Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-09-18
2001-06-12
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06246500
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to a method and apparatus for a communications system. In particular, the invention relates to a method for linearizing a signal transmitted over a communication link.
BACKGROUND OF THE INVENTION
Analog fiber-optic links with dynamic ranges free of spurious signals can be achieved using either direct or external modulation methods and can be assembled entirely from components that are commercially available. Using optical fiber to remotely locate radio frequency (RF) antennas for high performance communication links, however, can require a dynamic range of 125 dB·Hz or more in conjunction with a low noise figure (i.e., less than 5 dB).
External modulation of a carrier creates a modulated signal having a combination of high dynamic range and low noise figure more readily than direct modulation of the carrier because it permits the use of very low-noise solid-state lasers that cannot be modulated directly at RF frequencies. To extend the dynamic range beyond the 115 dB·Hz achievable using commercially available Mach-Zehnder lithium niobate modulators, an improvement in modulator linearity must be realized. The dynamic range of an external modulation link is limited by the nonlinearity of the modulator transfer function (i.e., the relationship between the optical output power and the signal voltage applied to the modulator). This nonlinearity causes a distortion in the modulated signal that increases with increasing signal voltage.
Electro-optic modulators utilizing the electroabsorptive effect can have very linear transfer functions. Unfortunately, electroabsorption modulators yield significant noise figures because they cannot operate at optical input powers greater than a few milliwatts. Communication links with lower noise figures are obtained using external modulation of higher CW optical carrier powers (i.e., at least 100 mW). Currently the only type of optical modulator that generally can operate at these higher power levels is a lithium niobate device based on the linear electro-optic effect (i.e., Pockels effect). Conversion of the linear modulation of the refractive index into a modulation of optical intensity is achieved using an interferometer or a directional coupler, however, either conversion method results in a nonlinear transfer function. The result of a nonlinear transfer function is the generation of harmonic and intermodulation distortions that degrade the modulated signal.
Dynamic ranges in analog optical communication links in excess of 115 dB·Hz have been achieved using specially designed electro-optic modulators that minimize one or more orders of harmonic and intermodulation distortion. Currently, however, improved dynamic ranges of approximately 125 dB·Hz using these linearized modulators have been achieved only for frequencies less than 1 GHz. In addition, linearization across more than an octave bandwidth requires precise balancing of the signal voltage levels on multiple electrodes in the custom modulator, thus providing a significant implementation challenge.
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for linearizing a signal transmitted over a communication link. The method and apparatus in one embodiment make use of a commercially available Mach-Zehnder electro-optic modulator having a single traveling-wave RF electrode and a single DC bias electrode. The DC bias is one parameter that is controlled to affect the relative levels of signal and distortion. In order to minimize both second and third order distortion, a second parameter must also be controlled. The halfwave voltage (V
&pgr;
) of a Mach-Zehnder modulator is a measure of the periodicity (i.e., sensitivity) of the modulator transfer function with respect to the modulator input voltage. Because the halfwave voltage is proportional to the optical wavelength, different wavelengths will have different transfer function periodicities. Multiple carriers at different wavelengths can be modulated and individually detected after spectral separation. If a specific ratio of optical powers (i.e., the second control parameter) at the different wavelengths is established, the detected signals can be combined to create a modulation transfer function having null second and third derivatives at a single DC bias voltage.
The present invention features a method of linearizing a signal transmitted over a communication link which includes the steps of supplying a first signal and a second signal at a first wavelength and a second wavelength, respectively. The first and second wavelengths can be optical wavelengths. The method also includes the steps of modulating each signal differentially with respect to the other signal to obtain respective modulated signals, detecting the modulated signals, and performing a linear operation on the detected signals. The linear operation in various embodiments includes summing or differencing the detected signals.
The invention also features a method of linearizing a received RF signal which includes the steps of supplying a first and a second signal at a first and a second wavelength, respectively, and modulating the signals differentially with respect to each other in response to the received RF signal to obtain respective modulated signals. The method includes the additional steps of detecting the modulated signals and performing a linear operation on the detected signals to generate a received linearized signal.
In another aspect, the invention features a system for providing a linearized signal over a communication link. The system includes a first and a second source producing a first and a second signal, respectively, at a first and a second wavelength, respectively. The system also includes a modulator, a first and a second detector, and a processor. The modulator is in optical communication with the source outputs and differentially modulates the first and second signals. Each detector is in optical communication with a respective modulator output and produces an electrical signal in response to the respective modulated signals. The processor is in electrical communication with the electrical signals from the detectors and performs a linear operation on them to generate a linearized output signal. In one embodiment, the system includes a wavelength division multiplexer in communication with the source outputs. In another embodiment, the system includes a wavelength division demultiplexer in communication with the modulator outputs.
The invention also features a modulator having a first modulator input, a second modulator input, a first splitter, a second splitter, and a signal electrode. The first splitter and the second splitter each have an input, and a first and a second output. The inputs of the first and the second splitter are in communication with the first and the second modulator inputs, respectively. The signal electrode is in close proximity to the first output of each splitter. The modulator can also include a first combiner and a second combiner, each combiner having a first and a second input, and an output. The first and second inputs of the first combiner are in communication with the first and second outputs of the first splitter, respectively. The first and second inputs of the second combiner are in communication with the first and second outputs of the second splitter, respectively.
The modulator can include a first ground electrode in close proximity to the signal electrode. In one embodiment, the modulator also includes a second ground electrode in close proximity to the signal electrode. In another embodiment, the modulator includes a first bias electrode located adjacent to the first outputs of the first and second splitters. In yet another embodiment, the modulator includes a second bias electrode located adjacent to the second output of the second splitter. In yet another embodiment, the modulator includes an optical element such that the outputs of the splitters are optically coupled to the respective inputs of the combiners.
REFERENCES:
patent:
Bello Agustin
Massachusetts Institute of Technology
Pascal Leslie
LandOfFree
Linearization of a broadband analog optical link using two... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Linearization of a broadband analog optical link using two..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Linearization of a broadband analog optical link using two... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2529974