Steady state operation : g = 1

Transcription

1 Laser = active medium + cavity L active medium R1 R Net gain per round trip L active medium R1 R2 losses gain (intrinsic) g Intensity after one round trip = = R1$ R2$ exp6 2^b-ah $ l@ I0 Steady state operation : g = 1 the optical gain is exactly balanced by the sum of all the losses 48 48

2 Cavity The optical cavity is nothing else than a Perot- Fabry interferometer...! o o o Only frequences, mnq, corresponding to the optical modes of cavities are authorized: o with: l g 1 = 1 - r1 l g 2 = 1 - r Cavity... longitudinal modes For a perfect cavity (100% reflectivity for the mirrors, g1=g2=1). The longitudinal modes (m=n=0) are separated by c/2l o 50 50

3 Resonator... optical modes spatial distribution of the intensity for different radial and angular modes The modes are denoted TEM pl where p and l are integers labeling the radial and angular mode orders Unstable / stable resonators l g 1 = 1 - r1 l g 2 = 1 - r2 stability condition 0 # g 1 $ g 2 #

4 Laser... all-lines laser He-Ne Energy [ev] Helium collisions Neon * * 1.15!m fast decay on strong visible transitions ( !m) 3.39!m nm A laser can work in a multi-line mode! To get only one single line, one must introduce in the cavity an element which induces losses for the different wavelengths except one... A dispersive element inside the cavity can force the laser to work on a single line Laser... single line operation Single line operation Idea: Introduce some losses at the different wavelengths except at 2 Argon laser m m m m m m This configuration is known as the Littrow configuration... (i.e. the wavelength 2 is refected back on the same axis...) 54 54

5 Laser... multimode operation gain curve (closely related to the line width profile) optical cavity modes We observe experimentally that the laser tends to work on several wavelengths... This is a bit paradoxical, because of the gain saturation Laser... multimode operation There is a saturation of the gain above the threshold... only one mode should lase! Why does the laser work on several modes? Hole burning (spatial and spectral) 56 56

6 Laser... spatial hole burning spatial hole burning The different modes use different parts of the active material Laser... spectral hole burning spectral hole burning In the case of an inhomogeneous broadening 58 58

7 Laser... single-frequency mode Laser... single-frequency operation To force the laser to work in a single-frequency mode, we can choose two different options: Ultra-short cavity + etalon Conventionnal cavity 60 60

8 Ring lasers A ring laser is a laser, with a resonator which has the form of a ring. In contrast to a standing-wave laser resonator, such a ring resonator allows for two different propagation directions of the intracavity light. In many cases, unidirectional operation (where light propagates only in one of the two possible directions) is enforced by introducing an element into the resonator which leads to different losses for the propagation directions; this can be, e.g., a Faraday rotator combined with a polarizing element (e.g. a Brewster surface of the laser crystal ). If unidirectional operation is achieved, there is no standing-wave interference pattern in the laser gain medium (except near reflection points), and consequently no spatial hole burning. Therefore, single-frequency-operation, is easily achieved. Particularly for solid-state bulk lasers, unidirectional ring laser designs can be considered as a standard approach to obtain stable single-frequency emission. from RP photonics Dye lasers A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. A dye laser consists of an organic dye mixed with a solvent, which may be circulated through a dye cell, or streamed through open air using a dye jet. A high energy source of light is needed to "pump" the liquid beyond its lasing threshold. The dyes used in these lasers contain rather large organic molecules which fluoresce when exposed to the proper frequency of light. Dyes will emit stimulated radiation when the molecules are in their singlet state. In this state, the molecules emit light via fluorescence, and the dye is quite clear to the lasing wavelength. Within a microsecond, or less, the molecules will change to their triplet state. In the triplet state, light is emitted via phoosphorescence, and the molecules begin to absorb the lasing wavelength, making the dye opaque, the dye must be circulated at high speeds to keep the triplet molecules out of the beam path

9 Dye lasers due to the lifetime of the triplet states, the laser does not work in CW regime Les lasers accordables (dye lasers) 64 64

