S. Ruschin*, D. Meshulach**
*Department of Electrical Engineering-Physical
Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
**School of Physics and Astronomy, Tel-Aviv University,
Tel-Aviv 69978
Abstract It is shown that it is possible
to accomplish near-complete power transfer functions in N coupled
waveguide arrays by controlling a set of N voltages applied to the
waveguides. Switching of the power from an excited center waveguide into
any output channel in a five waveguide array, and splitting of power into
any output combination of channels is demonstrated. In all cases, the cross
talk is better than -16 dB, and the power splitting efficiency ratio differs
from 1 on an amount of not more than 15%. The length of the device is arbitrarily
set. The sensitivity of the splitting functions to small random changes
of the applied voltages is investigated.
S. Ruschin
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract The use of graded index optical
elements in optical resonators may change significantly the transverse
and longitudinal profile of laser modes. This fact can be advantageous
in the design of optical resonators optimized for a given amplifying medium,
by matching the geometrical properties of the mode with the gain and saturation
profiles of the amplifier. Mode shaping is also of importance when a definite
profile is required for the outcoming beam related to a definite application
like machining or coupling to a fiber or other device. The main issue of
this presentation is the formulation of mathematical methods by means of
which an optical resonator can be synthesized according to a pre-required
profile shape, using either phase or amplitude graded optical elements.
Two formalisms will be presented which are useful for the cases the profile
is imposed on the transverse or longitudinal co-ordinates of the resonator.
In the first case a matrix formalism based on the orthogonal collocation
method is used [1].
In the case of profiling along the axial co-ordinate, the field is expanded
in axially displaced Gaussians, by means of the Gauss-Fourier method [2].
One main outcome of this research is the role of saturable gain on the
mode shape attained. Calculations show that this factor affects not only
the shape but also the possibility of single mode operation, even for cases
where empty cavity calculations show significant mode-loss differentiation.
References:
[1] S. Ruschin, Proc. SPIE Vol. 1834 (1992).
[2] S. Ruschin, J. Opt. Soc. of America A (to be published).
Modified Bessel nondiffracting beams
S. Ruschin
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract Solutions of the the scalar wave equation
are introduced based on modified Bessel functions. These functions are
radially non-oscillating, unbound and nondiffractive. Their propagation
constant is larger than that of vacuum meaning an inverse Guoy effect or
a phase velocity smaller than c. Physical realizations of the beams
are accomplished by apodizing the modified Bessel profiles by means of
suitable window functions. When Gaussian apodization functions are used,
slower axial decay is denoted as compared to the Gaussian and to the ordinary
Bessel counterparts. Super-Gaussian and circular apodization are also examined.
In the last case a persistent narrow axial lobe is encountered which has
also a non-decaying character.
Inversion of the Transverse Mode Calculation Problem
in Laser Resonators
S.Ruschin
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract Instead of calculating the transverse modal shape
for a given resonator configuration, we invert the problem and ask wether
a cavity can be developed based on the Orthogonal Collocation Method. The
formalism is designed which supports a given transverse modal structure.
This possibility can be advantageous in the design of optical resonators
optimized for a given amplifying medium, by matching the geometrical properties
of the mode with the gain and saturation profiles of the amplifier. A formalism
was developed based on the Orthogonal Collocation Method. The formalism
is based on the expansion of the field in a set of orthogonal functions
of number M, for which the propagation properties of each of its composing
elements are known. Sampling the field at M special points (collocation
points) results in the generation of an orthogonal MxM matrix which describes
the propagation of a wavefront on a resonator round-trip. By suitable matrix
operations, like diagonalization, the fundamental and higher order modes
are obtained, but more important, the inverse problem can be solved, namely,
the synthesis of a suitable optical element of graded transmissivity or
reflectivity, which will ensure that the cavity will support a pre-described
transverse mode shape.
Phase Velocity of Light Matching by Means of Axially
Displaced Gaussian Modes
S.Ruschin
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract Many physical fundamental phenomena in the
interaction between matter and light depend on the value and functional
dependence of the phase velocity of the electromagnetic wave. Notorious
examples are phase matching between electromagnetic waves and matter waves
in solids and plasmas, and among two electromagnetic waves interacting
via a optically non-linear medium. The purpose of this presentation is
to introduce a new mechanism of potential practical implications, able
to control the phase velocity of a propagating beam. The mechanism is based
on a the property which was recently demonstrated by the author that by
superposition of axially displaced Gaussian modes, the phase of the complex
amplitude of an optical field along its propagation axis can be determined.
