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Experimental research on an arbitrary pulse generation system for imaging VISAR
Optics & Laser Technology 43 (2011) 179–182
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Experimental research on an arbitrary pulse generation system for imaging VISAR Rui Zhang n, Mingzhong Li, Jianjun Wang, Wentao Duan, Feng Wang, Xiaoshi Peng, Xiaolin Tian Research Center of Laser Fusion, China Academy of Engineering Physics, Mail Box 919-988, Mianyang, Sichuan 621900, PR China
a r t i c l e in fo
abstract
Article history: Received 22 April 2010 Received in revised form 3 June 2010 Accepted 21 June 2010 Available online 8 July 2010
Imaging VISAR is an important diagnostic tool for a variety of shock-related phenomena in laser-driven experiments. To adapt to various types of shaped driven pulse, the imaging VISAR needs an illuminating light with good shaping capability. Here, a flexible laser probe system was experimentally studied. Being generated from a 1064-nm DFB laser, the continuous wave was modulated by a waveguide amplitude modulator driven by 10 GS/s arbitrary waveform generator. After being amplified by fiber amplifiers and Nd:YAG rod amplifiers, the signal pulse was frequency-converted to 532-nm green light by a thermally controlled LBO crystal with a final output energy larger than 10 mJ. Finally, the green light was coupled into a 1-mm core diameter, multimode fused silica optical fiber and propagated to the imaging VISAR. The probe laser could realize accurate pulse shaping with time resolution below 100 ps. Uniformity in intensity and capability of arbitrary pulse shaping provides great convenience for the analysis of experimental data. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Fiber laser Arbitrary waveform generator VISAR
1. Introduction Laser drivers could generate multiple kilojoules of energy in a nanosecond pulse and focus it to a tiny target [1]. Using a laser probe beam to illuminate the target, the imaging velocity interferometer system for any reflector (VISAR) could measure shock breakout times at temporal resolutions as low as 20 ps [2,3]. Therefore, the imaging VISAR is an important diagnostic tool for a variety of shock-related phenomena in laser-driven targets, including the motion of free surfaces, high-pressure state equation experiments and material characterization experiments [4]. Traditional laser system for imaging VISAR adopts frequencydoubled Q-switched Nd:YAG laser source with 10 ns pulse duration. However, it is difficult and hard to adjust the pulse shape, limiting the application of imaging VISAR, especially in physical experiments using a shaped main pulse with high contrast. The pulse shaping capability improves VISAR experiments by enhancing the probe laser intensity when the main laser pulse strikes on the target, thereby overcoming the X-ray ionizing effect. The free electron produced by the ionizing effect reduces the reflectivity of the probe laser, which would lead to the streak camera being unable to record the interference fringes. Therefore, developing a probe laser with good shaping capability for use in imaging VISAR is necessary.
n
Corresponding author. Tel.: +86 816 2485283. E-mail address: [email protected] (R. Zhang).
0030-3992/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2010.06.010
Among current technologies, fiber laser systems offer unique benefits as front end devices to meet the pulse shaping requirement. Fiber laser systems may be engineered for a high degree of robustness, reliability and ease of use. Large mode area (LMA) fiber amplifiers [5] could be used in the front end to boost the output power to the order of micro joules for subsequent launch into rod or slab amplifiers. In this article we describe a new type of laser probe beam for imaging VISAR and examples of data collected during recent experiments.