10 Q-switched laser Q-switching, sometimes known as giant pulse formation, is a technique used to produce a pulsed output beam. Compared to modelocking, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations. Q-switching is achieved by putting some type of variable attenuator, called "Qswitched" inside the optical resonator. When the attenuator is functioning, that's correspond to a decrease in the Q factor of the optical resonator. Active Q-switching Here, the Q-switch is an externally-controlled variable attenuator. This may be a mechanical device such as a shutter, chopper wheel, or spinning mirror/prism placed inside the cavity, or (more commonly) it may be some form of modulator such as an acousto-optic device or an electro-optic device a Pockels cell or Kerr cell. Passive Q-switching In this case, the Q-switch is a saturable absorber, a material whose transmission increases when the intensity of light exceeds some threshold. The material may be an ion-doped crystal like Cr:YAG, which is used for Q- switching of Nd:YAG lasers, a bleachable dye, or a passive semiconductor device. from Wikipedia Q-switched lasers mirror angle end mirror (100%) Flash lamp (pumping) laser used to probe the angular position of the output coupler Output coupler 0 detector pumping intensity time time Active medium Q cavity time Gain time detector Output power time time The output coupler is mounted on a rotating holder. Lasing is obtained only when there is no loss, e.g. when the output coupler is perfectly aligned with the end mirror

11 Q-switched lasers end mirror (100 %) laser crystal cut at bewster angle polarzation selector Electro-optic crystal output coupler The laser crystal is cut at the brewster angle and only one polarization can propagate without loss. The Q-switching is simply achieved using a Pockels cell (electro-optic crystal which can rotate the polarization...) Mode-locked lasers Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10!12 s) or femtoseconds (10!15 s). The basis of the technique is to induce a fixed phase relationship between the modes of the laser's resonant cavity. The laser is then said to be phase-locked or mode-locked. Interference between these modes causes the laser light to be produced as a train of pulses. Depending on the properties of the laser, these pulses may be of extremely brief duration, as short as a few femtoseconds. In a simple laser, each of these modes will oscillate independently, with no fixed relationship between each other, in essence like a set of independent lasers all emitting light at slightly different frequencies. The individual phase of the light waves in each mode is not fixed, and may vary randomly due to such things as thermal changes in materials of the laser. In lasers with only a few oscillating modes, interference between the modes can cause brating effects in the laser output, leading to random fluctuations in intensity; in lasers with many thousands of modes, these interference effects tend to average to a near-constant output intensity, and the laser operation is known as a c.w. or continuous wave

12 Mode-locked lasers If instead of oscillating independently, each mode operates with a fixed phase between it and the other modes, the laser output behaves quite differently. Instead of a random or constant output intensity, the modes of the laser will periodically all constructively interfere with one another, producing an intense burst or pulse of light. Such a laser is said to be mode-locked orphase-locked. These pulses occur separated in time by " = 2L/c, where " is the time taken for the light to make exactly one round trip of the laser cavity. This time corresponds to a frequency exactly equal to the mode spacing of the laser, #$ = 1/". The duration of each pulse of light is determined by the number of modes which are oscillating in phase (in a real laser, it is not necessarily true that all of the laser's modes will be phase0.44 locked). If there are N modes locked with a frequency separation #$, overall Dt = the N $ Do mode-locked bandwidth is N#$, and the wider this bandwidth, the shorter the pulse duration from the laser. In practice, the actual pulse duration is determined by the shape of each pulse, which is in turn determined by the exact amplitude and phase relationship of each longitudinal mode. Dt = 0.44 N $ Do time bandwidth product: Mode-locked lasers laser gain bandwidth cavity longitudinal modes intensity 6i = c 2L laser output spectrum N modes frequency I (t) max = E 20 $ N 2 Dt p = 70 1 Dy $ N 70

13 Mode-locked lasers!0 + #!!0!0 - #! Laser safety 72 72

14 Wavelengths of common lasers used at IPEQ Argon laser Excimer laser, chemical laser 3p 4 4p lasing 488 nm A= /sec levels 4p 3p 4 4s 72.3 nm A= /sec 73.1 nm A= /sec levels 4s 3p 5 3 P 0 1/2 3 P 0 3/

