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Rapid scanning autocorrelator for measurements of picosecond laser pulses
Volume 38, number 3
OPTICS COMMUNICATIONS
RAPID SCANNING AUTOCORRELATOR
1 August 1981
FOR MEASUREMENTS
OF PICOSECOND LASER PULSES H. HARDE
and H. BURGGRAF
Hochschule der Bundeswehr, Lasertechnik, 2000 Hamburg 70, FRC Received 6 April 1981
A rapid scanning autocorrelation interferometer for measurements of picosecond laser pulses is descrbed which uses a rotating prism as scanning device in one arm of the interferometer to permit continuous display of autocorrelation traces at audio frequencies on an oscilloscope. Scan widths of more than 500 ps with high linearity can be achieved. Autocorrelation measurements of picosecond pulses from a synchronously pumped mode-locked dye laser are presented.
The pulse width of ultrashort optical pulses can successfully be measured by means of an autocorrelation interferometer with second harmonic generation (SHG) [l] . This technique is well suited for use with mode-locked cw lasers according to the high repetition rate of these lasers and the relatively high SHG conversion efficiency under phasematched conditions. While autocorrelation measurements have long been employed for picosecond pulse diagnostics, only recently “real-time” techniques have emerged [2-61 to permit continuous display of laser pulses. In this contribution a simple fast scanning SHGautocorrelation interferometer is presented which operates at audiofrequencies. The autocorrelation function can easily be displayed on any high impedance oscilloscope and therefore allows to continuously monitor the pulse characteristics during experimental studies as well as to control the adjustments required to optimize the laser. The real-time aspect or quasi cw measurement is made possible by varying the pathlength in one interferometer arm via an circulating rooftop prism mounted on a rotating disk. This vibration-free scanning mechanism is compact in size and easily allows to realize a scan range of 500 ps and more with high scan linearity. The autocorrelation interferometer is illustrated schematically in f%. 1. A beam splitter divides the incoming train of laser pulses into a measuring and a reference beam with equal intensities. While the reference
beam travels to and is reflected by a stationary rooftop prism, the measuring beam traverses a combination of two rooftop prisms 90” twisted against one another and thus operating as a retroreflector. Relative time delay between both beams is achieved by mounting one prism of this combination on a rotating wheel and changing the path-length via an angular motion. The measuring beam is directed parallel but not collinear to the reference beam and both are then focused by a lens (focal length = 80 mm) into a 1 mm KDP crystal cut to produce type I SHG for 590 nm light at normal incidence. Sum-frequency generation, proportional only to the product of the two beam intensities is detected by a photomultiplier (RCA 1 P 28) at an angle bisecting the angle between the two fun&mental beams and therefore allows a background-free measurement of the autocorrelation function. Scattered light of the fundamental is suppressed by an UV bandpass filter in front of the photomultiplier. The autocorrelation function can be measured and displayed in two different ways. In the slow scanning mode a stepping motor with a reducing gear of about 100: 1 drives the prism wheel and by this controls the time delay between reference and measuring beam. The photomultiplier signal is digitized by a voltage-frequency converter (VFC) and stored by a multichannel analyser as a function of delay time. This mode is advantageous for calibration and digital recording and gives excellent signal-to-noise ratios, but it typically requires measuring times of a minute or even more. On the
0 030-4018/8 1/OOOO-0000/S 02.50 0 North-Holland Publishing Company
211
Volume 38, number 3
1 August 1981
OPTICS COMMUNICATIONS
Sl-6detection
Fig. 1. Schematic set-up of the autocorrelation interferometer: tube (PMT), voltage-frequency converter (VFC).
