ACKNOWLEDGMENTS

The authors would like to acknowledge the management of VIT

University especially our Chancellor Dr. G. Viswanathan for their

support and encouragement.

REFERENCES

1. G.R. Aiello and G.D. Rogerson, Ultra-wideband wireless systems,

IEEE Microwave Mag 4 (2003), 36–47.

2. K. Shambavi and Z.C. Alex, Design of printed multistrip monopole

antenna for UWB applications, Microwave Opt Technol Lett 53

(2011), 1750–1752.

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antennas comprised of series-fed printed-strip dipole pairs, IEEE

Trans Antennas Propag 48 (2000), 895–900.

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of a printed dipole antenna, IEEE Antennas Wireless Propag Lett 7

(2008), 1–4.

5. J.-S. Zhang and F.-J. Wang, Study of a double printed UWB dipole

antenna, Microwave Opt Technol Lett 50 (2008), 3179–3181.

VC 2012 Wiley Periodicals, Inc.

INVESTIGATION OF THE ARBITRARYWAVEFORM SEMICONDUCTOR LASERAS SEED LIGHT SOURCE FOR HIGHENERGY LASER

Hongyun Wang,1,2 Zhengshang Da,1 Baiyu Liu,1 and Hui Liu1,21 Xi’an Institute of Optics and Precision Mechanics, ChineseAcademy of Sciences, Xi’an 710119, People’s Republic of China;Corresponding author: liuhui3713@gmail.com2Graduate School of the Chinese Academy of Sciences, Beijing100049, People’s Republic of China

Received 26 May 2011

ABSTRACT: A pulse semiconductor laser modulated by arbitrary

shaping electrical waveform is produced and the generated optical pulsecan be taken as the seed resource for high-power laser facilities. Basedon ultrawide band microwave device and microstrip line transmission

delay application, an all-solid-state circuit for generating arbitrarymodulation pulse to modulate the semiconductor laser is fabricated. For

improving the semiconductor laser power, the output laser pulse is sentinto erbium-doped fiber master oscillator power amplifier architecturefor amplification. In the experiment, the output laser pulse can be

arbitrarily adjusted at 1547.9 nm center wavelength, less than 10 nsduration, lower than 100 kHz repetition rate, and 330 ps time domain

adjustment. VC 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett

54:751–755, 2012; View this article online at wileyonlinelibrary.com.

DOI 10.1002/mop.26652

Key words: semiconductor laser; pulse shaping; modulation; arbitrary

waveform generator

1. INTRODUCTION

Inertial confinement fusion (ICF) is a process where nuclear fusion

reactions are initiated by heating and compressing a fuel target,

typically in the form of a pellet that most often contains a mixture

of deuterium and tritium [1]. This physical program promises to

be a candidate technology for achieving controlled nuclear fusion,

which will provide a safe, clean, and virtually unlimited source of

energy. The ICF needs only to generate arbitrary output pulses for

technology demonstration purposes, but the input pulses must be

shaped to compensate for gain saturation in the power amplifier

[2, 3]. It is the requirement that temporally shaped optical pulses

must be generated and applied to the ICF high laser facility.

In the past three decades, programs such as Nova, Shiva, and

OMEGA have been engaged in this area. In the Nova facility, the

acquisition of arbitrary waveform laser pulse can be realized via

modulating the Q-switched pulse from output of the main oscilla-

tor by Pockels cell or optical waveguide modulator [4]. In the op-

tical experiment, the huge bulk of Pockels cells makes the calibra-

tion of the optical path difficult, and the high-voltage devices are

easy to degrade the dielectric performance and breakdown the

semiconductor unit resulting in low reliability in operation.

Researchers in OMEGA program obtain the optical waveform

based on an aperture-coupled stripline (ACSL) generator to modu-

late optical waveguide modulator [5, 6]. In the ACSL module, as-

sembly of multiple printed circuit boards (PCBs) are implemented;

therefore, the pulse shape precision, particularly its dynamic range

resulting from the mechanical tolerance in PCB fabrication and

assembling cannot be guaranteed. In addition, a uniquely designed

aperture is only for one desired pulse shape and energy, and an

ACSL module cannot be reconfigured once manufactured, which

makes it difficult to change the pulse shape in near real-time. For

obtaining optical pulse with adjustable temporal profile, the electri-

cal pulse facility is the top priority for being fabricated.

In this article, the method to achieve the generation of nano-

second electrical pulses with picosecond time resolution and

good dynamic range is investigated. The semiconductor laser

takes the ability to be directly modulated by altering the driven

current [7, 8]. Temporally shaped optical pulses can be produced

by applying shaped electrical waveforms to semiconductor laser.

This scheme has proved to be a convenient and practical method

to generate high-stability shaped seed pulses. The developed

system taken as seed optical source will be used for pulse shap-

ing in ICF front-end system.

