- Email: [email protected]
Diffraction filtered resonator for Rh6G dye laser transversely pumped by a copper vapor laser
15 January 1997
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 134 (1997) 149- 154
Diffraction filtered resonator for Rh6G dye laser transversely pumped by a copper vapor laser S.K. Dixit, S.R. Daulatabad, P.K. Shukla, R. Bhatnagar Luser
Programme.
Centrefi,r Advanced
Technology.
Indore-452013.
India
Received 21 May 1996; accepted 28 August 1996
Abstract The output from a transverse
pumped dye laser usually has poor beam quality and high divergence
asymmetry
along
horizontal and vertical directions. In this paper, it is demonstrated that diffraction filtered resonators can be applied to such small cross-section lasers with significant improvements in the output beam quality. A copper vapor laser pumped dye laser was operated with output divergence of 0.55 mrad in horizontal and vertical directions as compared to 5.2 mrad and 4.5 mrad respectively with a conventional plane-plane resonator.
1. Introduction
Diffraction filtered resonators have been applied to high gain, large aperture lasers to obtain diffraction limited beams [1,2]. In small cross-section high gain lasers like dye lasers, it is difficult to use these resonators. The output from transversely pumped dye lasers have poor beam quality and high divergence because of highly non-uniform gain and small size of the active medium which diffracts the radiation moving back and forth between the resonator mirrors. Lago et al. demonstrated diffraction limited symmetric output beams from a copper vapor laser pumped Rh6G dye laser using a stable, folded, grazing incidence cavity [3]. Recently Farahbod et al. reported the use of a self-filtering resonator to a nitrogen laser pumped Rh6G dye laser [4]. In this paper, we report 0030-4018/97/$17.00 PII
SOO30-4018(96)00575-5
Copyright
the use of diffraction filtered resonators [DFR] in a copper vapor laser [CVL] transverse pumped dye laser [DL]. A criteria for the selection of the filtering aperture in a gain medium of such a small cross-section is established. It is shown that the size of the filtering aperture in relation to the aperture offered by the gain medium itself is important in interpreting the results.
2. Experimental
arrangement
The experimental arrangement (Fig. 1) consists of a home made CVL 151 pumped dye laser. The CVL gives average output power of 10 W (5.5 kHz). In experiments, the laser was operated at 4.2 W (A = 510 nm) to minimize the amplified spontaneous
0 1997 Elsevier Science B.V. All rights reserved.
1.50
SK. Dixir ef 01./Optics
Communicufions
emission from the dye. The dye laser consists of a demountable dye cell (gain length 25 mm> developed in house with 5 mM solution of Rh6G in ethanol circulating at a flow rate of 4 lpm. The pump beam was focused by a cylindrical lens L ,(f= 10 cm> and its position from the dye cell was adjusted such that the output beam is almost circular. The resonator is a plane-plane generalized diffraction filtered resonator GDFR [I] consisting of plane mirrors M, (R = lOO%), and M, (R = 50%) and a hard aperture (H,) (Fig. 1). The resonator length was 27 cm. H, was placed close to the dye cell window and 7 cm away from mirror M,. H, of diameters varying from 0.15 mm to 0.6 mm were used. The divergence was estimated from the far-field intensity distribution recorded in the focal plane of a 1 m lens, L,, by using a slit-photomultiplier combination to scan the focal spot. The near field intensity distributions were recorded using a slit-PMT combination by imaging the intensity distribution at the output mirror, M, by a 1 : 1 imaging set-up. Since the geometrical divergence decides the bandwidth of the dye laser it is customary to estimate the divergence by measuring the spot size at the focal plane of a lens without collimating the output beam. In experiments, the divergence of the output beam for uncollimated and collimated cases were measured. For collimating the output beam, a lens, L, (f= 50 cm) placed approximately 50 cm from the aperture plane, was used. Rings around the central lobe were removed by another aperture placed in the output beam. The performance of GDFRs was
134 (1997) 149- 154
compared with that of the same resonator with the intracavity aperture removed.
