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Locking of a flashlamp-pumped dye laser at small injection powers
Volume 39, number 6
OPTICS COMMUNICATIONS
15 November 198 l
LOCKING OF A FLASHLAMP-PUMPED DYE LASER AT SMALL INJECTION POWERS S.K. BHATTACHARYYA, P. EGGETT and L. THOMAS SERC, Rutherford and Appleton Laboratories, Chilton, Didcot, Oxon O X l l OQX UK
Received 22 June 1981
Experiments have been carried out on the injection-locking of a pulsed dye laser to an argon-laser-pumped CW dye laser of 50 MHz linewidth without longitudinal mode matching. With the inclusion of a Fabry-Perot etalon within the ring laser arrangement of the pulsed laser, the initiation of injection locking and saturation occur at low injection powers.
1. Introduction Spectral narrowing of the output of a dye laser has normally been achieved by incorporating frequency selective elements in the laser cavity. Softer and McFarland [ 1] first replaced one of the cavity mirrors by a plane diffraction grating, Bradley et al. [2] included Fabry-Perot etalons, Walther and Hall [3] made use of a birefringent selective filter, and Sorokin et al. [4] used an arrangement to lock the dye laser to one of the sodium lines. With this type of approach, spectral widths as narrow as 100 MHz have been attained but generally at the expense of the power output. When a multistage system is used, incorporating an oscillator and amplifying stage, the efficiency is generally low [5]. In a technique introduced by Erickson and Szabo [6] the wavelength of a pulsed dye laser is controlled by injection of a stable narrow-band signal. The use of a pulsed gas laser as injection source by these workers, of a CW gas laser by Vrehen and Breimer [7], and of a CW dye laser by Turner et al. [8] demonstrated tire principle of the technique but yielded very low output pulse energies. Subsequently, Blit et al. [9], and Gibson and Thomas [10] reported experiments involving iniection from a CW dye laser and a krypton laser, respectively, in which pulses of linewidth less than 30 MHz and energies of tens of mJ were achieved. More recently, Trehin et al. [11] using a stabilised single-mode CW dye laser as injection source obtained smaller energy pulses, of linewidth 0 030-4018/81/0000-0000/$ 02.75 © 1981 North-Holland
identical to the injecting dye laser, 8 MHz, but frequency higher by about 7 MHz. These workers arranged that the cavity of the dye laser was tuned to the wavelength of the CW laser, and in the experiment of Blit et al. [9] care was taken to match both transverse and longitudinal modes of the injection and pulsed lasers. With these mode-matching arrangements, it was found that almost all the output pulse energy was channelled into the bandwidth of the injected radiation. Furthermore, full locking occurred with an injectiop_ input power of l mW, although the onset of locking occurred at much lower powers. No arrangements for mode matching the locked laser cavity to the injected radiation were made in the other experiments mentioned above. Vrehen and Breimer [7] made use of an injection power of 1 mW but, in general, inputs considerably in excess of this value were required. Unlike the findings of Blit et al. [9] and Trehin et al. [11], only partial locking was observed in these other experiments. The purpose of the present study was to examine further the characteristics of injection-locking without resorting to arrangements for transverse and longitudinal mode matching. The conditions for controlling the frequency and spectral properties of the major part of the output of a moderate-energy pulsed dye laser were thereby examined. The general optical arrangement was similar to that described by Gibson and Thomas [10] except that the injected radiation was obtained from a dye laser pumped by an argon ion laser. This permitted operation at wavelengths other 387
Volume 39, number 6
OPTICS COMMUNICATIONS
than 647.1 nm which corresponded to the krypton ion laser line they employed and, in particular, permitted the use of the high efficiency rhodamine 6 G dye.
