Optical multiplexer/demultiplexer (MUX/DeMUX) generally filters an optical signal with a specific wavelength component to spatially separated output channels in wavelength division multiplexing (WDM) optical transceivers for high-density optical interconnections in data center applications [1, 2]. Such optical filters fabricated on silicon-on-insulator (SOI) wafers are normally based on light wave interferences in microring resonators [3-5], multistage delayed Mach–Zehnder interferometers (DMZI) [6-11], or arrayed waveguide gratings (AWGs) [12-15].

Coarse WDM (CWDM) operating in the O-band regime has been actively investigated as a high-bandwidth and cost-efficient technique for use in optically pluggable transceivers with C-form pluggable and quad small form pluggable module form factors [2]. Unlike dense WDM systems, CWDM specifications have several advantages in terms of relaxed lasing wavelength accuracy and operating wavelength range, especially for the optical MUX/DeMUX. Furthermore, CWDM optical components must operate in a relatively wide spectral range of >60 nm because the four-channel grid is spaced 20 nm in a wavelength domain. Generally, optical MUX/DeMUX is operated based on multiple light wave interferences irrespective of each device scheme [6-11]. Consequently, it is challenging to keep the spectral response of the optical MUX/DeMUX constant unless the optical split and coupling behaviors in the device are stably preserved within a wide wavelength range.

Meanwhile, considering CWDM-based optical receivers, the polarization state of a signal is not normally preserved as the signal is transmitted through single-mode fibers (SMFs). Hence, the optical DeMUX located on the receiver side is a prerequisite for arbitrarily polarized optical signals [8, 15-17]. There have been approaches to handle WDM signals at the receiver end with depolarized or polarization diversity schemes. In the case of the silicon nitride (SiN) material platform, the optical split/coupling response and optical phase delays were depolarized by employing a relatively low refractive index contrast of the SiN waveguides in the CWDM-targeted DMZI scheme [8]. Although the SiN-based depolarized method was experimentally verified, there remained a lack of spectral uniformity between the two orthogonal polarization input signals. Moreover, the proposed method is hard to apply to the case of the Si wire waveguide structure due to the relatively high refractive index contrast. Additionally, SiN-based CWDM-targeted AWG was reported [15]. The device operated successfully irrespective of the input polarization state by locating several polarization rotators based on laterally asymmetric waveguide structures in the middle of each arrayed waveguide of the AWG. Nevertheless, the relatively large device footprint, complicated fabrication process of the polarization rotators, and lack of monolithic integration with other functional devices such as optical modulators and detectors are technical issues to be explored.

Considering the compactness of the device size and monolithic integration of the optical transceiver, Si wire-based optical DeMUX could be more favorable. Due mainly to the relatively large birefringence of the Si wire waveguides, the polarization problem has been managed by adopting a polarization compensation scheme for the CWDM-targeted four-channel DMZI device [16] and a polarization diversity architecture for the combination of the DMZI interleavers and eight-channel AWGs [17]. In [16], several polarization rotators based on laterally asymmetric waveguide shapes were intentionally situated in the center of all single-stage DMZIs to interchange the polarization state whenever the signal is propagated in each DMZI. This enables them to depolarize the spectral response irrespective of the input polarization state. Although the device is only configured with a Si wire waveguide, the Si core thickness was set to 340 nm to depolarize the optical split and coupling response of the bent-shaped directional coupler. However, this Si core layer thickness is not a standard value (i.e., 220 nm) and can be disadvantageous for lowering the propagation loss of the Si wire waveguides. Moreover, the formation of the polarization rotators requires high accuracy for a lithography process (i.e., electron beam lithography). In [17], a polarization-diversified 16-channel WDM filtering operation was reported using a polarization splitter rotator (PSR) and two identical optical DeMUXs. In this case, the DMZI-type optical interleaver was designed to operate for a wavelength range of <30 nm, so it was challenging to apply this method to the CWDM-targeted optical DeMUX.

