- Email: [email protected]
Design and fabrication of all-polymeric photonic waveguides in optical integrated circuits
Accepted Manuscript Title: Design and Fabrication of All-Polymeric Photonic Waveguides in Optical Integrated Circuits Authors: Abbas Madani, Hamid Reza Azarinia PII: DOI: Reference:
S0030-4026(17)30281-4 http://dx.doi.org/doi:10.1016/j.ijleo.2017.03.021 IJLEO 58944
To appear in: Received date: Revised date: Accepted date:
10-10-2014 1-11-2016 4-3-2017
Please cite this article as: Abbas Madani, Hamid Reza Azarinia, Design and Fabrication of All-Polymeric Photonic Waveguides in Optical Integrated Circuits, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2017.03.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design and Fabrication of All-Polymeric Photonic Waveguides in Optical Integrated Circuits Abbas Madani1, 2*, Hamid Reza Azarinia2 1
Institute for Integrative Nanosciences, Leibniz Institute for Solid State and Materials Research Dresden, Germany 2 Liquid Crystals & Photonics Group, ELIS Department, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium Email: [email protected] [email protected]
Abstract: In this work, the design and fabrication of the polymeric passive all optical waveguides, directional coupler and micro-ring resonator as tunable all pass flirting devices based on very interesting, simple, and inexpensive laser beam direct write lithography (LBDW) technique are reported. The LBDW approach is very robust and promising method for fabrication of the complex polymer optical devices. A continuous wave (CW) laser operating at 442 nm with a writing speed of 3.5 mm/s and a typical optical output power 120 mW is used in the laser beam direct writing system. These all passive optical devices have been fabricated based on optical ORMOCORE and ORMOCLAD negative photoresist which are inorganic-organic hybrid polymer, because of their very low losses at telecom wavelengths (1550 nm and 1300 nm), well controlled refractive indices, and ease of processing, good thermal stabilities, and good adhesion on a wide range of substrates. Optical characterization of these optical devices and the resulting optical waveguide and ring resonators structures are shown to ensure suitability for implementation in optical integrated circuits applications.
Keywords: polymeric optical waveguides, polymeric directional coupler, polymeric ring resonators, fiber tapered, laser beam direct write technique.
The passive and active optical devices such as photonic waveguides, directional coupler and optical micro ring resonators which can be defined as a physical structure that allows the confinement of light within its boundaries by total internal reflection (TIR) are the fundamental elements for optical integrated circuits (OICs) for guiding, transferring, switching or confinement electromagnetic waves in the optical spectrum [1-5]. Optical waveguides can be classfied based on their materials such as galss, polymer and semicoundctor, and/or to be single mode or multi mode and their configuration for example planar, strip or fiber waveguides and usual type of optical waveguides include optical fiber or rectangular waveguides as transmistion medium for optical communication systems [1, 5,6-14]. Directional couplers are disigned with two input ports and two output ports optical waveguides and gives this opportunity to switch light from one port to other port and vice versa or divide and distribute optical power [2,5-22]. Micro ring resonators proved a capability to trap, rout, and store light in a small volume and at well-defined spectral intervals [6 -32]. Up to now, these optical devices have been extensively investigated and fabricated from many different materials for difference application. Recently, it is well know that to have flexible and low cost optical devicesand integrated optics are becoming dramatically important and becoming highly demand and nececary in the near future. Polymer optical devices are good choice and can be expected to meet these requirements [22, 33, 34]. So far, several kind of materials are used in the fabrication of optical elements such as semiconductor for example Si, SiNx, SiO2 and or even lithiumniobate that have uses in both passive and active optical components. Optical fiber which is made of glass (SiO2) materials as the core layer is the most well know case of a special waveguide. Among the possible materials under investigation for fabricating OICs which mentioned above very briefly, recently photosensitive polymers show promising and popular materials and are increasingly becoming an attractive option in waveguides and ring resonators fabrication because of their low cost, their well-controlled refractive indices, their low-temperature fabrication procedures, their compatibility with a variety of substrates providing the possibility of
