Supercontinuum Pulse Source with Acousto-Optic Gating
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Solution Overview
Problem
Conventional supercontinuum lasers operating at high pulse repetition rates are inadequate for applications requiring lower pulse repetition rates, such as FLIM, where decay lifetimes exceed tens or hundreds of nanoseconds, due to the complexity of cavity design and difficulty in achieving lower repetition rates from mode-locked fibre oscillators.
Innovation Solution
An optical pulse source utilizing a microstructured optical fibre and a master oscillator power amplifier (MOPA) with a gating mechanism, such as an acousto-optic modulator, to control the pulse repetition rate from a few Hertz to 100 MHz, allowing for flexible generation of supercontinuum pulses at rates below 20 MHz, and a wavelength tunable optical bandpass filter for spectral control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If mode-locked fibre oscillators are used to generate supercontinuum pulses, then high pulse repetition rates (20MHz to 160MHz) are achieved, but the pulse repetition rate cannot be reduced below 20MHz due to cavity design complexity and high nonlinearity in long cavities
Solution Approach 1:
The system is divided into two independent parts: a mode-locked fibre oscillator that generates high repetition rate pulses, and a separate gating mechanism (acousto-optic modulator) that selectively gates pulses to achieve lower output repetition rates. This segmentation allows the oscillator to operate at its optimal high repetition rate while the gating mechanism provides flexible rate control without requiring complex cavity redesign.
Solution Approach 2:
An acousto-optic modulator is introduced as an intermediary component between the mode-locked fibre oscillator and the photonic crystal fibre. This intermediary gates the high repetition rate pulses from the oscillator to produce a lower repetition rate pulse train, enabling rate control without modifying the oscillator cavity design.
2Productivity
If high pulse repetition rates (20MHz to 160MHz) are used, then productivity is improved, but the pulse-to-pulse separation becomes too short for lifetime imaging applications where decay lifetimes exceed tens or hundreds of nanoseconds
Solution Approach 1:
The acousto-optic modulator is driven at a lower frequency to selectively gate pulses from the high repetition rate oscillator, creating a periodic pattern where only every Nth pulse is transmitted. This transforms the high repetition rate pulse train into a lower repetition rate train with sufficient pulse-to-pulse separation for lifetime imaging applications.
Solution Approach 2:
The gating mechanism pre-selects which pulses will be transmitted to the photonic crystal fibre based on the desired output repetition rate, ensuring that pulses are spaced appropriately for lifetime imaging before they enter the supercontinuum generation process.
3Adaptability or versatility
If the pulse repetition rate is reduced to below 20MHz, then suitability for lifetime imaging is improved, but it becomes difficult to achieve from mode-locked fibre oscillators due to high nonlinearity within long cavities
Solution Approach 1:
The system separates the pulse generation function (performed by the mode-locked fibre oscillator at high repetition rate) from the pulse rate control function (performed by the acousto-optic modulator). This allows low output repetition rates to be achieved without requiring the oscillator cavity to be designed for low repetition rates, avoiding the nonlinearity problems associated with long cavities.
Solution Approach 2:
The acousto-optic modulator serves as an intermediary that converts the high repetition rate output from the mode-locked fibre oscillator into a lower repetition rate pulse train, making low repetition rates easily achievable without modifying the oscillator design.
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
Enables the generation of higher energy supercontinuum pulses with adjustable pulse repetition rates, suitable for applications like STED microscopy, and flexible spectral profiles, enhancing the capability for fluorescence excitation and stimulated emission in microscopes.
Implementation Method 1
an optical pump laser source (31) operable to produce optical pulses
Implementation Method 2
pass through a microstructured fibre (35) to broaden a spectral bandwidth of the optical pulses thereby forming supercontinuum pulses
Implementation Method 3
The gating means (33) comprises an acousto-optic modulator
Data Source
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AI summary
An optical pulse source 30 comprising a DPSS pump laser 31, a photonic crystal fibre (PCF) 35 and acousto-optic modulator (AOM) gating means 33. The pump pulses are coupled through lenses 32 to the AOM 33, synchronised to the pump laser 31 and operable to gate the pump pulses to a reduced repetition rate Rr = Rf/N, where Rf is the pump laser fundamental frequency. The pulses from the AOM 33 are injected via optics 34 into the PCF 35. Propagation through the PCF 35 causes the pulses to broaden spectrally to produce supercontinuum pulses. An optical pulse source comprising the optical pulse source 30 and an acousto-optical tunable filter (AOTF) operable to convert supercontinuum pulses into wavelength variable output pulses is also provided. A method of scaling the energy of the optical supercontinuum pulses is also provided.