Download 50 GHz Velocity-matched, Broad Wavelength LiNbO3

~"'HEWLETT
~~PACKAAD
50 GHz Velocity-matched, Broad
Wavelength LiNbOs Modulator
with Multimode Active Section
David W. Dolfi, T. R. Ranganath
Instruments and Photonics Laboratory
HPL-92-61
June, 1992
integrated optics,
optical modulation,
Lithium Niobate
Internal Accession Date Only
To be publied in the Electronics Letters
© Copyright Hewlett-Packard Company 1992
A velocity-matched LiNbO s modulator is
described which achieves an electrical 3 dB
bandwidth of 44 Ghz and an optical 3 dB
bandwidth in excess of 50 Ghz. It also
utilizes multimode waveguides in the
active section to achieve excellent loss,
drive
voltage,
and contrast ratio
characteristics over a 1.3 to 1.55 um
wavelength range.
1
Introduction
Improved velocity matching in LiNb0 3 modulators has been achieved by several workers [l4] , using a combination of relatively thick (~ 1 p.m) Si0 2 buffer layers and thick (typically
~ 10 p.m) plated electrodes. Alternatively, a shielded ground plane above the substrate is
used in place of the thick electrodes [5]. Utilizing this combination, microwave indices ::;
2.5 have been achieved. However, increased electrode thickness and/or a shielded ground
plane lowers device impedance, which raises drive power requirements and causes mismatch
to most common 50 n sources. This decrease is partiall[ compensated by the thicker buffer
layer and by employing large gaps (typically 13-15 p.m between the hot and ground electrode(s). While both of these will help maintain a hig impedance, they also increase the
drive voltage, resulting in a larger voltage-length product relative to conventional modulators. To achieve closer velocity matching would require additional increases in electrode or
buffer layer thicknesses, resulting in even higher voltages.
We describe a modulator which also utilizes a thick electrode/buffer layer geometry. However,
it introduces two novel features, resulting in a device which achieves almost exact velocity
matching to the optical index, maintains high impedance, and has a voltage-length product
which compares favorably with previous devices of this type. In addition, it maintains this
performance over the whole range of wavelengths from 1.3 to 1.55 p.m. This latter property
is very important for optical measurement applications such as network analysis and optical
sampling.
2
Device design
The device is a Mach-Zehnder (M-Z) modulator, the cross section of which is shown in Fig.
1. The optical waveguides are either centered in the electrode gaps for x-cut geometry or
moved laterally to lie under the electrode edges for z-cut, as shown. Devices have been
fabricated on both orientations. Unlike previous designs, the structure utilizes a narrow
ground plane whose width is only slightly larger than either the hot electrode width or gap.
Decreasing the ground plane width increases both the impedance and the microwave velocity.
This introduces an extra degree of freedom which allows exact matching of the optical and
microwave index at a reasonably high impedance, without using excessively thick electrodes
or buffer layer, and with an actual reduction in the electrode gap relative to other designs
which employ infinite ground planes [1,5]. Referring to Fig. 1, the parameters are as follows:
hot electrode width W = 9 /-lm, ground electrode width W9 = 15 /-lm, and electrode gap G
= 10 p.m. The electrode and buffer layer thicknesses are 10 /-lm and 1 /-lm, respectively. The
substrate thickness is 500 p.m. Calculations predict that this structure has an impedance Z
~ 40 n and a microwave index n rn ~ 2.15, essentially equal to the optical index at 1.3 /-lm.
The second novel feature, shown in Fig 2, concerns the optical waveguides themselves. The
waveguides in the active section of the device have been fabricated with a high enough index
difference to be multimode at 1.3 utn. The input and output waveguide sections are reduced
in width so as to be single mode over the entire wavelength range from 1.3 to 1.55 p.m. The
symmetric, adiabatic nature of the y-junction connecting these two regions insures that no
coupling occurs to the higher order modes of the active section. Thus, even though the guides
in the active section are multimode over some part of the wavelength range of operation, only
their lowest order mode is excited. Because of the high index difference, this mode is very
well confined, reducing drive voltage and enabling efficient, low-loss operation over a much
1
greater wavelength range than that allowed by the strict use of singlemode guides. For the
devices fabricated here, the narrow and wide waveguide widths are 4 and 6 p.m, respectively.
The titanium (Ti) thickness is 1100 A (x-cut) and 1150 A (z-cut). Diffusion times are 6 and
8 hours for x and z cut, respectively. The diffusion temperature is 1050 °C.
3
Results
M-Z modulators were fabricated in both x and z cut LiNb0 3 as well as straight guides and
y-junctions (one-half of an interferometer). It was verified that the 6 p.m straight guides
supported at least two modes at 1.3 p.m wavelength. When y-junctions were excited at the 4
p.m end, however, only the well-confined, lowest order mode of the 6 p.m output guides was
observed at the output, independent of input alignment, verifying the principle of operation.