3. Estimation of the dye aperture A crucial factor controlling the behavior of the GDFR dye laser is the nature and the size of the aperture offered by the dye medium, D,. It is essential to estimate its size to understand its role vis a vis the filtering aperture H,. In order to estimate D,, mode calculations were performed on the planeplane resonator for various values of D, (without H,) to see for which value of D,, the calculated divergence and near field diameter match with experimentally observed values. The calculations were performed on uncollimated output (lens L, removed). Since the output can be made circular by suitably adjusting the pump beam focusing conditions, circular symmetry for D, was assumed. For convenience D, was assumed to be a single hard aperture located at the centre of the dye cell though in practice the dye aperture has a finite length. It may behave like a soft aperture with tapered attenuation towards the edge. Mode calculations were carried out using resonator design software, PARAXIA, developed by A.E. Siegman [Genesse Optics Software Inc]. The program was run on a Macintosh Power PC. The plane-plane cavity (without H,) used in the simulation is the same as that used in the experiment (Fig. 1). The resonator length was 27 cm and D, was located 9.5 cm from M , . The wavelength used is 590
FAR-FIELD MEASUFIEMENT I
I
2.5cm
Fig. 1. Experimental
arrangement
- CVL pumped GDFR dye laser.
S.K. Dixit ef al./Optics DIVERGENCE
[MAD]
NEAR-FIELD WIDTH
Communications
(a)
IMMl
Fig. 3. Calculated dye aperture, 0,. 2.6
151
134 (1997) 149-154
variation
of beam divergence
and width with
I 0.2
0.3 DYE GAIN MEDIUM
0.4 APERTURE
0.5 [Mt.ij
Fig. 2. Calculated intensity distributions for plane-plane resonator dye laser for 0, = 0.21 mm. (a) near-field (b) far-field. [Scale 0 = 0.5 mm].
nm, the emission peak of Rh6G. The calculations were started with a uniform plane wave falling on D, coming from M,. This wave was propagated several round-trips through the resonator till the mode is established i.e. amplitude and phase profiles do not change from one round-trip to the next. It was observed that this situation is reached after the second round-trip. The central lobe of near and far-field intensity distributions of this resonator mode were used to estimate the output width and divergence. Fig. 2 shows the variation of beam width (at I/e’> and divergence (at base) with various values of Da. The divergence and width increased as Da is reduced. This is consistent with the fact, that the central lobe, which decides the beam width and the divergence, increases as l/D,. At Da of 0.21 mm. the divergence was 5 mrad and width 0.94 mm. Typical computed near and far field intensity distributions for 0, = 0.21 mm are shown in Figs. 3a and 3b, respectively. The vertical lines mark the measuring scale, a, which is chosen as per convenience. Fig. 4 shows the experimental far-field and nearfield intensity distributions of an uncollimated beam from a plane-plane resonator dye laser in horizontal and vertical directions. The divergence is measured by scanning the spot at the focal plane of lens L,. These experimentally observed profiles are similar to those computed (Fig. 3) except for the flattening of
“I
I
0
7 *
6
8
1.5
2.0
Position tmm)
4
00
I
0.5
1.0
Position (mm) Fig. 4. Intensity distributions of uncollimated output from planeplane dye laser in far-field: (a) horizontal. (b) vertical directions, and in near-field: (c) horizontal, (d) vertical directions.
152
SK. Dixit et nl./Optics
Communications 134 11997) 149-154
observed profiles due to saturation. These profiles give geometrical divergence of 5.2 mrad and 4.5 mrad measured at base respectively in horizontal (Fig. 4a) and vertical (Fig. 4b) directions, indicating an almost circular beam. The near field intensity distributions give beam widths at l/e’ level of 1.0 mm in horizontal (Fig. 4c) and 0.96 mm in vertical directions (Fig. 4d). These values are very close to the calculated values of the output divergence and widths in both the planes. The gain medium can therefore be assumed to offer an effective aperture of 0.21 mm diameter placed at mid point of the dye cell.
