First-order Bragg Gratings Research Articles

Femtosecond laser direct-writing of high quality first-order Bragg gratings with arbitrary complex apodization by phase modulation.

Femtosecond laser direct-writing is an attractive technique to fabricate fiber Bragg gratings and to achieve through-the-coating inscription. In this article, we report the direct inscription of high-quality first-order gratings in optical fiber, without the use of an index-matching medium. A new alignment technique based on the inscription of weak probe gratings is used to track the relative position between the focal spot and fiber core. A simple and flexible method to precisely control the position of each grating plane is also presented. With this method, periodic phase modulation of grating structures is achieved and used to inscribe arbitrary apodization and phase profiles. It is shown that a burst of multiple laser pulses used to inscribe each grating plane leads to a significant increase in the grating strength, while maintaining low insertion loss, critical for many applications.

Bragg gratings inscribed in selectively inflated photonic crystal fibers

We report a method for effective fabrication of Bragg gratings in all-silica photonic crystal fibers (PCF). The problem of cladding-hole scattering in PCF grating inscription is avoided by selectively inflating a section of PCF, resulting a locally suspended-core fiber (SCF) region with relatively simple cladding structure. Hence, the inscription laser can laterally access to the core region with little loss. In the SCF regions with core diameter ranging from 2 to 4.5 μm, first-order Bragg gratings are fabricated by use of a phase mask and a focused infrared femtosecond laser with pulse energy as low as ~200 μJ. For the same grating period, samples with different core sizes exhibit different resonant wavelengths and spectral properties, which would enable a range of applications in grating-integrated PCF sensors and devices.

High-Temperature Strain Sensing Using Sapphire Fibers With Inscribed First-Order Bragg Gratings

Strain sensor designs and strain measurements based on single-crystal sapphire fibers with inscribed first-order fiber Bragg gratings for applications up to 600 °C are presented. We report on all the details of two different sensor designs; for instance, we show that the resolution of multimode sapphire fiber Bragg grating (SFBG) strain sensors is about $\Delta{l/l}=10^{-5}$ (10 $\mu\text{strain}$ ), which is comparable with state-of-the-art high-temperature sensors. We apply our sensors for the determination of the thermal expansion coefficients of high-temperature steel alloys, showing a good match to known values. Hence, we believe that SFBG sensors may represent a promising alternative to currently used non-optic-based strain-detecting devices.

Inscription of strong Bragg gratings in pure silica photonic crystal fibers using UV femtosecond laser pulses

We demonstrate our experimental results on inscription of strong first-order Bragg gratings in hydrogen-free and hydrogen-loaded pure silica photonic crystal fibers (PCFs) using high-intensity 160 femtosecond (fs) UV laser pulses at 266nm and a two-beam Talbot interferometer. Bragg reflectivities up to 37dB for the H2-loaded fiber as well as Bragg wavelength tuning over the telecom C-band using a single phase mask were achieved. The in-situ nonlinear filamentation of UV fs laser pulses, scattered by the fiber’s micro-structured cladding, is demonstrated by direct refractive index profile measurement across the cross section of a written fiber. The results of thermal annealing measurements for a H2-loaded PCF are also presented.

Fiber Bragg grating inscription based on optical filamentation of UV femtosecond laser pulses

We present the fabrication and characterization of strong first-order Bragg gratings operating around 1550nm in standard telecom fibers (SMF-28) by use of 160fs UV laser pulses at 266nm and a Talbot interferometer. Bragg reflectivities exceeding 50dB are achieved in both hydrogen-free and hydrogen-loaded fibers. In addition, Bragg wavelength tuning over the telecom C-band using a single phase mask is demonstrated. We also show that the formation of Bragg gratings, relying on a two-photon absorption process, is intimately accompanied by the optical filamentation process of UV fs laser pulses during nonlinear propagation in the fiber.

Inscription of first-order sapphire Bragg gratings using 400 nm femtosecond laser radiation

The paper describes the implementation of fiber Bragg gratings inscribed by femtosecond laser pulses with a wavelength of 400 nm. The use of a Talbot interferometer for the inscription process makes multiplexing practicable. We demonstrate the functionality of a three-grating multiplexing sensor and the temperature stability up to 1200 °C for a single first-order Bragg grating.

Demonstration of a compact temperature sensor based on first-order Bragg grating in a tapered fiber probe

We experimentally demonstrate an all-silica first-order fiber Bragg grating (FBG) for high temperature sensing by focused ion beam (FIB) machining in a fiber probe tapered to a point. This 61-period FBG is compact (~36.6 μm long and ~6.5 μm in diameter) with 200-nm-deep shallow grooves. We have tested the sensor from room temperature to around 500 °C and it shows a temperature sensitivity of nearly 20 pm/°C near the resonant wavelength of 1550 nm. This kind of sensor takes up little space because of its unique geometry and small size and may be integrated in devices that work in harsh environment or for detecting small objects.

