Vật lý Laser 2013 - Chương IV: Các loại laser và ứng dụng

LASER VÀ ỨNG DỤNG

TS. Nguyễn Thanh Phương

Bộ môn Quang học và Quang điện tử

Chương IV:

Các loại laser và ứng dụng

04/12/2013 3

Chương IV: Các loại laser và ứng dụng

04/12/2013 4

Chương IV: 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

và khuếch đại bên trog BCH:

- ngưỡng phát laser

- so sánh mode và lọc lựa mode

- bão hòa khuếch đại

- phương trình tốc độ của laser

IV.1. Laser rắn

Chương IV:


04/12/2013 6

- Dựa trên dịch chuyển của các ion kim loại (e.g.Cr3+) hoặc ion đất

hiếm (Nd3+, Ho3+). Các ion này phân bố trong môi trường tinh thể

hoặc thủy tinh với mật độ ~1019/cm3.

IV.1. Laser rắn

- Dịch chuyển laser xảy ra chủ yếu giữa các trạng thái điện tử nội,

chúng ít bị ảnh hưởng bởi tương tác giữa tạp chất cũng như mạng

tinh thể.

Suy giảm không bức xạ không đáng kể, dịch chuyển tương đối

“sắc nét”, vạch phổ không bị nở rộng do tương tác với tinh thể.

Giảm ngưỡng bơm

- Bơm quang học: Nd-lasers bơm bằng flash lamps (xung) hoặc laser

bán dẫn. cw-Ti:Sa-lasers are typically bơm bằng Ar+ lasers hoặc

Nd:YAG laser nhân đôi tần số (532 nm).

04/12/2013 7

- Bên cạnh laser bán dẫn, laser rắn ngày nay được sử dụng rộng rãi

và đạt hiệu quả thương mại nhất

• Nhỏ gọn, thuận tiện (1W Nd:YAG lasers không yêu cầu làm lạnh

bằng nước)

• Hiệu suất cao (nếu được bơm bằng laser bán dẫn)

• Công suất quang ra cao (TW), (e.g. Nd:glass lasers)

• Dịch chuyển bước sóng trong dải rộng (Ti:Sa: 660 to 986 nm)

• Xung cực ngắn (fs-Ti:Sa lasers)

• Tính ổn định cao (công suất, tần số)

IV.1. Laser rắn

04/12/2013 8

Laser Ruby

IV.1. Laser rắn

04/12/2013 9

• Laser ruby

- Tinh thể ruby: Cr3+ pha tạp trong tinh thể

sapphire (Al2O3)

- Truyền qua vùng màu hồng

IV.1. Laser rắn

- Hoạt động cả ở chế độ xung

và liên tục

- dài ~ 5-20 cm

~ 5-10 mm

- Chế tạo lần đầu năm 1960, laser

đầu tiên trong lịch sử! Không được

sử dụng nhiều nữa

- hiệu suất tổng cộng ~1%

04/12/2013 10

II.2.4. Một số loại khuếch đại laser

Là 1 laser rắn, đại diện hệ 3 mức năng lượng.

- Mức 1 là trạng thái cơ bản

- Mức 2 là kết hợp 2 mức năng lượng rất gần nhau, trạng thái thấp nhất

tương ứng với bước sóng đỏ 694,3 nm.

- Mức 3 là kết hợp của 2 dải có bước sóng trung tâm tương ứng 550 nm và

400nm.

• Laser ruby (tiếp) ...

~ 50ns

2A

E

~ 3ms

thermalization (~1ns)

~400nm

~550nm

electric dipole allowed only because of

interaction with crystal

energy converted into

heat

1R2R

04/12/2013 11

IV.1. Laser rắn


Dùng 1 đèn flash (ánh sáng trắng) kích thích Cr3+ từ 1 -> 3. Cr3+ phân rã

từ 3 -> 2 với thời gian 32 cỡ ps. Các nguyên tử này nằm lại ở 2 với thời

gian tsp 3 ms. Dịch chuyển không bức xạ được bỏ qua. Dịch chuyển này

nở rộng vạch đồng nhất với Dn 330 GHz.

