- Email: [email protected]
Frequency tunable, nanosecond duration pulses from flashlamp pumped dye lasers by pulsed Q-modulation
Volume 28, number 3
OPTICS COMMUNICATIONS
March 1979
FREQUENCYTUNABLE,NANOSECONDDURATIONPULSESFROM FLASHLAMPPUMPEDDYELASERSBYPULSEDQ-MODULATION P. EWART Blackett Laboratory, Imperial College of Science and Technology, London SW 7 2BZ, UK
Received 1 December 1978
A new technique is described for producing trains of nanosecond duration pulses from flashlamp pumped dye lasers, by pulsed modulation of the cavity flux with an electro-optic Q-switch. Spectrally narrow (0.005 nm), 2 ns duration pulses are obtained in 300 ns long trains over a wide spectral range at 5 Hz repetiion rate. Applications to other lasers systems is discussed.
1. Introduction
2. Apparatus
Short duration pulses are normally obtained from flashlamp pumped dye lasers by passive mode-locking with saturable absorbers. This technique which has been extensively reviewed [ 1,2] has two major advantages - very short pulses are produced (2-5 ps) and the method is cheap and relatively simple. There are however several disadvantages, notably a restricted range of operating wavelengths due to the need for matching amplifier and absorber transition crosssections. The pulses cannot be synchronized by external circuitry and it is often difficult to obtain reproducible pulse amplitudes as the system must operate very close to threshold. Also, nonlinear effects, particularly in the absorber dye, often lead to poor beam quality and poor spectral and temporal purity. Active modelocking of a pulsed dye laser was first achieved by acousto-optic loss modulation [3]. This device avoids the wavelength restrictions imposed by saturable absorbers although the pulse durations were relatively long. This report describes a novel technique for producing nanosecond duration pulses in trains similar to those obtained in modelocked op eration.
The laser cavity employed is similar to that used for multipass amplification of nitrogen laser pumped dye laser pulses [4]. In that work, trains of 2-4 nanosecond duration pulses were obtained from a flashlamp pumped dye laser. This was achieved by Qspoiling the cavity until a short duration pulse (from a N2-laser pumped dye laser) was injected, electrooptically, with sufficient intensity to saturate the amplifier medium after a few round trips. Gain saturation then ensured that the flux in the cavity did not build up except in the region of the injected pulse circulating in the cavity. In the present work a saturating pulse is produced from the initial uniform flux in the cavity by cavity dumping. The essential elements of the cavity are shown in fig. 1. The cavity is defined by a curved (3 m radius) 100% reflecting mirror, Ml, and a plane, partially transmitting mirror, M2, placed 1.8 m apart. The dye cell, pumped by two linear flashlamps in a double elliptical configuration was placed close to mirror Ml. The Q-switch, consisting of a GlanThompson polarizer prism and a Pockels cell was placed at various distances from the other laser mirror. The Pockels cell was driven by a rectangular quarter wave voltage pulse, of variable duration, from a pulse forming network activated by a Krytron switch. The rise time of the voltage step was approximately 1 ns and 379
Volume
28, number
3
Voltage
Ml
OPTICS COMMUNICATIONS
March
1979
Pulse
Laser Medium Fig. 1. Experimental
G.T. Prism
Pockels Cell
M2
arrangement.
was triggered by a delay unit to operate at a preselected time during the flashlamp pulse.
3.
