Phase Shifter Structure Research Articles

Cs-correction and electron holography: exciting TEM-methods for solving nanoquestions

The macroscopic properties of modern materials and nanostructured systems are determined by their atomic structure. Therefore, for a tailored design of materials with optimized properties, the structure-property relationships have to be investigated making a structural characterization in terms of the nanoquestions ″which atoms are where, which fields are around″ indispensable. These nanoquestions are tackled by well-elaborated characterization methods such as e.g. X-ray and neutron diffraction, SXM and AFM, and high-resolution transmission electron microscopy (HRTEM). However, every method has its own pros and cons: Despite the fact that diffraction methods are able to precisely determine lattice parameters, their spatial resolution is limited due to averaging over the illuminated area with a diameter of about 1 μm. Thus, specific structure modifications of real-world samples on the nanometer scale such as e.g. interfaces, grain boundaries, inclusions, and defects cannot be resolved locally. SXM and AFM are unrivalled in investigating surface structures up to atomic resolution; however, they have no access to the interior structure of the sample. HRTEM is the method of choice offering the unique possibility of spatially resolved characterization of the interior structure of real-world structures with atomic resolution. In the past, HRTEM was hampered by the spherical aberration (Cs) of the objective lens causing a blurring hence an averaging over 1..2 nm in the image plane. Since the late 90’s, hardware Cs-correctors [1,2] are available allowing the correction of aberrations up the 3rd order, which marks a giant step in HRTEM. Cs-correction reduces the blurring dramatically so that the signal piles up at atom positions well above noise. Despite this huge progress, the phase contrast problem of imaging still persists. Therefore, Cs-correction is combined with off-axis electron holography [3], where an interference pattern (″electron hologram″) is recorded in the TEM. By numerical reconstruction, the complete information of the electron wave is available quantitatively in amplitude and phase as well as in real space and in Fourier space. Additional beneficial capabilities are almost perfect zero-loss energy-filtering with about 10 -15 eV as well as linear imaging. All this allows a comprehensive processing and analysis of the recorded data: Residual aberrations also of higher orders, which cannot be completely avoided, can be corrected by an a-posteriori fine-tuning. Diffraction patterns are determined from areas of the size of a few unit cells allowing a local investigation of crystal orientation. Imaging in the light of single reflections enables the sophisticated analysis of diffracted waves. Moreover, large area phase-shifting structures like electric/magnetic domains and biological/polymer specimen can directly be investigated by electron holography. Therefore, modern TEM-methods cover a wide range of applications [4] from dopant-profiling in semiconductors and field measurements of small magnetic clusters up to holographic materials analysis with single atom sensitivity.

Photonic RF phase shifter and tunable photonic RF notch filter

A new photonic RF phase shifter structure for phased array antennas is presented. It is based on a single dual-output modulator and two optical switches and optical attenuators. This can realize continuous phase shifting of 0/spl deg/-360/spl deg/ without altering the signal amplitude. It has the advantages of wide bandwidth, fast response time, and fine tuning resolution. Experimental results demonstrate phase shifts over a 360/spl deg/ phase range, with RF signal power changes of less than 0.2 dB, which is in close agreement with predictions. A tunable photonic RF notch filter, which is based on the new phase shifter, is also presented. Experimental results demonstrate continuous tuning of the photonic notch filter over a wide tuning range, which covers the full free spectral range, which is in good agreement with predictions.

Distributed phase shifter with pyrochlore bismuth zinc niobate thin films

A monolithic Ku-band phase shifter employing voltage tunable Bi/sub 1.5/Zn/sub 1.0/Nb/sub 1.5/O/sub 7/ (BZN) thin film parallel plate capacitors is reported. BZN films were deposited by radio frequency magnetron sputtering on single-crystal sapphire substrates. A nine-section distributed coplanar waveguide loaded-line phase-shifter structure was designed. A differential phase shift of 175/spl deg/ was achieved with a maximum insertion loss of 3.5 dB at 15 GHz, giving a figure of merit /spl sim/50/spl deg//dB. To the best of our knowledge, this is the first demonstration of a monolithic tunable microwave circuit using BZN thin films.

