We demonstrate ultra-broadband optical parametric chirped-pulse amplification of 300-nm bandwidth pumped by a broadband pulse delivered from a diode-pumped, cryogenically-cooled Yb:YLF chirped-pulse amplification laser.
©2007 Optical Society of America
There has been rapid progress in the technologies of producing intense ultrashort laser pulses [1, 2]. An interaction of the intense light with matters can produce attosecond pulses through higher order harmonics in the soft x-ray region . Such attosecond pulses could be used as novel tools for seeing the motion of electrons inside atoms or molecule . However, the intense laser pulse with many cycles will generally produce a train of attosecond pulses, which may be limiting a number of pump-probe experiments . Thus a generation of single attosecond pulse is desired to explore fast electron dynamics with pump-probe techniques. For this reason, a few-cycle, carrier-envelop-phase locked laser pulse in the near infrared region is appropriate for the single attosecond pulse generation .
Optical parametric chirped-pulse amplification (OPCPA) is one of the most powerful techniques in the generation of a high-energy short duration laser pulses [7, 8]. Its major advantages include high gain, high contrast and high beam quality while maintaining ultra-broad spectral bandwidth. There are a number of efforts devoted to ultra-broadband amplification. A noncollinear OPCPA (NOPCPA) was discussed to extend over a bandwidth supporting sub-10-fs laser pulses . The NOPCPA geometry could compensate the group velocity mismatch between pump, signal and idler by adjusting a crossing angle between the pump and signal beams. Consequently, ultra-broadband amplification capable of producing few-cycle pulses is possible. Therefrom the generation of the few-cycle terawatt light pulse from the multi-stage NOPCPA has recently been reported .
As an another approach for ultra-broadband amplification, Limpert et al. reported an enhancement of the amplified bandwidth of OPCPA by using a broadband Ti:sapphire pump laser at degeneracy . The amplified bandwidth of Degenerate OPCPA (DOPCPA) is mainly determined by the bandwidth of the pump pulse whereas the bandwidth of NOPCPA is strongly affected by the change of a crossing angle between the signal and pump pulses and dependent on the chirp of the signal pulse. [12,13]. This modification in the spectrum of NOPCPA has disadvantages for pulse recompression and long-term operation. On the other hand, a configuration of DOPCPA could be simplified than that of NOPCA. This feature of DOPCPA is responsible for improving the long-term stability of the amplified bandwidth. Although the Ti:sapphire pump laser has a substantial bandwidth for ultra-broadband parametric amplification, an additional green pump laser for pumping Ti:sapphire itself is required. Therefore, the DOPCPA pumped by the T:sapphire laser becomes bulky and complex and its wall-plug efficiency is quite low. In contrast, Ytterbium (Yb) doped solid-state laser materials have received much attention because they have broad absorption and emission bandwidths, the low quantum defect and the simple electronic structures. These features are suitable for direct diode pumping and enable efficient, broadband and high repetition rate operations.
By taking these advantages of the Yb-doped solid-state media we demonstrated ultra-broadband DOPCPA by using a diode-pumped Yb: LiYF4 (YLF) chirped-pulse regenerative amplification system as the pump source. We have achieved ultra-broadband amplification in the spectral range of 300-nm around degeneracy, supporting to ~ 3 optical cycles.
2. Features of DOPCPA pumped by the Yb:YLF CPA laser
The DOPCPA pumped by the diode-pumped, cryogenically-cooled Yb:YLF CPA laser  has numerous advantages. First, a large emission cross section and high saturation intensity of cooled Yb:YLF enables to extract multi-millijoule output from only a regenerative amplifier. A wall-plug efficiency of Yb:YLF can be higher that that of Ti:sapphire since Yb:YLF is directly diode-pumped. Because a thermal conductivity of cooled Yb:YLF is also improved at the liquid nitrogen temperature, a kHz repetition rate operation of DOPCPA is feasible. Second, it generates a substantial broad bandwidth for the optical parametric amplification (OPA) of 300-nm at degeneracy. Its broad bandwidth also supports picosecond pump pulse generation. For efficient amplification and high fidelity of dispersion compensation in OPCPA, a femtosecond seed pulse is desired to stretch a few to several tens of picoseconds long. Therefore it is possible to obtain saturated parametric amplification in a nonlinear crystal with only a few millimeter long, resulting a low B-integral inside the crystal. This leads to reduce a risk of the damage of the crystal and other optical components. It is also possible to ensure overlap with the signal pulse duration precisely by adjusting the distance between the gratings of the pulse compressor, which improves the OPCPA conversion efficiency and reduces the total amount of super-fluorescence background. Third, a timing jitter between the signal and pump pulses can be strongly reduced since both pulses from the common oscillator are generated.
