1. Introduction

Using CMOS fabrication process and compact optical circuit obtained by tight light confinement in the Si wire waveguide, many types of devices has been demonstrated [1–4] which enable mass production.

In telecommunication systems or interconnects for data center and supercomputers, the polarization becomes random after propagated through a long optical fiber transmission line. Wavelength filters used in these systems need to be polarization insensitive. In this report, we describe a reflective grating device, which can convert the input polarization into another polarization of counter propagating light, which can be used to achieve polarization independence.

Bragg grating is one of the important devices for wavelength filtering [5–11]. We have demonstrated that the wavelength peaks and diffraction efficiencies of the TE and TM modes become the same when a polarization rotation Bragg diffraction [12–14] is used. The polarization independent peak wavelength is automatically achieved for polarization rotation diffraction so that the TE and TM diffraction peak is always the same irrespective of the waveguide size, which is not achieved by other schemes. We reported an experimental polarization rotation Bragg grating device using a rib waveguide structure and waveguide with non-vertical side wall. However, the diffraction efficiencies tend to be small for these grating structures even with a complex enhancement scheme [13]. Wide reflection band, such as that required in the fiber-to-the-home (FTTH) system, can be attained easily when grating with high diffraction efficiency is obtained. Compact grating can also be realized. In previous devices, we also needed to consider unwanted diffraction from the fundamental to the first order TE mode [14] which exists near the main diffraction wavelength.

In this report we describe a device using the diffraction from fundamental to the first order TE mode and polarization rotator that converts TM mode to the first order TE mode. The polarization rotating function is separated from the diffraction. In this way, we can use Bragg diffraction between the same polarizations. Bragg diffraction with high efficiency is expected by using diffraction between modes with the same polarization. In the previous devices [12–14] using diffraction between orthogonal polarizations, the small mode field overlap between different polarizations led to low diffraction efficiency. A single diffraction peak is obtained in this new scheme by incorporating the first order TE mode, which generated an additional peak in the previous devices [12–14], into the diffraction process. The polarization rotator used in our scheme requires waveguide structure non-symmetric in the depth directions. The air upper clad, rib waveguide and non-vertical side wall structures have been investigated [15–21]. We used a rib waveguide type. Separating polarization rotation from the diffraction allows these to be optimized individually.

In this report, firstly we suggest device structure and its function. Next, the grating characteristics are examined using the 3D-FDTD (finite difference time domain) method. And finally the measured results obtained from the fabricated device are described.

2. Polarization rotator and grating structures

The device structure is shown in Fig. 1. The rib waveguide type polarization rotator is used. This type is selected for the balance between high efficiency and fabrication easiness. An asymmetric grating is connected to the polarization rotator. The side wall grating is placed on both sides of the waveguide. The side wall corrugation can be fabricated simultaneously with the waveguide structure.

 

Fig. 1 Device structure.

Download Full Size | PDF

Grating structure required for conversion between the fundamental and first order TE modes can be deduced from the symmetries of the electric components of these fields. The integral used in calculating the coupling coefficient contains the product of two modes and refractive index perturbation generated by the grating. The fundamental and the first order modes are symmetrical and anti-symmetrical around longitudinal axis respectively. For this type of mode conversion, the grating on both sides should be shifted half period against each other [11] to generate anti-symmetric refractive index perturbation so that the coupling coefficient becomes non-zero.

The grating period is obtained by Λ = λ_B/(N_TE0 + N_TE1) where λ_B is the Bragg wavelength, N_TE0,TE1 is the effective index for the fundamental or first order TE mode.

The device operation is shown in Fig. 2.The fundamental TM light is converted to the first order TE mode by the polarization rotator. The fundamental TE light passes unchanged. The Bragg grating exchanges the fundamental and first order TE modes and converts them to the backward propagating lights. The backward first order TE mode is converted to the fundamental TM mode and ejected to the output. The backward fundamental TE mode is sent through to the output.

 

Fig. 2 Polarization-rotator and grating diffraction diagram.

Download Full Size | PDF

2.2 Device characteristics obtained by FDTD

The polarization rotator and grating characteristics were examined using the 3D-finite difference time domain (FDTD) method. A 220 nm thick waveguide is selected which is a standard thickness used in most manufacturing foundries. We designed the device for 1490 nm to be used in the FTTH system.

Example of calculated wavelength response of the polarization rotator is shown in Fig. 3. The rib step height was 70 nm. The rib waveguide was tapered from width of 450 to 550 nm. The taper length was 5.8 μm. The side terrace width was 105 nm and uniform along the whole length. The efficiency including insertion loss of the TM to the first order TE mode conversion was 0.6 dB at 1490 nm wavelength. The extinction ratio of the TM mode was 15 dB. The insertion loss of the fundamental TE mode was 0.1 dB. The TE1 peak is slightly shifted to shorter wavelength compared to TM0 bottom in Fig. 3(b) presumably due to higher loss at longer wavelength.

 

Fig. 3 (a) Polarization rotator structure and (b) wavelength response obtained by 3D-FDTD simulation.

