The lasing action in an unstable optical resonator is controlled as to frequency by limiting frequency selectivity to an axis-near beam. The control is provided preferably outside the cavity by means of active or passive elements. A passive element returns a frequency-controlled beam into the cavity, an active element injects a control beam at the desired frequency.

BACKGROUND OF THE INVENTION 
The present invention relates to lasers with unstable resonator cavities; 
and more particularly, the invention relates to line selection of and in 
such lasers. 
It is inherent in many laser media that they are capable of lasing at 
different frequencies. Moreover, the type of stimulation involved may 
result in multiple line lasing and resonating in the optical laser cavity. 
This is particularly true in chemical lasers in which the chemical 
reaction produces population inversions resulting in different 
transitions, i.e., emission of different frequencies. In many instances, 
multifrequency emissions are not wanted for a variety of reasons. Among 
them, for example, is the possibility of absorption of the most strongly 
developed lines by the environment through which the emitted laser beam 
propagates. 
Frequency selectivity of lasers in general and of unstable resonator 
cavities in particular has been dealt with in the past. See, for example, 
U.S. Pat. Nos. 4,123,149 and 3,928,817. These prior art devices have in 
common that the selectivity to be effective must cover the entire width of 
the laser beam, and uniformly so in order to be effective. This 
requirement poses considerable problems for the uniformity of the element 
or device which preforms the selection, and there are other practical 
limitations with large devices, particularly for lasers in which the 
lasing medium has a relatively large volume. 
SUMMARY DESCRIPTION OF THE INVENTION 
It is an object of the present invention to improve the frequency 
selectivity of lasers. 
It is a corresponding object of the present invention to improve the 
frequency selectivity of optical unstable resonators. 
It is a specific object of the present invention to improve the frequency 
selectivity of chemical or other lasers capable of multiline lasing. 
It is a further specific object of the present invention to improve the 
frequency selectivity of optical resonators which include an optical 
cavity defined by curved reflectors facing each other on a common axis, 
there being a scraper mirror disposed in between, having a particular 
aperture. 
The optical cavity and resonator in accordance with the further specific 
object is improved by including a frequency-selective element and device 
effective on and near the axis, but having an optically effective aperture 
significantly smaller than the aperture in the scraper mirror. Thus, in 
accordance with the principle of the invention, the reflectors defining 
the (primary) optical cavity of the resonator (and laser) are not made 
frequency selective; but control as to frequency selectivity is restricted 
to a beam or ray on and very near to the axis, having very little lateral 
extension in comparison to the width or diameter of the radiation beam as 
produced by and in the cavity as a whole. 
For practical reasons, the frequency-selective device or element is 
disposed outside the cavity, and one of the reflectors has a central 
aperture defining the effective aperture of the frequency selectivity. The 
frequency-selective device may be a passive one or an active one. A 
passive device is, basically, a frequency-selective reflector reflecting a 
ray that has left the cavity through the opening in one of the reflectors 
and is returned therethrough but as a single frequency beam (or a beam 
having only selected frequencies). In the case of active control, one may 
use an auxiliary laser emitting just one frequency or only particular 
frequencies and injecting that narrow control beam through the opening in 
the one reflector of the principal cavity. 
The principle of the invention is, thus, to eliminate relatively large 
surface configurations affecting a laser beam as a whole, in an attempt to 
render it frequency selective. By introducing the principle of single-ray, 
axis-near frequency control, the control operation needs to affect only a 
very small diameter ray or beam so that the device actually performing the 
function of frequency selection needs to be frequency selective only over 
a very small geometric area. 
If a control-beam-injecting laser (active control) is used, the invention 
can also be interpreted as a cascade as far as frequency control is 
concerned. A small frequency-selective laser controls the center beam of a 
large nonselective laser, by rendering the latter frequency selective by 
operation of the control. The small, injecting laser, in turn, may have 
been made selective by a passively acting means within the purview of this 
invention. 
The preferred embodiment of the invention, the objects and features of the 
invention, and further objects, features and advantages thereof, will be 
better understood from the following description taken in connection with 
the accompanying drawings.

