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  • 专利说明
  • 权利要求
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    • 专利摘要:

    公开号:NL2009996A
    申请号:NL2009996
    申请日:2012-12-17
    公开日:2013-07-09
    发明作者:Michal Nejbauer；Yuriy Stepanenko；Czeslaw Radzewicz；Pawel Wnuk；Piotr Skibinski；Bartlomiej Bialkowski
    申请人:Inst Chemii Fizycznej Polskiej Akademii Nauk；
    IPC主号:
    专利说明:

    Title: Method for producing tuneable narrow-bandwidth light pulses from a source of short light pulses.
    DESCRIPTION
    The invention relates to a method for producing tuneable narrow-bandwidth light pulses from a source of short light pulses, preferably from a femtosecond laser.
    Femtosecond lasers are more and more frequently used in the industry. The use of short light pulses in many femtosecond laser-based spectroscopic techniques leads to reduced spectral resolution, as the spectrum of such a pulse is broad due to fundamental Fourier limit. There are a number of spectroscopic methods where short broadband laser pulses are employed simultaneously with narrow-bandwidth laser pulses. The methods known at present include: time-resolved stimulated Raman spectroscopy, sum frequency generation surface spectroscopy, and coherent anti-Stokes Raman scattering. Generation of narrow-bandwidth pluses from the broadband ones, and not, on the contrary, seems to be less expensive and recently there has been growing need for invention of a method that would efficiently lead to spectral compression, and additionally would allow for reaching an arbitrary compression ratio and arbitrarily tuneable pulses.
    There are several methods for spectral compression of broadband laser pulses. The simplest one is to use optical narrow-bandwidth filters or to employ more advanced 4f optical systems, where undesired frequencies are physically eliminated from the spectrum by placing a slit in the system’s Fourier plane. Such a method of light conversion results in big losses of pulse energy, as only a fraction of the spectrum is being used. [S. Laimgruber, H. Schachenmayr, B. Schmidt, W. Zinth, and P. Gilch, Appl. Phys. B 85(4), 557-564 (2006)].
    The more narrow-bandwidth pulses are desired, the greater are losses in spectral conversion. These methods do not allow for generating broadly tuneable light pulses, as they offer only those frequencies that are present in the initial spectrum.
    To increase the light conversion efficiency and to allow for tuneability, optical parametric light conversion processes in nonlinear crystals are employed. Such a process is divided into 3 steps. In the first step, an intense narrow-bandwidth pulse of fixed frequency is generated, in the second step a spectrally broad white light pulse is generated, and in the third step a selected fraction of white light is parametrically amplified with a narrow-bandwidth pulse in a nonlinear crystal. A problem here is to select a narrow bandwidth of frequencies contained in white light. Due to the fact that the white light pulse is short, and the pump pulse is long, many frequencies are being simultaneously amplified in the optical parametric amplification process in a nonlinear crystal. The process yields broadband pulses.
    Generation of narrow-bandwidth light pulses requires a good selection of frequencies contained in the white light. This is typically achieved with sophisticated 4f systems, which yield, however, limited spectral compression. Two independent works [D. T. Co, J. V. Lockard, D. W. McCamant, and M. R. Wasielewski, Appl. Opt. 49, 1880 (2010) or S. A. Kovalenko, A. L. Dobryakov, and N. P. Ernsting, Rev. Sci. Instrum. 82, 063102 (2011)] reported spectral bandwidths of produced pulses as narrow as 25-30 cm'1. In line with higher spectral compression desired, the 4f system becomes bigger and impractical. A fundamental limit for improved selection of the light frequency fraction in the 4f systems is also the finite beam size.
    It turned out unexpectedly that a fraction of frequencies contained in the white light can be selected by stretching out the pulse in time. In the optical parametric amplification process only those frequencies are amplified, which overlap temporally with the pump pulse. A white light stretcher may be any dispersion medium (glass, optical fibre) or optical system employing dispersion element or elements (diffraction grating, prism).
    According to the invention, the method for generating tuneable narrow-bandwidth light pulses from a source of short light pulses, wherein a pulse obtained from the said source of short light pulses is divided into two parts, one of them is passed through an optical system generating the pump beam, whereas the other part is passed through an optical system generating the supercontinuum beam, is characterized in that the beam generated in the said optical system generating the supercontinuum beam is passed through a pulse stretcher, and subsequently - in an optical parametric amplifier -the white light pulses generated in the system are subject to optical parametric amplification using the pump beam generated in the system.
