CA1168339A

CA1168339A - Technique for synchronization of raman scattered radiation

Info

Publication number: CA1168339A
Application number: CA000389511A
Authority: CA
Inventors: Paul Rabinowitz; Bruce N. Perry
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1980-12-15
Filing date: 1981-11-05
Publication date: 1984-05-29
Also published as: EP0054429B1; AU542920B2; AU7849081A; JPS57124242A; EP0054429A1; DE3174823D1; US4361770A

Abstract

ABSTRACT OF THE DISCLOSURE
As shown in Figure 1, a device (10) provides a pair of intense (16) micron pulses through exit port (11) by stimulated Raman scattering of CO2 laser radiation (provided by CO2 lasers (12) and (15)) in parahydrogen (13) maintained in chamber (14) between spherical mirrors (16) and (17). The CO2 laser radiation is passed through lenses (20)and (25), combined by beam combiner (30), passes through entrance port (18), reflects between mirrors (16) and (17) and passes out through hole (21) to exit port (11).
Lenses (20)and (25) mode match the CO2 laser radiation to the structure formed by mirrors (16)and (17).

Description

~ .

2 This invention relates to optical pulse genera-

3 tion devices and particularly to devices employing Raman

4 scattering to generate hlgh intensity optical pulses.

5 BAC~GROUND OF THE INVENTION

6 It is useful for a number of physical and chemical

7 processes to have available highly synchronized pulsed

8 coherent radiation of differing optical frequencies. One

9 is often faced with the requirement to excite a molecule

10 or an atom with a combination oE frequencies within a

11 short time period in order to effect transitions to a given

12 state of a molecular system, e.g., to achieve selectivity

13 in an isotope separation process or to prepare a system for

14 a chemical reaction. Althougll many processes can be excited

15 stepwise and sequentially in time, it is usually the case

16 that the most efficient stepwise excitation for radiatively

17 connected states occurs with both radiation fields

18 simultaneously applied. This is because in most cases

19 collisional, radiative or transit-time decay reduces the

20 population of the available species for the second step

21 of excitation. The ability to synchronize two or more

22 laser sources can be the most significant factor in deter-

23 mining the efficiency of such processes.

24 In general, it is difficult to obtain a high

25 degree of synchronization between short pulses of laser

26 light. For electrical'y excited gaseous discharge lasers

27 which use thyratron control of the discharge firing,

28 the discharges are initiated within a few nanoseconds.

29 ~Iowever, subsequent fluctuations in the buildup time of

30 the optical pulses, which depend upon the resonant con-

31 ditions, the electrical current, and the gas compositions

32 and densities, produce typical overall interpulse jitter

33 of from 10 to 50 nanoseconds. For generation of synchron-

34 ized pulses shorter than 50 ns in duration, this degree of

35 jitter would be unacceptable.

36 We now consider the generation of optical pulses

37 through stimulated Raman scattering where the relative '~'~''~

r3 1 jitter between the output pulses i5 directly attributable 2 to -the jitter in the optical pump pulses which initlate 3 the process. When a strong excitation pulse (Pump Pulse) 4 is applied to a medium which is Raman responsive, stimu-5 lated scattering of the pump pulse results in the growth 6 of radiation (Stokes pulsa) which is downshifted by the 7 Raman frequency of the medium and propagates in the pump 8 direction. (In general, a Stokes pulse may grow in the 9 opposite direction, but the physical structures that we 10 shall consider do not allow such a pulse to develop.) The 11 copropagating Stokes pulse is initiated by spontaneous Raman 12 scattering and grows at an exponential rate that is pro-13 portional to the intensity of the pump pulse until pump 14 deplation occurs or the medium has been traversed ter-15 minating the process. Energetically, for every quantum of 16 energy appearing at the Stokes frequency a quantum oE
17 energy is removed from the pump and the energy difference 18 defined by the Raman frequency is absorbed by the medium.
19 This energy appears as an additional nonpropagating coherent 20 excitation of the medium, known as the Raman wave, which 21 provides a coupling mechanism between the two propagating 22 electromagnetic waves. In general, within the limits 23 imposed by dispersion, the Stokes pulse will copropagate 24 with the depleted pump radiation and will become most 25 intense at a point near the initial peak of the pump 26 radiation. There is an intensity threshold for the 27 appearance of the Stokes pulse. The threshold first occurs 28 near the peak of the pump pulse. Usually, because of the 29 limitations in gain or the presence of competing scattering 30 processes, the width of the Stokes pulse is considerably 31 less than that of the pump and the photon conversion 32 efficiency is much less than 100%.
33 If a third electromagnetic wave at a frequency 34 greater than the Raman frequency is injected colinearly 35 with the pump wave when the Raman wave is present, a fourth 36 electromagnetic wave shifted by the Raman frequency will be 37 generated even if the intensity of the third wave is in 3 ~ 3~:~

