- Email: [email protected]
Powerful dye laser oscillator-amplifier system for high resolution and coherent pulse spectroscopy
Volume
22, number
2
OPTICS COMMUNICATIONS
POWERFUL DYE LASER OSCILLATOR-AMPLIFIER HIGH RESOLUTION
August
1977
SYSTEM FOR
AND COHERENT PULSE SPECTROSCOPY *
M.M. SALOUR Department of Physicsand Gordon McKay Laboratory, Harvard University, Cambridge, Mass. 02138, USA Received
31 May 1977
A peak power output of 100 kW in the visible at a linewidth as low as 60 MHz has been laser oscillator followed by three single pass dye amplifier stages (or one double pass stage pumped by a single 1 MW nitrogen laser. The very high output power obtained in this laser coherent ultraviolet pulses by sum or second harmonic generation in non-linear crystals or vapor.
The vigorous development of a variety of broadly tunable pulsed lasers in the past few years has led to remarkable progress in the non-linear spectroscopy of atoms, molecules and crystals [e.g. 11. In particular, recent developments in high resolution multiphotonic spectroscopy with lasers have demonstrated that although the application of cw dye lasers (having a very narrow spectral width) is of great interest, their usefulness in multiphotonic spectroscopy must await the development of high power and narrow-band operation in such lasers. Furthermore, the problem of photochemical instability of coumarine and oxazine dyes presently limits the operating range of such dye lasers to the red and blue-green sections of the spectrum. The very high output power obtained in pulsed dye laser oscillator-amplifier systems has already been employed in a number of innovative spectroscopic experiments [2]. However, one drawback of such systems is that the output pulse is not quite Fourier-limited; in many coherent optics experiments it is essential to obtain Fourier-limited pulses, i.e., pulses whose coherence times are not shorter than their durations. While various authors have used different schemes for obtaining such pulses [3], none of these techniques have generated an output powerful enough to permit multiphotonic excitation of atoms and molecules.
* Supported
202
in part by the Joint Services Electronics
Program.
generated by using a cw dye and one single pass stage), system may be used to produce by third order mixing in atomic
In this paper we report the construction of a laser system consisting of a cw dye laser oscillator and a chain of synchronized pulsed amplifiers. We have shown that even if the cw dye laser delivers a very weak intensity in the spectral range under study, with a sufficient number of synchronized pulsed amplifiers one can obtain, at the end, intense 100 kW Fourier limited coherent laser pulses from as low as 1 mW of cw output power. We achieved this result by exciting the narrowband but lossy cw dye laser oscillator with only a small fraction of the available nitrogen laser pump light, and using the remainder to boost the laser intensity afterwards in a series of traveling wave dye laser amplifiers. Note that the dye amplifiers, operating in a highly saturated mode, will at the same time reduce amplitude fluctuations of the cw oscillator. Fig. 1 shows the scheme of such a laser system. A Spectra Physics Model 580 A cw dye laser was pumped by an Ar+ laser (Spectra Physics Model 164). The cw dye laser oscillator was frequency-stabilized to a pressure-tuned reference etalon in order to provide a highly linear frequency tuning mechanism for precision measurements. The pressure was monitored by a capacit&rce manometer (Baratron: MKS Model 310 BHS1000). The output of this stabilized, pressure-tuned cw dye laser oscillator (having a very long coherence time) was focused to a diameter of approximately 0.2 mm into a dye amplifier pumped by approximately 20% of the output of a one-megawatt nitrogen laser
Volume
22, number
August
OPTICS COMMUNICATIONS
2
1911
DELAY Fig.
