SINGLE SHOT AUTOCORRELATOR FOR MEASURING THE DURATION OF AN ULTRASHORT PULSE IN THE FAR FIELD

Abstract

A single shot autocorrelator for measuring duration of an ultrashort laser pulse in the far field, having a beam splitter to form two beams from an input ultrashort pulse: the reflected beam is firstly reflected by two mirrors mounted on a translation stage for adjusting time delay and subsequently a third mirror, and after focused by a spherical convex lens, enters a naturally grown strontium barium niobate crystal along the crystal z axis; the transmitted beam is firstly focused by a spherical convex lens, and after reflected by two mirrors, enters the crystal along the crystal z axis from opposite direction. The crystal is in the common focal regions of two spherical convex lenses and generates the transverse second harmonic pulse beam that is the autocorrelation signal to be recorded, which is imaged with an optical microscope onto a charge coupled device camera mounted perpendicular to the beams.

Claims

1. A far field single shot autocorrelator, comprising a beam splitter, a translation stage, two pairs of mirrors, one of the pairs being mounted on the translation stage for adjusting time delay, at least one spherical convex lens having a focal region, a naturally grown strontium barium niobate (SBN) crystal having a crystal z axis and being positioned in the focal region of the spherical convex lens, an optical microscope, and a charge coupled device (CCD) camera mounted perpendicular to beams, wherein the beam splitter splits an input ultrashort pulse P to form a reflected beam P.sub.r and a transmitted beam P.sub.t, the reflected beam P.sub.r is reflected by the pair of mirrors mounted on the translation stage for adjusting time delay, and enters the SBN crystal along the crystal z axis from a first direction, the transmitted beam P.sub.t is reflected by the other pair of mirrors and enters the SBN crystal along the crystal z axis from a second direction opposing the first direction, and the SBN crystal generates a transverse second harmonic pulse beam based on the reflected beam P.sub.r and the transmitted beam P.sub.t entering from opposing directions, the transverse second harmonic pulse beam is an autocorrelation signal to be recorded and is imaged with the optical microscope onto the charge coupled device (CCD) camera.

2. The far field single shot autocorrelator as described in claim 1, comprising the beam splitter 1, the translation stage 4, the first pair of mirrors 2 and 3, both mirrors mounted on the translation stage 4 for adjusting the time delay, a third mirror 5, the spherical convex lens 6, a spherical convex lens 8, having a common focal region as the spherical convex lens 6, the SBN crystal 7 being positioned in the common focal region of the spherical convex lenses 6 and 8, the second pair of mirrors 9 and 10, the optical microscope 11, and the charge coupled device (CCD) camera 12 mounted perpendicular to beams, wherein the beam splitter 1 splits an input ultrashort pulse P to form the two beams P.sub.r and P.sub.t, the reflected beam P.sub.r from the beam splitter 1 is firstly reflected by the first pair of mirrors 2 and 3 mounted on the translation stage 4 for adjusting the time delay, and subsequently reflected by the third mirror 5, and after focused by the spherical convex lens 6, enters the SBN crystal 7 along the crystal z axis from the first direction, the transmitted beam P.sub.t from the beam splitter 1 is firstly focused by the spherical convex lens 8, and after reflected by the second pair of mirrors 9 and 10, enters the SBN crystal 7 along the crystal z axis from the second direction opposing the first direction, and the SBN crystal 7 generates the transverse second harmonic pulse beam that is the autocorrelation signal to be recorded and is imaged with the optical microscope 11 onto the charge coupled device (CCD) camera 12.

3. The far field single shot autocorrelator as defined in claim 2, wherein the ultrashort pulses P.sub.r and P.sub.t are focused into the naturally grown SBN crystal 7 along the crystal z axis from the opposite directions using the spherical convex lenses 6 and 8, respectively.

4. The far field single shot autocorrelator as defined in claim 1, comprising the long focal length spherical convex lens A 1 having a focal region, a mirror A2, the beam splitter A3, the first pair of two mirrors A4 and A5, the SBN crystal A6 having the crystal z axis and being positioned in the focal region of the spherical convex lens A1, the translation stage A9, the second pair of mirrors A7 and A8 mounted on the translation stage A9 for adjusting the time delay, the optical microscope A10, and the charge coupled device (CCD) camera A11 mounted perpendicular to the beams, wherein an ultrashort pulse AP is firstly focused with the long focal length spherical convex lens A1, the focused pulse AP is reflected by the mirror A2, the beam splitter A3 splits the reflected focused pulse AP into the two beams AP.sub.r and AP.sub.t, the reflected beam AP.sub.r from the beam splitter A3 is firstly reflected by the first pair of mirrors A4 and A5, and enters the SBN crystal A6 along the first direction on the crystal z axis, the transmitted beam AP.sub.r from the beam splitter A3 is reflected by the second pair of mirrors A7 and A8 mounted on the translation stage A9 for adjusting the time delay, and enters the SBN crystal A6 along the crystal z axis from the second direction opposite to the first direction, the SBN crystal A6 generates the transverse second harmonic pulse beam that is the autocorrelation signal to be recorded, and is imaged with the optical microscope A10 onto the charge coupled device (CCD) camera A11.