2. PULSE-SHAPER ARCHITECTURE

The experimental setup of the all-solid-state optical pulse shaper

is schematically shown in Figure 1. The microcomputer unit

based on timing system is introduced in the system for taking

charge of the cooperation tasks. The essential part of the optical

pulse-shaping system is the arbitrary waveform generator (AWG),

which can produce highly stable electrical pulse at 100 kHz repe-

tition rate. To obtain high-contrast, high-precision, shaped optical

seed pulse, the semiconductor laser must be biased to the critical

state. To accomplish this condition, a direct current source is

used in the input of semiconductor. The semiconductor laser is

directly modulated by the electrical pulse to achieve optical pulse

with controllable profiles. However, the semiconductor lasers

were primarily developed for seeder applications at limited pulse

powers synchronously with the passage of optical pulses through

the erbium-doped fiber master oscillator power amplifier (MOPA)

[9, 10]. The entire optical path is designed with single-mode

polarization-maintaining fiber. The amplified pulse is injected into

the solid laser system for further amplification and frequency con-

version so as to obtain higher power.

Figure 1 Block diagram of the prototype pulse-shaping system

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 3, March 2012 751

3. ELECTRICAL SHAPING PULSE GENERATOR

3.1. The All-Solid-Stated CircuitThe proposed circuit is present in Figure 2; the transmission

lines used in microstrip line architecture consist of the pulse line

and the trigger line. The power GaAs FETs, which are com-

monly used in microwave amplifiers, are used in this project.

Each FET gate is fed with a direct current source to which a tap

resistor is connected, and the trigger pulse is transmitted through

the blocking capacitor directly to the gate. The gate bias V ini-

tially �8 V and the amplitude of the trigger pulse added to-

gether to decide the GaAs FET state. Originally, all the FETs

are biased in the off state because their cutoff voltage is nomi-

nally �2.2 V. To initiate operation, a trigger pulse generated

from avalanche transistor is launched on the trigger line and

propagates past each of the metal-semiconductor field effect

transistors (MESFETs), which has effect to make the gate-source

voltage exceed the cutoff voltage. Thus, a triggered voltage wave-

let with similar waveform but adverse polarity to the triggering

pulse is generated on the pulse line, and it travels to two opposite

directions. The wave traveling to the left termination is assimi-

lated by the resistor and the right traveling one becomes part of

the shaped pulse, which is being constructed from these wavelets.

The shaped pulse is built as shown in Figure 3. The first wave-

let becomes the rising edge of the initial shaped pulse and the last

one becomes the falling edge. The trigger line delay is 630 ps

between two FETs as compared to the 300 ps delay along the

pulse, and the temporal difference 330 ps is the designed time do-

main adjustment. Thus, after each wavelet is added to the shaped

pulse, it propagates entirely past the next FET, which is triggered

at such a time to add on the next part of the shaped pulse. In this

way, the pulse is assembled until the desired pulse shape has been

generated. At the end of shaping pulse output, the ripples from the

wavelet addition can be removed by pulse propagating through a

Figure 2 The unit pulse accumulating process

Figure 3 The final shaped pulse is constructed from scaled and

delayed replicas of the trigger pulse

Figure 4 Tapered line architecture. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com]

Figure 5 The comparing of the fist unit pulse and the last one. [Color

figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com]

Figure 6 Wavelength of semiconductor laser. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com]

752 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 3, March 2012 DOI 10.1002/mop

DC blocking capacitor or a maximally linear phase low-pass filter.

The electrical pulse is applied on a semiconductor laser and the

desired optical pulse is obtained. The pulse shape is varied by set-

ting a bias voltage on each chain of FETs.

3.2. Transmission Line DesignThe design of the transmission lines is of critical importance to

any all-solid-state circuit. Microstrip is a type of electrical trans-

mission line, which can be fabricated using PCB technology,

and it is used to convey microwave-frequency signals. In this

study, the microstrip line is fabricated for propagating the trig-

ger pulse and the shaped pulse with ultrafast rising and falling

edges, respectively. It is a fact that the pulse attenuation in am-

plitude is introduced in the propagation process. Circuit losses

are introduced by allowing the effective dielectric constant in

the telegraph equation to have an imaginary component, which

can be related to the substrate’s loss tangent and the conductor’s

ohmic losses. So, the linearly tapered microstrip line (LTML) is

applied for compensating the trigger pulse attenuation [11, 12].

Figure 4 illustrates the top view of LTML architecture.

With the designed LTML in all-solid-state shaping circuit, we

set the first unit GaAs FET gate bias and the last one at the same

value, and two unit pluses are generated from the pulse line,

which is shown in Figure 5. Figure 5 demonstrates the triggered

pulse with the similar amplitude. However, the last unit pulse

becomes wider than the first one resulting from the pulse fre-

quency dissipation in pulse propagation. Therefore, the LTML

design has been definitely supported by experimental results.

