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Bistable polarization switching in a continuous wave ruby laser
Volume 65, n u m b e r 6
OPTICS C O M M U N I C A T I O N S
15 March 1988
BISTABLE P O L A R I Z A T I O N S W I T C H I N G IN A C O N T I N U O U S WAVE RUBY LASER N.M. LAWANDY t Division of Engineering and Department of Physics, Brown University, Providence, R1 02912, USA
and R. Sohrab AFZAL e Department of Physics, Brown University, Providence, RI 02912, USA Received 2 September 1987; revised manuscript received 15 October 1987
We have observed bistability in the output power, polarization state, and mode volume of an argon-ion laser pumped single mode ruby laser at 6943 A. The laser operates in a radially confined mode which exhibits hysteresis and bistability only when the pump polarization is parallel to the c-axis.
1. Introduction
Since the first demonstration of laser action in optically pumped ruby, this laser system has been one of the most difficult to understand [ 1 ]. Although the optical pumping cycle of the ruby system is known, much of the complicated temporal dynamics of the lasers still remains a mystery [2,3]. Moreover, in the myriad of instability scenarios which exist in the formal structure of laser physics in isotropic media, one must include the optical anisotropy of the Cr 3+ :A1203 crystal. In addition to pulsed operation of ruby lasers, continuous wave ruby lasers have been demonstrated. The continuous wave systems have also exhibited self-pulsing instabilities for which no completely consistent explanation exist [4,5]. Moreover, it should also be mentioned that ruby was the first solid state material used for optical bistability [6]. These experiments used the dependence of the index of refraction contribution of the charge transfer bands on the relative populations of the 2A and 2E levels. t Supported by an Alfred P. Sloan fellowship, an NSF Young Investigator Award and NASA (NAG-5-526). 2 Supported by a NASA Graduate Student Researcher Fellowship.
452
Previously, bistable polarization switching has been observed in HeNe and semiconductor diode lasers. These experiments have demonstrated that birefringence due to applied magnetic fields and naturally occurring birefringence could result in polarization rotation [7,8] and switching [9,10]. These experiments on semiconductor diode lasers are the only ones which use the excitation (injection current) as a control parameter. The effects of bistable polarization switching are attributed to different temperature dependent gains for TE and TM and some spatial hole burning arguments. In addition to these measurements it has been shown theoretically and experimentally that when two polarization eigenstates lose there degeneracy due to an internal birefringence, bistable polarization switching may occur as the laser cavity resonant frequency is swept [ 11 ]. In this letter, we report the observation of a polarization bistable cw single mode ruby laser system which cannot be explained in terms of anisotropic gain and loss arguments. The effect is completely dependent on the parallel orientation of the pump polarization relative to the c-axis of ruby and disappears for the perpendicular orientation. Moreover, the sample is held at a constant temperature and the ruby thermal index coefficients rule out the possibility of
0 030-4018/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 65, number 6
OPTICS COMMUNICATIONS
any complicating swept resonance effets due to heating by the argon laser pump [ 12].
larization oriented parallel and perpendicular to the rod c-axis. The pump and the laser transition crosssection are larger for the perpendicular orientation. At 77 K, the pump absorption length products were measured and found to be 2.3 and 1.9 for the perpendicular and parallel orientations respectively. The output of the ruby laser for the perpendicular pump orientation was gaussian-like with a polarization orientation perpendicular to the c-axis, and exhibited smooth reversible behavior. The output power versus pump power is shown in fig. 2. When the pump polarization was oriented parallel to the c-axis the laser emission was hysteretic in power and polarization. As the pump power was gradually increased from threshold, the laser emission was found to also be oriented parallel to the caxis. When the pump power exceeded a critical value, the laser output spontaneously switches polarization and becomes oriented perpendicular to the c-axis. Cycling down the pump power reveals a hysteresis region in which the polarization and power output continuously return to the lower branch value. The results of this experiment are shown in fig. 3.
