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The dynamics of single-mode operation of high-pressure CO2 laser by means of saturable absorbers
Volume
THE
November
OPTICS COMMUNICATIONS
4, number 3
DYNAMICS
OF
LASER
SINGLE-MODE BY
MEANS
OPERATION OF
T. DETEMPLE
SATURABLE
OF
HIGH-PRESSURE
ABSORBERS
1971
CO2
*
and A. NURMIKKO**
Department
of Electrical Engineering and Computer Sciences and the Electronics Research Laboratory, University of California, Berkeley, California 04720, USA Received
1 October
1971
Single-longitudinal-mode operation of a pulsed, high-pressure, transversely excited CO2 laser by In this communication we report means of a saturable absorber (SF~) has been previously reported. the results of experiments performed to measure the frequencies of single-mode oscillation of the P(14) to P(22) transitions. These frequencies were measured with the use of a scanning Fabry-PBrot From these measurements and their interferometer and a conventional CO2 reference oscillator. correlation with the known frequencies of maximum transmission in the SF6 absorption spectrum, it is believed that single-longitudinal-mode operation is caused by selective gain discrimination.
Single-longitudinal-mode (SLM) operation of a pulsed, high-pressure CO2 laser, obtained through the use of an intracavity SF6 cell, has been previously reported [l]. One possible use of these high intensity, single frequency pulses would be as a source for investigating new phase coherent phenomena [2, 31. Many of these interactions are resonant in nature and, as such, are frequency sensitive. With this in mind, we have extended upon the previous experiment to determine specifically what the frequency of the SLM is. In the course of this investigation we have been able to infer that the mechanism responsible for the production of single mode operation is again discrimination. The experimental apparatus was arranged as shown in fig. 1. The high-pressure laser and pulser are identical with those used in the previous experiment [l]. The gas mixture in the laser tube consisted of 20 % CO2, 5 % N2 and 75% He at a total pressure of one atmosphere and was flowed slowly through the tube. The cavity was 245 cm long and was composed of a grating and a 10-m radius of curvature germamum reflector. Irises were used to restrict oscillations to the fundamental transverse mode.
* Research Sponsored by the Army Research Office, Durham Contract DAHC04-70-C-0011 and the Joint Services Electronics Program Contract F44620-71-C-0087. ** Present Address: Physics Department, Hebrew University, Jerusalem, Israel.
Fig. 1. Experimental apparatus. G, grating; I, iris; C, absorber cell; N, output mirror; TEA, high-pressure CO2 laser; CW, low-pressure reference laser; BS, beam splitter, SFP, scanning Fabry-Perot interferometer; L, NaCl lens; D, detector. The
absorber
cell
was
10 cm
long
with parallel,
Brewster-angle windows. The output pulses from the laser were analyzed with a Fabry-Perot interferometer which consisted of two high reflectivity germanium flats separated by 15 cm. A cw CO2 laser was used to align the interferometer and provide a frequency reference for the measurements. Extreme care was taken to ensure that the two optical beams were parallel and coaxial. The interferometer finesse was found to be in excess of 25 with a l-GHz free
spectral range. To ascertain the SLM frequency, the laser was fired repetitively at 5 pps while the interferometer spacing was slowly changed with a piezo electric translator. The pulses transmitted through the interferometer were with a liquid nitrogen cooled Ge : Au detector. After integra231
Volume 4, number 3
OPTICS COMMUNICATIONS
tion by a capacitor, the resulting detector signal was displayed on a Tektronix 454 oscilloscope operating in an X- Y-Z mode. The horizontal deflection was proportional to the interferometer spacing while the vertical deflection was proportional to the transmitted energy. The CRT beam intensity was gated on for a short time after the laser fired. This helped in reducing noise and improving trace definition. After a scan was recorded, the corresponding transmission of the cw reference laser, operating on the same rotational transition, was also recorded. By superimposing the data we could determine the frequency of the SLM* with respect to the cw reference oscillator . Fig. 2 shows an interferometer scan of the output of the two lasers operating on the P(16) transition. Operation on an SLM was obtained with 0.2 torr of SF6 in the cell. Single-mode operation is evident because of the narrowed width of the spectrum and lack of modulation observed in the time behavior of the laser output. In this case, an SLM frequency of approximately +200 MHz offset from the reference oscillator can be inferred. (The subsidiary peaks in the SLM spectrum are caused by interference effects in the germanium beam splitter.) Using the above techniques, we have measured the SLM frequencies on the P(14) to P(22) transitions of the 10.6 I_Lband of CO2**. We have found good correlation between these frequencies and the known frequencies of maximum transmission in the SF6 absorption spectrum [4]. We have also obtained SLM operation with the use of an intracavity, low-pressure CO2 amplifier alone and also with the simultaneous use of an SF6 * The reference oscillator was not stabilized to line center so there is a total uncertainty of +t25 MHz in the frequency
measurements.
