Guided-Wave Optical Wavelength-Manipulating Devices Using Electrooptic Effect...

Guided-Wave Optical Wavelength-Manipulating Devices Using Electrooptic Effect

Hiroshi MURATA and Yasuyuki OKAMURA

Osaka University, Graduate School Engineering Science, Department of Systems Innovation,

Area of Advanced Electronics and Optical Science

1-3 Machikaneyama, Toyonaka, Osaka 560-8531 Japan

Guided-wave devices with the first-order electrooptic effect (Pockels effect) and polarization-reversed structures have been

studied and developed for applications to optical wavelength/frequency manipulation. The electrooptic effect has potentials for

precise and continuous optical frequency manipulations with a tuning frequency range of ~100GHz and an operation wavelength

bandwidth of over 100nm. The authors have proposed and developed several novel electrooptic guided-wave devices, low-power

and low-chirping optical intensity modulators, optical single-sideband modulators operated with one driving RF signal, and optical

frequency shifters, using the polarization-reversal technology of ferroelectric materials. In this paper, the basic structures, the op-

eration principles, the analyses, the fabrications, and the experimental demonstrations of the proposed devices are reported. The

application of the developed electrooptic single-sideband modulators to radio-on-fiber systems for the transmission of microwave

mobile signals is also presented.

KEYWORD: electrooptic modulator, optical waveguide, LiTaO3, LiNbO3, polarization-reversal, optical integrated circuits, SSB

modulator, optical frequency shifter, radio-on-fiber system

1. Introduction

The first-order electrooptic (EO) effect (Pockels effect)

in ferroelectric optical materials, such as LiNbO3 and Li-

TaO3, has an extremely fast response time of less than pi-

cosecond order and an ultra-wide wavelength bandwidth of

over several hundred nanometers1,2). These characteristics

are very attractive in device applications for optical wave-

length/frequency manipulations in many optoelectronics

systems. With the use of the EO effect, precise and con-

tinuous control of optical frequency in a tuning frequency

range of ~100GHz and an operation wavelength bandwidth

of over 100nm is expected.

Recently, studies about the polarization reversal (domain

inversion) of ferroelectric optical materials and its applica-

tions to nonlinear optical devices have attracted consider-

able interest, and many excellent optical wavelength-

conversion devices have been reported 3-9).

Another interesting application of the polarization-

reversal technology is the EO device. The polarization-

reversed structure is applicable to several EO optical

modulators based on ferroelectric materials (LiNbO3, Li-

TaO3, KNbO3, etc.), and it enables us to construct novel

devices with advanced functions and perfomances10-12).

These devices are also attractive for applications to optical

wavelength/frequency manipulations[P1, P3].

We have proposed and developed several types of novel

EO guided-wave devices utilizing the polarization-reversal

technology. In this research project, we further studied the

proposed devices in terms of their applications to wave-

length/frequency manipulations and demonstrated their

performance. In this paper, the basic structures, the opera-

tion principles, the analyses, the fabrications, and the ex-

perimental demonstrations of the proposed devices are

reported. The application of the developed EO single-

sideband (SSB) modulators to the radio-on-fiber systems

for the transmission of quasi-millimeter-wave mobile sig-

nals using long silica optical fibers [P17] is also presented.

2. Background

2.1 Guided-wave EO modulators

EO guided-wave devices are very important in optoelec-

tronics systems, and a tremendous number of studies about

the EO functional devices have been published over the

last 30 years. Today, LiNbO3 guided-wave modulators are

widely used for long-haul optical fiber communication

systems and optical signal processing systems2, 13). Re-

cently, high-speed and low-voltage LiNbO3 traveling-wave

electrode intensity modulators operated at 40Gbps with a

half-wave voltage of ~1V have been reported14).

The EO effects cause an electrically induced index

change in optical materials, therefore, a basic EO modula-

tor is a phase modulator. By combining the EO effects

with several guided-wave elements, such as a Mach-

Zehnder interferometer, junctions, and directional cou-

plers, the electrically induced optical index change is con-

verted to other optical phenomena (interference, mode

conversion, coupling, etc.), and many useful EO devices

for optical intensity modulators/optical switches, mode

converters, and deflectors can be constructed.

By applying the polarization-reversal structure to these

EO guided-wave modulators, the following interesting

effects can be obtained.

a. Quasi-velocity-matching in traveling-wave modulation

b. Control of modulation polarity

c. Control of modulation depth and phase

Utilizing these attractive features, the following novel de-

vices with advanced functions and performances have been

realized.

1. Quasi-velocity-matched (QVM) EO phase modulator

2. Low-power and low-chirping EO intensity modulator

3. EO optical SSB modulator/optical frequency shifter

We previously proposed and developed the QVM EO

phase modulators10, 11). In this research project, we pro-

posed and developed low-power EO intensity modula-

tors[P13], EO SSB modulators[P3, P5, P8, P16, P17] and optical

frequency shifters[P18]. In the next subsection, a review of

QVM EO phase modulators is presented.

Photonics Based on Wavelength Integration and ManipulationIPAP Books 2 (2005) pp. 213–224

213

2.2 QVM EO phase modulator

The basic structure of the QVM EO phase modulator is

shown in Fig. 2-1. It consists of a single-mode waveguide

and traveling-wave coplanar electrodes formed on a ferro-

electric material substrate with a periodic polarization re-

versal. The length, L, of the polarization-reversed and

non-reversed region for the QVM is determined by the

following equation.

(2-1)

where fm is the designed modulation frequency, vg is the

group velocity of light propagating in the waveguide, and

vm is the phase velocity of a modulation wave traveling

along the electrodes. It should be noted that not the phase

velocity of the lightwave, but its group velocity must be

considered, because EO modulation is a three-wave mixing

process and the interaction among the lightwave, modula-

tion wave and modulated lightwave (sideband) should be

compensated by the periodic reversal.

