- Email: [email protected]
A widely tunable XeCl excimer laser
Volume 65, number 6
OPTICS COMMUNICATIONS
15 March 1988
A WIDELY TUNABLE XeCI EXCIMER LASER I.V. CHALTAKOV, N.I. M I N K O V S K I and I.V. T O M O V Faculty of Physics, Sofia University, BG- I126, 5 A. lvanov Blvd., Sofia, Bulgaria
Received 13 August 1987; revised manuscript received 12 November 1987
An extension of the tuning range up to 16.2 A of a convenient XeCI, UV preionized, electrodischarge laser is reported. Lasing on four new transitions, B( v= 0)-X ( v= 4,5) and B( v= 1) - X ( v= 6,7), with energies in the order of 1 /aJ was achieved. Additionally a continuous tuning between the transitions B( v= 0)-X (v= 0) and B( v= 1)-X (v= 7 ) was recorded.
I. Introduction A directly tuned excimer laser is a convenient source of narrow linewidth radiation for an injection-locked amplifier in order to produce wavelength tunable and powerful ultraviolet pulses. Thus extending the tuning range could be of particular interest for some applications such as spectroscopy and laser chemistry. Additionally the study o f the accessible linewidth o f the XeCI excimer laser is interesting for the development of a femtosecond amplifier in the ultraviolet. The emission spectra and a spectroscopic analysis of the spontaneous emission of the XeC1 molecule are reported in refs. [ 1,2]. Lasing experiments are also reported in ref. [2] resulting in generation on the B( v= 0 ) - X ( v = 1,2,3) transitions. Simultaneous emission on B ( v = 0 ) - X ( v = 0 , 1 , 2 , 3 ) transitions is reported in refs. [3] and [4], using discharge pumped devices. The B - X band of the XeCI molecule is vibrationally analyzed and the corresponding Franck-Condon factors are derived in ref. [ 5 ]. From the results of this work it can be seen that besides the mentioned transitions, already made to lase, there are some other transitions in the B - X band with high Franck-Condon factor values. These are B(v=0)-X(v=4,5,6) and B ( v = l ) - X ( v = 5 , 6 , 7 ) . They could be of interest from a point o f view o f extending the tuning range o f the XeC1 excimer medium. The estimation given in ref. [ 7] shows that the population density o f the B ( v = 1 ) state is nearly equal to that of the B ( v = 0 ) state. Thus in order to
extend the lasing o f the XeCI over those transitions a proper suppression o f the stronger transitions should be introduced in the laser cavity. In this work we report the results of our attempts to extend the tuning range of a transmission line driven, XeC1 electrodischarge laser, with ultraviolet preionization. In order to suppress lasing on the strongest transitions a simple diffraction grating optical cavity o f the grazing incidence type was used. The tuning range o f the XeC1 laser reported so far was 10 A [8]. Here we report an extension of this range up to 16.2 A. In fact additionally to the already reported lasing around the transitions B ( v = 0 ) - X ( v = 0 , 1 , 2 , 3 ) we recorded lasing on the transitions B ( v = 0 ) - X ( v = 4,5) and B ( v = 1 ) - X ( v = 6 , 7 ) , thus extending the tuning range o f the XeC1 laser from 3073 A up to 3089 A.
2. Experimental arrangement The laser used in the experiment is described elsewhere [ 9]. It is a convenient, transmission-line-driven, ultraviolet preionized electrodischarge device. With the gas mixture used (6 Torr HCI, 30 Torr Xe, 3 atm Ne) and R = 5% output coupler it is capable of producing 60 ns fwhm optical pulses and output energies of about 100 mJ. The experimental arrangement is shown in fig. 1. The one meter long laser cavity was composed of two fiat, dielectric coated mirrors M r ( R = 9 7 % ) , M 2 ( R = 9 9 % ) and a 3600 1/mm holographic grating. Additionally two 2 m m
0 030-4018/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
437
Volume 65, number 6
DI
OPTICS COMMUNICATIONS
LASER
D2
D6
d
15 March 1988
3
(a)
7 >-
M1
WI
W2 ~
M2
Q- 2& 306
SPECTROGRAPH
307
308
309
WAVELENGTH [nm] Fig. 1. Experimental arrangement.
