We constructed an optical parametric amplifier with BiBO crystals, which was injection seeded by a phase-modulated cw beam in the 1,040-1,070 nm region. Two-stage pre-amplification by Yb-doped fibers were implemented for stable injection to the OPA. The frequency chirp in the OPA pulse was actively controlled by adjusting the RF wave for the phase modulation and its synchronization to the OPA firing. Down/up chirps with up to 500 MHz shift were demonstrated. The output pulse energy was ~40 mJ, which is sufficient for future application of frequency conversion and coherent population transfer.
©2013 Optical Society of America
Tunable nanosecond (ns) pulsed lasers with single-longitudinal mode (SLM) operation are significantly useful for atomic and molecular spectroscopy, since detailed spectroscopic information can be obtained owing to the narrow bandwidth of the light sources and a variety of nonlinear spectroscopic approaches can be implemented due to the high peak power of the ns pulses [1,2]. Furthermore, high spectral coherence of the pulses allows us to realize rapid adiabatic passage (RAP), which drives coherent population transfer from an initial quantum state to a target state with 100% efficiency . So far, all the experiments on RAP in the ns regime have employed optical or static electric fields with varied strengths to induce “avoid crossings” of light-dressed states . If the spectral amplitudes and phases of ns pulses are actively controlled, as have already been realized by the pulse shaping technology in the ultrafast, i.e., femtosecond (fs) to picosecond (ps) time domain, applicability of RAP can be vastly expanded [4,5].
In the fs/ps pulse shaping, ultrashort pulses are spectrally dispersed and each frequency components are manipulated by using light modulating devices [4,5]. These schemes cannot be employed in the ns regime because of the narrow bandwidth of SLM pulses. In the use of continuous-wave (cw) light, on the other hand, highly sophisticated modulation of amplitudes and phases has been realized owing to recent developments in optical communications. In the present study, we incorporated the advanced cw modulation methodology with pulsed amplification by using an injection-seeded optical parametric amplifier (OPA), to obtain shaped ns pulses with high peak power. So far, there have been many reports on injection-seeded ns pulsed optical parametric oscillators (OPO) [6–9]. Though the OPOs afford narrow-bandwidth pulses with high peak power, it is highly demanding to couple an arbitrarily modulated seeding light with the optical cavity used in the OPOs. OPA will be an ideal approach in respect of its cavity-free operation. However, it has a drawback, i.e., difficulty in stable injection seeding by weak cw beams, and this may be the reason for scarcity of successful reports on narrow-band OPA [10,11].
To maintain stable injection-seeded OPA operation, we adopt a modular approach; the experimental setup is composed of an external cavity diode laser (ECDL) as a cw light source, an optical phase modulator (OPM) for providing appropriate modulation to the seeding light, a pair of Yb-doped optical fibers (YDF) for pre-amplification of the seeding beam, and an OPA to have final outputs with desired pulse energy. The OPA was pumped by the second harmonic of a Nd:YAG laser, and the wavelength of the seeding cw light was ca. 1,050 nm. The pre-amplification provided >200 W peak power of the seeding light, which was essential to sustain stable injection seeding in the OPA and to attain an output power at a desired level. In the present setup, BiBO was selected as nonlinear crystals because of its large nonlinear susceptibility, and the final output energy of ~20 mJ/pulse was obtained for each of the signal and idler waves. By appropriately adjusting the RF modulation frequency and depth as well as the timing between the RF wave and the OPA firing, we realized (almost) linear frequency chirps, with frequency shift up to 500 MHz during the 8 ns pulse width of the OPA. The time-dependent frequency shift was monitored in real time by employing optical-heterodyne (OH) measurements. In what follows, we describe the experimental setup, discuss its characteristics, in particular, about the control of frequency chirp, and conclude by mentioning its possible application.
