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Output-power and operation period of the CuII-laser in spherical and circularly slotted hollow cathodes
Volume 61, number 1
OPTICS COMMUNICATIONS
1 January 1987
OUTPUT-POWER AND OPERATION PERIOD OF THE CuII-LASER IN S P H E R I C A L A N D C I R C U L A R L Y S L O T T E D H O L L O W C A T H O D E S
H.J. EICHLER, J. H A M I S C H and S. S O N G Technische Unirersitat Berlin, Optisches lnstitut, 1000 Berlin 12, Fed. Rep. Germany
Received 21 July 1986
Two different types of copper hollow cathodes were tested for their geometric stability and usability for laser operation at 780 nm. Slotted hollow cathodes with a circular cross-section (4.2 mm diameter) yielded higher IR-laser power (4.5 mW cw for l0 cm active length) and could be operated for more than 45 hours with only 30% drop of the output power. Spherical hollow cathodes with 7.5 mm diameter resulted in lower power and operation period.
length of construction
1. Introduction The copper ion laser with interesting emission in the IR at 780 nm and in the UV at 260 nm was invented in 1974 [ 1 ]. Since then a lot o f work has been done to optimize the laser output, see e.g. refs. [ 2 ] and [ 3 ]. Usually the active medium is produced by sputtering copper from the cathode walls in a hollow cathode discharge. Rare gases (He for IR-emission and Ne for UV-emission) are used as buffer gases to maintain the gas discharge and to excite the copper ions by charge transfer collisions [ 4,5 ]. Typical hollow cathode geometries are cylindrical hollow cathodes for longitudinal discharges [6 ] and slotted hollow cathodes with rectangular cross-section for transversal discharges [ 7,8 ]. Although hollow cathode lasers are advantageous because of their simple construction, there are some problems resulting from different spatial distributions of cathode erosion by sputtering and o f copper condensation on the cathode and other parts of the discharge tube. This imbalance changes the cathode geometry [ 7,9 ] resulting in changing discharge characteristics and decreasing laser power. This problem can be solved in two different ways. Either one has to construct a discharge tube which allows the simple exchange of exhausted cathodes, or one has to prevent the cathode from changing its geometry. The latter would be the better solution, especially if the cathode stabilizes its geometry itself. I.e. one has to 0 030-401/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
active length Fig. 1. Spherical hollow cathode [9]. The sphere diameter is 7.5 mm and the aperture diameter is 1.5 ram. select a cathode geometry exhibiting an equilibrium between copper sputtering and copper deposition. Previous experiments by Koch [ 9 ] pointed out that a hollow sphere with two apertures has such a stable geometry. After operating a single cathode of this type (fig. 1 ) for 800 hours with dc current, there was no remarkable change in geometry. The two apertures make the cathode suitable for application in a laser by arranging several cathodes along the optical axis. Another solution to obtain a fairly stable geometry could be a slotted hollow cathode with a cross-section similar to that of the spherical cathode (fig. 2) as had been used by Schuebel for a He-laser [ 10].
2. Experimental To compare the laser power and the operation period o f the cathodes shown in fig. 1 and fig. 2 we 61
Volume 61, number 1
OPTICS COMMUNICATIONS
I January 1987
Jl Fig. 2. Slotted hollow cathode with circular cross-section (view in the direction of the optical axis). The bore diameter is 4.2 mm and the slot width is 0.85 mm in our experiment. built a discharge tube containing 6 cathode segments each 1.6 cm in length. The principle is shown in fig. 3, constructive details are given in fig. 4. The cathode segments with different geometry are inserted into an Al-tube, which has anode potential. Thus the test of the different cathode types is possible u n d e r similar conditions. The segments can be removed simply for exchange or inspection a n d then inserted reproducibly on the two adjusting pins, no readjusting of the discharge tube or of the resonator is necessary after that. The total active length is only sufficient for