5-fs, multi-mJ, CEP-locked parametric chirped-pulse amplifier pumped by a 450-nm source at 1 kHz

doi:10.1364/OE.16.014341

5-fs, multi-mJ, CEP-locked parametric chirped-pulse amplifier pumped by a 450-nm source at 1 kHz

S. Adachi, N. Ishii, T. Kanai, A. Kosuge, J. Itatani, Y. Kobayashi, D. Yoshitomi, K. Torizuka, and S. Watanabe »View Author Affiliations

Optics Express, Vol. 16, Issue 19, pp. 14341-14352 (2008)
http://dx.doi.org/10.1364/OE.16.014341

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Abstract

We report on the development of an optical parametric chirped-pulse amplifier at a 1-kHz repetition rate with a 5.5-fs pulse duration, a 2.7-mJ pulse energy and carrier-envelope phase-control. The amplifier is pumped by a 450-nm pulse from a frequency-doubled Ti:sapphire laser.

1. Introduction

Isolated attosecond XUV pulses have been produced via high harmonic generation (HHG) using high-energy few-cycle laser pulses [1

R. Kienberger, E. Goulielmakis, M. Uiberacker, A. Baltuska, V. Yakovlev, F. Bammer, A. Scrinzi, T. Wester-walbesloh, U. Kleineberg, U. Heinzmann, M. Drescher, and F. Krausz, “Atomic transient recorder,” Nature 427, 817–821 (2004). [CrossRef] [PubMed]

, 2

T. Sekikawa, A. Kosuge, T. Kanai, and S. Watanabe, “Nonlinear optics in the extreme ultraviolet,” Nature 432, 605–608 (2004). [CrossRef] [PubMed]

, 3

G. Sansone, E. Benedetti, F. Calegari, C. Vozzi, L. Avaldi, R. Flammini, L. Poletto, P. Villoresi, C. Altucci, R. Velotta, S. Stagira, S. De Silvestri, and M. Nisoli, “Isolated single-cycle attosecond pulses,” Science 314, 443–446 (2006). [CrossRef] [PubMed]

, 4

A. Kosuge, T. Sekikawa, X. Zhou, T. Kanai, S. Adachi, and S. Watanabe, “Frequency-resolved optical gating of isolated attosecond pulses in the extreme ultraviolet,” Phys. Rev. Lett. 97, 263,901 (2006). [CrossRef]

]. Such laser pulses are generated by several pulse compression techniques but with technological limitations. A gas-filled capillary was used to generate 0.5-mJ 5-fs pulses [5

M. Nisoli, S. DeSilvestri, O. Svelto, R. Szipocs, K. Ferencz, C. Spielmann, S. Sartania, and F. Krausz, “Compression of high-energy laser pulses below 5 fs,” Opt. Lett. 22, 522–524 (1997). [CrossRef] [PubMed]

], but it has been difficult to scale up the output energy of the few-cycle pulses toward a multi-mJ regime [6

J. Sung, J. Park, T. Imran, Y. Lee, and C. Nam, “Generation of 0.2-TW 5.5-fs optical pulses at 1 kHz using a differentially pumped hollow-fiber chirped-mirror compressor,” Appl. Phys. B 82, 5–8 (2006). [CrossRef]

]. Broadband frequency-doublingwas employed to generate 8-fs pulses at 400 nm, but further pulse shortening is difficult [7

X. Zhou, T. Kanai, D. Yoshitomi, T. Sekikawa, and S. Watanabe, “Generation of high average power, 7.5-fs blue pulses at 5 kHz by adaptive phase control,” Appl. Phys. B 81, 13–17 (2005). [CrossRef]

]. The approach of spectral broadening through filamentation has been used to generate a multi-mJ pulse, but the pulse width is limited to sub-10-fs regime [8

C. Hauri, A. Trisorio, M. Merano, G. Rey, R. Lopez-Martens, and G. Mourou, “Generation of high-fidelity, down-chirped sub-10 fs mJ pulses through filamentation for driving relativistic laser-matter interactions at 1 kHz,” Appl. Phys. Lett. 89, 151,125 (2006). [CrossRef]

]. Alternatively, the concept of optical parametric chirped-pulse amplification [9

A. Dubietis, G. Jonusauskas, and A. Piskarskas, “Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in BBO crystal,” Opt. Commun. 88, 437–440 (1992). [CrossRef]

] (OPCPA) has been recognized as a promising route to generate such high-intensity few-cycle laser pulses. Experimentally, sub-10 fs, multi-mJ to multi-tens-of-mJ OPCPA systems were already reported [10

N. Ishii, L. Turi, V. Yakovlev, T. Fuji, F. Krausz, A. Baltuska, R. Butkus, G. Veitas, V. Smilgevicius, R. Danielius, and A. Piskarskas, “Multimillijoule chirped parametric amplification of few-cycle pulses,” Opt. Lett. 30, 567–569 (2005). [CrossRef] [PubMed]

, 11

S. Witte, R. Zinkstok, A. Wolf, W. Hogervorst, W. Ubachs, and K. Eikema, “A source of 2 terawatt, 2.7 cycle laser pulses based on noncollinear optical parametric chirped pulse amplification,” Opt. Express 14, 8168–8177 (2006). [CrossRef] [PubMed]

, 12

F. Tavella, A. Marcinkevicius, and F. Krausz, “90 mJ parametric chirped pulse amplification of 10 fs pulses,” Opt. Express 14, 12,822–12,827 (2006). [CrossRef]

] with a relatively low repetition rate (10-30 Hz). However, higher repetition rate is naturally desirable to further exploit attosecond physics using high harmonics.

