A recent study published in Scientific Reports looks at how graphene-based saturable absorbers can be added to silicon-on-insulator (SOI) platforms to support both passive and active mode-locking in lasers.
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Background: How Mode-Locking Works
Mode-locked lasers create extremely short, precisely timed pulses of light. They’re used in many fields, including high-speed communications, gas detection, and medical imaging. These lasers can be mode-locked passively, using saturable absorbers, or actively, by modulating the losses in the laser cavity.
Traditional absorbers made from semiconductors have limits. They don’t work across a wide range of wavelengths and often respond too slowly. Graphene, a two-dimensional material, offers some advantages. It works over a broad range, reacts quickly (within about 200 femtoseconds), and doesn’t need much energy to reach saturation. Because of this, it’s being studied as a better alternative.
Integrating graphene with silicon photonics also makes it easier to build compact, fast laser systems using standard manufacturing techniques.
Research Overview: Integration and Fabrication
In this study, the authors tested how graphene-based saturable absorbers could be used on a silicon photonics platform to support both passive and active mode-locking of fiber lasers. To make the approach scalable and repeatable, they used a wafer-scale fabrication process that’s compatible with standard CMOS (chip-making) technology.
The graphene was grown using chemical vapor deposition (CVD) on a 6-inch wafer, then transferred to a silicon wafer with a 220 nm thick silicon layer over a 2 µm buried oxide (BOX) layer. The device was built to guide light through a 500 nm wide silicon waveguide. A doped silicon waveguide was included to act as a gate, allowing control over the graphene’s chemical potential using a bias voltage.
To test the absorber, the team connected it to a commercial erbium-doped fiber amplifier (EDFA) and ran it under different operating conditions. They measured how well it worked as a saturable absorber, focusing on key metrics like modulation depth and saturation power, and how these changed with different bias voltages.
They also used a range of optical tools to evaluate the system: Raman spectroscopy to check the graphene’s doping level, along with optical spectrum analyzers and autocorrelation measurements to look at pulse width and repetition rates under both passive and active mode-locking.
Key Findings: Impacts of Integrating Graphene
The graphene-based saturable absorber showed strong performance in both passive and active mode-locking setups. It reached an extinction ratio of 5 ± 0.2 dB and had an electro-optic bandwidth of 11.2 ± 0.7 GHz—both important for reliable, high-speed laser operation.
In passive mode-locking, the absorber relied on saturable absorption caused by Pauli blocking. This happens when intense light reduces graphene’s ability to absorb more photons. At a bias of -2 V, the absorber had a modulation depth of 1.3 % and a saturation power of 67.5 mW.
When the bias was increased to -3 V, these values rose to 2.7 % and 287 mW. This means the absorber’s response could be tuned by adjusting the voltage. Even with some two-photon absorption happening in the silicon waveguides, stable mode-locking was still achieved.
The system produced pulse trains at a repetition rate of 27.9 MHz in passive mode-locking, with pulse durations as short as 1.7 picoseconds. In active mode-locking, it reached higher speeds—repetition rates of 4 GHz and 10 GHz—showing that the absorber could handle fast optical modulation. This is especially useful in systems that need low-jitter timing, like high-speed analog-to-digital converters (ADCs).
Pairing the graphene absorber with an EDFA allowed precise control over the laser output. This combination meets the needs of advanced telecommunications and integrated photonic systems.
The results also show how much the bias voltage affects the absorber’s behavior. By tuning the voltage, researchers could change the modulation depth and saturation power, making use of graphene’s flexible optical properties. The integration on a silicon photonics platform also points to a scalable way to build high-speed laser systems.
Potential Applications in Photonics
The demonstrated approach is relevant to several application fields. In telecommunications, graphene-based absorbers could improve ultrafast data transmission and signal quality.
In biomedical imaging and spectroscopy, they may enhance resolution and acquisition speed. The integration process used in this work is compatible with established silicon photonics fabrication, offering a scalable method for incorporating ultrafast laser components into compact photonic systems.
The study also highlights the potential for further optimization through edge-coupled configurations, which could extend operational bandwidth and improve performance. These developments may support the realization of on-chip femtosecond laser sources, with implications for quantum photonics, high-resolution spectroscopy, and precision metrology.
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What’s Next
The researchers suggest improving the design of the graphene absorbers to extend their operational range and improve efficiency.
Investigating alternative waveguide materials and hybrid systems incorporating other 2D materials may also enhance performance. In parallel, long-term stability and reliability testing will be necessary to assess the suitability of such devices for commercial and field-deployed systems.
Journal Reference
Reep, T.,. et al. (2025). Graphene absorber on an SOI chip for active and passive mode locking of lasers. Sci Rep, DOI: 10.1038/s41598-025-93051-z, https://www.nature.com/articles/s41598-025-93051-z
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