Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2001-02-08
2002-06-25
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S008000, C385S014000, C430S321000
Reexamination Certificate
active
06411747
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a waveguide type optical device, such as a waveguide-type optical modulator and a waveguide-type optical switch, used in various optical systems including high-speed optical communication, optical switching network, optical information processing, and optical image processing.
BACKGROUND OF THE INVENTION
A waveguide-type optical modulator and a waveguide-type optical switch are important components to compose various optical systems including high-speed optical communication, optical switching network, optical information processing, and optical image processing. Especially a modulator using a LiNbO
3
substrate is a promising device since it has a smaller wavelength chirping in modulation than that of a semiconductor-system modulator, e.g., a modulator using a GaAs-system substrate.
Important parameters to determine the performance of LiNbO
3
optical modulator are drive power (or drive voltage), modulation bandwidth and insertion loss. Of these parameters, the modulation bandwidth and drive voltage are in trade-off relationship. Therefore, it is difficult to widen the modulation bandwidth as well as lowering the drive voltage. So, searches about waveguide-type optical modulator focus on the optimization of the trade-off relationship.
The bandwidth of waveguide-type optical modulator is mainly dependent on the kind, material and placement of electrode, and the permittivity of substrate. So, in order to widen the bandwidth of waveguide-type optical modulator, a traveling wave electrode is in wide use, and is formed as an extension of transmission line. Here, the characteristic impedance of electrode has to be equal to that of microwave power source and load. In this case, the modulation speed is restricted by the difference between the traveling times (or phase speeds or effective refractive indexes) of light wave and microwave. Meanwhile, as the traveling wave electrode structure used widely, there are two kinds of structures, i.e., an asymmetric strip line (hereinafter referred to as ‘ASL’) type or asymmetric coplanar strip (hereinafter referred to as ‘ACPS’) type electrode structure, and a coplanar waveguide (hereinafter referred to as ‘CPW’) type electrode structure.
The bandwidth of modulator is restricted by microwave attenuation &agr;, the speed discordance or effective refractive-index difference between light wave and microwave. To suppress the speed discordance, characteristic impedance and microwave attenuation, it is necessary to optimize the buffer-layer parameter and electrode parameter, particularly the width of signal electrode and the interval between signal electrode and earth electrode. However, even if the speed discordance could be suppressed, the bandwidth of modulator is restricted by microwave attenuation. So, to suppress the microwave attenuation is most important for realizing the wider bandwidth of modulation. Moreover, by reducing the microwave attenuation, the drive voltage in trade-off relationship with the bandwidth can be also controlled at the same time.
The microwave attenuation is caused by phenomena below.
(a) a loss in strip-line conductor that is a function of the form or structure of electrode (width of signal electrode, interval between signal electrode and earth electrode etc.), the resistivity of electrode material, buffer-layer parameter etc.
(b) a dielectric loss that is a function of the permittivity of LiNbO
3
substrate and tan &dgr; (loss tangent)
(c) a loss due to higher-order mode propagation
(d) a loss due to the impedance discordance between power-supply side characteristic impedance and load side characteristic impedance (normally, both characteristic impedances are matched into 50 &OHgr;)
(e) a loss in strip-line curved portion and tapered portion
(f) a loss due to a mounting package and external package including a loss in a connector, a feeder part of signal electrode, connection method or material thereof.
About the above phenomena (a), (b), (c) and (d), the optimization of electrode parameter and buffer-layer parameter has been considered to some extent. The inventor of this application also discloses an optical modulator that using a thick CPW electrode structure, a bandwidth as wide as 20 GHz and a drive voltage as low as 5V are obtained, in “A Wide Band Ti:LiNbO
3
Optical Modulator with A Conventional Coplanar Waveguide Type Electrode”, IEEE Photonics Technology Letters, Vol. 4, No. 9 (1992) (first prior art).
Adding to this, various optical modulators using ASL/ACPS type electrode structure or CPW electrode structure are suggested. The typical examples are disclosed in “Traveling-Wave Electro-Optic Modulator with Maximum Bandwidth-Length Product”, Applied Physics Letters, Vol. 45, No. 11, pp. 1168-1170 (1984) (second prior art), “20-GHz 3 dB-Bandwidth Ti:LiNbO
3
Mach-Zehnder Modulator”, International Conference, ECOC'90 pp. 999-1002 (1990) (third prior art), and “Highly Efficient 40-GHz Bandwidth Ti:LiNbO
3
Optical Modulator Employing Ridge Structure”, IEEE Photonics Technology Letters, Vol. 5, No. 1, pp. 52-54 (1993) (fourth prior art).
In general, an electric band (S21 characteristic) of modulator is represented as below.
&agr;=&agr;
0
(
f
)
½
L
where &agr; is a microwave loss (or microwave attenuation) of all electrodes [dB], &agr;
0
is a microwave attenuation constant [dB/{cm(GHz)
½
}], f is a frequency [GHz], and L is an electrode length [cm].
The above electric band (frequency for S21-characteristic of 6 dB) is restricted by the microwave attenuation constant &agr;
0
of electrode, and further influenced by the optical characteristic. Thus, the reduction of microwave attenuation constant &agr;
0
of electrode is restricted by the entire bandwidth of device. Meanwhile, the values of microwave attenuation constant &agr;
0
of electrode in the above prior arts are 0.45 (first prior art), 3.75 (second prior art), 0.5 (third prior art) and 0.75 (fourth prior art).
However, in order to construct a further high-speed communication system for, e.g., 40 Gb/s, it is necessary to realize an optical modulator with a wide modulator band of 30 GHz or wider and a low drive voltage of 3.5 V or lower. Therefore, the microwave loss has to be further reduced.
Referring to
FIGS. 1A and 1B
, an example of waveguide type optical device, which is disclosed in the first prior art, is explained below.
FIG. 1A
is a plan view showing the conventional waveguide type optical device, and
FIG. 1B
is a cross sectional view cut along the line G—G in FIG.
1
A.
In the conventional waveguide type optical device in
FIGS. 1A and 1B
, a titanium metal film strip is formed on a crystal substrate
101
with electro-optic effect, and, by internally-diffusing titanium into crystal of the crystal substrate
101
, an incidence-side Y-branch waveguide
102
, an emission-side Y-branch waveguide
103
and a phase shifter waveguide
104
are formed on the crystal substrate
101
. Namely, on the crystal substrate
101
, the two Y-branch waveguides to function as the incidence-side Y-branch waveguide
102
and emission-side Y-branch waveguide
103
, and the phase shifter waveguide (Mach-Zehnder interferometer type)
104
with two arms are provided.
Also, on the crystal substrate
101
, a buffer layer
105
composed of a dielectric material is formed. On the buffer layer
105
, a CPW type electrode structure composed of one signal electrode
106
(
107
) and two earth electrodes
108
and
109
is formed. On the incidence and emission sides of the waveguide, optical fiber mounts
110
a
and
110
b
, respectively, are provided. Further, to the optical fiber mounts
110
a
and
110
b
, optical fibers
111
a
and
111
b
, respectively, are connected.
In operation, optical field (ray of light) propagated through the optical fiber
111
a
passes through the optical fiber mount
110
a
, being input to the incidence-side Y-branch waveguide
102
, propagating through the phase shifter waveguide
104
and emission-side Y-branch wavegu
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