemes. These highly complex dynamical behaviors can be well controlled by properly adjusting the experimentally accessible parameters of perturbations. The ability to control the dynamical behavior of semiconductor lasers, together with the profound dynamical characteristics, not only provides valuable opportunities for the study of nonlinear laser dynamics but also opens up great possibilities for a wide range of novel applications.

We show that a self-mixing interferometer can supply not only to the usual optical-phase signal cos 2ks( $t$ ) of the external reflector displacement $s(t)$ , but also the attitude signal $P(a$ ) because of the dependence of power from the angle a between wavevector k and normal n to the target reflective surface. By proper interrogation of $P(a$ ) along two orthogonal components of tilt and yaw, the two angles can be measured simultaneously, together with the displacement, in a single self-mixing interferometer channel implemented by a single-laser diode and monitor photodiode combination.

A simple laser diode, without lenses, is demonstrated as a flow sensor. The diode is placed directly on the edge of the duct, exploiting the natural laser divergence. The flow-measurement is based on the Doppler shift induced by the scattering particles inside the fluid, read by the self-mixing effect inside the laser diode. The proposed approach was modeled by a numerical simulation, in order to find the best signal processing algorithm. Finally, a set of measurements have confirmed the sensor performances as flow-meter.

9 pages, 7 figures., Theory and experiments of single-mode ridge waveguide GaAs-AlGaAs semiconductor ring lasers are presented. The lasers are found to operate bidirectionally up to twice the threshold, where unidirectional operation starts. Bidirectional operation reveals that just above threshold, the lasers operate in a regime where the two counterpropagating modes are continuous wave. As the injected current is increased, a new regime appears where the intensities of the two counterpropagating modes undergo alternate sinusoidal oscillations with frequency in the tens of megahertz range. The regime with alternate oscillations was previously observed in ring lasers of the gas and dye type, and it is here reported and investigated in semiconductor ring lasers. A theoretical model based on a mean field approach for the two counterpropagating modes is proposed to study the semiconductor ring laser dynamics. Numerical results are in agreement with the regime sequence experimentally observed when the injected current is increased (i.e., bidirectional continuous-wave, bidirectional with alternate oscillations, unidirectional). The boundaries of the different regimes are studied as a function of the relevant parameters, which turn out to be the pump current and the conservative and dissipative scattering coefficients, responsible for an explicit linear coupling between the two counterpropagating field modes. By a fitting procedure, we obtain good numerical agreement between experiment and theory, and also an estimation for the otherwise unknown scattering parameters., The work of M. Sorel and A. Scirè was supported by the European Union under Marie Curie Fellowship Contracts HPMF-CT-1999-00083 and HPMF-CT-2000-00617.

This paper reviews the development of a new configuration of interferometry, Self-mixing Interferometry(SMI). SMI is attractive because it doesn't require any optical part external to the laser and can be employed in a variety of measurements, ranging from the traditional measurements related to optical path length like displacements, small-amplitude vibrations, velocity; to sensing of weak optical echoes, like return loss and isolation factor measurements; and also to a special feature for the interaction with the medium, measurements of physical parameters, like laser line width, coherence length, and the alfa factor. Because it is also a coherent detection scheme, the SMI operation is close to the quantum limit of received field and the minimum detectable amplitudes of 20 pm/√Hz or better are currently achieved upon operation on diffusive targets, whereas a corner cube allows half-wavelength counting mode or 0.5 μm resolution on a dynamic range up to 2 m. SMI has a compact setup, and is easy to deploy in the field and can interface a variety of experiments from MEMS testing to rotating machine vibration testing to pickup of biological motility as described in the paper.

The laser diode self-mixing (or feedback) interferometric technique is reviewed as a general tool for remote sensing applications. The operating principle is outlined, and the attainable performance is compared to conventional coherent detection. Applications to metrology and to new sensing schemes are described, experimental results are reported and the overall performance of the sensors are assessed.

Operation of a laser interferometer with a noncooperative target surface, and on a substantial span of displacement (1 m), is reported for the first time to our knowledge. In the paper, we first analyze the errors of an interferometer operating in the speckle pattern regime, then propose the technique of tracking the speckle relative maximum amplitude as a solution for problems so far preventing interferometric measurements of large displacements. Examples of the speckle-statistics improvements are reported. Residual errors are of the order of few wavelengths on a 1-m displacement, indicating that a 10/sup -6/-class instrument can be developed. The proposed approach has been implemented with an injection interferometer.