Overall, few reports have focused on all-Si wire-based polarization-manageable CWDM optical DeMUX. In this paper, we describe Si wire-based polarization-diversified CWDM optical DeMUX architecture consisting of hybrid mode conversion type PSR and two identical cascade-connected DMZI-type filters. In section II, the device structure of Si wire-based polarization-diversified CWDM optical DeMUX is shown, and its operation principle is explained. Section III describes the device fabrication and experimental characterization. The devices were fabricated on a 200 mm SOI wafer using a fabrication process based on 193 nm ArF-dry lithography technology. The measured optical DeMUX spectral response is theoretically analyzed in terms of group index, input polarization, and phase errors due to fabrication imperfections. The fabricated optical DeMUX exhibited a clear filtering response spaced 19 nm in the wavelength domain. By integrating with the hybrid mode conversion type PSR, the optical DeMUX experimentally demonstrated polarization-diversified filtering operation with a low insertion loss of <1 dB, spectral crosstalk of less than −20 dB, and polarization crosstalk of less than −20 dB for each peak wavelength channel grid in the CWDM wavelength range.

Figure 1 shows a schematic diagram of the polarization-diversified CWDM optical DeMUX. The device comprises the PSR for polarization diversity and the two identical optical DeMUX for filtering the optical signals for each orthogonally polarized input. The PSR comprises a hybrid mode conversion region based on rib-shaped Si wire waveguides and a mode split region based on a rectangular-shaped Si wire asymmetric directional coupler (ADC). It has been well known that the linearly polarized arbitrary states (i.e., the linear combination of TE₀₀ and TM₀₀ mode) can be spatially separated as two outputs with TE₀₀ modes [18, 19]. As explained in [18], the hybrid mode conversion type PSR is configured with shallow etched rib and normal channel waveguides whose configuration can easily be fabricated with the currently available standard complementary metal-oxide semiconductor (CMOS) foundry process.

Figure 1. Schematic diagram of polarization diversified CWDM optical DeMUX. CWDM, coarse wavelength division multiplexing; DeMUX, demultiplexer.

Figure 2 displays the numerically simulated mode excitation characteristics: Fig. 2(a) the cross-section dimension of the shallow etched Si wire waveguide and Fig. 2(b) the calculated equivalent indices (N_eq) for each excited mode distribution for the Si wire waveguide shown in Fig. 2(a) as a function of the waveguide width (W_rib). Packaged software based on a finite element method (FemSIM^TM; Synopsys, CA, USA) was used for implementing the simulation shown in Fig. 2(b). For the calculation model shown in Fig. 2(a), the Si core thickness (T_Si), thin layer thickness (T_Slab), and thin layer width (W_Slab) were set to 220, 90, and 100 nm, respectively. As shown in Fig. 2(b), the TE₀₀ mode signal maintains its electric field distribution regardless of the value of W_rib, whereas the TM₀₀ mode signal experiences hybrid mode excitation that simultaneously has both polarization states with TM₀₀ mode to TE₀₁ higher mode at a specific W_rib of around 0.55 μm. Consequently, only the TM₀₀-mode light can be coupled to the TE₀₁ mode by adjusting the values of W_rib and W_Slab. We used the TM₀₀- to TE₀₁-mode conversion to filter the two input polarization modes to spatially different output channels.

Figure 2. Numerically simulated mode excitation characteristics: (a) Cross-section dimension of shallow etched Si wire waveguide and (b) calculated equivalent indices (N_eq) for each excited mode distribution for Si wire waveguide shown in (a) as a function of waveguide width (W_rib).

Figure 3 shows the theoretical propagation characteristics in the mode split region in the PSR: 3(a) is the top view of the mode split area based on an ADC, 3(b) is a magnified view at around the beginning of the ADC, 3(c) is the specification of the ADC based on a linear taper, and 3(d) is the numerically simulated optical intensity distribution along the ADC region (λ_in = 1.31 μm). Packaged software based on a beam propagation method (BeamPROP^TM; Synopsys) was used for implementing the simulation shown in Fig. 3(d).

