Lasers Light Amplification by the Stimulated Emission of Radiation.

TUTORIAL OPTOELEKTRONIKACatatan Kuliah 2@ By Agus SupriyantoOptoelectronics Laboratory

Introduction to Lasers

Introduction to Lasers

Introduction to Lasers In popular science fiction movies during the 1950s, monsters were often portrayed that could emit lethal rays of light from their eyes (Figure 1)

but until the invention of the laser, such concentrated and powerful energy beams were only fantasy.

Now it is possible to modify, probe, or destroy matter using the highly focused radiation from energy sources known as lasers.

Ketika elektron berada di pita energi konduksi dan terjadi rekombinasi ke pita energi valensi, maka dapat terjadi emisi photon yang dikenal dengan spontaneous Emission Ketika elektron berada di pita energi konduksi dan diberi stimulasi atau injeksi photon maka terjadi rekombinasi ke pita energi valensi, sehingga dapat terjadi emisi photon yang dikenal dengan Stimulated Emission. Ketika elektron berada di pita energi valensi dan adanya injeksi photon maka terjadi eksitasi elektron ke pita energi konduksi yang dikenal dengan Stimulated Absorption

Sejarah Perkembangan Laser Albert Einstein may inadvertently have taken the initial step in laser development by realizing that two types of emission are possible. In an article published in 1917, he was the first to suggest the existence of stimulated emission.

Einstein's work, which became key to the development of quantum mechanics, holds that energy exists in discrete units or quanta and that atoms and molecules (and therefore everything else) are restricted to having only certain discrete amounts of energy.

A scientist at Columbia University, Charles H. Townes, was the first to succeed in amplification of stimulated radiation in the early 1950s, but his work centered around microwaves (with a much longer wavelength than visible light), and he termed his device a maser

Two Soviets, Nikolai Basov and Aleksander Prokhorov, shared the 1964 Nobel Prize in physics with Townes for their pioneering work on the principles underlying masers and lasers. Schawlow was awarded a share of the 1981 Nobel Prize in physics for his laser research.

GAS LASERS

JENIS LASERSIon lasers employ expensive plasma tubes constructed of graphite or beryllium oxide (BeO). Normally, a solenoid is placed around the tube. The magnetic field generated by the solenoid "squeezes" the plasma in order to increase the current density (current per unit area) in the active medium, providing for more efficient excitation. The large amount of current passing through the tube necessitates some type of cooling system. Normally, either water or forced airflow provides maintenance of stable operation temperatures

TABEL JENIS GAS LASERS

SOLID STATE LASER

SEMICONDUCTOR DIODE LASERS

Semiconductor diode lasers having sufficient power output to be useful in optical microscopy are now available from a host of manufacturers. In general these devices operate in the infrared region, but newer diode lasers operating at a variety of specific visible wavelengths are rapidly being developed

Diode lasers are fabricated utilizing a specialized type of semiconductor junction, and therefore share many of the advantages and characteristics of other semiconductors and solid-state devices. Although these lasers rely on electronic processes that take place in a solid semiconductor medium, the basic principles of laser action in diode lasers are no different from those controlling the operation of other (non-semiconductor) laser systems. In all lasers, it is necessary for energy transitions to occur among electrons in the lasing medium, and some of these must involve the emission of photons (categorized as optical transitions). In order for these transitions to result in emission of amplified light, the process of stimulated emission must predominate over either spontaneous emission or absorption. This situation is achieved under the conditions of a population inversion in the active medium, a process whereby the electron population of an upper energy level is induced to grow larger than that of a lower level.

Diode lasers are fabricated utilizing a specialized type of semiconductor junction, and therefore share many of the advantages and characteristics of other semiconductors and solid-state devices.

The simplest and most commonly utilized semiconductor is silicon, where each atom in the solid form shares electrons with four neighbors in a tetrahedral symmetry. Silicon is the example most often employed to illustrate various properties of semiconductors, although many important materials in diode laser applications are compounds of groups III and V elements, including gallium arsenide, indium phosphide, and indium arsenide. Even though the band arrangement is similar for all semiconductors, there are large differences in the band gap and in the distribution of electrons among the bands at specific temperature conditions.

