Self-injection locking - an efficient method to improve the spectral performance of semiconductor lasers without active stabilization - has already demonstrated its high potential for operation with single-longitude-mode fiber lasers. Recently, we demonstrated that self-injection locking of a conventional DFB laser through an external fiber optic ring cavity causes a drastic decrease of the laser linewidth and makes possible its direct application in a phase-sensitive optical time domain reflectometry (φ-OTDR) acoustic sensor system. Detection and localization of dynamic perturbations in the optical fiber were successfully demonstrated at the distance of 9270 m. However, the ability of the system to restore the perturbating frequency spectrum was not quantified. Here, we have evaluated the performance of a φ-OTDR system for acoustic/vibration measurements utilizing a conventional telecom DFB laser self-stabilized through an external PM optical fiber ring resonator. The use of PM fiber components prevents the polarization mode-hopping that is proved to be a major source of the laser instability, resulting in single frequency laser operation with 6 kHz linewidth. The laser diode current and the laser fiber configuration temperature both have been stabilized with accuracies better than 0.3%. All laser components have been placed into a special insulating box to protect the laser from external perturbations. Under these conditions, the typical duration of laser operation in self-maintaining stabilization regime is ~30 minutes. The laser long-term frequency drift is estimated to be less than ~30 MHz/min. This low-cost solution is directly compared with the use of a commercial, ultra-narrow linewidth (~ 100 Hz) fiber laser implemented into the same setup. Both systems are tested for measurement of the frequency of vibration applied to a fiber at a distance of 3500 m. The obtained SNR value higher than 6 dB demonstrates the ability of the DFB laser to be used in distributed measurements of vibrations with frequencies up to 5600 Hz with a spatial resolution of 10 meters.
Self-injection locking, an efficient method to improve the spectral performance of semiconductor lasers without active stabilization, has already demonstrated its high potential for operation with single-longitude-mode fiber lasers. Recently, we have demonstrated significant line-narrowing (more than 1000 times) of the conventional low-cost DFB laser locked to an external fiber optic ring resonator. However, dynamical behavior of such a laser exhibits mode-hopping making its applications for distributed acoustic sensing rather questionable. In order to explore capacity of the injection locked laser for a phase-OTDR, we have designed a simple configuration of the injection locking DFB laser and applied it for detection and localization of perturbations with a phase-OTDR based distributed vibration sensor. The conventional DFB laser locked at critical coupling regime through fiber optic ring resonator of 3.75 m length (Free Spectral Range is 54.5 MHz) delivers CW mode-hoping free radiation with a linewidth of about ~5.0 kHz, i.e. ~200 times narrower than the linewidth of free-running laser. In combination with the moving differential processing algorithm such a laser is capable to provide high SNR distributed measurements of vibrations and dynamic strain perturbations. The fiber under test comprises three sections of standard single mode fiber, with a total length of ~4.5 km. Perturbations have been locally implemented into the test fiber at two positions using a shaker and a piezoelectric stretcher, respectively. In the first case, perturbations of the fiber induced by the shaker at a frequency of 815 Hz have been recognized as a peak in the recorded and processed traces with a signalto- noise ratio (SNR) of 12 dB over a 10 m resolution cell. In the second case, dynamical strain induced by the fiber stretcher over 40 m at a frequency of 3 kHz is shown in a similar pronounced peak with a signal-to-noise ratio (SNR) of 11 dB. These signatures are similar to the results obtained with a commercial 1 kHz linewidth laser employed with the same phase- OTDR setup. We believe that proposed solution could be a basis for development of a cost-effective phase-sensitive OTDR for distributed sensing specified for the distance up to tens of kilometers.
We have presented fiber laser configuration with an injection locked DFB (IL-DFB) laser for application in phase sensitive optical time domain reflectometry (φ-OTDR). A low-loss fiber optical ring resonator (FORR) is used as a high finesse filter for the self-injection locking of the DFB laser.
We have used IL-DFB laser configuration for detection and localization of the vibrations in phase sensitive OTDR system. By applying moving differential processing algorithm for the Rayleigh backscattering traces we demonstrate accurate localization of the vibrations with a frequency ~50 Hz at a distance about 10 km with the same signal to noise ratio that is achievable with an ultra-narrow linewidth OEwaves laser (OE4020-155000-PA-00).
