Note: Descriptions are shown in the official language in which they were submitted.
Raman Laser
Backaround Of The Invention
This~ invention xelates to a Raman Laser and in
particular, but not exclusively to a Raman Laser which
provides "eyesafe" radiation.
Many applications of lasers, such as rangefinding
surveying, communication, terrain following, wire-avoidance
etc. require eyesafe laser sources before they can be
freely employed. One of the most successful class of solid
state lasers utilises the neodymium ion as the lasing
species. Although efficient, this laser normally has a
wavelength around l~m, an eye hazard at useful operating
powers and repetition rates. However this can be frequency
shifted to an eyesafe region of the spectrum around 1.5~m by
the Raman Effect, first observed by Sir Chandrasekhara
Vankata Raman in 1928.
The Raman Effect occurs when energy in the form of
photons incident on a molecular structure raises the energy
state of a molecule to an intermediate, or virtual state,
from which it makes a Stokes transition emitting a photon of
energy, termed a scattered photon. The scattered photon may
.: . :
have the same ener~y as the incident photon or alternatively
a higher or lower energy, (frequency), having been "~aman
shifted". For example, Raman shifting the Nd:YAG laser in
methane gas generates a laser beam at 1.54~m. Likewise
shifting in deuterium gas generates a laser beam at 1.56um.
A Raman laser, employing the Raman effect, can be
created by passing a laser beam, known as the pump beam,
through a cell containing a Raman active medium. At lower
powers the pump beam is normally focused to increase the
power density within the Raman active medium, thereby
enhancing the interaction, which is nonlinear, and
increasing the conversion efficiency. Many other geometries
are also possible, including collimated and waveguide
configurations.
The Raman conversion process is typically around 40%
efficient. Raman scattering is an inelastic process, i.e.
energy is deposited in the Raman medium at the end of the
interaction. Some 30% of the pump energy is deposited in
the gas during vibrational Raman scattering in methane. The
residual pump light which is unconverted exits the cell and
presents a remaining eye hazard.
Guaranteeing that a Raman shifted laser is eyesafe is
difficult. The design must be such that any hazardous light
is eliminated or is kept below an acceptable power level at
the exit of the system. This must be the case even if the
Raman laser should fail ~or any reason, allowing the pump
beam to pass through unmodified, or if the optics employed
are imperfectly manufactured or get damaged.
In order to better understand the problems associated
with present Raman lasers, these shall be discussed with
reference to Figure 1, which depicts a common Raman laser
arrangement using longitudinal pumping. In this arrangement
the laser source 1, which may be a Nd:YAG laser, produces a
pump beam 2. This passes through mirror 3, (which is
transmissive to the pump beam wavelength but reflective to
the Raman shifted wavelength), to cell 4, containing a Raman
medium 5, where part of the pump beam is frequency shifted.
The Raman and residual pump beams 6 then exit the cell 4 and
pass through a Raman output coupler 7, (which is a partial
reflector at the wavelength of the Raman beam, and totally
transmissive to the residual pump beam). This coupler is
spaced relative to mirror 3 such that it forms a resonator
and causes stimulated emission within the cell 4 of Raman
photons. Lens element 8 is provided in order to focus the
radiation within the cell to increase the conversion
efficiency as mentioned above. The Raman and residual pump
beams exit at 10 have been refocused by lens 9 and are then
separated and filtered by dielectric coated beam splitters
(not shown), and/or bulk absorbers (e.g. silicon). Such
methods are less than perfect because dielectric
mirrors/splitters can be imperfectly made, for example they
may contain pinhole defects, and both splitters and bulk
absorbers suffer stress at high incident beam intensities
and may fail. Also a method known as Four Wave Mixing can
generate other hazardous wavelengths in the forward
direction when, as in this simple geometry, the pump and
Raman beams are co-propogating. The splitters and bulk
absorbers must also cope with these.
It is important to minimise intensities wherever
possible in the simple longitudinally pumped Raman laser
design as described above, because if the Raman laser fails
for any reason (e.g. loss of gas, misalignment of Raman
resonator, damage to Raman resonator optics etc.) then the
pump beam is unconverted. It will be appreciated from
Figure 1, that if this happens the hazardous radiation
increases some threefold at the outlet of the system,
increasing stress on the output protective optics and/or
bulk absorber, and increasing the risk of unwanted radiation
escaping.
The object of the present invention is to provide a
Raman laser which overcomes the above mentioned problems.
Summary Of The Invention
According to the present invention there is provided
an axially pumped Raman laser comprising:
~ a pump laser source for providing a pump beam along an
optical path; a first optical element in the optical path
transmissive to the pump beam and partially reflective to
Raman shifted radiation of a desired wavelength;
a second optical element in the optical path
transmissive to pump beam and reflective to the Raman
shifted radiation.of a desired wavelength; -
a beam dump for receiving the pump beam passing
through the second optical element;
and a cell for containing a Raman active gas, wherein
said two optical elements form a resonator within the Raman
active gas and wherein the Raman shifted radiation is output
from the resonator through said first optical element.
.
,
An arrangement in accordance with the invention
provides a Raman beam which is output in a different
direction to the residual pump beam. This avoids the
problems associated with filtering the output to obtain the
Raman beam alone as is necessary with previous arrangements.
This also reduces the problems associated with failure of
components in such prior art arrangements.