Tunable optical narrowband filter using a gain-coupled phase-shift-controlled distributed feedback laser

In this paper, we have proposed and analyzed a new tunable optical narrowband filter using gain-coupled phase-shift-controlled distributed feedback (GC-PSC-DFB) laser diode. Coupled-mode equations are solved by using the transfer matrix method (TMM). The GC-PSC-DFB filters offer a stable single-mode bandpass output, similar to that obtained with phase-shifted index-coupled structures. However, GC structures do not suffer from the severe longitudinal spatial hole burning (SHB) that occurs in high-coupling quarter-wave-shifted DFB filters. This SHB can cause multimoded behavior for high-input signal power. Various filter parameters such as wavelength tuning range, side-mode suppression ratio (SMSR), and channel gain deviation have been investigated and discussed. Our results show that the GC DFB structures offer a wider tuning range of 23.3 /spl Aring/ compared with the similar index-coupled DFB structures with nearly steady bandwidth of 12 GHz, while maintaining 41.8-dB constant gain.

Resolution limit for two-dimensional crossed-grating patterns projected by a coherent beam

The resolution limit for two-dimensional crossed-grating patterns created by projecting mask objects by using a coherent beam has been investigated. We consider first two conventional mask types, a binary-amplitude mask and a two-level phase-shifting mask, in analyzing relationships between a diffraction-beam configuration and an image-intensity distribution. Then we derive, as a mask that overcomes the resolution limit of the conventional ones, a four-level phase-shifting structure with which the minimum image period can be reduced to square root(2)/2 time that of the two-level phase-shifting mask.

Analysis of a high power ferrite differential phase shift circulator operating above ferrimagnetic resonance

Abstract Magnetic losses limit the power handling/frequency performance of all ferrite phase shift and control components. At high power levels nonlinear loss is prevalent but this can be avoided using a technique called ‘mode segregation’. It is noted that this is achieved by biasing the ferrite above ferrimagnetic resonance. A combined magnetostatic/microwave finite element scheme is used to model this mode of operation in a differential phase shift structure used in air traffic control surveillance radars. Starting from the nonlinear material magnetisation curve, the magnetic state of the ferrite is evaluated as a prerequisite to calculating the full microwave solution. Above resonance operation avoids nonlinear loss but this study indicates that the differential phase shift is reduced and larger magnetic bias fields are required. Calculations agree well with the experimental data.

Transmission characteristics of a two-dimensional photonic crystal array of dielectric spheres using subterahertz time domain spectroscopy

The transmission characteristics of a two-dimensional millimeter-sized Si 3 N 4 sphere array system are measured and compared with calculations. The transmittance and the phase shift are measured by the subterahertz time domain spectroscopy and the calculation is based on the vector KKR formalism. Excellent agreement over a wide frequency range for the transmittance, phase shift, and photonic band structure has been obtained. The results show that, provided the system is well prepared geometrically and absorption can be suppressed, periodic arrays of dielectric spheres are very promising as photonic crystals.

Asymmetric ring-hybrid phase shifters and attenuators

A new structure of asymmetric ring-hybrid phase shifters and attenuators is presented. Each consists of an asymmetric ring hybrid and reflecting terminations, and it does not have any additional 90/spl deg/ phase delay line for utilizing symmetric reflecting terminations that conventional phase shifters use. To analyze these asymmetric ring-hybrid phase shifters, normalized impedance ratios NI/sub b/ and NI/sub d/ are introduced, and the possibilities to reduce the size of the reflecting terminations are presented. Using the new structure of the asymmetric ring-hybrid phase shifters, asymmetric ring-hybrid attenuators are synthesized. To analyze the attenuators, normalized resistance ratios NR/sub Lb/ and NR/sub Ld/ are introduced, so that the resistances in the reflection terminations can arbitrarily be determined. On the basis of the derived new structures, a uniplanar asymmetric ring-hybrid -135/spl deg/ phase shifter and a microstrip asymmetric 4-dB attenuator with 45/spl deg/ phase shift have been fabricated and measured. They show good agreement between measured and simulated results and they may be used for impedance transformers besides their original functions.