** The SF6 pressure used varied torr for these lines.
between 0.1 and 0.4
November 1971
o.oo~-T-300
-200
1
100
200
300
(MHz)
l’ig. 3. (a) Absorption spectra for pure SF6 and (b) net gain coefficient per pass for 1.0 torr cm of SF6 and assumed cavity loss and laser gain parameters. The frequency scale is relative to P(16) low-pressure line center.
cell. Because of these observations we infer that the mechanism responsible for SLM operation is one of selective gain discrimination. As an example of this, let us consider the results obtained with the P(16) transition. Fig. 3a shows a portion of the SF6 absorption spectrum near the center of the P(16) transition as determined by the high-resolution measurements of Hinkley [4] I. By combining the SF6 absorption spectrum, the estimated laser cavity losses, and the high-pressure laser gain profile, we find that the net gain coefficient per pass has a spectral distribution as shown in fig. 3b. (A homogeneous linewidth of 3 GHz is assumed [5] and an gain coefficient of the average, single-pass high-pressure laser was previously measured to be N 1.1 at the temporal peak.) The predicted gain maximum at + 200 MHz correlates very well with the observed SLM frequency offset
Frequency
Fig. 2. Transmitted signal through interferometer of reference oscillator (cw) and single-mode high-pressure CO2 oscillator (pulsed SLM). P(16) transition, 0.2 torr SF6, l-GHz free spectral range.
232
t The reference oscillator used in ref. [4] was not stabilized to line center so there is an uncertainty of f- 10 MHz in the location of the zero frequency. Additional data were kindly supplied by E. D. Hinkley.
Volume
4, number 3
OPTICS COMMUNICATIONS Table 1
Transition
P(22) P(20)
PG8)
P(16) P(l4)
Absorption minima a)
Relative
snub)
(MHz)
absorption coefficient
-20 +518 +372 +218 -13 -90 -300 +344 +236 +71 -35 -165 +200 +140 -20 -100
1 1.03 1.08 1.22 1 1 1.15 1.02 1.02 1.07 1 1.17 1 1.3 1 1.04
-40 +500
(MHz)
-100 -300 +240 +54 -60 +200 0
a) Ref. [4]; see also footnote 1, p.232. b) Ref. [4]. from line center of approximately + 200 MHz. In order to explain the suppression of other modes we use a simple gain discrimination model due to Sooy [6]. He shows that the final output intensity distribution from a saturable absorber Q-switched laser is approximately given by the intensity distribution just prior to bleaching of the absorber. As an example, let us assume that there are two equal amplitude modes present and that the gain remains constant in time. If the gain coefficient per pass differs between the two modes by as little as 0.1, then after 50 passes the intensity ratio of the two modes would be greater than 100. Consequently, because of the low gain and long mode buildup time (typically 0.75 - 3 ~_rset in our laser), only a small difference in the net single pass gain is required to allow only one mode to dominate. Qualitatively, we can now understand why, in the case of the P(16) transition, oscillations occur near the gain peak and are restricted to one longitudinal mode. Of course, use of a longer laser cavity would result in a closer frequency spacing of the modes and enhance the possibility that more than one mode would have nearly the same gain. Evidence for this spectrally narrowed, multimode operation has been recently reported [7].
