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We report a tunable optical pulse expander capable of producing narrowband long pulses, starting from a femtosecond optical parametric amplifier (OPA), which is based on chirp-matched frequency mixing technique. The device delivers nJ-level pulses with bandwidth of ~0.1 nm and duration of ~45 ps within its tuning range from 1000 nm to 1090 nm. The combination of a femtosecond OPA and a pulse expander may provide synchronized and both tunable femtosecond and tens of picoseconds pulses simultaneously, which should be promising in the applications where both temporal and spectral resolutions are needed.
©2006 Optical Society of America
1. Introduction
Quadratic optical nonlinearity has exhibited great prospects in femtosecond laser technologies due to its ease of experimental implementation [1, 2]. Of particular interest, frequency conversion with chirped femtosecond pulses has shown quite different behaviors comparing to that with unchirped pulses, which leaves us an additional controllable parameter [3–5]. Time microscope and time reversal of ultrafast waveforms have been demonstrated by using quadratic nonlinear processes with chirped pulses [5–7]. By matching the chirps of two interacting femtosecond pulses, the spectral acceptance bandwidth of frequency conversion may be enhanced significantly [8, 9]. This idea was recently extended to the experimental generation of narrowband picosecond pulses at either fundamental or harmonic wavelengths [10, 11], which was termed as pulse expander with functions of simultaneous temporal expanding and spectral narrowing, analogous to beam expander in spatial domain [12]. The expanded pulse can be accurately synchronized with the original femtosecond pulse, which is mainly of interest for pump-probe experiments such as time-resolved stimulated Raman spectroscopy [13] and broad-bandwidth sum-frequency generation (BBSFG) [14]. Besides, it is particularly important in constructing intense ultrashort pulse sources based on optical parametric chirped pulse amplification (OPCPA) where synchronization between a femtosecond pulse and a nanosecond or sub-nanosecond pulse is necessary [4].
Optical parametric amplification (OPA) makes femtosecond lasers tunable in a wide wavelength range, which has found a variety of applications in science and technology [1]. The high temporal resolution provided by femtosecond pulses, however, is obtained at a price of poor spectral resolution. In this paper, we report a pulse expander pumped by a femtosecond OPA, which delivers tunable narrowband picosecond pulses. Thus, the combination of the femtosecond OPA and the pulse expander may provide synchronized and both tunable femtosecond and picosecond pulses, which integrates high temporal and high spectral resolutions in one system. Since the output of a femtosecond OPA is typically of ~ 10–100 μJ, i.e., one order of magnitude lower than that of the laser sources used in the previous works [11, 12], efforts are made to improve the conversion efficiency of the pulse expander, especially at the second-harmonic generation (SHG) stage. The scheme of spectrally-noncritical phase-matching is applied to enhance the SHG of femtosecond pulses [15], which greatly reduces the required pumping pulse energy for the pulse expander.
2. Experimental results and discussions
The pulse expander is schematically shown in Fig. 1. The signal pulse from a CW seeded femtosecond OPA [16], tunable from 1010 nm to 1080 nm with pulse energy of ~25 μJ and duration of ~ 100 fs (typical spectrum and autocorrelation trace shown in Fig. 2(a)), was used as the initial laser source. The pulse was first frequency doubled in a 10-mm-thick 12% partially deuterated KDP crystal using a type-I phase-matching configuration, which delivered second-harmonic pulse energy of ~10 μJ. We demonstrated in our previous work that a partially deuterated KDP may support spectrally-noncritical phase-matching hence a larger acceptance bandwidth of SHG near 1 μm [15]. Thus, a longer crystal can be used to enhance the SHG that is beneficial to improving the conversion efficiency and stability of the pulse expander. The fundamental and its second harmonic components were separated by a beam splitter, and then stretched by independent gratings to ~70 ps and ~100 ps, respectively, with the same chirp parameter within the experimental accuracy. The two beams were finally focused by a 50-mm focal-length lens and overlapped in a 0.5-mm-thick BBO crystal noncollinearly with type-I phase-matching, and the delay between the two pulses was adjustable. The Rayleigh ranges of the focused beams were comparable to the crystal length, and the position of the crystal was set so that it slightly beyond the focusing points of the interacting beams to avoid damage. The narrowband long pulse was generated through parametric process in which the chirped second-harmonic pulse acted as a pump and the chirped fundamental pulse as a seeding signal. With the equal chirp parameter α, the instantaneous frequencies of the pump and signal pulses can be expressed as ω p(t)=2ω 0+αt and ω s(t)=ω 0+α(t-τ), respectively, where τ is the time delay between the two pulses. The generated difference frequency (i.e., the idler) is given by ω i=ω 0+απ with a bandwidth that to the first order comparable to the inverses of the chirped-pulse durations, which can be tuned either broadly by varying ω 0 or slightly by adjusting π.
Fig. 1. Schematic of the experimental set up. G1, G2, gratings; M, dichroic mirror; RM1-RM4, roof-mirror reflectors; L, achromatic lens; All unmarked mirrors are high reflective silver mirrors. Two half-wave plates are used to keep the grating diffraction plane parallel to the optical table.
