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Passive mode-locking of a large volume tea-CO2 laser using an unstable resonator configuration
Volume 14, number 2
OPTICS COMMUNICATIONS
June 1975
PASSIVE MODE-LOCKING OF A LARGE VOLUME TEA-CO2 LASER USING AN UNSTABLE RESONATOR CONFIGURATION P. LAVIGNE, J. GILBERT and J.L. LACHAMBRE Centre de Recherches pour la DPfense, Defence Research Establishment, Valcartier, Quebec, Canada Received 10 February 1975 An experimental investigation was made of the passive mode-locking of a uv-preionized CO2 laser operating in an unstable wavelength-selective resonator using SF6, BCl3 and N2 Fa as bleachable gases. Stable repetitive pulse trains with individual pulses having energies as high as 2 J and 2 ns duration have been obtained on a large number of P and R transitions of the 00” 1- lo”0 band at 10.6 Frn. Use of the selectively absorbing gas CzHsOH in the resonator has enabled the production of clean, mode-locked pulses by the saturation of N2 F4 without the need of an optical dispersive element.
1. Introduction
2. Experimental apparatus
Continued improvements in large aperture TEACO2 lasers have led to increased interest in their use for the generation of short powerful mode-locked pulses at 10.6 pm [l-5]. This communication describes the results of an experimental investigation of the passive mode-locking of an atmospheric pressure CO2 laser excited by uv preionization in an unstable line-selective resonator. In view of its inherent advantages of a large energy extraction volume, high radiance output beam and damage-free optics, the unstable resonator is extremely well suited for use with an active laser media of large cross-section. The three gases investigated (SF,, BC13 and N2F4) were found to be equally effective as intracavity absorbers. Wavelength selection by means of a diffraction grating has allowed the achievement of mode-locking on several rotational transitions in the P and R branches of CO;!. Regular trains of mode-locked pulses as short as 2 ns were obtained with good reproducibility at peak powers in excess of one gigawatt. This power is more than five times that previously reported with a mode-locked laser [3,4] of only 1-meter active length. NzF4 in particular was found to give rise to a clean mode:locked behaviour on line P(26) of the 00” l-l O”0 band in the absence of any selective element by inserting C2H,0H into the cavity. We believe this to be the first demonstration of passive mode-locking at 10.6 I.tm by the resonant saturation of N2F4.
The experimental arrangement used in this study is shown schematically in fig. 1. The active plasma region was 1 meter long with a rectangular cross-section of 5 X 5 cm2. It was provided with two individual sections of high-current discharge volume-stabilized by uv photoionization [6,7]. Each section comprises a solid anode, a wire-mesh cathode and a preionizing electrode formed by a two-dimensional array of discrete uv emitting sparks. The sections were mounted in tandem inside a rectangular plexiglass enclosure containing a flowing mixture of CO2-N2--He at atmospheric pressure with a relative gas composition normally near 20: 10: 70. In order to achieve a collimated output of high radiance, a positive confocal design was chosen for the unstable resonator with a long-radius concave mirror and a short-radius convex mirror spaced 5 m apart. The discharge enclosure was
194
G
P
C
M
TEA
Fig. 1. Diagram of experimental setup. G, spherical grating; P, annular coupler; C, absorber cell, M, convex totally reflecting mirror; D, detector.
