1. Introduction

Since silicon photonics has been a promising research theme since mid 2000’s, wavelength division multiplexing (WDM) multi/demultiplexers based on silicon waveguides have attracted much effort of researchers to improve their characteristics. The three major components of multi/demultiplexers are ring resonator, arrayed waveguide grating (AWG), and Echelle grating. Despite some advantages of the ring resonator and AWG [1–3], fabrication-induced wavelength errors have been a difficult problem to solve with the ring resonator and AWG, and researchers have considered the Echelle grating as a candidate for a commercial level silicon photonic transceiver component [4, 5].

Echelle grating filters consist of input/output waveguides, a slab waveguide region, and grating mirrors. While the width variation of waveguides of ring resonators and AWGs are susceptible to the fabrication-induced wavelength errors, the slab region in Echelle grating is structurally free from the width variation of waveguides.

A shortcoming of Echelle grating is to form a metallic mirror on the deep etched grating facets, which requires one or two additional lithography and RIE steps [5, 6]. Researchers have used total internal reflector (TIR) gratings, each saw tooth being a retro-reflector, to avoid forming a metallic layer on the deep vertical facets [7, 8].

Double reflections of the retro-reflector do not only increase reflection loss twice, but scattering loss in sharp angles is also significantly increased. If grating facets are deviated from 90°, loss due to higher mode generation is also increased twice, a typical angle of RIE facet being 87°~89° with respect to the horizontal plane.

Here, we present an Echelle grating that was designed and fabricated with a single reflection TIR in a 220 nm thick silicon slab waveguide, where the difference of effective refractive index between the silicon core and oxide claddings is Δn = ~1.4 for TE mode. Our single reflection TIR grating proves advantages of much easier fabrication than a metal mirror grating and smaller loss than a retro-reflector grating, though room for design tolerance is limited not only by a certain range of input/output angles but also by aberrations.

2. Design and fabrication

We designed 8 channels Echelle grating MUX/ DeMUX filter, targeted for photonic integrated circuit (PIC), on a 6 inch SOI wafer which has silicon thickness of 220 nm and buried oxide thickness of 3 μm. Our primary focus of designing is to match channel wavelengths between two separate MUX and DeMUX filters. Target specifications are channel spacing of 12 nm, adjacent channel crosstalk of <-15 dB, insertion loss of <3 dB to pass through one DeMUX filter and <6 dB to pass through two filters, MUX through DeMUX.

Figures 1(a) and 1(b) show some numbers of parameters to design our Echelle grating filter. There are 60 saw teeth in the circular arc of radius 2 mm which span an angle of 40° with respect to the center of the arc. Input and DeMUX output waveguides are aligned on the Rowland circle of radius 1 mm. A net footprint of the Echelle grating without input/output waveguides is 1.3x2.25 mm². Figure 1(c) shows a scanning electron microscope (SEM) image taken at the saw teeth of Fig. 1(b). To extract the parameter values shown in the Fig. 1, we used TE mode effective refractive index n_eff = 2.85 calculated theoretically for slab waveguide region at λ₀ = 1530 nm, and also used the diffraction equation,

 

Fig. 1 Mask diagrams of (a) two Echelle grating filters, MUX through DeMUX and (b) saw teeth of the grating, (c) scanning electron microscope (SEM) image of saw teeth

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The angles, α = 50° and β = 40°, were chosen for total internal reflection which had been proved in retro-reflector TIR gratings for 220 nm thick silicon slab waveguide [7]. The width of saw tooth, 21.9, μm was decided to put, at least, 60 teeth in the grating arc. The highest diffraction order allowed for 60 teeth within the arc is m = 5. The free spectral range (FSR) is calculated, 1250nm~1700nm. Since the critical angle of total internal reflection is θ_c = 30.6° for refractive indices, n_eff-slab = 2.85 and n_oxide = 1.45, it is theoretically possible to have more than 100 teeth and m = 10 to achieve a resolving power RP>1000. In the current PIC-targeted design, an angle α + β = 90° was chosen to have a wide room from the critical angle, which is safe, as far as the channel separation of <-20 dB for MUX through DeMUX is achieved. The effective refractive index of TM mode is calculated n_eff^TM = ~2.2 and the TM mode is ignored, because the TM mode at the wavelength, 1400~1600 nm, is diffracted outside DeMUX waveguides.

