LASER VÀ ỨNG DỤNG (TS. Nguyễn Thanh Phương) - CHƯƠNG 4 potx

- high pumping efficiency, because diode lasers at 810nm match Nd:YAG absorption bands very well ⇒ reduction of thermal load problems thermal lensing, thermal birefringence ⇒ improved to

TS Nguyễn Thanh Phương

Bộ môn Quang học và Quang điện tử

Các loại laser và ứng dụng

Nhắc lại: những yếu tố cấu thành laser

• tương tác giữa ánh sáng và vật chất

• đảo mật độ tích lũy

• môi trường khuếch đại thích hợp

• buồng cộng hưởng quang học

• tương tác giữa một buồng cộng hưởng quang học vàkhuếch đại bên trog BCH:

- ngưỡng phát laser

- so sánh mode và lọc lựa mode

- bão hòa khuếch đại

- phương trình tốc độ của laser

- we now want to get an overview about the different types of lasers, which

are practically relevant

IV.1 Laser rắn

optical and laser properties of ruby at room temperature

Neodymium Lasers

• crystal

- Nd:YAG is the most important material used for solid state laser systems

YAG stands for Yttrium-Aluminum-Garnet, Y 3 Al 2 O 12, a colourless, isotropic crystal For a Nd:YAG laser rod ~1% of the Y3+ ions is replaces by Nd3+

ions The YAG-structure is very stable from lowest to highest temperature,

its mechanical stability and workability (growing, grinding, polishing) as well

as the achievable optical quality are good

absorption spectrum of Nd:YAG

- level scheme of Nd:YAG

- "strongest" laser transition at

1064.1 nm

- lasing is mainly supported by the R2sub-level of the 4F3/2 level At room temperature ~40% of 4F3/2 atoms are in R2 (Ê Boltzmann)

- lower laser level is 4I11/2 with various sub-levels, which all give slightly different emission wavelength

- lower laser levels are thermally not populated, so inversion can easily

be achieved, even for cw-operation

- Nd:YAG is a four-level laser, it is homogeneously broadened

τ~240µs

fast, radiative decay

fast, radiative decay

non material processing (cw and pulsed lasers)

welding (Schweißen), marking, writing, drilling (Bohren) (sizes of few µm

possible), cutting

- illumination and ranging (military)

- medical, especially ophthalmology (Augenheilkunde)

- pumping of other lasers (e.g frequency doubled Nd:YAG for pumping of

Ti:Sa lasers) and non-linear optics (e.g frequency doubling [532 nm],

tripling [355 nm], quadrupling [256 nm], parametric conversion)

- Nd:glass lasers and corresponding amplifiers are also used for laser fusion

experiments

- cw-lasers are pumped by diode lasers at ~ 810nm or various types of

discharge lamps or filament lamps, pulsed lasers by flash lamps

- energy corresponding to non-radiative decays limits quantum efficiency to

~ 76% Excess power (~24%) is converted into heat, which has to be

dissipated Light not absorbed by the pump bands is also partially

converted into heat

- examples for pumping cw-Nd:YAG lasers

- discharge lamps (cw)

- discharge/filament tube

is mounted inside a flow

tube which carries the

- depending on requested power different "pump cavity" designs are used for discharge pumped lasers.

Elliptical cross sections are the basis for many of these geometries, where the discharge tube is located at one focus and the laser rod at the other

- thermal loading

pulsed Nd:YAG lasers as well as other solid-state systems can provide

very high peak powers (many GW) and large pulse energies (many joules)

Especially if lamps (~ 10 kW electric power each) are used for pumping,

thermal loading of the crystal is a serious, power-limiting issue

Absorption of pump plight outside the pump band, and heating due to

non-unity quantum efficiency

• will induce thermal lensing through temperature dependence of the

index of refraction This modifies the resonator geometry dynamically!

• thermal stress causes birefringence and can even lead to damage of

- slab geometry

a slab geometry provides a number of advantages over rod-designs:

• pumping is more homogeneous

• larger surface per volume (better heat removal)

• temperature gradients only in y-direction.

⇒ cartesian symmetry helps to avoid thermal stress induced

depolarization problems (laser emissions is already

polarized in the y-z plane due to Brewster cut of crystal)

- different slab geometries exist

multiple flash lamp design

single (dual) flash lamp design

- better gain uniformity

- better beam quality

- high pumping efficiency, because diode lasers at 810nm match Nd:YAG

absorption bands very well

⇒ reduction of thermal load problems (thermal lensing, thermal

birefringence)

⇒ improved total electrical-to-optical efficiency

- better pump beam quality: pump laser light can be focused into the gain

volume (especially for end-pumped systems)

- longer MTBF (mean time between failure): typically 10.000 h for diode

lasers vs a few hundred h to about 1000 h for discharge lamps

- operation simplified: reduced cooling requirements, no high voltage

"spikes", no UV-light which degrades crystal, optics and coolant

- a single diode laser can provide a few W cw-power (typically not

fundamental mode) Single transverse mode laser diodes with ~0.1 W up to

1 W output power exist Sometimes broad stripe diode lasers, 1D-arrays

("bars") or 2D-arrays can be used

- there are several geometries for optical pumping with laser diodes

• end pumped systems

(single and double)

- pump light can be matched to mode volume

• side pumping of a rod

- direct coupling (diodes close to amplifier)

- coupling with optics

- fiber coupling (!)

