Nondegenerate parametric generation of 2.2-mJ, few-cycle 2.05-m pulses using a mixed phase matching scheme Guibao Xu, Scott F. Wandel, and Igor Jovanovic Citation: Review of Scientific Instruments 85, 023102 (2014); doi: 10.1063/1.4865132 View online: http://dx.doi.org/10.1063/1.4865132 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Tunable 1.6–2m near infrared few-cycle pulse generation by filamentation Appl. Phys. Lett. 102, 191119 (2013); 10.1063/1.4807008 Tunable few-optical-cycle pulses with passive carrier-envelope phase stabilization from an optical parametric amplifier Appl. Phys. Lett. 90, 171111 (2007); 10.1063/1.2732834 High-gain multipass noncollinear optical parametric chirped pulse amplifier Appl. Phys. Lett. 86, 211120 (2005); 10.1063/1.1940132 Ultrabroad-band phase matching in two recently grown nonlinear optical crystals for the generation of tunable ultrafast laser radiation by type-I noncollinear optical parametric amplification J. Appl. Phys. 94, 1329 (2003); 10.1063/1.1591074 Optical parametric generation of femtosecond pulses up to 9 m with LiInS 2 pumped at 800 nm Appl. Phys. Lett. 78, 2623 (2001); 10.1063/1.1369386
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 023102 (2014)
Nondegenerate parametric generation of 2.2-mJ, few-cycle 2.05-μm pulses using a mixed phase matching scheme Guibao Xu, Scott F. Wandel, and Igor Jovanovica) Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
(Received 5 December 2013; accepted 27 January 2014; published online 12 February 2014) We describe the production of 2.2-mJ, ∼6 optical-cycle-long mid-infrared laser pulses with a carrier wavelength of 2.05 μm in a two-stage β-BaB2 O4 nondegenerate optical parametric amplifier design with a mixed phase matching scheme, which is pumped by a standard Ti:sapphire chirped-pulse amplification system. It is demonstrated that relatively high pulse energies, short pulse durations, high stability, and excellent beam profiles can be obtained using this simple approach, even without the use of optical parametric chirped-pulse amplification. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865132] I. INTRODUCTION
Few-cycle ultrashort laser pulses are an enabling tool for many areas of science, including physics, chemistry, biology, and materials science. Recent developments in ultrashort pulse laser technology make it possible to probe ultrafast dynamics with attosecond resolution.1, 2 Pulses of order 100 as can now be routinely produced by high-harmonic generation (HHG) in noble gases using compressed few-cycle laser pulses from carrier-envelope phase stabilized Ti:sapphire lasers. Further improvement of temporal resolution requires the production of extreme ultraviolet (XUV) supercontinuum radiation with higher photon energy (>100 eV). Within the tightly localized ionization region produced by the intense laser pulse, some of the liberated electrons are re-scattered on the ion core, leading to above-thresholdionized electron acceleration to kinetic energies of up to 10Up + Ip (Ref. 3) and in the case of electron-core recombination, HHG with photon energies of up to 3.17Up + Ip .4 Here, Ip denotes the atom’s ionization potential, and Up = E2 /4ω2 refers to the ponderomotive potential of the laser pulse, which depends on the electric field amplitude E and on the carrier frequency ω of the driver laser pulse. It is evident that both an increase in the drive pulse intensity and its wavelength can help generate higher harmonics. It has been demonstrated that the use of longer wavelength drive pulses can improve phase matching in HHG.5, 6 In addition, few-cycle mid-IR driving laser pulses reduce depletion of the neutral gas target by ionization before the main peak of the electric field, generating high-energy XUV photons.7 Another area of potential use for high-energy mid-IR ultrashort pulses is the novel direct particle acceleration scheme.8 For example, laser-driven particle acceleration in dielectric photonic structures9, 10 could enable very high acceleration gradients (>GV/m), even at modest laser peak powers. The use of long-wavelength drive lasers can help mitigate the problem of dielectric structure breakdown caused by a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2014/85(2)/023102/5/$30.00
