Mode-locked OPS Laser Resonator with Folded Optical Path
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Solution Overview
Problem
Mode-locked semiconductor lasers face limitations in pulse energy and pulse repetition frequency, making them unsuitable for multi-photon microscopy applications due to their short excited-state lifetime and high pulse repetition frequency, which restricts the wavelength tunability and efficiency of OPS-lasers.
Innovation Solution
A mode-locked OPS-laser resonator design with a selected length allowing multiple incidences of radiation on the gain-structure during each round trip, with a time between incidences less than twice the excited-state lifetime, enabling pulse energy build-up and adjusting the pulse repetition frequency to match that of solid-state lasers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If mode-locked semiconductor lasers operate with short resonators to match the short excited-state lifetime, then the pulse repetition frequency increases to several GHz, but the pulse energy becomes too low and the PRF is too high for multi-photon microscopy applications
Solution Approach 1:
The patent implements multiple periodic passes of the laser beam through the gain structure within each resonator round trip. By folding the optical path to include 2-4 passes through the gain medium, the system accumulates energy over multiple interactions while maintaining a resonator length suitable for multi-photon microscopy applications (50-150 MHz PRF).
Solution Approach 2:
The patent ensures continuous energy extraction from the gain medium by maintaining multiple beam passes through the active layers. This continuous interaction between the circulating radiation and the gain structure maximizes energy transfer efficiency while the resonator design ensures the time between successive passes remains less than twice the excited-state lifetime.
2Power
If the resonator length is extended to reduce pulse repetition frequency for multi-photon microscopy, then the time between round trips exceeds the excited-state lifetime, but mode-locking cannot be achieved in semiconductor lasers with long resonators
Solution Approach 1:
The patent introduces an additional spatial dimension by folding the optical path multiple times through the gain structure. Instead of simply extending the resonator length linearly, the beam is redirected through the gain medium multiple times in a compact configuration, effectively increasing the interaction length without proportionally increasing the resonator round-trip time.
Solution Approach 2:
The patent combines multiple beam passes through the gain structure into a single resonator round trip. By using optical folding elements to direct the beam through the active layers multiple times, the system merges several gain extraction opportunities into one continuous circulation cycle, maintaining mode-locking stability while achieving the desired pulse repetition frequency.
3Power
If multiple incidences on the gain structure are implemented during each round trip, then the pulse energy builds up effectively, but the device complexity increases
Solution Approach 1:
The patent designs the resonator to serve multiple functions simultaneously: the folding optical path not only increases the number of gain passes but also provides spatial separation of optical components, enables thermal management of the gain structure, and maintains a compact overall footprint suitable for laboratory and clinical applications.
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 design enhances the mode-locked output power and pulse energy, allowing for wavelength tunability and improved performance in multi-photon microscopy by increasing the number of gain-structure incidences per round trip, thereby optimizing the response of specific fluorophores.
Implementation Method 1
Means are provided for energizing the gain-structure, thereby causing radiation having a fundamental wavelength characteristic of the active layers of the gain-structure to circulate in the laser-resonator
Implementation Method 2
Mode-locking means are located in the laser-resonator for causing the circulating radiation to circulate as sequence of mode-locked pulses at a predetermined pulse-repetition frequency determined by the resonator length
Data Source
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AI summary
A laser (10) includes an optically pumped semiconductor OPS gain- structure (16). The apparatus has a laser-resonator (11) which includes a mode-locking device for causing the laser to deliver modelocked pulses. The resonator has a total length selected such that the mode-locked pulses are delivered at a pulse repetition frequency of about 100 MHz. An optical arrangement within the resonator provides that radiation circulating in the resonator makes a plurality of incidences on the OPS gain-structure with a time less than the excited-state lifetime of the gain-structure between successive incidences.