diameter apertures D~ and D 2 w e r e placed inside the cavity. Two flat MgF2 plates W~ and W2 were used for the laser chamber windows. W2 was oriented at a Brewster angle, thus avoiding generation build up between M t and W2. The grating was placed at a grazing angle of incidence (82 deg). This comparatively small angle was chosen to maintain the cavity losses as low as possible without a significant decrease in the angular dispersion. The scanning of the output wavelength was performed by means of rotating M 2 , which simultaneously serves as a feed-back coupler, reflecting the first order reflection from the grating, The zero order reflection was used for an output. As known, in the output of such cavity set up there is a large amount of superfluorescence content. However our aim was not to produce a high spectral purity radiation. When necessary, an output from the mirror M~ can be used, or a spatial filter can be employed in order to reject the divergent superfluorescence components. All spectra were recorded on ORWO WU-2 plates using a Q-24 spectrograph with a 25 ~tm input slot. Care was taken to ensure proper exposure in order to obtain blackening of the plates proportional to the incident radiation.
3. E x p e r i m e n t and results
In order to study the possibility of obtaining lasing on the mentioned weak transitions, the B-X band of the XeC1 molecule was scanned from 3055/~ up to 3095/~. This wavelength range includes the spectral lines of all transitions from B( v= 1) - X ( v= 0) up to B (v= 0 ) - X (v= 6). Since the estimated bandwidth 438
I0 In
9
g-
8
(b)
7
~
6' 5
4 zu.a
3
2 I
o 306
307
308
309
WAVELENGTH [nm]
Fig. 2. Microdensitometer traces (a) of the XeC1 fluorescence emission spectrum and (b) of the lasing spectra of the transitions B ( v = 0 ) - X ( v = 4,5 ) and B ( v = 1 ) - X ( v= 6,7 ), plotted together.
of the described laser cavity for a single-pass was about 0.4 /k, a scanning step of 0.1 ¢k was chosen. The spectra were recorded with two shots for each wavelength step. Besides lasing on the already known transitions B (v = 0 ) - X ( v= 0,1,2, 3), generation was obtained on four new transitions B ( v = 0 ) - X ( v = 4 , 5 ) and B(v= 1 ) - X ( v = 6 , 7 ) , identified on the basis of the results in ref. [ 5]. Microdensitometer traces of the spontaneous emission spectrum of the XeC1 molecule and of the mentioned transitions spectra, plotted together, in the case of lasing are shown in fig. 2a and fig. 2b respectively. A significant superfluorescence background is also present in those spectra. That is due mainly to the experimental arrangement used and to the very close (20 cm) separation of the spectrograph in respect to the laser output. A tuning width of about 0.3 /k was observed for the transitions
Volume 65, number 6
c ::,
OPTICS COMMUNICATIONS
12 '~0
8 6
\
4
I
3075
I-I
3076
t
3077
WAVELE.GTH {~.]
Fig. 3. Dependence of the output energy in the spectral region between the lines B(v= 1)-X(v= 7) and B(v=0)-X(v=0). B(v=0)-X(v=4,5) and B ( v = 1 ) - X ( v = 6 ) . A continuous lasing was o b t a i n e d when scanning the wavelength region between the transitions B(v=0)-X(v=0) a n d B ( v = 1 ) - X ( v = 7 ) , i.e. from 3077 A up to 3075 A. The d e p e n d e n c e o f the output intensity on the wavelength for this region is shown in fig. 3. As seen, within our scanning resolution o f 0.1 /~, a lasing c o n t i n u u m with two m a x i m a , corres p o n d i n g to the wavelengths o f the m e n t i o n e d transitions [ 5 ], is observed. The output energies for each o f the four new transitions were recorded with a cavity tuned in the center o f the corresponding spectral line, a n d were e s t i m a t e d relatively on the basis o f the blackening o f the recorded plates. As a reference source were used the traces owing to the B ( v = 0 ) - X ( v = 1,2) transitions, for which energies in the o r d e r o f 100 laJ were
15 March 1988
o b t a i n e d in the same arrangement. Thus a c o m p a r ison o f consistently recorded lasing spectra for all m e n t i o n e d transitions showed that the m a x i m u m energies o b t a i n e d on the four new wavelengths are in the o r d e r o f 1 pJ. The typical output pulse was about 15 ns ( f w h m ) long. This short pulse d u r a t i o n could be explained by the relatively long time, required for the generation to build up. In o r d e r to obtain higher energies, the buffer gas pressure was increased up to 4 atm. H o w e v e r in this case a strong lasing on B ( v = 0 ) - X ( v = 1,2) transitions built up regardless o f the position o f the tuning mirror. This caused a d e c r e m e n t o f the overall gain and thus the lasing on the weaker transitions was inhibited. An optical cavity with stronger dispersion is necessary for higher buffer gas pressure operation at the weaker transitions. In s u m m a r y we report an extension o f the tuning range o f an conventional, U V - p r e i o n i z e d XeCI laser up to 16.2 A. Lasing on four new transitions, B(v=0)-X(v=4,5) and B ( v = l ) - X ( v = 6 , 7 ) , was o b t a i n e d with energies in the o r d e r o f 1 pJ.