varied from 0.15 mm to 0.6 mm. A unifomr plane wave was assumed to be incident on Ha. The calculated near and far-field intensity distributions for Ha = 0.15 mm and 0.3 mm after the second roundtrip, when mode is established, are shown in Figs. 7a, b and 7c, d, respectively. The computed profiles resemble closely the observed profiles. The computed values of divergence and output width are plotted on the experimental curves (Fig. 6). A good match was observed between the computed and experimental values of divergence and beam width for all values of Ha. The analysis suggests that the resonator mode is shaped not only by the filtering aperture as in the empty resonator case but also by the aperture offered by the gain medium. In the
4. Results and discussions Fig. 5 shows the experimental near and far-field intensity distributions for H, of 0.15 mm and 0.3 mm diameters. All the distributions have smooth profiles. The divergence of the collimated beam, estimated from the width of central lobe at the base of the far-field intensity distribution was 0.55 mrad for H, of 0.15 mm (Fig. 5a) and 0.8 mrad for H, of 0.3 mm (Fig. 5b). The output beam was circular with near-field diameter [l/e* point] of 1.25 mm for H, = 0.15 mm (Fig. 5c) and 0.8 mm for H, = 0.3 mm (Fig. 5d). Similar intensity profiles were obtained for all apertures. Fig. 6 gives variation of output divergence and width as Ha was increased from 0.15 mm to 0.6 mm. The divergence increased from 0.55 mrad to 0.85 mrad as Ha increased from 0.15 mm to 0.4 mm. For Ha > 0.4 mm, the divergence remain fixed at 0.85 mrad. The output width decreased from 1.25 mm to 0.8 mm as Ha is increased from 0.15 mm to 0.3 mm. It increased to 0.94 mm for Ha = 0.4 mm and remains unchanged at 0.94 mm for all values of Ha > 0.4 mm. The decrease in beam diameter as Ha is increased from 0.15 to 0.3 mm is consistent with variation of the diameter of the central lobe in the far-field of the diffracted field as inverse of the aperture diameter. However the reverse trend observed for Ha beyond 0.3 mm needs explanation. In order to analyze the output in terms of relative role of Ha and Da in diffracting the resonator mode and shaping the output, calculations were performed on GDFR (Fig. 1) with Da = 0.21 mm. Ha was
401
1
,
0.2
0.0
0.4
0.6
0.6
1.0
Position (mm)
60-
w-
E
s
40-
d
b Mz-a
.s
20-
5 +
lo-
O1
,
/
,
,
,
,
,
0.0
02
0.4
0.6
0.6
1.0
12
I
1.4
1.6
Position (mm) Fig. 5. Intensity distributions of collimated GDFR dye laser. Far-field: (a) H, = 0.15 mm, (b) H, = 0.3 mm and near-field: (c) H,=O.15 mm,(d) H,=O.3 mm.
S.K. Dixit
er al. / Optics Communications
134 f 1997) 149-154
7.5 cm
Fig. 8. Experimental output.
Fig. 6. Variation aperture, Ha_
of beam divergence
and width
152
Ban
5cm
set up for double
7cm
pass GDFR
1
dye laser
with tiltering
present case [D, = 0.21 mm] for H, upto 0.4 mm, diffraction from both the apertures play a role in shaping the output. For Ha > 0.4 mm, the GDFR behaves as a plane-plane resonator and the mode is mainly influenced by the gain medium aperture. It should be noted that the nature of the beam coupled out of the resonator depends on the method of coupling and the external collimating optics used. To confirm this, an experiment was performed on a dye laser cavity shown in Fig. 8 where double pass output was taken out by a beamsplitter, BS, posi-
i Fig. 7. Calculated intensity distribution for H, = 0.15 mm, (a) near-field, (b) far field, H, = 0.3 mm (c) near field and (d) far-field. iScale (I = 0.25 mm].
tioned between the dye cell and the aperture H, (0.3 mm). A collimating mirror M, (f= 50 cm) was placed at a distance of 14 cm from the BS, as per the empty resonator considerations, so that the total distance, H,-M,-BS-M3 equals 50 cm. The output was expected to be reasonably collimated if the lensing effect of the gain medium is neglected which is in fact the case under our experimental conditions as the laser was operated at low pump power. To measure the divergence, the spot sizes were measured at two planes separated by 6 m. The measured spot diameters of 1 mm and 20 mm give a divergence of 3.2 mrad. On moving the collimating lens away from BS, the divergence starts decreasing and reaches a minimum value of 0.88 mrad. At this position, the collimating mirror was about SO cm away from the centre of the dye cell. This confirms that the output behaves as if it is diffracted by the dye medium aperture and not from the aperture H,. These experimental situations were also simulated. Figs. 9a, b show the calculated far-field intensity distributions for the collimating mirror placed at 14 cm and 42 cm from the BS, respectively. The divergence values estimated at the focal plane of the 1 m mirror M, placed beyond mirror M 3, were 3.4 mrad (Fig. 9a) and 0.92 mrad (Fig. 9b). These are close to the observed values. In these situations, the beam reflected from mirrors Mz arrives at the dye medium with a spot size much larger than the dye aperture. The output will behave as if it is diffracted only from the gain medium aperture and not by the filtering aperture. It is seen that the introduction of diffracting apertures within the dye laser resonator dramatically
154
S.K. Dixit et d/Optics
Communications
134 (1997) 149-154
5. Conclusions
Fig. 9. Calculated far field intensity distributions of output from double pass dye laser GDFR with collimating mirror M, located at (a) 14 cm from BS, (b) 42 cm from BS. [Scale (I = 0.25 mm].