multiphoton ionization; the fabrication tolerances for dielectric structures are simultaneously relaxed since they scale with the laser wavelength. Since the aperture of the structure is of order of the laser wavelength, larger apertures and the use of greater pulse energies is also possible by use of longer drive laser wavelength. Direct production of high-energy ultrashort mid-IR laser pulses is technically challenging due to the lack of broadband laser media and the general lack of technological maturity in this wavelength range. The mode-locked crystalline solid-state lasers based on Cr2+ -doped chalcogenides11, 12 and fiber oscillator/amplifiers based on Tm3+ -doped large mode area fiber13 are promising candidates; however, their level of maturity at present time is low. The available pulse durations and the accessible spectral range associated with those technologies are limited. A more flexible approach to access the mid-IR spectral region is the use of optical parametric amplification (OPA)14–18 and optical parametric chirped-pulse amplification (OPCPA).19–24 Broad gain bandwidth and favorable phase matching in certain nonlinear materials such as BiBO,14 BBO (β-BaB2 O4 ),15–18, 21 PPLN,19–21, 23 PPLT,21, 24 KTP, and KTA22 have been used to realize few-cycle midIR laser pulses. While many of the proposed and demonstrated schemes to date are scalable to higher (mJ-level) energies, for example, 8 mJ at 3.9 μm22 and 1.2 mJ at 2.1 μm,23 they frequently rely on complex and unique pump lasers and system designs that incorporate multiple lasers, especially for OPCPA. OPAs have also been used for production of mJ-level pulses; however, the lack of output stability16 and highly modulated beam profiles17 present issues for many intended applications. High-energy ultrafast OPAs often operate near degeneracy to simultaneously realize high gain and broad bandwidth. Even closer to degeneracy, three stages of amplification are usually required to obtain mJ-level energies. Here we report the parametric production of 2.2-mJ, ∼6-opticalcycle (42 fs) pulses at a central wavelength of 2.05 μm with 38% overall conversion efficiency when pumped by a standard Ti:sapphire laser. This performance is demonstrated far from parametric degeneracy and in the vicinity of the infrared absorption edge of BBO crystal. Further, the amplified
85, 023102-1
© 2014 AIP Publishing LLC
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 178.118.80.235 On: Wed, 02 Apr 2014 13:20:24
023102-2
Xu, Wandel, and Jovanovic
Rev. Sci. Instrum. 85, 023102 (2014)
high-energy pulses exhibit excellent beam profiles and energy stability that is an order of magnitude better than in prior work. This high pulse energy and peak power is realized in a simple design based on a compact two-stage OPA pumped by a standard commercial flashlamp-pumped Ti:sapphire laser. II. SYSTEM DESIGN
The schematic of our two-stage OPA setup is depicted in Fig. 1. The OPA is pumped by a commercial Ti:sapphire chirped-pulse amplification system (Trident X, Amplitude Technologies), producing 40-fs, up to 14-mJ pulses centered at 800 nm, with a full width at half maximum (FWHM) bandwidth of ∼25 nm at a repetition rate of 10 Hz. In our experiment, the pump pulse energy of ∼14 mJ is split by two beamsplitters (BS1 and BS2). The pulse transmitted by BS2 is used to produce the white-light continuum (WLC). After passing through a neutral density filter and an iris diaphragm, the pulse with ∼1 μJ energy is focused onto a 3-mm-thick sapphire plate to generate a stable single-filament continuum, and a half-waveplate is inserted in the beam path to rotate the polarization for injection into the first OPA stage. The WLC is collimated by a lens and then passed through a 10-mmthick ZnSe plate with dispersion of ∼273 fs2 /mm at 2.05 μm that acts as a pulse stretcher, allowing us to selectively amplify different portions of the WLC in consequent OPA stages. The stretched WLC pulse is directed to the first OPA crystal via a delay line, with the residual pump beam removed by a dichroic mirror. The ∼800-μJ pulse reflected by BS2 is focused onto the first OPA crystal by a focal lens with f = 500 mm. The position of the crystal is optimized for maximum conversion efficiency in the first OPA stage, but sufficiently far from the position where significant optical parametric generation could be produced.
