Flat-top Pulse Generation Research Articles

Generation of picosecond pulses with variable temporal profiles and linear polarization by coherent pulse stacking in a birefringent crystal shaper.

We report the study and demonstration of a new laser pulse shaping system capable of generating linearly polarized picosecond laser pulses with variable temporal profiles including symmetric intensity distributions, as well as non-symmetric distributions, which are highly desired by various applications. It is found that both high transmittance and high stability of the shaped pulse can be achieved simultaneously when crystals are set at a specific phase delay through fine control of the crystal temperature. Although multi-crystal pulse stacking with different configurations was reported before particularly for flattop pulse generation, this new configuration leads to new opportunities for many potential applications over a wide range of laser wavelengths, pulse repetition rate, time structures and power levels. A practical double-pass temporal shaping configuration that significantly reduces the number of crystals is also proposed in this paper as a result of present study.

Temporally flat top pulse generation from gain switched semiconductor lasers based on a polarization interferometer with variable transfer function

We propose the use of a polarization based interferometer with variable transfer function for the generation of temporally flat top pulses from gain switched single mode semiconductor lasers. The main advantage of the presented technique is its flexibility in terms of input pulse characteristics, as pulse duration, spectral bandwidth and operating wavelength. Theoretical predictions and experimental demonstrations are presented and the proposed technique is applied to two different semiconductor laser sources emitting in the 1550nm region. Flat top pulses are successfully obtained with input seed pulses with duration ranging from 40ps to 100ps.

Flattop pulse generation based on the combined action of active mode locking and nonlinear polarization rotation

A pulse-width-tunable 10GHz flattop pulse (FTP) train is generated based on the combined action of active mode locking and nonlinear polarization rotation pulse shaping. Although the setup was previously used for other applications, the mechanism of FTP generation based on it is first analyzed and confirmed in the experiment. An FTP with pulse width tunable from 12 to 20ps by changing polarization controllers is generated within the wavelength tuning range of 20nm. The generated pulse reveals good stability, with the side mode suppression ratio of 65dB, timing jitter of 92fs, and amplitude fluctuation of 0.36%.

光纤激光器中基于偏振控制器的脉宽可调平顶光脉冲生成技术

光纤激光器中基于偏振控制器的脉宽可调平顶光脉冲生成技术

In-Fiber Subpicosecond Pulse Shaping for Nonlinear Optical Telecommunication Data Processing at 640 Gbit/s

We review recent work on all-fiber (long-period fiber grating) devices for optical pulse shaping, particularly flat-top pulse generation, down to the subpicosecond range and their application for nonlinear switching (demultiplexing) of optical time-division multiplexed (OTDM) data signals in fiber-optic telecommunication links operating up to 640 Gbit/s. Experiments are presented demonstrating error-free 640-to-10 Gbit/s demultiplexing of the 64 tributary channels using the generated flat-top pulses for temporal gating in a Kerr-effect-based nonlinear optical loop mirror. The use of flat-top pulses has critical benefits in the demultiplexing process, including a significantly increased timing-jitter tolerance (up to ~500 fs, i.e., 30% of the bit period) and the associated improvement in the bit-error-rate performance (e.g., with a sensitivity increase of up to ~13 dB as compared with the use of Gaussian-like gating pulses). Long-period fiber grating pulse shapers with reduced polarization dependence are fabricated and successfully used for polarization-independent 640-to-10 Gbit/s demultiplexing experiments.

Flat-Top Pulse Generation by the Optical Fourier Transform Technique for Ultrahigh Speed Signal Processing

This paper reports on the generation of 1.6-ps full-width at half-maximum flat-top pulses by the optical Fourier transform technique, and the utilization of these pulses in a 320-Gb/s demultiplexing experiment. It is demonstrated how a narrow pulse having a 15-nm wide third-order super-Gaussian spectral intensity profile is mapped into a flat-top pulse resembling its spectrum by simple propagation in SMF. Theoretical and experimental descriptions are given on flat-top pulse generation, and an experimental validation of the systems performance of the pulses is carried out, demonstrating a benefit in terms of timing tolerance.

Design of periodic binary fiber gratings for single and multiple flat-top pulse generation

We demonstrate that a periodic binary fiber grating (PBFG) formed by a sequence of two alternating homogeneous layers can be designed to generate flat-top optical pulses by reflection. For this purpose, specific design criteria allow an exact PBFG reflectance that approaches a sinc function with linear phase. Our design provides large layer thicknesses (of around hundreds of microns) and it is robust under random defects in the structure, which represents an advantage from a technological viewpoint. Furthermore, the considered PBFG can be alternated with homogeneous fiber spans to generate precise sequences of flat-top pulses. Numerical examples of single and multiple flat-top pulse generation illustrate our results.

Flat-top pulse generation based on a fiber Bragg grating in transmission

We propose and analyze a flat-top pulse generator based on a fiber Bragg grating (FBG) in transmission. As is shown in the examples, a uniform period FBG properly designed can exhibit a spectral response in transmission close to sinc function (in amplitude and phase) in a certain bandwidth, because of the logarithm Hilbert transform relations, which can be used to reshape a Gaussian-like input pulse into a flat-top pulse.

Generation of flat-top picosecond pulses by coherent pulse stacking in a multicrystal birefringent filter

This paper deals with the pulse-shaping properties of birefringent filters that feature an optical layout similar to a Solc fan filter. A simple computational model is given that explains the pulse-shaping process in the fan filter in two steps: First, the input pulse is split into several mutually delayed replicas due to the birefringence of the crystals. Second, these replicas interfere at the output polarizer of the filter and form the shaped output pulse. Fine-tuning of the phases of the replicas of the input pulse is permitted by tuning the temperature of the crystals. A birefringent pulse shaper containing ten birefringent crystals was investigated experimentally. The shape of the output pulses was measured by means of a special cross-correlation technique. Although a variety of pulse shapes can be generated with the described filter, it is particularly well suited for generation of flattop pulses featuring a 20-ps-long plateau and rising and falling edges shorter than 2 ps.

Picosecond flat-top pulse generation by low-bandwidth electro-optic sinusoidal phase modulation

We report the first experimental demonstration to our knowledge of a microwave frequency upshifting system based on phase modulation. A sequence of flat-top optical and RF pulses at a repetition rate of 18.22 GHz, each with a FWHM time width of approximately 25 ps, is generated from a sinusoidal RF tone of only 3.680 GHz, in good agreement with our analytical and numerical calculations. A simple explanation of this technique based on Talbot effect theory is provided. The practical limitations and capabilities of the phase-modulation-based frequency upshifting approach for ultrabroadband RF waveform generation are also discussed.

Picosecond and sub-picosecond flat-top pulse generation using uniform long-period fiber gratings

We propose a novel linear filtering scheme based on ultrafast all-optical differentiation for re-shaping of ultrashort pulses generated from a mode-locked laser into flat-top pulses. The technique is demonstrated using simple all-fiber optical filters, more specifically uniform long period fiber gratings (LPGs) operated in transmission. The large bandwidth typical for these fiber filters allows scaling the technique to the sub-picosecond regime. In the experiments reported here, 600-fs and 1.8-ps Gaussian-like optical pulses (@ 1535 nm) have been re-shaped into 1-ps and 3.2-ps flat-top pulses, respectively, using a single 9-cm long uniform LPG.