improves the output beam quality. However this reduces the output power from 120 mW (5.2 mrad) without aperture to about 80 mW (0.8 mrad) and 40 mW (0.55 mrad) for diffracting apertures of 0.3 mm and 0.15 mm, respectively. This decrease in divergence is expected to further reduce the bandwidth of the conventional narrow band grating tuned dye laser of Littrow type or of Littman type. In many spectroscopy applications single longitudinal mode output is desired. With the present resonator length of 27 cm the output is expected to have multiple longitudinal modes. However with reduction in the resonator length by optimizing the components and reducing the dye cell length or choosing axially pumped dye cell, single longitudinal mode operation with this resonator should be possible.
The paper demonstrates that the diffraction filtered resonators can be applied to very small crosssection gain media such as dye laser to obtain symmetric, low divergence output beams. Divergence as low as 0.55 mrad could be obtained. It is shown experimentally and by resonator calculations that the gain medium aperture and the location of output collimating optics largely decides the output characteristics of the dye laser.
Acknowledgements
The authors gratefully wish to acknowledge the support of all members of the group in maintaining and operating the laser.
References [l] SK. Dixit, J.K. Mittal, B. Singh, P. Saxena and R. Bhatnagar, Optics Comm. 98 (1993) 91. [2] P. Pax and J. Weston, IEEE, J. Quantum Electron. 27 (1991) 1242. [3] A. Lago, G. Woehl and R. Rive, Appl. Phys. B 49 (1989) 73. (41 A.H. Farahbod and A. Hark, Optics Comm. 108 (1994) 84. [5] J.K. Mittal, B. Singh, P.K. Bhadani. L. Abhinandan and R. Bhatnagar, J. Phys. E: Sci. Instrum. 21 (1988) 388.
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 134 (1997) 149- 154
Diffraction filtered resonator for Rh6G dye laser transversely pumped by a copper vapor laser S.K. Dixit, S.R. Daulatabad, P.K. Shukla, R. Bhatnagar Luser
Programme.
Centrefi,r Advanced
Technology.
Indore-452013.
India
Received 21 May 1996; accepted 28 August 1996
Abstract The output from a transverse
pumped dye laser usually has poor beam quality and high divergence
asymmetry
along
horizontal and vertical directions. In this paper, it is demonstrated that diffraction filtered resonators can be applied to such small cross-section lasers with significant improvements in the output beam quality. A copper vapor laser pumped dye laser was operated with output divergence of 0.55 mrad in horizontal and vertical directions as compared to 5.2 mrad and 4.5 mrad respectively with a conventional plane-plane resonator.
1. Introduction
Diffraction filtered resonators have been applied to high gain, large aperture lasers to obtain diffraction limited beams [1,2]. In small cross-section high gain lasers like dye lasers, it is difficult to use these resonators. The output from transversely pumped dye lasers have poor beam quality and high divergence because of highly non-uniform gain and small size of the active medium which diffracts the radiation moving back and forth between the resonator mirrors. Lago et al. demonstrated diffraction limited symmetric output beams from a copper vapor laser pumped Rh6G dye laser using a stable, folded, grazing incidence cavity [3]. Recently Farahbod et al. reported the use of a self-filtering resonator to a nitrogen laser pumped Rh6G dye laser [4]. In this paper, we report 0030-4018/97/$17.00 PII
SOO30-4018(96)00575-5
Copyright
the use of diffraction filtered resonators [DFR] in a copper vapor laser [CVL] transverse pumped dye laser [DL]. A criteria for the selection of the filtering aperture in a gain medium of such a small cross-section is established. It is shown that the size of the filtering aperture in relation to the aperture offered by the gain medium itself is important in interpreting the results.