Applying the periodically polarization-reversed structure

to the traveling-wave modulators, velocity mismatching is

compensated similarly to that in QPM SHG devices.

Therefore, accumulative modulation over a long interac-

tion length is obtained at the designed frequency without

using specific electrodes or waveguides for velocity match-

ing15, 16). An example of the calculated characteristics of

the modulation index as a function of the interaction length

is shown in Fig. 2-2, where the propagation loss of the

traveling modulation signal was assumed to be zero. In the

QVM modulator, the modulation index increases almost

linearly with interaction length. Figure 2-3 is an example

of the calculated frequency dependence of the QVM

modulator, where the total interaction length, Lt was set to

Lt = 6L. The bandwidth of the modulation around the peak

modulation frequency is in a trade-off relationship with the

total interaction length. For example, if we set the modula-

tion frequency fm=16.4GHz, the light wavelength

=633nm, and the total interaction Lt=25mm, a 3dB modu-

lation bandwidth is calculated to be 6GHz for the QVM

guided-wave modulator with LiTaO3, a standard single-

mode waveguide, and thin coplanar waveguide (CPW)

electrodes (vm=6.47 107m/s, vg=1.36 108m/s, L=3.75mm).

Fig. 2-1. Basic structures of the QVM EO guide-wave phase

modulator.

Figure 2-4 shows the frequency dependence of the fabri-

cated QVM guided-wave optical phase modulator with z-

cut LiTaO310). It was designed for velocity matching be-

tween the modulation microwave of 16.4GHz and TM

guided mode of 633nm, and its total interaction length was

set to 25mm, where the corresponding 3dB modulation

bandwidth was 6GHz. These results were in good agree-

ment with the measured results10, 11).

Fig. 2-2. Calculated variation of the modulation index versus

interaction length for a velocity-matched, a QVM and a velocity-

mismatched modulator.

Fig. 2-3. Example of the calculated frequency dependence of the

modulation index of the QVM modulator. The total interaction

length was set to be Lt=6L.

Fig. 2-4. Measured frequency dependence of the modulation in-

dex of the light from the QVM LiTaO3 guided-wave modulator

with the operational power of 100mW.

gm

mvv

f

L11

2

1

214 Photonics Based on Wavelength Integration and Manipulation IPAP Books 2

3. Low-Power and Low-Chirping Intensity Modulator

3.1 Introduction

EO Mach-Zehnder waveguide intensity modulators are

very important and widely used in long-haul optical fiber

communication systems, radio-on-fiber systems, optical

measurements, and so on. In this section, novel EO Mach-

Zehnder intensity modulators with the polarization-

reversed structure[P13] are presented. Utilizing the polariza-

tion-reversal technology of ferroelectric materials in the

Mach-Zehnder intensity modulators, it is possible to

achieve low-power and low-voltage modulation character-

istics. In addition, push-pull modulation characteristics

with a complete balanced modulation depth are obtained

by using only one traveling electrode without the need for

precisely tuning. This leads to extremely low (~zero)-

chirping modulation characteristics. The basic structure of

the proposed device, the design, and the experimental

demonstrations are presented.

3.2 Device structure

Basic structures of the proposed EO intensity modulators

are shown in Fig. 3-1. The spontaneous polarization in one

straight waveguide of a Mach-Zehnder interferometer is

reversed by the polarization-reversal technique, while it is

not reversed in the other straight waveguide. In Fig. 3-1

(a), the two ground lines of symmetric coplanar electrodes

are set on the two straight waveguides of the Mach-

Zehnder interferometer, respectively, while in Fig. 3-1 (b),

two edges of the hot line are set on the two guides, respec-

tively. In these configurations, the direction of modulation

electric fields in the two guides are the same, however, the

signs of the index change in the two guides are opposite

because of the polarization reversal. Therefore, push-pull

modulation characteristics are obtained. The separation of

the hot and ground electrodes, which determines the

strength of modulation electric fields for a given voltage,

can be shortened compared to conventional single-

electrode Mach-Zehnder modulators, whereby the driving

voltage is lowered. In addition, chirping caused by the

slight difference in the EO index changes between the two

guides is negligible since the distributions of the modula-

tion electric field across the two waveguides are symmet-

ric. It is also possible to adopt this structure in periodi-

cally polarization-reversed schemes and to construct QVM

intensity modulators with band modulation characteristics[P13].

3.3 Experiments

The polarization-reversed intensity modulator in Fig. 3-

1(a) was fabricated using z-cut LiTaO3. Figure 3-2 shows

a typical example of the experimental results of the inten-

sity modulation in the fabricated polarization-reversed

modulator with the structure shown in Fig. 3-1(a). In the

experiments, the light source was a He-Ne laser of 633nm.

The measured half-wave voltage, V , was 2.4V, which was

in good agreement with the calculated value of 2.3V for

the proposed device with the 20mm-long electrodes, while

it was 4.4V in the Mach-Zehnder intensity modulator with

the conventional single electrodes of the same electrode

length. Using optimized device parameters and a long

(~30mm) electrode, the half-wave voltage is calculated to

be below 1V. A device with the structure shown in Fig. 3-

1(b) was also fabricated and its operation with a lower

driving voltage was also verified.

Fig. 3-1. Basic structures of the proposed EO guide-wave push-

pull intensity modulators with polarization reversal.

Fig. 3-2. Example of the modulated output from the fabricated

polarization-reversed intensity modulator.

3.4 Summary

Novel polarization-reversed Mach-Zehnder intensity

modulators are proposed and their operations are pre-

sented. We are now in the progress of designing and fab-

ricating low-power intensity modulators operated in the

IR-wavelength region used in optical fiber communica-

tions, and of measuring their chirping characteristics.