BS1
BS2
WP1
S1
S2
Delay 1
WLC LPF
C1
DM1
Delay 2 WP2
DM2 C2
We first used a collinear geometry in the first OPA to minimize the angular dispersion of the pre-amplified 2.05 μm pulse, when the OPA is seeded at 1.31 μm. The pre-amplified idler beam at 2.05 μm from the first OPA stage is subsequently used to seed the second OPA stage. A longpass filter (LPF) is used to remove the corresponding signal beam at 1.31 μm. A beam resizing telescope and another halfwaveplate are used to match the beam size and polarization in the second OPA. The majority of the pump laser pulse energy (∼12.4 mJ with beam diameter of ∼13.5 mm) is reflected by BS1 and directed to the second OPA via another delay line, without using any focusing lenses to minimize self-focusing and self-phase-modulation (SPM) effects on the pump beam in this part of the system. No obvious SPM and self-focusing in transport of the pump beam are observed since a relatively short delay line path (∼1.65 m) is used, and the calculated accumulated phase change of pump beam is ∼π /2. Finally, the amplified pulses at 2.05 μm, 1.31 μm, and the residual pump pulse are separated using two dichroic mirrors. For parametric amplification with ultrashort pulses, group-velocity mismatch (GVM) between interacting pulses is the primary factor that limits the interaction length over which parametric amplification takes place. The useful interaction length for parametric interaction is quantified by the pulse splitting length Ljp : Ljp =
τ , j = s, i, |1/υgj − 1/υgp |
(1)
where τ is the pump pulse duration and 1/υ gj − 1/υ gp is the GVM between signal/idler and pumps. Full-fledged crystal growth techniques available for BBO crystals simultaneously offer large aperture and high optical quality, resulting in scalability to high peak power. The relatively small group velocity mismatch among the three interaction waves in BBO allows the use of relatively thick crystals, even for 40-fs pump pulses, thus reducing the required pump intensity to realize high gain. The calculated GVM and pulse splitting lengths of type I and type II OPA for BBO crystal are shown in Fig. 2. Since the wavelength of 2.05-μm approaches the IR absorption edge of BBO, the Sellmeier coefficients for BBO crystal with improved accuracy near the IR absorption edge25 have been employed in this calculation. As a result, two BBO crystals with dimensions of 5 × 5 × 2.5 mm3 and 15 × 15 × 2.0 mm3 have been selected for the first stage and the second stage OPA, respectively, when we consider seeding OPA by 2.05 μm. The crystal with dimensions of 5 × 5 × 2.5 mm3 is cut at an angle θ = 19.8◦ for type I (ep → os + oi ) phase matching, and the crystal with dimensions of 15 × 15 × 2.0 mm3 is cut at an angle θ = 25.9◦ for type II (ep → es + oi ) phase matching. Both OPA crystals are coated for all three wavelengths present in the OPA process. III. EXPERIMENTS AND RESULTS
FIG. 1. Experimental setup of the two-stage OPA. Blue, 800 nm pump beam; green, white light continuum (WLC); red, 2.05 μm signal beam; and orange, 1.31 μm idler beam. BS, beamsplitter; WP, λ/2 waveplate; DM, dichroic mirror; S1, sapphire plate; S2, ZnSe plate; LPF, long-pass filter; C1, type I BBO crystal; and C2, type II BBO crystal.
To realize the most efficient amplification in this twostage OPA design employing BBO crystals, both the type I and type II phase matching schemes in BBO have been investigated. We first experimentally implemented a design that used same phase matching type (type I) in both OPA stages.
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Xu, Wandel, and Jovanovic 3.5
Type I s,p
(a)
Type I i,p
0
Type II s,p Type II i,p
-40
4
(b)
3 2
(a)
3.0
2.0 1.5 1.0 0.5 0.0 1800
2000
2200
Wavelength (nm) 0 1.6
1.8
2.0 2.2 2.4 2.6 Signal wavelength (µm)
2.8
3.0
FIG. 2. Calculated (a) group velocity mismatch (GVM) and (b) pulse splitting lengths for signal-pump and idler-pump as a function of signal wavelength for type I and type II BBO OPA pumped by 40-fs, 800 nm pulses. Solid line (black): pump-signal in type I OPA; dashed line (red): pump-idler in type I OPA; dotted line (blue): pump-signal in type II OPA; and dash-dotted line (magenta): pump-idler in type II OPA.
