Optical generation of millimeter-wave and sub-terahertz carrier through frequency 18-tupling

OPTICAL GENERATION OF MILLIMETER-WAVE AND SUB-TERAHERTZ CARRIERTHROUGH FREQUENCY 18-TUPLING

Seung Heon Han and Chang-Soo ParkGwangju Institute of Science and Technology, School ofInformation and Communications, 123 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, Republic of Korea; Corresponding author:[email protected]

Received 23 October 2012

ABSTRACT: We propose and experimentally demonstrate an opticalmillimeter-wave and sub-terahertz signal generation method based on

frequency 18-tupling using a power amplifier, an optical modulator, anda semiconductor optical amplifier (SOA). The power amplifier is

operated in the nonlinear region and generates a third-order harmonicfrequency. Next, frequency doubling by optical carrier suppression usingthe optical modulator is performed. Four-wave mixing of the SOA and

optical filtering produce two-tone waves whose frequency spacingcorresponds to 18-tupling of the input RF frequency. The experimental

results show that two idlers could be obtained at 1549.98 and 1550.85nm in the wavelength domain corresponding to the beat frequency of108 GHz from the 6 GHz RF signal. Because of the bandwidth

limitations of our available photodetectors, the phase noise of thebeat signal is measured at a lower frequency of 60 GHz for the input

of 3.3 GHz and is �74.60 dBc/Hz at an offset of 100 kHz. VC 2013

Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1677–1670,

2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/

mop.27605

Key words: four-wave mixing; millimeter wave; subterahertz; radioover fiber

1. INTRODUCTION

Millimeter-wave and sub-terahertz signal generation is important

for the development of broadband wireless communications, ra-

dio-over-fiber systems, and terahertz systems. However, electri-

cal signal generation of millimeter waves or sub-terahertz sig-

nals beyond 60 GHz and up to approximately 200 GHz is not

easy owing to the bandwidth limitations of electronic devices.

As an alternative, optical signal generation such as optical heter-

odyning [1], optical mode locking [1, 2], optical injection lock-

ing [3], and external modulation have been attempted because

of the lower electrical dependency and additional coverage

extension associated with the use of optical fibers. In particular,

the use of external modulation is preferred owing to the higher

signal purity, stability, and reliability of the system [4, 5].

Therefore, various techniques using external modulation com-

bined with other techniques have been attempted to demonstrate

high-order frequency multiplication [6–8]. Among these

schemes, the modulator and semiconductor optical amplifier

(SOA) combination is very attractive, because four-wave mixing

(FWM) of the SOA can optically generate a frequency that is a

multiple of the fundamental frequency [4, 5, 9].

In this article, we propose a millimeter-wave and sub-tera-

hertz signal generation method by optical 18-tupling. The basic

concept is similar to that in Ref. 4; however, to achieve a sim-

pler and more efficient structure, we use an electrical band-pass

power amplifier (E-BPA) and a single external modulator. From

the power amplifier, the third-harmonic electrical signal is gen-

erated with signal amplification. For frequency doubling through

optical carrier suppression, the modulator is operated at the min-

imum bias point of the transmission curve. A further increase in

frequency is achieved by using the FWM effect of the SOA. As

a result, two idlers whose spacing corresponds to 18 times the

fundamental frequency are obtained. The undesired harmonics

are removed by using a couple of optical interleavers, after

which the beat signal from the two idlers is detected by using a

photodetector.

2. PRINCIPLES

To generate harmonic frequencies of the input RF signal, an E-

BPA is selected and operated in the nonlinear region of the

transfer curve. The E-BPA has a passband frequency range,

because of the frequency characteristics of the internal transis-

tors and the matching network comprising a resistor-inductor-

capacitor circuit. Therefore, if a low-frequency signal below the

passband is injected with overdriving beyond the linear region,

output harmonic signals are generated. However, only the har-

monics in the passband of the E-BPA are amplified, whereas the

other harmonics are cancelled out. In our study, the third har-

monic (3fRF) in the passband of the input fRF signal is of interest.