Recently we have demonstrated a sub-7-fs, 1.5-mJ OPCPA system at a 1-kHz repetition rate with a 400-nm frequency-doubled Ti:sapphire laser pumping [13

S. Adachi, H. Ishii, T. Kanai, N. Ishii, A. Kosuge, and S. Watanabe, “1.5 mJ, 6.4 fs parametric chirped-pulse amplification system at 1 kHz,” Opt. Lett. 32, 2487–2489 (2007). [CrossRef] [PubMed]

]. The Ti:sapphire-based pump laser system has several advantages over those based on different laser media (Nd:YLF, Nd:YAG, etc). First, attainable pulse energy from the Ti:sapphire-based system is demonstrated to be much higher than those from the other systems. Actually, the output energy from a Ti:sapphire laser system can reach 25-30 mJ at 1 kHz [14

Y. Nabekawa, Y. Kuramoto, T. Togashi, T. Sekikawa, and S. Watanabe, “Generation of 0.66-TW pulses at 1 kHz by a Ti: sapphire laser,” Opt. Lett. 23, 1384–1386 (1998). [CrossRef]

] (c.f., ~10 mJ at 1 kHz with Nd:YLF [15

T. Fuji, N. Ishii, C. Teisset, X. Gu, T. Metzger, A. Baltuska, N. Forget, D. Kaplan, A. Galvanauskas, and F. Krausz, “Parametric amplification of few-cycle carrier-envelope phase-stable pulses at 2.1 mu m,” Opt. Lett. 31, 1103–1105 (2006). [CrossRef] [PubMed]

]). Second, if a broadband Ti:sapphire oscillator is used for seeding an OPCPA, a part of the oscillator output can be simultaneously amplified in the Ti:sapphire pump laser system. This assures the optical synchronization between the pump and seed pulses in the parametric amplification stages [9

, 16

C. Hauri, P. Schlup, G. Arisholm, J. Biegert, and U. Keller, “Phase-preserving chirped-pulse optical parametric amplification to 17.3 fs directly from a Ti: sapphire oscillator,” Opt. Lett. 29, 1369–1371 (2004). [CrossRef] [PubMed]

, 17

N. Ishii, C. Teisset, T. Fuji, S. Kohler, K. Schmid, L. Veisz, A. Baltuska, and F. Krausz, “Seeding of an eleven femtosecond optical parametric chirped pulse amplifier and its Nd3+ picosecond pump laser from a single broadband Ti: Sapphire oscillator,” IEEE J. Sel. Top. Quantum Electron. 12, 173–180 (2006). [CrossRef]

]. Another advantage of a Ti:sapphire-laser-based OPCPA is the capability to produce even shorter pulses (~5 fs) than demonstrated by tuning the pump wavelength, which is crucial to produce isolated attosecond pulses by the HHG process as discussed in Ref [1

]. In this letter, we report on the generation of 5-fs multi-mJ CEP-locked pulses at 1kHz from an OPCPA that is pumped by the second-harmonic (SH) of a spectrally-shifted Ti:sapphire laser. In section 2, we will show the detailed analysis of OPCPA’s bandwidth as a function of the pump wavelength. Then in section 3, the development of the OPCPA is described in the following manner. At first we describe the narrowband Ti:sapphire pump laser that produces 20-mJ 900-nm pulses at 1 kHz. The output was frequency-doubled to produce 15-mJ 450-nm pump pulses for the OPCPA. Second, our novel design of a stretcher/compressor system is discussed. Subsequently the experimental results of parametric amplification, pulse compression, and CEP stabilization is demonstrated.

2. Parametric gain spectra for several pump wavelengths

The generation of sub-5-fs pulses has been already demonstrated from noncollinear optical parametric amplifiers (NOPA), but with rather low pulse energies (~µJ) [18

A. Shirakawa, I. Sakane, M. Takasaka, and T. Kobayashi, “Sub-5-fs visible pulse generation by pulse-front-matched noncollinear optical parametric amplification,” Appl. Phys. Lett. 74, 2268–2270 (1999). [CrossRef]

, 19

M. Zavelani-Rossi, G. Cerullo, S. De Silvestri, L. Gallmann, N. Matuschek, G. Steinmeyer, U. Keller, G. Angelow, V. Scheuer, and T. Tschudi, “Pulse compression over a 170-THz bandwidth in the visible by use of only chirped mirrors,” Opt. Lett. 26, 1155–1157 (2001). [CrossRef]

, 20

A. Baltuska, T. Fuji, and T. Kobayashi, “Visible pulse compression to 4 fs by optical parametric amplification and programmable dispersion control,” Opt. Lett. 27, 306–308 (2002). [CrossRef]

]. Figure 1 shows calculated gain spectra of a type-I β -BaB₂O₄ (BBO) crystal for several pump wavelengths. In these experiments [18

, 19

, 20

A. Baltuska, T. Fuji, and T. Kobayashi, “Visible pulse compression to 4 fs by optical parametric amplification and programmable dispersion control,” Opt. Lett. 27, 306–308 (2002). [CrossRef]

], researchers exploited the combination of (1) SH of an ordinary Ti:sapphire laser (800 nm) as a pump pulse and (2) whitelight continuum generated with a glass plate as a seed pulse. In this case, the 400-nm pump gives broadest gain spectrum from 520 to 750 nm [Fig. 1(a)] supporting sub-5-fs operation, which is fully covered by the whitelight continuum seed [(b)]. In contrast, the spectrum of a typical broadband Ti:sapphire oscillator [(c)], when used as the seed source of OPCPA systems, has poor spectral overlap to the BBO’s spectral acceptance shown in (a). Therefore, we were obliged to modify the OPA configuration at the expense of broadest OPA gain bandwidth [(d)] in the previous report [13

]. Meanwhile, there is another solution for this problem: Shifting the pump wavelength itself [21

B. Zhao, X. Liang, Y. Leng, C. Wang, Y. Jiang, J. Du, and Z. Xu, “Optimization of optical parametric chirped pulse amplification with different pump wavelengths,” Opt. Commun. 259, 137–141 (2006). [CrossRef]

]. From our calculations following Ref [22

R. Danielius, A. Piskarskas, A. Stabinis, G. Banfi, P. Ditrapani, and R. Righini, “Traveling-wave parametric generation of widely tunable, highly coherent femtosecond light pulses,” J. Opt. Soc. Am. B 10, 2222–2232 (1993). [CrossRef]

], the pump wavelength of 450-500 nm is suitable to amplify the seed pulses from Ti:sapphire oscillators [see (c) and (e)-(g)], allowing to produce 5-fs pulses. We set the pump wavelength at 450 nm (corresponding fundamental wavelength of 900 nm) in order to get a moderate Ti:sapphire gain, although we could choose any pump wavelength within 450-500 nm since Ti:sapphire has a very broad gain bandwidth (typically 650-1050 nm, corresponding frequency-doubled range of 325-525 nm).

