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A magnetically polarized He-Ne laser
Volume
19, number
3
OPTICS COMMUNICATIONS
A MAGNETICALLY
POLARIZED
He-Ne
December
1976
LASER
M.D. CRISP Owens-Illinois, Inc., Corporate Technology, P.O. Box 103.5, Toledo, Ohio 43666, USA Received
4 August
1976
The performance of an internal mirror He-Ne laser that has a fraction of its capillary immersed in a strong transverse magnetic field will be described. The field is produced by permanent magnets which are built into the laser’s housing. The laser produces a light output which has less than 0.1% of its power in the modes polarized orthogonal to the magnetic field. The attainment of such a high degree of polarization requires that the magnetic field be oriented relative to naturally occurring anisotropies of the laser cavity. Misalignment not only lowers the degree of polarization, but also decreases the laser’s output.
The effects of a magnetic field on the operation of a He-Ne laser have been studied by several researchers. Most investigations have dealt with lasers whose entire length is immersed in a uniform magnetic field [l-3] . The papers by Sargent, Lamb and Fork [2,3] contain a fairly complete set of references on the subject. McMahon [4] has pointed out that a 500-1000 G transverse magnetic field applied to a portion of the plasma tube can result in “complete linear polarization” of the laser’s output. A detailed study of the effects of a transverse magnetic field which is applied to a fraction of the discharge capillary of a He-Ne laser operating at 6328 .&will be reported here. This study has culminated in a practical technique for polarizing a laser by incorporating permanent magnets into the laser’s housing as shown in figs. 1 and 2. In addition to magnetic polarization, the laser shown in figs. 1 and 2 has solder glass seals making a hermetically sealed device. The solder glass is impervious to water vapor and allows higher bakeout temperatures (typically 375’C) than the usual epoxy seals. For the sake of simplicity, the lasers studied in figs. 3,4 and 5 were filled with a 7 : 1 mixture of 3He : 2oNe, but other experiments were performed on lasers with different ratios and isotope mixtures without a significant change in the observations. It has been reported that cavity anisotropies favor certain directions of polarization over others in a 316
SILASTIC
RUBBER_,
i/< CiTNODE
FOAM PLASTICS,
BALLAST
MAGNET
YOKE 1
-ANODE
CLIP
RESISTORPRESS
0
1
2 SCALE
3
4
5
6
ANODE-:
FIT
’
INCHES
Fig. 1. This figure shows how the magnets are held in an iron yoke around the neck of the laser. The iaser’s envelope has a narrow extension which protects the capillary from convection effects. Not shown is the flat mirror which is attached to the other end of the capillary with solder glass and a hole drilled in the capillary to allow the electrical discharge to reach the cathode.
He-Ne laser’s output [.51. Rotating our laser about its beam’s axis so that the polarizing effect of the magnetic field worked with the cavity anisotropies was found to increase the degree of polarization by more than a factor of five. The total power output of the laser was also maximum when the magnetic field and cavity anisotropies were aligned. The unpolarized component of a laser’s output is plotted as a function of the angle between cavity anisotropies and the magnetic field in fig. 3. This angle is adjusted to give maximum degree of polarization in our magnetically polarized laser. For the laser studied in fig. 3 the power of the polarized component will be 2000 times larger than the unpolarized component (i.e., the extinction ratio will equal
Volume 19, number 3
OPTICS COMMUNICATIONS
December 1976
Fig. 2. This photograph shows the actual laser and its accompanying magnetic yoke. Also shown is a thick fiim ballast resistor that can be inserted into the yoke.
ANGLE OF ORIENTATION
Fig. 3. The fraction of a laser’s power that is in modes polarized orthogonal to the main output is plotted as a function of rotation angle of the laser about its beam’s axis. The reference angle is arbitrarily set so that 0” corresponds to the highest degree of polarization. Half of the laser’s capillary was subjected to a transverse magnetic field of 1000 G in this experiment.