integration
with
electronic
integrated
circuits
(ICs)
to
offer
enhance
functionality. Moreover, their potential applications in optical communications, optical interconnections, and integrated optics make these materials most important for researchers [5,
22, 34, 35]. Additionally polymer materials are better suited in board level electronics than other materials [35] and good thermal stabilities, ease of processing. There are several techniques for the fabrication of optical elements for example techniques of UV photolithography, electron beam lithography, nano-imprint lithography, ion implantation, and laser direct writing which are explored very extensively up to now. Photolithography techniques used to create smaller features by improving its resolution and is almost exclusively the process of choice to transfer patterns from a mask onto the targeted thin film [1-32]. Photolithography has been used in single and multimode waveguide, channel waveguides or even Y-branch power splitters directional coupler, ring resonators and many other optical elements fabrication by using commercially-available polymers or also by using many other materials. Several polymer optical components have been fabricated with this technique by using an inorganic-organic hybrid polymer capable of refractive index tuning [22, 33, 36]. Photolithography technique has been an applicable and great approach of fabricating microelectronic devices for many years [28-32]. But creating a suitable mask was very time consuming and expensive step. Electron beam lithography (EBL) is also other technique which can be provided a high-resolution pattern in which high-energy electrons (1 up to 100 keV) are focused to an extremely small beam. Optical waveguide components with large radius curves and sub-micron period grating structures, photonic crystal and ring resonators based on Si and other optical components for advanced laser systems have been be fabricated and explored by using EBL technique. The main disadvantage of this high-resolution technique is its long production time. In addition, there is a need for a vacuum due to the electron beams are charged particles, which causes more complex and expensive imaging systems. Nowadays, laser beam direct writing (LBDW) systems [22, 33, 37, 38] are rapidly growing as an attractive and good solution in the fabrication of 2D and 3D photonic waveguide components [38] because of many advantage of this technique compare to other technique such as in this technique, first of all to have a clean room is not very necessary which allows one to be fabricated them in a normal environment and this is the short process time which typically takes several seconds or maybe several minutes. It worth to nothing that very recently, it was found that ultrafast laser direct write 3D lithography technique may be offered unmatched resolution and flexibility for the fabrication and integration of micro-optical and photonic devices, for more information see refe.37.
In this work, the design and fabrication of the polymeric passive all optical waveguides, directional coupler and micro ring resonator as all pass flirting devices based on very interesting, simple, and inexpensive laser beam direct write lithography technique (LBDW) approach is reported. LBDW technique looks to be a very fascinating and promising fabrication approach for polymeric optical devices due to its parallel low cost, simplicity and enough-resolution properties. A low-power laser operating (120 mW) at 442 nm and with a typical a writing speed of 3.5 mm/s is used in the laser beam direct writing system. These all passive optical devices have been fabricated based on optical ORMOCORE and ORMOCLAD negative photoresist which are inorganic-organic hybrid polymer, because of their very low losses at telecom wavelengths (1550 nm and 1300 nm), well controlled refractive indices, and ease of processing, good thermal stabilities, and good adhesion on a wide range of substrates such as glass and silicon. Optical characterization of these optical devices and the resulting optical waveguide and ring resonators structures are shown to ensure suitability for implementation in OICs applications. The laser direct writing system used in this study is illustrated in Figure 1.
Fig.1.A schematic illustration of LBWD setup which is used in this experiment.
A few components are required in the construction of this system which lends itself to inexpensive initial setup costs. In this setup, we used linear polarized CW Nd-Yag laser (i.e.
