Mode-Locked Multi-Mode Laser RF Source for Low Phase Noise
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Solution Overview
Problem
Classic frequency synthesizers exhibit poor phase noise performance and limited frequency output, making them inadequate for high-speed telecommunications and applications requiring high RF frequencies, such as 5G and medical imaging, due to their reliance on external references and YIG-filter limitations.
Innovation Solution
A monolithically integrated multi-mode laser system with a distributed Bragg reflector (DBR) and electro-absorption modulator is used to generate tunable RF signals, employing phase and amplitude modulations to reduce phase noise and increase frequency stability, replacing external references with forced oscillations and self-active mode-locking techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If classic frequency synthesizers use external reference and YIG-filter, then frequency stability is maintained, but phase noise performance deteriorates to approximately -80 dBc/Hz at offset 10 kHz
Solution Approach 1:
The patent replaces the traditional electronic frequency synthesizer system (using YIG-filter and RF amplifier) with an optically-based frequency comb system. The optical frequency comb generator uses mode-locked lasers to produce a comb of optical frequencies, which are then converted to RF frequencies through optical-to-electrical conversion. This substitution of optical domain for electronic domain achieves superior phase noise performance while maintaining frequency stability through the inherent properties of the optical reference.
2Speed
If classic frequency synthesizers use YIG-filter and RF amplifier, then frequency output is limited, but adaptability to high-speed telecommunications deteriorates for frequencies above 1 GHz
Solution Approach 1:
The patent employs tunable mode-locked lasers that can generate optical frequency combs across a broad spectral range. By adjusting the laser tuning parameter and the optical-to-electrical conversion frequency, the system can generate RF frequencies from DC to THz ranges. This parameter tunability enables the system to adapt to various high-speed telecommunications applications including 5G and future THz communications, overcoming the fixed frequency limitations of YIG-filter-based synthesizers.
3Object-generated harmful factors
If multi-mode lasers increase cavity length to narrow linewidth, then phase noise characteristics improve, but device complexity and size increase
Solution Approach 1:
The patent introduces an optical frequency reference (such as an atomic or molecular transition) as an intermediary to stabilize the mode-locked laser comb. Instead of relying on a long optical cavity for stability, the comb frequencies are locked to the ultra-narrow linewidth optical reference through feedback control. This intermediary reference provides the necessary stability without requiring increased cavity length, thereby reducing device complexity while achieving excellent phase noise characteristics.
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 system achieves significantly reduced phase noise and increased frequency stability, enabling the generation of high-frequency RF signals up to THz levels with improved performance compared to traditional synthesizers, addressing the limitations of classic frequency synthesizers.
Implementation Method 1
The distributed Bragg reflector may be used as a filter to select laser output frequency
Implementation Method 2
The electro-absorption amplitude modulator may be applied to effectively control the number of output laser modes using its different absorption rate
Implementation Method 3
Phase tuning section in the laser set up may work for frequency tuning, which may function as the phase modulator in the DBR laser
Implementation Method 4
Chip level multi-mode lasers generate beat-notes at radio frequencies
Data Source
AI summary
Low phase noise signal generated in a small structure is required for communication and high-resolution imaging. A DBR based multi-mode laser is combined with mode-locking method to build frequency stabilized and tunable RF signal generator. The number of the output modes from each laser is adjusted using reflecting bandwidth of distributed Bragg reflector and electro-absorption (EA) modulator for amplitude control, while the phase section in integrated laser system provides frequency tuning. Mode-locking of 60 laser modes results in a highly frequency stable 10 GHz RF beat-notes with a calculated phase noise of −150 dBc/Hz at 10 kHz offset frequency.