Drive voltages of 1 em active length modulators were measured at both wavelengths. The
results are summarized in Table 1. The drive voltage scales roughly as ,\ and ,\2 for x and z
cut devices, respectively. The stronger dependence in the z-cut case is due to the sensitivity
of the overlap of the optical mode and fringing microwave field to the size of the mode,
and suggests that further reduction in the voltage-length product is possible by using even
higher titanium concentrations. Preliminary measurement of onloff. ratios at 1.3 p.m are well
in excess of 20 dB for both x and z cut. Fiber- to-fiber insertion loss, based on measurements
thus far on 1 em active length devices, is estimated to be < 3 dB for z-cut and < 4 dB for
x-cut, with some measurements as low as 2 dB for z-cut. The difference is mainly due to the
more symmetric mode of the z-cut devices, which matches more closely to singlemode fiber.
High frequency measurements have been performed on some of the x-cut devices. Fig. 3a
shows the frequency response of two such devices to 50 GHz, measured at 1.3 uti». The better
of the two devices has an electrical 3 dB bandwidth ~ 44 GHz. The optical 3 dB bandwidth
of both devices is > 50 GHz, and is estimated to be > 60 GHz by extrapolation of the
data. Fig. 3b shows the response of the faster device when optical and microwave signals
are counter-propagating. This induces a velocity mismatch which generates the sine-like
response shown. The dashed vertical line near 7 GHz represents the theoretically predicted
location of the first null for exact velocity matching. The small deviation of the actual null
from this line implies that the microwave and optical indices differ by less than 3.5 %. If the
device and microwave package were perfect in every other respect, this residual mismatch
would translate into an electrical 3 dB bandwidth of ~ 175 GHz.
4
Summary
In summary, a novel Mach-Zehnder modulator in LiNb03 is described. It utilizes a novel
microwave structure with narrow ground planes to achieve 50 GHz frequency response and almost exact velocity matching. In addition, a novel waveguide concept - the use of multimode
guides with excitation of only the lowest order mode - has been used in the interferometer
design to achieve good mode confinement over the entire wavelength range of 1.3 to 1.55
p.m, resulting in low optical insertion loss, no degradation in on/off ratio, and voltage-length
products as low as 8.3 volts-em for z-cut LiNb0 3 •
2
5
References
[1] M. Seino, N. Mekada, T. Namiki, and H, Nakajima, "33-GHz-cm broadband Ti:LiNb03
Mach-Zehnder modulator", Tech. dig. ECOC '89, Gothenburg, 1989, paper ThB22-5.
see also: M. Seino, N. Mekada, Y. Yamane, Y. Kubota, M. Doi, and T. Nakazawa,
"20 GHz 3 dB-bandwidth Ti:LiNb03 Mach-Zehnder modulator", Tech. dig. ECOC '90,
Amsterdam, 1990, postdeadline paper ThG1.4.
[2] H. Miyamoto, H. Ohta, K. Tabuse, H. Iwaoka, and Y. Miyagawa, "A broad-band
traveling-wave Ti:LiNb0 3 optical phase modulator", Japanese Jour. Appl. Phys., 1991,
30, pp. L383-L385.
[3] S.K. Korotky, J.J. Veselka, C.T. Kemmerer, W.J. Minford, D.T. Moser, J.E. Watson,
C.A. Mattoe, and P.L. Stoddard, "High-speed, low power optical modulator with adjustable chirp parameter", Tech. dig. IPR '91, Monterey, 1991, paper TuG2.
[4] H. Ohta, H. Miyamoto, K. Tabuse, and Y. Miyagawa, "Ti:LiNb03 Mach-Zehnder modulator using a buried traveling wave electrode", Tech. dig. OFC '92, San Jose, 1992,
paper ThG3.
[5] K. Kawano, T. Kitoh, H. Jumonji, T. Nozawa, and M. Yanagibashi, "New traveling-wave
Mach-Zehnder optical modulator with 20 GHz bandwidth and 4.7 V driving voltage at
1.52 p,m wavelength", Electron. Lett., 1989, 25, pp. 1382-1383.
3
Figures
Figure 1.
Cross-section of microwave coplanar structure for z-cut geometry. For x-cut,
waveguides are translated horizontally to the center of the electrode gaps. Parameter values
are given in text.
Figure 2. Geometry of M-Z interferometer. Widths of waveguide(s) are 4 and 6 pm in
input/output and active regions, respectively.
Fi~ure
3. Frequency response of 1 em x-cut devices is normal (a) and counter- propagating
(b) mode of operation. Vertical line in (b) represents the null frequency for exact velocity
matching.
Table 1. Voltage-length product (volts-em) of devices for different orientations and wavelengths. Active length = 1 em in all cases.
4
Au electrodes
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1--.1 G --
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buffer
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LiNb03 tb I
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IL waveguides:
substrate
Figure 1
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Figure 2
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10
30
20
40
50
20
25
Frequency [GHz]
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a.
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0
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15
Frequency [GHz]
(b)
Figure 3
~:
x-cut
z-cut
1.3 urn
10.4
8.4
1.55 urn
12.8
12.3
Table 1