Semiconductor lasers operate in a very different fashion, but also rely on electrical currents to produce the necessary population inversion. In these devices the inversion is produced between populations of current carriers (electrons and electron-hole pairs) in the plane of the junction between dissimilar regions of the semiconductor. Light emission in a semiconductor laser is concentrated in the junction plane by feedback from the cleaved ends of the crystal (Figure 9). The chip material has a high refractive index, and reflects enough light back into the crystal to achieve gain. The cleaved face can also be polished to control the reflectivity. Typically one end of the crystal is coated with a reflective material so that emission only occurs from a single end, as illustrated in Figure 9. PrototipeLaser Diode

Optical Power and Differential Quantum Efficiency The optical output power is where h depends on two factors: (1) the injection current efficiency accounting for the fraction of injected carriers contributing to the emission process (some the carriers can recombine in the undoped confinement regions where the carriers do not interact with the optical field), and the (2) the optical efficiency accounting for the fraction of generated photons that are transmitted out of the cavity. Note that the threshold current depends on the injection current as well as on the junction temperature Tjct. The differential quantum efficiency is then current dependent:

We see that h(I)ext can be negative if dith/di>1. The light output Pout vs. the injection current will have a negative slope.

OPERATING EFFICIENCYAn additional comparison of lasers can be made in terms of how efficiently typical lasers operate. The operating efficiency of a laser may be defined as its "optical output power, Plaser, divided by its electrical input power, Pin," as in Equations 1 and 2.Equation 1Operating efficiency = Plaser/PinorEquation 2% Efficiency = Plaser/Pin(100%)

EXAMPLE :Hitung Efisiensi Operasi dari : The operating plasma tube voltage of a 1-mW HeNe laser is 1500 V at 5 mA.

Temperature Dependence of Laser OutputThe current threshold for lasing in GaAs is strongly temperature-dependent, as shown in graphics. At low temperature (up to approximately 30 K) the threshold is fairly constant. Above 100 K, the threshold current density for laser operation increases rapidly with increasing temperature. At the cryogenic temperature of 77 K, the threshold current in a gallium arsenide laser is about one tenth that of the room temperature value. This means that cooling to cryogenic temperatures changes the operating and performance characteristics of the laser.

Kurva.Temperature dependence of threshold current

Gallium arsenide lasers emit radiation in the near infrared portion of the spectrum. The exact wavelength depends on the temperature at which the laser is operated. This is shown in Figure 6 which gives the wavelength of a gallium arsenide laser as a function of temperature. At temperatures above room temperature, the threshold for laser operation rises and it becomes difficult to operate the laser. However, gallium arsenide lasers have been operated over the range of temperatures from liquid helium temperature to room temperature.

Kurva.Temperature dependence of lasing wavelength

Spectral CharacteristicsOperation of a gallium arsenide laser is characterized by a threshold current. Figure 7 shows the peak pulsed power of a typical GaAs laser as a function of the peak current input. The threshold current for this diode is about 10 amperes. When the current through the device is relatively low, a broad spectrum of spontaneous emission with a bandwidth of around 100 nanometers is observed. When the current through the junction is increased stimulated emission will begin when the optical gain exceeds the losses. The threshold current density will depend on the temperature, on the absorption losses in the material, on the reflectivity of the diode surface, and on the doping of the material

KurvaPeak power output of laser diode as a function of peak input current.

Spatial CharacteristicsOne of the most important characteristics of gas lasers is the very small divergence of the emitted radiation. This characteristic is not shared by semiconductor lasers. The main reason is that light is emitted through the aperture defined by the small junction. Diffraction through the narrow dimensions of the junction causes the beam to spread into a broader angle than is observed with other types of lasers. Figure 9 illustrates the beam divergence of a typical GaAs laser. The emission from a gallium arsenide laser tends to be an elliptical beam with a full angle divergence around 20 in the direction perpendicular to the junction and around 5 in the direction parallel to the junction. These angles may vary considerably with individual lasers.

Other Types of Semiconductor Lasers

In addition to gallium arsenide lasers, a variety of other semiconductor lasers have been developed. Most of these lasers are in the laboratory development and have not reached commercial status. Table 3 gives some other semiconductor laser materials, and their wavelengths of operation. Those indicated by the letter I are pumped by injection, which is the direct-current flow described in this module for GaAs lasers

Semiconductor Laser

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