2. Experimental description The ruby laser under investigation was optically pumped by an amplitude stabilized argon-ion laser capable of delivering up to 8 W. The ruby rod with a chromium ion density of 5 × 10 ~8 cm -3 was 3.85 cm in length with one face coated with a high reflection coating for 6943 ,~ and the other optically polished. This length allows single longitudinal mode operation at 77 K. The rod was mounted in a liquid nitrogen cooled dewar with quartz windows. The pump beam was focused to a 70 ~tm spot into the dewar using a 20 focal length lens and could have its linear polarization rotated relative to the c-axis plane. The gaussian like mode output of the ruby laser was monitored using fast detectors and a Laser Precision radiometer through conventional polarizers. The experimental setup is shown in fig. 1. Experiments were performed with the pump po-
[]
15 March 1988
ARGON LASER - ( multi-line )
Polarization rotator Filter
j0
Radiometer
Lens f=20
~
Polarizer
to Vacuum AR coated windows
kN2 Ruby rod
Thermister
Cu cold finger DEWAR
Fig. 1. Experimental apparatus.
453
Volume 65, number 6
OPTICS C O M M U N I C A T I O N S []
cycle up
•
cycIe down
3. Discussion
300
g
s=
200
U
o
J II
U
100
U
#
II
II
II
_Em 0
-
II
•
0
i
i
i
i
2
4
6
8
10
Argon output power ( watts )
Fig. 2. Output power as a function of p u m p power for the p u m p polarization oriented perpendicular to the c-axis. The ruby rod is at 77 K and the emission is at 6943 A.
In addition to these quantitative effects, we have observed that the output mode size contracts when the switch occurs indicating that the laser mode diameter in the rod has expanded. Simultaneous to this, we observe that the transmitted argon laser spot shifts indicating that the pump beam is also affected in the transition from one polarization to the other.
[] •
parallel perpendicular
150
[]
vE
We have observed bistability in the polarization and output as a function of pump power in a single mode cw ruby laser. The effect occurs only when the pump radiation is polarized parallel to the c-axis of the ruby. In addition, the switching is accompanied by discontinuous jumps in both the laser and pump mode cross sections. The fact that the initial laser emission for low pump powers is polarized in the same direction as the pump indicates that the linearly polarized pump generates a higher gain than would be expected due to the single-photon cross sections. More important is the fact that the gain for this polarization is a factor of eight less then that of the perpendicular orientation relative to the c-axis. At this point in time we do not have a definitive explanation for the mechanisms which produce a higher gain in the expected lower gain laser polarization orientation. We speculate that two photon processes may be operative since only the parallel polarization orientation between the pump and laser fields would allow this process to drive the emission. Further evidence for this is the nonlinear output versus pump power curves which do not agree with what a single photon process predicts. This latter point however, could result from mode confinement effects which lead to a variable filling factor. Experiments are currently underway to determine if in fact two photon processes are responsible for the effects observed.
t~
100
[]
~" so o
•
References
°7
[] [] [] [] i
i
i
4
6
8
0
Argon power ( watts )
Fig. 3. Output power as a function of p u m p power for the pump polarized parallel to the c-axis. The higher output branch is polarized parallel to the p u m p polarization at the higher pump powers. The switch in polarization and output power occurred at a p u m p power of 6.5 W. The rod is at 77 K and the emission is at 6943 ~.
454
15 March 1988
[ 1 ] T.H. Maiman, Nature 187 (1960) 493. [2] 1. Freund, Appl. Phys. Lett. 12 (1964) 388. [31 C . L Tang, H. Statz and G. deMars, Appl. Phys. Lett. 16 (1963) 222. [4] M. Birnbaum, P.H. Wendzikowski and C.L. Fincher, Appl. Phys. Lett. 16 (1970) 436. [5] A. Szabo, J. Appl. Phys. 49 (1978) 533. [6] T.N.C. Venkatesan and S.L. McCall, Appl. Phys. Lett. 30 (1977) 282. [7] W. Culshaw and J. Kannelaud, Phys. Rev. 136 (1964) 1209. [8] W. Culshaw and J. Kannelaud, Phys. Rev. 141 (1966) 237. [9] Y.C. Chen and J.M. Liu, Appl. Phys. Lett. 46 (1985) 16. [ 10 ] G. Ropars, A. Le Floch, G. J6z6quel, R. Le Naour, Y.C. Chen and J.M. Liu, IEEE J. Q u a n t u m Electron. QE-23 (1987) 1027. [ 11 ] A. Le Floch and G, Ropars, Phys. Rev. Lett. 52 ( 1984 ) 918, [ 12] N.M. Lawandy and R.S. Afzal, Appl. Phys. B, accepted for publication.
