This document discusses different types of solid state lasers, including ruby lasers and neodymium lasers. Ruby lasers were one of the first lasers developed in the 1960s and operate using chromium ions in a ruby crystal. Neodymium lasers often use neodymium-doped yttrium aluminum garnet (Nd:YAG) crystals as the lasing medium and are pumped using flashlamps or laser diodes. Nd:YAG lasers can operate continuously or pulsed and are widely used for applications such as material processing, medicine, and pumping other lasers. The document also discusses pumping methods, thermal effects, and different crystal geometries used in solid state lasers.

1. LASER VÀ ỨNG DỤNG
TS. Nguyễn Thanh Phương
Bộ môn Quang học và Quang điện tử

2. Chương IV:
Các loại laser và ứng dụng

3. Chương IV: Các loại laser và ứng dụng
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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:
- phương trình tốc độ của laser
- ngưỡng phát laser
- bão hòa khuếch đại
- so sánh mode và lọc lựa mode
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5. Chương IV:
Các loại laser và ứng dụng
IV.1. Laser rắn

6. IV.1. Laser rắn
- 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.
- 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).
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7. IV.1. Laser rắn
- 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)
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8. IV.1. Laser rắn
Laser Ruby
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9. IV.1. Laser rắn
• Laser ruby
- Tinh thể ruby: Cr3+ pha tạp trong tinh thể
sapphire (Al2O3)
- Truyền qua vùng màu hồng
- Chế tạo lần đầu năm 1960, laser
đầu tiên trong lịch sử! Không được
sử dụng nhiều nữa
- Hoạt động cả ở chế độ xung
và liên tục
- dài ~ 5-20 cm
 ~ 5-10 mm
- hiệu suất tổng cộng ~1%
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10. II.2.4. Một số loại khuếch đại laser
• Laser ruby (tiếp) ...
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
 ~ 50ns
400nm.
energy converted into
heat
 ~ 3ms
2A
thermalization (~1ns)
E
~400nm
~550nm
R2 R1
electric dipole allowed only because of
interaction with crystal
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11. IV.1. Laser rắn
• Laser ruby (tiếp) ...
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
energy converted into
heat
 ~ 3ms
2A
thermalization (~1ns)
E
~400nm
~550nm
R2 R1
electric dipole allowed only because of
interaction with crystal
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12. IV.1. Laser rắn
• Laser ruby (tiếp) ...
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

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13. IV.1. Laser rắn
… Ruby (Rubin) continued
optical and laser properties of ruby at room temperature
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14. IV.1. Laser rắn
Neodymium Lasers
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15. IV.1. Laser rắn
• first realized with glass1961, with YAG 1964.
• crystal
- Nd:YAG is the most important material used for solid state laser
systems.
YAG
stands
for
YttriumAluminum-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.
absorption spectrum of Nd:YAG
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16. IV.1. Laser rắn
… Nd:Lasers continued
- level scheme of Nd:YAG
fast, nonradiative decay
 ~240µs
fast, nonradiative decay
- 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).
- "strongest" laser transition at
1064.1 nm
- 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 cwoperation.
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17. IV.1. Laser rắn
• applications
- material processing (cw and pulsed lasers)
welding, marking, writing, drilling (sizes of
few µm possible), cutting
- medical, especially ophthalmology
- illumination and ranging (military)
- 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
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18. IV.1. Laser rắn
• 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
arc
lampe
Wolframlamp
Diode laser
array
2 kW
500 W
1W
Useful power
100 W
5W
0.2 W
Laser power
8W
0.23 W
0.06 W
Conversion efficiency
0.4%
0.04%
6%
lifetime
400 h
100 h
5000 h
Total electrical
pumping power
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19. IV.1. Laser rắn
… discharge pumping continued
- discharge lamps (cw)
arc lamps
- typical electrical power: ~ 1…10 kW,
~ (100V, 50 A)
- arc length ~ 50mm
- lifetimes few 10h … ~ 1000 h
- discharge/filament tube
is mounted inside a flow
tube which carries the
coolant (liquid).
filament lamps
- typical electrical power: ~ 1kW,
- filament length ~ 50mm
- lifetimes ~100h
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20. IV.1. Laser rắn
… discharge pumping continued
- discharge lamps (pulsed)
• 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
• typical (large linear lamp):
16cm long, 13mm diameter, ~2kJ input
over 1ms @ ~1pps, lamp lifetime ~105
shots
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21. IV.1. Laser rắn
… discharge pumping continued
- 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 nonunity 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
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22. IV.1. Laser rắn
… discharge pumping continued
- 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
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23. IV.1. Laser rắn
… discharge pumping continued
- 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)
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24. IV.1. Laser rắn
… discharge pumping continued
- different slab geometries exist
single (dual) flash lamp design
multiple flash lamp design
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25. IV.1. Laser rắn
… discharge pumping continued
- 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
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26. IV.1. Laser rắn
• 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, 1Darrays ("bars") or 2D-arrays can be used.
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27. IV.1. Laser rắn
… diode pumping continued
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28. IV.1. Laser rắn
… diode pumping continued
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29. IV.1. Laser rắn
… diode pumping continued
- there are several geometries for optical pumping with laser diodes
• end pumped systems
(single and double)
- pump light can be matched to mode volume
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30. IV.1. Laser rắn
… diode pumping continued
• 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
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31. IV.1. Laser rắn
… diode pumping continued
• side pumping of a slab
- applies 2D-arrays or
densely-packed 1D-arrays
- diodes very close to slab,
no optics required
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32. IV.1. Laser rắn
… Nd:Lasers continued
- physical, optical, thermal properties
of Nd:YAG
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 unidirectional oscillation.
T. J. Kane and R. L. Byer, Opt. Lett. 10 (2), 65 (1985) ;
I. Freitag et al., Opt. Commun. 115, 511 (1995)
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33. IV.1. Laser rắn
• other solid state lasers
Name
Chemical form
Center
wavelength
(nm)
Range
(nm)
Temperature
Pumping
source
Efficiency
(%)
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34. IV.1. Laser rắn
… other solid state lasers continued
Tuning range for various transition metal solid state lasers
large tuning range
of Ti:Sa is basis
for ultra-short
pulse operation
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35. IV.1. Laser rắn
cw-Ti:Sa laser system Coherent MBR 110, tuning range
- further information regarding Ti:Sa lasers see 2.3.4
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36. Chương IV:
Các loại laser và ứng dụng
IV.1. Laser rắn
IV.2. Laser khí