References [ 1] C.A. Brau and J.J. Ewing, J. Chem. Phys. 63 (1975) 4640. [2] J. Tellinghuisen, J.M. Hoffman, G.C. Tisone and A.K. Hays, J. Chem. Phys. 64 (1976) 2484. [3] V.N. Ishchenko, V.N. Lisitsin and A.M. Razhev, Optics Comm. 21 (1977) 30. [4] R.C. Sze and P.B. Scott, Appl. Phys. Lett. 33 (1978) 419. [5] A. Sur, A.K. Hui and J. Tellinghuisen, J. Mol. Speetr. 74 (1979) 465. [6] O.L Bourne and A.J. Alcock, Appl. Phys. Lett. 42 (1983) 777. [7] O.L. Bourne and A.K. Alcock, Appl. Phys. B 32 (1983) 193. [ 8 ] T.J. Pacala, I.S. McDermid and J.B. Laudenslager, Technical Digest, CLEO 1986, paper TUR2. [9] I.V. Chaltakov, I.V. Tomov and Ch.G. Christov, J. Phys. E: Sci. Instr. 19 (1986) 1034.
439
OPTICS COMMUNICATIONS
15 March 1988
A WIDELY TUNABLE XeCI EXCIMER LASER I.V. CHALTAKOV, N.I. M I N K O V S K I and I.V. T O M O V Faculty of Physics, Sofia University, BG- I126, 5 A. lvanov Blvd., Sofia, Bulgaria
Received 13 August 1987; revised manuscript received 12 November 1987
An extension of the tuning range up to 16.2 A of a convenient XeCI, UV preionized, electrodischarge laser is reported. Lasing on four new transitions, B( v= 0)-X ( v= 4,5) and B( v= 1) - X ( v= 6,7), with energies in the order of 1 /aJ was achieved. Additionally a continuous tuning between the transitions B( v= 0)-X (v= 0) and B( v= 1)-X (v= 7 ) was recorded.
I. Introduction A directly tuned excimer laser is a convenient source of narrow linewidth radiation for an injection-locked amplifier in order to produce wavelength tunable and powerful ultraviolet pulses. Thus extending the tuning range could be of particular interest for some applications such as spectroscopy and laser chemistry. Additionally the study o f the accessible linewidth o f the XeCI excimer laser is interesting for the development of a femtosecond amplifier in the ultraviolet. The emission spectra and a spectroscopic analysis of the spontaneous emission of the XeC1 molecule are reported in refs. [ 1,2]. Lasing experiments are also reported in ref. [2] resulting in generation on the B( v= 0 ) - X ( v = 1,2,3) transitions. Simultaneous emission on B ( v = 0 ) - X ( v = 0 , 1 , 2 , 3 ) transitions is reported in refs. [3] and [4], using discharge pumped devices. The B - X band of the XeCI molecule is vibrationally analyzed and the corresponding Franck-Condon factors are derived in ref. [ 5 ]. From the results of this work it can be seen that besides the mentioned transitions, already made to lase, there are some other transitions in the B - X band with high Franck-Condon factor values. These are B(v=0)-X(v=4,5,6) and B ( v = l ) - X ( v = 5 , 6 , 7 ) . They could be of interest from a point o f view o f extending the tuning range o f the XeC1 excimer medium. The estimation given in ref. [ 7] shows that the population density o f the B ( v = 1 ) state is nearly equal to that of the B ( v = 0 ) state. Thus in order to
extend the lasing o f the XeCI over those transitions a proper suppression o f the stronger transitions should be introduced in the laser cavity. In this work we report the results of our attempts to extend the tuning range of a transmission line driven, XeC1 electrodischarge laser, with ultraviolet preionization. In order to suppress lasing on the strongest transitions a simple diffraction grating optical cavity o f the grazing incidence type was used. The tuning range o f the XeC1 laser reported so far was 10 A [8]. Here we report an extension of this range up to 16.2 A. In fact additionally to the already reported lasing around the transitions B ( v = 0 ) - X ( v = 0 , 1 , 2 , 3 ) we recorded lasing on the transitions B ( v = 0 ) - X ( v = 4,5) and B ( v = 1 ) - X ( v = 6 , 7 ) , thus extending the tuning range o f the XeC1 laser from 3073 A up to 3089 A.
