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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 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.

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.

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.

We apply the self-mixing method for the measurement of the linewidth enhancement factor of several types of semiconductor lasers. The α-factor value above threshold is determined by analysing the small perturbations that occur to the laser when it is subjected to moderate optical feedback, relying on the well-known Lang-Kobayashi equations. The method is applied to Fabry-Perot, VCSEL, External Cavity Laser (ECL), DFB, Quantum Cascade Laser. It is found that for some lasers the α-factor varies with the emitted power, and these variations can be correlated with variations in the laser linewidth.

This work reports on the fabrication, optimisation and characterisation of monolithic mode-locked lasers (MLLs) and colliding-pulse mode-locked (CPM) lasers with repetition rate in the range 10-60 GHz. The devices consist of double section split-contact ridge waveguide lasers fabricated in GaAs/AlGaAs double quantum well (DQW) material. For CPM devices, the saturable absorber section was fabricated with a coplanar ground-signal-ground (G-S-G) pad structure. The optimum saturable absorber size for efficient mode-locking is found experimentally. The fabricated devices are characterised in terms of operating regimes (Continuous Wave, Self-Pulsation, Mode-Locking) and the mode-locking signals were observed in the frequency domain using an external fast photodiode. In the case of CPM devices a mm-wave signal could also be extracted directly from the saturable absorber section of the laser using a microwave probe.

Semiconductor mode-locked lasers and colliding-pulse mode-locked lasers have proven to be effective sources of pulsed optical signals with repetition rate of several tens of GHz and above. Examples of application can be found in OTDM systems, radio-over-fiber networks, and millimeter-wave generation. This work reports on the characterisation of monolithic colliding-pulse passive mode-locked (CPM) lasers at 60 GHz in GaAs/AlGaAs double quantum well material, subjected to optical feedback. The characteristics of the optical-to-electrical converted signals are investigated both by means of an external fast commercial photodiode and by using the saturable absorber section of the device as an intra-cavity photodetector. The power of the electrical RF signal, linewidth, and central frequency stability are measured in unperturbed condition and under the effect of optical feedback. Measurements demonstrate the deterioration of the electrical properties of the signal as the optical feedback level is increased. The reduction of the stability region for mode-locking operation is also reported.

In this work we present experimental results on the auto-correlation and cross-correlation properties of the two counterpropagating modes of a monolithic semiconductor ring laser. The ring laser can operate in a bidirectional regime where the two modes have equal power, and also in a unidirectional regime where one of the modes is almost suppressed. Auto-correlation measurements, that are carried out using an unbalanced Mach-Zehnder fiber interferometer, allows to determine the coherence length and linewidth of the ring laser. Cross-correlation measurements are carried out using a modifed interferometric set-up, and they reveal that the two counterpropagating modes are phase-locked.

We present a new type of laser vibrometer based on the self—mixing interference effect in a laser diode. The self—mixing configuration allows for a practical set—up that is by far simpler than conventional laser vibrometer schemes and can be assembled at a low cost. The vibrometer principle relies on locking of the interferometer to half interferometric fringe, and on active phase—nulling technique by wavelength modulation, that permits to achieve an extended dynamic range still retaining a sensitivity in the sub—nm range. A prototype of the vibrometer has been designed and assembled, and it can operate on nearly all rough surfaces. The vibrometer noise floor is 100 pm/?Hz, the maximum measurable vibration amplitude is 600 ?m peak—to—peak, and the instrument covers the 0. 1 Hz—70 kHz frequency range of vibration. The self—mixing vibrometer can find application in modal analysis and noise and vibration measurements in industrial and scientific environments.

In this work, the injection-detection in a laser diode is studied as a special case of coherent detection scheme. Signal waveform, amplitude and signal-to-noise ratio are calculated as functions of target distance and total optical attenuation. Several semiconductor laser sources are considered (i.e. Fabry-Perot, DFB and external cavity), and signal dependence on the physical parameters of the sources is analyzed.

Semiconductor ring lasers have been designed and integrated with auxiliary coupled optical circuits to beat together the contra-rotating modes. The Sagnac effect predicts a beat frequency proportional to rotation rate, although no beat frequency has yet been observed. Calculations from theory predict a high lock-in frequency; improvements to the design and fabrication of the devices should reduce the lock-in frequency to a useful level.

We present the prototype of a laser feedback interferometer for displacement measurement with micrometric resolution and directional discrimination, which we propose for applications in industrial environment. The instrument incorporates a PC-interfaced electronic unit connected with the optical head, which is very compact thanks to the selected optical configuration and to the use of a semiconductor laser. Infrared as well as red emitting lasers have been used for operation on reflecting and diffusing targets.

In a previous paper, we proposed a new approach to the optical interconnection of telephone- card signals. Our approach was developed around a multimode-fiber star coupler as the optical bus, and standard optoelectronic components for transmitters and receivers. In this paper, we report on recent developments of two practical demonstrators of the above concept, namely: (1) an optical backplane for 24 X 24 I/O signals interconnection operating at 1.1 Gbit/s, with a transmitter and receiver fabricated in hybrid technology and fully connectorized to the star coupler; (2) an optical clock distribution capable of operating up to 4.5 Gbit/s with 25 dB), very little time unbalance between channels (< 15 ps), and an inherent bandwidth-length product of 40 Gbit/s(DOT)m as determined by the coupler and fiber pigtails.

A practical and cost-effective photonic backplane for the interconnection of signals from telephone circuits cards is presented. Each signal is a 20 Mbit/s stream and up to 24 or 48 signals are to be grouped into a 0.48 or 0.96 Gbit/sline. At this level of interconnection, the conventional copper-wire technology faces severe problems of layout congestion, speed limitations and delay matching which are all solved by the use of the photonic backplane technology. After an outline of the interconnection problem for telephone cards, we compare several approaches to the architecture of the photonic backplane in terms of achievable performances and technology requirements. As a consequence, we propose a new architecture based on a multimode star-coupler which offers several significant advantages: has a modular structure and no mechanical constraints of layout, the propagation delay is matched on substantial lengths, and last but not least, can be implemented by low-cost easily available components. A prototype of the backplane is described, which works at a bit rate of 2 Gbit/s and provides 25 dB S/N ratio in the interconnection of 12x12 input/output signals.

Liquid phase chromatography is a well known technique routinely used in analytical chemistry for assays and measurements of aminoacids 1,2. Basically, the solution is pumped at high pressure in a long capillary tube (the chromatographic column) to fraction out the constituents, is mixed to a suitable reactant (usually ninhydrine) so as to develop a spectral absorbance, and is finally analyzed in a flow cell by a colorimeter. With ninhydrine, the reaction product is DIDA (diketo-hydrindilidene-diketolhydrin diamine) which exhibits absorbance peaks at 440 nm (blue) and 570 nm (yellow) in a proportion dependent on the specific aminoacid (Fig. 1), while the amplitude of peaks is proportional to the aminoacid concentration in view of Lambert-Beer law. Besides the two measurement channels of absorbance, either of which or the sum of which is taken as the output signal, a third channel at the wavelength 690 nm at which DIDA is transparent (Ar = 0), is used internally as the reference to the first two. Thus, the colorimeter is actually a spectrophotometer with two fixed-wavelength channels, each referenced in wavelength. In this paper, we report on the design and engineering of a colorimeter aimed to medium/high performances, high reliability and low cost. Use of fiber optics as the beamsplitter of the optical channels is shown to give substantial advantages.