Volume 14, number 2
OPTICS COMMUNICATIONS
June 1975
Table 1 Comparative mode-locking performances Laser
Absorber Gas
Cell thickness
Pressure
(mm)
(torr)
Resonator magnification
Mode-locking Lasing transition (10.4 rm)
Pulse width (ns)
power (GW)
Output
Stability
energy (J)
SF6
1
20-40
5.7
P(20)
5
0.6
12
fair
SF6: He (1:2)
1
75-100
5.7
P(20)
2
1.0
10
good
5.7 11 G3.5
P(20) PC201 P(20)
2.5 2.5
0.5 0.4
8 6
5.1
PC201
2
0.25
6
5.7
PC261
2.5
0.08
1
BC13 BC13 BC13
30-40 30-40 -
Nz F4
100
N2 F4
150
+
CzHsOH
10
good good unsuccessful good fair
26
sealed at one end by the concave mirror and at the other by a NaCl window tilted about 5” from the plane perpendicular to the resonator axis. The laser emission was coupled out sideways through a 45” annular coupler placed near the convex mirror. A spherical grating of curvature radius 11 m, suitably masked by a 4.5 cm diameter aperture served as the concave mirror so as to restrain the operation of the laser within the absorption spectrum of the saturable absorber gas. Three distinct resonators were available to perform the experiments with optical magnifications of 3.5, 5.7 and 11.5 corresponding to experimentally measured output couplings of 80%, 90% and 96%, respectively. These were obtained by means of convex mirrors of radius of curvature 3 12 cm, 193 cm and 96 cm and aperture holes in the annular coupling ring of 16 mm, 9.5 mm and 6.5 mm diameter. The discharge was fired by 120 kV pulses supplied from a 3-stage Marx-bank generator connected through a voltage doubling circuit having a total energy of 600 J. Experimental observations indicated that the small-signal gain was fairly uniform across the section of the discharge with a va!ue of 0.04 cm-l at the center. With the employed optics, the system was capable of delivering 15 J pulses with a beam divergence of 0.6 mr. The absorption cell containing the saturable gas was inserted in the space between the annular coupling and the small convex mirror. The cell was constructed by
cementing NaCl windows to a Teflon tube at Brewster’s angle. Two versions of the cell were available with thicknesses 6 mm and 1 mm. The procedure adopted to obtain a mode-locked output was sensibly the same for each gas used. After first aligning the laser cavity for optimum laser output without the absorber gas, the cell was statically filled until laser action ceased. Finally the pressure inside the cell was slowly reduced to the point where laser action was observed to recommence. A reproducible mode-locked output (7 out of a group of 10 shots) generally occurred with the laser operating very close to the oscillation threshold. ‘Besides the usual alignment, a further fine adjustment accomplished by means of the discharge voltage and the laser gas mix was sometimes needed to maximize the consistency of operation. The mode-locked train was recorded with a photon drag detector and displayed in a Tektronix 7904 oscilloscope having a response time of - 0.8 ns. In general, all mode-locked train envelopes had the usual asymmetrical gain switch shape consisting of a sharp spike followed by a longer tail.
3. Experimental
results
The-experimental work described in this paper was done with a variety of optical cavities, absorber cell 195
Volume
14, number
2
OPTICS COMMUNICATIONS
thicknesses and gas pressures. For easier cross comparison of mode-locking performances achieved with the different absorber gases, a summary of experimental parameters is presented in table 1 together with pertinent experimental results. 3.1. Sulfur hexajluoride (SF6)
.
