Progress in chirped pulse optical parametric amplifiers - Springer Link

Appl. Phys. B 79, 693–700 (2004)

Applied Physics B

DOI: 10.1007/s00340-004-1614-3

Lasers and Optics

Progress in chirped pulse optical parametric amplifiers

r. butkus r. danielius a. dubietisu a. piskarskas a. stabinis

Department of Quantum Electronics, Vilnius University, Saul˙etekio Ave. 9, bldg. 3, 01222 Vilnius, Lithuania

Received: 24 May 2004/Revised version: 15 July 2004 Published online: 15 September 2004 • © Springer-Verlag 2004 ABSTRACT We discuss the main issues of optical parametric chirped pulse amplification and overview recent progress in the field. Although we distinguish between the two operating modes of modern chirped pulse parametric amplifiers, OPCPA and NOPA, we reveal that both represent the same technique and share a common concept. PACS 42.65.Yj;

1

42.65.Re

Introduction

There was remarkable progress in ultra-fast lasers during the last 20 years [1–3]. Recent developments in laser technology promise new challenging tasks and guidelines in ultra-fast optics [4] and in extreme intensities, opening fundamentally new domains of physics [5]. Rapid development of laser systems and increasing demand of new wavelengths, not supported by lasing media, boosted OPA/OPGs on the top of extensive research. Since the very first demonstrations of an optical parametric amplifier (OPA) and an optical parametric generator (OPG) carried out more than three decades ago [6, 7], the development of novel nonlinear techniques and the advent of efficient nonlinear crystals made it possible to demonstrate the full range of unique possibilities of OPA/OPGs producing pico- and femtosecond pulses broadly tunable from 0.2 up to 20 micrometers; see [8, 9] for extensive reviews. Nowadays, many fields of ultra-fast science, such as spectroscopy in physics, biology, chemistry and medicine, are hardly imagined without implementation of OPAs. Modern commercially available OPG/OPA systems could be matched to be pumped with a variety of ultra-short-pulse laser sources operating at different wavelengths [10]. Despite some disadvantages (lack of pump energy accumulation, limited aperture of the nonlinear crystals, tough requirements of pump and signal synchronization and losses introduced by the idler wave), an OPA is very promising due to extremely broad amplification bandwidth, high gain per single pass, energy combining of uncorrelated and incoherent pump beams, phase-conjugated output signal and idler u Fax: +370-5-2366-006, E-mail: [email protected]

pulses, high output-beam quality and high energy and intensity contrast ratio. Optical parametric amplification offers several advantages when compared to amplification in laser amplifiers, including absence of amplified spontaneous emission, low thermal effects and great wavelength flexibility. Although an OPA has to be operated with precise pump and signal timing, the spatial and temporal characteristics of the seed signal are not altered during the amplification process. Broad amplification bandwidth of the OPA (formally limited by the nonlinear crystal transparency range) and high gain achievable per single pass make these devices attractive for optical parametric chirped pulse amplifiers (OPCPAs). The OPCPA principle has been proposed and demonstrated for the first time by Dubietis et al. [11]. The idea to use an energetic nanosecond pump to amplify a chirped and stretched pulse and to boost ultra-short-pulse power to the terawatt (TW) level has been clearly formulated by the authors. Later on, an extensive development of the OPCPA technique has been continued by Ross et al. [12]. Other important work has been carried out by Wilhelm et al. [13], exploiting the extremely broad amplification bandwidth of noncollinear optical parametric amplifiers (NOPAs). Five years later this technique provided optical pulses shorter than 4 fs, as demonstrated by Baltuˇska et al. [14]. Eventually, most of the NOPAs share basic aspects with the OPCPA technique, since they involve chirped pulse amplification and compression. Nevertheless, there are two main distinctive features of the NOPA technique: (i) a white-light-continuum seed and a pump pulse are derived from the same femtosecond laser; (ii) the output pulse can be broadly tunable or compressed to a few-cycle limit. Practical implementation of a NOPA system is much simpler than that of a typical OPCPA setup; however, in the meantime its application is mostly limited to pump– probe spectroscopy. In this paper we overview the basic principles and development of optical parametric chirped pulse amplification based systems, covering the amplification of extremely short (few-cycle) and ultra-high-power (terawatt) optical pulses. 2