other hand the rapid scanning mode makes possible to continuously display the autocorrelation function on an oscilloscope. In this case the stepping motor is separated from the scanning device but a simple sqirrelcage motor as used for fans drives the prism wheel at about 30 Hz. Higher scanning frequencies can easily be achieved with higher speed of the motor or using more than one prism on the wheel. However, since the display time then becomes short compared with the human response time, for direct observation without automatic feedback control system a further increase of the scanning rate seems not necessary. The photomultiplier signal directly drives the vertical axis of the oscilloscope, while the horizontal axis is controlled by the internal time-base triggered by the scanning device. The photomultiplier load resistor (about 10 kC!) has to be adapted to the scan-speed to ensure sufficient bandwidth for the SHG-detection system. The scanning mechanism used for this autocorrelation interferometer is indicated in more detail in fig. 2. The measuring beam first travels to a rooftop prism mounted on a rotating wheel. This prism vertically displaces the beam and turns it round to a stationary rooftop prism by which the beam is horizontally displaced (out of drawing plane) and reflected back to the first 212
beam splitter (BS), rooftop prism (RTP), filter (0, photomultiplier
stationarv prism ’
[i;;;
:
’ ,’ a
R!3*’
w Fig. 2. Scanning mechanism schematic: prism height (h), radius (R), prism wheel diameter (D), angle of rotation (o), correction angle (IL), deviation from central beam (S), minimum beam displacement (E), beam diameter (2 we).
prism. After having passed this prism a second time, the beam propagates parallel but opposite to its initial direction independent of the particular position of the prism. According to this aspect the described set-up easily allows to vary the pathlength of the beam via an rotational motion particularly suited for rapid scanning
OPTICS COMMUNICATIONS
Volume 38, number 3
The change of pathlength A as a function rotary angle a! is given by the relation operation.
of
A = 4R {sin@ + $)
+p[1-n+(n2-sitl2a)l~2-cosa]}
(1)
where a is identical with the angle of incidence of the beam at the front surface of the rotating prism. R is the distance between the axis of rotation and the trailing edge of the prism, and the resultant vector forms an angle a + I,Lwith the vertical line. n represents the index of refraction of the prism and p = h/R gives the ratio of prism height h to the radius R. While the first addend in eq. (1) corresponds to the translational motion of the prism, the second originates from the rotational motion of the prism around the trailing edge and is identical to that contribution found for a beam transmitting a rotating plate of 2 h thickness and an refractive index n. The maximum continuous scan range of the described device is limited by rotary angles of the prism at which the beam can just be coupled in ((II_) and on the other hand still can traverse the rotating prism (a+). Within this scan width the beam has to be spatially displaced by a lowest amount E larger than the beam diameter 2 wo. A detailed geometrical analysis shows, that for clockwise rotation the angles, for which the delay line starts and stops operation, are given by
1 August 1981
Otherwise a_ or a, are found from eqs. (3) and (4) which may restrict the scan range. 4 and q’are dimensionless quantities with 4 = S/R and 4’ = (6 + wo)/R. 6 describes the deviation of beam height compared to a central beam at R cos J/ above rotating axis (see fig. 2). While eq.(2) determines the angular position at which the upper edge of the rotating prism just crosses the incoming beam, relation (3) demands at least a displacement E for the beam which is necessary to direct the beam into the stationary prism (see fig. 2). Eq. (4) takes into account, that for positive angles a beam can only be retroreflected by the rotating prism, until it hits the lower edge of the prism. Detailed analysis of eq. (1) shows, that within the scan range given by eqs. (2)-(4) very high scan linearity can be achieved if the correction angle $, which determines the position of the prism on the wheel, is optimized. Then a nonlinearity of the first term in eq. (1) is nearly compensated by the nonlinearity of the second term. Often, however, it can be of particular interest to have a maximum scan range or delay-time Arm, = [A(a+) - A(a_)]/c (c velocity of light) at constant overall diameter D of the prism wheel. Such an optimum configuration can be searched by variation of the parameters p, 4 and $. Evaluation of numerical calculations shows a strong dependence of the scan width on p as illustrated in fig. 3. The delay-time is plotted per unit wheel diameter
a _,+=arctan(,P;zc$)
[(p + cos $)2 + (p -sin
if for
the corresponding equality eG2R
At/O [pslcml
4’ + cos $
T arccos
(20
$@Iu2
angular range a+ -a_
the in-
qtcos$-cos(crt$)-psincu (
sin a cos a! + R (& - &y)1/2
1
(3)
can simultaneously be fulfilled and cr+ additionally satisfies the inequality
36
.’
