Laser Linewidth Control Using Phase Shifting Against Reflections
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Solution Overview
Problem
Laser diodes are sensitive to external reflections, which cause linewidth broadening and coherence collapse, affecting their performance in applications like optical communications and LIDAR, and existing solutions like optical isolators are bulky and not suitable for compact designs.
Innovation Solution
A laser apparatus with a sensor unit and phase shifter that measures frequency variations and controls the phase shift of the laser beam to mitigate the effects of external reflections, allowing for integration on the same substrate as the laser diode, eliminating the need for optical isolators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an optical isolator is arranged in front of the laser diode to shield external reflections, then the laser performance is improved by preventing linewidth broadening, but the device becomes bulky and cannot be integrated on the same substrate
Solution Approach 1:
The patent combines the laser diode, sensor unit, and phase shifter into a single integrated laser apparatus on the same substrate. The sensor unit detects frequency variations caused by external reflections, and the phase shifter compensates for these reflections by adjusting the phase of the laser beam, eliminating the need for a separate optical isolator.
Solution Approach 2:
The sensor unit continuously monitors frequency variations of the laser beam caused by external reflections and provides feedback to the control unit, which adjusts the phase shifter to compensate for the reflections. This active feedback mechanism replaces the passive optical isolator while maintaining laser performance.
2Device complexity
If external reflections are allowed to enter the laser diode, then the device structure is simplified, but the linewidth broadening reduces the laser performance
Solution Approach 1:
Instead of blocking external reflections with an optical isolator, the patent allows the reflections to enter the laser diode but uses the sensor unit to detect the frequency variations they cause. The phase shifter then converts this harmful effect into a controllable parameter by adjusting the phase to compensate for the reflections, thereby maintaining narrow linewidth without additional optical components.
3Reliability
If a phase shifter is used to control the phase of the laser beam to mitigate external reflections, then the linewidth is maintained narrow, but the device complexity increases
Solution Approach 1:
The phase shifter is integrated on the same substrate as the laser diode, and the sensor unit, phase shifter, and laser diode form a single laser apparatus. This integration reduces the overall device complexity compared to using a separate optical isolator while maintaining effective linewidth control through active phase adjustment.
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 or eliminates linewidth broadening, provides a more compact and stable laser solution, and achieves a narrower linewidth than conventional laser diodes, improving performance in applications such as optical communications and LIDAR.
Implementation Method 1
a phase shifter arranged in a path of a second portion of the laser beam, and configured to generate a phase shift of the second portion of the laser beam propagating through the phase shifter
Implementation Method 2
a laser diode configured to emit a laser beam, wherein the laser diode is further configured for frequency modulation of the laser beam; a sensor unit configured to receive a first portion of the laser beam, wherein the sensor unit is further configured for measuring variations of the frequency of the first portion of the laser beam
Data Source
Figure 1
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Figure 3A
AI summary
There is provided a laser apparatus comprising: a laser diode for emitting a laser beam, configured for frequency modulation of the laser beam; a sensor unit configured to receive a first portion of the laser beam, and for measuring variations of the frequency of the first portion; a phase shifter arranged in a path of a second portion of the laser beam, and configured to phase shift the second portion propagating through the phase shifter. The laser apparatus is configured to output the second portion having propagated through the phase shifter, and to acquire a plurality of measurements of variations of the frequency during frequency modulation at different phase shifts, and to control the phase shifter based on the plurality of measurements to control the variations of the frequency, whereby the laser apparatus may control a linewidth of the laser beam in dependence of an external reflection of the output laser beam, received by the laser apparatus.