Flat-Top Pulse Laser Transmitter for Reduced Self-Phase Modulation
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Solution Overview
Problem
Solid-state lasers suffer from self-phase modulation, which reduces coherence time and limits peak power, making them unsuitable for coherent LADAR applications due to unwanted spectral broadening and reduced spectral selectivity.
Innovation Solution
A laser transmitter is designed with a seed signal generator, electro-optic amplitude modulators to create flat-top pulse signals, and power amplifiers to amplify these signals, minimizing self-phase modulation by confining phase shifts to the rising and falling edges of pulses, thus maintaining coherence during the flat-top portion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If fiber lasers are used to achieve high pulse energy and compact architecture, then the laser transmitter becomes compact and efficient, but self-phase modulation occurs at relatively low peak powers reducing coherence time
Solution Approach 1:
The patent applies preliminary action by pre-shaping the pulse amplitude profile to a flat-top waveform before amplification. This is achieved through amplitude modulation of the seed signal using electro-optic modulators, creating a pulse with constant amplitude during the main duration. By preparing the pulse in this specific shape beforehand, the system prevents self-phase modulation from degrading coherence during the high-power amplification process, thus resolving the contradiction between achieving high pulse energy and maintaining long coherence time.
2Power
If peak power is increased to meet high pulse energy requirements, then the laser transmitter achieves required energy levels, but self-phase modulation increases causing spectral broadening and reduced spectral selectivity
Solution Approach 1:
The patent applies parameter changes by modifying the temporal amplitude profile parameter of the laser pulse from a conventional shape (such as Gaussian) to a flat-top profile. This parameter change is achieved through amplitude modulation that maintains constant amplitude during the pulse duration. By changing this specific parameter, the system enables high peak power operation while suppressing self-phase modulation, as the flat-top profile minimizes the time-dependent phase shifts that cause spectral broadening.
3Duration of action of stationary object
If long pulse duration is used to maintain coherence, then coherence time is extended, but pulse energy density decreases affecting LADAR performance
Solution Approach 1:
The patent applies parameter changes by optimizing the pulse temporal profile to a flat-top shape with a specific duration (e.g., 10 nanoseconds). This parameter optimization allows the pulse to maintain constant amplitude throughout its duration, maximizing the coherent portion of the pulse while concentrating sufficient energy density. The flat-top profile ensures that the entire pulse duration contributes to coherent interaction, unlike conventional profiles where only portions maintain coherence, thus simultaneously achieving long coherence time and high pulse energy density.
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 allows for significant scaling of fiber laser pulse power without coherence degradation, enhancing the capability of fiber lasers in coherent LADAR and other applications by reducing self-phase modulation, maintaining beam quality and ruggedness without requiring large-core fibers or complex phase correction loops.
Implementation Method 1
electro-optic amplitude modulators to create flat-top pulse signals
Implementation Method 2
the optical Kerr effect at relatively low peak powers. This parasitic, nonlinear effect manifests itself as self-phase modulation
Data Source
Figure 1~2
Figure 3A~3B
Figure 3C~3D
AI summary
A laser transmitter (100, 200) is provided that includes a seed signal generator (104, 204), an amplitude modulator (106, 206) and a power amplifier (108, 208). The seed signal generator is configured to generate a seed signal (116, 216) that has a continuous waveform. The amplitude modulator is configured to generate a flat-top pulse signal (118, 218) based on the seed signal. The power amplifier is configured to generate a laser output (102, 202) signal based on the flat-top pulse signal.