A new design of signal to noise ratio (SNR) measurement for single shot laser pulses, based on a stepped grating and an optical Kerr gate (OKG), is presented. A single shot laser pulse was simultaneously divided into multi-pulses in both time and space. Intensity-space distribution of laser multi-pulses, which was recorded with a CCD detector, was transformed into intensity-time distribution. Time resolution of 1.95 ps and detection time range of 42.9 ps were obtained, respectively. Moreover, optical spatial localization attenuators were introduced into optical path to acquire wide dynamic range in the SNR measurement and, as a result, the dynamic range has been extended remarkably.
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Recently, peak intensity of optical pulses produced in high-power laser systems has been able to reach 1022Wcm−2 [1–3]. At such a high peak intensity, SNR of laser pulses is of fundamental interest and yet of particular importance in many research and application fields, such as plasma physics, high-order harmonic generation, inertial confinement fusion, and quantum electrodynamics [4–6]. In general, laser systems generating optical pulses of high peak intensity operate at extremely low repetition rates, where pre-pulses or noises have strong impacts on the interaction of a main pulse with a target. Precisely obtaining detailed information of pre-pulses with picosecond time resolution in a large time range, e.g., about one hundred picoseconds, prior and posterior to the main pulse and, eventually, completely eliminating the pre-pulses are of profound significance in processing experimental data and explaining experimental results. Therefore, it is imperative to develop a device capable of measuring SNR of single shot laser pulses with wide dynamic range and large temporal window.
So far, a great deal of endeavor has been dedicated to the SNR measurements of optical pulses and, consequently, several single shot pulse measurement techniques [7–11] have been developed based on second-order and third-order correlation. The basic idea of the aforementioned techniques is to transform temporal shape of a pulse into spatial profile that can be analyzed with a plane array detector. Subsequently, an optical parametric amplifier correlator was implemented to measure high dynamic range contrast , and a third-order correlator was used to characterize few-cycle laser pulses . The application of the second-order and the third-order correlation techniques, however, is limited to a relatively fixed spectral range because of the requirement for phase matching in nonlinear crystals, accordingly, initiating an urgent requirement for SNR measurement techniques of single shot pulses with little spectral limitation. Fortunately, Mayer and Gires first observed the optical Kerr effect in 1964 . Duguay and Hansen devised an optical Kerr gate driven by ultrashort optical pulses in 1969 . Till now, optical Kerr gate has been widely used in investigation of various ultrafast optics phenomena such as self-diffraction , transient luminescence [17–20], light absorption, photoconductivity, photoimaging [21–23]. Albrecht et al. detected femtosecond pulses by using optical Kerr gate technology in several picoseconds window . One of the most distinct features of optical Kerr effect is that it occurs in all materials and has no limitation on spectral range. In this study, we developed a SNR measurement for single shot laser pulses with wide dynamic range and 40 ps temporal window.
2. Principle of Measurement
According to the gate scanning technology of signal collection, sampling gate is usually used to scan slowly through the signal. The gate scanning technology has been demonstrated to be valid merely for high repetition signal collection. Since it is invalid in collecting single shot pulses, one has to resort to other ways to tackle the problem. Toward this end, one of the feasible approaches is to divide a single pulse into multi-pulses. An ultrashort light pulse incident on a grating or a prism is chirped, with different frequencies distributing in spatial along the continuum temporal profile [25, 26]. The frequency distribution, in general, is nonlinear, leaving much inconvenience for data processing.
The principle of measurement is shown in Fig. 1 , in which N represents the number of the multiple light pulses. An optical Kerr gate is used to sample the multiple light pulses. Once the optical Kerr gate is open, the N light pulses are all sampled simultaneously at different positions in space (dotted line in Fig. 1), recorded by a plane array detector CCD (the right curve in Fig. 1). Hence, according to the corresponding space and time of the N light pulses, the SNR of the single shot pulse can be readily obtained using a spatio-temporal transformation.
