 Hello and welcome to the lecture on Injection Laser Characteristics. Learning Outcomes By the end of this session, student will be able to illustrate characteristics of semiconductor injection laser. You may pause the video here for reflection question before starting this topic. Try to write down the list, the properties of laser which may affect the laser performance. You have written the answer for the reflection spot question. When considering the use of injection laser for optical fiber communication, it is necessary to be aware of certain of its characteristics which may affect its efficient operation. The following are the major operating characteristics of the device that we will discuss in this session. First is threshold current temperature dependence, second is dynamic response, then third is frequency chirp, fourth is noise, fifth is reliability and the last is more hoping. Threshold current temperature dependence. Figure number 1 shows the variation in threshold current with temperature for two gain guided oxide insulated strip injection lasers. Both devices had strip widths of acrobsimately 20 micrometers but were fabricated from different material systems. For emission at wavelength of 0.85 micrometer and 1.55 micrometers for the materials. In general terms, the threshold current tends to increase with the temperature. The temperature dependence of the threshold current density being approximately exponential for most common structures and it is given by the formula JTH is directly proportional to the exponential of T by T0 ratio where T is the device absolute temperature and T0 is the threshold temperature coefficient which is a characteristics temperature describing the quality of the material but which is also affected by the structure of the device. The next characteristics is dynamic response. The dynamic behavior of the injection laser is critical especially when it is used in high bit rate optical fiber communication systems. The application of current step to the device results in a switch on delay often followed by high frequency damped oscillations known as relaxation oscillations. These transient phenomena occurs while the electron and photon populations within the structure come into equilibrium and are illustrated in the figure number 2. The switch on delay TD may last for 0.5 nanosecond and the RO that is relaxation oscillations for perhaps twice that period at data rates above 100 megabits per second. This behavior can produce a serious deterioration in the pulse shape hence reducing TD and damping the oscillations is highly desirable. The switch on or turn on delay is caused by initial built up of photon density resulting from stimulated emissions. It is related to minority carrier lifetime and the current through the device. Hence the switch on delay may be reduced by biasing the laser near threshold. The next characteristics is frequency chirp. The DC modulation of a single longitudinal mode semiconductor laser can cause a dynamic shift of a peak wavelength emitted from the device. This phenomena which results in dynamic line width broadening under the direct modulation of the injection current is referred as frequency chirping. Frequency chirping arises from gain induced variations in the laser refractive index due to the strong coupling between the free carrier density and the index of refraction which is present in any semiconductor structure. The number of techniques can be employed to reduce the frequency chirp. One approach is to bias the laser sufficiently above the threshold so that the modulation current does not drive the device below the threshold where the rate of change of optical output power varies rapidly with time. Unfortunately this strategy gives an extension ratio penalty. Another method to reduce frequency chirp involves the damping of the relaxation oscillations that can occur at turn on and turn off which results in large power fluctuations. This has been achieved for instance by shaping electrical drive pulses. Certain device structures also prove advantages for chirp reduction in particular quantum well lasers and multi electrode DFB lasers provide improved performance. Another important characteristics of injection laser is noise. Number of injection laser involves the noise behavior of the device. This is specially the case when considering analog transmission. The sources of noise are phase or frequency noise, instabilities in operation then reflection of light back into the device and the last mode partition noise. It is possible to reduce all the noises except phase or frequency noise by using mode stabilizer devices and optical isolators. Figure number three shows the spectral density of this phase or frequency noise has a characteristics represented by 1 by F to 1 by F square up to a frequency F of around 1 MHz. At frequencies above 1 MHz the noise spectrum is flat or white. It is associated with it is associated with quantum fluctuations which is a principal cause of line width broadening within semiconductor lasers. Mode partition noise is a phenomenon which occurs only in multi mode semiconductor lasers. The reducing number of modes in the multi mode fiber decreases the mode partition noise. Mode partition noise can also occur in single mode device as a result of residual side modes in the laser output spectrum. The next characteristics is reliability. The device reliability has been a major problem with the injection lasers and although it has been extensively studied not all the aspects of the failure mechanisms are fully understood. Nevertheless much progress has been made since the early days when the device lifetimes were very short of just few hours. The degradation behavior may be separated into two major processes known as catastrophic and gradual degradation. Gradual degradation is a result of mechanical damage of the mirror facets and leads to partial or complete laser failure. It may be limited by using the device in a pulsed mode. Gradual degradation mechanisms can be separated into two categories which are defect formation in active region and the other is degradation of the current confining junctions. These degradations are normally characterized by an increase in the threshold current for the laser which results in decrease in its external quantum efficiency. Next characteristics is a mode hopping. Figure 4a shows the single longitudinal mode output spectrum of a single mode laser while figure 4b shows mode of hopping to a longer wavelength as a current is increased above threshold is demonstrated by comparisons in a figure. Mode hopping behavior occurs in all single mode injection lasers and is consequence of increase in temperature of the device junction. The transition that is hoping from one mode to another is not a continuous function of the drive current but occurs suddenly over only 1 to 2 milliampere. Mode hopping alters the light output against current characteristics of the laser and is responsible for the kinks observed in the characteristics of many single mode devices. Stabilization against mode hopping and mode shift may be obtained with the adequate heat sinking or thermoelectric cooling. These are the references. Thank you.