Chirped pulse OPA concept

The chirped pulse parametric amplification technique originated from the chirped pulse amplification (CPA) concept proposed for high-energy laser amplifiers [15], which nowadays lies in the background of almost any ultra-short-

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Applied Physics B – Lasers and Optics

pulse laser system. The idea of CPA is to amplify an ultrashort laser pulse without the onset of self-action effects and intracavity material damage imposed by optical nonlinearities in the amplifier. This is done by stretching the pulse in time, which makes it possible to efficiently extract energy from the laser gain medium. Thereafter, the amplified pulse is recompressed to its original duration. Depending on configuration, several amplification stages could be involved. The OPCPA concept maintains all the basic requisites, namely pulse stretcher and compressor, except for the laser amplifier being replaced by the OPA (Fig. 1). The gain bandwidth of a conventional collinear OPA in the simplest case (monochromatic pump, plane-wave approximation and low pump depletion) can be derived by solving a set of truncated three-wave interaction equations [16]: ∂As 1 ∂As i ∂ 2 As =− + gs 2 + iσs A∗i Ap , ∂z u s ∂t 2 ∂t 1 ∂Ai i ∂ 2 Ai ∂Ai =− + gi 2 + iσi A∗s Ap , ∂z u i ∂t 2 ∂t ∂Ap 1 ∂Ap i ∂ 2 Ap =− + gp 2 + iσp As Ai , ∂z u p ∂t 2 ∂t

(1)

where As , Ai and Ap are complex amplitudes of signal, idler and pump waves, respectively. u , g and σ are the group velocity, group-velocity dispersion (GVD) and nonlinear coupling coefficients of signal, idler and pump waves denoted by the subscript. In this approximation, the gain bandwidth of the OPA in wavenumbers is given by [8] ∆ν = 0.53 Γ/L | u si | /c, u s = u i , (2) ∆ν = 0.58(Γ/L)1/4 | gs + gi |−1/2 /c,

us = ui ,

that, in the most general case, with pump pulse depletion accounted for, the amplification bandwidth can be estimated numerically. As will be discussed in Sect. 3, the amplification bandwidth can be notably extended by use of noncollinear geometry (see [9, 17] for details), promising more than a full octave gain bandwidth to be achieved [18, 19]. A unique feature of the OPA to amplify phase-modulated pulses has been experimentally demonstrated for the first time by Piskarskas et al. in 1986 [20]. It has been shown that phase modulation of the signal pulse is not considerably distorted during the amplification process if the intrinsic OPA gain bandwidth is larger when compared to the signal spectral width [21]. It has also been demonstrated that the frequency chirp in the idler pulse is reversed, pointing to phase conjugation between the signal and idler pulses. Further extension of this work, which relied on the basis of the modern OPCPA concept, has been demonstrated a decade ago by some of the present authors [11]. In the first demonstration, spectral broadening due to selfphase modulation along with temporal stretching of input 1.7-ps pulses centered at 1055 nm was performed in a singlemode fiber, yielding linearly chirped 155-cm−1 , 5-ps seed pulses at the input of the OPA. The pump pulse was regeneratively amplified and frequency doubled, and temporally shaped to match the signal pulse width. Seed pulses were amplified in a 8-mm-long type I phase-matching BBO crystal by factor 2 × 104 with no bandwidth reduction and no impact on the chirp linearity, and compressed down to 70 fs approaching the time–bandwidth limit (Fig. 2). Although stretching of the

