Frequency Modulation To Amplitude Modulation Research Articles

Experimental method of transfer function linewidth determination for laser amplifiers using FM-to-AM conversion.

We propose an experimental method for the determination of the transfer function linewidth of a laser amplifier. This technique is based on a transfer function scan using frequency modulation to amplitude modulation (FM-to-AM) temporal modulation measurement as a function of wavelength. Using this method, we show that the output spectrum of a laser amplifier in Q-switch mode is not representative of the transfer function of the amplifier.

Interference filters and their impact on FM-to-AM conversion: a method for investigation and characterization.

High-power nanosecond laser pulses are usually spectrally broadened via temporal phase modulations to tackle the issue of stimulated Brillouin scattering and to achieve optical smoothing of the focal spot. While propagating along the beamline, such pulses can undergo frequency modulation to amplitude modulation (FM-to-AM) conversion. This phenomenon induces modulations of the optical power that can have a strong impact on laser performance. Interference filters are specific FM-to-AM conversion contributors that lead to high-frequency modulations that cannot be measured using conventional means. We propose an indirect method to investigate for such FM-to-AM contributors using spectral measurements. Further analysis of the collected data makes the quantification of the defining parameters of interference filters possible. In turn, we show that it is possible to estimate the range of power modulations induced by interference filters.

Two-Dimensional Tunable and Temperature-Insensitive Lyot Filter for FM-to-AM Compensation

Utilizing polarization maintaining photonic crystal fiber (PM-PCF) with the low temperature coefficient of birefringence, a two-dimensional tunable and temperature-insensitive Lyot filter aiming to compensate the frequency modulation to amplitude modulation (FM-to-AM) conversion in high power laser facility is demonstrated. The Jones matrix is applied to analyze the relationship between optical characteristics of the filter and physical parameters (including amplitude ratio, phase delay, and susceptibility of the birefringence to temperature) of the polarization optical field. Both the transmission peak wavelength and extinction ratio of the spectral transmission are able to be changed simultaneously, hence, it shows more efficient FM-to-AM compensation ability. Besides, the transmission peak shift is about 18pm/°C with the PM-PCF configuration, which is about two orders of magnitude less than the normal polarization maintaining fiber (PMF) configuration. The demonstrated filter presents a practical application potential in large scale laser driven facility.

Nonlinear FM-AM conversion due to stimulated Brillouin scattering.

We report an effect potentially harmful occurring in regenerative amplifiers due to stimulated Brillouin scattering (SBS). Most high energy laser facilities use phase-modulated pulses to prevent the transverse SBS effect in large optical components and to smooth the focal spot on target. However, this kind of pulse format may undergo a detrimental effect known as frequency modulation to amplitude modulation (FM-AM) conversion in the presence of spectral distortions. In the present letter, we show experimentally and numerically, that SBS can also potentially be created in the regenerative amplifier located in the front-end. In this scenario, some of the side bands of the pulse reflected by regen end-cavity mirror may act as a seed for SBS in an optical component, if the pulse spectrum contains frequency components exactly separated by the Brillouin frequency shift. This self-seeded SBS induces amplitude modulation with a nonlinear dependence that may be detrimental during down-stream propagation. However, we show that a careful choice of the modulation frequencies can mitigate this effect.

Suppression of FM-to-AM conversion in a broadband Nd:glass regenerative amplifier with an intracavity birefringent filter.

In order to suppress the frequency modulation to amplitude modulation (FM-to-AM) conversion induced by a Nd:glass regenerative laser amplifier, a birefringent quartz crystal was designed by theoretical simulation and inserted into the regenerative amplifier. A broadband Nd:glass regenerative laser amplifier with a flattop region of ∼3.7 nm in the gain spectrum was developed. An all-fiber front-end system was used to generate a 2 ns shaped pulse with a spectral width of ∼0.5 nm, which was broadened by a two-stage phase modulator. Then, the laser pulse was amplified by a spectral-broadened regenerative amplifier. The amplified output temporal waveform modulation depth was reduced from a maximum of 47.7% to 17.3% at a center wavelength of 1053.328 nm. The experimental results show that the FM-to-AM conversion can be effectively suppressed in the spectral-broadened Nd:glass regenerative laser amplifier, which indicates that it has potential application as a preamplifier of large-scale laser facilities.

