Semiconductor Optical Communication Components and Devices Lecture 18: Introduction to Diode Lasers - I

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1 Semiconductor Optical Communication Components and Devices Lecture 18: Introduction to Diode Lasers - I Prof. Utpal Das Professor, Department of lectrical ngineering, Laser Technology Program, Indian Institute of Technology, Kanpur

2 Semiconductor Laser Materials For efficient light emission in lasers, direct band-gap semiconductors are required. As carriers recombine across the active region of the device, the wavelength of the light emitted is then dependent on the difference between the quasi Fermi energies and therefore proportional to this band-gap. X valley G valley nergy L valley For efficient electrical pump lasers, carrier mobilities in the semiconductor should also be high. In general, narrower band-gap minimums lead to higher carrier mobilities. However, too narrow a bandgap facilitates spontaneous thermally generated carriers. In semiconductor lasers this phenomenon manifests itself in the form of dark current, i.e., leakage current at the junctions. gi1 gd Light Hole gi2 Heavy Hole Split Band K

3 Direct and Indirect transition in Semiconductors - LD s - (k ) 1 Direct Band Gap c Absorption Recombination mission nergy + + Spontaneous mission g ph (k ) 2 ph v k g elec. khole 0 Reduced Plank s Constant Frequency of the Photon 0 ph k 0 Indirect Band Gap Recombination emission k p c OP g k k ph g p AP v a k k 0 h k

4 Defects in Laser Materials Crystals used in semiconductor lasers should be generally free of line dislocations as they may produce non-radiative recombination centers. Dislocation also creates traps that reduce electrical conductivity. Heterostructures are often used in laser structures to increase efficiency, where a smaller band-gap material is sandwiched between larger bandgap materials as described in the section on semiconductor basics. Normally the lattice constants are well matched between junctions and the substrate. Initially lasers were fabricated from high quality lattice matched layers from LP (equilibrium process could not grow good highly lattice mismatched epitaxial layers), mterial. With the advent of non-equilibrium growth processes such as MB and MOCVD, when the lattice mismatch strain can be contained, it produces one of the most efficient lasers till date. These are Strained Quantum well lasers, which until recently had been the work horse for Fiber-Amplifiers, although they are still used in certain older systems.

5 LASR: Light Amplification by Stimulated mission of Radiation LD LASR Diode nergy - - LD s Spontaneous mission BAR: No lateral confinement Gain Guided Index Guided + Stimulated mission: Light that is monochromatic (same wavelength) coherent (in phase) and polarized +

6 Other Feedback Mechanisms Over the years, numerous optical-cavity designs have evolved to couple out of laser diodes. The most widely used configuration is the classic Fabry-Perot Cavity mentioned above. But other resonant cavities have been devised for applications that require a highly coherent beam of light with a narrow band of frequencies. One of the simplest alternatives is to use a Reflection Grating as an external rear mirror. An antireflection coating on the back facet avoids excess Fresnel-reflection loss, and tilting the grating tunes the laser s output frequency. Two other important laser-diode cavity designs that use diffraction gratings 1. Distributed-Bragg-Reflection (DBR) laser. 2. Distributed-Feedback (DFB) laser. In the DFB laser, a grating structure placed alongside the active layer provides back reflected (diffracted) feedback only for a specific wavelength.

7 Front Mirror Active Region Front Mirror AR Coating xternal cavity GRATING BRAGG Grating Diode Laser Active Region Collimating Optics Distributed Feedback Laser (DFB) Active Region Rear mirror GRATING Distributed Bragg Reflector (DBR) Front mirror GRATING All the other wavelengths experience higher cavity losses and cannot reach threshold. The DBR laser applies the same concept, but the grating lies beyond the active layer and requires and index-guiding layer to optically link it to the gain region of the cavity. Both cavity arrangements (especially DFBs) are commercially used for fiber optic communications in the 1.3- and 1.55-μm spectral regions. In some devices, grating are applied to stabilize and/or control the output wavelength. In this fabrication step, holographic interference patterns are generated with lasers, obviating the use of masks and generating high accuracy gratings. Final step in the fabrication process include packaging and, in some cases, attaching fiber pigtails to allow flexibility in the direction of the laser output

8 Non-dge emitting Lasers All of the laser-diode structures discussed so far emit radiation from their edges. But another critically important class of laser diode radiates from its surface. The optical cavity of these kind of laser comes in two basic variety: Planar-Cavity Surface mitting Laser (PCSL) and Vertical-Cavity Surface mitting Laser (VCSL). Both are well suited for two-dimensional laser-diode array. Angle facet vertically deflect the light from an edge-emitting laser-diode, transforming it into PCSL (left). Cross section of a VCSL (right) reveals a laser vertical cavity with mirrors built of thin-film HR (High Reflector) stacks. Light Light Substrate Planar-Cavity Surface mitting Laser (PCSL) Vertical-Cavity Surface mitting Laser (VCSL)

9 PCSL and VCSL A PCSL essentially consists of an edge-emitting laser with an optical structure to redirect the optical beam through the surface. Therefore, these lasers are at least as long as the conventional edge-emitting variety and have the same elliptical far-field beam patterns. In a VCSL, however, the laser cavity is perpendicular to the active layer, which yields a circular beam pattern and makes the whole device extremely compact. A typical VCSL cavity spans about 6mm, creating very coherent light. Monolithic, two-dimensional arrays of PCSLS and VCSLS have tremendous potential for optoelectronic applications such as massive parallel processing, highspeed optical storage, and fiber optic network interconnects. Their development and the development of all modern laser diodes stand as elegant tributes to the extraordinary capabilities of semiconductor technologies.

10 Review Questions 1. Which of the semiconductors Si, Ge, GaAs, and GaP are suitable for the fabrication of diode lasers? Justify your conclusion. 2. An In 0.53 Ga 0.47 As semiconductor at 300K has parabolic conduction and valence bands [ c,v a k 2 ]. The effective masses of the electrons and holes in this material are m e *=0.06m o and m hh *=0.15m o, respectively. If the electron concentration peak is 0.5k B T above the bottom of the conduction band, then find the hole energy (ev) below the top of the valence band for efficient photon emission, assuming the density of states for the conduction and the valence bands to be the same. 3. What are the different varieties of Fabry-Perrot cavity diode lasers? 4. What are the configurations for which one would be able to get narrow linewidths for diode lasers? 5. Why is it essential that the diode laser output is emitted normal to the surface of the semiconductor substrate? What are the disadvantages associated with it?