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Polarization characteristics in optically pumped, far-infrared rectangular waveguide lasers
Volume 15, number 3
OPTICS COMMUNICATIONS
November/December 1975
POLARIZATION CHARACTERISTICS IN OPTICALLY PUMPED, FAR-INFRARED RECTANGULAR WAVEGUIDE LASERS Masanobu YAMANAKA, Hiroyuki TSUDA and Satoru MITANI* Departm ent of A pplied Physics, Osaka University, Yamada-Kami, Suita, Osaka 565, Japan Received 11 June 1975, revised manuscript received 18 August 1975
It has been shown that a linearly polarized, comparatively strong output power can be obtained from optically pumped, far-infrared rectangular waveguide lasers when the rectangular, metallic oversized waveguide is used as a tall guide instead of a standard guide.
Waveguide laser action has been demonstrated in the far-infrared (FIR) region in both electrical discharge lasers [1] and optically pumped lasers [ 2 - 5 ] . With the latter lasers, the F I R cavity can be a small metallic waveguide with end mirrors which compacts the instrument. As is well-known, when the pump beam is linearly polarized, the FIR output from open resonators is also almost always linearly polarized [6] ; i.e. the plane of polarization has a definite orientation for each FIR transition and is either parallel or perpendicular to the plane of polarization of the input beam. However, there is no detailed report so far on the polarization characteristics in FIR waveguide lasers. On the other hand, it is also well-known that a rectangular metallic waveguide used as a light pipe can transmit the linearly polarized FIR radiation with low attenuation and with a high degree of polarization [7]. Therefore, if the rectangular metallic waveguide is used as a laser resonator, we will get a linearly polarized output power from it. In this letter, it will be shown for the first time that a linearly polarized FIR output power can be obtained from rectangular metallic waveguide lasers. Three types of waveguide resonator, i.e., rectangular metallic, cylindrical dielectric [2,8] and cylindrical metallic [2,8] waveguide resonators, were examined in this study; the latter two cylindrical wave* Present address: Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka 571, Japan. 426
guide resonators were used for comparison. All waveguide resonators except the resonator III (see table 1) were set in a vacuum envelope which has been described in detail elsewhere [5]. The waveguide resonators I, II, IV and V had one fixed, flat copper mirror and one translatable copper plunger [ 3 - 5 ] as the other flat mirror. Both mirrors had a coupling hole at the center; the input and output holes were 1 and 3 mm, respectively. The waveguide resonator III had two fixed, flat copper mirrors with a coupling hole of 1.5 mm at the center. For tuning, the length o f this resonator was slightly varied by changing the temperature; in order to get a larger change, the resonator length was simply increased from 1 to 1.5 m. Both ends of all waveguide resonators were sealed by an NaC1 plate and a polyethylene sheet. NH 3 gas used as active medium was flowed through the resonator. The dimension and material of the resonators studied are shown in table 1. Commercially available rectangular and cylindrical waveguides were used. To test also the guiding effect which will be discussed later, we chose the rectangular waveguide of 20 m m × 40 mm. The interior surfaces of all waveguides studied were polished to be glossy; however, the quantitative study on the polarization characteristics was not made with respect to the condition of the interior waveguide surfaces. The pump beam from the cw 10.78-/am N 2 0 laser [9], whose power was of a few watts and degree of polarization was about 94%, was reflected once by an aluminized mirror, and converged by a
Volume 15, number 3
OPTICS COMMUNICATIONS
Table 1 Polarization characteristics in optically pumped FIR waveguide lasers. WAVE GUIDE
RESONATOR
CONFI GURATION
DIMENSION AND MATERIAL
DEGREE OF POLARIZATION AND POLARIZATION PLANE PUMPBEAM 81.5-pro FROM ~dT PUT BEAM OUTPUT HOLE RELATIVE
POLARIZATION pLANE
OUTPUT BEAM
2Ommx40mm Xlm COPPER 20ram X 40ram
90 %
-
-
100 %
I
( STRONG ) 87 %
92 %
xlm
COPPER 20minx/-~0 mm
(MEDIUM) O%
75%
x 1.5m ,',5"
COPPER
12mmlDxI rn
(MEDIUM) 82%
93%
0%
19%
1£ FUSEDOUART2
12mmlDxlm
COPPER
ZnSe lens (focal length being 25.4 cm) onto the input hole. Here, the degree of polarization (P) of the pump beam at the input hole was about 89%, which was measured by a near-infrared grid polarizer o f P > 99% at around 10tam. The output beam from the output hole at 10.78 tam, 81.5 tam or 263.4/am was detected after proper attenuation by a Golay detector being immediately behind the output hole. The degree of polarization of the FIR output beam was measured by a FIR grid polarizer o f P ~ 100%. In table 1, the measured degree of polarization for five configurations of the waveguide resonator and the polarization planes are summarized with the pump beam from the output hole and with the 81.5-tam output beam. With the configurations I and II, the degree of polarization P of the pump beam from the output hole was, as expected, almost the same as that of the pump beam. We observed that the degree of polarization of the output beam at 81.5-tam of the NH 3 laser
