GB1575033A

GB1575033A - Laser

Info

Publication number: GB1575033A
Application number: GB298077A
Authority: GB
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 1976-02-02
Filing date: 1977-01-25
Publication date: 1980-09-17
Also published as: FR2339972B1; FR2339972A1; DE2704273A1; CA1071745A

Description

(54) LASER (71) We, MASSACHUSETTS INSTI TUTE OF TECHNOLOGY, a Corporation organised and existing under the laws of the Commonwealth of Massachusetts, United States of America of 77 Massachusetts Avenue, Cambridge, Commonwealth of Massachusetts, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to lasers.

A typical example of existing methods for tuning the wavelength of a broad-band laser employs a laser resonator cavity consisting of a partially (or totally) reflecting mirror and one grating.

The grating is used in Littrow configuration at the oscillating wavelength, by which the ray at the oscillating wavelength is reflected back upon itself. For detailed description of a grating in Littrow configuration and other properties of a grating see, for example: Principles of Optics by Born and Wolf., Pergamon Press, New York (1959).

Such a resonator will provide regenerative feedback at the wavelength for which the grating angle is in Littrow with respect to the resonator's axis (determined by the direction normal to the resonator's fixed mirror). A broad-band amplifying medium placed within this resonator produces laser oscillation at the wavelength where the grating acts in Littrow, or in certain cases at a few closely spaced wavelengths near the peak of the grating's resolving band-width and determined by the various resonator modes which are generally spaced in frequency by 2L Kccp- ing the resonator mirror fixed and changing the grating angle changes the wavelength of the ray which will behave in Littrow as it propagates along the resonator axis. This then provides a means to wavelength tune a laser oscillator.

It is to be noted that such turning of the grating is in general used to provide coarse wavelength tuning over a wide region. Fine tuning of the laser is then obtained by keeping the grating angle fixed and changing the spacing between the mirror and the grating by a small amount. There are well known methods, such a piezoelectric tuning, where the latter can be achieved stably.

In other examples of existing methods of tuning, a laser cavity is employed in which a grating is fixed in non-Littrow position at least for some frequencies, and mirrors or Littrow gratings are placed to reflect rays of selected wavelengths diffracted by the grating, back upon themselves to the original grating, thence to the first mirror, see Figure 2, Osgood, Sackett and Javan, Measurement of vibrational-vibrational exchange rates for excited vibrational levels (2 S v 64) in hydrogen fluoride gas, The Journal of Chemical Physics, Vol. 60, No.4, 15 Fenruary 1974.

See also U.S. patent 3,928,817 and Friesem, Ganiel and Neumann, Simultaneous multiple wavelength operation of a tunable dye laser, Appl. Phys. Lett., Vol.23, No. 5, 1 September 1973.

Other arrangements for selection of wavelength or for simultaneous oscillation at a multiple wavelengths exist, for example those shown in U.S. patent 3,872,407 and in Lotem and Lynch, Double-wavelength laser, Appl. Phys. Lett., Vo. 27, No. 6, 15 September 1975. These and other prior art arrangements have disadvantages which the present invention overcomes.

According to one aspect of the invention there is provided a laser comprising an optical cavity defined by first and second reflecting optics, an amplifying medium within the cavity, means to restrict the laser beam to a common path for a band of wavelengths for the output of said laser, means including spectral dispersion apparatus disposed within the optical cavity, positioned to receive a beam of restricted cross section in said common path and to disperse said beam into spacially separated rays of different wavelength, said first and second reflecting optics constructed to reflect the beam regeneratively back and forth in said cavity along a regenerative path for amplification by said amplifying medium, a source of radiation separate from said amplifying medium for producing radiation at a selected frequency and disposed to transmit said radiation at a relatively weak intensity to follow the said regenerative path, and means to excite both said source and said amplifying medium whereby a given wavelength originating in said source can be present in said amplifying medium to determine the wavelength of radiation amplified by said amplifying medium. whereby radiation at a determined frequency and at a relatively high level of intensity can be produced at said output.