2. System layout 2.1. Technical route The probe laser system of imaging VISAR is responsible for providing a visible 7-ns shaped pulse at a wavelength different from the 1o, 2o and 3o of 1053 nm. Therefore, a distributed feedback (DFB) oscillator with 1064 nm wavelength is used. In the spatial amplification stage, Nd:YAG is adopted as the gain material. As high synchronization among probe laser and main laser is required, the two pulses are generated in the different channels of the same arbitrary waveform generator (AWG). Driven by an AWG with 10 GS/s sampling rate, a waveguide amplitude modulator shaped the continuous wave output from DFB laser and polarization stabilizer to the specified waveform. Then the pulse propagated through fiber amplifiers, acoustic optic modulators (AOMs), 10-mm large mode amplifiers, fiber
180
R. Zhang et al. / Optics & Laser Technology 43 (2011) 179–182
collimator, Pockels cell (PC) and Nd:YAG amplifiers. Finally, it was frequency-converted to 532 nm light and coupled to the 1 mm core multimode fused silica optical fiber. The probe laser was then delivered to the imaging VISAR diagnostic table. 2.2. Setup The VISAR laser is composed of pulse generation module, pulse amplification module, frequency conversion module, light coupling module and synchronization module. Fig. 1 shows a sketch of the system. The pulse generation module includes the laser oscillator, amplitude modulator, fiber amplifiers and AOMs. The oscillator is a DFB laser with thermal tuning range of 0.7 nm, which could realize precise control of the center wavelength to the maximum gain range of Nd:YAG amplifier. The DFB laser could output stable single mode with ultra narrow line width of 33 kHz. The optical S/N is better than 50 dB (50 pm resolution). The pulse amplification module includes one double-pass Nd:YAG rod amplifier, two single-pass Nd:YAG rod amplifiers, flash lamps and power supply. To suppress ASE generated by fiber power amplifier, Pockels cell and Glan polarizers are set behind the fiber collimator. The Nd:YAG rod amplifier adopts air cooling, which would greatly reduce the space taken. With single shot mode in full energy shot of facility and 0.05 Hz for imaging VISAR alignment, it has been proved that air cooling is sufficient. The frequency conversion module includes LBO crystal and precise temperature controller. The light coupling module includes two parts. The first part couples the light from the fiber power amplifier into the Nd:YAG rod amplifiers. The second part couples the frequency-converted
532 nm laser into the 1 mm core diameter, 0.37 numerical aperture, multimode fused silica optical fiber. The fiber-coupled arrangements have two advantages. The first part allows us to place the pulse generation module and associated injection optics in a remote location, which could avoid electromagnetic disturbance. The second part is a convenient way to produce spatially smooth source distribution over a well-defined numerical aperture. Spatial mode mixing through the long delivery path ( 30 m) of multimode fiber results in the formation of a random speckle pattern on the output surface of the delivery fiber. On average, the probe beam intensity is uniform across the output
Fig. 3. Output near field of the fiber power amplifier.
Fig. 1. Sketch map of the VISAR laser generation system.
Fig. 2. Fiber power amplifier.
R. Zhang et al. / Optics & Laser Technology 43 (2011) 179–182
face of the fiber. However, on very small spatial scales ( few mm), the beam is highly modulated. The spatial scale of the speckles is similar to the resolution limit of the optical system, while feature sizes of interest in experiments are 10–100 times larger than this. The synchronization module synchronized the AWG, AOMs, Pockels cell and flash lamps with the main laser pulse of facility. Being generated in one AWG, the probe laser pulse and the main
181
laser pulse have good synchronization relationship with pulse delay jitter below 10 picoseconds (rms).
3. Experimental results 3.1. Output of fiber laser system Fig. 2 shows the details of the fiber power amplifier, which could obtain fundamental transverse mode (Fig. 3) and amplify the 10 nJ energy to 2 mJ with pulse width 7 ns. The fiber laser could generate arbitrary waveform easily. Fig. 4 gives an example of the shaped waveform measured by a 8-GHz photoelectric cell and 6-GHz oscilloscope, which could be used in the stepped pulse experiments. 3.2. Final output Fig. 5 shows 2o waveform output from the 1 mm-core fiber, with pulse width 7 ns and energy larger than 5 mJ. 3.3. Experimental results of the imaging VISAR system
Fig. 4. Shaped waveform of the VISAR laser.
Fig. 6 shows the fringes obtained in experiments, which reflects the shock wave loading and slowdown processes. The probe laser has been set 1 ns ahead the main laser pulse. In the left graph of Fig. 6, a black area is generated synchronously with the 1-ns main laser pulse, which reflects the X-ray ionizing effect. If the effect gets worse, the measurement made by imaging VISAR may fail. In these areas, the intensity of probe laser for imaging VISAR could be enhanced by the powerful AWG and amplitude modulator. The experimental setting is as follows: hohlraum, sapphire window, and rectangular main laser pulse. The laser driver used 8 beams with 351 nm wavelength and 400-J energy for each beam. The experimental result indicates the two streak cameras could obtain very clear and continuous fringes. These fringes impressively reflect the loading and slowdown process in the sapphire sample. The probe laser could output stable energy, without modulation in time domain. Uniformity in intensity provides great convenience for the analysis of experimental data.
