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Single transverse mode operation of a pulsed volume excited atmospheric pressure CO2 laser using an unstable resonator
Volume 5, number 4
July 1972
OPTICS COMMUNICATIONS
SINGLE TRANSVERSE MODE OPERATION OF A PULSED VOLUME EXCITED ATMOSPHERIC PRESSURE CO, LASER USING AN UNSTABLE RESONATOR P.E. DYER, D.J. JAMES and S.A. RAMSDEN Department of Applied Physics, Universityof Hull, UK Received 17 april 1972
The operation of a volume excited TEA CO2 laser using an unstable resonator configuration is described. Considerable improvement over conventional single mode operation is achieved. The theoretical and experimental far field patterns rue compared.
Operation of volume excited TEA CO, lasers with large cross section allows high energy pulses to be extracted from compact devices. These systems, however, have high Fresnel numbers which, coupled with a gain > 3dB/m, leads to multimode output. To obtain lowest order transverse mode several methods may be employed, the most common being the use of apertures to reduce the Fresnel number. This method seriously reduces the output power and energy. We have investigated the operation of a volume excited atmospheric pressure CO, laser in an unstable resonator configuration. The theory of unstable resonators has been studied in some detail by Siegman [ 1,2] and experimental work to date shows that these resonators do exhibit good mode selectivity and that they should be particularly useful for high Fresnel number systems [3-61. The pulsed atmospheric pressure CO, laser used [7] employed a triggered cathode similar to that described by Dumanchin et al. [8]. Pulsed excitation was provided by a two stage Marx generator with a pulse forming network giving up to 70 kV voltage pulses with a rise-time of two microseconds. The current rise-time was approximately 200 nsec. The active volume had a square cross section of area 2 X 10m3 m2 and was 0.9 m long. Input energies up to 100 J/litre gave a uniform discharge throughout the volume apart from a region a few millimetres above the cathode which was avoided in the cavity designs used. Typical 236
gas flow rates were 13 l/min with flow ratios of 1: 1: 8 of CO, : N2 : He. Operated multimode with a stable cavity consisting of a 10 m concave mirror and 70% reflectivity Se-NaCl plane mirror the laser output was 12 J, 35 MW with an efficiency of 6% and a beam divergence of 2 mrad. The unstable resonator was formed by a 7.5 cm diameter convex mirror of gold coated brass having a radius of curvature 29 m and a plane mirror of diameter 2.85 cm separated by a distance of 1.3 m. The plane mirror consisted of a gold disc evaporated onto a sodium chloride substrate. The resonator was designed to have an equivalent Fresnel number of approximately 3.5 to obtain both good mode selection and a useful fraction of the energy in the central peak of the far field diffraction pattern. This value was chosen as a result of preliminary experiments with various unstable resonator configure tions. The laser output appeared as an annular section of an expanding spherical wave having a radius R’ at the plane mirror given by [3] : R’=(L2
+LR1)*,
where L is the cavity length and RI the radius of curvature of the convex mirror. The magnification, M, of the mirror system and the geometrical round trip loss factor S are given by [3] : M = (R’ + L)/(R’ - L)
OPTICS COMMUNICATIONS
Volume 5, number 4
July 1972
I ---
0.5
0.67
Experimental
points
Single
(theory)
mode
1.0
Fig. 1. Variation of normal&d energy contained within the first, second and third dark rings with l/M.
Angle Fig. 2. A typical oscilloscope trace of the far field energy density distribution (arbitrary scale).
and 6 = (1 -M-2). In these experiments R’ = 6.27m, M = 1.5 and 6 = 56%. Measurements were made of both the integrated and angular energy distribution in the far field pattern formed by a long focal length concave mirror. The integrated energy as a function of angle, 0, was measured using a 10 m radius of curvature concave mirror, a calibrated Griffin and George thermopile, and a variable circular aperture concentric with the far field pattern. For the lowest order single transverse mode the integrated energy is given by [8] :
Fig. 3. Normalised energy density in the far field as a function of 20.