Among the three gases tested, SF, is the one that has received the most attention in the literature as a saturable absorber for passive mode-locking of CO2 lasers. To carry out the present experiment, we chose the P(20) line of the 00” 1- 1O”0 CO;! transition which lies near the center of the v3 fundamental vibration band of SF,. The best overall mode-locked performance was with the short 1 mm cell filled with a mixture of SF, and He under high pressure using the 5.7 magnification resonator. When only SF, was present in the cell, mode-locked operation was obtained with only moderate stability in the narrow pressure range 20-40 torr. In this case, the most intense pulses in the train had a peak power of 600 MW and a width of 5 ns (fwhm). The addition of He to the absorber resulted in an appreciable improvement in output stability and a much narrower pulse. The most stable condition corresponding to the minimum obtainable pulse duration occurred with a 2 : 1 mixture of He and SF, at a total pressure of 75-100 torr. At these conditions the pulse train displayed a clean structure of single isolated pulses similar to that observed by Fortin et al. [3]. It consisted essentially of 3 intense pulses spaced 33 ns apart with a peak power estimated to be 1 GW. The total energy in the train was in excess of 10 J and the duration of the individual pulses was within the value l-2 ns. For higher gas pressures be-
196
June 1975
yond 77 torr no further reduction in pulse length took place but there was a noticeable decrease in the stability of the laser. It may be noted that pulses of varying duration in the range 2-5 ns could be generated by adjusting the He-SF6 mixture ratio. Modelocked behaviour was observed on several of the P and R branch rotational lines. 3.2. Boron trichloride (BCl,) To simplify the experimental procedure all the experimental investigations were carried out with pure BCl,. A large number of different P and R vib-rot lines of the CO2 have produced good mode-locking but only the strong P(20) line was chosen for detailed examination. With the grating tuned on the P(20) line, the best conditions for a reproducible mode-locked operation were found using the 6 mm thick cell filled with about 30 torr of BCl,. Clean pulse trains were observed with resonators having an optical magnification of 5.7 and 11. The most intense single pulses had an energy in the order of 1 J and halfamplitude duration of 2.5 ns. The total energy in the train was roughly 8 J and the cavity with the smaller magnification generally tended to yield more energetic pulses. A typical oscillogram is shown in fig. 2a. When the magnification was decreased to 3.5, the pulse train was seen to deteriorate and mode-locked operation corresponding to a single pulse in the cavity could no longer be obtained. Attempts to mode-lock the laser in this condition have been uniformly unsuccessful. In general, the output of the laser consisted of a superposition of overlapping trains of very short pulses that seemed to be generated independently by various portions of the laser as a result of multimode oscillations.
Volume 14, number 2
OPTICS COMMUNICATIONS
To verify that this type of behavior was occurring, an experiment was set up in which the laser emission in different parts of the beam cross-section was simultaneously displayed on two oscilloscopes using two photon drag detectors each fitted with a 1 mm pinhole. This was done by passing the beam through a beam splitter with each detector positioned so as to provide a continuous monitor of the transmitted and reflected beams over the entirety of their cross-sections. Typical results are shown in fig. 3 of two such simultaneous recordings of laser emission versus time obtained at
June 1975
the center and periphery of the beam. The dissimilarity in time shape provides clear evidence that the recorded trains originate from different sets of off-axial modes emitting into different parts of the beam. When monitoring the same area of the beam with both detectors identical pulse patterns were consistently obtained. The experiment was repeated under stable locking conditions using optical resonators of larger optical magnification (5.7 and 11). A thorough scanning of the beam indicated that the compared pulse trains had essentially the same shape, thus allowing
Fig. 3. Simultaneous time display of a multiply mode-locked output in two parts of the beam using BC13 as intracavity absorber.
Volume
14, number
2
OPTICS COMMUNICATIONS
