Optical frequency combs based on mode-locked lasers have revolutionised the field of metrology and precision spectroscopy by providing precisely calibrated optical frequencies and coherent pulse trains. Amplification of the pulsed output from these lasers is very desirable, as nonlinear processes can then be employed to cover a much wider range of transitions and wavelengths for ultra-high precision, direct frequency comb spectroscopy. Therefore full repetition rate laser amplifiers and enhancement resonators have been employed to produce up to microjoule-level pulse energies. Here we show that the full frequency comb accuracy and resolution can be obtained by using only two frequency comb pulses amplified to the millijoule pulse energy level, orders of magnitude more energetic than what has previously been possible. The novel properties of this approach, such as cancellation of optical light-shift effects, is demonstrated on weak two-photon transitions in atomic rubidium and caesium, thereby improving the frequency accuracy up to thirty times.Comment: 16 pages (including supporting material), 6 figures and 2 table
We characterize the saturable absorption of the second (E22) electronic transition of a sample of single‐walled carbon nanotubes and use it to mode‐lock an ytterbium fiber ring laser. The modulation depth of ∼15% was found to be similar to the corresponding E11 transition (∼13%), but the saturation intensity (∼220 MW cm–2) about an order of magnitude larger (∼10 MW cm–2). We achieved a 15 MHz mode‐locked pulse train with an output pulse duration of 6.5 ps. For comparison we also demonstrate stable mode‐locking on the E11 transition, of the same nanotubes, with an erbium fiber ring laser, producing 1.1 ps pulses. Using the E22 transition should enable the use of carbon nanotube saturable absorbers at shorter wavelengths than currently possible with the E11 transition, which are limited by the smallest achievable nanotube dimensions. (© 2011 by Astro Ltd., Published exclusively by WILEY‐VCH Verlag GmbH & Co. KGaA) (© 2011 by Astro Ltd., Published exclusively by WILEY‐VCH Verlag GmbH & Co. KGaA)
We recently demonstrated that the spectroscopic accuracy and resolution of optical frequency combs can be obtained from a series of Ramsey-like measurements using only two amplified frequency comb pulses at variable delays. In this work we present a comprehensive analytical framework of this Ramsey-comb method in both time and frequency domains. It is shown that as opposed to traditional forms of spectroscopy, the signal analysis can be performed purely in the time domain, based on the temporal phases of the individual Ramsey signals. We give a detailed description of the robust fitting algorithm relying solely on this phase information and discuss special features such as an insensitivity to (transition-independent) spectral line-broadening mechanisms and constant phase shifts, e.g., due to the ac Stark effect from the excitation pulses themselves. The precision and resolution of the Ramsey-comb fitting method is assessed via numerical simulations, including cases of transition-dependent broadening mechanisms and phase shifts.
We report on an 880 nm quasi-continuously pumped Nd:YVO 4 grazing-incidence "bounce" amplifier, operating at a 300 Hz repetition rate. More than 70 dB small signal gain is achieved with a single crystal. Combined with fast programmable modulators, high-contrast and near-diffraction-limited pulse sequences at the 100 μJ level are produced and can be tailored in terms of pulse duration, amplitude, and a temporal spacing well into the microsecond range. This system could significantly improve extreme-UV comb generation based on parametric amplification and harmonic upconversion of two near-IR comb laser pulses. © 2012 Optical Society of America OCIS codes: 140.3530, 140.3280. An increasing number of research groups have developed terawatt intensity, ultrafast optical parametric chirped-pulse amplifier systems because their considerable potential in fields such as attoscience and highenergy physics (see, for example, [1][2][3]). Recently we showed that such a system is also well suited to perform frequency comb metrology in the extreme-UV by amplifying and upconverting two consecutive pulses from a near-IR frequency comb laser [4]. This approach requires a carefully synchronized pump pulse for each frequency comb pulse that is amplified in the parametric amplifier.Up to now, these have been generated by applying beam splitters and a fixed delay line in the pump laser. Here we present a more versatile and general approach, using an ultrahigh-gain amplifier combined with fast modulators. The system employs a grazing-incidence "bounce" amplifier, based on Nd 3 -doped gain material (Nd:YVO 4 ), which benefits in particular from high-peak-power quasicontinuous-wave (QCW) diode pumping [5]. Hundreds of microjoules of amplified pulse energies have been reported for picosecond pulses with small signal gains of around 40 and 60 dB for single and double slab modules, respectively [6]. To the best of our knowledge, all reported Nd 3 -doped bounce amplifiers so far (see, for example, [5-9]) have been pumped by 808 nm light. Despite a slightly lower absorption coefficient, direct pumping in the upper-band laser level at 880 nm is a promising alternative thanks to the higher quantum efficiency and lower thermal distortion [10].In this Letter we present, to the best of our knowledge, the first Nd:YVO 4 bounce amplifier pumped at 880 nm. Together with spectral clipping and the combination of fast electro-optical and high-contrast acousto-optical modulators, the system produces near-diffraction-limited pulse sequences with widely tunable timings, intensities, and pulse lengths. Figure 1 shows a simplified sketch of the experimental setup. The master oscillator is a home-built high-power Nd:YVO 4 laser, mode locked with a semiconductor saturable absorber mirror and pumped with 24 W at 880 nm. It provides 0.25 nm spectral bandwidth in a 126 MHz pulse train with 5 W average output power; the repetition rate is locked via a piezo-mounted mirror.A slit on a translation stage close to the Fourier plane of a 4f -grating-system was used...
Abstract:We demonstrate the generation of phase-stable mJ-pulse pairs at programmable inter-pulse delays up to hundreds of nanoseconds. A detailed investigation of potential sources for phase shifts during the parametric amplification of the selected pulses from a Ti:Sapphire frequency comb is presented, both numerically and experimentally. It is shown that within the statistical error of the phase measurement of 10 mrad, there is no dependence of the differential phase shift over the investigated inter-pulse delay range of more than 300 ns. In combination with nonlinear upconversion of the amplified pulses, the presented system will potentially enable short wavelength (<100 nm), multi-transition Ramsey-frequency comb spectroscopy at the kHz-level. high-harmonic generation inside a femtosecond enhancement cavity," Phys. Rev. Lett. 94, 193201 (2005).
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