Semrock Master Catalog 2018

Polarizers
Mirrors NIR Filters
Edge
Filters
Dichroic
Beamsplitters
Laser-line
Filters
Laser Diode
Filters
TECHNICAL NOTE
Technical Note: Laser Damage Threshold
Laser damage to optical filters is strongly dependent on many factors, and thus it is difficult to guarantee the performance of
a filter in all possible circumstances. Nevertheless, it is useful to identify a Laser Damage Threshold (LDT) of pulse fluence or
intensity below which no damage is likely to occur.
P
Ppeak
Pulsed vs. continuous-wave lasers: Pulsed lasers emit light in a series of
pulses of duration t at a repetition rate R with peak power P peak
. Continuouswave
(cw) lasers emit a steady beam of light with a constant power P. Pulsedlaser
average power P avg
1/R
and cw laser constant power for most lasers typically
P
range from several milliWatts (mW) to Watts (W). The table at the end of this
avg
time
Note summarizes the key parameters that are used to characterize the output
of pulsed lasers.
The table below summarizes the conditions under which laser damage is expected to occur for three main types of lasers.
Units: P in Watts; R in Hz;
diameter in cm; LDT LP
in J/cm 2 .
Note: l spec
and t spec
are the
wavelength and pulse width,
respectively, at which LDT LP
is specified.
* The cw and quasi-cw cases
are rough estimates, and
should not be taken as
guaranteed specifications.
Type of Laser Typical Pulse
Properties
Long-pulse τ ~ ns to µs
R ~ 1 to 100 Hz
cw
Quasi-cw
Continuous
output
τ ~ fs to ps
R ~ 10 to 100 MHz
When Laser Damage is Likely
P avg
λ
R x (π/4) x diameter > 2 λ x Ʈ
x
Ʈ LDTLP
spec spec
P
W λ
(π/4) x diameter > ~10,000
2 J
x λ x LDTLP*
spec
P avg
W λ
(π/4) x diameter > ~10,000
2 J
x λ x LDTLP*
spec
Long-pulse lasers:
Damage Threshold Long Pulse is generally specified in terms of pulse fluence for “long-pulse lasers.” Because the time
between pulses is so large (milliseconds), the irradiated material is able to thermally relax—as a result damage is generally
not heat-induced, but rather caused by nearly instantaneous dielectric breakdown. Usually damage results from surface or
volume imperfections in the material and the associated irregular optical field properties near these sites. Most Semrock
filters have LDT LP
values on the order of 1 J/cm 2 , and are thus considered “high-power laser quality” components. An
important exception is a narrowband laser-line filter in which the internal field strength is strongly concentrated in a few
layers of the thin-film coating, resulting in an LDT LP
that is about an order of magnitude smaller.
cw lasers: Damage from cw lasers tends to result from thermal (heating) effects. For this reason the LDT CW
for cw lasers
is more dependent on the material and geometric properties of the sample, and therefore, unlike for long-pulse lasers, it is
more difficult to specify with a single quantity. For this reason Semrock does not test nor specify LDT CW
for its filters. As a
very rough rule of thumb, many all-glass components like dielectric thin-film mirrors and filters have a LDT CW
(specified as
intensity in kW/cm 2 ) that is at least 10 times the long-pulse laser LDT LP
(specified as fluence in J/cm 2 ).
Quasi-cw lasers: Quasi-cw lasers are pulsed lasers with pulse durations τ in the femtosecond (fs) to picosecond (ps) range,
and with repetition rates R typically ranging from about 10 – 100 MHz for high-power lasers. These lasers are typically
mode-locked, which means that R is determined by the round-trip time for light within the laser cavity. With such high
repetition rates, the time between pulses is so short that thermal relaxation cannot occur. Thus quasi-cw lasers are often
treated approximately like cw lasers with respect to LDT, using the average intensity in place of the cw intensity.
Example: Frequency-doubled Nd:YAG laser at 532 nm. Suppose τ = 10 ns, R = 10 Hz, and Pavg = 1 W. Therefore D = 1 x
10 –7 , E = 100 mJ, and Ppeak = 10 MW. For diameter = 100 μm, F = 1.3 kJ/cm 2 , so a part with LDT LP
= 1 J/cm 2 will likely be
damaged. However, for diameter = 5 mm, F = 0.5 J/cm 2 , so the part will likely not be damaged.
Notch
Filters
Lamp Clean-up
Filters
Symbol Definition Units Key Relationships
τ Pulse duration sec τ = D / R
R Repetition rate Hz = sec -1 R = D / τ
D Duty cycle dimensionless D = R x τ
P Power Watts = Joules / sec P peak
= E / τ; P avg
= P peak
x D; P avg
= E x R
E Energy per pulse Joules E = P peak
x τ; E = P avg
/ R
A Area of laser spot cm 2 A = (π / 4) x diameter 2
I Intensity Watts / cm 2 I = P /A; I peak
= F / τ; I avg
= I peak
x D; I avg
= F x R
F Fluence per pulse Joules / cm 2 F = E / A; F = I peak
x τ; F = I avg
/ R
More
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Try out Semrock’s Laser Damage Threshold Calculator at www.semrock.com/ldt-calculator.aspx