Direct temporal intensity measurement of ultrashort optical pulses using third-harmonic-generation based triple correlation

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(2000).

5. L.J. Gamble, W.M. Diffey, S.T. Cole, R.L. Fork, and D.K. Jones, “Simultaneous mea- surement of group delay and transmission for a one dimensional photonic crystal,” Op- tics Express 5,267-272 (1999).

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Ultra Broadband Gereration and Measurements TBA, Presider

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Direct Temporal intensity Measurement

of Ultrashort Optical Pulses Using Third-Harmonic-Generatlon based Triple Correlation

Tzu-Ming Liu, Yin-Chieh Huang, Gia-Wei Chern, Chih-Jie Lee,* Yu-Chueh Hung,* Chi-Kuang Sun,** Graduate Institute of Electro-

Optical Engineering, National Taiwan

University, Taipei 1061 7, TAIWAN R.O.C.;

Email: [email protected]; *Department of Electrical Engineering, National Taiwan

University, Taipei 10617, TAIWAN R.O.C.;

**Graduate Institute of Electro-Optical

Engineering and Department of Electrical

Engineering, National Taiwan University, Taipei

10617, TAIWAN R.O.C.

Because electronic devices (streak camera etc.)

are too slow to measure temporal evolution of

ultrashort optical pulses, many techniques were developed to retrieve temporal pulse shape. Most of them extract the temporal intensity either by assuming an analytic pulse shape (autocorrela- tion) or with the help of spectral measurements (including FROG’ and SPIDER.’) AU of these techniques rely on either interference or various nonlinear effect including second harmonic gen- eration and optical Kerr effect. However, in the later 1960 it was shown that a triple correlation is sufficient to determine the temporal intensity of a laser pulse3 with a direct mathematical cal- culation. In this report, we demonstrate direct temporal intensity measurement of ultrashort optical pulses using a novel third-harmonic-gen- eration (THG) based triple correlation method. Using THG process in a single GaN thin film, the optical pulse intensity profile from a mode- locked Cr:forsterite laser was directly obtained with the background free triple correlation trace without any spectral information and pulse- shape assumption. This is different from the pre- vious demonstration4 where triple correlation was obtained through a complex combination of second-harmonic generation and sum-frequen- cy-generation. In order to retrieve the correspon- ding phase information, a simple genetic algo- rithm was also developed for the first time based on the direct optical spectrum measurement with improved O(n) complexity than O(nZ) in.’

An optical pulse with intensity I(t) can be ex- panded in frequency domain v as

as shown in Fig. 1. By varying the time delay be- tween pulses t , and t2 and recording the selected THG signals with a detector, the background- free triple correlation

I ( t ) =

1

= Ij(v)lexp(ia(v) - i2xvt)dv (1)

G3 ( q , t Z ) =

I

+

was directly measured. Then we can use bispec- trum G 3 (vI,vZ), the Fourier transform of triple correlation function, to calculate li(v)l and a(v),’

and thus determine the I(t) using equation (1). Figure 2 shows two examples of the obtained triple correlation traces of ultrashort laser pulses where Il(v)l and a ( v ) are magnitude and phase of

the optical pulse intensity I(t) in the spectral do- main. After beam splitting, three mutual parallel and equal distance laser pulses were focused on the THG crystal and THG signal with three fun- damental frequency photon contributed from three individual pulses was spatially selected with an iris according to momentum conservation law

n

w

---

.

30 0 -200 -100 0 100 200 300

Delay 11 (ps) Delay ~l (fs)

CMPl laser.

Fig. 2. Two THG triple correlation traces of optical pulses from a modelocked Cr:forsterite

1 1 z 0.0 U N -0 0.0 L

.

I

from a modelocked Cr:forsterite laser using a GaN thin film as the THG crystal. The laser wavelength was centered around 1230 nm. In or- der to distinguish the THG signals from other photoluminescence signals through multi-pho- ton absorption, we use a CCD based spectrome- ter as the detector. In one of the measurement, we distorted out laser pulseshape intentionally by misarranging the laser cavity (Fig. 2(a)). Diago- nally symmetric traces were obtained, which is the characteristic of triple correlation. Their corresponding recovered temporal intensity are calculated and shown in Figure 3(a) and (b) re- spectively (solid line), whose mathematical auto- correlations agree with the measured one based on a SHG autocorrelator.

In order to retrieve the corresponding phase information, a simple genetic algorithm was also developed for the first time based on the direct obtained optical pulse temporal intensity profile as shown in Figure 3 with the aid of a direct opti- cal spectrum measurement (not shown here). This simple algorithm has improved O(n) com- plexity compared with most other iterative algo- rithm with O(n’) complexity.’ Thus retrieved temporal phase is also shown in Figure 3 (dotted line).

