High wavelength laser device

A laser having a glass enclosure placed in an optical cavity receives a pump beam and generates at least one first-order and one second-order Stokes wave. The construction is such that an optical waveguide placed in the gas enclosure is used to select the first and the second-order Stokes wave.

BACKGROUND OF THE INVENTION 
The invention concerns a high wavelength laser device and more particularly 
a powerful laser tuneable at high wavelengths. 
Tuneable lasers are known in the industry such as, for example, a laser 
based on a sapphire crystal doped with titanium which emits in a domain 
covering a window between 0.6 and 1.1 microns. 
Devices are also known which include a Raman cell excited by a pump laser. 
The description of such a device can be found in French patent application 
no. 89 09303, filed on Jul. 11, 1989. Such a Raman cell is constituted of 
an enclosure containing a pressurized gas which can be hydrogen, deuterium 
or methane. This gas cell excited by a pump laser, for example a laser 
based on a sapphire crystal doped with titanium, enables a wave (Stokes 
wave) to be generated by non-linear interaction, whose wavelength can be 
tuned within a window between approximately 0.7 and 2 microns. 
However certain applications may require a laser emitting at high 
wavelengths, such as the visible wavelengths, and tuneable in large 
wavelength ranges. 
SUMMARY OF THE INVENTION 
The invention therefore concerns a high wavelength laser, characterized by 
the fact that it comprises: 
a pump laser source emitting a pump beam (Fp) of a given wavelength; 
a pressurized gas enclosure in which is transmitted the pump beam (Fp), the 
gas contained in the enclosure being excited by the pump beam (Fp) and 
giving rise by the Raman effect to at least a first-order Stokes wave and 
a second-order Stokes wave; 
an optical cavity, comprising a first input reflection device and a second 
output reflection device, in which is placed the gas enclosure, the second 
reflection device also serving as an output mirror transparent, over at 
least part of its surface, to wavelengths in excess of the first-order 
Stokes wavelength. 
The invention also concerns a high wavelength laser characterized by the 
fact that it comprises: 
a pump laser source emitting a pump beam (Fp) of given length; 
a pressurized gas enclosure in which is transmitted the pump beam (Fp), the 
gas contained in the enclosure being excited by the pump beam and giving 
rise by the Raman effect to the creation of at least one first-order 
Stokes wave and one second-order Stokes wave; 
an optical wave guide situated in the gas enclosure in the path of the pump 
beam, this optical wave guide being made in such a way as to select at 
output a Stokes wave of at least second order.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention uses a powerful primary wave acting as a pump wave and a tank 
filled with a gas under high pressure in which the pump wave is made to 
act to create a multi-Stokes Raman process. 
FIG. 1 therefore shows a tuneable laser 1 emitting a pump beam Fp towards 
an optical cavity 3, 4 in which is located a pressurized enclosure 2 
filled with a gas. The mirrors 3 and 4 of the optical cavity are such that 
the mirror 3 allows the beam Fp to penetrate into the cavity but reflects 
all wavelengths towards the interior of the cavity. The mirror 4 reflects 
the first-order Stokes wave but lets the light beams of wavelengths 
exceeding that of the first order Stokes wave pass through. 
As an example, the pump laser is based on a sapphire crystal doped with 
titanium and the Raman cell is constituted of an enclosure in which is 
compressed a gas which can be hydrogen, deuterium or methane. 
The sapphire laser doped with titanium is known to cover the spectral 
domain from 0.65 .mu.m to 1 .mu.m in continuous regime and from 0.6 .mu.m 
to 1.1 .mu.m in pulsed regime. In the latter case, a source is obtained 
which is perfectly suitable for the realization of non-linear 
interactions, among which can be mentioned the stimulated Raman effect. 
The combination of a sapphire laser doped with titanium and a gas cell 
(such as hydrogen, deuterium or methane) enables a wave covering the 
window 1-9 .mu.m to be generated by non-linear interaction. Knowing 
moreover that the tuneability domain of the pump laser covers the 0.6- 1.1 
.mu.m window, a source is thus made which is tuneable from an extreme 
component of the visible to the domain of the middle infrared. 