37. 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)
• work well down (even cw) into the deep blue or UV (ArI, copper 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)
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38. IV.2. Laser khí
• pumping
there are different methods of pumping.
- most common type is based on a continuous or pulsed discharge
- optical pumping (with another laser)
- 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 rotationalvibrational states is generated.
- chemical lasers: inversion is generated through a chemical reaction
• 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.
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39. IV.2. Laser khí
• 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")
A*  BLaser  A  B*
LASER
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).
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40. IV.2. Laser khí
HeNe Lasers
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41. IV.2. Laser khí
• level scheme
- He and Ne are mixed at a ratio of ~
5:1 in a discharge tube
- He is excited to high lying states, from
which it decays rapidly to the 1s2s
level
- He 1s2s states are meta-stable, as
transition to the ground state
corresponds to a l=0l=0 tran-sition,
S=1S=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.
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42. IV.2. Laser khí
… gas lasers: HeNe continued
- some transitions and laser lines in HeNe
not allowed
not allowed
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43. IV.2. Laser khí
… gas lasers: HeNe continued
• 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!
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44. IV.2. Laser khí
… gas lasers: HeNe continued
- HeNe laser lines
- laser activity covers many lines between 543 nm (green)
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
and
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45. IV.2. Laser khí
… gas lasers: HeNe continued
• common HeNe laser parameters
wavelength
transition
Typ. Output power
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46. IV.2. Laser khí
… gas lasers: HeNe continued
• 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.
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47. IV.2. Laser khí
Ar-Ion (Ar+) Lasers
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48. IV.2. Laser khí
• gas lasers: ArI (Argon-Ion laser)
- inversion is achieved by a twostep mechanism
1. ionization of Ar-atoms in the
discharge tube:
Ar  e   EKIN   Ar   e   slow 
(fast)
collisions
2. excitation of Ar-ions in the
discharge tube:
Ar   e   E KIN  
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
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49. 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
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50. IV.2. Laser khí
• gas lasers: ArI (Argon-Ion laser) continued
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51. IV.2. Laser khí
• gas lasers: ArI (Argon-Ion laser) continued
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52. IV.2. Laser khí
• gas lasers: ArI (Argon-Ion laser) continued
• applications
- Ar+ lasers have been extensively used as pump lasers for dye lasers and
cw-TiSa lasers.
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.
- holography (but see comment above)
- medical applications (but see comment above)
- laser light shows (but see comment above)
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53. IV.2. Laser khí
• emission wavelength of various nobel gas lasers
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54. IV.2. Laser khí
Excimer Lasers
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55. IV.2. Laser khí
• excimers
- excimers are diatomic molecules which do not posses a stable
electronic ground state. They only exist as excited dimers.
- 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
& laser emission
the system ground state (i.e. two
atoms). The lower laser level is
effectively unpopulated.
- many dimers provide gas laser
activity, e.g. ArF, KrF, XeF, HgCl,
NaXe, Xe2Cl, …
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56. IV.2. Laser khí
… excimer lasers continued
- excimer lasers are typically pumped by
(i) an electron beam
(current 5-50 kA, 5-500 A/cm2)
(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.
- repetition rates are in the few
Hz to ~100 Hz range
- 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)
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57. IV.2. Laser khí
… excimer lasers continued
- 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.
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58. IV.2. Laser khí
… excimer lasers continued
• excimer parameters
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59. IV.2. Laser khí
… excimer lasers continued
• common excimer laser parameters
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60. IV.2. Laser khí
N2 Lasers
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61. IV.2. Laser khí
• N2 laser active medium
- N2 is a gas-laser medium which provides three different types of laser
activity described below
• lasers based on transitions between
different electronic states (emission
in the blue-to-UV range)
• 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)
20
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62. IV.2. Laser khí
• 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
3P state. The lower laser level is long lived (~30 µs, increase to ~10 ms
g
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.
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63. IV.2. Laser khí
• 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).
- typical N2 laser parameters
- 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
04/12/2013
63

