1. Speaker: Tzung Da Jiang Adviser : Dr. Ja-Hon Lin
Use of commercial grade light emitting diode in auto-correlation measurements of femtosecond and picosecond laser pulses at 1054 nm
Speaker: Tzung Da Jiang
Adviser : Dr. Ja-Hon Lin

2. Outline Introduction Theoretical background
Characterization of AlGaAs LEDs for non-linear photo-current
Real time auto-correlation of laser pulses
Conclusion

3. Introduction
The generation of ultra-short laser pulses [1] and their applications [2] require reliable measurement pulse parameters like :
duration
shape
frequency chirp
Laser pulses are generally characterized using auto- correlation methods based on second harmonic generation (SHG) in non-linear crystals followed by a linear detector :
Photo-multiplier tube (PMT)
charge coupled device (CCD) cameras
[1] T. Kobayashi, A. Baltuska, Meas. Sci. Technol. 13 (2002)1671.
[2] G.A. Mourou, C.P.J. Barty, M.D. Parry, Phys. Today 51 (1998) 22.

4. Advantages of semiconductor detector
Two-photon absorption (TPA) in commercial grade semiconductor devices has drawn considerable interest as a substitute for quadratic intensity response of SHG in certain non-linear crystals.
The advantages of the semiconductor photodiodes and light emitting
diodes are :
Available off-the-shelf and are quite inexpensive
Relatively insensitive to incident light polarization and wavelength
Does not require any phase matching condition
Non-hygroscopic
Their optical and electrical properties are integrated in a single unit
No spectral filtering effects

5. Introduction In this paper, we present the characterization:
Use AlGaAs based LED as TPA detector for autocorrelation measurement.
Measurements of 200 fs and 30 ps laser pulses at 1054 nm wavelength.
We have investigated:
Measure lifetime of the LED.
Measure pulse duration and small amount of frequency chirp.
Modify IAC signals enhance twofold sensitivity for detection chirp.
Use time calibration method to estimate the pulse duration and the frequency chirp.
Discuss different time calibration methods about their suitability.

6. Theoretical background
The two-photon absorption based induced photo-current (ITPA) may be described as :
 (ampere/watt²) : the two-photon induce photo-current response
P : the pulse peak power
In a practical situation, for two-photon absorption the condition of TPA (2h > Eg > h) is necessary, but not sufficient.
The response may vary linearly, if
impurities
defects
present in the semiconductor diode

7. Signal of two-photon absorption
The two- photon induced current in a semiconductor diode as a function of
relative time delay () between the two pulses, would then represent the second order auto-correlation function S2()
k : the intensity ratio of the two beams
E(t) = [I(t)]½ exp[i(t)] : the electric field
I(t) : the pulse intensity envelope function
(t) is the phase function

8. Signal of two-photon absorption
The above auto-correlation may also be expressed in the form,
where
and
The phase function may be expressed in the form,
: linear chirp
: quadratic chirp
: cubic chirp
p: FWHM duration

9. Advantage of MOSAIC
Though the IAC signals are widely used to detect and estimate the frequency chirp in laser pulses, they are not very sensitive to the magnitude and order of chirp :
The technique of modified spectrum autointerferometric-
correlation (MOSAIC) can enhance the sensitivity toward the presence
of the frequency chirp. The reason is:
The envelope function G2(s) and F22(s) can be very useful in sensitive detection.
chirp-free pulses : E(t) and E*(t) are same
chirped pulses : E(t) and E*(t) are different

10. Principle of MOSAIC
MOSAIC signal generated from S2() by filtering out the cos 0 term and increasing the weight factor of the cos20 term by a factor of two, can be expressed as
The locus of the interference minima of this signal expressed below:
g2 and f22 are the normalized
G2(s) and F22(s)

11. Two-photon induced response with 30 ps laser pulse
Pulse duration : 30ps
Peak emission wavelength : 660nm
Oscillator : 100 MHz cw
mode-locked Nd:fluorophosphate
Pump : single-shot, flash lamp
From the log–log plots (with slope 2) in (a), it is clearly seen that the induced
photo-current varies quadratically for a substantial range of incident laser power.
The output current got saturated at 0.8 A.

12. Two-photon induced response with 200 fs laser pulse
Pulse duration : 200fs
Peak emission wavelength : 660nm
Oscillator : 100 MHz cw
mode-locked Nd:fluorophosphate
pump : single-shot, flash lamp
From the log–log plots (with slope 2) in (a), it is clearly seen that the induced photo-current varies quadratically for a substantial range of incident laser power.
No such effect was observed for 200 fs laser pulses upto an output of 1.0 A for an average power of 80 mW.

13. Two-photon induced response for different bias voltages
It is observed that while the response increases with bias voltage, its quadratic nature remains the same.
This is perhaps due to the fact that the capacitance of the LED junction becomes smaller in the reverse bias condition

14. Estimate its life-time as a two-photon detector
The lifetime in this case is defined
as the number of laser pulses after
which the photo-current reduces
to half the initial value.
It is clearly seen that the induced
photocurrent decreases with an
increase in the number of laser pulses .
decreases slowly:(1.2 kW pulsed, 24 mW average, 21011 lifetime )
decreases faster :(3.5 kW pulsed, 70 mW average , 61011 lifetime )
It may be mentioned here that, the decrease in photo-current with laser exposure is not due to any drift of the alignment condition of the incident laser beam on the LED.
Normalised LED signal
Number of laser pulses

15. Experimental setup Pulse width:200 fs and 30 ps wavelength :1054 nm
LED materal:AlGaAs based

16. Determine the pulse duration
FWHM of IAC signal:165(mv)
FWHM spatial width:
165(mv)*0.78(m/mv)=129(m)
FWHM temporal width=
The actual laser pulse duration:
205fs (FWHM)

17. Determine the actual pulse duration
Parameter:
Wavelength: 1054nm
Fringe number:98
temporal duration of IAC
signals :
FWHM:
981054  10^-9/3  10^8=345(fs)
Pulse duration:
345/1.92=180(fs)
The number of fringes over a fixed time interval remains the same at different
temporal locations of the IAC signal, even if the response of the delay line is non- linear.

18. Detect the frequency chirp
Autocorrelation signal
Ramp signal time(s)
Using fast Fourier transform and appropriate digital filters derived the envelope function g2 and f22
The peak amplitude of the difference signal was 0.06, which corresponds to an estimate of linear chirp () of 0.20.

19. Intensity auto-correlation of multi-picosecond laser pulses
Parameter:
Pulse duration :30ps
Pulse energy :20J
Output:
Pulse duration :33ps
Issue:
Need larger amount of pulse energy.
Use higher pulse energy, device may get damaged.

20. Conclusion
The intensity response is observed to be quadratic over a wide range of incident laser intensity range.
The lifetime of the LED has been estimated at two different intensity exposure levels.
The LED has been used to determine
pulse duration
small amount frequency chirp
Suitability of these LEDs to record intensity auto-correlation in multi-picosecond regime is demonstrated using 30 ps laser pulses.

21. Introduction
The IAC signal for detection of a small amount of chirp present in the pulse is twofold enhancement in sensitivity compared to:
A.K. Sharma