IR-laser action. With regard to future applications in an UV-laser, which requires more cathodes, we constructed the tube avoiding hydro.:arbons, i.e. adhesives and plastic seals. This would :~e advantageous for the U V - m i r r o r lifetime [9]. ,ikewise with regard to an UV-laser application we chose the d i m e n s i o n s of the cathodes as indicated in fig. 1 and fig. 2: According to ref. [ 11 ] the o p t i m u m bore diameter in a cathode similar to fig. 2 is 6 m m in the IR and 4 m m in the UV. We chose 4.2 m m diameter, which is approximately the mean diameter of the spherical cathode, thus the slotted cathode and the spherical cathode (having the same d i m e n s i o n s
i?ii iiiii
c~
®
~-16mrn~ \l d
Fig. 4. Designof the cathode inset mounted in the discharge tube. The figure shows a spherical cathode inserted, a, aluminium tube; b, anode flange; c, anode case; d, cathode (the hollow sphere is pressed together from 2 parts); e, adjusting pin', f, O-ring or aluminium seal; g, polished alumina ring and copper edges (insulation and seal): h, alumina ring (insulation); i, nut: k, bore for water cooling; 1, hollow cathode discharge: m, obstructed discharge. as in ref. [9 ]) have approximately the same surface a n d the same volume. Therefore the spherical cathodes are much smaller in our experiments than in the
¢)
db
®
®
m-
¢)
(D
Fig. 3. Discharge tube containing separately exchangeablecathode segments (principle). The figure shows a set of 6 spherical cathodes inserted. 62
Volume 61, number 1
OPTICS COMMUNICATIONS
experiment of Van der Hoeven [ 12 ], who demonstrated IR-laser action in 5 hollow spheres, each having a 12 m m sphere diameter and a 5 m m aperture. Both types of cathodes have the same length of construction (16 m m ) . But the active length of the spherical cathode is only half the slotted cathode active length, because no hollow cathode discharge operates in the two cones next to the sphere. The cathode inset shown in fig. 4 consists of the anode flange (sealed by an O-ring or Al-seal), the anode case and the cathode. While Koch [9] manufactured the spherical cathode by sputtering an initiaUy cylindrical cathode for 200 hours in a dc current discharge, we mounted the spherical cathodes from two half spheres. The anode flange and the cathode are pressed together by a central nut and sealed by copper edges on a polished alumina ring. The anode case is pressed onto the anode flange. It is exchangeable for the purpose of testing different anode types and of cleaning the cap between the cathode and the anode case. The gap protects the alumina insulation from copper deposition by means of an obstructed discharge. The cathode is water cooled. The water inlet is a tube inside the bore, which is screwed onto the cathode together with a water connection (not shown in fig. 4). The power supply generates regulated current pulses with 15 ms width. Also regulated dc current is possible. The current of each cathode is individually stabilized. The maximum voltage is 500 V. The resonator consists of two internal mirrors each having a transmission of 0.2% at 780.8 nm. The biggest diameter of the TEMoo-mode amounts to 0.7 m m at a cathode aperture.
3. Results First the stable cathodes proposed by Koch [9] were tested. Lasing is not possible in 6 hollow spheres having dimensions as given in fig. 1 (7.5 m m diameter, 1.5 m m aperture, 4.8 cm active length). Maximum peak current was 25 A corresponding to a current density of 2.4 A/cm 2. Further we tested spherical cathodes with the same sphere diameter of 7.5 m m but with bigger apertures of 2 mm, 2.8 m m and 3.2 mm. Only the latter resulted in lasing as shown in fig. 5. At a current density of 3 AJcm 2 the
s
1 January 1987
e [mW]
4
/
~
6 stotfed ¢clfhodes
96mm clctive [encjth
3 st0ffed cofhodes 48 mm Qctive Length 6 spher]cclt cQthodes _4n...
2
~.jJ
/ 1
-~
• -
o / = z,8 mm ~ctive length
./"
/ ~
t~
I
S J [A&oth.] 3 J[A/cmq
Fig. 5. IR laser power of spherical hollow cathodes ( 3.2 mm aperture) and of slotted hollow cathodes of the same active length respectively of the same length of construction versus current density. He pressure is 27 mbar, Ne pressure is 0.2 mbar, pulse width is 15 ms, repetition frequency is 8 Hz.
