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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.

In thermal detectors, radiant power is dissipated in an absorber and the resulting minute temperature increase is measured by one of several temperature‐sensing mechanisms well known in electronics. In the thermal infrared, thermal detectors are widely used as non‐contact thermometers and thermographs, for industrial, engineering and biomedical applications. All thermal detectors are made of two independent parts: a power dissipation section and a temperature sensor. This chapter considers the intrinsic limit of detectivity expected for an ideal thermal detector, assuming that the thermal response is maximized and that all the parasitic contributions of any other disturbance are made negligible. A non‐contact thermometer is based on the measurement of the radiant power emitted by a target surface, which is assumed to follow the blackbody (or gray‐body) emission law.

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.

Adding a plain photodiode to a commercial green laser pointer, we obtain a high resolution self-mixing interferometer, capable of detecting minute vibrations down to ≈4.4-nm (rms) resolution, or measuring displacements with half-wavelength (λ/2=266 nm) increments.

Signal processing is a fundamental component of almost any sensor-enabled system, with a wide range of applications across different scientific disciplines. Time series data, images, and video sequences comprise representative forms of signals that can be enhanced and analysed for information extraction and quantification. The recent advances in artificial intelligence and machine learning are shifting the research attention towards intelligent, data-driven, signal processing. This roadmap presents a critical overview of the state-of-the-art methods and applications aiming to highlight future challenges and research opportunities towards next generation measurement systems. It covers a broad spectrum of topics ranging from basic to industrial research, organized in concise thematic sections that reflect the trends and the impacts of current and future developments per research field. Furthermore, it offers guidance to researchers and funding agencies in identifying new prospects., 48 pages, https://iopscience.iop.org/article/10.1088/1361-6501/ac2dbd

Using a glass-clad Ti:sapphire crystal fiber (CF) as the gain medium, we demonstrate for the first time a diode-pumped, 25-mW tunable laser with a 180-nm tuning range. Active medium is a 2.4-cm long, a-cut Ti:sapphire CF we have drawn with an elliptical core of 13- $\mu \text{m}$ diameter in the [0001] axis and 15.5- $\mu \text{m}$ diameter in the [ $1\bar {1}00$ ] axis. The CF is cladded with silica and has high efficiency (88%) of coupling to the pump, a diode laser emitting 1-W at 520-nm. A blazed grating provides a continuous smooth tuning on the wavelength range from 700 to 880 nm.

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.

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.

A laser-diode self-mixing vibrometer with a new, minimum part-count analogue processing of fringe signal is developed. It detects ambient vibrations with high sensitivity and is demonstrated to be an excellent burglary intrusion detector, on a range up to several meters.

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.

Lack of lung surfactant in preterm newborns leads to respirator distress, a major cause of death. Non-invasive medical cares are developed, based on real time measurement of flow surfactant instilled in the newborn. We propose a new optical flow sensor based on selfmixing interferometr, and develop a dedicated prototype for measuring surfactant flow rate in a small diameter duct.

Respiratory distress syndrome is a major cause of death of preterm newborns. This is due to the absence of lung surfactant. Non-invasive medical cares are in development, which need to know in real time the flow of surfactant instilled in the newborn. This work proposes the use of an optical flow sensor based on self-mixing interferometry. A dedicated prototype is realized for measuring surfactant flow rate in a small diameter duct, and its performances are evaluated.

Interferometric techniques have been demonstrated valuable in a number of different experiments involving the measurement of biological signals, because they provide unparalleled sensitivity, contactless operation and are applicable to a variety of biological signals involving kinematic measurands. In particular, self-mixing interferometry (SMI) is attracting interest because of the very simple experimental setup and low cost. We discuss recent progresses of SMI in the field of vibrometry, pressure waves and flow, and THz detection.

Full-field optical coherence tomography with ultra-precise positioning of the PZT by polynomial hysteresis compensation enables depth-dependent spectral data extraction from raw OCT interferometric data. Spectral analysis could quantify molecular/chemical composition of the tissue constituents.

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.

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.

We present a new optical instrument for vibrations measurement, based on a method to read the frequency modulation of a laser diode self-mixing interferometer. The laser source, when exposed to back-injection of its emitted power, is perturbed in frequency and amplitude (self-mixing effect). When the optical back-injection level is low, the self-mixing effect introduces a frequency and amplitude modulation (FM and AM) of the emitted optical field. We demonstrated with a prototype instrument that the performances of FM modulation, in terms of resolution, sensitivity and bandwidth, show an improvement of two order of magnitude with respect to well-known AM self-mixing interferometer.

