A multi-shot, or S-on-1, test differs from a single-shot test in that it uses
a series of laser shots, or pulses, per testing site as opposed to a single
shot. The common number of shots per site, or S, is between 10 and
1000. Multi-shot tests provide a better prediction of the real-world performance
of the optic, and allow LIDT testers to avoid a phenomenon
called the infant mortality realm,12 When using between 1 and 10 shots
per site, test results are non-deterministic and have high levels of statistical
variation; this causes the range of shots per site to be known as
the infant mortality realm. When S is greater than 10, the test results are
more deterministic and predictable. Therefore, when around 100 shots
per site are used, enough information can be gathered to predict the
long-term performance of the optic. However, using more shots per site
requires longer and more expensive LIDT testing.
Section 15.5:
Damage Detection Methods
Testing results may significantly differ depending on the detection
method used to evaluate damage and there is currently no industry
consensus on what method to use. While microscopy is the most common
detection method used to identify damage, there are several other
detection methods including scattered light diagnostics, plasma spark
monitoring, and topography analysis.
Differential Interference Contrast Microscopy
Nomarski-type differential interference contrast (DIC) microscopy is the
most common method used for laser damage detection following ISO
21254. DIC microscopy enhances image contrast in transparent samples
by utilizing interferometry, allowing for viewing defects that would otherwise
be difficult to identify,13 Once images of optic are taken from before
and after testing, damage can be identified using human judgement
or image processing techniques. Test results may vary widely when using
human judgment due to subjective identification of damage by the
operator, whereas image processing algorithms detect damage with no
human error. However, even with image processing, false positives can
result from vignetting, non-uniform illumination, or misalignment. In addition
to confirming the presence of damage, DIC microscopy can also
determine the dimensions of the defects.
Scattered Light Diagnostics
Another common detection method defined in ISO 21254 is scattered
light diagnostics. This method uses light scattered from the target site to
determine the existence and characteristics of laser-induced damage,13
In scattered light diagnostics, a probe beam (often a HeNe laser) illuminates
the target site and any scattered signal difference significantly
greater than the background noise implies the existence of damage on
the optic (Figure 15.8). The probe beam itself is blocked before reaching
the detector so that only scattered light is detected from damage sites.
In a standard setup used for scattered light diagnostics, the larger the
solid angle of the detector, the more sensitive the measurement (Figure
15.9). One disadvantage of this method is that it is heavily dependent on
the amount of background noise. This dependency can be overcome by
taking multiple measurements and averaging the results, increasing the
gain of the detector, or filtering out the background noise.
Plasma Spark Monitoring
Plasma spark monitoring is another method used to detect laser damage.
Laser-induced damage often results in plasma generation on the
optical surface from non-resonant optical breakdown (called a plasma
spark), which causes plasma scalds to form around the damage site.
Identifying plasma sparks or scalds is a clear indication of damage to
an optic,13 Plasma scalds have a relatively even surface area - this makes
them difficult to detect through microscopy or scattered light diagnostics.
However, the plasma spark itself can be detected during LIDT testing
by using a collecting lens to focus light from any plasma sparks onto
a detector (Figure 15.10). To detect damage, scattered light from the test
laser is filtered out and the response time of the detector must be shorter
than the duration of the plasma spark, which typically reaches its maximum
value in about 100 ns.
48 +44 (0) 1904 788600 | Edmund Optics®
Scatter signal (V)
0.17
0.16
0.15
0.14
Damage onset
0 20 40 60 80 Shot
number (N)
Figure 15.8: Drastic change in scatter signal after laser-induced damage
onset
Photo detector Beam block Probe laser
Sample
Lens
Figure 15.9: Schematic of a typical scattered light diagnostics setup
for LIDT testing
Detector
Filter
Sample
Test Laser
Lens
Figure 15.10: Schematic of a typical plasma spark monitoring setup
for LIDT testing