Lasers and Applications Unit 11: Specific lasers Contents
Introduction ........................................................................................ 3 Learning outcomes ............................................................................ 3 HeNe laser ........................................................................................... 4 Ar ion laser.......................................................................................... 9 Erbium doped fibre amplifier (EDFA)........................................... 13 Ti:sapphire laser............................................................................... 16 2 Unit 11 Specific lasers Introduction
In this unit we shall review a number of specific laser types, including two gas lasers (the HeNe and the Ar ion lasers), an optical fibre amplifier (the Erbium Doped Fibre Amplifier or EDFA), and a solid state laser (the Ti:sapphire laser). The aim is to become familiar with the design, components and characteristics of these devices, and to find out how the various theoretical schemes that we discussed in the previous chapters are implemented in practice. There are several other laser types that are equally important but will not be discussed in these notes because the section in the textbook describing these lasers is adequate. These lasers are listed at the end of this unit under Additional Lasers. You should read these lasers in your textbook. Learning outcomes
After studying this unit you will be able to · · · · · · discuss the characteristics of the HeNe laser discuss the characteristics of the argon ion laser discuss the characteristics of the erbium doped fibre amplifier discuss the characteristics of the Ti:sapphire laser discuss the basic characteristics of the CO2, Nd:YAG and the excimer laser discuss the mode of operation of the free electron laser. Unit 11 Specific lasers 3 HeNe laser
The helium neon (HeNe) laser is a gas laser. In gas lasers the active material is a gas enclosed typically in a glass or ceramic tube. Excitation is usually achieved by running a current through the gas. There are three basic types of gas lasers: 1. neutral atom 2. ion 3. molecular lasers. The HeNe laser is a neutral atom gas laser. The HeNe laser was the first gas laser to be made and the first ever laser to operate as a continuous wave (CW) laser (the ruby laser was/is a pulsed laser). Since the early 1960s, when the HeNe was developed by Ali Javan and colleagues, millions HeNe lasers have been manufactured and used in countless applications. They owed their popularity to the following few factors: · · · · · they emit visible radiation they operate in CW mode they are highly collimated the are relatively inexpensive, and they are reasonably safe to use. In spite of all these fine qualities, HeNe lasers are nowadays replaced by diode lasers in most applications. HeNe lasers contain a mixture of two neutral gases, helium and neon. In fact, it would probably be more accurate to call this laser the neon laser as it is only the neon atoms that actually take part in the radiative transitions. The helium is only there to help excite the neon atoms. The energy diagram of the He and the Ne atoms is shown below: 4 Unit 11 Specific lasers Fig. 11.1 Simplified energy level diagram of the He and Ne atoms. The figure only shows those energy levels that play a major role in the red (8=632.8nm) and near infrared (8=1.15:m, 8=3.39:m) emission of the HeNe laser. (from F.G. Smith and T. A. King Optics and Photonics, Wiley, ISBN 0471489255, p: 282) The excitation or pumping of the HeNe laser is a rather convoluted process. It usually takes place in a d.c. discharge created by first applying a high voltage (10kV) for a short time to ionise the gas, after which a lower voltage (2-4 kV) maintains the low operating current (typically a few mA). Electrons in the electric discharge are released at the cathode and are accelerated by the applied voltage towards the anode. The moving electrons collide with the gaseous atoms and as a result some of the He and Ne atoms are excited into higher lying states. "So why de we need the He?" you ask. It turns out that it is much easier to excite the He by electric collisions than the Ne atoms, and importantly, the excited He atoms can pass on the excitation energy to the unexcited Ne atoms. This is how it works: · · · the collision with electrons excites the He atoms into one of two metastable states; since these states have long lifetimes, very soon the population of the excited He atoms builds