Uniform Bragg Gratings Research Articles

Ultra-Short Fiber Bragg Grating Used for Spectral Analysis of Guided Light in Single-Mode Fibers

An ultra-short fiber Bragg grating with a grating length of 0.2 mm and constant grating period (uniform FBG) is proposed as an integrated dispersive element for spectral analysis in a single-mode glass fiber. This dispersive element is used to set up a fiber optical spectrometer that demonstrated an average spectral resolving power of 825, a pixel resolution of 0.02 nm/pixel, and a 40 nm bandwidth covering the 810-850 nm wavelength range. Furthermore, the dependence of this dispersive element on light polarization was examined, which induces a reduction in the wavelength accuracy up to ±0.05 nm. Finally, the investigated spectrometer to function well as a part of a readout device for fiber Bragg grating sensors is presented. Short uniform Bragg gratings as dispersive elements allow the manufacturing of miniaturized, compact spectrometers without additional focusing components between the FBG and detector.

Femtosecond Laser Modification of Silica Optical Waveguides for Potential Bragg Gratings Sensing.

The optimum femtosecond laser direct writing of Bragg gratings on silica optical waveguides has been investigated. The silica waveguide has a 6.5 × 6.5 µm2 cross-sectional profile with a 20-µm-thick silicon dioxide cladding layer. Compared with conventional grating inscribed on fiber platforms, the silica planar waveguide circuit can realize a stable performance as well as a high-efficiency coupling with the fiber. A thin waveguide cladding layer also facilitates laser focusing with an improved spherical aberration. Different from the circular fiber core matching with the Gaussian beam profile, a 1030-nm, 400-fs, and 190-nJ laser is optimized to focus on the top surface of the square silica waveguide, and the 3rd-order Bragg gratings are inscribed successfully. A 1.5-mm long uniform Bragg gratings structure with a reflectivity of 90% at a 1548.36-nm wavelength can be obtained. Cascaded Bragg gratings with different periods are also inscribed in the planar waveguide. Different reflection wavelengths can be realized, which shows great potential for wavelength multiplexing-related applications such as optical communications or sensing.

Advancing the sensitivity of integrated epoxy-based Bragg grating refractometry by high-index nanolayers.

In this Letter, we report on significantly improved surrounding RI sensitivity of epoxy polymer waveguide Bragg grating sensors. Uniform Bragg gratings were generated inside flat rectangular epoxy waveguides near the cutoff regime using standard phase mask excimer laser writing. Thickness controlled nanolayers of high-index titanium dioxide were deposited homogeneously on the waveguide sensor's surface area by repeated reactive sputter processing. Maximum Bragg wavelength shifts as high as 74.22 nm, as well as maximum sensitivities around 523 nm/RI unit corresponding to a minimum RI resolution of 1.9⋅10-6, could be obtained by employing a ∼75nm thick titanium dioxide coating.

On-chip optical narrowband reflector based on anti-symmetric Bragg grating.

We propose an on-chip optical narrowband reflector (NBR) based on two cascaded Bragg gratings (BGs). A π phase shifted anti-symmetric Bragg grating (π-PS-ASBG) and a rear uniform Bragg grating (UBG), are in-line connected. The π-PS-ASBG provides a hybrid mode resonance between the even- and odd TE (TE0 and TE1) modes, while the UBG is used as a rear reflector to reflect the TE0 mode that transmitted from the π-PS-ASBG. Different from traditional UBG, the reflection bandwidth decreases when the coupling coefficient increases. The calculated 3-dB bandwidth is 0.16 nm when the whole grating length is 400 µm. The proposed NBR can be applied in the cases requiring narrow reflection such as narrow linewidth semiconductor lasers.

A Hybrid Transverse Mode Resonance DFB Semiconductor Laser

We propose and theoretically analyze a hybrid transverse mode resonance DFB semiconductor laser which consists of one active section with π -phase shifted anti-symmetric Bragg grating ( π -ASBG) and two passive sections with uniform Bragg grating. The hybrid light resonance along with mode conversion between the fundamental transverse electric (TE0) mode and first order transverse electric (TE1) mode is formed by the π-ASBG. The emitted light from one facet with the TE0 or TE1 mode can be controlled by tuning the injection currents of the passive sections. As an example, a laser with ridge waveguide is designed and simulated. The transverse mode with the same wavelength is tuned by changing the injection currents of the passive sections. The suppression ratio between the two transverse modes reaches 18.4 dB and 18.7 dB when the TE0 and TE1 modes are emitted, respectively. A simple method for equivalently realizing the grating in the proposed laser was also analyzed. Therefore, as the transverse mode tunable light source, the proposed laser may benefit the future mode division multiplexing (MDM) systems for reconfigurability and more flexibility.