Modulation
mechanism and results
Fig. 2 shows the normal output of the flashlamp pumped laser recorded by a fast photodiode (Instrument Technology Limited) and oscilloscope (Tektronix 5 19). Operation of the Q-switch at approximately the quarter wave voltage produces a modulation of the initially uniformly distributed flux inside the cavity in the following manner. On the first complete round trip after application of the quarter wave voltage, flux initially between the Pockels cell and mirror Ml, passes through the Pockels cell twice and so is efficiently dumped from the cavity by the polarizing prism. Flux initially between the Pockels cell and mirror M2, on the other hand, passes only once through the Pockels cell before reaching the polarizing prism and so only about half of this flux is dumped. The contrast in flux levels induced in this way is maintained on subsequent round trips with an overall reduction in intensity. At some later time the cavity Q is restored to its original high value by switching off the voltage on the Pockels cell. The flux, now with a modulation peak, begins to build up again. If the net gain of the system is sufficiently large and the modulation peak to background contrast is also large, then gain saturation by the peak to background contrast is also large, then gain saturation by the peak will maintain and enhance the modulation resulting in a single burst of flux circulating in the cavity. This operation is illustrated in fig. 2(b), where the laser has been allowed to establish its normal lasing level before switching the modulator. The cavity 380
Fig. 2. Oscillograms of laser output: (a) 100 ns/division. Normal output of flashlamp pumped dye laser. (b) 100 ns/division. Effect of a 30 ns Q-modulation pulse on normal laser operation. Note the rapid build up of the short duration pulses. (c) 100 m/division. Quasi-modelocked operation obtained by pulsed Q-modulation at the start of the laser pulse. (d) 5 ns/division. Individual pulses from near the middle of the pulse train showing smooth profiles and a very low background light level. spoiled in this case for 30 ns corresponding to round trips. This was found to be the optimum duration for the Q-switching pulse. It was almost im-
Q was 2%
Volume 28, number 3
OPTICS COMMUNICATIONS
possible to obtain pulse trains with Q-spoiling pulses of less than 24 ns and for very long pulses, 2 100 ns, the pulse trains were slow to develop and showed poor modulation depth. Operating the Q-spoiling for a halfintegral number of round trips ensures that the impressed modulation is not adversely affected by the second switching action. Fig. 2(b) shows the very rapid build up of the pulses following restoration of the cavity Q to its original value. The large small signal gain of flashlamp pumped dye lasers allows the switching to be carried out near the beginning of the pumping pulse. This produces a pulse train with 100% modulation lasting for practically the whole duration of the normal laser pulse as shown in fig. 2(c). The individual pulses shown in fig. 2(d) have smooth, almost gaussian profiles with durations of 2 to 4 ns. The duration of the initial modulation is determined by the distance between the Pockels cell and mirror M2, but is limited by the rise time of the Pockels cell itself. Pockels cells are available with risetimes in the 100 ps range so it may be possible to produce very short pulses. The pulse duration can be varied to some degree by adjusting the position of the Q-switch in the cavity. The shortest pulses were however obtained by placing the gain medium close to one mirror. This indicates that pulse shortening by gain saturation plays an important role in the operation of the system. A useful feature of the pulses emitted is their fixed timing relative to the Pockels cell trigger (jitter is 2 1 ns) which facilitates synchronization with external circuitry. It is relatively easy to obtain TEMoo mode outputs from the system with its associated high quality beam profile and low divergence. Insertion of two FabryPerot etalons (with 5 pm and 200 pm spacings) narrowed the spectral output to 0.005 nm. Pulse trains were thus obtained over the whole tuning ranges of Rhodamine dyes. Since there is no wavelength dependent factor inherent in the system the device operates over the spectral range, covered by flashlamp pumped dye lasers [ 51. The narrowed spectral bandwidth encompassed -50 longitudinal modes which if perfectly locked with zero phase difference would give a much narrower pulse than those observed. (Pulse duration is given approximately by (cavity round trip time)/number of modes locked [2] . This would correspond to a pulse duration of -220 ps). Further spectral narrowing is possible to obtain Fourier trans-
March 1979
form limited pulses which must then be ideally modelocked [5]. The temporal coherence properties of such pulses are very useful for applications in nonlinear spectroscopy. (It is worth noting that a pulse duration of 3 ns in a cavity of 12 ns round trip time corresponds to 4 phase locked cavity modes. With a mode spacing of 80 MHz such a pulse would have a bandwidth of 320 MHz. Such pulses were in fact obtained in the multipass or regenerative amplifier system of ref. [4]. The pulse trains produced were remarkably stable and reproducible even when the laser was operated at 5 Hz repetition rate. This rate was limited by the power capabilities of the laser power supply. Trains of single pulses were obtained even well above threshold. At still higher pumping rates double or triple pulses were produced. Peak powers of approximately 4 kW were obtained in the short pulses, using Rhodamine 6G in ethanol. It should be possible to obtain high power single pulses by using two 100% mirrors and cavity dumping a second time after a single pulse becomes established in the cavity.
