Abstract

Nonlinear optical microscopy is a new and rapidly growing technique within which ultrafast laser technology finds a wide range of applications. Pulse widening, due to the microscope optics, is an issue of major concern for nonlinear excitation efficiency. We herewith describe a novel, simple and inexpensive autocorrelation technique to characterize the laser temporal behavior at the microscope focal plane. The method is based on a wavefront-division lateral shearing interferometer which is inserted into the microscope optical path like an ordinary filter, while a spatially uniform fluorescent specimen is observed. The two-photon excited fluorescent image provides the second-order autocorrelation curve.

© 2004 Optical Society of America

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References

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  25. F. Quercioli, A. Ghirelli, B. Tiribilli, M. Vassalli, "Autocorrelator for multiphoton microscopy," in Optical Sensing , B. Culshaw, A. G. Mignani and R. Riesenberg eds., Proc. SPIE 5459 (to be published).
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Appl. Opt. (3)

Appl. Phys. B (2)

R. Wolleschensky ,T. Feurer, R. Sauerbrey, U.Simon, "Characterization and optimization of a laser-scanning microscope in the femtosecond regime," Appl. Phys. B 67, 87-94 (1998).
[CrossRef]

H. Mashiko, A. Suda, K. Midorikawa, "All-reflective interferometric autocorrelator for the measurement of ultra-short optical pulses," Appl. Phys. B 76, 525-530 (2003).
[CrossRef]

J. Microsc. (4)

F. Cannone, G. Chirico, G. Baldini, A. Diaspro, "Measurement of the laser pulse width on the microscope objective plane by modulated autocorrelation method," J. Microsc. 210, 149-157 (2003).
[CrossRef] [PubMed]

M. Müller, J. Squier, R. Wolleschensky, U. Simon, G.J. Brakenhoff, "Dispersion precompensation of 15 femtosecond optical pulses for high numerical aperture objectives," J. Microsc. 191, 141-150 (1998).
[CrossRef] [PubMed]

C. Millard, D. N. Fittinghoff, J. A. Squier, M. Müller, A. L. Gaeta, "Using GaAsP photodiodes to characterize ultrashort pulses under high numerical aperture focusing in microscopy," J. Microsc. 193, 179-181 (1999).
[CrossRef]

G.J. Brakenhoff, M. Müller, J. Squier, "Femtosecond pulse width control in microscopy by Two-photon absorption autocorrelation," J. Microsc. 179, 253-260 (1995).
[CrossRef]

Microsc. Res. Tech. (2)

F. Quercioli, A. Ghirelli, B. Tiribilli, M. Vassalli, "Ultracompact autocorrelator for multiphoton microscopy," Microsc. Res. Tech. 63, 27-33 (2004).
[CrossRef]

A. Diaspro, M. Corosu, P. Ramoino, and M. Robello, "Adapting a compact confocal microscope system to a two-photon excitation fluorescence imaging architecture," Microsc. Res. Tech. 47, 196-205 (1999).
[CrossRef] [PubMed]

Opt. Commun. (2)

D. Welford and B.C. Johnson, "Real time monitoring of CW mode-locked dye laser pulses using a rapidscanning autocorrelator," Opt. Commun. 45, 101-104 (1983).
[CrossRef]

K. Wada, H. Fukuta, R. Kawashima, N. Kurahashi, T. Matsuyama, H. Horinaka, M. Okuno, A. Watanabe and Y. Cho, "Simple real-time fringe-resolved autocorrelator for measuring picosecond optical pulses," Opt. Commun. 214, 343-351 (2002).
[CrossRef]

Opt. Lett. (6)

Optical Sensing (1)

F. Quercioli, A. Ghirelli, B. Tiribilli, M. Vassalli, "Autocorrelator for multiphoton microscopy," in Optical Sensing , B. Culshaw, A. G. Mignani and R. Riesenberg eds., Proc. SPIE 5459 (to be published).