In the above arrangement Raman conversion takes place
and the hazardous residual pump beam, which is unwanted,
exits the cell and is absorbed in the beam dump. The Raman
beam exits the cell in the reverse direction relative to the
pump beam, and can be directed to the output of the system.
In this arrangement the "eyesafe" Raman radiation is
generated in the opposite direction to the incident
hazardous pump beam. Should the Raman laser fail for any
reason, such as those detailed above, then the increased
hazardous radiation is absorbed in the beam dump (e.g.
ceramic or KG3 glass). This dump can be capable of handling
very great stress levels. Furthermore the stress levels on
the output optics are actually reduced during a failure
mode.
Preferably the Raman laser further comprises a lens
element for focusing the pump beam in the cell. This
increases the power density within the cell enhancing the
nonlinear interaction and thereby increasing the conversion
efficiency. It is also advantageous if a beam splitter is
employed arranged to direct the Raman beam emerging from the
cell off the optical path extending from the pump source to
the cell. This prevents the Raman beam being directed back
to the laser source.
A problem in Raman shifting in methane is that the
Raman scattering competes with a process known as Stimulated
Brillouin Scattering (SBS). The SBS interaction can
backscatter some of the hazardous pump beam in the same
direction as the output beam~ The Raman laser design can be
chosen to eliminate this competing process almost entirely
but some low levels of hazardous radiation may still remain.
Also, the Raman mirrors and input optics will backscatter
some small, but worrying, amounts of hazardous radiation.
.
In order to overcome the problems mentioned
immediately above it is advantageous if the pump beam is
circularly polarized prior to being incident on the cell,
for then any backscattered pump light, either due to SBS or
Fresnel reflection from Raman optics (or possibly damaged
optics), will return with the opposite rotation of circular
polarisation, and this can be removed by including a beam
~ ~ .
splitter and quarter wave plate in the path of the Raman
beam. The wave plate can be designed to act as a quarter
wave plate for the pump beam but to be neutral to the Raman
beam, this is aided by the near half integral relatianship
of the pump to Raman wavelengths of 1.06,um to 1.54~m
encountered in many applications.
Any hazardous light fre~uencies being generated by
Four Wave Mixing will be generated primarily in the
backwards direction and will be absorbed in the beam dump.
Another important benefit occurs from the use of
circularly polarized pump laser light. Packaging of Raman
cells into short lengths is generally desired but is often
restricted because power densities on optics increase as the
cell is made shorter in a tight focused design. If linearly
polarized pump light were used then a standing wave could be
established in the Raman resonator. The electric fields of
the forward and backward travelling Raman beams within the
Raman resonator would interfere constructively with each
other, the two field strengths would add at certain points,
and the intensity would be four times that of a single beam.
This can be particularly problematic within the dielectric
coatings of the.Raman resonator mirrors, leading to high
stress and possible failure. However, if the pump beam is
, . . . .
- 9
made circularly polarized then the Raman beam is also
generated circularly polarized. No standing wave is
established within the Raman resonator and the intensities
are lower by a factor of two.
Brief Descri~tion Of The Drawinqs
Figure 1 depicts a prior art Raman laser arrangement
described above.
., .
Figure 2 is a schematic illustration of a Raman laser
~in accordance with one embodiment of thé present invention
for providing eyesafe indication. This is given by way of
example only and is described below.
Detailed Descri~tion Of The Drawinqs
The arrangement of Figure 2 comprises a Nd:YAG laser
20 which provides a pump beam 21 of wavelength l,um. This
pump beam is polarized by polarizer 22 and is directed by
beam splitter 23 through a quarter wave plate 24 such that
it is circularly polarized. The beam is then focused by the
lens 25 and passes through a mirror 26, (which is nominally
100% transmissive to the pump wavelength), through a cell 27
containing methane gas 28, through~a mirror 29, ~which
-- 10 --
again, is totally transmissive to the pump beam), to an
absorbing beam dump 30.
The pump beam on passing through the cell 27 causes
excitation of the methane molecules to an intermediate
state, from where they undergo a Stokes transition which
causes then to emit photons of a different wavelength,
producing the Raman beam. The mirror 29 is totally
reflective to the Raman beam generated, and the mirror 26 is
partially` reflective to the Raman beam generated causing
lasing action within the cavity. The Raman beam exits
through the mirror 26. This beam is refocused by the lens
25 and passes unaffected through plate 24, (which is not a
quarter wave plate to the Raman beam as the wavelength of
this beam is considerably different from that of the pump
beam). ~he Raman beam then passes to an output 31 via beam
splitter 23, which is dielectrically coated to pass Raman
shifted light but to reflect hazardous light, and then
through a silicon bulk absorber 33 which removes any small
remaining intensities of hazardous radiation.
In the cell 27 there also occurs Stimulated Brillouin
Scattering (SBS) which backscatters some of the hazardous
pump beam in the same direction as the output beam. Also
some components of the pump beam are reflected by the
optics. However, because this backscattered radiation has
undergone a reflection, it is circularly polarized in the
opposite sense to the pump beam, and when passing through
the quarter wave plate becomes plane polarized and is
prevented from passing to the output 31 by beam splitter 23
and is further prevented from reaching the pump laser by
polariser 22 permitting it to be absorbed by beam dump 32.
'
~ `
., ~