(3)

where 1/u si = 1/u s − 1/u i is the group-velocity mismatch between the signal and idler pulses, gs,i = ∂ 2 ks,i /∂ω2 |ω=ωp and L is the medium (interaction) length. Γ is the parametric gain factor ωs ωi Ip Γ = deff 2 , (4) ε0 n s n i n p c3 where Ip is the pump intensity, deff is the effective nonlinear coefficient, ωs and ωi are the frequencies of the signal and idler waves, respectively, and n denotes the refractive index of the waves labeled by the subscript. A simple estimation for a type I phase-matching KDP crystal at degeneracy (λp = 527 nm, Ip = 30 GW/cm2 , L = 1 cm) yields the gain bandwidth as broad as 1900 cm−1 ; more examples for BBO and KDP crystals are provided in [8]. It has to be noted

FIGURE 1

Principle scheme of chirped pulse parametric amplification

FIGURE 2 Schematic (a) and experimental results (b, c) of the first OPCPA demonstration [11]

BUTKUS et al.


695

seed signal pulse was accomplished by means of dispersion in an optical fiber, pulse stretching by several orders of magnitude using a grating-pair setup to match (sub)nanosecond pump pulses was suggested. 3

OPCPA: towards extremely high-power pulses

With growing interest in the development of ultrahigh-power (TW to PW), ultra-short-pulse laser systems, OPCPA has been identified as a feasible amplification technique [12, 22] and became an attractive alternative to existing broadband Ti:sapphire laser systems [23]. In recent years there was rapid progress in design of OPA-based systems towards more stable, more powerful and shorter pulses in the TW peak power range [24–28]. There are several peculiarities in high output power OPCPA systems, which distinguish them from low-energy systems described in Sect. 4. Usually, such powerful systems are pumped by nanosecond or sub-nanosecond Nd:glass or Nd:YAG laser pulses. The seed pulse is mostly taken from a separate properly synchronized femtosecond Ti:sapphire laser. The OPA is operated around the degeneracy, using several amplification stages. Typically, the last amplifier stage employs a nonlinear crystal, which can be grown in large dimensions to withstand high pump energies, e.g. KDP or LBO. Thus high total gain and high output pulse energies can be obtained, but the achieved compressed pulse duration is not less than a few hundreds of femtoseconds to date. Large dimensions of lasing media required to extract reasonable energy impose strict limitations on the repetition rate; the highest repetition rate in today’s powerful OPCPA systems does not exceed 10 Hz [23], while the most powerful systems operate in a single-shot regime. The progress towards high output powers in OPCPA systems is illustrated in Fig. 3. The plotted dots represent the highest experimental achievements in this field; the details are

Reference

Year

λp , nm

λs , nm

τp , ns

τs , ns

Dubietis et al., [11]

1992

527

1055

0.008

0.005

Ross et al., [12]

1997

527

1053

0.5

0.5

Ross et al., [22]

2000

527

1054

0.6

0.3

2.5

Yang et al., [24]

2002

532

1064

0.5

0.15

Jovanovic et al., [26]

2002

532

1054

8.5

Yoshida et al., [27]

2003

532

1053

Waxer et al., [28]

2003

527

1054

FIGURE 3

Progress in OPCPA: towards high output powers

listed in Table 1. Further, we briefly discuss peculiarities of every system. The pioneering work in this field [11], described in Sect. 2, stands as a starting point in the plotted trend. In 1997, a detailed project was presented by Ross et al. [12], offering several practical designs of OPCPA configurations and promising output powers above 10 PW. Since then, no one has achieved the anticipated powers; however, there has been significant progress towards them. The major challenge is the compression of the nanosecond OPCPA output down to the sub-20-fs range. More than 1 TW of output power has been achieved in an OPCPA system with two amplifiers [22]. A total energy gain of 1010 and output pulse energies of 0.5 J have been reported using LBO and KDP crystals. Amplified pulses were recompressed to 300 fs. The continuous development of the latter laser system promises much higher output energies [29]. A powerful OPCPA system has been demonstrated by Yang

G

η, %

E out , J (E comp , J)

τout , fs

Pout , TW

2 × 104

3

70

9 × 10−4∗)

2 × 1011

30

10−4 (6.5 × 10−5 ) 400 (224)

21

11000∗∗)

1010

20

0.5 (0.4)

300

1.3

5

4 × 1010

18

0.9 (0.57)

155

3.67

3

0.51

6 × 107

6

0.031 (0.015)

310

0.05

9.5

3

0.29

108

23

0.065 (n.a.)

