Segmented DFB Laser Grating for Return Light Noise Reduction
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Solution Overview
Problem
Wavelength division multiplexing communication systems face challenges in achieving high single-mode yield and low laser linewidth due to return-light-induced noise and increased linewidth, particularly in λ/4 phase shift DFB-LDs, which require optical isolators and result in high power consumption and unstable light intensity distribution.
Innovation Solution
A semiconductor laser design with current-injection and current-non-injection diffraction gratings, along with phase shifters at their boundaries, reduces the impact of reflected return light, maintaining single-mode oscillation and high side mode suppression ratio (SMSR) regardless of reflection phase or intensity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If anti-reflection coatings are applied to both end surfaces of λ/4 phase shift DFB-LD, then single-mode oscillation is achieved with high SMSR, but reflected return light enters the active layer causing linewidth increase and noise
Solution Approach 1:
The diffraction grating is segmented into three distinct regions: current-injection diffraction grating (active region), current-non-injection diffraction gratings (passive regions). This segmentation allows the active region to generate light while the passive regions act as reflectors that redirect return light away from the active layer, thereby reducing return-light-induced noise while maintaining single-mode oscillation with high SMSR
Solution Approach 2:
Different regions of the diffraction grating are assigned different functional qualities: the current-injection region provides light generation and amplification, while the current-non-injection regions provide reflection and light redirection. This local differentiation of function allows simultaneous achievement of high SMSR through single-mode oscillation and reduced noise through return light management
2Power
If current injection is increased to achieve higher output, then light output increases, but return light from end surface reflection becomes more problematic
Solution Approach 1:
The patent converts the harmful effect of return light into a beneficial effect by using the current-non-injection diffraction gratings to intentionally reflect return light in a controlled manner. Instead of allowing return light to randomly re-enter the active layer and cause noise, the passive grating regions redirect it constructively, thereby reducing noise while allowing higher current injection for increased light output
3Object-affected harmful factors
If optical isolator is added to prevent return light entry, then return light impact is reduced, but device complexity and power consumption increase
Solution Approach 1:
The patent extracts the return light reflection function from external optical isolators and integrates it directly into the semiconductor laser structure through current-non-injection diffraction gratings. This eliminates the need for separate optical isolator components, reducing device complexity while effectively managing return light to prevent noise and linewidth increase
Solution Approach 2:
The current-non-injection diffraction gratings serve as an intermediary element between the active layer and the external environment. These passive grating regions mediate the interaction with return light by reflecting it in a controlled manner, preventing direct re-entry into the active layer without requiring external optical isolators
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 design effectively minimizes the impact of reflected return light, stabilizing single-mode oscillation and preventing SMSR degradation, while reducing power consumption and maintaining high light output, even with increased current injection.
Implementation Method 1
a distributed feedback laser diode (hereinafter, referred to as DFB-LD) that determines an oscillation wavelength using a diffraction grating provided in a semiconductor chip
Implementation Method 2
the Bragg wavelength determined by a diffraction grating period
Implementation Method 3
a phase shift region (phase shifter) for shifting a diffraction grating phase by π
Implementation Method 4
front and rear cleaved end surfaces covered with anti-reflection coatings
Data Source
AI summary
A semiconductor laser according to the present invention includes an active layer, a guide layer laminated on the active layer, a diffraction grating formed along a light emission direction in the guide layer, an upper electrode provided above the guide layer, and a lower electrode provided below the active layer. The diffraction grating includes a current-injection diffraction grating and current-non-injection diffraction gratings provided both in front of and in back of the current-injection diffraction grating. Phase shifters are individually provided at a central portion of the current-injection diffraction grating and at boundaries between the current-injection diffraction grating and the current-non-injection diffraction gratings. The upper electrode is provided above the current-injection diffraction grating and is not provided above the current-non-injection diffraction gratings.