2. Experimental
arrangement
The experimental arrangement (Fig. 1) consists of a home made CVL 151 pumped dye laser. The CVL gives average output power of 10 W (5.5 kHz). In experiments, the laser was operated at 4.2 W (A = 510 nm) to minimize the amplified spontaneous
0 1997 Elsevier Science B.V. All rights reserved.
1.50
SK. Dixir ef 01./Optics
Communicufions
emission from the dye. The dye laser consists of a demountable dye cell (gain length 25 mm> developed in house with 5 mM solution of Rh6G in ethanol circulating at a flow rate of 4 lpm. The pump beam was focused by a cylindrical lens L ,(f= 10 cm> and its position from the dye cell was adjusted such that the output beam is almost circular. The resonator is a plane-plane generalized diffraction filtered resonator GDFR [I] consisting of plane mirrors M, (R = lOO%), and M, (R = 50%) and a hard aperture (H,) (Fig. 1). The resonator length was 27 cm. H, was placed close to the dye cell window and 7 cm away from mirror M,. H, of diameters varying from 0.15 mm to 0.6 mm were used. The divergence was estimated from the far-field intensity distribution recorded in the focal plane of a 1 m lens, L,, by using a slit-photomultiplier combination to scan the focal spot. The near field intensity distributions were recorded using a slit-PMT combination by imaging the intensity distribution at the output mirror, M, by a 1 : 1 imaging set-up. Since the geometrical divergence decides the bandwidth of the dye laser it is customary to estimate the divergence by measuring the spot size at the focal plane of a lens without collimating the output beam. In experiments, the divergence of the output beam for uncollimated and collimated cases were measured. For collimating the output beam, a lens, L, (f= 50 cm) placed approximately 50 cm from the aperture plane, was used. Rings around the central lobe were removed by another aperture placed in the output beam. The performance of GDFRs was
134 (1997) 149- 154
compared with that of the same resonator with the intracavity aperture removed.
3. Estimation of the dye aperture A crucial factor controlling the behavior of the GDFR dye laser is the nature and the size of the aperture offered by the dye medium, D,. It is essential to estimate its size to understand its role vis a vis the filtering aperture H,. In order to estimate D,, mode calculations were performed on the planeplane resonator for various values of D, (without H,) to see for which value of D,, the calculated divergence and near field diameter match with experimentally observed values. The calculations were performed on uncollimated output (lens L, removed). Since the output can be made circular by suitably adjusting the pump beam focusing conditions, circular symmetry for D, was assumed. For convenience D, was assumed to be a single hard aperture located at the centre of the dye cell though in practice the dye aperture has a finite length. It may behave like a soft aperture with tapered attenuation towards the edge. Mode calculations were carried out using resonator design software, PARAXIA, developed by A.E. Siegman [Genesse Optics Software Inc]. The program was run on a Macintosh Power PC. The plane-plane cavity (without H,) used in the simulation is the same as that used in the experiment (Fig. 1). The resonator length was 27 cm and D, was located 9.5 cm from M , . The wavelength used is 590
FAR-FIELD MEASUFIEMENT I
I
2.5cm
Fig. 1. Experimental
arrangement
- CVL pumped GDFR dye laser.
S.K. Dixit ef al./Optics DIVERGENCE
[MAD]
NEAR-FIELD WIDTH
Communications
(a)
IMMl
Fig. 3. Calculated dye aperture, 0,. 2.6
151
134 (1997) 149-154
variation
of beam divergence
and width with
I 0.2
0.3 DYE GAIN MEDIUM
0.4 APERTURE
0.5 [Mt.ij
Fig. 2. Calculated intensity distributions for plane-plane resonator dye laser for 0, = 0.21 mm. (a) near-field (b) far-field. [Scale 0 = 0.5 mm].