Figure 3. Theoretical propagation characteristics in mode split region in polarization splitter rotator (PSR): (a) top view of mode split area based on asymmetric directional coupler (ADC), (b) magnified view around ADC, (c) specification of ADC based on linear taper, and (d) numerically simulated optical intensity distribution along ADC region (λ_in = 1.31 µm).

Usually, an ADC makes it extremely difficult for the TE₀₀-mode light to couple to a neighboring waveguide due to severe mode mismatching between the two coupled waveguides. In this case, when the relatively narrow waveguide width is appropriately designed to suffer from the index matching between TE₀₀ mode and TE₀₁ mode, the TE₀₁-mode light converted from the TM₀₀ mode can be coupled to the adjacent output channel as the TE₀₀ mode. As for a fixed value of the narrow waveguide width for the ADC, the above-mentioned mode matching is satisfied only for a specific wavelength of light, which makes the operating wavelength range of the PSR narrower. As reported in [17], this drawback can be further overcome by using a tapered ADC scheme. As shown in Fig. 3(c), a relatively broad range of wavelength components satisfy the above-mentioned mode matching by gradually widening the narrow waveguide width from W_2a to W_2b, leading to a wider operating range of the PSR. As shown in Fig. 3(d), when the waveguide parameters are set as those shown in Fig. 3(c), only the TE₀₁-mode light suffers from directional coupling to the neighboring channel, whereas the TE₀₀-mode light goes straight without any interaction at the ADC. Consequently, the two orthogonally polarized inputs can be separated as the TE₀₀ mode. We theoretically confirmed the similar trends of the hybrid mode conversion in Fig. 2 and the mode split in Fig. 3 in the CWDM operating spectral range.

Each TE₀₀-mode output from the PSR is incident on each DMZI-based optical DeMUX, depending on the initial polarization state before launching into the PSR. As shown in Fig. 1, the DMZI-based optical DeMUX comprises three optical delay lines where each amount of path difference is adjusted to get constructive interference at each CWDM channel grid, together with the two 3 dB multimode interference (MMI) couplers. It is noted that the optical DeMUX needs to operate in a wavelength range of >70 nm. Thus, as an optical coupler, we adopted MMI couplers rather than directional couplers that show sinusoidal coupling behaviors [20]. In the optical DeMUX area, the Si wire waveguide width, except for the MMI coupler, was set to W_CH = 350 nm. Figure 4 shows the numerically simulated equivalent index (N_eq) and group index (N_gr) for the Si wire waveguide with W_CH = 350 nm and T_Si = 220 nm. The packaged software based on a finite element method (FemSIM^TM; Synopsys) was used for implementing the simulation shown in Fig. 4.

Figure 4. Numerically simulated equivalent index (N_eq) and group index (N_gr) for Si wire waveguide with W_CH = 350 nm and T_Si = 220 nm.

As for the TE₀₀ mode, N_eq and N_gr were roughly estimated to be 2.5 and 4.5, respectively. In the case of TM₀₀ mode, the parameters mentioned above were approximately 2.0 for N_eq and 4.7 for N_gr, whereas the TM₀₀-mode case also tends to show a relatively larger dispersion of each parameter. These tendencies explicitly indicate that the Si wire-based optical DeMUX cannot handle the arbitrary polarization state of light. Hence, the PSR is adopted in Fig. 1. In each optical DeMUX, ∆L was set to 5 μm, which enables us to have the channel gap spaced by 19 nm at λ = 1.3 μm. Each optical DeMUX can process both polarization states of the signal with low insertion loss and crosstalk as long as the PSR properly handles the arbitrarily polarized light since the MMI coupler typically maintains a balanced optical split ratio close to 50:50. Notably, the two identical optical DeMUXs are located centro-symmetrically. Therefore, the filtering relation can be determined, as shown in Fig. 1, making the two optical DeMUX outputs easier to optically couple to the single photodiode (PD) array where the two outputs are counter-propagated to the single PD.

