Tuan Guo

Reflective tilted fiber Bragg grating refractometer based on strong cladding to core recoupling

A novel in-fiber structure for power-referenced refractometry with the capability to measure surrounding refractive index (SRI) as low as 1.33 is proposed and demonstrated. A short optical fiber stub containing a weakly tilted Bragg grating is spliced to another fiber with a large lateral offset. The reflection from this structure occurs in two well-defined wavelength bands, the Bragg reflected core mode and the cladding modes. The cladding modes reflect different amounts of power as the SRI changes, while the core-mode reflection from the same weakly tilted FBG remains unaffected by the SRI. The power reflected in the core mode band can be used as a reliable reference to cancel out any possible power fluctuations. The proposed refractometer with improved sensitivity for low SRI measurement together with the tip-reflection sensing feature, is a good candidate for sensing in chemical and biological applications.

Tilted fiber grating accelerometer incorporating an abrupt biconical taper for cladding to core recoupling

We demonstrate a compact power-referenced fiber-optic accelerometer using a weakly tilted fiber Bragg grating (TFBG) combined with an abrupt biconical taper. The electric-arc-heating induced taper is located a short distance upstream from the TFBG and functions as a bridge to recouple the TFBG-excited lower-order cladding modes back into the fiber core. This recoupling is extremely sensitive to microbending. We avoid complex wavelength interrogation by simply monitoring power change in reflection, which we show to be proportional to acceleration. In addition, the Bragg resonance is virtually unaffected by fiber bending and can be used as a power reference to cancel out any light source fluctuations. The proposed sensing configuration provides a constant linear response (nonlinearity < 1%) over a vibration frequency range from DC to 250 Hz. The upper vibration frequency limit of measurement is determined by mechanical resonance, and can be tuned by varying the sensor length. The tip-reflection sensing feature enables the sensor head to be made small enough (20~100 mm in length and 2 mm in diameter) for embedded detection. The polymer-tube-package makes the sensor sufficiently stiff for in-field acceleration measurement.

Highly sensitive bending sensor based on Er³⁺-doped DBR fiber laser

A short cavity Er(3+)-doped distributed-Bragg-reflector (DBR) fiber laser with a low polarization beat frequency has been demonstrated for bending measurement. The polarization beat frequency of the DBR laser is extremely sensitive to bending and can measure curvature changes as small as 1.8 x 10(-2) m(-1). Excellent agreement between experimental and theoretical results was obtained for bending curvatures from 0 m(-1) to 58.8 m(-1) with corresponding changes in beat frequency from 18.6 MHz to 253 MHz. The sensor is insensitive to temperature fluctuations and has a temperature coefficient of the beat frequency of -25.4 kHz/degrees C, making the temperature compensation unnecessary in most practical applications. The very low beat frequency of the DBR fiber laser makes frequency down-conversion unnecessary. This can greatly simplify the demodulation scheme and thus, allow the realization of low-cost but highly sensitive optical bending sensor systems.

Beat-frequency adjustable Er³⁺-doped DBR fiber laser for ultrasound detection

A compact low beat-frequency dual-polarization distributed Bragg reflector (DBR) fiber laser whose beat frequency can be varied, for high-frequency ultrasound detection has been proposed and experimentally demonstrated. The laser was fabricated in small birefringent commercial erbium-doped fiber. It operated in a robust single-longitude mode with output power of more than 1 mW and high signal-to-noise ratio better than 60 dB. Induced birefringence to the fiber during the UV inscription process is small (~10(-7)) and consequently the laser beats at a low frequency of ~20 MHz which is at least one order of magnitude smaller than previously reported results, making frequency down-conversion unnecessary. The beat frequency can be adjusted by controlling the side-exposure time of the UV light irradiating the gain cavity, providing a simple approach to multiplex a large number of DBR fiber lasers of different frequencies in series using frequency division multiplexing (FDM) technique. The proposed DBR fiber laser is also temperature insensitive, making it a good candidate for hydrophone applications.

Linearly chirped and weakly tilted fiber Bragg grating edge filters for in-fiber sensor interrogation

Power-referenced in-fiber edge filters based on a linearly chirped and weakly tilted fiber Bragg gratings (TFBG) are proposed and experimentally demonstrated. A strongly chirped TFBG with a polymer coating provides a smooth transfer function with up to 0.7 dB/nm of discrimination over the C-band. The slope profile of UV-induced chirped TFBG can be easily tailored by adjusting the tilt angle using only one chirped phase mask. Interrogators based on chirped TFBG can provide temperature-self-compensated, broadband, robust and cost-effective in-fiber demodulation for FBG sensing applications.

Ultrasound detection using a tunable low beat-frequency Er3+-doped DBR fiber laser

A compact low-beat-frequency dual-polarization distributed Bragg reflector (DBR) fiber laser for high-frequency ultrasound detection has been demonstrated. The laser was fabricated in high germanium concentration, small-core erbium-doped fiber with very small birefringence. Induced birefringence to the fiber during the UV inscription process is small (~10-7) because of the small fiber core (4.2-μm) and consequently the laser beats at a low frequency of ~20 MHz, making frequency down-conversion unnecessary. The beat frequency can be adjusted by controlling the side-exposure time of the UV light irradiating the gain cavity, providing a simple approach to multiplex a large number of DBR fiber lasers of different frequencies in series using frequency division multiplexing (FDM) technique.

Ultrasound detection using low beat frequency distributed Bragg reflector Er-doped fiber laser

Dual-polarization distributed Bragg reflector fiber laser (26 mm length) in Er-doped fiber with beat-frequency of ∼20 MHz (significantly lower than ∼1 GHz reported earlier) that is capable of detecting 16 MHz ultrasound has been demonstrated.

Optical fiber refractometer with improved sensitivity based on an offset tilted fiber Bragg grating

Highly sensitive fiber-optic refractometer with an over-offset tilted fiber Bragg grating configuration is proposed based on strong cladding-core recoupling. Reflection with two well-defined bands performs an improved refractive index measurement combining with power self-calibration property.

Tilted fiber Bragg grating-based accelerometer

A power-referenced fiber-optic accelerometer is proposed by using a weakly tilted fiber Bragg grating (TFBG) combined with a lateral-misaligned splice structure. The recoupling of lower-order cladding modes (ghost modes), excited by the TFBG, to the fiber core via a slightly lateral-offset slice is sensitive to vibration and can be unambiguously determined by monitoring the power change in reflection, avoiding complex wavelength interrogation. However, the Bragg resonance is virtually unaffected by fiber bending and it can be used as a power reference to cancel out any light source fluctuations. Combining with a polymer tube package, the novel sensing configuration presents a linear and reproducible response for acceleration measurement.