Optical Transmitter for QKD With Injection-Seeding Pulse Control
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Solution Overview
Problem
Conventional QKD systems face limitations in tunability, requiring multiple modulators and being incompatible with photonic integration, and are limited to narrow wavelength ranges, restricting their use in applications like MDI QKD and classical communication systems.
Innovation Solution
A multi-modal laser with a wavelength tuneable laser and a modulator for phase control, enabling modulator-free wavelength tuneable optical pulses by using injection seeding and gain switching, allowing wide wavelength tunability and phase encoding without external modulators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If multiple intensity modulators and phase modulators are used in series to achieve required extinction levels, then the extinction ratio is improved, but the device complexity and system bulk increase
Solution Approach 1:
The patent extracts the modulation function from separate external modulators and integrates it directly into the laser cavity through gain-switching. The laser medium itself performs the intensity and phase modulation that previously required separate modulator components, thereby achieving the required extinction ratio without the complexity of multiple series modulators.
Solution Approach 2:
The patent merges the functions of the laser source, intensity modulator, and phase modulator into a single integrated system. By combining these functions in the laser cavity through gain-switching, the system eliminates the need for separate modulator components while maintaining the required modulation performance for QKD.
2Manufacturing precision
If LiNbO3 technology is used for modulators, then the extinction ratio is improved, but compatibility with photonic integration is lost
Solution Approach 1:
The patent replaces the mechanical/electrical modulator system (LiNbO3 based) with an optical gain-switching mechanism implemented directly in the laser cavity. This substitution eliminates the need for incompatible LiNbO3 modulators while achieving the same modulation function through the laser medium's gain characteristics, enabling photonic integration compatibility.
3Adaptability or versatility
If conventional tuneable lasers are directly modulated, then wavelength tunability is achieved, but pulse performance and modulation bandwidth are reduced
Solution Approach 1:
The patent segments the laser into a master laser for wavelength tuning and a slave laser for high-speed pulse generation. The master laser provides wavelength selectivity through injection seeding, while the slave laser performs gain-switching at high speeds. This segmentation allows each component to optimize its function without compromising the other.
Solution Approach 2:
The patent uses the master laser output as an intermediary to seed the slave laser cavity. This intermediary injection provides wavelength control to the pulse-generating slave laser, enabling the slave laser to operate at high modulation bandwidths while maintaining wavelength tunability through the master laser's injection seeding.
4Stability of the object's composition
If DFB lasers with Bragg grating are used, then single wavelength stability is improved, but wavelength tunability is limited to narrow ranges
Solution Approach 1:
The patent makes the laser system multi-functional by combining a DFB slave laser (for stable single-mode operation and pulse generation) with a tunable master laser (for wavelength selection). The slave laser maintains its stable single-wavelength operation through the Bragg grating, while the master laser provides wide wavelength tunability through injection seeding, making the overall system both stable and tunable.
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 provides a compact, high-speed, and widely tuneable QKD system that supports phase encoding, compatible with photonic integration, enabling flexible use in various QKD protocols and classical communication systems, and supports quantum repeaters and quantum internet applications.
Implementation Method 1
The photons injected into the cavity cause the slave laser to seed via stimulated emission rather than spontaneous emission
Implementation Method 2
a modulator for controlling a phase shift between successive pulses output from the transmitter
Data Source
AI summary
An optical transmitter for a Quantum Key Distribution (QKD) system, the transmitter comprising: a multi-modal laser; a wavelength tuneable laser arranged to inject light into a cavity of the multi-modal laser, so as to cause the multi-modal laser to output light at a selected wavelength for use in generating pulses to be output by the transmitter; and a modulator for controlling a phase shift between successive pulses output from the transmitter.