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Spatial transmission of the picosecond optical shutter
Volume 11, number 3
OPTICS COMMUNICATIONS
SPATIAL
TRANSMISSION
OF THE PICOSECOND
July 1974
OPTICAL
SHUTTER
G.B. GILLMAN and S.A. RAMSDEN Department
of Applied Physics, University of Hull, Hull, HlJ6 7RX, UK
Received 4 June 1974 Both longitudinal and transverse modes of operation of the picosecond optical shutter have been investigated. An optimum power level for operation in the longitudinal mode is predicted and experimentally confirmed. The power dependence gives rise to a periodic variation of spatial transmission.
It was shown by Mayer and Gires [l] that powerful optical pulses can be used instead of electrical pulses to induce birefringence in liquids with high optical Kerr coefficients such as CS,. This technique has been extended by Duguay and Hansen to the picosecond regime and has resulted in an ultra-fast light gate [2]. For application in a high speed camera system knowledge of the spatial transmission properties is required and this paper presents such a study. When a powerful plane polarised optical field of peak amplitude E passes through an isotropic dielectric it induces changes in the refractive index which affect the propagation of a second weaker beam. If the pulse width is much longer than the relaxation time of the induced refractive index change then the resultant birefringence varies with the field according to [l,
21
tin,,-6q =&qE
-2
)
where 6n is the refractive index change for probe light parallel or perpendicular to E and nzB is the optical
Kerr coefficient. The resultant differential phase retardation S@in a cell of length L cm is given by 6$ = n(L/X)n,,
E2 ,
where h is the wavelength of the probe beam in cm. In this work a mode locked Nd:YAG laser was used for which the pulse width, as determined by two-photon fluorescence [3], was 25 psec. In comparison, the molecular relaxation time of 2 psec of the CS, may be ignored [2] and when the cell is placed between crossed polarisers the transmission is given by
T= sin2 (nLnzBE2/h), or
(1)
T = sin2 (nLn2&‘/XZ), where P is the peak power and Z is the wave impedance of the carbon disulphide. This is a sinusoidal function and therefore peak transmission will occur at specific values of L or P. With n2B = 2.2 X 10p20 m2V2 [4], Z = 236 !J and a 2 cm cell, peak transmission occurs at 250 MW/cm2 for probe light of wavelength X = 0.531.1. Both transverse and longitudinal switching has been used as shown in fig. 1. The Nd:YAG laser had an integral dye cell and gave a highly reproducible train of TEMOu mode locked pulses. By means of a Pockels cell and laser triggered spark gap a single pulse was switched out from the train, beam expanded and double passed through an amplifier to provide approximate ly 500 MW/cm2 peak power over a diameter of 0.75 cm at the cell. Green probe light was derived from the fundamental by second harmonic generation in a KDP crystal, separated out by means of a dichroic mirror, reduced to 0.2 cm diameter, and used to investigate the transmission of the central area of the switching fundamental beam, The KDP crystal was detuned so that the green pulse itself did not introduce any birefringence. Transmission was monitored by means of two HP4220 pin diodes. The green and fundamental were made time coincident at the cell by means of a variable optical delay. 309
Volume
11, number
OPTICS COMMUNICATIONS
3
DETECTOR
VARIABLE
[HP42201
Fig. 1. Experimental
versus power density for a 2 cm cell is shown in fig. 2a; each point on the graph is an average of 5 shots, the error bars representing the maximum variation of transmission. The expected peak in transmission is seen to correspond to the theoretical figure of 2.50 MW/cm2. Transmission does not fall to Transmission
July 1974
DELAY
arrangement
zero at 500 MW due to the finite diameter of the probe beam and the 5” angle of incidence. Transmission measurements could not be made with longer cells because of the oblique angle of incidence. A 1 cm cell had a peak transmission at about 500 MW/cm2. The peak transmission of the gate was usually about 4 %. (b)
7
i
I
200
LOO
POWER
Fig. 2. Transmission
310
(MW
of optical
600
200
cmZl
shutter.
400
POWER
(a) Longitudinal
mode of operation.
(b) Transverse
(MW
cm21
mode of operation.