The proposed polarization-reversal scheme can be com-

bined with several velocity-matching techniques. For ex-

ample, combined with the etched ridge waveguide struc-

ture and the thick electrodes15), base-band, low-power and

low-chirping modulators are obtained. When this structure

is combined with the QVM technique described in section

2.2, band-operated, low-power and low-chirping modula-

tors are obtained[P13]. It is also possible to apply this struc-

ture to the EO SSB modulators with the periodically po-

larization-reversed structures with quarter-spatial shifts[P12].

These high-performance SSB modulators are presented in

detail in section 5.

215Photonics Based on Wavelength Integration and Manipulation IPAP Books 2

4. EO SSB Modulator with Polarization Reversal

4.1 Introduction

High-speed optical single-sideband (SSB) modulation by

microwave and millimeter-wave signals is very useful in

many optoelectronics/microwave photonics applications

such as long-haul optical fiber communication systems,

radio-on-fiber systems for the transmission of mobile RF

signals, optical measurements, and high-resolution coher-

ent spectroscopy. Several guided-wave EO SSB modula-

tors have been proposed and implemented17-19). The opera-

tion principle of these SSB modulators is based on the

mixing of two balanced phase-modulated light beams

driven by a pair of microwave signals with a phase shift of

/220). Therefore, these devices have complicated struc-

tures composed of two or more EO phase modulators, and

a pair of RF modulation signals and their fine tuning are

required for SSB modulation.

In this section, we present the demonstration of a novel

EO SSB modulator by means of the control technique of

EO modulation in the QVM by the polarization-reversed

structures[P3]. Utilizing the technique of controlling the

modulation phase via the spatial shift of the periodic po-

larization-reversal pattern in the QVM traveling-wave

modulators, balanced phase modulation characteristics

with a /2 modulation phase shift are obtained with only

one traveling-wave electrode and one driving RF signal

without tuning. This SSB modulator can be operated in the

very high frequency range of ~100GHz because there is no

restriction on the interaction length by use of the QVM.

4.2 Structure and operation principle

The basic device structure of the proposed EO SSB

guided-wave modulator is shown in Fig. 4-1. It consists of

a Mach-Zehnder waveguide and traveling-wave electrodes

fabricated on a periodically polarization-reversed ferro-

electric material substrate. LiNbO3, LiTaO3, or KNbO3 are

applicable for the ferroelectric material substrate. Two

periodically polarization-reversed structures for the QVM

are formed along two straight optical waveguides of a

Mach-Zehnder interferometer. The polarization-reversal

periods are the same, 2L, in the two guides, however, pat-

terns of reversal are spatially shifted by a quarter period,

L/2, relative to each other. The length of the reversed and

nonreversed region, L, is determined using equation (2-1).

We have found that the modulation phase in QVM EO

modulation with the periodic polarization reversal is con-

trollable by shifting the spatial position of the periodically

polarization-reversal structure along the propagation direc-

tion. Utilizing this interesting feature, balanced modula-

tion characteristics with a quadrature phase shift are ob-

tained in the two arms of the Mach-Zehnder waveguide

when the proposed device is driven by a RF modulation

signal near the designed frequency. Adjusting the elec-

trode length to be a moderate length, an optical quarter-

wavelength delay between the two arms is also obtained by

supplying a DC voltage between the electrodes. In this

manner, the EO SSB modulation characteristics are ob-

tained [P3, P6, P8].

By using an X-junction waveguide for the Y-junction

waveguide of the output side of the Mach-Zehnder inter-

ferometer, both upper single-sideband (USB) modulated

light and lower single-sideband (LSB) modulated light can

be obtained at the same time.

Fig. 4-1. Basic structure of the prototype EO SSB modulator

using the polarization-reversal scheme.

4.3 Analysis

In Fig. 4-1, the lightwave propagating in each straight

waveguide of the Mach-Zehnder interferometer is modu-

lated at the same time by the RF modulation wave travel-

ing along the electrodes. Each waveguide has the periodi-

cally polarization-reversed structure with the same period

but a spatial shift of L/2. They correspond to the EO phase

modulator shown in Fig. 4-2. The modulation indices in

these two phase modulators are analyzed.

In the traveling-wave modulators, the electrically in-

duced index change that the light sees at point y, n(y, t0),

through the EO effect is expressed as follows.

(4-1)

where nm is the maximum index change by an electric

field, t0 is the time when the light is incident at the edge of

the electrode, and L is defined by equation (2-1). There-

fore, the modulation indices of the light from the two EO

phase modulators in Fig. 4-2 are calculated as follows.

(4-2)

(4-3)

]/2sin[

}]/)/{(2sin[),(

0

00

Lytfn

vyvytfntyn

mm

mgmm

)2cos(4

),()1(

),(),(

0

)1(

0

1

0

2

00

tfLnN

dyktyn

dyktyndyktyn

mm

LN

LN

N

L L

L

a

)2sin(4

),()1(

),()1(

),(),(

0

2/)12(

0

1

2/)12(

2/)32(

0

2/

0

2/3

2/

00

tfLnN

dyktyn

dyktyn

dyktyndyktyn

mm

LN

LN

N

LN

LN

N

L L

L

b


where k and are the wave number and the wavelength of

light in vacuum, respectively, and N is the total number of

the polarization-reversed and nonreversed regions. The

magnitudes of the modulation indices a and b are the

same, but their phases are shifted by /2. By adjusting the

spatial pattern of the periodic polarization-reversal struc-

ture, the phase of the modulation can be controlled arbi-

trarily.

In addition, by setting the interaction length as odd mul-

tiples of L, it is possible to achieve an optical phase shift

(optical delay) of or . Therefore, the equivalent

optical circuit of the proposed EO modulator is shown in

Fig. 4-3; this is the SSB modulation scheme with three

discrete phase modulators, as invented by Hartley20).