In the experiments, only about 1.5 μJ of the pump pulse and 3 μJ of the signal pulse reached into the final BBO crystal, mainly limited by the loss of the optics used. The pump intensity was estimated to be about 1.5 GW/cm2. The narrowband idler pulse with energy of ~10 nJ was obtained, corresponding to the pump-to-idler conversion efficiency of ~0.7%. Typical spectrum and autocorrelation trace of the generated narrowband pulse, measured by Ando AQ-6315A spectrum analyzer and a home-built real-time autocorrelator, respectively, are shown in Fig. 2(b). The obtained bandwidth of 0.12 nm and pulse duration of 45 ps (assuming a Gaussian pulse shape) corresponds to a time-bandwidth product of 1.4. The initial femtosecond pulse from the OPA has a bandwidth of ~20 nm, thus bandwidth reduction by a factor over 160 has been demonstrated. In principle, temporal duration of the idler pulse should be identical to the signal pulse in the ideal case, noting the degenerate feature of the nonlinear process involved. However, the actual duration of the idler pulse obtained is shorter than that of the signal pulse, which can be attributed mainly to the pulse shortening by GVM in the pulse expander [12]. In addition, a minor reason might be that the chirp parameters of the fundamental and its second-harmonic pulses were not perfectly matched in the experiments.
Fig. 2. Typical spectra of the generated pulses at ~1053 nm: (a) OPA, and (b) pulse expander. Insets: the corresponding autocorrelation traces.
Fig. 3. Tunable spectra from OPA (a–c) and pulse expander (d–f). The spectra were measured at the short-wavelength tuning edge, middle, and long-wavelength tuning edge of OPA and pulse expander, respectively.
In the works using an input with fixed central wavelength for a pulse expander, the generated narrowband pulse can be slightly tunable within the bandwidth of the initial source by adjusting the relative delay between the interacting pulses [10, 12]. In such a case, however, the tuning range is quite limited, typically ~10 nm. Based on the tunability of the OPA used in our case, however, the pulse expander can be tuned in a much broader range, which is promising in spectroscopy applications. The experimental results are shown in Fig. 3. Tuning of the pulse expander was accomplished mainly by tuning the OPA and correspondingly adjusting the orientations of the partially deuterated KDP and BBO crystals for phase-matching. Besides, slight adjustment of the optical delay line may enhance the tunable range of the pulse expander. The tuning range of the pulse expander from 1000 nm to 1090 nm was obtained with the input from 1010 nm to 1080 nm, within which the pulse expander delivered narrowband pulses with energy of ~10 nJ, bandwidth of ~0.1 nm and duration of ~45 ps. With accurate calibrations, the whole system can potentially be adjusted automatically which is under implementation. In principle, generation of longer narrowband pulses, e.g., over 100 ps, is feasible at the price of lower conversion efficiency, if larger stretching ratios hence gratings with larger aperture are adopted.
We note that the tuning range of the pulse expander matches well with the gain bandwidth of Yb:glass or Yb:YAG, thus the narrowband pulses from the pulse expander may potentially be amplified further in these laser media if necessary. A commercially-available femtosecond Ti:sapphire regenerative amplifier may deliver pulse energy from several hundred microjoules to mJ-level that is sufficient to pump two femtosecond OPAs simultaneously. The two intrinsically synchronized OPAs can be tuned independently, and one of them can be used to seed the pulse expander. Thus synchronized and independently tunable femtosecond and tens of picoseconds (potentially hundreds of picoseconds) optical pulses can be provided by an integrated system, which should be promising in temporal- and spectral-resolved spectroscopic studies.
Precise chirp-matching between the pump and signal pulses is the major factor in obtaining the narrowest bandwidth of the idler pulse. In the experiments, this was accomplished by fixing the stretcher for the signal pulse (i.e., the fundamental) while adjusting arm length of the stretcher for the pump pulse (i.e., the second-harmonic), with the help of monitoring the idler spectrum. The sensitivity of the idler bandwidth on deviation of the stretcher arm length from its optimum position was measured and the results are given in Fig. 4, which shows centimeter-magnitude deviation is tolerable in obtaining sub-nanometer bandwidth of the idler pulse. Besides, we should also point out that once optimized, the stretchers do not need to be re-adjusted during tuning and still quite satisfactory output can be obtained.
Fig. 4. The measured bandwidth of idler pulse as a function of the detuning of grating position (G2) around the optimum position for chirp matching.
3. Conclusion
In conclusion, we have established a tunable optical pulse expander, initiating from a femtosecond optical parametric amplifier, which is based on chirp-matched frequency mixing technique. The device delivers nJ-level pulses with bandwidth of ~0.1 nm and duration of ~45 ps within its tuning range from 1000 nm to 1090 nm. The combination of a femtosecond OPA and a pulse expander can provide synchronized and both tunable femtosecond and tens of picoseconds (even hundreds of picoseconds) pulses simultaneously, which may find applications where both temporal and spectral resolutions are needed.
Acknowledgements
This work was partially supported by the Natural Science Foundation of China (grant Nos. 60538010, 10335030 and 10376009), and the Science and Technology Commission of Shanghai (grant Nos. 05JC14005 and 05SG02).
References and links
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