With a semiclassical model of the optical amplifier, we calculate the evolution of noise along optically amplified lines in presence of nonlinear interaction between signal and amplified spontaneous emission, amplifier saturation, fiber dispersion and vacuum field fluctuations associated with attenuation of the fiber. The noise figure (NF) is calculated for two representative cases of long and short-distance links with different numbers of optical amplifiers and for several fiber dispersions. Results indicate that the optimum number of optical amplifiers is limited by the increase of nonlinear effects with signal power.

We review the major features of laser diode feedback interferometry for sensing applications. The ensembles of the feedback effects on the optical field can be exploited for several non-contact measurements such as displacement, distance, vibration amplitude and frequency. Methods of signal processing are also discussed. On resume les characteristiques principaux de l'interferometrie par diffusion dans le laser a semi-conducteur. Les effects dus a l'injection de la radiation diffusee par le cible dans la cavite du laser peuvent etre utilises pour des mesures de deplacement, de distance, d'amplitude et de frequence de vibration. On illustre aussi les methodes de traitement du signal.

In this paper, we analyze the phenomena arising when a monomode semiconductor laser is subjected to external injection from another laser. The system stability is investigated as a function of detuning and of the relative injected power. Different regimes, spanning from phase locking to chaos and coherence collapse, are described by analytical and numerical methods for weak and moderate injection. Previous theoretical studies are extended by describing the inverse transition from chaos to stability and by deriving the final locking condition. Also, further investigation on the coherence collapse regime has been performed. Besides contributing to the exploration of an interesting fundamental phenomenon, the results of this analysis are useful for different applications, including coherent detection and chaotic cryptography.

Using a laser-diode feedback interferometer, we show how to reconstruct without ambiguity the displacement waveform of an external target from a single interferometric signal. We present the underlying theory with numerical simulations and report an example of actual reconstruction from experimental data. Reconstruction accuracy is on the order of tens of nanometers for displacements of a few micrometers.

We have analyzed the failure data of a standard telecommunication optical fiber under static bending stress. Experimental data have been collected on a 468 day period, observing more than 7000 turns, with a bend radius ranging from 1.25 to 6 mm. The statistical analysis has been carried out by both the least-square and the maximum likelihood method. We have found that data are fitted by a Weibull distribution, as expected theoretically. Moreover, we have determined the scale and shape parameters and tested their dependence on the bend radius. According to our results, we conclude that in sensor applications silica fibers can be safely bent down to a radius less than 5 mm.

Self-mixing interferometry is a noncontact method well suited for measuring a variety of biological signals, like blood pressure wave at wrist and thorax (the optical stethoscope), blood velocity in vein and in external circulation, THz echoes from skin, ear drum vibration, and oculomotor reflex measurements. In this review, after presenting the underlying theory and the main developments of self-mixing, we analyze the applications to biosignal measurement reported so far, and illustrate potentialities and perspectives of the technique.

We show that, in studying eye-head coordination, the rotation of the head can be measured with the aid of a fiber optics gyroscope mounted on a helmet worn by the subject, and that the performances of this approach are considerably better than those of currently used sensors. After presenting the specifications required in head rotation measurements, we report details on the gyroscope used and on its interconnection to the acquisition system, and discuss experimental results of head rotation measurements made by the gyroscope and the potentiometer sensor.

A new method for the measurement of the linewidth enhancement factor of semiconductor lasers is presented, based on the interferometric self-mixing effect. It is a fast and easy to perform method that does not require radio frequency nor optical spectrum measurements. A small fraction of the emitted light is backreflected into the laser cavity by a remote target driven by a sine waveform. The mixing of the returned and the lasing fields generates a modulation of the optical output power in the form of an interferometric waveform, with a shape that depends on the optical feedback strength and the linewidth enhancement factor /spl alpha/, according to the well-known Lang-Kobayashi theory. We show that the value of /spl alpha/ can be retrieved from a simple measurement of two characteristic time intervals of the interferometric waveform. Experimental results obtained on different laser diodes show an accuracy of /spl plusmn/6.5%.

