Denis Penninckx

Compensation of frequency modulation to amplitude modulation conversion in frequency conversion systems

Frequency modulation to amplitude modulation (FM-to-AM) conversion is an important issue that can prevent fusion ignition with high power lasers such as the Laser MegaJoule (LMJ). On LMJ, most of the FM-to-AM conversion is expected in the so-called frequency conversion and focusing system, which is a nonlinear system. However, we propose linear transfer functions to compensate the effect of frequency conversion on FM-to-AM conversion. We show that most of AM distortion can be reduced by practical systems: for beam intensity up to 3 GW/cm(2), the FM-to-AM conversion level can be divided by at least 2, and we almost cancel intensity modulation for intensities below 1 GW/cm(2).

Definition, meaning, and measurement of the polarization extinction ratio of fiber-based devices

We clarify the definition of the polarization extinction ratio-also called polarization cross talk--of fiber-based devices. Its strong wavelength dependence, even for simple devices such as a single-fiber patchcord, is highlighted. Thus white sources may not be used for most measurements. We also explain the weakness of measurements with a rotating polarizer and a monochromatic source. Only a polarimeter may be used if accuracy is required. We report measurements of connections (including two connectors and one mating sleeve) and show the importance of the mating sleeve in the result. Finally, we define the validity domain of the standardized method, which uses both a white source and a rotating polarizer.

Axis alternation for signal propagation over polarization-maintaining fibers

When a signal is split over both axes of polarization-maintaining fibers (PMFs), alternation of axes is well known to cancel the impact of the differential group delay, e.g., in filtering functions such as modulators based on Kerr effect and a Sagnac loop. This solution seems not of concern if the signal remains on one axis as is the case for polarization-maintained signal transportation. As opposed to the common belief, we show experimentally and numerically that alternating the slow and the fast axes is also important to avoid signal distortions in that case. As a matter of fact, strong distortions of polarized signals are observed when the signals are transported with PMF because of the finite extinction ratio of connectors.

Distortion cancellation of frequency converted pulses with simple linear signal processing and application to frequency modulation to amplitude modulation conversion in high power lasers

It is known that a linear filter may be easily compensated with its inverse transfer function. However, it was shown that this approach could also be valid even for such a complex nonlinear system as frequency conversion. As a matter of fact, it is possible to at least partly precompensate for distortions occurring within, or even downstream from, frequency conversion crystals with a simple linear optical filter set upstream. In this paper, we give the theoretical background and derive the optimum precompensation filter from simple analytical formulas even in the case of saturation. We first show the relevance of our approach for Gaussian pulses: the pulse may be short or not and chirped or not, and the same linear precompensation filter may be used as long as saturation is not reached. We then study the case of phase-modulated pulses, as can be found on high power lasers such as lasers for fusion. We show that previous experimental results are in perfect agreement with these calculations. Finally, justified by our simple analytical formulas, we present a rigorous parametrical study giving the distortion reduction for any second and third harmonic generation system in the case of phase-modulated pulses.

Paradox in the measurement of FM to AM conversion in high power lasers

High power lasers such as the Laser MegaJoule (LMJ) under construction near Bordeaux (France) require phase modulations of the pulses in order to smooth the beam and to avoid stimulated Brillouin scattering in large optics which may cause damages. For anti-Brillouin, the phase modulation chosen for the LMJ is a sine-wave at a frequency of 2 GHz and a modulation index of 7 inducing a spectral broadening of 32 GHz. For smoothing, the broadening is larger because the phase modulation is a 14.25 GHz sine-wave having a modulation index of 5. Anti-Brillouin is always activated while smoothing may be activated or not (see the optical spectral power distributions (SPD).

Numerical optimization of large-scale fiber-optic splitting tree uniformity

In order to split the optical power of a signal into many fibered outputs, large-scale optical splitting trees (splitters) are used. In most applications, the optical power should remain constant from output to output. Large-scale splitting trees (typically 1 : 32 or 1 : 128) may be composed of a single-stage or of cascaded small splitters (typically 1 : 2). In the former case, power uniformity is determined by the design and manufacturing tolerances. In the latter case, considered here, uniformity may be optimized by a proper choice of the positions of the small splitters in the tree. Simulations show that less than 1-dB uniformity may be reached.

Laser damage measurement of thick silica plates using a new laser injection scheme

Some silica plates of high power nanosecond lasers may be a few centimeter thick for instance because they should sustain vacuum. Measuring laser-induced damage thresholds at the output surface of these thick silica plates is a complex task because non-linear laser propagation effects may occur inside the plate which prevents knowing accurately the fluence at the output. Two non-linear effects have to be considered: stimulated Brillouin scattering (SBS) and Kerr effect. SBS is mainly driven by the spectral power density of the pulses: if the spectral power density is below a threshold, SBS is negligible. Thus, spectral broadening is required. Kerr effect depends on the instantaneous intensity. Hence, a smooth temporal shape without overshoots is required. However, both conditions (wide spectrum and no overshoots) are impossible to fulfill with standard lasers. As a matter of fact, an injected laser has a smooth temporal profile but is spectrally narrow. Without injection, the laser is multimode yielding a wide spectrum but a chaotic temporal profile. We solved the problem by phase-modulating a continuous-wave seeder of our laser (patent pending). The phasemodulation frequency is adjusted to a multiple of the inverse of the round-trip time of the laser cavity. The laser pulses have a wide spectrum to suppress SBS and do not exhibit temporal overshoots to reduce Kerr effects. During the presentation, we will show the features of the laser pulses and laser-induced damage measurements of thick silica plates using this scheme.

Experimental measurements of frequency transfer function due to smoothing by spectral dispersion

In order to avoid propagation nonlinearities (Kerr effect, Raman and Brillouin scattering) and optical damage, nanosecond high power lasers such as the Laser MegaJoule (LMJ) amplify quasi-monochromatic pulses. But they generate a static speckle pattern in the focal spot. This speckle pattern needs to be smoothed in order to lower high intensity peaks which are detrimental during the propagation and the interaction with the plasma in the target. Different techniques are implemented to smooth the intensity nevertheless all high power lasers carry at least smoothing by spectral dispersion. It consists in broadening the spectrum through a phase modulator and focusing the different wavelengths at slightly different positions using a diffractive element such as a grating. In the temporal domain, it has been theoretically shown that the pulse power is thus filtered between near field and far field [1, 2]. The filtering allows techniques such as “picket fence” to increase conversion efficiency [1] and reduces detrimental effects of unwanted intensity distortions called FM-AM conversion [2, 3]. Here, to the best of our knowledge we show the first experimental measurement of the frequency transfer function of this filtering. Measurements are in perfect agreement with the numerical calculations.

Linear precompensation of phase-modulated nanosecond pulse distortions in second-harmonic generation

The first experimental demonstration of linear precompensation of a non-linear transfer function due to frequency conversion is reported. We show the effective precompensation with an interferometric filter of phase-to-amplitude modulations conversion due to second-harmonic generation.