4. SYSTEM PERFORMANCE AND ANALYSIS

4.1. The Characteristic of Input–OutputThe AWG is varied by setting a chain of bias voltage on GaAs

FETs gate. The temporal-shaped pulse generated from AWG is

taken as a current source for modulating the semiconductor

laser. In this study, we used a semiconductor laser at 1547.8 nm

central wavelength, which is shown in Figure 6, and the peak

power is 8 mW. There are two typical optical waveforms shown

in Figures 7 and 8. Figures marked (a) are the electrical pulse

and the (b) are the optical pulse output from the semiconductor

laser. Measurements are made by the LeCory oscillograph (40

GHz sampling frequency and 11 GHz bandwidth) and a photo

detector (40 GHz bandwidth). Figure 7(a) is a ladder pulse and

Figure 8(a) is a triangle pulse; Figures 7(b) and 8(b) are their

corresponding optical pulses, which have the same temporal pro-

file as the electrical ones. These figures illustrate that the shaped

optical pulses agree well with the electrical waveform with

excellent performance and predictability.

Comparing the electrical and optical pulses from (a) and (b),

we can observe that the glitch in electrical pulses is eliminated

when sending the pulses into the electro-optic waveguide modu-

lator. In the Fourier transform domain, the glitch is the high-fre-

quency components of the signal, and the modulator’s perform-

ance is viewed as a low-pass filter. The high-frequency

components of the electrical pulse cannot make response on the

waveguide modulator. Therefore, the optical pulse becomes

much smooth.

Figure 7 The ladder pulse. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

Figure 8 The triangle pulse. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

Figure 9 Square optical pulse is amplified in EDFA with aberrance

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 3, March 2012 753

4.2. Obtaining High Power Square PulseFigure 9 shows that a square optical pulse is amplified with

aberrance in all-fiber-based MOPA architecture [13]. Suppose

that at the time of t > 0, the amplifier pumped by continuous

light is in the steady state and at t ¼ 0, a square signal arrives

to the amplifier. If the pumped light power is high enough,

the small signal gain will be caused at the front-edge of the

optical pulse. However, if the input pulse peak power is

higher with long duration, the state variables of amplifier will

be evacuated within a short time because of the stimulated

radiation process. The gain obtained by the back-edge pulse

Gb is far less than that by the front-edge Gf, which leads to

amplitude distortion for optical pulse [14, 15]. Thus, the opti-

cal pulse generated from amplifier is not a square waveform

any more.

On the basis of the discussed characteristic of transient gain

saturation in the fiber amplifier, we can adjust the AWG to gen-

erate a pulse profile that can drive the semiconductor laser to

give an output of the laser’s waveform as shown in Figure

10(a). An amplified square optical pulse is obtained by injecting

the laser pulse into MOPA architecture. Figure 11 demonstrates

a single square pulse obtained by the gain in the erbium-doped

fiber amplifiers [16, 17], and the peak power of the amplified

optical pulse is up to 1 kW. The experimental results are well in

agreement with the theoretical analysis. The amplified optical

pulse takes the ability to apply to high-power laser-shaping

system.

4.3. Stability of SystemTo satisfy the stability of high-power laser output, the seeder

pulse needs to be guaranteed at a stable degree significantly. To

summarize the accumulated production of arbitrary shaping

pulse technology research, the following skills can be applied to

enhance the specifications of arbitrary shaping electrical pulse:

selecting high-quality, high-frequency avalanche diodes to gen-

erate pulse, input and output impedance matching study, high-

frequency property of shaping circuit, improving the technology

of circuit, applying tapered line, and radio frequency PCB mak-

ing process. These approaches can enhance the bandwidth, mod-

ulation precision in time domain, pulse-shaping ability, and

reduce the top square wave ripple drastically. Figure 11 shows

the addition of testing optical pulse together within 3 min at the

100-kHz repetition frequency, and the results demonstrate the

stability of the optical pulse.

5. CONCLUSION

In this article, a new type of arbitrary waveform semiconductor

laser is successfully assembled and operated. The produced high

bandwidth, temporally shaped laser pulses can be taken as seed

resource for high-power laser facilities. For improving the laser

power, the MOPA technology is introduced for amplifying the

semiconductor laser pulse. On the basis of transient gain charac-

teristic of optical pulse in all-fiber MOPA technology, we can

obtain a square optical pulse by altering the driven pulse wave-

form. The measured pulse shapes agree well with our models.

This kind of semiconductor laser with arbitrary waveform repre-

sents an attraction technology for obtaining a seeder for solider

laser amplification, which takes high priority in the investigation

of high-power laser in ICF facilities.

