Abstract We introduce the simulation of a photonic crystal slab with a square lattice, whose basis elements are layered cylinders of an averaged refractive index <n>. We compare it with a similar photonic crystal with a basis of the same size and a refractive index matching to the average of the layered ones. Even when this is such a simple system with internal structure, we have found an interesting phenomenology: an increase in band gaps, flattening of the bands, degeneracy nodes, etc. We also introduce additional methods to fine tuning the design and analyze the inclusion of a plain cylinder defect within the slab.
We discuss the complex dispersion relation of a one dimensional metallo-dielectric photonic crystal, produced by a
dielectric photonic crystal with extremely thin metallic inserts with the same periodicity. We have carried out the
analytical and numerical analysis. Also, we show a method to avoid the problem of solving the complicated system of
transcendental equations of the dispersion relation that was proposed previously for us and we extended it to the oblique
incidence, i.e., for calculating transversal electric and magnetic modes. Moreover, we demonstrated a metallic band gap
not only at the bottom but also at high frequencies.
In this paper we propose the fabrication, implementation, and testing of a novel fiber optic sensor based on Multimode
Interference (MMI) effects for independent measurement of curvature and temperature. The development of fiber based
MMI devices is relatively new and since they exhibit a band-pass filter response they can be used in different
applications. The operating mechanism of our sensor is based on the self-imaging phenomena that occur in multimode
fibers (MMF), which is related to the interference of the propagating modes and their accumulated phase. We
demonstrate that the peak wavelength shifts with temperature variations as a result of changes in the accumulated phase
through thermo-optics effects, while the intensity of the peak wavelength is reduced as the curvature increases since we
start to loss higher order modes. In this way both measurements are obtained independently with a single fiber device.
Compared to other fiber-optic sensors, our sensor features an extremely simple structure and fabrication process, and
hence cost effectiveness.
We present the numerical modeling of the interaction between a spatial soliton and a surface plasmon polariton under
leak and strong coupling in the following two cases: at metal/dielectric/Kerr structures and metal/Kerr structures in 1D.
Here, we solved the vectorial and nonlinear wave equation using a novel iterative method based in self-autoconsistency,
and we found two kinds of nonlinear stationary solutions called odd and even modes. On the other hand, the propagation
of the stationary solutions is performed for the metal/Kerr system, and quantitatively it shows that odd modes are more
stable than even modes when the spatial soliton and surface plasmon are strongly coupled. Also, we analyzed the
influence of the dielectric layer between the metal and Kerr media, and we discuss their implication and feasibility for
applications in photonic nanodevices. Additionally, the advantages and disadvantages of the numeric method used to
obtain the stationary solutions are discussed. The results obtained in this work are reproducible and contributes with new
information for the development of power-tunable photonic nanocircuits based in nonlinear plasmonic waveguides.
We investigated numerically the TM electric field solutions of a dielectric slab formed by a photorefractive crystal with
diffusion-type nonlinearity and limited by two metallic films. This study allows us the analysis of nonlinear surface
optical waves as nonlinear solutions of the photorefractive crystal slab. Additionally, we analyzed the influence of these
nonlinear solutions to excite surface plasmon-polariton waves at the metallic interfaces. In this case, the coupling
between plasmons and nonlinear solutions it is possible because only TM electromagnetic waves are supported by a
metal-dielectric planar waveguide. Here, we solved the vectorial and nonlinear wave equation using an iterative method
based in self-autoconsistency. With this algorithm, the coupling between the waveguide modes and the surface plasmon-polariton
waves are systematically investigated. The results obtained in this work are reproducible and contributes with
new information for the design of tunable plasmonic devices based in nonlinear photorefractive crystals.