us to assume that the laser emission was concentrated in a single transverse mode. When the resonator was purposely misaligned a multimode situation was again found to take place. It may be pointed out that no perceptible change in the radial intensity distribution of the radiation could be observed in going from a stable to an unstable mode-locking condition. 3.3. Tetrajluorohydrazine (N2Fq) With N2F4 [8] as intracavity absorber, two types of experiments were performed. In the first experiment the laser cavity and associated optics were the same as for BCl, and SF6 with the frequency selection being accomplished by means of a grating. Under these conditions the N2F4 absorber was observed to passively mode-lock several rotational lines of the P and R branches of the 10.41.tm band. For the P(20) line, on which the majority of the tests were done, the most stable pulsing occurred at pressures of 100 torr in the 6 mm thick cell using the resonators with the largest magnification parameters (5.7 and 11). An oscilloscope trace of a typical mode-locked train is depicted in fig. 2b. The energy of the entire train averaged approximately 6 J and the energy of the largest amplitude pulse reached about 500 mJ. The recorded pulse halfwidth was in the order of 2 ns. One contrasting feature of the mode-locked emission with N2F4 was a significant increase in the number of pulses in the train. This longer pulse train is attributed in part to the fact that when fully saturated the residual absorption of N2F4 is larger than in SF, and BCl,. The applicability of this explanation was verified by examining the transmission characteristics of N2F4 using a short, intense pulse of 2 ns duration produced by a CO2 laser amplifier chain [9]. The saturable absorption data indicated the linear absorption at the higher intensities to be in fact quite large, reaching a constant level of about 0.003 cm-l torr-l . In the second experiment, the spherical grating was replaced by a spherical reflecting mirror and an additional cell filled with C2H50H was inserted in the cavity between the laser discharge and the annular coupler. This cell, which was made of glass, had a thickness of 10 cm and was closed at both ends by NaCl windows arranged at a small oblique angle of incidence with respect to the perpendicular to the laser axis. The experiment was conducted using the 6 mm 198
June 1975
cell for N2F4 and a resonator having an optical mag nification of 5.7. Mode-locking operation was achieved on P(26) only and the best performance was obtained with 26 torr of C2H50H and 150 torr of N2F4. In contrast to the first experiment where use was made of a dispersive cavity, the output pulse train comprised only 5 pulses of 2.5 ns duration and the total energy in the train did not exceed 1 J. When C2H50H was absent from the resonator, spurious laser oscillations took place in the R branch of 001-020 band. Since CzH,OH has an intense absorption band centered around 9.6 mm, the absorption bandwidth of the N2F4/C2H50H combination appears to be comparable to the bandwidth of the CO2 such that oscillations at wavelengths other than 10.6nm are suppressed.
4. Conclusion In summary, we have described initial experiments with a passively mode-locked TEA laser in an unstable cavity and have demonstrated the possibility of effectively mode-locking large Fresnel number systems to produce intense short pulses in the gigawatt range with a very low beam divergence. The simplicity of the method makes it very attractive for use in experiments in generation of high temperature plasmas by optical breakdown in gases. The limits of the technique are far from being reached; more intense pulses of shorter duration appear possible without serious material problem damage using more optimum cells and high absorber gas pressures. Another conclusion that can be drawn from the previous results is that gas combination N2F4/C2H50H looks like a promising candidate for an efficient optical isolator in a high gain CO;! laser chain.
Acknowledgements The authors are indebted to Dr. R. Suart of the Propulsion Division for suggesting the use of N2F4 as a saturable filter and for his help in supplying the N2F4 gas used in the initial phase of this experiment. The technical assistance of Mr. A. Deslauriers is also gratefully acknowledged.
Volume 14, number 2
OPTICS COMMUNICATIONS
References [ 11 O.R. Wood, R.L. Abrams and T.L. Bridges, Appl. Phys. Lett. 17 (1970) 376. [2] F. Rheault, J.L. Lachambre, J. Gilbert, R. Fortin and M. Blanchard, Can. J. Phys. 50 (1972) 1876. [3] R. Fortin, F. Rheault, J. Gilbert, M. Blanchard and J.L. Lachambre, Can. J. Phys. 57 (1973) 414. [4] M.C. Richardson, Appl. Phys. Lett. 25 (1974) 31. (51 A.J. Alcock and A.C. Walker, Appl. Phys. Lett. 25 (1974) 299.
June 1975
161H. Seguin and J. Tulip, Appl. Phys. Lett. 21 (1972) 414. 171 M.C. Richardson, A.J. Alcock, K. Leopold and P. Burtyn, J. Quant. Electron. QE-9 (1973) 2. 181 R.D. Suart, 4th Conf. on Chemical and Molecular Lasers, paper No. TA-1, St. Louis, Missouri (1974); R.D. Suart, NzF4 as a saturable absorber of CO2 laser radiation, DREV TN-21 18174. I91 F. Rheault, J.L. Lachambre, J. Gilbert, R. Fortin and M. Blanchard, VII Intern. Quantum Electronics Conf., paper No. Q-2, Montreal (19721.