green laser) for autofocusing and other CW UV laser for writing with a wavelength at 442 nm and with typical optical output power 120 mW. A 20X objective lens is fine-tuned with a manual actuator so that the spot size, or focal waist, is as small as possible (less than ≈2 μm) to provide the best pattern resolution. It should note that by optimization all of the optical elements used in our LBWD setup (as shown schematically in Figure.1), it is possible to increase the resolution of our system down to 1 μm. Moreover, it is worth to note that based on the mechanical properties of the x-y-stage transporting the coated substrate with ormocore and ormoclad, the writing speed of our setup was limited to 3.5 mm/s. It worth mentioning that we used linear polarized laser beam in order to write our photonic elements and it was not very critical to use in plane or out of plane polarization but what was important that the laser has linear polarization. Very recently, it was found and studied that the polarization of laser beam is very important on the feature size or on the other words on the resolution of patterning and writing. For example S. Rekštytė and et al. [39] experimentally and theoretically demonstrated that polarization effects, partially in 3D nanolithography, can successfully be applied to fine tune the feature sizes in the structuring of photoresist which is coated on the substrate. The process of fabrication the optical elements of this paper, includes four main steps. First of all, the substrate is cleaned to make sure maximal adhesion of the polymers to the substrate. Second of all, a first layer of lower cladding (ORMOCLAD) material is deposited on the substrate and cured by UV lamp. Finally, the optical pattern is written to the substrate by using laser beam direct writing, and the development of the core layer material. As it is wellkwon that the uniformity and adhesion of the film to the substrate depend highly how well can be cleaned. Hence, they need to be cleaned with great attention. For cleaning of them, first, substrates are immersed in a beaker containing acetone for a couple of the minutes in an ultrasonic bath followed by 3 minutes ultrasonic bath in isopropyl alcohol. Substrates are then blow dried with compressed nitrogen, followed by baking in an oven at 100°C for two hours. The substrates are now ready for other fabrication processes. ORMOCLAD as a negative photoresist layer is deposited to the cleaned silicon substrate by spin on coating. This produces a film that is uniform a cross the substrate. Thin film thickness is depends on the spin speed, the viscosity of the material, and the ramping acceleration of photo resist spinner. After the cladding layer is spun on to the substrate around 8 μm, it is placed under UV lamp which emits a peak wavelength at 442 nm for rapid curing for a couple of minutes. The cladding layer is then baked
on a hotplate to ensure maximum adhesion to the substrate as well as bake out any remaining solvents. The cured cladding layer remains on the substrate and ready for the core layer that will be waveguide patterned. After the ORMOCORE as a core material is spun coated on top of the cladding layer, the LBWD system is used to write the waveguide, or even ring resonators. The fabricated one array of polymeric waveguides are shown in Fig. 1 which consists of a layer ORMOCLAD of 5 μm top of the substrate, ORMOCORE (as a second layer on substrate) waveguides of 2.5 μm in height and five centimeters in length. In the last step, a hard baking might be performed at temperatures of 150 °C for a couple of minutes to increase the adhesion to the substrate depending on the final application for the waveguides.
Fig. 2. Optical images of an array of the polymer waveguides. Insert, optical image of polymer waveguide with higher magnification.
For fabrication of directional coupler and ring resonators, we can follow the same process, the only diffrence is that after spin coating clad layer, we can write two waveguides instead on one waveguide with only 2 μm gap between them by following the same process which is expalined obve that is shown that in figure 3. We can repeat the same fabrication procees for ring resonators, it means that; after one layer of ORMOCLAD about 8 μm is coated by means of spinner top of the substrate which is cured by using UV light around 2 min, then second layer of ORMOCORE of 2.5 μm is coated on first layer (ORMOCLAD) by means on spinner (4500 rpm at 45 s). After that we wrote a polymer straight waveguides and one ring (in racetrack ring shape) by LBDW technique. The fabricated microring device is shown in Fig. 3 which consists of a
layer ORMOCLAD of 5 μm top of the substrate and ORMOCORE (as a second layer on substrate) waveguides of 2.5 μm in height with a coupling gap distance of 900 nm between the micro ring and the straight waveguide.
Fig. 3. Optical image of a fabricated polymeric ring resonator based on all pass filter
It is also possible to write very easily other bus waveguide on top of the ring resonators by following and repeating the same approach to have an add/drop microring resonators that is shown in figure 4.
Fabrication and optical characterization of this add/drop filter has already
reported it in this paper [7]
Fig. 4. Optical image of a polymeric micro ring resonator with add/drop configuration.
By following this technique, it was also possible to write three ring resonators between these two optical bus waveguides for slowing light or many other interesting applications which is shown in figure 6.
Fig. 5. One array of optical images a polymeric ring resonators
One more interesting application of ring resonators is to use them as sensor, one simple idea is to make add/drop filter on top of the copper (Cu) instead of glass and then by heating the substrate, it is possible to shift the resonance modes. For this one, we can deposit on thick layer of Cu on top of the galls by using electron deposition and then we can repeat the same process which is explained that at above. This process is illustrated in figure 6 (a) and the one real optical image of this sensor is shown in figure 6 (b). (a)
Glass
(b)
Glass + Cu
Fig.6 (a) Following step to write add/drop microring on top of the Cu, (b) optical image of add/drop microring resonators on top of the Cu
For optical characterization of our optical devices, we start first quick tests of optical wave guiding by using a simple butt coupling experimental setup which is shown in figure. 7. In this setup we have used a tapered fiber which is made of by CO2 laser in our lab.
