1.

Introduction

The demand for high aggregate capacity and bandwidth for optical interconnection and communication requires the development of wavelength division multiplexing (WDM) systems. Many different waveguide WDM techniques have been proposed, mainly including planar concave grating (PCG), arrayed waveguide grating (AWG), ring resonators, and Mach–Zehnder interferometers.^1–5 PCG and AWG have been widely researched and have made great progress due to their low cost, large fabrication tolerance, and high performance. We consider the PCG structure because of its significant size advantage over AWG.^5–7 A typical PCG structure has a Gaussian-like spectral response with high sensitivity to small spectral shifts, which seriously restricts its practical application in dense or coarse WDM systems.⁷ A flat spectral response is desirable for large spectral shift tolerance that can be realized by using a multimode interference (MMI) coupler section in connection to the PCG planar waveguide.^8–10

There are various optical waveguides with different materials for realizing PCG devices, including silica, semiconductors, and polymers. In silica-on-silicon³ and InP,¹¹ a large bending radius is generally required due to low refractive index (RI) contrast waveguide, which results in relatively large device size and low-density on-chip integration. In silicon-on-insulator,⁵ one can realize ultracompact PCG because of the high RI contrast waveguides and associated sharp waveguide bending. However, expensive lithography techniques such as deep UV or E-beam lithography are usually required for silicon photonic device fabrication due to the submicron cross-section dimension.^12,13 The silicon waveguide is not suitable for the application in visible to near infrared below the 1100-nm wavelength.

Polymer has the obvious advantages of a wide spectral transparent window, high flexibility, and low-cost fabrication.^14,15 SU-8 polymer in particular is an attractive photosensitive polymeric material for the fabrication of planar lightwave circuits because of its easy fabrication and relatively good chemical and thermal stabilities.^16–20 To support cost-effective integration with polymer-based optical interconnection and to facilitate minimal interface loss with fiber-based optical communication networks, we consider SU-8 polymer for the implementation of the PCG wavelength demultiplexer in this paper. For large RI contrast, we use polymer waveguide with an air cladding.^17,20

In this paper, we present four-channel PCG wavelength demultiplexers on SU-8 polymer waveguides with a flattened spectral response. The flattened spectral response is accomplished by using an optimized MMI as the PCG planar waveguide input for all spectrally separated channels. The mode field distribution at the PCG planar waveguide input is controlled by adjusting the waveguide taper width coupled to the MMI instead of the MMI width or length.⁹ The devices are realized by using SU-8 polymer waveguides fabricated by a one-step UV lithography process, offering lower cost and wider spectral band selection flexibility. The PCG wavelength demultiplexer with the desired flattened spectral response has been demonstrated.

2.

Theory

Figure 1 shows the schematic diagram of the MMI-PCG based on conventional Rowland circle geometry.²¹ The input and output waveguides are positioned along the Rowland circle with a radius $R$. The grating facets are located on a grating circle with a radius equal to the diameter of the Rowland circle. The grating circle and Rowland circle touch at the center of the grating. There is a planar waveguide region between the input/output channel waveguides and the concave grating. The input light diverges toward the grating in this planar free propagation region. The concave grating on the grating circle diffracts back and focuses the light into the output waveguides on the Rowland circle. The positions of the input and output waveguides are determined by the grating equation⁵

Fig. 1

Schematic diagram of MMI-PCG.

The MMI aperture located on the input port of the PCG planar waveguide is used as a mode shaper to form a flat field distribution. It consists of a taper with width $w$ and a multimode section with length $L$ and width $W$, as shown in Fig. 2. The input taper of the MMI is located at the center of the MMI for a symmetric interference. According to the selfimaging principle of MMI, an input field profile is reproduced in single or multiple images along the propagation direction of the MMI waveguide.²² The location $z$ of the first $N$ images of the input field distribution can be calculated from

Fig. 2

Schematic of an MMI aperture for a PCG planar waveguide.