Type I phase matching has ∼23% higher effective nonlinearity and ∼3.5× broader phase matching bandwidth, which is favorable for achieving high parametric gain and shorter pulse durations, respectively, in comparison with type II phase matching. However, the phase matching angles of secondharmonic generation (SHG) of signal and idler are relatively close to the type I OPA phase matching angle (θ = 21.3◦ for SHG of 2.05 μm and θ = 20.4◦ for SHG of 1.31 μm). The parasitic SHG process significantly reduces the output pulse energy and degrades the beam quality. After replacing the type I BBO with type II BBO in the second OPA stage, the parasitic SHG of both signal and idler pulses are significantly suppressed because of the relatively large difference between the OPA and the SHG phase matching angles and incorrect polarization of the signal pulse for the SHG process. The amplified 2.05-μm pulse exhibits a slightly elliptical beam profile in a collinear OPA configuration, which we believe originates from the tilted pulse front from imperfect WLC production and/or transport. We experimentally determined that by employing a small (0.4◦ ) tilt between the seed and the pump in both OPA stages allows one to simultaneously achieve the highest efficiency with the best amplified beam quality. It is well-known that OPAs with noncollinear geometry can achieve broad phase-matching bandwidth at the cost of introducing idler angular dispersion,26 as also observed in our experiment (Fig. 3(a)). Idler angular dispersion can be suppressed by adjusting the pulse front tilt of signal and/or pump pulses,27, 28 but a more complicated design is needed to avoid additional unwanted nonlinear effects. We therefore chose to seed the OPA directly using 2.05-μm pulses from WLC, effectively eliminating the spatial chirp of amplified 2.05-μm pulses (Fig. 3(b)). In the second OPA, we produced mid-IR laser pulses at 2.05 μm with an average pulse energy of 2.2 mJ, excellent beam quality (Fig. 4, inset), and energy stability of 1.1% RMS measured over 30 min. This high energy stability achieved
(b)
2.5
1
2400
1800
2000
2200
2400
Wavelength (nm)
FIG. 3. Spectra of amplified 2.05 μm pulses measured at different transverse locations of the beam (blue, left; red, center; and black, right) for (a) 1.31μm-seeded OPA and (b) 2.05-μm-seeded OPA.
even with the use of a flashlamp-pumped Ti:sapphire system is possible both because of the relatively high energy stability of the pump laser (
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 023102 (2014)
Nondegenerate parametric generation of 2.2-mJ, few-cycle 2.05-μm pulses using a mixed phase matching scheme Guibao Xu, Scott F. Wandel, and Igor Jovanovica) Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
(Received 5 December 2013; accepted 27 January 2014; published online 12 February 2014) We describe the production of 2.2-mJ, ∼6 optical-cycle-long mid-infrared laser pulses with a carrier wavelength of 2.05 μm in a two-stage β-BaB2 O4 nondegenerate optical parametric amplifier design with a mixed phase matching scheme, which is pumped by a standard Ti:sapphire chirped-pulse amplification system. It is demonstrated that relatively high pulse energies, short pulse durations, high stability, and excellent beam profiles can be obtained using this simple approach, even without the use of optical parametric chirped-pulse amplification. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865132] I. INTRODUCTION
Few-cycle ultrashort laser pulses are an enabling tool for many areas of science, including physics, chemistry, biology, and materials science. Recent developments in ultrashort pulse laser technology make it possible to probe ultrafast dynamics with attosecond resolution.1, 2 Pulses of order 100 as can now be routinely produced by high-harmonic generation (HHG) in noble gases using compressed few-cycle laser pulses from carrier-envelope phase stabilized Ti:sapphire lasers. Further improvement of temporal resolution requires the production of extreme ultraviolet (XUV) supercontinuum radiation with higher photon energy (>100 eV). Within the tightly localized ionization region produced by the intense laser pulse, some of the liberated electrons are re-scattered on the ion core, leading to above-thresholdionized electron acceleration to kinetic energies of up to 10Up + Ip (Ref. 3) and in the case of electron-core