maximum output power is 2.2 mW. This power is nearly independent of the He pressure (15-27 mbar) and the Ne pressure (0-6 mbar). The output power of the slotted cathodes is shown in fig. 5. For comparison, measurements were made for the same length of construction and for the same active length. In both cases the power is higher (4.5 mW saturation power for about I 0 cm active length) than in the hollow spheres. The voltage in the slotted discharge is much higher, thus the maximum obtainable current density is limited by the power supply. The threshold current density is lower, so that power saturation can be obtained with 6 cathodes again. Low current densities are very favourable because of less arcing tendency. Therefore cw lasing is impossible with the spherical cathodes whereas there is no problem with the slotted cathodes yielding the same power as shown in fig. 5 for a pulsed discharge. In addition to the comparison of the laser power we tested the geometrical stability of the two cathode types. Fig. 6 shows the apertures of the spherical cathodes in dependence on the time in a pulsed discharge. In the beginning the diameters of the apertures on both sides of each cathode diminished in the same way. After 9 hours, corresponding to about 1 hour dc current, the apertures are to small for lasing. In the following slight asymmetries built up more and 63
Volume 61, number 1
OPTICS COMMUNICATIONS
1 ,lanuar~ 1987
??2n
+ Fig. 8. Slotted hollow cathode with an anode blade inserted.
i
i
i
i
10
20
30
40
~ I2
50 t ~ ] r
t[h] Fig. 6. Two different apertures in spherical hollow cathodes versus time in a pulsed discharge (average from 6 cathodes). more leading to a blocking o f each cathode o n one side. The apertures on the other side decreased m o r e slowly and then grew again. In a d d i t i o n fig. 6 shows the average d i a m e t e r d from the apertures o f both sides o f the cathodes which approaches the original d i a m e t e r o f 1.5 mm. The slotted cathodes were tested 45 hours with dc
current. During this time the bore d i a m e t e r diminished slightly from 4.2 to 4.1 mm. The slot width was enlarged rapidly, becoming bigger than the bore d i a m e t e r in the center o f the cathodes. Fig. 7 shows the laser power o f 6 slotted cathodes in the original state and after 45 hours. The voltage is growing with growing slot width and threshold current density is diminishing. Thus with the d e f o r m e d cathode 3.3 m W saturation laser power can be obtained, which is 30% less than at the beginning. Recent experiments show that the distortion o f the slotted hollow cathode can be influenced by the anode geometry. The enlargement o f the slot width can be stopped by inserting an anode blade into the slot as shown in fig. 8.
4. Conclusions
@[mW]
The spherical hollow cathode with 7.5 m m d i a m eter, which shows very, good geometrical stability as a single cathode [9], is not so stable within the arrangement o f several cathodes. The geometry' o f the slotted hollow cathode with circular cross-section is unstable too, but this hardly affects the laser power. M o r e o v e r it seems that the stability o f the slotted cathode can be i m p r o v e d by a p r o p e r choice o f the anode geometry. The higher laser power o f the slotted cathodes is o b t a i n e d in pulsed (15 ms) and in cw operation due to the low threshold current densities.
center edge
~hours dE ischar
f/Kgm.IAl O,B
{5
Fig. 7. IR laser power of 6 slotted hollow cathodes showing different shapes at the beginning and after 45 hours in a dc discharge. 64
References
2 [ 1] L. Csillag, M. Jzinossy,K. R6zsa and T. Salamon, Phys. Lett. A 50 (1974) 13
Volume 6 l, number 1
OPTICS COMMUNICATIONS
[2] D.C. Gerstenberger, R. Solanki and G.J. Collins, IEEE J. Quantum Electron. QE-16 (1980) 820 [3] H.J. Eichler, H. Koch, R. Molt, J.L. Qiu and W. Martin, Appl. Phys. B 26 (1981) 49 [4] B.E. Warner, K.B. Persson and G.J, Collins, J. Appl. Phys. 50 (1979) 5694 [5] H. Koch and H.J. Eichler, J. Appl. Phys. 54 (1983) 4939 [6] H.J. Eichler, H. Koch, J. Salk and Ch. Skrobol, Optics Comm. 34 (1980) 228
1 January 1987
[7] B. Auschwitz, H.J. Eichler and W. Wittwer, IEEE J. Quantum Electron. QE-17 (1981) 546 [8] M. Yang, Appl. Phys. B 32 (1983) 127 [9] H.J. Eichler, H. Koch and R. Tornow, Proc. 6th Int. Congr. Laser 83 (1984) 14 [ 10] W.K. Schuebel, Appl. Phys. Lett. 30 (1977) 516 [ 11 ] R. Solanki, W.M. Fairbank and G.J. Collins, IEEE J. Quantum Electron. QE-16 (1980) 1292 [ 12] C.J. Van der Hoeven and E.G. Jones, Optics Comm. 52 (1984) 292.