We report what, to our knowledge, is the first example of laser feedback interferometer capable of measuring displacements of arbitrary form using a single interferometric channel. With a GaAlAs laser diode we can measure 1.2-m displacements, with interferometric resolution, simply by means of the backreflection from the surface (reflective or diffusive) under test. The operation is performed at moderate (i.e., not very weak) levels of feedback, such that a two-level hysteresis is found in the amplitude modulated signal. This is shown to allow the recovery of displacement without sign ambiguity from a single interferometric signal. Experimental results are reported, which are found to be in good agreement with the underlying theory. Performances of the developed feedback interferometer are finally presented. >

With a figure-of-eight winding of monomode fiber, we have been able to build multiturn (>100) coils which can be easily trimmed to match the condition of exactly one wavelength-per-turn birefringence. As an example, we report a 45/spl deg/ Faraday rotator intended for an all-fiber optical isolator at /spl lambda/=1300 nm.

We present an optical vibrometer especially designed to measure small amplitude (hundreds pm to several μm) vibrations at very low frequency, down to 0.01 Hz. The instrument is based on a Self-Mixing Interferometer of the fringe-lock type, to achieve high sensitivity and a wide dynamic range. As compared to previous design based on the same principle, our vibrometer is free from fringe-lock loss due to thermal drift of the laser wavelength; this result is obtained by exploiting a separate DC feedback loop acting on the Laser Diode bias. Experiments show a very good long-term stability, glitch-free operation, with rms error of 100-pm/sqrt(Hz) over a bandwidth from less than 20 Hz to 20 kHz.

We have analyzed the behavior of a semiconductor laser subjected to increasing external injection. Numerical simulations show the well-known nonlinear modulation and locking regimes, followed by an intermediate chaotic region that precedes definitive locking to the external source at significantly higher injection levels. >

Self-mixing interferometry (SMI) is a new scheme of interferometry having several advantages compared to standard configurations: it can work with no optical part external to the laser, is self-aligned, is immune to external stray light, and can measure also on target with diffusing surfaces. In this paper, an overview of the several measurements already demonstrated with self-mixing is presented, with application tthe measurements of kinematic quantities (displacement, vibration. etc.) as well physical (linewidth, alfa factor, etc.)

New results on the polarization properties of a matched fiber-coil (i.e. one beat length per turn) immersed in a magnetic field are described. It is shown that both components H/sub y/ and H/sub z/, perpendicular to the coil axis, can be simultaneously measured. Sensing coils of 1-cm diameter, with multiple turns of fiber, were fabricated. Using 100-turn heads, a sensitivity of 0.01 G was achieved for the vectorial fiber sensor. >

We present a theoretical and experimental research of the linewidth enhancement factor or α factor in multiple-longitudinal mode semiconductor lasers. In this work, several methods originally developed for single-mode lasers have been adapted for their use with multiple-longitudinal mode lasers and applied to each mode of several Fabry-Perot lasers at a time. The concepts of material LEF and device LEF are compared and discussed.

The design, realization and characterization of a new type of laser instrument for non-contact measurement of differential displacements are presented. The operating principle is based on two distinct vibrometers that use semiconductor lasers in the self-mixing interferometric configuration. The differential vibration is obtained by electronic subtraction of the vibration signals supplied by the two channels of the vibrometers. The prototype instrument is made by two compact optical heads (5 cm length, 1 cm diameter) and an electronic unit. The working distance can be varied between 10 and 50 cm, and operation is guaranteed on any diffusive surface. The maximum measurable vibration is in excess of 100 μm, and noise-equivalent differential vibration equals 20 nm. The instrument can be used to measure the differential vibrations of two metal samples kept into contact, revealing the hysteresis cycle in the micro-slip and gross-slip regimes.