5. The far field single shot autocorrelator as defined in claim 4, wherein the ultrashort pulses AP is focused by the spherical convex lens A l before two beams formed by the beam splitter AP.sub.r and AP.sub.t entering the SBN crystal A6 along the crystal z axis from the opposite directions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 depicts a far field single shot autocorrelator in accordance with a first embodiment of the present invention;

[0014] FIG. 2 depicts a far field single shot autocorrelator in accordance with a second embodiment of the present invention;

[0015] FIG. 3A depicts a graph of the near field beam spatial profile of an ultrashort laser pulse emitting directly from a chirped pulse amplification laser system measured with a CCD camera;

[0016] FIG. 3B depicts a graph of the autocorrelation trace of the pulse shown in FIG. 3A measured with a far field single shot autocorrelator in accordance with the first embodiment; and FIG. 3C depicts a graph of the autocorrelation curve extracted from the autocorrelation trace shown in

[0017] FIG. 3B with black dots representing experimental data and solid line representing fitting curve of experimental data with the Gaussian function.

[0018] FIG. 4A depicts a graph of the near field beam spatial profile of an ultrashort laser pulse clipped with a knife edge left and right in the plane perpendicular to the beam propagation direction measured with a CCD camera; FIG. 4B depicts a graph of the autocorrelation trace of the pulse shown in FIG. 4A measured with a far field single shot autocorrelator in accordance with the first embodiment; and FIG. 4C depicts a graph of the autocorrelation curve extracted from the autocorrelation trace shown in FIG. 4B with black dots representing experimental data and solid line representing fitting curve of experimental data with the Gaussian function.

[0019] FIG. 5A depicts a graph of the far field pattern of an ultrashort laser pulse with a pulse front tilt angle of 3.8 in air in the near field of the beam introduced with a 60 (N-SF11 Schott glass) prism with minimum deviation configurations simulated with the software VirtualLab FUSION (www.lighttrans.com); FIG. 5B depicts a graph of the autocorrelation trace of the pulse shown in FIG. 5A measured with a far field single shot autocorrelator in accordance with the first embodiment; and FIG. 5C depicts a graph of the autocorrelation curve extracted from the autocorrelation trace shown in FIG. 5B with black dots representing experimental data and solid line representing fitting curve of experimental data with the Gaussian function.

DETAILED DESCRIPTION OF THE INVENTION AND EMBODIMENTS

[0020] Although specific embodiments of the present invention are described with reference to the drawings, it should be understood that such embodiments are by way example only merely illustrative of but a small number of the many possible specific embodiments which can represent applications of the present invention. Various changes and modifications obvious to one skilled in the art the present invention pertains are deemed to be within the spirit, scope and contemplation of the present invention as further defined in appended claims.

[0021] Single shot autocorrelators characterizes ultrashort pulses, when time resolution through other methods (direct photodiode measurement, streak cameras etc.) is inadequate or pulse repetition rate is so low that multi shot autocorrelators cannot work effectively. The present invention is to measure the pulse duration of an ultrashort pulse in the far field by detecting the signal of transverse second harmonic generation by the nonlinear effect of two counter counter-propagating fundamental beams () coupled in media with randomly oriented ferroelectric domains, such as an as-grown strontium barium niobate (SBN) crystal. Advantages of the far field single shot autocorrelator of the present invention are immune to the influence of intensity modulations in the near field of the beam spatial profile on the measurement, able to obtain the global pulse duration in the presence of spatial-temporal coupling in fundamental beams, of stable physicochemical properties with the use of media for second harmonic generation, low nonlinear optical conversion threshold and free of background in the autocorrelation trace.

[0022] FIG. 1 shows one of the preferred embodiments of the present invention. The single shot autocorrelator working in the far field utilizes a beam splitter 1 to form two beams P.sub.r and P.sub.t from an input ultrashort pulse P. The reflected beam P.sub.r from the beam splitter 1 is firstly reflected by two mirrors 2 and 3 mounted on a translation stage 4 for adjusting the time delay and subsequently the third mirror 5, and after focused by a spherical convex lens 6, enters a naturally grown strontium barium niobate (SBN) crystal 7 along the crystal z axis. The transmitted beam P.sub.t from the beam splitter 1 is firstly focused by a spherical convex lens 8, and after reflected by two mirrors 9 and 10, enters the naturally grown SBN crystal 7 along the crystal z axis from the opposite direction. The position of SBN crystal 7 is in the common focal regions of two spherical convex lenses 6 and 8. The transverse second harmonic pulse beam generated by the SBN crystal is the autocorrelation signal to be recorded, which is imaged with an optical microscope 11 onto a charge coupled device (CCD) camera 12 mounted perpendicular to the beams. The spherical lenses 6 and 8 both have a focal length of 300 mm, the microscope objective is at 4 magnification and the model of CCD camera is SP620U; Spiricon Ltd.

