Presentation on theme: "1.5 Types of lasers Lasers may be classified according to several criteria: We will somewhat arbitrarily look at lasers based on whether the gain medium."— Presentation transcript:
1.5 Types of lasers Lasers may be classified according to several criteria: We will somewhat arbitrarily look at lasers based on whether the gain medium is a gas, liquid, or solid. Whether the gain medium is a gas, liquid, or solid Fixed frequency or tuneable Excitation source Emission range (UV to IR) Laser power CW or pulsed Gas: HeNe laser, excimer lasers, CO 2 lasers Liquid: dye lasers Solid: Nd:YAG laser, Ti:sapphire laser Diode (semiconductor) lasers
Types of lasers Summary of some common lasers LaserPulsed or CW * TuneableEmission wavelengths HeNeCWNo632.8 nm (1.15 μm) Ar ionCWNo351, 455, 458, 466, 477, 488, 497, 502, 515, 529 nm N2N2 PNo337 nm ExcimerP No ** 190 – 350 nm CO 2 BothYes10.6, 9.6 μm ** Dye lasersBothYes365 – 930 nm Nd:YAGBothNo1064 nm (532, 355, & 266 nm) *** AlexandritePYes720 – 800 nm Ti:sapphireCWYes670 – 1100 nm Diode lasersBothYesUV to mid-IR * Typical mode of operation ** Tuneable over a narrow range (or depending on particular laser) *** 2 nd, 3 rd etc. harmonics
HeNe laser The helium-neon (HeNe) laser The HeNe laser is a common, relatively low power and low cost CW laser. The gain medium is a helium-neon gas mixture. The HeNe is a 4-level laser with the following energy levels: Lasing occurs between excited energy levels of neon, but helium is necessary to transfer pump energy (from an electric or microwave discharge) to the neon atoms. Helium atoms are excited by collisions with electrons to various excited states, of which the 2 1 S 0 and 2 3 S 1 states are metastable and long-lived because radiative transition back to the 1 1 S ground state is forbidden.
HeNe laser Ne has ground configuration 1s 2 2s 2 2p 6 and excited states 2p 5 ns 1 and 2p 5 np 1 (with n>2). The 2p 5 ns 1 levels are much-longed lived than the 2p 5 np 1 levels, so it is feasible to produce a population inversion. Lasing may occur between various energy levels, but the 632.8 nm line (red) is most commonly used for HeNe lasers. The near-IR lines at 3.39 μm are not often used but interfere with lasing at 633 nm by depleting the population of the upper state. This effect can be minimised using cavity mirrors that only reflect red light, allowing lasing to take place at 633 nm only. Decay from 2p 5 3p 1 to 2p 5 3s 1 is rapid, but 2p 5 3s 1 is a long-lived state. 2p 5 3s 1 Ne atoms can lose this energy to wall collisions – for this reason, HeNe lasers have narrow tubes.
Excimer lasers “Excimer”: excited dimer(E.g., He 2 ) “Exciplex”: excited complex (dissimilar atoms)(E.g., ArF) Excimers and exciplexes are molecules characterised by a dissociative ground state, but by a bound potential for an excited electronic state: Since the lower state is very short-lived, a population inversion can also be achieved relatively easily. Excimer lasers are pulsed, high power lasers
Excimer lasers E.g., ArF (193 nm), KrF (248 nm), XeF (351 nm), KrCl (222 nm), XeCl (308 nm), XeBr (282 nm) Noble gas-halogen exciplexes are useful for many laser applications. (While technically exciplex lasers, these lasers are usually called excimer lasers.) An electric discharge is used to pump the laser. Note that the excimer laser can be changed by exchanging the gas mixture (along with the HR and OC). Although the ground energy level is short-lived, in some cases the lower level potential may be very slightly bound, allowing some tuneability of the laser.
CO 2 laser The carbon dioxide laser CO 2 has 3 normal modes of vibration – the symmetric stretch (ν 1 ) at 1354 cm -1, the bending vibration (ν 2 ) at 673 cm -1, and the assymetric stretch (ν 3 ) at 2396 cm -1. Since the lower state is very short-lived, a population inversion can also be achieved relatively easily. The CO 2 laser is a high power infrared laser of high efficiency that may be pulsed or CW. It lases between vibrational levels of CO 2.
CO 2 laser A He:CO 2 :N 2 mixture is used. Very high laser (up to kW) powers can be achieved. Pumping is achieved by electric discharge – some CO 2 molecules are directly excited, together with efficient transfer from excited N 2 to CO 2. Transitions between vibrational levels also involve rotational transitions, giving rise to a relatively large number of closely spaced emission lines – the laser can be tuned between these transitions.