2000 : 1 and the degree of polarization [6] will equal 0.9995) if the orientation angle is set equal to 0”. Although the origin of the cavity anisotropies is unknown and therefore not under experimental control, good polarization was obtained on a number of different laser tubes. Extinction ratios larger than 500 : 1 (and usually larger than 1000 : 1) were consistently obtained for the 23 experimental laser tubes which were studied in great detail. The degree of polarization was studied as a function of the fraction of the laser’s capillary which was immersed in the magnetic field. The experiment, which was performed with the laser optimally oriented with respect to its cavity anisotropies, showed the degree of polarization increasing monotonically until one-half of its capillary was immersed in the magnetic field. Increasing the amount of capillary in the magnetic field to three quarters of the discharge length did not result in a further increase in degree of polarization. The components of the laser’s output power that are polarized parallel to and orthogonal to the mag netic field are plotted as a function of the magnetic field strength in fig. 4. It is seen from this graph that a magnetic field of 750 G applied to one-half of the capillary is strong enough to produce a well-polarized laser. In none of the 23 experimental tubes was it 317
Volume 19, number 3
OPTICS COMMUNICATIONS
December 1976
t--628 MHz--i FREQUENCY
COMPONENT
200
400
MAGNETIC
600 FIELD
800 IN
1000
GAUSS
Fig. 4. The fraction of a laser’s output that is polarized parallel to and perpendicular to the magnetic field is plotted as a function of the strength of the field which is applied to half of its capillary. The laser was oriented for optimum polarization during this experiment.
found that a magnetic field of 100 G would produce an extinction ratio as large as 100 : 1 as reported by Mas et al. [7]. It is possible that the particular laser which they studied had an unusually large cavity asymmetry. Unless great care was taken to keep the laser at a constant temperature, there were small thermally induced cavity length changes which result in longitudinal modes moving through the gain curve. A scanning spherical mirror interferometer was used to analyze the spectral output of a magnetically polarized laser. When the spectrum was observed over a sufficiently long time to include the passage of several longitudinal modes, it was found that saturation effects are present in the magnetically polarized laser’s output. Fig. 5 illustrates that the saturation effects can be eliminated by simply removing the magnets. A power spectrum similar to the one observed in the upper part of fig. 5 was observed for a laser of the same cavity parameters which was polarized with a Brewster window. The laser studied in fig. 5 had two longitudinal modes excited at all times. It is believed that the saturation effects seen in fig. 5 occur when these two modes are 318
Fig. 5. The power spectrum of a polarized laser shows saturation effects (lower curve) which can be eliminated by removing the magnets (upper curve). The output power was observed as a function of frequency with a scanning spherical mirror interferometer. The observation was made over a sufficiently long time to include several thermally induced passages of longitudinal modes through the gain curve. This laser’s cavity separation was 23.9 cm, the frequency difference between its longitudinal modes was 628 MHz, and it was operating at an output power of 2 mW.
interacting with one or two common velocity groups of 2oNe atoms. Removal of the magnetic field allows the two longitudinal modes to become orthogonally polarized. This decouples them and eliminates the saturation effect. Observation of the power spectrum of the component of light polarized perpendicular to the magnetic field reveals that it is most intense when the longitudinal modes lie symmetrically on each side of the Doppler curve. According to fig. 5, this is the condition required for maximum power saturation of the component polarized parallel to the field. A thorough understanding of these phenomena, as well as other properties of the magnetically polarized laser, would require the application of Lamb’s theory 181. The electrical and noise characteristics of the laser tube are altered by placing part of its capillary in a magnetic field. The magnetic field causes the electrons to move along curved paths resulting in more collisions per unit length of capillary. This lowers the mean energy that the electrons gain from the electric field so that, in many aspects, the changes are equivalent to those that would occur if the pressure were increased [9]. At normal filling pressures, the magnetic field raises the striking and operating voltages, lowers the current threshold for plasma oscillations and decreases the output power of the laser. The lowering of the threshold for plasma oscillation can be compensated for
