On-Chip Optical Dispersion Management for Femtosecond Pulse Generation

Dispersive materials properties are a fundamental part of any optical system, and needs to be carefully managed or compensated for in applications spanning spectroscopy, sensing, and communications. Optical pulses in particular are necessarily composed of multiple frequency components, meaning that dispersion management is vital to maintaining high peak power pulses with short time durations. Carefully engineering the dispersive properties of a nanostructured material is one way to manage dispersion in a controllable manner, and can make dispersive elements orders of magnitude smaller than using bulk materials. This offers the ability to sculpt the temporal profile of light with unprecedented control on a chip.<br/>The ability to generate ultrashort, broadband and high-peak-power optical pulses on-chip has been a long-sought-after goal. However, all demonstrations to date rely on a table-top pulse laser source which increases the system complexity, size, and cost, and thus hinders practical applications. In addition, optical pulses can be generated via microresonator frequency comb sources through coupling a continuous-wave laser into a high-quality-factor microresonator. These devices are limited by their low efficiencies (usually &lt;2%), comb-line power and high repetition rates. Therefore, high-power and controllable integrated pulse sources are still missing, a major roadblock for fully integrated nonlinear photonic circuits.<br/>Here we generate femtosecond laser pulses using an integrated electro-optic time lens. Whereas a spatial lens system can focus a collimated laser beam to a spot at its focal plane, a time lens system can compress CW light to a short pulse at a proper dispersive focal length. The key components to build such a spatial/temporal lens system are an aperture, a lens which induces a quadratic phase in space/time, and a diffraction/dispersion medium representing the focal length of the imaging system. In our EO time lens, these roles are played by a single amplitude modulator, one phase modulator, and a dispersive medium (chirped Bragg grating). In the frequency domain, we generate an optical comb spectrum with a flat-top envelope. Without the dispersive medium, the pulse is 16.6 ps long.<br/>To integrate dispersion management on-chip, we designed a chirped Bragg grating based on a nanofin photonic crystal in LN. By linearly chirping the Bragg period from 406.5 nm to 414.5 nm over a length L, we generate the desired group delay dispersion by modifying to total grating length. On a second chip with the chirped grating, we compress the pulse to its bandwidth limit. Fabry-Perot fringes in reflection show spectrally-varying FSR, which we use to compute the group delay. By varying the length of the grating we determine the optimal length for the desired GDD. The generated frequency comb still shows high power and a flat-top spectrum after compression owing to the low loss nature of the grating. Finally, the autocorrelator trace demonstrates a pulse width is 545 fs using a Bragg grating length of 2.25 mm, near the transform limited value. This represents a 25,000 times reduction in the required path length. The loss of our Bragg grating is estimated to be 0.033 dB/mm. Our results represent a tunable, robust and low-cost integrated pulsed light source with CW-to-pulse conversion efficiencies an order of magnitude higher than achieved with previous integrated sources. The novel pulse generator can find applications from ultrafast optical measurement to networks of distributed quantum computers.