Optical Modulation Circuit for OFDM Bandwidth Compression
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Solution Overview
Problem
Conventional optical OFDM transmission systems require high-speed Fourier transform circuits and multiple modulators, leading to difficulties in achieving high-speed operation and control, and result in a wider occupied band due to the use of intensity or SSB modulators, which complicates wavelength-multiplexing and necessitates guard bands, thereby inefficiently utilizing frequency bands.
Innovation Solution
An optical modulation circuit that generates modulator drive signals for phase-shift keying and intensity modulation across optical subcarriers, using IQ-modulators to reduce the bit rate and band requirements, and applies double-sideband carrier suppression modulation to narrow the occupied band and suppress chromatic dispersion and PMD effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If an intensity modulator or SSB modulator is used to modulate continuous light with an OFDM signal, then optical OFDM transmission can be implemented, but the occupied band is expanded
Solution Approach 1:
The patent replaces intensity modulation and SSB modulation with phase modulation using IQ-modulators. This substitution changes the modulation mechanism from amplitude-based to phase-based, enabling the optical carrier to be modulated in the phase domain while maintaining spectral efficiency. The IQ-modulators generate multiple optical subcarriers through phase modulation, avoiding the bandwidth expansion issues associated with intensity and SSB modulation approaches.
2Productivity
If the symbol rate is increased to narrow channel spacing for more WDM multiplexes, then transmission capacity is enlarged, but chromatic dispersion and PMD influence becomes more noticeable
Solution Approach 1:
The patent segments the high-rate data stream into multiple lower-rate optical subcarriers, each operating at a symbol rate that is tolerant to chromatic dispersion and PMD. By dividing the total transmission capacity across multiple subcarriers with spacing equal to the symbol rate, the system achieves high transmission capacity while each individual subcarrier operates at a lower symbol rate that is less susceptible to dispersion effects.
Solution Approach 2:
The patent transitions from single-carrier transmission to multicarrier transmission, adding the frequency dimension to the transmission system. By distributing data across multiple frequency channels (subcarriers) with orthogonal spacing, the system achieves high capacity through frequency diversity while each subcarrier operates at a lower symbol rate that is more tolerant to chromatic dispersion and PMD.
3Measurement precision
If more power is required to obtain necessary receiver sensitivity, then receiver sensitivity is improved, but nonlinear optical effects are produced in the fiber
Solution Approach 1:
The patent segments the total optical power across multiple subcarriers, so that each subcarrier operates at a lower power level. This segmentation in the frequency domain allows the system to achieve necessary receiver sensitivity through the combined signal strength of multiple subcarriers while keeping the power per subcarrier low enough to avoid nonlinear optical effects such as self-phase modulation, four-wave mixing, and cross-phase modulation.
4Productivity
If the number of modulators is increased to generate multiple optical subcarriers, then transmission capacity is increased, but device complexity is increased
Solution Approach 1:
The patent employs IQ-modulators that can generate multiple optical subcarriers with different frequencies and phases through a single device. The IQ-modulator structure, combined with optical frequency shifting mechanisms, allows one modulator to perform the function of multiple modulators by generating multiple orthogonal subcarriers simultaneously, thereby increasing transmission capacity without proportionally increasing the number of modulators.
Solution Approach 2:
The patent combines multiple modulation functions into a unified IQ-modulator system that generates multiple optical subcarriers. By merging the modulation of multiple subcarriers into a single integrated modulator structure with optical frequency shifting, the system achieves high transmission capacity while reducing the overall device complexity compared to using separate modulators for each subcarrier.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces the number of oscillators and drive frequencies needed, narrows the occupied band, and effectively suppresses chromatic dispersion and nonlinear optical effects, improving the efficiency of optical transmission systems.
Implementation Method 1
optical IQ-modulators (4-1 to 4-n) for generating optical subcarriers on which phase-shift keying is superimposed for every division of the optical multicarrier
Implementation Method 2
an increase in symbol rate results in a more noticeable influence of a chromatic dispersion and a polarization mode dispersion (PMD) in a transmission fiber
Implementation Method 3
an increase in symbol rate results in a more noticeable influence of a chromatic dispersion and a polarization mode dispersion (PMD) in a transmission fiber
Implementation Method 4
suppressing a nonlinear optical effect produced in the fiber such as self phase modulation (SPM), four wave mixing (FWM), or cross phase modulation (XPM)
Implementation Method 5
suppressing a nonlinear optical effect produced in the fiber such as self phase modulation (SPM), four wave mixing (FWM), or cross phase modulation (XPM)
Data Source
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AI summary
An optical modulator and an optical transmission system convert continuous light of a multiple wavelength light source, which generates the continuous light with a fixed and complete phase but different frequencies, to a modulator driving signal so as to generate a light subcarrier with each frequency at the center and modulate the continuous light to the light subcarrier by using the modulator driving signal. In the case where an optical modulation is carried out by an optical IQ-modulator, transmitting data, for example, is converted to two parallel data A(t) and B(t), an I phase signal, in which the data A(t)+B(t) are modulated with a clock signal with a frequency ω, and a Q phase signal, in which the data A(t)-B(t) are modulated with a clock signal with a π/2 phase shifted, are generated, and the I phase signal and the Q phase signal are applied to electrodes of the optical IQ-modulator, respectively.