Ti:Sapphire Ultrafast Amplifiers Offer Superior Performance, Reliability and Versatility

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Nov 15, 2013 (3 years and 8 months ago)

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DSF2059, 010108

Laser Focus World, CHINA

Ti:Sapphire supremacy.doc, Rev.
4
.01

1



Ti:Sapphire

Ultrafast Amplifiers Offer Superior Performance,
Reliability and Versatility


Ti:Sapphire and the
B
irth of
U
ltrafast
L
aser
S
ystems


Since
its introduction

in 1960
the
laser

ha
s

become the enabling tool for a multit
ude of
scien
tific

and commercial applications.

Demand for convenient, widely t
u
nable lasers led
to the development of the solid
-
state Titanium
-
doped sapphire (Ti:S) laser in 1986,
offering an alternative to the less desirable dye laser. Due to its wide gai
n

bandwidth,
high thermal conductivity, and excellent optical and mechanical properties, today Ti:S is
the most widely
-
used gain medium for ultrafast and tunable lasers. The development of
Kerr
-
lens modelocked (KLM) Ti
:
S oscillators in the early 1990s ushe
red in a new era of
ultrafast research by enabling simple short pulse (~100fs) generation.
Ti:S
also addressed

the subsequent need for increasingly higher pulse energies when the first CPA (chirped
pulse
amplifier
) lasers were introduced in the mid 1990’s
.

A

key advantage of Ti:S based
ultrafast
systems is the

availability of

convenient solid
-
state pump lasers

(frequency
-
doubled
,

Neodymium
(
Nd
)

doped
,

gain medium) which currently dominate

both the CW
and pulsed

Ti:S pump market

today
.


I
t is estimated that,

A
sia
, thanks to its vibrant economies,

now
accounts for ~
25% of the
global market for Ti:
S based ultrafast systems
.

China, in particular, has become one of
the most important laser markets with strong growth in recent years
,

thanks to relatively
robust go
vernment funding conditions.

DSF2059, 010108

Laser Focus World, CHINA

Ti:Sapphire supremacy.doc, Rev.
4
.01

2

Application
Driven Ultrafast Laser Technology: the Most Recent T
rends


In the past decade, the development of commercial ultrafast lasers ha
s

been driven
by
application

requirements
. At the forefront of ultrafast scientific l
asers are modelocked
Ti:S oscillators typically produc
ing

pulse duration
s from

10fs
to
1
0
ps
, with energies
ranging from 1nJ
-

20nJ at repetition rate between 70

and
90 Mhz. T
hese
oscillators have
multiple application
s

such as
mulitphoton excitation (
MPE
)

microscopy,

teraherz (TH
z)
spectroscopy and imaging,
ultrafast spectroscopy
, an
d

seeding ultrafast amplifier
systems.



Higher
pulse energies
are achieved

thr
ough

amplification

of
these o
scillators.
CPA
-
based

system
s

are used

in order to avoid
optical
comp
onent damage throughout the
amplification process
,

while preserving
a

short amplified pulse

width
.

Several

levels of
amplification are
commercially
available. For applications requiring high duty cycle
,

such as
ultrafast spectroscopic techniques in the liq
uid and solid phase
,

ultrafast
micromachining,
and multiple OPA pumping,
regenerative amplification
pumped

by a
CW solid
-
state pump

lasers

is

well establish
ed. For example, Coherent’s RegA 9050
amplifier
,

pumped by a Verdi CW DPSS laser can t
ypically

gener
ate

pulse energies up to
10uJ with repetition rates up to
300
k
Hz repetition

rate and pulse duration
below 40fs
.


In order to increase the pulse energ
ies

even further
, the

amplifier must be

design
ed to
operate at

lower
repetition rate
s

and higher pump inte
nsity
. This helps

avoid
serious
limit
at
i
ons from

thermal effects in the Ti:S amplifier.
Pulse
d,

solid
-
state pump lasers are
used at r
epetition rate
s

from 1 to
1
0 kHz, leading to

ultrafast

amplified
pulse
energies
betwe
en 1
and
7

mJ

at 800nm.


F
urther ampl
ification (up to 20 mJ) at similar repetition rate

(up to

20 kHz) can be
achieved using cryogenic cooling
of the Ti:S crystal to
significantly limit increased
thermal lensing effects
. For example,
Coherent
’s

Legend Elite Cryo

M
PA

s
ystem

offers
multipass po
wer amplification of the Legend Elite
regenerative amplifier
output
.



Figure 1: Typical Coherent Legend Elite cryogenically cooled Ti:S ultrafast laser system
.


The
milijoule

(mJ) energy level amplifiers

discussed above

are ty
pically used for
ultrafast
spectroscopy,
OPA pumping,
micromachining, etc.
More recent
research directions

that
demand higher energies and lower pulse widths

include

high harmonic generation,
attosecond studies, THz generation in air
, etc.