Analysis on FM-to-AM conversion of SSD beam induced by etalon effect in a high-power laser system

FM-to-AM (frequency modulation-to-amplitude modulation) conversion caused by nonuniform spectral transmission of broadband beam is harmful to high-power laser facility. Smoothing by spectral dispersion (SSD) beam is a special broadband beam for its monochromatic feature at the given time and space on the near field. The traditional method which uses the optical spectral transfer function as filters cannot accurately describe its AM characteristics. This paper presents the theoretical analysis of the etalon effect for SSD beam. With a low-order approximation, the analytic model of the temporal shape of SSD beam is obtained for the first time, which gives the detailed AM characteristics at local and integral aspects, such as the variation of ripples width and amplitude in general situation. We also analyze the FM-to-AM conversion on the focal plane; in the focusing process, the lens simply acts as an integrator to smooth the AM of SSD beam. Because AM control is necessary for the near field to avoid optics damage and for the far field to ensure an optimal interaction of laser–target, our investigations could provide some important phenomena and rules for pulse shape control.

Random and pseudo-random phase modulations for FM-to-AM reduction in high power lasers

Frequency modulation to amplitude modulation (FM-to-AM) conversion is an important issue that can prevent fusion ignition with high power lasers such as with the Laser Megajoule (LMJ). A way to reduce FM-to-AM conversion is to change the phase modulation (currently sinusoidal). In this paper, we study the case of random phase modulation. We show, with numerical simulation modeling, FM-to-AM conversion induced by different transfer functions, in which such a modulation format, in some cases, may significantly reduce FM-to-AM conversion, particularly in the case of amplitude filtering. Phase filtering may create high overshoots, but the occurence probability is low, and, on average, random phase modulation is advantageous compared with sinusoidal modulation. We also demonstrate an equivalent smoothing efficiency with the LMJ facility with an important reduction of FM-to-AM conversion compared with the current solution. Taking particular random draws (pseudo-random modulation), we exhibit optimal performance.

Suppression of amplitude modulation induced by polarization mode dispersion using a multi-degree-of-freedom fiber filter

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.

A fiber-based polarization–rotation filter utilized to suppress the FM-to-AM effect in a large-scale laser facility

The frequency modulation to amplitude modulation (FM-to-AM) effect is one of the few remaining scientific issues in a large-scale laser facility which could prevent fusion ignition. A fiber-based polarization–rotation filter which consists of a fiber polarizer, a section of polarization-maintaining fiber, a half-wave plate and a polarization beam splitter is proposed to suppress the FM-to-AM effect. Theoretical analysis and experimental demonstration in the preamplifier module of the SG-III laser facility prove the effectiveness of this filter.

Suppression of FM-to-AM Conversion in Broadband Third-Harmonic Generation of Nd:Glass Laser

Frequency modulation to amplitude modulation (FM-to-AM) conversion is important for controlling temporal shapes of high-power Nd:glass lasers in the application of fusion ignition. To suppress this FM-to-AM conversion effect in the process of broadband third-harmonic generation of Nd:glass laser, we report an efficient frequency-tripling scheme based on mixing narrowband and broadband pulses. Numerical results show that the FM-to-AM conversion in frequency conversion process can be suppressed effectively. For a required third-harmonic bandwidth of 1 THz, the defined relative modulation α in pulse intensity is as large as 180% in the conventional baseline design, while it will be reduced to only about 20% by using our proposed scheme.

Weak etalon effect in wave plates can introduce significant FM-to-AM modulations in complex laser systems

The conversion of frequency modulation to amplitude modulation (FM-to-AM) effect is harmful to the high power laser facility based on the phase modulation technique. The FM-to-AM effect of phase modulation pulse induced by the weak etalon effect in wave plates was investigated theoretically and experimentally. A bulk phase modulator with a modulation frequency of 9.2GHz was employed. The numerical simulation results show that the FM-to-AM effect with a temporal modulation depth of 2.5% and 29.7% on the top of the pulse shape was induced by the weak etalon effect in half-wave plate with thickness of 1mm and residual reflectance ratio of 0.5% for 1 pass and 12 passes respectively. On the same condition, the temporal modulation depth is 3.0% and 23.4% respectively in the experiment. The results are in good agreement with numerical simulation results. To our knowledge, it is the first time to introduce the weak etalon effect in wave plates for a complex phase modulation laser system.

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).