Figure 4-4 shows calculated dependences of the output

sideband intensity on modulation frequency and magnitude

of the modulation index, where the designed frequency

was fm=15GHz, the interaction length was Lt=7L (N=7),

and the optical phase shift was . Around the designed

frequency, the +1st side-band component is enhanced,

while the -1st side-band component is almost completely

suppressed. The calculated 3dB bandwidth of the SSB

modulation was about 4GHz.

Fig. 4-2. QVM EO phase modulators with periodic polariza-

tion reversal. In each modulator, the reversal period is the same,

2L, but the spatial position is shifted by L/2.

Fig. 4-3. Block diagram for SSB modulation using three discrete

phase modulators.

Fig. 4-4. Calculated frequency responses of the +1st and -1st

modulated side-band intensities.

4.4 Design and fabrication

The parameters of the designed prototype EO SSB modu-

lator are listed in Table 4-1. In the device fabrication,

first, the periodically polarization-reversed structures with

a spatial shift of L/2 were fabricated on a z-cut LiTaO3

substrate by the pulse-voltage application method. The

period of polarization reversal was set at 8.2mm

(L=4.1mm), which was designed for SSB modulation for

light wavelength ~650nm and modulation frequency

fm=15GHz. The calculated 3dB modulation bandwidth was

4GHz. Next, a Mach-Zehnder optical waveguide was fab-

ricated on the polarization-reversed substrate by the pro-

ton-exchange method using benzoic acid. The width and

the depth of the waveguide core were about 3.0 m and

0.8 m, respectively. After all fabrication processes, the

waveguides were thermally annealed at 400 centigrade

degree for one hour in order to reduce the propagation

losses of the waveguide and to recover the Pockels effect,

which may have been degraded by the proton-exchange

processes. Finally, after sputtering of a 0.1 m-thick SiO2

buffer layer, 1.7 m-thick Al asymmetric coplanar elec-

trodes were formed on the waveguide by thermal vapor

deposition and standard photolithography techniques. The

length of the electrode on the waveguide (the interaction

length) was 28.7mm. The width of the hot electrode was

15 m and the spacing between the hot and the ground

electrodes was 30 m, where the intrinsic impedance was

calculated as 50 .

Table I. Parameters of the prototype SSB modulator.

Substrate material z-cut LiTaO3

Designed operation frequency fm 15GHz

3dB operation bandwidth f 4GHz

Designed light wavelength ~650nm

Period of polarization reversal 2L 8.2mm

Electrode length Lt 28.7mm

Electrode spacing d 30 m

Hot electrode width w 15 m

Intrinsic impedance Z 50

4.5 Experiments

Optical spectra from the fabricated device were measured

using a scanning Fabry-Perot interferometer. The light

source was a CW He-Ne laser of 633nm wavelength and

was irradiated onto the device as TM light. Examples of

the measured optical spectra are shown in Fig. 4-5, where

the modulation microwave frequency and the modulation

power were 17GHz and +24dBm, respectively. By chang-

ing the DC bias voltage for the optical delay from 0V to

17V, the enhanced optical modulation sideband was

clearly switched from the upper to the lower. The meas-

ured frequency dependence of the optical SSB modulation

is shown in Fig. 4-6. The achievement of band modulation

characteristics near the designed frequency was confirmed.

The peak operation frequency of the band modulation in

the fabricated device was about 16GHz, which was slightly

shifted from the designed frequency of 15GHz, however,

the 3dB modulation bandwidth was in good agreement

with the designed value of 4GHz.


4.6 Summary

The novel EO SSB modulator was proposed and its ba-

sic operations were demonstrated. The operation band-

width of the proposed device can be enlarged over several-

fold by using the chirped periodic polarization-reversal

structure[P8]. The applications of the fabricated SSB modu-

lator to radio-on-fiber systems are presented in the next

section.

LSB

-1st sideband

USB

+1st sideband

carrier

17GHz17GHz

LSB

-1st sideband

USB

+1st sideband

carrier

17GHz17GHz

Fig. 4-5. Optical frequency spectra measured using scanning

Fabry-Perot interferometer. The modulation frequency and the

driving microwave power were 17GHz and +24dBm, respec-

tively. The DC bias voltage was +17V for (a) and 0V for (b).

8 10 12 14 16 18 20

modulation frequency (GHz)

3dB bandwidth

f = 4GHz

Fig. 4-6. Measured frequency dependence of the fabricated

SSB modulator. The operation microwave power and the DC

bias voltage were +20dBm and 17V, respectively.

5. Application of the SSB Modulator to ROF Systems

5.1 Introduction

Optical SSB modulators operated in microwave and

millimeter-wave frequency ranges have received much

interest for their potential applications in many microwave-

photonics systems, particularly, the radio-on-fiber (ROF)

systems for the microwave and millimeter-wave signal

transmissions and distributions in mobile broadband com-

munication networks.

In the ROF systems for the transmission of quasi-

millimeter-wave and millimeter-wave frequency signals,

there is a severe problem concerning the power penalty of

the transmitted signals caused by the chromatic dispersion

of the silica fibers when conventional optical intensity

modulators are used for microwave-lightwave converters21,

22). In order to overcome the power penalty, the applica-

tion of the EO SSB modulators has been proposed and

implemented17-19). These SSB modulators are essentially

based on balanced optical modulations using two or three

discrete EO phase modulators with RF driving circuits

supplying the phase-shifted signals to achieve balanced

modulation. Therefore they have relatively complicated

device structures and driving circuits, and delicate RF

phase adjustments are needed.

We have proposed and demonstrated novel EO SSB

modulators by using the polarization-reversal technologies

of ferroelectric materials[P3, P6, P8]. Utilizing the interesting

characteristics of the quasi-velocity-matched traveling-

wave modulators, a pair of balanced modulations is easily

obtained with only one modulation signal without the need

for any RF circuits.

