Presentation on theme: "Speaker: Tzung Da Jiang Adviser : Dr. Ja-Hon Lin"— Presentation transcript:
1Speaker: 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 nmSpeaker: Tzung Da JiangAdviser : Dr. Ja-Hon Lin
2Outline Introduction Theoretical background Characterization of AlGaAs LEDs for non-linear photo-currentReal time auto-correlation of laser pulsesConclusion
3IntroductionThe generation of ultra-short laser pulses  and their applications  require reliable measurement pulse parameters like :durationshapefrequency chirpLaser 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 T. Kobayashi, A. Baltuska, Meas. Sci. Technol. 13 (2002)1671. G.A. Mourou, C.P.J. Barty, M.D. Parry, Phys. Today 51 (1998) 22.
4Advantages 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 emittingdiodes are :Available off-the-shelf and are quite inexpensiveRelatively insensitive to incident light polarization and wavelengthDoes not require any phase matching conditionNon-hygroscopicTheir optical and electrical properties are integrated in a single unitNo spectral filtering effects
5Introduction 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.
6Theoretical background The two-photon absorption based induced photo-current (ITPA) may be described as : (ampere/watt²) : the two-photon induce photo-current responseP : the pulse peak powerIn a practical situation, for two-photon absorption the condition of TPA (2h > Eg > h) is necessary, but not sufficient.The response may vary linearly, ifimpuritiesdefectspresent in the semiconductor diode
7Signal of two-photon absorption The two- photon induced current in a semiconductor diode as a function ofrelative time delay () between the two pulses, would then represent the second order auto-correlation function S2()k : the intensity ratio of the two beamsE(t) = [I(t)]½ exp[i(t)] : the electric fieldI(t) : the pulse intensity envelope function(t) is the phase function
8Signal of two-photon absorption The above auto-correlation may also be expressed in the form,whereandThe phase function may be expressed in the form,: linear chirp: quadratic chirp: cubic chirpp: FWHM duration
9Advantage of MOSAICThough 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 presenceof 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 samechirped pulses : E(t) and E*(t) are different
10Principle of MOSAICMOSAIC signal generated from S2() by filtering out the cos 0 term and increasing the weight factor of the cos20 term by a factor of two, can be expressed asThe locus of the interference minima of this signal expressed below:g2 and f22 are the normalizedG2(s) and F22(s)
11Two-photon induced response with 30 ps laser pulse Pulse duration : 30psPeak emission wavelength : 660nmOscillator : 100 MHz cwmode-locked Nd:fluorophosphatePump : single-shot, flash lampFrom the log–log plots (with slope 2) in (a), it is clearly seen that the inducedphoto-current varies quadratically for a substantial range of incident laser power.The output current got saturated at 0.8 A.
12Two-photon induced response with 200 fs laser pulse Pulse duration : 200fsPeak emission wavelength : 660nmOscillator : 100 MHz cwmode-locked Nd:fluorophosphatepump : single-shot, flash lampFrom 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.
13Two-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
14Estimate its life-time as a two-photon detector The lifetime in this case is definedas the number of laser pulses afterwhich the photo-current reducesto half the initial value.It is clearly seen that the inducedphotocurrent decreases with anincrease in the number of laser pulses .decreases slowly:(1.2 kW pulsed, 24 mW average, 21011 lifetime )decreases faster :(3.5 kW pulsed, 70 mW average , 61011 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 signalNumber of laser pulses
15Experimental setup Pulse width:200 fs and 30 ps wavelength :1054 nm LED materal:AlGaAs based
16Determine 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)
17Determine the actual pulse duration Parameter:Wavelength: 1054nmFringe number:98temporal duration of IACsignals :FWHM:981054 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 differenttemporal locations of the IAC signal, even if the response of the delay line is non- linear.
18Detect the frequency chirp Autocorrelation signalRamp signal time(s)Using fast Fourier transform and appropriate digital filters derived the envelope function g2 and f22The peak amplitude of the difference signal was 0.06, which corresponds to an estimate of linear chirp () of 0.20.
19Intensity auto-correlation of multi-picosecond laser pulses Parameter:Pulse duration :30psPulse energy :20JOutput:Pulse duration :33psIssue:Need larger amount of pulse energy.Use higher pulse energy, device may get damaged.
20ConclusionThe 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 determinepulse durationsmall amount frequency chirpSuitability of these LEDs to record intensity auto-correlation in multi-picosecond regime is demonstrated using 30 ps laser pulses.
21IntroductionThe IAC signal for detection of a small amount of chirp present in the pulse is twofold enhancement in sensitivity compared to:A.K. Sharma