4. Conclusion
Fig. 5. 2o waveform of the 1 mm-core output fiber.
A probe laser system for imaging VISAR has been developed and studied. The system was based on electro-optical amplitude modulation to generate shaped pulse, which has good synchronization relationship with the main laser pulse. The probe laser is quite robust and stable, with energy fluctuation below 5% and pulse width variation below 1%. Compared to the traditional probe laser for imaging VISAR, this type of probe laser could
Fig. 6. Fringes obtained by a pair of streak camera detectors reflecting the shock wave loading and slowdown process.
182
R. Zhang et al. / Optics & Laser Technology 43 (2011) 179–182
obtain strict single-longitudinal mode with powerful pulse shaping ability. With the flexible probe laser and imaging VISAR, the function to measure velocity history of shock wave front movement in transparent materials was added to the laser facility used for Inertial Confinement Fusion research.
Acknowledgment This work was supported by the National Natural Science Foundation of China under Grant no. 60878058.
References [1] Miller GH. The National Ignition Facility. SPIE 2004;5341:1–8. [2] Barker LM, Hollenbach RE. Laser interferometer for measuring high velocities of any reflecting surface. J Appl Phys 1972;43:4669–75. [3] Malone RM, Frogget BC, Kaufman MI, Watts PW, Bell PM, Celeste JR, et al. Design of an imaging VISAR diagnostic for the Nation Ignition Facility (NIF). SPIE 2004;5173:26–37. [4] Celliers PM, Bradley DK, Collins GW, Hicks DG, Boehly TR, Armstrong WJ. Lineimaging velocimeter for shock diagnostics at the OMEGA laser facility. Rev Sci Instrum 2004;75:4916–29. [5] Dawson JW, Beach R, Jovanovic I, Wattellier B, Liao Z, Payne SA, et al. Large flattened mode optical fiber for reduction of non-linear effects in optical fiber lasers. SPIE 2004;5335:132–140.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Experimental research on an arbitrary pulse generation system for imaging VISAR Rui Zhang n, Mingzhong Li, Jianjun Wang, Wentao Duan, Feng Wang, Xiaoshi Peng, Xiaolin Tian Research Center of Laser Fusion, China Academy of Engineering Physics, Mail Box 919-988, Mianyang, Sichuan 621900, PR China
a r t i c l e in fo
abstract
Article history: Received 22 April 2010 Received in revised form 3 June 2010 Accepted 21 June 2010 Available online 8 July 2010
Imaging VISAR is an important diagnostic tool for a variety of shock-related phenomena in laser-driven experiments. To adapt to various types of shaped driven pulse, the imaging VISAR needs an illuminating light with good shaping capability. Here, a flexible laser probe system was experimentally studied. Being generated from a 1064-nm DFB laser, the continuous wave was modulated by a waveguide amplitude modulator driven by 10 GS/s arbitrary waveform generator. After being amplified by fiber amplifiers and Nd:YAG rod amplifiers, the signal pulse was frequency-converted to 532-nm green light by a thermally controlled LBO crystal with a final output energy larger than 10 mJ. Finally, the green light was coupled into a 1-mm core diameter, multimode fused silica optical fiber and propagated to the imaging VISAR. The probe laser could realize accurate pulse shaping with time resolution below 100 ps. Uniformity in intensity and capability of arbitrary pulse shaping provides great convenience for the analysis of experimental data. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Fiber laser Arbitrary waveform generator VISAR
1. Introduction Laser drivers could generate multiple kilojoules of energy in a nanosecond pulse and focus it to a tiny target [1]. Using a laser probe beam to illuminate the target, the imaging velocity interferometer system for any reflector (VISAR) could measure shock breakout times at temporal resolutions as low as 20 ps [2,3]. Therefore, the imaging VISAR is an important diagnostic tool for a variety of shock-related phenomena in laser-driven targets, including the motion of free surfaces, high-pressure state equation experiments and material characterization experiments [4]. Traditional laser system for imaging VISAR adopts frequencydoubled Q-switched Nd:YAG laser source with 10 ns pulse duration. However, it is difficult and hard to adjust the pulse shape, limiting the application of imaging VISAR, especially in physical experiments using a shaped main pulse with high contrast. The pulse shaping capability improves VISAR experiments by enhancing the probe laser intensity when the main laser pulse strikes on the target, thereby overcoming the X-ray ionizing effect. The free electron produced by the ionizing effect reduces the reflectivity of the probe laser, which would lead to the streak camera being unable to record the interference fringes. Therefore, developing a probe laser with good shaping capability for use in imaging VISAR is necessary.