In a preferred embodiment the spectral dispersion apparatus comprises a pair of similar dispersive elements, e.g. diffraction gratings provided within the laser cavity, the first dispersive element being arranged to disperse the beam of restricted cross-section coming from the first reflection optics to produce a dispersed beam consisting of angularly separated rays of different wavelengths, the second dispersive element being arranged to receive the dispersed beam and to effect further dispersion so that the dispersed beam from the second dispersive element consists of rays of different wavelength extending substantially parallel to each other and mutually spaced according to their wavelengths.

The source is preferably another amplifying medium through which the dispersed beam from the dispersion apparatus passes.

the rays of different wavelengths passing through spatially separated portions of this other amplifying medium. and the second reflecting optics being arranged to reflect the rays back through that medium to the dispersion apparatus. Accordingly. their amplification of the rays occurs substantially in different regions of the other amplifying medium, thus limiting their coupling via that amplifying medium.

In certain embodiments the amplifying medium produces multiple wavelengths from different transitions of a gas. e.g. from a given molecular rotation-vibration band, and the spatial separation of the rays serves to limit collisional coupling. In other embodiments the amplifying medium produces multiple wavelengths sufficiently close in frequency to be coupled to the same transition by homogeneous broadening and the spatial separation of rays serves to limit coupling of the wavelengths via such broadening, e.g., as in pressure-broadened gas lasers and in dye lasers.In all such cases, parallelism of the spatially separated rays while propagating through the amplifying medium, achieved by placing the amplifying medium following both first and second spectral dispersive elements as above-mentioned, facilitates the design and provides uniformity in the conditions for rays at the various wavelengths.

According to another aspect of the invention there is provided a laser comprising an optical cavity defined by first and second reflecting optics, a first amplifying medium within the cavity and excitable to produce lasing conditions, means to restrict the laser beam to a common path for all wavelengths in the region of said first reflecting optics, means including spectral dispersion apparatus disposed within the optical cavity positioned to receive a beam of restricted cross section in said common path from said first reflecting optics and to disperse said beam to a plurality of paths in which rays Qf different wavelength are laterally spaced apart, said paths passing through said first amplifying medium whereby rays differing in wavelength can be subjected to amplification in different spatially separated portions of said first amplifying medium, said second reflecting optics being arranged to reflect regeneratively said rays back along said paths, in laterally spaced apart condition through said portions of said first amplifying medium for further amplification, thence from said spectral dispersion apparatus along said common path to said first reflecting optics. said laser including a second amplifying medium disposed in the common beam path between said first reflecting optics and said spectral dispersion apparatus, in which common beam path said rays are in a nonresolved, overlapping relation, and means to excite both said first and said second amplifying medium wherebu a given wavelength amplified in a portion of said first amplifying medium can be present in said common beam path to cause excitation of laser oscillation at said wavelength in said second amplifying medium.

By exciting both amplifying media. a wavelength produced in the amplifying medium in the path of the dispersed beam can be present in the common beam path to cause further excitation of laser oscillation at the wavelength in the amplifying medium there. In certain such embodiments the amp lifying medium in the path of the dispersed beam is adapted to produce multiple wavelengths. Advantageously, e.g. where the amplifying medium in the path of the dispersed beam and its excitation system is pulsed at low power, its excitation is triggered first in time, followed with a predetermined delay by triggering of excitation of the lasing process in the common path according to an injection phenomenon caused by the weak radiation from the amplifying medium in the path of the dispersed beam.

In the embodiments mentioned, in certain instances, modulating means, either active or passive, may be disposed in the common beam path between the first reflecting optics and the first dispersive element, arranged to modulate in unison the different wavelengths or to provide one or multiple injection frequencies for amplifying medium elsewhere in the resonator.

There may also be provided one or more apertures disposed in the path of the dispersed beam positioned to transmit a selected ray or rays whereby the wavelength of laser oscillation is determined. In preferred embodiments a set of these apertures is adjustably positioned across the said path for wavelength tuning.

In various embodiments, colinear output of the numerous wavelengths is provided through the first reflecting optics of the cavity or by zero order diffraction from the dispersion apparatus within the cavity that receives colinear beams from the first reflecting optics.