~ 50ns

2A

E

~ 3ms

thermalization (~1ns)

~400nm

~550nm

electric dipole allowed only because of

interaction with crystal

energy converted into

heat

1R2R

04/12/2013 12

IV.1. Laser rắn


Loại Bước sóng

(nm)

Công suất đỉnh Độ rộng xung

Xung bình thường 694,3 100 kW < 0,5 ms

Q-switch 694,3

10 – 50 MW 10 – 20 ns

Mode-locking 694,3

~ GW 10 – 30 ps

CW 694,3

1 mW

04/12/2013 13

… Ruby (Rubin) continued

optical and laser properties of ruby at room temperature

IV.1. Laser rắn

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Neodymium Lasers

IV.1. Laser rắn

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• crystal

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

systems.

• first realized with glass1961, with YAG 1964.

absorption spectrum of Nd:YAG

IV.1. Laser rắn

YAG stands for Yttrium-

Aluminum-Garnet, Y3Al2O12, 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.

04/12/2013 16

… Nd:Lasers continued

- level scheme of Nd:YAG

- "strongest" laser transition at

1064.1 nm

- 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

- lasing is mainly supported by

the R2 sub-level of the 4F3/2

level. At room temperature

~40% of 4F3/2 atoms are in R2 (

Boltzmann).

- lower laser level is 4I11/2 with

various sub-levels, which all

give slightly different emission

wavelength.

~240µs

fast, non-

radiative decay

fast, non-

radiative decay

IV.1. Laser rắn

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• applications

- material processing (cw and pulsed lasers)

welding, marking, writing, drilling (sizes of

few µm possible), cutting

- 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

IV.1. Laser rắn

- medical, especially ophthalmology

- illumination and ranging (military)

04/12/2013 18

• discharge pumping

- 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

IV.1. Laser rắn

arc

lampe

Wolframlamp Diode laser

array

Total electrical

pumping power

2 kW 500 W 1 W

Useful power 100 W 5 W 0.2 W

Laser power 8 W 0.23 W 0.06 W

Conversion efficiency 0.4% 0.04% 6%

lifetime 400 h 100 h 5000 h

04/12/2013 19

- discharge lamps (cw)

… discharge pumping continued

- discharge/filament tube

is mounted inside a flow

tube which carries the

coolant (liquid).

- typical electrical power: ~ 1…10 kW,

~ (100V, 50 A)

- lifetimes few 10h … ~ 1000 h

- arc length ~ 50mm

arc lamps

filament lamps

- typical electrical power: ~ 1kW,

- lifetimes ~100h

- filament length ~ 50mm

IV.1. Laser rắn

04/12/2013 20


• typical (small linear lamp): 60mm long, 4mm diameter,

10J input over 300µs @ 10pps for a 60mm long, 6

mm diameter rod Q-switched output 100…200mJ,

lamp lifetime ~106 shots

- discharge lamps (pulsed)

• typical (large linear lamp):

16cm long, 13mm diameter, ~2kJ input

over 1ms @ ~1pps, lamp lifetime ~105

shots

IV.1. Laser rắn

04/12/2013 21


- 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 the

crystal.

Reduction of problems arising from thermal loading requires

• uniform pumping

• good heat removal

IV.1. Laser rắn

04/12/2013 22

- thermal birefringence

Light transmitted through a pair of

crossed polarizers with a Nd:glass

rod in between. Light is injected

by a second laser, and only a

single flash lamp pulse is applied.

polarization of light is

dynamical and spatially

dependent, i.e. light is

depolarized


IV.1. Laser rắn

04/12/2013 23

- 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)


IV.1. Laser rắn

04/12/2013 24

- different slab geometries exist


multiple flash lamp design

single (dual) flash lamp design

IV.1. Laser rắn

04/12/2013 25

- disk geometry

beneficial for ultra-high power pulsed solid-state laser systems like those

used for laser fusion (at Lawrence Livermore National Laboratory):


- better cooling,

- larger aperture ( >70cm )

- better gain uniformity

- better beam quality

IV.1. Laser rắn

04/12/2013 26

• diode pumping

- 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.