Fig. 7. Schematic of the horizontal setup for coupling light into the bus waveguide.
To couple light into the bus waveguide and received the signal from the output port of the waveguide using another tapered fiber. The signal has been monitored using an optical spectrum analyzer therefore we could analyses the output from other port of the waveguide which is shown in figure 8. As it is shown from figure 8 that this polymer optical waveguide in the laterally coupled geometry for wavelength 1550 nm works very well.
Fig. 8. The output of polymeric optical waveguide.
For optical characterization of optical polymeric ring resonators, first of all in order to the better coupling what we have done, the gap (which is at 900 nm) of the fabricated micro ring resonator is then filled with nitrobenzene liquid by micropipette (which has large dependence of the refractive index on temperature) for increasing efficiently coupling to ring waveguide. We used a horizontal butt-coupling setup which is shown in figure. To couple light into the bus waveguide and received the signal from the output port of the waveguide using another tapered fiber. The signal has been monitored using an optical spectrum analyzer therefore we could analyses the output from other port of top waveguide which is shown in figure 9. As it is shown from figure 9 by blue color that the micro ring resonator in the laterally coupled geometry for wavelength 1550 nm have band width (Δλ) 0.8 nm, and by increasing the temperature of the step, we can change the refractive index and as a result the resonance mode is shifted to longer wavelength which is clear in figure 10 by green color.
Fig. 9. The normalized output of polymeric optical ring resonator, blue one before heating up the sample, the green one after heating up the sample.
In conclusion, the several type of passive optical devices such as waveguides, directional coupler and micro ring resonators of optical devices have been fabricated based on inorganicorganic hybrid polymers materials by using laser beam direct write technique and these optical devices have been successfully characterized. Laser beam direct write technique is shown to be a suitable fabrication technique for polymer integrated optics due to the enough resolution, highly simple and low-cost provided. Optical characterizations of these optical devices have been successfully performed and the resulting optical waveguide and ring resonators structures are shown to ensure suitability for implementation in Optical Integrated Circuits applications.
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S0030-4026(17)30281-4 http://dx.doi.org/doi:10.1016/j.ijleo.2017.03.021 IJLEO 58944
To appear in: Received date: Revised date: Accepted date:
10-10-2014 1-11-2016 4-3-2017
Please cite this article as: Abbas Madani, Hamid Reza Azarinia, Design and Fabrication of All-Polymeric Photonic Waveguides in Optical Integrated Circuits, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2017.03.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design and Fabrication of All-Polymeric Photonic Waveguides in Optical Integrated Circuits Abbas Madani1, 2*, Hamid Reza Azarinia2 1
Institute for Integrative Nanosciences, Leibniz Institute for Solid State and Materials Research Dresden, Germany 2 Liquid Crystals & Photonics Group, ELIS Department, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium Email: [email protected] [email protected]
Abstract: In this work, the design and fabrication of the polymeric passive all optical waveguides, directional coupler and micro-ring resonator as tunable all pass flirting devices based on very interesting, simple, and inexpensive laser beam direct write lithography (LBDW) technique are reported. The LBDW approach is very robust and promising method for fabrication of the complex polymer optical devices. A continuous wave (CW) laser operating at 442 nm with a writing speed of 3.5 mm/s and a typical optical output power 120 mW is used in the laser beam direct writing system. These all passive optical devices have been fabricated based on optical ORMOCORE and ORMOCLAD negative photoresist which are inorganic-organic hybrid polymer, because of their very low losses at telecom wavelengths (1550 nm and 1300 nm), well controlled refractive indices, and ease of processing, good thermal stabilities, and good adhesion on a wide range of substrates. Optical characterization of these optical devices and the resulting optical waveguide and ring resonators structures are shown to ensure suitability for implementation in optical integrated circuits applications.
Keywords: polymeric optical waveguides, polymeric directional coupler, polymeric ring resonators, fiber tapered, laser beam direct write technique.