[0023] FIG. 2 shows an alternative embodiment of the present invention, where the ultrashort pulse AP is firstly focused with a long focal length spherical convex lens A1 and then reflected by a mirror A2. With a beam splitter A3, two beams AP.sub.r and AP.sub.t are formed from the focused pulse AP. The reflected beam AP.sub.r from the beam splitter A3 is firstly reflected by two mirrors A4 and A5, and enters a naturally grown SBN crystal A6 along the crystal z axis. The transmitted beam AP.sub.t from the beam splitter A3 is reflected by two mirrors A7 and A8 mounted on a translation stage A9 for adjusting the time delay, and enters the naturally grown SBN crystal A6 along the crystal z axis from the opposite direction. The position of SBN crystal A6 is in the focal region of the spherical convex lens A1. The transverse second harmonic pulse beam generated by the SBN crystal is the autocorrelation signal to be recorded, which is imaged with an optical microscope A10 onto a charge coupled device (CCD) camera A11 mounted perpendicular to the beams.

[0024] The spatial width of the sum frequency signal z.sub.FWHM is related to the temporal pulse duration .sub.FWHM of the ultrashort pulse, which is usually calibrated by introduction of a delay t into one of the replicas, and the transverse second harmonic signal shifts a certain amount z along the transverse direction. By making a hypothesis about the temporal profile of the ultrashort pulse to be measured, the pulse duration is given by

[00001]${}_{\mathrm{FWHM}}=K.Math.z_{\mathrm{FWHM}}.Math.\frac{.Math..Math.t}{.Math..Math.z},$

[0025] where K is a form factor depending on the shape of the fundamental pulse to be measured, e.g., K=(1.414).sup.1 for a Gaussian pulse. The autocorrelator is calibrated by introducing a series of temporal delays with a micrometer in one arm, and the peak shifts of the autocorrelation traces are recorded. By performing linear fitting of the calibration curve, the calibration factor is obtained, which is one pixel of CCD camera corresponding to 18.86 fs.

[0026] Referring to FIG. 3A, the near field beam spatial profile of an ultrashort laser pulse emitting directly from a chirped pulse amplification laser system is show, which is measured with a CCD camera (WinCamD-LCM4; DataRay Inc.). The laser system is a Yb:KGW femtosecond laser system (PHAROS from Light Conversion) with maximum pulse average power of 4W, a repetition rate of 40 kHz, and a wavelength of 1028 nm. The beam profile is approximately a Gaussian shape with a FWHM of 2.5 mm. The nominal Fourier-transform-limited pulse duration is 223 fs. The nominal pulse duration was calibrated with a commercial scanning autocorrelator (PulseCheck; APE GmbH) during the initial deployment of the laser apparatus. The polarization direction of the fundamental pulse beam is in the x-z plane. Referring to FIG. 3B, there is shown the autocorrelation trace of the pulse shown in FIG. 3A measured with a far field single shot autocorrelator in accordance with the first embodiment. Referring to FIG. 3C, there is shown the autocorrelation curve extracted from the autocorrelation trace shown in FIG. 3B with black dots representing experimental data and solid line representing fitting curve of experimental data with the Gaussian function. The FWHM of the fundamental pulse is 248 fs by assuming a Gaussian temporal shape. The discrepancy between the pulse duration measured by FFSSA and the nominal one by the laser manufacturer of Light Conversion is 11.2%, and it may originate from the dispersion of the pulse and imperfect imaging of the autocorrelation trace onto the CCD camera caused by the limited depth of field.

[0027] FIG. 4A shows the near field beam spatial profile of an ultrashort laser pulse clipped with a knife edge left and right in the plane perpendicular to the beam propagation direction, which is measured with a CCD camera (WinCamD-LCM4; DataRay Inc.). FIG. 4B shows the autocorrelation trace of the pulse shown in FIG. 4A measured with a far field single shot autocorrelator in accordance with the first embodiment. FIG. 4C shows the autocorrelation curve extracted from the autocorrelation trace shown in FIG. 4B with black dots representing experimental data and solid line representing fitting curve of experimental data with the Gaussian function. The measured pulse duration is 243 fs, which agrees well with the pulse duration without beam clip in the near field.

[0028] FIG. 5A shows the far field pattern of an ultrashort laser pulse with a pulse front tilt angle of 3.8 in air in the near field of the beam introduced with a 60 (N-SF11 Schott glass) prism with minimum deviation configurations, which is simulated with the commercial wave optical modeling software VirtualLab FUSION (www.lighttrans.com). FIG. 5B shows a graph of the autocorrelation trace of the pulse shown in FIG. 5A measured with a far field single shot autocorrelator in accordance with the first embodiment. FIG. 5C shows a graph of the autocorrelation curve extracted from the autocorrelation trace shown in FIG. 5B with black dots representing experimental data and solid line representing fitting curve of experimental data with the Gaussian function. With the help of VirtualLab FUSION, the global pulse duration is calculated to be 625 fs in the focus, and the measured pulse duration with the far field single shot autocorrelator is 681 fs.