32
”
28
..
2L
”
20
I/.._....__. 0.2
J/). 1+cos(cx+ -(Asin%p sin 2a
(4)
OL
,_._ 0.6
0.8
1.0
12
1L
1.6
1.8
2.0 p
Fig. 3. Calculated delay-time per unit wheel diameter as a function of prism height to radius for a scanning device with E/D= 0.02 and n = 1.5.
213
Volume
38, number
OPTICS
3
COMMUNICATIONS
as a function of p for a fixed ratio E/D= 0.02 and an index of refraction II = 1.5. While the dashed curve was found for values of 4 and $ yielding maximum delay, the solid line represents a configuration, for which 4 is still optimized to give a large scan range but $ is determined to give the best linearity. It is interesting to see that up to the absolute maximum delay-time both curves are nearly identical. For some p-values the corresponding optimized parameters are specified in fig. 3. The first value in brackets gives the maximum deviation from linearity within the scan range (in %), at second and third position are listed the values for Q and $ ($ in degrees). According to these numerical results it is obvious to choose a configuration for the prism wheel at which the maximum of the curves is approximated. This maximum will slightly be reduced in height and shifted towards larger p-values for increasing ratios E/Dand vice versa. Nevertheless the calculations presented in fig. 3 can be used for practical sizing and give a conception what scan range and linearity can be expected. Our first autocorrelation measurements were performed with a scanning device of 370 ps scan range with an overall diameter D = 80 mm and paying respect to a prism height available, p was determined to p = 0.7. In order to control the linearity, the central part of the scan range (270 ps) was interferometrically calibrated. With a correction angle J/ = 18” maximum deviations from a regression line are less than 1.2 % (see fig. 4), while for the whole scan range of 370 ps the nonlinearity is smaller than 2.5 %.
1 August
1981
Typical autocorrelation traces of 600 nm picosecond pulses from a synchronously pumped mode-locked dye laser are shown in fig. 5. The laser was only operating with a tuning wedge as frequency selective element in the cavity. Due to the nature of actively modelocked lasers well defied Fourier-limited pulses are only produced, if the mode separation frequency of the dye laser is matched to the modulation frequency used to acoustooptically mode-lock the Ar+ pump laser. Already very small changes in the length of the dye laser cavity show dramatic variations in the pulse structure. Fig. 5a gives an example for a cavity length mismatch AL in which the dye laser cavity is 20 pm too short with regard to an optimum length. Autocorrelation traces corresponding to pulses with a noisy substructure can be observed. If the cavity length is more correctly matched (see fig. 5b), the secondary peaks move to larger delay-times and then disappear completely. Evaluating this trace by a symmetric two-sided exponential [3] the laser pulse width is determined to be rp = 1.2 ps. The effect of lengthening the dye cavity with a mismatch of 20 pm and 60 pm is shown in figs. 5c and 5d respectively. The pulses are strongly broadened with increasing mismatch but the system is still operating in a relatively stable configuration [7-91. This sequence of photos demonstrates the advantage of a fast scanning autocorrelation interferometer to quickly and accurately permit optimization of the synchronously pumped dye laser output. Since reliable autocorrelation traces can still be generated from only
200 -
150 -
100 -
SO-
Fig. 4. Interferometrically line (dotted curve).
214
calibrated
delay-time
of the autocorrelator
versus rotary
angle (solid line) with corresponding
regression
Volume 38, number 3
OPTICS COMMUNICATIONS
AL= +2tlt,tm
1 August 1981
In conclusion we can say that the described scanning mechanism offers a number of valuable and practical features. It consists of very cheap optical and mechanical components easily set-up and aligned as scanning device. Further on it gives an excellent degree of linearity over a large scan range as to be seen on fig. 4. Scan widths of 500 ps and more can simply be realized and permit fast scanning autocorrelation measurements of mode-locked ion laser pulses. Comparing this scan mechanism with other techniques such as using a rotating plate or an audio speaker the prism wheel offers a very favourable ratio of maximum scan range to mechanical size and therefore makes it appropriate for use in even compact experimental set-ups. The dimensions of our autocorrelation interferometer are only 27 cm X 15 cm X 15 cm. Finally it should be mentioned, that the described device can also be employed for scanning rates up to several hundred Hz permitting autocorrelation measurements for automatic control and tracking of the laser. The valuable technical assistance of J. Pfuhl and G. Neufeld is gratefully acknowledged.