n
Corresponding author. Tel.: +86 816 2485283. E-mail address: [email protected] (R. Zhang).
0030-3992/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2010.06.010
Among current technologies, fiber laser systems offer unique benefits as front end devices to meet the pulse shaping requirement. Fiber laser systems may be engineered for a high degree of robustness, reliability and ease of use. Large mode area (LMA) fiber amplifiers [5] could be used in the front end to boost the output power to the order of micro joules for subsequent launch into rod or slab amplifiers. In this article we describe a new type of laser probe beam for imaging VISAR and examples of data collected during recent experiments.
2. System layout 2.1. Technical route The probe laser system of imaging VISAR is responsible for providing a visible 7-ns shaped pulse at a wavelength different from the 1o, 2o and 3o of 1053 nm. Therefore, a distributed feedback (DFB) oscillator with 1064 nm wavelength is used. In the spatial amplification stage, Nd:YAG is adopted as the gain material. As high synchronization among probe laser and main laser is required, the two pulses are generated in the different channels of the same arbitrary waveform generator (AWG). Driven by an AWG with 10 GS/s sampling rate, a waveguide amplitude modulator shaped the continuous wave output from DFB laser and polarization stabilizer to the specified waveform. Then the pulse propagated through fiber amplifiers, acoustic optic modulators (AOMs), 10-mm large mode amplifiers, fiber
180
R. Zhang et al. / Optics & Laser Technology 43 (2011) 179–182
collimator, Pockels cell (PC) and Nd:YAG amplifiers. Finally, it was frequency-converted to 532 nm light and coupled to the 1 mm core multimode fused silica optical fiber. The probe laser was then delivered to the imaging VISAR diagnostic table. 2.2. Setup The VISAR laser is composed of pulse generation module, pulse amplification module, frequency conversion module, light coupling module and synchronization module. Fig. 1 shows a sketch of the system. The pulse generation module includes the laser oscillator, amplitude modulator, fiber amplifiers and AOMs. The oscillator is a DFB laser with thermal tuning range of 0.7 nm, which could realize precise control of the center wavelength to the maximum gain range of Nd:YAG amplifier. The DFB laser could output stable single mode with ultra narrow line width of 33 kHz. The optical S/N is better than 50 dB (50 pm resolution). The pulse amplification module includes one double-pass Nd:YAG rod amplifier, two single-pass Nd:YAG rod amplifiers, flash lamps and power supply. To suppress ASE generated by fiber power amplifier, Pockels cell and Glan polarizers are set behind the fiber collimator. The Nd:YAG rod amplifier adopts air cooling, which would greatly reduce the space taken. With single shot mode in full energy shot of facility and 0.05 Hz for imaging VISAR alignment, it has been proved that air cooling is sufficient. The frequency conversion module includes LBO crystal and precise temperature controller. The light coupling module includes two parts. The first part couples the light from the fiber power amplifier into the Nd:YAG rod amplifiers. The second part couples the frequency-converted
532 nm laser into the 1 mm core diameter, 0.37 numerical aperture, multimode fused silica optical fiber. The fiber-coupled arrangements have two advantages. The first part allows us to place the pulse generation module and associated injection optics in a remote location, which could avoid electromagnetic disturbance. The second part is a convenient way to produce spatially smooth source distribution over a well-defined numerical aperture. Spatial mode mixing through the long delivery path ( 30 m) of multimode fiber results in the formation of a random speckle pattern on the output surface of the delivery fiber. On average, the probe beam intensity is uniform across the output
Fig. 3. Output near field of the fiber power amplifier.
