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Ultrahigh-speed imaging by parametric image amplification
1 July 1995
OPTICS COMMUNICATIONS Optics Communications 118( 1995) 25-27
ELSEVIER
Ultrahigh-speed imaging by parametric image amplification F. Devaux, E. Lantz Laboratoire
d’optique
P.M. Dufjieux. U.R.A. 214 CNRS, Vniversitkde
Franche-Comtk
25030 Besancon, cedex, France
Received 30 January 1995
Abstract By picosecond
parametric image amplification
we performed
successivepictures of a light pulse propagatingon a diffusing
screen.
Ultrahigh speed imaging techniques are now capable of picosecond and subpicosecond framing times. They are very useful for photography of light in flight [ 11, time gated imaging through scattering media [ 2-81 or study of ultrafast optical phenomema [9]. Several methods already exist for ultrafast imaging, but each of them has its own limitations. For example in the picosecond range, streak cameras are limited to one spatial dimension. Kerr cells [ lo] can perform a bidimensional image in real time with picosecond framing time, but they can not be used for the detection of low light levels. Holographic techniques permit, for example, very precise visualisation of propagating wavefronts but are not real time techniques [ 111. With picosecond pulses, we recently performed the parametric amplification of a monochromatic nearinfrared image in one pass in a type II KTP crystal [ 12,131, Quasi-phase-matching occurred for a cone of signal wavevectors of the order of 20 mrad in both transverse directions, so that experimentally 70 per 70 points were resolved in the amplified image for a mean gain of 15 dB. In this letter we show how parametric image amplification acts also as an ultrafast shutter that permits imaging with a picosecond exposure time. Fig. 1 gives a schematic lay-out of the experimental set-up. An infrared pulse (A = 1.064 pm) is delivered 0030-4018/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10030-4018(95)00212-X
by a Nd:YAG laser and frequency doubled in a KDP crystal. The remaining infrared light is separated from the green one and illuminates a diffusing screen obliquely. The diffused light is horizontally polarized and an inverted microscope objective forms the image of the screen in a KTP crystal whose cross-section is 4 by 4 mm. The frequency-doubled pulse is used as the pump. This pump is superimposed on the signal in the crystal after synchronisation by means of a delay line. A dichroic mirror eliminates the pump beam after crossing the crystal. Finally, the amplified image is formed on a single-shot CCD camera. Since signal and idler waves have orthogonal polarisations in a type II crystal, a Glan polarizer in front of the detector permits the idler to be selected and the signal to be eliminated with a very good rejection rate (30 to 60 dB) . Fig. 2.1 represents the image of the light spot scattered on the screen without parametric amplification. Since light in flight is not time resolved because of the too long integration time of the CCD camera, the spot size is determined by the intersection between the light beam and the screen. Figs. 2.2, 2.3 and 2.4 represent amplified images of the infrared pulse, in flight from right to left, for three different positions of the delay line. Although many wave vectors of the diffused light are not amplified because they are not phase-matched,
26
F. Devaux, E. Lam / Optics Communications I18 (I 995) 25-27
effect occurs like in classical photography. On Fig. 3, the mean width of the profiles is about 90 ps, giving a duration of 55 ps for the infrared pulse. This duration is compatible with theory of frequency doubling [ 14 J. In a second experiment, a mirror perpendicular to the screen cut the upper half of the infrared beam (see insert in Fig. 4). Fig. 4.1 is taken without parametric amplification: light in flight is not resolved and we can see the shadow of the mirror on the screen. With par-
Fig. 1. Experimental set-up. The pump and the idler, denoted respectively p and i, are polarised perpendicularly to the figure plane. The signal, denoted s, is polarised in the figure plane.