Based on the theoretical investigations, the polarization-diversified CWDM optical DeMUX was fabricated by 193 nm ArF-dry lithography technology on a 200 mm SOI wafer with a 220-nm-thick Si layer and a 2-μm-thick buried oxide layer. The parameters for the waveguides for the PSR and the DMZI-based optical DeMUX were set to be identical to those considered in the theoretical analyses. Figure 5 shows the top view of the fabricated polarization-diversified Si wire optical DeMUX. The entire scheme is the same as shown in Fig. 1. The chip size was measured to be 720 µm long and 210 µm wide, including the PSR and the two DMZI-based optical DeMUXs. As seen in Fig. 5, dummy tiles for the waveguide and metal layers were properly located to achieve process stability in fully integrated photonics fabrication.

Figure 5. Top view of fabricated polarization-diversified Si wire optical demultiplexer (DeMUX).

Figure 6 shows the experimental setup for measuring the transmission spectra for the fabricated devices. As a light source, we used a tunable laser whose tuning range is set from 1,270 to 1,330 nm. The transmission spectra of the fabricated devices were characterized by using an optical vector analyzer (OVA) (OVA 5013; LUNA Innovations,VA, USA) that can analyze the amplitude and phase information of the signal. By analyzing the Jones matrix of the output signals, the transmission spectra via optical interactions from arbitrary linearly polarized input to each kind of output (i.e., TE₀₀-mode input to TE₀₀-mode output, TM₀₀-mode to TE₀₀-mode, TE₀₀-mode to TM₀₀-mode, and TM₀₀-mode to TM₀₀-mode) through SMFs was analyzed [21]. The measurements were made on an air-suspension-based optical die that has sufficient tolerance for external physical vibration. The temperature around the chip was kept constant. Also, the OVA we used periodically implements a pre-programed calibration process to compensate for the temperature variations. Thus, a stable measurement condition for analyzing the Jones matrix of SMF can be preserved.

Figure 6. Experimental setup for measuring the transmission spectra for fabricated devices. Inside a fabricated Si photonics chip there are channel waveguides (L) and bend waveguides (P). The channel waveguides are used to estimate coupling losses between lensed SMFs and inverse tapered Si-wire waveguides and to compensate for the wavelength dependence of Jones matrix between the lensed fibers and the inverse tapered regions on the fabricated chip. The bend waveguides work as a linear polarizer to find the polarization axis of fabricated CWDM optical DeMUX (D). SMF, single-mode fibers; CWDM, coarse wavelength division multiplexing; DeMUX, demultiplexer.

Usually, the Jones matrix measured by the OVA indicates a cascade of individual Jones matrices between the input SMF, the device under test (DUT) and the output SMF, which does not match with the Jones matrix only for the DUT [22]. To factor out the Jones matrix for the DUT, as a reference scheme, we characterized the bend waveguide (P) working as a polarizer where only the TM₀₀ mode tends to be strongly attenuated, thus enabling the Jones matrix of the DUT to be specified by verifying the eigenvector for the TE₀₀ mode. As another reference scheme, the channel waveguide (L) was also characterized for calibrating the coupling losses between the lensed SMFs and the DUT, and for compensating for the wavelength dependence of the Jones matrix between the lensed fibers and the inverse tapered regions on the fabricated chip. For the measurement of the transmittance of the fabricated device, we subtracted the coupling losses at the two facets and the propagation loss of the reference S-bend-shaped waveguides to figure out the excessive losses in the fabricated device.

In this work, besides the polarization-diversified optical DeMUX scheme, we fabricated and characterized a single optical DeMUX without the PSR to analyze only the spectral response of the optical DeMUX. The device design is identical to that shown in Fig. 1.