Optical Shop Testing (1)

M. V. Mantravadi, "Lateral Shearing Interferometers" in Optical Shop Testing , D. Malacara ed. (Wiley, New York, 1992).

Quantum Electron. (1)

S. M. Kobtsev, S.V. Smirnov, S.V. Kukarin, V. B. Sorokin, "Femtosecond autocorrelator based on swinging birifrangent plate," Quantum Electron. 31, 829-833 (2001).
[CrossRef]

Rev. Sci. Instrum. (1)

P. Wasylczyk, "Ultracompact autocorrelator for femtosecond laser pulses," Rev. Sci. Instrum. 72, 2221-2223 (2001).
[CrossRef]

Science (1)

W. Denk, J.H. Strickler, and W.W. Webb, "Two-photon laser scanning fluorescence microscopy," Science 248, 73-76 (1990).
[CrossRef] [PubMed]

Other (2)

F. Quercioli, B. Tiribilli, M. Vassalli, A. Ghirelli, "Microscopio confocale multifotone con autocorrelatore basato su un interferometro a spostamento laterale e relativo metodo di caratterizzazione temporale," Italian patent FI2003A000261 (2003).

.C. Diels and W. Rudolph, Ultrashort Laser Pulse Phenomena: Fundamentals, Techniques, & Applications on a Femtosecond Time Scale (Academic, San Diego, 1996), Chap. 8.

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Figures (8)

Fig. 1.
Fig. 1.

Typical autocorrelator set-up. The device is made up of three main subsystems: a nonlinear-detector D, an interferometer I, and a control electronics E. The longitudinal displacement of the corner reflector R produces a temporal delay ∆t among the two output beam replicas (red lines). A possible lateral spatial shift s can also arise due to a reflector transverse displacement. PMT: photomultiplier tube; F: emission filter; N: non-linear medium; O: objective; BS: beam splitters; R: corner reflectors; A: actuator; (fx, fy, zf), (x, y, zo): spatial coordinates at the objective front and back focal planes respectively; w: beam width in the x direction.

Fig. 2.
Fig. 2.

Effect of the beams lateral shift s on the second-order autocorrelation. Computed curves for an unchirped Gaussian pulse ε(t) = exp(-t2) and an (arbitrarily chosen) optical angular frequency ω=70. (a) Superposed beams (s = 0); the standard second order interferometric autocorrelaton curve results. (b) Separated beams (s ≥ 2w); the curve corresponds to the intensity autocorrelation with background. (c) Side by side beams (s = w); this curve was computed for two spatially uniform beams with a rectangular outline along the x coordinate.

Fig. 3.
Fig. 3.

Principle of operation of a Lateral Shearing Interferometer. (a) When an input plane wave propagates perpendicularly to the lateral shift s direction, no optical path difference between the two beam replicas occurs. (b) When the propagation direction forms an angle α with the perpendicular, an optical path difference c ∆t = s sinα arises.

Fig. 4.
Fig. 4.

Lateral shearing autocorrelation technique for ultrafast laser microscopy. UL: ultrafast laser system; SH: scanning head; SD: short-pass dichroic; X,Y: x and y scanning galvanometers; LSI: lateral shearing interferometer; O: Objective; N: spatially uniform fluorescent material; α: x scanning angle (parallel to the shear direction); f: objective focal length; ∆t: pulse temporal delay at P; F: emission filter; PMT: photomultiplier tube.

Fig. 5.
Fig. 5.

Wavefront-division LSI. (a) Side view; P1, P2: glass plates of thickness d and refractive index n; 2φ: angle among the plate normals. Plate P1 displaces the input beam (black line) towards the left by s/2 (red line), plate P2 towards the right (blue line). (b) Front view; solid black line is the input beam circumference. Light red and blue semicircles represent the output displaced beams. Solid colors show the beam fractions inside the objective rear pupil.

Fig. 6.
Fig. 6.

Experimental set-up. (a) Nonlinear optical microscope (in the forefront, the Nikon TE2000U stand). (b) The epifluorescence filter holder was removed and the LSI was put in its place while N was located onto the microscope specimen stage. The red arrow indicates the excitation laser beam direction. (c) Wavefront-division LSI. The device shown here is made up of two 10 mm plates rotated by 20 degrees. (d) Spatially uniform fluorescent specimen N.

