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The Significance of Silicon Photonics Modulators in Optical Interconnection Systems

Key Takeaways

  • The silicon photonics modulator plays an important role in optical interconnection systems, as it modulates the light beam that propagates in the optical waveguide or in free space. 

  • Silicon optical modulators can generally be placed into two categories: the direct absorption type and those relying on embedded phase shifters. 

  • The plasma dispersion effect is most widely used in silicon to achieve phase modulation—the other method that can be used is called thermal modulation. 

Cables transferring data

When you want to drive faster and your vehicle does not permit you to do so, how do you feel? Frustrated? Impatient? So, what do you do about it? If you are like me, you drive at the maximum possible speed available to you and try to oppress your urge to go faster. 

A similar problem arises when using copper interconnects for inter-and intra-connections in circuit boards, chips, and microprocessors. Fast communication data rates are a requirement in modern communication technologies, however, the transmission bandwidth of copper interconnects poses a barrier to establishing speed. To mitigate this problem, optical data transmission can be leveraged, as it has been revolutionized with the introduction of silicon photonics technology. Optical interconnects within an integrated circuit use light to maximize bandwidth with minimum energy per bit and device footprint. 

The silicon photonics modulator is a significant device used in optical interconnection systems. This modulator regulates the light beam that originates in the optical waveguide or in free space. There are various ways in which light can be modified and different categories of silicon photonics optical modulators available. In this article, we will break down the structure of silicon photonics modulators.

Why are Silicon Photonics Modulators Advantageous? 

When silicon acts as the waveguide material, an optical modulator is called a silicon photonics modulator. Silicon photonics modulators utilizing silicon-based technology achieved tremendous popularity in optical communication systems due to their gigahertz bandwidth modulation, reduced energy, and smaller size.  Silicon optical modulators generally come in two categories: direct absorption types and those relying on embedded phase shifters. Direct absorption-type silicon photonics modulators rely on effects such as the Franz–Keldysh effect or the quantum-confined Stark effect (QCSE). 

Optical Mechanisms in Silicon Photonics Modulators 

Optical modulators are devices that produce changes in the optical density or refractive index of a material, thus, modifying different parameters of the light beam. Depending on the change, optical modulators are divided into amplitude, phase, or polarization modulators. The linear electro-optic (Pockels) effect is the traditional means of optical modulation, however, the centrosymmetric structure of crystalline silicon obstructs the linear electro-optic effect in it. 

The plasma dispersion effect is most widely used in silicon to achieve phase modulation and changes the real and imaginary part of the refractive index leading to electro refraction and electroabsorption, respectively. This effect is related to the concentration of electrons and holes in the silicon semiconductor. 

Thermal modulation is another method of optical modulation—it varies the concentration of holes and electrons, and thereby the refractive index of the material. In thermal modulation, the refractive index of waveguide material is altered by locally applying temperature in a controllable manner. 

Silicon Photonics Phase Shifters

(a) PIN diode (b) SISCAP

Silicon photonics phase shifters

Silicon photonics phase shifters employ mechanisms such as carrier depletion and carrier accumulation in PIN diodes and silicon-insulator-silicon capacitors (SISCAP), respectively. In both types of phase shifters, the concentration of free charge carriers is varied to change the refractive index of the material.  

In the PIN type phase shifter, the depletion region is enlarged by increasing the reverse bias applied, thereby removing the carriers from the waveguide. In SISCAP type phase shifters, carriers of opposite polarity concentrate on the two sides and in the immediate neighborhood of the insulation region. This increases the concentration of free carriers in the waveguide, when the bias voltage applied is increased. 

Irrespective of the type, silicon photonics modulators satisfy the bandwidth requirements of all modulation formats with low drive voltages and insertion losses. 

Silicon Photonics Modulator Design

Silicon photonics modulator design is governed by basic waveguide technology. The earliest design of silicon photonics modulators consisted of a silicon guiding layer fabricated on a doped silicon substrate. The silicon substrate formed the lower waveguide boundary and exhibited a reduced refractive index through the plasma dispersion effect. The incorporation of silicon-on-insulator offers stronger optical confinement in optical modulators.

The first plasma dispersion silicon photonics modulator was a p+-n-n+ modulator (shown in the figure below) proposed by Soref and Bennet. This modulator is composed of a single-mode silicon rib waveguide and is polarization independent. The interaction length of the plasma dispersion silicon modulator for a 𝜋 radian phase shift is less than 1mm. The modulator contains a buried block of silicon dioxide below the waveguide material. This acts as the lower waveguide boundary in this modulator design. The retained contact to the buried n+ substrate allows vertical current flow. 

p+-n-n+ modulator

The p+-n-n+ modulator

Using silicon as a photonics material is a remarkable advancement in the electronics industry, as it has helped establish increased communication rates, gigahertz bandwidth, reduced energy per bit, and reduced footprints. The optical interconnects are the most outstanding achievement of silicon-based photonic technology, and silicon photonics modulators are the foundation of these interconnections. Apart from the plasma dispersion effects, various novel design approaches have been introduced in silicon photonics modulators over the years, providing significant improvements in parameters such as power consumption, modulation depth, and modulation bandwidth.

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