3. Experiments and Results
The experimental setup is schematically shown in Fig. 2 . One single shot laser pulse, 800 nm, 200 fs and 300 μJ, generated from a Ti:sapphire regenerative amplifier (Spitfire, Spectra Physics Co.), was split into two pulses, signal pulse and gate pulse, and optical anisotropy in the optical Kerr material was created by the gate light. The ratio of the beam splitter was 10:1, and horizontal size of the beam was 30 mm. The signal light beam was expanded with a beam expander and then passed through a slit (0.2 mm width) to produce a slender light beam. The beam was spatially homogeneous, with an error less than 1%. A stepped grating was used to divide the slender light into N light beams (N=22 in this paper) successively arraying in space with a fixed delay time Δt=1.95 ps (see Fig. 2 inset). The grating was made by a stack of glass of 1.1 mm width tightly pressed. Parasitic reflections existed in the measured profile by transmission gratings. The signal spectrum was not modified by the device. In the optical path of signal light, two crossed polarizers, P1 and P2, were placed before and behind the Kerr material, respectively, forming an OKG. The gate light pulse, polarized at an angle of 45° to the polarization direction of the polarizer P1, after propagating through an optical delay line, was focused on and coupled with the N light beams in the optical Kerr material, CS2 with thickness of 1 mm, and then blocked. An ultrashort light pulse incident on the stepped grating was divided into multiple ultrashort light pulses, distributing in space with constant time delay Δt between adjacent pulses. There were light scattered from the edge with large angles when the Ker medium was placed at the focusing point of lens L1, nevertheless, as the gating pulse arrived, the Kerr medium spatially filtered the scattered light. The threshold intensity level was about 109 Wcm−2, signal and gate pulses were precisely overlapped at the Kerr gate. When the gate light pulse arrived at the optical Kerr material, the OKG was opened. At the same time, the N light pulses crossing through the optical Kerr material were sampled and recorded in space by CCD.
The time resolution of the setup was determined by the laser pulse width (200 fs in our case), the fixed delay time Δt (1.95 ps) which is determined by the grating and the opening duration of the OKG. The full width half maximum of the dynamic response curve of CS2 is 570 fs, as shown in Fig. 3 . Therefore, the time resolution of the setup was limited to be 1.95 ps and the detection time range is calculated to be 42.9 ps, taking into account of the number of the steps of the grating, N=22.
According to the principle of measurement shown in Fig. 1, for a single shot laser pulse, once a noise pulse appears before the main pulse, it is captured and recorded by the CCD. We rectified the shape of gating pulses, suppressing as strongly as possible the pre-pulses and post-pulses, enhancing the S/N ratio of gating by four orders of magnitude. Moreover, the intensity of gating pulses can be adjusted to reduce noise to an extreme extent, forming a situation that only the main pulse can open the gate. In order to confirm this point, an artificial pre-pulse was generate in Fig. 4(a) , the two bright spots in the image are the main pulse (left) and the pre-pulse (right) gated by the OKG, respectively. The intensity-time distribution curve, as displayed in Fig. 4(b), was obtained by the spatio-temporal transformation of Fig. 4(a). The feature after the main pulse at ~20 ps may be explained in terms of the pedestal of ma d by a reflector with single-side coating. When a light pulse was incident on the non-coating surface of the reflector, the reflected pulse on this surface formed a pre-pulse, which was much weak compared to the main pulse due to the low reflectance of the non-coating surface. The transmitted light pulse was reflected on the coating surface, forming the main pulse, which was delayed in time with respect to the pre-pulse by 9.6 ps. The reflectances of the two sides of the mirror considered, the calculated intensity ratio of the main pulse to the pre-pulse was 23. By changing the time delay between the signal pulse and gate pulse, the pre-pulse and the main pulse were captured simultaneously. As shown in pulse. The time interval between the two pulses measured in the experiment was 9.75 ps and, meanwhile, the intensity ratio of main pulse to the pre-pulse measured in the experiment was 29, indicating that the experimental results are in excellent agreement with the theoretical prediction.