    Preferably, the pass of the supercontinuum beam through the pulse stretcher unit comprises, in turn: directing the supercontinuum beam by a mirror to a diffraction grating, and subsequently to a retroreflector, then again to the diffraction grating, reflection of the beam from the mirror, back to the diffraction grating, to the retroreflector, again to the diffraction grating, and subsequently to the mirror, interception of the beam by the mirror and guiding the beam to the optical parametric amplifier unit.
    In such a case, the supercontinuum beam is preferably directed from the diffraction grating to the retroreflector of from the retroreflector to the diffraction grating by reflecting it from one or more mirrors.
    In a preferred embodiment of the invention, the said retroreflector is a prism retroreflector. The retroreflector is also called corner reflector or corner cube. In the embodiment of the invention presented below, a specific variant of the retroreflector - the prism retroreflector is preferably used, but any retroreflector (corner reflector) can be used, not necessarily the prism one.
    Preferably, the said source of short light pulses is a femtosecond laser.
    Preferably, the optical parametric amplification of white light pulses in the optical parametric amplifier comprises the pass of the pump and the white light beams through a nonlinear crystal and reflection of these beams back to that crystal by a mirror placed directly behind the crystal. The said mirror is placed just behind the crystal, and turning the beams back allows for compensation for the walk-off of the pump beam. The pump beam moves through the BBO crystal (birefringent crystal) as the extraordinary ray, so after passing the crystal it is macroscopically laterally displaced yielding a parallel displaced beam. On the other hand, the white light moves as the ordinary ray and is not displaced. In the case of a single pass through the crystal, the pump beam does not overlap spatially the white light beam in the crystal any longer and the optical parametric amplification process stops to occur. When the beams are reflected back, they both pass the same paths in the crystal, but in the opposite direction. An equally effective amplification occurs. To allow for separation of the incoming beams from the outgoing ones, the said flat mirror reflects the beams slightly downwards (alternatively one can also reflect upwards), which allows for interception of the beams by a second mirror, placed at a different height.
    Preferably, the said nonlinear crystal is a p-BaB204 crystal.
    Preferred embodiment of the invention
    The present invention is now explained more in detail in a preferred example of embodiment, with reference to the accompanying figures, wherein:
    Fig. 1 shows schematic diagram for generating narrow-bandwidth laser pulses from a pulse broadband laser using a pulse stretcher as a spectral filter for white light; wherein: 100 - source of short light pulses, (a) - white light generation, (c) - pulse stretcher unit, (b) - narrow-bandwidth pump pulses, (d) - optical parametric amplifier, 105 - tuned narrow-bandwidth laser pulses,
    Fig. 2 shows an example of embodiment of the invention, with a femtosecond laser as a pulse source (1 - mirror, 2 - mirror, 3 - diffraction grating, 4 - mirror, 5 -retroreflector, 6 - mirror, 7 - mirror, 8 - mirror, 21 - beam splitter plate, 22 - half-wave plate, 23 - Porro prism, 24 - diffraction grating, 25 - diaphragm, 26 - sapphire plate, 27 -p-BaB204 nonlinear crystal, 28 - lens, 29 - achromatic half-wave plate, 30 - dichroic mirror, 31 - cylindrical telescope, 32 - telescope; the lines indicate the paths of laser beams: continuous line - 1030 nm beam path, dashed line - 515 nm narrow-bandwidth pump path, dotted line - supercontinuum path and further the path of the amplified beam of narrow-bandwidth tuneable pulses; the arrows indicate that a precise movement of an optical element is possible; two slashes in the beam path indicate optical delay line), and
    Fig. 3 shows typical pulse spectra obtained with the invention. An average spectral width for pulses shown in Fig. 3 is 10 cm'1.
    Fig. 2 shows an example of embodiment of the invention. The pulse source 100 is a Pharos (Light Conversion) femtosecond laser, providing pulses with pulse duration 180 fs, pulse energy 200 pj and the central wavelength 1030 nm. The laser beam is divided in a beam splitter plate (beam splitting means) 21a with 97:3 splitting ratio. The lower energy fraction of the pulse is directed to the section (b) generating the supercontinuum beam: the beam passes through a 2 mm aperture 25 and a positive lens 28a with 50 mm focal length. A 3 mm thick sapphire plate 26 is placed at the focus to generate white light in a nonlinear process. The white light beam is collimated by a concave mirror with a curvature radius -150 mm and directed to a pulse stretching unit (c). The pulse stretcher unit (c) is composed of a diffraction grating 3 with 2000 grooves/mm, a prism retroreflector 5 that turns back the beam while changing its height, and three flat mirrors 2, 4, 6.