Volume
19, number
3
OPTICS
COMMUNICATIONS
by lowering the laser tube filling pressure by 10% to 20%. Some insight into the laser’s operation can be obtained by considering the Zeeman splitting of the gain curve in that portion of the capillary which is subject to the magnetic field. A field of 1000 G is strong enough to cause frequency shifts that are greater than the 6328 A Doppler half width. This causes a frequency mismatch between the AM= 21 transitions of atoms in the high field region and the zero field region. The resultant lowering of the total gain for the Ml = _+1 transitions would cause the laser to prefer to operate in a linearly polarized mode with the electric field vector parallel to the magnetic field. Aligning the magnetic field relative to inherent cavity loss anisotropies can be expected to further increase the polarization preference. However, arguments based upon considerations of the single pass gain seem inadequate for explaining the high degree of polarization obtained for tubes operating well over the lasing threshold. A complete description of this magnetically polarized laser would require the application of Lamb’s semi-
December
1976
classical theory of a multimode laser [Sl to a laser with a fraction of its capillary in a magnetic field. The analysis should explain the onset of polarization related saturation effects which are observed when the magnetic field gradually increases from zero to 1000 G
References [ 11 R. Paananen, CL. Tang and H. Statz, Proc. IEEE 5 1 (1963) 63. (21 M. Sargent III, W.E. Lamb, Jr. and R.L. Fork, Phys. Rev. 164 (1967) 436. [ 3) M. Sargent III, W.E. Lamb, Jr. and R.L. Fork, Phys. Rev. 164 (1967) 450. [4] D.H. McMahon, Rev. Sci. Instr. 40 (5) (1969) 727. [S] A. Javan, E.A. Ballik and W.L. Bond, J. Opt’. Sot. Am. 52 (1962) 96. [6] M. Born and E. Wolf, Principles of Optics (Pergamon, New York, 196.5), p. 44. [7] G. Mas, H. Blancher and J. Roig, Appl. Opt. 13 (1974) 2771. [8] W.E. Lamb, Jr., Phys. Rev. A 134 (1964) 1429. (91 J.S. Townsend and E.W.B. Gill, Phil. Mag. 26 (1938) 290.
319
19, number
3
OPTICS COMMUNICATIONS
A MAGNETICALLY
POLARIZED
He-Ne
December
1976
LASER
M.D. CRISP Owens-Illinois, Inc., Corporate Technology, P.O. Box 103.5, Toledo, Ohio 43666, USA Received
4 August
1976
The performance of an internal mirror He-Ne laser that has a fraction of its capillary immersed in a strong transverse magnetic field will be described. The field is produced by permanent magnets which are built into the laser’s housing. The laser produces a light output which has less than 0.1% of its power in the modes polarized orthogonal to the magnetic field. The attainment of such a high degree of polarization requires that the magnetic field be oriented relative to naturally occurring anisotropies of the laser cavity. Misalignment not only lowers the degree of polarization, but also decreases the laser’s output.
The effects of a magnetic field on the operation of a He-Ne laser have been studied by several researchers. Most investigations have dealt with lasers whose entire length is immersed in a uniform magnetic field [l-3] . The papers by Sargent, Lamb and Fork [2,3] contain a fairly complete set of references on the subject. McMahon [4] has pointed out that a 500-1000 G transverse magnetic field applied to a portion of the plasma tube can result in “complete linear polarization” of the laser’s output. A detailed study of the effects of a transverse magnetic field which is applied to a fraction of the discharge capillary of a He-Ne laser operating at 6328 .&will be reported here. This study has culminated in a practical technique for polarizing a laser by incorporating permanent magnets into the laser’s housing as shown in figs. 1 and 2. In addition to magnetic polarization, the laser shown in figs. 1 and 2 has solder glass seals making a hermetically sealed device. The solder glass is impervious to water vapor and allows higher bakeout temperatures (typically 375’C) than the usual epoxy seals. For the sake of simplicity, the lasers studied in figs. 3,4 and 5 were filled with a 7 : 1 mixture of 3He : 2oNe, but other experiments were performed on lasers with different ratios and isotope mixtures without a significant change in the observations. It has been reported that cavity anisotropies favor certain directions of polarization over others in a 316
SILASTIC
RUBBER_,
i/< CiTNODE
FOAM PLASTICS,
BALLAST
MAGNET
YOKE 1
-ANODE
CLIP
RESISTORPRESS
0
1
2 SCALE
3
4
5
6
ANODE-:
FIT
’
INCHES
Fig. 1. This figure shows how the magnets are held in an iron yoke around the neck of the laser. The iaser’s envelope has a narrow extension which protects the capillary from convection effects. Not shown is the flat mirror which is attached to the other end of the capillary with solder glass and a hole drilled in the capillary to allow the electrical discharge to reach the cathode.