199
OPTICS COMMUNICATIONS
June 1975
PASSIVE MODE-LOCKING OF A LARGE VOLUME TEA-CO2 LASER USING AN UNSTABLE RESONATOR CONFIGURATION P. LAVIGNE, J. GILBERT and J.L. LACHAMBRE Centre de Recherches pour la DPfense, Defence Research Establishment, Valcartier, Quebec, Canada Received 10 February 1975 An experimental investigation was made of the passive mode-locking of a uv-preionized CO2 laser operating in an unstable wavelength-selective resonator using SF6, BCl3 and N2 Fa as bleachable gases. Stable repetitive pulse trains with individual pulses having energies as high as 2 J and 2 ns duration have been obtained on a large number of P and R transitions of the 00” 1- lo”0 band at 10.6 Frn. Use of the selectively absorbing gas CzHsOH in the resonator has enabled the production of clean, mode-locked pulses by the saturation of N2 F4 without the need of an optical dispersive element.
1. Introduction
2. Experimental apparatus
Continued improvements in large aperture TEACO2 lasers have led to increased interest in their use for the generation of short powerful mode-locked pulses at 10.6 pm [l-5]. This communication describes the results of an experimental investigation of the passive mode-locking of an atmospheric pressure CO2 laser excited by uv preionization in an unstable line-selective resonator. In view of its inherent advantages of a large energy extraction volume, high radiance output beam and damage-free optics, the unstable resonator is extremely well suited for use with an active laser media of large cross-section. The three gases investigated (SF,, BC13 and N2F4) were found to be equally effective as intracavity absorbers. Wavelength selection by means of a diffraction grating has allowed the achievement of mode-locking on several rotational transitions in the P and R branches of CO;!. Regular trains of mode-locked pulses as short as 2 ns were obtained with good reproducibility at peak powers in excess of one gigawatt. This power is more than five times that previously reported with a mode-locked laser [3,4] of only 1-meter active length. NzF4 in particular was found to give rise to a clean mode:locked behaviour on line P(26) of the 00” l-l O”0 band in the absence of any selective element by inserting C2H,0H into the cavity. We believe this to be the first demonstration of passive mode-locking at 10.6 I.tm by the resonant saturation of N2F4.
The experimental arrangement used in this study is shown schematically in fig. 1. The active plasma region was 1 meter long with a rectangular cross-section of 5 X 5 cm2. It was provided with two individual sections of high-current discharge volume-stabilized by uv photoionization [6,7]. Each section comprises a solid anode, a wire-mesh cathode and a preionizing electrode formed by a two-dimensional array of discrete uv emitting sparks. The sections were mounted in tandem inside a rectangular plexiglass enclosure containing a flowing mixture of CO2-N2--He at atmospheric pressure with a relative gas composition normally near 20: 10: 70. In order to achieve a collimated output of high radiance, a positive confocal design was chosen for the unstable resonator with a long-radius concave mirror and a short-radius convex mirror spaced 5 m apart. The discharge enclosure was
194
G
P
C
M
TEA
Fig. 1. Diagram of experimental setup. G, spherical grating; P, annular coupler; C, absorber cell, M, convex totally reflecting mirror; D, detector.