itself sufficient to produce threshold gain for corlversion to Stokes radiation. ~he fourth wave is produced by scattering from the Raman wave already present and therefore does not require a high gain needed to build from noise to threshold.
This physical scatteriny process is commonly called four wave mixing. Four wave mixing occurs only where the Raman wave overlaps the new pump (third) wave both in time ar.d space within the limits of the transient response time of the medium. Therefore, the fourth wave (new Stokes wave) is completely overlapped (synchronized) with the Stokes wave generated in response to the pump wave. However, it can be seen that if the new pump (third) wave has a duration which is long compared with the first pump, the overall efficiency for the four wave mixing process will be small because only a small fraction of the total energy of the new pump will be converted. Additionally, in the presence of jitter, the two pump peaks may be displaced in time, further degrading the conversion efficiency.
For many useful Raman media, the stimulated Raman gain at practical pump laser intensities is insufficient for the Stokes radiation to reach threshold when starting from quantum noise in a single focused pass through the mediumO
It has been shown in U.S. Patent 4,245,171 which issued to ~essrs. Rabinowitz and Stein and which is entitled "Device for Producing High-powered Radiation Employing Stimulated Raman Scattering in an Off-Axis Path Between a Pair of Spherical Mirrors" and also in an article entitlçd "Efficient tunable H2 Raman Laser" by Messrs. Rabinowitz, Stein, Brickman and Kaldor, which appeared in Applied Ph~sics Letters, 35 (10), November 15, 1979 at page 739, that a multiple pass cell may be used to increase the cumulative gain so that threshold may be reached with pump intensities far below those required for a single pass device.

, ~ ~

3l.~ 3~

It was also found, as described in an article entitled "Controllable Pulse Compression in Multiple-Pass-Cell Raman Laser", by Messrs, Peery, srickman~ Stein, Treacy and Rabinowitz, which appeared in ~ptics Letters, Volume 5, No. 7, July 1980 at page 288, that in such a multiple pass cell, the energy in the Stokes pulse is compressed in time in com-parison with the pump pulse as a result of ray crossings associated with the geometry of the multiple pass cell. In such a device, provided the lengt:h of the pump pulse extends over several passes of the cell, a forward propaga-ting pump wave will intersect itself many times on each pass, and the intersection regions of the pump will be spaced at regular time intervals as indicated in Table I of the Perry et al article on page 289. Duriny the initial buildup of the Stokes radiation from noise, the beam crossings have only small effect on the Stokes growth and threshold is reached near the peak of the pump pulseafter a number of passes. The resulting Stokes pulse then interacts strongly at crossings with the intersecting pump radiation as allowed by the cell geometry. This interaction has two effects~ First, energy is extracted from regions of the pump that are spaced away from the region where threshold is first reached by the time intervals between crossings. This suppresses the growth of copropagating Stokes radiation in those regions. Second, as the Stokes wave continues to grow, the threshold region broadens until its width is limited by the pump depletion produced by the crossings. Thus, a single Stokes pulse develops and has a width determined by the time difference between the peak and the nearest crossing (approximately 8/3 passes of the multiple pass cell). This pulse extracts energy from regions of the pump which would not have reached threshold in a device lacking crossings, but having the same cumulative gain. For strong interactions to occur at the crossings, it is necessary that the gain due to the intersecting pump be high. Such gain is maximized for a 3,!~3 given intensity of the intersecting pump when the Raman gain is isotropic, and when the length of the crossing region is maximized.
In an article entitled "16 - ~m Genera-tion by CO2-Pumped Rotational Raman Scattering in H2" by Messrs.
Byer and Trutna, appearing in Optics Letters, Volume 3, ~o a October 1978 at page 144, an apparatus was described in which four wave mixing occurs in a multiple pass cell. A Nd:Yag laser is employed to generate a Raman wave in parahydrogen to initiate Stokes radiation from a CO2 laser pulse which is simultaneously passed therethrough. The Nd:Yag laser pulse is short in time compared to the single pass transit time of the multiple pass cell.
BRIEF DESCRIPTION OF T~IE INVENTION
In accordance with the present invention an effi-cient device for improving the degree of synchronization between optical pulses is achieved through the proper combina-tion of the physical phenomena discussed above, namely the stimulated Raman scattering, four wave mixing and pulse com-pression in a multiple pass cell.
The apparatus of this invention is responsive to a pair of at least partially overlapping optical pulses for generating a second pair of optical pulses. The apparatus includes a medium capable of producing stimulated scattering of radiation occupying a volume and reflectors for defining a multiple pass optical path through the volume which inter-sects itself. The apparatus also includes optics for direct-ing a first of said first pair of optical pulses along the optical path and optics for directing a second of said first pair of optical pulses along the optical path. Ea~h of the first pair of optical pulses have a full width at half maximum equal to or greater than the transit time of each of said optical pulses for one pass of the multiple pass optical path defining structure.