ACKNOWLEDGMENTS

This work was supported by the Chinese Academic of Sciences

National Defense Innovation Project (No. 0729941213). The

author thanks Prof. DA and Prof. LIU for their guidance on all-

solid-state circuit test and fabrication. The experiment MOPA tech-

nology implemented by Dr. Gao and Dr. Zhu validated author’s

theory analysis. The author thanks all these people collectively for

their valuable contributions and would like to express particular

appreciation to the teams that designed and built the system.

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pression of matter to super-high densities: Thermonuclear (CTR)

applications, Nature 239 (1972), 139–142.

2. J.H. Campbell, Special issue: Beamlet laser project. ICF Quarterly

Report, CA, 1995, pp. 42–44; Vol. 5.

3. Y. Zhu, J.D. Zuegel, and J.R. Marciante, Distributed waveform

generator: A new circuit technique for ultra-wideband pulse gener-

ation, shaping and modulation, IEEE J Solid-State Circuits 44

(2009), 808–823.

Figure 10 Obtained square optical pulse. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

Figure 11 Demonstration of the seeder stability. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

754 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 3, March 2012 DOI 10.1002/mop

4. R.B. Wilcox, Photoconductive switch pulse-shaping device for

Nova master oscillator, Laser Part Beams 4 (1986), 141–143.

5. M.D. Skeldon, Optical pulse-shaping system based on an electro-

optic modulator driven by an aperture-coupled-stripline electrical-

waveform generator, Opt Soc Am 19 (2002), 2423–2426.

6. A.V. Okishev, M.D. Skeldon, R.L. Keck, et al., All-solid-state opti-

cal pulse shaper for the OMEGA laser fusion facility, Adv Solid

State Lasers Opt Soc Am 34 (2000), 112–115.

7. L. Wang, X.D. Lin, Z.M. Wu, et al., Current-driven state-bistability

and power-bistability in a DFB semiconductor laser subject to opti-

cal injection, Laser Phys 20 (2010), 1957–1960.

8. M.S. Alias, S. Shaari, and S.M. Mitani, Optimization of electro-op-

tical characteristics of GaAs-based oxide confinement VCSEL,

Laser Phys 20 (2010), 806–810.

9. N.Md. Yusoff, M.H. Abu Bakar, S.J. Sheih, et al., Gain-flattened

erbium-doped fiber amplifier with flexible selective band for optical

networks, Laser Phys 20 (2010), 1747–1751.

10. M. Li, J. Ma, L.Y. Tan, et al., Investigation of the irradiation effect

on erbium-doped fiber amplifiers composed by different density er-

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Davis, A simplified analytic CAD model for linearly tapered

microstrip lines, IEEE MIT-S Int Microwave Symp Dig 2003, Phil-

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53 (2004).

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laser pulses through the quantum nonlinear medium with resonant

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VC 2012 Wiley Periodicals, Inc.

DESIGN OF AN INDOOR REPEATERANTENNA WITH HIGH ISOLATION USINGMETAMATERIALS

Youngki Lee, Jeageun Ha, and Jaehoon ChoiDepartment of Electronics and Communications Engineering,Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul133-791, Korea; Corresponding author: choijh@hanyang.ac.kr

Received 26 May 2011

ABSTRACT: In this article, an innovative indoor repeater antennawith high isolation is proposed. The necessary impedance bandwidth isobtained using the coupling between the main and parasitic patches. Ahigh isolation between transmitting and receiving antennas is achievedusing metamaterial. The fabricated indoor repeater antenna has aVSWR less than 1.5, a gain higher than 9.6 dBi, and an isolationbetween the transmitting and receiving antennas greater than 80 dBover the WiBro band from 2.3 to 2.5 GHz. VC 2012 Wiley Periodicals,

Inc. Microwave Opt Technol Lett 54:755–761, 2012; View this article

online at wileyonlinelibrary.com. DOI 10.1002/mop.26651

Key words: microstrip patch antenna; indoor repeater antenna;metamaterial; isolation

1. INTRODUCTION

Repeaters have been widely used in wireless communication

systems to improve the service quality at the cell edges, in shad-

owed areas, and in buildings. The use of repeaters is a cost-

effective solution for extending coverage in areas with low sig-

nal levels. On-frequency repeaters [1–4] use frequency more

effectively; however, the strong coupling between the receiving

and transmitting antennas severely degrades the signal quality.

As the input and output frequencies are the same for a channel-

selective repeater, the requirement for antenna isolation becomes

critical. Excessive feedback from the transmitting antenna to the

receiving antenna causes magnitude and phase errors in the

Figure 1 Structure of the proposed indoor repeater antenna: (a) side view

of the repeater antenna, (b) top view of the repeater antenna, (c) top view of

the metamaterial unit cell, and (d) side view of the metamaterial cell

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 3, March 2012 755