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Polarisation of 3 μm laser emission in YAlO3:Er
Volume 61, number 4
OPTICS COMMUNICATIONS
l 5 FebruaD, 1987
POLARISATION OF 3/~m LASER EMISSION IN YAIO3:Er M. S T A L D E R and W. L O T H Y Institute of Applied Physics, University of Bern, Sidlerstrasse .5, CH-3012 Bern, Switzerland
Received 5 November 1986
The polarisation properties of the 3/~m laser emission from YAIO3:Erat room temperature have been measured for rods oriented in two different crystallographic directions as well as with different dopant concentrations.
3 ~m emission from erbium doped crystal lasers has been obtained already in 1967 [ 1 ]. Recently, this wavelength has obtained much attention due to its potential for medical applications [2,3] and a number of papers have appeared describing 3/zm lasing in Er:YAG [4] and in YA103:Er [ 5 - 8 ] . Laser emission from YA103 crystals is polarised because of the orthorombic D~6(pnma) unit cell and the monoclinic CsEr 3+ site symmetry [ 9 ]. Depending on the dopant concentration more than one line from the transitions between 4111/2and 4113/2manifolds can oscillate [ 5,7,8] and the various laser lines can be differently polarised. As yet, however, no measurements of the polarisation properties have been reported. In our letter we describe the polarisation properties o f room temperature laser emission from YA103:Er laser crystals. Laser rods with their axis II to the crystallographic b-direction and with dopant concentrations o f 10% and 50% as well as a rod with the axis Ila and an Er concentration of 20% have been used in the experiment. The experimental arrangement is shown in fig. 1. The laser crystal is fixed in a m o u n t revolving around the rod axis. Since the absorption o f light in YA103:Er depends on the polarisation with respect to the crystallographic axes [10], we used a helical flashlamp to avoid variations of p u m p light absorption with the angle of rotation. The laser resonator is formed by dielectric mirrors, one with reflectivity R1 = 100% and radius of curvature o f 2m, the other mirror a fiat of R2 = 95%. The polarisation is selected by an intracavity Brewster window made from sap274
M 1
L
B
M 2
F
M
0 ~
D
....
Fig. 1. Experimental arrangement. M~, M2: resonator mirrors; B: Brewster plate; L: laser head with water cooled helical flashlamp and YAIO3:Errod; F: lens; M: monochromator; D: InAs photodiode; O: oscilloscope. phire with its optical axis adjusted parallel or perpendicular to the plane of incidence in order to avoid birefringence. The Brewster window with a refractive index of about 1.72 at 2.8 gtm [ I 1 ] has practically no losses for p-polarisation (E-vector II plane of incidence) and 43% loss per round trip for s-polarisation. This is enough to raise the threshold for spolarised laser action in the given resonator by a factor of about 7 with respect to p-polarisation. Therefore s-polarised laser emission is practically suppressed. The laser emission is focused with a lens of 6 cm focal length onto the slit o f a 20 cm grating m o n o c h r o m a t o r with a resolution of about + 1 nm. The transmitted light is detected with an InAs photodiode and monitored with a storage oscilloscope. In a preliminary experiment the threshold energies o f different laser lines have been measured as a function of the temperature in the range of 15-60°C. The result is shown in fig. 2. The threshold energy Eth is given in relative units with respect to E,h at 16°C. The laser rod is a YAIO3:Er (20%) crystal with its 0 030-401/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 61, number 4
OPTICS COMMUNICATIONS
15 February 1987
ap
..c 1,5
o~oJ
I
10 °
l
;ZO°
I
I
30 °
40 °
/
I
50 °
I
60 °
Temperature
Fig. 2. Laser threshold as a function of temperature in the range of 16°C-60°C in a YAIO3:Er (20%) crystal with its axis in the crystallographic a-direction. The laser was operated without the Brewster plate and oscillated simultaneously at wavelengths of • 2.92 pro, • 2.79 #m and • 2.73 #m.