The frequency doubling method by optical carrier-suppressed

modulation implemented in the modulator is a well-known tech-

nique [10]. For an input signal of EinðtÞ ¼ E0 exp ½j2pf0tþju0ðtÞ�, the output signal is given by

EoutðtÞ � E0exp½j2pðf0 þ fmÞtþ ju0ðtÞ�þ exp½j2pðf0 � fmÞtþ ju0ðtÞ�

�; (1)

where E0 is the amplitude of the incident light source, f0 is the

frequency of light, fm is the modulation frequency applied to the

modulator, and u0 is the phase noise component of light. The

carrier signal is suppressed under the bias condition at the mini-

mum point of the transmission curve, and other higher terms are

also ignored. Therefore, the frequency spacing between two

peaks is 2fm (= 6fRF, where fm is equal to 3fRF in our case), cor-

responding to frequency sextupling.

Next, the frequency spacing is further multiplied by using the

FWM effect of the SOA, which is used as a nonlinear medium. In

general, if three optical fields with frequencies f1, f2, and f3 are

injected into the nonlinear medium, a fourth field with frequency f4given by the relation f4 ¼ f1 6 f2 6 f3 is generated; this process

is called FWM. Among these signals, the signal with

f4 ¼ f1 þ f2 � f3 has a large peak amplitude. In fact, the degener-

ate FWM process of f1 ¼ f2 is dominant in the SOA. Thus, two

fields that are commonly called ‘‘idlers’’ are determined by f4 ¼2f1 � f3. From the above frequency sextupling, f1 ¼ f0 þ 3fRF and

f3 ¼ f0 � 3fRF. Therefore, two idler waves of f0 � 9fRF and f0 þ9fRF are obtained from the SOA that have a frequency spacing of

18fRF, which represents frequency 18-tupling of the input fRF.

3. EXPERIMENTAL RESULTS AND DISCUSSION

Figure 1 shows the experimental setup to demonstrate frequency

18-tupling. From the input frequency fRF, a third-harmonic sig-

nal is generated from the E-BPA and applied to the modulator.

This input third-harmonic signal is split into two and linked to

dual electrodes of the Mach–Zehnder modulator, with a relative

phase difference of p/2. For carrier suppression, the modulator

is biased at the minimum point of the transmission curve. The

output is optically amplified with an erbium-doped fiber ampli-

fier (EDFA). To remove undesired amplified spontaneous emis-

sion (ASE) noise from the output, an optical band-pass filter

(OBPF) is placed before the SOA. Undesired side modes

included in the output of the SOA are removed by using two

25/50 and 50/100 GHz optical interleavers. Next, the idlers and

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DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 1677

beat frequency are measured with optical and RF spectrum

analyzers.

The center wavelength of the tunable laser source (TLS)

used is 1550.42 nm, with a narrow linewidth of 200 kHz, and

its output power is þ5.0 dBm. The power amplifier (model:

NB00506) has a frequency range of 18–40 GHz and its gain is

24 dB. The input RF signal applied to the amplifier is 6 GHz.

The passband range was selected by considering the third-

Figure 1 Experimental setup of optical millimeter-wave generation using electrical and optical nonlinearities. TLS: tunable laser source, SG: signal

generator, E-BPA: electrical band-pass power amplifier, SPL: power splitter, MZM: Mach–Zehnder modulator, EDFA: erbium-doped fiber amplifier,

OBPF: optical band-pass filter, SOA: semiconductor optical amplifier, IL: optical interleaver, OSA: optical spectrum analyzer, PD: photodetector, ESA:

electrical spectrum analyzer

Figure 2 Electrical output spectrum of the electrical power amplifier

for a 6 GHz input microwave signal

Figure 3 Optical spectrum of the 36 GHz two-tone waves from the

optically amplified and band-pass filtered modulation signal

Figure 4 Optical spectrum of the FWM after SOA at a bias current

of 200 mA

Figure 5 The optical spectrum obtained with optical filtering using

optical interleavers for 108 GHz signal generation



1678 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 DOI 10.1002/mop

harmonic frequency. The RF spectrum generated from the am-

plifier is shown in Figure 2. Low- and high-order harmonic fre-

quencies except for the third-order harmonic frequency were

suppressed because of low- and high-cutoff characteristics,

respectively. The peak power of the third-order harmonic fre-

quency (3fRF) was enhanced to þ0.36 dBm.