Schematic of the present experimental setup is shown in Fig. 1 . The coupled power of ~8 mW from a home-built ECDL, operating in the wavelength region of 1,040-1,070 nm, was amplified to ~1 W by passing through an polarization-maintained (PM) YDF (15 μmϕ core, 130 μmϕ clad, 2 m long; Nufern PLMA-YDF-15/130) pumped by the 3 W output from a laser diode (LD; Lumics, LU0975T100) emitting at 976 nm. The amplified cw light was then delivered into an acousto-optic modulator (AOM; Crystal Technology, LLC 3200-1113), driven by a pulsed RF field at 200 MHz. This AOM cut out near IR pulses with a duration of 20 ns (FWHM) from the cw light at a repetition rate of 15 kHz. The resultant pulsed light, frequency shifted by 200 MHz, was fed into an OPM (EOSPACE, PM-AV5-40-PFU-PFU-105). The modulator can phase-modulate the near IR light with modulation frequency up to 40 GHz. It consists of a LiNbO3 waveguide, and needs the RF field strength of only 2.6 V for providing modulation depth of π radian. So, the output from an arbitrary waveform generator (AWG) was used directly as the RF wave to drive the modulator. The modulated pulses were further amplified by passing through the second PM YDF pumped by the 3 W output from another LD. The output energy was 4 μJ/pulse, corresponding to the peak power of 200 W. The pulsed beam with 1.8-2.0 mm diameter was finally delivered into an OPA, which consisted of two BiBO crystals (type I, 5 mm square aperture with 15 mm long). The OPA was pumped by a 3.2 mm diameter beam of the second harmonic of an injection-seeded Nd:YAG laser, which operated at a repetition rate of 30 Hz. The repetition rate was set sub-harmonics of the OPM modulation frequency, so as to synchronize the pump firing to the modulation wave within 1 ns. The 532 nm pump pulse was also synchronized to the seeding pulse. To reject unwanted back reflection, several optical isolators (OI) were installed in the optical pass. A number of half-wave plates and polarizers were used for controlling the polarization direction of light beams (not shown in Fig. 1).
For the measurements of frequency components of the OPA output, we employed the OH technique [2,12]. A non-diffracted cw beam from the AOM was passed through another AOM (Brimrose, GPM-400-100), which was operated at 400 MHz. The resultant diffracted near IR beam was merged with a trace fraction of the OPA pulse, and monitored by a high-speed photo detector (PD) with 1.2 GHz bandwidth. The PD output was directly recorded by a 40 GS/s digital oscilloscope with 3.5 GHz bandwidth. The digitized time-domain signals were transferred to a personal computer for further data analysis.
The present two-stage pre-amplification configuration has two advantages. The first stage provided a strong cw beam for the reference in the OH measurements. In the second stage, the pulsed input was amplified so that the LD pumping power can be greatly reduced than the case for amplifying cw lights, while keeping the output peak power at the desired level. The output power from the first fiber was kept below the limit to the OPM input. The pumping power to the second fiber was limited so as to avoid unwanted stimulated Brillouin scattering induced by the SLM light field .
3. Results and discussion
When maximum pumping pulses of 140 mJ at 532 nm were delivered, the total OPA output energy reached ~40 mJ/pulse. Since the wavelengths of the signal (i.e., seeding; 1,050 nm) and the idler (1,078 nm) beams are quite close, the pulse energy was ~20 mJ for each. The resulting total conversion efficiency was ~27%. The value is consistent with that calculated with χ(2) of BiBO , dimensions of the crystals used in the OPA, and the seeding field strength. Even at this level of pumping, no optical parametric generation (OPG) was observed when no seed was introduced, indicating quite high threshold for OPG in near degenerate mode without cavity. The pulse duration of the OPA output was 8 ns (FWHM), which was slightly shorter than those of the 532 nm pump pulse (12 ns) and the seeding pulse (20 ns).
The OH signals of the OPA outputs, with and without the phase modulation to the seeding beam, are shown in the upper panels of Fig. 2 . The high coherence of the OPA pulse is evident as the full modulation of the signals within the pulse envelope. The beat frequency is certainly the sum of the RF frequencies (600 MHz) fed into the two AOMs, when the seeding beam was not modulated [Fig. 2(a)]. When the phase modulation was applied, the beat frequency changed in time, as shown in Figs. 2(b) and 2(c). Shifts in the time-dependent (or instantaneous) frequencies were obtained from the observed OH traces by a Fourier-transform (FT) analysis, which was similar to those developed in Refs [2,12]. They are shown in the lower panels of Fig. 2.
The OPA output without the phase modulation shows frequency fluctuation ranging ± 10 MHz during the 8 ns pulse [Fig. 2(d)]. On the other hand, the frequency of the seeding beam before OPA was observed as constant within ± 1 MHz during the 20 ns pulse width by the OH measurements (not shown). The larger fluctuation in the OPA may arise from the interference to the beat analysis by the high-frequency components from the sharp rising and falling edges of the OPA pulse envelope . No further systematic change (i.e., frequency chirp) exceeding the fluctuation of ± 10 MHz was observed in the OPA output. The FT analysis showed that the bandwidth of the OPA pulse was 70 MHz (FWHM). This value is slightly larger than that of a FT-limited Gaussian pulse with 8 ns duration, 55 MHz.