350

< 0.18

1

0.7

0.025

6 × 106

14

0.005 (n.a.)

470

< 0.01

Ep , J 0.003 1490

Crystal; L c , mm

p.r.r., Hz

BBO ; 8

1.5

LBO ; 1.5 LBO ; 6.8 KDP ; 28.6 LBO ; 20 KDP ; 30 LBO ; 15 LBO ; 18 KDP ; 30 BBO ; 15 BBO ; 15 BBO ; 10 BBO ; 15 BBO ; 15 LBO ; 25 LBO ; 23

n.a. n.a. n.a. 10 6 5

λp - pump wavelength, λs - seed wavelength, τp - pump pulse duration, τs - seeded pulse duration, E p - pump pulse energy, G - gain, η - pump-to-signal energy conversion efficiency, E out and E comp - output pulse energy before and after compression, respectively, τout - output pulse duration, Pout - output power, L c crystal length, p.r.r. - pulse repetition rate. ∗) a pioneering OPCPA experiment; ∗∗) project in prospect TABLE 1

Progress in (sub-)ns-pulse-pumped chirped pulse OPA

696


et al. [24]. Chirped pulses were amplified from 20 pJ to 900 mJ in two LBO optical parametric preamplifiers and in a final KDP OPA pumped by 5-J frequency-doubled (532 nm) pulses extracted from Nd:YAG–Nd:glass hybrid amplifiers. With a total gain of 4 × 1010, 155-fs output pulses of 579-mJ energy with a peak power of 3.67 TW were obtained after the compression stage. Further improvements over the latter system have led to the most powerful OPCPA system designed to date, delivering femtosecond pulses with 16.7-TW peak power [25]. Another powerful OPCPA system is under development employing a terawatt iodine laser system as a pump laser [30]. There are numerous OPCPA systems aiming at relatively compact, stable and efficient table-top sub-TW operation. A relatively simple system was designed to replace a conventional Ti:sapphire regenerative-amplifier-based CPA system [23]. Broadband oscillator pulses at 1054 nm were amplified to 31 mJ and then compressed to 310 fs. Long pulse durations were attributed to spherical and chromatic aberrations in the stretcher setup, and the repetition rate of 10 Hz is the highest figure achieved in today’s powerful OPCPA systems. Another extension of this work was based on a novel hybrid chirped pulse amplification scheme that used a single pump pulse and combined OPA and laser amplification [26]. Two BBO crystal based OPAs served as a preamplifier, and a final amplification stage was realized in a 10-cm-long Ti:sapphire crystal in four-pass configuration. While the pump-to-signal conversion efficiency after the two-stage OPA has not exceeded 1%, after the final amplification stage it was increased to 29%. The Fourier-transform limit of the amplified output spectrum was calculated to be 30 fs; however, no compression was accomplished. A laser system based on two BBO OPA crystals was developed providing a total gain of 108 and an energy-conversion efficiency of ∼ 23% [27]. The seed pulse at 1053 nm was amplified in single-pass geometry under 10-nslong pump pulses. The suppression of prepulses was addressed as an important issue for the future CPA-based power amplifiers. And, the most efficient OPCPA system designed to date was demonstrated to deliver 5-mJ pulses at 5-Hz repetition rate with a pump-to-signal conversion efficiency of 29% [28]. Carefully performed spatiotemporal pump pulse shaping served as the key element for highly efficient OPA operation and resulted in a highly stable output pulse. Considering the aforementioned techniques, an even more efficient OPCPA system is under development [31]. There are also several projects under development concerning the implementation of the OPCPA technique that remained without proper temporal characterization using BBO [32, 33] and KD∗ P crystals [34]. An important property of OPA, which has been rarely exploited, is that OPA permits the use of multiple pump beams if each of them satisfies the phase-matching condition with a single signal beam. This possibility is predefined by the nature of the parametric amplification process; the phase difference between the pump and signal pulses is transferred to the idler pulse, thus compensating for random differences between these two. Originally, multiple-beam-pumped OPA was proposed for narrowing the spectral and spatial bandwidth of the emitted parametric superfluorescence [35, 36]. In seeded OPA,