recombination, HHG with photon energies of up to 3.17Up + Ip .4 Here, Ip denotes the atom’s ionization potential, and Up = E2 /4ω2 refers to the ponderomotive potential of the laser pulse, which depends on the electric field amplitude E and on the carrier frequency ω of the driver laser pulse. It is evident that both an increase in the drive pulse intensity and its wavelength can help generate higher harmonics. It has been demonstrated that the use of longer wavelength drive pulses can improve phase matching in HHG.5, 6 In addition, few-cycle mid-IR driving laser pulses reduce depletion of the neutral gas target by ionization before the main peak of the electric field, generating high-energy XUV photons.7 Another area of potential use for high-energy mid-IR ultrashort pulses is the novel direct particle acceleration scheme.8 For example, laser-driven particle acceleration in dielectric photonic structures9, 10 could enable very high acceleration gradients (>GV/m), even at modest laser peak powers. The use of long-wavelength drive lasers can help mitigate the problem of dielectric structure breakdown caused by a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2014/85(2)/023102/5/$30.00
multiphoton ionization; the fabrication tolerances for dielectric structures are simultaneously relaxed since they scale with the laser wavelength. Since the aperture of the structure is of order of the laser wavelength, larger apertures and the use of greater pulse energies is also possible by use of longer drive laser wavelength. Direct production of high-energy ultrashort mid-IR laser pulses is technically challenging due to the lack of broadband laser media and the general lack of technological maturity in this wavelength range. The mode-locked crystalline solid-state lasers based on Cr2+ -doped chalcogenides11, 12 and fiber oscillator/amplifiers based on Tm3+ -doped large mode area fiber13 are promising candidates; however, their level of maturity at present time is low. The available pulse durations and the accessible spectral range associated with those technologies are limited. A more flexible approach to access the mid-IR spectral region is the use of optical parametric amplification (OPA)14–18 and optical parametric chirped-pulse amplification (OPCPA).19–24 Broad gain bandwidth and favorable phase matching in certain nonlinear materials such as BiBO,14 BBO (β-BaB2 O4 ),15–18, 21 PPLN,19–21, 23 PPLT,21, 24 KTP, and KTA22 have been used to realize few-cycle midIR laser pulses. While many of the proposed and demonstrated schemes to date are scalable to higher (mJ-level) energies, for example, 8 mJ at 3.9 μm22 and 1.2 mJ at 2.1 μm,23 they frequently rely on complex and unique pump lasers and system designs that incorporate multiple lasers, especially for OPCPA. OPAs have also been used for production of mJ-level pulses; however, the lack of output stability16 and highly modulated beam profiles17 present issues for many intended applications. High-energy ultrafast OPAs often operate near degeneracy to simultaneously realize high gain and broad bandwidth. Even closer to degeneracy, three stages of amplification are usually required to obtain mJ-level energies. Here we report the parametric production of 2.2-mJ, ∼6-opticalcycle (42 fs) pulses at a central wavelength of 2.05 μm with 38% overall conversion efficiency when pumped by a standard Ti:sapphire laser. This performance is demonstrated far from parametric degeneracy and in the vicinity of the infrared absorption edge of BBO crystal. Further, the amplified
85, 023102-1
© 2014 AIP Publishing LLC
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023102-2
Xu, Wandel, and Jovanovic
Rev. Sci. Instrum. 85, 023102 (2014)
high-energy pulses exhibit excellent beam profiles and energy stability that is an order of magnitude better than in prior work. This high pulse energy and peak power is realized in a simple design based on a compact two-stage OPA pumped by a standard commercial flashlamp-pumped Ti:sapphire laser. II. SYSTEM DESIGN