Figure 3 shows the cross-section of a small SU-8 (Microchem) ridge waveguide with an air top cladding and OE4141 (Dow Corning) photopolymer bottom cladding on a Si substrate. The refractive indices of SU-8 and OE4141 are 1.575 and 1.494, respectively, at the 1310 nm wavelength. The optical channel waveguides for the device input and output have $2.5\text{-}\mu \mathrm{m}$ width and $2\text{-}\mu \mathrm{m}$ height, satisfying the single-mode condition calculated by the finite-difference method. All waveguide structures are designed for transverse electric (TE) polarization in this paper. The inset in the right corner of Fig. 3 shows the calculated fundamental (${\mathrm{TE}}_{00}$) mode profile. All the PCG input and output channel waveguides have a bend radius of $300\mu \mathrm{m}$. The output channel waveguide has a $300\mu \mathrm{m}$ long waveguide taper that changes width from a $5\text{-}\mu \mathrm{m}$ aperture to $2.5\mu \mathrm{m}$ wide waveguide. The width $w$ of the $300\mu \mathrm{m}$ long input taper of MMI has been optimized as discussed below.

Fig. 3

Cross-section of an SU-8 strip waveguide. The insert shows the calculated fundamental mode profile for the $2.5\mu \mathrm{m}$ wide and $2\mu \mathrm{m}$ high waveguide.

3.

Design of Multimode Interference and Planar Concave Grating

The length $L$ of the MMI aperture is chosen at two image locations for flat field output in this paper because of its better flat top performance and larger tolerance compared with other $N$ image locations. The flat field width of the MMI aperture should be comparable to the width of the PCG output aperture to create a sufficiently flat image. A $10\mu \mathrm{m}$ wide $W$ MMI aperture with about a $5\mu \mathrm{m}$ wide flat top profile (close to the width of the PCG output aperture) are the best compromise for achieving a low loss and flat spectral response. The decrement in width $W$ will produce a lower loss but have a narrower spectral response. The increment in width $W$ will have a wider spectral response but significantly increase the loss. The required length $L$ of a $10\mu \mathrm{m}$ wide MMI aperture for the two image positions is calculated to be $60.8\mu \mathrm{m}$ from Eq. (2).

The simulated field distribution for the $10\mu \mathrm{m}$ wide and up to $122\mu \mathrm{m}$ long MMI couplers with input taper widths $w$ of 4.0, 6.0, 7.0, 7.7, and $9.0\mu \mathrm{m}$ are shown in Fig. 4. Two images of the input field profile are clearly reproduced at the $L=60.8\mu \mathrm{m}$ position (dotted line). A line scan of the field distribution at the two image positions of these MMI apertures is shown in Fig. 5. The field distribution of these two images in combination changes from a twofold image with a center dip to a Gaussian-like curve by varying the input taper width $w$ from 4.0 to $9.0\mu \mathrm{m}$. The spectral response of the device can be obtained from the convolution of the waveguide mode field profile of the PCG output aperture (black solid line) with the imaged field distributions of the MMI aperture on the output plane.

Fig. 4

(a) Schematic geometry of MMI aperture. (b)–(f) Simulated field distribution of $10\mu \mathrm{m}$ wide MMIs with input taper widths $w$ of $4.0\mu \mathrm{m}$, $6.0\mu \mathrm{m}$, $7.0\mu \mathrm{m}$, $7.7\mu \mathrm{m}$, and $9.0\mu \mathrm{m}$, respectively.

Fig. 5

Mode field distribution at the two image positions of MMI apertures for different input taper widths $w$.

The PCG is designed around a central wavelength $\lambda$ of 1310 nm. The diffraction order $m$ is chosen to be 10 in this study, which results in a free spectral range of about 130 nm. The total internal reflection facet is applied to the PCG grating. The incident and diffracted angles are chosen to be 65 deg and $-25\mathrm{deg}$, respectively. The calculated grating period $d$ is $17.4\mu \mathrm{m}$ from Eq. (1). The $5\mu \mathrm{m}$ wide tapered output waveguides are spaced $10\mu \mathrm{m}$ apart on the Rowland circle. The designed central wavelengths of the four output channels are 1280, 1300, 1320, and 1340 nm, with 20 nm spectral separations. The Rowland circle radius $R$ is then $613.6\mu \mathrm{m}$.