The second reflecting optics may comprise an extended plane mirror constructed to reflect substantially parallel rays constituting the dispersed beam back upon themselves to the mentioned pair of dispersive elements, or a long focal length concave reflector, e.g., 10 times longer than the optical path in the resonator.

Accordingly, the critical resonator components (e.g. gratings, mirrors, etc.) can be kept at a fixed angle with respect to the laser axis and locked in position. Coarse frequency tuning is then accomplished by moving the above-mentioned aperture or apertures within the resonator, e.g., by translation, in a manner which is considerably less critical than turning a grating about its axis as in the previous Littrow configured arrangements.

Such configuration lends itself to a rugged mechanical design free from microphonic and jitter effects.

The invention provides a new way to tune the frequency of a broad-band laser oscillator over a wide region. In one embodiment, the invention makes it possible to operate the broad-band laser on a number of frequencies simultaneously, relatively strongly, with each of the frequencies controllable independently, while the laser beams corresponding to the various simultaneously oscillating frequencies can all leave the laser oscillator colinearly. In this case, these independently controllable laser beams will be overlapping at the laser output along their propagation paths-optimally, the basic dispersion spread will determine the extent of their overlap.

The colinearity of such output beams of many different frequencies is an important feature. This will obviate in certain cases the need to use an alternate method in which several lasers, each oscillating at the different frequencies, are used and their outputs combined into a colinear direction through cumbersome use of several beam splitters. The independent controllable multi-frequency operation of the device, with a colinear output, will make it possible to excite or probe simultaneously several transitions of an atomic or molecular system. Multi-quantum excitation has important application in laser initiated chemistry, molecular photodissociation, molecular or atomic photoionization, isotope separation, and others.

Further background to the invention will now be given with reference to Figures 1 to 3 of the accompanying drawings.

Fig. 1 is a diagrammatic view of a multiple grating optical path and Figs. 2 and 3, are diagrammatic views of alternative lasers employing the optical path of Fig. 1; Referring to Fig. 1, consider a ray at a wavelength Al incident on a grating along the AB direction. The ray will be diffracted from the grating in various orders. As an example, consider grating 1 which is blazed so that most of the energy is diffracted in the first order. Supposing this grating is to diffract the ray at the wavelength Al in the direction BCl.

(Note that AB direction is not in Littrow at A1.) Consider now a second ray at an appreciably different wavelength, X2, to be incident on the same grating, again along the same AB direction. For this ray, the diffracted ray will be along a path BC2 different from BC1.

A second grating 2 which may be an exact replica of the first grating may then be placed at some distance from the first grating and parallel to the first grating. The separation between the two gratings is selected so that, for a given beam size, the Al and X2 rays incident on the second grating are resolved and nonoverlapping. Inspection shows that, for the two gratings parallel to each other, the two rays diffracted from the 2nd grating will follow directions C1D1 and C2D2 which are parralel to one another.

Consider now another beam at an intermediate wavelength Al (between Al and X2, say Al 3 X1 3 X2), to be again incident on grating 1 along the common path AB. The diffracted ray at the wavelength X1 will follow the paths AB, BC1, C1D1. Note that C1D1 is parallel to the other two rays in the CD region.

Consider now a plane N perpendicular to the paths of the rays diffracted from grating 2. The intercept of the Al, X1, and A2 rays on this plane follow a direction perpendicular to the CD path, defined as the y axis. If the wavelength of a ray incident along the fixed AB path is continuously tuned from X1 to A2, after diffraction from the first and the second gratings, its intercept with the fixed plane will continuously move along the y direction from al to a2.

Referring to Fig. 2, a resonator is constructed by placing a long planar reflecting mirror 3 perpendicular to the CD path, and another reflecting mirror 4 perpendicular to the common AB path. An aperture member 5 defining aperture 6 bounded by blocking walls is disposed in front of the long mirror, adjustable by micrometer screw 9. The wavelength region where the resonator can provide high-Q regenerative feedback now depends upon the position of the aperture 6 along the y direction. Further, by moving the aperture along the y direction from, say, al position to a2 position in Fig. 1 the resonator is coarse frequency tuned from A1 to X2, the extended mirror 3 regeneratively reflecting the wavelength back upon itself wherever the aperture 6 is positioned.

