We demonstrate a stable and low noise mode-locked multi-wavelength ring laser for potential uses in optical sensors. The
ring laser cavity contains an all fiber phase modulator operating at 26.2 kHz, a Fabry-Perot comb filter, a band-pass filter
and a small core EDF, and passively mode-locked pulses at 5.62MHz rate are generated. This laser has a relative intensity
noise (RIN) value under -105 dB/Hz up to 1 GHz, and under -140 dB/Hz over 1 GHz. High peak power, high speed and
high temporal resolution of mode-locked multi-wavelength laser can be useful for diverse applications including
multiplexed interrogation of distributed sensor, spectroscopy and multi-spectral bio imaging.
We demonstrate a novel wavelength division multiplexed polarimetric fiber laser sensor array for current sensing. A tunable Fabry-Perot filter is used for discriminating each sensor from the multiplexed signals, and a frequency demodulation technique based on a phase-locked loop circuit is applied for signal processing.
Basic principles and several key applications of wavelength tunable acousto-optic filters are described. The devices utilize periodic coupling of guided and/or dissipating modes in optical fibers provided by traveling acoustic wave in the same fiber. The wavelength tuning is realized by tuning the applied acoustic frequency, and proper phase matching condition selects filtered optical wavelength. Application of the acousto-optic filters to gain flattening of fiber amplifiers, wavelength selection, optical switching, and non-reciprocal filtering will be discussed.
Optical fiber sensors have been developed for more than two decades and are now in practical use for many different applications. Their advantageous characteristics, such as high sensitivity, immunity to electromagnetic interference, small size, environmental stability, and multiplexing capability, make it possible to measure new physical parameters and to place the sensors in new environment
We report a new type of gyroscope using a fiber laser with a simple configuration. It consists of a Fabry-Perot type fiber laser and an interferometer which is used to combine tow output beams from both mirrors of the laser. The rotation rate is obtained from the change of the phase difference between the two output beams. The experimental results are in good agreements with theoretical predictions.
An anti-reflection coated GaAlAs laser diode was used as a gain medium in a mode-locked fiber laser gyroscope to obtain stable mode-locked optical pulses without gain competition. The time intervals between the pulses could be measured with much improved accuracy using an electronic counter. The rms noise equivalent rotation rate was 3.4 deg/hr/(root)Hz and the long-term drift was less than 200 deg/hr.
We describe the results of experimental investigations of a bidirectional Er-doped fiber ring laser for its output power and the rotation rate dependent beat signal. Enhanced gyroscope beat signal is demonstrated by using an AM mode- locked Er-doped fiber ring laser.
Intrinsic Fabry-Perot optical fiber sensors were embedded to the tensile side of the 20 cm by 20 cm by 150 cm cement concrete structures. The sensors were attached to the reinforcing steels and then, the cement concretes were applied. It took 30 days for curing the specimens. After that, the specimens were tested with 4-point bending method by a universal testing machine. Strains were measured and recorded by the strain gauges embedded near optical fiber sensors. Output data of fiber sensor showed good linearity to the strain data from the strain gauges up to 2000 microstrain. The optical fiber sensors showed good response after yielding of the structure while embedded metal film strain gauges did not show any response. We also investigated the behavior of the optical fiber sensor when the specimens were broken down. In conclusion, the optical fiber sensors can be used as elements of health monitoring systems for cement concrete infra-structures.
Signals from fiber-optic interferometric sensors fabricated with conventional
communication fibers may be lost due to the random fluctuation of the polarization
states of two interfering optical waves.
A mode locked fiber laser gyroscope (MLFLG)  has a laser cavity formed with a fiber amplifier with a planar mirror at one end and a Sagnac loop interferometer at the other end, as shown in figure 1. The Sagnac interferometer functions as a loop mirror, and a phase modulator in the interferometer is used to modulate the reflectivity of the loop mirror.
Fiber laser sensors based on rare-earth doped fiber lasers have potential advantages over conventional fiber optic interferometric sensors with their simplicity in both the optical configuration and the electronic signal processing.[1,2] A ring laser gyroscope(RLG) built with rare-earth doped fiber amplifier has not been seriously investigated due to a few potential problems including gain competition between the two laser signals oscillating in opposite directions.
Intracavity phase modulation in a fiber-optic ring laser gyro can provide 'optical dithering' to reduce the effects of frequency locking while retaining optical reciprocity in the cavity. We show that the use of two antiphased phase modulators placed symmetrically within the fiber cavity can provide uniformly distributed dithering. A modulation index of 2.4 theoretically eliminates the zero-order lock-in band around zero frequency, while use of a high modulation frequency puts higher-order lock-in bands outside the beat frequency dynamic range. Push-pull modulation allows for high modulation frequency with minimum dynamic perturbation of the cavity resonant behavior. We describe results with an experimental Brillouin fiber optic gyro operating at 1.3-micron wavelength using push-pull modulation together with a novel synthetic heterodyne detection scheme for sensing rotation rate and direction. A ten-fold reduction of the width of the zero-order lock-in band is observed. We also demonstrate that the observed frequency bias at zero rotation rate is caused by the Kerr effect due to the power imbalance between the two oppositely directed circulating lasers.
The polarization and modal properties of fiber lasers are theoretically and experimentally analyzed. The polarization properties of the fiber laser is advantageously applied to a novel form of fiber laser sensor.
Recent research has indicated that broadband fiber sources are a viable alternative to superluminescent diodes for fiber-optic gyro applications. The key issues in using such sources are possible source configurations, the effects of feedback, source power output, and the stability of such sources in the gyro system. Both Nd:silica and Er:silica sources are discussed with an emphasis on superfluorescent fiber sources pumped by laser diodes. In both media, 100 mW of pump power can produce more than 10 mW of source power with a spectral width in excess of 10 nm. Furthermore, such sources have consistently produced mean wavelength thermal coefficients of less than 10 ppm/C.
The characteristics of Er-doped superfluorescent fiber sources are simulated for pump wavelengths in the 980 nm and 1.48 ?m pump absorption bands. Because absorption near 980 nm occurs to a short-lived pump state, while absorption near 1.48 ?m occurs to the long-lived upper-laser state, sources pumped in these wavelength regions have different characteristics. Both pump bands are found to have optimal pump wavelengths for stability and power (976 nm and 1.475 ?m). While both pump bands are efficient, the 1.48 ?m pump band has a lower threshold pump power level and a higher slope efficiency. On the other hand, the 980 nm pump band produces broader spectra and permits the use of shorter fiber lengths. These and other source characteristics are discussed in detail.
Stable, broadband, long-wavelength sources are required for accurate fiber sensors such as the fiberoptic gyroscope. The Er-doped superfluorescent fiber source and wavelength-swept fiber laser, which emit near 1 .55 m and can be pumped near 980 nm, are excellent candidates for this application. We discuss the design of such sources, their efficiency, pump source requirements, and the spectra they produce. The spectrum sensitivity to environmental factors such as temperature is also briefly discussed.
The spectral thermal stability of broadband rare-earth-doped fiber sources makes them attractive for fiber sensor applications. We quantify the mean wavelength variation of both Nd- and Er-doped fiber sources operating as superfluorescent fiber lasers. Besides the intrinsic variation of such sources the effects of pump power and pump wavelength are also considered since both the power and wavelength of conventional laser diode pump sources are temperature sensitive. Other types of rare-earth-doped sources are also briefly considered.