A new technique of phase velocity matching in electrooptic modulators was demonstrated. The results show that the phase velocity mismatch due to material dispersion in traveling-wave LiNbO_3 optical waveguide modulators can be greatly reduced by breaking the modulation transmission line into short segments and connecting each segment to its own surface dipole antenna. The array of antennas is then illuminated by the modulation signal from below at the proper angle to produce a delay from antenna to antenna that matches the optical waveguide's delay. A phase modulator 25 mm in length with five antennas and five transmission line segments was operated from 4.6 to 13 GHz with a maximum phase modulation sensitivity of over 100°/W^(1/2)

Bridges, William B.

Schaffner, J. H.

Sheehy, Finbar T.

English

Caltech Authors - Main

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 3, NO. 2, FEBRUARY 1991 133 
Wave-Coupled LiNbO, Electrooptic 
Modulator for Microwave and 
Millimeter- W ave Modulation 
William B. Bridges, Finbar T. Sheehy, and James H. Schaffner 
Abstract-The phase velocity mismatch due to material dispersion in 
traveling-wave LiNbO, optical waveguide modulators may be greatly 
reduced by breaking the modulation transmission line into short seg- 
ments and connecting each segment to its own surface dipole antenna. 
The array of antennas is then illuminated by the modulation signal from 
below at the proper angle to produce a delay from antenna to antenna 
that matches the optical waveguide's delay. A phase modulator 25 mm 
in length with five antennas and five transmission line segments was 
operated from 4.6 lo 13 GHz with a maximum phase modulation 
sensitivity over IOO"/W'/~. 
INTRODUCTION 
RAVELING-WAVE optical waveguide modulators in T LiNbO, have shown frequency responses greater than 20 
GHz. Unfortunately, both high frequency response and high 
sensitivity are difficult to realize in the same device. The mis- 
match in phase velocities between the optical wave and the 
microwave signal due to material dispersion in LiNbO, limits 
the interaction length and hence the sensitivity. This paper 
describes a new technique to overcome the material dispersion 
limitation, and as an added benefit, utilizes a waveguide medium 
to introduce the modulation signal. This latter feature may prove 
as important as the former in extending modulator operation to 
the millimeter wave range. 
Alferness et al. [l]  overcame the dispersion limitation by 
dividing the modulation electrodes into sections whose length 
produces a 180" differential phase error from end to end at the 
center of a passband. The ends of one section are then connected 
to the next by transposed leads to correct the phase by 180". In 
this way the average phase velocity of the modulating wave 
follows the optical phase velocity. Unfortunately, the length of 
each section results in a loss in sensitivity. 
Schaffner [2] recently described a technique that also divides 
the electrodes into sections, but each section is shorter than the 
length that produces 180" phase error. The next section is 
connected to the previous section through a stub transmission 
line that provides a phase delay equal to 360" minus the phase 
error suffered in the short modulator section. As in [l], the 
average modulating wave velocity is equal to the optical veloc- 
ity, but the freedom to choose shorter sections means that less 
penalty per section is suffered. 
Manuscript received August 30, 1990; revised November 21, 1990. 
Preliminary results of this paper were presented in paper OE 7.2 at the 
LEOS Annual Meeting, Boston, MA, November 7, 1990. This work was 
supported by a grant from the Caltech President's Fund and a grant from 
Rome Air Development Center. 
W. B. Bridges and F. T. Sheehy are with the Division of Engineering and 
Applied Science, California Institute of Technology, Pasadena, CA 91 125. 
J. H. Schafier is with Hughes Research Laboratories, Malibu, CA 
90265. 
IEEE Log Number 9042486. 
Fig. 1. Schematic drawing of two dipole antennas and connected transmis- 
sion line sections on the surface of a waveguide E-0  modulator. 
We report still another technique to realize phase matching on 
the average. As in [2], the modulator electrodes are divided into 
short sections, but each section is connected to its own surface 
antenna as shown in Fig. 1. This array of antennas is then 