2. Experimental arrangement The laser used in the experiment is described elsewhere [ 9]. It is a convenient, transmission-line-driven, ultraviolet preionized electrodischarge device. With the gas mixture used (6 Torr HCI, 30 Torr Xe, 3 atm Ne) and R = 5% output coupler it is capable of producing 60 ns fwhm optical pulses and output energies of about 100 mJ. The experimental arrangement is shown in fig. 1. The one meter long laser cavity was composed of two fiat, dielectric coated mirrors M r ( R = 9 7 % ) , M 2 ( R = 9 9 % ) and a 3600 1/mm holographic grating. Additionally two 2 m m
0 030-4018/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
437
Volume 65, number 6
DI
OPTICS COMMUNICATIONS
LASER
D2
D6
d
15 March 1988
3
(a)
7 >-
M1
WI
W2 ~
M2
Q- 2& 306
SPECTROGRAPH
307
308
309
WAVELENGTH [nm] Fig. 1. Experimental arrangement.
diameter apertures D~ and D 2 w e r e placed inside the cavity. Two flat MgF2 plates W~ and W2 were used for the laser chamber windows. W2 was oriented at a Brewster angle, thus avoiding generation build up between M t and W2. The grating was placed at a grazing angle of incidence (82 deg). This comparatively small angle was chosen to maintain the cavity losses as low as possible without a significant decrease in the angular dispersion. The scanning of the output wavelength was performed by means of rotating M 2 , which simultaneously serves as a feed-back coupler, reflecting the first order reflection from the grating, The zero order reflection was used for an output. As known, in the output of such cavity set up there is a large amount of superfluorescence content. However our aim was not to produce a high spectral purity radiation. When necessary, an output from the mirror M~ can be used, or a spatial filter can be employed in order to reject the divergent superfluorescence components. All spectra were recorded on ORWO WU-2 plates using a Q-24 spectrograph with a 25 ~tm input slot. Care was taken to ensure proper exposure in order to obtain blackening of the plates proportional to the incident radiation.
3. E x p e r i m e n t and results
In order to study the possibility of obtaining lasing on the mentioned weak transitions, the B-X band of the XeC1 molecule was scanned from 3055/~ up to 3095/~. This wavelength range includes the spectral lines of all transitions from B( v= 1) - X ( v= 0) up to B (v= 0 ) - X (v= 6). Since the estimated bandwidth 438
I0 In
9
g-
8
(b)
7
~
6' 5
4 zu.a
3
2 I
o 306
307
308
309
WAVELENGTH [nm]
Fig. 2. Microdensitometer traces (a) of the XeC1 fluorescence emission spectrum and (b) of the lasing spectra of the transitions B ( v = 0 ) - X ( v = 4,5 ) and B ( v = 1 ) - X ( v= 6,7 ), plotted together.