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Ti:Sapphire supremacy.doc, Rev.
4
.01

3

For application
s demanding yet e
ven higher

ultrafast pulse

energy
,
Ti:S amplifier
s


ranging

from

tens

of
milijoule
s


up to several
J
oules at much lower
repetition rate

(from 1

to
10Hz)

are commercially available
.
The pump source
s

f
or such amplifiers

are typically

low rep
etition
-
rate,
Q
-
switched Nd:YAG laser
s
.
At these energy levels, the
application
s
are

dominated by plasma physics, X
-
ray generation, high energy physics and military
applications.



Typical K
ilohertz (
1
-
10
kHz)
U
ltrafast
A
mplifier
C
onfiguration
s


The major
ity of ultrafast amplifiers sold today fall into the 1 to 10mJ, 1 to 10kHz
category. A trend towards increasing pulse energy and decreasing pulse

width has put
new demands on ultrafast oscillator and amplifier design.




















Figure 2: Principl
e of operation of a chirp
ed

pulse amplifier


A reliable and effective
,

short pulse
,

Ti:S

CPA
,

kHz ultrafast amplifier system
must
start
with a
very stable
,

wideband, Ti:S modelocked oscillator
. One box
oscillator
designs
,

with integrated pump lasers
,

offer

new levels of stability and ease of use. There are two
main
technologies used
commercially
for dispersion compensation

in oscillator
s
.
Most
oscillators

are
prism based

and offer the highest level of flexibility. In many amplifier
applications
,

this flexib
ility is not requ
ired and
an

alternative

dispersion compensation

method,

using chirped mirrors

provides broad bandwidth for the well defined requirement
of amplifier seeding.

In
the
simplest
amplifier
configuration
s
, the
se

broadband
oscillator
pulse
s

are p
ositively
chirped in a stretcher,

amplified in a regenerative amplifier (RGA) and/or a multipass
amplifier

(MPA)
,

before
b
e
ing

recompressed
in a
pulse compressor.
Not

so long ago,
such

a

laser system would have required an entire laboratory and several
exp
erts

to build,
maintain, and operate
.

T
oday’s commercially available high power amplifiers can fit

comfortably

on a single
optic
al table,
require

little maintenance and can be used by
most
t
Short pulse from
oscillator
t
Pulse
stretcher
t
t
Ti:S amplifier
Small beam size,
LOW peak power
t
t
Pulse
compressor
Larger beam
size, HIGH
peak power
t
Short pulse from
oscillator
t
Pulse
stretcher
t
t
Ti:S amplifier
Small beam size,
LOW peak power
t
t
Pulse
compressor
Larger beam
size, HIGH
peak power
DSF2059, 010108

Laser Focus World, CHINA

Ti:Sapphire supremacy.doc, Rev.
4
.01

4

research
students as well as production
personnel

with limited lase
r experience. The
ultrafast
amplifier

system
has evolved from itself being the focus of research

to be
com
ing

a productive and reliable research tool.





Figure
3
:

Coherent Legend Elite series, seeded by a Micra
-
5 Ti:S oscillato
r offers
a
compact solution for pulse
width
s

ranging from sub
-
25fs to 2 ps, energies between 1
-
10mJ, at r
epetition rates from 1 to 10kHz
b)

typical table layout for commercial CPA amplifier systems e.g. Coherent Legend Elite Series


While multipass amplifi
er configurations are in use by some manufacturers

today
, the
regenerative amplifier is by far the most widely used
,

due to its clear advantages
over the
multipass designs with regards to

beam quality, energy and beam pointing stability.



















Figure
4
:

Block diagram and t
ypical beam quality after amplification from a:
a)

regenerative
amplifier
(RGA)

b)

multipass amplifier

(no RGA amplifier prior the multipass)
.


Ti:Sapph
HR
DPSS Pulsed
pump @ 527nm
HR
Ejector
Injector
Seed pulses
FROM
stretcher
REGENERATIVE AMPLIFIER
Amplified pulses
TO
compressor
Ti:Sapph
HR
DPSS Pulsed
pump @ 527nm
HR
Ejector
Injector
Seed pulses
FROM
stretcher
REGENERATIVE AMPLIFIER
Amplified pulses
TO
compressor
HR
DPSS Pulsed
pump @ 527nm
HR
Seed pulses
from stretcher
MULTIPASS AMPLIFIER
Amplified
pulses
TO
compressor
Ti:Sapph
HR
DPSS Pulsed
pump @ 527nm
HR
Seed pulses
from stretcher
MULTIPASS AMPLIFIER
Amplified
pulses
TO
compressor
Ti:Sapph
Ti:Sapph
HR
DPSS Pulsed
pump @ 527nm
HR
Ejector
Injector
Seed pulses
FROM
stretcher
REGENERATIVE AMPLIFIER
Amplified pulses
TO
compressor
Ti:Sapph
HR
DPSS Pulsed
pump @ 527nm
HR
Ejector
Injector
Seed pulses
FROM
stretcher
REGENERATIVE AMPLIFIER
Amplified pulses
TO
compressor
Ti:Sapph
HR
DPSS Pulsed
pump @ 527nm
HR
Ejector
Injector
Seed pulses
FROM
stretcher
REGENERATIVE AMPLIFIER
Amplified pulses
TO
compressor
Ti:Sapph
HR
DPSS Pulsed
pump @ 527nm
HR
Ejector
Injector
Seed pulses
FROM
stretcher
REGENERATIVE AMPLIFIER
Amplified pulses
TO
compressor
HR
DPSS Pulsed
pump @ 527nm
HR
Seed pulses
from stretcher
MULTIPASS AMPLIFIER
Amplified
pulses
TO
compressor
Ti:Sapph
HR
DPSS Pulsed
pump @ 527nm
HR
Seed pulses
from stretcher
MULTIPASS AMPLIFIER
Amplified
pulses
TO
compressor
Ti:Sapph
DSF2059, 010108