2. Experimental system The optical configuration illustrated in fig. 1 made use of a ring laser arrangement to prevent feedback of the pulsed output into the CW laser, as proposed by Ganiel et al. [5]. A suitably placed expanding telescope enabled the diameter of the CW beam to be varied relative to that of the dye cell. The majority of the measurements were made with a pulsed dye laser cavity length of 10 m, corresponding to a longitudinal mode spacing of 30 MHz. The CW dye laser providing the injection radiation employed a transverse flow dye-jet pumped by the output of an argon laser. The jet consisted of a solution of 3 X 10 - 3 M rhodamine 6 G in ethylene glycol circulated at a rate of 0.9 ~ min - I (1.5 X 10 - 5 m 3 s - 1 ) through a 10 #m pore filter. A controllable wedge was used to tune the resonant cavity within the broadband output and a temperature-stabilised plane-parallel electronically-tunable Fabry-Perot etalon, together with a 0.11 mm thickness solid etalon, provided a linewidth of 50 MHz near the 600 nm chosen wavelength.
.j> flashlamp pulsedyeloser
Coxial
J
50 %/'J~:
> ~
>
--
Outputbeam
Telescope
Argon laser
Fig. l. Arrangement of ring laser (coaxial tlashlamp pumped dye) with injection from a CW dye laser pumped by an argon laser. 388
15 November 1981
The pulsed dye laser used in tile measurements incorporated a xenon-filled coaxial flashlamp, driven by a Marx-Bank circuit providing a pulse duration of about 0.3/as, as described previously by Gibson and Thomas [10]. To obtain tile free-running broadband laser output centred at about 600 nm, a dye solution o f 5 X 10 - 5 M rhodamine 6 G in a 1 : 1 water-isopropanol mixture was used, this solution being circulated at a flow rate of about 4 ~ rain 1 through a 0.45/am pore filter.
3. Experimental results Preliminary measurements were carried out with only the wedge and 0.11 mm solid etalon included within the cavity of the CW dye laser, for which the output had a spectral width of about 45 GHz (0.05 nm at 600 nm). Very efficient injection-locking was observed for injection powers down to about 2 mW with a marked reduction in the broad-band emission. This was to be expected since with a cavity length of 10 m an overlap of the injection spectrum with a longitudinal mode frequency would be expected to occur always. With the reduction in the injection spectrum to 50 MHz, brought about by the inclusion of the electronically tunable Fabry-Perot etalon in the CW dye laser cavity, it was found that injection-locking was less complete and a smaller reduction in the broadband emission was observed. Examples of the spectra obtained with the narrow-band injection using a I m grating spectrograph with a broad slit width are shown in (a) and (c) of fig. 2; (b) and (d) represent the resuits with no injection-locking but with the CW signal shown superimposed for reference. The reduction in the intensity of the broad-band emission during injection-locking is noticeable and the narrow-band emission unresolved by the slit width was sufficiently strong to over expose tile film. In order to estimate the intensity in the narrow-band emission, a 5/~m spacing Fabry-Perot etalon of finesse about 17 acting as a filter centred on the injected line was inserted between the output mirror and the power meter; this restricted the broad-band output to about 2.0 nm. Fig. 3 then shows the pulse energy measured witlfin this pass-band as a function of the injected power; the free-running pulse energy of 12 mJ corresponds to zero injection power. These results refer to a charging voltage of 18
Volume 39, number 6
OPTICS COMMUNICATIONS
CI
b
C
I
I 5 nm
Fig. 2. Spectra of pulsed laser obtained using a grating spectrograph: (a) (c) with injection-locking, (b) (d) without injectionlocking but with injection radiation shown for reference. kV for the pulsed dye laser and a cavity length o f 10 m. It is seen that even with an injection power input of 0.5 roW, a narrow-band o u t p u t o f about 3 mJ is obtained. Furthermore, the narrow-band output saturates at about 18 mJ for an injection power near 2 mW. A similar saturation effect was found by Turner et al. [8] but at a considerably larger injection power.