The dual-electrode optical modulator (model: T.DEH1.5-10)

used has a 6-dB bandwidth of 18 GHz. Owing to the band-pass

characteristics of both the E-BPA and the modulator, only the

third-order harmonic was modulated. The modulator was dc-bi-

ased at the minimum point (þ5.52 V) of the transmission curve

to obtain the carrier-suppressed modulation signal.

The output from the modulator is amplified by using an

EDFA to induce the FWM effect inside the SOA. The OBPF

used to remove the ASE noise has a bandwidth of 0.5 nm. The

output spectrum is shown in Figure 3. The two peak powers are

approximately �6 dBm. The frequency spacing between the two

tones appeared to be approximately 36 GHz as a result of fre-

quency sextupling of the fundamental 6 GHz RF signal.

These two-tone waves enter the SOA. The bias current of the

SOA is 200 mA, and its gain is approximately 20 dB at 200

mA. These two-tone waves are used to generate two idler waves

due to the FWM effect in the SOA. As shown in Figure 4, two

idler waves are observed at wavelengths of 1549.98 and 1550.85

nm, respectively. The wavelength spacing is approximately

0.869 nm, corresponding to a frequency of 108 GHz, which is

18 times the fundamental frequency. The other waves are fil-

tered out by creating a suppression ratio of 15 dB between the

peak idler wave and the undesired modes using the two cas-

caded optical interleavers (25/50 GHz and 50/100 GHz), as

shown in Figure 5.

For phase noise measurement of the proposed method, fre-

quency 18-tupling was attempted on an input RF signal of 3.3

GHz using the same devices and following the same procedures.

By frequency sextupling, two tones with a frequency spacing of

19.998 GHz were obtained. After FWM and optical filtering,

two-tone idler waves with 60 GHz spacing were obtained, as

shown in Figure 6. The suppression ratio between the peak idler

wave and the undesired modes was approximately 15 dB. The

beat signal and phase noise are shown in Figure 7. The peak

power and carrier-to-noise power ratio of the 60 GHz milli-

meter-wave signal generated are �49.12 dBm and 20.26 dB,

respectively. The phase noise was measured to be approximately

�74.60 dBc/Hz at an offset of 100 kHz.

4. CONCLUSION

Optical millimeter-wave and sub-terahertz signal generation

based on frequency 18-tupling (108 GHz from 6 GHz) using an

E-BPA and a SOA was experimentally demonstrated. Because

of the bandwidth limitations of our available photodetectors,

beating between the two idlers was attempted at the lower fre-

quency of 3.3 GHz and was successfully generated at 60 GHz.

Figure 6 The optical spectrum of (a) the FWM after SOA at a bias

current of 200 mA and (b) optical filtering using optical interleavers for

60 GHz signal generation

Figure 7 The electrical spectrum and phase noise of the generated 60

GHz (59.994 GHz) millimeter-wave signal



DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 1679

The measured phase noise of the beat signal appeared to be

�74.60 dBc/Hz at an offset of 100 kHz.

ACKNOWLEDGMENT

This research was supported by The Ministry of Knowledge Econ-

omy, Korea, under the Information Technology Research Center

support program supervised by the National IT Industry Promotion

Agency (NIPA-2012-H0301-12-2005).

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VC 2013 Wiley Periodicals, Inc.