The PCG simulation is performed based on the scalar diffraction theory.⁵ The field distribution at the end of the MMI is calculated by BeamPROP (Rsoft), which is used as the input field of the PCG planar waveguide. The incident waveguide field on the grating facets and the diffracted field on the Rowland circle are calculated according to the Kirchhoff–Huygens diffraction formula. The spectral response of each channel can be obtained by calculating the overlap integral between the mode field profile of the PCG output aperture and the diffracted and focused field of the PCG on the Rowland circle. The grating facets are chipped and blazed²³ for better performance (higher peak transmission and lower crosstalk). The grating reflection is set to 100% for these simulations. The simulated spectral response of the MMI-PCGs shows the change from a center dip shape to a Gaussian-like shape by varying the width of the input taper from 4.0 to $9.0\mu \mathrm{m}$. Figure 6(a) shows the simulated spectral response of the MMI-PCGs with four different input taper widths (4.0, 6.0, 7.7, and $9.0\mu \mathrm{m}$). We get a flat spectral response at $7.7\mu \mathrm{m}$ of the input taper width. If the taper width is 4.0 or 6.0 μm smaller than the optimized taper width $7.7\mu \mathrm{m}$, the device indicates a center dip spectral response. If the taper width is $9.0\mu \mathrm{m}$ wider than $7.7\mu \mathrm{m}$, the device approaches a Gaussian-like spectral response. The simulated transmission spectrum of the MMI-PCG with $7.7\mu \mathrm{m}$ wide input taper is shown in Fig. 6(b). The device exhibits a central channel loss of $-6.2\mathrm{dB}$, a crosstalk of $-36.6\mathrm{dB}$, and an on-chip loss nonuniformity of 0.3 dB.

Fig. 6

Simulation spectral response of (a) MMI-PCGs with variation of input taper width $w$ and (b) MMI-PCG with $7.7\mu \mathrm{m}$ wide input taper.

4.

Fabrication

To verify the device design, MMI-PCG devices with different input taper widths $w$ were fabricated on SU-8 polymer. Only a one-step UV lithography process is needed, which greatly simplifies the fabrication process.^17,19 We first spin-coated an $8\mu \mathrm{m}$ thick OE4141 cladding layer on top of a cleaned Si substrate and cured it by flood UV exposure. It has a very good optical property and is thick enough to avoid waveguide mode leakage to the Si substrate. A $2\mu \mathrm{m}$ thick SU-8 polymer film was then formed on the cladding layer by a spin-coating process. The Si wafer with an SU-8 core layer was prebaked with two steps (1 min @ 65°C and 1 min @ 95°C) on a hotplate to evaporate the solvent. Then, the patterns on the photomask were transformed to the SU-8 film by i-line UV photolithography. The Si wafer with the exposed SU-8 film was then postbaked (1 min @ 65°C and 2 min @ 95°C) to finish the polymer cross-linking process. Following development and rinsing, the final MMI-PCG device with the SU-8 planar and channel waveguides was obtained. A short hard baking time (1 min @ 65°C and 5 min @ 150°C) is preferred to smooth the waveguide sidewalls and reduce the scattering loss while keeping the sidewall vertical.¹⁷