November 1971
Table 1 lists the SLM frequencies that have been detected in this experiment. All SLM frequencies have been correlated with SF6 absorption minima. The P(14), P(16) and P(22) transitions oscillate in a single mode at a reproducible frequency. P(18) and P(20) also give rise to oscillations in a single mode but the exact frequency changes from pulse to pulse with occasional multifrequency pulses being observed. By looking for a beat note with a fast detector and oscilloscope we have determined that single frequency oscillation is the most probable event (80% of the time). When multifrequency operation occurred, the resulting beat note was in excess of 100 MHz. In such cases, the depth of modulation was small indicating that the second frequency was only incipient. Presumably, for these cases the exact frequency of oscillation is determined by the laser cavity tuning as well as the gain distribution. We feel that the utility of this technique lies not only in the ability to suppress all but one axial mode but also to stabilize that mode to a known frequency [in the case of P(14), P(16) and P(22)]. By using other gases and gas mixtures one should be able to obtain single frequency operation on all the transitions and, in addition, to tune that frequency over the entire laser bandwidth. The authors wish to thank GTE Sylvania Inc., Mountain View, California, for making available some of the optical components used in this experiment. REFERENCES [l] A. Nurmikko, T. DeTemple and S. E. Schwarz, Appl. Phys. Letters 18 (1971) 130. [2] F. A. Hopf, G. L. Lamb, C. K. Rhodes and M. 0. Scully, Phys. Rev. A3 (1971) 758. [3] A. V. Nurmikko and S. E. Schwarz, Opt. Commun. 2 (1971) 416. [4] E. D. Hinkley, Appl. Phys. Letters 16 (1971) 351. [5] T. J. Bridges, H.A. Haus and P. W. Hoff, IEEE J. Quantum Electron. QE-4 (1968) ,777. [6] W. R. Sooy, Appl. Phys. Letters 7 (1965) 36. [7] J. Gilbert and J. L. Lachambre, Appl. Phys. Letters 18 (1971) 187.
233
THE
November
OPTICS COMMUNICATIONS
4, number 3
DYNAMICS
OF
LASER
SINGLE-MODE BY
MEANS
OPERATION OF
T. DETEMPLE
SATURABLE
OF
HIGH-PRESSURE
ABSORBERS
1971
CO2
*
and A. NURMIKKO**
Department
of Electrical Engineering and Computer Sciences and the Electronics Research Laboratory, University of California, Berkeley, California 04720, USA Received
1 October
1971
Single-longitudinal-mode operation of a pulsed, high-pressure, transversely excited CO2 laser by In this communication we report means of a saturable absorber (SF~) has been previously reported. the results of experiments performed to measure the frequencies of single-mode oscillation of the P(14) to P(22) transitions. These frequencies were measured with the use of a scanning Fabry-PBrot From these measurements and their interferometer and a conventional CO2 reference oscillator. correlation with the known frequencies of maximum transmission in the SF6 absorption spectrum, it is believed that single-longitudinal-mode operation is caused by selective gain discrimination.
Single-longitudinal-mode (SLM) operation of a pulsed, high-pressure CO2 laser, obtained through the use of an intracavity SF6 cell, has been previously reported [l]. One possible use of these high intensity, single frequency pulses would be as a source for investigating new phase coherent phenomena [2, 31. Many of these interactions are resonant in nature and, as such, are frequency sensitive. With this in mind, we have extended upon the previous experiment to determine specifically what the frequency of the SLM is. In the course of this investigation we have been able to infer that the mechanism responsible for the production of single mode operation is again discrimination. The experimental apparatus was arranged as shown in fig. 1. The high-pressure laser and pulser are identical with those used in the previous experiment [l]. The gas mixture in the laser tube consisted of 20 % CO2, 5 % N2 and 75% He at a total pressure of one atmosphere and was flowed slowly through the tube. The cavity was 245 cm long and was composed of a grating and a 10-m radius of curvature germamum reflector. Irises were used to restrict oscillations to the fundamental transverse mode.