Volume 14, number 2
OPTICS COMMUNICATIONS
June 1975
Table 1 Comparative mode-locking performances Laser
Absorber Gas
Cell thickness
Pressure
(mm)
(torr)
Resonator magnification
Mode-locking Lasing transition (10.4 rm)
Pulse width (ns)
power (GW)
Output
Stability
energy (J)
SF6
1
20-40
5.7
P(20)
5
0.6
12
fair
SF6: He (1:2)
1
75-100
5.7
P(20)
2
1.0
10
good
5.7 11 G3.5
P(20) PC201 P(20)
2.5 2.5
0.5 0.4
8 6
5.1
PC201
2
0.25
6
5.7
PC261
2.5
0.08
1
BC13 BC13 BC13
30-40 30-40 -
Nz F4
100
N2 F4
150
+
CzHsOH
10
good good unsuccessful good fair
26
sealed at one end by the concave mirror and at the other by a NaCl window tilted about 5” from the plane perpendicular to the resonator axis. The laser emission was coupled out sideways through a 45” annular coupler placed near the convex mirror. A spherical grating of curvature radius 11 m, suitably masked by a 4.5 cm diameter aperture served as the concave mirror so as to restrain the operation of the laser within the absorption spectrum of the saturable absorber gas. Three distinct resonators were available to perform the experiments with optical magnifications of 3.5, 5.7 and 11.5 corresponding to experimentally measured output couplings of 80%, 90% and 96%, respectively. These were obtained by means of convex mirrors of radius of curvature 3 12 cm, 193 cm and 96 cm and aperture holes in the annular coupling ring of 16 mm, 9.5 mm and 6.5 mm diameter. The discharge was fired by 120 kV pulses supplied from a 3-stage Marx-bank generator connected through a voltage doubling circuit having a total energy of 600 J. Experimental observations indicated that the small-signal gain was fairly uniform across the section of the discharge with a va!ue of 0.04 cm-l at the center. With the employed optics, the system was capable of delivering 15 J pulses with a beam divergence of 0.6 mr. The absorption cell containing the saturable gas was inserted in the space between the annular coupling and the small convex mirror. The cell was constructed by
cementing NaCl windows to a Teflon tube at Brewster’s angle. Two versions of the cell were available with thicknesses 6 mm and 1 mm. The procedure adopted to obtain a mode-locked output was sensibly the same for each gas used. After first aligning the laser cavity for optimum laser output without the absorber gas, the cell was statically filled until laser action ceased. Finally the pressure inside the cell was slowly reduced to the point where laser action was observed to recommence. A reproducible mode-locked output (7 out of a group of 10 shots) generally occurred with the laser operating very close to the oscillation threshold. ‘Besides the usual alignment, a further fine adjustment accomplished by means of the discharge voltage and the laser gas mix was sometimes needed to maximize the consistency of operation. The mode-locked train was recorded with a photon drag detector and displayed in a Tektronix 7904 oscilloscope having a response time of - 0.8 ns. In general, all mode-locked train envelopes had the usual asymmetrical gain switch shape consisting of a sharp spike followed by a longer tail.
3. Experimental
results
The-experimental work described in this paper was done with a variety of optical cavities, absorber cell 195
Volume
14, number
2
OPTICS COMMUNICATIONS
thicknesses and gas pressures. For easier cross comparison of mode-locking performances achieved with the different absorber gases, a summary of experimental parameters is presented in table 1 together with pertinent experimental results. 3.1. Sulfur hexajluoride (SF6)
.