illuminated from the substrate side by a plane wave at an angle 
8 with respect to the surface normal, the angle chosen so that 
the phase velocity of the incident modulating wave in the 
direction of the optical waveguide is equal to that of the optical 
wave. That is, 0 = sin-'(n,/n,) where no  is the optical 
refractive index and n, is the modulation refractive index. (A 
similar angle phase matching was demonstrated by Kaminow et 
al. [3] for a bulk LiNbO, electrooptic modulator.) Thus, each 
antenna receives the proper phase to match the phase velocity of 
the optical wave. As the wave from the antenna travels down the 
short transmission line section, a phase error develops, but this 
is kept small by making the section short. 
Since no physical connections are made to the antennas, no 
parasitic circuit elements are introduced that limit scaling to 
higher frequencies. Likewise, the effect of loss in the transmis- 
sion line segments is minimized since the array of line segments 
is fed from the side and not the end. In return for these 
advantages, we must develop an efficient method of coupling 
energy into the antennas from a waveguide feed. 
EXPERIMENTAL PHASE MODULATOR 
Fig. 2 shows a photolithographic mask for five modulator 
electrode segments with attached antennas designed for a 12 
GHz center frequency. The overall modulator length is 25 mm. 
A simple phase modulator was built rather than a Mach-Zehnder 
amplitude modulator; however, the same mask should also work 
if applied to one arm of a M-Z. The antennas were configured 
as "two half waves in phase," and the modulator electrodes 
were also a half wave long at 12 GHz. In designing both the 
antennas and the transmission lines an effective refractive index 
of 3.8 was taken as the proper average of LiNbO, and air for 
1041-1135/91/0020-0133$01.00 01991 IEEE 
I 
1000 : 
- 8 0 0 :  
I 
P 
2 6 0 0 :  
; 4 0 0 :  
m 
2 0 0 :  
134 
iC-- 25mm -4 
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 3, NO. 2, FEBRUARY 1991 
f 
I 
-_-_ J 
- USB ' - - 
LSE - 11.50GHz 
Fig. 4. Oscilloscope display of scanning Fabry-Perot (SFP) output with 
11.50 GHz narrow deviation phase modulation. The He-Ne laser output was 
single frequency (large pips at the free spectral range (FSR) spacing). The 
phase modulation sidebands are the smaller pips. The apparent spacing of 
1.5 GHz is due to the aliasing by the SFP. 
1200 3 
Fig. 2. Photolithographic mask for five antennas-plus-transmission lines 
covering a 25 mm long optical waveguide. Magnified details are shown in 
the lower two drawings. 
I I YE.3  
WR-90 
WAVEGUIDE 
Fig. 3. Schematic side view of an experimental modulator showing LiNbO, 
wafer with antennas, input and output lenses, a wedged block of high 
dielectric constant material, quarter-wave matching layers, and microwave 
waveguide. The waveguide feed was changed to WR-137 for 4.6-7 GHz 
measurements. 
electrodes on the dielectric surface. The optical waveguide was 
formed by titanium diffusion, and was about 6 pm in size. 
As shown in Fig. 3, the LiNbO, plate was attached to a 
wedge of dielectric with E = 30. A wedge was used to introduce 
the modulating wave into the block at the calculated optimum 
angle of 23" since the critical angle for LiNbO, is about 9". 
Two matching layers of intermediate dielectric constant were 
introduced to minimize the reflection at the interface. The beam 
from a single-frequency He-Ne laser at 633 nm was coupled in 
and out with microscope objectives. The optical power was kept 
low to minimize optical damage. 
The pattern of a dipole antenna on the surface of a dielectric is 
strongly directed into the dielectric; for E = 30, there is virtually 
no coupling to the air side. The pattern has a strong lobe in the 
direction of the critical angle (9" here). We have used the theory 
given by Engheta et al. [4] to estimate the pattern at the desired 
angle of 23", and find that it is only about 0.5 dB down from the 
peak. 
We used a scanning Fabry-Perot interferometer to detect the 
phase modulated signal. Fig. 4 shows the display for modulation 
4 0  6 b  8 b  160 12'0 140 
0 0  t 7 7 1 1 7 , n  ~ , ~ , ' ' I ~ ~  , , ' I , ' ' ' '  " " " " '  " " " " ' ~  
Frequency (GHz) 
Fig. 5.  Modulator sensitivity in degrees of phase modulation per watt''' of 
microwave drive power as a function of drive frequency. 
at 11.50 GHz. Since the free spectral range of 2 GHz is much 