multiple-beam pumping yielded a number of benefits if compared to a single-beam pump. Energy combining by the use of multiple-beam-pumped OPA was demonstrated by some of the present authors; the amplified signal energy exceeded that of the single pump pulse, and almost 80% conversion efficiency has been reported with no impact on the pulse temporal profile [37]. As a more general case, an OPA pumped by several incoherent and uncorrelated (second and third harmonics of an Nd:YAG laser) has been demonstrated [38]. Another relevant feature of the multiple-beam-pumped OPA has been demonstrated recently, showing that multiple pump beams used in a proper geometry could notably extend the amplification bandwidth in the case of the chirped broadband seed [39]. The multiple beam pump approach has been considered to be feasible also for pumping periodically poled lithium niobate (PPLN) optical parametric oscillators by mutually incoherent fiber lasers whose beams intersect along a phase-matching cone [40]. The major benefit emerging from the use of the multiplebeam pump is that the synchronized incoherent low-tomedium power pump sources can be used to amplify a single signal beam, thus potentially increasing the repetition rate. On the other hand, many low-energy pump sources can replace one high-energy pump laser, usually used in powerful OPCPA systems. Summarizing, the aforementioned scheme could be potentially useful in desirable systems with pulses of high energy, low duration but high repetition rate, yielding very high average output power. 4

NOPA: towards ultimate short and tunable light pulses

Although OPA intrinsically exhibits a broad amplification bandwidth, defined by the temporal walk-off between the signal and idler pulses, it becomes too narrow to support ultimately short pulses maintaining a reasonable gain, however. Direct generation of shortest pulses in collinear OPA geometry without compression utilities of 14.5 fs has been demonstrated [41]. It has been widely recognized that OPA may exhibit an extremely broad amplification bandwidth at the degeneracy, and in some other particular cases predefined by the choice of nonlinear crystal and wavelength [42, 43]. Another method to expand the amplification bandwidth was proposed for multipass OPA, where either each pass was independently tuned to amplify a different spectral portion of the continuum seed source [44] or by the use of several slightly phase-mismatched pump beams in a single pass [39]. Application of noncollinear geometry enables one to create a degeneracy-like situation over all the tuning range. This feature of the noncollinear interaction has been considered both theoretically and experimentally by a number of research groups [45–48], and a combination of this approach with seeding by a femtosecond white-light continuum and a prism compressor resulted in a versatile device capable of delivering sub-20-fs pulses tunable in the visible, which became known as NOPA [13]. Later on, several groups contributed to its further development, mainly in the direction of pulse shortening. The main issue addressed was adjusting the negative GVD of the compressor to compensate for the continuum chirp as well as for the positive GVD of the optical components used. Minimization of the amount of dispersive material

BUTKUS et al.