up; when the excited He atoms collide with the Ne atoms they transfer their energy to the Ne atoms. Unit 11 Specific lasers 5 · If we look at the energy diagram on Fig 11.1, we can see that the He energy levels are (nearly) resonant with two of the energy levels of the Ne atom. This resonant energy transfer is a relatively efficient way of exciting the Ne atoms The energy transfer is symbolically represented as: He* + Ne => He + Ne* where He* (Ne*) is an excited He (Ne) atom. Thus, pumping of the Ne atoms is achieved indirectly via the He gas. Although lasing action may be observed in a gas containing only Ne atoms (i.e. without He gas), the pumping efficiency is greatly increased when helium `buffer gas' is added. If we choose the He/Ne ratio correctly, which is about 10 parts of He to 1 part of Ne, considerable amount of energy can be transferred from the electric discharge to the Ne atoms via the He atoms. The upshot of all this is that there will be more excited Ne atoms in the 5s state then in the 4p or 3p states, or in other words, it is possible to achieve population inversion between two Ne energy levels, as is shown on the figure 11.1. (In spite of all the talk about "efficient energy transfer", the HeNe laser is a very inefficient device. More than 99% of the input energy is wasted as heat. More about this later.) The HeNe laser is a four-level system. For efficient operation, we must make sure that the population of the final state in the laser transition is rapidly depleted. This means that electrons from the lower lasing level should be `encouraged' to decay rapidly to the ground state. In the HeNe laser, as well as in many other gas lasers, this can be achieved by increasing the probability of collisions between the Ne atoms and the walls of the discharge tube. How? By reducing the tube diameter, atoms "see" more of the walls and collide with it more often. Such collisions allow the excited Ne to lose energy and relax to the ground state and hence increase the gain of the system. It is not surprising therefore that the gain of the HeNe laser is found to be inversely proportional to the tube radius; the narrower the discharge tube, the higher the gain. In most HeNe lasers the tube diameter is not larger than a few millimetres. (An additional benefit of the small tube diameter is that the emission is restricted to the TEM00 mode; higher order transverse modes cannot oscillate in very narrow tubes.) As well as the emission lines shown in Figure 11.1, there are over 100 other known lasing transitions in the HeNe system, but of these only a few are of any practical importance. The best-known line is the red at 8=632.8nm, although the gain of some of the other transitions can be much larger. For example, the gain at 8=3.39:m in the near infrared is so great that it is possible to obtain `laser-like' emission even without mirrors. That is, a single pass through the tube is enough to produce enough amplified spontaneous emission to produce reasonable emission. (Similar effects were discussed when we talked about superluminescent LEDs.)
6 Unit 11 Specific lasers Commercial HeNe lasers usually emit at one of the following wavelengths: 8=543nm (green), 8=611.9nm (orange), 8=632.8nm (red) and several infrared lines (8=1.15:m, 8=1.523:m and 8=3.39:m). The red line (8=632.8nm) is by far the strongest visible line and the only visible wavelength with output powers above a few milliwatts. HeNe lasers emitting in the green are becoming popular, as they have no competition from diode lasers. (P.S. What do you call an environmentally friendly HeNe laser emitting in the green? A GreNe! Get it? A `greenie'.) Since the HeNe can lase at several (many) wavelengths, in order to limit the emission to a single wavelength, oscillations at the unwanted wavelengths are suppressed. As we discussed in Unit 7, one way of achieving this is by using specific mirrors for each wavelength range. For example, the transmission spectra of mirrors used for a HeNe laser emitting at 8=632.8nm, are shown below. As you see, the mirror transmission is very low at 632.8nm but changes rapidly at other wavelengths. (Low transmission means high reflectivity.) Fig 11.2 Typical multiplayer dielectric mirrors used in HeNe lasers emitting at 632.8nm The laser mirrors are made of alternating layers of dielectric materials having high and low refractive index deposited onto a glass or quartz substrates. Materials such as TiO2 (n=2.5) and SiO2 (n=1.5) are often used with HeNe lasers. These mirrors have negligible absorption losses (