Alexander Siekiera

Numerical Modeling of Intracavity Spectral Broadening of Raman Fiber Lasers

Raman fiber lasers (RFLs) are efficient light sources at frequencies where no other comparable all-solid-state sources are available. Especially if fiber Bragg gratings (FBGs) with narrow bandwidths are used, the bandwidth of the Stokes light is strongly broadened by Kerr nonlinearities like four-wave mixing (FWM), and self- and cross-phase modulation (SPM, XPM). In this letter, we discuss an exact numerical model to calculate the spectral behaviour of RFLs and show its application to determine the effective reflectivity of the FBGs. The model is based on a combination of the nonlinear Schroedinger equation including dispersion, FWM, SPM, and XPM with a shooting method to solve the power steady-state equations for RFLs. Numerical results are in good agreement with measurements.

Short 17-cm DBR Raman Fiber Laser With a Narrow Spectrum

We present experimental results on the lasing of a short distributed Bragg reflector (DBR) Raman fiber laser (RFL) with short cavity length. The high finesse laser cavity with an effective length of 17 cm was realized by direct inscription of two Bragg gratings into a short piece of highly nonlinear polarization maintaining Raman fiber. It is pumped by an Ytterbium fiber laser at 1100 nm with up to 8 W pump power. The Stokes wave at 1151 nm is generated within the DBR RFL with 700-mW output power and a narrow spectral full-width at half-maximum bandwidth of 60 pm. Due to the short cavity length, the number of oscillating longitudinal modes is strongly reduced to less than 24 modes. The short DBR RFL has a low lasing threshold of 4.1 W. Its spectral properties and temporal stability are investigated. Distinct longitudinal cavity modes could be observed within the radio-frequency intermode-beat spectrum detected with a fast photodiode. The measured mode separation of 607 MHz validates the short effective length of 17 cm.

Characterization of short PM Raman fiber lasers with a small spectral bandwidth

Linearly polarized Raman fiber lasers using 3 m and 6 m of a novel fiber are discussed. Experimental and theoretical results for output powers at watt levels and spectral widths in the 0.1 nm range are compared.

Characterization of a narrowband Raman MOPA with short master oscillator

Raman fiber lasers (RFL) with a narrow spectral bandwidth are important light sources, e.g. for applications that require second-harmonic generation. Non-linear optical processes within the fiber like four-wave mixing (FWM) lead to spectral broadening, due to the generation of frequency components outside of the initial laser bandwidth. This mainly results from FWM of the longitudinal modes generated in the laser cavity. Consequently a reduced number of modes with increased frequency spacing, which is obtained by using a short laser cavity, is expected to reduce the spectral broadening. We investigate a Raman master oscillator power amplifier (MOPA) configuration, consisting of a RFL with a very short cavity length of 17 cm and a subsequent Raman amplifier of 55 m length. Pumping with 8 W power of an Ytterbium-doped fiber laser at 1100 nm, the Stokes wave at 1151 nm is generated, yielding 0.7 W Stokes power with a narrow full-width at half-maximum bandwidth of 60 pm at the MO output. From the measured frequency spacing of 607 MHz between the longitudinal modes a maximum number of 24 modes can be calculated to oscillate within the bandwidth of the MO output. The residual pump power is used to amplify the Stokes wave in a subsequent Raman amplifier. At the MOPA output the amplified Stokes power is 4.3 W with a strongly broadened bandwidth of 328 pm. Thus our results show that even for a Stokes wave with only few longitudinal modes at the MO output, strong FWM induced spectral broadening occurs within the amplifier.

Fiber-Bragg-grating writing in highly nonlinear PM fibers for Raman fiber lasers

We describe the fabrication of fiber-Bragg-gratings (FBG) in highly nonlinear polarization maintaining (PM) fibers with tight constraints concerning their spectral bandwidth and maximum reflectivity. The experimental setup for the inscription process and writing results are presented.

Frequency tuning of microwave filter at 60 GHz by laser ablation

The transfer characteristic of microwave filters realized as microstrip circuits is depending on geometrical properties of the structured conducting layers as well as on the material properties of the used substrate. Due to variations of material properties and manufacturing tolerances a certain deviation of target to realized transfer function has to be tolerated. In this article a method for the subsequent tuning of filter characteristics using an excimer laser is presented. By laser ablation of the metal thin film layer, material removal with a minimum size in the range of 5 mm is achieved. Index Terms – microwave filter tuning, microstructuring, excimer laser ablation