IV.1. Laser rắn

04/12/2013 27

… diode pumping continued

IV.1. Laser rắn

04/12/2013 28


IV.1. Laser rắn

04/12/2013 29


- there are several geometries for optical pumping with laser diodes

• end pumped systems

(single and double)

- pump light can be matched to mode volume

IV.1. Laser rắn

04/12/2013 30


• 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

IV.1. Laser rắn

04/12/2013 31


• side pumping of a slab

- applies 2D-arrays or

densely-packed 1D-arrays

- diodes very close to slab,

no optics required

IV.1. Laser rắn

04/12/2013 32

… Nd:Lasers continued

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

IV.1. Laser rắn

04/12/2013 33

• other solid state lasers

IV.1. Laser rắn

Name Chemical form Center

wavelength

(nm)

Range

(nm)

Temperature Pumping

source

Efficiency

(%)

04/12/2013 34

Tuning range for various transition metal solid state lasers

large tuning range

of Ti:Sa is basis

for ultra-short

pulse operation

… other solid state lasers continued

IV.1. Laser rắn

04/12/2013 35

cw-Ti:Sa laser system Coherent MBR 110, tuning range

- further information regarding Ti:Sa lasers see 2.3.4

IV.1. Laser rắn

IV.1. Laser rắn

Chương IV:


IV.2. Laser khí

• gas lasers: general - gas lasers are very important historically. (Besides the ruby laser) they

where the first lasers to be used, and they have been cultivated very

well. There are still some advantages over other types of lasers, but

most of the gas lasers are being replaced by semiconductor or solid

state lasers. - advantages • gas lasers can be powerful

(few 10 W cw in the optical domain, e.g. ArI laser)

• gas lasers exist for many different wavelength, and different gas

species may be combined to give a large variety of laser types

- problems

all gas lasers have very low efficiency (typically 10-3 or less), poor

stability (power and frequency), can not easily be frequency controlled,

and typically have poor beam quality (all compared to solid-state or

semiconductor lasers)

• work well down (even cw) into the deep blue or UV (ArI, copper laser)

IV.2. Laser khí

04/12/2013 37

04/12/2013 38

• pumping

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.

IV.2. Laser khí

04/12/2013 39

• the role of collisions

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")

* *Laser LASERA 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).

IV.2. Laser khí

04/12/2013 40

HeNe Lasers

IV.2. Laser khí

04/12/2013 41

• level scheme

IV.2. Laser khí

- He is excited to high lying states, from

which it decays rapidly to the 1s2s

level

- He and Ne are mixed at a ratio of ~

5:1 in a discharge tube

- He 1s2s states are meta-stable, as

transition to the ground state

corresponds to a l=0l=0 tran-sition,

S=1S=0 corresponds to a spin-flip,

i.e. is strongly forbidden.

population accumulates in the He

1s2s state

- He 1s2s collides with ground state Ne

and transfers the full excitation energy

to the Ne atom in a nearly resonant

exchange collision.

… gas lasers: HeNe continued

not allowed

not allowed

- some transitions and laser lines in HeNe

IV.2. Laser khí

04/12/2013 42

04/12/2013 43

• typical setup

- 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!

IV.2. Laser khí


- HeNe laser lines

- laser activity covers many lines between 543 nm (green) and

3.39µm (IR) with output powers of up to a few mW.

Popular and commercially available lines are:

543 nm

594 nm

612 nm

633 nm

1523 nm

IV.2. Laser khí


04/12/2013 44

04/12/2013 45

• common HeNe laser parameters

IV.2. Laser khí

wavelength transition Typ. Output power


• application

Red HeNe lasers have many industrial and scientific uses.