Design of chirped-coupling sidewall Bragg gratings for narrow linewidth distributed feedback lasers.

A chirped-coupling sidewall Bragg grating is proposed to mitigate the nonlinear effects that deteriorate the linewidth in DFB lasers operating at a high power and current regime. This novel grating geometry is analyzed and simulated to extract the optimal design parameters in terms of intra-cavity field distribution. Compared to lasers with uniform Bragg gratings, devices fabricated with a chirped-coupling grating are shown to operate in stable single mode operation over a wider current and power range and exhibit linewidth as narrow as 100kHz.

Femtosecond Laser Written Plane-by-Plane Bragg Grating Sensors in Bioresorbable Phosphate Optical Fibres

We report on the realization of different types of optical sensors in a bioresorbable phosphate glass optical fiber using a femtosecond laser operating at 517 nm. We inscribed fiber Bragg grating-based optical filters such as uniform Bragg gratings, chirped gratings, and Fabry–Perot cavities, using the plane-by-plane direct write inscription. The gratings were characterized in reflection and tested as sensors for temperature and relative humidity. We studied the stability and resilience of the gratings when inserted in high humidity environments (>95%) for 52 h and observed the evolution of the grating characteristics during this period.

A Chirp-Managed Tunable DBR Laser Based on Novel Cascaded Gratings

In this paper, a novel tunable DBR laser based on novel cascaded gratings is proposed for chirp management in high-speed direct modulation applications. The cascaded gratings, i.e., a tilted π phase-shift grating followed by a normal uniform Bragg grating, can provide a narrow-band reflection spectrum with a sharp falling slope on the longer wavelength side, so that the optical spectrum shaping and the detuned-loading effects can be enhanced. Simulation results indicate that the wavelength tuning range of the laser is about 10 nm and the 3-dB modulation bandwidth reaches up to 25 GHz. By simply adjusting the phase current and thus changing the detuned frequency, the extinction ratio and the frequency chirp can be optimized, which is helpful to improve the dispersion tolerance and reduce the dispersion power penalty.

Mathematical Model for Diagnosing Strains by an Optical Fiber Sensor with a Distributed Bragg Grating According to the Solution of a Fredholm Integral Equation

A mathematical model for the diagnostics of strain distribution along a optical fiber sensor (light guide) with a base distributed weakly reflecting fiber Bragg grating is developed. In the initial, undeformed state, the base Bragg grating has a constant period along the light guide. The base uniform Bragg grating is transformed to a nonuniform one as a result of emergence of nonuniform axial strains along the light guide. These changes in the period of Bragg grating lead to corresponding informative changes in reflection coefficients for various lengths of the waves entering into the optical fiber. As a result, the problem on finding the required density distribution function of axial strains along the optical fiber is reduced to solution of the Fredholm integral equation of the 1st kind by using the measured values of derivatives of reflection coefficients at the exit of the light guide. The results of numerical modeling of the spectra of coefficients of reflection from the nonuniform deformed Bragg grating are presented for various model and real distribution laws of diagnosed strains along the light guide.

Electron beam lithography on uneven resist for uniform Bragg gratings in GaSb based distributed-feedback lasers

Nanofabrication of metallic Bragg gratings vertically connected to the AlGaAsSb ridges for distributed-feedback (DFB) lasers based on AlGaAsSb/InGaAsSb was successfully conducted in this work. Owing to the densely distributed 4-μm wide ridges on the surface, electron beam lithography on the spin-coated PMMA suffered from severely uneven thickness of the resist, significantly deteriorating the grating quality. To overcome this difficulty, a special e-beam exposure strategy was developed by optimizing the dose distribution on the non-flat PMMA to achieve uniform Bragg gratings throughout the whole wafer surface. Single mode operation at room temperature of the fabricated DFB lasers was achieved at the designed lasing wavelength around 1973 nm, indicating the high quality of the gratings generated on extremely uneven PMMA. This exposure method provided us with a novel approach to pattern uniform nanostructures on the resist with uneven thickness as a whole.

