Educação matemática pela arte
Gusmão, Lucimar Donizete
2013-08-28
Visualization of Optical Continuum Evolution Along a Nonlinear Waveguide 
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Date
2006-05-25
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We describe the first non-destructive measurements of the evolution of the optical continuum along a non-linear waveguide. By utilizing near-field microscopy, the spectral variation can be imaged along and across the waveguide with sub-wavelength resolution.
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Mills, J.D., Chaipiboonwong, Tipsuda, Charlton, M.D.B., Zoorob, Majd, Netti, M.C., Baumberg, J. J. and Brocklesby, W.S. (2006) Visualization of Optical Continuum Evolution Along a Nonlinear Waveguide. In, CLEO 2006, Long Beach, CA USA, 22 - 25 May 2006. (Submitted).
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Visualization of Optical Continuum Evolution Along a
Nonlinear Waveguide
J. D. Mills1, T. Chaipiboonwong1, M. D. B Charlton2,3, M.E.Zoorob3, M.C.Netti3,
J. J. Baumberg3,4, and W. S. Brocklesby1 1Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK, 2School of Electronics And Computer Science, University of
Southampton, SO17 1BJ, UK, 3Mesophotonics Ltd, Chilworth Science Park, Southampton, SO16 7NP, UK, 4Dept of Physics and Astronomy, University Of Southampton, SO17 1BJ, UK
jm3@orc.soton.ac.uk
Abstract: We describe the first non-destructive measurements of the evolution of the optical continuum along a non-linear waveguide. By utilizing near-field microscopy, the spectral variation can be imaged along and across the waveguide with sub-wavelength resolution. © 2006 Optical Society of America OCIS codes: (180.5810) Scanning microscopy; (190.7110) Ultrafast nonlinear optics
Continuum generation by ultrafast laser pulses using nonlinear effects in waveguides provide an important new broadband light source for many areas of physics [1-3]. To date, all study of these sources has been via their output only, after propagating a certain distance in the nonlinear medium [4,5]. However, the recent development of planar waveguide devices for supercontinuum generation makes possible a new insight into the generation process. Using a Near-field Scanning Optical Microscope (NSOM), the spectrum of the generated light can be sampled along the length and width of the device via its evanescent field while the continuum is being generated, allowing observation of the build–up of the spectrum in a manner previously impossible. The waveguide utilized in these experiments was chosen from a set of rib waveguides on a Mesophotonics Ltd. supercontinuum generation chip. It consists of a Ta2O5 stripe of length 6mm, width 4µm, and height 0.5µm, on a layer of SiO2, grown on a silicon wafer. A schematic is shown in Figure 1(a). Laser pulses from a Coherent Mira oscillator of duration 80fs, wavelength 800nm, energy per pulse 2.1nJ at a repetition rate of 76MHz were focused into the waveguide. The evanescent field of the optical mode which extended out into the air above the guide for ~100nm was sampled using an uncoated NSOM fiber probe of ~80nm tip diameter, held at a fixed distance of 20nm
Fig. 1. (a) The NSOM probe is positioned at a height of ~20nm above the waveguide in order to sample the local spectrum via the evanescent field. (b) NSOM measured continuum growth as it evolves along the length of the waveguide (log scale). The spectra shown were obtained at
0.5mm intervals along the central axis of the guide. Spectra shown at positions 0mm and 6mm show the pre-waveguide laser spectrum and post- waveguide output respectively. All other spectra shown from 0.5mm to 5.5mm represent NSOM evanescent-field measurements.