++
3o
As expected, the transfer of the 5/am etalon to within the pulsed laser cavity reduced the spectral width o f the free-running output being measured. A set o f measurements with the etalon in this position showed a similar dependence of pulse energy output on injection power, fig. 4. These results refer to a charging voltage of 19 kV. It is to b e noted that in this case, saturation occurred at an injection power of only 0.2 mW. Fig. 5, which refers to a charging voltage of 18 kV but with various ages of dye solution, shows that the pulse energy in the saturation condition is directly proportional to that in the free-lasing condition. It is difficult to estimate accurately the proportion of energy at the injection wavelength in the saturation condition. As pointed out by Ganiel et al. [5], the output of a free-running ring laser will be emitted equally in opposite directions but with injection the division of output is more complicated. The injection might exist for only a limited part o f the pulse and be superseded by free-lasing radiation [5,12]. Ganiel et al. [5] have pointed out that owing to the persistence of the asymmetrical distribution o f the excited singlet states set up within the lasing medium during injection, the output during the free-lasing part of the pulse will consist o f two beams of unequal intensity; that in the direction of the output beam in fig. 1 will be the smaller. The proportion of the pulse energy locked to the injection wavelength is, therefore, great-
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15 November 1981
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2
3
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0)2 0~ Injection power, mW
014
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Fig. 3. Dependence of the pulse energy output in a bandwidth of 2.0 nm on injected power for a charging voltage of 18 kV.
Fig. 4. Dependence of the total pulse energy on injected power with a 5 #m spacing Fabry-Perot etalon included within the laser cavity, for a charging voltage of 19 kV. 389
Volume 39, number 6
OPTICS COMMUNICATIONS
curs at a substantially smaller threshold p o w e r than the 20 mW value f o u n d previously with a similar ringlaser arrangement e m p l o y i n g a mixture o f sulphar h o d a m i n e B and cresyl violet [10]. F u r t h e r m o r e , with the inclusion o f a suitable Fabry-Perot etalon within the pulsed laser c a v i t y , to reduce the spectral w i d t h o f the free-running o u t p u t , the threshold power for injection locking and that required for saturation were very small. With this configuration the proportion o f the pulse laser energy locked to the injection wavelength was greater than 30%.
120 IiO Ioo 90
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15 November t 981
/ /
50
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30
40
50
60
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80
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Fig. 5. Variations of the total pulse energy in the saturated injection condition and the corresponding free-running pulse energy, for a charging voltage of 18 kV and various ages of dye solution with a 5 pm spacing Fabry-Pcrot etaton included within the cavity.
er than the 30% implied by the c o m p a r i s o n o f the full and broken lines in fig. 5.
4. Conclusions It is f o u n d that injection locking o f a dye laser incorporating r h o d a m i n e 6 G as the active m e d i u m oc-
390
[1] D .D. Solfer and B.B. McFarlane, Appl. Phys. Lett. 10 (1967) 266. [2] D.J. Bradley, A.J.I:. Durrant, G.M. Gate, M. Moore andt P.I). Smith, IEEE J. Quantum Electron. QE4 (1968) 707. [3] H. Walther and J.L. Hall, Appl. Phys. Lett. 17 (1970) 239. 14[ P.P. Sorokin, J.R. Lankar, V.L. Morussi and A. Lurio, Appl. Phys. Lett. 15 (1969) 179. [5] U. Ganiel, A. Ifardy and 1). Troves, It';1!;I'~J. Quantum Flectron QEI2 (1976) 704. [6] L.E. Erickson and A. Szabo, Appl. Phys. Lett. 18 (1971) 433. [7] Q.It.F. Vrehen and A.J. Breimer, Optics Comm. 4 (1972) 416. [8] J.J. Turner, E.I. Moses and C.L. Tang, Appl. Phys. Left. 27 (1975) 441. [9] S. Blit, U. Ganiel and D. Trcves, Appl. Phys. 12 (1977) 69. [10] A.J. Gibson and L. Thomas, J. Phys. D. Appl. Phys. 11 (1978) L59. [ 11 ] I:. Trehin, 1". Biraben and B. Cagnac, Optics Comm. 31 (1979) 76. [ 12] J.E. Bjorkholm and H.G. Danielmeyer, Appl. Phys. Lett. 15 (1969) 171.