4. Conclusions A novel system has been developed which provides trains of nanosecond duration pulses from flaihlamp pumped dye lasers in a quasi-modelocked operation. The short pulses are well synchronized to external triggers and have been generated at 5 Hz repetition rate. A useful feature of the device is its ability to generate these short pulses over the entire visible spectral range by using different dyes. Of perhaps greater significance is the possibility of using the technique to modulate or even modelock other high gain systems such as excimer, exciplex and other lasers operating in spectral regions which make passive modelocking difficult. These short duration, narrow bandwidth, tunable laser pulses may be useful in resonance scattering experiments, pollution monitoring and ranging, reaction kinetics studies, time resolved holography and nonlinear spectroscopy. Additional note. Soon after submission of this article, a technique, broadly similar in principle to that described here, was reported by Yung S. Liu [6]. In this work short (-20 ns) trains of nanosecond duration pulses were produced in a laser pumped dye laser. 381
Volume 28, number
3
OPTICS COMMUNICATIONS
Acknowledgements It is a pleasure to acknowledge the experimental assistance of Dr. J. Vukusic. The author is also grateful to Professor D.J. Bradley for provision of equipment and the Science Research Council for personal support in the fQrm of an Advanced Fellowship.
References [ 1] D.J. Bradley, in Ultrashort light pulses, ed. S.L. Shapiro (Springer-Verlag, Berlin 1977).
382
March
1979
[2] P.W. Smith, M.A. Duguay and E.P. Ippen, Progress in Quant. Electron., Vol. 3, part 2, 1974. [3] C.M. Ferrar, I.E.E.E.J. Quant. Electron. QE-5 (1969) 550. [4] P. Ewart and J.M. Catherall, Optics Comm. 27 (1978) 439 [5] P. Ewart, to be published. [6] Y.S. Liv, Optics Lett. 3 (1978) 167.
OPTICS COMMUNICATIONS
March 1979
FREQUENCYTUNABLE,NANOSECONDDURATIONPULSESFROM FLASHLAMPPUMPEDDYELASERSBYPULSEDQ-MODULATION P. EWART Blackett Laboratory, Imperial College of Science and Technology, London SW 7 2BZ, UK
Received 1 December 1978
A new technique is described for producing trains of nanosecond duration pulses from flashlamp pumped dye lasers, by pulsed modulation of the cavity flux with an electro-optic Q-switch. Spectrally narrow (0.005 nm), 2 ns duration pulses are obtained in 300 ns long trains over a wide spectral range at 5 Hz repetiion rate. Applications to other lasers systems is discussed.
1. Introduction
2. Apparatus
Short duration pulses are normally obtained from flashlamp pumped dye lasers by passive mode-locking with saturable absorbers. This technique which has been extensively reviewed [ 1,2] has two major advantages - very short pulses are produced (2-5 ps) and the method is cheap and relatively simple. There are however several disadvantages, notably a restricted range of operating wavelengths due to the need for matching amplifier and absorber transition crosssections. The pulses cannot be synchronized by external circuitry and it is often difficult to obtain reproducible pulse amplitudes as the system must operate very close to threshold. Also, nonlinear effects, particularly in the absorber dye, often lead to poor beam quality and poor spectral and temporal purity. Active modelocking of a pulsed dye laser was first achieved by acousto-optic loss modulation [3]. This device avoids the wavelength restrictions imposed by saturable absorbers although the pulse durations were relatively long. This report describes a novel technique for producing nanosecond duration pulses in trains similar to those obtained in modelocked op eration.
