Narrow-linewidth lasers can now be stabilised more effectively without adding optics. A method using an electronic ‘Smith predictor’ within existing feedback loops suppresses phase noise, commonly known as servo bumps, and improves performance. This is accomplished with only one Field Programmable Gate Array (FPGA), offering minimal hardware overhead for applications in quantum precision measurement and information science.

The standard method for stabilising narrow-linewidth lasers, key tools in quantum physics and precision measurement, often creates unwanted noise called “servo bumps”. Researchers devised a new technique that tackles this issue by integrating an electronic ‘Smith predictor’ into existing systems; it anticipates and cancels out these disturbances within the feedback loop itself.

The Shanghai Institute of Optics and Fine Mechanics team developed this technique, essentially cruise control for lasers, anticipating needed adjustments than reacting after disturbances occur. This approach avoids adding extra optical components, simplifying stabilisation with minimal additional hardware. Their system utilises a Field Programmable Gate Array (FPGA), akin to LEGO bricks allowing flexible circuit reconfiguration without needing entirely new hardware each time. But can this purely electronic solution effectively suppress servo bumps and unlock the full potential of next-generation quantum devices. Further details on their implementation follow.

Electronic Smith predictor suppresses laser noise for quantum sensing applications

David Szczerba collaborated with colleagues and Technology () to develop a Smith predictor that reduced peak servo bumps by 14 decibels (dB), a level previously unattainable without extra optical parts. This suppression is crucial for high-resolution quantum sensing applications, as even small noise spikes can obscure weak signals or introduce errors in measurements. A purely electronic solution addresses time delays within existing Pound-Drever-Hall feedback loops using just one Field Programmable Gate Array (FPGA), simplifying laser stabilisation and minimising hardware overhead compared to previous methods reliant on complex optics.

When deployed with Rydberg atom sensors, it eliminated a 3.7 dB noise bump near 140kHz, improving signal-to-noise ratio by 2.46 dB, demonstrating its practical benefit beyond fundamental performance metrics. The Radio Frequency (RF) spectrum of the locked laser also improved significantly, with amplitude at the servo bump frequency attenuated by an impressive 18 decibels, exceeding the initial peak suppression figure.

FPGA Implementation Offers Promise But Lacks Broad Performance Characterisation

This FPGA-based solution improves laser stability for sensitive quantum experiments but offers limited insight into its broad applicability. Real-world scenarios often involve varying laser powers or initial noise levels, yet performance under these conditions remains unspecified. Previous attempts to tackle servo bumps frequently involved adding complex optical elements and increasing cost; however, quantifying trade-offs between hardware simplification achieved here versus potential limitations in achievable suppression is still needed.

Acknowledging that detailed performance across a wide range of laser conditions isn’t available is fair given the considerable variation found in real-world quantum setups regarding power and existing noise levels. The method addresses time delays, a source of unwanted phase noise known as servo bumps, through electronics rather than additional optical components; it represents a novel implementation within the Pound-Drever-Hall feedback loop.

This research demonstrated a method to reduce laser instability, specifically, “servo bumps” caused by time delay in frequency stabilization, by implementing a Smith predictor within the electronic control loop using a Field-Programmable Gate Array (FPGA). This approach suppresses phase noise and improved performance at frequencies of 377kHz, 408kHz and 448kHz with reductions exceeding 18 decibels compared to previous methods.

By addressing this issue electronically instead of adding more optics, researchers achieved simplification and flexibility in controlling lasers used for quantum experiments. The authors note that further work is needed to fully characterise performance across diverse operating conditions.

👉 More information
🗞 Predictive electronic feedback control for narrow-linewidth lasers
✍️ Jiatian Liu, Xuan Qi, Xiayang Fan, Shenchao Jin and Yuan Sun (Chinese Academy of Sciences)
🧠 ArXiv: https://arxiv.org/abs/2610.31396

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