of the described laser cavity for a single-pass was about 0.4 /k, a scanning step of 0.1 ¢k was chosen. The spectra were recorded with two shots for each wavelength step. Besides lasing on the already known transitions B (v = 0 ) - X ( v= 0,1,2, 3), generation was obtained on four new transitions B ( v = 0 ) - X ( v = 4 , 5 ) and B(v= 1 ) - X ( v = 6 , 7 ) , identified on the basis of the results in ref. [ 5]. Microdensitometer traces of the spontaneous emission spectrum of the XeC1 molecule and of the mentioned transitions spectra, plotted together, in the case of lasing are shown in fig. 2a and fig. 2b respectively. A significant superfluorescence background is also present in those spectra. That is due mainly to the experimental arrangement used and to the very close (20 cm) separation of the spectrograph in respect to the laser output. A tuning width of about 0.3 /k was observed for the transitions
Volume 65, number 6
c ::,
OPTICS COMMUNICATIONS
12 '~0
8 6
\
4
I
3075
I-I
3076
t
3077
WAVELE.GTH {~.]
Fig. 3. Dependence of the output energy in the spectral region between the lines B(v= 1)-X(v= 7) and B(v=0)-X(v=0). B(v=0)-X(v=4,5) and B ( v = 1 ) - X ( v = 6 ) . A continuous lasing was o b t a i n e d when scanning the wavelength region between the transitions B(v=0)-X(v=0) a n d B ( v = 1 ) - X ( v = 7 ) , i.e. from 3077 A up to 3075 A. The d e p e n d e n c e o f the output intensity on the wavelength for this region is shown in fig. 3. As seen, within our scanning resolution o f 0.1 /~, a lasing c o n t i n u u m with two m a x i m a , corres p o n d i n g to the wavelengths o f the m e n t i o n e d transitions [ 5 ], is observed. The output energies for each o f the four new transitions were recorded with a cavity tuned in the center o f the corresponding spectral line, a n d were e s t i m a t e d relatively on the basis o f the blackening o f the recorded plates. As a reference source were used the traces owing to the B ( v = 0 ) - X ( v = 1,2) transitions, for which energies in the o r d e r o f 100 laJ were
15 March 1988
o b t a i n e d in the same arrangement. Thus a c o m p a r ison o f consistently recorded lasing spectra for all m e n t i o n e d transitions showed that the m a x i m u m energies o b t a i n e d on the four new wavelengths are in the o r d e r o f 1 pJ. The typical output pulse was about 15 ns ( f w h m ) long. This short pulse d u r a t i o n could be explained by the relatively long time, required for the generation to build up. In o r d e r to obtain higher energies, the buffer gas pressure was increased up to 4 atm. H o w e v e r in this case a strong lasing on B ( v = 0 ) - X ( v = 1,2) transitions built up regardless o f the position o f the tuning mirror. This caused a d e c r e m e n t o f the overall gain and thus the lasing on the weaker transitions was inhibited. An optical cavity with stronger dispersion is necessary for higher buffer gas pressure operation at the weaker transitions. In s u m m a r y we report an extension o f the tuning range o f an conventional, U V - p r e i o n i z e d XeCI laser up to 16.2 A. Lasing on four new transitions, B(v=0)-X(v=4,5) and B ( v = l ) - X ( v = 6 , 7 ) , was o b t a i n e d with energies in the o r d e r o f 1 pJ.
References [ 1] C.A. Brau and J.J. Ewing, J. Chem. Phys. 63 (1975) 4640. [2] J. Tellinghuisen, J.M. Hoffman, G.C. Tisone and A.K. Hays, J. Chem. Phys. 64 (1976) 2484. [3] V.N. Ishchenko, V.N. Lisitsin and A.M. Razhev, Optics Comm. 21 (1977) 30. [4] R.C. Sze and P.B. Scott, Appl. Phys. Lett. 33 (1978) 419. [5] A. Sur, A.K. Hui and J. Tellinghuisen, J. Mol. Speetr. 74 (1979) 465. [6] O.L Bourne and A.J. Alcock, Appl. Phys. Lett. 42 (1983) 777. [7] O.L. Bourne and A.K. Alcock, Appl. Phys. B 32 (1983) 193. [ 8 ] T.J. Pacala, I.S. McDermid and J.B. Laudenslager, Technical Digest, CLEO 1986, paper TUR2. [9] I.V. Chaltakov, I.V. Tomov and Ch.G. Christov, J. Phys. E: Sci. Instr. 19 (1986) 1034.
439