Pieter L. Swart

Long-period grating Michelson refractometric sensor

This paper presents a single-probe Michelson interferometer that uses mode coupling in a long-period grating to establish the two optical paths in a single optical fibre. The interferometer phase shift depends on the refractive index of the material that surrounds the fibre probe, with the phase sensitivity directly proportional to the probe length. A simple phenomenological model explains the experimental results obtained with a liquid level sensor and a refractive index sensor. Its temperature sensitivity depends on the type of fibre that constitutes the probe. With a 45 mm long fibre probe, it is −2.5 and 12.8 degrees °C−1 for normal single mode fibre (SMF28) and germanium–boron co-doped fibre (PS1500), respectively.

Temperature-insensitive fiber Bragg grating accelerometer

A cantilever beam and fiber Bragg grating is used to measure acceleration. The cantilever induces strain on the grating, resulting in a Bragg wavelength modification that is subsequently detected. The output signal is insensitive to temperature variations and for a temperature change from -20/spl deg/C to 40/spl deg/C, the output signal fluctuated less than 5% without any temperature compensation schemes. Because the accelerometer does not utilize expensive and complex demodulation techniques, it is potentially cost effective. For the experimental system, a linear output range of 8 g could be detected.

Interferometric distributed optical-fiber sensor

An interferometric technique is described for detecting and locating perturbations along an optical fiber. This distributed sensor, based on a modified fiber-ring interferometer, has a position-dependent response to time-varying disturbances such as strain or temperature. These disturbances cause a phase shift that is detected and converted to spatial information. The sensor consists of two parts, namely, a reflecting-fiber-ring interferometer and a differentiating-ring interferometer. The reflecting ring consists of a fiber ring with one port of the coupler connected to a reflector. Consequently the output port of the reflecting-ring interferometer is the same as the input port. Because it is an inherent zero-path-imbalanced system, a short-coherence-length source such as a light-emitting diode can be used. Any time-varying perturbation on the fiber in the ring results in a detector signal proportional to the product of the rate-of-phase change caused by the perturbation and the distance of the perturbation relative to the center of the fiber ring. The second part of the system, a differentiating-ring interferometer, consists of the same fiber-ring interferometer modified only slightly. The output of this part of the sensor is proportional only to the rate of phase change as a result of the unknown perturbation and contains no distance information. By dividing the output of the reflecting-ring interferometer by the output of the differentiating-ring interferometer, we determine disturbance location. Results obtained with a 155-m distributed fiber sensor are discussed.

Non-contact torsion sensor using fibre Bragg gratings

Non-contact measurement of torsion on a rotating shaft was done with fibre Bragg gratings and involved the use of graded index lenses for transmitting optical information between the light source and the gratings, and back to a spectrum analyser. Special Bohler steel with high yield strength was used for the shaft. For the experimental measurements, one end of the shaft was clamped in a lathe chuck, and the other end was fixed to a brake system, allowing the application of torque on the rotating shaft. Up to 97 N m of torque was applied to the shaft rotating at 190 rpm, and the results were very close to the expected values. Whereas torsion changes the differential-mode wavelength of the gratings in the proposed configuration, this wavelength difference is insensitive to temperature. For the particular system investigated experimentally, the differential wavelength change was 7.4 pm N−1 m−1, and the corresponding temperature sensitivity was −0.547 pm °C−1.

Fourier-transform design and electron cyclotron resonance plasma-enhanced deposition of lossy graded-index optical coatings

A Fourier-transform synthesis technique is applied to the design of inhomogeneous refractive-index optical thin films with dispersion of refractive index and absorption taken into account. Using measured properties of SiO(x) N(y), we have designed a broadband reflector and two three-line filters with high reflectance. One of the three-line filters was manufactured by electron cyclotron resonance plasma-enhanced chemical vapor deposition from a mixture of SiH(4), N(2), O(2), and Ar. Good agreement between design and measured performance proves the feasibility of applying the Fourier-transform technique to the design of reflection filters even in the case of highly dispersive, absorbing material, if its optical properties are well characterized.

Properties and applications of electron cyclotron plasma deposited SiOxNy films with graded refractive index profiles

Abstract In the experiments in the present study, electron cyclotron resonance plasma enhanced chemical vapour deposition was used for the deposition of SiOxNy films from O 2 Ar , N2 and SiH 4 Ar mixtures. Optical properties of the films were studied by means of transmission spectroscopy in the wavelength range 200–2600 nm. Refractive indices ranged between 1.5 and 3.5 (measured at 632.8 nm) for different film compositions. The extinction coefficient showed a strong blue shift with decrease of silane content in the gas mixture. Optical interference filters with continuously varying refractive index profiles were designed, and manufactured by our computer controlled electron cyclotron resonance system. Designs which take into account dispersion of refractive index, and absorption in the constituent SiOxNy material, differ only significantly from designs in which it is neglected at wavelengths shorter than 700 nm.

