We have achieved a high compression ratio by stimulated Brillouin scattering (SBS) consisting of two long cells. A 13-ns Nd:YAG laser pulse was temporally compressed to about 160-ps phase-conjugated pulse in heavy fluorocarbon FC-40 liquid at a 1064 nm wavelength. The maximum reflectivity of SBS process was over 80 % without an optical damage. The compressed pulse brightness was about 65-fold higher than that of the incident pulse.
© 2009 Optical Society of America
A stimulated Brillouin scattering-phase conjugation mirror (SBS-PCM) is a useful tool for improving the beam quality of high-peak-power laser systems in various liquids, gases and solids [1–6]. Excellent SBS-liquid media, such as heavy fluorocarbons [7, 8] and tetrachlorides, require the fine filtration of dissolved impurities and solid micro-particles. The SBS compression of laser pulses has been investigated theoretically and experimentally for various laser wavelengths in a range of liquids, gases and solids [9–14]. An SBS compressor enhances the peak intensity of pulses while maintaining their high energy, and can extend the laser pulse duration to within 1 ns. This method is very simple and can be used to achieve high brightness. In order to obtain picosecond pulses, the mode-locking scheme with a regenerative amplifier is used to apply. This scheme generally requires a complicated configuration of optics. The SBS pulse compressor is similar to a SBS-PCM, and uses a very long SBS cell (or two long SBS-cells). The optical layout for the SBS-PC is also similar to that of a SBS phase conjugation mirror, where the reflected pulse is compressed automatically by the SBS cell.
We previously reported that a 8 ns Nd:YAG laser pulse was temporally compressed to a SBS phase conjugation pulse of 600 ps in a 1.7-m-long cell using a FC-75 liquid. In high energy operation using FC-75 as the SBS medium, it was possible to convert a 25-J, 25-ns pulse from a Nd:glass laser into a 0.5-ns high contrast pulse with 88 % efficiency . With an SBS-generator filled with SiCl4, (30 % SBS reflectivity) a compressed pulse with duration around 350 ps in a 1-µm laser system was produced . The phonon lifetime τB depends inversely on the pump wavelength. The relaxation time for CCl4 is about 150 ps at 532-nm wavelength, and in water is about 130 ps at 355 nm wavelength . The SBS compression for several wavelengths in various liquids was reported, a 5-ns, 532-nm pulse was compressed to 200 ps using CCl4, and a 5-ns, 355-nm pulse was compressed to 200 ps in water. A pulse width of 350 ps was temporally compressed to 90 ps in a SBS filled with FC-72 liquid at the 248 nm wavelength from a KrF laser [19, 20].
At the magnetic confinement plasma facility, plasma diagnostics by Thomson scattering of a laser beam is a routine tool for measurement of electron temperature and density profiles [21–23]. Since the pulse width of the laser system corresponds to the spatial resolution in the LIDAR Thomson scattering diagnostics[24, 25] the pulse width must be 300 ps or less to obtain 10 cm of spatial resolution. As for the SBS pulse compressor, although there is a room for further investigation, it seems to be possible to compress to shorter pulses for LIDAR Thomson scattering diagnostics in ITER, if an appropriate SBS medium is used.
In this study, we have demonstrated a temporal compressor with high compression ratio by stimulated Brillouin scattering, which consists of two long cells. For a two-cell SBS system at higher laser power, nonlinear processes including optical breakdown, self-focusing, and thermal heating can disrupt the SBS process. For the first time to our knowledge, we have demonstrated over 80 % efficient compression of a 13-ns pulse to about 160 ps at 1064 nm. The compressed pulse brightness was about 65-fold higher than that of the incident pulse.
