We present recent developments regarding fiber Bragg gratings for kilowatt-level fiber lasers. First, we show that writing grating reflectors through the fiber coating using an ultrafast laser improves reliability and enables higher pump power handling. The use of ultrafast laser technology also offers more options to produce gratings in larger core fibers. Finally, we show that Raman suppression gratings are a good solution for SRS mitigation with their large (<20 dB) rejection over 15 nm and low reflectivity at Raman wavelengths, and negligible insertion loss at the laser wavelength.
A CW kilowatt fiber laser numerical model has been developed taking into account intracavity stimulated Raman scattering (SRS). It uses the split-step Fourier method which is applied iteratively over several cavity round trips. The gain distribution is re-evaluated after each iteration with a standard CW model using an effective FBG reflectivity that quantifies the non-linear spectral leakage. This model explains why spectrally narrow output couplers produce more SRS than wider FBGs, as recently reported by other authors, and constitute a powerful tool to design optimized and innovative fiber components to push back the onset of SRS for a given fiber core diameter.
Fourier domain mode locked (FDML) lasers provide high sweep rates, broad tuning ranges, and high output powers for
optical coherence tomography (OCT) systems. However, presently-known FDML lasers at 1300 nm have relatively
short coherence lengths, limiting the size of samples that can be imaged. Furthermore, FDML lasers produce only one
useable sweep direction. We report FDML coherence length extension by incorporating advanced dispersion
compensation modules (DCMs). DCMs eliminate group velocity dispersion in the cavity, doubling coherence lengths
and ensuring uniform axial resolution over the imaging range. Additionally, forward and backward sweeps are nearly
identical, removing the need for external buffering stages.
Gain equalization of an amplifier is performed by introducing spectrally designed Bragg gratings in the
mid-stage of a dual-stage erbium-doped fiber amplifier. The long-haul performances of the amplifier are
evaluated using a 50 km recirculating loop. The results show a clear improvement of the transmission
quality when equalizing the gain.
In this paper, we show that Bragg gratings can greatly contribute to enhance the performances of today's optical amplifiers.
some of the applications of Bragg gratings in optical amplifiers such as gain equalization, gain stabilization and dispersion
compensation will thus be reviewed.
Gain equalization of an EDFA is performed by introducing spectrally designed all-fiber filters in the mid-stage of a dual-stage fiber amplifier. Two types of filters are studied: a cascade of narrow-band Bragg gratings for discrete equalization of a finite number of channels and a wide-band Bragg grating performing equalization over the whole 1539 nm to 1557 nm range. In future work, it is planned to use the discrete Bragg grating configuration to simultaneously perform Dispersion compensation (D), Equalization (E) and Stabilization of gain (S) and Channel dropping (C) in a dual-stage EDFA. Integration of these functions will result in a high performance amplifier called the DESC-EDFA.
We present measurements of the atmospheric refractive index structure parameter Cn2 and the inner scale lo made with a SLS-20 scintillometer manufactured by Scintec GmbH. The measurements are compared to measurements of scintillation made with a simple in-house built scintillometer and to Cn2 derived from measurements of CT2 made with differential temperature probes. We found that the combination of lo and Cn2 data provided by the SLS-20 allowed us to calculate the log-amplitude variance of the scintillation measured with the in-house scintillometer with good agreement. Poor agreement was found between the SLS-20 and temperature probe Cn2 data primarily because of the limited frequency response of the probes.