Some comments about the recently published Optics Letters paper "Quadrature detection for self-mixing interferometry," by Wu and Shu, Opt. Lett.43, 2154 (2018)OPLEDP0146-959210.1364/OL.43.002154, are provided.

We present an overview of the applications of self-mixing interferometer (SMI) to tasks of interest for mechanical engineering, namely high-resolution measurement of linear displacements, measurements of angles (tilt, yaw, and roll), measurements of subnanometer vibrations, and absolute distance, all on a remote target—representative of the tool-carrying turret of a tool-machine. Along with the advantages of SMI—compactness, low cost, minimum invasiveness, ease of use, and good accuracy, we illustrate the typical performance achieved by the basic SMI sensors, that is, the versions requiring a minimum of signal processing and discuss special features and problems of each approach.

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.

Solid-state photomultipliers (SSPM) made by multielement SPAD (single-photon avalanche detector) are nowadays jousted to replace the traditional, vacuum-tube photomultipliers (PMT) based on the photocathode and dynode-chain technology. We revisit the milestones and the conceptual steps leading to single-photon detectors, from the PMT to the SSPM. Then, we discuss state-of-the-art performances of the two detectors and point out that SSPMs are equalling or even surpassing PMTs for response time and sensitivity, while are still lagging for acceptance area, linearity and dark current. We finally compare the detectors in applications to pulse spectrometry, fast waveform analysis, and photon counting.

We analyze the random errors occurring in interferometric measurements because of the speckle pattern regime, when the remote target is a diffusing surface. First, we review the statistical properties of speckle and discuss amplitude fading that is affecting the self-mixing interferometer (SMI) signal and methods to alleviate it. Second, we derive intra-speckle phase errors using the bivariate conditional probability, and find that the noise-equivalent-displacement for small displacement Δ is proportional to the ratio of Δ to speckle longitudinal size sl. Last, we extend the analysis to inter-speckle displacements (Δ > s1) and, after deriving speckle systematic and random errors, show that operation up to meters on a diffusing surface target is possible with a small (≈ λ) error. Results are mainly focussed on SMI, yet they have general validity for any configuration of interferometry.

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.

A novel method for very high resolution measurement of roll angle on a transparent plate is developed theoretically and tested experimentally. The new optical configuration is based on the interferometric readout of phase shift accumulated on the double passage through half wave plate, together with a careful control of polarization state by means of quarter wave plate, and optimizing the tilt of the folding mirror. Sensitivity to roll angle is greatly enhanced and a gain coefficient exceeding 700 is found theoretically, based on Jones' matrix analysis, with a 6-fold increase respect to previous results. In the experimental setup, at the optimum 36° incidence to retroreflector, we measured a gain coefficient of 340. Correspondingly, with an interferometer phase meter resolving 0.01°, a roll-angle resolution 0.1-arc sec is attained.

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.

In this paper, we first introduce mutual and self-coupling and related phenomena in laser diodes, and discuss how the chaos paradigm has been unveiled as a refinement of classical Adler's locking equation describing coupling. Then, we take the ICL (injected coupled laser) system as a reference and illustrate the newly found regimes of injection modulations, labile locking, periodicity and multiperiodicity, chaos, opening and closing bifurcations, up to the final locking of the system. Analysis is carried out by Lamb and Kobayashi equations, in good agreement with experiments. The concept of synchronization is then developed and schemes devised to implement it on an ICL system are discussed. We then show that two cryptography schemes easily follow from synchronization, namely CM (chaos masking) and CSK (chaos shift keying), and describe some implementations of them as well as the results of a sensitivity analysis. After that, we broaden the range of applicability of coupled phenomena showing they are robust against change of parameters and configurations. As a preferred, minimum part-count scheme useful for engineering implementation, we then introduce and evaluate the DOF (delayed optical feedback) configuration, basically a self-mixing scheme operating at a high level of (self)-coupling. For the DOF system, we run through the paradigm of high level dynamics, synchronization and cryptography variants (CM and CSK) again, and find that this system is well suited to all application criteria. We describe practical and in-field implementation of cryptography schemes, CM as well as the PIC (photonics integrated circuit) devices reported so far. A system analysis is then presented, with a summary of experiments on variants of basic cryptography scheme, and some general considerations about cryptography codes and robustness. Finally, we briefly digress to illustrate the recent field of non-cryptography applications of optical chaos systems, such as random number generation, distance measurement, and microwave photonics.