Bragg grating inscription in PMMA optical fibers using 400-nm femtosecond pulses

In this Letter, we report the fast growth of high quality uniform Bragg gratings in trans-4-stilbenemethanol (TS)-doped poly(methyl methacrylate) (PMMA) step-index optical fibers. Grating manufacturing was obtained using a 400nm femtosecond pulsed laser and a 1060-nm-period uniform phase mask. For 20mW mean laser beam power, the grating reflectivity reaches 98% in ∼60 s.

Reflection and Transmission Spectra of Multimode Interference Devices With Bragg Gratings

We numerically investigate the reflection and transmission spectra of multimode interference (MMI) devices incorporating uniform and sampled Bragg gratings with emphasis on their applications to an add-drop multiplexer (ADM). In numerical analysis, a rectangular index modulation is assumed as the grating and a 2×2 MMI device is treated to operate as a nearly two-mode interference (TMI) device, which is frequently used as an alternative to conventional directional couplers that have a disadvantage of large size. Although the first two guided modes are used in the TMI device, it is especially important to overlap their reflection bands. Therefore, a design method for attaining a sufficient overlap and some considerations are given. The spectral response of the TMI device with a uniform Bragg grating is analyzed by a combination of the mode-propagation method and the transfer-matrix method and discussed on the basis of the state of overlap. In the case of a sampled Bragg grating, it is found that although many modes are excited in the MMI section, multiple reflection bands (multi-channel) are created as expected. The treated device can be applied to single- and multi-channel ADMs.

Analysis of the Possibilities for Using a Uniform Bragg Grating in a Tunable Dispersion Compensator

Abstract The article presents a tunable fibre optic dispersion compensator system, consisting of a specially designed cantilever beam and a uniform Bragg grating. It analyses the group delay and dispersion characteristics in the case that there is no apodization of the grating and also for a grating with apodization used for modulation of the refractive index. Various apodization parameters were tested, along with their effects on the dispersion characteristics of the entire system properties. It is demonstrated in the paper that the apodization parameter affects the compensator’s group delay characteristic. The finite elements method was used to design a compensator of such a shape that enabled chirp to be induced in a grating of a specified shape. A new design is presented for the system, in which the dispersion properties are tuned by the maximum value of the heterogeneous deformation of the compensator. The paper also includes results showing the effect of the maximum value of heterogeneous stress of the grating on the dispersion characteristics of the proposed construction.

Fabrication of a third-order polymer chirped waveguide Bragg grating with tapered core size by contact lithography

A polymer chirped waveguide Bragg grating (CWBG) that uses uniform grating structures on a tapered waveguide core is designed and demonstrated. Third-order uniform grating structures were formed in the bottom cladding by inexpensive contact lithography and then a channel waveguide with tapered core size was fabricated on top of it to realize the CWBG. The reflection spectrum of the fabricated polymer CWBG was measured, and the 3 dB bandwidth is 2.9 nm, which is about 6.5 times the 3 dB bandwidth (0.45 nm) of the fabricated uniform waveguide Bragg grating. In addition, the measured reflectivity is larger than 16 dB. The reflection spectrum of the fabricated CWBG was numerically simulated, which agreed well with the experimental result.

Tunable nanophotonic delay lines using linearly chirped contradirectional couplers with uniform Bragg gratings

We demonstrate an integrated tunable optical delay line in grating-assisted contradirectional couplers using a CMOS-compatible photonic technology. The input signal is delayed through dispersive Bragg gratings and distributedly coupled to the drop port of the coupler without backreflections. This add-drop design enables monolithic integration of grating-based delay lines without using optical circulators. The gratings are formed by slab perturbations in rib waveguides, with the index chirping realized by linearly tapering the rib widths. Both the pitch and size of the gratings are constant through the entire coupler, for a higher tolerance to fabrication errors. Continuous tuning of the optical group delay of up to 96 ps has been obtained, with a low insertion loss of less than 2 dB and a negative chromatic dispersion of -11 ps/nm that allows for bit rates of up to almost 100 Gb/s at the maximal delay. The device has a small footprint of 0.015 mm2, and can be used for on-chip optical buffering, dispersion compensation, and pulse compression.