Among the three gases tested, SF, is the one that has received the most attention in the literature as a saturable absorber for passive mode-locking of CO2 lasers. To carry out the present experiment, we chose the P(20) line of the 00” 1- 1O”0 CO;! transition which lies near the center of the v3 fundamental vibration band of SF,. The best overall mode-locked performance was with the short 1 mm cell filled with a mixture of SF, and He under high pressure using the 5.7 magnification resonator. When only SF, was present in the cell, mode-locked operation was obtained with only moderate stability in the narrow pressure range 20-40 torr. In this case, the most intense pulses in the train had a peak power of 600 MW and a width of 5 ns (fwhm). The addition of He to the absorber resulted in an appreciable improvement in output stability and a much narrower pulse. The most stable condition corresponding to the minimum obtainable pulse duration occurred with a 2 : 1 mixture of He and SF, at a total pressure of 75-100 torr. At these conditions the pulse train displayed a clean structure of single isolated pulses similar to that observed by Fortin et al. [3]. It consisted essentially of 3 intense pulses spaced 33 ns apart with a peak power estimated to be 1 GW. The total energy in the train was in excess of 10 J and the duration of the individual pulses was within the value l-2 ns. For higher gas pressures be-
196
June 1975
yond 77 torr no further reduction in pulse length took place but there was a noticeable decrease in the stability of the laser. It may be noted that pulses of varying duration in the range 2-5 ns could be generated by adjusting the He-SF6 mixture ratio. Modelocked behaviour was observed on several of the P and R branch rotational lines. 3.2. Boron trichloride (BCl,) To simplify the experimental procedure all the experimental investigations were carried out with pure BCl,. A large number of different P and R vib-rot lines of the CO2 have produced good mode-locking but only the strong P(20) line was chosen for detailed examination. With the grating tuned on the P(20) line, the best conditions for a reproducible mode-locked operation were found using the 6 mm thick cell filled with about 30 torr of BCl,. Clean pulse trains were observed with resonators having an optical magnification of 5.7 and 11. The most intense single pulses had an energy in the order of 1 J and halfamplitude duration of 2.5 ns. The total energy in the train was roughly 8 J and the cavity with the smaller magnification generally tended to yield more energetic pulses. A typical oscillogram is shown in fig. 2a. When the magnification was decreased to 3.5, the pulse train was seen to deteriorate and mode-locked operation corresponding to a single pulse in the cavity could no longer be obtained. Attempts to mode-lock the laser in this condition have been uniformly unsuccessful. In general, the output of the laser consisted of a superposition of overlapping trains of very short pulses that seemed to be generated independently by various portions of the laser as a result of multimode oscillations.
Volume 14, number 2
OPTICS COMMUNICATIONS
To verify that this type of behavior was occurring, an experiment was set up in which the laser emission in different parts of the beam cross-section was simultaneously displayed on two oscilloscopes using two photon drag detectors each fitted with a 1 mm pinhole. This was done by passing the beam through a beam splitter with each detector positioned so as to provide a continuous monitor of the transmitted and reflected beams over the entirety of their cross-sections. Typical results are shown in fig. 3 of two such simultaneous recordings of laser emission versus time obtained at
June 1975
the center and periphery of the beam. The dissimilarity in time shape provides clear evidence that the recorded trains originate from different sets of off-axial modes emitting into different parts of the beam. When monitoring the same area of the beam with both detectors identical pulse patterns were consistently obtained. The experiment was repeated under stable locking conditions using optical resonators of larger optical magnification (5.7 and 11). A thorough scanning of the beam indicated that the compared pulse trains had essentially the same shape, thus allowing
Fig. 3. Simultaneous time display of a multiply mode-locked output in two parts of the beam using BC13 as intracavity absorber.
Volume
14, number
2
OPTICS COMMUNICATIONS