1. Components
of the frequency-stabilized
cw dye laser oscillator
(Molectron W-1000). The emerging amplified light beam (80 W) was collimated and sent through a direct vision prism, which dispersed the background of amplified spontaneous emission. This background was eliminated by a pinhole of 0.3 mm diameter. The beam was then focused into the second stage amplifier, which was pumped by approximately 30% of the output of the same nitrogen laser. The output of the second stage amplifier was again collimated, and filtered through another direct vision prism, which also compensated the beam wander introduced by the first prism. The filtered beam was finally focused to about 0.3 mm diameter into the third stage dyti amplifier, which was pumped by approximately 50% of the output of the same pump laser. The amplified output (100 kW) emerged with a near-diffraction-limit divergence of about 0.3 mrad. The three dye amplifiers were constructed from closed cuvettes with magnetic stirrers. Their windows were tilted by 3” and were anti-reflectively coated to avoid optical feedback. The N, laser output was divided geometrically with highly reflecting dielectrically
with three single pass synchronized
pulsed
amplifiers.
coated mirrors, and was focused into the cuvette dye cells by cylindrical quartz lenses of 80 mm focal length. The active diameter of the focused nitrogen beam in each cuvette was adjusted to avoid broadening resulting from amplifier gain saturation. This was important for our purpose, since any such broadening would contribute to a bandwidth at the amplifier output which would be larger than the Fourier-transform limit of the pulse. The adjustments were made either by rotating or by defocusing the quartz cylindrical lens to avoid gain saturation. Moreover, in order to maximize the length of the output pulse, the pump light for the amplifiers was optically delayed so that it arrived slightly later than the input from the cw oscillator. With one mW of cw dye laser output and a single-pass gain of 103 to 104 in each amplifier stage, an output power of 100 kW was measured at the third stage amplifier when pumped with the one-megawatt N, laser. As was noted earlier, changes in the intensity of the cw beam were unimportant because of the gain saturation in each amplifying stage. A slight change in the intensity of the amplified output was observed, 203
Volume 22, number 2
OPTICS COMMUNICATIONS
August 1977
Pressure swept
etalon
I
To
lllllll
Telescope
be Amplifier I
Echelle Grotina
-
1 Nitrogen
ll/llllOutput
Fig. 2. Components of the frequency-stabilized ized pulsed amplifiers.
cw dye laser oscillator with one double pass followed by one single pass synchron-
however, when the cw dye laser was mode-hopping; it was thus necessary to operate the cw dye laser with its frequency “locked” to the side of the transmission window of a Fabry-Perot interferometer. In this arrangement of single pass stages of amplification, the length of the output pulse of the third stage amplifier was only slightly shorter than the pulse length of the N2 laser. Thus the length and shape of the output pulse were essentially dictated by the duration and shape of the pump laser pulse. Various techniques could be used to alter the duration of the pump pulse, for example by changing the alignment of the rear mirror, the gas pressure, or the repetition rate, but never204
theless the length and shape of the output pulse could be varied only with difficulty. To obtain more intense Fourier limited optical pulses of controllable shape and duration, we employed a technique first demonstrated by Bolger, Baede and Gibbs [3], who removed the output reflecting mirror of a Hansch type nitrogen-pumped dye laser, thus converting it into a double-pass amplifier, When amplifying a cw dye laser, they obtained intense and highly coherent pulses, shorter than the pumping laser pulses, whose time dependence was a complicated function of various parameters, including the cellgrating distance, the telescope imaging, the input pow-
Volume
22, number
August
OPTICS COMMUNICATIONS
2
er, and the pulse length and intensity of the N21aser. . In our experimental investigation, the output of this double pass amplifier, which generated intense (5OOW), tunable and Fourier limited short pulses, was further amplified by another dye amplifier operating in a single pass mode while being pumped by 50% of the output of the 1 MW nitrogen laser (fig. 2). Suitable spectral and spatial filters were inserted between the two dye amplifiers in order to remove the undesirable amplified spontaneous emission of the double pass dye amplifier I (fig. 2) from dye amplifier II. In this configuration, stable pulses of 8.5 kW output power and 4 ns duration were obtained. It is important to note that because of the steep rise in the gain of the nitrogen laser-pumped dye and the large amplification crosssection, pulse shaping in the double-pass amplifier can easily occur when the intensity of the evolving pulse
1977
is larger than the saturation level and when the pulse rises steeply from an exponential or hard truncated leading edge. In both experimental configurations (figs. 1 and 2), it was desirable for our purpose that the output be collinear with the cw beam. For this reason, the dye cell cuvet ‘es were all anti-reflectively coated. The output.pulse could be separated from the cw carrier, however, by a reflection on the dye cell window. The two versions of the cw dye laser oscillator with synchronized pulsed amplifiers discussed above can be employed to generate pairs of intense, Fourier-limited, and coherent pulses by sending the output of either laser system into a highly stabilized optical delay line (fig. 3a) whose total transit time T is longer than the duration of the initial pulse (T > T). In such a case one can lock the phases of the two pulses by using inter-
IO nsec/div
( b) (0)
CONFOCAL FAEIRY- PEROT
‘0
rl
r2
r3
r4
--it+
1.5 nsec (cl
(d) Fig. 3. (a) Optical delay line for delay by T ns of a 7 ns pulse (T > 7). (b) The output of the optical delay line of fig. 3a, for input from the dye laser of fig. 1. (c) Optical delay line for delay by Tns of a 7 ns pulse (T < 7). (d) The output of the optical delay line of fig. 3c, for input from the dye laser of fig. 1.
