GB2113905A

GB2113905A - Conversion of laser frequencies

Info

Publication number: GB2113905A
Application number: GB08121156A
Authority: GB
Inventors: Dr P F Browne
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-07-09
Filing date: 1981-07-09
Publication date: 1983-08-10
Also published as: GB2113905B

Abstract

An optical cavity which comprises a beam expansion section (as in an unstable resonator) and also a beam reduction section (which may be the expansion section used in reverse) has considerable advantages for achieving non-linear effects if the amplifying medium is in the beam expansion section and the non-linear medium in the beam reduction section, and specifically near to the axis of the cavity. One of many possible realizations of such a cavity is described in the specification, and its advantages described for off- resonance stimulated Raman scattering. The advantages for achieving high laser beam intensity are (i) geometrical intensification by a factor M<2> of the amplified expanded beam, where M is the magnification of the reducer considered as a telescope, (ii) trapping of the stimulated emission within a high Q cavity so that the desired non-linear process becomes the dominant loss mechanism, and (iii) the possibility of automatic mode- locking by using an intracavity bleachable absorber at the laser frequency. <IMAGE>

Description

SPECIFICATION A cavity for conversion of laser frequencies by non-linear processes This invention relates to laser sources, and in particular to systems in which the emission from one laser is used to optically pump a second laser.

An example would be stimulated emission from CH3F at wavelength 496 ,um in response to pumping by a suitable line of a CO2 laser with wavelength near 10.6 ,um. Laster action on numerous lines in the far-infrared has now been achieved by pumping various molecules in this manner. The pump beam raises the molecule to the first excited vibrational state (transition iej in figure 1) and specifically to a particular rotational level j for this state. When population inversion is established between the rotational levels j and k (figure 1) of the excited vibrational state stimulated emission will build up at the frequency jk of this transition.In principle it should be possible to achieve the Wik photon for one w1 pump photon, but actual conversion efficiencies fall far short of this limit. There are several reasons for the poor efficiency. The populations of the rotational levels thermalize at a rate of about 108 S-1 torr1, and so the pressure must be low in order that this time constant should exceed the build-up time for the stimulated emission. Low pressure means a small absorption coefficient for pump radiation, and hence a long path length in the emitting gas. The need to tune the pump radiation to the frequency Wii and to match the bandwidths also are constraints which contribute to inefficient conversion.

If, however, one exposes the pumped medium M to sufficiently high intensity I of pump radiation (frequency W) it is possible to excite the molecules directly from the level i to the final level k by a single transition involving two photons--absorption of a photon of frequency a) and emission of a photon of frequency e.)', where fi ke The process is stimulated Raman scattering. The cross-section is enhanced in the case of resonance in the sense WNO/J and w'-w,,.

One defines a pump offset a ,j and an emission offset 6'=w'--wj,, so that conservation of energy implies that 6=6'. For systems employing this pumping mechanism see references (1-5). The advances are substantial.

In the first place, population inversion between levels j and k no longer is required so that collisional relaxation rates are now unimportant and the pressure of the pumped gas can be increased. Actually, this in itself does not increase the absorption coefficient for pump radiation because, due to pressure broadening, this coefficient is independent of pressure. But the absorption coefficient for pump radiation is proportional to the intensity I' of far-infrared emission. (Also, the gain coefficient for the farinfrared emission is proportional to the intensity I of the pump beam). Hence the effective length of medium M can be reduced by a high value of l'l.

Secondly, exact resonances o"wij and Ct k no longer are required. Not only does this alleviate a constraint on the pump source, or alternatively provide the facility of tunable w' by varying W, but it also means that the growing population of molecules in level j do not reabsorb the farinfrared emission at frequency '.

The two-photon process does not depend on the existence of a level j which provides a near resonance ~cl)iJ although the cross-section is enhanced when there is such a level. One relies of the enhancement when w' co (as is true for farinfrared emission and CO2 laser pumping) because the cross-section is proportional also to '. When '~cos the two-photon process can be employed for frequency conversion without the resonance condition. It then amounts to parametric frequency conversion in a non-linear medium. It then amounts to parametric frequency conversion in a non-linear medium. For systems operating under non-resonance conditions see references (6, 7).