In order to generate a laser pulse with the high-energy and ultra-broad bandwidth around 1-μm, we use a frequency-doubled pump laser pulse at a center wavelength of 508.5-nm with its bandwidth of 3-nm. By proper choosing a phase-matching angle of a nonlinear crystal, ultra-broad phase-matching bandwidth is meet. The calculated phase-matching bandwidth in the DOPCPA scheme is shown in Fig. 1. As shown in Fig. 1, a bandwidth enhancement can be achieved by utilizing the broadband pump. The calculation shows that a phase matching bandwidth could be accomplish from 900-nm to 1130-nm by using a type-I BBO crystal (θ=23.8°) at a crossing angle between the signal and pump pulses of α=0.6°.
3. Experiment and results
The experimental setup of ultra-broadband parametric amplification with a single-stage DOPCPA is shown in Fig. 2. A seed pulse (400-mW average power and 80-MHz repetition rate) from the mode-locked Ti:sapphire oscillator (Tsunami; Spectra-Physics Inc.) operating at a center wavelength of 1017-nm is split into two beams. One is temporally stretched with a 1.2-km-long, polarization-maintained single-mode fiber up to ~ 1-ns and then amplified in the cryogenically-cooled Yb:YLF regenerative amplifier. The regenerative amplifier cavity is similar to ones presented previously . The laser crystal was 20-at.% Yb:YLF with a thickness of 2-mm and a 5-mm × 5-mm cross section. A fiber-coupled laser diode (LD, Jenoptik; JOLD-140-CAXF-6A) beam with an emission wavelength of 940-nm was focused to 800-μm diameter on the crystal. A maximum output pulse energy of up to 30-mJ was obtained at a LD pump power of 93-W with a pulse duration of 4 ms. The amplified chirped pulse was then compressed by two parallel, gold-coated, 1100-grooves/mm, ruled gratings. The duration of the compressed pulse was 1.4-ps. A fraction of the compressed pulse was down-collimated to a 3-mm diameter by a Galilean telescope. The pulse was then frequency doubled in a 7-mm-long, type-I BBO crystal (θ=23.8°) for pumping the parametric amplifier. An output pulse energy of the frequency-doubled pump pulse was measured to be 2.2-mJ at a fundamental laser intensity of 36 GW/cm2 which corresponded to an energy conversion efficiency of ~ 40 %. A duration and bandwidth of the pulse were measured to be 2.4-ps and 3-nm at the full width half maximum (FWHM), respectively. At present the laser system is operated at a 10-Hz repetition rate which is limited by the capability of the LD driver.
Another seed pulse from the oscillator was converted into the white light continuum (WLC) through a photonic crystal fiber (PCF) to use as a signal pulse of DOPCPA. The PCF had a length of 15-cm and core diameter of 4.7-μm with a zero-dispersion wavelength of around 1030-nm (Crystal Fibre; NL-4.7-1030). The laser pulse with an energy of 2.8-nJ and duration of 80-fs was focused into the PCF. The generated white light was then stretched to ~2-ps by a piece of glass with an optical pass length of 10.5-cm. The energy of the white light seed pulse injected into the parametric amplifier was ~ 0.75-nJ.
A BBO crystal is cut at 23.8° and arranged type-I collinear phase matching with an internal crossing angle between the seed and pump pulses of 0.6°. The seed pulse was loosely focused to 1.0 mm diameter on the crystal with an intensity of 48 kW/cm2. The OPA pump pulse was down-collimated to a 1.2-mm diameter with an energy and intensity of up to 2.1-mJ and 76.9 GW/cm2, respectively. Laser energies and power spectra were measured by a thermopile power meter (Coherent; PM-10) and fiber optic spectrometer (Ocean Optics; USB-2000-VIS-NIR and NIR-512), respectively.