Proceeding now to the detailed description of the drawings, FIG. 1 shows an 
unstable, optical resonator cavity 10 along an optical resonator axis 11 
and being comprised of a concave reflector 12 and a convex reflector 13. 
The gain medium is depicted by 18. These elements are supplemented by a 
so-called scraper mirror 15 having an aperture 16. The width of that 
aperture in the scraper mirror is representatively given by a dimension D, 
transversely to optical axis 11. By way of example, dimension D may be the 
diameter of a circular, square-shaped or rectangular aperture or opening. 
The convex mirror 13 is provided with a central bore or aperture 14 whose 
width is given by a diameter dimension d. The aperture or bore 14 has 
preferably a circular cross-section, but it may be rectangular or 
square-shaped. It is now significant that dimension d. is substantially 
smaller than dimension D; possibly being one or more orders of magnitude 
apart. The large dimension D permits utilization of extended gain regions. 
The aperture 14 is centrally traversed by the optical axis 11. 
An optical control element, or system 20, is disposed outside the optical 
cavity proper, but can be deemed a portion thereof under some 
circumstances. This control element injects or selects a beam of a single 
or of a few frequencies to be transmitted through bore 14 into cavity 10. 
Several examples for this device 20 will be given below. Suffice it to say 
that a beam of a single frequency or limited numbers of frequencies is 
injected through bore 14 into the cavity. That beam is very narrow on 
account of the dimensions of bore 14 and propagates right on axis 11 
toward reflector 12. 
The device 20 includes at least one reflecting surface which participates 
in the transmission of the frequency-selected and controlled ray on axis 
11 into (or back into) cavity 10. This reflecting surface of and in device 
20 may define a second resonating cavity, together with the center of 
reflector 12, to be attuned to the particular line to be selected. That 
cavity extension can be understood to extend radially from axis 11 at a 
distance therefrom, in the plane of reflector 13, of less than d/2. As far 
as the width of bore or opening 14 is concerned, (dimension d), that width 
should be about equal to the width of one Fresnel zone. For maximum 
practicability, a larger opening is not needed; a smaller opening may be 
optically impractical. The origin of the particular Fresnel zone cone is 
the optical center point of reflector 12. 
The lasing medium 18 contained in cavity 10 is for example, a mixture of 
diluent and excited DF or HF, or CO, or CO.sub.2, etc. It would be more 
appropriate to say that the optical cavity 10 is being passed through, or 
flowed through, by a gas or mixture of gases containing at least one 
laser-active component. It should be noted, however, that the particular 
lasing mechanism is not of any direct significance for practicing the 
preferred embodiment of the invention. The invention deals particularly 
with frequency selectivity of an optical resonator. Particulars of the 
optical gain medium are incidental, except that the invention can be 
applied in all those cases in which the lasing medium is capable of 
spontaneous emission at more than one line. 
As far as the frequency-selective control is concerned, consider the center 
axis region of the cavity. Due to the presence of bore 14, reflector 13 
does not reflect any radiation at its center. Thus, the center beam (any 
center beam) in the cavity propagating toward reflector 12 is exclusively 
determined by the radiation from controller 20. Due to diffraction at bore 
14, that center beam assumes a slightly diverging wave front so that a 
portion, when reflected by mirror 12, will not leave cavity 10 again 
through opening 14, but will be reflected by mirror 13. Multiple, 
gain-producing reflections occur until the beam is captured by the scraper 
mirror and reflected out of the system. Radiation following this pattern 
will dominate so that, indeed, the output beam has the selected frequency 
or frequencies only. Other frequencies may still establish some gain, but 
from an overall point of view, the functionality of the device is improved 
by limiting operation to a single one, or to but a few, of the possible 
lines. Furthermore, in some types of lasers, the energy in the nonselected 
lines is transferred to the selected lines, thus retaining their energy 
but utilizing it more favorably. 
The control element 20 can be any of a variety of devices. These may be 
classified as passive and active devices. The former use light from cavity 
10, the latter inject a particular frequency into the cavity via bore 14. 