where C is a constant and z. = (2trMB/h) X (radius of output mirror). Using this expression the normalised energy within each dark ring is plotted against l/M (fig. 1) for the first three dark rings together with the experimental points for l/M = 0.67. The total output energy was 70% of that obtained from a stable cavity operating multimode under the same conditions and 25% of this energy was contained in the central peak. To measure the angular energy distribution the far field pattern formed by a 4 m radius of curvature concave mirror was magnified X 20 by an ancillary optical system and scanned in 1 mm steps with a Mullard pyroelectric detector, type F362, having an aperture 0.6 mm in diameter. For these measurements the laser beam was attenuated by a factor X 1.5. Using a slow time base and beam modulation the energy distribution was recorded directly on successive shots using a Tektronix 556 oscilloscope as shown in fig. 2. Although difficulty was initially experienced in maintaining cavity alignment accurately enough to prevent beam steering, this was overcome by rigidising the mirror mounts and the final shot to shot reproducibility was better than 15%. As shown in fig. 3 the data obtained agrees quite well with the distribution expected for a lowest order single transverse mode; the slight deviation observed may be due to amplitude variations across the mirror. From bum patterns on exposed Polaroid film a measurement was made of the radius R’ of the ex237
Volume 5. number 4
p m
I n
OPTICS COMMUNICATIONS
(bl
Fig. 4. (a) Typical pulse shape when operated multimode. (b) Laser pulse shape when operated single mode with the unstable resonator.
panding wavefront which was in good agreement with the calculated value. Figs. 4a and 4b show typical pulse shapes, observed by attenuating the laser beam and focusing it onto a photon drag detector, for stable and unstable resonator cavities respectively. When operated single mode in the unstable resonator configuration = 60% of the energy was in a pulse of width 70 nsec (fwhm) whereas with a stable cavity = 65% of the energy was in a tail lasting for = 2.5 psec. The maximum energy from the unstable resonator was limited by mirror damage to 3.5 J single mode. The peak power of 3 X lo7 W was, however, twice that of the stable resonator operating multimode under the same conditions. When the stable resonator was operated with a 10 m concave mirror and 50% reflecting germanium mirror to give approximately the same round trip loss as the unstable resonator and apertured down to give a single mode the energy was reduced to
238
July 1972
0.33 J and the peak power to = lo6 W. In conclusion, a pulsed volume excited atmospheric pressure laser operated single mode with an unstable resonator has been shown to give a much higher brightness than a stable cavity operated under the same conditions. There is also some improvement in the pulse shape with less energy in the tail. With the present system giving a peak power of 30 MW, beam divergence x 0.2 mrad and a brightness 2 X 1013 W cm2 sr-l the maximum energy that can be extracted is limited by damage to the gold mirror on the sodium chloride substrate and work to improve this is in progress. We would like to acknowledge the technical assistance of R. Bosomworth and the departmental workshops. This work is supported by a grant from the Science Research Council and one of us (P.E.D.) acknowledges the receipt of a S.R.C. Research Studentship during the course of this work.
References (11 A.E. Siegman, Proc. IEEE 53 (1965) 277. [2] A.E. Siegman and H.Y. Miller, Appl. Opt. 9 (1970) 2729. [3] Yu.A. Anan’ev, N.A. Sventsitskaya and V.E. Sherstobitov, Soviet Phys. JETP 28 (1969) 69. [4] W.F. Krupke and W.R. Sooy, IEEE J. Quantum Electron. QE-5 (1969) 575. [S] J.P. Reilly, Avco Everett Research Laboratory Report No. 421 (1971). [6] E.T. Gerry, IEEE Spectrum 56 (1970) 51. [ 71 P.E. Dyer, D.J. James and S.A. Ramsden, to be published. [ 81 R. Dumanchin, J.C. Farcy, M. Michon and J. Rocca-Serra, Sixth International Quantum Electronics Conference, Kyoto (1970). [9] M. Born and E. Wolf, Principles of optics (Pergamon Press, London, 1964) p. 414.