We first measured the OPA gain at various pump laser intensities by using a photodiode coupled with a set of calibrated neutral density filters. Figure 3 shows the amplifier gain as a function of pump intensity. A maximum single-pass gain of more than 1.6×105 was achieved with a pump intensity of 51 GW/cm2. Saturated amplification was observed with the pump intensity in excess of 20 GW/cm2. We have obtained a conversion efficiency of 9.6 % into the signal, resulting in an amplified signal energy of 83-μJ.
Because the amplifier started saturation at the low pump intensity of ~ 20 GW/cm2 (corresponding to the 0.54 mJ energy), the residual pump energy of more than ~ 1.6-mJ could be used for pumping a second stage OPA to achieve the higher conversion efficiency.
Figure 4 shows the measured amplified spectrum of DOPCPA at the pump intensity of 38.5 GW/cm2. Strong signal depletion was observed at around the center wavelength of 1020-nm due to the back-conversion process. In order to amplify ultra-broad signal pulse, the duration of the pump pulse was slightly lengthened to ~ 3-ps to ensure overlap with the signal pulse duration by adjusting the distance between the compressor gratings. The spectrum ranging from 900-nm to 1200-nm was amplified, corresponding to a calculated, transform-limited pulse duration of 10.9-fs (3.2 optical cycles). This result was reasonably agreed with the calculated one as shown in Fig. 1. The measured spectrum was however slightly broader than that of the calculation which may be due to the presence of pump beam divergence .
By using a compact, diode-pumped Yb:YLF CPA laser as a pump source for OPCPA, we have achieved ultra-broadband amplification around degeneracy. A careful control of a pump pulse duration is a key role to enhance the amplification bandwidth with a high optical-to-optical conversion efficiency. A spectrum ranging from 900-nm to 1200-nm is amplified in a type-I BBO crystal, supporting a pulse duration of ~ 10-fs.
A potential scalability of the Yb:YLF CPA laser pumped DOPCPA architectures described in this paper to higher energies and shorter pulse duration is an important issue considering peak powers to the multi-terawatt level. To scale the system to peak powers above one terawatt, a higher pump energy and broader amplification bandwidth are required.
A post amplifier will be added to boost the output of the Yb:YLF regenerative amplifier to 100 mJ level with a LD pump power of 300-W. Using a frequency-doubled pump pulse with an energy of ~ 40-mJ, a signal is expected to amplify to an energy of ~ 12-mJ from the two-stage OPA, assuming a conversion efficiency of 30%. The throughput of the OPA compressor can be improved to ~95% by employing a combination of negative chirped stretcher and positive chirped bulk glass compressor . In order to obtain shorter pulse duration less than 10-fs, two broadband pump beams are employed to enlarge the amplified bandwidth more than 400-nm . Figure 5 shows a calculation of the amplified bandwidth pumped by the two broadband pulses. The bandwidth of the pump laser is assumed to be 6-nm. The phase matching bandwidth can be accomplished from 820-nm to 1230-nm at two crossing angles between the signal and pump pulses of α=0.5° and 1.0° inside the type-I BBO crystal, respectively. Regenerative pulse shaping can be used to counter gain narrowing during regenerative amplification of Yb:YLF . It should be noted that the compressibility of the white light continuum generated in the PCF is greatly affected by the input pulse duration and fiber length . In order to compress down to the few-cycle pulse duration, we will replace the mode-locked Ti:sapphire oscillator with the self-mode-locked Yb:glass or Yb:KYW oscillators (< 30-fs) and the long fiber (15 cm) with the short one (< 5-cm) in the future study. By combining these modifications, the compressed pulse would have an energy of ~ 11-mJ with a duration of ~ 6-fs, resulting in a peak power of nearly 2-TW.
A higher repetition rate operation is also desired for attosecond applications. No thermal loading on the parametric process and the liquid-nitrogen temperature operation of Yb:YLF  will allow us to operate the system at a kHz repetition rate while maintaining the multi-terawatt peak power. The combination of modern high-power lasers and OPCPA will open a new route to the generation of the intense few-cycle pulses.
We thank N. Miyanaga of Institute of Laser Engineering, Osaka University for his stimulating discussions. This work was partly supported by the MATSUO FUNDATION.
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