OPTICS C O M M U N I C A T I O N S
15 March 1988
BISTABLE P O L A R I Z A T I O N S W I T C H I N G IN A C O N T I N U O U S WAVE RUBY LASER N.M. LAWANDY t Division of Engineering and Department of Physics, Brown University, Providence, R1 02912, USA
and R. Sohrab AFZAL e Department of Physics, Brown University, Providence, RI 02912, USA Received 2 September 1987; revised manuscript received 15 October 1987
We have observed bistability in the output power, polarization state, and mode volume of an argon-ion laser pumped single mode ruby laser at 6943 A. The laser operates in a radially confined mode which exhibits hysteresis and bistability only when the pump polarization is parallel to the c-axis.
1. Introduction
Since the first demonstration of laser action in optically pumped ruby, this laser system has been one of the most difficult to understand [ 1 ]. Although the optical pumping cycle of the ruby system is known, much of the complicated temporal dynamics of the lasers still remains a mystery [2,3]. Moreover, in the myriad of instability scenarios which exist in the formal structure of laser physics in isotropic media, one must include the optical anisotropy of the Cr 3+ :A1203 crystal. In addition to pulsed operation of ruby lasers, continuous wave ruby lasers have been demonstrated. The continuous wave systems have also exhibited self-pulsing instabilities for which no completely consistent explanation exist [4,5]. Moreover, it should also be mentioned that ruby was the first solid state material used for optical bistability [6]. These experiments used the dependence of the index of refraction contribution of the charge transfer bands on the relative populations of the 2A and 2E levels. t Supported by an Alfred P. Sloan fellowship, an NSF Young Investigator Award and NASA (NAG-5-526). 2 Supported by a NASA Graduate Student Researcher Fellowship.
452
Previously, bistable polarization switching has been observed in HeNe and semiconductor diode lasers. These experiments have demonstrated that birefringence due to applied magnetic fields and naturally occurring birefringence could result in polarization rotation [7,8] and switching [9,10]. These experiments on semiconductor diode lasers are the only ones which use the excitation (injection current) as a control parameter. The effects of bistable polarization switching are attributed to different temperature dependent gains for TE and TM and some spatial hole burning arguments. In addition to these measurements it has been shown theoretically and experimentally that when two polarization eigenstates lose there degeneracy due to an internal birefringence, bistable polarization switching may occur as the laser cavity resonant frequency is swept [ 11 ]. In this letter, we report the observation of a polarization bistable cw single mode ruby laser system which cannot be explained in terms of anisotropic gain and loss arguments. The effect is completely dependent on the parallel orientation of the pump polarization relative to the c-axis of ruby and disappears for the perpendicular orientation. Moreover, the sample is held at a constant temperature and the ruby thermal index coefficients rule out the possibility of
0 030-4018/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 65, number 6
OPTICS COMMUNICATIONS
any complicating swept resonance effets due to heating by the argon laser pump [ 12].
larization oriented parallel and perpendicular to the rod c-axis. The pump and the laser transition crosssection are larger for the perpendicular orientation. At 77 K, the pump absorption length products were measured and found to be 2.3 and 1.9 for the perpendicular and parallel orientations respectively. The output of the ruby laser for the perpendicular pump orientation was gaussian-like with a polarization orientation perpendicular to the c-axis, and exhibited smooth reversible behavior. The output power versus pump power is shown in fig. 2. When the pump polarization was oriented parallel to the c-axis the laser emission was hysteretic in power and polarization. As the pump power was gradually increased from threshold, the laser emission was found to also be oriented parallel to the caxis. When the pump power exceeded a critical value, the laser output spontaneously switches polarization and becomes oriented perpendicular to the c-axis. Cycling down the pump power reveals a hysteresis region in which the polarization and power output continuously return to the lower branch value. The results of this experiment are shown in fig. 3.
