Fast, stable gas sensing under harsh conditions is hugely important for industrial safety, environmental monitoring, and medical testing.
Many established optical gas-sensing approaches require careful optical-path adjustment and alignment and can struggle in environments with strong electromagnetic interference (EMI). Fiber sensors, by contrast, are inherently EMI-immune and avoid free-space alignment.
The "Sandwich" Sensor
The research team has developed an ultra-compact, low-finesse Fabry-Pérot interferometer formed entirely in fiber.
The sensing cavity is 150 μm long with a volume of <1.4 nL, designed to be small enough for rapid response while remaining reliable and repeatable.
The offset-core sandwich structure is central to the device's mechanics. A 60 μm thin-diameter fiber (TDF) section is fusion-spliced between two single-mode fibers (SMFs) with a deliberate lateral offset.
Finite-element analysis (FEA) was used to optimize the geometry, including an offset of about 45 μm, which the researchers chose as a practical trade-off between coupling efficiency and mechanical strength.
Detecting Gases with the Fabry-Pérot Interferometer
The sensor uses photothermal spectroscopy. Acetylene absorbs modulated pump light, heats the gas via non-radiative relaxation, and changes the cavity's refractive index. That refractive-index change shifts the Fabry-Pérot interference signal, which is read out as a phase-sensitive measurement.
Both pump and probe fields are treated as Gaussian beams emerging from the fiber core. The pump is sinusoidally modulated so the photothermal effect is periodic, producing a clean signal that can be demodulated at harmonics.
To target an acetylene absorption line, the researchers used a distributed feedback (DFB) laser at 1530.37 nm as the pump and amplified it with an erbium-doped fiber amplifier (EDFA) to 280 mW output.
A narrow-linewidth 1555 nm probe laser was set at the interferometer’s Q-point for maximum phase sensitivity.
The OSFP sensor sat in a small gas cell at room temperature and atmospheric pressure, and the pump was modulated at 1 kHz.
A sawtooth scan across the absorption feature enabled 2f detection, and the modulation depth was set to 100 mV to maximise signal-to-noise ratio. The readout used balanced photodetection followed by lock-in amplification to suppress common-mode noise and stabilise the measurement.
Performance and Future Work
The system detected acetylene at parts-per-billion levels, reporting a minimum detection limit of 0.39 ppm (390 ppb) with an integration time of 450 s (from Allan–Werle analysis).
During the course of an hour, the reported signal fluctuations stayed within 4 %. The sensor showed a six-order-of-magnitude dynamic range, a response time of about 2.1 s, and strong linearity (reported as 0.999 in the paper’s performance summary).
The key finding is practicality - this is a compact, all-fiber photothermal interferometric gas sensor made with straightforward cleaving and fusion splicing. There is very little complication: no micro- or nano-printing, no delicate free-space alignment, and no sophisticated feedback control, but acetylene detection sensitivity remains high.
The authors position their device as a strong candidate for integration in tight spaces and harsher environments where fiber’s EMI immunity is valuable.
Journal Reference
Niu, C. et al. (2026). All-fiber offset-core sandwich-structured gas sensor based on photothermal spectroscopy detection. Optics Express, 34(4), 6476-6485. DOI: 10.1364/OE.589730
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