Figure 7 shows the microscope images of the fabricated MMI-PCG device. A series of strip waveguides was also fabricated on the same chip for reference. Figure 8 shows the cross-section view of the fabricated SU-8 strip optical waveguide with $2.5\mu \mathrm{m}$ width and $1.7\mu \mathrm{m}$ height, which is slightly thinner than our design. The 1310-nm input light from the light source fiber was butt-coupled to the input waveguide through a tapered lens fiber with a focus spot size of about $2.5\mu \mathrm{m}$. A $60\times$ microscope objective lens was used to collect the output light from the end of the output optical waveguides, and was connected to an infrared camera (Electrophysics Microviewer 7290). The insert picture shows the mode profile of the waveguide, indicating a single mode propagation in the fabricated SU-8 optical waveguide. The propagation loss of a straight SU-8 waveguide was about $-0.6\mathrm{dB}/\mathrm{mm}$ measured by the conventional cut back method at the 1310 nm wavelength.

Fig. 7

Microscope images of the fabricated MMI-PCG device.

Fig. 8

Microscope images of the cross-section view of the SU-8 optical waveguide. The insert shows the mode profile of the fabricated waveguide.

5.

Measurement and Discussion

A broadband superluminescent diode light source was used to characterize the fabricated MMI-PCG devices. A tapered lens fiber was used to butt-couple the input light from the light source to the input channel waveguide. Another tapered lens fiber was used to collect the output light from the end of output channel waveguides, and was connected to an optical spectrum analyzer (Agilent 86142B). The input fiber was aligned and fixed to the input channel waveguide of an MMI-PCG by maximizing the output power to the second output channel waveguide (counted from the long wavelength). Then, we aligned the output fiber to each output channel waveguide and recorded its spectral response. The same procedure was used for the measurement of all other devices.

The measured spectral response of the fabricated MMI-PCG devices with four different input taper widths (4.0, 6.0, 7.0, and $8.0\mu \mathrm{m}$) is shown in Fig. 9(a). The losses are normalized to a reference straight waveguide. The MMI-PCG devices with 4.0 and $6.0\mu \mathrm{m}$ wide input tapers show the center dip spectral response, while the one with the $8.0\mu \mathrm{m}$ wide input taper shows a near Gaussian-shaped spectral response. The device with the $7.0\mu \mathrm{m}$ wide input taper presents the most flattened spectral response, which is slightly different from simulation results, due most likely to the small mismatch between the fabricated MMI width and waveguide height and the values used in the design simulation.

Fig. 9

Measurement spectral response of (a) MMI-PCG with variation of input taper width $w$ and (b) MMI-PCG with $7.0\mu \mathrm{m}$ wide input taper.

Figure 9(b) shows the measured transmission spectrum of the fabricated MMI-PCG device with the $7.0\mu \mathrm{m}$ wide input taper. The on-chip loss for the central wavelength is about $-14.8\mathrm{dB}$. There is about a $-3\mathrm{dB}$ loss resulting from the flat top response. The remaining loss is believed to be mainly due to grating facet imperfections such as nonverticality, corner rounding, and sidewall roughness. The crosstalk of the present fabricated device is $-22\mathrm{dB}$, and the on-chip loss nonuniformity is 2.5 dB. Compared with the simulation results, there is about 10 nm of spectral shift in the transmission spectrum toward shorter wavelengths. This is believed to be caused by the thinner planar waveguide of the fabricated PCG and its slightly smaller effective RI of planar waveguide mode than the simulation. Thus, we have successfully achieved a flattened spectral response by adjusting the input taper width $w$ of the MMI aperture structure.

6.

Conclusion

In summary, we have demonstrated four-channel PCG wavelength demultiplexers with a flattened spectral response based on SU-8 polymer waveguides. The flattened spectral response to all spectrally separated channels is accomplished by using an optimized MMI as the input aperture to the PCG planar waveguide. The mode field distribution at the planar waveguide input aperture is controlled by adjusting the width $w$ of the input taper connected to the MMI. The devices were realized on SU-8 polymer waveguides by one-step UV lithography. Experimental results confirm that the shape of spectral response can be controlled by adjusting the input taper width. The PCG-based wavelength demultiplexer with a flattened spectral response can be widely used for dense or coarse WDM systems to support various optical interconnection and communication applications.