July 1972
OPTICS COMMUNICATIONS
SINGLE TRANSVERSE MODE OPERATION OF A PULSED VOLUME EXCITED ATMOSPHERIC PRESSURE CO, LASER USING AN UNSTABLE RESONATOR P.E. DYER, D.J. JAMES and S.A. RAMSDEN Department of Applied Physics, Universityof Hull, UK Received 17 april 1972
The operation of a volume excited TEA CO2 laser using an unstable resonator configuration is described. Considerable improvement over conventional single mode operation is achieved. The theoretical and experimental far field patterns rue compared.
Operation of volume excited TEA CO, lasers with large cross section allows high energy pulses to be extracted from compact devices. These systems, however, have high Fresnel numbers which, coupled with a gain > 3dB/m, leads to multimode output. To obtain lowest order transverse mode several methods may be employed, the most common being the use of apertures to reduce the Fresnel number. This method seriously reduces the output power and energy. We have investigated the operation of a volume excited atmospheric pressure CO, laser in an unstable resonator configuration. The theory of unstable resonators has been studied in some detail by Siegman [ 1,2] and experimental work to date shows that these resonators do exhibit good mode selectivity and that they should be particularly useful for high Fresnel number systems [3-61. The pulsed atmospheric pressure CO, laser used [7] employed a triggered cathode similar to that described by Dumanchin et al. [8]. Pulsed excitation was provided by a two stage Marx generator with a pulse forming network giving up to 70 kV voltage pulses with a rise-time of two microseconds. The current rise-time was approximately 200 nsec. The active volume had a square cross section of area 2 X 10m3 m2 and was 0.9 m long. Input energies up to 100 J/litre gave a uniform discharge throughout the volume apart from a region a few millimetres above the cathode which was avoided in the cavity designs used. Typical 236
gas flow rates were 13 l/min with flow ratios of 1: 1: 8 of CO, : N2 : He. Operated multimode with a stable cavity consisting of a 10 m concave mirror and 70% reflectivity Se-NaCl plane mirror the laser output was 12 J, 35 MW with an efficiency of 6% and a beam divergence of 2 mrad. The unstable resonator was formed by a 7.5 cm diameter convex mirror of gold coated brass having a radius of curvature 29 m and a plane mirror of diameter 2.85 cm separated by a distance of 1.3 m. The plane mirror consisted of a gold disc evaporated onto a sodium chloride substrate. The resonator was designed to have an equivalent Fresnel number of approximately 3.5 to obtain both good mode selection and a useful fraction of the energy in the central peak of the far field diffraction pattern. This value was chosen as a result of preliminary experiments with various unstable resonator configure tions. The laser output appeared as an annular section of an expanding spherical wave having a radius R’ at the plane mirror given by [3] : R’=(L2
+LR1)*,
where L is the cavity length and RI the radius of curvature of the convex mirror. The magnification, M, of the mirror system and the geometrical round trip loss factor S are given by [3] : M = (R’ + L)/(R’ - L)
OPTICS COMMUNICATIONS
Volume 5, number 4
July 1972
I ---
0.5
0.67
Experimental
points
Single
(theory)
mode
1.0
Fig. 1. Variation of normal&d energy contained within the first, second and third dark rings with l/M.
Angle Fig. 2. A typical oscilloscope trace of the far field energy density distribution (arbitrary scale).
and 6 = (1 -M-2). In these experiments R’ = 6.27m, M = 1.5 and 6 = 56%. Measurements were made of both the integrated and angular energy distribution in the far field pattern formed by a long focal length concave mirror. The integrated energy as a function of angle, 0, was measured using a 10 m radius of curvature concave mirror, a calibrated Griffin and George thermopile, and a variable circular aperture concentric with the far field pattern. For the lowest order single transverse mode the integrated energy is given by [8] :
Fig. 3. Normalised energy density in the far field as a function of 20.
