Laser diode | EPFL Graph Search

A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction. Driven by voltage, the doped p–n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generates light with the same phase, coherence and wavelength. The choice of the semiconductor material determines the wavelength of the emitted beam, which in today's laser diodes range from infrared to the ultraviolet (UV) spectrum. Laser diodes are the most common type of lasers produced, with a wide range of uses that include fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray disc re

High resolution differential laser interferometry for the VLTI (Very Large Telescope Interferometer)

Olivier Boris Scherler

One method for locating extrasolar planets is to observe the lateral movement of a star in the sky caused by a planet in orbit around it. In order to detect this displacement, the angular position of the star has to be measured with high accuracy. This technique is called astrometry. The Very Large Telescope Interferometer (VLTI) is operated by the European Southern Observatory and located at the Paranal Observatory in Chile. The purpose of the PRIMA instrument (Phase Referenced Imaging and Micro-arcsecond Astrometry) of the VLTI is to perform high-resolution astrometric measurements and high-resolution imaging of faint stars using white light interferometry, by combining the light collected by two telescopes. In order to allow the detection of extrasolar planets, the astrometric measurement has to be performed with micro-arcsecond accuracy. In astrometric mode the PRIMA instrument observes two targets at the same time: the object of scientific interest, and a bright reference star. The angular position of the science object relative to the reference star is obtained by monitoring the differential optical path travelled by the light of each object in two separate white-light interferometers. The aim of this work was to develop a high-resolution laser metrology based on superheterodyne interferometry, with an accuracy of 5 nm over a differential optical path of 100 mm. Moreover the laser source had to be stabilised on an absolute frequency reference, in order to ensure the long-term stability and calibration required to achieve the target performance. Superheterodyne interferometry allowed the direct measurement of the differential optical path using two heterodyne interferometers working with two different frequency shifts. The differential phase measurement between the two interferometers was obtained by electronic mixing of the two heterodyne signals, leading to the differential optical path needed for the astrometric measurement

Institut de Microtechnique, Université de Neuchâtel2006

Encapsulation hermétique pour systèmes hydro- et thermo-sensibles

Frank Seigneur

The quantity of MEMS manufacturers has considerably been increasing these last years. This doesn't mean that the introduction of such products on the market has become easier. A packaging operation is needed to obtain a usable product out of a microsystem. The packaging ensures protection but also interconnection with the outside. Too foten, during the conception of a microsystem, the engineer usually does not think about packaging method, which increases costs and time to obtain a product. This thesis is a conception tool to help those who need to realise a microsystem requiring protection against gas and vapours. Some microsystems are sensitive to humidity and temperature. Up to now, to obtain an hermetic package, the easiest solution was soft soldering of a cap on a base. This operation was often done in an oven, which may heat the microsystem to temperatures it can not withstand. In this thesis we describe methods which allow the design and manufacturing of hermetic packages together with a low sealing temperature. In a first phase, material permeability is defined, as well as the notion of package hermeticity and leak measurement methods. The conclusion is that the only materials which allow long-term hermeticity (up to 10 years) are ceramics, glasses and metals. Usual methods for hermetic sealing of packages are then described. Soft soldering by the mean of a laser diode, which is used in this thesis, allows to obtain an hermetic package, together with a low thermal budget. The advantage of the laser diode over another type of laser is its low power density, as well as the possibility to continuously control its power during operation. The power can be controlled during soldering depending on the measured temperature, for example by the mean of a pyrometer. In order to design a package, it is necessary to formalize the requirements in terms of functions that need to be fulfilled by the package. Functional analysis is used to formalize these requirements. This iterative method needs to be done during the conception of the microsystem, but can also be done when the design of the microsystem is already done. In order to reduce reconception phases, most of the functions need to be found at an early stage of the package design. Some methods which allow to find these functions are described. A thermal modelling method is also proposed. This method allows to find an appropriate model to each situation. The first step consists in finding orders of magnitude. The model is then refined by the mean of an electrical analogy or by the mean of numerical simulations when necessary. Several demonstrators have been realized during this thesis. The feasibility of an LTCC (Low Temperature Cofired Ceramic) package for a micromirrors array is demonstrated. The manufacturing of this package is based on screen-printing technology. LTCC packages are suitable for small to medium series of complex microsystems. For complex devices, standard ceramic packages are more expensive than LTCC-based ones, and may not provide the required flexibility. The proposed method is adapted to manufacturing of hermetic cells containing rubidium, for miniature atomic clocks. Sealing time and temperature are drastically reduced compared to anodic bonding, which reduces evaporation of the rubidium during the sealing operation. Some improvements are needed, mainly adhesion of the metallization and atmosphere control during the sealing. The thermal modelling method is illustrated in an example of laser curing of epoxy glue. In order to determine the behaviour of the system, dimensional analysis is used together with experimentation. Versatility of the laser diode is also shown in this project. Finally, some technical points related to the use of a laser diode for soft soldering are also described. Recommendations are given regarding wettability of the solder and reduction of the heating time. Notably, pre-tinning the surfaces with solder is found to be preferable to the use of preforms.