The passive and active optical devices such as photonic waveguides, directional coupler and optical micro ring resonators which can be defined as a physical structure that allows the confinement of light within its boundaries by total internal reflection (TIR) are the fundamental elements for optical integrated circuits (OICs) for guiding, transferring, switching or confinement electromagnetic waves in the optical spectrum [1-5]. Optical waveguides can be classfied based on their materials such as galss, polymer and semicoundctor, and/or to be single mode or multi mode and their configuration for example planar, strip or fiber waveguides and usual type of optical waveguides include optical fiber or rectangular waveguides as transmistion medium for optical communication systems [1, 5,6-14]. Directional couplers are disigned with two input ports and two output ports optical waveguides and gives this opportunity to switch light from one port to other port and vice versa or divide and distribute optical power [2,5-22]. Micro ring resonators proved a capability to trap, rout, and store light in a small volume and at well-defined spectral intervals [6 -32]. Up to now, these optical devices have been extensively investigated and fabricated from many different materials for difference application. Recently, it is well know that to have flexible and low cost optical devicesand integrated optics are becoming dramatically important and becoming highly demand and nececary in the near future. Polymer optical devices are good choice and can be expected to meet these requirements [22, 33, 34]. So far, several kind of materials are used in the fabrication of optical elements such as semiconductor for example Si, SiNx, SiO2 and or even lithiumniobate that have uses in both passive and active optical components. Optical fiber which is made of glass (SiO2) materials as the core layer is the most well know case of a special waveguide. Among the possible materials under investigation for fabricating OICs which mentioned above very briefly, recently photosensitive polymers show promising and popular materials and are increasingly becoming an attractive option in waveguides and ring resonators fabrication because of their low cost, their well-controlled refractive indices, their low-temperature fabrication procedures, their compatibility with a variety of substrates providing the possibility of
integration
with
electronic
integrated
circuits
(ICs)
to
offer
enhance
functionality. Moreover, their potential applications in optical communications, optical interconnections, and integrated optics make these materials most important for researchers [5,
22, 34, 35]. Additionally polymer materials are better suited in board level electronics than other materials [35] and good thermal stabilities, ease of processing. There are several techniques for the fabrication of optical elements for example techniques of UV photolithography, electron beam lithography, nano-imprint lithography, ion implantation, and laser direct writing which are explored very extensively up to now. Photolithography techniques used to create smaller features by improving its resolution and is almost exclusively the process of choice to transfer patterns from a mask onto the targeted thin film [1-32]. Photolithography has been used in single and multimode waveguide, channel waveguides or even Y-branch power splitters directional coupler, ring resonators and many other optical elements fabrication by using commercially-available polymers or also by using many other materials. Several polymer optical components have been fabricated with this technique by using an inorganic-organic hybrid polymer capable of refractive index tuning [22, 33, 36]. Photolithography technique has been an applicable and great approach of fabricating microelectronic devices for many years [28-32]. But creating a suitable mask was very time consuming and expensive step. Electron beam lithography (EBL) is also other technique which can be provided a high-resolution pattern in which high-energy electrons (1 up to 100 keV) are focused to an extremely small beam. Optical waveguide components with large radius curves and sub-micron period grating structures, photonic crystal and ring resonators based on Si and other optical components for advanced laser systems have been be fabricated and explored by using EBL technique. The main disadvantage of this high-resolution technique is its long production time. In addition, there is a need for a vacuum due to the electron beams are charged particles, which causes more complex and expensive imaging systems. Nowadays, laser beam direct writing (LBDW) systems [22, 33, 37, 38] are rapidly growing as an attractive and good solution in the fabrication of 2D and 3D photonic waveguide components [38] because of many advantage of this technique compare to other technique such as in this technique, first of all to have a clean room is not very necessary which allows one to be fabricated them in a normal environment and this is the short process time which typically takes several seconds or maybe several minutes. It worth to nothing that very recently, it was found that ultrafast laser direct write 3D lithography technique may be offered unmatched resolution and flexibility for the fabrication and integration of micro-optical and photonic devices, for more information see refe.37.