Applications of Multimode Interference (MMI) effects in optical devices have been increased today due their excellent
properties and easy fabrication. Incorporation of these effects in optical fiber has been achieved through a single-mode -
multimode - single-mode (SMS) fiber structure showing high sensitivity to bending-loss phenomenon. The latter has
been efficiently implemented in pressure sensing application such as is presented in this work. Basically, the SMS
structure is embedded in a pressure-sensitive membrane to convert pressure in a mechanical displacement resulting in an
attenuation of the transmitted intensity proportionally to the applied pressure. Under this configuration, an all-fiber
pressure sensor with high sensitivity and repeatability is obtained into a pressure range from -13 psi to +13 psi. The max
pressure range can be varied to 140 psi with our configuration when the membrane thickness is changed. Important
features of the proposed all-fiber MMI pressure sensor are its easy-fabrication and low-cost since an inexpensive
instrumentation is required.
We report the fabrication of a saturable absorber made of a novel polymer SU8 doped with Single Wall Carbon
Nanotubes (SWCNTs). A passive mode-locked ring cavity fiber laser was built with a 100 μm thick SU8/SWCNT film
inserted between two FC/APC connectors. Self-starting passively mode-locked lasing operation was observed at 1572.04
nm, with a FWHM of 3.26 nm. The autocorrelation trace was 1.536 ps corresponding to a pulse-width of 871 fs. The
time-bandwidth product was 0.344, which is close enough to transform-limited sech squared pulses. The repetition rate
was 21.27 MHz, and a maximum average output power of 1 mW was also measured.
A novel all-fiber Multimode Interference (MMI) liquid level sensor is proposed and demonstrated. We show that MMI
effects can be effectively applied for multiplexed liquid level sensing, and by selecting an adequate fiber, discrete and
continuous level sensing is feasible. Using a standard 105/125 multimode fiber a simple discrete level sensor was
fabricated, which can also can also discriminate the refractive index of the liquid. When a specialty fiber, know as No-
Core fiber is used, both continuous and discrete level sensing can be achieved. We can also modify at will the
continuous level range by increasing the No-core fiber by the appropriate length, while retaining the ability to determine
the refractive index of the liquid during the level measurement. The MMI liquid level sensors are not only inexpensive,
but their fabrication is quite simple.
In this work an investigation into the viability of a dual gas sensor based on correlation spectroscopy using a single
Fabry-Perot Interferometer (FPI) is presented. Here, based on sensor response simulations, it is demonstrated that the
commonly considered undesirable effect due to multiple internal reflections of the FPI's mirror substrate can be used to
increase the sensing capability of the system. Usually designers tend to minimize these reflections to improve the FPI
transmission spectrum. However we let them to occur in order to used them as a part of the modulation system of the
sensor which allows us to detect two gases simultaneously using a single FPI.
A nonlinear stack is one of the handier photonic crystals where new schemes and
methodologies can be tested. Nonlinear Stacks have shown the presence of
switching, chirping and bistability, but in practice it is hard to find nonlinear
material with the adequate physical and mechanical properties. Metallic
Nanoparticles are well known to have strong nonlinearities and their composites
show the desired nonlinear properties. The nonlinearities are Kerr when
described Quantum Mechanically and field amplitude, when described classically.
In this report we describe the band gap of such classical composite stack.
Spatial and temporal solitons are at the core of many physical, geological, biological, transmission and information
processing and other problems. However, in most cases we have focused on their steady behavior, and therefore on
homogeneous media and their single soliton eigenvalues spectrum. This has been done even in the case of an all optical
simultaneous loss and amplification, where we have assumed stability of those eigenvalues. However, the transient
behavior has received little attention, often disregarded under a generic pulse reshaping or experimentally diafragmed as
often occurs in large amplifiers. But such transient behavior can be frozen in a periodic nonhomogenous media, tandems,
where such behavior corresponds to the soliton convergence in each tandem media, producing a regular but not steady
behavior. We discuss the resonant pulse propagation in a two level atom media tandem, described by a real convergence
and a Kerr intensity dependent nonlinearity, described by a complex convergence.