References
Fig. 5. Autocorrelation traces of picosecond pulses from a synchronously pumped mode-locked dye laser displayed on an oscilloscope. AL is the mismatch of dye cavity length from optimum conffntration.
small samples of the laser beam with inputs of only a few mW average power, the pulse characteristics can continuously be monitored and adjustments can be controlled during experimental studies.
[ 11 See, e.g., E.P. Ippen and C.V. Shank, in: Ultrashort light pulses, ed. S.L. Shapiro (Springer, Heidelberg, 1977) p. 83. [2] R.L. Fork and F.A. Beisser, Appl. Optics 17 (1978) 3534. [3] K.L. Sala, G.A. Kennay-Wallace and G.E. Hall, IEEE J. Quantum Electron QE-16 (1980) 990. [4] G.A. Kenney-Wallace, L.A. Hunt, K.L. Sala, in: Picosecond Phenomena II, eds. R.M. Hochstrasser, W. Kaiser and C.V. Shank (Springer, Heidelberg 1980) p. 203. [S] Y. Esheda, T. Yajema and Y. Tanaka, Jap. J. Appl. Phys. 19 (1980) L 289. [6] Z.A. Yasa and N.M. Amer, Optics Comm. 36 (1981) 406. [ 71 D.M. Kim, J. Kuhl, R. Lambrich and D.v.d. Linde, Optics Comm. 27 (1978) 123. [ 81 J. Kuhl, H. Klingenberg and D.v.d. Linde, Appl. Phys. 18 (1979) 279. [9] C.P. Ausschnitt, R.K. Jaln and J.P. Heritage, IEEE J. Quantum Electron. QE-15 (1979) 912.
215
OPTICS COMMUNICATIONS
RAPID SCANNING AUTOCORRELATOR
1 August 1981
FOR MEASUREMENTS
OF PICOSECOND LASER PULSES H. HARDE
and H. BURGGRAF
Hochschule der Bundeswehr, Lasertechnik, 2000 Hamburg 70, FRC Received 6 April 1981
A rapid scanning autocorrelation interferometer for measurements of picosecond laser pulses is descrbed which uses a rotating prism as scanning device in one arm of the interferometer to permit continuous display of autocorrelation traces at audio frequencies on an oscilloscope. Scan widths of more than 500 ps with high linearity can be achieved. Autocorrelation measurements of picosecond pulses from a synchronously pumped mode-locked dye laser are presented.
The pulse width of ultrashort optical pulses can successfully be measured by means of an autocorrelation interferometer with second harmonic generation (SHG) [l] . This technique is well suited for use with mode-locked cw lasers according to the high repetition rate of these lasers and the relatively high SHG conversion efficiency under phasematched conditions. While autocorrelation measurements have long been employed for picosecond pulse diagnostics, only recently “real-time” techniques have emerged [2-61 to permit continuous display of laser pulses. In this contribution a simple fast scanning SHGautocorrelation interferometer is presented which operates at audiofrequencies. The autocorrelation function can easily be displayed on any high impedance oscilloscope and therefore allows to continuously monitor the pulse characteristics during experimental studies as well as to control the adjustments required to optimize the laser. The real-time aspect or quasi cw measurement is made possible by varying the pathlength in one interferometer arm via an circulating rooftop prism mounted on a rotating disk. This vibration-free scanning mechanism is compact in size and easily allows to realize a scan range of 500 ps and more with high scan linearity. The autocorrelation interferometer is illustrated schematically in f%. 1. A beam splitter divides the incoming train of laser pulses into a measuring and a reference beam with equal intensities. While the reference