1. K.W. DeLong et al. “Frequency-resolved op-

tical gating with the use of second-harmonic generation,”J Opt. Soc. Am. B 11,2206-2215 (1994).

C. Iaconis and LA. Walmsley,“Spectral phase interferometry for direct electric-field recon- struction of ultrashort optical pulses,” Opt. Lett. 23,792-794 (1998).

E.I. Blount and J.R. Klauder “Recovery of laser intensity from correlation data,” J. Appl.

T. Feurer et al. “Measuring the temporal in-

tensity of ultrashort laser pulses by triple cor- relation,” Appl. Pbys. B 66,163-168 (1998).

2.

3.

PbyS. 40,2874-2875 (1969).

4.

CMP2 4 0 0 pm

Novel adaptive scheme for acoustwptic gain equalization and ultrabroadband amplification

T. Rupp, B. Schenkel, C.P. Hauri, G. Steinmeyer, U. Keller, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH Honggerberg HPT, CH-8093 Zurich, Switzerland; Email: [email protected]

Adaptive control is an extremely powerful tool for optical pulse shaping.’ For amplifier systems, e.g., phase shaping was employed for compres- sion of ultrashort pulses’ and optimized high- harmonic g e n e r a t i ~ n . ~ In this paper, we will in- troduce a method for amplitude shaping that allows for an adaptive spectral redistribution of amplifier gain. Rather than previous attempts to overcome the detrimental gain narrowing effect (e.g., the use of antiresonant etalons4) our method does not introduce a spectrally depen- dent loss. Our alternative approach uses gain shaping by generation of a suitable spatial pump profile. In contrast to previous demonstra- tions,5’6 our concept can be used adaptively to produce a particular spectral profile of the am- plified pulse and is based on acousto-optic imag- ing of the pump laser.

Our setup is depicted in Fig. 1. The pump

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CMP2 Fig. 1. Schematic setup of the acousto-optic gain shaper. Note that only one pass is shown for simplicity. AWG: arbitrary waveform generator. Shifter: frequency-shifter to convert AWG output into the 100-MHz range, RF-Amp: radio-frequency power amplifier, AOD: acousto-optic deflector, Osc: Kerr-lens modelocked Tisapphire laser producing sub-10-fs pulses, PC: Pockels cell used as pulse pick- er, Pump: Nd:YLF laser, producing 6 mJ pulse energy at 1 kHz repetition rate, TkSa: 1-cm Tksapphire

crystal, OSA optical spectrum analyzer.

source is a Q-switched NdYLF laser operating at 523.5 nm, producing 6-mJ pulses at a repetition rate of 1 kHz. The pulses from this laser are fed into an acousto-optic deflector (AOD) with a center frequency of 100 MHz and a bandwidth (3dB) of 50 MHz, operating at efficiencies of <70% by deflection into the first order. We use an arbitrary waveform generator and an rf amplifier to write an arbitrary acousto-optic grating into the AOD. With the first-order Bragg angle being a function of grating period, control of the fre- quency distribution of the driver wave allows to generate a desired image of the pump beam. One example of a generated spatial beam profile is shown in Fig. 2a. This distribution has been gen- erated by applying two consecutive sinusoidal functions linearly chirped in frequency. The de- flected and the non-deflected beams are used to pump a 1-cm long Tisapphire crystal from two opposing sides. The 6-pass amplifier is seeded

distance (um) 0.5 0.70 0.75 0.80 0.85 0.90 wavelength (pm) 0.6 0.7 0.8 0.9 1.0 wavelength (pm)

CMP2 Fig. 2. a: Spatial beam profile used in the shaping experiments. b: Amplified output spectrum with shaping (solid line) and without shaping (dashed line). c: Tisapphire oscillator spectrum used as seed.

with pulses from a sub-10-fs Ti:sapphire laser (see Fig. 2c). The spatial dispersion of the differ- ent frequency components of the pulses is pro- duced by a 4fsystem consisting of 10-degree sili- ca prisms andf= 375 mm lenses.

An example for the spectral shaping capabili- ties of our setup is illustrated in Fig. 2b. Operat- ing the AOD with a single-frequency rf wave at 100 MHz, i.e. disabling the shaping effect, gain narrowing reduces the spectral width of the am- plified pulse to 65 nm. Enabling gain shaping with the spatial profile in Fig. 2a broadens the output spectrum to a FWHM of 90 nm, an im- provement of nearly 40%. Proper dispersion compensation provided, such a spectrum direct- ly supports a 13.5-fs pulse. So far, we reached a net gain of 4000 and output energies in the pJ- range with shaping enabled. As our technique can be adapted within a wide range of spectral shapes, it appears promising for the generation of sub- 10-fs pulses directly from Ti:sapphire ampli- fiers. A further application may be the optimiza- tion of the pulse profile for high-harmonic gen- eration or hollow-fiber compression.

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