Fiber Bragg Grating External-Cavity Laser for Narrow Linewidth
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Solution Overview
Problem
Existing narrow linewidth semiconductor laser devices face challenges with large form factors, thermal and acoustic noise, and manufacturing difficulties, particularly in achieving compact and environmentally stable designs with high performance.
Innovation Solution
A compact narrow linewidth semiconductor device is developed, featuring a gain element with anti-reflection and high-reflectivity coatings, integrated with a fiber Bragg grating and thermoelectric coolers, which provides a high reflectivity slope ratio and side lobe suppression to achieve ultra-narrow linewidth laser light with reduced noise sensitivity and ease of manufacturing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a very long laser cavity is used to achieve narrow linewidth, then the linewidth is reduced, but the device suffers from thermal and acoustic noise and has a large form factor
Solution Approach 1:
The laser cavity is segmented into two distinct parts: a short semiconductor gain chip cavity and a separate external fiber Bragg grating reflector. This segmentation allows the optical path length to be effectively long (for narrow linewidth) while the physical device volume remains small (reducing thermal and acoustic noise sensitivity).
Solution Approach 2:
An optical intermediary (the fiber Bragg grating) is introduced to provide the reflective function at a distance from the gain chip. This intermediary enables the system to achieve the optical equivalent of a long cavity without requiring a physically long structure, thus resolving the contradiction between linewidth and form factor.
2Measurement precision
If a very long laser cavity is used to achieve narrow linewidth, then the linewidth is reduced, but the device suffers from thermal and acoustic noise
Solution Approach 1:
By segmenting the cavity into a compact gain chip and a separate external grating reflector, the physical structure is minimized, reducing the device's susceptibility to thermal and acoustic noise while maintaining the optical path length needed for narrow linewidth operation.
3Measurement precision
If a grating is integrated in a PLC to achieve narrow linewidth, then the linewidth is reduced, but the device has large form factor and is difficult to manufacture
Solution Approach 1:
The invention merges the semiconductor gain chip with a fiber Bragg grating in a compact external cavity configuration. This combination achieves narrow linewidth performance while using well-established, easily manufacturable technologies (semiconductor chip fabrication and fiber grating writing) rather than complex PLC integration processes.
4Measurement precision
If a grating is integrated in a PLC to achieve narrow linewidth, then the linewidth is reduced, but the device has large form factor
Solution Approach 1:
The laser system is segmented into a compact semiconductor gain chip and an external fiber grating, allowing the optical cavity to be effectively long (for narrow linewidth) while the physical footprint remains small, thus resolving the form factor contradiction.
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 solution enables the production of compact, environmentally stable semiconductor devices with single mode ultra-narrow linewidth laser light, minimizing thermal and acoustic noise while maintaining a small form factor and improving manufacturing feasibility.
Implementation Method 1
A fiber Bragg grating is formed in an optical fiber which is positioned to receive the output of the gain element and return a portion of said output back into the gain element
Implementation Method 2
gain element, such as quantum well, quantum dot or bulk waveguide laser chip with anti-reflection coating (ARC) on a first end of the gain element
Implementation Method 3
high-reflectivity coating (HRC) on a second end of the gain element
Implementation Method 4
The substrate can be positioned on top of one or more thermoelectric coolers (TECs) to control the temperature profile of the device
Data Source
AI summary
A novel narrow linewidth laser device is disclosed that includes a gain element, such as a quantum well, quantum dot or bulk waveguide laser chip and a fiber Bragg grating formed in an optical fiber positioned to receive the output from a first end of the gain element and return a portion of said output back into the gain element. The fiber Bragg grating is constructed so that its power reflectivity profile has a ratio of reflectivity slope over reflectivity at the 3 dB point below the reflectivity peak on the red side (longer wavelength side) of the grating larger than a value of 2/nm. The operating wavelength of the device may be tuned thermally, electrically, or thermo-electrically to be on the red side of the fiber Bragg grating reflectivity profile, preferably, but not necessarily, at the 3 dB point below the reflectivity peak or lower. In another embodiment, a second grating is optically coupled to a second end of the gain element and has a reflectivity profile that overlaps at least a portion of the reflectivity profile of the front end fiber Bragg grating.