Analysis of radiation mode effects on oscillating properties of DFB lasers

The effects of radiation mode on the oscillating properties of distributed feedback (DFB) lasers with second-order corrugations are analyzed for designing a new type of DFB laser. A formulation based on the transfer matrix technique is applied to calculating Streifer's /spl zeta/-terms added to the coupled-wave equations. These terms represent the effects of radiation and evanescent modes. This formulation greatly simplifies the analysis of distributed resonance along multilayered waveguide structures with arbitrary-shaped second-order corrugations. The effects of vertical resonance are also incorporated into the formulation. Various types of DFB lasers with phase-shifted second-order corrugations are analyzed using this method. It is found that the phase shift and the blaze of the corrugations greatly affect the longitudinal mode selectivity. A new phase-shift DFB laser structure with two complementary blazing regions connected at the shift is proposed. It is demonstrated that this structure has small radiation loss resulting in low-threshold performance despite employing second-order corrugations.

Wavelength-tunable optical filters with phase-shifted DFB structures

Wavelength-tunable optical filters with phase-shifted DFB structures

A polarization-independent grating resonator

A polarization-independent optical resonator based on a novel phase-shifted grating structure is proposed and analyzed. Its application as a polarization-independent optical wavelength filter with ultranarrow bandwidth and fine tunability is described.

A new active structure of high frequency wide band differential phase shifters

The design and performance of a wide band active GaAs IC 180° differential phase shifter are presented. A two-stage GaAs IC was developed containing a microwave differential amplifier and a matched common-gate input. The active phase shifter achieved a 10° phase unbalance and an isolation at the output ports of better than 10 dB in the 1-10-GHz band. A large percentage of the GaAs chip contains active devices thereby providing a very large operating bandwidth and a reduced surface area. This active phase shifter presents an interesting alternative to the passive rat-race or an equivalent coupler for radio mobile communications. © 1996 John Wiley & Sons, Inc.

A polarization-independent distributed Bragg reflector based on phase-shifted grating structures

A polarization-independent narrow-band Bragg reflector based on a novel phase-shifted grating structure is proposed and analyzed. Operation and design principles for the proposed grating structure as an polarization-independent optical filter are described. Robustness of the polarization-independent filter against modal birefringence and polarization effects in grating couplings and modal losses are examined and verified. Tunability of the polarization-independent filter is also studied and demonstrated.

Analysis of nonuniform nonlinear distributed feedback structures: generalized transfer matrix method

A new method for the analysis of almost-periodic, nonuniform, nonlinear distributed feedback (NLDFB) structures is presented. A grating segmentation technique used for linear DFB devices is combined with the analytic solutions corresponding to a strictly periodic, uniform NLDFB device. The method is demonstrated for tapered, chirped, and phase-shifted structures. New results describing the operation of single- and multiple-phase shifted NLDFB are reported. NLDFB structures with a axially-varying effective Kerr index are also considered. >

Improved spectral characteristics of MQW-DFB lasers by incorporation of multiple phase-shifts

The single-mode stability of strained-layer MQW-DFB lasers with electron-beam written gratings containing zero, one and three phase-shifts, and with a relatively large coupling strength, has been investigated theoretically and experimentally. The fabricated lasers with the multiple phase-shifted (MPS) structure exhibited a higher degree of stability than the /spl lambda//4-shifted lasers, and a considerably improved stability for both categories with phase-shifts compared to the nonshifted lasers was obtained. These results were in good agreement with our simulations. An investigation of the linewidths of the phase-shifted lasers is also presented. The MPS lasers had a significantly lower linewidth floor (down to 600 kHz) than those with one phase-shift. There was no degradation of the side-mode suppression-ratio correlated to the linewidth floor or rebroadening. A possible explanation for the lower linewidth floors obtained for the MPS lasers is that this structure is less sensitive to inhomogeneous carrier fluctuations. Longitudinal fluctuations in the shape of the photon density distribution will thereby be suppressed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Inspection of optical phase-shifting masks with an automated electron-beam system