    The beam moves in the unit (c) as follows. The supercontinuum beam is directed by the mirror 2 at small angle upwards (in a plane perpendicular to the plane of the drawing), and passes under the mirror 6, and subsequently by the following elements, consecutively, 3—>4—>5—>4—>3—>6. The mirror 6 reflects the beam back, guiding it slightly downwards - the beam passes consecutively through the elements 3—>4—>5—>4—>3—>2—>1. Placed at an appropriate height, the mirror 1 intercepts the beam and guides it to the section (d) - the optical parametric amplifier. The desired range of frequencies contained in the white light spectrum is selected by rotating the grating around the beam’s point of incidence. In this section, the white light beam is stretched out in time - it is chirped with a group delay dispersion > 1.0x106 fs2.
    A narrow-bandwidth pulse to be used as a pump in the optical parametric generator is generated from the remaining fraction of the femtosecond pulse in section (a) that is used for sum frequency mixing of two copies of pulses with opposite chirps. First, the high energy femtosecond pulse is divided in a beam splitter 21b in a 50:50 ratio. One pulse copy is directed to a pulse stretcher composed of a transmission diffraction grating 24a with 900 grooves/mm and two Porro prisms 23b-23c.
    Appropriate distances in the system ensure that the pulse is stretched out up to duration of about 5 ps (FWHM). Such a unit gives the laser pulse a negative group delay dispersion. The other pulse copy is directed to the pulse stretcher 24b consisting of a diffraction grating with 900 groves/mm, a positive lens 28b of focal length 200 mm, a Porro prism 23a and a flat back mirror. This unit gives a positive group delay dispersion, and the distances are adjusted so that the stretching is identical as in the previous unit. So prepared beams are focused by positive lenses with focal lengths 500 mm and crossed in a 1.5 mm thick β-Β3Β204 (BBO) nonlinear crystal. A parametric sum frequency generation taking place in the crystal, results in a narrow-bandwidth pulse with a spectral width 5 cm'1, 58 pJ energy, and 515 nm central wavelength.
    The narrow-bandwidth laser pulse beam is subsequently collimated with a +300 mm lens and divided by a beam splitter in a 80:20 ratio. The lower energy fraction of the pulse is focused with a +500 mm lens in a 4 mm thick BBO crystal. The white light beam prepared earlier is focused using a +400 mm lens, and after reflection from the dichroic mirror, collinearly overlapped with the pump beam (a beam of narrow-bandwidth laser pulses centered at 515 nm). It is also necessary to make sure that the pulses from both beams overlap temporally in the crystal, which is accomplished by inserting an optical delay line in the pump beam path. In the crystals of the first and the second stage, the white light frequencies which overlap with the pump pulse, are parametrically amplified.
    The amplification takes place at the expense of the pump pulse energy as a result of a nonlinear optical parametric mixing of 3 waves (type I difference frequency generation). On passing the I stage crystal, both beams are turned back by the flat mirror 7, placed directly behind the crystal, which allows increasing the gain. Reflection of both beams at a small angle allows for interception of the amplified beam by the mirror 8, placed at different height and for directing it to the II amplification stage that is accomplished in a 6 mm thick BBO crystal. Before entering the crystal, the size and the spatial shape of the beam being amplified are adjusted by 2 telescopes so as to obtain a high efficiency of the amplification process and, at the same time, a good beam quality factor.
    Likewise, the remaining fraction of the pump beam is telescoped to an appropriate size, assuring an optimal overlap with the beam being amplified. Both beams overlap spatially with each other at a small angle in the crystal (the angle is used for easier separation of the two beams). Finally, tuneable narrow-bandwidth (~ 10 cm'1) laser pulses are obtained and smoothly tuned in the tuning range 610 - 985 nm with energies > 3 pj. The pulses can be tuned by rotating the grating in the white light stretcher unit. Typical spectra of generated pulses are shown in Fig. 3.