EPFL2009

Realization and study of InAs/GaAs quantum dot superluminescent diodes emitting at 1.3µm

Marco Rossetti

A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons and valence band holes in all three spatial directions, thus creating fully discrete energy levels. The confinement in the InAs/GaAs material system is generated by the bandgap difference for the two materials (∼ 0.4 eV for InAs, and ∼ 1.4 eV for GaAs) which provides a minimum of potential energy for both electrons and holes inside the InAs nanoparticle. The appearance of new nanoscale effects modifies the electro-optical properties of such a system which makes possible the improvement of existing device performance or even fascinating novel device applications. For the creation of coherent structures with very high purity and high density, as it is required for application in light emitting devices, self-assembling is a very important pathway. The quantum dot creation mostly relies on the relaxation of the strain accumulated during the growth of lattice mismatched materials, which happens in a non-equlibrium condition. This results in a statistical distribution of all the parameters characterizing the InAs nanocrystals as size, composition and strain, whose direct manifestation is the inhomogeneously broadened light emission deriving from quantum dot ensembles. At the dawning of the quantum dot development the broadening was considered as a limiting factor for application in semiconductor lasers. Today, it is considered favorably for all the applications where a broad gain spectrum is required as optical amplifiers, monolithic and external-cavity tunable lasers, and superluminescent diodes. The objective of this thesis is the application of InAs/GaAs self-assembled quantum dots to the active region of high power and broad-band superluminescent diodes emitting in the 1.3 μm wavelength region. Superluminescent diodes are edge-emitting semiconductor light sources based on the amplification of spontaneous emission along a waveguide where optical gain is achieved through current injection. They combine the high power and brightness of laser diodes with the low coherence of LEDs. The latter is a fundamental feature for the application in optical coherence tomography medical imaging, where the image resolution is related to the spectral bandwidth of the light source used. The achievement of bandwidths larger than 100 nm are demonstrated in this work through the optimization of molecular beam epitaxy growth conditions. This may be done optimizing the statistical size-dispersion of the dot ensemble, or through the use of different dot layers emitting at slightly different wavelengths. The large bandwidth emission is obtained also due to the peculiar energy structure of this material, for which a multi-state emission appears under particular injection conditions. As a term of comparison, best commercial single-chip superluminescent diodes for the imaging application at 1.3 μm show bandwidths around 60 nm. Moreover, the discrete nature of the energy levels in InAs quantum dots together with the implementation of a GaAs-based technology can be beneficial for the device temperature stability (the competing technology, based on InP, suffers from strong thermal degradation). The analysis and understanding of the device temperature characteristics will be detailed in the last chapter of the manuscript. Even though standard devices show an important temperature dependence, a better stability may be achieved through the use of p-doped active regions. In the last few years quantum dot devices have shown outstanding properties if compared to the competing quantum well technology. Low threshold currents, high temperature stability and low chirp in lasers, large bandwidths and low gain saturation in amplifiers have been demonstrated. However, in spite of the large interest among the scientific community, many of the microscopic processes at the origin of the electro-optical characteristics of this material are not completely understood yet. The physics of the quantum dot active material, will be modeled in this work through the use of systems of rate equations with different complexity depending on the system they are applied to. We will provide a mean-field rate equation model allowing to get insights about carrier dynamics in QD lasers. Also, we will develop two different traveling-wave rate equation models, one for the modeling of the L-I characteristics and the other for the modeling of the spectral characteristics of quantum dot superluminescent diodes. Considering the carrier and photon density distributions across the device cavity is essential in superluminescent diodes, where the high single pass gain results in large non-homogeneities of photon and carrier distributions. The work is motivated by an industrial cooperation with EXALOS AG, a leading company in the development, manufacturing, and sales of broadband superluminescent diodes.

EPFL2007