The laser cavity employed is similar to that used for multipass amplification of nitrogen laser pumped dye laser pulses [4]. In that work, trains of 2-4 nanosecond duration pulses were obtained from a flashlamp pumped dye laser. This was achieved by Qspoiling the cavity until a short duration pulse (from a N2-laser pumped dye laser) was injected, electrooptically, with sufficient intensity to saturate the amplifier medium after a few round trips. Gain saturation then ensured that the flux in the cavity did not build up except in the region of the injected pulse circulating in the cavity. In the present work a saturating pulse is produced from the initial uniform flux in the cavity by cavity dumping. The essential elements of the cavity are shown in fig. 1. The cavity is defined by a curved (3 m radius) 100% reflecting mirror, Ml, and a plane, partially transmitting mirror, M2, placed 1.8 m apart. The dye cell, pumped by two linear flashlamps in a double elliptical configuration was placed close to mirror Ml. The Q-switch, consisting of a GlanThompson polarizer prism and a Pockels cell was placed at various distances from the other laser mirror. The Pockels cell was driven by a rectangular quarter wave voltage pulse, of variable duration, from a pulse forming network activated by a Krytron switch. The rise time of the voltage step was approximately 1 ns and 379
Volume
28, number
3
Voltage
Ml
OPTICS COMMUNICATIONS
March
1979
Pulse
Laser Medium Fig. 1. Experimental
G.T. Prism
Pockels Cell
M2
arrangement.
was triggered by a delay unit to operate at a preselected time during the flashlamp pulse.
3.
Modulation
mechanism and results
Fig. 2 shows the normal output of the flashlamp pumped laser recorded by a fast photodiode (Instrument Technology Limited) and oscilloscope (Tektronix 5 19). Operation of the Q-switch at approximately the quarter wave voltage produces a modulation of the initially uniformly distributed flux inside the cavity in the following manner. On the first complete round trip after application of the quarter wave voltage, flux initially between the Pockels cell and mirror Ml, passes through the Pockels cell twice and so is efficiently dumped from the cavity by the polarizing prism. Flux initially between the Pockels cell and mirror M2, on the other hand, passes only once through the Pockels cell before reaching the polarizing prism and so only about half of this flux is dumped. The contrast in flux levels induced in this way is maintained on subsequent round trips with an overall reduction in intensity. At some later time the cavity Q is restored to its original high value by switching off the voltage on the Pockels cell. The flux, now with a modulation peak, begins to build up again. If the net gain of the system is sufficiently large and the modulation peak to background contrast is also large, then gain saturation by the peak to background contrast is also large, then gain saturation by the peak will maintain and enhance the modulation resulting in a single burst of flux circulating in the cavity. This operation is illustrated in fig. 2(b), where the laser has been allowed to establish its normal lasing level before switching the modulator. The cavity 380
Fig. 2. Oscillograms of laser output: (a) 100 ns/division. Normal output of flashlamp pumped dye laser. (b) 100 ns/division. Effect of a 30 ns Q-modulation pulse on normal laser operation. Note the rapid build up of the short duration pulses. (c) 100 m/division. Quasi-modelocked operation obtained by pulsed Q-modulation at the start of the laser pulse. (d) 5 ns/division. Individual pulses from near the middle of the pulse train showing smooth profiles and a very low background light level. spoiled in this case for 30 ns corresponding to round trips. This was found to be the optimum duration for the Q-switching pulse. It was almost im-
Q was 2%
Volume 28, number 3
OPTICS COMMUNICATIONS
possible to obtain pulse trains with Q-spoiling pulses of less than 24 ns and for very long pulses, 2 100 ns, the pulse trains were slow to develop and showed poor modulation depth. Operating the Q-spoiling for a halfintegral number of round trips ensures that the impressed modulation is not adversely affected by the second switching action. Fig. 2(b) shows the very rapid build up of the pulses following restoration of the cavity Q to its original value. The large small signal gain of flashlamp pumped dye lasers allows the switching to be carried out near the beginning of the pumping pulse. This produces a pulse train with 100% modulation lasting for practically the whole duration of the normal laser pulse as shown in fig. 2(c). The individual pulses shown