In this section, optical SSB modulators with the polariza-

tion-reversed structure designed for ROF systems[P17] are

presented. Optical SSB modulation around the designed

modulation frequency of 26GHz was demonstrated, and a

side-band suppression ratio of over 15dB was presented.

In addition, the demonstration of optical fiber transmission

of the quasi-millimeter-wave signals using the SSB modu-

lators was also reported. The power penalty of the trans-

mitted signal was drastically reduced and the usefulness of

the proposed SSB modulator was clarified[P17].

5.2 Device structure

The structure of the guided-wave EO SSB modulators

newly designed for the application to the ROF system is

shown in Fig. 5-1. The periodically polarization-reversed

structures were applied to a traveling-wave EO guided-

wave Mach-Zehnder modulator, in order to achieve both

quasi-velocity-matching and control of the modulation

phase. Therefore, it is possible to realize a pair of balanced

modulation with a modulation phase shift of /2 around the

designed frequency by means of a simple structure.

In this work, the advanced configuration of the modula-

tion electrodes and the Mach-Zehnder interferometer

waveguide was adopted, where the two edges of the hot

electrode of the symmetric coplanar electrodes were set

onto the two straight optical waveguides of the Mach-

Zehnder interferometer, as shown in the cross-sectional

view of the device. In this configuration, the distributions

of the modulation electric fields at the two parallel optical

waveguides of the Mach-Zehnder interferometer are com-

pletely symmetric in cross section. Therefore, the magni-

tudes of the modulation index of the light passing through

the two waveguides are exactly the same and a large side-

band suppression ratio is expected in the SSB operation.

(a) (b)


Fig. 5-1. Structure of EO SSB modulators for 26GHz ROF sys-

tem applications.

5.3 Fabrication and experiments

The fabrication processes are essentially the same as

those for the device presented in the previous section. The

period of polarization reversal, 2L, was set to be 4.88mm

for the optimum modulation frequency of 26GHz and the

light wavelength of 1.3~1.55 m. The width and the depth

of the proton-exchanged waveguide, which was designed

to be a single-mode guide for lightwave of 1.3~1.55 m,

were set to 4 m and 1.2 m, respectively. The electrode

length, the width of the hot electrode, and the electrode

gaps were 29.28mm, 30 m and 22 m, respectively. In this

device, the intrinsic impedance of the electrodes was de-

signed to be 40 in order to have relatively small electrode

gaps and to realize low driving microwave power. After

all fabrication processes, the waveguides were thermally

annealed at 400 centigrade degree for one hour in order to

reduce the propagation losses of the waveguide and to re-

cover the Pockels effect.

Examples of the modulated light spectra measured using

an optical spectrum analyzer are shown in Fig. 5-2, where

the modulation frequency and the input power were

26GHz and +23dBm, respectively. By changing the DC

bias voltage, the optical sideband component was clearly

switched between the upper and lower ones. The meas-

ured sideband suppression ratio was over 15dB.

An example of the measured frequency dependence of

the fabricated modulators is shown in Fig.6-3. The band

modulation operations around the designed frequency of

26GHz were confirmed. We believe that the dip in the

frequency response at 23GHz was due to the microwave

resonance mode in the LiTaO3 substrate13); it can be elimi-

nated by adjusting the substrate thickness or width, which

was confirmed by calculation using microwave simulators.

Table II. Parameters of the SSB modulator for the ROF system.




Designed light wavelength 1300~1550nm

Period of domain-inversion 2L 4.88mm





Single-mode silica fibers and microwave K-connectors

were connected and fixed to both ends of the Mach-

Zehnder waveguide and the coplanar electrodes of the fab-

ricated modulators, respectively. Then, the SSB modulator

module as shown in Fig. 5-4 was fabricated for the ROF

system experiments.

Fig. 5-2. Modulated light spectra from the fabricated SSB modu-

lator. The modulation frequency and the input power were

26GHz and +23dBm, respectively. The DC voltage was -5V for

(a) and 40V for (b).

Fig. 5-3. Measured frequency dependence of the fabricated EO

SSB modulator.

(a)

(b)


Fig. 5-4. The fabricated SSB modulator module.

5.4 Applications to the Radio-On-Fiber system

The fabricated SSB modulator module was applied to

the ROF systems and its performance for the transmission

of quasi-millimeter-wave signals over long silica fibers

was tested.

The experimental setup for the ROF system is shown in

Fig. 5-5. Lightwave from a 1.55 m DFB laser diode was

modulated using the fabricated SSB modulator module

driven by a quasi-millimeter-wave signal of 26GHz. The

modulated light was transmitted through long silica single-

mode fibers with a length of 1~20km. The transmitted

light was amplified by an erbium-doped optical fiber am-

plifier, and then detected using an ultra high-speed

photodetector.

In the ROF system, the span of fiber length, Lc, where

the detected power cancellation by fiber chromatic disper-

sion may occur, is given by a following equation22).

(5-1)

where c is the speed of light in vacuum, D is the dispersion

parameter of the optical fiber, is the light wavelength,

and fm is the modulation frequency. For a standard silica

single-mode fiber used in the experiments with

D=17ps/nm/km at the wavelength of 1.55 m, the corre-

sponding fiber length where the detected power cancella-

tion occurs is calculated to be Lc=11km for fm=26GHz.

Figure 5-6 shows the experimental results of the de-

tected 26GHz signal power versus fiber length. For com-

parison, the characteristics of the detected signals modu-

lated by a double side-band modulator are also plotted.

The power penalty of the detected 26GHz signal power at

Lc=11km was clearly shown in the DSB modulation case.

However, it was drastically suppressed when using the

SSB modulator. Therefore, the excellent performance of

the SSB modulator in the application to a ROF system was

successfully confirmed.

5.5 Summary

The demonstration of the EO SSB modulator at 26GHz

and its application to the ROF system were reported. The

power penalty of the detected 26GHz signal caused by the

fiber chromatic dispersion was significantly reduced com-

pared to those of the DSB modulation schemes. The fur-

ther improvement of the performance of the proposed EO

SSB modulators and the attempts at operation in millime-

ter-wave frequency ranges are presented in the next sec-

tion.