The present invention is a type of cavity which is particularly suitable for frequency conversion (via either the stimulated Raman effect or parametric frequency conversion) using intracavity pumping. Essentially, it achieves a very high value of l'l and automatically ensures that fi )Jk (S') is sufficiently large to avoid reabsorption by molecules in the final state k. An example of the cavity is shown in figure 2. Here the electrodes E provide a transverse discharge to excite CO2 gas at high pressure, resulting in laser action at some line near 10.6 ym. The cavity consists of a beam expander section (say mirrors M1, M2, followed by a beam reducer section (say mirrors M3, M4). The expansion and reduction is accompanied by considerable amplification.Holes in the centre of M3 and M2 feed the high intensity reduced beam (or a fraction of it) back to the expander section for reamplification. The medium to be pumped M is placed in the feed-back channel (see figure 2). It is exposed to a particularly high pump intensity for three reasons:: (i) since M1, M2, M3 and M4 are totally reflecting at the pump frequency w, the pump beam is completely trapped and builds up in intensity until the desired non-linear process within M becomes the dominant loss mechanism. (ii) reduction of the beam, after expansion as in an unstable resonator, greatly intensifies the beam. (iii) Since the medium M essentially is a bleachable filter within the CO2 gain medium, it will tend to mode-lock the pump radiation, and the factor by which the duration of the pulse is decreased is roughly the factor by which its intensity is increased. Due to the above three effects extremely high intensities of the pump beam within the medium M may be expected. For experiments concerning intracavity pumping see references (8, 9).

The system depicted in figure 2 is but an example. The principle is to have a beam expander section followed by a beam reducer section with feed back of the reduced beam through the medium to be pumped to the axis of the expander section. The optics of the expander and the reducer and their distance apart have not been specified, and will depend on the application. For example M, and M2 might have a common focus at F,2 whilst M4 and M3 have a common focus at F34. The distance F,2 F34 remains to be determined by the application. The magnification of the beam expander and demagnification of the beam reducer also is a free parameter.How the radiation generated within the nonlinear medium M is coupled out also will depend on circumstances; in figure 2 it is assumed that the mirror M4 is partially transmitting at frequency eft)' whilst totally reflecting at frequency a). The principle also can be executed by means of lenses.

The cavity involves two media, the medium M in which the non-linear process occurs and a medium which provides the gain for the pump beam. The process of conversion has been described when both media are gaseous and windows w (figure 2) are required to separate them. But in principle either medium may be gaseous, liquid or solid.

For tunability one may replace mirror M, by a rotatable grating.

References 1. H. R. Fetterman. H. R. Schlossberg and J.

Waldman, Submillimeter lasers optically pumped off-resonance, Optics Communications 6 (1872)156.

2. T. Y. Chang and J. D. McGee, Off-resonant infrared laser action in NH3 and C2H4 without population inversion, Applied Physics Letters 29 (1976) 725.

3. S. J. Petuchowski, A. T. Rosenberger and T. A. DeTemple, Stimulated Raman emission in infrared excited gases, IEEE Journal of Quantum Electronics, Q-E13 (1977)476.

4. J. D. Wiggins, Z. Drozdowicz and R. J.

Temkin, Two-photon transitions in optically pumped submillimeter lasers, IEEE Journal of Quantum Electronics Q E14 (1978)23.

5. A. De Martino, R. Frey, F. Pradere and J.

Ducuing, Tunable far-infrared Raman generation, Infrared Physics 18 (1978) 551.

6. A. De Martino, R. Frey, F. Pradere and J.

Ducuong, Powerful tunable infrared and far-infrared Ram an sources, Infrared Physics 19(1979)247.

7. A. De Martino, R. Frey and F. Pradere, Near- to far-infrared tunable Raman laser, IEEE Journal of Quantum Electronics Q E16(1980) 1184.

8. G. A. Koepf, CW operation of an intra cavity pumped molecular submillimeter wave laser, IEEE Journal of Quantum ElectronicsQ-E13 (1977) 732.

9. H. Hirose and S. Kon, Intracavity pumped far-infrared lasers by TE CO2 laser, Japanese Journal of Applied Physics 19 (1980)1131.

