Integrated 2.0 μm femtosecond laser amplifier for advanced accelerator applications
- Gener8 Inc., Sunnyvale, CA (United States)
We proposed a novel new architecture for a mode-locked laser oscillator/amplifier system at 2050 nm that is designed to achieve 300 fs pulse widths at a repetition rate of 119 MHz with an average power of greater than 1 W. The system is designed for application to free-electron laser (FEL)-based light sources and for the recently demonstrated micro-fabricated Dielectric Laser Accelerators (DLAs). The laser design uses hybrid-integration to directly incorporate carrier phase envelope (CEP) stabilization in the femtosecond modelocked laser oscillator cavity. This design eliminates all mechanical adjustments or moving parts that are currently used in CEP systems and require periodic alignment using mechanical motion. Most of the large free-space optical components currently used in femtosecond laser systems are replaced with waveguide chips that implement all the optical functions required for long term stability in a small compact footprint. One of key design features of our technology is the ability to lock the laser modelocking repetition rate to an external clock frequency. The design will also implement Carrier Envelop Pulse stabilization (CEP) to control the phase drift of the carrier wave relative to the peak of the pulse due to phase jitter or drift of the laser cavity. These capabilities are critical for all of the anticipated DOE applications. A large design-of-experiments in a single chip having periodic and chirped gratings fabricated in silicon nitride and silica was assembled in an array of 450 waveguide variations supporting wavelengths from 780 nm to 2100 nm. A second wafer fabrication run was implemented after results from the first run showed that the lithography did not produce grating duty cycles and feature sizes as designed. This was corrected nearly perfectly in the second fabrication run, and the improvements were confirmed through microscopic and spectroscopic analysis. Using an optical vector analyzer, periodic gratings were characterized for insertion loss, reflectivity and transmission as a function of wavelength and grating geometry in order to identify the best design parameters for functional chirped gratings. In the process of analyzing the periodic gratings, a technique to derive very accurate values for the effective refractive index of the silicon nitride strip waveguides was developed. Also, an unexpected secondary scattering mechanism was observed. A detailed investigation of the secondary peak’s behavior led to the hypothesis that its appearance is a consequence of scattering from the components of the high contrast, segmented features of the third-order grating designs. This explanation is testable in subsequent wafer trials using the analysis methodology developed here by designing purely sinusoidal sidewall modulations along the waveguide that would suppress the scattering and reduce the Bragg reflection losses. Chirped gratings for wavelengths from 1550 nm to 2100 nm were investigated with the OVA for insertion loss and for chromatic dispersion as a function of wavelength and versus propagation distance. Negative and positive chirps ranging from 5 ps/nm to 80 ps/nm for 4 cm long gratings were characterized and confirmed as capable of providing stretching and compression factors of nearly 1000 for ultrashort pulses that are 300 fs wide with transform-limited 12 nm bandwidth. This performance is on-par with much larger and more complex free-space and fiber-based approaches. Compared to free-space or fiber pulse stretchers and compressors, this highly versatile material system and compact waveguide technology demonstrates considerable potential to meet or exceed the performance of conventional alternatives while reducing size, complexity and cost.
- Research Organization:
- Gener8 Inc., Sunnyvale, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0017822
- OSTI ID:
- 1484767
- Type / Phase:
- SBIR (Phase I)
- Report Number(s):
- DOE-Gener8-17822
- Country of Publication:
- United States
- Language:
- English
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