1-WATT HIGHLY LINEAR HBT POWERAMPLIFIER DRIVER INTEGRATEDCIRCUIT WITH LOW CURRENTCONSUMPTION

S. Joo,1 S. Han,2 M. Song,1 H. Kim,1 J. Ham,1 M. Seo,1

M. Kim,1 C. Park,1 and Y. Yang1

1 Microwave Circuits and Systems Laboratory, School ofInformation and Communication Engineering, SungkyunkwanUniversity, Suwon 440-746, South Korea2 Samsung Electro-Mechanics, Suwon 443-743, South Korea;Corresponding author: [email protected]

Received 31 October 2012

ABSTRACT: This article presents a power amplifiers (PA) driver IC

which has a parallel combined structure with a dual bias network. EachPA cell for parallel combination has an optimized bias to achieve both

lower current consumption and IM3 cancellation at the same time, so that

the overall PA driver IC has an improved linearity with a low currentconsumption. An incomplete Wilkinson divider including an input

matching capability was proposed. This was adopted to prevent a stabilitydegradation which can be caused by a parallel combination. Theproposed PA driver IC was designed. This was implemented using a 2-lm

InGaP/GaAs heterojunction bipolar transistor (HBT) to have an outputpower of 1 Watt. The quiescent currents for the upper and lower PA cellswere optimized to have the best compromised performances with

quiescent current levels of 100 and 150 mA, respectively. By theimplemented PA driver IC, a gain of 17.25 dB and a high power-added

efficiency of 54.15% at a P1dB of 31 dBm were achieved at a centerfrequency of 900 MHz. For a two-tone signal having a tone-spacing of 1MHz, it also exhibited a high output IP3 of 54.28 dBm at an output

power level of 20 dBm. The average output power was 22.36 dBm towhich the IMD3 level should be lower than �60 dBc. VC 2013 Wiley

Periodicals, Inc. Microwave Opt Technol Lett 55:1680–1683, 2013; View

this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27639

Key words: linear power amplifier IC; linear PA driver IC; RFamplifier IC

1. INTRODUCTION

Recently, the modulated signals used in the major wireless com-

munication systems, such as long-term evolution (LTE), have

high peak-to-average power ratio. This is mainly due to an

increased demand on higher data rate. Therefore, linearity

requirement becomes very stringent for the power amplifiers

(PAs) or their driver amplifiers (PA drivers). The PA drivers are

required to be operated with an even higher linearity.

Moreover, their operating points for the average output power

should be significantly backed off from their peak power levels.

Due to this, the power efficiency can get seriously degraded.

Therefore, both high linearity and efficiency have been pursued

at the same time through the recent research [1–3].

InGaP/GaAs heterojunction bipolar transistors (HBT’s) are

suited for high-power RF circuits due to their high power den-

sity and good linearity characteristics. Hence, most of the hand-

set PA’s and many PA drivers for the base transceiver or relay

systems have been realized using InGaP/GaAs HBT technology.

In [4] for linearity improvement, a parallel combination of two

amplifier cells with constant voltage and constant current bias

circuits was proposed. In [5] for efficiency improvement, a dual-

chain HBT PA with a dual mode operation for the low-power

and high-power regions was proposed.

Generally, PA driver IC’s have high quiescent current for

almost class-A operation due to a stringent requirement of high

linearity. Due to this, they have been inefficient. In this article, a

new PA driver IC using InGaP/GaAs HBT technology, which has

a parallel combined structure with dual bias network, was pro-

posed. One of the two amplifier cells for parallel combination has

significantly lower quiescent current compared to that of another

cell for the improvement in both linearity and efficiency. Phases

of the intermodulation distortion (IMD) terms generated from the

two amplifier cells can be aligned not to be directly added up

using appropriate lower bias level of one amplifier cell. More-

over, an incomplete Wilkinson divider, having an input matching

capability as well, is also proposed to avoid the stability degrada-

tion. This can be caused by a feedback loop from a parallel com-

bination and imperfect isolation of the transistor.

2. CIRCUIT CONFIGURATION

2.1. Dual Biasing SchemeFigure 1 shows a schematic diagram of the proposed PA driver

IC and its application circuits. It has two amplifier cells, Q1 and



1680 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 DOI 10.1002/mop

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Optical generation of millimeter-wave and sub-terahertz carrier through frequency 18-tupling