in fig. 2(d) have smooth, almost gaussian profiles with durations of 2 to 4 ns. The duration of the initial modulation is determined by the distance between the Pockels cell and mirror M2, but is limited by the rise time of the Pockels cell itself. Pockels cells are available with risetimes in the 100 ps range so it may be possible to produce very short pulses. The pulse duration can be varied to some degree by adjusting the position of the Q-switch in the cavity. The shortest pulses were however obtained by placing the gain medium close to one mirror. This indicates that pulse shortening by gain saturation plays an important role in the operation of the system. A useful feature of the pulses emitted is their fixed timing relative to the Pockels cell trigger (jitter is 2 1 ns) which facilitates synchronization with external circuitry. It is relatively easy to obtain TEMoo mode outputs from the system with its associated high quality beam profile and low divergence. Insertion of two FabryPerot etalons (with 5 pm and 200 pm spacings) narrowed the spectral output to 0.005 nm. Pulse trains were thus obtained over the whole tuning ranges of Rhodamine dyes. Since there is no wavelength dependent factor inherent in the system the device operates over the spectral range, covered by flashlamp pumped dye lasers [ 51. The narrowed spectral bandwidth encompassed -50 longitudinal modes which if perfectly locked with zero phase difference would give a much narrower pulse than those observed. (Pulse duration is given approximately by (cavity round trip time)/number of modes locked [2] . This would correspond to a pulse duration of -220 ps). Further spectral narrowing is possible to obtain Fourier trans-
March 1979
form limited pulses which must then be ideally modelocked [5]. The temporal coherence properties of such pulses are very useful for applications in nonlinear spectroscopy. (It is worth noting that a pulse duration of 3 ns in a cavity of 12 ns round trip time corresponds to 4 phase locked cavity modes. With a mode spacing of 80 MHz such a pulse would have a bandwidth of 320 MHz. Such pulses were in fact obtained in the multipass or regenerative amplifier system of ref. [4]. The pulse trains produced were remarkably stable and reproducible even when the laser was operated at 5 Hz repetition rate. This rate was limited by the power capabilities of the laser power supply. Trains of single pulses were obtained even well above threshold. At still higher pumping rates double or triple pulses were produced. Peak powers of approximately 4 kW were obtained in the short pulses, using Rhodamine 6G in ethanol. It should be possible to obtain high power single pulses by using two 100% mirrors and cavity dumping a second time after a single pulse becomes established in the cavity.
4. Conclusions A novel system has been developed which provides trains of nanosecond duration pulses from flaihlamp pumped dye lasers in a quasi-modelocked operation. The short pulses are well synchronized to external triggers and have been generated at 5 Hz repetition rate. A useful feature of the device is its ability to generate these short pulses over the entire visible spectral range by using different dyes. Of perhaps greater significance is the possibility of using the technique to modulate or even modelock other high gain systems such as excimer, exciplex and other lasers operating in spectral regions which make passive modelocking difficult. These short duration, narrow bandwidth, tunable laser pulses may be useful in resonance scattering experiments, pollution monitoring and ranging, reaction kinetics studies, time resolved holography and nonlinear spectroscopy. Additional note. Soon after submission of this article, a technique, broadly similar in principle to that described here, was reported by Yung S. Liu [6]. In this work short (-20 ns) trains of nanosecond duration pulses were produced in a laser pumped dye laser. 381
Volume 28, number
3
OPTICS COMMUNICATIONS
Acknowledgements It is a pleasure to acknowledge the experimental assistance of Dr. J. Vukusic. The author is also grateful to Professor D.J. Bradley for provision of equipment and the Science Research Council for personal support in the fQrm of an Advanced Fellowship.
References [ 1] D.J. Bradley, in Ultrashort light pulses, ed. S.L. Shapiro (Springer-Verlag, Berlin 1977).
382
March
1979
[2] P.W. Smith, M.A. Duguay and E.P. Ippen, Progress in Quant. Electron., Vol. 3, part 2, 1974. [3] C.M. Ferrar, I.E.E.E.J. Quant. Electron. QE-5 (1969) 550. [4] P. Ewart and J.M. Catherall, Optics Comm. 27 (1978) 439 [5] P. Ewart, to be published. [6] Y.S. Liv, Optics Lett. 3 (1978) 167.