3¢.~

1 In the preferred embodiment of this invention 2 the full width at half maximum of each of -the first 3 pair of optical pulses is at least 8/3rds of the transit 4 time of the respective pulses for one pass of the multiple 5 pass optical path defining structure.

7 For a more complete understanding of the 8 invention reference should be made to the following 9 detailed description and drawings wherein:
Fig. 1 is a view partially in cutaway isome-11 tric form, partially i.n schernatic form,, showing 12 a device constructed in accordance with the 13 teachings of this invention.
14 Fig. 2, 3 and 4 are graphs of intensity of various pulses in various position within the 16 apparatus of Fig. 1 which demonstrate the opera-17 tion of the apparatus of this invention. In 18 Fig. 3 and 4 dashed lines are used to show the 19 shape of the original pulses as shown in Fig. 2 prior to depletion in accordance with this inven~
21 tion.
22 Fig. 5 and 6 are 3-axis graphs showing the 23 efficiency of the system of Fig. 1 as a function 24 of the time separation between input pulses and the relative intensity of one of the input pulses.
26 Fig. 7 is a 3-axis graphs showing the efficiency 27 of a prior art system for generating pairs of 2~ optical pulses.

Referring now to Fig. 1, we see a device 10 which 31 provides a pair of high-powered 16 micron radiation pulses 32 (Pl and P2 in Figs. 2, 3 and 4) through an exit port 11 by 33 stimulated Rarnan scattering of CO2 laser radiation (pro-34 vided by CO2 lasers 12 and 15) in parahydrogen 13 main-35 tained in a chamber 14 between a pair of spherical mirrors 36 16 and 17. The CO2 laser radiation from the CO2 lasers 37 12 and 15 is passed through lenses 20 and 25 and combined ti ~ .3 1 ky a 4eam combiner 30 (which could be a diffraction 2 gra-ting, dichroic beam splitter or prism). The combined 3 beam passes through an entrance port 13, a hole 19 in the 4 spherical mirror 16, reflects between the mirrors 16 and 17 5 a~ shown in Fig. 1, and passes out through a hole 21 in the 6 spherical mirror 17 to the exit port 11. The structure 7 defined by the mirrors 16 and 17 shall be referred to as 8 a multiple pass cell. The lenses 20 and 25 mode match the 9 radiation from the CO2 lasers 12 and 15 respectively -to 10 the multiple pass cell.
11 The spherical mirrors in the pxeferred embodi-12 ment of this invention have the same radius of curvature 13 and are mounted with their concave reflecting surfaces 14 facing each other, thereby defining an optical axis in-15 cluding their centers of curvature with the optical axis 16 so defined intersecting the concave reflecting surfaces of 17 the spherical mirrors 16 and 17 preferably through the 18 centers thereof. The center of curvature of the concave 19 reflecting surface of ;the spherical mirror 16 is located 20 between the concave reflecting surface of the spherical 21 mirror 17 and its center of curvature. In a like fashion, 22 the center of curvature of the concave reflecting surface 23 of the spherical mirror 17 is located between the concave 24 reflecting surface of the spherical mirror 16 and its center 25 of curvature.
26 Thus the path traveled by the combined radiation 27 through the multiple pass cell intersects itself repeatedly 28 prior to exiting at the exit port 11.
29 In the preferred embodiment of this invention, 30 the combined power provided by the CO2 lasers 12 and 15 is 31 40 megawatts in the TEM O mode. The pulse length of each 32 of the pulses is 100 nanoseconds. In order to employ these 33 pulses in the chambers 10 the pulses are circularly polarized 34 in the same sense by devices, not shown in the preferred 35 embodiment of this invention. The spacing between the mir-36 rors 16 and 17 is 373 centimeters.

