Raman laser

This invention provides a Raman laser wherein a light beam from a laser source (1) passes through a first chamber (3) and experiences Raman scattering, and then passes through a second chamber (9) and experiences further Raman scattering, such that the output of the Raman laser is frequency shifted. The first and second chambers may be distinct from each other enabling different types and pressures of gases to be utilised, or may be one and the same (20) providing a particularly compact arrangement. A Raman laser in accordance with the invention is particularly suitable for the production of light consisting of multiple rotational Raman orders, and particular embodiments enable frequency switching to be achieved and also provides arrangements which reduce the problems of boresight stability and gas circulation encountered with previous designs.

BACKGROUND OF THE INVENTION 
This invention relates to a Raman laser and in particular but not 
exclusively a Raman laser for producing multiple rotational Raman orders. 
There are only certain materials or combinations of materials convenient 
for use in a laser, and consequently only a limited number of wavelength 
bands which can conveniently be obtained by lasing action and each of 
these is usually narrow. In certain applications it is desirable to use 
other frequencies, to be able to switch between different frequencies, or 
to broaden the frequency spectrum of the band. 
In 1928 Sir Chandrasekhara Vankata Raman observed an effect now known as 
Raman scattering. This 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 energy as the incident photon or alternatively a 
higher or lower energy. To have a higher or lower energy the energy value 
of the molecule must have changed. The molecule can obtain or release this 
energy in the form of vibrational or rotational energy. Because distinct 
vibrational and rotational energy levels exist scattered photons also have 
distinct energy values and therefore the incident beam of photons is in 
effect frequency shifted by "scattering". This effect is most commonly 
achieved using molecular gases such as H.sub.2, D.sub.2, CH.sub.4 or 
CO.sub.2, which are commonly referred to as being Raman active. 
Raman scattered light consists of vibrationally scattered and rotationally 
scattered components which are side bands of the incident laser frequency. 
The molecule has a larger separation between vibrational energy states 
than rotational energy states and thus "vibrational shifts" in frequency 
are greater than "rotational shifts" in frequency. One of the prime uses 
of the Raman effect has so far been in analytical chemistry whereby the 
change in frequency gives an indication of the energy level structure of a 
molecule. 
The most efficient conversion of light using the Raman effect requires that 
a laser beam incident on the molecular gas is above the threshold 
intensity for stimulated Raman scattering (SRS). When moderate powered 
lasers are employed Raman scattering can be enhanced by focusing the pump 
beam in the gas by means of lenses or mirrors. However the use of focusing 
optics makes it difficult to maintain boresight stability as movement of 
these optics will deflect the beam. 
SUMMARY OF THE INVENTION 
According to a first aspect of the present invention there is provided a 
Raman laser comprising a first and a second chamber each containing a 
Raman active gas, wherein the first chamber is arranged to receive an 
incident coherent pump beam which experiences forward Raman scattering on 
passing through the first chamber, and wherein the second chamber is 
arranged to receive an output beam from the first chamber which 
experiences further forward Raman scattering on passing through the second 
chamber the Raman laser further comprising a steering optic by which the 
pump beam entering the first chamber, and the Raman shifted beam exiting 
the second chamber are deflected. 
By forward Raman scattering it is meant that the beam passes through the 
chamber once on each pass as opposed to experiencing recursive Raman 
scattering. The Raman laser beam is generated by SRS in the same direction 
as the pump laser beam. Since scattering is in the forward direction the 
Raman gain is insensitive to the pump laser linewidth suiting the Raman 
device to a wide range of lasers. 
By employing a Raman laser in accordance with the first aspect of the 
present invention the incident input beam and output beam are deflected by 
the same optical element, the steering optic. This can be suitably 
arranged such that the Raman laser is insensitive to small deviations in 
the incident beam direction and also so that the exact orientation of the 
laser is immaterial to the incident and output beams remaining parallel. 
Preferably the steering optic has two reflective faces arranged orthogonal 
to each other the first reflecting the incedent pump beam to the first 
chamber and the second receiving the output beam from the second chamber 
and reflecting it in the same direction as the incident beam. 
