We discuss a fiber Bragg grating laser sensor in which the measurement of an intermodal beating frequency is used for the Bragg gratings (BG) interrogation. The linear cavity of the laser is formed by one reference BG at one side and one or several sensing BGs at the opposite side of the cavity. For this report we used two sensing BGs. In the laser with one BG at each cavity side the beating frequency is defined by the distance between BGs. If two sensing BGs are used, the beating frequency is defined by both the distances between the reference BG and each of the sensing BGs and by a ratio between sensing BGs reflections at the laser wavelength. So, when the reflection of sensing BGs is shifted by temperature or strain, the beating frequency is changed. We discuss the experimental results obtained for the laser sensor with the total cavity length equal to 4277 m; the distance between the sensing BGs was 47 m. A RF spectrum analyzer with 100-kHz bandwidth was used that allows the measurement of three first harmonics. We found that the beating frequency can be moved from the value corresponding to the 4277-m cavity to the value corresponding to the 4230-m cavity by shifting the maximum reflection of the sensing BGs. The narrowest spectrum equal to 52 Hz was obtained when the tuned delay was incorporated into the cavity. A shift of the maximum BG reflection less then 0.05 nm was detected.
We present a design of an Er-doped fiber amplifier (EDFA) with high amplification. The purpose of the device is amplification of low power pulses from laser diodes to powers sufficiently high for investigations of optical fiber nonlinear processes. The optical amplifier consists of two stages. As the first stage we use the reflecting configuration where a signal is amplified twice in the same Er-doped fiber. The second stage works as a high power amplifier. The input pulses have pulse duration of 1 ns - 100 ns, and wavelength 1549.1 nm. The maximum amplification of this design is 50 dB. We obtained the output pulses with maximum power of 50 W. These pulses were used successfully to investigate the NOLM and SRS.
A multipoint fiber laser sensor, which consists of two cavities coupled based in three Bragg gratings of fiber optics and interrogated by the longitudinal mode beating frequency is presented. We used one Bragg grating (reference) and two Bragg gratings (sensors), which have the lowest reflection wavelength. The reference grating with the two sensors grating make two cavities: first one is the internal cavity which has 4230 m of length and the next one is the external cavity which has 4277 m of length. Measuring the laser beating frequency with a radio frequency (rf) analyzer for a resonance cavity and moving the frequency peaks when the another cavity are put in resonance, we prove that the arrangement can be used as a two points sensor for determining the difference of temperature or stress between these two points. On the other hand, one principal peak and three harmonics with bandwidths of 52 Hz were obtained with the rf analyzer. Their bandwidth was controlled by an intra-caivty fiber Optical Delay Line (ODL) and was measured with the rf analyzer.
The fiber Raman amplifier employs the intrinsic properties of silica fiber to obtain the amplification, thus they use the transmission fiber as the amplification medium, where the gain is created along the transmission line. The amplification is realized by Stmulated Raman Scattering (SRS). This nonlinear process occurs when a sufficiently powerful pump is within the same fiber as the signal. In this paper, we showed experimental and numerical analysis of SRS in optical fibers. We obtain a continuous spectral when we plot the energy content in each Stokes sublines with the wavelength, which are self-pump between them. The numerical results are in agreement with the experimental results, just as the waveform in the time of optical fiber end and the energy is transferred from the signal pump to the Stokes sublines. With the simulations, we can obtain several parameters of this optical amplifier like the optical fiber effective length to obtain the amplification.
We investigate longitudinal mode beating frequency at the output of a linear Er-Doped fiber laser formed by a reference Bragg grating at one side of cavity and several Bragg gratings at another side when one of them is pulled. We detect the distance between the pulled and reference gratings by mode beating frequency measurements. (<i>Summary only available</i>)