An amplifying medium 7 is provided with a broad amplification band-width extending at least from X1 to A2. For a laser to be oscillated on a single tunable frequency, the amplifying medium can be placed in either the common arm AB, or in the region BC or in the CD region. A more convenient location for this is the AB region. The frequency tuning of the tunable wavelength At is then obtained by moving the aperture 6.

Referring to the resonator of Fig. 3, regenerative feedback is provided simultaneously at several wavelengths, by providing several separate apertures along the y axis; specifically the figure shows a system tunable at two different wavelengths chosen by two apertures, 5a, 5b.

The cross-section of the common arm beam AB is restricted, as by limiting aperture member 10, to restrict the point of incidence of rays from mirror 4, to ensure well defined multi-wavelength operation. (In place of the aperture member, the beam aperture may be similarly restricted by limiting the length of the grating 1, or limiting the size of mirror 4.) A basic feature of this multi-wavelength resonator is that it provides regions, such as BC and CD, where the directions for regenerative feedback at two different wavelengths are spatially resolved and nonoverlapping. By placing the amplifying medium 7a, 7b in such regions, highly troublesome coupling of two (or several) oscillating wavelengths by the amplifying medium is avoided.Such coupling effects arise from a variety of nonlinear effects, for instance homogeneous broadening of a single transition as in dye lasers or high pressure gas lasers, or collisional coupling of different transitions in a given rotation vibration band of a gas. In either case there is a tendency for the energy to be concentrated mainly in one wavelength and deprived from another, an effect which can be diminished or entirely avoided by causing (as in Fig. 3), the rays at different wavelengths to occup different regions in the amplifying medium placed within the resonator.Placement of the amplifying medium 7a in the path CD has the further advantage that the various wavelengths are parallel, and of equal path length through the medium. (In contrast, with the coupling effects mentioned, it is realized that diffusion coupling between spaced points in the medium, being relatively time dependent, will not defeat the isolation here achieved, particularly if relatively short pulses are employed.) Another advantage of the arrangement of Fig. 3 is that the simultaneously oscillating frequencies can all be coupled out of the resonator colinearly by partially transmitting mirror 4 in the common AB arm. The coupling can also be obtained colinearly via zeroorder diffraction from the first grating, via the arrow in dotted lines.The zero order diffraction is one for which the angle of diffracted ray with respect to the normal to the grating is exactly the same as the incident angle but it occurs on the opposite side of the normal to the grating, i.e., the diffracted angle is exactly the negative of the incident angle. Since, in this arrangement, the angle of incidence of common arm AB is the same irrespective of the wavelengths (i.e., the rays ocrresponding to the different wavelengths are all incident along the AB path), the zero order diffraction from the first grating occurs colinearly for all wavelengths, along the dotted line path in Fig. 3.

The above resonator is used to obtain an independently controllable multi-frequency laser, using a molecular rotation vibration band. For this, the amplifying medium is placed in the BC or CD region. At a low gas pressure, the independently frequencies will consist of oscillations at the different rotation-vibration transisitions within the band. At elevated pressures where collision broadening in the amplifying medium results in overlapping of all of the transitions within the band, continuous frequency tuning can be obtained over the entire band.

For modulation of all frequencies a modulator 18 is placed in the common arm AB, either an active modulator, e.g., an electrooptic modulator, or a passive modulator, e.g.

a saturable absorbing medium, for forming short pulses.

Embodiments of the invention will now be described by way of example with reference to Figures 4-6 of the accompanying drawings, in which: Fig. 4 is a diagrammatic view, of a laser according to the invention employing two gratings and two amplifying media; Fig. 5 is a diagrammatic view of another laser according to the invention employing a pair of oppositely directed prisms for dispersing the beam within the optical cavity, and Fig. 6 is a diagrammatic view of an injection locked laser according to the invention.

Referring to Fig. 4, here amplifying medium 7d with low power pulsed excitation source 20, is providedin the parallel CD arm, while an additional amplifying medium 7e provided with high power pulsed excitation source 22 and subject by itself to coupling difficulties is placed in the common arm AB.