axis Ua. The laser is operated without the Brewster plate and oscillates simultaneously at wavelengths of 2.92 gm, 2.79 /~m and 2.73 #m. The three wavelengths are identical to those observed by Kaminskii et al. in different rods with various dopant concentrations [ 5,7,8 ]. To our knowledge this is for the first time that simultaneous lasing at room temperature has been achieved on all the three lines. Fig. 2 shows that the laser threshold rises with temperature increasing from 16°C to 60°C by as much as 80% in the case of 2.73 #m. Therefore in all of the following experiments the temperature of the crystals has been kept constant at 16 ° C. With the Brewster plate inserted the laser rod is rotated with respect to the plane of incidence of the Brewster plate and the threshold is measured at different angles for each wavelength. The measurement is performed over a total angle of 180 ° and in view of the symmetry of the resulting values averaged in one quadrant of the symmetric figure. These values are indicated in figs. 3-5. The solid lines in these figures represent a fit of the measured values completed to 360 °. The result for a b-rod with 50% dopant concentration is shown in fig. 3. The threshold is measured in intervals of 5 °. The arrows termed by a and c indicate the position with a-axis and c-axis respectively parallel to the plane of incidence. The radius is inversely proportional to the lasing threshold; the circles denoted by 1, 2 and 4 represent the same values of threshold pump energy, double threshold pump energy and the fourfold energy respectively in all the
Fig. 3. Polarisation dependence of the laser threshold of a b-rod with 50% dopant concentration. The arrows termed by a and c indicate the position with the crystallographic a-axis and c-axis respectively parallel to the plane of incidence. The radius is inversely proportional to the lasing threshold; the circles denoted by 1, 2 and 4 represent the same values of threshold pump energy, double threshold pump energy and the fourfold energy respectively. The wavelengths are labelled with • for 2.92/zm, • for 2.79pm and • for 2.73 pro.
figures. Thus the relative threshold energies of the different graphs can be compared. From the figure, it can be seen that only one laser line at 2.92 #m is found and that minimum laser threshold results in an arrangement with the c-axis parallel to the plane of incidence. Fig. 4 shows the result for an a-rod with 20% dopant concentration. In this case the lowest threshold is found for the 2.73/zm emission polarised LIc. At angles near the b-axis the threshold of the 2.73 #m laser line becomes unstable. Lasing occurs only in a limited energy range; with excess pump energy 2.73 # m emission is suppressed. 2.79/zm emission occurs with a polarisation lib and 2.92/~m emission IIc. Fig. 5 shows the result for a b-rod with 10% dopant concentration. In this case the lowest threshold is found for the 2.73/tm emission polarised IIc. 2.79 #m emission has not been observed, however, a new wavelength at 2.763 #m has been found with minim u m threshold for a polarisation of about + 30 ° from the a-axis. Although the wavelength is close to the 2.7608 # m emission described in ref. [6] and assigned to the 4111/2X2 ~ 4I 13/2Y3 transition, the laser lines are not identical. With the energies of the levels measured at room temperature the emission at 2.763 a m is tentatively assigned to a transition from 4111/2X5 275
Volume 61, number 4
OPTICS COMMUNICATIONS
15 February 1937
ther introduction of wavelength selective absorbing filters into the laser r e s o n a t o r it is h o p e d to excite e v e n o t h e r t r a n s i t i o n s o f the 4 I I t 2 "-' 41132 manifblds. E x p e r i m e n t s to look [br new laser lines are in progress. In c o n c l u s i o n we h a v e s h o w n that s i m u l t a n e o u s laser oscillation can be a c h i e v e d on three different lines o f the 4Ijl 2 -~4113/,'- m a n i f b l d s at r o o m t e m p e r a ture. T h e p o l a r i s a t i o n p r o p e r t i e s o f a 50% b - r o d , a 20% a - r o d a n d a 10% b-rod h a v e b e e n m e a s u r e d . In the e m i s s i o n o f the 10% b-rod o p e r a t e d at r o o m t e m p e r a t u r e the t r a n s i t i o n 4I I j/2X 5 tO 41j3/2Y> at a wavelength o f 2.763 p m has b e e n o b s e r v e d for the first time. Fig. 4. Polarisation dependence of the laser threshold of an a-rod with 20% dopant concentration. The arrows termed by b and c indicate the position with the crystallographic b-axis and c-axis respectively parallel to the plane of incidence. The wavelengths are labelled with III for 2.92 ~m, • for 2,79,um and • for 2.73 ,um. tO 4II3/2Y5. To o u r k n o w l e d g e this is the first observ a t i o n o f this laser line. As in t h e o t h e r rods 2.92 tzm e m i s s i o n o c c u r s w i t h a p o l a r i s a t i o n IIc. With the knowledge of the polarisation properties it is possible to c o n t r o l l e d suppress u n d e s i r e d laser e m i s s i o n w i t h s i m p l e p o l a r i s i n g e l e m e n t s . W i t h fur-
Fig. 5. Polarisation dependence of the laser threshold of a b-rod with 10% dopant concentration. The arrows termed by a and c indicate the position with the crystallographic a-axis and c-axis respectively parallel to the plane of incidence. The wavelengths are labelled with II for 2.92 ktm, • for 2.763/tin and • for 2.73 //m.