It is well known that back-reflections in single mode semiconductor lasers usually lead to line broadening and amplitude increase of noise. We will show that a particular choice of the laser bias, which we have called the crossover working point, is helpful to mitigate these effects. Starting the analysis using a self-mixing interferometer, we demonstrate a decrease of the disturbances as high as 40dB in the weak and moderate feedback regimes. We also extend the analysis for incoherent and coherent feedback for the case of high leve l of injection arising from an all-fiber setup. Two different semiconductor laser devices have been tested to verify the analysis predictions, with good agreement with experimental results Keywords: Self-mixing interferometry, semiconductor la ser, optical isolation, optical feedback 1. INTRODUCTION It is well known that back-reflected signa ls affect the operation of a semiconductor laser used as a transmitter in a fiber optic communication system, impairing both the optical line width and phase noise. For these reasons, insertion of an optical isolator is mandatory between the source and the fiber. With reference to a self-mixing interferometric technique, where back-reflected waves are used to advantage, we have been able to carry out a detailed analysis of the back-reflection effect, and show criteria for significant reduction. The principle of self-mixing interferometer was demonstrated many years ago in a He-Ne laser source [1], but it was only in the nineties [2-6] that the method underwent a sharp technical development with the advent of semiconductor lasers suitable to self-mixing. Nowadays the self-mixing inte rferometric technique can be regarded as well established. Radiation emitted by the laser source propagates toward a m oving target where it can be back-reflected or back-scattered in the direction of the front facet of the laser source. Here a fraction of the field is allowed to re-enter into the laser cavity where it is coherently mixed with the lasing field, thus generating a modulation of both the amplitude and the frequency of the radiation emitted by the laser in form of a cosine function, cos(2ks), or of a distorted cosine F(2ks) at middle-high injection, where s is the path length between the laser and the target and k = 2 S /O is the wave number. In this paper we also extend the analysis to the case of a coherent feedback setup for high level of injection, different from that of a self-mixing interferometer, and to the case of an incoherent feedback setup where the forth-and-back path in the external cavity is longer than the coherence length of the source.

The adverse effects of back-reflected waves on a diode laser used as a transmitter can be substantially mitigated if the current biasing is set at a specific value, the self-mixing crossover which can be found by a simple setup. A de-sensitization of the reflection effects by a factor of 25 dB has been observed.

International audience; Round-Robin measurements on the linewidth enhancement factor are carried out in many laboratories participating to EU COST 288 Action. Up to 7 different techniques are applied to DFB, VCSELs, QCL, and QD lasers, and results are compared.

We demonstrate the feasibility of piezoelectric actuation of waveguide devices in silica-on-silicon. Relative to the commonly employed technique of thermal expansion by power dissipation in a resistive element, the piezo actuation drastically reduces drive-power consumption and improves switching time, from milliseconds to microseconds. We report experimental results using a discrete piezoelement, and discuss how the technique can be implemented with integrated technologies for the simultaneous fabrication of a large number of actuators on a wafer.

En: Semiconductor Lasers and Laser Dynamics II; Daan Lenstra, Markus Pessa, Ian H. White, In this work we numerically calculate the linewidth of a single–mode monolithic Semiconductor Ring Laser (SRL) operating in the unidirectional regime. A new expression for the SRL linewidth is derived from the conventional Henry’s formula, and the importance of different physical parameters is discussed. In particular, the linewidth is mainly determined by the SRL diameter, because the waveguide bending loss have a very strong dependence on the diameter. We show that circular SRLs with diameter smaller than 180 µm are unlikely to be operated CW. As a general rule, the linewidth decreases for increasing SRL diameter, and it varies from 15 MHz for 200 µm diameter down to 1 MHz for 2000 µm diameter., We have numerically analysed the performance of a monolithic Semiconductor Ring Laser with a circular cavity. Based on the obtained results, optimum device design rules have been identified, with the purpose of either reducing the threshold current, or increasing the emitted power. The linewidth of the SRL has been calculated by adapting the Henry’s formula to the case of CW unidirectional operation. Calculated values for the linewidth lie between 1 MHz and 14 MHz, and show a dependence on the device radius. Large radius SRLs may exhibit linewidths well below 1 MHz if proper design rules are adopted, while smaller radius devices are not likely to have very narrow linewidth.

This paper deals with linewidth enhancement factor measurements performed with a new method based on the self-mixing optical feedback interference effect capable of measuring the /spl alpha/ value in operating conditions above threshold. Measurements are performed on a commercial ECL in Littman configuration, emitting 40 mW maximum power, with wavelength tunable between 830 and 870 nm.

An instrument for non-contact measurement of differential displacements is realized based on two self-mixing laser diode interferometers. Stress-strain hysteresis cycles in micro- and gross-slip regimes of two metal samples kept into contact are measured.