600
Volume
11,
number 3
OPTICS COMMUNICATIONS
Fig. 3. Example of spatial variation of transmission when shutter is operated in the longitudinal mode (2 cm cell at 750 MW/
cm*). Correction for absorption in the Polaroid filters and losses in the optics yielded a value of 40 % f 4 % for the transmission of the cell itself which is thought to be due mainly to the finite area sampled and to a lesser
July 1974
extent dispersion [.5] and depolarisation effects [6]. The active length of the cell in the cross switching mode is proportional to the width of the beam which in this case was 0.75 cm. The power is also non-linear across the cell. A graph of transmission versus power density (fig. 2b) shows no maximum as the power required is in excess of the 500 MW available. Owing to the sinusoidal nature of eq. (1) more than one value of power or length will produce maximum transmission. Thus at high powers rings of transmission should occur. To confirm this the green was ex-: panded to provide uniform illumination at the cell and a film was placed behind the shutter to record the spatial transmission. One photograph showing the ring structure that can be obtained is shown in fig. 3 and the shape of the microdensitometer trace (fig. 4) agrees qualitatively with the calculated transmission for a gaussian beam. In the cross switching mode the power density was not high enough to produce any peaks in transmission and thus the spatial profile in the vertical direction shows a gaussian profile and in the horizontal dimension a measure of the fourth-order correlation function [7]. Despite the non-uniform transmission of the shutter it has been successfully used to obtain high resolution picosecond framing photographs of laser produced plasmas and these results will be published later. One of us (G.B. Gillman) was supported by an S.R.C. Research Studentship and equipment for this work was provided by the Paul Instrument Fund of the Royal Society.
\ -?L!‘L!‘.: :I‘l_ Referewes
I
I
I
I
I
[l] G. Mayer and F. Gires, Compt. Rend. 258 (1964) 2039. [2] M.A. Duguay and J.W. Hansen, Appl. Phys. Letters 15 (1969) 192. [3] J.A. Giordmaine, P.M. Rentzepis, S.L. Shapiro, Appl. Phys. Letters 11 (1967) 216. [4] M. Paillette, Compt. Rend. 262B (1966) 264. 151 M.R. Topp, P.M. Rentzepis, Chem. Phys. Lett. 19 (1973) 162.
I
I
\
I
\
/
I
I
/
/
I
1
\
’
\
’ ’ 1
I
\
I
\
I
‘.
I’
\
\
\
\
\
RADIUS
Fig. 4. Microdensitometer trace of fig. 3 (solid line) compared with calculated transmission (broken line).
[61 C. Varma, 162.
[71
P.M. Rentzepis,
Chem. Phys. Lett.
19 (1973)
J.w. Hansen and M.A. Duguay, S.M.P.T.E. 80 (1971) 73.
311
OPTICS COMMUNICATIONS
SPATIAL
TRANSMISSION
OF THE PICOSECOND
July 1974
OPTICAL
SHUTTER
G.B. GILLMAN and S.A. RAMSDEN Department
of Applied Physics, University of Hull, Hull, HlJ6 7RX, UK
Received 4 June 1974 Both longitudinal and transverse modes of operation of the picosecond optical shutter have been investigated. An optimum power level for operation in the longitudinal mode is predicted and experimentally confirmed. The power dependence gives rise to a periodic variation of spatial transmission.
It was shown by Mayer and Gires [l] that powerful optical pulses can be used instead of electrical pulses to induce birefringence in liquids with high optical Kerr coefficients such as CS,. This technique has been extended by Duguay and Hansen to the picosecond regime and has resulted in an ultra-fast light gate [2]. For application in a high speed camera system knowledge of the spatial transmission properties is required and this paper presents such a study. When a powerful plane polarised optical field of peak amplitude E passes through an isotropic dielectric it induces changes in the refractive index which affect the propagation of a second weaker beam. If the pulse width is much longer than the relaxation time of the induced refractive index change then the resultant birefringence varies with the field according to [l,
21
tin,,-6q =&qE
-2
)
where 6n is the refractive index change for probe light parallel or perpendicular to E and nzB is the optical
Kerr coefficient. The resultant differential phase retardation S@in a cell of length L cm is given by 6$ = n(L/X)n,,
E2 ,
where h is the wavelength of the probe beam in cm. In this work a mode locked Nd:YAG laser was used for which the pulse width, as determined by two-photon fluorescence [3], was 25 psec. In comparison, the molecular relaxation time of 2 psec of the CS, may be ignored [2] and when the cell is placed between crossed polarisers the transmission is given by
T= sin2 (nLnzBE2/h), or
(1)