276
We w o u l d like to t h a n k H.P. Weber, P. H e r r e n and M. F r e y for t h e i r s t i m u l a t i n g interest and support for o u r work, H.J. W e d e r we t h a n k for technical assistance. T h i s w o r k was s u p p o r t e d in part by the Swiss C o m m i s s i o n for the E n c o u r a g e m e n t o f Scientific Research.
References [I l M. Robinson and DP. Devor, Appt. Phys, Left, l0 11967) 167 [2] V.P, Gapotsev, S.M. Matitsin, A.A. lsineev and V.B. Kravchenko, Optics and Laser Technology 14 ( 1982 ) 189. [3] M.L. Wolbarsht, IEEE J. Quantum Electron. QE-20 ( 1984 ! t427. [4] E.V. Zharikov, V.I. Shekov, L.A. Kulevskii, T.N. Murina, V,V, Osiko, A.M. Prokhorov, A,D. Savel'ev, V.V. Smirnov~ B.P. Starikov and M.l. Timoshechkin, Sov. J. Quant. Electron. 4 (1975) 1039. [5 t A.A. KaminskiL T.I. Butaeva, A.O. Ivanov, l,V. Mochaiov, A.G. Petrosian, G.I. Rogov and V,A. Fedorov, Soy, Tech. Phys. Lett. 2 (1976) 308. [ 6 ] A.A. Kaminskii, V.A, Fedorov and I.V, Mochalov, Soy, Phys Dokl, 25 (1980) 744, [ 7 ] A.A. Kaminskii, V.A. Fedorov, A.O. lvanov, l.V. Mochalov and L.I. Krutova, Soy. Phys. Dokt. 27 (1982) 725. [8] A.A. Kaminskii, Soy, Phys. Dokl. 27 (1982) 1039, [9] R. Diehl and G. Brandt, Mat. Res. Bull. 10 (1975) 85, [ 10] M, Dgtwyler. W. Lfithy and H.P. Weber, to appear in IEEE J. of Quantum Electronics. [ t I ] I.H. Malitson, J. Opt, Soc. Am. 52 ( t 962 ) 1377.
OPTICS COMMUNICATIONS
l 5 FebruaD, 1987
POLARISATION OF 3/~m LASER EMISSION IN YAIO3:Er M. S T A L D E R and W. L O T H Y Institute of Applied Physics, University of Bern, Sidlerstrasse .5, CH-3012 Bern, Switzerland
Received 5 November 1986
The polarisation properties of the 3/~m laser emission from YAIO3:Erat room temperature have been measured for rods oriented in two different crystallographic directions as well as with different dopant concentrations.
3 ~m emission from erbium doped crystal lasers has been obtained already in 1967 [ 1 ]. Recently, this wavelength has obtained much attention due to its potential for medical applications [2,3] and a number of papers have appeared describing 3/zm lasing in Er:YAG [4] and in YA103:Er [ 5 - 8 ] . Laser emission from YA103 crystals is polarised because of the orthorombic D~6(pnma) unit cell and the monoclinic CsEr 3+ site symmetry [ 9 ]. Depending on the dopant concentration more than one line from the transitions between 4111/2and 4113/2manifolds can oscillate [ 5,7,8] and the various laser lines can be differently polarised. As yet, however, no measurements of the polarisation properties have been reported. In our letter we describe the polarisation properties o f room temperature laser emission from YA103:Er laser crystals. Laser rods with their axis II to the crystallographic b-direction and with dopant concentrations o f 10% and 50% as well as a rod with the axis Ila and an Er concentration of 20% have been used in the experiment. The experimental arrangement is shown in fig. 1. The laser crystal is fixed in a m o u n t revolving around the rod axis. Since the absorption o f light in YA103:Er depends on the polarisation with respect to the crystallographic axes [10], we used a helical flashlamp to avoid variations of p u m p light absorption with the angle of rotation. The laser resonator is formed by dielectric mirrors, one with reflectivity R1 = 100% and radius of curvature o f 2m, the other mirror a fiat of R2 = 95%. The polarisation is selected by an intracavity Brewster window made from sap274
M 1
L
B
M 2
F
M
0 ~
D
....