OPTICS COMMUNICATIONS
15 November 198 l
LOCKING OF A FLASHLAMP-PUMPED DYE LASER AT SMALL INJECTION POWERS S.K. BHATTACHARYYA, P. EGGETT and L. THOMAS SERC, Rutherford and Appleton Laboratories, Chilton, Didcot, Oxon O X l l OQX UK
Received 22 June 1981
Experiments have been carried out on the injection-locking of a pulsed dye laser to an argon-laser-pumped CW dye laser of 50 MHz linewidth without longitudinal mode matching. With the inclusion of a Fabry-Perot etalon within the ring laser arrangement of the pulsed laser, the initiation of injection locking and saturation occur at low injection powers.
1. Introduction Spectral narrowing of the output of a dye laser has normally been achieved by incorporating frequency selective elements in the laser cavity. Softer and McFarland [ 1] first replaced one of the cavity mirrors by a plane diffraction grating, Bradley et al. [2] included Fabry-Perot etalons, Walther and Hall [3] made use of a birefringent selective filter, and Sorokin et al. [4] used an arrangement to lock the dye laser to one of the sodium lines. With this type of approach, spectral widths as narrow as 100 MHz have been attained but generally at the expense of the power output. When a multistage system is used, incorporating an oscillator and amplifying stage, the efficiency is generally low [5]. In a technique introduced by Erickson and Szabo [6] the wavelength of a pulsed dye laser is controlled by injection of a stable narrow-band signal. The use of a pulsed gas laser as injection source by these workers, of a CW gas laser by Vrehen and Breimer [7], and of a CW dye laser by Turner et al. [8] demonstrated tire principle of the technique but yielded very low output pulse energies. Subsequently, Blit et al. [9], and Gibson and Thomas [10] reported experiments involving iniection from a CW dye laser and a krypton laser, respectively, in which pulses of linewidth less than 30 MHz and energies of tens of mJ were achieved. More recently, Trehin et al. [11] using a stabilised single-mode CW dye laser as injection source obtained smaller energy pulses, of linewidth 0 030-4018/81/0000-0000/$ 02.75 © 1981 North-Holland
identical to the injecting dye laser, 8 MHz, but frequency higher by about 7 MHz. These workers arranged that the cavity of the dye laser was tuned to the wavelength of the CW laser, and in the experiment of Blit et al. [9] care was taken to match both transverse and longitudinal modes of the injection and pulsed lasers. With these mode-matching arrangements, it was found that almost all the output pulse energy was channelled into the bandwidth of the injected radiation. Furthermore, full locking occurred with an injectiop_ input power of l mW, although the onset of locking occurred at much lower powers. No arrangements for mode matching the locked laser cavity to the injected radiation were made in the other experiments mentioned above. Vrehen and Breimer [7] made use of an injection power of 1 mW but, in general, inputs considerably in excess of this value were required. Unlike the findings of Blit et al. [9] and Trehin et al. [11], only partial locking was observed in these other experiments. The purpose of the present study was to examine further the characteristics of injection-locking without resorting to arrangements for transverse and longitudinal mode matching. The conditions for controlling the frequency and spectral properties of the major part of the output of a moderate-energy pulsed dye laser were thereby examined. The general optical arrangement was similar to that described by Gibson and Thomas [10] except that the injected radiation was obtained from a dye laser pumped by an argon ion laser. This permitted operation at wavelengths other 387
Volume 39, number 6
OPTICS COMMUNICATIONS
than 647.1 nm which corresponded to the krypton ion laser line they employed and, in particular, permitted the use of the high efficiency rhodamine 6 G dye.