along with the application of a small apex angle prism compressor and chirped mirrors has led to generation of sub-8-fs pulses [49, 50]. Precompression of the seed pulse prior to amplification and compression using prism/grating or prism/chirped mirror combinations enabled the production of sub-5-fs pulses [51, 52]. The pulse-front-matched pump geometry has been realized in a NOPA system providing sub-10-fs pulses tunable in the visible and the near infrared (IR) [51], with even shorter pulses of 6.1-fs duration tunable in the visible (from 550 to 700 nm). The same group reported a similar NOPA system one year later, delivering pulses as short as 4.7 fs [52]. The development of low-energy OPCPA-based systems (NOPA) towards ultimate short pulses is depicted in Fig. 4 with a more detailed list of achievements presented in Table 2. Usually, white light continuum seeded noncollinear BBO OPAs are pumped by conventional frequency-doubled 1-kHz Ti:sapphire amplifiers. High-pulse-repetition-rate pump sources deliver low energy but relatively high average output power. It should be noted that in the tunable systems, many parameters vary within the tuning range. These parameters include gain, energy-conversion efficiency, output pulse energy and pulse duration. Performing several improvements over the latest works, the parametric amplification generation of a continuous, simultaneously phase-matched 250-THz spectrum was demonstrated [14]. The resultant visible–near-IR signal wave pulses were compressed down to 3.9-fs duration, which is the shortest figure to date, obtained in a modern NOPA configuration (Fig. 5). These improvements included seed precompression, pump pulse stretching and utilization of gain bandwidth broadening due to introduced angular dispersion of the pump beam. Feedback with iterative optimization based on secondharmonic generation (SHG) frequency-resolved optical gating (FROG) was applied to match the spectral phase of the

697

Progress in OPCPA/NOPA systems: towards ultimate short pulses. The full dots represent systems based on BBO crystals and open dots represent OPCPA systems with periodically poled materials

FIGURE 4

parametrically amplified chirped pulse for achieving optimal compression. The improved compressor consisted of a pair of chirped mirrors and a grating dispersion line with a micromachined flexible mirror. Along with an obvious advantage of broad amplification bandwidth, application of noncollinear geometry creates problems as well. The most important is the limitation on the applicable pump energy due to the small spot size within which the pulses are overlapped in time. This limits the output pulse energy to ∼ 10 µJ and even less if the spectral content is to be preserved within the entire beam. Another shortcoming is that the use of the idler wave is not as straightforward due to spatial dispersion of the idler beam. At least two solutions for the removal of dispersion and spatial chirp have been suggested so far, employing either a diffraction

Reference

Year

λp nm

τp , fs

Ep , µJ

η, %

E out , µJ (E comp , µJ)

Dubietis et al., [11] Yakovlev et al., [60] Wilhelm et al., [13] Cerullo et al., [49] Shirakawa et al., [61] Cerullo et al., [50] Shirakawa et al., [51] Cerullo et al., [62] Shirakawa et al., [52] Armstrong et al., [63] Zavelani-Rossi et al., [64] Baltuˇska et al., [14] Clark-MXR, Inc., [58] Light Conversion Ltd., [59]

1992 1994 1997 1997 1998 1998 1998 1999 1999 2001 2001 2002 1999 2003

527 800 405 390 395 390 395 390 395 387 390 395 387 400

8000 100 65 180 150 180 150 180 150 120 180 > 150 150 150

3000 250 40 7 100 10 130 10 100 75 10 50 200 500

3 20 25 15 3 20 4 20 6 7 20 10 5 10

100 (65) 50 (n.a.) 11 (n.a.) 1 (n.a.) 3 (n.a.) 2 (1.8) 5 (0.5) 2 (n.a.) 6 (5) 5 (2) 2 (n.a.) 5 (0.5) n.a. (10) n.a. (50)

Galvanauskas et al., [54] Rotermund et al., [55] Petrov et al., [56]

1998 2002 2003

786 1064 1064

5000 850 1000

20 2.4 8.5

1000 (600) 20 (13) 85 (55)

680 320 270

0.9 0.04 0.2

I. Jovanovic et al., [57]

2003

532

700

6.5

45 (22)