Advantageously the steering optic is arranged to be oscillated in use such 
as to cause the beams passing through the chambers to move within the 
chambers, and the output beam deflected by the steering optic to remain 
stationary. This provides a means of oscillating the beam within the 
chambers such as to prevent hot spots developing in the gas contained 
within the chambers, within areas of high intensity light. Because heat 
changes the density and refractive incidence of the gas, the gas defocuses 
the light passing through the Raman laser if hot spots occur. Therefore by 
employing such an oscillated steering optic the necessity to provide a 
device for circulating the gas within the chamber, used with previous 
designs, is eliminated as are the problems associated with providing a 
device for circulating the gas within a sealed chamber. 
In some applications it is advantageous if the steering optic can be 
displaced into and out of the incident beam such that an output beam from 
the Raman laser can be switched from the pump beam frequency to the Raman 
shifted frequency. This provides a way of easily switching the output 
frequency between that of the pump beam and that of the Raman shifted 
frequency. 
In accordance with a second aspect of the invention there is provided a 
Raman laser comprising a first and a second chamber each containing a 
Raman active gas, the first chamber being arranged to receive a coherent 
pump beam which experiences forward Raman scattering on passing through 
the first chamber, and the second chamber being arranged to receive an 
output beam from the first chamber which experiences further forward Raman 
scattering on passing through the second chamber, the Raman laser further 
comprising a filter between the first and second chambers which is 
transmissive to rotationally scattered Raman radiation and the pump beam, 
and is non-transmissive to vibrationally scattered Raman radiation. 
Employing this aspect of the invention encourages a greater proportion of 
rotational scattering in the second chamber as the rotationally scattered 
light from the first chamber is of the correct nature to stimulate 
rotational scattering in the second chamber. Preventing the vibrational 
scattered component from entering the second chamber reduces or eliminates 
"seeded" generation of vibrational scattering in the second chamber and 
therefore encourages multiple rotational orders. 
In accordance with a third aspect of the invention there is provided a 
Raman laser comprising a first and a second chamber each containing a 
Raman active gas, the first chamber being arranged to receive a coherent 
pump beam which experiences forward Raman scattering on passing through 
the first chamber, and the second chamber being arranged to receive an 
output from the first chamber which experiences further forward Raman 
scattering on passing through the second chamber, the Raman laser further 
comprising a device for circularly polarising the pump beam incident on 
the first chamber, and altering the polarisation of the beam emerging from 
the first chamber such that it enters the second chamber elliptically 
polarised. By employing the third aspect of the invention rotational 
scattering is encouraged because circularly polarised light favours 
rotational scattering to vibrational scattering. Thus the rotational 
output from the first chamber is enhanced. The rotationally scattered 
light from this first chamber is of the correct nature to stimulate 
rotational Raman scattering in the second chamber and the conversion of 
the beam to the elliptically polarised beam provides a linearly polarised 
component which takes part in a four wave mixing process (FWM), described 
below with reference to the accompanying drawings. This process is only 
possible when a linearly polarised component is present. This process 
produces multiple Stoke and anti-Stokes Raman orders providing a broader 
frequency spectrum. 
The change in polarisation between the two chambers is preferably achieved 
by a double reflection which takes places in a corner cube arrangement, 
this corner cube arrangement also serving to reflect the beam back through 
the second chamber parallel to the first. 
It is advantageous if any two, or all three of the aspects of the 
invention, or combination thereof, described above are combined together 
in a single Raman laser. 
It is advantageous if in accordance with any aspect of the invention the 
first and second chambers are parallel to each other and further 
comprising a reflector arranged to receive radiation emerging from the 
first chamber and reflect it through the second chamber parallel to the 
radiation in the first chamber, and preferably this reflector arrangement 
comprises a corner cube. 
In accordance with any aspect of the invention it can be advantageous if 
the two chambers are distinct from one another, enabling different gases 
or different gas pressures to be used in each chamber. Alternatively it 
can be advantageous if the two chambers are one and the same chamber 
wherein the beam emerging from the chamber is deflected back through the 
chamber in the opposite direction. This arrangement eliminates the need to 
have two chambers separately controlled.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1 a circularly or near circularly polarised beam from a 
laser source 1 is incident on a focusing optic 2 which focuses the 
radiation within a first chamber 3 of the Raman laser containing a volume 
of Raman active gas 4. Light leaving the first chamber 3 passes through a 
filter 5 and a second lens 6, the lenses 2 and 6 being afocal making the 
net optical power zero. The light then passes through the quarter 
wave-plate 7, a first lens 8, a second chamber 9 filled with a second 
volume of gas 10, before leaving through the second lens 11. Again lenses 
7 and 11 associated with the second chamber 9 are afocal. (It is possible 
that lenses 6 and 8 could be combined in one single more powerful lens). 