Waveguides coupling have been widely studied; however, nanowaveguides of high refraction index contrast open the
opportunity of studying the nonlinear dynamics of coupled waveguides, in particular those filled with metallic
nanaoparticles composites. Those composites show a Quantum Mechanical Kerr Nonlinearity and a classical field
amplitude nonlinearity that are compared by using a iterative WKB to introduce the field nonlinearity and based in the
ensuing M matrix. The produced nonlinear supermodes show a confinement of the pulse in the waveguides and a
breaking of the coupling at small and large core waveguides.
We report on the optofluidic tuning of MMI-based bandpass filters. It is well known that MMI devices exhibit their
highest sensitivity when their diameter (D) is modified, since they have a D<sup>2</sup> wavelength dependence. In order to
increase the MMF diameter we use a special fiber, called No-Core fiber, which is basically a MMF with a diameter of
125 μm with air as the cover. Therefore, when this No-Core fiber is immersed in liquids with different refractive indexes,
as a result of the Goes-Hänchen shift the effective width (fundamental mode width) of the No-Core fiber is increased,
and thus the peak wavelength is tuned. A tunability of almost 40 nm in going from air (n=1.333) to ethylene glycol
(n=1.434) was easily obtained, with a minimum change in peak transmission, contrast, and bandwidth. Moreover, since
replacing the entire liquid can be difficult, the device was placed vertically and the liquid was covering the No-Core fiber
in small steps. This provided similar amount of tuning as before, but a more controllable tuning mechanism.
We report on a novel tuning mechanism to fabricate an all-fiber tunable laser based on multimode interference (MMI)
effects. It is well known that the wavelength response of MMI devices exhibits a linear dependence when the length of
the multimode fiber (MMF) section. Therefore, tuning in the MMI filter is achieved using a ferrule (capillary tube of 127
μm diameter) filled with a liquid with a higher refractive index than that of the ferrule, which creates a variable liquid
MMF. This liquid MMF is used to increase the effective length of the MMI filter and tuning takes place. Using this
simple scheme, a tuning range of 30 nm was easily achieved, with very small insertion losses. The filter was tested
within a typical Erbium doped fiber (EDF) ring laser cavity, and a tunable EDF laser covering the full C-band was
demonstrated. The advantage of our laser is of course the simplicity of the tunable MMI filter, which results in an
inexpensive tunable fiber laser.
We report on a novel all-fiber refractometer sensor based on multimode interference (MMI) effects. The operating
mechanism is based on the self-imaging phenomena that occur in the multimode fiber (MMF) section, which basically
replicates the field at the input of the MMF on the output of the MMF for a specific wavelength. However, the
longitudinal position of this image is highly dependent on the MMF diameter (D), since there is D<sup>2</sup> dependence on the
longitudinal position of this image. For the refractive index measurement a section of no-core multimode fiber, whose
cladding is air, is surrounded by the liquid sample. The liquid sample now works as the cladding medium and as a result
of the Goes-Hanchen shift the effective width (fundamental mode width) of the No-Core fiber is increased. As a result,
the maximum coupling resulting from the imaging phenomena occurs at a different wavelength, and this can be used to
measure the refractive index of the liquid. Using this scheme we can achieve a resolution on the order of 1x10<sup>-5</sup> for a
refractive index range from 1.333 to 1.434. The device was used here to measure refractive index in liquids, but can also
be applied for measuring concentration of liquids. These sensors are promising and attractive in chemical and
biotechnological applications because of their high sensitivity, immunity to electromagnetic interference, and compact
The Erbium doped fiber laser (EDFL) has demonstrated to be the ideal source for optical communications due to its
operating wavelength at 1550 nm. Such wavelength matches with the
low-loss region of silica optical fiber. This fact has
caused that the EDFL has become very important in the telecomm industry. This is particularly important for Dense
Wavelength Division Multiplexing (DWDM) which demands the use of single emission sources with different emission
wavelengths. In the long run, this increases the capacity of transmission of information without the necessity to increase
the infrastructure, which makes tunable laser sources an important component in DWDM applications. Many techniques
for tuning have been demonstrated in the state of the art and we can mention, for example, the ones using birefringence
plates, bulk gratings, polarization modified elements, fiber Bragg gratings, and very recently the use of multimode
interference (MMI) effects. The MMI consists in the reproduction of single images at periodic intervals along the
propagation direction of a multimode optical fiber, taking into account that these single images come from a single mode
Here, a compact, tunable, erbium-doped fiber laser is experimentally demonstrated. The mechanism for tuning is based
on the multimode interference self-imagining effect, which results in a tunable range of 12 nm and optical powers of
1mW within the region of 1549.78-1561.79nm.