where C is a constant and z. = (2trMB/h) X (radius of output mirror). Using this expression the normalised energy within each dark ring is plotted against l/M (fig. 1) for the first three dark rings together with the experimental points for l/M = 0.67. The total output energy was 70% of that obtained from a stable cavity operating multimode under the same conditions and 25% of this energy was contained in the central peak. To measure the angular energy distribution the far field pattern formed by a 4 m radius of curvature concave mirror was magnified X 20 by an ancillary optical system and scanned in 1 mm steps with a Mullard pyroelectric detector, type F362, having an aperture 0.6 mm in diameter. For these measurements the laser beam was attenuated by a factor X 1.5. Using a slow time base and beam modulation the energy distribution was recorded directly on successive shots using a Tektronix 556 oscilloscope as shown in fig. 2. Although difficulty was initially experienced in maintaining cavity alignment accurately enough to prevent beam steering, this was overcome by rigidising the mirror mounts and the final shot to shot reproducibility was better than 15%. As shown in fig. 3 the data obtained agrees quite well with the distribution expected for a lowest order single transverse mode; the slight deviation observed may be due to amplitude variations across the mirror. From bum patterns on exposed Polaroid film a measurement was made of the radius R’ of the ex237
Volume 5. number 4
p m
I n
OPTICS COMMUNICATIONS
(bl
Fig. 4. (a) Typical pulse shape when operated multimode. (b) Laser pulse shape when operated single mode with the unstable resonator.
panding wavefront which was in good agreement with the calculated value. Figs. 4a and 4b show typical pulse shapes, observed by attenuating the laser beam and focusing it onto a photon drag detector, for stable and unstable resonator cavities respectively. When operated single mode in the unstable resonator configuration = 60% of the energy was in a pulse of width 70 nsec (fwhm) whereas with a stable cavity = 65% of the energy was in a tail lasting for = 2.5 psec. The maximum energy from the unstable resonator was limited by mirror damage to 3.5 J single mode. The peak power of 3 X lo7 W was, however, twice that of the stable resonator operating multimode under the same conditions. When the stable resonator was operated with a 10 m concave mirror and 50% reflecting germanium mirror to give approximately the same round trip loss as the unstable resonator and apertured down to give a single mode the energy was reduced to
238
July 1972
0.33 J and the peak power to = lo6 W. In conclusion, a pulsed volume excited atmospheric pressure laser operated single mode with an unstable resonator has been shown to give a much higher brightness than a stable cavity operated under the same conditions. There is also some improvement in the pulse shape with less energy in the tail. With the present system giving a peak power of 30 MW, beam divergence x 0.2 mrad and a brightness 2 X 1013 W cm2 sr-l the maximum energy that can be extracted is limited by damage to the gold mirror on the sodium chloride substrate and work to improve this is in progress. We would like to acknowledge the technical assistance of R. Bosomworth and the departmental workshops. This work is supported by a grant from the Science Research Council and one of us (P.E.D.) acknowledges the receipt of a S.R.C. Research Studentship during the course of this work.
References (11 A.E. Siegman, Proc. IEEE 53 (1965) 277. [2] A.E. Siegman and H.Y. Miller, Appl. Opt. 9 (1970) 2729. [3] Yu.A. Anan’ev, N.A. Sventsitskaya and V.E. Sherstobitov, Soviet Phys. JETP 28 (1969) 69. [4] W.F. Krupke and W.R. Sooy, IEEE J. Quantum Electron. QE-5 (1969) 575. [S] J.P. Reilly, Avco Everett Research Laboratory Report No. 421 (1971). [6] E.T. Gerry, IEEE Spectrum 56 (1970) 51. [ 71 P.E. Dyer, D.J. James and S.A. Ramsden, to be published. [ 81 R. Dumanchin, J.C. Farcy, M. Michon and J. Rocca-Serra, Sixth International Quantum Electronics Conference, Kyoto (1970). [9] M. Born and E. Wolf, Principles of optics (Pergamon Press, London, 1964) p. 414.