3. OPCPA development

The overview of our OPCPA system is depicted in Fig. 2. A broadband Ti:sapphire oscillator (Venteon OS version, Nanolayers GmbH) with octave-spanning spectrum, pumped with a 4-W green laser (Verdi V6, Coherent), was used as the master laser of our system. The spectral components at 570 and 1140 nm were utilized for the CEP stabilization with an f -to-2 f setup [23

D. Jones, S. Diddams, J. Ranka, A. Stentz, R. Windeler, J. Hall, and S. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000). [CrossRef] [PubMed]

], while the spectral component of 600-1100 nm was sent to a stretcher system as a seed pulse of the OPCPA. Small portion (~4 %) of the seed pulse was reflected with a beam splitter (BS) and was injected to a Ti:sapphire regenerative amplifier pumped by a 4-mJ frequency-doubled Nd:YLF laser (Evolution 6, Spectra Physics).

3.1. Pump laser system

Since the pulse width of the injected pulse to the regenerative amplifier was about 10 fs, it must be stretched to ≿ 100 ps to extract energies from Ti:sapphire amplifiers without any optical damage. In our system the stretching was realized by limiting the amplification bandwidth of the regenerative amplifier to ~4 GHz at 900 nm, corresponding to the pulse width of ~100 ps assuming Gaussian spectrum. For this purpose, a three-fold birefringent filter (also known as Lyot filter) with a thickness ratio of 1:4:16, and a solid etalon (FSR≃500 GHz, Finesse=2.8 at 900 nm) were inserted in the regenerative amplifier cavity as depicted in Fig. 3. The pulse width only with the birefringent filter was measured to be about 2 ps (Δλ≃0.5 nm at 900 nm) after ~30 round-trips in the regenerative amplifier, and that both with the birefringent filter and etalon was 75 ps. The amplified seed pulse was rejected from the regenerative amplifier after ~30 round-trips, where the amplification was saturated by controlling the amplifier gain (i.e., the pump pulse energy).

Fig. 1. Calculated parametric gain spectra of a BBO crystal for several pump wavelengths. Noncollinear and phase-matching angles for the calculations are optimized to achieve broadest gain bandwidth for each pump wavelength.

Fig. 2. Diagram of OPCPA system. BPF, band-pass filter; BS, beam splitter; CEP, carrier-envelope phase; SHG, second-harmonic generation; AOPDF, acousto-optic programmable dispersive filter; OPA, optical parametric amplifier.

Fig. 3. Schematic of narrowband Ti:sapphire regenerative amplifier. PBS, polarization beam splitter; FR, Faraday rotator; QR, Quartz rotator; TFP, thin-film polarizer; PC, Pockels cell; BRF, birefringent filter; DM, dichroic mirror.

The dotted curve in Fig. 4 shows the temporal profiles of the regenerative amplifier output measured with an oscilloscope with sub-ns resolution. We observed pre/post-pulses and long pedestal (5-10 ns) superimposed onto the main pulse. The pre/post-pulses were due to the amplification of the previous/next pulses of the main one in the oscillator pulse train, respectively. They were separated by ±2.5 ns from the main pulse, which corresponds to the difference of round-trip times between the oscillator (12.5 ns) and regenerative amplifier (10 ns). Meanwhile, the pedestal was due to amplified spontaneous emission (ASE). These unwanted pulses could not be separated from the main one by means of a Pockels cell in the regenerative amplifier, which has a relatively slow (6 ns) response time. The pre-pulse and the precedent portion of the pedestal is much more detrimental, because in the subsequent Ti:sapphire power-amplification stages they are always amplified with unsaturated gain, and consequently the contrast between the pre-pulses and main pulse becomes much worse. In the previous report [13

], the regenerative amplifier output was sent to a saturable absorption filter (ITF-83IR, Sigma koki) in order to get rid of these unwanted pulses before power-amplification, which resulted in the improvement of the contrast by one order of magnitude. As a result, the second-harmonic generation (SHG) conversion efficiencies after power-amplifiers were ~60 %, while the efficiencies without the saturable absorption filter were only 40 % since considerable portion of the output energy was attributed to the ASE (the SHG conversion efficiency is a good indicator of the temporal/spatial quality in our experiments). Nonetheless, the throughput of the filter was very low (~10 %), and the degradation occurred quickly due to the radiation of the regenerative amplifier output. Therefore, we needed to move the filter constantly to avoid the degraded spots. Recently, we replaced the saturable absorber to a fast pulse-slicing unit (UPC, Leysop Ltd; rise time: 200 ps, flat top: 250-300 ps, fall time: 1.5-2 ns). The solid curve in Fig. 4 corresponds to the temporal profile of the regenerative amplifier outputs with the pulse slicer. The output pulse shows a very clean temporal profile (note that the ringing in the trailing edge is an artifact for the impulsive response of our oscilloscope). The throughput of the main pulse was >95 %, and the average power after the pulse slicer was 50 µJ (in Fig. 4 the relative intensities are normalized by their average power to be comparable with each other).

Fig. 4. Temporal profiles of the regenerative amplifier outputs with (solid curve, 50 µJ) and without (dotted curve, 110 µJ) the pulse slicer.

