Optical fiber sensing based on reflection laser spectroscopy

Abstract

An overview on high-resolution and fast interrogation of optical-fiber sensors relying on laser reflection spectroscopy is given. Fiber Bragg-gratings (FBGs) and FBG resonators built in fibers of different types are used for strain, temperature and acceleration measurements using heterodyne-detection and optical frequency-locking techniques. Silica fiber-ring cavities are used for chemical sensing based on evanescent-wave spectroscopy. Various arrangements for signal recovery and noise reduction, as an extension of most typical spectroscopic techniques, are illustrated and results on detection performances are presented.

Keywords: Fiber Bragg gratings; Pound-Drever-Hall method; fiber resonator; frequency locking; high-birefringence fiber; laser-frequency modulation.

Figure 1.

RF-modulation-based FBG interrogation set-up. PD: photodiode; DBM: double-balanced mixer; BT: bias-tee.

Figure 2.

Experimental set-up: P-rotators: polarization rotators; splitter: polarization splitter; EC laser: extended cavity laser.

Figure 3.

Mixer output line shapes over a laser-frequency scan around the PM Bragg resonances with a 45° linear polarization state.

Figure 4.

Longitudinal strain response for PM FBG in both the Fast and Slow axes of the fiber (1.264 ± 0.02 pm/με and 1.282 ± 0.009 pm/με for slow and fast axes respectively). (grey), Fast and ▴ (black), Slow axes.

Figure 5.

Temperature response for PM FBG in both the Fast and Slow axes of the fiber (10.326 ± 0.02 pm/°C and 11.858 ± 0.2 pm/°C for slow and fast axes respectively). (grey), Fast and ▴ (black), Slow axes.

Figure 6.

(a) Time response of the laser-locked system when a sine voltage is applied to the PZT (8 Hz) attached to the FBG and a periodic temperature change created with the PID controller (0.5 Hz). (b) FFT spectrum of Figure 6a (50 mHz resolution bandwidth). The traces were shifted by 30 dB for sake of clarity.

Figure 7.

Noise spectral density of the FBG-resonator locking signal for different excitation frequencies in the SM-fiber cavity: (a) a sharp peak is evident at 1.2 kHz with a noise increase towards low frequencies and spurious oscillations due to harmonics of the AC line frequency; (b) the system is capable of detecting deformations down to 2.4 Hz.

Figure 8.

Pound-Drever-Hall interrogation set-up of the HiBi FBG cavity. PD: photodiode; DBM: double-balanced mixer; LF: low-frequency; HF VCO: high-frequency voltage-controlled oscillator; PBS: polarizing cube beamsplitter.

Figure 9.

Transmission of the PM FBG resonator for a wide laser sweep (∼0.2 nm).

Figure 10.

Narrow laser frequency scan equivalent to about one cavity free spectral range (FSR) with RF sidebands at 12 MHz (upper graph). Two peaks appear well separated in frequency by 50 MHz. The PDH signals are also recorded for both polarization eigenmodes.

Figure 11.

Response to dynamic strain of fast and slow axes cavity modes in laser-locked condition for a 10 nε signal applied to the intra-cavity fiber.

Figure 12.

Sketch of the flexural beam acceleration transducer.

Figure 13.

Signals from the two accelerometers with a mechanical pulse train applied to the base. On the left, the strain response of the fiber-optic beam sensor. On the right, the display readout of the K2.

Figure 14.

Schematic diagram of the accelerometer’s head. Stainless steel cantilevers are clamped together using aluminum plates. All cantilevers have the same dimensions and nominal resonant frequencies of about 1.5 kHz.

Figure 15.

Pound-Drever-Hall error signal obtained by 60 MHz demodulation of the reflected field from a PS FBG.

Figure 16.

The acceleration noise spectral density along one axis of the accelerometer. A known deformation is applied by a PZT attached to the accelerometer and aligned with the measurement direction to enable the conversion of the sensor’s voltage signal into acceleration.

Figure 17.

Reflection spectrum of an optical cavity made from two low reflectance FBGs spaced by 10 mm. The insert shows the laser emission spectrum as a red dashed line.

Figure 18.

Left: Response of the FBG transducer (top) and the PZT to a plucked E4 string. Right: The Fourier transform of the respective complete waveforms shows the fundamental frequency at 326.8 Hz and overtones up to 12 kHz.

Figure 19.

Schematic of the fiber-ring resonator. PC: polarization controller; EAB: evanescent-access block; LPF: low-pass filter.

Figure 20.

Locking of the laser to the fiber cavity resonance. Top: cavity transmitted power for free-running (black solid line) and locked laser with different low and high servo gains (gray and dotted line, respectively). Bottom: PDH error signal in unlocked (black solid line) and locked cases (gray and dotted line, respectively).

Figure 21.

Cavity transmission signals in different cases. Left: free-running laser case with no sample on the EAB (black line) and with a sample (glycerol diluted at 99.5% with D₂O) causing a small index overlay (gray line); right: laser-locked condition with the sample on the EAB.