November/December 1975
was always larger than that of the pump beam from the output hole. This may be due to the fact that the laser action takes place intensively with the electric field polarized in the preferred direction [6]. The polarization plane with the 81.5-tam output beam was, as expected, perpendicular [6] to the polarization plane of the pump beam, as shown in table 1. P of the 81.5-tam output beam was almost 100% with the configuration I, but P = 92% with the configuration II. Although the absolute output power was not measured in this study, the relative 81.5-tam output power with the configuration I was about ten times stronger than that with the configuration II. This is clearly understood by the fact that when the rectangular, metallic oversized waveguide is used as a tall guide in the far-infrared, it has about half the attenuation coefficient of the standard guide, for example, with TEm 0 modes [10]. The observed P = 92% with the configuration II shows that a small part of the output power is simultaneously oscillating as a tall guide because of its low attenuation. Meanwhile, the polarization plane with the 263.4-tam output beam was, as also expected, parallel [6] to the polarization plane of the pump beam. Since the 263.4-tam output power was about one order lower than that of the 81.5-tam transition, the degree of polarization with both configurations at 263.4-tam was almost 100%. The relative 263.4-tam output power with the configuration II, however, was stronger than that with the configuration I, for in this case the configuration II acted as a tall guide with the 263.4-tam transition. The rectangular waveguide resonator III was set at an angle of 45 ° against the polarization plane of the pump beam. The degree of polarization was nearly 0% with the pump beam from the output hole. This is due to the fact that when the direction of polarization of the incident beam is neither perpendicular nor parallel to the metallic plane of incidence, the reflected beam is elliptically polarized [7], resulting in complete depolarization after multiple reflections in the resonator. P of the 81.5-tam output beam was 75%, and the direction of polarization was parallel to the broad wall of the rectangular waveguide. This means that the tall guide action is dominant because of its low attenuation. Naturally, when this 1.5-m waveguide resonator was set at angles of 90 ° and 0 ° against the polarization plane of the pump beam, similar results were obtained with the configurations I and II, respectively. 427
Volume 15, number 2
OPTICS COMMUNICATIONS
With the three configurations I, II and III, clearly defined tuning curves were observed at 81.5 t~m and/or 263.4 pm by changing the resonator length. Although the rectangular waveguide used here is greatly oversized, both the tuning curve observed and the polarization characteristics with the configuration III indicate apparently that true guiding takes place rather than light pipe action [11 ]. If we consider the oversized waveguide resonator used here to be an open resonator, the diffraction loss at the open resonator, for example, at 263.4/am is too large to lase. This also confirms the above-mentioned guiding effect. To confirm how the rectangular metallic waveguide resonator is excellent in order to get the linearly polarized FIR output power, two types of cylindrical waveguide resonator made from fused quartz and copper were examined. With the waveguide resonator IV, the reflectivity of fused quartz at around 10 pm is so low that only the pump beam along the resonator axis may remain in the quartz waveguide resonator; thus the observed P = 82% of the pump beam from the output hole is a reasonable value. The 81.5-pro output beam had a relatively high degree of polarization of 93%, and the polarization plane of the output beam was perpendicular to the pump beam. This shows that the rectangular metallic waveguide laser has better polarization characteristics than that of the quartz waveguide. On the other hand, with the copper waveguide resonator V, the pump beam from the output hole was completely depolarized as observed with the configuration III; and it led to a low degree of polarization at 81.5/am. Moreover, the direction of polarization of the output beam for the strongest mode was scarcely perpendicular to the pump beam.