The non-linear effect is observed in solids, liquids and gases. In 
particular, the greatest Raman shift has been observed in pressurized 
hydrogen. It should be remembered that the Stokes wave generated is 
related to the pump wavelength and to .DELTA..GAMMA. (Raman frequency 
shift) by the relationship: 
EQU .lambda.s=(.lambda.p.sup.-1 -.DELTA..GAMMA.).sup.-1 
We see therefore that any modification in the pump wavelength is 
accompanied by a Stokes emission at a particular wavelength. Thus, in the 
spectral domain covered by the pump wave, it is possible to generate 
wavelengths from 0.6 to 13 .mu.m. 
In the table below are cited the first two orders of Stokes wavelengths 
which can be obtained from the tuneability domain characterizing the 
emission from a pulsed sapphire laser doped with titanium. The gain 
coefficient is also determined as a function of the pressure and the 
nature of the gas filling the Raman cell. The Raman gain coefficient is 
obtained from the relationship: 
##EQU1## 
In this relationship, c is the speed of light in a vacuum, 
his Planck's constant/2, 
.DELTA.N is the density of molecules characterizing the population 
inversion, between the initial and final states, participating in the 
Raman process of inelastic diffusion, 
.omega..sub.S represents the angular frequency of the Stokes wave, 
.omega..sub.R is the width half-way up the profile of the ray defining the 
Raman gain, 
and .differential..GAMMA./.differential..OMEGA. characterizes the 
differential efficient section of the Raman diffusion. 
The gain at the Stokes frequency .nu..sub.s =c/.lambda..sub.s is then 
obtained from the classic relationship: 
EQU G=g.sub.R I.sub.p L 
where Ip is the intensity of the pump wave (I.sub.p =P.sub.p /A.sub.eff), L 
the length of interaction and A.sub.eff is an effective surface taking 
into account the overlap integrals between spatial modes at angular 
frequencies .omega..sub.s and .omega..sub.p. 
In the assembly considered, we use the Stokes wave generated by the primary 
pump wave, which after a certain course in the non-linear medium behaves 
like a new pump wave triggering a new mechanism of power transfer from 
this Stokes wave (.lambda..sub.s1) towards another Stokes component 
located at the wavelength: 
EQU .lambda.s2=(.lambda.s1-.DELTA..GAMMA.).sup.-1 
In the table below, we have therefore calculated the position of the first 
and second Stokes rays as a function of the nature of the Raman medium and 
the primary pump wavelength (.lambda.p). 
______________________________________ 
Lambda 
Pump 1st Stokes 
Gain 2nd Stokes 
Gain 
(.mu.m) 
(.mu.m) (cm/GW) (.mu.m) (cm/GW) 
______________________________________ 
Hydrogen (100 amagats) 
0.6 0.7992 2.416 1.1966 1.613 
1.1 2.026 0.953 12.806 0.15 
Deuterium (100 amagats) 
0.6 0.731 0.471 0.935 0.368 
1.1 1.6383 0.21 3.208 0.1073 
Methane (60 amagats) 
0.6 0.7273 0.966 0.923 0.7613 
1.1 1.619 0.4338 3.07 0.2289 
______________________________________ 
It should be noted that the use of hydrogen, with the extreme spectral 
components emitted by the sapphire pump laser doped with titanium, enables 
Stokes waves between 1.1966 and 12.8 .mu.m to be generated. 
One of the advantages of the pump laser lies in the fact that this type of 
tuneable laser can be pumped optically via a YAG:Nd laser doubled in 
frequency and itself pumped by diode lasers. The basic diagram of the 
source is therefore represented in FIG. 2 by the association of the 
different parts: 
tuneable source pump laser 
tuneable sapphire:titanium laser 
Raman cell 
The primary pump laser can operate in tuneable regime. The pump laser is a 
YAG laser doped with Nd3+ pumped by diode lasers and doubled in frequency 
emitting a consistent wave at 0.53 .mu.m. In this case, the wave emitted 
by the sapphire laser doped with titanium 1 is in pulsed form and can be 
spectrally tuned using a network-type dispersive assembly constituting one 
of the cavity mirrors or a system of double refracting filters inserted in 
the cavity. 