Fig. 1. Experimental arrangement. M~, M2: resonator mirrors; B: Brewster plate; L: laser head with water cooled helical flashlamp and YAIO3:Errod; F: lens; M: monochromator; D: InAs photodiode; O: oscilloscope. phire with its optical axis adjusted parallel or perpendicular to the plane of incidence in order to avoid birefringence. The Brewster window with a refractive index of about 1.72 at 2.8 gtm [ I 1 ] has practically no losses for p-polarisation (E-vector II plane of incidence) and 43% loss per round trip for s-polarisation. This is enough to raise the threshold for spolarised laser action in the given resonator by a factor of about 7 with respect to p-polarisation. Therefore s-polarised laser emission is practically suppressed. The laser emission is focused with a lens of 6 cm focal length onto the slit o f a 20 cm grating m o n o c h r o m a t o r with a resolution of about + 1 nm. The transmitted light is detected with an InAs photodiode and monitored with a storage oscilloscope. In a preliminary experiment the threshold energies o f different laser lines have been measured as a function of the temperature in the range of 15-60°C. The result is shown in fig. 2. The threshold energy Eth is given in relative units with respect to E,h at 16°C. The laser rod is a YAIO3:Er (20%) crystal with its 0 030-401/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 61, number 4
OPTICS COMMUNICATIONS
15 February 1987
ap
..c 1,5
o~oJ
I
10 °
l
;ZO°
I
I
30 °
40 °
/
I
50 °
I
60 °
Temperature
Fig. 2. Laser threshold as a function of temperature in the range of 16°C-60°C in a YAIO3:Er (20%) crystal with its axis in the crystallographic a-direction. The laser was operated without the Brewster plate and oscillated simultaneously at wavelengths of • 2.92 pro, • 2.79 #m and • 2.73 #m.
axis Ua. The laser is operated without the Brewster plate and oscillates simultaneously at wavelengths of 2.92 gm, 2.79 /~m and 2.73 #m. The three wavelengths are identical to those observed by Kaminskii et al. in different rods with various dopant concentrations [ 5,7,8 ]. To our knowledge this is for the first time that simultaneous lasing at room temperature has been achieved on all the three lines. Fig. 2 shows that the laser threshold rises with temperature increasing from 16°C to 60°C by as much as 80% in the case of 2.73 #m. Therefore in all of the following experiments the temperature of the crystals has been kept constant at 16 ° C. With the Brewster plate inserted the laser rod is rotated with respect to the plane of incidence of the Brewster plate and the threshold is measured at different angles for each wavelength. The measurement is performed over a total angle of 180 ° and in view of the symmetry of the resulting values averaged in one quadrant of the symmetric figure. These values are indicated in figs. 3-5. The solid lines in these figures represent a fit of the measured values completed to 360 °. The result for a b-rod with 50% dopant concentration is shown in fig. 3. The threshold is measured in intervals of 5 °. The arrows termed by a and c indicate the position with a-axis and c-axis respectively parallel to the plane of incidence. The radius is inversely proportional to the lasing threshold; the circles denoted by 1, 2 and 4 represent the same values of threshold pump energy, double threshold pump energy and the fourfold energy respectively in all the
Fig. 3. Polarisation dependence of the laser threshold of a b-rod with 50% dopant concentration. The arrows termed by a and c indicate the position with the crystallographic a-axis and c-axis respectively parallel to the plane of incidence. The radius is inversely proportional to the lasing threshold; the circles denoted by 1, 2 and 4 represent the same values of threshold pump energy, double threshold pump energy and the fourfold energy respectively. The wavelengths are labelled with • for 2.92/zm, • for 2.79pm and • for 2.73 pro.
