A simple method for the simultaneous inscription of two spectrally separated fiber Bragg gratings (FBGs) with a femtosecond laser at the same spatial spot is presented. The inscription setup consists of the following elements, i.e., an amplified near-IR femtosecond laser, two identical Phase-Masks (PM), two cylindrical focusing lenses, a negative defocusing spherical lens and a 50%:50% beam splitter. The inscription beam is divided into two equal beams each containing ~50% of the energy by means of a beam splitter. Each beam is then focused on the same spatial spot of the optical fiber through a 2140 nm period PM and a cylindrical lens. One beam is focused from one side, while the other beam is focused on the same spot, but from the opposite side. This ensures the simultaneous inscription of two FBGs at the same spatial spot. The wavelength separation is achieved by defocusing one of the beams with a negative spherical lens. The transmission and reflection spectrum of the two FBGs are measured. The center Bragg wavelength of the shorter FBG is ~1548.6 nm, and the center Bragg wavelength of the longer FBG is ~1553.9 nm or ~1550.4 nm when using a defocusing lens of −400 mm or −1000 mm, respectively, in the second inscription beam path. The measured transmission dip of all FBGs is greater than −4 dB.
An array of four fiber Bragg gratings (FBGs) is inscribed at the same spot with a single uniform phase-mask (PM). The inscription setup consists of an 800 nm femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. The wavelength tunability of the center Bragg wavelength is achieved by a defocusing lens, and by PM translation. An FBG is inscribed, followed by three cascaded FBGs, which are inscribed exactly at the same spot, only after a movement of the PM. The second order Bragg grating array at ~1.55 µm transmission spectra of these four FBGs is measured, showing a spectral shift of ~1.8 nm between each one, and a total spectral shift of ~5.4 nm. The transmission dip of each FBG is approximately −6 dB. The third order Bragg grating array at ~1.04 µm transmission spectra shows a transmission dip of approximately −3 dB, a wavelength separation of ~1.2 nm between each one, and a total wavelength shift of ~3.6 nm.
Two slightly shifted wavelength gratings are inscribed one over the other in a single mode fiber by shifting the phasemask between two positions. The inscription setup includes a NIR femtosecond laser, a phase-mask, a defocusing spherical lens and a cylindrical focusing lens. A first fiber Bragg grating (FBG) is inscribed, while the second overlapping FBG is inscribed only after a slight shift of the phase-mask, enabling a slight wavelength shift. The transmission spectrum of the complex structure is like that of a phase shifted grating, while the inscription process is very fast and simple compared to other standard methods. A high-quality phase shifted grating with two −20 dB transmission dips, a 15 dB transmission peak with a 30 pm transmission bandwidth at 3 dB is achieved. It is also observed that this phase shifted grating structure is birefringent.
Night-vision systems in vehicles are a new emerging technology. A crucial problem in active (illumination-based)
systems is distortion of images by saturation and blooming, due to strong retro-reflections from road signs. In this
work we quantified this phenomenon. We measured the Mueller matrices and the polarization state of the reflected light
from three different types of road signs commonly used. Measurements of the reflected intensity were taken also with
respect to the angle of reflection. We found that different types of signs have different reflection properties. It is
concluded from our measurements that the optimal solution for attenuating the retro-reflected intensity is using a linear
horizontal polarized light source and a linear vertical polarizer. Unfortunately, while the performance of this solution is
good for two types of road signs, it is less efficient for the third sign type.
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