Back Cover: Thin film optimization design of organic–inorganic hybrids for waveguide high‐rejection optical filters (Phys. Status Solidi RRL 8/2011)

Carlos M. S. Vicente et al. (see pp. 280–282) have fabricated high-rejection optical filters based on first-order Bragg gratings which were inscribed in channel waveguides through UV-laser writing in transparent, low surface roughness (<0.1 nm), and low-loss (1.5 ± 0.2 dB cm–1, 532 nm) thin films of organic–inorganic hybrids with controlled refractive index values stable under aging. The high-rejection figure of merit (∼24 dB) of these optical filters is the best value found until now for organic–inorganic hybrids, reinforcing the potential of sol–gel technology in the fabrication of cost-effective integrated optics devices for the next generation of optical networks.

Small core rib waveguides with embedded gratings in As2Se3 glass

Low-loss shallow-rib waveguides were fabricated using As2Se3 chalcogenide glass and polyamide-imide polymer. Waveguides were patterned directly in the As2Se3 layer by photodarkening followed by selective wet etching. Theory predicted a modal effective area of 3.5-4 microm2, and this was supported by near-field modal measurements. The Fabry-Perot technique was used to estimate propagation losses as low as ~0.25 dB/cm. First-order Bragg gratings near 1550 nm were holographically patterned in some waveguides. The Bragg gratings exhibited an index modulation on the order of 0.004. They were used as a means to assess the modal effective indices of the waveguides. Small core As2Se3 waveguides with embedded Bragg gratings have potential for realization of all-optical Kerr effect devices.

Azimuthal mode discrimination in radially chirped concentric-circle-grating distributed feedback lasers

We report a theoretical investigation analyzing the threshold modes of a concentric-circle-grating (CCG) distributed feedback laser with a radially chirped first-order Bragg grating. A numerical coupled-mode analysis of a chirped CCG laser shows improved azimuthal mode discrimination for chirped gratings with linear, quadratic, and square-root radial dependence. Negatively chirped gratings, in which the grating period decreases with radius, result in improved threshold discrimination between the circularly symmetric fundamental mode and higher order modes; positively chirped gratings, in which the grating period increases with radius, result in decreased threshold for higher order modes in general. Also, the intensity for the fundamental mode at the grating center can be reduced by an order of magnitude using linearly chirped gratings.

Production of submicrometer period Bragg gratings in optical fibers using wavefront division with a biprism and an excimer laser source

Biprisms are used as a method of wavefront division with an excimer laser source to holographically write first-order Bragg gratings in single-mode optical fibers. Gratings with a period of 556 nm and peak reflectivity of 62% are produced using single pulses from a krypton fluoride (KrF) laser. A comparison is made between grating production with line-narrowed and nonline narrowed lasers.

Excimer laser writing of submicrometre period fibre Bragg gratings using phase-shifting mask projection

Submicrometre period, first-order Bragg gratings have been written in optical fibre, for the first time to the authors knowledge, by projecting a phase-shifting mask. Gratings with a peak reflectivity of 85% at a Bragg wavelength of 1530 nm were recorded using single pulses from a broadband KrF excimer laser. >

Spatial-phase-locked electron-beam lithography and x-ray lithography for fabricating first-order gratings on rib waveguides

The two basic structural elements of the integrated resonant channel-dropping filter are the rib waveguides and the first-order Bragg gratings. Spatial-phase-locked electron-beam lithography is used to write the Bragg grating patterns with a coherence better than λ0/150, and x-ray nanolithography is used to transfer the gratings onto optically patterned rib waveguides. These technologies are successfully combined to demonstrate a process by which channel-dropping filters and related devices can be fabricated.

8-wavelength DBR laser array fabricated with a single-step Bragg grating printing technique

An 8-wavelength distributed Bragg reflector (DBR) array for narrow channel wavelength division multiplexing (WDM) has been fabricated with a new technique for printing first-order Bragg gratings using a phase mask and a conventional incoherent source. All the distributed gratings were printed in a single photolithographic step with a slightly modified mask aligner. The DBR's excellent wavelength control for channels separated by as little as 0.8 nm is described. Many advanced photonic devices relying on gratings like quarter-wave shifted distributed feedback (DFB) lasers and wavelength division multiplexing (WDM) components can potentially be manufactured with this technique in a simple and cost-effective way. >

RIE etching of deep Bragg grating filters in GaInAsP/InP

An RIE (CH4/H2 process is presented for the production of up to 1 µm-deep first-order Bragg gratings with a grating constant of A = 0.23µm in InP and GaInAsP. A Bragg grating filter on a GaInAsP/InP rib waveguide showed a stopband width as large as 5nm at −10dB and a nearly rectangular shape of the filter function.