2. Experimental results
The optical layout of the temporal SBS compressor is shown in Fig. 1. The laser used in the experiments was a linearly polarized single frequency Q-switched Nd:YAG oscillator with TEM00 mode operation. The beam quality was 1.3 times diffraction-limited. The single shot output energy amplified by a glass rod amplifier was over 1 J in 13-ns pulse duration. The laser light was introduced into the SBS cell through a variable attenuator using a combination of a half-wave plate and a thin-film polarizer. The reflected laser beam was separated using a dielectric thin-film polarizer and a quarter-wave plate. We fabricated two long SBS cells as the liquid phase conjugation mirror. Both had a length of 1.5 meters and a diameter of 4 cm and the windows used were specially made with an antireflection coating at 1064 nm. The cell was filled up with the heavy fluorocarbon FC-40 liquid. The absorption coefficient at 1064 nm for the FC-40 liquid is <1×10-5 cm-1. Before the filling process, we filtered the liquid FC-40 by using Millipore filters with pore size of 0.025 µm. This filtering is important to increase the optical breakdown threshold. The incident laser beam was about 9.5 mm and was expanded to a diameter of 1.8 cm by using a beam expander consisting of a convex and a concave lens-pair. Experiments on SBS compression were performed using a combined generator-amplifier system. The circularly polarized laser beam was transmitted through the 150-cm long SBS cell, and was focused into the other 150-cm long generator using a lens (focal length=300, 750 and 1000 mm). The pulse shapes of the pump and reflected beams were monitored using a Hamamatsu R1328U-51 biplanar phototube (rise time; 55 ps) and a Tektronix CSA7404 signal analyzer (analog bandwidth; 4 GHz and rise time; 100ps). The reflected energy was directly measured using an energy meter. The SBS backward reflectivity from the FC-40 liquid was also compared with the total reflection from a 99.5-% conventional thin-film mirror.
A FC-40 liquid with a fast SBS relaxation time was selected from a lot of heavy fluorocarbon. The phonon lifetime τB and Brillouin bandwidth ΔνB depend inversely on the pump wavelength assuming that τB∝λ- 2p and ΔνB=1/2πτB. For FC-40, the calculated value of ΔνB and the phonon lifetime τB at 1064 nm are approximately 410 MHz and 240 ps, respectively. The SBS gain coefficient of FC-40 was calculated to be 1.5 to 2 cm/GW. Because SBS originates from the leading edge of the propagating pulse, an optimum reflection is expected for the spatial length of the pulse, (c/2n) τπ, where τπ is the pulse width (FWHM) and n is the refractive index of FC-40. For a 13-ns fundamental pulse and a refractive index n=1.28, the interaction length Lin was estimated 169 cm. The optical path length of the Brillouin amplifier, L=300 cm was chosen over 216 cm (l=169 cm, n=1.28) to satisfy the condition L>Lin.
The laser pulse was focused into a single SBS cell using a focal length of 30 cm to measure the reflectivity of conventional SBS phase conjugate mirror. The SBS reflectivity characteristic was shown by the solid line of Figure 2. A maximum SBS reflectivity of 80 % was obtained at an incident energy of about 1 J. The SBS threshold was higher than FC-75 at about 10mJ. No optical damage of the SBS cell was observed during the experiment when a focal length of 30 cm was used.
The SBS compression reflectivity at 1064-nm wavelength for several focal lengths is shown in Fig. 2. The incident laser beam was expanded to a diameter of 1.8 cm by using a beam expander. A maximum intrinsic SBS reflectivity of over 75% was obtained at an incident energy of about 1 J. The expected SBS threshold is about 10–15 mJ because of SBS gain coefficient of 2 cm/GW. The Brillouin generator-amplifier system could be optimized by the insertion of a partial reflectance mirror or neutral density filter between the two cells. Thereby, this system allowed to control of the energy entering the Brillouin generator to prevent optical breakdown. Our results confirm that the SBS process is effective when the coherence length of the optical pulse is equal to or longer than the length of the Stokes field region. In practice, the coherence length of the SBS pump pulse should be longer than the interaction length. The bandwidth of the laser oscillator used here was about 150 MHz and the coherence length Lc was about 200 cm. Total interaction length L in the SBS cell was 300 cm.