- They are widely used in laboratory, because of relatively low cost

and ease of operation compared to other visible lasers producing

beams of similar quality in terms of spatial coherence (a single

mode gaussian beam) and long coherence length

- however since about 1990 semiconductor lasers have offered a

lower cost alternative for many such applications.

- A consumer application of the red HeNe laser is the LaserDisc

player, made Pioneer. The laser is used in the device to read the

optical disk.

04/12/2013 46

IV.2. Laser khí


04/12/2013 47

Ar-Ion (Ar+) Lasers

IV.2. Laser khí

• gas lasers: ArI (Argon-Ion laser)

- inversion is achieved by a two-

step mechanism 1. ionization of Ar-atoms in the

discharge tube:

KINAr e E Ar e slow

2. excitation of Ar-ions in the

discharge tube:

KINAr e E

Ar excited e slow

- the ArI-laser is a four level laser, it provides cw-operation

- ArI lasers provide output powers of up to a few 10 W (multiline),

and can be operated single line in the green (514 nm) and in the

blue (488 nm) and at other slightly different wavelength

collisions

(fast)

04/12/2013 48

IV.2. Laser khí

• gas lasers: ArI (Argon-Ion laser) continued

- common ArI laser lines [nm]:

454, 457, 465, 472 477, 483,

488, 496, 502, 514, 520, 568

04/12/2013 49

IV.2. Laser khí


04/12/2013 50

IV.2. Laser khí


04/12/2013 51

IV.2. Laser khí

04/12/2013 52

• applications

- 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)

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.

IV.2. Laser khí


04/12/2013 53

• emission wavelength of various nobel gas lasers

IV.2. Laser khí

04/12/2013 54

Excimer Lasers

IV.2. Laser khí

04/12/2013 55

- 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

- 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, Xe2Cl, …

& laser emission

IV.2. Laser khí

04/12/2013 56

(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)

IV.2. Laser khí

… excimer lasers continued

04/12/2013 57


- 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.

IV.2. Laser khí

04/12/2013 58

• excimer parameters

IV.2. Laser khí


04/12/2013 59

• common excimer laser parameters

IV.2. Laser khí


04/12/2013 60

N2 Lasers

IV.2. Laser khí

04/12/2013 61

• N2 laser active medium

- N2 is a gas-laser medium which provides three different types of laser

activity described below

• vibration-rotation lasers based on

transitions between different

vibrational states of the same

electronic state (emission: 3µm -

300µm)

• rotation lasers based on transitions

between different rotational states of

the same vibrational and electronic

state (emission: 25µm – 1mm)

• lasers based on transitions between

different electronic states (emission

in the blue-to-UV range)

20

IV.2. Laser khí

04/12/2013 62

• N2 laser active medium

- all transitions to the A 3S+u state (0.75µm…1.24µm) are self-terminating:

transition to the electronic ground state X 1S+u is not dipole allowed

(intercombination line).

Lasers based on the A 3S+u as the lower laser level can therefore only be

operated as pulsed lasers.

- visible laser activity can be observed between the C 3Pu state and the B 3Pg state. The lower laser level is long lived (~30 µs, increase to ~10 ms

due to interaction with atoms) so that these lasers are self-terminating as

well.

- if the laser active gas medium is not quickly exchanged between

subsequent laser pulses the repetition rate has to be limited to ~100 Hz

in order to allow the population to decay back to the electronic ground

state.

IV.2. Laser khí

04/12/2013 63

• N2 lasers

- N2 lasers provide very large

amplification (2.2 / cm for the 337 m

line), so that the inversion is fully

depleted by a single trip through the

amplifier. Therefore these lasers can

be operated without mirrors (typically,

at least one mirror is used).

- lasers are typically operated with pure

N2 at a pressure between a few 1

mbar and ~1 bar.

- beam profile quality and coherence

length is poor.

- applications: pump laser for dye lasers, spectroscopy

- typical N2 laser parameters

IV.2. Laser khí

04/12/2013 64

CO2 Lasers

IV.2. Laser khí

04/12/2013 65

• CO2 laser active medium

- 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 !!)