" ~

1 As seen ln Fig. 1, the radiation from the CO2 2 laser 12 is directed through the hole 19 in the mirror 16 to 3 strike the mirror 17 at the point numbered 2. It should be 4 appreciated that the point 2 and the center of the hole 19 5 are equidistant from the optical axis. In the example shown, 6 the radiation passes between the mlrrors 16 and 17 twenty-one 7 times before exiting. The number of passes between the 8 mirrors 16 and 17 is determined by the radius of curvature 9 of the mirrors 16 and 17 and the spacing therebetween.
10 The distance between the mirrors is 373 centimeters; the 11 radius of curvature is 203 centimeters.
12 In operation, as pointed out above, the pulses 13 Pl and P2 from the CO2 lasers 12 and 15 are synchronized 14 as well as practical by prior art synchronization tech-15 ni~ues. Notwithstanding the above, jitter in the order of 16 magnitude of 50 nanoseconds is still present. Thus the 17 pulses Pl and P2 are separated in time but the leading edge 18 of the second pulse P2 overlaps the peak of the first pulse Threshold is reached first by Stokes wave Sl 21 (see Fig. 3), close to the peak of the pump Pl. The small 22 leading edge of P2 scatters by four wave mixing from the 23 Raman wave produced by the stimulated scattering of Sl, 24 and generates a small but macroscopic wave S2, which is 25 synchronized to Sl, see Fig. 3. Through the ray crossing 26 interactions spaced at regular intervals, each of the 27 Stokes waves Sl and S2 continues to grow by extracting 28 energy from its own pump wave, while retaining a high degree 29 of synchronization with the other, see Fig. 4. In this 30 case the centroids of the Stokes pulses Sl and S2 will be 31 closer than the centroids of the pump pulses Pl and P2. In 32 contrast, if Pl and P2 had been passed through separate 33 multiple pass cells or passed through the same multiple pass 34 cell at substantially different times, the temporal beha 35 viour of S2 would be determined by the intensity and 36 te~.poral c~a:^acte--istics of P2 only, ..~d slmilarly Sl would 37 depend only on Pl.

- 9 ~

1 We have performed a mathematical analysis of the 2 preferred embodiment of the invention. The results demon-3 strate the improved synchronization obtainable with the 4 invention. We define a merit factor, n, called the frac-tional synchronized energy which is twice the common energy 6 of the ~wo Stokes pulses divided by the total Stokes 7 energy at the device output. The common energy is 8 defined as the integral over tirne of the lesser of the two 9 Stokes intensities at each time. A plot of n as a func-10 tion of pump time separation ancl relative intensity of 11 the pump pulses is shown in Fig. 5 ancl 6 for a pair of 12 pump pulses processed in the apparatus of -the preferred 13 embodiment of the invention. This should be compared with 14 the value of n in Fig. 7 that is obtained when the same two 15 pump pulses are processed by two independent multiple pass 16 cells, with the cells as described in the aforementioned 17 article entitled "Efficient tunable H2 Raman Laser".
18 Notice, for example, that for two equally intense pulses 19 delayed by 46 ns, typical of jitter that one may expect 20 from spark gaps, the synchronized energy fraction with in-21 dependent multiple pass cells is 0.10 while with the inven-22 tion its value is 0.83, an improvement of more than eight 23 fold. Overall, for any value of relative intensities of 24 the pump pulses and/or any relative delay which maintains 25 a degree of pump overlap there is improvement in the amount 26 of synchronized energy with this invention.
27 It has been found, in general, that in order to 28 achieve high conversion efficiencies, the Raman gain should 29 be sufficiently high so that threshold is reached after 30 about half the number of passes of the multiple pass cell, 31 and the gain should bereasonably isotropic for a strong 32 interaction at the ray crossings. The conditions of tem-33 perature and pressure to achieve isotropic gain in various 34 Raman media is well known. In the preferred embodiment, 35 parahydrogen is the Raman medium, the temperature is 77K
36 and the pressure is 440 Torr. In the preferred embodiment, 37 the gain is about 18db per pass for the stronger Stokes 3 ~ts~