• achievable: optical cw-pumping

at ~10kW, cw-output typical 100W, up to ~1kW

A MISER oscillator (Monolithic

Isolated Single-mode End-pumped Ring), or alternatively, an NPRO

(Non-Planar Ring-Oscillator):

the crystal itself constitutes the

amplifier, optical resonator, and optical diode to enforce uni-

directional oscillation

T J Kane and R L Byer, Opt Lett 10 (2), 65 (1985) ;

I Freitag et al., Opt Commun 115, 511 (1995)

- physical, optical, thermal properties

of Nd:YAG

Tuning range for various transition metal solid state lasers

large tuning range

of Ti:Sa is basis for ultra-short pulse operation

- further information regarding Ti:Sa lasers see 2.3.4

IV.1 Laser rắn

IV.2 Laser khí

there are different methods of pumping

- chemical lasers: inversion is generated through a chemical reaction

- gas-dynamical lasers: through fast adiabatic expansion, the gas is

transferred to a non-equilibrium state It approaches a new equilibrium at lower temperature, but for some gases and transitions the lower laying

rotational vibrational states re-thermalize faster than some excited

rotational vibrational state: transient inversion between

rotational-vibrational states is generated

- optical pumping (with another laser)

- most common type is based on a continuous or pulsed discharge

• general features

- gas lasers are among the most powerful (cw and pulsed) lasers

However, the beam profile, linewidth, stability, and tuneability can

typically not compete with dye lasers, solid state lasers, or diode lasers

collisions (between electrons and laser atoms, between gas atoms or

between laser atoms and the containing walls) play an important role for

gas lasers

- collisions with e- transfer atomic population into the upper laser level

- collisions between atoms can transfer energy from one atom of some

other atomic species to the laser atoms ("collisions of the second kind")

A + B → +A B

These processes are effective if the collision is almost resonant, i.e the

laser atom needs about the same amount of energy for excitation as the

atom A can deliver through de-excitation during the collision.

- collisions with the wall can help to transfer atoms from the lower laser

level to the electronic ground state if a spin-flip is required (which can

not be provided by a fast radiative (i.e electric dipole) transition)

HeNe Lasers

collisions with wall

For further information refer to sec 2.3.5

- HeNe lasers are typically based on capillary discharge tubes The small

diameter provides effective de-excitation to the electronic ground state

through collisions with the wall It further also provides transversal mode selection, so that HeNe lasers typically run in fundamental Gaussian

mode

- two concepts for discharge tubes exist: (i) smaller tubes are typically

sealed with the end caps formed by the mirrors There is no user access

to the mirrors! (ii) Alternatively separate discharge tubes with Brewster

windows are used, which provide "polarization selection"

- the shortest HeNe

lasers (~20cm) provide

single axial mode

oscillation!

Ar-Ion (Ar + ) Lasers

- discharge parameters:

30-150 A/cm2, ~300 K

- efficiency (wall plug-to-optical) only ~0.1%

→ larger Ar+ lasers generate a couple of

10 kW of thermal load !

transition wavelength [nm]

- Ar+ lasers have been extensively used as pump lasers for dye lasers

and cw-TiSa lasers

- holography (but see

comment above)

- medical applications

(but see comment

above)

- laser light shows (but

see comment above)

- typical Ar + laser parameters

They are now being replaced by all-solid-state laser system, which are

based on frequency doubled NdYag lasers (1064 nm → 532 nm), that

are more compact, much more efficient, cheaper, more stable and

typically provide better beam profiles

Excimer Lasers

- because the electronic ground

state is unstable, the laser atoms

dissociate immediately after they

have reached the lower laser level

Excimer lasers are effectively four

level lasers with a very fast (~ps)

decay from the lower laser level to

the system ground state (i.e two

atoms) The lower laser level is

effectively unpopulated

- excimers are diatomic molecules which do not posses a stable

electronic ground state They only exist as excited dimers

- many dimers provide gas laser

activity, e.g ArF, KrF, XeF , HgCl,

NaXe, Xe 2 Cl, …

& laser emission

(i) an electron beam

(current 5-50 kA, 5-500 A/cm2)

- excimer lasers are typically pumped by

- repetition rates are in the few Hz

to ~100 Hz range

(ii) a pulsed gas discharge (power

densities of discharge ~ 200 MW/dm3,

1 dm3 typical discharge volume) and

emit pulses with temporal width on the

order of 10 ns

- excimer lasers are based on molecular

electronic transitions Excimer lasers

therefore provide tuneable laser activity

in the deep blue-to-UV wavelength range

(down to below 100 nm)

