LASER (basic) Light Amplification by Stimulated Emission of Radiation 雷射 (激光)

倪其焜 中央研究院原子與分子科學研究所 Part I: How does a laser work? -Ei / kT Boltzmann distribution: Pi = Di xe

Pi > Pj when Ei < Ej Interaction between light and atoms (molecules) absorption

emission

Stimulated emission -Ei / kT Boltzmann distribution: Pi = Di x e

Pi > Pj when Ei < Ej

In average:

absorption Population inversion

Pi > Pj if Ei > Ej

Stimulated emission Design

(Population inversion) Gain medium

High reflector Output coupler Rear mirror Cavity

Intra cavity laser beam > output laser beam At least three energy levels are required

1

laser

Pump 2

3 Design

Optical switch Gain medium

High reflector Output coupler Rear mirror Laser modes of operation

• Continuous wave operation (CW)

I

time • Pulsed operation

I

time At least three energy levels are required

1

laser Pump 2

3 Design

Gain medium

High reflector Output coupler Rear mirror

Gain medium

Output coupler

High reflector Rear mirror Part II: More details of lasers Cavity: stable cavity unstable cavity

Standing waves Laser modes

Cylindrical transverse mode Rectangular transverse patterns TEM(pl) mode patterns TEM(mn) Different modes: different frequencies

Single mode laser A single mode laser is called a single frequency laser. The linewidth can be very small and the coherent length is long.

Multimode laser A multimode operation causes the linewidth to be wide:a multiple of the mode spacing of the resonator. Polarization Definition: direction of the electric field oscillation Linear polarization horizontal polarization vertical polarization Circular polarization

right

left Electric field of a laser pulse Beam Divergence

• Definition: a measure for how fast a laser beam expands far from its focus • lower-divergence beam is preferable • divergence of a laser beam is proportional to its wavelength and inversely proportional to the diameter of the beam at its narrowest point. • The divergence of good-quality laser beams is modeled using the mathematics of by Gaussian beams Gaussian beams

• Beam divergence:

zw      wz 0 Tunability

broadband

laser Pump 2

3 Time–bandwidth Product

(Fourier ) transform limit Gaussian-shaped pulses : wx t = 0.44 intensity intensity time frequency (Fourier ) transform limit

10 fs 1000 cm-1 1 ps 10 cm-1 1 ns 0.01 cm-1 10ns 0.001 cm-1 . . . CW very small CW 0.00001 cm-1 Specifications • CW laser Power: 100 W = 100 J/s Intensity: 100 W/cm2 = 100 J/(s cm2) Linewidth: 0.02 cm-1 Mode: 80% TEM(00) mode Beam diameter: 3 mm Energy stability: 1% Pointing stability: Specifications • Pulsed laser Energy: 400 mJ/pulse Pulsed width: 5 ns Repeatition rate: 100 Hz Fluence: 1000 mJ/cm2 Power: 100 W = 100 J/s Intensity: 100 W/cm2 = 100 J/(s cm2) Time gitter: 2 ns Linewidth: 0.2 cm-1 Mode: Energy stability Pointing stability Part III: Different types of lasers Types and operating principles • Gas lasers • Chemical lasers • Excimer lasers • Solid-state lasers • Fiber lasers • Semiconductor lasers • Dye lasers • Free electron lasers

Excimer laser • ultraviolet lasers • excimer = excited dimer • generates nanosecond pulses • Wavelength:

Excimer Wavelength F2 (fluorine) 157 nm ArF (argon fluoride) 193 nm KrF (krypton fluoride) 248 nm XeBr (xenon bromide) 282 nm XeCl (xenon chloride) 308 nm XeF (xenon fluoride) 351 nm Excimer laser

Energy state of excimer lasers Excimer laser

Spectra for different types of excimer lasers Excimer lasers

Widely used in semiconductor industries Specifications: Nitrogen laser • operating in the ultraviolet range (typically 337.1 nm, 357.6 nm is weak) • Gain medium: nitrogen gas • easy to build and operate • The wall-plug efficiency is low

