10 kW
1 kW
100 W
10 W
www.edmundoptics.eu/LO 45
Continuous Wave Lasers:
Damage from continuous wave (CW) lasers is typically a result of thermal
effects caused by absorption in the optic’s coating or substrate,2 Cemented
optical components, such as achromats, tend to have lower CW
damage thresholds because of absorption or scattering in the cement.
To understand a CW LIDT specification, it is necessary to know the
laser’s wavelength, beam diameter, power density, and intensity profile
(e.g., Gaussian or flat top). LIDT for CW lasers is specified in units of
power per area, typically in W/cm2. For example, if a 5 mW, 532 nm
Nd:YAG laser with a flat top beam is used with a beam diameter of 1
mm, then the power density is:
Therefore, if the LIDT specified for an optic is lower than 0,64 W/cm2
then the user risks optical damage at 532 nm. An extra factor of 2 would
need to be added if using a Gaussian beam.
Pulsed Lasers:
Pulsed lasers emit discrete pulses of laser energy at a given repetition
rate, or frequency (Figure 15.3). The energy per pulse is directly proportional
to the average power and inversely proportional to the repetition
rate of the laser (Figure 15.4).
Damage from short nanosecond laser pulses is typically due to dielectric
breakdown of the material resulting from exposure to the high electric
fields in the laser beam,3 Dielectric breakdown occurs when a current
flows through an electrical insulator because the applied voltage exceeds
the material’s breakdown voltage. For longer pulse widths or high repetition
rate laser systems, laser-induced damage may result from a combination
of thermally-induced damage and dielectric breakdown. This occurs
because the pulse duration is still on the order of the time duration
of electron-lattice dynamics, which is responsible for thermally-induced
damage. These thermal processes are negligible for ultrashort pulses of
about 10ps or less,4 In this case, nonlinear excitation of electrons from
the valence band to the conduction band, through mechanisms such as
multiphoton absorption, multiphoton ionization, tunnel ionization, and
avalanche ionization, leads to damage5.
LIDT for pulsed lasers is specified as a fluence with units of J/cm2 as
opposed to power density. It is important to recognize that while J/cm2
does not contain a unit of time, the damage threshold is dependent on
pulse duration. In most cases, the LIDT fluence value will increase as the
pulse duration increases. To understand a pulsed LIDT specification, it is
necessary to know the laser’s wavelength, beam diameter, pulse energy,
pulse duration, repetition rate, and intensity profile (e.g., Gaussian or flat
top). The relationship between the fluence of a pulsed laser, the pulse
energy, and the beam diameter is defined by:
For example, a flat top Q-switched (pulsed) laser with a pulse energy of
10 mJ, pulse duration of 10 ns, and a beam diameter of 10 μm will have
the following fluence:
P (W)
Ppeak
τ 1/Repetition Rate
Pavg
Time (s)
Figure 15.3: The pulses of a pulsed laser are temporally separated by
the inverse of the repetition rate
1000
100
10
1
0.1
0.1 1 10
Pulse Energy (J)
Repetition
Rate (MHz)
1 W
Figure 15.4: The combination of pulse energy and repetition rate
determine the total power of a laser
15.1
15.2
15.3
15.4
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