- excimer lasers provide large amplification (~0.1/cm) Typically, excimer

lasers provide large peak power (MW-GW) and pulse energy (~J)

- due to large amplification excimer lasers do not require low loss cavities

Consequently the emission features poor beam profile quality and

modest coherence length

- excimer lasers are or have been used for

• pump sources for pulsed dye lasers

• LIDAR systems (Light Detection And Ranging)

• material processing and surface cleaning

• due to the large peak powers and energies excimer lasers have also

been used in non-linear optics to generate deep-UV coherent radiation

through high-order frequency conversion in laser generated plasmas

Today these lasers can often be replaced ultra-short (fs) pulse laser

systems (e.g Ti:Sa-based) which provide significantly higher peak

powers because their pulses are much shorter

CO 2 Lasers

- CO2 is a gas-laser medium which provides vibrational-rotational laser

activity In can be operated in cw- as well as in pulsed mode CO2 lasers are the most powerful cw lasers at all (~100 kW cw !!)

2

- the CO2 laser is discharge pumped, discharge contains also N2 and He

- CO2 laser feature very high efficiency (quantum efficiency: 45%,

electrical-to-optical efficiency: up to 30%)

upper laser level life time: 1µs … 1 ms

excitation through N2is very efficient (it is almost resonant, corresponding

• typical CO 2 laser parameters

IV.1 Laser rắn

IV.2 Laser khí

IV.3 Laser bán dẫn

Semiconductor Lasers

Courtesy of Sacher Lasertechnik

- history: semiconductor lasers were first realized in 1962

- first semiconductor lasers could only be operated at low temperature, in

1970 first cw-semiconductor lasers were operated at room temperature

• high efficiency: typically the differential efficiency ( ΔPout/ΔPin above

threshold) is ~50%

- "pro's" of semiconductor lasers

• simple pumping: current injection

- semiconductor lasers rely on solid state physics Most common type is

diode laser, which applies physics of semiconductor diode (pn-junction)

• very compact: typical dimension is 100µm × 100µm × 500µm for typical 10mW …100mW (single transverse mode) or up to few 10 W for

transverse multimode lasers

• available at almost all wavelength between ~400nm and ~2µm

• diode lasers are relatively cheap: diode "chip" ranges between few Euro (i.e for consumer electronics) and few 1000 Euro, mostly depending on (i) production volume, (ii) wavelength, (iii) power

• to make a diode laser from a laser diode, current and temperature

stabilization electronics as well as opto-mechanics have to be added

(total cost between 10.000 and 20.000 Euro for a scientific diode laser)

• good tuneability: typically, diode lasers are tuneable by a few % of the central wavelength

• very agile: fast frequency modualtion via current modulation (up to GHz)

- very sensitive to optical, electrical, and electrostatic damage

(anyone who has ever build a diode laser has "killed" a laser diode)

- pour beam quality: elliptic, e.g 1x3 or larger aspect ratio, and astigmatic, distortion, side lobes

- large line width: ~MHz typically, can be reduced by orders of magnitude; active stabilization requires large (~MHz) control bandwidth

- strong dependence on current and temperature (e.g ~100 GHz/K and

30 GHz / mA for a single transverse laser diode at 850nm): for a

spectroscopy laser temperature stabilization at mK level is required and the current source has to be ultra-low noise (typically few µARMS at diode currents of 100mA for a laser diode with few mW output)

⇒ most spectroscopy applications require active frequency stabilization

- based on the recombination between electrons pumped into the

conduction band and holes in the valence band During this process a

photon is spontaneously emitted, or is created by a stimulated emission

process

quasi-Fermi-energy of …

… conduction band

… valence band

- in thermodynamical equilibrium the (quasi-) Fermi energy related to the

electrons in the conduction band (FL) and of the holes in the valence

band (FV) are identical

If the Fermi-energy lays in between the conduction and valence band,

an undoped "semiconductor" is an isolator

For the conduction band the quasi Fermi-energy gives the energy of

highest laying level which is populated by an electron (T=0 K)

For the valence band the quasi Fermi-energy gives the energy of

highest laying level which is populated by a hole (T=0 K)

- pn-junction lasers

• with no voltage applied the

quasi-Fermi-levels are degenerate No

inversion is achieved (at T=0K)

If FL-FV>Eg inversion is generated in

the junction zone, and electrons in

the conduction band and holes in the

valence band can recombine

• typical and common semiconductors

are GaAlAs (~800nm), InGaAsP (1.3µm -1.5µm), GaInP (670 nm)

pn-junction, no bias

• with voltage applied in forward

direction thermal non-equilibrium is

established and the degeneracy of

quasi-Fermi-levels is removed in the

junction zone

pn-junction, forward bias