He-Ne laser’s output [.51. Rotating our laser about its beam’s axis so that the polarizing effect of the magnetic field worked with the cavity anisotropies was found to increase the degree of polarization by more than a factor of five. The total power output of the laser was also maximum when the magnetic field and cavity anisotropies were aligned. The unpolarized component of a laser’s output is plotted as a function of the angle between cavity anisotropies and the magnetic field in fig. 3. This angle is adjusted to give maximum degree of polarization in our magnetically polarized laser. For the laser studied in fig. 3 the power of the polarized component will be 2000 times larger than the unpolarized component (i.e., the extinction ratio will equal
Volume 19, number 3
OPTICS COMMUNICATIONS
December 1976
Fig. 2. This photograph shows the actual laser and its accompanying magnetic yoke. Also shown is a thick fiim ballast resistor that can be inserted into the yoke.
ANGLE OF ORIENTATION
Fig. 3. The fraction of a laser’s power that is in modes polarized orthogonal to the main output is plotted as a function of rotation angle of the laser about its beam’s axis. The reference angle is arbitrarily set so that 0” corresponds to the highest degree of polarization. Half of the laser’s capillary was subjected to a transverse magnetic field of 1000 G in this experiment.
2000 : 1 and the degree of polarization [6] will equal 0.9995) if the orientation angle is set equal to 0”. Although the origin of the cavity anisotropies is unknown and therefore not under experimental control, good polarization was obtained on a number of different laser tubes. Extinction ratios larger than 500 : 1 (and usually larger than 1000 : 1) were consistently obtained for the 23 experimental laser tubes which were studied in great detail. The degree of polarization was studied as a function of the fraction of the laser’s capillary which was immersed in the magnetic field. The experiment, which was performed with the laser optimally oriented with respect to its cavity anisotropies, showed the degree of polarization increasing monotonically until one-half of its capillary was immersed in the magnetic field. Increasing the amount of capillary in the magnetic field to three quarters of the discharge length did not result in a further increase in degree of polarization. The components of the laser’s output power that are polarized parallel to and orthogonal to the mag netic field are plotted as a function of the magnetic field strength in fig. 4. It is seen from this graph that a magnetic field of 750 G applied to one-half of the capillary is strong enough to produce a well-polarized laser. In none of the 23 experimental tubes was it 317
Volume 19, number 3
OPTICS COMMUNICATIONS
December 1976
t--628 MHz--i FREQUENCY
COMPONENT
200
400
MAGNETIC
600 FIELD
800 IN
1000
GAUSS
Fig. 4. The fraction of a laser’s output that is polarized parallel to and perpendicular to the magnetic field is plotted as a function of the strength of the field which is applied to half of its capillary. The laser was oriented for optimum polarization during this experiment.