Descriptive text relating to drawings Figure 1: Energy level diagram showing (a) the singlephoton two-transition excitation, and (b) the twophoton single-transition excitation. Case (b), stimulated Raman scattering or parametric frequency conversion, is shown under near resonance conditions, the pump frequency (cos) differing from ,j by S, and the emitted frequency (s9') differing from Cl)sk by suss Figure 2: The medium m is within the cavity of the pumping laser, shown as a transversely excited gas discharge system with electrodes E (but the pumping laser may be of any type, with gas, liquid or solid gain medium).The pumped medium m is positioned between beam expander and beam reducer sections of the cavity, one being formed by totally reflecting mirrors M1, M2 and the other by totally reflecting mirrors M3, M4. All mirrors are totally reflecting at the pump frequency co, but M4 is partially transmitted for the radiation of frequency c,)' generated by a non-linear process within m. m is contained by transmitting windows W when a gas or liquid. The pump beam, after reduction, is fed back through m to the axis of the expander section for recycling and further amplification.

Claims (Filed on 26/1/83) 1. An optical cavity comprising a beam expansion section and a beam reduction section, the beam expansion section containing an amplifying medium and the beam reduction section containing a medium in which it is desired to achieve a non-linear process with acceptable efficiency. In the expansion section amplification by stimulated emission draws energy from an increasing volume of medium and outweighs the reduction of intensity that would result simply from increased cross-sectional area. In the reduction section the amplified expanded beam (which essentially is what would emerge from a laser with an unstable resonator) recontracts, with increase of intensity due to decreasing crosssectional area. This very high intensity beam of small cross-sectional area then enters the medium where the non-linear effect is sought.

The non-linear effect, for example frequency conversion, may attenuate the original beam.

After emerging from the non-linear medium the original beam can be reamplified in the expander section.

2. An optical cavity, a special case of that described in claim 1 above, in which the expansion of the beam and the reduction of the beam use the same optical system with opposite directions of propagation. Such a system is realized when the output from an unstable resonator (or a portion thereof) is reflected back into the optical system which constitutes the unstable resonator.

3. An optical cavity of the type described in claims 1 and 2 above in which the non-linear

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. and the reducer and their distance apart have not been specified, and will depend on the application. For example M, and M2 might have a common focus at F,2 whilst M4 and M3 have a common focus at F34. The distance F,2 F34 remains to be determined by the application. The magnification of the beam expander and demagnification of the beam reducer also is a free parameter. How the radiation generated within the nonlinear medium M is coupled out also will depend on circumstances; in figure 2 it is assumed that the mirror M4 is partially transmitting at frequency eft)' whilst totally reflecting at frequency a). The principle also can be executed by means of lenses. The cavity involves two media, the medium M in which the non-linear process occurs and a medium which provides the gain for the pump beam. The process of conversion has been described when both media are gaseous and windows w (figure 2) are required to separate them. But in principle either medium may be gaseous, liquid or solid. For tunability one may replace mirror M, by a rotatable grating. References 1. H. R. Fetterman. H. R. Schlossberg and J. Waldman, Submillimeter lasers optically pumped off-resonance, Optics Communications 6 (1872)156. 2. T. Y. Chang and J. D. McGee, Off-resonant infrared laser action in NH3 and C2H4 without population inversion, Applied Physics Letters 29 (1976) 725. 3. S. J. Petuchowski, A. T. Rosenberger and T. A. DeTemple, Stimulated Raman emission in infrared excited gases, IEEE Journal of Quantum Electronics, Q-E13 (1977)476. 4. J. D. Wiggins, Z. Drozdowicz and R. J. Temkin, Two-photon transitions in optically pumped submillimeter lasers, IEEE Journal of Quantum Electronics Q E14 (1978)23. 5. A. De Martino, R. Frey, F. Pradere and J. Ducuing, Tunable far-infrared Raman generation, Infrared Physics 18 (1978) 551. 6. A. De Martino, R. Frey, F. Pradere and J. Ducuong, Powerful tunable infrared and far-infrared Ram an sources, Infrared Physics 19(1979)247. 7. A. De Martino, R. Frey and F. Pradere, Near- to far-infrared tunable Raman laser, IEEE Journal of Quantum Electronics Q E16(1980) 1184. 8. G. A. Koepf, CW operation of an intra cavity pumped molecular submillimeter wave laser, IEEE Journal of Quantum ElectronicsQ-E13 (1977) 732. 9. H. Hirose and S. Kon, Intracavity pumped far-infrared lasers by TE CO2 laser, Japanese Journal of Applied Physics 19 (1980)1131. Descriptive text relating to drawings Figure 1: Energy level diagram showing (a) the singlephoton two-transition excitation, and (b) the twophoton single-transition excitation. Case (b), stimulated Raman scattering or parametric frequency conversion, is shown under near resonance conditions, the pump frequency (cos) differing from ,j by S, and the emitted frequency (s9') differing from Cl)sk by suss Figure 2: The medium m is within the cavity of the pumping laser, shown as a transversely excited gas discharge system with electrodes E (but the pumping laser may be of any type, with gas, liquid or solid gain medium).The pumped medium m is positioned between beam expander and beam reducer sections of the cavity, one being formed by totally reflecting mirrors M1, M2 and the other by totally reflecting mirrors M3, M4. All mirrors are totally reflecting at the pump frequency co, but M4 is partially transmitted for the radiation of frequency c,)' generated by a non-linear process within m. m is contained by transmitting windows W when a gas or liquid. The pump beam, after reduction, is fed back through m to the axis of the expander section for recycling and further amplification. Claims (Filed on 26/1/83)