•Widely used in MALDI mass spectrometer

•Recently replaced by 3rd of Nd:YAG Nitrogen laser (LTB laser technik) Ion lasers • uses an ionized gas as its lasing medium • the energy level transitions that contribute to laser action come from ions • Ex. Argon ion laser; Kr ion laser, …

This argon-ion laser emits blue- green light at 488/514 nm. Large ion laser requires large amount of cooling water and electric power Recently replaced by Nd:YAG laser and diode lasers

Coherent Melles Griot Ar ion laser (Coherent) Ar ion laser (Coherent) Helium–neon Lasers He: Ne= 10:1

• optical output power levels ranging from 0.5 to 50 mW • small devices He-Ne laser He-Ne 雷射波長、功率表

雷射波長 功率範圍 偏振態 外觀/種類 應用舉例 備註 632.8nm 0.5-17mW 線 圓柱型 光學試驗準直、工業雷射監 提供普通 紅光 17-35mW 方形 測、基因芯片、醫療儀器等 實驗室用

電源和

0.5-17mW 园 圓柱型 OEM專

0.5-1mW 線 一體化外觀 工業醫療、質量檢測等 用的小型

電源。並 0.5-1mW 線 高穩頻系列 高品質校準、干涉光源 根據情况

0.5-1mW 線 高穩頻一體化 工業干涉光源 接受特殊

的定制 543.5nm 0.2-1mW 線 圓柱型 醫療儀器、物理科研、工業 綠光 材料激發、檢測等 0.2-2mW 园 圓柱型 594.1nm 0.2-1mW 線 圓柱型 黃光 0.2-2mW 园 圓柱型 611.9nm 0.2-1mW 線 圓柱型 橘光 0.2-2mW 园 圓柱型 1523nm 0.2-1mW 線 圓柱型 紅外 0.2-2mW 园 圓柱型 Nd-YAG laser • Nd:YAG (neodymium-doped yttrium aluminium

garnet; Nd:Y3Al 5O 12) • operate in both pulsed and continuous mode • optically pumped using a flashtube or laser diodes • typically emit light with a wavelength of 1064 nm, in the infrared Nd-YAG laser

Energy level structure and common pump and laser transitions of the trivalent neodymium ion in Nd3+:YAG. Nd-YAG laser

Schematic of Nd:YAG Laser

Flash lamp pump Diode pump

Nd-YAG laser (continuum) • pulsed Nd-YAG laser (continuum) Nd-YAG laser (continuum) Nd-YAG laser (CW)

Output Characteristics1, 2 Millennia Prime Output Power 5, 6, 10, 15 W Wavelength 532 nm Spatial Mode: TEM00 Beam Quality (M2) <1.1 Beam Diameter (1/e²) 4 2.3 mm ±10 % Beam Divergence <0.5 mrad Polarization >100:1 vertical Power Stability ±1% Beam Pointing Stability 2 mrad/°C Noise <0.04% rms Boresight Tolerance Near field ±0.25 mm Far field <3 mrad Dye laser • uses an organic dye as the lasing medium • wavelength tunability

Organic dye

Pump by laser 2nd Nd:YAG 2 or Excimer

3 Dye laser

532 nm from 2nd Nd:YAG

Gain medium (dye) Output coupler

Grating Dye laser (continuum) Ring dye laser (CW, single mode laser) Ti : Sapphire laser

• lasers based on a Ti:sapphire gain medium

• Ti:Al2O 3 lasers, titanium-sapphire lasers • tunable lasers • emit red and near-infrared light from 650 to 1100 nanometers • operate most efficiently at wavelengths near 800 nm • generate CW-ns-fs ultrashort pulses • pumped with another laser with a wavelength of 514 to 532 nm Mode-locked oscillators • generate ultrashort pulses with a typical duration between 10 femtoseconds and a few picoseconds • pulse repetition frequency around 70 to 90 MHz • pumped with a continuous-wave laser beam (Ex. Ar laser, frequency-doubled Nd:YVO4 laser) • output power : 0.5 to 1.5 watt Tunable continuous wave lasers