The schematic of our two-stage OPA setup is depicted in Fig. 1. The OPA is pumped by a commercial Ti:sapphire chirped-pulse amplification system (Trident X, Amplitude Technologies), producing 40-fs, up to 14-mJ pulses centered at 800 nm, with a full width at half maximum (FWHM) bandwidth of ∼25 nm at a repetition rate of 10 Hz. In our experiment, the pump pulse energy of ∼14 mJ is split by two beamsplitters (BS1 and BS2). The pulse transmitted by BS2 is used to produce the white-light continuum (WLC). After passing through a neutral density filter and an iris diaphragm, the pulse with ∼1 μJ energy is focused onto a 3-mm-thick sapphire plate to generate a stable single-filament continuum, and a half-waveplate is inserted in the beam path to rotate the polarization for injection into the first OPA stage. The WLC is collimated by a lens and then passed through a 10-mmthick ZnSe plate with dispersion of ∼273 fs2 /mm at 2.05 μm that acts as a pulse stretcher, allowing us to selectively amplify different portions of the WLC in consequent OPA stages. The stretched WLC pulse is directed to the first OPA crystal via a delay line, with the residual pump beam removed by a dichroic mirror. The ∼800-μJ pulse reflected by BS2 is focused onto the first OPA crystal by a focal lens with f = 500 mm. The position of the crystal is optimized for maximum conversion efficiency in the first OPA stage, but sufficiently far from the position where significant optical parametric generation could be produced.
BS1
BS2
WP1
S1
S2
Delay 1
WLC LPF
C1
DM1
Delay 2 WP2
DM2 C2
We first used a collinear geometry in the first OPA to minimize the angular dispersion of the pre-amplified 2.05 μm pulse, when the OPA is seeded at 1.31 μm. The pre-amplified idler beam at 2.05 μm from the first OPA stage is subsequently used to seed the second OPA stage. A longpass filter (LPF) is used to remove the corresponding signal beam at 1.31 μm. A beam resizing telescope and another halfwaveplate are used to match the beam size and polarization in the second OPA. The majority of the pump laser pulse energy (∼12.4 mJ with beam diameter of ∼13.5 mm) is reflected by BS1 and directed to the second OPA via another delay line, without using any focusing lenses to minimize self-focusing and self-phase-modulation (SPM) effects on the pump beam in this part of the system. No obvious SPM and self-focusing in transport of the pump beam are observed since a relatively short delay line path (∼1.65 m) is used, and the calculated accumulated phase change of pump beam is ∼π /2. Finally, the amplified pulses at 2.05 μm, 1.31 μm, and the residual pump pulse are separated using two dichroic mirrors. For parametric amplification with ultrashort pulses, group-velocity mismatch (GVM) between interacting pulses is the primary factor that limits the interaction length over which parametric amplification takes place. The useful interaction length for parametric interaction is quantified by the pulse splitting length Ljp : Ljp =
τ , j = s, i, |1/υgj − 1/υgp |
(1)
where τ is the pump pulse duration and 1/υ gj − 1/υ gp is the GVM between signal/idler and pumps. Full-fledged crystal growth techniques available for BBO crystals simultaneously offer large aperture and high optical quality, resulting in scalability to high peak power. The relatively small group velocity mismatch among the three interaction waves in BBO allows the use of relatively thick crystals, even for 40-fs pump pulses, thus reducing the required pump intensity to realize high gain. The calculated GVM and pulse splitting lengths of type I and type II OPA for BBO crystal are shown in Fig. 2. Since the wavelength of 2.05-μm approaches the IR absorption edge of BBO, the Sellmeier coefficients for BBO crystal with improved accuracy near the IR absorption edge25 have been employed in this calculation. As a result, two BBO crystals with dimensions of 5 × 5 × 2.5 mm3 and 15 × 15 × 2.0 mm3 have been selected for the first stage and the second stage OPA, respectively, when we consider seeding OPA by 2.05 μm. The crystal with dimensions of 5 × 5 × 2.5 mm3 is cut at an angle θ = 19.8◦ for type I (ep → os + oi ) phase matching, and the crystal with dimensions of 15 × 15 × 2.0 mm3 is cut at an angle θ = 25.9◦ for type II (ep → es + oi ) phase matching. Both OPA crystals are coated for all three wavelengths present in the OPA process. III. EXPERIMENTS AND RESULTS
FIG. 1. Experimental setup of the two-stage OPA. Blue, 800 nm pump beam; green, white light continuum (WLC); red, 2.05 μm signal beam; and orange, 1.31 μm idler beam. BS, beamsplitter; WP, λ/2 waveplate; DM, dichroic mirror; S1, sapphire plate; S2, ZnSe plate; LPF, long-pass filter; C1, type I BBO crystal; and C2, type II BBO crystal.