beam travels to and is reflected by a stationary rooftop prism, the measuring beam traverses a combination of two rooftop prisms 90” twisted against one another and thus operating as a retroreflector. Relative time delay between both beams is achieved by mounting one prism of this combination on a rotating wheel and changing the path-length via an angular motion. The measuring beam is directed parallel but not collinear to the reference beam and both are then focused by a lens (focal length = 80 mm) into a 1 mm KDP crystal cut to produce type I SHG for 590 nm light at normal incidence. Sum-frequency generation, proportional only to the product of the two beam intensities is detected by a photomultiplier (RCA 1 P 28) at an angle bisecting the angle between the two fun&mental beams and therefore allows a background-free measurement of the autocorrelation function. Scattered light of the fundamental is suppressed by an UV bandpass filter in front of the photomultiplier. The autocorrelation function can be measured and displayed in two different ways. In the slow scanning mode a stepping motor with a reducing gear of about 100: 1 drives the prism wheel and by this controls the time delay between reference and measuring beam. The photomultiplier signal is digitized by a voltage-frequency converter (VFC) and stored by a multichannel analyser as a function of delay time. This mode is advantageous for calibration and digital recording and gives excellent signal-to-noise ratios, but it typically requires measuring times of a minute or even more. On the
0 030-4018/8 1/OOOO-0000/S 02.50 0 North-Holland Publishing Company
211
Volume 38, number 3
1 August 1981
OPTICS COMMUNICATIONS
Sl-6detection
Fig. 1. Schematic set-up of the autocorrelation interferometer: tube (PMT), voltage-frequency converter (VFC).
other hand the rapid scanning mode makes possible to continuously display the autocorrelation function on an oscilloscope. In this case the stepping motor is separated from the scanning device but a simple sqirrelcage motor as used for fans drives the prism wheel at about 30 Hz. Higher scanning frequencies can easily be achieved with higher speed of the motor or using more than one prism on the wheel. However, since the display time then becomes short compared with the human response time, for direct observation without automatic feedback control system a further increase of the scanning rate seems not necessary. The photomultiplier signal directly drives the vertical axis of the oscilloscope, while the horizontal axis is controlled by the internal time-base triggered by the scanning device. The photomultiplier load resistor (about 10 kC!) has to be adapted to the scan-speed to ensure sufficient bandwidth for the SHG-detection system. The scanning mechanism used for this autocorrelation interferometer is indicated in more detail in fig. 2. The measuring beam first travels to a rooftop prism mounted on a rotating wheel. This prism vertically displaces the beam and turns it round to a stationary rooftop prism by which the beam is horizontally displaced (out of drawing plane) and reflected back to the first 212
beam splitter (BS), rooftop prism (RTP), filter (0, photomultiplier
stationarv prism ’
[i;;;
:
’ ,’ a
R!3*’
w Fig. 2. Scanning mechanism schematic: prism height (h), radius (R), prism wheel diameter (D), angle of rotation (o), correction angle (IL), deviation from central beam (S), minimum beam displacement (E), beam diameter (2 we).
prism. After having passed this prism a second time, the beam propagates parallel but opposite to its initial direction independent of the particular position of the prism. According to this aspect the described set-up easily allows to vary the pathlength of the beam via an rotational motion particularly suited for rapid scanning
OPTICS COMMUNICATIONS
Volume 38, number 3
The change of pathlength A as a function rotary angle a! is given by the relation operation.
