Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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"LASER RESONATOR"
This invention relates to an improved laser
and in particular it relates to a laser of the
general type which uses prism reflectors of the
total reflection type, such as Porro prisms.
Lasers employing Porro prism reflectors, for
example the crossed Porro laser, are finding favour
due to their insensitivlty to mechanical shocks.
Such lasers are the subject of patents, such as
Ferranti Australian Patent No. 466,196 and International
Laser Systems Inc. (ILS), USA Patent No. 3,924,201.
These patents describe systems having a total reflector
at each end of the laser cavity and take their output
from a polariser.
We have now found that a more advantageous
laser of the type using a prism-type total reflector
at each end is possible using a type of prism described
in Opto-Electronics, Volume 5, page 255 (1973)
under the title "Compound TIR prism for polarisation-
selective resonators".
Such a prism consists of four flat surfaces,
three of which are inclined at 90 to each other
and the fourth inclined at 45 to one of the roof
- edges formed by the other three sides. A ray entering
the surface will undergo four reflections, each
at 45 lncidence, before being reflected along
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its originaL path. Provided the refractive index
of the prism exceeds 2 each reflection will be
total and no energy will be lost. The four reflections
are so arranged that the parallel and perpendicularly
polarised components at one reflection are interchanged
at the next reflection. This causes the phase
shift that occurs at one total internal reflection
to be reversed at the next reflection and because
there are an even number of reflections the overall
phase shift due to total internal reflection is
zero. However, due to image reversal a phase shift
of 180 occurs in one component, hence the prism
is equivalent to a half wave plate. A plane polarised
beam, entering the prism with its plane of polarisation
inclined at àn angle ~ with the X axis, will emerge from
the prism with its plane of polarisation rotated by 2~.
A Porro prism operates differently from the
compound TIR prism in that only two total internal
reflections occur, and the phase shift occurring
at each reflection add rather than cancel. The total
resultant phase shift between S and P components from a
Porro prism is given by the following expression
phase shift = ~ -~ 4tan~l~[1 - 2/(n2)])
where n is the refractive index of the Porro prism.
The fact that the phase shift is not ~ or a multiple
thereof leads to some performance Limitations in
lasers using Porro prisms.
In the drawings forming part of this specification
FIG. 1 is a perspective view of a compound
TIR prism,
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FIG. 2 illustrates schematically an in-line
laser according to this invention, and
FIG. 3 illustrates similarly a folded design.
Referring first to FIGo 1 which illustrates
a TIR prism it will be seen that the surface A
which forms the entry and exit surface for the
rays and which is normal to the laser cavity axis
when in use and we thus term the "normal" surface
has the surfaces B and C extending rearwardly from
it at 90 while the surfaces B and C themselves
are at 90 the one to the other, the surface D
extending rearwardly from the surface A at a 45
angle with the surfaces B, C and D merging at the
roof line R.
i5 As referred to earlier herein, provided the
refractive index of the prism exceeds ~2, each
reflection will be total. The four reflections
are such that the S and P components of the beam
are interchanged as the beam traverses the prism
as shown in the illustration. This allows the
phase shifts that occur between S and P components
on total internal reflection to cancel, giving
no phase shift due 'LO this cause.
There is a second effect that produces a phase
shift and it is the image reversal that occurs
when the beam is reflected by the prism. It causes
an eff-ective phase shift of 180 in that component
of the radiation that is polarised parallel to
the Y - axis, hence the phase shift produced by
the prism is the same as that produced by a half
wave plate.
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A plain polarised beam, entering the prism
with its plane of polarisation inclined at an angle
o is the X dXi.S shown in ~IG. 1, will emerge from the
prism with its plane of polarisation rotated by 2a.
Referring now to FIGS. 2 and 3, in which similar
reference characters are used for corresponding
parts, a pair of compound TIR prisms 2 and 3 are
used which are placed one at each end of the resonant
cavity, and as in the case of the crossed Porro
laser either an in-line configuration as shown
` in FIG. 2 or a folded configuration as shown in
FIG. 3 can be used. The compound TIR prism 2 at
the Pockels cell end of the laser is orientated
at 45 so that the reflected beam receives a 90
rotation in its plane of polarisation. The Q-switch
is designated 4 and its voltage generator 5. This
rotation leads to perfect hold-off in the Q-spoiled
state without the need for bias to be applied to
the Pockels cell Q-switch. This allows a fairly
simple driving circuit for the Pockels cell, especially
when compared to the Porro case in which bias is
required in order to achieve perfect hold-off.
Further, the elimination of the bias from the Pockels
cel] is likely to lead to more reliable operation
since possible electrical leakage problems will
be avoided.
The compound TIR prism at the other end is
disposed adjacent to the laser rod 6 and is used
to control the output coupling by rotation about
an a~is parallel to the laser beam. At an orientation
oE ~, the effective reflectivity is given by
R = cos (2~)
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which can be varied between 0 and 100%. This range
exceeds that obtainable with a Porro and leads
to some performance advantages for the compound
TIR prism e.g. in very high gain lasers where very
low reflectivities are desired or where the intracavity
power levels are required to be kept very low.
The laser rod 6 is powered in any normal manner
such as by a flash lamp 7 powered by a supply 8.
The output is at 9.
In each case the laser rod is designated
and the polariser 10 but in FIG. 3, because of
the folding, a mirror 11 is included in the cavity.
The 45 face of the compound TIR prisms 2
and 3 is shown dotted to readily differentiate
that reflective face.
From the foregoing it will be realised that
by replacing the Porro prisms of earlier systems
with TIR prisms, an improved laser results which
has low energy loss, firstly because the TIR prisms
have the four required reflections internally of
the prism and secondly because the perpendicularly
polarised components at one reflection are interchanged
at the next reflection. The system also lends itself
to very rugged mechanical construction because the
resonator is relatively unsensitive to alignment errors
provided that the retro-reflecting planes of the prism
are arranged to be substantially perpendicular to each
other.
Also, use of compound TIR prisms in polarisation
coupled lasers introduces further advantages over
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the use of Porro prisms. ~oremost are the performance
advantages of perEect hold-off without the need
to bias the Pockels cell and of extending the range
of output coupling available. Another advantage
arises due to the independence of performance on
refractive index, a feature that allows the choice
of prism material to be made on such grounds as
thermal stability, optical quality, low absorption
coefficient, high damage thresholds, etc, rather
than on refractive index as in the case of Porro
prisms. Improved reliability and efficiency will
result from this freedom in choosing prism properties.
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