By predetermined delay 24 it is ensured that excitation source 22 for the common arm fires after pulsing excitation source 20 for the CD arm, but while radiation produced by excitation 20 persists in the resonator. The injection effects of X1 and A2 produced separately in arm CD force oscillation at both and X2 in the high power medium 7e, despite tendencies to couple via the amplifying medium.

In another mode of operation the excitation of amplifying medium 7d by itself can be kept below the threshold for oscillation. The mere presence of small gain in that medium and the very weak radiation associated with it will be sufficient to trigger the amplifying medium 7e at wavelength determined by the gain characteristics of the 7d amplifying medium. In still another embodiment both the 7d and 7e amplifying medium can be placed in a path where the diffracted rays are spatially resolved according to their wavelength.

As shown, the laser of Fig. 4 is constructed as a CO2 laser for operation in the 10.6 u band. The adjustable apertures 5a and 5b are translated parallel to plane mirror 3 to positions corresponding for instance to wavelengths of the p(18) and p(20) transitions (blocking the wavelength of the p(16) transistion).

Amplifying medium 7d may be a gas laser at a low pressure and of low power and the high power system 7e may comprise a high pressure gas laser employing a photoionization method to produce a uniform high density plasma gain medium. In other embodiments the amplifying medium 7d can operate in CW, or the amplifying medium 7e may be pulsed so that gain exists in both gain media 7d and 7e simulataneously.

Referring to Fig. 5, other dispersive means can be employed, e.g. the parallel prisms 41 and 42, which are oppositely directed, the first prism 41 refracting the common beam to the first refracted path P1 and the second prism refracting the beam to refracted path P2, thence to mirror 3 for regenerative reflection. The amplifying media in the refracted paths P1 and P2 are employed in accordance with principles mentioned above in connection with the embodiment of figure 4.

An external source of radiation can also be employed to advantage. According to the embodiment of Fig. 6, a laser cavity similar to that of Fig. 3 is employed. The output of an external laser 90, preferably after passing through isolator 92, enters the cavity through the first mirror and locks laser oscillation produced by amplifying medium 94 or 96.

Output is obtained through beam splitter 87.

One advantage offered by the resonator cavity for injection purposes lies in the many resonator modes offered by the arrangement. Even further resonator modes can be obtained in certain cases by using an unstable laser construction, e.g. by use of convex mirrors. The many resonator modes assures that a resonant path is found by rays of the desired wavelength despite variations in the optical properties of the resonator, e.g. variation in the refractive index of the amplifying medium, etc.

WHAT WE CLAIM IS: 1. A laser comprising an optical cavity defined by first and second reflecting optics, an amplifying medium within the cavity, means to restrict the laser beam to a common path for a band of wavelengths for the output of said laser, means including spectral dispersion apparatus disposed within the optical cavity, positioned to receive a beam of restricted cross section in said common path and to disperse said beam into spacially separated rays of different wavelength, said first and second reflecting optics constructed to reflect the beam regeneratively back and forth in said cavity along a regenerative path for amplification by said amplifying medium, a source of radiation separate from said amplifying medium for producing radiation at a selected frequency and disposed to transmit said radiation at a relatively weak intensity to follow the said regenerative path, and means to excite both said source and said amplifying medium whereby a given wavelength originating in said source can be present in said amplifying medium to determine the wavelength of radiation amplified by said amplifying medium, whereby radiation at the determined frequency and at a relatively high level of intensity can be produced at said output.

2. The laser of claim 1 wherein said first reflecting optics are partially transparent enabling a colinear output of various wavelengths to propagate there through.

