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

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

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.

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

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.

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

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

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

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

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)

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.

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.

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

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

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

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)

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.

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.

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.

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.

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