 Hello and welcome to the lecture on Semiconductor Injection Laser. Learning outcome of this session? At the end of this session, student will be able to illustrate Semiconductor Injection Laser. Before starting this topic, try to recall and write down the list of different semiconductor materials used in optical sources. Semiconductor Injection Lasers are also called as Injection Laser DIO or Simply Injection Laser. Stimulated emission by the recombination of injected carriers is encouraged in semiconductor injection laser by the provision of an optical cavity in the crystal structure in order to provide the feedback of photons. The feedback of photons gives avalanche multiplication of photons, in other words, light amplification. This gives the injection laser several major disadvantages over other semiconductor sources, for example LEDs, used for optical communications. The advantages of injection laser over LEDs are as follows. High radiance due to the amplifying effect of stimulated emissions, injection lasers generally supply millivets of optical output power. The second advantage over LEDs is narrow line width on the order of 1 nanometer or less which is useful in minimizing the effect of material dispersion. Modulation capabilities which at present extend up to the gigahertz range and will be improved upon. The last advantage, good spatial coherence which allows the output to be focused by a lens into a spot which has greater intensity than the dispersed unfocused emission. This permits efficient coupling of optical output power into the fiber even for fibers with low numerical aperture. In several different ways the efficiency of injection laser can be defined. The total efficiency that is external quantum efficiency which can be defined as it is the ratio of total number of output photons to the total number of injected electrons. If we write the mathematical equation of the total efficiency that is external quantum efficiency we get the formula P by I into EG, where P is optical power emitted from the device I is the current, E is a charge on an electron and HF is a photon energy, EG is a band gap energy expressed in electron volts. The another definition for efficiency of injection laser is the external power efficiency of the device is given by new equivalent efficiency equals to P E by P into 100 where P equals to I into V is a electrical input power using the equation one we can find out new equivalent equal to new total into EG by V into 100. Early injection lasers had the form of fabric parot cavity often fabricated in gallium arsenide which is a semiconductor with electro luminescent properties at the appropriate wavelength. The basic structure of this gallium arsenide homo junction device is shown in figure number one where the cleaved ends of the crystal acts as a partial mirrors in order to encourage stimulated emissions in the cavity when electrons are injected into the P type region. However, these devices had a high threshold current due to their lack of carrier containment and proved inefficient light sources. The high threshold current requirement dictates that these devices to operate in pulse mode in order to minimize the junction temperature and thus avoid the damage. The improved carrier containment and thus lower threshold current were achieved using hetero junction structures. The double hetero junction injection laser fabricated provide both carrier and optical confinement on the both sides of the PN junction giving the injection laser a greatly enhanced performance. However, in order to provide reliable operation of double hetero junction injection laser it was necessary to provide further carrier and optical confinement which lead to the introduction of strip geometry double hetero junction laser configurations. The double hetero junction laser configuration structure provides optical confinement in vertical direction through the refractive index step at the hetero junction interfaces but lasing takes place across the whole width of the device. This situation is illustrated in figure number two. Figure number two shows the broad area aluminum gallium arsenide double hetero junction laser where the sides of cavity are simply formed by the roughening of the edges of the device in order to reduce unwanted emissions in these directions and limit the number of horizontal transverse modes. However, the broad emission area creates several problems including difficult heat sinking, lasing from multiple filaments in relatively wide area and broad emission area emits unsuitable light for efficient coupling to cylindrical fibers. To overcome these problems associated with broad emission area while reducing the required threshold current, laser structures in which the active region does not extend to the edges of the device were developed. A common technique involved the introduction of strip geometry to the structure to provide optical containment in the horizontal plane. The structure of aluminum gallium arsenide double hetero junction strip contact laser is shown in figure number three where the major current flows through the device and hence the active region is within the strip. Generally, the strip is formed by creation of high resistance areas on either side. The strip therefore act as a guiding mechanism which overcomes the major problem of broad area device. The optical output and far field emission pattern are also illustrated in figure number three. The output beam divergence is typically 45 degrees perpendicular to the plane of junction and 9 degrees parallel to it. Such structures have active regions which are planar and continuous. The typical output spectrum of broad area injection laser is shown in figure number four A. It does not consist of a single wavelength but consists of a series of wavelength peaks corresponding to different longitudinal modes which are in the plane of junction along the optical cavity within the structure. The spacing of these modes is dependent on the optical cavity length. These wavelengths are generally separated by a few tenths of a nanometer and the laser is said to be a multimode device. However, figure four A also indicates some broadening of longitudinal modes. Due to sub-peaks caused by higher order horizontal transverse modes. These higher order lateral modes may exist in the broad area device due to the unrestricted width of active region. The correct strip geometry inhibits the occurrence of higher order lateral modes by limiting the width of optical cavity leaving only a single lateral mode which gives the output spectrum shown in figure number four B, where only the longitudinal modes may be observed. This represents the typical output spectrum for a good multimode injection laser. These are the references. Thank you.