1351.2 cm-1 (7.5 µm)

2396.4 cm-1 (4.2 µm)

- CO2 normal modes

672.2 cm-1 (14.9 µm)

two-fold degenerate, upper

index gives resulting angular

momentum, l=n2, n2-2,…1 or 0

IV.2. Laser khí

04/12/2013 66

… CO2 laser active medium continued

- 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 N2 is very

efficient (it is almost

resonant, corresponding N2

state is meta-stable)

lower laser level is depleted

through collisions, especially

through collisions with He.

IV.2. Laser khí

04/12/2013 67

- many different technical realizations of CO2 lasers exist.

• "sealed-off" lasers (up to 60 W/m, many

1000 h of continuous operation) addition of H2O + H2 or H2 + O2 causes

CO to react to CO2.

• lasers with a slow, longitudinal N2-flow (~80 W / m)

flow removes dissociation products (CO, O2)

• lasers with a fast N2-flow

(few 10 kW cw): a fast (~300m/s)

flow guarantees fast exchange of

active volume. This is important

especially for high power lasers: the

fast flow provides convection to

remove the heat which would other-

wise limit the excitation density

IV.2. Laser khí


04/12/2013 68

• waveguide lasers (20 W / m)

"optical" fields can be guided in waveguides rather than between

mirrors. This allows a compact setup.

• transversally excited atmospheric pressure (TEA) laser

uses a short (<1µs) electric pulse between transversally oriented

electrodes. This allows high pressure and consequently larger

power (peak power MW to GW, energy many 10 J / liter ).

IV.2. Laser khí


- cutting and welding

- lower power level lasers are used for engraving.

- Some examples of medical uses are laser surgery, skin resurfacing ("laser

facelifts") , treat certain skin conditions

- fabricating microfluidic devices from it, with channel widths of a few hundred

micrometers.

- Because the atmosphere is quite transparent to infrared light, CO2 lasers are also

used for military rangefinding using LIDAR techniques.CO2 lasers are used in the

Silex process to enrich uranium.

04/12/2013 69

IV.2. Laser khí


• applications

04/12/2013 70

• typical CO2 laser parameters

IV.2. Laser khí


IV.1. Laser rắn

Chương IV:


IV.2. Laser khí

IV.3. Laser bán dẫn

04/12/2013 72

Semiconductor Lasers


Courtesy of Sacher Lasertechnik

Communication & Optical Storage

(transmitter lasers, R/W lasers)

Material processing (welding of

plastic materials, soldering and

annealing of metals)

Pumping of solid-state and fiber

lasers

Medical equipment (hair removal,

surgery, dentistry, ophthalmology, PDT)

Lighting & Display applications

Measuring equipment (incl. sensors)

Science and research

. . .

Semiconductor lasers Laser Focus World, Issue 1, Vol. 47, Jan 2011

Application (II)

Laser Transmitter modules

for Fiber Optics

Communication

Diode laser for optical data

storage

Application (III)

Plastics welding with

diode lasers [ Leister

Process Technologies]

Heat Treatment of Metal

Material processing

Application (IV)

Diode lasers for

optical pumping

system

Application (V)

Application (VI)

Low-cost surgery module @ 980nm

Red LD for

Photo-dynamic

Therapy

. . .

Diode Laser accupunture

Application (VII)

Frequency Stabilized Diode Laser @ 670 nm for Shifted Excitation Raman Difference Spectroscopy

History (I)

Ch. Townes N.G. Basov A. M. Prokhorov Zh. Alferov H. Kroemer

1962: Groups at GE, IBM, and MIT's Lincoln Lab. simultaneously

develop GaAs laser and first semiconductor laser desmontrated (in

cryogenically cooled, pulsed operation).

Oct. 1962: N. Holonyak Jr. (GE Co. Lab. in Syracuse, N.Y) publishes

his work on the "visible red" GaAsP laser diode.