nm, the emission peak of Rh6G. The calculations were started with a uniform plane wave falling on D, coming from M,. This wave was propagated several round-trips through the resonator till the mode is established i.e. amplitude and phase profiles do not change from one round-trip to the next. It was observed that this situation is reached after the second round-trip. The central lobe of near and far-field intensity distributions of this resonator mode were used to estimate the output width and divergence. Fig. 2 shows the variation of beam width (at I/e’> and divergence (at base) with various values of Da. The divergence and width increased as Da is reduced. This is consistent with the fact, that the central lobe, which decides the beam width and the divergence, increases as l/D,. At Da of 0.21 mm. the divergence was 5 mrad and width 0.94 mm. Typical computed near and far field intensity distributions for 0, = 0.21 mm are shown in Figs. 3a and 3b, respectively. The vertical lines mark the measuring scale, a, which is chosen as per convenience. Fig. 4 shows the experimental far-field and nearfield intensity distributions of an uncollimated beam from a plane-plane resonator dye laser in horizontal and vertical directions. The divergence is measured by scanning the spot at the focal plane of lens L,. These experimentally observed profiles are similar to those computed (Fig. 3) except for the flattening of
“I
I
0
7 *
6
8
1.5
2.0
Position tmm)
4
00
I
0.5
1.0
Position (mm) Fig. 4. Intensity distributions of uncollimated output from planeplane dye laser in far-field: (a) horizontal. (b) vertical directions, and in near-field: (c) horizontal, (d) vertical directions.
152
SK. Dixit et nl./Optics
Communications 134 11997) 149-154
observed profiles due to saturation. These profiles give geometrical divergence of 5.2 mrad and 4.5 mrad measured at base respectively in horizontal (Fig. 4a) and vertical (Fig. 4b) directions, indicating an almost circular beam. The near field intensity distributions give beam widths at l/e’ level of 1.0 mm in horizontal (Fig. 4c) and 0.96 mm in vertical directions (Fig. 4d). These values are very close to the calculated values of the output divergence and widths in both the planes. The gain medium can therefore be assumed to offer an effective aperture of 0.21 mm diameter placed at mid point of the dye cell.
varied from 0.15 mm to 0.6 mm. A unifomr plane wave was assumed to be incident on Ha. The calculated near and far-field intensity distributions for Ha = 0.15 mm and 0.3 mm after the second roundtrip, when mode is established, are shown in Figs. 7a, b and 7c, d, respectively. The computed profiles resemble closely the observed profiles. The computed values of divergence and output width are plotted on the experimental curves (Fig. 6). A good match was observed between the computed and experimental values of divergence and beam width for all values of Ha. The analysis suggests that the resonator mode is shaped not only by the filtering aperture as in the empty resonator case but also by the aperture offered by the gain medium. In the
4. Results and discussions Fig. 5 shows the experimental near and far-field intensity distributions for H, of 0.15 mm and 0.3 mm diameters. All the distributions have smooth profiles. The divergence of the collimated beam, estimated from the width of central lobe at the base of the far-field intensity distribution was 0.55 mrad for H, of 0.15 mm (Fig. 5a) and 0.8 mrad for H, of 0.3 mm (Fig. 5b). The output beam was circular with near-field diameter [l/e* point] of 1.25 mm for H, = 0.15 mm (Fig. 5c) and 0.8 mm for H, = 0.3 mm (Fig. 5d). Similar intensity profiles were obtained for all apertures. Fig. 6 gives variation of output divergence and width as Ha was increased from 0.15 mm to 0.6 mm. The divergence increased from 0.55 mrad to 0.85 mrad as Ha increased from 0.15 mm to 0.4 mm. For Ha > 0.4 mm, the divergence remain fixed at 0.85 mrad. The output width decreased from 1.25 mm to 0.8 mm as Ha is increased from 0.15 mm to 0.3 mm. It increased to 0.94 mm for Ha = 0.4 mm and remains unchanged at 0.94 mm for all values of Ha > 0.4 mm. The decrease in beam diameter as Ha is increased from 0.15 to 0.3 mm is consistent with variation of the diameter of the central lobe in the far-field of the diffracted field as inverse of the aperture diameter. However the reverse trend observed for Ha beyond 0.3 mm needs explanation. In order to analyze the output in terms of relative role of Ha and Da in diffracting the resonator mode and shaping the output, calculations were performed on GDFR (Fig. 1) with Da = 0.21 mm. Ha was
401
1
,
0.2
0.0
0.4
0.6
0.6
1.0
Position (mm)
60-
w-
E
s
40-
d
b Mz-a
.s
20-
5 +
lo-
O1
,
/
,
,
,
,
,
0.0
02
0.4
0.6
0.6
1.0
12
I
1.4
1.6
Position (mm) Fig. 5. Intensity distributions of collimated GDFR dye laser. Far-field: (a) H, = 0.15 mm, (b) H, = 0.3 mm and near-field: (c) H,=O.15 mm,(d) H,=O.3 mm.