205
Volume
22, number
2
OPTICS COMMUNICATIONS
ference between two direct and delayed cw beams [4]. Great care must be taken, however, to properly achieve this result, not only because of the low intensity of the cw oscillating wave, but also because any external perturbation can make this task nearly impossible. Fig. 3b shows the output of such a stabilized delay line observed on a Tektronix 590 scope, when the output of the laser system of fig. 1 was employed to generate two phase-coherent pulses. An extension of this technique to multiple phase-coherent pulses is clearly possible, by sending the initial pulse into a confocal resonator, for instance [5]. If, however, the total transit time Tin the optical delay line is much shorter than the pulse duration 7, one then obtains an output consisting of a succession of light pulses. Fig. 3d illustrates this situation when the intense r = 8 ns output pulse of the laser system of fig. 1 was delayed by T = 1.5 ns in the optical delay line of fig. 3c, and then observed on a Tektronix 661 sampling oscilloscope. The first peak corresponds to the direct reflection of the incident pulse on the mirror M, of the delay line of fig. 3c, and the following echoes correspond to the successive outgoing waves after each complete path of the interferometer. This device could be applied in time resolved spectroscopy to saturate a transition or an absorbing environment with the first reflected beam, and the successive echoes could then be used to analyze the time evolution of the perturbation due to the first reflected pulse. Such an experiment would require as intense a saturable beam as possible. Moreover, it would be desirable that the successive echoes have intensities adequate for signal/noise ratio but not large enough to perturb the medium.
206
August
1911
In addition to coherent optics, the new dye laser oscillator-amplifier systems discussed above could have a number of important applications in high resolution spectroscopy of atoms and molecules. The advantage of these systems of cw dye laser oscillator with pulsed dye amplifiers over the pulse dye laser oscillator with two-stage dye amplifiers (with time coincidence between signal and pump pulses), is not only that the pulses are Fourier limited, but also that the collinearity of the cw beam with the output pulse can be used in experimental situations. Furthermore, since we have been able to obtain 100 kW output power with as low as one mW output from the single mode cw oscillator, this system could not only generate very intense Fourier limited pulses throughout the visible range (certainly from 4200 to 7800 A where an intense pump source such as a nitrogen laser and a signal source are available), but could also generate pulses of a few hundred watts for wavelengths down to 2500 A by efficient sum or second harmonic conversion in nonlinear crystals or by third-order mixing in atomic vapor.
References 111 H. Walther, ed., High resolution
spectroscopy with tunable dye lasers (Springer, Berlin, 1975). 121 R. Wallenstein and T.W. HHnsch, Opt. Commun. 14 (1975) 353, and references therein, 18 131 B. Bolger, L. Baede and H.M. Gibbs, Opt. Commun. (1976) 67; 19 (1976) 346, and references therein. Phys. Rev. Letters 141 M.M. Salour and C. Cohen-Tannoudji, 38 (1977) 757. is1 R. Teets, J. Eckstein and T.W. HLnsch, Phys. Rev. Letters 38 (1977) 760.