Light Amplification by Stimulated Emission of Radiation

Nobel prize winners (pioneers of semiconductor lasers)

History (II)

04/12/2013 81

1963: H. Kroemer of the University of California, Santa Barbara, R.

Kazarinov & Zh. Alferov team of the Ioffe Physico-Technical Institute

in St. Petersburg, independently propose ideas to build

semiconductor lasers from semiconductor heterostructures.

Spring 1970: Zh. Alferov’s group at the Ioffe Physico-Technical

Institute St. Peterburg, Russia and M. Panish and I. Hayashi at Bell

Lab. produce the first CW room-temperature semiconductor lasers,

paving the way toward commercialization of fiber optics

communications.

1972: Ch. Henry at Bell Lab. invents the QW laser, which has very

low lasing threshold than conventional diode lasers, and more

efficient.

History (III)

1975: Engineers at Laser Diode Labs Inc. develop the first

commercial CW semiconductor laser operating at room temperature.

1975: First QW laser operation made by Jan P. Van der Ziel, R.

Dingle, R. C. Miller, W. Wiegmann, and W.A. Nordland Jr. The lasers

are actually developed in 1994.

1976: First demonstration, at Bell Labs, of a CW semiconductor laser

at room temperature at a wavelength beyond 1 µm, the forerunner of

sources for long-wavelength lightwave systems.

1994: The first quantum cascade laser (QCL) - is invented at Bell

Lab. by J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson

and A. Y. Cho (changing the thickness of the semiconductor layers

can change the laser’s wavelength; room-temperature operation and

power and tuning ranges features are ideal for remote sensing of

gases in the atmosphere.

History (III)

1994: The first demonstration of a quantum dot laser with high

threshold density was reported by N. N. Ledentsov of A.F. Ioffe

Physico-Technical Institute in St. Petersburg

Jan. 1997: S. Nakamura, S. P. Den Baars and J. S. Speck at

University of California, Santa Barbara, announce the development of

GaN laser emitting at blue-violet light in pulsed operation.

Sep. 2006: J. Bowers and colleagues at the University of

California, Santa Barbara, and M. Paniccia, director of Intel’s Photonics

Technology Lab. in Santa Clara, California, announced that they have

built the first electrically powered hybrid silicon laser using standard

silicon manufacturing processes. The breakthough could lead to low-

cost, terabit-level optical data pipes inside future computers.

04/12/2013 84

• general

• high efficiency: typically the differential efficiency ( DPout/DPin

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


04/12/2013 85

… pro's of semiconductor lasers continued

• 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)


04/12/2013 86

- "con's" of semiconductor lasers

- 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


04/12/2013 87

• principle of operation

- 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


04/12/2013 88

… principle of operation continued

- 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).


04/12/2013 89


- 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


p-n homojunction & heterojunction



Biased p-n homojunction & heterojunction



04/12/2013 92


• front and rear end of the

semiconductor can provide the

optical feedback if appropriately

reflection coated. Then the laser

diode provides laser operation and

can be considered a diode laser.

• especially in spectroscopy

applications at least one of the

ends is AR-coated and feedback is provided by external,

frequency selective elements. Then, the chip functions as an

amplifier only and should be termed laser diode


04/12/2013 93

• pumping

different methods of pumping semiconductor lasers exist

- optical pumping

- electron beam pumping (with high energy

electrons generated by electron gun)

- current injection

(term: injection laser or diode laser)

This is the most common application

Inversion can be created in a thin

layer (~1µm) of the pn-junction. Laser

emission therefore always features

large divergence angles (few 10 deg

HWHM) at least in the direction

normal to the junction.


