Abstract
Polarization mode dispersion (PMD) in fibers for high-power lasers can induce significant frequency modulation to amplitude modulation (FM-to-AM) conversion. However, existing techniques are not sufficiently flexible to achieve efficient compensation for such FM-to-AM conversion. By analyzing the nonuniform transmission spectrum caused by PMD, we found that the large-scale envelope of the transmission spectrum has more serious impacts on the amount of AM. In order to suppress the PMD-induced FM-to-AM conversion, we propose a novel tunable spectral filter with multiple degrees of freedom based on a half-wave plate, a nematic liquid crystal, and an axis-rotated polarization-maintaining fiber. Peak wavelength, free spectral range (FSR), and modulation depth of the filter are decoupled and can be controlled independently, which is verified through both simulations and experiments. The filter is utilized to compensate for the PMD-induced FM-to-AM conversion in the front end of a high-power laser facility. The results indicate that, for a pulse with phase-modulation frequency of 22.82 GHz, the FM-to-AM conversion could be reduced from 18% to 3.2% within a short time and maintained below 6.5% for 3 h. The proposed filter is also promising for other applications that require flexible spectral control such as high-speed channel selection in optical communication networks.
Highlights
In high-power laser facilities designed for inertial confinement fusion (ICF), temporal shaping of a single longitude mode laser is utilized in the front end in combination with an intensity modulator to obtain a smooth pulse envelope in the time domain
The line width of a single longitude mode laser is usually narrow, which may cause transverse Brillouin scattering in large-aperture optical elements and make the smoothing of the speckle focusing on the target challenging
The results show that the FM-to-AM conversion caused by Polarization mode dispersion (PMD) can be reduced from 18% to 3.2% at a modulation frequency of 22.82 GHz
Summary
In high-power laser facilities designed for inertial confinement fusion (ICF), temporal shaping of a single longitude mode laser is utilized in the front end in combination with an intensity modulator to obtain a smooth pulse envelope in the time domain. The result shows that the large-scale envelope is the main factor that induces the strong AM It can be compensated by using a linear spectrum filter. Considering that the PMD-induced FMto-AM conversion is time-varying, a positive, closed-loop control is implemented for the proposed filter to achieve a fast and efficient real-time suppression of the FM-to-AM conversion in the PM fiber front end. It is believed to be the first time to use the spectrum tracking method to suppress the PMD-induced FM-to-AM conversion of a complex fiber system and achieve an effective long-term compensation. The results show that the FM-to-AM conversion caused by PMD can be reduced from 18% to 3.2% at a modulation frequency of 22.82 GHz. The modulation of below 6.5% for 3 h indicates the outstanding adjustability and environmental suitability of the filter. It has a great potential application in many engineering areas
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