Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-10-06
2001-12-11
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S199200, C359S245000, C385S002000, C385S040000
Reexamination Certificate
active
06330098
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an external optical modulator. Although the present invention is suitable for a wide scope of application, it is particularly suitable for simultaneously modulating two independent optical channels in one modulator.
2. Discussion of the Related Art
An external optical modulator (EOM) is an important component in wavelength division multiplexed (WDM) optical transmission systems. In such systems, an electrical signal representing the information to be transmitted is applied to the EOM, which then modulates a continuous wave (CW) laser beam that propagates the modulated light through the transmission system. One of the advantages of external optical modulation over directly modulating the laser is that data can be transmitted relatively chirp-free, thereby reducing errors in long distance transmission. Chirp is an instantaneous change in optical frequency, which accompanies the process of directly modulating the laser diode. Chirp interacts with the dispersion profile of the transmission fiber to severely limit the distance over which error-free data transmission is possible. External optical modulators can be designed with little or no chirp, enabling a much higher transmission distance than can be obtained with direct modulation.
In a conventional optical modulator, the dual electrode design concept was developed so that the data and its complement can be separately applied to the two electrodes of the dual electrode modulator. This is also known as differential driving. By adjusting the relative amplitudes of the data and its complement the chirp parameter of the modulator can be fixed to any desired value between −∝ and +∝.
Conventionally, externally modulated WDM transmission systems have employed one EOM per optical channel. Further, the EOM is one of the more expensive optical components employed in the system. It would therefore be advantageous to provide an apparatus wherein two or more optical channels can be modulated with one EOM.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to an external modulator that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide an apparatus that allows for the operation of two optical channels on one modulator.
A further object of the present invention is to provide an economically more feasible apparatus for modulating more than one optical channels in a single modulator.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, a Mach-Zehnder interferometer-type modulator for externally modulating two independent optical signals with first and second electrical signals, the modulator includes a first electrode receiving the first electrical signal, a second electrode receiving the second electrical signal, a first optical signal path co-propagating the first optical signal with the first electrical signal and counter-propagating the first optical signal with the second electrical signal, to generate a first modulated optical signal corresponding to the first optical signal modulated with the first electrical signal, and a second optical signal path co-propagating the second optical signal with the second electrical signal and counter-propagating the second optical signal with the first electrical signal, to generate a second modulated optical signal corresponding to the second optical signal modulated with the second electrical signal.
In another aspect of the present invention, a Mach-Zehnder interferometer-type modulator for externally modulating first and second optical signals with first and second electrical signals, the modulator includes a first electrode receiving the first electrical signal, a second electrode receiving the second electrical signal, a first laser, coupled to the first optical signal path, for generating the first optical signal, a second laser, coupled to the second optical signal path, for generating the second optical signal, a first optical port receiving the first optical signal, a second optical port receiving the second optical signal, a first terminating resistor coupled to a second end of the first dual electrode, and a second terminating resistor coupled to a second end of the second dual electrode, wherein the first and second terminating resistors are disposed diagonally to each other.
In another aspect of the present invention, a Mach-Zehnder interferometer-type modulator for externally modulating first and second optical signals with first and second electrical signals, respectively, the modulator includes a first dual electrode having first and second ends, the first end receiving a first electrical signal for propagation in a first direction and the second end receiving a second electrical signal for propagation in a second direction opposite to the first direction, a second dual electrode substantially isolated from the first dual electrode and having first and second ends, the first end receiving a third electrical signal for propagation in a third direction and the second end receiving a fourth electrical signal for propagation in a fourth direction opposite to the third direction, a first optical signal path for propagating a first optical signal in the first direction and the third direction to modulate the first optical signal with the first electrical signal and the third electrical signal, and a second optical signal path for propagating a second optical signal in the second direction and the fourth direction to modulate the second optical signal with the second electrical signal and the fourth electrical signal.
In a further aspect of the present invention, a wavelength division multiplexed optical transmission system, comprising a Mach-Zehnder interferometer-type modulator for externally modulating two independent optical signals with first and second electrical signals, the modulator includes a first electrode receiving the first electrical signal, a second electrode receiving the second electrical signal, a first optical signal path co-propagating the first optical signal with the first electrical signal and counter-propagating the first optical signal with the second electrical signal, to generate a first modulated optical signal corresponding to the first optical signal modulated with the first electrical signal, and a second optical signal path co-propagating the second optical signal with the second electrical signal and counter-propagating the second optical signal with the second electrical signal, to generate a second modulated optical signal corresponding to the second optical signal modulated with the second electrical signal.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only are not restrictive of the invention, as claimed.
REFERENCES:
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patent: 5101450 (1992-03-01), Olshansky
patent: 5303079 (1994-04-01), Gnauck et al.
patent: 5408544 (1995-04-01), Seino
patent: 5787211 (1998-07-01), Gopalakrishnan
patent: 5812306 (1998-09-01), Mizrahi
A. H. Gnauck et al., “Dispersion Penalty Reduction Using an Optical Modulator with Adjustable Chirp,” IEEE Photonics Technology Letters, vol. 3, No. 10, Oct. 1991, pp. 916-918.
G. K. Gopalakrishnan et al., “Performance and Modeling of Broadband LiNbO3Traveling Wave Optical Intensity Modulators,” Jour. Of Lightwave Technology, vol. 12, No. 10, Oct. 1994, pp. 1807
Codeon Corporation
Epps Georgia
Morgan & Lewis & Bockius, LLP
Spector David N.
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