In this work, the design and fabrication of the polymeric passive all optical waveguides, directional coupler and micro ring resonator as all pass flirting devices based on very interesting, simple, and inexpensive laser beam direct write lithography technique (LBDW) approach is reported. LBDW technique looks to be a very fascinating and promising fabrication approach for polymeric optical devices due to its parallel low cost, simplicity and enough-resolution properties. A low-power laser operating (120 mW) at 442 nm and with a typical a writing speed of 3.5 mm/s is used in the laser beam direct writing system. These all passive optical devices have been fabricated based on optical ORMOCORE and ORMOCLAD negative photoresist which are inorganic-organic hybrid polymer, because of their very low losses at telecom wavelengths (1550 nm and 1300 nm), well controlled refractive indices, and ease of processing, good thermal stabilities, and good adhesion on a wide range of substrates such as glass and silicon. Optical characterization of these optical devices and the resulting optical waveguide and ring resonators structures are shown to ensure suitability for implementation in OICs applications. The laser direct writing system used in this study is illustrated in Figure 1.
Fig.1.A schematic illustration of LBWD setup which is used in this experiment.
A few components are required in the construction of this system which lends itself to inexpensive initial setup costs. In this setup, we used linear polarized CW Nd-Yag laser (i.e.
green laser) for autofocusing and other CW UV laser for writing with a wavelength at 442 nm and with typical optical output power 120 mW. A 20X objective lens is fine-tuned with a manual actuator so that the spot size, or focal waist, is as small as possible (less than ≈2 μm) to provide the best pattern resolution. It should note that by optimization all of the optical elements used in our LBWD setup (as shown schematically in Figure.1), it is possible to increase the resolution of our system down to 1 μm. Moreover, it is worth to note that based on the mechanical properties of the x-y-stage transporting the coated substrate with ormocore and ormoclad, the writing speed of our setup was limited to 3.5 mm/s. It worth mentioning that we used linear polarized laser beam in order to write our photonic elements and it was not very critical to use in plane or out of plane polarization but what was important that the laser has linear polarization. Very recently, it was found and studied that the polarization of laser beam is very important on the feature size or on the other words on the resolution of patterning and writing. For example S. Rekštytė and et al. [39] experimentally and theoretically demonstrated that polarization effects, partially in 3D nanolithography, can successfully be applied to fine tune the feature sizes in the structuring of photoresist which is coated on the substrate. The process of fabrication the optical elements of this paper, includes four main steps. First of all, the substrate is cleaned to make sure maximal adhesion of the polymers to the substrate. Second of all, a first layer of lower cladding (ORMOCLAD) material is deposited on the substrate and cured by UV lamp. Finally, the optical pattern is written to the substrate by using laser beam direct writing, and the development of the core layer material. As it is wellkwon that the uniformity and adhesion of the film to the substrate depend highly how well can be cleaned. Hence, they need to be cleaned with great attention. For cleaning of them, first, substrates are immersed in a beaker containing acetone for a couple of the minutes in an ultrasonic bath followed by 3 minutes ultrasonic bath in isopropyl alcohol. Substrates are then blow dried with compressed nitrogen, followed by baking in an oven at 100°C for two hours. The substrates are now ready for other fabrication processes. ORMOCLAD as a negative photoresist layer is deposited to the cleaned silicon substrate by spin on coating. This produces a film that is uniform a cross the substrate. Thin film thickness is depends on the spin speed, the viscosity of the material, and the ramping acceleration of photo resist spinner. After the cladding layer is spun on to the substrate around 8 μm, it is placed under UV lamp which emits a peak wavelength at 442 nm for rapid curing for a couple of minutes. The cladding layer is then baked
on a hotplate to ensure maximum adhesion to the substrate as well as bake out any remaining solvents. The cured cladding layer remains on the substrate and ready for the core layer that will be waveguide patterned. After the ORMOCORE as a core material is spun coated on top of the cladding layer, the LBWD system is used to write the waveguide, or even ring resonators. The fabricated one array of polymeric waveguides are shown in Fig. 1 which consists of a layer ORMOCLAD of 5 μm top of the substrate, ORMOCORE (as a second layer on substrate) waveguides of 2.5 μm in height and five centimeters in length. In the last step, a hard baking might be performed at temperatures of 150 °C for a couple of minutes to increase the adhesion to the substrate depending on the final application for the waveguides.
Fig. 2. Optical images of an array of the polymer waveguides. Insert, optical image of polymer waveguide with higher magnification.