We report for the first time, a single mode, tunable, double-clad ytterbium-fiber (YDF) laser emitting in a
wavelength range between 976 and 985 nm that operates using the re-imaging effect that occurs in multimode
interference (MMI) devices. The system consists of an YDF with bare fiber cleaved ends. The forward end of this fiber is
fusion spliced to a piece of 3 m of Samarium-doped- single-mode fiber with absorption measured at 980 nm of 0.3 dB/m,
and at 1030 nm of 6 dB/m. The other end of the Sm<sup>+3</sup> doped single-mode fiber is spliced to a 16.2 mm long multimode
fiber (MMF) in order to induce the MMI self-imaging effect. From simulations, we found that, at this particular length,
for the MMF, the light exiting will exhibit a maximum transmission for the 980 nm wavelength, while keeping a
minimum for the 1030 nm wavelength. Near to the MMF facet, at a distance between 0 and 100 µm, we place a dichroic
mirror which also helps in the selection of the wavelength emission. We calculated that 10 dB gain generated at 980 nm
is enough to build up a laser since the total round-trip cavity losses are estimated to be 8.8 dB, whereas for the unwanted
1030nm get more than 60dB insertion loss in this setup. At the end, there is more than 1 dB for the effective gain at the
preferred wavelength emission range which is enough to promote lasing at around 980 nm.
We demonstrate the use of an area selective zinc in-diffusion technique as a simple and efficient technique for the
fabrication of integrated photonic devices. In this work, the zinc in-diffusion process has a two fold application. It is well
known that the diffusion of zinc in InP follows an interstitial-substitutional diffusion mechanism. This provides a
concentration dependent diffusion profile, which allows us to control the sharpness of the diffusion front by controlling
the background doping concentration of the semiconductor wafer. By controlling the zinc depth combined with a sharp
diffusion front, the insertion losses of the devices can be minimized. In addition, this results in selective definition of p-n
junctions across the semiconductor wafer and therefore offers the potential for integration with electronic devices. Using
this technique an integrated 2x2 Mach-Zehnder modulator/switch was fabricated. The semiconductor wafer is based on
InGaAsP multiple quantum wells. To selectively define p-n regions for the contacts, we use a 200-nm thick silicon
nitride mask during the diffusion. The Mach-Zehnder structure is then patterned using photolithography and dry etching.
After a cyclotene planarization process, p-type contacts are deposited on top of the diffused regions by evaporation and
lift-off. Our experimental results demonstrate that on-chip losses on the order of 4-dB are obtained, which is
significantly lower compared to the use of isolation trenches. The device response as a modulator requires an additional
insertion loss of 3-dB for voltage controlled operation, with an extinction ratio better than 16 dB. In the case of electrical
current operation, better than 20 dB extinction ratio was obtained with only 8 mA.
A very simple and cost-effective technique for wavelength tuning a fiber laser using multimode interference effects is
demonstrated. The tuning mechanism relies on the self-imaging effect which occurs in multimode waveguides. The
tuning mechanism consists of a section of multimode fiber (MMF) spliced to a single mode fiber (SMF), with a
broadband mirror located at the other end of the MMF. The signal coming out of the SMF will be imaged within the
MMF at a very specific location. Therefore, if the length of the MMF is slightly shorter than this length, the image will
be formed in free space. By placing the mirror at this position, the light is reflected back through the MMF and SMF.