15 Argon laser Excimer laser, chemical laser The upper laser levels (there are several, clumped tightly together) are about 20 ev above the ground state of the argon ion or nearly 36 ev above the ground state of the argon atom. Obviously, this is a very high energy which will require a large pump energy to build-up a high population of ions in that high-energy state. The dynamics of the argon ion are good for CW laser action in that the lifetime of the lower level is very short compared to the upper level. This allows population inversion to be maintained so long as a large pump energy is available (and all argon lasers need large pump energies - most have between 10 to 70A continuously through the discharge!). The short lifetime of the lower lasing level lead to another problem though in that ions in that energy state (i.e. having just emmitted a coherent photon) drop rapidly to the ground state of argon ion. This is a large jump and results in the spontaneous emission of a 74-nm extreme-ultraviolet photon (the energy had to go somewhere, right?). From an efficiency standpoint, this means an excited ion at the upper lasing level loses about 2 ev of energy in producing the coherent photon then loses 18 ev in spontaneous emission of that UV photon. These dynamics limit efficiency severely. As well the extreme UV light from that emission can damage many optical materials so mirrors and windows in an argon laser must be built to withstand such punishment! There are three major mechanisms which raise the argon ion to the upper laser levels. One is energy transfer from an electron to a ground-state argon ion. Another is collisional transfer of energy from an electron to the argon ion in an excited metastable state similar to the way in which helium pumps neon's levels in the HeNe laser. The third is decay of higher levels produced by electron excitation. All three effects combine to pump the argon ions to the upper levels for lasing. from Pofessor Mark Csele's Homebuilt Lasers Page Argon laser Excimer laser 76 76

16 Argon laser... single mode Excimer laser He-Ne laser Excimer laser A 1400 V high voltage, DC power supply maintains a glow discharge or plasma in a glass tube containing an optimal mixture (typically 5:1 to 7:1) of helium and neon gas. The discharge current is limited to about 5 ma by a 91 kw ballast resistor. Energetic electrons accelerating from the cathode to the anode collide with He and Ne atoms in the laser tube, producing a large number of neutral He and Ne atoms in excited states. He and Ne atoms in excited states can deexcite and return to their ground states by spontaneously emitting light. This light makes up the bright pink-red glow of the plasma that is seen even in the absence of laser action. Energy [ev] Helium collisions Neon * * 1.15!m fast decay on strong visible transitions ( !m) 3.39!m nm

17 Helium-Neon laser Excimer laser He-Ne laser Excimer laser 80 80

18 Excimer laser Excimer laser The term excimer is short for 'excited dimer', while exciplex is short for 'excited complex'. An excimer laser typically uses a combination of an inert gas (Ar, Kr, or Xe) and a reactive gas (fluorine or chlorine). Under the appropriate conditions of electrical stimulation, a pseudo-molecule called an excimer (or in case of noble gas halides, exciplex) is created, which can only exist in an energized state and can give rise to laser light in the UV range. Laser action in an excimer molecule occurs because it has a bound (associative) excited state, but a repulsive (disassociative) ground state. This is because noble gases such as Xe and Kr are highly inert and do not usually form chemical compound. However, when in an excited state (induced by an electrical discharge or high-energy electron beams, which produce high energy pulses), they can form temporarily-bound molecules with themselves (dimers) or with halogens (complexes) such as Fl and Cl. The excited compound can give up its excess energy by undergoing spontaneous or stimulated emission, resulting in a strongly-repulsive ground state molecule which very quickly (on the order of a picosecond) dissociates back into two unbound atoms. This forms a population inversion between the two states Le laser excimer ArF, KrF,... Excimer laser, chemical laser 82 82