2. Experimental system The optical configuration illustrated in fig. 1 made use of a ring laser arrangement to prevent feedback of the pulsed output into the CW laser, as proposed by Ganiel et al. [5]. A suitably placed expanding telescope enabled the diameter of the CW beam to be varied relative to that of the dye cell. The majority of the measurements were made with a pulsed dye laser cavity length of 10 m, corresponding to a longitudinal mode spacing of 30 MHz. The CW dye laser providing the injection radiation employed a transverse flow dye-jet pumped by the output of an argon laser. The jet consisted of a solution of 3 X 10 - 3 M rhodamine 6 G in ethylene glycol circulated at a rate of 0.9 ~ min - I (1.5 X 10 - 5 m 3 s - 1 ) through a 10 #m pore filter. A controllable wedge was used to tune the resonant cavity within the broadband output and a temperature-stabilised plane-parallel electronically-tunable Fabry-Perot etalon, together with a 0.11 mm thickness solid etalon, provided a linewidth of 50 MHz near the 600 nm chosen wavelength.
.j> flashlamp pulsedyeloser
Coxial
J
50 %/'J~:
> ~
>
--
Outputbeam
Telescope
Argon laser
Fig. l. Arrangement of ring laser (coaxial tlashlamp pumped dye) with injection from a CW dye laser pumped by an argon laser. 388
15 November 1981
The pulsed dye laser used in tile measurements incorporated a xenon-filled coaxial flashlamp, driven by a Marx-Bank circuit providing a pulse duration of about 0.3/as, as described previously by Gibson and Thomas [10]. To obtain tile free-running broadband laser output centred at about 600 nm, a dye solution o f 5 X 10 - 5 M rhodamine 6 G in a 1 : 1 water-isopropanol mixture was used, this solution being circulated at a flow rate of about 4 ~ rain 1 through a 0.45/am pore filter.
3. Experimental results Preliminary measurements were carried out with only the wedge and 0.11 mm solid etalon included within the cavity of the CW dye laser, for which the output had a spectral width of about 45 GHz (0.05 nm at 600 nm). Very efficient injection-locking was observed for injection powers down to about 2 mW with a marked reduction in the broad-band emission. This was to be expected since with a cavity length of 10 m an overlap of the injection spectrum with a longitudinal mode frequency would be expected to occur always. With the reduction in the injection spectrum to 50 MHz, brought about by the inclusion of the electronically tunable Fabry-Perot etalon in the CW dye laser cavity, it was found that injection-locking was less complete and a smaller reduction in the broadband emission was observed. Examples of the spectra obtained with the narrow-band injection using a I m grating spectrograph with a broad slit width are shown in (a) and (c) of fig. 2; (b) and (d) represent the resuits with no injection-locking but with the CW signal shown superimposed for reference. The reduction in the intensity of the broad-band emission during injection-locking is noticeable and the narrow-band emission unresolved by the slit width was sufficiently strong to over expose tile film. In order to estimate the intensity in the narrow-band emission, a 5/~m spacing Fabry-Perot etalon of finesse about 17 acting as a filter centred on the injected line was inserted between the output mirror and the power meter; this restricted the broad-band output to about 2.0 nm. Fig. 3 then shows the pulse energy measured witlfin this pass-band as a function of the injected power; the free-running pulse energy of 12 mJ corresponds to zero injection power. These results refer to a charging voltage of 18
Volume 39, number 6
OPTICS COMMUNICATIONS
CI
b
C
I
I 5 nm
Fig. 2. Spectra of pulsed laser obtained using a grating spectrograph: (a) (c) with injection-locking, (b) (d) without injectionlocking but with injection radiation shown for reference. kV for the pulsed dye laser and a cavity length o f 10 m. It is seen that even with an injection power input of 0.5 roW, a narrow-band o u t p u t o f about 3 mJ is obtained. Furthermore, the narrow-band output saturates at about 18 mJ for an injection power near 2 mW. A similar saturation effect was found by Turner et al. [8] but at a considerably larger injection power.