A substantially circularly polarised beam from the source 1 is focused by 
the lens 2 in the Raman active gas 4 contained in the chamber 3 such that 
the intensity of radiation is sufficient to overcome the required 
threshold value for stimulated rotational Raman scattering to occur. 
Because the incident beam is substantially circularly polarised, and the 
pressure of the gas 4 is maintained at a relatively low pressure, 
considerable rotational Raman scattering occurs. 
Light leaving the chamber 3 passes through the filter 5 which only permits 
those frequencies to pass which have been generated by rotational Raman 
scattering. The lens 6 refocuses the radiation before it passes through a 
quarter wave-plate 7. This causes the beam to become more elliptically 
polarised and the beam is then focused by lens 8 in the gas 10 contained 
within chamber 9. In chamber 9 the linearly polarised component of the 
elliptically polarised beam takes part in a FWM process where multiple 
Stokes and anti-Stokes rotational transitions are made generating multiple 
rotational Raman orders. This process is possible when the beam has both 
components of incident wavelength and wavelengths generated by rotational 
Raman scattering. Because the filter 5 has removed frequencies 
corresponding to vibrational Raman scatting the gain due to rotational 
Raman scattering in the chamber 9 exceeds the gain due to vibrational 
scattering, and the output consists primarily of multiple rotational Raman 
orders. 
FIG. 2 illustrates a modification of the apparatus illustrated in FIG. 1. 
In this embodiment light from the source 1 is deflected by planar surface 
15 of steering optic 13 into the first chamber 3. In this arrangement the 
chamber 3 is sealed by the lenses 2 and 6, but otherwise the chamber 
performs the same function as in the FIG. 1 arrangement. In the FIG. 2 
arrangement filtering is achieved by a coating applied to lenses 6, 8 and 
corner cube 14 and the incident, substantially circularly polarised light 
beam is converted to elliptically polarised light during the two total 
internal reflections it experiences on passing through the corner cube 14. 
The light then passes through chamber 9 which performs the same function 
as in the FIG. 1 arrangement. The beam is then deflected off a second 
planar surface 12 of the steering optic 13 in a direction co-linear with 
the original incident beam. In employing this arrangement the alignment of 
the chambers 3 and 9 and the corner cube 14 are not critical for even if 
these deviate slightly from the position shown the output beam will still 
be co-linear with the input beam, provided chambers 3 and 9 are rigidly 
fixed together. Furthermore if it is wished to switch the output beam 16 
between having a modified frequency and a frequency of the incident beam, 
then this is achieved by displacing the steering optic in the direction of 
the arrow 17 by any suitable conventional type of actuator (not shown). 
The steering optic 13 when in the position shown, is oscillated further by 
actuator 24 such that the focal point of the beam formed by lenses 2 and 8 
within the chambers 3 and 9 is moved between laser shots so preventing the 
generation of hot spots within the gas. 
FIG. 3 depicts a particularly advantageous arrangement wherein the 
functions of the first and second chamber of FIGS. 1 and 2 are performed 
by the single chamber 20. Only two lens elements 21 and 22 are required 
and one filter element 23. Again the steering optic 13 can be moved in the 
direction of arrow 17 to perform the switching function and in use is 
oscillated by actuator 24 in the same manner as described with reference 
to FIG. 2 above. 
Although the FIGS. 2 and 3 arrangements have been described with reference 
to the production of multiple Raman orders, it will be appreciated that by 
removing the filtering components a conventional Ramar laser function may 
be performed with the advantages that an arrangement is provided in which 
the alignment of the chamber or chambers is not critical, the beam can be 
switched, and defocusing of the beam can be avoided while oscillating the 
input beam to avoid the generation of hot spots within the chamber or 
chambers.