390

0.06

0.5 ns 1 ns 1 ns 6 ns

τout , fs 70 30 16 11 14 7.2 6.1 7.5 4.7 7 5.7 4 10 7.9

Pout , GW

Crystal; L c , mm

Tunability range, nm

0.9 < 1.6 < 0.7 < 0.1 < 0.2 0.25 0.08 < 0.26 1 0.29 < 0.35 0.125 1 6

BBO ; 8 BBO ; 7 BBO ; 2 BBO ; 1 BBO ; 1 BBO ; 1 BBO ; 1 BBO ; 1 BBO ; 1 BBO ; 1 BBO ; 1 BBO ; 1 n.a. BBO ; n.a.

n.t. 1100-1300 475-750∗) 545-650 550-690 n.t. 550-700∗) n.t. n.t. n.t. n.t. n.t. 470-650 490-750

PPLN ; 5 PPKTP ; 15 PPKTP ; 5 PPKTP ; 7 PPKTP ; 15

n.t. n.t. n.t. n.t.

λp - pump wavelength, τp - pump pulse duration, E p - pump pulse energy, η - pump-to-signal energy conversion efficiency, E out and E comp - output pulse energy before and after compression respectively, τout - output pulse duration, Pout - output power, L c - crystal length, n.t. - not tunable. ∗) denotes a tunable idler wave also TABLE 2 Progress in ultra-short-pulse-pumped chirped pulse OPA

698


repetition rate. A similar setup with a PPKTP crystal has been demonstrated operating far from degeneracy and achieving 20 µJ of amplified signal pulse energy and 3.3 × 105 gain at 1573 nm [55]. A repetition rate of 1 kHz was achieved thanks to an all-solid-state laser system. The refinement of the latter system has led to development of a two-stage PPKTP OPCPA system with 85-µJ and 270-fs output pulses [56]. In a different setup [57], a PPKTP crystal in a collinear OPA configuration was used as a preamplifier for an Nd:glass laser based CPA system. Due to rapid development of knowledge and technology and increasing demand in various fields of applications, commercial NOPA systems became available. A commercial tunable sub-30 fs and up to 10 µJ energy per pulse NOPA system is available from Clark-MXR, Inc. [58]. An energyscalable NOPA system, Topas-white, designed by Light Conversion Ltd. [59], offers even shorter and the most powerful ultra-short broadly tunable pulses to date. Topas-white delivers pulses as short as 7.9 fs with a maximum energy of 50 µJ. Pumped by a Ti:sapphire laser, it produces a whitelight seed and therefore a signal wave can be tuned within the 490– 750-nm range (Fig. 6). This device employs a twostage parametric amplifier with a 1-mm-thick BBO crystal, a diffraction grating based signal stretcher with a fixed phase mask for GVD control and a compact, low-loss positive GVD pulse compressor. A substantial increase in output power is achieved by using tilted pump pulses, which increases the interaction area and thus the applicable pump pulse energy. Diverse modifications of this product extend the tuning range even further into the infrared region. An important characteristic of few-optical-cycle pulses is the stability of the carrier envelope phase, which originates from frequency fluctuations of phase-locked modes that comprise the pulse spectrum. Although it has been demonstrated that a white-light continuum pulse, which is usually

FIGURE 5 Results of the shortest achieved pulses in NOPA [14]: a SHG FROG traces after adaptive phase correction, b bandwidth of amplified signal and c autocorrelation trace of 3.9-fs pulse

grating [51] or a two-prism sequence [53]; however, the use of complex adaptive optics introduces considerable energy losses as well. Considerable efforts utilizing the OPCPA technique, which cannot be straightforwardly addressed as streaming towards high output powers or ultimate short pulses, are directed towards more compact and low-power systems. These include systems where the use of periodically poled materials, such as LiNbO3 (PPLN) or KTiPO4 (PPKTP), instead of the more common BBO, is preferred. High pulse energies were obtained in a setup proposed by Galvanauskas et al. [54] demonstrating several novel approaches – an Er-doped fiber laser was used as a seed source at 1550 nm, being electronically synchronized with the pump laser (alexandrite amplifier). A PPLN crystal was used to deliver 1-mJ pulses at 10-Hz

