Researchers have successfully integrated a graphene-based absorber onto a silicon photonics platform to achieve both active and passive mode-locking in fiber lasers, marking a significant step forward in laser technology.
This innovation paves the way for scalable, integrated mode-locked laser systems, critical in applications such as telecommunications, gas spectroscopy, and biomedical fields.
The study demonstrated passive mode-locking at a remarkable 28 MHz repetition rate, producing ultra-short pulses of just 1.7 picoseconds. In parallel, active mode-locking capabilities were achieved at frequencies of 4 GHz and 10 GHz, broadening the potential of laser systems.
The researchers utilized wafer-scale manufacturing techniques in imec's 300 mm fabrication facility, underscoring the scalability of their approach. Raman Spectroscopy confirmed that the transferred graphene exhibited a doping level ranging from 6 x 10^12 to 10 x 10^12 cm^-2, which is conducive for optimal laser performance.
A notable feature of the manufactured graphene absorber is its electro-optic bandwidth of 11.2 ± 0.7 GHz alongside a good extinction ratio of 5 ± 0.2 dB. These characteristics enhance the potential for dynamic tunability in laser systems.
The study also highlighted the intricate relationship between bias voltage and modulation depth in the absorbers. For instance, at a bias of -2 V, a modulation depth of 1.3% was measured, improving to 2.7% at -3 V, showcasing the versatility of the graphene-based absorbers.
In constructing their passively mode-locked laser setup, the researchers employed a total cavity length of about 7 m. The achieved pulse width was a concise 1.7 ps, along with a 10 dB optical bandwidth of 2.6 nm, presenting an output power of 3 dBm.
Transitioning to active mode-locking, the laser cavity fluctuated around a fundamental repetition rate of 18 MHz, with average output powers reaching 1.0 dBm at 4 GHz and 1.3 dBm at 10 GHz. These outputs are critical for applications requiring high-speed optical signals, such as low-jitter clock sources for ADCs.
Overall, the integration of graphene absorbers on silicon platforms represents a leap in fiber laser technology, offering new pathways for high-speed, efficient laser systems. Enhanced performance may be realized with continued refinements, such as utilizing edge-couplers and employing gain-flattening filters to broaden optical bandwidth.
This innovative work not only demonstrates the robust capabilities of graphene but also sets a precedent for future advancements in integrated laser technology, promising more efficient and compact solutions that can meet the escalating demands within various scientific and industrial fields.