Figures 2(b) and 2(c) show the OH signals with the phase modulated seeding beam, taken by applying a sinusoidal RF field at 45 MHz with the modulation depth of π radian. The two traces correspond to different setting for relative timing between the OPA pulse firing and the modulation RF wave, as schematically shown in Fig. 3 . The time-dependent frequency shifts [Figs. 2(e) and 2(f)] exhibit much larger changes of ~300 MHz than that without modulation. The changes are almost linear against the time. Their amplitudes are almost the same but the directions are opposite, i.e., increasing and decreasing in frequency, respectively. The results are explained as follows. When applying the time-dependent phase, where m represents the modulation depth and is the modulation frequency, the instantaneous frequency is given as, where is the carrier frequency. Because the OPA pulse duration was shorter than the RF cycle, (~22 ns), a part of the sinusoidally changing appeared within the time window of the OPA pulse. Thus, by appropriately adjusting the timing between the OPA pulse and the modulation wave, as shown in Fig. 3, we obtained the frequency-up and -down chirped pulses, as observed in Figs. 2(e) and 2(f). Since the time window covered ca. in the present condition, the frequency shift during the OPA pulse is approximated to be: ~240 MHz, which is in accord with the observation.
Degree of frequency chirp in the OPA pulse can be controlled easily by adjusting the modulation depth and frequency. When the same value of = 45 MHz was adopted while the value of m was increased to 1.9π, the OH signal was observed as shown in Fig. 4(a) . The corresponding time-dependent frequency shift [Fig. 4(c)] exhibits the change of > 500 MHz. The value is ca. × 1.9 of that with m = π, as expected. When the doubled modulation frequency (90 MHz) was applied with m kept to π, the OH signal was obtained as shown in Fig. 4(b). In this case, ~11 ns was only slightly larger than the OPA pulse duration, and the time window covered almost full cycle of the RF wave. Then, the time-dependent shift cannot be approximated to be linear, and exhibits a sinusoidal curve as observed in Fig. 4(d). The amplitude is ~500 MHz, which is close to the value of In the previous study, the frequency chirp in the OPO output was controlled by carefully adjusting the detuning of the injection-seeding frequency relative to the free-running OPO frequency . The maximum chirp rate was reported to be ~2 MHz/ns, and the total shift then realized was ≤ 60 MHz. The present study achieved much larger chirp (~70 MHz/ns) and total shift (~500 MHz).
We demonstrated injection-seeded operation of an OPA in near degenerate mode with two BiBO crystals. The seeding beam was pre-amplified to have a peak power of > 200 W by using a pair of Yb-doped fibers. The final output energy of the OPA was ~20 mJ/pulse for each of the signal and idler waves. The injection-seeding beam was phase-modulated so that the frequency chirp in the OPA pulse was actively controlled. By properly adjusting the parameters of the RF wave for the phase modulation and the timing between the RF wave and the OPA firing, almost linear chirp was realized with its frequency shift up to 500 MHz. This OPA will be a suitable light source for high-resolution atomic and molecular spectroscopy. Its high peak power allows us to easily apply frequency conversion, e.g., harmonic generation and sum frequency mixing, to have intense narrow-band pulses in the visible and UV region. It will also be used to execute nonlinear optical processes, e.g., two-photon absorption and stimulated Raman scattering. A particularly promising application is the realization of chirped adiabatic Raman passage (CARP), which was originally proposed in the ultrafast regime . The present OPA is designed to produce the chirped signal and idler waves, of which energy difference is in the range from 0 to 400 cm−1 (or 13 THz). Thus, by using the two beams as pump and Stokes pulses for CARP experiments, we will realize coherent population transfer in intra- and inter-molecular low-frequency Raman transitions. This will be significantly useful for precise spectroscopic investigation and detailed study on state-specific dynamics.
We thank Prof. Kiyofumi Muro of Chiba University for his advices on the design of the EDCL and Mr. Shouji Sugito of Equipment Development Center, IMS, for his contribution in the construction of the EDCL. This work was partly supported by grants-in-aid from MEXT Japan (#20050032, #22018031, and #22245004), the RIKEN-IMS joint program on “Extreme Photonics,” and Consortium for Photon Science and Technology.
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