Principles of Operations
|1. Monochromatic- Monochromatic means single color – Lasers emit a light that consists of a very narrow spectral range.|
|2. Directional- This refers to light divergence over long distances. A laser is collimated and will travel long distances without the beam spreading.|
|3. Coherent- All of the light waves emitted by a laser are in phase with each other. All the peaks and valleys we would see on a scope would be perfectly in line with each other.|
A Lase media is required, either a solid state material (crystal) or gas. The Lase media is Stimulated by “pumping” energy into the Lase media atoms causing them to release amplified coherent light energy which we utilize as a laser beam.
PUMP TECHNOLOGIES – There are two basic “pumps” or excitation mechanisms.
|Optical – Is used in most solid state lasers. Wavelength specific light is energized into a solid material, usually some form of man made crystal. The pump light is generated from special flash lamps or laser diodes or diode arrays.|
|Electrical– This type of pump uses either a DC or RF signal to excite atoms. These are used mostly in gas lasers. Electrical pumps usually consist of 2 electrodes in the center of the optical cavity.|
|These excitation mechanisms cause the electrons in an atom to absorb energy and move to a higher energy level; upon obtaining this elevated energy state they release it in order to return to ground state. The energy is released in the form of a photon, which is a short wave of light.|
These types of lasers must contain some form of an optical cavity. There are several types of optical cavities, but they all have the same principle behind them. There are 2 basic components that form an optical cavity.
|1. High Reflector- Consists of mirror that will reflect over 99.9% of the light that comes in contact with it. It is generally a concave mirror. The mirror material is dependant upon what type of laser media is used.|
|2. Output Coupler- The output coupler is almost the same as the high reflector except it is partially transparent to allow some laser light to emit through it. This “leftover” light is what we utilize as our laser beam.|
|These mirrors are aligned inside the cavity in a way that allows the released photons to “bounce” back and forth between them. As the photons are moving through the cavity the photons come into contact with other excited atoms. This is when stimulated emission occurs.|
STIMULATED EMISSION – Now we have used the excitation mechanism to excite atoms and cause the electrons in these atoms to release photons. These photons are gathering optical gain by “bouncing” back and forth in the optical cavity and gathering photons from other atoms. As these incident photons come into contact with an excited atom they stimulate those atoms to emit a photon which is identical to the incident photon. When the optical gain reaches a certain point a population inversion occurs, which means there are more excited atoms than non-excited atoms. At this point stimulated emission occurs and we get a usable laser beam emitting from the output coupler.
NOTE– In order to speed up the emission process laser manufacturers have implemented certain features that keep the excited atoms just below stimulated emission or, as in solid state lasers, use an electro-optical assembly to attenuate the beam.
|Electro-optics– In solid state lasers an optical excitation mechanism is used. With most solid-state lasers some form of electro or acousto-optic assembly is used to attenuate the beam until the laser is ready to be fired. This means the laser is always “on”. The general concept of both types of these attenuators is when a signal is applied to them the beam passes through with little or no refraction. When a signal is removed the crystal, usually a liquid crystal sandwiched between optical materials, the refraction index inside the crystal changes and the laser beam cannot pass through the optical assembly. This is a brief definition and there are many types of optical attenuators used, but most operate on this general principle.|
Semiconductor Lasers (Direct Diode Lasers, and Fiber Lasers):
A series of semiconductor diodes are optically coupled (diode arrays, or optical fibers) which in combination produce laser energy densities required for various industrial material processes.
|Laser action (with the resultant monochromatic and coherent light output) can be achieved in a p-n junction formed by two doped gallium arsenide layers. The two ends of the structure need to be optically flat and parallel with one end mirrored and one partially reflective. The length of the junction must be precisely related to the wavelength of the light to be emitted. The junction is forward biased and the recombination process produces light as in the LED (incoherent). Above a certain current threshold the photons moving parallel to the junction can stimulate emission and initiate laser action.|
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.
Most diode lasers are based on crystal wafers of group III-V compounds from the periodic chart of elements. Those fabricated from gallium arsenide and its derivatives typically lase at wavelengths between 660 and 900 nanometers, and those utilizing indium phosphide-based compounds produce wavelengths between 1300 and 1550 nanometers.
As the technology surrounding the diode laser has evolved, dramatic improvements have been made in the efficiency, spectral characteristics, and functional lifetime of the devices. A primary objective in design of these lasers is preventing internal loss of radiation due to excessive beam spread from the small junction, where gain occurs. Through various techniques for confining the beam, not only is the efficiency and output power of the laser maximized, but also certain other characteristics of the beam are affected in a desirable manner.
Examples of High Power, High Brightness Direct Diode Laser Arrays:
Single-mode, rare-earth-doped fiber lasers sources provide high electrical-to-optical efficiency (up to 39% for Yb-doped fiber amplifiers), small-signal gains as high as 105, and low-threshold operation. The devices can achieve diffraction-limited beam quality (M2 = 1) that is defined by the refractive-index profile of the fiber and is thus insensitive to thermal or mechanical fluctuations or optical power level. The glass host broadens the optical transitions in the rare-earth ion dopants, yielding continuous tunability; moreover, the variety of possible rare-earth dopants such as Yb, Er, and Tm yields broad wavelength coverage in the near-IR spectral region. Fiber lasers can be diode pumped and further offer low heat dissipation and facile heat removal (high surface-area-to-volume ratio) and room-temperature operation. They also require no consumables other than electrical power.
Recent advances have enabled dramatic power scaling of continuous-wave (CW) and pulsed fiber sources, bringing the benefits of this technology to a wide range of applications previously dominated by other laser systems: materials processing, lidar, and nonlinear frequency conversion, for example. These developments have led to a surge of interest in fiber-based laser systems for both industrial and military use.
High-power fiber sources incorporate double-clad fiber (see figure 1), in which the rare-earth-doped core is surrounded by a much larger and higher-NA inner cladding. Light from high-power multimode pump diode arrays can be launched efficiently into the inner cladding, but the pump light is absorbed only in the core, retaining the benefits of a single mode gain region.