428
November/December 1975
In conclusion, the experiment has shown that a linearly polarized FIR output power can be obtained in spite of a metallic waveguide resonator if the rectangular metallic oversized waveguide is used as a tall guide. To obtain, however, higher FIR output power with linear polarization under a constant pump power a smaller rectangular waveguide than that adopted in this study will be recommended; then the confinement effect [5 ] of the pump beam will become large, resulting in higher output power.
References [ 1 ] H. Steffen and F.K. Kneub/ihl, 1EEE J. Quantum Electron. QE-4 (1968) 992. [2] D.T. Hodges and T.S. Hartwick, Appl. Phys. Lett. 23 (1973) 252. [3] D.T. Hodges and T.S. Hartwick, IEEE Trans. Microwave Theory Tech. MTT-22 (1974) 1118. [4] M. Yamanaka and H. Yoshinaga, IEEE Trans. Microwave Theory Tech. MTT-22 (1974) 1117. [5] A. Tanaka, A. Tanimoto, N. Murata, M. Yamanaka and H. Yoshinaga, Japan, J. Appl. Phys. 13 (1974) 1491. [6] T.Y. Chang, IEEE Trans. Microwave Theory Tech. MTT-22 (1974) 983. [7] T.O. Poehler and R. Turner, Appl. Opt. 9 (1970) 971. [8] A. Tanaka, Dr. Eng. Thesis (Osaka University, 1975). [9] A. Tanaka, A. Mori, Y. Homma and M. Yamanaka, Japan. J. Appl. Phys. 13 (1974) 2009. [ 10] D.J. Kroon and J.M. Van Nieuwland, Spectroscopic Techniques for Far Infra-red, Submillimetre and Millimetre Waves, ed. D.H. Martin (North-Holland Publish-
ing Company-Amsterdam 1967), Ch. 7. [II] B.J. Batt, H.L. Bradley, A. Doswell and D.J. Harris, IEEE Trans. Microwave Theory Tech. MTT-22 (1974) 1089.
OPTICS COMMUNICATIONS
November/December 1975
POLARIZATION CHARACTERISTICS IN OPTICALLY PUMPED, FAR-INFRARED RECTANGULAR WAVEGUIDE LASERS Masanobu YAMANAKA, Hiroyuki TSUDA and Satoru MITANI* Departm ent of A pplied Physics, Osaka University, Yamada-Kami, Suita, Osaka 565, Japan Received 11 June 1975, revised manuscript received 18 August 1975
It has been shown that a linearly polarized, comparatively strong output power can be obtained from optically pumped, far-infrared rectangular waveguide lasers when the rectangular, metallic oversized waveguide is used as a tall guide instead of a standard guide.
Waveguide laser action has been demonstrated in the far-infrared (FIR) region in both electrical discharge lasers [1] and optically pumped lasers [ 2 - 5 ] . With the latter lasers, the F I R cavity can be a small metallic waveguide with end mirrors which compacts the instrument. As is well-known, when the pump beam is linearly polarized, the FIR output from open resonators is also almost always linearly polarized [6] ; i.e. the plane of polarization has a definite orientation for each FIR transition and is either parallel or perpendicular to the plane of polarization of the input beam. However, there is no detailed report so far on the polarization characteristics in FIR waveguide lasers. On the other hand, it is also well-known that a rectangular metallic waveguide used as a light pipe can transmit the linearly polarized FIR radiation with low attenuation and with a high degree of polarization [7]. Therefore, if the rectangular metallic waveguide is used as a laser resonator, we will get a linearly polarized output power from it. In this letter, it will be shown for the first time that a linearly polarized FIR output power can be obtained from rectangular metallic waveguide lasers. Three types of waveguide resonator, i.e., rectangular metallic, cylindrical dielectric [2,8] and cylindrical metallic [2,8] waveguide resonators, were examined in this study; the latter two cylindrical wave* Present address: Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka 571, Japan. 426