S.K. Dixit
er al. / Optics Communications
134 f 1997) 149-154
7.5 cm
Fig. 8. Experimental output.
Fig. 6. Variation aperture, Ha_
of beam divergence
and width
152
Ban
5cm
set up for double
7cm
pass GDFR
1
dye laser
with tiltering
present case [D, = 0.21 mm] for H, upto 0.4 mm, diffraction from both the apertures play a role in shaping the output. For Ha > 0.4 mm, the GDFR behaves as a plane-plane resonator and the mode is mainly influenced by the gain medium aperture. It should be noted that the nature of the beam coupled out of the resonator depends on the method of coupling and the external collimating optics used. To confirm this, an experiment was performed on a dye laser cavity shown in Fig. 8 where double pass output was taken out by a beamsplitter, BS, posi-
i Fig. 7. Calculated intensity distribution for H, = 0.15 mm, (a) near-field, (b) far field, H, = 0.3 mm (c) near field and (d) far-field. iScale (I = 0.25 mm].
tioned between the dye cell and the aperture H, (0.3 mm). A collimating mirror M, (f= 50 cm) was placed at a distance of 14 cm from the BS, as per the empty resonator considerations, so that the total distance, H,-M,-BS-M3 equals 50 cm. The output was expected to be reasonably collimated if the lensing effect of the gain medium is neglected which is in fact the case under our experimental conditions as the laser was operated at low pump power. To measure the divergence, the spot sizes were measured at two planes separated by 6 m. The measured spot diameters of 1 mm and 20 mm give a divergence of 3.2 mrad. On moving the collimating lens away from BS, the divergence starts decreasing and reaches a minimum value of 0.88 mrad. At this position, the collimating mirror was about SO cm away from the centre of the dye cell. This confirms that the output behaves as if it is diffracted by the dye medium aperture and not from the aperture H,. These experimental situations were also simulated. Figs. 9a, b show the calculated far-field intensity distributions for the collimating mirror placed at 14 cm and 42 cm from the BS, respectively. The divergence values estimated at the focal plane of the 1 m mirror M, placed beyond mirror M 3, were 3.4 mrad (Fig. 9a) and 0.92 mrad (Fig. 9b). These are close to the observed values. In these situations, the beam reflected from mirrors Mz arrives at the dye medium with a spot size much larger than the dye aperture. The output will behave as if it is diffracted only from the gain medium aperture and not by the filtering aperture. It is seen that the introduction of diffracting apertures within the dye laser resonator dramatically
154
S.K. Dixit et d/Optics
Communications
134 (1997) 149-154
5. Conclusions
Fig. 9. Calculated far field intensity distributions of output from double pass dye laser GDFR with collimating mirror M, located at (a) 14 cm from BS, (b) 42 cm from BS. [Scale (I = 0.25 mm].
improves the output beam quality. However this reduces the output power from 120 mW (5.2 mrad) without aperture to about 80 mW (0.8 mrad) and 40 mW (0.55 mrad) for diffracting apertures of 0.3 mm and 0.15 mm, respectively. This decrease in divergence is expected to further reduce the bandwidth of the conventional narrow band grating tuned dye laser of Littrow type or of Littman type. In many spectroscopy applications single longitudinal mode output is desired. With the present resonator length of 27 cm the output is expected to have multiple longitudinal modes. However with reduction in the resonator length by optimizing the components and reducing the dye cell length or choosing axially pumped dye cell, single longitudinal mode operation with this resonator should be possible.
The paper demonstrates that the diffraction filtered resonators can be applied to very small crosssection gain media such as dye laser to obtain symmetric, low divergence output beams. Divergence as low as 0.55 mrad could be obtained. It is shown experimentally and by resonator calculations that the gain medium aperture and the location of output collimating optics largely decides the output characteristics of the dye laser.
Acknowledgements
The authors gratefully wish to acknowledge the support of all members of the group in maintaining and operating the laser.
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