Reducing TE and TM Polarization Dependency in SOI Waveguides

Key Takeaways

• What is silicon photonics-enabled communication? 

• TE and TM polarization in SOI waveguides.

• How to reduce polarization dependency in SOI waveguides. 

Silicon photonics integrated circuits satisfy the speed and bandwidth demands of modern communications and are used in a wide range of applications in telecommunication, data centers, high-performance computing systems, optical communications, etc.

In optical communications, silicon photonics elements on silicon-on-insulator (SOI) platforms are often used along with optical fibers. When SOI photonic integrated circuits are coupled with optical fibers, a difference in polarization is experienced. The optical fiber utilizes a random polarization state, whereas the polarization state of light can be either transverse-electric (TE) or transverse-magnetic (TM) polarization in SOI waveguides. Most of the time, the difference in the polarization states between optical fibers and SOI photonic integrated circuits causes a loss of signal integrity in communication systems. To maintain reliability and signal integrity in communications, achieving polarization insensitivity in SOI photonic integrated circuits is necessary.

In this article, we will discuss the polarization states in SOI waveguides and explore some options to manipulate the polarization states in SOI photonic devices.
 

Silicon Photonics-Enabled Communication

Silicon photonics is a promising technology for optical communications due to its low cost, excellent processing control, high volume processing, and compatibility with CMOS technology. In silicon photonics integrated circuits or chip-enabled communication, data is transferred into light pulses using lasers. These lasers are usually made of direct bandgap materials such as indium phosphide or gallium arsenide. The lasers are integrated into the silicon photonic integrated circuits to drive the photonic elements within the chip.

In downstream silicon photonic components, the light pulses from lasers are combined into a single signal using multiplexers. The single signal is transmitted across the optical fibers to a silicon-based receiver. At the receiver, demultiplexers are employed to divide the signal back to separate channels. The light is transformed back to data using photodetectors. 

TE and TM Polarization in SOI Waveguides

In communication systems supported by SOI photonic chips, the light travels through optical fibers, fiber arrays, waveguide structures, etc. An SOI waveguide is a promising component used for guiding light in silicon photonic integrated circuit-based communication systems. The SOI waveguide structures are fabricated with silicon core and silicon dioxide or air cladding. The high contrast between the refractive indices of the core and cladding induces birefringence in SOI waveguides. Due to the birefringence property, there is a significant dependency on the polarization state of light in SOI waveguides.

The polarization state of light in SOI waveguides can be described based on the orientation of the electric and magnetic fields of incident light in the waveguide relative to the plane of incidence.  The polarization state of light in SOI waveguides can be of two types, either TE or TM polarization.

1. Transverse-electric (TE) polarization - In TE polarized light, the electric field is perpendicular to the plane of incidence. The magnetic field of TE polarized light aligns itself to the plane of incidence. 

2. Transverse-magnetic polarization - In TM polarized light, the magnetic field is perpendicular to the plane of incidence. The electric field of TE polarized light aligns itself to the plane of incidence. 

The Polarization Sensitivity of SOI Waveguides 

The polarization sensitivity of SOI waveguides limits the integration with optical fibers. The light in SOI waveguides may be either TE or TM polarized, whereas, in optical fibers, the light takes a random polarization state. It can be summarized that an SOI waveguide is a polarization-sensitive component and the optical fiber is a polarization-insensitive element.

Whenever SOI waveguides are integrated with optical fibers, the transmission of light from the polarization-sensitive component to the polarization-insensitive structure degrades the signal-to-noise ratio (SNR) of the communication system. The signal integrity and reliability of communication are compromised if measures are not taken to eliminate the polarization sensitivity of silicon photonic integrated circuits. 

Reducing Polarization Dependency in SOI Waveguides

In the upcoming section, we will discuss a few of the methods used for eliminating TE and TM polarization sensitivity in silicon photonic elements. 

Achieve Nonbirefringence

The primary method to reduce polarization dependency is to achieve nonbirefringence in SOI waveguides. Nonbirefringent SOI waveguides can be achieved by optimizing the waveguide dimension. This method of reducing polarization-dependency works well for large SOI ridge waveguides. Controlling the oxide cladding-induced stress can help minimize nonbirefringence in thinner SOI waveguides. 

Polarization Diversity Technology

Another technique to eliminate polarization sensitivity in ultra-compact SOI nanowire waveguides is to use polarization diversity technology. In polarization diversity technology, devices such as polarization beam splitters and polarization rotators are utilized for making SOI nanowire waveguides insensitive to polarization. Polarization beam splitters are used to separate the input lights into TE polarized and TM polarized beams. Either TE or TM polarized beams are then converted into other polarizations to form two beams of the identical polarization state. The two beams are then allowed to enter the two identical photonic integrated circuits separately. One of the two outputs obtained from the photonic integrated circuits is converted back to the original polarization using a polarization rotator. The two orthogonal beams obtained are combined with a polarization beam combiner. The aforementioned polarization diversity circuit helps in converting the TM polarization to TE polarization and vice-versa in SOI nanowire-based photonic integrated circuits.

TE and TM polarization dependency in SOI waveguides limits their practical applications in modern communication circuits. Altering the SOI waveguide dimensions or employing a polarization diversity circuit is effective in minimizing polarization dependency. Cadence’s software can help you in modifying SOI waveguide dimensions or designing polarization beam splitters or polarization rotators in polarization diversity circuits.

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