Figure 7 shows the measured transmission spectra for the DMZI-type optical DeMUX for the input polarization of Fig. 7(a) TE₀₀ mode and Fig. 7(b) TM₀₀ mode. As shown in Fig. 7(a), when the TE₀₀-mode signal was spatially filtered out corresponding to the CWDM grid, the wavelength spacing between Ch-2 and Ch-3 was estimated to be 18.7 nm, which agrees well with the numerically simulated value of 19 nm. The set value of ∆L is why each filtering grid tends to deviate slightly from the discrete CWDM channel grids (i.e., 1,271, 1,291, 1,311, and 1,331 nm). If we optimize the value of ∆L to 4.7 μm, the channel spacing is nearly matched with 20 nm, satisfying the CWDM standard condition. Due to the limited tuning range of the optical source in the OVA, the measured wavelength range was as wide as 60 nm. However, the wavelength range could be extended by using widely tunable lasers. We did not observe any spectral degradation of the filter response in terms of insertion loss and spectral crosstalk since the MMI couplers used in each DMZI of the optical DeMUX have a nearly constant 3 dB split ratio over a 60-nm-wide spectral range.

Figure 7. Measured transmission spectra for DMZI-type optical DeMUX for input polarization of (a) TE₀₀ mode and (b) TM₀₀ mode. DMZI, delayed Mach–Zehnder interferometers; DeMUX, demultiplexer.

As shown in Fig. 7(b), as the input polarization is set to be TM₀₀ mode, considerable spectral deterioration was observed from the viewpoint of insertion loss, the relation between the output channel, filtering grid, and channel spacing. These spectral tendencies originating from the Si wire waveguide’s large birefringence (see Fig. 4) signify why the PSR should be required in the CWDM optical DeMUX scheme.

We analyzed the measured spectra through spectral fitting analysis based on an analytical calculation model [23] and clarified the phase deviation from the ideal state due to fabrication imperfections. Figure 8 shows the analytically calculated transmission spectra for the DMZI-type optical DeMUX for the input polarization of Fig. 8(a) TE₀₀ mode and Fig. 8(b) TM₀₀ mode. Good agreement was found between the simulated and measured spectral response, which validates the robustness and stability of the currently used measurement system.

Figure 8. Analytically calculated transmission spectra for DMZI-type optical DeMUX for input polarization of (a) TE₀₀ mode and (b) TM₀₀ mode. DMZI, delayed Mach–Zehnder interferometers; DeMUX, demultiplexer.

As a result, the excessive phase deviations for the first-stage, upper, and lower second-stage DMZIs were characterized as −0.08π [rad], −0.08π [rad], and −0.07π [rad], respectively. As shown in Fig. 8(a), the estimated phase deviations were low enough to show a Gaussian-shaped clear filter response with low spectral crosstalk at each peak wavelength. As mentioned in Fig. 7(b), one spectral deterioration was the birefringence of the Si wire waveguides. However, the birefringence of the Si wire waveguide usually does not cause an abrupt increase in the excess loss of the device. When it comes to determining the main reason for the degradation of the excess loss for the TM₀₀-mode input, as shown in Fig. 8(b), we clarified that the equivalent index differences in each MMI region caused severe excess loss of >1.5–4.2 dB together with an imbalanced MMI split ratio between the two outputs by 47%.

Finally, we characterized the PSR-integrated optical DeMUX. Figure 9 shows the measured transmission spectra of TE₀₀-mode input to TE₀₀-mode output for Ch-1–Ch-4 and of TM₀₀-mode input to TE₀₀-mode output for Ch-5–Ch-8. We superimposed the spectra for each polarization input case to clarify the spectral comparison. As shown in Fig. 5, the TE₀₀-mode input signal is spatially separated to the upper output of the PSR without any mode conversion within the PSR and then launches into the upper side optical DeMUX. By contrast, the TM₀₀-mode input signal is spatially separated to the lower output of the PSR via hybrid mode conversion from TM₀₀ mode to TE₀₁ mode, and its mode is split as TE₀₀ mode in the ADC. Unlike the case shown in Fig. 7, the input signal exhibited nearly the same spectral response in Fig. 9, regardless of the input polarization state.

Figure 9. Measured transmission spectra of TE₀₀-mode input to TE₀₀-mode output for Ch-1–Ch-4 and of TM₀₀-mode input to TE₀₀-mode output for Ch-5–Ch-8.