22, number
2
OPTICS COMMUNICATIONS
POWERFUL DYE LASER OSCILLATOR-AMPLIFIER HIGH RESOLUTION
August
1977
SYSTEM FOR
AND COHERENT PULSE SPECTROSCOPY *
M.M. SALOUR Department of Physicsand Gordon McKay Laboratory, Harvard University, Cambridge, Mass. 02138, USA Received
31 May 1977
A peak power output of 100 kW in the visible at a linewidth as low as 60 MHz has been laser oscillator followed by three single pass dye amplifier stages (or one double pass stage pumped by a single 1 MW nitrogen laser. The very high output power obtained in this laser coherent ultraviolet pulses by sum or second harmonic generation in non-linear crystals or vapor.
The vigorous development of a variety of broadly tunable pulsed lasers in the past few years has led to remarkable progress in the non-linear spectroscopy of atoms, molecules and crystals [e.g. 11. In particular, recent developments in high resolution multiphotonic spectroscopy with lasers have demonstrated that although the application of cw dye lasers (having a very narrow spectral width) is of great interest, their usefulness in multiphotonic spectroscopy must await the development of high power and narrow-band operation in such lasers. Furthermore, the problem of photochemical instability of coumarine and oxazine dyes presently limits the operating range of such dye lasers to the red and blue-green sections of the spectrum. The very high output power obtained in pulsed dye laser oscillator-amplifier systems has already been employed in a number of innovative spectroscopic experiments [2]. However, one drawback of such systems is that the output pulse is not quite Fourier-limited; in many coherent optics experiments it is essential to obtain Fourier-limited pulses, i.e., pulses whose coherence times are not shorter than their durations. While various authors have used different schemes for obtaining such pulses [3], none of these techniques have generated an output powerful enough to permit multiphotonic excitation of atoms and molecules.
* Supported
202
in part by the Joint Services Electronics
Program.
generated by using a cw dye and one single pass stage), system may be used to produce by third order mixing in atomic
In this paper we report the construction of a laser system consisting of a cw dye laser oscillator and a chain of synchronized pulsed amplifiers. We have shown that even if the cw dye laser delivers a very weak intensity in the spectral range under study, with a sufficient number of synchronized pulsed amplifiers one can obtain, at the end, intense 100 kW Fourier limited coherent laser pulses from as low as 1 mW of cw output power. We achieved this result by exciting the narrowband but lossy cw dye laser oscillator with only a small fraction of the available nitrogen laser pump light, and using the remainder to boost the laser intensity afterwards in a series of traveling wave dye laser amplifiers. Note that the dye amplifiers, operating in a highly saturated mode, will at the same time reduce amplitude fluctuations of the cw oscillator. Fig. 1 shows the scheme of such a laser system. A Spectra Physics Model 580 A cw dye laser was pumped by an Ar+ laser (Spectra Physics Model 164). The cw dye laser oscillator was frequency-stabilized to a pressure-tuned reference etalon in order to provide a highly linear frequency tuning mechanism for precision measurements. The pressure was monitored by a capacit&rce manometer (Baratron: MKS Model 310 BHS1000). The output of this stabilized, pressure-tuned cw dye laser oscillator (having a very long coherence time) was focused to a diameter of approximately 0.2 mm into a dye amplifier pumped by approximately 20% of the output of a one-megawatt nitrogen laser
Volume
22, number
August
OPTICS COMMUNICATIONS
2
1911
DELAY Fig.