65
OPTICS COMMUNICATIONS
1 January 1987
OUTPUT-POWER AND OPERATION PERIOD OF THE CuII-LASER IN S P H E R I C A L A N D C I R C U L A R L Y S L O T T E D H O L L O W C A T H O D E S
H.J. EICHLER, J. H A M I S C H and S. S O N G Technische Unirersitat Berlin, Optisches lnstitut, 1000 Berlin 12, Fed. Rep. Germany
Received 21 July 1986
Two different types of copper hollow cathodes were tested for their geometric stability and usability for laser operation at 780 nm. Slotted hollow cathodes with a circular cross-section (4.2 mm diameter) yielded higher IR-laser power (4.5 mW cw for l0 cm active length) and could be operated for more than 45 hours with only 30% drop of the output power. Spherical hollow cathodes with 7.5 mm diameter resulted in lower power and operation period.
length of construction
1. Introduction The copper ion laser with interesting emission in the IR at 780 nm and in the UV at 260 nm was invented in 1974 [ 1 ]. Since then a lot o f work has been done to optimize the laser output, see e.g. refs. [ 2 ] and [ 3 ]. Usually the active medium is produced by sputtering copper from the cathode walls in a hollow cathode discharge. Rare gases (He for IR-emission and Ne for UV-emission) are used as buffer gases to maintain the gas discharge and to excite the copper ions by charge transfer collisions [ 4,5 ]. Typical hollow cathode geometries are cylindrical hollow cathodes for longitudinal discharges [6 ] and slotted hollow cathodes with rectangular cross-section for transversal discharges [ 7,8 ]. Although hollow cathode lasers are advantageous because of their simple construction, there are some problems resulting from different spatial distributions of cathode erosion by sputtering and o f copper condensation on the cathode and other parts of the discharge tube. This imbalance changes the cathode geometry [ 7,9 ] resulting in changing discharge characteristics and decreasing laser power. This problem can be solved in two different ways. Either one has to construct a discharge tube which allows the simple exchange of exhausted cathodes, or one has to prevent the cathode from changing its geometry. The latter would be the better solution, especially if the cathode stabilizes its geometry itself. I.e. one has to 0 030-401/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
active length Fig. 1. Spherical hollow cathode [9]. The sphere diameter is 7.5 mm and the aperture diameter is 1.5 ram. select a cathode geometry exhibiting an equilibrium between copper sputtering and copper deposition. Previous experiments by Koch [ 9 ] pointed out that a hollow sphere with two apertures has such a stable geometry. After operating a single cathode of this type (fig. 1 ) for 800 hours with dc current, there was no remarkable change in geometry. The two apertures make the cathode suitable for application in a laser by arranging several cathodes along the optical axis. Another solution to obtain a fairly stable geometry could be a slotted hollow cathode with a cross-section similar to that of the spherical cathode (fig. 2) as had been used by Schuebel for a He-laser [ 10].
2. Experimental To compare the laser power and the operation period o f the cathodes shown in fig. 1 and fig. 2 we 61
Volume 61, number 1
OPTICS COMMUNICATIONS
I January 1987
Jl Fig. 2. Slotted hollow cathode with circular cross-section (view in the direction of the optical axis). The bore diameter is 4.2 mm and the slot width is 0.85 mm in our experiment. built a discharge tube containing 6 cathode segments each 1.6 cm in length. The principle is shown in fig. 3, constructive details are given in fig. 4. The cathode segments with different geometry are inserted into an Al-tube, which has anode potential. Thus the test of the different cathode types is possible u n d e r similar conditions. The segments can be removed simply for exchange or inspection a n d then inserted reproducibly on the two adjusting pins, no readjusting of the discharge tube or of the resonator is necessary after that. The total active length is only sufficient for IR-laser action. With regard to future applications in an UV-laser, which requires more cathodes, we constructed the tube avoiding hydro.:arbons, i.e. adhesives and plastic seals. This would :~e advantageous for the U V - m i r r o r lifetime [9]. ,ikewise with regard to an UV-laser application we chose the d i m e n s i o n s of the cathodes as indicated in fig. 1 and fig. 2: According to ref. [ 11 ] the o p t i m u m bore diameter in a cathode similar to fig. 2 is 6 m m in the IR and 4 m m in the UV. We chose 4.2 m m diameter, which is approximately the mean diameter of the spherical cathode, thus the slotted cathode and the spherical cathode (having the same d i m e n s i o n s
