Semiconductor Lasers PDF

Semiconductor Diode laser: Definition: It is specifically fabricated p-n junction diode. This diode emits laser light when it is forward biased. Principle: When a p-n junction diode is forward biased, the electrons from n – region and the holes from the p- region cross the junction and reco...

Semiconductor Diode laser: Definition: It is specifically fabricated p-n junction diode. This diode emits laser light when it is forward biased. Principle: When a p-n junction diode is forward biased, the electrons from n – region and the holes from the p- region cross the junction and recombine with each other. During the recombination process, the light radiation (photons) is released from a certain specified direct band gap semiconductors like Ga-As. This light radiation is known as recombination radiation. The photon emitted during recombination stimulates other electrons and holes to recombine. As a result, stimulated emission takes place which produces laser. Construction:  Figure shows the basic construction of semiconductor laser. The active medium is a p-n junction diode made from the single crystal of gallium arsenide. This crystal is cut in the form of a platter having thickness of 0.5μmm. The platelet consists of two parts having an electron conductivity (n-type) and hole conductivity (p-type). The photon emission is stimulated in a very thin layer of PN junction (in order of few microns). The electrical voltage is applied to the crystal through the electrode fixed on the upper surface. The end faces of the junction diode are well polished and parallel to each other. They act as an optical resonator through which the emitted light comes out. Working: Figure shows the energy level diagram of semiconductor laser.  When the PN junction is forward biased with large applied voltage, the electrons and holes are injected into junction region in considerable concentration The region around the junction contains a large amount of electrons in the conduction band and a large amount of holes in the valence band. If the population density is high, a condition of population inversion is achieved. The electrons and holes recombine with each other and this recombination’s produce radiation in the form of light. When the forward – biased voltage is increased, more and more light photons are emitted and the light production instantly becomes stronger. These photons will trigger a chain of stimulated recombination resulting in the release of photons in phase. The photons moving at the plane of the junction travels back and forth by reflection between two sides placed parallel and opposite to each other and grow in strength. After gaining enough strength, it gives out the laser beam of wavelength 8400 o A. The wavelength of laser light is given by Where Eg is the band gap energy in joule. Characteristics: 1. Type: It is a solid state semiconductor laser. 2. Active medium: A PN junction diode made from single crystal of gallium arsenide is used as an active medium. 3. Pumping method: The direct conversion method is used for pumping action 4. Power output: The power output from this laser is 1mW. 5. Nature of output: The nature of output is continuous wave or pulsed output. 6. Wavelength of Output: gallium arsenide laser gives infrared radiation in the wavelength 8300 to 8500o A. Advantages: 1. It is very small in dimension. The arrangement is simple and compact. 2. It exhibits high efficiency. 3. The laser output can be easily increased by controlling the junction current 4. It is operated with lesser power than ruby and CO2 laser. 5. It requires very little auxiliary equipment 6. It can have a continuous wave output or pulsed output. Disadvantages: 1. It is difficult to control the mode pattern and mode structure of laser. 2. The output is usually from 5 degree to 15 degree i.e., laser beam has large divergence. 3. The purity and monochromacity are power than other types of laser 4. Threshold current density is very large (400A/mm2). 5. It has poor coherence and poor stability. Application: 1. It is widely used in fiber optic communication 2. It is used to heal the wounds by infrared radiation 3. It is also used as a pain killer 4. It is used in laser printers and CD writing and reading.  Introduction Why are we here today? Overview of semiconductor laser diodes Answer key questions Learn Why do we care? Take away knowledge to our future employers To serve our scientific interests To give me a good grade!  Questions to be Answered What is a semiconductor laser diode? What does it look like? How do they work? What are they made of? What are their properties? What are their uses? How much do they cost? Where can you get them? What is a semiconductor laser diode? A semiconductor laser diode is a device capable of producing a lasing action by applying a potential difference across a modified pn- junction. This modified pn-junction is heavily doped and contained within a cavity thus providing the gain medium for the laser. A feedback circuit is also implemented in order to control the amount of current sent to the laser diode.  What do they look like? Semiconductor laser diodes come in many shapes and sizes. They maybe round, square, or rectangular, and have a few to many leads. There are many reasons for the different shapes including function, power output, specifications, etc. What do they look like? The above photo shows a variety of low-power semiconductor laser diodes set in several different types of casings.  What do they look like? When in use, the S.L.D. is normally mounted in a laser diode module. The S.L.D. is driven and its current controlled by the drive circuit. A lens provides the desired beam shape.  What do they look like? The photo below shows a typical module-mounted S.L.D. with driver circuitry. The above photo shows a green semiconductor laser diode set in a module and with driver circuitry attached.  What do they look like? The left photo shows a high-power semiconductor laser diode set in a module. The middle shows a pigtailed S.L.D. module package for optical communication. The right photo shows a receptacle S.L.D. module package also for optical communication. What do they look like? These S.L.D.s are examples of high-power “bar array diodes”. Clockwise from bottom left: 5 element CW array, 150W, actively cooled 60W QCW array, conductively cooled 30W CW array, conductively cooled 2500W QCW stacked array, actively cooled  What do they look like? This diagram shows a close up view of a typical semiconductor laser diode. It can be seen that the S.L.D. consists of a laser diode, a photo diode, and connecting leads and pins. All of this is housed in a protective metal casing. A clear screen allows the beam to be emitted.  