Optical: systems and elements – Optical modulator
Reexamination Certificate
2001-07-02
2003-07-08
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
C359S245000
Reexamination Certificate
active
06590691
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical modulators and drivers used in high speed optical transmission systems and more particularly to a hybridly integrated driver and external modulator device designed to have low voltage and power requirements.
2. Description of the Related Art
The expected growing demand in the telecommunications field for higher frequency broadband communications systems has spawned and sustained the significant efforts in the development of medium haul and long haul fiber high speed optical transmission systems. Some of the conventional limitations of such systems include optical fiber loss, fiber dispersion and optical noise. With the advent of optical amplifiers that amplify the optical signal without first converting it to an electrical signal, optical loss is ostensibly no longer the limiting factor. However, one of the most significant system limitations remains fiber dispersion. In particular, when high frequency laser light sources are modulated to carry a signal, they produce pulses having large wavelength shifts about their center transmission wavelength, called “chirp.” When applied to a dispersive medium such as optical fiber, the chirped pulses can become severely distorted when they reach a remote receiver, which can be tens or hundreds of kilometers away. Moreover, the severity of the dispersion problem increases with the spectral linewidth of the laser light source. In most modern medium and long haul systems that operate at a wavelength of 1.55 &mgr;m, fiber dispersion effects can thus become significant.
The two types of modulating schemes typically used in optical transmission systems are direct modulation and external modulation. In direct modulation, the electrical signal is combined with a bias and applied directly to the laser source. In this way, the signal directly modulates the laser gain and thus the optical intensity output of the source. While simple to design, since conventional semiconductor laser direct modulation techniques result in large linewidth (due to frequency chirping), direct modulation is not usually employed in medium and long-haul links at presently-considered high bit rates (e.g. 10 Gbps and above). Rather, external modulation techniques that result in lower dispersion penalty due to the narrow linewidth of the sources are presently preferred.
Among external modulators, electro-absorption modulators (EAM's) and electro-optic modulators (EOM's) are used in high-speed links. Electro-absorption modulators are compact in size, require low driving voltages, have wide bandwidths and can potentially be integrated with lasers and drive electronics. Unfortunately, electro-absorption modulators also tend to result in chirping of the laser output (although not as severe as in direct modulation) and thus are typically only used for span lengths less than 80 Km at 10 Gbps and for even smaller spans at higher bit rates. For high-speed links with span distances greater than approximately 80 Km, traveling-wave electro-optic modulators are the modulators of choice. EOM's can be phase modulators or intensity modulators, also called Mach-Zehnder modulators (MZM's). EOM's are fabricated in several material systems such as GaAs, InP, LiNbO
3
(lithium niobate) and polymers, but LiNbO
3
modulators are the most extensively used because they tend to have the best performance and reliability.
One of the major challenges in using conventional traveling-wave, electro-optic modulators is that they typically require high drive voltages. This problem is more severe at high bit rates because traveling-wave EOM's have a bandwidth versus drive voltage tradeoff. Thus, high-speed EOM's suffer from an even higher drive voltage requirement than other types. These higher drive voltages result in higher power dissipation in the modulator and driver circuits. Moreover, as bit rate requirements increase, the driver circuits that generate these high drive voltages to process these bit rates become increasingly difficult and expensive to manufacture.
Two important reasons for the high drive voltage and power requirements are: 1) non-idealities in the packages and interconnections between the traveling-wave modulator and the driver circuit; and 2) performance limitations in the driver and modulator due to the constraints of having standardized 50 &OHgr; interface impedances. It should be understood that the requirement of having 50 &OHgr; interface impedances stems from the use of industry standard 50 &OHgr; connectors, cables and test equipment. It is also understood that the standardized impedance constraint need not necessarily by 50 &OHgr;Other standardized impedance constraints are possible, such as 75 &OHgr;.
Several attempts at addressing the high-voltage problem at these high frequencies have been made. For example, in Noguchi et al., “Millimeter-Wave Ti:LiNbPO
3
Optical Modulators,” Journal of Lightwave Technology”. Vol. 16, No. 4, April, 1998, the authors describe a Mach-Zehnder-type optical modulator with coplanar waveguide (CPW) electrodes, wherein a titanium-diffused waveguide is formed in a z-cut lithium niobate substrate in a ridge structure. As stated therein, an objective of the authors was to reduce conductor loss to lower the voltage requirement while maintaining velocity and 50 ohm impedance matching. While this design improved the performance of the modulator over other designs, it still included a 50 ohm impedance requirement as a design constraint.
The above-described drawbacks are present in the two common architectures for driving electrical signals in traveling-wave modulators, namely, single-drive and dual-drive modulation, which are now described in detail.
Single-Drive Modulators
Single-drive traveling-wave electro-optic modulators have just one electrical signal input, as opposed to dual-drive modulators that have two electrical inputs with the electrical signals 180 degrees out of phase, and which are described next. Single-drive modulators are easier to use than dual-drive versions and are most commonly employed in commercial systems. Single-drive modulators can be fabricated from LiNbO
3
(x-cut or z-cut), GaAs, InP or polymers. This discussion applies to single-drive modulators of all types, regardless of technology or fabrication methods.
FIG. 1
shows a conventional configuration for modulating an electrical signal onto an optical carrier using a single-drive modulator. In operation, an electrical input signal
12
, which can be analog or digital, is first amplified with a packaged driver
30
and then modulated with a packaged modulator
20
onto an optical fiber
46
carrying light of a particular wavelength that is sourced by a continuous light source, such as a laser (not shown). Through the electro-optic effect in the modulator chip
22
, the electrical signal is effectively converted into an optical signal that can be transmitted for long distances on the fiber
46
to an optical receiver.
The traveling-wave electro-optic modulator chip
22
and the driver chip
32
are enclosed in their own dedicated packages
20
,
30
, respectively. The interconnection between the driver circuit chip
32
and the traveling-wave modulator chip
22
is made through (a) a transition board
36
on the driver package
30
; (b) a high frequency output connector
40
on the driver package
30
; (c) an input connector
42
on the modulator package connected by a 50 &OHgr; coaxial cable
44
; and (d) a transition board
24
on the modulator package
20
. Each of the transition boards is connected to the chips via wire bonds and has planar transmission lines.
More particularly, the signal
12
is fed into the driver chip
32
of the driver package
30
from a first transition board
34
via wire bonds
38
, and from the driver chip to a second transition board
36
via bond wires
39
. The second transition board includes planar 50 &OHgr; transmission lines with input/output bond pads. The output of the transition board
36
goes to a 50 &OHgr;
Logan, Jr. Ronald T.
Nagra Amit S.
Woodard Nathan G.
Epps Georgia
Hasan M.
Phasebridge, Inc.
Pritzkau Michael
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