Since the self-imaging is wavelength dependent, the position of the re-imaging point will depend on the wavelength, and
the laser wavelength can then be tuned by varying the distance between the MMF facet and the broadband mirror. To
obtain a stable system and easy to align mechanism an integrated fiber gripper was fabricated on silicon wafer. This
novel tuning device was incorporated into a double-clad Ytterbium-doped fiber laser (DCYDF), and the tuning
characteristics were evaluated by varying the distance between the broadband mirror and the output facet of the MMF.
The mirror was moved in 25 microns steps, and the optical power and spectrum measured at every step. A tunability of
12.24 nm was measured with this implementation, and the laser system was shown to be very robust and highly stable.
We believe that further improvement in our system will lead to a wider tuning range.
We demonstrate an integrated 1x3 optical switch that operates using the principle of carrier-induced refractive index change in InGaAsP multiple quantum wells. The core of the switch relies on a beam-steering concept which allows us to steer the optical beam to any of three output waveguides. The device is relatively simple, since current is only applied to two electrodes for complete operational control. The device integration is achieved using an area-selective zinc in-diffusion process that is used to channel the currents into the multiple quantum wells, thereby enhancing the efficiency of the carrier-induced effects. This results in a low electrical power consumption, allowing the switch to be operated uncooled and under d.c. current conditions. The crosstalk between channels is better than -17 dB over a range of 50 nm centered at 1565 nm.
A tunable multimode interference (MMI) coupler that operates by modifying the phase of the multiple images that are
formed around the midpoint of the MMI section is demonstrated. The phase change is achieved by current injection, and
therefore minimizing current spreading is crucial for optimal operation. A zinc in-diffusion process has been
implemented to selectively define p-i-n regions and effectively regulate the current spreading by controlling the depth of
the zinc doping. Using this process a tunable 3-dB MMI coupler has been fabricated. Our initial results show that the
device can be easily tuned all the way from a 90:10 to a 30:70 splitting ratio of the optical power transmitted through the
two output ports. We believe that further improvement on the device fabrication will lead to a more symmetric tuning
response of the device. Nevertheless, the initial results are very encouraging since, to our knowledge, this degree of
tuning has never been experimentally demonstrated in similar MMI devices. Furthermore, this device processing
technique can easily be applied to a wide variety of semiconductor photonic switches that operate on MMI effects.
We propose a robust, multi-mode interferometer-based, 2x2 photonic switch, which demonstrates high tolerance to typical fabrication errors and material non-uniformity. This tolerance margin is dependent upon the properties inherent to the MMI design and benefits from the high symmetry of the switch. The key design parameter of the device is to form a pair of well defined self-images from the injected light in the exact center of the switch. In allowing the index modulated regions to precisely overlap these positions, and by creating identical contact features there, any refractive index change induced in the material due to electrical isolation will be duplicated in both self-images. Since the phase relation will remain unchanged between the images, the off-state output will be unaltered. Similarly, offset and dimension errors are reflected symmetrically onto both self-images and, as a result, do not seriously impact the imaging. We investigate the characteristics of the switch under different scenarios using the finite difference beam propagation method. Crosstalk levels better than -20 dB are achievable over a wavelength range of 100 nm when utilizing this configuration. Polarization independence is maintained during device operation.
We present a fast and efficient numerical model for Yb<sup>3+</sup>-doped fiber lasers based on shooting method. The algorithm is based on the assumption of a starting value for the slope efficiency and the evaluation of the pump power threshold. The starting value of slope efficiency is related to initial conditions through the boundary conditions, and it is subsequently optimized by iteration. The method ensures a fast and efficient convergence of the solution of the coupled first-order differential equations that describes the evolution of pump and signal powers in a Yb<sup>3+</sup>-doped fiber laser. The results of the numerical solution are compared with experimental and published data giving a good agreement.