us to assume that the laser emission was concentrated in a single transverse mode. When the resonator was purposely misaligned a multimode situation was again found to take place. It may be pointed out that no perceptible change in the radial intensity distribution of the radiation could be observed in going from a stable to an unstable mode-locking condition. 3.3. Tetrajluorohydrazine (N2Fq) With N2F4 [8] as intracavity absorber, two types of experiments were performed. In the first experiment the laser cavity and associated optics were the same as for BCl, and SF6 with the frequency selection being accomplished by means of a grating. Under these conditions the N2F4 absorber was observed to passively mode-lock several rotational lines of the P and R branches of the 10.41.tm band. For the P(20) line, on which the majority of the tests were done, the most stable pulsing occurred at pressures of 100 torr in the 6 mm thick cell using the resonators with the largest magnification parameters (5.7 and 11). An oscilloscope trace of a typical mode-locked train is depicted in fig. 2b. The energy of the entire train averaged approximately 6 J and the energy of the largest amplitude pulse reached about 500 mJ. The recorded pulse halfwidth was in the order of 2 ns. One contrasting feature of the mode-locked emission with N2F4 was a significant increase in the number of pulses in the train. This longer pulse train is attributed in part to the fact that when fully saturated the residual absorption of N2F4 is larger than in SF, and BCl,. The applicability of this explanation was verified by examining the transmission characteristics of N2F4 using a short, intense pulse of 2 ns duration produced by a CO2 laser amplifier chain [9]. The saturable absorption data indicated the linear absorption at the higher intensities to be in fact quite large, reaching a constant level of about 0.003 cm-l torr-l . In the second experiment, the spherical grating was replaced by a spherical reflecting mirror and an additional cell filled with C2H50H was inserted in the cavity between the laser discharge and the annular coupler. This cell, which was made of glass, had a thickness of 10 cm and was closed at both ends by NaCl windows arranged at a small oblique angle of incidence with respect to the perpendicular to the laser axis. The experiment was conducted using the 6 mm 198
June 1975
cell for N2F4 and a resonator having an optical mag nification of 5.7. Mode-locking operation was achieved on P(26) only and the best performance was obtained with 26 torr of C2H50H and 150 torr of N2F4. In contrast to the first experiment where use was made of a dispersive cavity, the output pulse train comprised only 5 pulses of 2.5 ns duration and the total energy in the train did not exceed 1 J. When C2H50H was absent from the resonator, spurious laser oscillations took place in the R branch of 001-020 band. Since CzH,OH has an intense absorption band centered around 9.6 mm, the absorption bandwidth of the N2F4/C2H50H combination appears to be comparable to the bandwidth of the CO2 such that oscillations at wavelengths other than 10.6nm are suppressed.
4. Conclusion In summary, we have described initial experiments with a passively mode-locked TEA laser in an unstable cavity and have demonstrated the possibility of effectively mode-locking large Fresnel number systems to produce intense short pulses in the gigawatt range with a very low beam divergence. The simplicity of the method makes it very attractive for use in experiments in generation of high temperature plasmas by optical breakdown in gases. The limits of the technique are far from being reached; more intense pulses of shorter duration appear possible without serious material problem damage using more optimum cells and high absorber gas pressures. Another conclusion that can be drawn from the previous results is that gas combination N2F4/C2H50H looks like a promising candidate for an efficient optical isolator in a high gain CO;! laser chain.
Acknowledgements The authors are indebted to Dr. R. Suart of the Propulsion Division for suggesting the use of N2F4 as a saturable filter and for his help in supplying the N2F4 gas used in the initial phase of this experiment. The technical assistance of Mr. A. Deslauriers is also gratefully acknowledged.
Volume 14, number 2
OPTICS COMMUNICATIONS
References [ 11 O.R. Wood, R.L. Abrams and T.L. Bridges, Appl. Phys. Lett. 17 (1970) 376. [2] F. Rheault, J.L. Lachambre, J. Gilbert, R. Fortin and M. Blanchard, Can. J. Phys. 50 (1972) 1876. [3] R. Fortin, F. Rheault, J. Gilbert, M. Blanchard and J.L. Lachambre, Can. J. Phys. 57 (1973) 414. [4] M.C. Richardson, Appl. Phys. Lett. 25 (1974) 31. (51 A.J. Alcock and A.C. Walker, Appl. Phys. Lett. 25 (1974) 299.
June 1975
161H. Seguin and J. Tulip, Appl. Phys. Lett. 21 (1972) 414. 171 M.C. Richardson, A.J. Alcock, K. Leopold and P. Burtyn, J. Quant. Electron. QE-9 (1973) 2. 181 R.D. Suart, 4th Conf. on Chemical and Molecular Lasers, paper No. TA-1, St. Louis, Missouri (1974); R.D. Suart, NzF4 as a saturable absorber of CO2 laser radiation, DREV TN-21 18174. I91 F. Rheault, J.L. Lachambre, J. Gilbert, R. Fortin and M. Blanchard, VII Intern. Quantum Electronics Conf., paper No. Q-2, Montreal (19721.
199