• with extremely narrow linewidths tunable over a wide range Ti : Sapphire laser (Newport) • 3900S CW Tunable Ti:sapphire Laser

• Up to 3.5 W TEM00 output for high-power applications • Tunable from 675 to 1100 nm • Computer-optimized Z-fold cavity for maximum power and ease of alignment • Sealed cavity design and patented nitrogen purge option minimize maintenance and enable smoother tuning through wavelengths overlapping water absorption bands • Etalon option available for narrow linewidth applications Ti : Sapphire laser (Newport)

Applications

•SpectroscopyFiber laser research •Telecommunications research •Semiconductor studies

Available Products

Model 3900S- 675 nm 3900S- 700 nm 3900S- 790 nm 3900S- 1000 nm 3900S- 1050 nm Ti : Sapphire laser (Newport) Output 3900S- 675 3900S- 700 3900S- 790 3900S- 1000 3900S- 1050 Characteristi nm nm nm nm nm cs Average 600 mW;800 600 mW; 950 1.0 W; 1.7 W; 400 mW; 500 400 mW; 500 Power mW mW; 1.2 W 2.2 W mW mW 700-1000 nm; 700-1000 nm; 700-1000 nm; 700-1000 nm; 700-1000 nm; Tuning Range 950-1000 nm; 950-1000 nm; 950-1000 nm; 950-1000 nm; 950-1000 nm; 675-750nm 675-750nm 675-750nm 675-750nm 675-750nm <40 GHz; <40 GHz; <40 GHz; <40 GHz; <40 GHz; Linewidth <15 GHz; <1 <15 GHz; <1 <15 GHz; <1 <15 GHz; <1 <15 GHz; <1 GHz GHz GHz GHz GHz Noise < 1% < 1% < 1% < 1% < 1% Frequency < 3% < 3% < 3% < 3% < 3% Drift Spatial Mode TEMoo TEMoo TEMoo TEMoo TEMoo >100:1 >100:1 >100:1 >100:1 >100:1 Polarization Horizontal Horizontal Horizontal Horizontal Horizontal Beam Diameter 0.95 mm 0.95 mm 0.95 mm 0.95 mm 0.95 mm (1/e²) Beam Divergence, < 1 mrad < 1 mrad < 1 mrad < 1 mrad < 1 mrad full angle Tsunami Ultrafast Lasers (mode-locked Ti:Sapphire laser), Newport

Industry’s widest tuning range of 700 to 1080 nm Average output power > 3 W High peak power >450 kW at 800 nm (typical) Tsunami Ultrafast Lasers (mode-locked Ti:Sapphire laser), Newport Broadband Tsunami Specifications

Output Tsunami Tsunami Tsunami Tsunami Characteristic Broadband fs Broadband ps Broadband fs Broadband ps s 10 W Pump 10 W Pump 5 W Pump 5 W Pump

Tuning Range 700–1000 nm 700–1000 nm 710–980 nm 710–980 nm

>1.4 W at 800 >1.5 W at 800 >0.7 W at 800 >0.7 W at 800 Average Power nm nm nm nm

Pulse Width <100 fs <2–100 ps <100 fs <2–100 ps

>170 kW at >85 kW at 800 Peak Power - - 800 nm nm

Pulse Energy ~14 nJ ~15 nJ ~8 nJ ~8 nJ

Repetition Rate 80 MHz 80 MHz 80 MHz 80 MHz (nominal) Tsunami Ultrafast Lasers (mode-locked Ti:Sapphire laser), Newport Optical Parametric Oscillators • coherent light sources based on parametric amplification within an optical resonator • light source similar to a laser • based on optical gain from parametric amplification in a nonlinear crystal • wide wavelength tunability • requires a pump source with high optical intensity and relatively high spatial coherence Optical Parametric Oscillators

crystal

mirror mirror grating crystal

mirror mirror Optical Parametric Oscillators Fiber lasers • the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, pras eodymium, and thulium.