a gain of 7.5 dB is obtained. The number of resolved points is 70 per 70, corresponding to a resolution of 60 p,m. This resolution is a constant, while the resolution on the screen can be chosen by adjusting the magnification of the imaging system. Since the beam and the screen form a slight angle, the geometrical distortion of the pulse can be neglected. Each image can be directly time located by calibrating the successive positions of the delay line, 3 cm spaced each other, namely 100 ps time delayed. As the idler is generated only during the 35 ps of the amplification process, all the diffused light reaching the crystal before or after the pump pulse is not amplified and is cut off by the cross polarizer. Hence, a very good shutter is obtained. Fig. 3 represents four superimposed profiles of Fig. 2. From the image profile formed by the idler, the infrared pulse duration can be estimated. The duration of the pump pulse, measured with a streak camera, is 35 ps. It gives the exposure time of the imaging process. During this time, the infrared pulse was propagating. So a fuzzy
Fig. 2.2.1: Light pulse on the screen, without parametric amplification. 2.2, 2.3 and 2.4: Amplified images of the pulse for three positions of the delay line. Time scaling is deduced from these positions and fits the spatial scale X. Y is the second spatial coordinate.
Time scale from poshon of the delay line (ps)
Fig. 3. Superimposed profiles of the images 2.1,2.2,2.3 and 2.4.
F. Devawr, E. Lontz /Optics
Communications
I18 (1995) 25-27
21
figure that a small part of the light is transmitted trough the mirror, with a delay of few picoseconds. To summarise, parametric image amplification permits real time imaging with an exposure time given by the pump pulse duration, here 35 ps and a gain of 7.5 dB. This method appears to be a new method which permits picosecond real time imaging of wide bidimensional low light level phenomena, with perfect rejection of the light before and after the exposure time. This method combines a good resolution (60 pm), in comparison with Raman imaging ( 150 pm), and the obtention of an amplified image, unlike the methods based on the upconversion or Kerr gating. One of the most important application of this technique could be imaging through scattering media. Indeed, the gain can be increased, for example by using a larger crystal, and accounting for the rejection rate of the polariser, the sensitivity required for this application can be reached. References
Fig. 4. 4.1: Light beam cut in part by a mirror perpendicular to the screen without amplification. 4.2 to 4.5: Amplified image of the pulse in flightat four successivetimes. When the pulse reaches the mirror, the lower part of the pulse is still propagating in the same direction while its upper part is propagating after reflection in the opposite direction (4.3 to 4.5).
ametric amplification, the pulse propagation is displayed (Figs. 4.2 to 4.5). As soon as the pulse reaches the mirror (Fig. 4.3)) we can see its lower part still propagating in the same direction while its upper part is propagating in the opposite direction after reflection on the mirror (Figs. 4.4 and 4.5). It appears on this
[ 11 N. Abramson, Appl. Optics 22 ( 1983) 215. [2] C. Yan and J.C. Diels, Appl. Optics 31 (1992) 6869. [3] G.W. Faris and M. Banks, Optics Len. 19 (1994) 1813. [4] M. Bashkansky, C.L. Adler and J. Reintjes, Optics Le.tt. 19 ( 1994) 350. 15] J.C. Hebden, R.A. Kruger and KS. Wong, Appl. Optics 30 (1991) 788. 161 M.R. Hee, J.A. Izatt, J.M. Jacobson, J.G. Fujimoto and E.A. Swanson, Optics Lett. 18 ( 1993) 950. [7] K.M. Yoo, Q. Xing and R.R. Alfano, Optics Lett. 16 (1991) 1019. [8] M.D. Duncan, R. Mahon, L.L. Tankersley and J. Reintjes, Optics Lett. 16 (1991) 1868. [9] A.J. Alcock, C. Demichelis and M.C. Richardson, IEEE J. Quantum Electron. QE-6 ( 1970) 622. [ lo] M.A. Duguay and A.T. Mattick, Appl. Optics 10 (1971) 2162. [ II] H. Chen, Y. Chen, D. Dilworth, E. L&h, J. Lopez and J. Valdmanis, Optics L&t. 16 (1991) 487. [ 121 F. Devaux, E. Lantz, A. Lacourt, D. Gindre, H. Maillotte, P.A. Doreau and T. Laurent, Picosecond parametric amplification of a monochromatic image, Nonlinear Optics, to be published. [ 131 F. Devaux and E. Lantz, Optics Comm. 114 (1995) 295. [ 141 Y. Wang, B. Luther-davies, Y.H. Chuang, R.S. Craxton and D.D. Meyerhofer, Optics Lett. 16 (1991) 1862.