found that a magnetic field of 100 G would produce an extinction ratio as large as 100 : 1 as reported by Mas et al. [7]. It is possible that the particular laser which they studied had an unusually large cavity asymmetry. Unless great care was taken to keep the laser at a constant temperature, there were small thermally induced cavity length changes which result in longitudinal modes moving through the gain curve. A scanning spherical mirror interferometer was used to analyze the spectral output of a magnetically polarized laser. When the spectrum was observed over a sufficiently long time to include the passage of several longitudinal modes, it was found that saturation effects are present in the magnetically polarized laser’s output. Fig. 5 illustrates that the saturation effects can be eliminated by simply removing the magnets. A power spectrum similar to the one observed in the upper part of fig. 5 was observed for a laser of the same cavity parameters which was polarized with a Brewster window. The laser studied in fig. 5 had two longitudinal modes excited at all times. It is believed that the saturation effects seen in fig. 5 occur when these two modes are 318
Fig. 5. The power spectrum of a polarized laser shows saturation effects (lower curve) which can be eliminated by removing the magnets (upper curve). The output power was observed as a function of frequency with a scanning spherical mirror interferometer. The observation was made over a sufficiently long time to include several thermally induced passages of longitudinal modes through the gain curve. This laser’s cavity separation was 23.9 cm, the frequency difference between its longitudinal modes was 628 MHz, and it was operating at an output power of 2 mW.
interacting with one or two common velocity groups of 2oNe atoms. Removal of the magnetic field allows the two longitudinal modes to become orthogonally polarized. This decouples them and eliminates the saturation effect. Observation of the power spectrum of the component of light polarized perpendicular to the magnetic field reveals that it is most intense when the longitudinal modes lie symmetrically on each side of the Doppler curve. According to fig. 5, this is the condition required for maximum power saturation of the component polarized parallel to the field. A thorough understanding of these phenomena, as well as other properties of the magnetically polarized laser, would require the application of Lamb’s theory 181. The electrical and noise characteristics of the laser tube are altered by placing part of its capillary in a magnetic field. The magnetic field causes the electrons to move along curved paths resulting in more collisions per unit length of capillary. This lowers the mean energy that the electrons gain from the electric field so that, in many aspects, the changes are equivalent to those that would occur if the pressure were increased [9]. At normal filling pressures, the magnetic field raises the striking and operating voltages, lowers the current threshold for plasma oscillations and decreases the output power of the laser. The lowering of the threshold for plasma oscillation can be compensated for
Volume
19, number
3
OPTICS
COMMUNICATIONS
by lowering the laser tube filling pressure by 10% to 20%. Some insight into the laser’s operation can be obtained by considering the Zeeman splitting of the gain curve in that portion of the capillary which is subject to the magnetic field. A field of 1000 G is strong enough to cause frequency shifts that are greater than the 6328 A Doppler half width. This causes a frequency mismatch between the AM= 21 transitions of atoms in the high field region and the zero field region. The resultant lowering of the total gain for the Ml = _+1 transitions would cause the laser to prefer to operate in a linearly polarized mode with the electric field vector parallel to the magnetic field. Aligning the magnetic field relative to inherent cavity loss anisotropies can be expected to further increase the polarization preference. However, arguments based upon considerations of the single pass gain seem inadequate for explaining the high degree of polarization obtained for tubes operating well over the lasing threshold. A complete description of this magnetically polarized laser would require the application of Lamb’s semi-
December
1976
classical theory of a multimode laser [Sl to a laser with a fraction of its capillary in a magnetic field. The analysis should explain the onset of polarization related saturation effects which are observed when the magnetic field gradually increases from zero to 1000 G
References [ 11 R. Paananen, CL. Tang and H. Statz, Proc. IEEE 5 1 (1963) 63. (21 M. Sargent III, W.E. Lamb, Jr. and R.L. Fork, Phys. Rev. 164 (1967) 436. [ 3) M. Sargent III, W.E. Lamb, Jr. and R.L. Fork, Phys. Rev. 164 (1967) 450. [4] D.H. McMahon, Rev. Sci. Instr. 40 (5) (1969) 727. [S] A. Javan, E.A. Ballik and W.L. Bond, J. Opt’. Sot. Am. 52 (1962) 96. [6] M. Born and E. Wolf, Principles of Optics (Pergamon, New York, 196.5), p. 44. [7] G. Mas, H. Blancher and J. Roig, Appl. Opt. 13 (1974) 2771. [8] W.E. Lamb, Jr., Phys. Rev. A 134 (1964) 1429. (91 J.S. Townsend and E.W.B. Gill, Phil. Mag. 26 (1938) 290.
319