Electron cyclotron resonance plasma enhanced chemical vapour deposition and optical properties of SiOx thin films

Abstract Silicon–oxygen alloys ranging from amorphous silicon to silicon dioxide have been deposited from electron cyclotron resonance SiH4/O2/Ar plasmas onto radio frequency biased substrates at room temperature. Hundred Watt microwave power, a working pressure of 2 mTorr, a total gas flow rate of 20 sccm, and 13.5 Watt radio frequency bias power were used. Refractive indices of 3.8 to 1.48 were measured by ellipsometry at a wavelength of 632.8 nm. Deposition rates for different SiH4/(SiH4+O2) gas flow ratios were between 4.3 and 7.5 nm min−1. Fourier transform infrared spectroscopy revealed a gradual evolution of the chemical composition of the films from amorphous silicon to silicon dioxide with change in gas phase composition. For silicon dioxide films, the centre frequency of the main Si–O stretching band is 1060 cm−1, whereas full width at half maximum is approximately 94 cm−1. Presence of Si–OH bonds is not detectable for any of the films. The amount of hydrogen in the films is considerably less than that in similar layers deposited by radio frequency plasma enhanced chemical vapour deposition, and for amorphous silicon it does not exceed 1.5×1022 cm−3. The presence of a single band at 486 cm−1 in the Raman spectrum of the a-Si film confirmed its amorphous structure.

Rugate filter manufacturing by electron cyclotron resonance plasma-enhanced chemical vapor deposition of SiNx

Electron cyclotron resonance plasma-enhanced chemical va- por deposition (ECR-PECVD) is used to deposit thin films of SiN x of different composition from mixtures of N2 and 30% SiH4 in Ar onto dif- ferent substrates. Measured values of the complex refractive index over the wavelength range of interest are used in the design and computer- controlled fabrication of one- and two-band rugate optical interference filters. The excursion of the continuously varying refractive index is cho- sen to be between 0.8 and 1.2, and window functions and matching layers at both sides are employed for sidelobe suppression. Measured reflection patterns of filters agreed well with simulated results. Filters designed for a center wavelength of 1000 nm and a total thickness of 2.1 mm, have a reflectance of 97% in the stopband and a bandwidth of typically 240 nm. The two-band filter is designed for stopbands at 1000 and 770 nm with reflectance of 98 and 96% and bandwidths of 150 and 120 nm, respectively. The total thickness of this filter is 3.33mm. Filters are successfully deposited on optical fiber ends using one of the fibers for in situ monitoring of the deposition process.

A new algorithm for layer thickness and index step estimation in multi-layer hetero-epitaxial structures

Closed-form mathematical expressions are derived in this work enabling one to obtain both thickness and refractive index steps from the measured optical reflectance of an arbitrary number of layers on a substrate. The theory is based on the assumption that the normalized index steps are small, making this new technique particularly appropriate to the non-destructive analysis of hetero-epitaxial layers. Despite the fact that the derivation is quite involved, experimental data can be analyzed with very modest computer requirements. If the reflectance data is available at equi-spaced wavenumber intervals, each data point is pre-processed by an algebraic transform requiring one addition, one subtraction, and a division, followed by a fast Fourier transform. It is shown theoretically that the Fourier spectrum of the transformed reflectance possesses spectral peaks only at positions corresponding to the interfaces, and in the same sequence as the actual positions of the interfaces in the multilayer structure. The structural determination is therefore totally unambiguous, requiring no time consuming interactive procedures. It is also shown that the relative refractive index steps between adjacent layers may be calculated directly from the amplitude of the discrete Fourier components by direct substitution. The thickness and refractive index estimation technique is demonstrated by both simulated and experimental data on AlGaAs-GaAs structures comprising up to five layers on a substrate. It is demonstrated by these examples that the thickness dynamic range of the method is very wide, ranging in these examples from 1 to 50. The minimum thickness which can be resolved is determined by the frequency resolution of the Fourier transform, which for AlGaAs-GaAs is approximately 150 to 200 nm. However, if this new method is used in conjunction with a curvefit procedure, for the thin layers, it is possible to improve the resolution. Analysis of the simulated data shows that the accuracy in the determination of thickness and refractive index is better than 3% in most instances.

Comments on: Wavelength-tunable add/drop multiplexer for dense wavelength division multiplexing using long-period gratings and fiber stretchers

Abstract We have designed and configured a novel wavelength-tunable add/drop multiplexer (ADM) device that employs long-period gratings (LPGs) and piezoelectric ceramic fiber stretchers. The modeling for this ADM predicts that 50 International Telecommunication Union (ITU) dense wavelength division multiplexing (DWDM) channel signals could be selected in the wavelength range from 1526.25 to 1563.75 nm with 0.75-nm channel spacing, which covers the whole C-band gain spectrum of erbium-doped fiber amplifiers. The cross-talk between channels is less then −39 dB, and the total insertion loss of the device is about 0.24 dB which includes the splicing losses, the mode mismatching loss, and the expected losses in the two side-by-side coupled fibers. The tuning of add/drop channels within a limited region can be provided by changing the voltage, which is applied to the piezoelectric ceramic fiber stretchers. This technique is suitable for the in-line implementation of optical communication networks.