++
3o
As expected, the transfer of the 5/am etalon to within the pulsed laser cavity reduced the spectral width o f the free-running output being measured. A set o f measurements with the etalon in this position showed a similar dependence of pulse energy output on injection power, fig. 4. These results refer to a charging voltage of 19 kV. It is to b e noted that in this case, saturation occurred at an injection power of only 0.2 mW. Fig. 5, which refers to a charging voltage of 18 kV but with various ages of dye solution, shows that the pulse energy in the saturation condition is directly proportional to that in the free-lasing condition. It is difficult to estimate accurately the proportion of energy at the injection wavelength in the saturation condition. As pointed out by Ganiel et al. [5], the output of a free-running ring laser will be emitted equally in opposite directions but with injection the division of output is more complicated. The injection might exist for only a limited part o f the pulse and be superseded by free-lasing radiation [5,12]. Ganiel et al. [5] have pointed out that owing to the persistence of the asymmetrical distribution o f the excited singlet states set up within the lasing medium during injection, the output during the free-lasing part of the pulse will consist o f two beams of unequal intensity; that in the direction of the output beam in fig. 1 will be the smaller. The proportion of the pulse energy locked to the injection wavelength is, therefore, great-
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15 November 1981
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2
3
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0.1
0)2 0~ Injection power, mW
014
0!5
I njecfion power, mW
Fig. 3. Dependence of the pulse energy output in a bandwidth of 2.0 nm on injected power for a charging voltage of 18 kV.
Fig. 4. Dependence of the total pulse energy on injected power with a 5 #m spacing Fabry-Perot etalon included within the laser cavity, for a charging voltage of 19 kV. 389
Volume 39, number 6
OPTICS COMMUNICATIONS
curs at a substantially smaller threshold p o w e r than the 20 mW value f o u n d previously with a similar ringlaser arrangement e m p l o y i n g a mixture o f sulphar h o d a m i n e B and cresyl violet [10]. F u r t h e r m o r e , with the inclusion o f a suitable Fabry-Perot etalon within the pulsed laser c a v i t y , to reduce the spectral w i d t h o f the free-running o u t p u t , the threshold power for injection locking and that required for saturation were very small. With this configuration the proportion o f the pulse laser energy locked to the injection wavelength was greater than 30%.
120 IiO Ioo 90
=s //
80
~- 7o
/
60-
15 November t 981
/ /
50
References eL 30 20 I0 0
I
~
I
L
I
I
I0
20
30
40
50
60
h
I
70
80
Pulse loser output without injection locking, mJ
Fig. 5. Variations of the total pulse energy in the saturated injection condition and the corresponding free-running pulse energy, for a charging voltage of 18 kV and various ages of dye solution with a 5 pm spacing Fabry-Pcrot etaton included within the cavity.
er than the 30% implied by the c o m p a r i s o n o f the full and broken lines in fig. 5.
4. Conclusions It is f o u n d that injection locking o f a dye laser incorporating r h o d a m i n e 6 G as the active m e d i u m oc-
390
[1] D .D. Solfer and B.B. McFarlane, Appl. Phys. Lett. 10 (1967) 266. [2] D.J. Bradley, A.J.I:. Durrant, G.M. Gate, M. Moore andt P.I). Smith, IEEE J. Quantum Electron. QE4 (1968) 707. [3] H. Walther and J.L. Hall, Appl. Phys. Lett. 17 (1970) 239. 14[ P.P. Sorokin, J.R. Lankar, V.L. Morussi and A. Lurio, Appl. Phys. Lett. 15 (1969) 179. [5] U. Ganiel, A. Ifardy and 1). Troves, It';1!;I'~J. Quantum Flectron QEI2 (1976) 704. [6] L.E. Erickson and A. Szabo, Appl. Phys. Lett. 18 (1971) 433. [7] Q.It.F. Vrehen and A.J. Breimer, Optics Comm. 4 (1972) 416. [8] J.J. Turner, E.I. Moses and C.L. Tang, Appl. Phys. Left. 27 (1975) 441. [9] S. Blit, U. Ganiel and D. Trcves, Appl. Phys. 12 (1977) 69. [10] A.J. Gibson and L. Thomas, J. Phys. D. Appl. Phys. 11 (1978) L59. [ 11 ] I:. Trehin, 1". Biraben and B. Cagnac, Optics Comm. 31 (1979) 76. [ 12] J.E. Bjorkholm and H.G. Danielmeyer, Appl. Phys. Lett. 15 (1969) 171.