less than the modulation frequency, there is an aliasing every 2 
GHz, and the sidebands appear to be at 11.50 - 5 x 2.0 = 1.5 
GHz. The modulator phase deviation can be deduced from the 
amplitude of the sidebands relative to the carrier for narrow 
deviation phase modulation from the formula [5] 
This measurement technique has the advantage of not requiring a 
fast photodetector and is thus equally effective for 4.6-13 GHz 
modulation, 60 GHz modulation, or any other frequency. 
Fig. 5 shows the phase deviation normalized to the square 
root of the microwave drive power as a function of modulation 
frequency. The gaps at 6, 8, 10, and 12 GHz are regions where 
measurements were difficult since the sidebands fall on top of the 
aliased carriers in the SFP display. The structure on this curve 
has several possible sources: antenna and electrode resonance, 
reflections within the wedge, and at the LiNb0,-air 
interface, . . . However, we obtain a reasonable fit with a simple 
model of the antenna and open-circuited transmission line 
impedances, and modulation resulting from forward and re- 
flected waves on the line [6]. There are two resonance peaks in 
the response: one around 6 GHz where the antenna elements and 
transmission line segments are each a quarter wave in length, 
and another around 12 GHz (the design value) where they are a 
half wave in length. The absolute value of 100°/W'/2 is not too 
much less than conventional microwave modulators [7]. 
A number of different feed wedges was used to establish the 
effect of illumination angle on performance. Fig. 6 shows the 
BRIDGES et al.: LiNbO, ELECTROOPTIC MODULATOR 
-\* 
135 
array of 20 antennaftransmission-line segments of one-sixth the 
dimensions indicated in Fig. 2. 
ACKNOWLEDGMENT 
The authors wish to acknowledge the technical support of R. 
L. Joyce and R. E. Johnson, and many valuable discussions with 
R. W. Terhune. 
00 10 0 20 0 30 0 
0 0  j # # , ! 8 8 # # 8 , 1 8 m -  I , ,  I ~ “ “ ~ ~ “ I ’ ’ ” ’  
Angle of Incidence (Degrees) 
Fig. 6. Average modulator response as a function of angle of incidence of 
modulating signal. 
average response over the 9- 12 GHz range at each illumination 
angle. The response peaks at about 23” as expected, confirming 
the phase-velocity-matching picture. 
SUMMARY 
We have demonstrated a new technique of phase velocity 
matching in electrooptic modulators by illuminating an array of 
antennas each connected to its own short section of modulating 
electrode. Promising sensitivity results of 100” /W ‘1’ were ob- 
tained at 12 GHz. 
Note Added in Proof: We have recently obtained approxi- 
mately 80”/W1/* phase modulation at 61 GHz with an 18 mm 
111 
121 
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[51 
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161 
REFERENCES 
R. C. Alferness, S. K. Korotky, and E. A. J. Marcatili, “Veloc- 
ity-matching techniques for integrated optic traveling wave switch 
modulators,” IEEE J. Quantum. Electron., vol. QE-20, pp. 
301-309. Mar. 1984. 
J. H. Schaffner, “Analysis of a millimeter wave integrated 
electro-optic modulator with a periodic electrode,” in Proc. 
SPIE OE-LASE Conf. 1217, paper 13, Los Angeles, CA, Jan. 
I. P. Kaminow, T. J. Bridges, and M. A. Pollack, “A 964-GHz 
traveling-wave electro-optic light modulator,” Appl. Phys. 
Lett.,  vol. 16, pp. 416-418, June 1, 1970. 
N. Engheta, C. H. Papas, and C. Elachi, “Radiation patterns of 
interfacial dipole antennas,” Radio Sci., vol. 17, pp. 1557-1566, 
Nov.-Dec. 1982. 
S e e ,  for example, Reference Data for Radio Engineers. 5th Ed. 
New York: Howard Sams, 1968, Section 21-7. 
F. T. Sheehy, unpublished calculations. 
D. Erasme, D. A. Humphreys, A. G. Roddie, and M. G. F. 
Wilson, “Design and performance of phase reversal traveling 
wave modulators,” J. Lightwave Technol., vol. 6, pp. 933-936, 
June 1988. 
16-17, 1990, pp. 101-110. 


Wave-coupled LiNbO_3 electrooptic modulator for microwave and millimeter-wave modulation

1217, paper 13,

Analysis of a millimeter wave integrated electro-optic modulator with a periodic electrode,” in

Design and performance of phase reversal traveling wave modulators,”

Radiation patterns of interfacial dipole antennas,”

https://authors.library.caltech.edu/9866/1/BRIieeeptl91.pdf

Wave-coupled LiNbO_3 electrooptic modulator for microwave and millimeter-wave modulation

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