For fabrication of directional coupler and ring resonators, we can follow the same process, the only diffrence is that after spin coating clad layer, we can write two waveguides instead on one waveguide with only 2 μm gap between them by following the same process which is expalined obve that is shown that in figure 3. We can repeat the same fabrication procees for ring resonators, it means that; after one layer of ORMOCLAD about 8 μm is coated by means of spinner top of the substrate which is cured by using UV light around 2 min, then second layer of ORMOCORE of 2.5 μm is coated on first layer (ORMOCLAD) by means on spinner (4500 rpm at 45 s). After that we wrote a polymer straight waveguides and one ring (in racetrack ring shape) by LBDW technique. The fabricated microring device is shown in Fig. 3 which consists of a
layer ORMOCLAD of 5 μm top of the substrate and ORMOCORE (as a second layer on substrate) waveguides of 2.5 μm in height with a coupling gap distance of 900 nm between the micro ring and the straight waveguide.
Fig. 3. Optical image of a fabricated polymeric ring resonator based on all pass filter
It is also possible to write very easily other bus waveguide on top of the ring resonators by following and repeating the same approach to have an add/drop microring resonators that is shown in figure 4.
Fabrication and optical characterization of this add/drop filter has already
reported it in this paper [7]
Fig. 4. Optical image of a polymeric micro ring resonator with add/drop configuration.
By following this technique, it was also possible to write three ring resonators between these two optical bus waveguides for slowing light or many other interesting applications which is shown in figure 6.
Fig. 5. One array of optical images a polymeric ring resonators
One more interesting application of ring resonators is to use them as sensor, one simple idea is to make add/drop filter on top of the copper (Cu) instead of glass and then by heating the substrate, it is possible to shift the resonance modes. For this one, we can deposit on thick layer of Cu on top of the galls by using electron deposition and then we can repeat the same process which is explained that at above. This process is illustrated in figure 6 (a) and the one real optical image of this sensor is shown in figure 6 (b). (a)
Glass
(b)
Glass + Cu
Fig.6 (a) Following step to write add/drop microring on top of the Cu, (b) optical image of add/drop microring resonators on top of the Cu
For optical characterization of our optical devices, we start first quick tests of optical wave guiding by using a simple butt coupling experimental setup which is shown in figure. 7. In this setup we have used a tapered fiber which is made of by CO2 laser in our lab.
Fig. 7. Schematic of the horizontal setup for coupling light into the bus waveguide.
To couple light into the bus waveguide and received the signal from the output port of the waveguide using another tapered fiber. The signal has been monitored using an optical spectrum analyzer therefore we could analyses the output from other port of the waveguide which is shown in figure 8. As it is shown from figure 8 that this polymer optical waveguide in the laterally coupled geometry for wavelength 1550 nm works very well.
Fig. 8. The output of polymeric optical waveguide.
For optical characterization of optical polymeric ring resonators, first of all in order to the better coupling what we have done, the gap (which is at 900 nm) of the fabricated micro ring resonator is then filled with nitrobenzene liquid by micropipette (which has large dependence of the refractive index on temperature) for increasing efficiently coupling to ring waveguide. We used a horizontal butt-coupling setup which is shown in figure. To couple light into the bus waveguide and received the signal from the output port of the waveguide using another tapered fiber. The signal has been monitored using an optical spectrum analyzer therefore we could analyses the output from other port of top waveguide which is shown in figure 9. As it is shown from figure 9 by blue color that the micro ring resonator in the laterally coupled geometry for wavelength 1550 nm have band width (Δλ) 0.8 nm, and by increasing the temperature of the step, we can change the refractive index and as a result the resonance mode is shifted to longer wavelength which is clear in figure 10 by green color.
Fig. 9. The normalized output of polymeric optical ring resonator, blue one before heating up the sample, the green one after heating up the sample.
In conclusion, the several type of passive optical devices such as waveguides, directional coupler and micro ring resonators of optical devices have been fabricated based on inorganicorganic hybrid polymers materials by using laser beam direct write technique and these optical devices have been successfully characterized. Laser beam direct write technique is shown to be a suitable fabrication technique for polymer integrated optics due to the enough resolution, highly simple and low-cost provided. Optical characterizations of these optical devices have been successfully performed and the resulting optical waveguide and ring resonators structures are shown to ensure suitability for implementation in Optical Integrated Circuits applications.
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