of
A = 4R {sin@ + $)
+p[1-n+(n2-sitl2a)l~2-cosa]}
(1)
where a is identical with the angle of incidence of the beam at the front surface of the rotating prism. R is the distance between the axis of rotation and the trailing edge of the prism, and the resultant vector forms an angle a + I,Lwith the vertical line. n represents the index of refraction of the prism and p = h/R gives the ratio of prism height h to the radius R. While the first addend in eq. (1) corresponds to the translational motion of the prism, the second originates from the rotational motion of the prism around the trailing edge and is identical to that contribution found for a beam transmitting a rotating plate of 2 h thickness and an refractive index n. The maximum continuous scan range of the described device is limited by rotary angles of the prism at which the beam can just be coupled in ((II_) and on the other hand still can traverse the rotating prism (a+). Within this scan width the beam has to be spatially displaced by a lowest amount E larger than the beam diameter 2 wo. A detailed geometrical analysis shows, that for clockwise rotation the angles, for which the delay line starts and stops operation, are given by
1 August 1981
Otherwise a_ or a, are found from eqs. (3) and (4) which may restrict the scan range. 4 and q’are dimensionless quantities with 4 = S/R and 4’ = (6 + wo)/R. 6 describes the deviation of beam height compared to a central beam at R cos J/ above rotating axis (see fig. 2). While eq.(2) determines the angular position at which the upper edge of the rotating prism just crosses the incoming beam, relation (3) demands at least a displacement E for the beam which is necessary to direct the beam into the stationary prism (see fig. 2). Eq. (4) takes into account, that for positive angles a beam can only be retroreflected by the rotating prism, until it hits the lower edge of the prism. Detailed analysis of eq. (1) shows, that within the scan range given by eqs. (2)-(4) very high scan linearity can be achieved if the correction angle $, which determines the position of the prism on the wheel, is optimized. Then a nonlinearity of the first term in eq. (1) is nearly compensated by the nonlinearity of the second term. Often, however, it can be of particular interest to have a maximum scan range or delay-time Arm, = [A(a+) - A(a_)]/c (c velocity of light) at constant overall diameter D of the prism wheel. Such an optimum configuration can be searched by variation of the parameters p, 4 and $. Evaluation of numerical calculations shows a strong dependence of the scan width on p as illustrated in fig. 3. The delay-time is plotted per unit wheel diameter
a _,+=arctan(,P;zc$)
[(p + cos $)2 + (p -sin
if for
the corresponding equality eG2R
At/O [pslcml
4’ + cos $
T arccos
(20
$@Iu2
angular range a+ -a_
the in-
qtcos$-cos(crt$)-psincu (
sin a cos a! + R (& - &y)1/2
1
(3)
can simultaneously be fulfilled and cr+ additionally satisfies the inequality
36
.’
32
”
28
..
2L
”
20
I/.._....__. 0.2
J/). 1+cos(cx+ -(Asin%p sin 2a
(4)
OL
,_._ 0.6
0.8
1.0
12
1L
1.6
1.8
2.0 p
Fig. 3. Calculated delay-time per unit wheel diameter as a function of prism height to radius for a scanning device with E/D= 0.02 and n = 1.5.
213
Volume
38, number
OPTICS
3
COMMUNICATIONS
as a function of p for a fixed ratio E/D= 0.02 and an index of refraction II = 1.5. While the dashed curve was found for values of 4 and $ yielding maximum delay, the solid line represents a configuration, for which 4 is still optimized to give a large scan range but $ is determined to give the best linearity. It is interesting to see that up to the absolute maximum delay-time both curves are nearly identical. For some p-values the corresponding optimized parameters are specified in fig. 3. The first value in brackets gives the maximum deviation from linearity within the scan range (in %), at second and third position are listed the values for Q and $ ($ in degrees). According to these numerical results it is obvious to choose a configuration for the prism wheel at which the maximum of the curves is approximated. This maximum will slightly be reduced in height and shifted towards larger p-values for increasing ratios E/Dand vice versa. Nevertheless the calculations presented in fig. 3 can be used for practical sizing and give a conception what scan range and linearity can be expected. Our first autocorrelation measurements were performed with a scanning device of 370 ps scan range with an overall diameter D = 80 mm and paying respect to a prism height available, p was determined to p = 0.7. In order to control the linearity, the central part of the scan range (270 ps) was interferometrically calibrated. With a correction angle J/ = 18” maximum deviations from a regression line are less than 1.2 % (see fig. 4), while for the whole scan range of 370 ps the nonlinearity is smaller than 2.5 %.