Figure 5 shows the schematic of a 6-pass Ti:sapphire amplifier and 1-pass Ti:sapphire amplifiers, followed by frequency-doubling. The output from the pulse slicer was injected to the 6-pass Ti:sapphire amplifier pumped by a frequency-doubled Nd:YLF laser (Evolution 30, Spectra Physics; 20 mJ), and an 900-nm radiation with 5-mJ pulse energy was obtained. This output was divided into two parts (1.5 and 3.5 mJ), and subsequently each of them was amplified in the 1-pass power-amplifiers pumped by four frequency-doubled Nd:YLF (527 nm) lasers (Darwin, Quantronix, 20-25 mJ; Evolution HE, Coherent inc., 45 mJ; See Fig. 5 for the detail), and 900-nm radiations of 5 mJ and 20 mJ were obtained. In all of the Ti:sapphire amplifiers, the Ti:sapphire crystals (see Fig. 5 for the thicknesses) were water-cooled and placed in the air. After amplification, 5-mJ and 20-mJ pulses were converted to SH (450 nm) pulses of 4 mJ and 15 mJ, respectively, by two 1-cm-long antireflection(AR)-coated LiB ₃O₅ (LBO) crystals with the peak laser intensity of ~3 GW/cm². Figure 6 displays far-field image of the loosely-focused 15-mJ 450-nm pump pulse at OPA crystal, showing excellent beam quality. It is noteworthy that the SHG conversion efficiencies reached as high as 75-80 % owing to the excellent temporal and spatial profiles of the Ti:sapphire output. The shot-to-shot fluctuations of the fundamental (900 nm) and its SH (450 nm) pulses were 0.7 and 1.1 % RMS error, respectively.

3.2. Stretcher and compressor system

The stretcher system consisted of a grating pair (1200 grooves/mm) and a prism pair (SF57, Schott) to give a negative chirp, which was compensated by a positive chirp in a bulk material (SF57, 250 mm; Schott) compressor after amplification. In addition, an acousto-optic programmable dispersion filter (AOPDF; Dazzler UWB-600-900, Fastlite), containing a 30-mm-long TeO₂ crystal, was inserted after the stretcher for adaptive chirp control. Table 1 shows lower-order dispersions of main dispersive elements of the stretcher-compressor system. We didn’t include the dispersion of other optics in the system, eg, OPA crystals, since they were negligibly small compared to the main dispersive elements. The whole stretcher and compressor system was designed (1) to eliminate third-order dispersion (TOD) and (2) to balance inevitable negative forth-order dispersion (FOD) and small positive group-delay dispersion (GDD). Consequently, residual group-delay in the whole spectral range could be compensated by the AOPDF (its tuning range is 3.5 ps) as shown in Fig. 7. The duration of the stretched seed pulse is calculated to be 50 ps. The throughputs of each stretcher element (grating pair, prism pair, and Dazzler) were 60 %, 60 %, and 30 %, respectively. We thus obtained the overall stretcher throughput of 0.6×0.6×0.3≃10 %, resulting in ~0.1 nJ seed pulse energy.

Fig. 5. Schematic of a 6-pass Ti:sapphire amplifier and 1-pass Ti:sapphire amplifiers, followed by frequency-doubling crystals. DM, dichroic mirror; BS, beam splitter; LBO, LiB₃O₅ crystal.

Fig. 6. Far-field image of 15-mJ, 450-nm pump pulses.

Table 1. Lower-order dispersions at the center wavelength of 770 nm.

	GDD (10³ fs²)	TOD (10³ fs³)	FOD (10³ fs⁴)
Grating pair	-49	71	-159
Prism par	-17	-118	-231
TeO₂(AOPDF)	16	10	6
SF57 glass	60	37	15
Total	10	0	-369

3.3. Parametric amplification

The spectrum of the OPCPA seed pulse is depicted in Fig. 8(a) with a thin curve, showing a sharp peak at 670 nm. The seed pulse was pre-amplified with a 2-pass Ti:sapphire amplifier in advance of a parametric amplifier. A Ti:sapphire crystal has maximum gain around 800 nm, and the gain is relatively low at shorter wavelength range. The 2-pass Ti:sapphire pre-amplifier gain was measured to be ~100 (10×10) at 800 nm and ~3 at 670 nm. This pre-amplifier flattened the seed pulse spectrum as shown in Fig. 8(a) with a thick curve (the peak at 670 nm is moderate now), and consequently signal-to-superfluorescence contrast of OPCPA output became better as is discussed later.

Fig. 7. Residual group-delay of the stretcher-compressor system, which is within the AOPDF tuning range (hatched area) of 3.5 ps.

Fig. 8. (a) OPCPA seed spectra before (thin curve) and after (thick curve) Ti:sapphire preamplifier. Dotted curve shows 10-times magnification of the thin curve. (b) Calculated OPA gain spectrum with a 450-nm pump pulse.

Figure 9 shows a schematic of a 3-stage parametric amplifier. The 4-mJ SH pulse was divided into two parts, and each of them was loosely focused to a two-stage optical parametric pre-amplifier (pre-OPA). Their beam diameters at crystals were about 250 and 300 µmFWHM, respectively. Typical OPA gain for each stage was set at about 10³ though much higher single-pass gain could be achieved. Such a rather low single-pass gain below saturation (the conversion efficiency was as small as 4 % even for the second stage) allowed to obtain the seed pulse after the pre-OPA with good signal-to-superfluorescence contrast. Keeping good signal-to- superfluorescence contrast in the pre-OPA stages is crucial to suppress the superfluorescence in the next power-OPA stage. We obtained the contrast of more than 100 in energy, while it had been ~20 without the Ti:sapphire pre-amplifier. At the final stage, the power-OPA was pumped with the 15-mJ SH pulses with the diameter of 700 µm. The noncollinear and phase-matching angles were set at 3.0° (internal) and 27.3°, respectively. Fig. 8(b) shows calculated OPA gain spectrum with a 450-nm pump pulse. All of the crystals for the OPA stage were 5-mm-thick broadband-AR-coated (650-930 nm) type-I BBO.

Fig. 9. Schematic of a 3-stage parametric amplifier. Solid curve, seed/signal beam; Dashed curve, pump beams.

Fig. 10. Temporal profiles of OPCPA outputs for moderate (solid curve) and too high (dotted curve) OPA gain.

