Laser operation

10.3.1 Optical amplification

The fact that electromagnetic radiation (including visible light)from induced (stimu-lated) emission is coherent with the absorbed radiation it can be used to create coherent radiation sources. Depending on the frequency range such a device is called either a laser (Light Amplification by Stimulated Emission of Radiation) or amaser (Microwave Amplification by Stimulated Emission of Radiation⁴).

As we saw for frequencies in the visible range and above the induced emission is usually negligible. The ratio of the number of transitions per unit time with reordering (10.2.1) and using that B₁₂=B₂₁: In thermal equilibrium N₂ < N₁, but in an open medium (the gain medium) that is not thermal equilibrium using special non-thermal external excitation it is possible to

4In modern usage ”light” broadly denotes electromagnetic radiation of any frequency, not only visible light, hence we can talk about infrared laser, ultraviolet laser, X-ray laser, and so on. Because the microwave predecessor of the laser, the maser, was developed first, devices of this sort operating at microwave and radio frequencies are referred to as ”masers” rather than ”microwave lasers” or ”radio lasers”.

artificially reverse this so that the population of the higher level will be the larger one, i.e. N₂ > N₁. Such excitation is called “pumping”. This is called population inversion.

A small perturbation of such a metastable system can start the de-excitation process (spontaneous emission) whose end result will be the equilibrium state. This perturba-tion can be caused by a light of suitable frequency. Because of the populaperturba-tion inversion the probability of induced emission (proportional to N2→1) is much larger than the prob-ability of absorption and as we saw the emission will be coherent with the perturbation.

This process is an optical amplification: a small intensity light enters the gain medium and a higher intensity coherent light leaves it. The gain medium therefore is itself an optical amplifier.

10.3.2 Laser operation

A laser which produces light by itself is technically an optical oscillator rather than an optical amplifier as suggested by the acronym. When anoptical amplifier is placed inside anoptical resonator⁵, one obtains alaser oscillator. This optical resonator (see Fig.10.1) usually consist of two parallel mirrors and the coherent light beam of the optical amplifier travels to and fro between these in both direction. This way a if the amplification (gain) is larger than the resonator losses (caused by absorption and diffraction) the power of the light bouncing between the two mirrors increases exponentially. As more and more atoms will go back to the lower energy state the gain will decrease, until the losses overcome the gain. The level of gain equal to the losses is the laser treshold.

Simple 2 level systems cannot be used in lasers, because the pumping source not only provides energy for excitations, but at the same type it creates the induced emission itself, effectively negating the pumping effect. At least 3 levels are required as seen in Fig. 10.2 a. The pumping creates a popular inversion on level E₃ with short occupation life time, from which the electrons almost immediately decay onto the metastable level E₂. In a 3 level system a perturbation of frequency (E₂−E₁)/h starts the laser process.

3 level lasers work only in pulsed operation mode, because pumping is a linear process, therefore it cannot compensate for the exponential process of the laser emission⁶.

In a 4 level system (Fig.10.2b) the laser process occurs between the metastable level E₃ and the short lifetime levelE₂. This is the model of a continuous operation mode laser, because in this case the ground level may have a linear filling up rate from electrons that participated in the laser process therefore the linear pumping process can maintain a steady (dynamic equilibrium) state.

5The optical resonator is sometimes referred to as an ”optical cavity”, but this is a misnomer: lasers use open resonators as opposed to the literal cavity that would be employed at microwave frequencies in a maser.

6Naturally any laser can be used in pulse operation mode by either switching it on and off or by using pulsed pumping, but 3 level lasers cannot work in continuous operation mode.

Figure 10.1: Components of a typical laser. The ”high mirror” is a perfect mirror, while the ”output coupler” (OC) is only partially reflecting, the reflectivity required depends on the gain medium. E.g. for He-Ne lasers the reflectivity must be at least 99%, while nitrogen lasers have extremely high gain and do not require any OC at all.

Figure 10.2: Simplest laser level structures. a) 3 level laser, b) 4 level laser

10.3.3 Types of lasers

There are very different types of lasers, possessing properties like parallelism, monochro-matic radiation, high average or peak power, very short pulse length. In size they vary from microscopic semiconductor lasers to the building sizes ystem used for laser research.

Gas lasers

the gain medium is a gas (e.g. C O₂) or gas mixture (e.g. He-Ne). He-Ne lasers are able to operate at a number of different wavelengths, however the vast majority of gas lasers are engineered to lase at 633 nm; these relatively low cost but highly coherent lasers are extremely common in optical research and educational labora-tories. Commercial carbon dioxide (CO₂) lasers can emit many hundreds of watts in a single spatial mode which can be concentrated into a tiny spot. This emission is in the thermal infrared at 10.6µ m; such lasers are regularly used in industry for cutting and welding. The efficiency of a CO2 laser is unusually high: over 10%.

Chemical lasers

these lasers are powered by a chemical reaction permitting a large amount of energy to be released quickly. Such very high power lasers are especially of interest to the military, however continuous wave chemical lasers at very high power levels, fed by streams of gasses, have been developed and have some industrial applications.

Solid-state lasers

the gain medium is a crystalline or glass rod containing impurity ions (the accepted terminology is that they aredoped with this impurities, usually calleddopants) that provide the required energy states. In fact the first working laser was a ruby laser, made from ruby (chromium-doped corundum). The population inversion is actu-ally maintained in the ”dopant”, such as chromium or neodymium. These materials are pumped optically using a shorter wavelength than the lasing wavelength, often from a flashtube or from another laser. These lasers are also commonly frequency doubled, tripled or quadrupled, in so-called ”diode pumped solid state” or DPSS lasers. Under second, third, or fourth harmonic generation these produce 532 nm (green, visible), 355 nm and 266 nm (UV) beams. This is the technology behind the bright laser pointers particularly at green (532 nm) and other short visible wavelengths. Some dped crystal (e.g. Ti:GaS) have very broad amplification spec-tral range that allows generation of very short pulses ub the range of femtoseconds (10¹⁵sec).

Semiconductor lasers

are diodes which are electrically pumped. Recombination of electrons and holes created by the applied current introduces optical gain. Reflection from the ends of the crystal form an optical resonator, although the resonator can be external to the semiconductor in some designs. Commercial laser diodes emit at wavelengths

from 375 nm to 3500 nm. Low to medium power laser diodes are used in laser pointers, laser printers and CD/DVD players. Laser diodes are also frequently used to optically pump other lasers with high efficiency. The highest power industrial laser diodes, with power up to 10 kW (70dBm) are used in industry for cutting and welding. They serve also as light sources for fiber optic communication systems that are the technical foundation of the internet.

Fiber lasers

These are olid-state lasers or laser amplifiers where the light is guided due to the total internal reflection in a single mode optical fiber. Pump light can be used more efficiently by creating a fiber disk laser, or a stack of such lasers. Fiber lasers have a fundamental limit in that the intensity of the light in the fiber cannot be so high that optical nonlinearities induced by the local electric field strength can become dominant and prevent laser operation and /or lead to the material destruction of the fiber. This effect is called photodarkening.

Free electron lasers’

or FELs, generate coherent, high power radiation, that is widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to soft X-rays. They have the widest frequency range of any laser type. While FEL beams share the same optical traits as other lasers, such as coherent radiation, FEL operation is quite different. Unlike gas, liquid, or solid-state lasers, which rely on bound atomic or molecular states, FELs use a modulated relativistic electron beam as the lasing medium, hence the term free electron.