FIG. 2 represents a detailed example of embodiment of the device of the 
invention. This device includes a diode laser 10 emitting a beam of 
wavelength 0.808 .mu.m pumping a YAG11 crystal. The laser emission is then 
doubled in frequency 11 and a beam of wavelength 0.532 .mu.m is obtained, 
enabling the sapphire laser 1 doped with titanium to be pumped and to emit 
a pump beam towards the Raman cavity 2. The Raman cavity is a pressurized 
gas enclosure whose faces 3 and 4 constitute the optical cavity. These 
faces 3 and 4 have reflecting properties such as those of the mirrors 3 
and 4 in FIG. 1. 
In FIG. 3 we have marked the position of the Stokes wavelengths emitted by 
a hydrogen Raman cell as a function of the pump wavelength emitted by the 
sapphire laser doped with titanium. At the right-hand side of the figure 
is obtained, from the pump wavelength emitted by a sapphire laser doped 
with titanium, the first-order Stokes wavelength emitted by the Raman 
cell. At the left of the figure is obtained, from the first-order Stokes 
wave, the emitted second-order Stokes wavelength. It can for example be 
observed that the use of a pump source centered at .lambda.p=1 .mu.m 
generates a Stokes ray at 1.71 .mu.m in hydrogen. This emission can itself 
give rise to the creation of a second Stokes ray at 5.197 .mu.m. 
FIG. 4 represents an example of embodiment of the optical cavity 3, 4 and 
of the Raman cell 2. 
The Raman cell placed before the primary tuneable source (sapphire laser 
doped with titanium) is constituted of a hermetically closed enclosure 
under high pressure. This cell is constituted of a cavity with two mirrors 
3, 4. The first mirror 3 is highly reflective for the spectral domain 
between wavelengths of 0.6 .mu.m and 13 .mu.m. The second mirror 4 
presents the same characteristics with an output opening 40. The pump wave 
Fp is injected via an opening 30 in the mirror 3. 
The openings 30 and 40 are not both situated in the direction of the pump 
beam exciting the gases in the enclosure 2. The pump wave thus undergoes 
multiple reflections between the two mirrors 3, 4 until total depletion 
occurs. The use of metallized mirrors enables a multipassage achromatic 
system to be made. In this case, the injected pump power level must simply 
be modified to cover the spectral domain proper to the emission covered by 
the first or second Stokes order. FIGS. 5 to 8 represent another example 
of embodiment of the Raman cell. The mirror 3 has the same properties as 
before. Only the output mirror 4 has sectors (sector 1, sector 2) of 
different spectral reflectivities. 
FIGS. 7 and 8 represent, as an example, diagrams of reflectivities of the 
sectors 1 and 2 of the mirror 4. 
Thus, by a rotation of the output mirror 4, it is possible to determine the 
Raman component and the wavelength which can be emitted. Each radial 
sector enables the reflectivity to be adapted to a given spectral domain. 
The successive reflections are established in a plane which is retained. 
The wave emitted is collinear with the pump wave and a control of the 
parameters of the cavity can be obtained by moving the mirror 4 with 
respect to the mirror 3 using an electrically-controlled mobile assembly. 
The use of a true confocal cavity can also be considered. 
FIG. 9 represents another mode of embodiment of the invention in which a 
hollow dielectric wave guide 6 is placed in the Raman cell. In this case 
the coupling between the TEM.sub.00 mode of the primary pump wave and the 
EH.sub.11 mode of the wave guide structure is optimized. 