Therefore, the integrated PSR operated properly as the mode converter and splitter only for the TM₀₀-mode input signal. In this case, the insertion loss was measured to be <1.0 dB for the worst case. Spectral crosstalk was measured to be as low as −20 dB for the peak filtering grid of each four-output channel. An available wavelength range of 5 nm was estimated when the spectral crosstalk of −15 dB is permitted in which the power penalty by the incoherent crosstalk of less than −15 dB could be less than 0.3 dB for the bit error rate of 10⁻¹² [24]. The wavelength range of 5 nm will be able to cover a temperature variation of ±35-degrees considering a temperature-dependent refractive index change of 0.07 nm/degree for Si wire waveguides. In Fig. 9, spectral discrepancies between the two identical optical DeMUXs were mainly caused by the random phase fluctuations in each optical DeMUX due to fabrication imperfections. However, the phase deviations were small enough to have a polarization-diversified spectral response for the orthogonally polarized two input signals. A more advanced lithography process such as ArF-immersion [17] or unique design optimization to increase fabrication tolerance and production yield [11] is required to further reduce the spectral discrepancy due to the phase deviations.

Meanwhile, the spectral crosstalk and the channel isolation ratio of the fabricated optical DeMUX could be further improved by optimizing the DMZI device scheme since the current filter scheme is designed to have Gaussian-shaped spectra. Mathematically, its filter spectrum for each output is close to sinc-function shaped. Thus, the insertion loss and corresponding crosstalk tend to get slightly worse as the operating wavelength deviates from each CWDM grid wavelength. The best way to further improve the channel isolation and spectral crosstalk is to make the DMZI filter design flat-topped by cascade connecting other optimized DMZIs with different optical path lengths and phase adjusters [16, 20], which make the above-mentioned factors much lower by an order of 10-dB because relatively wider wavelength components can be phase matched. In this case, a slightly bigger footprint and a fine fabrication process or phase control technology for accurate phase matching is additionally required.

A CWDM-targeted polarization-diversified four-channel optical DeMUX comprising a hybrid mode conversion type PSR and DMZI-type optical DeMUX was theoretically analyzed, and its operability was experimentally verified. The Si wire-based PSR-integrated DMZI-type optical DeMUX was fabricated with a CMOS foundry fabrication process, and an optical vector analyzer was employed to calibrate its transmission spectra. As for the TE₀₀-mode input signal, the single DMZI-based optical DeMUX showed a Gaussian-shaped CWDM-like filter spectra with a very low insertion loss of <0.5 dB and spectral crosstalk of less than −20 dB in a more than 60-nm-wide wavelength range, since the optical split behavior was stably preserved by using the MMI couplers in the device. However, such operability was eliminated by launching the TM₀₀-mode input signal that can be formed through the SMF before launching into the optical receiver end.

By integrating the PSR with the two identically designed optical DeMUXs, the CWDM-like filter spectral response was maintained irrespective of the input polarization state of the signal, validating that the PSR operated properly as the mode converter only for the TM₀₀ mode to change to the TE₀₁ mode in the rib waveguide region. Additionally, the copropagating TE₀₀- and TE₀₁-mode signals were spatially separated in the TE₀₀-mode form. Consequently, the output signal maintained the filter spectral shapes nearly constant irrespective of the input polarization state, together with an extremely low insertion loss of <1.0 dB, polarization-dependent loss of <1.0 dB, and interchannel imbalance of <1.0 dB, suppressing unwanted wavelength and polarization crosstalk from neighboring channels of less than −20 dB at the peak transmission channel grid.

The authors declare no conflicts of interest.

Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.

This work was supported by a grant from the Electronics and Telecommunications Research Institute (ETRI) funded by the ICT R&D program of MSIT/IITP [2019-0-00002, Development of Optical Cloud Networking Core Technology]. This work was supported by a research grant from the University of Suwon in 2022.

Electronics and Telecommunications Research Institute (ETRI) grant funded by the ICT R&D program of MSIT/IITP [2019-0-00002, Development of Optical Cloud Networking Core Technology].