19 Excimer laser Excimer laser The wavelength of an excimer laser depends on the molecules used, and is usually in the ultraviolet Excimer Wavelength Relative Power (mw) Ar2 126 nm Kr2 146 nm F2 157 nm 10 Xe2 172 and 175 nm ArF 193 nm 60 KrF 248 nm 100 XeBr 282 nm XeCl 308 nm 50 XeF 351 nm 45 KrCl 222 nm 25 Cl2 259 nm N2 337 nm Chemical laser Excimer laser, chemical laser A chemical laser is a laser that obtains its energy from a chemical reaction Chemical lasers can achieve CW output with power reaching to MW levels. They are used in industry for cutting and drilling. Common examples of chemical lasers are the chemical oxygen iodine laser (COIL), all gas-phase iodine laser (AGIL), and the HF laser and deuterium fluoride laser, both operating in the mid-infrared region. Origin of the CW chemical HF/DF laser Very quickly, deuterium was dropped in favor of hydrogen, since it is far less costly and more readily available. However, later it was realized that HF produces infrared radiation in the 2.6 to 3.1 µm waveband, a region of the spectrum absorbed by water vapor in the atmosphere. Interest was renewed in DF, which produces radiation in the 3.7 to 4.2 µm band, which passes easily through the atmosphere

20 Les lasers chimiques Excimer laser, chemical laser Chemical laser Excimer laser, chemical laser 86 86

21 CO2 laser Excimer laser, chemical laser The carbon dioxide laser (CO 2 laser) was one of the earliest gas lasers to be developed (invented by Kumar Patel of Bell labs in 1964), and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available. CO2 lasers are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO 2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.4 and 10.6 micrometers% CO2 Lasers Excimer laser, chemical laser Nitrogen N2 Carbon Dioxyde CO2 O C O Asymmetric strech Symmetric strech Bending CO2 molecule 0.3 collisions * [ R10 ] 9.6!m [ P10 ] * 10.6!m Laser transitions O O C asymmetric stretch mode C O O 0.2 bending mode Pumping Fast decay collisions O C O symmetric stretch mode 0.1 Fast decay 0.0 Ground level 88 88

22 CO2 laser Excimer laser, chemical laser Because of the high power levels available (combined with reasonable cost for the laser), CO 2 lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving. They are also very useful in surgical procedures because water (which makes up most biological tissue) absorbs this frequency of light very well. Some examples of medical uses are laser surgery, skin resurfacing ("laser facelifts") (which essentially consist of burning the skin to promote collagen formation), and dermabrasion. Also, it could be used to treat certain skin conditions by removing embarrassing or annoying bumps, podules, etc. Researchers are experimenting with using CO 2 lasers to weld human tissue, as an alternative to traditional sutures YAG lasers Nd:YAG (neodymium-doped yttrium aluminium garnet; Nd:Y 3 Al 5 O 12 ) is a crystal that is used as a lasing medium for solid state laser [DPSS - Diode-Pumped Solid- State]. The dopant, triply ionized neodynium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet (YAG), since they are of similar size. Generally the crystalline host is doped with around 1% neodymium by atomic percent. Laser operation of Nd:YAG was first demonstrated by Geusic et al. at Bell Laboratories in Nd:YAG lasers are optically pumped using a flashlamp or laser diodes. They are one of the most common types of laser, and are used for many different applications. Nd:YAG lasers typically emit light with a wavelength of 1064 nm. Nd:YAG lasers operate in both pulsed and continuous mode. Pulsed Nd:YAG lasers are typically operated in the so called Q-switching mode. Ref.: Wikipedia 90 90

23 YAG laser Pumping bands nonradiative decay Pumping 1.06!m laser transition nonradiative decay ground state Ti:sapphire laser Ti:sapphire lasers (also known as Ti:Al 2 O 3 lasers, titanium-sapphire lasers, or simply Ti:sapphs) are tunable lasers which emit red and near-infrared light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulses. Lasers based on Ti:sapphire were first constructed in Titanium-sapphire refers to the lasing medium, a crystal of sapphire (Al 2 O 3 ) that is doped with titanium ions. A Ti:sapphire laser is usually pumped with another laser with a wavelength of 514 to 532 nm, for which argon-ion lasers (514.5 nm) and frequency-doubled Nd:Yag, Nd:YLF, and Nd:YVO lasers ( nm) are used. Ti:sapphire lasers operate most efficiently at wavelengths near 800 nm

24 Ti:sapphire laser collisonal relaxation optical pumping * * * tunable laser output collisonal relaxation Semiconductor laser Excimer laser, chemical laser 94 94

25 Temporal coherence Spatial coherence 96 96

26 Cavité P-F Cavité P-F 98 98

27 Cavité P-F Lamb dip