We have employed a new injection locking DFB laser configuration for detection and localization of the perturbations in phase sensitive OTDR system. The spectral performance of available DFB laser source has been significantly improved with implementation of self-injection locking mechanism. To provide the effect, a part of the optical radiation emitted by the laser is returned back into the laser cavity through an external fiber optic ring resonator. Self-injection locking of DFB laser coupled with the ring cavity in the under-coupled, critically coupled, and over-coupled regimes has been tested bringing us to the conclusion that the best locked laser stability and narrower linewidth is observed with the critical coupling. With the laser operating in this regime an accurate localization of 50 Hz harmonic perturbation with the spatial resolution of ~10 m at the distance of 9270 m is experimentally demonstrated with phase sensitive OTDR technique.
We study self-injection locking of DFB laser through ring fiber optic resonator in the different regimes. Experimentally measured transmitted and reflected from fiber cavity powers at undercoupled, critically coupled, and overcoupled regimes are properly coincide with the calculated values. Significant reduction of locked laser linewidth was observed. For the free running laser, the full-width at half-maximum linewidth was equal to 2.55 MHz assuming that line shape is Lorentzian. Meanwhile the self-injection locked DFB laser demonstrates linewidth equal to 2.5 KHz for undercoupled and critically coupled regimes and about two times more for overcoupled one.
We present detailed analysis of the algorithm for adjustment of double resonance in short-length Brillouin ring fiber laser. Adapted laser cavity is simultaneously resonant for the pump and Stokes radiations. Demonstrated approach is equally useful for design of single mode fiber lasers with ultra-narrow optical spectra, Q-switched Brillouin fiber lasers as well as for the applications which required high power fiber resonators free from stimulated Brillouin scattering.
Single longitudinal-mode Brillouin fiber lasers are very promising for many applications, such as coherent optical communication, interferometric sensing, coherent radar detection, and microwave photonics. In the lasers with doublyresonant cavities Stokes wave is generated by a short fiber ring cavities that are simultaneously resonant for pump and Stokes radiation. Therefore, such lasers emit coherent light in two laser lines separated by the Brillouin shift. We report a simple procedure allowing precise adjustment of the Brillouin fiber ring cavities for doubly-resonant operation and demonstrate two completely passive solutions enabling perfect stabilization of such cavities for generation of doublefrequency light. In our experiments, the first laser configuration is stabilized through self-injection locking mechanism implemented to the laser cavity with DFB semiconductor pump laser. Second configuration comprises a nonlinear fiber mirror based on the population inversion dynamical gratings induced in low-absorbed Er-doped fiber. The pump-to- Stokes conversion efficiency of ~40% in both cases and the Stokes linewidth of <500Hz for the first case and < 100Hz for the second are successfully demonstrated.
We report two completely passive solutions enabling perfect stabilization of the Brillouin lasers with doubly-resonant cavities. The first laser configuration was stabilized through self-injection locking mechanism implemented to the laser cavity with DFB semiconductor pump laser. Second configuration included a nonlinear fiber mirror based on the population inversion dynamical gratings induced in low-absorbed Er-doped fiber. In both cases, the operating wavelength of the pump laser occurred to be locked to the ring resonance frequency leading to effective generation of Brillouin Stokes radiation. The pump-to-Stokes conversion efficiency of ~40% and the Stokes linewidth of <500Hz have been successfully demonstrated with both laser configurations.
We have demonstrated suppression of low-frequency fluctuations of backscattered Rayleigh radiation in distributed fiber
optical sensors with chaotic single-longitudinal mode DFB and multi-longitudinal mode FP lasers subjected by
incoherent optical feedback. Significant decreasing of Rayleigh power variations up to 15-20 dB for 10-1000 Hz
frequency interval was recorded for both chaotic lasers. It was shown that chaotic DFB laser also efficiently restrain
stimulated Brillouin scattering in the test fiber. The results have important consequences for distributed fiber optical
sensors, which utilized Rayleigh signals.