Design of arbitrary waveform generator for shaping high power laser system

LIU Hui，LIU Bai-yu，BAI Yong-lin，OUYANG Xian，GOU Yong-sheng，ZHENG Jin-kun

(1．Key Laboratory of Ultra-fast Photoelectric Diagnostics Technology，Xi'an Institute of Optics and Precision Mechanics，Chinese Academy of Sciences，Xi'an 710119，China;2．State Key Laboratory of Transient Optics and Photonics，Xi'an Institute of Optics and Precision Mechanics，Chinese Academy of Sciences，Xi'an 710119，China;3．Graduate University of Chinese Academy of Sciences，Beijing 100049，China)

1 Introduction

In the mid of 1960s，the concept of achieving the fusion between deuterium and tritium by a high power laser pulse is put forward. The laser heats the pellet instantly and the generated inertia recoil produces high temperature and high pressure，which leads to nuclear fusion to occur. This process is called Inertia Confinement Fusion( ICF) . The achievement of ICF needs that the ablation pressure resulting from the laser pulse irradiating the pellet must increase according to certain rules. Therefore，there are high requirements for a laser pulse shape system. The new ICF driver demands that multi-channel laser pulses must be with arbitrary shaping ability，besides each channel＇s waveform is the same so as to make power balance. The arbitrary laser pulse can be acquired by modulating theQ-switched pulse output from a master oscillator with pockel cells or optical waveguide modulators.

The advanced approach to adjust the laser pulse shape is to utilize the integrated optics technology［1，2］. By injecting the shaping electrical pulse into the optical waveguide modulator，and then the laser pulse shape can be determined by the electrical pulse. The arbitrary waveform generator takes advantage of pure electronic devices with simple structure，excellent specifications，dependable performance，and the lower cost. It can generate an ultrawide band low voltage pulse with high stability. For the reason that the laser driver shape in ICF is particular with the narrow width and wide dynamic range，it is the key technology to generate this kind of electrical pulse. Consequently，the research for generating arbitrary waveform technology plays a significant role in the ICF driver design.

2 Design of pulse shaping circuit

2．1 The basic unit circuit

Fig.1 Principle of generating unit pulse.

The basic unit circuit is show in Fig.1. The onoff state of GaAs FET is determined by triggering pulse and the direct current bias together. The pinch-off voltage of GaAs generally is ～2.2 V［3］.When the triggering pulse does not come to the gate port，the GaAs is on off state which is can be shown in Fig.1( a) . At the time that the triggering pulse propagates to the gate，the voltage that the pulse amplitude and the bias add together is higher than pinch-off value. As is shown in Fig. 1( b) ，the GaAs is on an open state. There is a basic unit electrical pulse with same amplitude and waveform but opposite polarity as triggering pulse. This unit pulse propagates along the transmission line into two different directions. In order to eliminate the reflection generating from the generated pulse，a 50 Ω load is set to assimilate the left-travel sub-pulse. The righttravel sub-pulse as a basic unit pulse makes a part of the shaping electrical pulse.

2．2 Pulses accumulation

Fig. 2 demonstrates the process of each sub-pulse accumulating together［4］. Each GaAs FET consists of a basic unit circuit. When the triggering pulse arrives to the first GaAs，it is in an on state and a unit pulse is obtained propagating forward along transmission line. After a certain delay time，the triggering pulse comes to the second GaAs，it is open too. We obtain the second unit pulse which can be accumulated with the first one to form a widther pulse. The delay time between the adjacent unit pulses is designed by controlling the length difference of pulse transmission line. The triggering pulse triggers GaAs FETs in turn and generates several independent basic unit pulses to accumulate together. Thus，the shaping electrical pulse is obtained.

Fig.2 Pulses accumulating theory.

3 Pulse transmission line design

At present，the micro-strip technology is widely used in the transmission line field which can be executed by a gerber file format. Furthermore，it can be integrated with some passive and active microwave devices. With the quasi-TEM mode［5］，the phase velocity，propagation constant and characteristic impedance of micro-strip can be obtained by analyzing quiescent or quasi - quiescent wave equations. In the ultra-fast pulse generator，the micro-strip technology is utilized in signal generating and transmitting circuits，which is taken for reducing the effects that high-frequency distribution parameters result in. Thus，the front-edge and the end-edge of electrical pulse can be improved［6］. The structure of micro-strip line is shown in Fig.3. A strip single line and the ground plane are separated by the dielectric. The characteristic impedance of microstrip is controllable by altering the copper thickness and the width of signal line or the width of dielectric. And the design precision is within the scope of 5%.

Fig.3 Micro-strip line structure

The parameters of micro-strip are given in Fig.3，therefore the characteristic impedance can be computed according to the formula below［7］

The propagation delay time is

In the formula (1) and (2) ，εris the relative dielectric constant of the material that is made of PCB board;his the thickness of the dielectric layer;Wandtare the width and thickness of the zonal wire，respectively. In the design，50 Ω micro-strip line is used for generating ultra-fast electrical pulse.According to formula (1) ，selection to high frequency PCB material and the width of micro-strip can be realized; based on formula (2) ，we can confirm the length of delay line.

4 Experimental results and analysis

The AWG adopting GaAs FET as switch device takes full advantage of voltage controlling current and onoff characteristics. Each channel of basic unit pulse amplitude is controlled by adjusting the corresponding gate bias. The multi-channel unit pulses with different amplitudes are added independently，and the dif-form electrical pulses waveform are generated. And then，injecting the electrical pulse into optical waveguide modulator，a laser pulse with the same waveform as electrical one is output. The experiment results are shown in Figs.4 and 5; ( a) is the electrical pulse and the ( b) is the laser pulse output from an optical waveguide modulator. They are with the same shape but different polarities.

The amplitude of pulse generated by electrical generator is too low to apply to the optical waveguide modulator. Therefore，an ultra-wide bandwidth voltage amplifier is used at the end of output interface to enlarge the signal pulse. Finally，the shaped and amplified electrical pulse is applied to the electrical waveguide modulator and the exactly same shape optical pulse is generated and applied to the high-power laser shaping system. The experiment results are satisfied.

Fig.4 Square waveform.

Fig.5 Ladder waveform.

5 Conclusions

An ultra-wide bandwidth electrical shaping pulse generator consisting of GaAs and micro-strip line is proposed. The generator can generate arbitrary waveform pulse within the width of 10 ns and amplitude of 5 V. The pulse is applied to the optical waveguide modulator and the ideal shape optical pulse is generated. The electrical shaping technology can be also used in radar communication，radar positioning，information ware，complex analog radar signals and optical communications areas.

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