1. An optical cavity comprising a beam expansion section and a beam reduction section, the beam expansion section containing an amplifying medium and the beam reduction section containing a medium in which it is desired to achieve a non-linear process with acceptable efficiency. In the expansion section amplification by stimulated emission draws energy from an increasing volume of medium and outweighs the reduction of intensity that would result simply from increased cross-sectional area. In the reduction section the amplified expanded beam (which essentially is what would emerge from a laser with an unstable resonator) recontracts, with increase of intensity due to decreasing crosssectional area. This very high intensity beam of small cross-sectional area then enters the medium where the non-linear effect is sought.

medium is transferred from intracavity location to extracavity location. For example, after the amplified expanded beam is reduced it might pass through a small central hole in one of the mirrors comprising the cavity and thence enter the nonlinear medium.

4. The claim embraces any optical system by means of which is put into practice the principle of beam expansion in an amplifying medium followed by beam reduction followed by passage through a non-linear medium, feedback being ensured by enclosing the whole in an optical cavity. Figures 2, 3 and 4 depict three possible optical systems as examples.

4. Several realizations of the optical cavity described in claims 1,2 and 3 above are envisaged, one being mentioned in the description and depicted in figure 2.

5. Several types of non-linear process are envisaged. As an example, frequency conversion by stimulated Raman scattering was mentioned in the description, and in particular conversion of the 10.6 ,um radiation from CO2 lasers into farinfrared radiation emitted when the laser beam passes through gases such as CH3F. Generation of high order harmonics of the laser beam would be another application.

New claims or amendments to claims filed on 13 April 1983 Superseded claims 1-5 New or amended claims:

1. An optical cavity which comprises a beam expansion section and a beam reduction section to be used in succession, and within which is contained (i) an amplifying medium through which the radiation beam passes during expansion, and (ii) a non-linear medium through which the radiation beam passes after reduction.

In completing a round trip of the cavity, the radiation beam first is expanded whilst being amplified (as in a laser employing an unstable resonator), then the radiation beam is reduced (with or without further amplification by stimulated emission), then the reduced beam (having been intensified in both the expander and reducer sections) is passed through the non-linear medium, and finally the beam (or a portion of it) is returned to the first stage for repeat of the cycle.

The term 'non-linear medium' referes to any medium in which it is desired to effect a nonlinear process, such as double or multiple-photon transitions. The beam may be returned to the initial stage directly as in the example depicted in figure 2, via a second transit through the nonlinear medium as in the example shown in figure 3, or via a complete reverse pass through the optical system as in the example shown in figure 4.

2. As a special case of the optical cavity described in claim 1, an optical cavity in which the expansion and reduction sections are one and the same, an example being shown in figure 3.

3. As a special case of the cavity described in claim 1, an optical cavity in which the amplifying medium and the non-linear medium are one and the same.