To fully explore the SNR measurements for single shot optical pulses, we focused our attention on the dynamic range of the setup, which was limited by the detector CCD’s, 16 bits. Extinction ratio is meant by the ratio of the intensity of light passing through the closed Kerr gate to the light intensity before going into the gate. In the experiment, the extinction ratio for the Glan prism is 10−6, i.e., the extinction ratio for Kerr gate is 10−6. Inasmuch as the S/N of laser pulses was not very high, the measured result was the S/N of the laser pulses, rather than the dynamic range of the system. For one Kerr gate system, the dynamic range of the system is 106.
In order to increase the dynamic range, a series of optical spatial localization attenuators were implemented to solely reduce the main pulse intensity to an unsaturated level on the CCD, giving rise to an intensity of the main comparable to that of the pre-pulse. Figure 5 shows the detection result of laser pulse in which only the main pulse was weakened by a factor of 40. The intensity ratio of the main pulse to the pre-pulse was retrieved to be 28.8, suggesting a result consistent with that in Fig. 4, as well as a dramatic extension of the dynamic range. Although attenuators can be employed to enlarge the dynamic range of a CCD, ultimately, the dynamic range is limited by the extinction ratio of the OKG.
In conclusion, based on a stepped grating and an OKG, a new SNR measurement of single shot laser pulses has been demonstrated. A stepped grating, composed of a stack of glass slices, was utilized to divide a single shot pulse into multi-pulses separated both in time and space. As the OKG was opened, the multi-pulses were sampled simultaneously at different positions in space and, then, recorded by a plane array detector CCD. The intensity-space distribution of laser multi-pulses was transformed into the intensity-time distribution. As a consequence, time resolution of 1.95 ps and detection time range of 42.9 ps have been obtained, respectively. In addition, the dynamic range of the SNR measurements for single shot optical pulses was considerably increased by reducing merely the intensity of the main pulse.
This work was financially supported by the National Natural Science Fund Committee- Chinese Academy of Engineering Physics Joint Fund (NSAF, No. 10576037) and the National Natural Science Foundations of China (No. 60978038, 60808023).
References and links
1. J. Bromage, S. W. Bahk, D. Irwin, J. Kwiatkowski, A. Pruyne, M. Millecchia, M. Moore, and J. D. Zuegel, “A focal-spot diagnostic for on-shot characterization of high-energy petawatt lasers,” Opt. Express 16(21), 16561–16572 (2008). [PubMed]
2. V. Yanovsky, V. Chvykov, G. Kalinchenko, P. Rousseau, T. Planchon, T. Matsuoka, A. Maksimchuk, J. Nees, G. Cheriaux, G. Mourou, and K. Krushelnick, “Ultra-high intensity- 300-TW laser at 0.1 Hz repetition rate,” Opt. Express 16(3), 2109–2114 (2008). [CrossRef]
3. S.-W. Bahk, P. Rousseau, T. A. Planchon, V. Chvykov, G. Kalintchenko, A. Maksimchuk, G. A. Mourou, and V. Yanovsky, “Characterization of focal field formed by a large numerical aperture paraboloidal mirror and generation of ultra-high intensity (1022 W/cm2),” Appl. Phys. B 80(7), 823–832 (2005). [CrossRef]
4. G. A. Mourou, C. P. J. Barry, and M. D. Perry, “Ultrahigh-Intensity Lasers: Physics of the extreme on a tabletop,” Phys. Today 51(1), 22–28 (1998). [CrossRef]
5. V. I. Kukulin and V. T. Voronchev, “Pinch-based thermonuclear D3He fusion driven by a femtosecond laser,” Physics of Atomic Nuclei 73(8), 1376–1383 (2010). [CrossRef]