OPTICS COMMUNICATIONS Optics Communications 118( 1995) 25-27
ELSEVIER
Ultrahigh-speed imaging by parametric image amplification F. Devaux, E. Lantz Laboratoire
d’optique
P.M. Dufjieux. U.R.A. 214 CNRS, Vniversitkde
Franche-Comtk
25030 Besancon, cedex, France
Received 30 January 1995
Abstract By picosecond
parametric image amplification
we performed
successivepictures of a light pulse propagatingon a diffusing
screen.
Ultrahigh speed imaging techniques are now capable of picosecond and subpicosecond framing times. They are very useful for photography of light in flight [ 11, time gated imaging through scattering media [ 2-81 or study of ultrafast optical phenomema [9]. Several methods already exist for ultrafast imaging, but each of them has its own limitations. For example in the picosecond range, streak cameras are limited to one spatial dimension. Kerr cells [ lo] can perform a bidimensional image in real time with picosecond framing time, but they can not be used for the detection of low light levels. Holographic techniques permit, for example, very precise visualisation of propagating wavefronts but are not real time techniques [ 111. With picosecond pulses, we recently performed the parametric amplification of a monochromatic nearinfrared image in one pass in a type II KTP crystal [ 12,131, Quasi-phase-matching occurred for a cone of signal wavevectors of the order of 20 mrad in both transverse directions, so that experimentally 70 per 70 points were resolved in the amplified image for a mean gain of 15 dB. In this letter we show how parametric image amplification acts also as an ultrafast shutter that permits imaging with a picosecond exposure time. Fig. 1 gives a schematic lay-out of the experimental set-up. An infrared pulse (A = 1.064 pm) is delivered 0030-4018/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10030-4018(95)00212-X
by a Nd:YAG laser and frequency doubled in a KDP crystal. The remaining infrared light is separated from the green one and illuminates a diffusing screen obliquely. The diffused light is horizontally polarized and an inverted microscope objective forms the image of the screen in a KTP crystal whose cross-section is 4 by 4 mm. The frequency-doubled pulse is used as the pump. This pump is superimposed on the signal in the crystal after synchronisation by means of a delay line. A dichroic mirror eliminates the pump beam after crossing the crystal. Finally, the amplified image is formed on a single-shot CCD camera. Since signal and idler waves have orthogonal polarisations in a type II crystal, a Glan polarizer in front of the detector permits the idler to be selected and the signal to be eliminated with a very good rejection rate (30 to 60 dB) . Fig. 2.1 represents the image of the light spot scattered on the screen without parametric amplification. Since light in flight is not time resolved because of the too long integration time of the CCD camera, the spot size is determined by the intersection between the light beam and the screen. Figs. 2.2, 2.3 and 2.4 represent amplified images of the infrared pulse, in flight from right to left, for three different positions of the delay line. Although many wave vectors of the diffused light are not amplified because they are not phase-matched,
26
F. Devaux, E. Lam / Optics Communications I18 (I 995) 25-27
effect occurs like in classical photography. On Fig. 3, the mean width of the profiles is about 90 ps, giving a duration of 55 ps for the infrared pulse. This duration is compatible with theory of frequency doubling [ 14 J. In a second experiment, a mirror perpendicular to the screen cut the upper half of the infrared beam (see insert in Fig. 4). Fig. 4.1 is taken without parametric amplification: light in flight is not resolved and we can see the shadow of the mirror on the screen. With par-
Fig. 1. Experimental set-up. The pump and the idler, denoted respectively p and i, are polarised perpendicularly to the figure plane. The signal, denoted s, is polarised in the figure plane.
