Demodulator Using Polarization Control for Phase Shift Keying
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Solution Overview
Problem
Existing demodulators for phase modulation signals in optical fiber communication systems face challenges in achieving high accuracy branching ratios over a wide wavelength range, leading to phase difference errors and increased costs due to the use of non-polarizing beam splitters, which also result in misalignment of interference lights and increased device complexity.
Innovation Solution
A demodulator configuration utilizing a polarization converting unit, a polarization separating unit, and an interference light generating unit, which includes a beam displacer or polarization beam splitter to align and multiplex signal lights with high accuracy, reducing phase difference errors and simplifying the optical system, thereby reducing costs and skew.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If non-polarizing beam splitters are used for branching signal light and local oscillator light, then the demodulator can be constructed, but the branching ratio accuracy deteriorates over wide wavelength range and phase difference errors increase
Solution Approach 1:
The patent changes the operating parameter from non-polarizing beam splitting to polarizing beam splitting, where the branching ratio is determined by polarization state rather than wavelength. This allows accurate branching ratio control across wide wavelength ranges by maintaining fixed polarization properties of the beam splitter.
Solution Approach 2:
The patent applies different polarization states to different signal components (I-channel and Q-channel signals have orthogonal polarizations), allowing the beam splitter to branch different components with high precision based on their local polarization properties rather than relying on wavelength-dependent non-polarizing splitting.
2Reliability
If non-polarizing beam splitters are used, then the demodulator can operate, but phase difference errors increase due to polarization-dependent phase shifts
Solution Approach 1:
The patent changes from non-polarizing beam splitting to polarizing beam splitting, eliminating the harmful polarization-dependent phase shifts that occur in non-polarizing splitters. The polarizing beam splitter introduces controlled 90-degree phase shifts only for the intended polarization separation, not random phase errors.
Solution Approach 2:
The patent introduces polarization control elements (wave plates, polarizers) as intermediaries to manage phase relationships. These elements provide precise phase control to compensate for and correct phase difference errors, ensuring accurate demodulation.
3Ease of manufacture
If non-polarizing beam splitters are used for multiplexing, then interference light can be generated, but the branching ratio requires high accuracy over wide wavelength range which increases cost
Solution Approach 1:
The patent changes the multiplexing mechanism from non-polarizing beam splitting to polarizing beam splitting combined with polarization rotation. This allows standard polarizing beam splitters (which are cheaper and more stable) to be used, eliminating the need for expensive non-polarizing splitters with tight tolerance specifications.
Solution Approach 2:
The patent uses standard polarizing beam splitters and polarization control elements that are commercially available and inexpensive, replacing the need for specialized non-polarizing beam splitters that require high precision manufacturing and are therefore costly.
4Ease of operation
If non-polarizing beam splitters are used, then the system can function, but interference lights become misaligned and device complexity increases
Solution Approach 1:
The patent changes from non-polarizing to polarizing beam splitting, where the polarization state serves as an additional control parameter. This allows precise alignment and separation of interference lights through polarization filtering, simplifying the optical path arrangement and reducing system complexity.
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
The proposed solution enables low-cost, high-accuracy demodulation of phase shift modulated signals with reduced skew and phase difference errors, resulting in a more efficient and cost-effective optical communication module.
Implementation Method 1
a polarization converting unit that converts polarizations of the signal light and the local oscillator light
Implementation Method 2
a polarization separating unit that separates the polarization-converted signal light into multiple signal lights that interfere at mutually different phase relationships
Implementation Method 3
each separated signal light being in a polarization state mutually orthogonal
Implementation Method 4
multiple interference lights by converting and separating polarization of the multiplexed beams generated by the polarization multiplexing unit
Data Source
AI summary
In a free space optical system type demodulator of a phase shift keying signal, if a half beam splitter is used as a non-polarizing optical branching unit that is used when generating beams corresponding to I and Q channels or when multiplexing an interference light, control of a power branching ratio is difficult, and it is necessary to suppress phase shifts that are different depending on a polarization state of an input state, and thereby the demodulator becomes high cost. Moreover, since directions of branched lights are different, it is difficult to suppress a skew of the demodulator. In the present invention, the non-polarizing optical branching unit that is used when generating the beams corresponding to the I and Q channels and when multiplexing the interference light is realized using polarization rotating elements and polarization separating elements. Moreover, branched beams are substantially aligned.