There has been great interest lately in using optical masks that contain phase-shifting structures in order to print smaller features than is possible with a standard binary mask. Because of the many varieties under investigation and the complexity of the phase interactions, no definitive experimental work concerning the effects of various types of defects yet exists. However, it is known that, under some circumstances, defects in a phase-shifting mask (PSM) will cause a larger deviation in the printed linewidth than a defect of equal size on a binary mask. For 0.25 μm lithography done with a 248 nm stepper using an ‘‘etched Levinson’’ or alternating PSM, it is thought that phase defects as small as 80 nm need to be detected. At present, the only inspection system capable of meeting the size requirements is based on electron optics (KLA SEMSpec). However, this type of system, which was designed to inspect x-ray masks, requires that the defects exhibit either a topographical or material anomaly in order to be visible and that the mask be conductive. While some types of phase-shifting masks satisfy the contrast formation requirements, none are conductive. This can be remedied by depositing a metallic layer over the surface. However, in order for inspection to proceed at high speed, the defect contrast and signal level must be high. In this article we elucidate the physics of electron-beam image formation of conductively coated phase-shifting masks for various types and thicknesses of coatings. Both theoretical and experimental results are presented. Since adding and removing the conductive coating must be done without damaging the mask, only certain types of coatings and processes are allowable. A list of the most promising coatings is presented along with the supporting experimental evidence. By inspecting a phase-shifting mask that has intentionally written defects of various types, it is shown that defects as small as 0.1 μm can be reliably found.

A new structure of microwave ultrawide-band differential phase shifter

A new structure for TEM transmission line ultrawide-band differential phase shifters is proposed. This consists of a cascade of two-ports, each of which is a single coupled section with parallel transmission lines connected to each other at one end. The section lengths and coupling coefficients are different. The results of numerical synthesis have been tabulated for phase shifters of 90 degrees differential phase shift. The proposed structure has the advantage of a lower coupling coefficient and improved phase-frequency characteristic in comparison with the other stepped phase shifters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Polarization- and thickness-dependent near fields of small phase-shifting structures

The spatial distribution of amplitude and phase in the diffraction near field behind phase-shifting structures of infinite length and small rectangular cross section (</= lambda(2), lambda is the wavelength) is investigated for incident, linearly polarized plane waves. The near fields are computed by an inverse Fourier transformation of the calculated angular spectra; 3-cm microwave measurements yield good agreement. The computed near fields are compared to the structure images in the range of geometrical optics. Quantitative relations are derived for the deviations between the corresponding angular spectra, depending on polarization of the incident wave and structure thickness.

Optical filtering as an alternative method to phase shifting

Starting from the spatial frequency spectra of phase-shift structures sufficient filters are derived to enhance the resolution and the contrast of line imaging in photolithography. It is shown, that common mask structures in connection with suitable filters yield similar results like phase-shift techniques.

Distributed feedback lasers with distributed phase-shift structure

Distributed feedback (DFB) lasers with distributed phase-shift (DPS) structures are suggested for reducing the spatial hole burning in conventional phase-shifted DFB lasers. Solving the coupled-mode equations subject to suitable boundary conditions, we have determined the threshold gain condition of a symmetric distributed phase-shift distributed feedback (DPSDFB) laser as well as the internal photon intensity distribution. The results show that by distributing the phase shift to a greater extent, the spatial hole burning can be reduced significantly and a large threshold gain difference between lowest gain oscillation modes can be maintained over a large phase-shift range. Moreover, a properly designed DPSDFB laser does not pose extra requirements on its fabrication.