a gain of 7.5 dB is obtained. The number of resolved points is 70 per 70, corresponding to a resolution of 60 p,m. This resolution is a constant, while the resolution on the screen can be chosen by adjusting the magnification of the imaging system. Since the beam and the screen form a slight angle, the geometrical distortion of the pulse can be neglected. Each image can be directly time located by calibrating the successive positions of the delay line, 3 cm spaced each other, namely 100 ps time delayed. As the idler is generated only during the 35 ps of the amplification process, all the diffused light reaching the crystal before or after the pump pulse is not amplified and is cut off by the cross polarizer. Hence, a very good shutter is obtained. Fig. 3 represents four superimposed profiles of Fig. 2. From the image profile formed by the idler, the infrared pulse duration can be estimated. The duration of the pump pulse, measured with a streak camera, is 35 ps. It gives the exposure time of the imaging process. During this time, the infrared pulse was propagating. So a fuzzy
Fig. 2.2.1: Light pulse on the screen, without parametric amplification. 2.2, 2.3 and 2.4: Amplified images of the pulse for three positions of the delay line. Time scaling is deduced from these positions and fits the spatial scale X. Y is the second spatial coordinate.
Time scale from poshon of the delay line (ps)
Fig. 3. Superimposed profiles of the images 2.1,2.2,2.3 and 2.4.
F. Devawr, E. Lontz /Optics
Communications
I18 (1995) 25-27
21
figure that a small part of the light is transmitted trough the mirror, with a delay of few picoseconds. To summarise, parametric image amplification permits real time imaging with an exposure time given by the pump pulse duration, here 35 ps and a gain of 7.5 dB. This method appears to be a new method which permits picosecond real time imaging of wide bidimensional low light level phenomena, with perfect rejection of the light before and after the exposure time. This method combines a good resolution (60 pm), in comparison with Raman imaging ( 150 pm), and the obtention of an amplified image, unlike the methods based on the upconversion or Kerr gating. One of the most important application of this technique could be imaging through scattering media. Indeed, the gain can be increased, for example by using a larger crystal, and accounting for the rejection rate of the polariser, the sensitivity required for this application can be reached. References
Fig. 4. 4.1: Light beam cut in part by a mirror perpendicular to the screen without amplification. 4.2 to 4.5: Amplified image of the pulse in flightat four successivetimes. When the pulse reaches the mirror, the lower part of the pulse is still propagating in the same direction while its upper part is propagating after reflection in the opposite direction (4.3 to 4.5).
ametric amplification, the pulse propagation is displayed (Figs. 4.2 to 4.5). As soon as the pulse reaches the mirror (Fig. 4.3)) we can see its lower part still propagating in the same direction while its upper part is propagating in the opposite direction after reflection on the mirror (Figs. 4.4 and 4.5). It appears on this
[ 11 N. Abramson, Appl. Optics 22 ( 1983) 215. [2] C. Yan and J.C. Diels, Appl. Optics 31 (1992) 6869. [3] G.W. Faris and M. Banks, Optics Len. 19 (1994) 1813. [4] M. Bashkansky, C.L. Adler and J. Reintjes, Optics Le.tt. 19 ( 1994) 350. 15] J.C. Hebden, R.A. Kruger and KS. Wong, Appl. Optics 30 (1991) 788. 161 M.R. Hee, J.A. Izatt, J.M. Jacobson, J.G. Fujimoto and E.A. Swanson, Optics Lett. 18 ( 1993) 950. [7] K.M. Yoo, Q. Xing and R.R. Alfano, Optics Lett. 16 (1991) 1019. [8] M.D. Duncan, R. Mahon, L.L. Tankersley and J. Reintjes, Optics Lett. 16 (1991) 1868. [9] A.J. Alcock, C. Demichelis and M.C. Richardson, IEEE J. Quantum Electron. QE-6 ( 1970) 622. [ lo] M.A. Duguay and A.T. Mattick, Appl. Optics 10 (1971) 2162. [ II] H. Chen, Y. Chen, D. Dilworth, E. L&h, J. Lopez and J. Valdmanis, Optics L&t. 16 (1991) 487. [ 121 F. Devaux, E. Lantz, A. Lacourt, D. Gindre, H. Maillotte, P.A. Doreau and T. Laurent, Picosecond parametric amplification of a monochromatic image, Nonlinear Optics, to be published. [ 131 F. Devaux and E. Lantz, Optics Comm. 114 (1995) 295. [ 141 Y. Wang, B. Luther-davies, Y.H. Chuang, R.S. Craxton and D.D. Meyerhofer, Optics Lett. 16 (1991) 1862.