The broadband AR coatings on BBO crystals were critically important since high gain operation of OPA leads to the ”oscillation” of amplified pulses inside the crystal due to multiple internal reflections [24

F. Tavella, K. Schmid, N. Ishii, A. Marcinkevicius, L. Veisz, and F. Krausz, “High-dynamic range pulse-contrast measurements of a broadband optical parametric chirped-pulse amplifier,” Appl. Phys. B 81, 753–756 (2005). [CrossRef]

]. For example, considering a non-coated OPA crystal with 6 % reflectivity (typical value) for single surface, the relative intensity of the amplified pulse (here assuming an OPA gain as 10³) after double-reflection is 10³×0.06²=3.6. Meanwhile, the time taken for the double-reflection in a 5-mm-thick BBO crystal is about 50 ps, which is shorter than the pump pulse width (75 ps). Accordingly, the double-reflected pulse, which is rather more intense than the seed pulse, is re-seeded after 50 ps, and then the OPCPA output would be a pulse train with 50-ps separation. This re-seeding problem can be avoided by the broadband AR coating (<1 %) on the BBO crystals. However, it is noted that if the single OPA gain becomes too high (≿10⁴), the double-reflection causes the same problem, even with the AR coating. Figure 10 depicts the temporal profiles of OPCPA outputs with moderate (solid curve) and too high (dotted curve) OPA gain, acquired by a sampling oscilloscope (TDS8000, Tektronix; attached a 50-GHz electrical sampling module, 80E01) with a high-speed photodetector (Model 1001 60-GHz, Newfocus), where the overall time resolution was 8 ps. The post pulse due to the double-reflection appeared with 50-ps separation.

In the two-stage pre-OPA, the seed pulse energy was amplified from 0.1 nJ to 0.1 mJ (10 ³×10³=10⁶ gain), and the signal energy of 2.7 mJ was obtained after the power-OPA, followed by the pulse compression. The pump-to-signal conversion efficiency in the power-OPA was about 17% with 15-mJ pump pulse. The shot-to-shot intensity fluctuation of the OPCPA output was 3.3 % RMS error. Figure 11 displays the OPCPA spectra pumped at 450 nm for slightly different phase-matching angles (in the order of 10^-2 degree) [(b)-(d)], as well as the previous result obtained with 400-nm pump pulse [(a)] [13

]. The spectrum was substantially broadened by shifting the pump wavelength to 450 nm, sufficient to support 5-fs pulse generation.

Fig. 11. Measured OPCPA spectra pumped with (a) 400-nm pump pulse and (b)–(d) 450-nm pump pulse with slightly different phase-matching angles.

3.4. Pulse compression

After amplification, the output pulses were collimated with a diameter of ~2 cm FWHM before compression by the SF57 block. The B-integral for the output pulses in the SF57 block is estimated to be as small as <0.2 π radian, and in the experiments, no significant evidence of phase distortion (eg, spectral modulation or spatial pattern distortion) was observed after the compressor. The compressed pulses were characterized by a SPIDER apparatus [25

C. Iaconis and I. Walmsley, “Self-referencing spectral interferometry for measuring ultrashort optical pulses,” IEEE J. Sel. Top. Quantum Electron . 35, 501–509 (1999). [CrossRef]

], and its result was directly feedbacked to the AOPDF phase setting. Figure 12 shows spectral and temporal profiles of the compressed OPCPA output pulses after the adaptive phase control. In Fig. 12(a) the spectral phase was almost flat over the whole OPCPA spectral range of >130 THz after the huge compression ratio of 10⁴ (50 ps : 5 fs). Consequently, the reconstructed pulse width was 5.5 fs, which is almost equal to the transform-limited pulse width [Fig. 12(b)]. Small peaks in the pedestal of the temporal profile are mainly due to the twin-peaked structure in the spectral domain which is caused by the overall OPA gain spectrum, and not from the residual spectral phase distortion. These satellite peaks would be suppressed further if the system is operated with narrower spectrum [dotted curve in Fig. 12(b)].

Fig. 12. Spectral and temporal profiles of recompressed OPCPA output pulse. (a) Solid curve, spectral intensity; Dotted curve, spectral phase. (b) Thick curve, temporal intensity; Thin curve, transform-limited pulse shape; Dotted curve corresponds to the transform-limited pulse shape for the spectrum in Fig. 11(d).

3.5. CEP stabilization

The spectral components at 570 and 1140 nm from the oscillator were utilized for the f-to-2f spectral interferometry, and beat signal was detected by an avalanche photodiode. The phase and frequency of the beat signal were compared to f _rep/4 reference (f _rep: oscillator repetition frequency) by a digital comparator, and the error signal after a loop filter was feedbacked to an acousto-optic modulator placed into the pump beam path of the oscillator in order to compensate the phase shift. The CEP beat note with a SNR of ~35 dB in a 100-kHz resolution bandwidth was observed by a spectrum analyzer (E4403B, Agilent), and the CEP of the oscillator was verified to be stabilized for several hours. The seed pulses amplified by the OPCPA had an identical CEP since the trigger signal for the Ti:sapphire pump system was generated by the rational frequency division (/80,000) of the oscillator repetition rate.

The evaluation of the CEP drift induced in the OPCPA was implemented with another f -to- 2f interferometer placed after the compressor. The small portion (~20 µJ) of the OPCPA output was focused in a 2-mm-thick CaF₂ to obtain octave-spanning spectrum. The SH of 900-nm spectral component was generated with a 1-mm-thick BBO crystal, and both the fundamental and SH were directed to a spectrometer (HR4000, Ocean Optics) after a polarizer. Figure 13 shows the f-to-2f signal which was acquired around 450 nm with 4-msec CCD exposure time, e.g.,~4 pulses are exposed in a single spectrum. The CEP of the OPCPA output was stable over 30 sec except for a slow drift. The phase fluctuation was 0.30π rad. RMS error for the 4-msec integration without taking into account the slow drift. The slow drift might be explained by the beam-pointing fluctuation directing to the stretcher [26

M. Kakehata, H. Takada, Y. Kobayashi, K. Torizuka, H. Takamiya, K. Nishijima, T. Homma, H. Takahashi, K. Okubo, S. Nakamura, and Y. Koyamada, “Carrier-envelope-phase stabilized chirped-pulse amplification system scalable to higher pulse energies,” Opt. Express 12, 2070–2080 (2004). [CrossRef] [PubMed]

], and it would be easily compensated by providing feedback to the relative delay in the first f -to-2f interferometer. It is noted that our stretcher-compressor system has much smaller GDD (ϕ ₂=6×10⁴ fs²) than the previously reported CEP-stabilized chirped-pulse amplification systems, e.g., ϕ ₂=3.6×10⁶ fs² [26

]. This relatively small GDD results in a better CEP stability because the beam-pointing fluctuation induces a change in GDD and thus, a shift in CEP.

