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Ultrasensitive Strain Sensing by Using Two Parallel Structured Fabry--Perot Interferometers in Cascaded Connection

An optical fiber in-line Fabry--Perot interferometric device is presented for ultrasensitive strain sensing by use of vernier effect. The device consists of two parallel structured Fabry--Perot cavities in cascaded connection, formed by two pairs of femtosecond laser inscribed in-fiber reflection mirrors. The cavity lengths and their difference can be accurately controlled, which enables an extremely large amplification factor when vernier effect is used and the strain sensitivity achieved is ∼236.32 pm/μϵ. Moreover, the temperature cross sensitivity realized is as low as ∼29.2 nϵ/°C. The device is featured with ultra-compact size, robust structure, easy fabrication, convenient operation and good reproducibility, which makes it highly attractive in many sensing applications where extremely large strain sensitivity is critically required.