FIGURE 6 Operational characteristics of TOPAS-white: a tuning curve, b autocorrelation trace of 7.9-fs pulse and c spectrum

BUTKUS et al.


employed to seed NOPA, is fully phase coherent [65], and this coherence is completely preserved in the parametric amplification process [66], the problem of the stability of the carrier envelope phase still remained open. This shortcoming is attributed mainly to phase instability of the pump laser source. Baltuˇska et al. [67] suggested an all-optical method for control of the carrier envelope phase by means of a phase link between signal, idler and pump waves in the parametric amplifier. This latest achievement promises challenging applications of few-cycle optical pulses, such as generation of high-order harmonics and production of X-ray pulses with attosecond duration [68]. 5

Conclusions

In this review we briefly discussed a 10-year progress in the field of chirped pulse parametric amplification. Although we distinguish between the two operating modes of modern chirped pulse parametric amplifiers, OPCPA and NOPA, we reveal that both represent the same technique and share a common concept. The most outstanding experimental achievements reported to date are 120-fs, ∼ 2-J pulses for the OPCPA, and 4-fs, 0.5-µJ pulses for the NOPA, respectively. Naturally, the major challenge is to combine these two techniques and attain powerful few-femtosecond pulses employing a noncollinear multistage OPA and high-power pump lasers. Very recently, an OPCPA system pumped by 5-ps pulses has been modeled numerically [69] and demonstrated experimentally [70], yielding ∼ 17-fs, 85-µJ pulses. Another relevant step towards demonstrating the advantages of a combination of OPCPA and NOPA approaches reports noncollinear amplification of 110-THz spectral bandwidth pulses up to 8 mJ at 800 nm, using a stretched broadband seed signal derived from a Ti:sapphire mode-locked oscillator and amplified in BBO crystal pumped by synchronized sub-nanosecond pulses of an Nd:YAG laser [71]. We foresee that in the near future these results will be improved, bringing the OPCPA-based systems to the top of the list of high-peak-power and ultimately short-pulse lasers for applications in various fields of research, for instance attosecond and related science. REFERENCES 1 G. Mourou: Appl. Phys. B 65, 205 (1997) 2 S. Backus, C.G. Durfee III, M.M. Murnane, H.C. Kapteyn: Rev. Sci. Instrum. 69, 1207 (1998) 3 U. Keller: Nature 424, 831 (2003) 4 T. Brabec, F. Krausz: Rev. Mod. Phys. 72, 545 (2000) 5 G. Mourou, T. Tajima: Phys. Rev. Spec. Top. 5, 031301 (2002) 6 S.A. Akhmanov, A.I. Kovrigin, A.S. Piskarskas, V.V. Fadeev, R.V. Khokhlov: JETP Lett. 2, 191 (1965) 7 K.P. Burneika, M.V. Ignatavichus, V.I. Kabelka, A.S. Piskarskas, A.Yu. Stabinis: JETP Lett. 16, 257 (1972) 8 R. Danielius, A. Piskarskas, A. Stabinis, G.P. Banfi, P. Di Trapani, R. Righini: J. Opt. Soc. Am. B 10, 2222 (1993); see also R. Byer, A. Piskarskas (eds.): ‘Feature Issue on Optical Parametric Oscillation and Amplification’, J. Opt. Soc. Am. B 9, 1656–1791 (1993) and 10, 2148– 2243 (1993) 9 G. Cerullo, S. De Silvestri: Rev. Sci. Instrum. 74, 1 (2003) 10 A. Piskarskas: Opt. Photon. News 7, 25 (1997) 11 A. Dubietis, G. Jonuˇsauskas, A. Piskarskas: Opt. Commun. 88, 437 (1992) 12 I.N. Ross, P. Matousek, M. Towrie, A.J. Langley, J.L. Collier: Opt. Commun. 144, 125 (1997)

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

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