Fig. 13. f-to-2f spectral interferometric signal of OPCPA output.

4. Conclusion

In this paper, we have demonstrated the development of 5.5-fs, 2.7-mJ, and CEP-locked parametric chirped-pulse amplifier at 1 kHz. The narrowband Ti:sapphire pump laser system generated a 20-mJ, 900-nm pulse, which was frequency-doubled to a 15-mJ, 450-nm pump pulse. The stretcher system consisted of a grating pair and a prism pair giving a negative chirp, which was compensated by a positive chirp in a bulk material compressor after 3-stage parametric amplifier. The CEP of the OPCPA output was verified to be stabilized. It is noteworthy that our 20-mJ, 900-nm pump laser system can be also applied to pump infrared OPCPA system (e.g. [15

], at 2 µm)). The pump pulse energy is expected to be 25-30 mJ if the pump wavelength is set back to 800 nm, which is optimum for Ti:sapphire amplification [14

Y. Nabekawa, Y. Kuramoto, T. Togashi, T. Sekikawa, and S. Watanabe, “Generation of 0.66-TW pulses at 1 kHz by a Ti: sapphire laser,” Opt. Lett. 23, 1384–1386 (1998). [CrossRef]

The output pulse energy is almost doubled from the previous report [13

], demonstrating the energy scalability toward a multi-mJ level. This scalable output with few-cycle pulse duration is enough to produce an isolated attosecond pulse by the HHG process with unprecedentedly higher intensity than with other conventional pulse compression schemes. Therefore we expect that our CEP-locked terawatt-class few-cycle laser system offers an ideal light source for the HHG experiments in the attosecond timescale, including the XUV experiments with nonlinear optical processes.

Acknowledgment

N. Ishii is grateful to a JSPS fellowship.

References and links

1.	R. Kienberger, E. Goulielmakis, M. Uiberacker, A. Baltuska, V. Yakovlev, F. Bammer, A. Scrinzi, T. Wester-walbesloh, U. Kleineberg, U. Heinzmann, M. Drescher, and F. Krausz, “Atomic transient recorder,” Nature 427, 817–821 (2004). [CrossRef] [PubMed]
2.	T. Sekikawa, A. Kosuge, T. Kanai, and S. Watanabe, “Nonlinear optics in the extreme ultraviolet,” Nature 432, 605–608 (2004). [CrossRef] [PubMed]
3.	G. Sansone, E. Benedetti, F. Calegari, C. Vozzi, L. Avaldi, R. Flammini, L. Poletto, P. Villoresi, C. Altucci, R. Velotta, S. Stagira, S. De Silvestri, and M. Nisoli, “Isolated single-cycle attosecond pulses,” Science 314, 443–446 (2006). [CrossRef] [PubMed]
4.	A. Kosuge, T. Sekikawa, X. Zhou, T. Kanai, S. Adachi, and S. Watanabe, “Frequency-resolved optical gating of isolated attosecond pulses in the extreme ultraviolet,” Phys. Rev. Lett. 97, 263,901 (2006). [CrossRef]
5.	M. Nisoli, S. DeSilvestri, O. Svelto, R. Szipocs, K. Ferencz, C. Spielmann, S. Sartania, and F. Krausz, “Compression of high-energy laser pulses below 5 fs,” Opt. Lett. 22, 522–524 (1997). [CrossRef] [PubMed]
6.	J. Sung, J. Park, T. Imran, Y. Lee, and C. Nam, “Generation of 0.2-TW 5.5-fs optical pulses at 1 kHz using a differentially pumped hollow-fiber chirped-mirror compressor,” Appl. Phys. B 82, 5–8 (2006). [CrossRef]
7.	X. Zhou, T. Kanai, D. Yoshitomi, T. Sekikawa, and S. Watanabe, “Generation of high average power, 7.5-fs blue pulses at 5 kHz by adaptive phase control,” Appl. Phys. B 81, 13–17 (2005). [CrossRef]
8.	C. Hauri, A. Trisorio, M. Merano, G. Rey, R. Lopez-Martens, and G. Mourou, “Generation of high-fidelity, down-chirped sub-10 fs mJ pulses through filamentation for driving relativistic laser-matter interactions at 1 kHz,” Appl. Phys. Lett. 89, 151,125 (2006). [CrossRef]
9.	A. Dubietis, G. Jonusauskas, and A. Piskarskas, “Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in BBO crystal,” Opt. Commun. 88, 437–440 (1992). [CrossRef]