3. The laser of claim 1 or 2 wherein said amplifying medium is disposed along said common path whereby its amplifying effects can be fully effective regardless of the specific frequency in said band width that is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. to Figures 4-6 of the accompanying drawings, in which: Fig. 4 is a diagrammatic view, of a laser according to the invention employing two gratings and two amplifying media; Fig. 5 is a diagrammatic view of another laser according to the invention employing a pair of oppositely directed prisms for dispersing the beam within the optical cavity, and Fig. 6 is a diagrammatic view of an injection locked laser according to the invention. Referring to Fig. 4, here amplifying medium 7d with low power pulsed excitation source 20, is providedin the parallel CD arm, while an additional amplifying medium 7e provided with high power pulsed excitation source 22 and subject by itself to coupling difficulties is placed in the common arm AB. By predetermined delay 24 it is ensured that excitation source 22 for the common arm fires after pulsing excitation source 20 for the CD arm, but while radiation produced by excitation 20 persists in the resonator. The injection effects of X1 and A2 produced separately in arm CD force oscillation at both and X2 in the high power medium 7e, despite tendencies to couple via the amplifying medium. In another mode of operation the excitation of amplifying medium 7d by itself can be kept below the threshold for oscillation. The mere presence of small gain in that medium and the very weak radiation associated with it will be sufficient to trigger the amplifying medium 7e at wavelength determined by the gain characteristics of the 7d amplifying medium. In still another embodiment both the 7d and 7e amplifying medium can be placed in a path where the diffracted rays are spatially resolved according to their wavelength. As shown, the laser of Fig. 4 is constructed as a CO2 laser for operation in the 10.6 u band. The adjustable apertures 5a and 5b are translated parallel to plane mirror 3 to positions corresponding for instance to wavelengths of the p(18) and p(20) transitions (blocking the wavelength of the p(16) transistion). Amplifying medium 7d may be a gas laser at a low pressure and of low power and the high power system 7e may comprise a high pressure gas laser employing a photoionization method to produce a uniform high density plasma gain medium. In other embodiments the amplifying medium 7d can operate in CW, or the amplifying medium 7e may be pulsed so that gain exists in both gain media 7d and 7e simulataneously. Referring to Fig. 5, other dispersive means can be employed, e.g. the parallel prisms 41 and 42, which are oppositely directed, the first prism 41 refracting the common beam to the first refracted path P1 and the second prism refracting the beam to refracted path P2, thence to mirror 3 for regenerative reflection. The amplifying media in the refracted paths P1 and P2 are employed in accordance with principles mentioned above in connection with the embodiment of figure 4. An external source of radiation can also be employed to advantage. According to the embodiment of Fig. 6, a laser cavity similar to that of Fig. 3 is employed. The output of an external laser 90, preferably after passing through isolator 92, enters the cavity through the first mirror and locks laser oscillation produced by amplifying medium 94 or 96. Output is obtained through beam splitter 87. One advantage offered by the resonator cavity for injection purposes lies in the many resonator modes offered by the arrangement. Even further resonator modes can be obtained in certain cases by using an unstable laser construction, e.g. by use of convex mirrors. The many resonator modes assures that a resonant path is found by rays of the desired wavelength despite variations in the optical properties of the resonator, e.g. variation in the refractive index of the amplifying medium, etc. WHAT WE CLAIM IS:

1. A laser comprising an optical cavity defined by first and second reflecting optics, an amplifying medium within the cavity, means to restrict the laser beam to a common path for a band of wavelengths for the output of said laser, means including spectral dispersion apparatus disposed within the optical cavity, positioned to receive a beam of restricted cross section in said common path and to disperse said beam into spacially separated rays of different wavelength, said first and second reflecting optics constructed to reflect the beam regeneratively back and forth in said cavity along a regenerative path for amplification by said amplifying medium, a source of radiation separate from said amplifying medium for producing radiation at a selected frequency and disposed to transmit said radiation at a relatively weak intensity to follow the said regenerative path, and means to excite both said source and said amplifying medium whereby a given wavelength originating in said source can be present in said amplifying medium to determine the wavelength of radiation amplified by said amplifying medium, whereby radiation at the determined frequency and at a relatively high level of intensity can be produced at said output.

determined by said source.

4. The laser of any of the foregoing claims wherein said source comprises a further amplifying medium disposed within said optical cavity.

5. The laser of any of the claims 1 to 3 wherein said source is disposed outside of said optical cavity.

6. The laser of claim 5 including an isolator disposed between said cource and means introducing said relatively weak radiation from said source into said cavity.

7. The laser of any of the foregoing claims wherein said source comprises a CW source.

8. The laser of any of the foregoing claims wherein said amplifying medium comprises excitable gas.

9. The laser of claim 8 wherein said gas comprises carbon dioxide,

10. The laser of any of the foregoing claims wherein both said source and said amplifying medium are of the pulsed operation type and wherein the means to excite includes a delay means for delaying the excitation of said amplifying medium until the weak radiation is present therein as a result of excitation of said source.