T = sin2 (nLn2&‘/XZ), where P is the peak power and Z is the wave impedance of the carbon disulphide. This is a sinusoidal function and therefore peak transmission will occur at specific values of L or P. With n2B = 2.2 X 10p20 m2V2 [4], Z = 236 !J and a 2 cm cell, peak transmission occurs at 250 MW/cm2 for probe light of wavelength X = 0.531.1. Both transverse and longitudinal switching has been used as shown in fig. 1. The Nd:YAG laser had an integral dye cell and gave a highly reproducible train of TEMOu mode locked pulses. By means of a Pockels cell and laser triggered spark gap a single pulse was switched out from the train, beam expanded and double passed through an amplifier to provide approximate ly 500 MW/cm2 peak power over a diameter of 0.75 cm at the cell. Green probe light was derived from the fundamental by second harmonic generation in a KDP crystal, separated out by means of a dichroic mirror, reduced to 0.2 cm diameter, and used to investigate the transmission of the central area of the switching fundamental beam, The KDP crystal was detuned so that the green pulse itself did not introduce any birefringence. Transmission was monitored by means of two HP4220 pin diodes. The green and fundamental were made time coincident at the cell by means of a variable optical delay. 309
Volume
11, number
OPTICS COMMUNICATIONS
3
DETECTOR
VARIABLE
[HP42201
Fig. 1. Experimental
versus power density for a 2 cm cell is shown in fig. 2a; each point on the graph is an average of 5 shots, the error bars representing the maximum variation of transmission. The expected peak in transmission is seen to correspond to the theoretical figure of 2.50 MW/cm2. Transmission does not fall to Transmission
July 1974
DELAY
arrangement
zero at 500 MW due to the finite diameter of the probe beam and the 5” angle of incidence. Transmission measurements could not be made with longer cells because of the oblique angle of incidence. A 1 cm cell had a peak transmission at about 500 MW/cm2. The peak transmission of the gate was usually about 4 %. (b)
7
i
I
200
LOO
POWER
Fig. 2. Transmission
310
(MW
of optical
600
200
cmZl
shutter.
400
POWER
(a) Longitudinal
mode of operation.
(b) Transverse
(MW
cm21
mode of operation.
600
Volume
11,
number 3
OPTICS COMMUNICATIONS
Fig. 3. Example of spatial variation of transmission when shutter is operated in the longitudinal mode (2 cm cell at 750 MW/
cm*). Correction for absorption in the Polaroid filters and losses in the optics yielded a value of 40 % f 4 % for the transmission of the cell itself which is thought to be due mainly to the finite area sampled and to a lesser
July 1974
extent dispersion [.5] and depolarisation effects [6]. The active length of the cell in the cross switching mode is proportional to the width of the beam which in this case was 0.75 cm. The power is also non-linear across the cell. A graph of transmission versus power density (fig. 2b) shows no maximum as the power required is in excess of the 500 MW available. Owing to the sinusoidal nature of eq. (1) more than one value of power or length will produce maximum transmission. Thus at high powers rings of transmission should occur. To confirm this the green was ex-: panded to provide uniform illumination at the cell and a film was placed behind the shutter to record the spatial transmission. One photograph showing the ring structure that can be obtained is shown in fig. 3 and the shape of the microdensitometer trace (fig. 4) agrees qualitatively with the calculated transmission for a gaussian beam. In the cross switching mode the power density was not high enough to produce any peaks in transmission and thus the spatial profile in the vertical direction shows a gaussian profile and in the horizontal dimension a measure of the fourth-order correlation function [7]. Despite the non-uniform transmission of the shutter it has been successfully used to obtain high resolution picosecond framing photographs of laser produced plasmas and these results will be published later. One of us (G.B. Gillman) was supported by an S.R.C. Research Studentship and equipment for this work was provided by the Paul Instrument Fund of the Royal Society.
\ -?L!‘L!‘.: :I‘l_ Referewes
I
I
I
I
I
[l] G. Mayer and F. Gires, Compt. Rend. 258 (1964) 2039. [2] M.A. Duguay and J.W. Hansen, Appl. Phys. Letters 15 (1969) 192. [3] J.A. Giordmaine, P.M. Rentzepis, S.L. Shapiro, Appl. Phys. Letters 11 (1967) 216. [4] M. Paillette, Compt. Rend. 262B (1966) 264. 151 M.R. Topp, P.M. Rentzepis, Chem. Phys. Lett. 19 (1973) 162.
I
I
\
I
\
/
I
I
/
/
I
1
\
’
\
’ ’ 1
I
\
I
\
I
‘.
I’
\
\
\
\
\
RADIUS
Fig. 4. Microdensitometer trace of fig. 3 (solid line) compared with calculated transmission (broken line).
[61 C. Varma, 162.
[71
P.M. Rentzepis,
Chem. Phys. Lett.
19 (1973)
J.w. Hansen and M.A. Duguay, S.M.P.T.E. 80 (1971) 73.
311