10.	N. Ishii, L. Turi, V. Yakovlev, T. Fuji, F. Krausz, A. Baltuska, R. Butkus, G. Veitas, V. Smilgevicius, R. Danielius, and A. Piskarskas, “Multimillijoule chirped parametric amplification of few-cycle pulses,” Opt. Lett. 30, 567–569 (2005). [CrossRef] [PubMed]
11.	S. Witte, R. Zinkstok, A. Wolf, W. Hogervorst, W. Ubachs, and K. Eikema, “A source of 2 terawatt, 2.7 cycle laser pulses based on noncollinear optical parametric chirped pulse amplification,” Opt. Express 14, 8168–8177 (2006). [CrossRef] [PubMed]
12.	F. Tavella, A. Marcinkevicius, and F. Krausz, “90 mJ parametric chirped pulse amplification of 10 fs pulses,” Opt. Express 14, 12,822–12,827 (2006). [CrossRef]
13.	S. Adachi, H. Ishii, T. Kanai, N. Ishii, A. Kosuge, and S. Watanabe, “1.5 mJ, 6.4 fs parametric chirped-pulse amplification system at 1 kHz,” Opt. Lett. 32, 2487–2489 (2007). [CrossRef] [PubMed]
14.	Y. Nabekawa, Y. Kuramoto, T. Togashi, T. Sekikawa, and S. Watanabe, “Generation of 0.66-TW pulses at 1 kHz by a Ti: sapphire laser,” Opt. Lett. 23, 1384–1386 (1998). [CrossRef]
15.	T. Fuji, N. Ishii, C. Teisset, X. Gu, T. Metzger, A. Baltuska, N. Forget, D. Kaplan, A. Galvanauskas, and F. Krausz, “Parametric amplification of few-cycle carrier-envelope phase-stable pulses at 2.1 mu m,” Opt. Lett. 31, 1103–1105 (2006). [CrossRef] [PubMed]
16.	C. Hauri, P. Schlup, G. Arisholm, J. Biegert, and U. Keller, “Phase-preserving chirped-pulse optical parametric amplification to 17.3 fs directly from a Ti: sapphire oscillator,” Opt. Lett. 29, 1369–1371 (2004). [CrossRef] [PubMed]
17.	N. Ishii, C. Teisset, T. Fuji, S. Kohler, K. Schmid, L. Veisz, A. Baltuska, and F. Krausz, “Seeding of an eleven femtosecond optical parametric chirped pulse amplifier and its Nd3+ picosecond pump laser from a single broadband Ti: Sapphire oscillator,” IEEE J. Sel. Top. Quantum Electron. 12, 173–180 (2006). [CrossRef]
18.	A. Shirakawa, I. Sakane, M. Takasaka, and T. Kobayashi, “Sub-5-fs visible pulse generation by pulse-front-matched noncollinear optical parametric amplification,” Appl. Phys. Lett. 74, 2268–2270 (1999). [CrossRef]
19.	M. Zavelani-Rossi, G. Cerullo, S. De Silvestri, L. Gallmann, N. Matuschek, G. Steinmeyer, U. Keller, G. Angelow, V. Scheuer, and T. Tschudi, “Pulse compression over a 170-THz bandwidth in the visible by use of only chirped mirrors,” Opt. Lett. 26, 1155–1157 (2001). [CrossRef]
20.	A. Baltuska, T. Fuji, and T. Kobayashi, “Visible pulse compression to 4 fs by optical parametric amplification and programmable dispersion control,” Opt. Lett. 27, 306–308 (2002). [CrossRef]
21.	B. Zhao, X. Liang, Y. Leng, C. Wang, Y. Jiang, J. Du, and Z. Xu, “Optimization of optical parametric chirped pulse amplification with different pump wavelengths,” Opt. Commun. 259, 137–141 (2006). [CrossRef]
22.	R. Danielius, A. Piskarskas, A. Stabinis, G. Banfi, P. Ditrapani, and R. Righini, “Traveling-wave parametric generation of widely tunable, highly coherent femtosecond light pulses,” J. Opt. Soc. Am. B 10, 2222–2232 (1993). [CrossRef]
23.	D. Jones, S. Diddams, J. Ranka, A. Stentz, R. Windeler, J. Hall, and S. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000). [CrossRef] [PubMed]
24.	F. Tavella, K. Schmid, N. Ishii, A. Marcinkevicius, L. Veisz, and F. Krausz, “High-dynamic range pulse-contrast measurements of a broadband optical parametric chirped-pulse amplifier,” Appl. Phys. B 81, 753–756 (2005). [CrossRef]
25.	C. Iaconis and I. Walmsley, “Self-referencing spectral interferometry for measuring ultrashort optical pulses,” IEEE J. Sel. Top. Quantum Electron . 35, 501–509 (1999). [CrossRef]
26.	M. Kakehata, H. Takada, Y. Kobayashi, K. Torizuka, H. Takamiya, K. Nishijima, T. Homma, H. Takahashi, K. Okubo, S. Nakamura, and Y. Koyamada, “Carrier-envelope-phase stabilized chirped-pulse amplification system scalable to higher pulse energies,” Opt. Express 12, 2070–2080 (2004). [CrossRef] [PubMed]