1. Components
of the frequency-stabilized
cw dye laser oscillator
(Molectron W-1000). The emerging amplified light beam (80 W) was collimated and sent through a direct vision prism, which dispersed the background of amplified spontaneous emission. This background was eliminated by a pinhole of 0.3 mm diameter. The beam was then focused into the second stage amplifier, which was pumped by approximately 30% of the output of the same nitrogen laser. The output of the second stage amplifier was again collimated, and filtered through another direct vision prism, which also compensated the beam wander introduced by the first prism. The filtered beam was finally focused to about 0.3 mm diameter into the third stage dyti amplifier, which was pumped by approximately 50% of the output of the same pump laser. The amplified output (100 kW) emerged with a near-diffraction-limit divergence of about 0.3 mrad. The three dye amplifiers were constructed from closed cuvettes with magnetic stirrers. Their windows were tilted by 3” and were anti-reflectively coated to avoid optical feedback. The N, laser output was divided geometrically with highly reflecting dielectrically
with three single pass synchronized
pulsed
amplifiers.
coated mirrors, and was focused into the cuvette dye cells by cylindrical quartz lenses of 80 mm focal length. The active diameter of the focused nitrogen beam in each cuvette was adjusted to avoid broadening resulting from amplifier gain saturation. This was important for our purpose, since any such broadening would contribute to a bandwidth at the amplifier output which would be larger than the Fourier-transform limit of the pulse. The adjustments were made either by rotating or by defocusing the quartz cylindrical lens to avoid gain saturation. Moreover, in order to maximize the length of the output pulse, the pump light for the amplifiers was optically delayed so that it arrived slightly later than the input from the cw oscillator. With one mW of cw dye laser output and a single-pass gain of 103 to 104 in each amplifier stage, an output power of 100 kW was measured at the third stage amplifier when pumped with the one-megawatt N, laser. As was noted earlier, changes in the intensity of the cw beam were unimportant because of the gain saturation in each amplifying stage. A slight change in the intensity of the amplified output was observed, 203
Volume 22, number 2
OPTICS COMMUNICATIONS
August 1977
Pressure swept
etalon
I
To
lllllll
Telescope
be Amplifier I
Echelle Grotina
-
1 Nitrogen
ll/llllOutput
Fig. 2. Components of the frequency-stabilized ized pulsed amplifiers.
cw dye laser oscillator with one double pass followed by one single pass synchron-
however, when the cw dye laser was mode-hopping; it was thus necessary to operate the cw dye laser with its frequency “locked” to the side of the transmission window of a Fabry-Perot interferometer. In this arrangement of single pass stages of amplification, the length of the output pulse of the third stage amplifier was only slightly shorter than the pulse length of the N2 laser. Thus the length and shape of the output pulse were essentially dictated by the duration and shape of the pump laser pulse. Various techniques could be used to alter the duration of the pump pulse, for example by changing the alignment of the rear mirror, the gas pressure, or the repetition rate, but never204
theless the length and shape of the output pulse could be varied only with difficulty. To obtain more intense Fourier limited optical pulses of controllable shape and duration, we employed a technique first demonstrated by Bolger, Baede and Gibbs [3], who removed the output reflecting mirror of a Hansch type nitrogen-pumped dye laser, thus converting it into a double-pass amplifier, When amplifying a cw dye laser, they obtained intense and highly coherent pulses, shorter than the pumping laser pulses, whose time dependence was a complicated function of various parameters, including the cellgrating distance, the telescope imaging, the input pow-
Volume
22, number
August
OPTICS COMMUNICATIONS
2
er, and the pulse length and intensity of the N21aser. . In our experimental investigation, the output of this double pass amplifier, which generated intense (5OOW), tunable and Fourier limited short pulses, was further amplified by another dye amplifier operating in a single pass mode while being pumped by 50% of the output of the 1 MW nitrogen laser (fig. 2). Suitable spectral and spatial filters were inserted between the two dye amplifiers in order to remove the undesirable amplified spontaneous emission of the double pass dye amplifier I (fig. 2) from dye amplifier II. In this configuration, stable pulses of 8.5 kW output power and 4 ns duration were obtained. It is important to note that because of the steep rise in the gain of the nitrogen laser-pumped dye and the large amplification crosssection, pulse shaping in the double-pass amplifier can easily occur when the intensity of the evolving pulse
1977
is larger than the saturation level and when the pulse rises steeply from an exponential or hard truncated leading edge. In both experimental configurations (figs. 1 and 2), it was desirable for our purpose that the output be collinear with the cw beam. For this reason, the dye cell cuvet ‘es were all anti-reflectively coated. The output.pulse could be separated from the cw carrier, however, by a reflection on the dye cell window. The two versions of the cw dye laser oscillator with synchronized pulsed amplifiers discussed above can be employed to generate pairs of intense, Fourier-limited, and coherent pulses by sending the output of either laser system into a highly stabilized optical delay line (fig. 3a) whose total transit time T is longer than the duration of the initial pulse (T > T). In such a case one can lock the phases of the two pulses by using inter-
IO nsec/div
( b) (0)
CONFOCAL FAEIRY- PEROT
‘0
rl
r2
r3
r4
--it+
1.5 nsec (cl
(d) Fig. 3. (a) Optical delay line for delay by T ns of a 7 ns pulse (T > 7). (b) The output of the optical delay line of fig. 3a, for input from the dye laser of fig. 1. (c) Optical delay line for delay by Tns of a 7 ns pulse (T < 7). (d) The output of the optical delay line of fig. 3c, for input from the dye laser of fig. 1.