Fig. 5-5. Experimental setup for the measurement of the ROF

performance.

Fig. 5-6. Detected 26GHz signal power as a function of the opti-

cal fiber length.

6. Millimeter-Wave EO SSB Modulator

6.1 Introduction

The application of the EO SSB modulator with the po-

larization-reversal structures to the ROF system for 26GHz

mobile signal transfers was successfully demonstrated and

presented in the previous section. In this research project,

we have also tried to develop millimeter-wave EO SSB

modulators for ROF applications of millimeter-wave mo-

bile systems of ~40GHz[P16].

6.2 Device structure and design

Figure 6-1 shows the structure of the EO SSB modulator

designed for millimeter-wave ROF system applications.

The operation frequency was set at ~40GHz, which is one

of the mobile communication frequency bands in the mil-

limeter-wave region. Therefore the period of polarization

reversal, 2L, was set to be 3.28mm. In order to further

improve the SSB modulation performance, the rounded

part of the coplanar electrodes was modified so that the

effective lengths of the modulation electrodes for the two

straight waveguides were exactly the same, as shown in

Fig. 6-2. This configuration results in perfectly balanced

phase modulations with the same modulation index, and it

22

m

cfD

cL


is expected to exhibit a higher sideband suppression ratio

in SSB modulation compared to the previous devices pre-

sented in sections 4 and 5. In addition, in order to reduce

the DC bias voltage for the control of optical bias, a nonpe-

riodic polarization-reversed region was defined as shown

in Fig. 6-1. The length of the nonperiodic region was ad-

justed to be an integer multiple of the polarization period,

2nL (n: integer), therefore, millimeter-wave modulation

characteristics were not affected by this region.

Fig. 6-1. Structure of the millimeter-wave EO SSB modulators.

Fig. 6-2. Conventional (a) and new (b) electrode configurations

for achieving perfectly balanced modulation characteristics and a

high SSB extinction ratio.

6.3 Experiments

The designed device was fabricated using z-cut LiTaO3.

The fabrication processes are almost the same as those

adopted for the previous devices. Figure 6-3 shows exam-

ples of the optical frequency spectra measured from the

fabricated modulator driven by a 38GHz millimeter-wave

signal. The upper or the lower modulated sideband com-

ponent was clearly suppressed by changing the DC bias

voltage for an optical phase shift, and a good SSB modula-

tion performance was confirmed around the designed mil-

limeter-wave frequency. A sideband suppression ratio of

over 30dB was obtained, which was greatly improved from

the value of 15dB of the previous device.

The measured frequency dependence of the SSB modula-

tor is shown in Fig. 6-4. The peak modulation frequency

was shifted to slightly higher than the designed value of

40GHz. However, the 3dB operation bandwidth, 6GHz,

was in good agreement with the designed one.

6.4 Summary

The millimeter-wave EO SSB modulators were designed

and their experimental results were presented. The excel-

lent optical SSB modulation characteristics with an extinc-

tion ratio of over 30dB were demonstrated. The applica-

tions to millimeter-wave signal ROF systems are now un-

der way.

Table III. Parameters of the millimeter-wave SSB modulator.


Designed operation frequency fm ~40GHz


Designed light wavelength 1300~1550nm

Period of domain-inversion 2L 3.28mm

Length of the periodically polarization-reversed region for MM-wave modulation

19.68mm

Length of the polarization-reversed region for optical bias

9.84mm





Fig. 6-3. Modulated light spectra from the fabricated SSB modu-

lator. The modulation frequency and the input power were

38GHz and 18dBm, respectively. The DC bias voltage was 1.9V

for (a), and +14.8V for (b).

Fig. 6-4. Measured frequency response of the fabricated millime-

ter-wave EO SSB modulator.

(a)

(b)

(a) (b)


7. EO Optical Frequency Shifter

7.1 Introduction

An optical frequency shifter (OFS) is an important de-

vice in many optoelectronics applications: for example,

optical fiber communication systems, particularly wave-

length division multiplexing (WDM) systems for precise

control of the laser frequency to the grid ones; optical sig-

nal processing systems; optical measurements for hetero-

dyne detections; laser spectroscopy; laser cooling; and

photochemistry. However, the operation frequency of a

commercially available OFS using acoustooptic effects is

in the range of several hundred MHz. Therefore, a high-

speed OFS with the operation frequency exceeding the

GHz range is expected in many application fields.

We have proposed a novel EO high-speed OFS/carrier

suppressed SSB modulator that has a new guided-wave

optical circuit, which we named the “3-branch waveguide

interferometer” and the polarization-reversal technology of

ferroelectric materials[P18]. This OFS is compact and can

be operated in very high frequency range (1~100GHz)

with one RF modulation signal and DC bias voltages. A

continuous sweep of optical frequency over a range of sev-

eral GHz is possible, and the direction of frequency shift

(upward or downward) can be switched by tuning the bias

voltage. In this section, the operation principle, analysis,

and demonstration are presented.

7.2 Structure and operation principle

Figure 7-1 shows the basic structure of the proposed

EO OFS. It consists of a 3-branch waveguide interferome-

ter composed of two 3-branching waveguides with three

straight waveguides, periodically polarization-reversed

structures, traveling-wave symmetric coplanar line elec-

trodes for RF modulation, and DC bias electrodes for the

control of optical delays. These are fabricated on a ferro-

electric material substrate. One hot and two ground elec-

trodes of the symmetric coplanar line are located on the

three straight guides, as shown in the cross-sectional view

in Fig. 7-1. Therefore, three light waves are modulated

simultaneously by only one RF modulation signal. The

optical frequency of output light is shifted from the input

one by the driving RF frequency. The output power is

controllable by the driving RF signal power.

