Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-09-13
2004-07-27
Jackson, Jerome (Department: 2815)
Coherent light generators
Particular active media
Semiconductor
C257S013000, C257S017000, C257S019000, C257S014000, C257S021000, C257S097000
Reexamination Certificate
active
06768754
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of photonics, and more particularly to low-dimensionality semiconductor laser devices capable of emitting different frequencies.
BACKGROUND OF THE INVENTION
Presently, very efficient and compact laser sources can be obtained using semiconductor laser diodes based on 2-dimensional quantum well(s) in their active gain region. Such state-of-the-art semiconductor laser diodes can produce hundreds of milliwatts of laser light emitted over a narrow range of wavelengths of a few nanometers (nm) or smaller. Typically, to obtain a different wavelength, a distinct laser diode must be fabricated with the appropriate quantum well(s) in its active region. For several applications, a wide range of wavelengths are necessary. This limits the usefulness of semiconductor laser diodes based on quantum wells because the 2-dimensional density-of-states of the electronic structure results in a gain spectrum which can be tuned at most by tens of nanometers using external cavities, or using integrated tuning elements.
The current state-of-the-art technology used to obtain laser sources tunable over hundreds of nanometers using external cavity configurations with a solid-state crystal such as a Ti-Sapphire lasers (Ti-Saph lasers), or with dyes mixed in a liquid medium (Dye lasers). These lasers have major limitations because they are not compact and are very inefficient since they have to be aligned and optically pumped with another powerful laser operated at shorter wavelengths.
There exists a real need for compact and efficient lasers, tunable over a broad range of wavelengths for multimedia and telecommunication applications, as well as for diagnostic and research/development tools. New applications will also emerge with the development and availability of such laser sources.
It is therefore an object of the invention to provide an apparatus and method capable of generating laser light tunable over a wide range of wavelengths in a compact and efficient way.
SUMMARY OF THE INVENTION
Unlike the density-of-state of bulk material and of quantum wells, the electronic configuration of low-dimensional nano-structures, herein defined as quantum wires (one- or quasi-one dimensional structures) or quantum dots (zero or quasi-zero dimensional structures), will allow the saturation of their reduced density-of-states over a wide range of energies because the total number of available states is orders of magnitude smaller than for quantum wells. This will permit the production of population inversions and lasing over wide range of wavelengths. Also, it is possible to produce Self-Assembled Quantum Dots (QD) by epitaxy using highly strained semiconductors, and to have good
1
o control over their zero-dimensional density-of-state. Such quantum dots can be grown in a laser diode configuration with conventional techniques, and the carriers will be injected electrically in the QD laser diode. To obtain the tunability in such a QD laser diode having a wide gain spectrum, an external cavity is used. The resulting QD tunable external cavity (QD-TEC) laser retains the efficiency and convenience of conventional semiconductor laser diodes, and yet is tunable over hundreds of nanometers by choosing the low-dimensional electronic structure of the QD and the optical properties of the external cavity.
Accordingly in a broad aspect the invention provides a laser system comprising a laser diode with low dimensional quantum structures for emitting light over a wide range of wavelengths, a wavelength-selective element for selecting a wavelength of interest emitted by said laser diode, and an external cavity resonant at a wavelength selected by said wavelength-selective element so that the system generates laser light at said selected wavelength.
The wavelength-selective element used to tune the laser output may consist of an a diffraction grating, a prism, a birefringent element, an etalon, or a dispersive element.
One dimensional or quasi-one-dimensional structures can be obtained from coupled zero- or quasi-zero dimensional structures, or from other techniques which can produce quantum wires.
In operation the application of an electric field causes charged-carriers to be injected from contact layers into an active region of a semiconductor heterostructure containing quantum dots or quantum wires. Then photons originating from the radiative recombination of the charged carriers in the active region are emitted. The photons are confined in the cavity designed with tunable wavelength-selective elements which are adjusted to support a lasing output over the selected wavelengths.
The laser diode and the wavelength-selective element are preferably located within the external cavity in such a way that the laser light is emitted from the laser diode passes through the wavelength-selective element and resonates within the external cavity by passing one or several times through the laser diode and the wavelength-selective element, to finally exit out of the external cavity through one or several outputs. In a preferred embodiment the laser diode is a quantum dot (QD) laser diode.
The external cavity may be formed either in part from a facet of the laser diode, and/or in part from the said wavelength-selective element as an output-coupler, and/or from specially designed optical components as high reflectors, and/or folding mirrors, and/or output couplers.
The QD laser diode preferably comprises multiple layers of semiconductor materials including a least one quantum dot layer in an active region between an electron emitter layer, allowing the injection of electrons towards the quantum dots, and a hole emitter layer, allowing the injection of holes towards the quantum dots. The composition and doping of the materials is chosen so that the relative optical constants, bandgaps, and conductivity of the layers establish an effective guiding of the optical modes in a cavity formed perpendicular to the plane of the layers, as well as efficient carrier injection when an electric field is applied with the proper forward-bias polarity.
In the case where multiple quantum dot layers are used in the active region, barriers separate the quantum dot layers. The electron and hole emitter layers are preferably doped n-type and p-type respectively to act as a reservoir of charged carriers and to conduct the current necessary for the operation under bias. The electron and/or hole emitter layers can be composed of several layers or regions to vary the composition and/or doping, to optimize the optical and electrical properties of the QD laser diode.
The active region is preferably not doped to minimize loses of the guided optical modes. Intermediate layers with chosen bandgap and doping can also be introduced between the active region and the emitter layers to tailor the optical guiding and the optical and electrical properties of the laser diode. The current injection and the optical mode guided in the QD laser diode material are preferably confined laterally to tailor the electrical, thermal, and optical characteristic of the QD-TEC laser. The current injection in the QD laser diode material might preferably be confined longitudinally to tailor the electrical, thermal, and optical characteristic of the QD-TEC laser. The longitudinal confinement of the optical mode guided in the QD laser diode material is preferably adjusted by changing the reflectivity of a front and a back facet individually to tailor the electrical, thermal, and optical characteristic of the QD-TEC laser. It might be preferable to regulate the temperature and/or remove excess heat generated by the operation of the QD laser diode with the help of a temperature regulating device.
The wavelength-selective element is preferably designed to be adjustable to a bandpass over the gain spectrum of the QD-TEC laser. For the wavelengths selected in the bandpass, lasing will be achieved from a net optical gain which will be obtained before the photon escape the cavity, whereas the wavelengths outside the bandpass will not lase becaus
(Marks & Clerk)
National Research Council of Canada
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