Figure 1: Setup of a simple fiber laser. Pump light is launched from the left- hand side through a dichroic mirror into the core of the doped fiber. The generated laser light is extracted on the right-hand side. Fiber laser

Figure 6: Level scheme of thulium (Tm3+) ions in ZBLAN fiber, showing how excitation with an 1140-nm laser can lead to blue fluorescence and laser emission.

ZBLAN: ZrF4- BaF2- LaF3 -AlF3- NaF (Heavy metal fluoride glasses) Fiber laser (light conversion)

With external OEM version enclosure

Laser 593 L x 360 W x 520 L x 300 W x head 212 H 175 H Semiconductor Lasers • wavelengths from 375 nm to 1800 nm • based on semiconductor gain media • optical gain is usually achieved by stimulated emission at an interband transition under conditions of a high carrier density in the conduction band. Semiconductor Lasers

Chemical lasers • convert chemical energy into laser light • Powers: up to the megawatt level • In mid- or near-infrared region • wavelength : 2.7-2.9 µm • continuous wave laser • e.g. hydrogen-fluoride (HF) lasers:

H + F2 → HF (v = 3, 4)

Carbon dioxide laser

e- Carbon dioxide laser

Schematic setup of a sealed-tube carbon dioxide laser. The gas tube has Brewster windows and is water-cooled. CO2 laser (COHERENT)

•CO 2 laser tube: CO2 laser 切割鋼板 CO2 Laser Pumped CH3OH Far Infrared Laser Free electron lasers • laser devices where light amplification occurs by interaction with fast electrons in an undulator • FELs use a relativistic electron beam as the lasing medium which moves freely through a magnetic structure, hence the term free electron. • has the widest frequency range of any laser type • can be widely tunable Setup of an undulator, as used in a free electron laser. The periodically varying magnetic field forces the electron beam (blue) on a slightly oscillatory path, which leads to emission of radiation. Free electron lasers

mage 1. Electrons are released from the source at the lower left, and are accelerated in a linear accelerator (linac). After emerging from this linac, the electrons pass into a laser cavity which has a wiggler at its center. This wiggler causes the electrons to oscillate and emit light which is captured in the cavity, and used to induce new electrons to emit even more light. Part IV: Extension of laser wavelength Extension of laser wavelength

• Nonlinear optics w1+w2=w3 w1 Sum frequency w3 Generation of UV, VIS, w2

For example: Nd:YAG laser: 1064 nm +1064 nm → 532 nm 1064 nm+ 532 nm → 355 nm 532 nm + 532 nm → 266 nm 532 nm + 355 nm → 213 nm Extension of laser wavelength

Different frequency w1-w2=w3 w1 w3 Generation of IR, w2 For example: Nd:YAG laser: 532 nm -780 nm → 1673 nm 800 nm-1064 nm → 3224 nm Extension of laser wavelength

Raman shift: Generation of VUV, UV, VIS, IR

Gas or liquid

Stock shift and anti stock shift Extension of laser wavelength

Four wave mixing (tripling): Generation of VUV

Gas Part V: Laser Safety Laser safety

For ”how dangerous the laser is”

• Class I/1 is inherently safe, usually because the light is contained in an enclosure, for example in CD players. • Class II/2 is safe during normal use; the blink reflex of the eye will prevent damage. Usually up to 1 mW power, for example laser pointers. • Class IIIa/3R lasers are usually up to 5 mW and involve a small risk of eye damage within the time of the blink reflex. Staring into such a beam for several seconds is likely to cause damage to a spot on the retina(視網 膜 ).

• Class IIIb/3B can cause immediate eye damage upon exposure. • Class IV/4 lasers can burn skin, and in some cases, even scattered light can cause eye and/or skin damage. Many industrial and scientific lasers are in this class. Laser Safety Laser safety

•Reflection from Ring, Watch, Necklace, Wall, Window

•To align laser beam: do not watch laser beam, use a piece of paper, IR card Laser safety

Potential hazards from non radiation

• High electric voltage • Large capacitor • Toxic gases

• UV laser: generation of O3