The SBS compressed-pulse duration using several different focal lengths is shown in Fig. 3. The temporal-pulse duration for the different focal lengths was compressed from 13 ns to less than 1 ns at an incident energy of about 1 J. At an input energy of about 100 mJ, the compressed pulse width was different depending on the focal length of the SBS generator. The compressed pulse widths for focal lengths f=300 mm and f=1000 mm were about 7 ns and 3 ns, respectively. In the case of the 1000-mm focal length, the compressed pulse width was 0.88 ns at an incident energy of 1.2 J. The maximum compression ratio is about 15-fold higher than that of the incident pulse of 13.5 ns. In Fig. 3, typical shapes of compressed pulses are shown. A slow falling time of the compressed pulse was observed owing to the reduced intensity in the peripheral part of the spatial profile. The pump beam was highly compressed near the center of the beam, while the compression effects was much less at the wings. The pulse shape of the reflected beam was a mixed pulse that combines a fast rising time at the center and a slow falling time at the wing of the pulse.
Next, the beam expansion optics was removed, and the beam diameter was estimated to be 9.5 mm. The SBS compression reflectivity at 1064 nm wavelength using several different focal lengths is shown in Fig. 4. A maximum SBS reflectivity of over 80% using focal lengths f=750 mm and f=1000 mm was obtained at an incident energy of over 1 J. For an incident energy of about 100 mJ, a SBS reflectivity from 60 % to 70% was obtained. The SBS reflectivity characteristics is similar to the conventional SBS generation used for the f=300 mm focal length lens.
The compressed pulse duration without an expander for focal lengths f=750 mm and f=1000 mm are shown in Fig. 5. At an input energy of about 100 mJ, the compressed pulse duration for f=750 mm and f=1000 mm were 1.2 ns and 0.8 ns, respectively. In the case of the 1000 mm focal length, the minimum compressed pulse width achieved about 160 ps at an incident energy of 1 J. The pulse durations for focal lengths f=750 mm and f=1000 mm were 220 ps and 160 ps, respectively. Figure 5 shows the typical pulse shapes for both focal lengths. The brightness was defined in a product of the SBS compression pulse ratio and reflectivity, if a spot size for the SBS-compressor return beam was similar to that of the incident beam. The maximum compression ratio for the reflected pulse width of 160 ps is 81-fold higher than that of for the incident pulse of 13 ns. The compressed pulse brightness was about 65-fold higher than that of the incident pulse, considering SBS reflectivity of 80%.
The phase-conjugation fidelity in terms of the beam quality was defined using the spot size of the far-field pattern of the SBS-return pulse. Figure 6 shows (a) the NFP: near-field pattern and (b) FFP: far-field pattern of the compressed-pulse beam. The NFP of the SBS compressed pulse shows a good flattop pattern as a result of the spatial filtering effect of the SBS process. The reflection from the SBS-PCM was initiated at the beam waist so that the diffracted fraction of the higher spatial frequency of the incident beam felt a weak reflectance. A typical spot size for the SBS-compressor return beam was 1 to 1.3 times diffraction-limited and similar to that of the incident beam. This trend was emphasized in the longer focal-length lens (f=2040 mm). For high-average-power operation of a PC mirror using FC-40, careful maintenance of the conditions is required. With an increasing numbers of incident pulses, the heat absorption of the long cell under repeated laser irradiation increases the probability of optical breakdown.
The beam diameter at the SBS amplifier was about 9.5 mm and was expanded to 1.8 cm by using x 2 beam expander. Figure 7 shows that the compressed pulse width depends on the pump intensity of the SBS amplifier, when the SBS generator was used with the f=1000mm focal lens. The Stokes pulse duration depends on the pumping peak intensity. The compressed minimum pulse width reached to within 200 ps at about 100 MW/cm2. The compressed pulse width τs is simply given as τs=2.3τB/IpgBL, where Ip is the pump pulse intensity, and gB and L correspond to the SBS gain coefficient and interaction length, respectively . The calculated curve is shown in Fig.7. For maximum pumping, the predicted pulse width of 180 ps is in agreement with the measured 160 ps. The temporal profiles were measured on the large-area biplanar phototube covering the whole beam diameter. A slow falling-time of the compressed pulse was generally observed due to the reduced intensity in the wings of the spatial profile. In experiment results on low pumping intensity of about 30 MW/cm2 in Fig.7, the pump beam such as Gaussian spatial mode was only highly compressed near the center of the beam, while in the wings the compression effect was much less. Thus the pulse shape of reflected beam was a mixed pulse that had a combination of fast rising-time in the center and slow falling-time in the wings of the beam (Fig. 3). The central part by the inner 5-mm of diameter aperture is compressed close to the minimum pulse width. On the other, the pulse shapes shown in Fig.5 did not observe the slow falling-time in the wings of the pulse. The varying pulse width over the spatial beam distribution was suppressed when the SBS compression process reach the saturation condition at about 100 MW/cm2.