1 August
1981
Typical autocorrelation traces of 600 nm picosecond pulses from a synchronously pumped mode-locked dye laser are shown in fig. 5. The laser was only operating with a tuning wedge as frequency selective element in the cavity. Due to the nature of actively modelocked lasers well defied Fourier-limited pulses are only produced, if the mode separation frequency of the dye laser is matched to the modulation frequency used to acoustooptically mode-lock the Ar+ pump laser. Already very small changes in the length of the dye laser cavity show dramatic variations in the pulse structure. Fig. 5a gives an example for a cavity length mismatch AL in which the dye laser cavity is 20 pm too short with regard to an optimum length. Autocorrelation traces corresponding to pulses with a noisy substructure can be observed. If the cavity length is more correctly matched (see fig. 5b), the secondary peaks move to larger delay-times and then disappear completely. Evaluating this trace by a symmetric two-sided exponential [3] the laser pulse width is determined to be rp = 1.2 ps. The effect of lengthening the dye cavity with a mismatch of 20 pm and 60 pm is shown in figs. 5c and 5d respectively. The pulses are strongly broadened with increasing mismatch but the system is still operating in a relatively stable configuration [7-91. This sequence of photos demonstrates the advantage of a fast scanning autocorrelation interferometer to quickly and accurately permit optimization of the synchronously pumped dye laser output. Since reliable autocorrelation traces can still be generated from only
200 -
150 -
100 -
SO-
Fig. 4. Interferometrically line (dotted curve).
214
calibrated
delay-time
of the autocorrelator
versus rotary
angle (solid line) with corresponding
regression
Volume 38, number 3
OPTICS COMMUNICATIONS
AL= +2tlt,tm
1 August 1981
In conclusion we can say that the described scanning mechanism offers a number of valuable and practical features. It consists of very cheap optical and mechanical components easily set-up and aligned as scanning device. Further on it gives an excellent degree of linearity over a large scan range as to be seen on fig. 4. Scan widths of 500 ps and more can simply be realized and permit fast scanning autocorrelation measurements of mode-locked ion laser pulses. Comparing this scan mechanism with other techniques such as using a rotating plate or an audio speaker the prism wheel offers a very favourable ratio of maximum scan range to mechanical size and therefore makes it appropriate for use in even compact experimental set-ups. The dimensions of our autocorrelation interferometer are only 27 cm X 15 cm X 15 cm. Finally it should be mentioned, that the described device can also be employed for scanning rates up to several hundred Hz permitting autocorrelation measurements for automatic control and tracking of the laser. The valuable technical assistance of J. Pfuhl and G. Neufeld is gratefully acknowledged.
References
Fig. 5. Autocorrelation traces of picosecond pulses from a synchronously pumped mode-locked dye laser displayed on an oscilloscope. AL is the mismatch of dye cavity length from optimum conffntration.
small samples of the laser beam with inputs of only a few mW average power, the pulse characteristics can continuously be monitored and adjustments can be controlled during experimental studies.
[ 11 See, e.g., E.P. Ippen and C.V. Shank, in: Ultrashort light pulses, ed. S.L. Shapiro (Springer, Heidelberg, 1977) p. 83. [2] R.L. Fork and F.A. Beisser, Appl. Optics 17 (1978) 3534. [3] K.L. Sala, G.A. Kennay-Wallace and G.E. Hall, IEEE J. Quantum Electron QE-16 (1980) 990. [4] G.A. Kenney-Wallace, L.A. Hunt, K.L. Sala, in: Picosecond Phenomena II, eds. R.M. Hochstrasser, W. Kaiser and C.V. Shank (Springer, Heidelberg 1980) p. 203. [S] Y. Esheda, T. Yajema and Y. Tanaka, Jap. J. Appl. Phys. 19 (1980) L 289. [6] Z.A. Yasa and N.M. Amer, Optics Comm. 36 (1981) 406. [ 71 D.M. Kim, J. Kuhl, R. Lambrich and D.v.d. Linde, Optics Comm. 27 (1978) 123. [ 81 J. Kuhl, H. Klingenberg and D.v.d. Linde, Appl. Phys. 18 (1979) 279. [9] C.P. Ausschnitt, R.K. Jaln and J.P. Heritage, IEEE J. Quantum Electron. QE-15 (1979) 912.
215