The operation principle of the proposed OFS is shown in

Fig. 7-2. The input lightwave irradiated onto the device is

divided into three parts of equal powers by the first 3-

branching waveguide, and the three divided light waves

propagate through the three straight waveguides. By the

RF signal supplied to the traveling-wave electrodes, the

three light waves are modulated with the same modulation

depth, but their modulation phases are set to have a differ-

ence of +2 /3 or -2 /3 by controlling the modulation depth

and phase in the traveling-wave modulators with the peri-

odic polarization-reversal scheme, as is in the case of the

SSB modulators[P3, P8]. Optical delays of /3 are also im-

posed on these modulated light waves by the successive

DC bias electrodes. Finally, the three light waves are

combined and interfered by the second 3-branching

waveguide, where the upper (or lower) sideband compo-

nents are in phase and enhanced, while the carrier and the

other sideband are mutually phase shifted by 2 /3 and

suppressed by the interference. Therefore, a frequency-

shifted light wave is obtained.

Fig. 7-1. Basic structure of the proposed OFS.

Fig. 7-2. Schematic of the operation principle of the OFS.

Fig. 7-3. Calculated frequency response of the OFS.

7.3 Analysis

The proposed OFS essentially performs a band operation

since it utilizes QVM traveling-wave modulation with pe-

riodic polarization reversal. Its frequency response was

analyzed by the impulse response method. Figure 7-3

shows an example of the calculated frequency responses of

+1st and -1st modulation sidebands under the appropriate


optical delays. In this case, the +1st modulation sideband

component was enhanced, and the carrier and the -1st side-

band were almost completely suppressed.

The light propagation characteristics of the 3-branching

waveguides were also analyzed using the BPM. The pa-

rameters of the 3-branch waveguide for dividing light

waves into three parts of equal powers were determined in

the analysis and the 3-branch waveguide interferometer

was designed.

7.4 Experiments

The fabrication processes are essentially the same as

those adopted for the device presented in the previous sec-

tions. Z-cut LiTaO3 was used as the substrate material.

The period of polarization reversal, 2L, was set to be

8.2mm, which was designed for the operation of light

waves of ~650nm and the operation frequency of 15GHz.

The 3-branch waveguide interferometer was fabricated by

the proton-exchange method with benzoic acid. In the 3-

branching waveguides, the width of the waveguide was set

to be 3.3 m for the center guide and 4.2 m for the outer

two guides, and the branching angle between the center

and the outer waveguides was 1/200rad. These parameters

were determined by BPM analysis in order to have the

exactly equal power splitting ratio. The length of the RF

modulation electrodes, Lt, was set to be 20.5mm (5L).

Figure 7-4 shows examples of the optical frequency

spectra measured from the fabricated OFS using a scan-

ning Fabry-Perot interferometer. By changing the DC bias

voltage, the +1st or -1st optical sideband components was

enhanced, and the carrier and the other sideband were

clearly suppressed. Figure 7-5 shows the measured fre-

quency dependence of the fabricated device. The peak

operation frequency and the 3dB bandwidth were 15GHz

and 5GHz, respectively, which were in good agreement

with the designed values. Therefore, the basic operations

of the proposed OFS were confirmed.

Table IV. Parameters of the proto-type OFS.



3dB operation bandwidth f 5.5GHz

Designed light wavelength ~650nm

Period of polarization-reversal 2L 8.2mm

Center straight waveguide width wg2 3.3 m

Outer straight waveguide widths wg1, wg3 4.2 m

Branching angle 1/200rad

Waveguide separation s1, s2 30 m

RF electrode length Lt 20.5mm

RF electrode spacing d 18.5 m

RF hot electrode width w 26 m


7.5 Summary

A novel EO OFS was proposed and its basic operations

were demonstrated. The operation bandwidth of the pro-

posed OFS can be enlarged over several-fold using the

chirped periodic polarization-reversal structure[P8]. The

application of the fabricated OFS to coherent laser spec-

troscopy of standard gases is now under way.

Fig. 7-4. Optical frequency spectra from the fabricated OFS.

( fm=15GHz, =633nm )

Fig. 7-5. Measured frequency response of the fabricated OFS.

8. Conclusion

Several EO guided-wave devices utilizing polarization-

reversal structures have been proposed and developed for

optical frequency/wavelength manipulation. Their basic

operations were demonstrated and the potential of the EO

guided-wave devices with the polarization-reversal

schemes was confirmed. The applications of the proposed

EO SSB modulators to ROF systems of quasi-millimeter-

wave signal transfer were also carried out and their useful-

ness was verified. We also studied EO guided-wave de-

vices with polarization-reversal structures using novel

stoichiometric LiTaO3[P10], which will be reported in the

near future.

Acknowledgments

The authors would like to thank Professors S. Yama-

moto, T. Kobayashi and S. Urabe of Osaka University, and

Drs. Enokihara, H. Sasai and H. Shiomi for their helpful

advice and encouragement. We also thank Mr. T. Doi, H.

Fukuchi, K. Hirosawa, Y. Toyoshima, A. Ishikawa, T.

Iwamoto, and K. Kaneda for their assistance in the analy-

ses and the experiments.


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Publications

Journal papers

[P1] H. Murata, A. Morimoto, T. Kobayashi, and S. Yamamoto: “Optical

pulse generation by using quasi-velocity-matched guided-wave elec-

trooptic phase modulator,” Optical and Quantum Electronics, vol.33,

pp.785-794 (July 2001)

[P2] A. Enokihara, H. Yajima, M. Kosaki, H. Murata and Y. Okamura:

“Guided-wave electro-optic modulator using resonant electrode of

coupled microstrip lines,” Electron.Lett., vol.39, pp.1671-1673 (Nov.

2003).