How do they work? This diagram shows the inside construction of the semiconductor laser diode. It can be seen that the laser diode emits in two directions, sending one as the output, and the other as a feedback to the photo diode.  How do they work? This diagram shows how the LD and the PD are connected. The photo diode optically senses the amount of light sent from the laser diode, this is called feedback. This feedback is sent to a drive circuit which regulates the amount of current that the laser diode sees. The laser diode is very sensitive, and needs to be operated at 80% of its absolute maximum ratings.  How do they work? A typical laser consists of three things: a Pump, a Gain Medium, and a Cavity. The diagram below shows the gain medium and the cavity for a generic laser. The pump would send energy into the gain medium and this would excite the electrons and holes within it. This process then gets amplified within the cavity and lasing takes place.  How do they work? This diagram shows a cross section of the inside of the laser diode. The L.D. consists mainly of a pn-junction separated by a thin active layer. The sides of the L.D. form the mirrors of the cavity and the beam is sent in the two opposite directions. A potential difference creates the pumping action and starts the lasing process.  How do they work? Here is another look at how the laser diode creates the lasing action. This diagram clearly shows how the cavity is formed by cleaving and polishing the ends and roughening the sides.  How do they work? Now looking back at the schematic of the S.L.D., we can see how the laser diode sends its output to the photo diode. The feedback mechanism is the most important feature of the S.L.D., without it, there would be no way to regulate the current and the device would burn itself out very quickly.  What are they made of? Semiconductor laser diodes can be made from many different types of semi- conducting materials including several elements found in groups III and V from the periodic table. A few examples of these materials include: GaAlAs, AlGaInP, InGaAsP. The major reasons for using one material over another are for the specifications needed, especially desired wavelength. What are their properties? The properties of semi- conductor laser diodes can be measured in various ways. The most common terms to describe them are given in the next two slides. These properties are used to place S.L.D.s in various classes such as: high/low power, wavelength, high temperature, etc.  What are their properties? Here is a list of the most important properties of semiconductor laser diodes. S.L.D. properties can be broken up into three general categories: Electrical, Optical, and Temperature. Electrical: Optical: Temperature: Threshold current Light output power Peak wavelength Operating current Slope efficiency Laser threshold Operating Voltage Beam Divergence Peak wavelength  What are their properties? Electrical: Wall-plug efficiency Duty Cycle Series resistance Optical: Continuous/Pulsed Wave Far field pattern Fast axis Slow axis Spectral width Near field pattern Polarization ratio Temperature: Active/Conductively cooling Slope efficiency Thermal impedance  What are their properties? S.L.D.s can range in wavelengths such as Red (635-670 nm) and IR (780, 800, 900, 1,550 nm, etc). Green (~550 nm), blue (~450 nm), and violet (~400 nm) laser diodes have been produced in various research labs but until recently, only operated at liquid nitrogen temperatures, had very limited life ( N4. When flashtube or laser diode supplies light energy to the active medium (Nd:YAG crystal), the lower energy state (E1) electrons in the neodymium ions gains enough energy and moves to the pump state or higher energy state E4. The lifetime of pump state or higher energy state E4 is very small (230 microseconds (µs)) so the electrons in the energy state E4 do not stay for long period. After a short period, the electrons will fall into the next lower energy state or metastable state E3 by releasing non-radiation energy (releasing energy without emitting photons). The lifetime of metastable state E3 is high as compared to the lifetime of pump state E4. Therefore, the electrons reach E3 much faster than they leave E3. This results in an increase in the number of electrons in the metastable E3 and hence population inversion is achieved. After some period, the electrons in the metastable state E3 will fall into the next lower energy state E2 by releasing photons or light. The emission of photons in this manner is called spontaneous emission. The lifetime of energy state E2 is very small just like the energy state E4. Therefore, after a short period, the electrons in the energy state E2 will fall back to the ground state E1 by releasing radiationless energy. When photon emitted due to spontaneous emission is interacted with the other metastable state electron, it stimulates that electron and makes it fall into the lower energy state by releasing the photon. As a result, two photons are released. The emission of photons in this manner is called stimulated emission of radiation. When these two photons again interacted with the metastable state electrons, four photons are released. Likewise, millions of photons are emitted. Thus, optical gain is achieved. Spontaneous emission is a natural process but stimulated emission is not a natural process. To achieve stimulated emission, we need to supply external photons or light to the active medium. The Nd:YAG active medium generates photons or light due to spontaneous emission. The light or photons generated in the active medium will bounce back and forth between the two mirrors. This stimulates other electrons to fall into the lower energy state by releasing photons or light. Likewise, millions of electrons are stimulated to emit photons. The light generated within the active medium is reflected many times between the mirrors before it escapes through the partially reflecting mirror. Advantages of Nd:YAG laser  Low power consumption  Nd:YAG laser offers high gain.  Nd:YAG laser has good thermal properties.  Nd:YAG laser has good mechanical properties.  The efficiency of Nd:YAG laser is very high as compared to the ruby laser. Applications of Nd:YAG laser Military Nd:YAG lasers are used in laser designators and laser rangefinders. A laser designator is a laser light source, which is used to target objects for attacking. A laser rangefinder is a rangefinder, which uses a laser light to determine the distance to an object. Medicine Nd: YAG lasers are used to correct posterior capsular opacification (a condition that may occur after a cataract surgery). Nd:YAG lasers are used to remove skin cancers. Manufacturing Nd:YAG lasers are used for etching or marking a variety of plastics and metals. Nd:YAG lasers are used for cutting and welding steel.