To make use of the wide incident angle and the large grating arc, it is important to remove spherical aberration, different focal points for different angular positions of the grating arc. An easy way to remove the spherical aberration is to place each saw tooth at the position calculated by the diffraction equation, Eq. (1), rather than at the position of uniform distance. Let z_iⁱⁿ and z_i^out be distances from the input waveguide to i th saw tooth and from i th saw tooth to a output waveguide at the center of DeMUX output, respectively.

In order to minimize the fabrication-induced wavelength shifts from chip to chip or wafer to wafer, it is important to form input/output waveguides and the saw teeth by the same mask. If they are formed by two different masks, the mask alignment of one layer to another in photolithography may cause a significant fabrication-induced wavelength shifts. Our Echelle grating filter was fabricated by one consecutive step of photolithography and RIE to form waveguides and the grating saw teeth by the same mask. After the RIE step, a 2 μm thick PECVD oxide layer of top cladding was deposited over whole wafer to finish fabrication. Compared with metal mirror grating, fabrication is much simple and cost-effective for PIC integration.

3. Measurements and theoretical calculation

The spectral curves for 8 channels were taken by injecting amplified spontaneous emission (ASE) of EDFA into the input waveguide. Light from DeMUX output waveguides was collected by a lens-tip single mode fiber and sent to an optical spectrum analyzer to record output spectra as shown in Fig. 2(a) . Noise level is high in the weak intensity region of ASE. All spectral curves are normalized by the spectral curve of ASE of EDFA taken from the reference waveguide located near input and output waveguides as shown in Fig. 1(a).

 

Fig. 2 (a) 8 channel spectra of Echelle grating DeMUX, and (b) transmission spectrum of MUX through DeMUX, where 5 channels are shown within the strong ASE intensity region of EDFA.

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Figure 2(b) shows a spectrum transmitted through two Echelle filters, MUX through DeMUX, where 5 channels within the strong ASE region are shown. The average insertion loss in the strong intensity region collected from 10 samples is 2.4dB, and the adjacent channel crosstalks are in 15dB~18dB to pass through DeMUX. The average insertion loss to pass through MUX/DeMUX is 5.3 dB, in which additional loss, ~0.5 dB, is attributed to channel wavelength mismatch. The channel spacing of 11.9 nm was measured. Our specifications are extracted from the peaks in the strong intensity region shown in Fig. 2(b). The peak height variation between the channels in center and outside is ~1.5 dB, attributed mainly to color aberration which needs a further optimization using experimental data of dn/dω.

The spectral features in Fig. 2(a) can be reproduced by the theoretical calculation of wave equation, E = E₀exp (ik₀n_effz_i^{in + out}), propagating from the input waveguide to saw teeth, and then to DeMUX output waveguides. Figure 3 shows the theoretical reproduction of the measured spectra of Fig. 2(a). The coordinates of all saw teeth which were calculated to remove the spherical aberration were taken into account. Experimental value, n_eff = 2.839, of effective refractive index was used.

 

Fig. 3 Theoretical reproduction of 8 channel spectra of Echelle grating DeMUX filter.

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In conclusion, we designed and fabricated single reflection TIR 8 channel Echelle grating filters which are featured by a low average insertion loss 2.4 dB measured at 1520~1570 nm, adjacent channel crosstalk 15-18 dB, and simple fabrication process for easy integration with PIC. A transmission spectrum through two Echelle filters, MUX through DeMUX, showed an accumulative insertion loss 5.3 dB and channel separation >20 dB. Output spectra were accurately reproduced by a theoretical calculation of wave function.