figures. Thus the relative threshold energies of the different graphs can be compared. From the figure, it can be seen that only one laser line at 2.92 #m is found and that minimum laser threshold results in an arrangement with the c-axis parallel to the plane of incidence. Fig. 4 shows the result for an a-rod with 20% dopant concentration. In this case the lowest threshold is found for the 2.73/zm emission polarised LIc. At angles near the b-axis the threshold of the 2.73 #m laser line becomes unstable. Lasing occurs only in a limited energy range; with excess pump energy 2.73 # m emission is suppressed. 2.79/zm emission occurs with a polarisation lib and 2.92/~m emission IIc. Fig. 5 shows the result for a b-rod with 10% dopant concentration. In this case the lowest threshold is found for the 2.73/tm emission polarised IIc. 2.79 #m emission has not been observed, however, a new wavelength at 2.763 #m has been found with minim u m threshold for a polarisation of about + 30 ° from the a-axis. Although the wavelength is close to the 2.7608 # m emission described in ref. [6] and assigned to the 4111/2X2 ~ 4I 13/2Y3 transition, the laser lines are not identical. With the energies of the levels measured at room temperature the emission at 2.763 a m is tentatively assigned to a transition from 4111/2X5 275
Volume 61, number 4
OPTICS COMMUNICATIONS
15 February 1937
ther introduction of wavelength selective absorbing filters into the laser r e s o n a t o r it is h o p e d to excite e v e n o t h e r t r a n s i t i o n s o f the 4 I I t 2 "-' 41132 manifblds. E x p e r i m e n t s to look [br new laser lines are in progress. In c o n c l u s i o n we h a v e s h o w n that s i m u l t a n e o u s laser oscillation can be a c h i e v e d on three different lines o f the 4Ijl 2 -~4113/,'- m a n i f b l d s at r o o m t e m p e r a ture. T h e p o l a r i s a t i o n p r o p e r t i e s o f a 50% b - r o d , a 20% a - r o d a n d a 10% b-rod h a v e b e e n m e a s u r e d . In the e m i s s i o n o f the 10% b-rod o p e r a t e d at r o o m t e m p e r a t u r e the t r a n s i t i o n 4I I j/2X 5 tO 41j3/2Y> at a wavelength o f 2.763 p m has b e e n o b s e r v e d for the first time. Fig. 4. Polarisation dependence of the laser threshold of an a-rod with 20% dopant concentration. The arrows termed by b and c indicate the position with the crystallographic b-axis and c-axis respectively parallel to the plane of incidence. The wavelengths are labelled with III for 2.92 ~m, • for 2,79,um and • for 2.73 ,um. tO 4II3/2Y5. To o u r k n o w l e d g e this is the first observ a t i o n o f this laser line. As in t h e o t h e r rods 2.92 tzm e m i s s i o n o c c u r s w i t h a p o l a r i s a t i o n IIc. With the knowledge of the polarisation properties it is possible to c o n t r o l l e d suppress u n d e s i r e d laser e m i s s i o n w i t h s i m p l e p o l a r i s i n g e l e m e n t s . W i t h fur-
Fig. 5. Polarisation dependence of the laser threshold of a b-rod with 10% dopant concentration. The arrows termed by a and c indicate the position with the crystallographic a-axis and c-axis respectively parallel to the plane of incidence. The wavelengths are labelled with II for 2.92 ktm, • for 2.763/tin and • for 2.73 //m.
276
We w o u l d like to t h a n k H.P. Weber, P. H e r r e n and M. F r e y for t h e i r s t i m u l a t i n g interest and support for o u r work, H.J. W e d e r we t h a n k for technical assistance. T h i s w o r k was s u p p o r t e d in part by the Swiss C o m m i s s i o n for the E n c o u r a g e m e n t o f Scientific Research.
References [I l M. Robinson and DP. Devor, Appt. Phys, Left, l0 11967) 167 [2] V.P, Gapotsev, S.M. Matitsin, A.A. lsineev and V.B. Kravchenko, Optics and Laser Technology 14 ( 1982 ) 189. [3] M.L. Wolbarsht, IEEE J. Quantum Electron. QE-20 ( 1984 ! t427. [4] E.V. Zharikov, V.I. Shekov, L.A. Kulevskii, T.N. Murina, V,V, Osiko, A.M. Prokhorov, A,D. Savel'ev, V.V. Smirnov~ B.P. Starikov and M.l. Timoshechkin, Sov. J. Quant. Electron. 4 (1975) 1039. [5 t A.A. KaminskiL T.I. Butaeva, A.O. Ivanov, l,V. Mochaiov, A.G. Petrosian, G.I. Rogov and V,A. Fedorov, Soy, Tech. Phys. Lett. 2 (1976) 308. [ 6 ] A.A. Kaminskii, V.A, Fedorov and I.V, Mochalov, Soy, Phys Dokl, 25 (1980) 744, [ 7 ] A.A. Kaminskii, V.A. Fedorov, A.O. lvanov, l.V. Mochalov and L.I. Krutova, Soy. Phys. Dokt. 27 (1982) 725. [8] A.A. Kaminskii, Soy, Phys. Dokl. 27 (1982) 1039, [9] R. Diehl and G. Brandt, Mat. Res. Bull. 10 (1975) 85, [ 10] M, Dgtwyler. W. Lfithy and H.P. Weber, to appear in IEEE J. of Quantum Electronics. [ t I ] I.H. Malitson, J. Opt, Soc. Am. 52 ( t 962 ) 1377.