As a consequence, the phase velocity of light along the axis is affected.
Examples of increase and decrease of of the phase velocity are presented.
The advantage of the method is that it is based on a pure optical effect
and is not dependent on dispersive and anisotropy properties of the propagating
medium.
Field-induced waveguided devices in LiNbO3:
New developements
S.Ruschin
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract Field-induced waveguides were demonstrated two
decades ago, but did not attracted much further attention due to the high
voltages required for their operation. Advantageous features of these devices
are fabrication simplicity and high extinction ratio when used as a modulating
means. Recently, a series of improvements have lowered the operating voltages
to within practical values. This was achieved by confining the radiation
in the depth direction by means of a planar waveguide and by narrowing
the gap between the electrodes which delimit the electrical field. In a
further development we demonstrated in such a configuration a field
induced beating effect, when buried modes were excited. The beats were
induced by a modulating voltage of 0.8 Volts on a 15mm interaction length,
meaning voltage-length product significantly lower as compared to conventional
LiNbO3 modulators. These parameters allow shorter device lengths
and present evident advantages in large-bandwidth modulation at microwave
frequencies.
Improved Near Field Method for Refractive Index Measurement
of Optical Waveguides
D. Brooks and S. Ruschin
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract Refractive index profiles of waveguides are deduced
from near-field intensity distributions of guided modes. To improve the
accuracy of refractive index profiles, some measurements are taken with
the center peak of the mode intensity profile blocked, allowing the increase
of the optical power at the waveguide boundaries with a corresponding increase
in the signal-to-noise ratio. Additional improvements reported here are
related to the subsequent signal processing, which is performed mainly
in the spatial frequency domain. Smooth and reliable profiles of channel
waveguides are obtained, the method being very general and not presuming
the functional dependence of n(x,y).
Visible waveguide laser based on dye-doped sol-gel
glass film
I. Finkelstein, S. Ruschin, Y. Sorek and R. Reisfeld
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract We report laser action in thin films of glass
produced by the sol-gel method. The low temperature at which the glass
and films were processed allows the incorporation of organic dyes that
amplify light when optically pumped. XXX dye-doped films where prepared
that were pumped by frequency-doubled Nd:YAG laser light focused by means
of a cylindrical lens. Fresnel back- reflections at the edge of the film
provided sufficient feedback to allow laser action. A clear threshold effect
was observed at pumping energies of XX microjoules/pulse. Laser action
was also evidenced by a pulse narrowing effect . The developement of good
quality optical waveguides based on thin films prepared by the sol-gel
method [1], permitted us to report recently [2] the measurement of high-gain
coefficients in dye- doped planar waveguides. The main advantage of the
sol-gel method relies on the low temperature of processing of the films.
This property allows the incorporation of dye molecules into the the glass
matrix, which, by conventional methods would decompose due to the high
temperature involved.
Device for Laser Source Identification
S.Ruschin
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract A compact device for the characterization of
laser incoming radiation based on a novel concept is presented. The device
has the ability of performing simultaneously the following tasks: (1) To
identify positively the incoming radiation as laser light (2) To determine
the wavelength of the radiation. (3) To determine the direction of the
radiation. Being based on low-cost components, these devices are expected
to find applications in laser safety as well as in laser games and entertainment
systems. A prototype of the device was built and tested.
M. Keselbrener and S. Ruschin
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract We descibe a mirror, based on the frustrated
total internal reflection effect, in conjunction with a Fox-Smith interferometer.
The device has a performance which is very sensitive to frequency, and
has potential applications in laser resonators.