* Research Sponsored by the Army Research Office, Durham Contract DAHC04-70-C-0011 and the Joint Services Electronics Program Contract F44620-71-C-0087. ** Present Address: Physics Department, Hebrew University, Jerusalem, Israel.
Fig. 1. Experimental apparatus. G, grating; I, iris; C, absorber cell; N, output mirror; TEA, high-pressure CO2 laser; CW, low-pressure reference laser; BS, beam splitter, SFP, scanning Fabry-Perot interferometer; L, NaCl lens; D, detector. The
absorber
cell
was
10 cm
long
with parallel,
Brewster-angle windows. The output pulses from the laser were analyzed with a Fabry-Perot interferometer which consisted of two high reflectivity germanium flats separated by 15 cm. A cw CO2 laser was used to align the interferometer and provide a frequency reference for the measurements. Extreme care was taken to ensure that the two optical beams were parallel and coaxial. The interferometer finesse was found to be in excess of 25 with a l-GHz free
spectral range. To ascertain the SLM frequency, the laser was fired repetitively at 5 pps while the interferometer spacing was slowly changed with a piezo electric translator. The pulses transmitted through the interferometer were with a liquid nitrogen cooled Ge : Au detector. After integra231
Volume 4, number 3
OPTICS COMMUNICATIONS
tion by a capacitor, the resulting detector signal was displayed on a Tektronix 454 oscilloscope operating in an X- Y-Z mode. The horizontal deflection was proportional to the interferometer spacing while the vertical deflection was proportional to the transmitted energy. The CRT beam intensity was gated on for a short time after the laser fired. This helped in reducing noise and improving trace definition. After a scan was recorded, the corresponding transmission of the cw reference laser, operating on the same rotational transition, was also recorded. By superimposing the data we could determine the frequency of the SLM* with respect to the cw reference oscillator . Fig. 2 shows an interferometer scan of the output of the two lasers operating on the P(16) transition. Operation on an SLM was obtained with 0.2 torr of SF6 in the cell. Single-mode operation is evident because of the narrowed width of the spectrum and lack of modulation observed in the time behavior of the laser output. In this case, an SLM frequency of approximately +200 MHz offset from the reference oscillator can be inferred. (The subsidiary peaks in the SLM spectrum are caused by interference effects in the germanium beam splitter.) Using the above techniques, we have measured the SLM frequencies on the P(14) to P(22) transitions of the 10.6 I_Lband of CO2**. We have found good correlation between these frequencies and the known frequencies of maximum transmission in the SF6 absorption spectrum [4]. We have also obtained SLM operation with the use of an intracavity, low-pressure CO2 amplifier alone and also with the simultaneous use of an SF6 * The reference oscillator was not stabilized to line center so there is a total uncertainty of +t25 MHz in the frequency
measurements.
** The SF6 pressure used varied torr for these lines.
between 0.1 and 0.4
November 1971
o.oo~-T-300
-200
1
100
200
300
(MHz)
l’ig. 3. (a) Absorption spectra for pure SF6 and (b) net gain coefficient per pass for 1.0 torr cm of SF6 and assumed cavity loss and laser gain parameters. The frequency scale is relative to P(16) low-pressure line center.
cell. Because of these observations we infer that the mechanism responsible for SLM operation is one of selective gain discrimination. As an example of this, let us consider the results obtained with the P(16) transition. Fig. 3a shows a portion of the SF6 absorption spectrum near the center of the P(16) transition as determined by the high-resolution measurements of Hinkley [4] I. By combining the SF6 absorption spectrum, the estimated laser cavity losses, and the high-pressure laser gain profile, we find that the net gain coefficient per pass has a spectral distribution as shown in fig. 3b. (A homogeneous linewidth of 3 GHz is assumed [5] and an gain coefficient of the average, single-pass high-pressure laser was previously measured to be N 1.1 at the temporal peak.) The predicted gain maximum at + 200 MHz correlates very well with the observed SLM frequency offset
Frequency
Fig. 2. Transmitted signal through interferometer of reference oscillator (cw) and single-mode high-pressure CO2 oscillator (pulsed SLM). P(16) transition, 0.2 torr SF6, l-GHz free spectral range.
