Ultracompact Lens‐Less “Spectrometer in Fiber” Based on Chirped Filament‐Array Gratings

Abstract

Femtosecond laser irradiation is applied to a single-mode optical fiber to embed a filament array through the silica cladding and guiding core and form chirped Bragg gratings. Unlike a planar-shaped refractive index modification, the long and uniform filament facilitates efficient optical scattering into azimuthally narrowed radiation modes, external and transverse to the fiber cladding. Chirping of the grating period further provides spectral focusing. The combined spectral and azimuthal focusing permits lens-less recording of bright and high-resolution spectra spanning across most of the visible band with a low-cost charged coupled device camera. The flexible point-by-point writing enables fiber tapping of light with engineered spectral and geometric focusing properties, permitting the design of new compact photonic devices based on the all-fiber spectrometer.

Non-destructive technique for characterization fiber Bragg gratings via diffraction profile

“…We called them positive 𝑚 = 1 and negative 𝑚 = −1 parts of the diffraction order. This phenomenon links to the elongated ellipsoid form of individual grating points, as described in references 32,22,33,[5][6][7]34 . Both maxima are oriented perpendicularly to the long axis of the ellipsoidal.…”

“…In all these applications, the typical optical responses of the fiber Bragg grating are a reflection and transmission spectrum. However, with the growing diversity in applications of fiber Bragg grating in certain sensing [2][3][4] and spectrometric fields, [5][6][7][8][9] there is an increasing need for and importance of practical techniques involved in the manufacturing process providing the spatial information of the grating.…”

In‐Fiber Switchable Polarization Filter Based on Liquid Crystal Filled Hollow‐Filament Bragg Gratings

“…Relatively strong (≈15 dB) and narrow (≈1 nm at 3 dB) stopbands have aligned except for a small birefringence of the capillary shape that shifted the Bragg peaks by Δλ = 250 pm between the perpendicular (90°, blue line) and parallel (0°, red line) polarization states as defined with respect to the capillary axis (Figure 1). The filament "antenna" geometry enabled a relatively strong radiation and cladding mode coupling, [29,30] the latter contributing to an ≈8 dB loss extending to a short wavelength from the ≈1 nm Bragg resonance (Figure 2b). Otherwise, a smaller loss of ≈3 dB seen on the longer wavelength side of the Bragg resonance (1551-1560 nm) can be attributed to optical scattering from irregular patterning and nano-scale surface roughness in the capillary arrays.…”

“…In the case of focusing on cylindrical-shaped fiber, our group has harnessed surface aberration of glass plates to create long and uniform laser filaments that pierce the core waveguide. This method enabled low-contrast refractive index modification [29,30] or micro-explosion of nano-capillaries, which formed into fiber Bragg gratings (FBGs) when assembled in arrays. [31,32] The hollow-filament tracks provided high aspect ratio nanoholes with tunable hole diameters from 200 to 500 nm that extended fully through the core and cladding zones.…”

Compact fiber optic spectrometers formed by chirped diffraction gratings inscribed with a femtosecond laser platform

“…Indeed, focusing of femtosecond laser pulses deep into the fiber's core creates micro-voids surrounded by a shell of densified silica [3] that can form an FBG when disposed periodically along the optical fiber with point-by-point distances of the order of ~0.5-1 µm or more. In addition, such micro-voids can form arrays of sub-wavelength antennas [4] that exhibit wavelength dispersive properties of diffraction gratings. Mie scattering [5] can be further exploited by varying the periodicity of such gratings in order to focus a given input spectrum directly on the photodetector array, thus eliminating the need of any focusing optics.…”