In the case of an active device, a narrow beam of the selected frequency 
or frequencies is injected into cavity 10 through opening 14. As stated, 
some diffraction occurs and a slightly diverging control beam will reach 
reflector 12 and be returned. Most of the control beam will not leave the 
cavity but will miss opening 14 on return and be reflected back by 
reflector 13. Each traversal of cavity 10, increases the gain until the 
beam is captured by the scraper mirror. 
If device 20 is a passive device, the control beam is produced from 
radiation that has left cavity 10 through opening 14. That radiation is 
not yet frequency-selected, but device 20 is presumed to return (reflect) 
radiation having only (or predominantly) the selected frequency or 
frequencies. 
A central portion of the wave front of this passively produced control beam 
leaves opening 14 for cavity 10 and will be returned by reflector 12 
through opening 14, towards the reflecting surface in device 20. That 
portion traverses the gain medium several times and is, thus, 
significantly augmented. Moreover, this particular reflecting surface in 
passive device 20 resonates optically with reflector 12. 
On each passage of the center beam through opening 14, some diffraction 
occurs and a portion of the beam is, so to speak, laterally branched off 
so that a dynamic equilibrium is maintained in the center region of the 
cavity. Since the diameter d of bore 14 is considerably smaller than the 
width of the laser medium 18 in cavity 10 and the diameter D of aperture 
16 in the scraper mirror 15, the center beam will experience many 
gain-increasing reflections between the mirrors 12 and 13 before such a 
beam is deflected by mirror 15. 
FIG. 2 illustrates a passive device in the form of a convex (spherical) 
Littrow grating 21. FIG. 2a illustrates the grating in front view. The 
device is oriented in such a way that a Littrow frequency is returned on 
and along axis 11. The returned beam will be slightly wider than the 
diameter of aperture 14; but only an axis-near, single-frequency component 
is returned into the cavity. It should then be noted that this convexity 
of device 21 introduces an additional diverging component into the 
returned beam, augmenting the diffraction at opening 14. However, device 
21 could have a planar surface as diffraction suffices to obtain 
divergence of the control beam. 
FIG. 3 illustrates an alternative, somewhat simpler and less exacting 
frequency selection. The element 22 is a spherical mirror with a thin 
coating 23, attuned to the frequency to be selected. The coating 22 is of 
the .lambda./2 type and will suppress lines outside its return response 
range. This type of element has a relatively large bandwidth and is 
suitable only if the other lasing lines of the gain medium 18 are well 
outside this band. 
FIG. 4 illustrates a selecting device 24 in front view. The device includes 
several Littrow elements 25. These elements are arranged on a slightly 
convex surface of a carrier element 26. The purpose of this arrangement is 
to simulate a spherical surface of this boundary for the secondary central 
optical resonator cavity, the other boundary being the central portion of 
reflector 12. This convex arrangement of Littrow elements compensates for 
the fact that axis-near beams, leaving cavity 10 and traversing opening 
14, also have a converging component. Thus, Littrow elements outside the 
dead center portion have to have a slightly different angle in order to 
ensure uniform blazing angles and, thus, proper frequency selectivity. 
The passive device shown in FIG. 5 is of particular simplicity; it includes 
a spherical mirror 27, having a high nonselective reflectivity or a 
frequency-selective surface. In addition, the device includes an adsorber 
cell 28 which attenuates unwanted frequencies. One will choose such an 
adsorbing device in cases in which the frequency selectivity does not have 
to be restricted to a single frequency. 
By way of example, it may be the purpose of the laser beam to be generated, 
to traverse the atmosphere over a long distance with little or no 
attenuation. Thus, the laser should emit radiation of only those 
frequencies which will not be attenuated by the atmosphere. Certain 
wavelengths of the frequencies which the medium 18 is capable of 
producing, will be suitable in this regard; others will be strongly 
attenuated by water, i.e., moisture. Consequently, the laser should not 
waste its energy content by lasing at these frequencies; the production of 
gain should be restricted to those frequencies which will not be adsorbed 
by atmospheric moisture. In furtherance of this objective, the center beam 
of the laser should be controlled to be limited to these desired 
frequencies. The cell 28 in FIG. 5 may be filled with water vapor. The 
multifrequency beam from cavity 10 along an axis 11 will pass through cell 
28, and some wavelengths will be attenuated, others will not or to a 
lesser degree. The cell 28 will, therefore, automatically select all 
frequencies which will not, or only very little, be attenuated by 
atmospheric moisture. 