2. Experimental description The ruby laser under investigation was optically pumped by an amplitude stabilized argon-ion laser capable of delivering up to 8 W. The ruby rod with a chromium ion density of 5 × 10 ~8 cm -3 was 3.85 cm in length with one face coated with a high reflection coating for 6943 ,~ and the other optically polished. This length allows single longitudinal mode operation at 77 K. The rod was mounted in a liquid nitrogen cooled dewar with quartz windows. The pump beam was focused to a 70 ~tm spot into the dewar using a 20 focal length lens and could have its linear polarization rotated relative to the c-axis plane. The gaussian like mode output of the ruby laser was monitored using fast detectors and a Laser Precision radiometer through conventional polarizers. The experimental setup is shown in fig. 1. Experiments were performed with the pump po-
[]
15 March 1988
ARGON LASER - ( multi-line )
Polarization rotator Filter
j0
Radiometer
Lens f=20
~
Polarizer
to Vacuum AR coated windows
kN2 Ruby rod
Thermister
Cu cold finger DEWAR
Fig. 1. Experimental apparatus.
453
Volume 65, number 6
OPTICS C O M M U N I C A T I O N S []
cycle up
•
cycIe down
3. Discussion
300
g
s=
200
U
o
J II
U
100
U
#
II
II
II
_Em 0
-
II
•
0
i
i
i
i
2
4
6
8
10
Argon output power ( watts )
Fig. 2. Output power as a function of p u m p power for the p u m p polarization oriented perpendicular to the c-axis. The ruby rod is at 77 K and the emission is at 6943 A.
In addition to these quantitative effects, we have observed that the output mode size contracts when the switch occurs indicating that the laser mode diameter in the rod has expanded. Simultaneous to this, we observe that the transmitted argon laser spot shifts indicating that the pump beam is also affected in the transition from one polarization to the other.
[] •
parallel perpendicular
150
[]
vE
We have observed bistability in the polarization and output as a function of pump power in a single mode cw ruby laser. The effect occurs only when the pump radiation is polarized parallel to the c-axis of the ruby. In addition, the switching is accompanied by discontinuous jumps in both the laser and pump mode cross sections. The fact that the initial laser emission for low pump powers is polarized in the same direction as the pump indicates that the linearly polarized pump generates a higher gain than would be expected due to the single-photon cross sections. More important is the fact that the gain for this polarization is a factor of eight less then that of the perpendicular orientation relative to the c-axis. At this point in time we do not have a definitive explanation for the mechanisms which produce a higher gain in the expected lower gain laser polarization orientation. We speculate that two photon processes may be operative since only the parallel polarization orientation between the pump and laser fields would allow this process to drive the emission. Further evidence for this is the nonlinear output versus pump power curves which do not agree with what a single photon process predicts. This latter point however, could result from mode confinement effects which lead to a variable filling factor. Experiments are currently underway to determine if in fact two photon processes are responsible for the effects observed.
t~
100
[]
~" so o
•
References
°7
[] [] [] [] i
i
i
4
6
8
0
Argon power ( watts )
Fig. 3. Output power as a function of p u m p power for the pump polarized parallel to the c-axis. The higher output branch is polarized parallel to the p u m p polarization at the higher pump powers. The switch in polarization and output power occurred at a p u m p power of 6.5 W. The rod is at 77 K and the emission is at 6943 ~.
454
15 March 1988
[ 1 ] T.H. Maiman, Nature 187 (1960) 493. [2] 1. Freund, Appl. Phys. Lett. 12 (1964) 388. [31 C . L Tang, H. Statz and G. deMars, Appl. Phys. Lett. 16 (1963) 222. [4] M. Birnbaum, P.H. Wendzikowski and C.L. Fincher, Appl. Phys. Lett. 16 (1970) 436. [5] A. Szabo, J. Appl. Phys. 49 (1978) 533. [6] T.N.C. Venkatesan and S.L. McCall, Appl. Phys. Lett. 30 (1977) 282. [7] W. Culshaw and J. Kannelaud, Phys. Rev. 136 (1964) 1209. [8] W. Culshaw and J. Kannelaud, Phys. Rev. 141 (1966) 237. [9] Y.C. Chen and J.M. Liu, Appl. Phys. Lett. 46 (1985) 16. [ 10 ] G. Ropars, A. Le Floch, G. J6z6quel, R. Le Naour, Y.C. Chen and J.M. Liu, IEEE J. Q u a n t u m Electron. QE-23 (1987) 1027. [ 11 ] A. Le Floch and G, Ropars, Phys. Rev. Lett. 52 ( 1984 ) 918, [ 12] N.M. Lawandy and R.S. Afzal, Appl. Phys. B, accepted for publication.