An analysis of out-coupling in a laser shows an optimum way of subtracting more output power by choosing an appropriate cavity arrangement from a high-power fiber laser. This investigation consisted in resolving analytically the effect of different cavities in our laser system and one thing that outcome was to know that a fiber laser can operate with high efficiency even with high losses in one end of the cavity (e.g. at an external diffraction grating), only if the feedback in the out-coupling end is low. Moreover, it was also found that is possible to improve the output power by reducing the feedback in the out-coupling end. Parameters considered in this resolved method are 0.1 NA, 10 μm diameter core, 200 μm inner-cladding diameter and 10 dB small-signal absorption. The fiber laser was doped with ytterbium and lases at 1080 nm, when pumped at 915 hm. The maximum pump power was set to 10 W.
We report on an integrated 1 x 4 optical switch that operates using the principle of carrier-induced refractive index change in InGaAsP multiple quantum wells. The device is very simple, requiring only the currents applied to two electrodes for complete operational control. An area-selective zinc in-diffusion process is used to channel the current into the multiple quantum wells, thereby enhancing the efficiency of the carrier based effects. As a result, the electrical power consumption of the device is significantly reduced, allowing the switch to be operated uncooled and under d.c. current conditions. Our initial 1 x 4 switch exhibits a -8 dB crosstalk between channels. However, improvements on the switch design and better control during the device fabrication process will significantly enhance this value.
We propose a new structure for an integrated variable optical attenuator using InGaAsP multiple quantum wells. The principle of operation relies on the self-imaging properties of multimode interference (MMI) waveguides. The device consists of a MMI region that is 12 μm wide by 350 μm long, with input and output waveguides that are 2 μm wide. The dimensions of the MMI are calculated such that an image of the input field is produced at the output waveguide. The last statement is true as long as the phase relation between the modes in the MMI section is kept constant. Therefore, by selectively perturbing the refractive index within the MMI section, the phase relation of the modes is altered, thereby modifying the interference properties at the output of the device. We present numerical simulations using the Finite-Difference Beam Propagation Method (FD-BPM), and demonstrate that optical attenuation is possible by selectively modulating the refractive index of a narrow region within the MMI section. A dynamic range of -37 dB can be easily obtained at a wavelength of 1.55 μm with a device insertion loss of 0.3 dB. The effects of electro-absorption on the device performance are also investigated.
We demonstrate the use of an area selective zinc in-diffusion technique to fabricate an integrated InP/InGaAsP Mach-Zehnder optical switch. The zinc in-diffusion process using a semi-sealed open-tube diffusion furnace was characterized to enable the creation of p-n junctions at a precise depth in selected areas of the device sample. The method is simple, yet highly controllable and reproducible; with the crystal quality remaining intact after the diffusion process is complete. Using this technique an integrated 1 x 2 Mach-Zehnder optical switch has been fabricated. Our preliminary devices show a switch contrast ratio of 12 dB with a voltage swing of ±2.5 volts. Improving our fabrication process will further optimize the performance of the switch. Nevertheless, very good electrical isolation is obtained between the contacts, which demostrates the potential of the technique for the fabrication of Photonic Integrated Circuits.
We report an optical switch that is based on the beam steering of an optical waveguide formed by injection of electrons in a p-i-n slab waveguide structure. The structure consists of an undoped InGaAsP multiple quantum well (MQW) layer, with a total thickness of 0.28 μm that is sandwiched between n-doped InP cladding layers. Zinc is diffused into the top cladding layer through a silicon nitride mask to form the p-regions on top of which a pair of 10 um wide parallel titanium-zinc-gold contact stripes are deposited by evaporation and lift-off. The gap between the stripes is 20 μm wide and the device is cleaved to a length of 800 um. Electrical currents are injected through the electrodes and a laser beam is launched into the middle of the gap region. The injected electrons accumulate in the MQW layer and spread sideways by diffusion. The regions that are saturated with electrons experience a decrease in refractive index and surround a narrow high index region effectively forming a channel waveguide. By carefully controlling the current ratio through the two parallel stripes, the waveguide can be shifted, thereby steering the guided laser beam.
We have developed a program for phase shift measurement on fringes pattern. Our program has capability to measure displacement of vertical fringes, tilted, curvy, and circular fringes, because of the use of pattern correlation.