Multimode Interference (MMI) device is a useful optical component for optical power splitter/combiner and router
applications. In this paper we present high precision calculation results on the optimum position of self-images in an
MMI and their variation due to wavelengths, for WDM applications. We show that the commonly used MMI self-image
position calculation methods, using the beat length of two lowest order modes or effective MMI width approximation,
lead to significant deviations from the optimum self-image position. We calculate the optimum position of the self-image
by finding the maximum value of overlap integral of total MMI field, comprised of all MMI modes, with the total field at
the input of the MMI device. In addition, for the optimum output power coupling distance for MMI, we calculate the
overlap integral of the total MMI field with the output waveguide field. Both these methods give approximately the same
optimum length. We obtain up to 60 um difference in optimum self-image position for a Si MMI (width =15 ~ 30 um),
and refractive index difference of 0.02 between core and cladding, from the approximations based methods. We also
calculate the variation of this image position in 1.50 um to 1.60 um wavelength region. We show that the optimum image
position is strongly dependent on wavelength, with up to 100 um variations in this wavelength range. In addition, we
show that there is a significant variation in this self image position with MMI widths, at points where a new power
carrying mode is added.
We propose novel tool employing both low coherence interferometer and spectrally resolved reflectometer sensor. We discuss application of this novel tool for measurements of the narrow high aspect ratio structures. We demonstrate that the visible reflectance spectrum of such structures allows us to extend range of interferometer to measure depth trenches with diameter from 2 μm to 1 mm, with reproducibility 10 nm - 100 nm depending on range of the thin film thickness. We also present of this novel tool for measurement of ultra-thin coated pressure sensor membranes. Application of an auxiliary spectral reflectometer allows correcting for systematic errors of low coherence interferometer which can be as large as 1.5 - 2 μm.
We presented novel tool employing both low coherence interferometer, and spectrally resolved reflectometer sensor. We discuss compatibility of this metrology with high resolution Raman spectroscopy. We present measurements of the stability of the Raman spectrometer indicating that system is capable to measure stress in silicon with reproducibility corresponding to 1 MPa and below. We propose integrated tool for simultaneous measurement of stress and displacement of the micro-machined electromechanical devices. Furthermore we propose Raman system configuration allowing measurement of all independent stress tensor components on submicron scale.
The most commonly employed tools for wafer thickness and topography metrology are based on capacitance method, which due to physical size of probes, and may not be suitable for direct measurement of multi-layer non-conductive wafers or Micro Electromechanical Systems (MEMS) structures. Recently developed that low coherence interferometry provides solution, which overcomes limitations of these methods. Selected MEMS applications including characterization of deep (high aspect) trenches and membrane structures have been also developed. The above listed applications were limited to measurements of relative distance between two optical interfaces in material transparent at the wavelength of probing radiation. Absolute distance gauging by fiber optic low coherence interferometer is difficult due to large thermal drift (of the order of 0.04 mm/K). We demonstrate that this drift is a result of thermal changes of refractive index of fiber optic glass. We present solution eliminating this drift is based on introduction of the additional reference plane in the signal arm of the Michelson interferometer. Use of this reference plane eliminates influence of changes of refractive index of glass fibers on result of measurement and improves thermal stability of low coherence interferometer by three orders of magnitude.
KEYWORDS: Near field optics, Extremely high frequency, Near field, Heterodyning, Switches, Field effect transistors, Receivers, Ultrafast phenomena, Gallium arsenide, Fiber optics
A novel technique, that combines optical heterodyning and near-field optics, was developed for highly localized of millimeter waves in ultrafast devices. The technique relies on evanescent coupling of the interfering laser to a small are of the device, by means of a near-field fiber optic probe. The applicability of the technique was first validated by measurements on heterojunction photo transistors up to 100GHz. Later, scanned measurements at 63GHz were performed on two ultrafast device structures, namely low temperature GaAs photoconductive switches and InP-based high electron mobility transistors. The response characteristics were rich in structures that revealed important details of the device dynamics.
Subpicosecond electrical pulses have been generated from low temperature grown GaAs (LT-GaAs) coplanar stripline photoconductive switches by illumination of 160 fs optical pulses at 1.55 micrometers , which corresponds to below bandgap radiation. The device optical response was attributed to two photon absorption (TPA) as confirmed by the observed quadratic dependence of the DC photocurrent on average incident illumination. At a bias of 39 V and 5.16 mW average illumination a 2.6 (mu) A photocurrent was measured which corresponded to a 0.5 mA/W switch responsivity. For similar switch geometries fabricated onto LT-GaAs and illuminated by above bandgap radiation, responsivities of a few mA/W have been measured. Therefore TPA switch triggering has retained acceptable responsivity levels. The contribution of TPA to the device photoresponse was also confirmed by the dependence of the photocurrent on incident polarization direction; the TPA coefficient exhibited a strong anisotropy with respect to polarization. The device generated electrical pulses (impulse photoresponse) were measured using double sliding-contact photoconductive sampling techniques. Impulse photoresponses with full widths at half maximum of 451 fs were observed which are, to date, the fastest responses measured from LT-GaAs switches at this wavelength. Fourier transformation of the time domain data revealed electrical transients with 3 dB bandwidths of 190 GHz. The feasibility of back illumination of these devices at 1.55 micrometers was also demonstrated. Based on these highly encouraging results freely positionable photoconductive switches were fabricated and at present they are being tested.
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