Research Of Optical Switches Based On Negative Photoresist SU-8

Information technology is the forerunner of today's industry. Internet is the most important part of the modern information technology. People need more and more bandwidth and capacity with the development of the society. With the development of the bandwidth and capacity, the present network develops towards all optical network ( AON ) . AON is high-speed and wide-bandwidth communication network. Dense wavelength division multiplexing (DWDM) technology is used on the artery to enlarge the capacity. Optical add drop multiplexing (OADM) and optical cross connecting (OXC) are used at the crossing. Among all kinds of AON devices, OXC and OADM are the kernels of the AON. It becomes an urgent task to research the all optical OXC and OADM for performing large capacity communication network. Optical switches and optical switches array are the essential techniques of OXC and OADM. Thus optical switches play a decisive role in the development of present AON.Optical switches are widely used in OADM and OXC,monitoring and protecting of the network, testing of optical communication devices. Different applications propose different requirements. This promotes many development directions of optical switches. At present mechanical switches and waveguide switches are commonly used. Waveguide switches will be the leading role in the optical switches market because of its advantages of no moving components, easy integrating, low power consuming and so on. Most polymer organic materials are low cost, easy processing and have higher thermo-optic or electro-optic coefficient compared with inorganic materials. Polymer waveguide devices attract more and more researchers. Many developed countries are now researching polymer waveguide devices and have made some important improvements. Some major characters of these polymer devices are getting ahead of the inorganic ones. At the 2008 OFC conference, Fuji Xerox Corporation reported that their switching speed of a polymer electro-optic switch reached 6ns. At the 2009 OFC conference a polymer 2×2 thermo-optic switch with less than 10mW power consuming is reported. The major work of this paper is the research of polymer thermo-optic and electro-optic switches based on SU-8 2005 (abbreviated to SU-8 afterwards). The main contents of this paper are listed below.1 With the planar waveguide theory, the characteristic equation of rectangle waveguide and ridge waveguide are analyzed, the bending waveguide design method is proposed. These can guide the design of the waveguide. The theories of Mach-Zehder interferometer (MZI) and multimode interference - Mach-Zehder interferometer (MMI-MZI) optical switches are introduced. The thermo-optic and electro-optic effects are analyzed. The design methods of coplanar waveguide traveling wave electrode and push-pull micro strip line electrode are proposed. These works lay the theory foundation for structure design and performance testing of thermo-optic switches and electro-optic switches.2 The polymer material SU-8 is introduced in detail. It is a kind of ultraviolet cross-linkable photorisist. The absorption spectrum of this material is tested. From the spectrum we can see that there exist two absorption valleys at 1310nm and 1550nm optical communication windows. So SU-8 is suitable for optical communication. As it is a kind of photorisit, it can be used to fabricate waveguide by simple ultraviolet photolithography process. We select SU-8 as the core material and the mixture of poly-methyl-methacrylate–co-glyciclyl methacrylate (PMMA-GMA) and bis-phonel-A epoxy. With this structure, we design 1×N series MMI splitters, optimize the MMI region parameters, and simulate the whole structure by beam propagation method (BPM). Then we optimize the fabrication process of SU-8. Pre-bake: 60℃10minites, 90℃20minites. UV exposed: 4minintes. Post-bake: 65℃10minites 95℃20minites. Development: 40seconds. With these optimized parameters, we successfully fabricated the splitters and tested the output near-field profile. The above work paves way for research of thermo-optic switches and electro-optic switches. 3 A 2×2 MMI-MZI thermo-optic switch based on SU-8 is designed. By analyzing the characteristics of polarization and coupling loss, we select the waveguide core width and thickness to be both 4μm. By analyzing the influence of the cladding layer thickness to substrate radiation loss, we select the cladding layer thickness to be 3μm. To design the MMI region, firstly, we can obtain the relationship among MMI region length, width and the two input/output waveguides gap by the MMI theory. Then by analyzing the crosstalk between the two input/output waveguides, we select the gap to be 10μm. The MMI region width and length are 30μm and 650μm, respectively. We study the influence factors of the switching power and select the heating electrode to be 5μm wide and 1.2cm long, respectively. The gap between the two arms in the modulating region is 20μm and the lengths of the two arms are 1.2cm. With the optimized parameters above, we simulate the cross and bar states of the thermo-optic switch by 3D-BPM. From the simulation result we can see the two output port crosstalks at the two states are -44.6dB and -37.1dB, respectively. According to the optimized SU-8 fabrication process we fabricate the designed thermo-optic switch successfully. Properly power is applied on the heating electrode and the output near-field profile can be obtained at the cross state and bar state. By testing the output optical power with different heating powers on the electrode, we can obtain that the switching power is 7.5mW and the crosstalks at the two states are-18dB and -20dB, respectively. A 0.4 KHz square wave is applied on the heating electrode to test the switching speed. The tested switching time is about 0.4ms. Then we improve the switch structure. The MMI region length and width are changed to be 45μm and 1427μm, respectively. The gap between the two input waveguides is 15μm. The gap between the two modulating arms is enlarged to be 60μm. The same testing method is used to the improved switch. Its switching power is about 6.9mW. Crosstalks at the two states are -22dB and -18dB, respectively. Switching time is also about 0.4ms. At last we analyze the testing result of the two switches.4 Three electro-optic materials, cross-linkable PMMA-AMA doped with chromophore AJC146, organic-inorganic hybrid material and SU-8 doped with chromophore DR1 (DR1/SU-8), are simply introduced. Three different structures electro-optic switches are designed with these three materials. Taking the real situation into account, we select the third design. We synthesize the guest-host electro-optic material by ourselves. From the atomic force microscope (AFM) picture of the spun coated film with DR1/SU-8, we can see that the root-mean-square (rms) roughness is only 2.728nm within 10μm×10μm. By testing its absorption spectrum, we can see that there exist two absorption valleys at 1310nm and 1550nm communication windows. Then the thermogravimetric analysis (TGA) curve is tested and its thermal stability is very good. So this material is very suitable for optical communication devices. We fabricated the electro-optic switch using DR1/SU-8 as the core material, SU-8 as cladding and aluminium as electrode. At first, we fabricate the waveguide by the reactive ion etching (RIE) process. After several experiments, we find it difficult to obtain the designed devices. So we improve the fabrication process. Firstly, An SU-8 groove is fabricated. Secondly, DR1/SU-8 is injected to it. Thirdly, the plane part of DR1/SU-8 is removed by RIE. Fourthly, upper cladding is spun coated. At last CPW electrodes are fabricated. The traveling loss of the waveguide is tested to be 2.0dB/cm by cut-back method. Then we test the switching speed of this device. A 100 KHz square wave signal is applied on the electrode and the response signal shows its rising time and falling time to be 36.68ns and 66.69ns, respectively. Higher frequency such as 500 KHz, 1 MHz, 2 MHz square wave and 5 MHz, 10 MHz sine wave are also tested and the response signals are obtained. But we find that the source square wave has a 40ns rinsing/falling time. In this way, we can deduce that the real switching time is faster than the display value on the oscilloscope.