64. IV.2. Laser khí
CO2 Lasers
04/12/2013
64

65. IV.2. Laser khí
• 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 !!)
- CO2 normal modes
1351.2 cm-1 (7.5 µm)
672.2 cm-1 (14.9 µm)
two-fold degenerate, upper
index gives resulting angular
momentum, l=n2, n2-2,…1 or 0
2396.4 cm-1 (4.2 µm)
04/12/2013
65

66. IV.2. Laser khí
… CO2 laser active medium continued
- the CO2 laser is discharge pumped, discharge contains also N2 and He.
excitation through N2 is very
efficient
(it
is
almost
resonant, corresponding N2
state is meta-stable)
upper laser level life time: 1µs
… 1 ms
lower laser level is depleted
through collisions, especially
through collisions with He.
- CO2 laser feature very high efficiency (quantum efficiency: 45%,
electrical-to-optical efficiency: up to 30%)
04/12/2013
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67. IV.2. Laser khí
… CO2 laser active medium continued
- many different technical realizations of CO2 lasers exist.
• lasers with a slow, longitudinal N2-flow (~80 W / m)
flow removes dissociation products (CO, O2)
• "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 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 otherwise limit the excitation density
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67

68. IV.2. Laser khí
… CO2 laser active medium continued
• 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 ).
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69. IV.2. Laser khí
… CO2 laser active medium continued
• applications
- 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, CO 2 lasers are also
used for military rangefinding using LIDAR techniques.CO2 lasers are used in the
Silex process to enrich uranium.
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70. IV.2. Laser khí
… CO2 laser active medium continued
• typical CO2 laser parameters
04/12/2013
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71. Chương IV:
Các loại laser và ứng dụng
IV.1. Laser rắn
IV.2. Laser khí
IV.3. Laser bán dẫn

72. IV.3. Laser bán dẫn
Semiconductor Lasers
Courtesy of Sacher Lasertechnik
04/12/2013
72

73. Laser Focus World, Issue 1, Vol. 47, Jan 2011
Semiconductor lasers
 Communication & Optical Storage
(transmitter lasers, R/W lasers)
 Medical equipment (hair removal,
surgery, dentistry, ophthalmology, PDT)
 Material processing (welding of
plastic materials, soldering and
annealing of metals)
 Lighting & Display applications
 Measuring equipment (incl. sensors)
 Pumping of solid-state and fiber
lasers
 ...
 Science and research

74. Application (II)
Laser Transmitter modules
for Fiber Optics
Communication
Diode laser for optical data
storage

75. Application (III)
Material processing
Plastics welding with
diode lasers [ Leister
Process Technologies]
Heat Treatment of Metal

76. Application (IV)
Diode lasers for
optical pumping
system

77. Application (V)

78. Application (VI)
Red LD for
Photo-dynamic
Therapy
Low-cost surgery module @ 980nm
Diode Laser accupunture
...

79. Application (VII)
Frequency Stabilized Diode Laser @ 670 nm for Shifted Excitation Raman Difference Spectroscopy

80. History (I)
Light Amplification by Stimulated Emission of Radiation
Ch. Townes
N.G. Basov A. M. Prokhorov
Zh. Alferov
H. Kroemer
Nobel prize winners (pioneers of semiconductor lasers)
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.

81. History (II)
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.
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82. 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.

83. 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 lowcost, terabit-level optical data pipes inside future computers.