1 wave. Also, each pump wave must have a Eull width at half 2 maximum of at least the length of a single pass through the 3 multiple pass cell in order to achieve a practical, effi-4 cient device. In the preferred embodiment, the length of 5 each pump wave is at least 8/3 passes but less than the 6 total number of passes of the structure. If the pumps have 7 disparate lengths and one of the pump waves is shorter than 8 8/3 passes, then the photon conversion to synchronized 9 Stokes radiatioM cannot be complete. Fur-thermore, if a 10 pump pulse is shorter than one pass, the maximum realiza-11 ble synchronized conversion efficiency will be less than 12 37%.
13 Although this invention has been described with 14 respect to its preferred embodiments, it should be under~
15 stood that many variations and modifications will now be 16 obvious to those skilled in the art, and it is preferred, 17 therefore, that the scope of the invention be limited 18 not by the specific disclosure herein, but only by the 19 appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS.

1. Apparatus responsive to a first pair of at least partially overlapping optical pulses for generating a second pair of optical pulses, said apparatus including:
a medium capable of producing stimulated scat-tering of radiation occupying a volume;
means for defining a multipass optical path through said volume, said optical path intersecting itself;
means for directing a first of said first pair of optical pulses along said optical path, said first of said first pair of optical pulses having a full width at half maximum equal to or greater than the transit time of said first of said first pair of optical pulses for one pass of said multipass optical path defining means; and means for directing a second of said first pair of optical pulses along said optical path, said second of said first pair of optical pulses having a full width at half maximum equal to or greater than the transit time of said second of said first pair of optical pulses of one pass of said multipass optical path defining means thereby providing said second pair of optical pulses.

2. The apparatus as defined in Claim 1 in which said means for defining a multiple pass optical path includes:
a first spherical mirror having a first concave reflecting surface which is a section of a sphere having a first predetermined radius and a first center of curva-ture;
a second spherical mirror having a second concave reflecting surface which is a section of a sphere having a second predetermined radius and a second center of curva-ture; and means for mounting said first and second spherical mirrors with said first and second concave reflecting sur-faces facing each other, defining an optical axis including said first and second centers of curvature, said optical axis intersecting said first and second concave reflecting surfaces.

3. The apparatus as defined in Claim 2 in which said first center of curvature is located between said second center of curvature and said second concave reflecting surface; and said second center of curvature is located between said first center of curvature and said first concave reflecting surface.

4. The apparatus as defined in Claim 3 in which said medium capable of producing stimulated scattering is hydrogen and each of said first and second pair of optical pulses are generated by CO2 lasers.

5. The apparatus as defined in Claim 1 in which the first of said first pair of optical pulses has a full width at half maximum equal to or greater than 8/3rds the transit time of said first pair of optical pulses for one pass of said multipass optical path defining means and said second of said first pair of optical pulses has a full width at half maximum equal to or greater than 8/3rds of the transit time of said second of said first pair of optical pulses for one pass of said multiple pass optical path defining means.

6. The apparatus as defined in Claim 5 in which said means for defining a multiple pass optical path in-cludes:
a first spherical mirror having a first concave reflecting surface which is a section of a sphere having a first predetermined radius and a first center of curva-ture;
a second spherical mirror having a second concave reflecting surface which is a section of a sphere having a second predetermined radius and a second center of curvature; and means for mounting said first and second spheri-cal mirrors with said first and second concave reflecting surfaces facing each other, defining an optical axis in-cluding said first and second centers of curvature, said optical axis intersecting said first and second concave reflecting surfaces.

7. The apparatus as defined in Claim 6 in which said first center of curvature is located beween said second center of curvature and said second concave reflecting surface; and said second center of curvature is located between said first center of curvature and said first concave reflecting surface.

8. The apparatus as defined in Claim 7 in which said medium capable of producing stimulated scattering is hydrogen and each of said first and second pair of optical pulses are generated by CO2 lasers.