A monolithically integrated eight-channel optical multiplexer (Mux) with a channel spacing is presented based on a silicon-on-insulator rib waveguide and an asymmetric Mach–Zehnder interferometer. All channels were optimized independently with integrated heaters. The fully tuned Mux shows an adjacent channel isolation of , an excess loss of , and a channel uniformity of over a wavelength span. In addition, the phase tuning efficiency for different interlevel dielectric layer thicknesses and thermal crosstalk were investigated.
© 2008 Optical Society of America
Wavelength division multiplexing (WDM) is a technology of transmitting multiple optical signals of different wavelengths on a single mode fiber, which increases the transmission capacity and enhances flexibility of network configurations in optical communication systems. Recently, WDM devices based on silicon have attracted a great deal of attention, because they offer tremendous potential for low-cost and highly integrated optical components using conventional complementary metal-oxide semiconductor (CMOS) manufacturing technology [1, 2, 3]. A multiplexer (Mux) plays an important role in WDM systems. Silicon-based optical Muxes can be categorized into Mach–Zehnder interferometers (MZIs)  and arrayed-waveguide gratings (AWGs) [5, 6, 7]. Here, we demonstrate an eight-channel optical Mux that integrates seven asymmetric Mach–Zehnder interferometers (AMZIs) on a silicon-on-insulator (SOI) substrate. The AMZI consists of a splitter and a coupler connected to each other with different lengths of waveguides. The difference in length between the two arms dictates the period of the interferometric pattern at the output. The AMZIs can be cascaded as interleavers, increasing the number of transmitting channels. The thermo-optic effect was used to compensate phase mismatch, which can be caused by fabrication tolerances or temperature instability by using aluminum thin film heaters deposited on one arm of each AMZI; the thermo-optic coefficient for silicon is . To our knowledge, this is the first report of a based on cascaded AMZIs in an SOI with active tuning; while such a Mux has been reported in silica-on-silicon , the advantage of an SOI-based Mux is a smaller footprint.
Figure 1 shows a schematic of the eight-channel Mux consisting of three stages of AMZIs. Each AMZI is composed of a multimode interferometer (MMI) and a MMI. The third stage has a difference in length between the two arms of , the second stage of , and the first of . The channel spacing and the difference in arm length are related with the following equation:2 shows a cross sectional view of an SOI rib waveguide on which thin metal film (Al) heaters were deposited to form the heater. The width , height , and rib etch depth of the waveguide were measured to be 0.55, 0.5, and , respectively. The waveguide was designed to be compatible with a modulator for integration , and all waveguides employed a lateral taper with an width at input and an length for both ends to reduce coupling loss. An interlayer dielectric layer (ILD), which provides electrical insulation and planarization, was placed between the heater and waveguide to vary their separation. The cross sectional area of the Al layer is , and the respective heater lengths for stages 1, 2, and 3 are 797.2, 608.6, and . The footprint of the whole device is .
A superluminescent light emitting diode (SLED) was used as a broadband light source for testing; a tapered single mode fiber with a mode field diameter of was used for coupling; and a linear polarizer and a polarization controller were interleaved between the light source and input fiber to set the desired polarization. An optical spectrum analyzer (OSA) was used to measure the output spectrum. All waveguides were antireflection coated (ARC) on both end facets to prevent undesired spectral modulations from the etalon formed between the fiber and waveguide facet.
The propagation loss of straight waveguides was measured to be for TE polarization with the cutback method . The waveguide supports only a single TE mode. The excess loss, channel spacing, and extinction ratio of a single stage AMZI was measured to be , , and over the wavelength from 1537 to , respectively. Figure 3a shows the phase tuning efficiencies of the heater employed in stage 1 for different ILD thicknesses as a function of applied power to the heater. The phase tuning efficiency can be defined by the following equation:3b shows the tuning efficiency, herein defined as the power needed for the π-phase shift for each stage. Each stage shows similar characteristics in terms of tuning efficiency for different ILD thicknesses. On average, of power was consumed to change the π-phase shift for each stage. In addition, by measuring the phase shift caused by adjacent heaters for all ILD thicknesses, we found that thermal cross talk was negligible for all cases ( was needed to produce a π-phase shift using a heater on an adjacent MZI).
Figure 4a shows the spectra of all outputs of the eight-channel Mux before the phase mismatch compensation, and Fig. 4b shows the spectra of the fully optimized Mux after heater tuning. All heaters were wire bonded to a test board [printed circuit board (PCB)] and connected to the power supply through it, which is controlled by a LABVIEW program. Applied powers to the heaters for stages 2 and 3 were 13 and , respectively, and thus total applied power was . The measured excess loss and the adjacent channel isolation of the optimized Mux were 2.6 and , respectively. The channel uniformity, defined as the difference between the best and the worst cases in excess loss over all channels, was over 1537 to . The loss slowly decreased as the wavelength increased owing to the wavelength dependence of the MMI, which was proved in simulation. In practice, the wavelength dependence is a key factor in determining the operating wavelength range. The channel spacing and bandwidth were and , respectively.
We successfully demonstrated an eight-channel optical Mux based on an AMZI and an SOI rib waveguide. Thermal tuning was used to compensate phase mismatch due to fabrication, and a total power of was consumed to optimize all the channels. The fully tuned Mux shows a low waveguide propagation loss and high adjacent channel isolation. Furthermore, the smaller the separation between the waveguide and heater, the better the tuning efficiency, with each stage showing similar characteristics for all ILD thicknesses. In addition, thermal cross talk for the design of this device is negligible.
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