04/12/2013 94

• homojunction lasers

- consist of p- and n-doped zones of identical semiconductor

material; first laser diodes realized

- threshold current density of homojunction lasers is

~100kA/cm2 at room temperature, at room temperature

operation therefore only in pulsed mode

Homojunction lasers can be operated in cw-mode at low

temperatures (few 10 K)

Homojunction lasers were soon replaced by heterojunction

lasers, where different host material was used for the

different layers


04/12/2013 95

• gain-guided double heterostructure lasers

- two structure boundaries are used:

Ga1-xAlxAs-GaAs and GaAs-Ga1-yAlyAs to

reduce threshold current (density)

(ii) provides a wave guide like confinement for the vertical direction due

the relatively larger index of refraction of Ga1-xAlxAs

optical loss in non-active region is reduced

threshold current is reduced

threshold current density ~1 kA/cm2

- this design

(i) avoids diffusion of electrons and holes

out of the active area so that the active zone is better localized

threshold current is reduced

(iii) thin (~10µm) wide top electrodes confine the gain region in

horizontal direction, so that transverse single mode operation can

be achieved (gain-guiding)


04/12/2013 96

… gain-guided double heterostructure lasers continued


04/12/2013 97

• index-guided double heterostructure lasers

- the active region is confined in horizontal direction by a diode oriented

such that it is biased in reverse direction under operating conditions.

This forces the injection current into the active region, and it provides

wave guide like confinement in the horizontal direction. Both decreases

threshold current (density)

Index-guided heterostructure lasers have proven to work well. They

provide threshold currents as low as 10 mA and feature transverse

single mode operation.


04/12/2013 98

• quantum well heterostructure lasers

- quantum well lasers use a multilayer sandwich of very thin

heterostructure layers (e.g. GaAs-Ga1-xAlxAs, each structure ~5nm).

This way the active area is confined vertically to ~30nm, which is less

than the de-Broglie wavelength of the electrons

This design further reduces the threshold current. Threshold current is

less dependent on temperature, so that quantum well lasers can also

provide high output powers (~100mW, single transverse mode) at room

temperature.


04/12/2013 99


• Distributed Bragg Reflector (DBR) lasers

- transverse single mode operation of diode lasers can be achieved by

transverse and horizontal confinement (gain-guiding, index-guiding, ridge

waveguides)

- DBR lasers use an on-chip periodic structure outside the active region

which acts like a volume phase grating (Bragg diffraction) and selects

one wavelength (longitudinal mode) for operation.

reflector section

gain section

ridge

waveguide

metallization

burried

grating

typical emission spectrum

04/12/2013 100

• DFB and DBR lasers

- transverse single mode operation of diode lasers can be achieved by

transverse and horizontal confinement (gain-guiding, index-guiding,

ridge waveguides)

- DBR (Distributed Bragg Reflector) laser use an on-chip periodic

structure outside the active region which acts like a grating and selects

one wavelength (longitudinal mode) for operation

- DFB (Distributed Feed Back) laser use an on-chip periodic structure

inside the active region which acts like a grating and selects one

wavelength (longitudinal mode) for operation.

- DFB and DBR provide single mode operation without any additional

external elements, but they can by far not be coarsly tuned as well as

"regular" lasers.


04/12/2013 101

• Distributed Feedback (DFB) lasers

- DFB (Distributed Feed Back) laser use an on-chip periodic structure

inside the active region which acts like a grating and selects one

wavelength (longitudinal mode) for operation.

- DFB and DBR provide single mode operation without any additional

external elements, but they can by far not be coarsely tuned as well as

extended cavity diode lasers (sometimes also called: “external cavity DL” ).

ridge

waveguide

metallization

burried

grating

typical emission spectrum


04/12/2013 102

• single-frequency diode lasers: Littrow lasers

- for spectroscopic applications lasers have to be operated in single-

frequency mode, i.e. in single axial and transverse mode. Gain-guided

and index guided double heterostructure lasers guaranty TEM00 mode

operation, but single axial mode operation has to established through

frequency selective components in an extended (also: “external”) cavity.