To realize the most efficient amplification in this twostage OPA design employing BBO crystals, both the type I and type II phase matching schemes in BBO have been investigated. We first experimentally implemented a design that used same phase matching type (type I) in both OPA stages.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 178.118.80.235 On: Wed, 02 Apr 2014 13:20:24
Xu, Wandel, and Jovanovic 3.5
Type I s,p
(a)
Type I i,p
0
Type II s,p Type II i,p
-40
4
(b)
3 2
(a)
3.0
2.0 1.5 1.0 0.5 0.0 1800
2000
2200
Wavelength (nm) 0 1.6
1.8
2.0 2.2 2.4 2.6 Signal wavelength (µm)
2.8
3.0
FIG. 2. Calculated (a) group velocity mismatch (GVM) and (b) pulse splitting lengths for signal-pump and idler-pump as a function of signal wavelength for type I and type II BBO OPA pumped by 40-fs, 800 nm pulses. Solid line (black): pump-signal in type I OPA; dashed line (red): pump-idler in type I OPA; dotted line (blue): pump-signal in type II OPA; and dash-dotted line (magenta): pump-idler in type II OPA.
Type I phase matching has ∼23% higher effective nonlinearity and ∼3.5× broader phase matching bandwidth, which is favorable for achieving high parametric gain and shorter pulse durations, respectively, in comparison with type II phase matching. However, the phase matching angles of secondharmonic generation (SHG) of signal and idler are relatively close to the type I OPA phase matching angle (θ = 21.3◦ for SHG of 2.05 μm and θ = 20.4◦ for SHG of 1.31 μm). The parasitic SHG process significantly reduces the output pulse energy and degrades the beam quality. After replacing the type I BBO with type II BBO in the second OPA stage, the parasitic SHG of both signal and idler pulses are significantly suppressed because of the relatively large difference between the OPA and the SHG phase matching angles and incorrect polarization of the signal pulse for the SHG process. The amplified 2.05-μm pulse exhibits a slightly elliptical beam profile in a collinear OPA configuration, which we believe originates from the tilted pulse front from imperfect WLC production and/or transport. We experimentally determined that by employing a small (0.4◦ ) tilt between the seed and the pump in both OPA stages allows one to simultaneously achieve the highest efficiency with the best amplified beam quality. It is well-known that OPAs with noncollinear geometry can achieve broad phase-matching bandwidth at the cost of introducing idler angular dispersion,26 as also observed in our experiment (Fig. 3(a)). Idler angular dispersion can be suppressed by adjusting the pulse front tilt of signal and/or pump pulses,27, 28 but a more complicated design is needed to avoid additional unwanted nonlinear effects. We therefore chose to seed the OPA directly using 2.05-μm pulses from WLC, effectively eliminating the spatial chirp of amplified 2.05-μm pulses (Fig. 3(b)). In the second OPA, we produced mid-IR laser pulses at 2.05 μm with an average pulse energy of 2.2 mJ, excellent beam quality (Fig. 4, inset), and energy stability of 1.1% RMS measured over 30 min. This high energy stability achieved
(b)
2.5
1
2400
1800
2000
2200
2400
Wavelength (nm)
FIG. 3. Spectra of amplified 2.05 μm pulses measured at different transverse locations of the beam (blue, left; red, center; and black, right) for (a) 1.31μm-seeded OPA and (b) 2.05-μm-seeded OPA.
even with the use of a flashlamp-pumped Ti:sapphire system is possible both because of the relatively high energy stability of the pump laser (
