for the sqfe use of lqsers - LIGO

APPENDIX
which has an output of three 20 ns pulses, each
separated by l0O ns.
This is not a repetitively pulsed laser in the usual
sense (that is, one having a continuous train of pulses
lasting on the order of 0.25 s or more with the pulses
being reasonably equally spaced). The inaabeam
visible MPE per pulse is given as the product of n-rla
and the MPE for a single pulse (from Table 5), or the
MPE per pulse is
MPE/Pulse : H < 1n-rla; lMpE; J' cm-2
= 3-l/a (5 x l0-?)
= 3.8x l0-7 J.cm-2
Example 7: Repetitively Pulsed Pulse Groups. Find
the MPE for an Argon laser (488 nm) used in a
pulse.code-modulated (pcm) communications link.
The laser presents l0a "words" per second (that is,
10" pulse groups per second) and each word consists
of five 20 ns pulses spaced at coded intervals such
that each pulse group lasts no longer than I ps.
The effective prf of the pulse train is equal to the product
of the number of words per second and the
number of pulses per word, or 50 kHz. Since this is
greater than 15 kHz, the cw or average power limitation
applies. Thus, the corresponding MPE from
Table 5 or Fig.4 for a nonmodulated laser and an
exposure duration of 0.25 s is:
or
MPE:11 = 1.8 l3l4x10-3 J - cm-z
I R,l/4 Y l n-l
MPE : E( ""' ^ '" W cm-2
I
=2.55x10-3W.cm-2
This value is compared with the average irradiance of
the laser which is obtained from the effective duty
factor of the pulse train and the peak power. The
duty factor is defined as the ratio of the pulse width
(r) to the period, which can also be expressed as
t x f. In this example the effective duty factor is
20nsx50kHz=0.001 and hence the average irradiance
is 0.001 times the peak inadiance.
83.2 Determining When to Use the Extended-
Source MPEs.
The intrabeam MPEs are used in all situations of
intrabeam viewing of thc direcl beam or specularly
62
rcflected beam, except for close viewing of laser
diodes or diode arrays. The intrabeam MPEs are also
used when viewing an extended source at a distance
greater than r I ll's .
83.2.1 Extended-Source MPEs Application.
The extended-source MPEs are applied only in the
spectral region of 0.4 to 1.4 pm where the source size
is significantly larger than a "point;'and wherc the
conesponding retinal image in the viewer's eye is
definitely not a "minimal spot". Diffuse reflections
arc extended sources at close viewing distances;
therefore, depending upon environmental considerations
and the laser, it may be necessary to consider
the extended-source MPEs. Class I and 2 lasers are
not capable of producing hazardous diffuse
reflections, and only the direct ocular inrabeamviewing
MPEs arE applied (exc€pt in the case of
intrabeam viewing of semiconductor diode lasers and
laser arrays), Class 4 lasers are always capable of
producing hazardous diffuse reflections at close viewing
distances. Class 3 lasers will not pmduce hazardous
diffuse reflections for exposure times