Using Noise to Enhance Optical Sensing
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Using noise to enhance optical sensing:
Like many researchers in the field of optics, [AMOLF physicist Said] Rodriguez works with resonant systems that can detect tiny changes in their environment. A typical optical sensor is based on a cavity, an empty space with laser light resonating between two mirrors. The resonance frequency depends on what happens in and around the cavity. "For example, a gas flowing through the cavity changes the resonance frequency, but so does a change in temperature or pressure," Rodriguez explains. "A typical detector measures this change in the resonance frequency as a change in intensity of light that comes out of the cavity. However, intensity fluctuations, i.e., noise, always disturb the measurement. The most common way to reduce the deleterious effect of noise is to average the signal over a long period of time. This limits the detection speed, while in the vast majority of applications there is great value in sensing as fast as possible. Moreover, the detection speed is always limited by noise; even if all classical (e.g. thermal) noise is suppressed, quantum noise remains."
While most optical sensors are linear-the light that comes out is a linear function of what went in-Rodriguez proposes an optical sensing scheme based on nonlinearity, which means that photons can effectively interact with each other inside the sensor. "Inside the optical cavity, we add a material that influences the resonating light in a non-linear way. The light that comes out is not a linear function of what went in, but it is bistable: for a given input, the output has two possible values," he says. "Due to inherent noise in the system, the output of the sensor flips randomly between those two values. When the resonance frequency of the cavity changes (e.g. because a particle enters the cavity) this flipping pattern changes as well."
Analyzing the statistics of the flipping pattern reveals the change in the resonance frequency. Since noise increases the flipping rate between the two values, and a greater flipping rate means less time is needed to acquire sufficient statistics, this means that noise makes the sensor faster. Rodriguez: "In conventional sensors increased noise increases the time needed to detect something entering the cavity, but in this sensor the detection is faster when there is more noise. That is really remarkable."
Reference:
Said R.K. Rodriguez. Enhancing the Speed and Sensitivity of a Nonlinear Optical Sensor with Noise, Physical Review Applied (2020). (DOI: 10.1103/PhysRevApplied.13.024032)
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