(a) To spectrum analyzer
NSOM tapered fiber probe Continuum out
0mm
6mm
Ta2O5 waveguide 0.5µm x 4µm x 6mm
SiO2
Laser pulses in
y x
x
(b)
from the guide surface by shear-force feedback [6]. The light collected by the tip was detected with a high-resolution CCD-based spectrometer. Figure 1(b) shows the spectrum of the continuum as it evolved along the length of the waveguide. The development of the continuum along the guide is clearly visible. The spectra shown were obtained along the central axis of the guide by sampling the local spectrum via the evanescent field at intervals of 0.5mm. The laser spectrum itself, having a FWHM of 12nm is included in the figure at position 0mm for reference. Additionally, the waveguide output is shown at position 6mm. As can be seen, measurement at the guide output does not show the detailed spectral variation recorded by NSOM local sampling, which we have found to cycle over lengths of ~5µm along the guide, caused by interference between modes. Both the input and output spectra were recorded using the same spectrometer as the NSOM-acquired evanescent field data. By repeating these measurements with different input powers we have studied how the continuum broadens under various conditions. Figure 2 gives an example of how the spectrum of the generated light is observed to evolve on a much smaller length-scale. Here, measurements are shown in a direction along the guide. Spectral variation across the guide is also considerable as a consequence of modal beating. With the NSOM probe positioned at 3mm along the guide’s central axis, spectra were collected by stepping 100nm intervals along an overall length of 2µm. In this figure the spectrum is seen to broaden and narrow on a length scale of ~400nm, which is approximately the wavelength of the light in the guide. We are currently extending these studies to include guides with greater confinement in order to simplify the modal structure and to seek understanding of the evolution of spectra in the regime of supercontinuum. Fig. 2. Small-scale spatial variation of the spectrum measured at 100nm intervals, at a distance 3mm from the front end. The data was acquired by
scanning along the central axis of the guide. The spectra are seen to broaden and narrow on a length scale of ~400nm. The spatial variation was repeatable over periods of ~1 hour, demonstrating that the laser intensity and coupling into the guide was extremely stable, and did not contribute
to the variations observed. The capacity to visualize the development of nonlinear processes along waveguide devices with NSOM will not only enable a much better understanding of the important design properties of such devices, but also assist in the development of theory. This NSOM-based technique should also lend itself to more complex analysis of the nonlinear process, such as localized phase measurement or sub-wavelength-scale FROG analysis of the evolving pulse. [1] R. Holzwarth, T. Udem, T.W. Hansch, J.C. Knight, W.J. Wadsworth, P.S.J. Russell, “Optical frequency synthesizer for precision spectroscopy”, Phys. Rev. Lett. 85, 2264-2267, (2000) [2] B.R. Washburn, S.A. Diddams, N.R. Newbury, J.W. Nicholson, M.F. Yan, C.G. Jorgensen, “Phase-locked, erbium-fiber-laser-based frequency comb in the near infrared”, Opt. Lett. 29, 250-252 (2004) [3] D.J. Jones, S.A. Diddams, J.K. Ranka, A. Stentz, R.S. Windeler, J.L. Hall, S.T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis”, Science 288, 635-639 (2000) [4] Q. Cao, X. Gu, E. Zeek, M. Kimmel, R. Trebino, J. Dudley, R.S. Windeler, “Measurement of the intensity and phase of supercontinuum from an 8-mm-long microstructure fiber” Appl. Phys. B. 77, 239-244 (2003) [5] T. Hori, N. Nishizawa, T. Goto, M. Yoshida, “Experimental and numerical analysis of widely broadened supercontinuum generation in highly nonlinear dispersion-shifted fiber with a femtosecond pulse”, J. Opt. Soc. Am. B. 21, 1969-1980 (2004) [6] Khaled Karrai, Robert D. Grober, “Piezoelectric tip-sample distance control for near field optical microscopes”, Appl. Phys. Lett. 66, 1842- 1844, (1995).
400nm
J. D. Mills1, T. Chaipiboonwong1, M. D. B Charlton2,3, M.E.Zoorob3, M.C.Netti3,
J. J. Baumberg3,4, and W. S. Brocklesby1 1Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK, 2School of Electronics And Computer Science, University of
Southampton, SO17 1BJ, UK, 3Mesophotonics Ltd, Chilworth Science Park, Southampton, SO16 7NP, UK, 4Dept of Physics and Astronomy, University Of Southampton, SO17 1BJ, UK
jm3@orc.soton.ac.uk
Abstract: We describe the first non-destructive measurements of the evolution of the optical continuum along a non-linear waveguide. By utilizing near-field microscopy, the spectral variation can be imaged along and across the waveguide with sub-wavelength resolution. © 2006 Optical Society of America OCIS codes: (180.5810) Scanning microscopy; (190.7110) Ultrafast nonlinear optics
Continuum generation by ultrafast laser pulses using nonlinear effects in waveguides provide an important new broadband light source for many areas of physics [1-3]. To date, all study of these sources has been via their output only, after propagating a certain distance in the nonlinear medium [4,5]. However, the recent development of planar waveguide devices for supercontinuum generation makes possible a new insight into the generation process. Using a Near-field Scanning Optical Microscope (NSOM), the spectrum of the generated light can be sampled along the length and width of the device via its evanescent field while the continuum is being generated, allowing observation of the build–up of the spectrum in a manner previously impossible. The waveguide utilized in these experiments was chosen from a set of rib waveguides on a Mesophotonics Ltd. supercontinuum generation chip. It consists of a Ta2O5 stripe of length 6mm, width 4µm, and height 0.5µm, on a layer of SiO2, grown on a silicon wafer. A schematic is shown in Figure 1(a). Laser pulses from a Coherent Mira oscillator of duration 80fs, wavelength 800nm, energy per pulse 2.1nJ at a repetition rate of 76MHz were focused into the waveguide. The evanescent field of the optical mode which extended out into the air above the guide for ~100nm was sampled using an uncoated NSOM fiber probe of ~80nm tip diameter, held at a fixed distance of 20nm
Fig. 1. (a) The NSOM probe is positioned at a height of ~20nm above the waveguide in order to sample the local spectrum via the evanescent field. (b) NSOM measured continuum growth as it evolves along the length of the waveguide (log scale). The spectra shown were obtained at
0.5mm intervals along the central axis of the guide. Spectra shown at positions 0mm and 6mm show the pre-waveguide laser spectrum and post- waveguide output respectively. All other spectra shown from 0.5mm to 5.5mm represent NSOM evanescent-field measurements.