205
Volume
22, number
2
OPTICS COMMUNICATIONS
ference between two direct and delayed cw beams [4]. Great care must be taken, however, to properly achieve this result, not only because of the low intensity of the cw oscillating wave, but also because any external perturbation can make this task nearly impossible. Fig. 3b shows the output of such a stabilized delay line observed on a Tektronix 590 scope, when the output of the laser system of fig. 1 was employed to generate two phase-coherent pulses. An extension of this technique to multiple phase-coherent pulses is clearly possible, by sending the initial pulse into a confocal resonator, for instance [5]. If, however, the total transit time Tin the optical delay line is much shorter than the pulse duration 7, one then obtains an output consisting of a succession of light pulses. Fig. 3d illustrates this situation when the intense r = 8 ns output pulse of the laser system of fig. 1 was delayed by T = 1.5 ns in the optical delay line of fig. 3c, and then observed on a Tektronix 661 sampling oscilloscope. The first peak corresponds to the direct reflection of the incident pulse on the mirror M, of the delay line of fig. 3c, and the following echoes correspond to the successive outgoing waves after each complete path of the interferometer. This device could be applied in time resolved spectroscopy to saturate a transition or an absorbing environment with the first reflected beam, and the successive echoes could then be used to analyze the time evolution of the perturbation due to the first reflected pulse. Such an experiment would require as intense a saturable beam as possible. Moreover, it would be desirable that the successive echoes have intensities adequate for signal/noise ratio but not large enough to perturb the medium.
206
August
1911
In addition to coherent optics, the new dye laser oscillator-amplifier systems discussed above could have a number of important applications in high resolution spectroscopy of atoms and molecules. The advantage of these systems of cw dye laser oscillator with pulsed dye amplifiers over the pulse dye laser oscillator with two-stage dye amplifiers (with time coincidence between signal and pump pulses), is not only that the pulses are Fourier limited, but also that the collinearity of the cw beam with the output pulse can be used in experimental situations. Furthermore, since we have been able to obtain 100 kW output power with as low as one mW output from the single mode cw oscillator, this system could not only generate very intense Fourier limited pulses throughout the visible range (certainly from 4200 to 7800 A where an intense pump source such as a nitrogen laser and a signal source are available), but could also generate pulses of a few hundred watts for wavelengths down to 2500 A by efficient sum or second harmonic conversion in nonlinear crystals or by third-order mixing in atomic vapor.
References 111 H. Walther, ed., High resolution
spectroscopy with tunable dye lasers (Springer, Berlin, 1975). 121 R. Wallenstein and T.W. HHnsch, Opt. Commun. 14 (1975) 353, and references therein, 18 131 B. Bolger, L. Baede and H.M. Gibbs, Opt. Commun. (1976) 67; 19 (1976) 346, and references therein. Phys. Rev. Letters 141 M.M. Salour and C. Cohen-Tannoudji, 38 (1977) 757. is1 R. Teets, J. Eckstein and T.W. HLnsch, Phys. Rev. Letters 38 (1977) 760.