Fig. 7.
Fig. 7.

A (pseudocolor encoded) two-photon excited fluorescence image of N is taken and shown on the microscope display. The intensity profile (black curve) along the y direction is constant (apart from the small bending at the edges of the field) while, along the x direction (the LSI shear direction), it is proportional to the second-order interferometric autocorrelation curve, as clearly shown in the enlarged image detail.

Fig. 8.
Fig. 8.

Measured second-order autocorrelation curve. (a) Experimental data. (b) Comparison with the theoretical curve for a Gaussian pulse. The absolute value of the relative error is plotted.

Equations (23)

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E T ( x , y , z o , t ) = E ( x , y , z o , t ) + E ( x + s , y , z o , t Δt )
E T ( x , y , z o , t ) = u ( x , y , z o ) ε ( t ) exp ( iωt ) + u ( x + s , y , z o ) ε ( t Δt ) exp [ ( t Δt ) ]
E T ( f x , f y , z f , t ) = u ( f x , f y , z f ) ε ( t ) exp ( iωt ) + u ( f x + f y , z f ) exp ( i 2 πs f x ) ε ( t Δt ) exp [ ( t Δt ) ]
G 2 ( Δt ) = + dt + + d f x df y u ( f x , f y , z f ) 4 ε ( t ) + ε ( t Δt ) exp ( i 2 πs f x ) exp ( iωΔt ) 4
G 2 ( Δt ) = + + df x df y u ( f x , f y , z f ) 4 ×
× + dt [ ε ( t ) 4 + ε ( t Δt ) 4 + 4 ε ( t ) 2 ε ( t Δt ) 2 ] +
+ { 2 exp ( iωΔt ) + + df x df y u ( f x , f y , z f ) 4 exp ( i 2 πs f x ) ×
× + dt ε * ( t ) ε ( t Δt ) [ ε ( t ) 2 + ε ( t Δt ) 2 ] + c . c } +
+ { exp ( i 2 ωΔt ) + + df x df y u ( f x , f y , z f ) 4 exp ( i 2 π 2 s f x ) ×
× + dt ε * 2 ( t ) ε 2 ( t Δt ) + c . c }
+ + df x df y u ( f x , f y , z f ) 4 exp [ i 2 π ( xf x + yf y ) ] 1 { u ( f x , f y , z f ) 4 }
1 { u ( f x , f y , z f ) 4 } = 1 { u ( f x , f y , z f ) 2 } 1 { u ( f x , f y , z f ) 2 } =
= u ( x , y , z o ) u * ( x , y , z o ) u ( x , y , z o ) u * ( x , y , z o ) =
C ( x , y )
G 2 ( Δt ) = C ( 0,0 ) + dt [ ε ( t ) 4 + ε ( t Δt ) 4 + 4 ε ( t ) 2 ε ( t Δt ) 2 ] +
+ { C ( s , 0 ) exp ( iωΔt ) 2 + dt ε * ( t ) ε ( t Δt ) [ ε ( t ) 2 + ε ( t Δt ) 2 ] + c . c } +
+ { C ( 2 s , 0 ) exp ( i 2 ωΔt ) + dt ε * 2 ( t ) ε 2 ( t Δt ) + c . c }
u ( x , y , z o ) = { f ( y ) ; for x < w / 2 0 ; for x w / 2 }
C ( s , 0 ) = C ( 0,0 ) × { 3 4 s w 3 3 2 s w 2 + 1 ; for s w 1 4 s w 3 + 3 2 s w 2 3 s w + 2 ; for w < s < 2 w 0 ; for s 2 w }
G 2 ( Δt ) / C ( 0,0 ) = + dt [ ε ( t ) 4 + ε ( t Δt ) 4 + 4 ε ( t ) 2 ε ( t Δt ) 2 ] +
+ 0.25 { 2 exp ( iωΔt ) + dt ε * ( t ) ε ( t Δt ) [ ε ( t ) 2 + ε ( t Δt ) 2 ] + c . c }
Δt = OPD / c = s sin α / c ,
s = 2 d sin φ ( 1 cos φ n 2 sin 2 φ )

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