The transmitted fluence at the waist in the SBS generator was estimated to be over 120 J/cm2 because the transmitted energy was about 200 mJ (incident energy of 1 J and SBS reflectivity of over 80 %). This value is consistent with the damage threshold for long pulse operation because the breakdown threshold of heavy fluorocarbon for a 1 ns pulse was measured to be over 100 J/cm2. This value did not exceed the damage threshold of 360 J/cm2 resulting from the scaling law on pulse width. The damage threshold using a long focal lens for a long-pulse mode cannot be estimated by only the critical incident intensity. The maximum incident energy is available for operation up to and over 3 J for a 13-ns pulse when the SBS reflectivity is 80%. A higher energy operation without breakdown can be achieved using a longer focal length and pulse duration and with rearrangement of the SBS generator-amplifier separation system. In this experiments on pulse compression in FC-40, there is a possibility that the process of stimulated Raman scattering (SRS) competes with SBS. At maximum input energy SRS is very weak and does not influence SBS compression because the temporal fluctuation of the SBS back-scattering pulse did not observe. It seems that the nonlinear effect, such as self-phase modulation and self-focusing, are not caused because the nonlinear refractive index of 1×10-7 cm2GW-1 for Fluorinert liquid12 is low and the beam quality of SBS compression pulse is similar to that of the incident pulse. It will be necessary to measure a nonlinear effect in the future.
From the UV to the visible region, the shorter pulse is expected to compress due to the shorter phonon lifetime of heavy fluorocarbon liquids and the perfluoropolyether liquids . The phonon lifetime depends inversely on the pump wavelength. The SBS temporal compression at 266 nm in FC-40 liquid will expect to generate the shortest pulse duration near the phonon lifetime.
The fluctuation of the pulse width greatly depends on the focusing condition of the SBS generator. The focusing condition depends on the input beam quality of shot to shot and the environmental condition of SBS amplifier. When the SBS cell happen a vibration of the enclosed liquid and the convection flow by an environmental temperature, the focusing performance of the SBS generator decreases. The SBS phase-conjugation ability corrected these perturbations, but it is necessary the SBS cell have good insulation for focusing condition of the SBS generator.
In summary, we successfully demonstrated an FC-40 liquid as a SBS compressor for the short pulse generation. A 13-ns Nd:YAG laser pulse was temporally compressed to a 160 ps phase-conjugated pulse at 1064 nm wavelength. The SBS maximum reflectivity was over 80 % without any damage. The brightness of the compressed pulse was about 65-fold higher than that of the incident pulse. This damage-free operation using a two-cell system of an FC-40 liquid as a superior phase-conjugate material could lead to the construction of a laser-diode-pumped, all-solid-state laser system with much higher brightness. The application of the SBS pulse compression can be expanded, because FC-40 offers stable performance even at high incident energies of a few-J for high repetition rate. The generation of a few hundred ps pulse is useful for precise material processing instead of mode locking laser systems. It seems to be possible to compress ever to shorter pulses for LIDAR Thomson scattering diagnostics in ITER, if an appropriate SBS medium is chosen for high average power operation.
Part of this work is supported in a part by Grant-in Aid for Scientific Researches on Priority Area “Advanced diagnostics for burning plasma” from Ministry of Education, Culture, Sports, Science and Technology (No. 20026011), has been performed under the JT-60 Collaborative Research Program, and has been performed under the Institute of Laser Engineering-Osaka university Collaborative Research Program.
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