FIG. 5 introduces still another aspect; the particular cell 28 does not 
have any critical dimensions in any direction transverse to the beam 
propagation and traversal. However, it would be highly impractical to 
insert such a cell into cavity 10, and having a width equal to the width 
of the beam resonating in the cavity. The direct exposure to the radiation 
in cavity 10 would render the device extremely hot. Limiting the frequency 
control to a narrow, axis-near beam, outside cavity 10, ensures ready 
dissipation of a relatively small amount of absorbed radiation and makes 
such control practical. 
FIG. 6 illustrates an active device. In particular, this figure illustrates 
a laser 30 which includes a lasing medium 31, a semitransparent mirror 32, 
and a frequency-selective, e.g., Littrow, grating 33. The lasing medium 31 
should be the same or the same kind as medium 18 in FIG. 1 so that lasing 
conditions are identical as far as the media are concerned. Reflector 33 
singles out the desired frequency and laser 30 emits, in fact, a 
monochromatic beam. This narrow beam is injected into cavity 10 through 
aperture 14 and serves as prime control beam. Diffraction at aperture 14, 
possibly augmented by a slightly diverging component of the beam itself 
ensures that a portion of the frequency selected control beam is captured 
by the cavity 10 and will not leave again through opening 14. 
FIG. 7 illustrates an active device analogous to FIG. 5. One may say that 
the passive reflector 27 has been replaced by a laser cooperating with an 
absorber cell 38. This particular laser 35, is basically a small version 
of the principal laser. There is a lasing medium 36, a semitransparent 
mirror 37, and a rear reflector 39. Laser 35 may have no frequency 
selectivity at all; frequency selectively is provided by cell 38, 
containing the particular frequency-selective absorber material for 
frequency-controlling the center beam as injected and returned to the 
laser cavity 10 (FIG. 1). The cell 38 can also be located within the laser 
cavity mirror 39 and 37. 
The various figures illustrate various ways of controlling the center beam 
in the laser cavity by means of devices outside the cavity, but being 
disposed and effective directly on the axis; the effectiveness is obtained 
through the aperture or opening 14 which determines the lateral or width 
dimension of the injected or returned control beam. The control element 20 
does not have to have comparable, small dimensions. It should be as small 
as possible, simply for reducing the difficulties in obtaining a high 
degree of accuracy (frequency selectivity) over a particular area. On the 
other hand, the smaller the effective diameter and cross section of device 
20, the more accurately it has to be positioned and retained on the axis. 
It can thus be seen that there is a trade-off in making the device 20 
larger than needed as far as the size of aperture 14 is concerned, but 
only to the extent needed for ensuring retention of its position on and in 
relation to axis 11 within reasonable tolerances. 
In principle, there is no need for placing the control of the center beam 
outside the cavity and/or directly in line with axis 11. Rather, one may 
use for example, a very small filter or filter cell inside cavity 10, and 
having width dimensions comparable to the dimension of bore 14 in FIG. 1; 
but in this case one will not need such a bore. Alternatively, one may 
place a very small, 45-degree mirror into the cavity and on axis 11; and 
one may inject or introduce, otherwise, a control beam laterally. In all 
of these instances, the respective element and device inside cavity 10 
must have very small dimensions transverse to axis 11 to effectively 
restrict control to the axis' beam. This was found to be less practical 
from a point of view of mounting; and so the arrangement of the type shown 
in FIG. 1 is clearly preferred and demand to be the best mode. 
The invention is not limited to the embodiments described above; but all 
changes and modifications thereof, not constituting departures from the 
spirit and scope of the invention, are intended to be included.