This hollow guide 6 is constituted either of a capillary of glass 
containing fluorine or a chalcogenide material, depending on the gas and 
the spectral domain covered, or of a metal guide. The guide is delocalized 
with respect to the injection window 7 constituted of a film of silica 
treated against reflection between 0.6 and 1.1 .mu.m. If spectral 
components are required which are at great distances in the middle 
infrared, the output window 8 can be constituted of a film of barium 
fluoride (BaF.sub.2). The input windows 7 and 8 do not then constitute an 
optical cavity in this mode of embodiment. 
The length of the hollow guide 6 is optimized in order to obtain at output 
a Stokes wave of wavelength higher than the first order. This optimization 
leads to a length for the hollow guide 6 of the order of a meter. 
FIG. 10 represents a variant embodiment of FIG. 9 in which the hollow guide 
is made in the form of two hollow guides 6' and 6" placed end to end in 
the direction of the pump beam. The two guides 6' and 6" are made of 
different materials. The first guide 6' is of a material with low 
absorption in the spectral domain covered by the first Stokes (e.g. 
between 0.8 and 2.1 .mu.m). The second guide 6" is made of a material 
which is absorbent outside the spectral domain 2.1-12 .mu.m; it 
constitutes an efficient guide for waves in this spectral domain. 
As an example, for these examples of wavelengths, the materials of which 
the guides are made are: 
for the first guide 6': glass containing fluorine 
for the second guide 6": tube of BeO. 
The geometry of the guides 6' and 6" is also optimized to improve the 
conversion yield for the first and second-order Stokes waves. 
For the examples given above the dimensions of the guides are as follows: 
For the guide 6': 
Internal diameter: 15 .mu.m 
Length: approx. 1 m 
For the guide 6": 
Internal diameter: 100 .mu.m 
Length: approx. 1 m 
Using a metal guide, it is possible to adapt a configuration with a wide 
spectral domain of emission. In this case an optical guide structure is 
used with adiabatic coupling of the mode, made using a spacing between the 
walls of the guide which varies linearly with distance. This enables the 
dimension of the mode to be modified and its monomode nature to be 
preserved over a wide spectral domain. An extension of this principle can 
be adapted to a cylindrical hollow guide using a conical capillary. The 
wavelength emitted can in this case be controlled via an adjustment of the 
pump power used. This structure is shown in FIG. 11; The dependence of the 
pump and Stokes powers is represented in FIGS. 12a and 12b as a function 
of the level of incident power at the input to the Raman guide. 
FIG. 12a is a diagram showing the conversion of a pump wave (curve Fp) into 
a Stokes wave Fs.sub.1. FIG. 12b is a diagram showing the conversion of a 
pump wave (Fp) to obtain a first-order Stokes wave (Fs.sub.1) and then a 
second-order Stokes wave (Fs.sub.2). 
According to the above description, the invention thus concerns a tuneable 
laser based on a pump laser which is itself tuneable over a spectral 
domain covering for example a window from 0.6 to 1.1 .mu.m. This laser, 
from a Raman-type non-linear process and according to a multi-Stokes 
mechanism, then generates new frequencies with a spectral shift 
corresponding to the Raman shift of the gas used. 
For example, from a sapphire laser doped with titanium and a Raman cell 
containing hydrogen under high pressure, it is possible to obtain an 
emission stretching from 0.6 .mu.m to 1.3 .mu.m. 
Such a source has certain advantages over the other types of tuneable 
sources based on parametric effects (parametric oscillator): 
higher tuneability domain, from the visible to the infrared 
wider transparency domain of the Raman medium 
greater resistance to light intensity and higher optical damage threshold; 
in addition optical damage (optical breakdown) is not irreversible since 
the medium in a gaseous form is naturally regenerated 
natural phase matching, not requiring the use of a particular angle of 
attack and rotation of the cell. This automatic phase matching specific to 
the stimulated Raman effect minimizes the dependence of the non-linear 
interaction on temperature (with respect to variations close to the 
ambient temperature) 
collinear emission whatever the wavelength generated. 
It is clear that the above description is a non-restrictive example and 
that other variants can be envisaged within the framework of the 
invention. The numerical examples and the nature of the materials 
indicated have been given only to illustrate the description.