Fig. 1. Sketch map of the VISAR laser generation system.
Fig. 2. Fiber power amplifier.
R. Zhang et al. / Optics & Laser Technology 43 (2011) 179–182
face of the fiber. However, on very small spatial scales ( few mm), the beam is highly modulated. The spatial scale of the speckles is similar to the resolution limit of the optical system, while feature sizes of interest in experiments are 10–100 times larger than this. The synchronization module synchronized the AWG, AOMs, Pockels cell and flash lamps with the main laser pulse of facility. Being generated in one AWG, the probe laser pulse and the main
181
laser pulse have good synchronization relationship with pulse delay jitter below 10 picoseconds (rms).
3. Experimental results 3.1. Output of fiber laser system Fig. 2 shows the details of the fiber power amplifier, which could obtain fundamental transverse mode (Fig. 3) and amplify the 10 nJ energy to 2 mJ with pulse width 7 ns. The fiber laser could generate arbitrary waveform easily. Fig. 4 gives an example of the shaped waveform measured by a 8-GHz photoelectric cell and 6-GHz oscilloscope, which could be used in the stepped pulse experiments. 3.2. Final output Fig. 5 shows 2o waveform output from the 1 mm-core fiber, with pulse width 7 ns and energy larger than 5 mJ. 3.3. Experimental results of the imaging VISAR system
Fig. 4. Shaped waveform of the VISAR laser.
Fig. 6 shows the fringes obtained in experiments, which reflects the shock wave loading and slowdown processes. The probe laser has been set 1 ns ahead the main laser pulse. In the left graph of Fig. 6, a black area is generated synchronously with the 1-ns main laser pulse, which reflects the X-ray ionizing effect. If the effect gets worse, the measurement made by imaging VISAR may fail. In these areas, the intensity of probe laser for imaging VISAR could be enhanced by the powerful AWG and amplitude modulator. The experimental setting is as follows: hohlraum, sapphire window, and rectangular main laser pulse. The laser driver used 8 beams with 351 nm wavelength and 400-J energy for each beam. The experimental result indicates the two streak cameras could obtain very clear and continuous fringes. These fringes impressively reflect the loading and slowdown process in the sapphire sample. The probe laser could output stable energy, without modulation in time domain. Uniformity in intensity provides great convenience for the analysis of experimental data.
4. Conclusion
Fig. 5. 2o waveform of the 1 mm-core output fiber.
A probe laser system for imaging VISAR has been developed and studied. The system was based on electro-optical amplitude modulation to generate shaped pulse, which has good synchronization relationship with the main laser pulse. The probe laser is quite robust and stable, with energy fluctuation below 5% and pulse width variation below 1%. Compared to the traditional probe laser for imaging VISAR, this type of probe laser could
Fig. 6. Fringes obtained by a pair of streak camera detectors reflecting the shock wave loading and slowdown process.
182
R. Zhang et al. / Optics & Laser Technology 43 (2011) 179–182
obtain strict single-longitudinal mode with powerful pulse shaping ability. With the flexible probe laser and imaging VISAR, the function to measure velocity history of shock wave front movement in transparent materials was added to the laser facility used for Inertial Confinement Fusion research.
Acknowledgment This work was supported by the National Natural Science Foundation of China under Grant no. 60878058.
References [1] Miller GH. The National Ignition Facility. SPIE 2004;5341:1–8. [2] Barker LM, Hollenbach RE. Laser interferometer for measuring high velocities of any reflecting surface. J Appl Phys 1972;43:4669–75. [3] Malone RM, Frogget BC, Kaufman MI, Watts PW, Bell PM, Celeste JR, et al. Design of an imaging VISAR diagnostic for the Nation Ignition Facility (NIF). SPIE 2004;5173:26–37. [4] Celliers PM, Bradley DK, Collins GW, Hicks DG, Boehly TR, Armstrong WJ. Lineimaging velocimeter for shock diagnostics at the OMEGA laser facility. Rev Sci Instrum 2004;75:4916–29. [5] Dawson JW, Beach R, Jovanovic I, Wattellier B, Liao Z, Payne SA, et al. Large flattened mode optical fiber for reduction of non-linear effects in optical fiber lasers. SPIE 2004;5335:132–140.