Propagation Property of Femtosecond Laser Pulses in Air

Propagation Property of Femtosecond Laser Pulses in Air

Propagation Property of Femtosecond Laser Pulses in
Air
Jingle Liu, Jianming Dai, and X.-C. Zhang
Center for Terahertz Research, Rensselaer Polytechnic Institute, Troy, New York
Abstract

2000

The evaluation of the 100fs pulse propagation in air was
done by using golden mirrors to reflect the laser pulse
back and forth to increase the propagation distance along
with using a
FROG to measure the pulse duration.

1500
1000

Spectrum of the Hurricane laser pulse

500

1.0

0
650

700

Introduction

950

Fig. 2 Spectrum of 10 femtoseconds laser pulse. The
central wavelength is 798nm., The HMFW is 99nm.
The central wavelength would be used to calculate the
time period of the fringes later appearing in the
autocorrelator. By being aware of the fringe period and
the number of fringes, we can obtain the HMFW of the
pulse autocorrelation.
Interferometric fringes after 12.4m propagation

Interferometric fringes after 3.0m propagation
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
40

0.0
-0.2
Intensity (a.u.)

Intensity (a.u.)

Terahertz time-domain spectroscopy has long been
applied in the fields of semiconductor, chemical, and
biological characterization. Standoff distance THz
sensing and imaging is expected to play a role in the
new generation of security screening, remote sensing,
biomedical imaging, and NDT [1]. To avoid the
significant water absorption in air [2], it is crucial to
employ the THz wave generation and detection in air
[3,4]. We proposed that an amplified femtosecond
laser can be used to generate a THz wave locally near
a target in ambient air by focusing intense optical
pulses to induce air plasma at stand-off distance.

750 800 850 900
Wavelength (nm)

50

60

70
80
Time (fs)

90

-1.4
40

100

0.4
0.2
0.0
760

780
800
820
Wavelength (nm)

The spectrum and central wavelength are obtained by
FROG. Below is a measured time vs frequency
spectrogram; the pulse
duration in time domain was measured by the FROG
pulse temperal duration after 9m
through
time-frequency spectrogram after 9m
the 2D spectral phase retrieval.
1.0
0.8

50

60

70
80
Time (fs)

90

100

Intensity (a.u.)

0.00

-0.04
-0.06

Understanding femtosecond laser pulse propagation
properties and precise phase control in air are crucial
to
realizing standoff distance THz sensing and imaging.

10fs laser pulse propagation in air
The evaluation of the pulse propagation in air was done
by using golden mirrors to reflect the laser pulse back
and forth to increase the propagation distance along
with using a spectrometer and a broadband optical
autocorrelator to measure the spectrum and pulse
duration. The pulse duration was measured at several
distances to
evaluate how the femtosecond pulse evolves in the air.

60

70
80
Time (fs)

90

100

Acknowledgment

This work was supported in part by the Bernard M.
Gordon Center for Subsurface Sensing and Imaging
Systems, under the Engineering Research Centers
Program of the National Science Foundation. The project
fits in level 1
Fundamental Science. R1

0.4
0.2

-300 -200 -100

0

100

200

300

Time (fs)

-0.01

pulse temperal duration after 105m

time-frequency spectrogram after 105m

-0.02

1.0
40

0.8
50

60

70
80
Time (fs)

90

100

Fig. 3 Pulse interferometric fringes after different
propagation distances

Amplitude (a.u.)

Fig. 1 Schematics of experimental setup for THz wave
generation and detection in air

50

0.6

1. Extend the propagation distance up to 200m or
400m.
2. Pre-set the negative chirp of the femtosecond laser
pulse to compensate for the large air dispersion for
broadband optical pulses.
3. Adjust the parameters of the pulse to control the
standoff distance THz wave generation and detection in
air.
4. Apply THz standoff distance technology to remote
sensing and imaging of biological and chemical
samples.

0.0

-0.03
-0.08
40

The properties of femtosecond laser pulse propagation
over a long distance (up to 100m) were studied for two
different pulses with 10fs and 100fs initial chirp-free
pulse durations. Air dispersion is the major factor
causing the laser pulse chirp. The quantitative results
provided by this study are very helpful for the future
control of laser propagation over a long distance and
ultimately THz standoff distance sensing and imaging.

Interferometric fringes after 28m propagation
0.00

-0.02

840

Fig. 4 Spectrum of Hurricane femtosecond laser. The
central wavelength is 798nm., The HMFW is 9nm.

-0.8
-1.0

Summary

Future Plan

0.6

-0.6

-1.2

Interferometric fringes after 21m propagation

Intensity (a.u.)

-0.4

0.8

Amplitude (a.u.)

Intensity (a.u.)

air were investigated with 10fs optical pulses from a
Ti:sapphire oscillator and 100fs optical pulses from a
Ti:sapphire amplifier, respectively. For the 10fs pulse,
the dispersion in the air has a severe effect on the
pulse duration due to the broad bandwidth while the
100fs pulse duration does not undergo significant
change over its 100
meter propagation in the air.

Intensity (a.u.)

Abstract
Propagation properties of femtosecond laser pulses in

100fs laser pulse propagation

Spectrum of Femtosecond laser

0.6
0.4
0.2
0.0

Above are the interferometric fringes measured by the
autocorrelator at four different distances, 3.0m, 12.4m,
21.0m and 28.0m.
It can be easily noted that as the propagation distance
increases, the shape of the autocorrelation fringes has
become more distorted and the edge tails are no longer
horizontal. At a distance of 28m, the distortion has
become very severe. This is because the chirp by air
dispersion has a dominant effect on this ultra short 10fs
laser pulse with a band width as broad as 100nm. The
severe chirp effect can be explained by experimentally
measured
air
refractive
indexes
for
different
wavelengths (i.e. from 750nm to 850nm) [5].

-300 -200 -100

0

100

200

300

Time (fs)

Fig. 5. Pulse time-frequency spectrogram and pulse
duration
after 9m and 105m propagation in the air.
The results show that the pulse duration changes very
little within 100 meters. The frequency-time profile after
100 meters remains basically the same as it is after 9
meters. The effect of air dispersion on the 100fs pulse is
very small.

Conclusion

Compared to the 10fs laser pulse, the 100fs pulse, with a
relatively narrow bandwidth of 9nm, keeps the pulse
duration from broadening too much over a long distance.
This is ideal for standoff distance THz generation and
detection.

References
[1] H. Zhong, A. Redo, Y. Chen, and X.-C. Zhang, Joint 30 th
International Conference on Infrared and Millimeter
Waves,1, 42 (2005)
[2] Jing Xu, Kevin Plaxco, S. James Allen, J. Chem.Phys,
124, 036101 (2006)
[3] Jianming Dai, Xu Xie, X.-C. Zhang, Physical Review
Letters, 97, 103903 (2006)
[4] Xu Xie, Jianming Dai, X.-C. Zhang, Physical Review
Letters, 96, 075005 (2006)
[5] J. Zhang, Z.H. Lu, L.J. Wang, Source: Optics Letters,
30, 3314 (2005)

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