84. IV.3. Laser bán dẫn
• general
- semiconductor lasers rely on solid state physics. Most common
type is diode laser, which applies physics of semiconductor diode
(pn-junction)
- "pro's" of semiconductor lasers
• simple pumping: current injection
• high efficiency: typically the differential efficiency ( DPout/DPin
above threshold) is ~50%
• 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
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85. IV.3. Laser bán dẫn
… 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)
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85

86. IV.3. Laser bán dẫn
- "con's" of semiconductor lasers
- 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
- very sensitive to optical, electrical, and electrostatic damage
(anyone who has ever build a diode laser has "killed" a laser diode)
- 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
stabilization
applications
require
active
frequency
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87. IV.3. Laser bán dẫn
• 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
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88. IV.3. Laser bán dẫn
… 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.
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).
If the Fermi-energy lays in between the conduction and valence
band, an undoped "semiconductor" is an isolator.
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89. IV.3. Laser bán dẫn
… principle of operation continued
- pn-junction lasers
• with no voltage applied the quasi-Fermilevels are degenerate. No inversion is
achieved (at T=0K)
• 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, no bias
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.
pn-junction, forward bias
• typical and common semiconductors
are GaAlAs (~800nm), InGaAsP (1.3µm -1.5µm), GaInP (670 nm)
04/12/2013
89

90. IV.3. Laser bán dẫn
… principle of operation continued
p-n homojunction & heterojunction

91. IV.3. Laser bán dẫn
… principle of operation continued
Biased p-n homojunction & heterojunction

92. IV.3. Laser bán dẫn
… principle of operation continued
• 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
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93. IV.3. Laser bán dẫn
• 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.
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94. IV.3. Laser bán dẫn
• 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
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95. IV.3. Laser bán dẫn
• gain-guided double heterostructure lasers
- two structure boundaries are used:
Ga1-xAlxAs-GaAs and GaAs-Ga1-yAlyAs to
reduce threshold current (density)
- 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
(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/cm 2
(iii) thin (~10µm) wide top electrodes confine the gain region in
horizontal direction, so that transverse single mode operation can
be achieved (gain-guiding)
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96. IV.3. Laser bán dẫn
… gain-guided double heterostructure lasers continued
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96

97. IV.3. Laser bán dẫn
• 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.
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98. IV.3. Laser bán dẫn
• 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.
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99. IV.3. Laser bán dẫn
• 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.
typical emission spectrum
reflector section
burried
grating
gain section
metallization
ridge
waveguide
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100. IV.3. Laser bán dẫn
• 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.
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101. IV.3. Laser bán dẫn
• 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.
metallization
typical emission spectrum
burried
grating
ridge
waveguide
- 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” ).
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102. IV.3. Laser bán dẫn
• single-frequency diode lasers: Littrow lasers
- for spectroscopic applications lasers have to be operated in singlefrequency 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)
grating in Littrow
configuration
short focal length
(few mm) lens with
large numerical
aperture (0.5..0.6)
grating has large line density so that
• only one diffraction order exists
• 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%)
• 0-order provides laser output
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103. IV.3. Laser bán dẫn
… single-frequency diode lasers: Littrow lasers continued
- the Littrow wavelength follows from the grating equation

d
 sin  IN  sin OUT
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  IN  OUT
so that

 sin  ,  IN  OUT
2d
so that, if Q~45 deg is required, then d ~ 2
For a 850 nm laser this corresponds to 1660 lines / mm
04/12/2013
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104. IV.3. Laser bán dẫn
… single-frequency diode lasers: Littrow lasers continued
- 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.
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105. IV.3. Laser bán dẫn
… single-frequency diode lasers: Littrow lasers continued
Courtesy of Sacher Lasertechnik
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105

106. IV.3. Laser bán dẫn
• 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
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107. IV.3. Laser bán dẫn
… single-frequency diode lasers: Littrow lasers continued
Courtesy of Sacher Lasertechnik
04/12/2013
107

108. IV.3. Laser bán dẫn
• 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)
- 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).
D
C
E
M
G
laser diode
collimator
etlaon
curved cavity mirror
glass plate to pick off
some (4%) of light
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)
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109. IV.3. Laser bán dẫn
… diode laser with resonant optical feedback continued
- note that the cavity only feeds back light in
case of resonance
- 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
D laser diode
C collimator
the cavity resonance frequencies
E etlaon
- these lasers provide narrower linewidth (few
M curved cavity mirror
G glass plate to pick off
10 kHz), and reduced phase noise at high
some (4%) of light
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).
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110. IV.3. Laser bán dẫn
• 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
curved cavity mirror
MP
MC
planar coupling mirror
HCD Hänsch-Couillaud detector
G
stabilization electronics
- this setup combines good absolute and continuous tuneability
of grating diode lasers with narrow linewidth of diode lasers
with resonant optical feedback.
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111. IV.3. Laser bán dẫn
• grating enhanced
external cavity diode laser
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