i?ii iiiii
c~
®
~-16mrn~ \l d
Fig. 4. Designof the cathode inset mounted in the discharge tube. The figure shows a spherical cathode inserted, a, aluminium tube; b, anode flange; c, anode case; d, cathode (the hollow sphere is pressed together from 2 parts); e, adjusting pin', f, O-ring or aluminium seal; g, polished alumina ring and copper edges (insulation and seal): h, alumina ring (insulation); i, nut: k, bore for water cooling; 1, hollow cathode discharge: m, obstructed discharge. as in ref. [9 ]) have approximately the same surface a n d the same volume. Therefore the spherical cathodes are much smaller in our experiments than in the
¢)
db
®
®
m-
¢)
(D
Fig. 3. Discharge tube containing separately exchangeablecathode segments (principle). The figure shows a set of 6 spherical cathodes inserted. 62
Volume 61, number 1
OPTICS COMMUNICATIONS
experiment of Van der Hoeven [ 12 ], who demonstrated IR-laser action in 5 hollow spheres, each having a 12 m m sphere diameter and a 5 m m aperture. Both types of cathodes have the same length of construction (16 m m ) . But the active length of the spherical cathode is only half the slotted cathode active length, because no hollow cathode discharge operates in the two cones next to the sphere. The cathode inset shown in fig. 4 consists of the anode flange (sealed by an O-ring or Al-seal), the anode case and the cathode. While Koch [9] manufactured the spherical cathode by sputtering an initiaUy cylindrical cathode for 200 hours in a dc current discharge, we mounted the spherical cathodes from two half spheres. The anode flange and the cathode are pressed together by a central nut and sealed by copper edges on a polished alumina ring. The anode case is pressed onto the anode flange. It is exchangeable for the purpose of testing different anode types and of cleaning the cap between the cathode and the anode case. The gap protects the alumina insulation from copper deposition by means of an obstructed discharge. The cathode is water cooled. The water inlet is a tube inside the bore, which is screwed onto the cathode together with a water connection (not shown in fig. 4). The power supply generates regulated current pulses with 15 ms width. Also regulated dc current is possible. The current of each cathode is individually stabilized. The maximum voltage is 500 V. The resonator consists of two internal mirrors each having a transmission of 0.2% at 780.8 nm. The biggest diameter of the TEMoo-mode amounts to 0.7 m m at a cathode aperture.
3. Results First the stable cathodes proposed by Koch [9] were tested. Lasing is not possible in 6 hollow spheres having dimensions as given in fig. 1 (7.5 m m diameter, 1.5 m m aperture, 4.8 cm active length). Maximum peak current was 25 A corresponding to a current density of 2.4 A/cm 2. Further we tested spherical cathodes with the same sphere diameter of 7.5 m m but with bigger apertures of 2 mm, 2.8 m m and 3.2 mm. Only the latter resulted in lasing as shown in fig. 5. At a current density of 3 AJcm 2 the
s
1 January 1987
e [mW]
4
/
~
6 stotfed ¢clfhodes
96mm clctive [encjth
3 st0ffed cofhodes 48 mm Qctive Length 6 spher]cclt cQthodes _4n...
2
~.jJ
/ 1
-~
• -
o / = z,8 mm ~ctive length
./"
/ ~
t~
I
S J [A&oth.] 3 J[A/cmq
Fig. 5. IR laser power of spherical hollow cathodes ( 3.2 mm aperture) and of slotted hollow cathodes of the same active length respectively of the same length of construction versus current density. He pressure is 27 mbar, Ne pressure is 0.2 mbar, pulse width is 15 ms, repetition frequency is 8 Hz.