guide resonators were used for comparison. All waveguide resonators except the resonator III (see table 1) were set in a vacuum envelope which has been described in detail elsewhere [5]. The waveguide resonators I, II, IV and V had one fixed, flat copper mirror and one translatable copper plunger [ 3 - 5 ] as the other flat mirror. Both mirrors had a coupling hole at the center; the input and output holes were 1 and 3 mm, respectively. The waveguide resonator III had two fixed, flat copper mirrors with a coupling hole of 1.5 mm at the center. For tuning, the length o f this resonator was slightly varied by changing the temperature; in order to get a larger change, the resonator length was simply increased from 1 to 1.5 m. Both ends of all waveguide resonators were sealed by an NaC1 plate and a polyethylene sheet. NH 3 gas used as active medium was flowed through the resonator. The dimension and material of the resonators studied are shown in table 1. Commercially available rectangular and cylindrical waveguides were used. To test also the guiding effect which will be discussed later, we chose the rectangular waveguide of 20 m m × 40 mm. The interior surfaces of all waveguides studied were polished to be glossy; however, the quantitative study on the polarization characteristics was not made with respect to the condition of the interior waveguide surfaces. The pump beam from the cw 10.78-/am N 2 0 laser [9], whose power was of a few watts and degree of polarization was about 94%, was reflected once by an aluminized mirror, and converged by a
Volume 15, number 3
OPTICS COMMUNICATIONS
Table 1 Polarization characteristics in optically pumped FIR waveguide lasers. WAVE GUIDE
RESONATOR
CONFI GURATION
DIMENSION AND MATERIAL
DEGREE OF POLARIZATION AND POLARIZATION PLANE PUMPBEAM 81.5-pro FROM ~dT PUT BEAM OUTPUT HOLE RELATIVE
POLARIZATION pLANE
OUTPUT BEAM
2Ommx40mm Xlm COPPER 20ram X 40ram
90 %
-
-
100 %
I
( STRONG ) 87 %
92 %
xlm
COPPER 20minx/-~0 mm
(MEDIUM) O%
75%
x 1.5m ,',5"
COPPER
12mmlDxI rn
(MEDIUM) 82%
93%
0%
19%
1£ FUSEDOUART2
12mmlDxlm
COPPER
ZnSe lens (focal length being 25.4 cm) onto the input hole. Here, the degree of polarization (P) of the pump beam at the input hole was about 89%, which was measured by a near-infrared grid polarizer o f P > 99% at around 10tam. The output beam from the output hole at 10.78 tam, 81.5 tam or 263.4/am was detected after proper attenuation by a Golay detector being immediately behind the output hole. The degree of polarization of the FIR output beam was measured by a FIR grid polarizer o f P ~ 100%. In table 1, the measured degree of polarization for five configurations of the waveguide resonator and the polarization planes are summarized with the pump beam from the output hole and with the 81.5-tam output beam. With the configurations I and II, the degree of polarization P of the pump beam from the output hole was, as expected, almost the same as that of the pump beam. We observed that the degree of polarization of the output beam at 81.5-tam of the NH 3 laser
November/December 1975
was always larger than that of the pump beam from the output hole. This may be due to the fact that the laser action takes place intensively with the electric field polarized in the preferred direction [6]. The polarization plane with the 81.5-tam output beam was, as expected, perpendicular [6] to the polarization plane of the pump beam, as shown in table 1. P of the 81.5-tam output beam was almost 100% with the configuration I, but P = 92% with the configuration II. Although the absolute output power was not measured in this study, the relative 81.5-tam output power with the configuration I was about ten times stronger than that with the configuration II. This is clearly understood by the fact that when the rectangular, metallic oversized waveguide is used as a tall guide in the far-infrared, it has about half the attenuation coefficient of the standard guide, for example, with TEm 0 modes [10]. The observed P = 92% with the configuration II shows that a small part of the output power is simultaneously oscillating as a tall guide because of its low attenuation. Meanwhile, the polarization plane with the 263.4-tam output beam was, as also expected, parallel [6] to the polarization plane of the pump beam. Since the 263.4-tam output power was about one order lower than that of the 81.5-tam transition, the degree of polarization with both configurations at 263.4-tam was almost 100%. The relative 263.4-tam output power with the configuration II, however, was stronger than that with the configuration I, for in this case the configuration II acted as a tall guide with the 263.4-tam transition. The rectangular waveguide resonator III was set at an angle of 45 ° against the polarization plane of the pump beam. The degree of polarization was nearly 0% with the pump beam from the output hole. This is due to the fact that when the direction of polarization of the incident beam is neither perpendicular nor parallel to the metallic plane of incidence, the reflected beam is elliptically polarized [7], resulting in complete depolarization after multiple reflections in the resonator. P of the 81.5-tam output beam was 75%, and the direction of polarization was parallel to the broad wall of the rectangular waveguide. This means that the tall guide action is dominant because of its low attenuation. Naturally, when this 1.5-m waveguide resonator was set at angles of 90 ° and 0 ° against the polarization plane of the pump beam, similar results were obtained with the configurations I and II, respectively. 427
Volume 15, number 2
OPTICS COMMUNICATIONS
With the three configurations I, II and III, clearly defined tuning curves were observed at 81.5 t~m and/or 263.4 pm by changing the resonator length. Although the rectangular waveguide used here is greatly oversized, both the tuning curve observed and the polarization characteristics with the configuration III indicate apparently that true guiding takes place rather than light pipe action [11 ]. If we consider the oversized waveguide resonator used here to be an open resonator, the diffraction loss at the open resonator, for example, at 263.4/am is too large to lase. This also confirms the above-mentioned guiding effect. To confirm how the rectangular metallic waveguide resonator is excellent in order to get the linearly polarized FIR output power, two types of cylindrical waveguide resonator made from fused quartz and copper were examined. With the waveguide resonator IV, the reflectivity of fused quartz at around 10 pm is so low that only the pump beam along the resonator axis may remain in the quartz waveguide resonator; thus the observed P = 82% of the pump beam from the output hole is a reasonable value. The 81.5-pro output beam had a relatively high degree of polarization of 93%, and the polarization plane of the output beam was perpendicular to the pump beam. This shows that the rectangular metallic waveguide laser has better polarization characteristics than that of the quartz waveguide. On the other hand, with the copper waveguide resonator V, the pump beam from the output hole was completely depolarized as observed with the configuration III; and it led to a low degree of polarization at 81.5/am. Moreover, the direction of polarization of the output beam for the strongest mode was scarcely perpendicular to the pump beam.
428
November/December 1975
In conclusion, the experiment has shown that a linearly polarized FIR output power can be obtained in spite of a metallic waveguide resonator if the rectangular metallic oversized waveguide is used as a tall guide. To obtain, however, higher FIR output power with linear polarization under a constant pump power a smaller rectangular waveguide than that adopted in this study will be recommended; then the confinement effect [5 ] of the pump beam will become large, resulting in higher output power.
References [ 1 ] H. Steffen and F.K. Kneub/ihl, 1EEE J. Quantum Electron. QE-4 (1968) 992. [2] D.T. Hodges and T.S. Hartwick, Appl. Phys. Lett. 23 (1973) 252. [3] D.T. Hodges and T.S. Hartwick, IEEE Trans. Microwave Theory Tech. MTT-22 (1974) 1118. [4] M. Yamanaka and H. Yoshinaga, IEEE Trans. Microwave Theory Tech. MTT-22 (1974) 1117. [5] A. Tanaka, A. Tanimoto, N. Murata, M. Yamanaka and H. Yoshinaga, Japan, J. Appl. Phys. 13 (1974) 1491. [6] T.Y. Chang, IEEE Trans. Microwave Theory Tech. MTT-22 (1974) 983. [7] T.O. Poehler and R. Turner, Appl. Opt. 9 (1970) 971. [8] A. Tanaka, Dr. Eng. Thesis (Osaka University, 1975). [9] A. Tanaka, A. Mori, Y. Homma and M. Yamanaka, Japan. J. Appl. Phys. 13 (1974) 2009. [ 10] D.J. Kroon and J.M. Van Nieuwland, Spectroscopic Techniques for Far Infra-red, Submillimetre and Millimetre Waves, ed. D.H. Martin (North-Holland Publish-
ing Company-Amsterdam 1967), Ch. 7. [II] B.J. Batt, H.L. Bradley, A. Doswell and D.J. Harris, IEEE Trans. Microwave Theory Tech. MTT-22 (1974) 1089.