A Method for Measurement the Coupling Coefficient of
Coupled Optical Waveguides
D. Brooks, D. Scarlat and S. Ruschin
Department of Electrical Engineering-Physical Electronics,
Faculty of Engineering, Tel-Aviv University,
Tel-Aviv 69978, Israel
Abstract A new method for direct measurement of the coupling
coefficient of coupled optical waveguides is presented. The method consists
of imaging the field at the output facet of the device, while the excitation
at the input facet is performed by illuminating it with a planar wavefront,
and scanning the angle of incidence of the front with respect to the input
facet. A useful and simple relationship of Fourier-type exists between
the coefficients of the normal-modes excited by this procedure and the
spatial Fourier Transform of these modes. From this relationship, the coupling
coefficients are readily deduced. Only one pair of coupled guides running
from facet-to facet is required. The coupling coefficient of coupled Titanium
indiffused LiNbO3 waveguides was experimentally measured by
this technique and the results compared favorably with another known method.
The method is simple and requires microcomputer assisted CCD camera and
minimal standard optical equipment. The method is especially adequate for
automatic industrial online nondestructive measurement and characterization
of related Integrated Optic devices.
Light amplification in a dye-doped glass planar waveguide
Y. Sorke and R. Reisfeld
Department of Inorganic Chemistry, Hebrew University,
Jerusalem 91904, Israel.
I. Finkelstein and S. Ruschin
Department of Electrical Engineering-Physical Electronics,
Tel-Aviv University, Tel-Aviv 69978, Israel.
Abstract Glass waveguide films were prepared from titania
and modified silica using the sol-gel method and doped by the laser dye
rhodamine B. The guided and amplif ied fluorescence (pumped by a double
frequency Nd-YAG laser) was coupled out either by static grating written
on the film or by a prism. The gain of the emitted sduperradiance was determined
from the amplified spontaneous emission intensity dependence ont he pumped
strip length. A maximum net gain of 54 dB/cm was measured.
Proton-exchanged periodically segmented channel waveguides
in lithium niobate
D. Nir, S. Ruschin, A. Hardy and D. Brooks
Department of Electrical Engineering - Physical Electronics,
Faculty of Engineering, Tel-Aviv University, Ramat-Aviv, Israel
Abstract Annealed proton-exchanged periodically segmented
channel waveguides were fabricated in Z-cut lithium niobate. the propagation
losses as a function of the duty cycle and the segmentation period are
exported. Waveguiding was observed at large segmentation periods up to
10 muem. For duty cycle > 0.5 the propagation losses are nearly the
same as measured for a continuous waveguide with the same width, namely
~1.45 dB/cm. With decreasing duty cycle, the mode expands in the transverse
directions.
Asymmetric Y-junction wavelength demultiplexer based
on segmented waveguides
Z. Weissman, D., Nir, S. Ruschin, and A. Hardy
Faculty of Engineering, Tel-Aviv University, Ramat-Aviv,
Israel
Abstract An asymmetric Y-junction wavelength demultiplexer,
with a segmented branch, was realized in Ti:LiNbO1. The device was fabricated
in a single-step lithographical process. Wavelength demultiplexing operation
in the range 6.33 nm lambda 834 nm is demonstrated with extinction ratios
up to 30 dB. Insertion loss vary between a negligible value (far away from
cutoff) up to 7dB (near cutoff). Possible applications of the device are
considered.
Periodically segmented waveguides in Ti:LiNbB3
D. Nir, Z. Weissman, S. Ruschin, and A. Hardy
Faculty of Engineering, Tel-Aviv University, Ramat-Aviv,
Israel
Abstract Periodically segmented waveguides with periods
of few tens of micrometers and nonconnected segments are studied experimentally
in Ti:LiNbO3. The near-field patterns were found to expand with decreasing
duty cycle but to be independent of the segmentation period. The measured
segmentation losses vary between 0.5 and 2.5 dB/cm, depending on the period
and the duty cycle. These results agree with a simple numerical model,
based on two loss mechanisms: waveguide segmentation and surface relief
related to Ti indiffusion.
Buried modes in combined Ti diffused and Li outdiffused
LiNbO3 slab waveguides
E. Arad, S. Ruschin, and D. Nir,
Faculty of Engineering, Tel-Aviv University, Ramat-Aviv,
Israel
Abstract Buried waveguiding planar modes of well-defined
profiles were observed in samples undergoing simultaneous Ti indiffusion
and Li outdiffusion in a water vapor atmosphere. Each of these modes was
distinctly excited by end-fire fiber coupling and suffered no loss by metal
loading. An index profile which fits the mode shapes measured, and a mechanism
is suggested based on the influence of a Ti-rich layer compound on the
Li outdiffusion process.
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