    权利要求:
    Claims (14)
    [1]
    A method for generating adjustable narrow bandwidth light pulses from a source of short light pulses, wherein a pulse obtained from the light source of short light pulses is divided into two parts, one of which is transmitted by an optical system (a) which generates a pump bundle, while the other part is guided through an optical system (b) for generating the supercontinuous bundle, characterized in that the bundle generated in the optical system (b) that generates the supercontinuous bundle is passed through a pulse widening unit (c) and then - in an optical parameter amplifier (d) - the white light pulses generated in the system (c) are subjected to optical parameter amplification using the pump bundle generated in the system (a).
    [2]
    Method according to claim 1, characterized in that the passage of the supercontinuous beam through the pulse widening unit (c) comprises directing: from the supercontinuous beam through a mirror (2) to a diffraction grating (3) and then to a retro-reflector (5) , then to the diffraction grating (3), reflection of the beam from the mirror (6) back to the diffraction grating (3) to the retro-reflector (5), again to the diffraction grating (3) and then to the mirror (2), as well as interception of the beam by the mirror (1) and guiding the beam to the optical parameter amplifier unit (d).
    [3]
    Method according to claim 2, characterized in that the supercontinuous beam is directed from the diffraction grating (3) to the retroreflector (5) or from the retroreflector (5) to the diffraction grating (3) by reflection via one or more mirrors (4) ).
    [4]
    Method according to claim 2 or 3, characterized in that the retro-reflector (5) is a prism retro-reflector (5).
    [5]
    Method according to one of the preceding claims, characterized in that the source of short light pulses is a femtosecond laser.
    [6]
    Method according to one of the preceding claims, characterized in that the optical parameter amplification of white light pulses in the optical parameter amplifier (d) comprises the passage of the pump and the white light beams through a non-linear crystal and reflection of these beams back to that crystal through a mirror (7) that is placed directly behind the crystal.
    [7]
    Method according to claim 6, characterized in that the non-linear crystal is a p-BaB 2 O 4 crystal.
    [8]
    A system for generating adjustable narrow bandwidth light pulses from a source of short light pulses, which system comprises: A beam splitting element for - during use - splitting a pulsed beam obtained from the source of short light pulses into two parts, an optical system (a) that receives one of the two bundle parts for generating a pump bundle, an optical system (b) that receives the other of the two bundle parts for generating a supercontinuous bundle, characterized in that the system further comprises a pulse widening unit (c) for receiving the supercontinuous beam generated in the optical system (b) for generating a beam of white light pulses, as well as an optical parameter amplifier (d) for receiving the white light pulses generated in the system (c) and subjecting the beam of white light pulses to an optical parameter gain using the generated in the system (a) pump bundle.
    [9]
    A system according to claim 8, characterized in that the passage of the supercontinuous beam through the pulse widening unit (c) comprises directing: from the supercontinuous beam through a mirror (2) to a diffraction grating (3) and then to a retro-reflector (5) , then to the diffraction grating (3), reflection of the beam from the mirror (6) back to the diffraction grating (3) to the retro-reflector (5), again to the diffraction grating (3) and then to the mirror (2), as well as interception of the beam by the mirror (1) and guiding the beam to the optical parameter amplifier unit (d).
    [10]
    System according to claim 9, characterized in that the supercontinuous beam is directed from the diffraction grating (3) to the retroreflector (5) or from the retroreflector (5) to the diffraction grating (3) by reflection through one or more mirrors (4) ).
    [11]
    A system according to claim 9 or 10, characterized in that the retro-reflector (5) is a prism retro-reflector (5).
    [12]
    The system according to any of claims 8 to 11, characterized in that the source of short light pulses is a femtosecond laser.
    [13]
    A system according to any of claims 8 to 12, characterized in that the optical parameter gain of white light pulses in the optical parameter amplifier (d) comprises the passage of the pump and the white light beams through a non-linear crystal and reflection of these bundles back to that crystal through a mirror (7) placed directly behind the crystal.
    [14]
    A system according to claim 13, characterized in that the non-linear crystal is a β-Β3Β204- ^ ί3ΐ3ΐ.
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    同族专利:
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    引用文献:
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    US20090244695A1|2008-03-27|2009-10-01|Andrius Marcinkevicius|Ultra-high power parametric amplifier system at high repetition rates|
    法律状态:
    2016-09-07| MM| Lapsed because of non-payment of the annual fee|Effective date: 20160101 |
    优先权:
    申请号 | 申请日 | 专利标题
    PL397704A|PL219206B1|2012-01-04|2012-01-04|Method for obtaining narrow band tunable light pulses from a source of short light pulses|
    PL39770412|2012-01-04|
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