Acoustic Response of a Sinusoidally Perturbed Hard-Walled Duct

Acoustic wave propagation in hard-walled ducts is of interest in many fields including vehicle design, musical instruments acoustics, and architectural and environmental noise-control. For the case of small sinusoidal perturbation of the cross-section, it is possible to derive simple though approximate analytical formulas of its plane wave acoustic reflection and transmission spectral response that resembles the optical situation of uniform Bragg gratings. The proof is given here, starting from the “horn equation” and then exploiting the coupled-modes theory. Examples of the results obtained with these analytical formulas are shown for some sinusoidally perturbed ducts and compared to results obtained through a numerical method, revealing a very good agreement.

Generation of a Zone Chirp in Uniform Bragg Grating is a Way of Obtaining Double Functionality of a Sensor

Abstract This paper presents a method of using a sensor with uniform Bragg grating with appropriately generated zone chirp. The presented method can be used for measuring two physical quantities, namely strain and temperature. By providing the same temperature sensitivity and different sensitivity to strain of two parts of a sensor, and experimental measurement of qualities of the proposed system and its calibration (experimental determination of sensitivity), verification of the results obtained from laboratory tests and the possibility of its practical implementation has been confirmed. The sensor grating was placed in such a way that its half was in the zone of a variable value of axial strain caused by changes of the cross-section of the sample. The other half, however, was in the zone of a constant cross-section of the sample and of constant value of strain, caused by the force stretching the sample. The obtained errors of non-linearity of processing characteristics for measuring strain and temperature of the proposed system were 2.7% and 1.5% respectively, while coefficients of sensitivity to strain and temperature were 0.77 x 10-6 m/e and 4.13 x 10-12 m/K respectively. The maximum differences between the values obtained from the indirect measurement and the set values were 110 μe for strain and 3.8°C for temperature, for a strain of 2500 μe and a temperature of 40°C.

An analysis on mode selection by V–I transmission matrix in DBR laser with asymmetric fiber gratings

The V–I Transmission Matrix Method (VITMM) which is well known in the microwave engineering field was firstly applied to analyze the output spectra of the Distributed Bragg Reflector (DBR) laser, formed by asymmetric fiber gratings as resonator mirrors. One mirror is the uniform Bragg grating and the other is chirped grating. A theoretical model of grating was established, and then a numerical simulation of the mode selection in DBR laser with asymmetric fiber gratings was presented. Simulation results show that VITMM, with calculation error less than 0.1%, could save the calculation time compared to the Rouard method. In the experiment, the setup design of the single-longitudinal-mode laser output at 1544.7nm was carried out, and the result, which lasted about 10min, observed on an optical spectrum analyzer, demonstrates the feasibility of VITMM to address the mode output issues of DBR fiber laser.

Finite element analysis of a novel weak-pressure sensor based on fiber Bragg gratings in photonic crystal fibers

The fiber Bragg grating has been widely used in sensors. We have studied the spectral properties of uniform Bragg gratings in photonic crystal fibers under transverse pressure. By the finite element method, the relation between the birefringence and the pressure was simulated in bare photonic crystal fibers. The results show that the birefringence is changed even under weak pressure. A new method based on polarization-dependent loss is presented for measuring pressure. The maximal amplitude of the polarization-dependent loss varies linearly in the pressure range of 0 to 4 MPa with a slope of 0.75 MPa−1.

Discretely tunable single‐ and dual‐wavelength fiber ring laser using a fiber Bragg gratings based Fabry‐Perot filter

AbstractA Fabry‐Perot filter is realized by inscribing two identical 1 mm uniform Bragg gratings separated by 7.7 mm space in the hydrogen‐loaded photosensitive optical fiber. It is then incorporated with a tunable chirped fiber Bragg grating into a fiber ring cavity. Fourteen single‐wavelength lasings from 1552.186 nm to 1553.453 nm with average 97.5 pm spacing, and 13 pairs of dual‐wavelength lasings within the above wavelength range are achieved stably. This kind of laser will be very useful in applications requiring close‐spacing and tunable single‐ or dual‐wavelength components and systems. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2170–2173, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23620