OCIS Codes
(140.7090) Lasers and laser optics : Ultrafast lasers
(190.4970) Nonlinear optics : Parametric oscillators and amplifiers
(320.5520) Ultrafast optics : Pulse compression

ToC Category:
Ultrafast Optics

History
Original Manuscript: July 7, 2008
Revised Manuscript: August 20, 2008
Manuscript Accepted: August 21, 2008
Published: August 29, 2008

Citation
S. Adachi, N. Ishii, T. Kanai, A. Kosuge, J. Itatani, Y. Kobayashi, D. Yoshitomi, K. Torizuka, and S. Watanabe, "5-fs, multi-mJ, CEP-locked parametric chirped-pulse amplifier pumped by a 450-nm source at 1 kHz," Opt. Express 16, 14341-14352 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-19-14341

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References

R. Kienberger, E. Goulielmakis, M. Uiberacker, A. Baltuska, V. Yakovlev, F. Bammer, A. Scrinzi, T. Westerwalbesloh, U. Kleineberg, U. Heinzmann, M. Drescher, and F. Krausz, "Atomic transient recorder," Nature 427, 817-821 (2004). [CrossRef] [PubMed]
T. Sekikawa, A. Kosuge, T. Kanai, and S. Watanabe, "Nonlinear optics in the extreme ultraviolet," Nature 432, 605-608 (2004). [CrossRef] [PubMed]
G. Sansone, E. Benedetti, F. Calegari, C. Vozzi, L. Avaldi, R. Flammini, L. Poletto, P. Villoresi, C. Altucci, R. Velotta, S. Stagira, S. De Silvestri, and M. Nisoli, "Isolated single-cycle attosecond pulses," Science 314, 443-446 (2006). [CrossRef] [PubMed]
A. Kosuge, T. Sekikawa, X. Zhou, T. Kanai, S. Adachi, and S. Watanabe, "Frequency-resolved optical gating of isolated attosecond pulses in the extreme ultraviolet," Phys. Rev. Lett. 97, 263,901 (2006). [CrossRef]
M. Nisoli, S. DeSilvestri, O. Svelto, R. Szipocs, K. Ferencz, C. Spielmann, S. Sartania, and F. Krausz, "Compression of high-energy laser pulses below 5 fs," Opt. Lett. 22, 522-524 (1997). [CrossRef] [PubMed]
J. Sung, J. Park, T. Imran, Y. Lee, and C. Nam, "Generation of 0.2-TW 5.5-fs optical pulses at 1 kHz using a differentially pumped hollow-fiber chirped-mirror compressor," Appl. Phys. B 82, 5-8 (2006). [CrossRef]
X. Zhou, T. Kanai, D. Yoshitomi, T. Sekikawa, and S. Watanabe, "Generation of high average power, 7.5-fs blue pulses at 5 kHz by adaptive phase control," Appl. Phys. B 81, 13-17 (2005). [CrossRef]
C. Hauri, A. Trisorio, M. Merano, G. Rey, R. Lopez-Martens, and G. Mourou, "Generation of high-fidelity, down-chirped sub-10 fs mJ pulses through filamentation for driving relativistic laser-matter interactions at 1 kHz," Appl. Phys. Lett. 89, 151,125 (2006). [CrossRef]
A. Dubietis, G. Jonusauskas, and A. Piskarskas, "Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in BBO crystal," Opt. Commun. 88, 437-440 (1992). [CrossRef]
N. Ishii, L. Turi, V. Yakovlev, T. Fuji, F. Krausz, A. Baltuska, R. Butkus, G. Veitas, V. Smilgevicius, R. Danielius, and A. Piskarskas, "Multimillijoule chirped parametric amplification of few-cycle pulses," Opt. Lett. 30, 567-569 (2005). [CrossRef] [PubMed]
S. Witte, R. Zinkstok, A. Wolf, W. Hogervorst, W. Ubachs, and K. Eikema, "A source of 2 terawatt, 2.7 cycle laser pulses based on noncollinear optical parametric chirped pulse amplification," Opt. Express 14, 8168-8177 (2006). [CrossRef] [PubMed]
F. Tavella, A. Marcinkevicius, and F. Krausz, "90 mJ parametric chirped pulse amplification of 10 fs pulses," Opt. Express 14, 12,822-12,827 (2006). [CrossRef]
S. Adachi, H. Ishii, T. Kanai, N. Ishii, A. Kosuge, and S. Watanabe, "1.5 mJ, 6.4 fs parametric chirped-pulse amplification system at 1 kHz," Opt. Lett. 32, 2487-2489 (2007). [CrossRef] [PubMed]
Y. Nabekawa, Y. Kuramoto, T. Togashi, T. Sekikawa, and S. Watanabe, "Generation of 0.66-TW pulses at 1 kHz by a Ti: sapphire laser," Opt. Lett. 23, 1384-1386 (1998). [CrossRef]
T. Fuji, N. Ishii, C. Teisset, X. Gu, T. Metzger, A. Baltuska, N. Forget, D. Kaplan, A. Galvanauskas, and F. Krausz, "Parametric amplification of few-cycle carrier-envelope phase-stable pulses at 2.1 mu m," Opt. Lett. 31, 1103-1105 (2006). [CrossRef] [PubMed]
C. Hauri, P. Schlup, G. Arisholm, J. Biegert, and U. Keller, "Phase-preserving chirped-pulse optical parametric amplification to 17.3 fs directly from a Ti: sapphire oscillator," Opt. Lett. 29, 1369-1371 (2004). [CrossRef] [PubMed]
N. Ishii, C. Teisset, T. Fuji, S. Kohler, K. Schmid, L. Veisz, A. Baltuska, and F. Krausz, "Seeding of an eleven femtosecond optical parametric chirped pulse amplifier and its Nd3+ picosecond pump laser from a single broadband Ti: Sapphire oscillator," IEEE J. Sel. Top. Quantum Electron. 12, 173-180 (2006). [CrossRef]
A. Shirakawa, I. Sakane, M. Takasaka, and T. Kobayashi, "Sub-5-fs visible pulse generation by pulse-frontmatched noncollinear optical parametric amplification," Appl. Phys. Lett. 74, 2268-2270 (1999). [CrossRef]
M. Zavelani-Rossi, G. Cerullo, S. De Silvestri, L. Gallmann, N. Matuschek, G. Steinmeyer, U. Keller, G. Angelow, V. Scheuer, and T. Tschudi, "Pulse compression over a 170-THz bandwidth in the visible by use of only chirped mirrors," Opt. Lett. 26, 1155-1157 (2001). [CrossRef]
A. Baltuska, T. Fuji, and T. Kobayashi, "Visible pulse compression to 4 fs by optical parametric amplification and programmable dispersion control," Opt. Lett. 27, 306-308 (2002). [CrossRef]
B. Zhao, X. Liang, Y. Leng, C. Wang, Y. Jiang, J. Du, and Z. Xu, "Optimization of optical parametric chirped pulse amplification with different pump wavelengths," Opt. Commun. 259, 137-141 (2006). [CrossRef]
R. Danielius, A. Piskarskas, A. Stabinis, G. Banfi, P. Ditrapani, and R. Righini, "Traveling-wave parametric generation of widely tunable, highly coherent femtosecond light pulses," J. Opt. Soc. Am. B 10, 2222-2232 (1993). [CrossRef]
D. Jones, S. Diddams, J. Ranka, A. Stentz, R. Windeler, J. Hall, and S. Cundiff, "Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis," Science 288, 635-639 (2000). [CrossRef] [PubMed]
F. Tavella, K. Schmid, N. Ishii, A. Marcinkevicius, L. Veisz, and F. Krausz, "High-dynamic range pulse-contrast measurements of a broadband optical parametric chirped-pulse amplifier," Appl. Phys. B 81, 753-756 (2005). [CrossRef]
C. Iaconis and I. Walmsley, "Self-referencing spectral interferometry for measuring ultrashort optical pulses," IEEE J. Sel. Top. Quantum Electron. 35, 501-509 (1999). [CrossRef]
M. Kakehata, H. Takada, Y. Kobayashi, K. Torizuka, H. Takamiya, K. Nishijima, T. Homma, H. Takahashi, K. Okubo, S. Nakamura, and Y. Koyamada, "Carrier-envelope-phase stabilized chirped-pulse amplification system scalable to higher pulse energies," Opt. Express 12, 2070-2080 (2004). [CrossRef] [PubMed]

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