232
t The reference oscillator used in ref. [4] was not stabilized to line center so there is an uncertainty of f- 10 MHz in the location of the zero frequency. Additional data were kindly supplied by E. D. Hinkley.
Volume
4, number 3
OPTICS COMMUNICATIONS Table 1
Transition
P(22) P(20)
PG8)
P(16) P(l4)
Absorption minima a)
Relative
snub)
(MHz)
absorption coefficient
-20 +518 +372 +218 -13 -90 -300 +344 +236 +71 -35 -165 +200 +140 -20 -100
1 1.03 1.08 1.22 1 1 1.15 1.02 1.02 1.07 1 1.17 1 1.3 1 1.04
-40 +500
(MHz)
-100 -300 +240 +54 -60 +200 0
a) Ref. [4]; see also footnote 1, p.232. b) Ref. [4]. from line center of approximately + 200 MHz. In order to explain the suppression of other modes we use a simple gain discrimination model due to Sooy [6]. He shows that the final output intensity distribution from a saturable absorber Q-switched laser is approximately given by the intensity distribution just prior to bleaching of the absorber. As an example, let us assume that there are two equal amplitude modes present and that the gain remains constant in time. If the gain coefficient per pass differs between the two modes by as little as 0.1, then after 50 passes the intensity ratio of the two modes would be greater than 100. Consequently, because of the low gain and long mode buildup time (typically 0.75 - 3 ~_rset in our laser), only a small difference in the net single pass gain is required to allow only one mode to dominate. Qualitatively, we can now understand why, in the case of the P(16) transition, oscillations occur near the gain peak and are restricted to one longitudinal mode. Of course, use of a longer laser cavity would result in a closer frequency spacing of the modes and enhance the possibility that more than one mode would have nearly the same gain. Evidence for this spectrally narrowed, multimode operation has been recently reported [7].
November 1971
Table 1 lists the SLM frequencies that have been detected in this experiment. All SLM frequencies have been correlated with SF6 absorption minima. The P(14), P(16) and P(22) transitions oscillate in a single mode at a reproducible frequency. P(18) and P(20) also give rise to oscillations in a single mode but the exact frequency changes from pulse to pulse with occasional multifrequency pulses being observed. By looking for a beat note with a fast detector and oscilloscope we have determined that single frequency oscillation is the most probable event (80% of the time). When multifrequency operation occurred, the resulting beat note was in excess of 100 MHz. In such cases, the depth of modulation was small indicating that the second frequency was only incipient. Presumably, for these cases the exact frequency of oscillation is determined by the laser cavity tuning as well as the gain distribution. We feel that the utility of this technique lies not only in the ability to suppress all but one axial mode but also to stabilize that mode to a known frequency [in the case of P(14), P(16) and P(22)]. By using other gases and gas mixtures one should be able to obtain single frequency operation on all the transitions and, in addition, to tune that frequency over the entire laser bandwidth. The authors wish to thank GTE Sylvania Inc., Mountain View, California, for making available some of the optical components used in this experiment. REFERENCES [l] A. Nurmikko, T. DeTemple and S. E. Schwarz, Appl. Phys. Letters 18 (1971) 130. [2] F. A. Hopf, G. L. Lamb, C. K. Rhodes and M. 0. Scully, Phys. Rev. A3 (1971) 758. [3] A. V. Nurmikko and S. E. Schwarz, Opt. Commun. 2 (1971) 416. [4] E. D. Hinkley, Appl. Phys. Letters 16 (1971) 351. [5] T. J. Bridges, H.A. Haus and P. W. Hoff, IEEE J. Quantum Electron. QE-4 (1968) ,777. [6] W. R. Sooy, Appl. Phys. Letters 7 (1965) 36. [7] J. Gilbert and J. L. Lachambre, Appl. Phys. Letters 18 (1971) 187.
233