- the most simple approach is the "extended cavity" diode laser design,

where light is fed back from a grating in Littrow configuration ( 6.2)

• 0-order provides laser output

• for a given orientation of the grating

the first diffraction order is diffracted

right back into the diode laser only for

one specific wavelength (typically

30%)

• only one diffraction order exists

grating has large line density so that grating in Littrow

configuration

short focal length

(few mm) lens with

large numerical

aperture (0.5..0.6)


04/12/2013 103

… single-frequency diode lasers: Littrow lasers continued

- the Littrow wavelength follows from the grating equation

sin sinIN OUTd

where denotes the wave length, d the distance between two

adjacent grating lines, and QIN and QOut the incidence angle and the

exit angle of the first diffraction order. For a Littrow setup the geometry requires so that IN OUT

sin ,2

IN OUTd

so that, if Q~45 deg is required, then

For a 850 nm laser this corresponds to 1660 lines / mm

~ 2d


04/12/2013 104


- the extended cavity design reduces the laser linewidth: as the cavity

length is increased (from few 100µm to few cm) the intrinsic laser

linewidth is decreased according to DnLASER ~ 1/L3

A typical grating laser line width is 100 kHz – 1 MHz (over 1..10 ms), the

intrinsic linewidth is significantly smaller (in the kHz range)

- continuous tuning range of AR-coated laser diode in a grating setup is a

few GHz, with special mechanical design 50 GHz…100 GHz. With special

care taken for mechanical tuning, tuning of current (and temperature) the

diode laser can provide continuous scanning all through its gain

bandwidth.

- absolute tuning range of AR-coated laser diodes in a grating setup

depends on laser diode, but will typically be ~1-5% of central wavelength.

Non-AR coated laser diodes have to be temperature tuned for wavelength

tuning, and achieve significantly smaller absolute tuning ranges.


04/12/2013 105




04/12/2013 106

• single-frequency diode lasers: Littman lasers

- the Littman design is an alternative external cavity design.

In the Littman setup the first diffraction order is retro-reflected by an

additional mirror. Frequency tuning is now achieved by tilting the planar

retro-mirror.

- Littman setups

• make use of the grating twice, i.e. the grating provides larger selectivity

• the output beam does not move as the laser is tuned


04/12/2013 107




04/12/2013 108

• single-frequency diode lasers:

diode laser with resonant optical feedback

- grating lasers have relatively large line

width because the corresponding cavity

finesse is low (“effective reflection" from

grating 30% max)

D laser diode

C collimator

E etlaon

M curved cavity mirror

G glass plate to pick off

some (4%) of light

- one can use an external, resonant

cavity with much higher finesse as

optical reference. Light coupled into the cavity will be

coupled back to the diode once per

round trip (resonant optical feedback).

Thanks to the phase-intensity coupling

( large line width enhancement factor,

Henry's alpha-parameter) the laser emission will phase lock

to the reflected light (self-injection locking)


04/12/2013 109

… diode laser with resonant optical feedback continued

- note that the cavity only feeds back light in

case of resonance

D laser diode

C collimator

E etlaon

M curved cavity mirror

G glass plate to pick off

some (4%) of light

- the cavity also acts like a low pass filter

which suppresses high-frequency phase

noise. For fast phase noise it acts like a fly

wheel, to which the laser is locked / locks

itself

The laser frequency is very close to one of

the cavity resonance frequencies

- these lasers provide narrower linewidth (few

10 kHz), and reduced phase noise at high

Fourier frequencies. They are more easy

to phase lock, but they are harder to operate

and they provide smaller absolute (few nm) as well as continuous

tuning ranges (~100 MHz).


04/12/2013 110

• single-frequency diode lasers: grating enhanced external

cavity diode laser

D laser diode

COL collimator

GRT volume holographic

transmission grating

OD optical diode

HWP half wave plate

MF

MP

MC planar coupling mirror

HCD Hänsch-Couillaud detector

G stabilization electronics

curved cavity mirror

- this setup combines good absolute and continuous tuneability

of grating diode lasers with narrow linewidth of diode lasers

with resonant optical feedback.


04/12/2013 111

• grating enhanced

external cavity diode laser