11. The laser of any one of the foregoing claims wherein said source comprises means for simultaneously producing radiation at a plurality of spaced apart, discrete frequencies at relatively weak intensities, thereby determining a plurality of wavelengths of radiation amplified by said amplifying medium.

12. The laser of claim 11 wherein said means for producing radiation at a plurality of frequencies are disposed in a dispersed path in said cavity following said spectral dispersion apparatus.

13. The laser of claim 11 or 12 wherein both said amplifying medium and said means for producing radiation at a plurality of frequencies comprise excitable gas, the gas of said amplifying medium being at a relatively high gas pressure.

14. The laser of any of the foregoing claims including modulating means disposed in the common beam path between said first reflecting optics and said spectral dispersion apparatus, in which path rays of differing wavelength are in a non-resolved, overlapping relation, said modulating means being arranged to modulate in unison different wavelengths.

15. A laser comprising an optical cavity defined by first and second reflecting optics, a first amplifying medium within the cavity and excitable to produce lasing conditions, means to restrict the laser beam to a common path for all wavelengths in the region of said first reflecting optics, means including spectral dispersion apparatus disposed within the optical cavity positioned to receive a beam of restricted cross section in said common path from said first reflecting optics and to disperse said beam to a plurality of paths in which rays of different wavelength are laterally spaced apart, said paths passing through said first amplifying medium whereby rays differing in wavelength can be subjected to amplification in different spatially separated portions of said first amplifying medium, said second reflecting optics being arranged to reflect regeneratively said rays back along said paths, in laterally spaced apart condition through said portions of said first amplifying medium for further amplification, thence from said spectral dispersion apparatus along said common path to said first reflecting optics, said laser including a second amplifying medium disposed in the common beam path between said first reflecting optics and said spectral dispersion apparatus, in which common beam path said rays are in a nonresolved, overlapping relation, and means to excite both said first and said second amplfying medium whereby a given wavelength amplified in a portion of said first amplifying medium can be present in said common beam path to cause excitation of laser oscillation at said wavelength in said second amplifying medium.

16. The laser of claim 15 wherein said amplifying medium through which said dispersed rays pass is of a type productive of detrimental coupling of different wavelengths when said wavelengths are colinear, the passage of said dispersed rays through said spatially separated portions enabling amplification at the different wavelengths without detrimental coupling via the amplifying ,edium.

17. The laser of claim 15 or 16 wherein said portions of amplifying medium through which said dispersed rays pass comprise adjacent portions of a volume of gas having a characteristic given rotation-vibration band having different transitions from which different wavelengths can emit when said medium is energized, the wavelengths subject to detrimental collisional coupling, and the distance of apatial separation of said sispersed rays in said volume of gas being of predetermined value to limit collisional coupling.

18. The laser of any of the claims 15 to 17 wherein the amplifying wavelength characteristic of said first and of said second amplifying medium are cooperatively predetermined such that multiple wavelengths characteristically produced by said first medium are wavelengths which are susceptible to coupling effects in said second amplifying medium in the absence of supportive radiation at said wavelength produced by said first amplifying medium and passing through said second amplifying medium.

19. The laser of any of the claims 15 to 18 wherein separate pulsed excitation sources are provided for said first and said second amplifying medium, and means for delaying excitation of said second amplifying medium relative to said first amplifying medium to ensure presence of rays of said wavelengths in said second medium during initiation of excitation of said second medium.

20. The laser of any of the foregoing claims wherein an aperture means are disposed in said cavity where the beam consists of said dispersed rays, and are arranged to transmit a ray of a selectable wavelength and block adjacent wavelengths whereby the wavelength of laser oscillation may be determined.

21. The lase of any of the claims 15 to 18 wherein said means to excite are operable to excite both said first and said second amplifying medium to lasing levels.

22. The laser of any of the claims 15 to 18 wherein said exciting means are operable to excite said first amplifying medium to a condition below lasing level and to excite said second amplifying medium to lasing level.

level.