maximum output power is 2.2 mW. This power is nearly independent of the He pressure (15-27 mbar) and the Ne pressure (0-6 mbar). The output power of the slotted cathodes is shown in fig. 5. For comparison, measurements were made for the same length of construction and for the same active length. In both cases the power is higher (4.5 mW saturation power for about I 0 cm active length) than in the hollow spheres. The voltage in the slotted discharge is much higher, thus the maximum obtainable current density is limited by the power supply. The threshold current density is lower, so that power saturation can be obtained with 6 cathodes again. Low current densities are very favourable because of less arcing tendency. Therefore cw lasing is impossible with the spherical cathodes whereas there is no problem with the slotted cathodes yielding the same power as shown in fig. 5 for a pulsed discharge. In addition to the comparison of the laser power we tested the geometrical stability of the two cathode types. Fig. 6 shows the apertures of the spherical cathodes in dependence on the time in a pulsed discharge. In the beginning the diameters of the apertures on both sides of each cathode diminished in the same way. After 9 hours, corresponding to about 1 hour dc current, the apertures are to small for lasing. In the following slight asymmetries built up more and 63
Volume 61, number 1
OPTICS COMMUNICATIONS
1 ,lanuar~ 1987
??2n
+ Fig. 8. Slotted hollow cathode with an anode blade inserted.
i
i
i
i
10
20
30
40
~ I2
50 t ~ ] r
t[h] Fig. 6. Two different apertures in spherical hollow cathodes versus time in a pulsed discharge (average from 6 cathodes). more leading to a blocking o f each cathode o n one side. The apertures on the other side decreased m o r e slowly and then grew again. In a d d i t i o n fig. 6 shows the average d i a m e t e r d from the apertures o f both sides o f the cathodes which approaches the original d i a m e t e r o f 1.5 mm. The slotted cathodes were tested 45 hours with dc
current. During this time the bore d i a m e t e r diminished slightly from 4.2 to 4.1 mm. The slot width was enlarged rapidly, becoming bigger than the bore d i a m e t e r in the center o f the cathodes. Fig. 7 shows the laser power o f 6 slotted cathodes in the original state and after 45 hours. The voltage is growing with growing slot width and threshold current density is diminishing. Thus with the d e f o r m e d cathode 3.3 m W saturation laser power can be obtained, which is 30% less than at the beginning. Recent experiments show that the distortion o f the slotted hollow cathode can be influenced by the anode geometry. The enlargement o f the slot width can be stopped by inserting an anode blade into the slot as shown in fig. 8.
4. Conclusions
@[mW]
The spherical hollow cathode with 7.5 m m d i a m eter, which shows very, good geometrical stability as a single cathode [9], is not so stable within the arrangement o f several cathodes. The geometry' o f the slotted hollow cathode with circular cross-section is unstable too, but this hardly affects the laser power. M o r e o v e r it seems that the stability o f the slotted cathode can be i m p r o v e d by a p r o p e r choice o f the anode geometry. The higher laser power o f the slotted cathodes is o b t a i n e d in pulsed (15 ms) and in cw operation due to the low threshold current densities.
center edge
~hours dE ischar
f/Kgm.IAl O,B
{5
Fig. 7. IR laser power of 6 slotted hollow cathodes showing different shapes at the beginning and after 45 hours in a dc discharge. 64
References
2 [ 1] L. Csillag, M. Jzinossy,K. R6zsa and T. Salamon, Phys. Lett. A 50 (1974) 13
Volume 6 l, number 1
OPTICS COMMUNICATIONS
[2] D.C. Gerstenberger, R. Solanki and G.J. Collins, IEEE J. Quantum Electron. QE-16 (1980) 820 [3] H.J. Eichler, H. Koch, R. Molt, J.L. Qiu and W. Martin, Appl. Phys. B 26 (1981) 49 [4] B.E. Warner, K.B. Persson and G.J, Collins, J. Appl. Phys. 50 (1979) 5694 [5] H. Koch and H.J. Eichler, J. Appl. Phys. 54 (1983) 4939 [6] H.J. Eichler, H. Koch, J. Salk and Ch. Skrobol, Optics Comm. 34 (1980) 228
1 January 1987
[7] B. Auschwitz, H.J. Eichler and W. Wittwer, IEEE J. Quantum Electron. QE-17 (1981) 546 [8] M. Yang, Appl. Phys. B 32 (1983) 127 [9] H.J. Eichler, H. Koch and R. Tornow, Proc. 6th Int. Congr. Laser 83 (1984) 14 [ 10] W.K. Schuebel, Appl. Phys. Lett. 30 (1977) 516 [ 11 ] R. Solanki, W.M. Fairbank and G.J. Collins, IEEE J. Quantum Electron. QE-16 (1980) 1292 [ 12] C.J. Van der Hoeven and E.G. Jones, Optics Comm. 52 (1984) 292.
65