Download Full Size | PDF

Figure 4 shows an example of calculated wavelength response of the polarization rotator and grating combination (Fig. 1). The grating length was 50 μm and corrugation depth of 150 nm was used. The corrugation period was 354 nm. The grating waveguide width was 550 nm. A rectangular grating corrugation was used. The TE-to-TM and TM-to-TE rotation diffraction wavelength peaks have the same wavelength characteristics as expected. The differences of the peak heights and wavelengths between polarizations are below detectable amount (0.2 dB and 0.2 nm respectively). The excess loss of the grating was 0.1~0.2 dB. The full width peak width, which we define as a separation between first zeros around the peak, is Δλ = 2λ_B²[|K²-(π/L)²|]^1/2/[π(n_g^TE0 + n_g^TE1)], where λ_B is the Bragg wavelength, K is the coupling coefficient, L is the grating length, n_g^TE0 and n_g^TE1 are group indices for the TE fundamental and first order modes respectively. The polarization independent peak width can be obtained.

 

Fig. 4 Wavelength response of grating combined with the polarization rotator (Fig. 1) obtained by 3D-FDTD simulation. The grating length is 50 μm. The waveguide width is 550 nm and corrugation depth of 150 nm is used.

Download Full Size | PDF

The coupling coefficients of the grating are shown in Fig. 5 for 220 nm thick and 550 nm wide waveguide. The coupling coefficient was obtained by matching results of coupled mode equations and 3D-FDTD simulation. The coupling coefficient increases with deeper grating corrugation. The coupling coefficient is more than 10 times larger than that for our previous polarization rotator gratings.

 

Fig. 5 Grating coupling coefficient as function of corrugation depth.

Download Full Size | PDF

3. Experiment

As shown in Fig. 6, a pairs of Bragg gratings and polarization rotators are connected to the two outputs of the polarization beam splitter (PBS). The Si core thickness was 220 nm. The same design as that described in the previous section was used for the Bragg grating except for the length. A 500 μm long Bragg grating is used in the experiment. We used long grating so that diffraction can be observed even if the grating efficiency became low due to rounding of the grating corrugation. A 100 nm tip wide and 100 μm long inverse taper waveguide termination is connected to the end of the Bragg grating. This tapered waveguide termination can reduce the unwanted reflection at the waveguide end to −45 dB which is indicated by 3D-FDTD simulation. The polarization rotator has the same design as that described in the previous section. We used a directional coupler type PBS [22]. The coupler waveguide width and gap were 600 nm and 300 nm respectively. The length of the PBS is 20 μm. The PBS excess losses obtained by 3D-FDTD simulation for the 1490 nm wavelength TE and TM modes were 0.05 dB for both polarizations. The polarization extinction ratio was 17 dB. Elements are connected with 450 nm wide waveguides and 20 μm radius was used at curved waveguides. The insertion losses of elements such as PBS were estimated by measurement using each device fabricated separately.

 

Fig. 6 Device structure used in the experiment.

Download Full Size | PDF

A light is injected into one input of the PBS. The separated TE and TM modes are sent to different polarization rotators and Bragg gratings. The diffracted and polarization rotated lights are sent back to the PBS, combined at PBS and ejected from the opposite port.

The device was fabricated using SOI (Silicon on insulator) wafer and standard fabrication process used in the foundry. The waveguide pattern was written in resist layer by photo lithography. The Si layer was etched by RIE (reactive ion etching) to obtain waveguides. Two step mask alignments and etching processes were used to obtain rib waveguide structure.

In the measurement, super luminescent diode (SLD) was used as a wide band light source. The input polarization was controlled and injected to the sample by polarization maintaining fiber (PMF). The output is introduced into an optical spectrum analyzer through PMF and the polarization was determined by using a polarizer. The lensed fibers are used. The inverse width taper is used as spot size converters (SSC) at the waveguide facets. The propagation losses of 1.9 and 3.0 dB/cm obtained using 440 nm wide reference waveguides, the SSC coupling losses of 7 and 4 dB/facet for the TE and TM modes respectively were taken into account. The chip length is 4.5 mm. The measured wavelength response is shown in Fig. 7. The diffraction peak wavelengths of the TE and TM polarization were the same within 0.1 nm. The peak height difference between polarizations was 0.5 dB and −2.5 dB peak height was obtained for the TE mode. The flat-top wavelength peak indicates a high diffraction efficiency of the Bragg grating. The grating coupling coefficient is estimated to be 0.08 μm⁻¹. This slightly smaller value than expectation could be explained by rounding effect of the grating shape. The value is more than 5 times larger than those obtained in our previous experiments. This results indicates thet grating length of 50 μm may be long enough. The polarization extinction ratio of the PBS was 15 dB in the experiment. The round trip excess losses of 1.5 dB and 1 dB were generated at polarization rotator and PBS respectively, which can be reduced by improved design such as a tapered rib–channel junction or width waveguide.

 

Fig. 7 Measured wavelength response.

Download Full Size | PDF

4. Conclusion

We have reported polarization independent Bragg grating wavelength filter with high efficiency. A rib waveguide polarization rotator and antisymmetric grating structure for fundamental to first order diffraction are used to generate the polarization rotation Bragg diffraction. The diffraction efficiencies and peak wavelengths become the same for two orthogonal input polarizations. Strong diffraction is easily attained. The grating characteristics were examined using 3D-FDTD. The diffraction peaks are almost the same for the TE and TM modes. A device was fabricated and tested. A 220 nm thick waveguide was used. Polarizations rotating Bragg diffraction and polarization beam splitter are used to achieve polarization independent wavelength filtering. The diffraction peak wavelengths of the TE and TM polarization were the same within 0.1 nm. The polarization dependent peak height difference was 0.5 dB.