(a) To spectrum analyzer
NSOM tapered fiber probe Continuum out
0mm
6mm
Ta2O5 waveguide 0.5µm x 4µm x 6mm
SiO2
Laser pulses in
y x
x
(b)
from the guide surface by shear-force feedback [6]. The light collected by the tip was detected with a high-resolution CCD-based spectrometer. Figure 1(b) shows the spectrum of the continuum as it evolved along the length of the waveguide. The development of the continuum along the guide is clearly visible. The spectra shown were obtained along the central axis of the guide by sampling the local spectrum via the evanescent field at intervals of 0.5mm. The laser spectrum itself, having a FWHM of 12nm is included in the figure at position 0mm for reference. Additionally, the waveguide output is shown at position 6mm. As can be seen, measurement at the guide output does not show the detailed spectral variation recorded by NSOM local sampling, which we have found to cycle over lengths of ~5µm along the guide, caused by interference between modes. Both the input and output spectra were recorded using the same spectrometer as the NSOM-acquired evanescent field data. By repeating these measurements with different input powers we have studied how the continuum broadens under various conditions. Figure 2 gives an example of how the spectrum of the generated light is observed to evolve on a much smaller length-scale. Here, measurements are shown in a direction along the guide. Spectral variation across the guide is also considerable as a consequence of modal beating. With the NSOM probe positioned at 3mm along the guide’s central axis, spectra were collected by stepping 100nm intervals along an overall length of 2µm. In this figure the spectrum is seen to broaden and narrow on a length scale of ~400nm, which is approximately the wavelength of the light in the guide. We are currently extending these studies to include guides with greater confinement in order to simplify the modal structure and to seek understanding of the evolution of spectra in the regime of supercontinuum. Fig. 2. Small-scale spatial variation of the spectrum measured at 100nm intervals, at a distance 3mm from the front end. The data was acquired by
scanning along the central axis of the guide. The spectra are seen to broaden and narrow on a length scale of ~400nm. The spatial variation was repeatable over periods of ~1 hour, demonstrating that the laser intensity and coupling into the guide was extremely stable, and did not contribute
to the variations observed. The capacity to visualize the development of nonlinear processes along waveguide devices with NSOM will not only enable a much better understanding of the important design properties of such devices, but also assist in the development of theory. This NSOM-based technique should also lend itself to more complex analysis of the nonlinear process, such as localized phase measurement or sub-wavelength-scale FROG analysis of the evolving pulse. [1] R. Holzwarth, T. Udem, T.W. Hansch, J.C. Knight, W.J. Wadsworth, P.S.J. Russell, “Optical frequency synthesizer for precision spectroscopy”, Phys. Rev. Lett. 85, 2264-2267, (2000) [2] B.R. Washburn, S.A. Diddams, N.R. Newbury, J.W. Nicholson, M.F. Yan, C.G. Jorgensen, “Phase-locked, erbium-fiber-laser-based frequency comb in the near infrared”, Opt. Lett. 29, 250-252 (2004) [3] D.J. Jones, S.A. Diddams, J.K. Ranka, A. Stentz, R.S. Windeler, J.L. Hall, S.T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis”, Science 288, 635-639 (2000) [4] Q. Cao, X. Gu, E. Zeek, M. Kimmel, R. Trebino, J. Dudley, R.S. Windeler, “Measurement of the intensity and phase of supercontinuum from an 8-mm-long microstructure fiber” Appl. Phys. B. 77, 239-244 (2003) [5] T. Hori, N. Nishizawa, T. Goto, M. Yoshida, “Experimental and numerical analysis of widely broadened supercontinuum generation in highly nonlinear dispersion-shifted fiber with a femtosecond pulse”, J. Opt. Soc. Am. B. 21, 1969-1980 (2004) [6] Khaled Karrai, Robert D. Grober, “Piezoelectric tip-sample distance control for near field optical microscopes”, Appl. Phys. Lett. 66, 1842- 1844, (1995).
400nm
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