Laser Focus World, CHINA

Ti:Sapphire supremacy.doc, Rev.
4
.01

5

Within the range of pulse energies obtained in 1 to 10kHz amplifiers, different c
ooling
strategies are typically used for the Ti:S crystals in such systems. Coherent’s Legend
Elite amplifiers utilize water
-
cooling of the Ti:S for systems with amplified pulse energy
<2mJ. For systems with energies up to ~
7
mJ, thermo
-
electric cooling (TE
C) of the rod is
the preferred technique
,

thanks to its simplicity and cost effectiveness. Such high pulse
energy is only useful if other key laser
characteristics

such as the beam quality, stability
and pulse width are maintained during amplification. In
particular, this requires
extremely careful design of the TE
-
cooled crystal housing, the optical cavity of the
regenerative amplifier, the stretcher/compressor and the amplifier platform itself.

H
igh
efficiency
, stability and compactness

are

enabled by adv
anced design features such as an
integrated pump laser, thermal stabilization of the regenerative amplifier
, and

conversion
efficiency of pump energy to regenerative amplifier outp
ut approaching 35%
.




Figure
5
:

Typical

IR (80
0nm)

Coherent Legend Elite USX
:

a)

far field beam profile quality
b)

autocorrelation pulse width trace (deconv.=~22fs), and
c
)

long term power stability



Ultrafast System
s
: Complete S
olution
s

now Commercially A
vailable


Previously,
almost

every innovatio
n

in ultraf
ast amplifier laser systems

came

from

research

laboratories and unive
rsities. In recent years,
significant
R&D
and product
improvemen
t efforts

have accelerated technology transfer and new
product
developments
,

to offer complete
commercial
soluti
ons to a wide variety of ever more
demanding applications
. These demanding applications include

carrier

envelope phase
(
CEP
)

stabilization in attosecond physics, amplitude and phase modulation for
pulse
shaping and optimization, multiple OPA system options

for

ultrafast spectroscopic

research, pulse synchronization in particle accelerator facilities
, etc
.

A wide variety of
laser diagnostics are
also
now commercially available
to improve user productivity
such
as
single
-
shot
autocorrelator
s
, phase analyzer
s
,

beam profilers, power and energy meters,
etc.


While integration appears to be a major trend in the laser design itself
,

users benefit when
the las
e
r suppl
i
er is fully
i
ntegrated

and able to

offer versatility in the designs,

as well as
efficient
,
knowledg
eable

support
for

all
the
sub components of the ultrafast amplifier
system
.

DSF2059, 010108

Laser Focus World, CHINA

Ti:Sapphire supremacy.doc, Rev.
4
.01

6



Figure
6
: Coherent’s vertically integrated ultrafast product portfolio


Choosing an Ultrafast Amplifier
: System Versatility is the Key to Economical
Ultrafast
Laser S
ystems fo
r Long Term Productivity and R
esearch.



Customers now have a multitude of choices in Ti:S amplifiers. There are several elements
which, in combination with the price, establish the value proposition. U
sers now
typically
consider
:


-

Technical specifications

-

Stability and reliability

-

Ease of use and maintenance

-

Sub
-
system
i
ntegration

(pumps, osc
illator, post
-
amplifiers
,
harmonics &
OPA
,
pulse shapers, etc.
)

-

Modularity and scalability

(upgrade pathways)

-

Service and support network

kn
owledge and availability.

-

Price/Value ratio


C
ommercial customers for high power
ultrafast
amplifier systems
typically
require turn
-
key fully integrated system
s

for minimum user
-
laser interaction
. An

increasing number of
scientific and research customers
a
re
opting for the benefits of
integrated, yet
highly
flexible system
s

that
also
have the capacity

to incorporate

significant
future

upgrades

(e.g. further amplification stages)
.

This approach enhances the long term value of the
original investment



DSF2059, 010108

Laser Focus World, CHINA

Ti:Sapphire supremacy.doc, Rev.
4
.01

7

Althou
gh other gain media will
find a niche in ultrafast applications, T
i:S
based
amplifiers remain the system of choice for the overwhelming majority of scientific
applications
,

due to their unsurpassed combination of performance capability, versatility
and val
ue.