6. P. M. McKenty, V. N. Goncharov, R. P. J. Town, S. Skupsky, R. Betti, and R. L. McCrory, “Analysis of a direct-drive ignition capsule designed for the National Ignition Facility,” Phys. Plasmas 8(5), 2315–2322 (2001). [CrossRef]
8. A. Brun, P. Georges, G. Le Saux, and F. Satin, “Single-shot characterization of ultrashort light pulses,” J. Phys. D Appl. Phys. 24(8), 1225–1233 (1991). [CrossRef]
9. J. Collier, C. Hernandez-Gomez, R. Allott, C. Danson, and A. Hall, “A single-shot third-order autocorrelator for pulse contrast and pulse shape measurements,” Laser Part. Beams 19(2), 231–235 (2001). [CrossRef]
10. D. Zhang, L. Qian, P. Yuan, H. Zhu, S. Wen, and C. Xu, “Fiber-array-based detection scheme for single-shot pulse contrast characterization,” Opt. Lett. 33(17), 1969–1971 (2008). [CrossRef]
11. R. C. Shah, R. P. Johnson, T. Shimada, and B. M. Hegelich, “Large temporal window contrast measurement using optical parametric amplification and low-sensitivity detectors,” Eur. Phys. J. D 55(2), 305–309 (2009). [CrossRef]
13. F. Tavella, K. Schmid, N. Ishii, A. Marcinkevičius, L. Veisz, and F. Krausz, “High-dynamic range pulse-contrast measurements of a broadband optical parametric chirped-pulse amplifier,” Appl. Phys. B 81(6), 753–756 (2005). [CrossRef]
14. G. Mayer and F. Gires, “Action d′une onde lumineuse intense sur l′indice de refraction des liquids,” Compt. Rend. Acad. Sci. (Paris) 258, 2039–2042 (1964).
15. M. A. Duguay and J. W. Hansen, “An ultrafast light gate,” Appl. Phys. Lett. 15(6), 192–194 (1969). [CrossRef]
16. L. H. Yan, J. J. Yue, J. H. Si, and X. Hou, “Influence of self-diffraction effect on femtosecond pump-probe optical Kerr measurements,” Opt. Express 16(16), 12069–12074 (2008). [CrossRef]
17. S. Kinoshita, H. Ozawa, Y. Kanematsu, I. Tanaka, N. Sugimoto, and S. Fujiwara, “Efficient optical Kerr shutter for femtosecond time-resolved luminescence spectroscopy,” Rev. Sci. Instrum. 71(9), 3317–3322 (2000). [CrossRef]
18. R. Nakamura and Y. Kanematsu, “Femtosecond spectral snapshots based on electronic optical Kerr effect,” Rev. Sci. Instrum. 75(3), 636–644 (2004). [CrossRef]
19. J. Takeda, K. Nakajima, S. Kurita, S. Tomimoto, S. Saito, and T. Suemoto, “Femtosecond optical Kerr gate fluorescence spectroscopy for ultrafast relaxation processes,” J. Lumin. 87–89, 927–929 (2000). [CrossRef]
20. J. Takeda, K. Nakajima, S. Kurita, S. Tomimoto, S. Saito, and T. Suemoto, “Time-resolved luminescence spectroscopy by the optical Kerr-gate method applicable to ultrafast relaxation processes,” Phys. Rev. B 62(15), 10083–10087 (2000). [CrossRef]
21. K. Minoshima, T. Yasui, E. Abraham, H. Matsumoto, G. Jonusauskas, and C. Rullière, “Three-dimensional imaging using a femtosecond amplifying optical Kerr gate,” Opt. Eng. 38(10), 1758–1762 (1999). [CrossRef]
23. L. Wang, P. P. Ho, C. Liu, G. Zhang, and R. R. Alfano, “Ballistic 2-d imaging through scattering walls using an ultrafast optical kerr gate,” Science 253(5021), 769–771 (1991). [CrossRef]
24. H.-S. Albrecht, P. Heist, J. Kleinschmidt, D. V. Lap, and T. Schröder, “Single-shot measurement of ultraviolet and visible femtosecond pulses using the optical Kerr effect,” Appl. Opt. 32(33), 6659–6663 (1993). [CrossRef]
25. E. B. Treacy, “Measurement and Interpretation of Dynamic Spectrograms of Picosecond Light Pulses,” J. Appl. Phys. 42(10), 3848–3858 (1971). [CrossRef]
26. M. R. Topp, “Oscilloscope display of picosecond fluctuations in light intensity,” Opt. Commun. 14(1), 126–130 (1975). [CrossRef]