[P3] H. Murata, and Y. Okamura: “Guided-wave electro-optic single side-

band modulators by use of polarization-reversed structures,” The Re-

view of Laser Engineering (in Japanese), vol. 32, No. 3, pp.186-190

(March, 2004).

[P4] A. Ishikawa, H. Murata, T. Tanaka, H. Shiomi, Y. Okamura, and S.

Yamamoto: “Positive and Negative Optical Responses in High-

Electron Mobility Transistors and Their Applications to Optically

Controlled Microwave Oscillators,” Japanese Journal of Applied

Physics, vol.43, No.3, pp.997-1001 (March, 2004).

International conferences

[P5] H.Murata, H. Fukuchi, K. Kinoshita, A. Morimoto, T. Kobayashi,

and S. Yamamoto: “Quasi-velocity-matched guided-wave optical

modulator with resonant electrodes for integrated ultrashort pulse

generators,” The Conference on Lasers and Electro-Optics 2001

(CLEO2001), Baltimore, CThL10, pp.442-443 (May 10, 2001).

[P6] H. Murata, T. Doi, T. Kobayashi, and S. Yamamoto: “Novel elec-

trooptic SSB modulator/optical frequency shifter using periodically

domain-inverted structure,” The 4th Pacific Rim Conference on La-

sers and Electro-Optics (CLEO/Pacific Rim2001), Chiba, ME2-5,

Vol.1, pp.106-107 (July 16, 2001).

[P7] H. Murata, H. Fukuchi, T. Doi, T. Kobayashi, and S. Yamamoto:

“Guided-wave electrooptic LiTaO3 modulators using domain-inverted

structure,” The 2nd International Laser, Lightwave and Microwave

Conference (ILLMC2001), Shanghai, pp.133-135, H-2 (November 7,

2001).

[P8] H. Murata, T. Doi, T. Kobayashi, and S. Yamamoto: “Novel guided-

wave single-sideband electrooptic modulators by using periodically

domain-inverted structure,” The Conference on Lasers and Electro-

Optics 2002 (CLEO2002), Long Beach, CTuK14, pp.204-205 (May

21, 2002).

[P9] H. Murata, T. Doi, K. Hirosawa, and S. Yamamoto: “Optical single-

sideband modulation by using periodically domain-inverted LiTaO3

guided-wave modulators,” The Microwave Photonics Conference

2002 (MWP2002), Hyogo, W4-3, pp.53-56 (November 6, 2002).

[P10] A. Ishikawa, H. Murata, and S. Yamamoto: “Quasi-velocity-

matched guided-wave electrooptic modulator by using stoichiometric

domain-inverted LiTaO3,” The sixth International Topical Workshop

on Contemporary Photonic Technologies (CPT2003), Tokyo, PDP-5,

p.5 (January 16, 2003).

[P11] H. Murata and S. Yamamoto: “Optical modulator with polarization-

reversed structures (invited talk),” The 1st NIMS International Con-

ference -Material Solutions for Photonics-, Tsukuba, II-11, pp.45-46

(March 18, 2003).

[P12] H. Murata, S. Yamamoto, H. Sasai, and A. Enokihara: “Novel

guided-wave electrooptic single-sideband modulator by using peri-

odically domain-inverted structure for a long wavelength operation,”

The Optical Fiber Communication Conference 2003 (OFC2003), At-

lanta, MF53 (March 24, 2003).

[P13] H. Murata, K. Kaneda, S. Yamamoto: “Low-power and low-chirp

guided-wave electrooptic intensity modulator by use of domain-

inverted structure,” The Conference on Lasers and Electro-Optics

2003 (CLEO2003), Baltimore, CWA-19 (June 4, 2003).

[P14] H. Murata, K. Hirosawa, S. Yamamoto, H. Sasai, and A. Enokihara:

“26GHz optical single-sideband modulation by using guided-wave

electrooptic modulator with periodically polarization-reversed struc-

tures,” The 10th International Workshop on Femtosecond Technology

(FST2003), Chiba, TP-3, p.152 (July 17, 2003).

[P15] A. Ishikawa, H. Murata, T. Tanaka, H. Shiomi, and S. Yamamoto:

“Positive and negative optical responses in high-electron mobility

transistors and their applications to optically controlled microwave

oscillators,” The 5th Pacific Rim Conference on Lasers and Electro-

Optics (CLEO/Pacific Rim2003), Taipei, THP-(2)-4, p.510 (Decem-

ber 18, 2003).

[P16] H. Murata, K. Kaneda, and Y. Okamura: “38GHz optical single-

sideband modulation by using guided-wave electrooptic modulators

with periodic polarization-reversal,” The Conference on Lasers and

Electro-Optics 2004 (CLEO2004), San Francisco, CThT10 (May 20,

2004).

[P17] H. Murata, Y. Okamura, H. Sasai, and A. Enokihara: “Demonstra-

tion of 26GHz signal optical fiber transmission by use of guided-

wave electrooptic single-sideband modulators with periodically po-

larization-reversed schemes,” The IEEE MTT-S International Micro-

wave Symposium (IMS2004), Fort Worth, Texas, IFTH-60, vol.3,

pp.2059-2062 (June 10, 2004).

[P18] H. Murata, T. Iwamoto, and Y. Okamura: “Novel electrooptic opti-

cal frequency shifters by use of a 3-branch waveguide interferometer

and polarization-reversal structures,” The Ninth Optoelectronics and

Communications Conference/Third International Conference on Op-

tical Internet (OECC/COIN2004), Yokohama, 14F3-3, pp.564-565,

(July 14, 2004).

[P19] H. Murata, and Y. Okamura: “Millimeter-wave electrooptic Single-

Sideband modulators using polarization-reversed structures (invited

talk),” The 8th International Symposium on Ferroic Domains and Mi-

cro- to Nanoscopic Structures (ISFD-8), S08-Thp05, p.20, Tsukuba

(August 26, 2004).


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