Optoelectronics: Emerging Technology Focused on Light-detecting Devices

CHAPTER 1

General Theory of Optoelectronics

As we discussed before, optoelectronics is a specific discipline of electronics that focuses explicitly on emitting or light-detecting devices. 

Optoelectronic devices refer to components used to detect or emit electromagnetic radiation, typically in the visible and near-infrared (NIR) regions of the electromagnetic spectrum.

Each of these functions exploits the photoelectric effect of materials, also known as light-matter interaction. The premise of this was established by Albert Einstein, who postulated that light was quantised, formed of photons instead of continuous waves.

Source: Introduction to Optoelectronics and Photonics by Jordan Edmunds

The underlying mechanisms of all optoelectronic devices are based on the photovoltaic effect, which refers to the emission of electrons from material by photons.

When a light beam strikes a photoelectric material, photon energy may be absorbed by electrons in the material’s crystal lattice. Provided this energy exceeds the electron’s energy bandgap; it is ejected from the material.

The same general principle works in the inverse to produce light from electrical signals.

CHAPTER 2

The Difference Between Optoelectronics, Electro-Optics, and Photonics Explained

The study of light is an incredibly complex and intersectional field that concerns various schools of thought.

Photonics and optics, for example, are two undoubtedly interlinked yet distinct areas of science. Likewise, optoelectronics and electro-optics are entirely separate yet related entities.

It can be easy to lose track of the vernacular when considering these various fields from such a broad vantage point. Furthermore, these terms are often used interchangeably when they have distinct meanings that are worth noting. They're defined as the following:

Optoelectronics - described as "a device that responds to optical power, emits or modifies optical radiation or utilizes optical radiation for its internal operation” or “any device that functions as an electrical-to-optical or optical-to-electrical transducer.” [source]

Electro-optics - these are known as “use of applied electrical fields to generate and control optical radiation" [source]. It's also worth noting that a warning is provided for this term because electro-optic (E-O) is frequently used as a synonym for optoelectronic, which is incorrect.

Photonics - Photonics is known as "technology for generating and harnessing light, whose quantum unit is the photon." This definition is the broadest of the three terms listed. [source]

Let's break it down further...

A simple way to distinguish between optics and photonics is that both are concerned with manipulating light. Still, electro-optics is involved with electrically manipulating devices and systems to produce desired light properties, while photonics is concerned with manipulating light to produce a required electrical signal.

Before exploring the types of optoelectronic devices available today, it is worth  distinguishing what is meant by electro-optics compared to optoelectronics. There is some disagreement on the word usage, as  mentioned.

Some argue that the two are synonymous, but this is not entirely accurate.

As the name would suggest, electro-optics is more closely linked to the field of optics. Digging deeper, electro-optics typically refers to methods and devices used to moderate the characteristics of light via an electric effect, such as electronically adjusting the refractive indices of optical devices.

Optoelectronics is significantly more varied than that.

Now that we've covered the basic terminology let's dive into the different types of optoelectronic devices on the market.

CHAPTER 3

Types of Optoelectronic Devices You Need To Know and Their Applications

Today, optoelectronic devices are primarily based on semiconductors like silicon (Si), which exhibit electronic properties between those of a conductor and an insulator based on the structure of the energy bandgap in the material.

Though the relationship between optoelectronics and semiconductors is not mutually exclusive, they form the basis of most optoelectronic systems used in consumer, industrial, and military products.

These include but are not limited to:

• Photodiodes
• Photovoltaics (or solar cells)
• Photoresistors
• Light-emitting diodes (LEDs)
• Encoder sensor integrated circuits (ICs)
• Laser diodes
• Optical fibres

Photodiodes

In short, a photodiode is a semiconductor light sensor that consists of an active P-N junction and generates a current or voltage when light falls on the junction, according to ELPROCUS.

This type of device has three "modes" it can be used in:

1. forward biased as an LED
2. reverse biased as a photodetector
3. photovoltaic as a solar cell

Photodiodes are used in numerous applications such as medical equipment, cameras, industrial equipment, and safety equipment.

Photovoltaics (or solar cells)

A solar cell or photovoltaic cell is a device that converts the sun's energy directly into electricity. [source] Since sunlight is composed of photons when it falls on to a solar cell, it produces a current and a voltage which generates electric power.

Applications for photovoltaics include telecommunication systems, ocean navigation aids, rural electrification, and more.

Photoresistors

These devices are light-controlled variable resistors, also known as light-dependent resistors (or LDRs).

According to All About Circuits, when a photoresistor is placed someone very dark, its resistance is very high (in the megaohms). In contrast, when illuminated, its resistance reduces significantly (depending on the light's intensity, it can be hundreds of ohms).

In terms of application, photoresistors are used primarily in light-sensitive switching devices. 

Light-Emitting Diodes (or LEDs)

A light-emitting diode (LED) is a P-N semiconductor diode where the recombination of electrons and holes produces a photon, which is an effect called electroluminescence.

The uses of LEDs vast because they produce less heat, consume less power, and last longer than incandescent lamps.

Typical applications can range anywhere from computer components and medical devices to instrument panels, household appliances, watches, and more. 

Encoder sensor integrated circuits (ICs)

These are a complete system-on-chip (SoC) integrated circuit sensor that is utilized as the heart of an optical encoder to convert rotary or linear motion into electrical signals that can be used to determine speed, rate, velocity, distance, position or direction. 

A typical application will use one or more of these sensors as feedback to the controller in a motion control system.  

Encoder Sensors IC’s feature a monolithic array of active photodiodes that are utilized to convert the light signal into electrical signals but in addition to this, it has most of the peripheral circuitry to condition and manipulates the electrical analog signals. 

Some of the most complex encoder IC’s may contain up to 10 different subsystems to form a cohesive system for precise motion detection.

For example, it may contain a close-loop feedback input driver, a temperature sensor, analog, and digital outputs and short-circuit protection circuitry which together, form a cohesive system to provide a turn-key solution to the customer’s need.

Laser Diodes

Laser diodes are semiconductor laser devices that convert electrical energy into light energy. They are very similar in form and operation to LEDs.

A LASER diode (also known as VCSEL – Vertical Cavity Surface Emitting LASER) has many benefits such as:

• 70 times more efficient than traditional LEDs. 
• Single optical wavelength, high coherence light, which minimizes signal loses over long distance fiber communications. 
• Unparalleled reliability, typically sees around 1M operating hours vs about 100k operating hours for LEDs.
• Because of the nature of VCSEL light and due to leaps and bounds in semiconductor manufacturing processes and controls, VCSEL die are faster than their LED counterparts.

Note: In the industry today, 10 Gbps VCSELs are very common in telecommunications applications. The VCSEL components that TT Electronics manufactures are 2.5 Gbps. Typically LEDs are in the megahertz range.  So the speed difference is orders of magnitude.

Again, there are numerous uses for this device, including but not limited to, military applications, CD players, surgical procedures, and long-distance communication.

Optical Fibre

Optical fibre is used with optoelectronic devices to transmit information through modulated light. Typical uses of optical fibres are telecommunications, sensors, bio-medicals, and fibre lasers.

A common question that comes up is:

"I see optoelectronics are used in fibre optics, do you have any examples how this is better than traditional fibre optics?"

Here are four reasons why fiber is used over electrical...

1. Optical signals are not susceptible to EMI/EMC interference, nor do they cause it.
2. Signals of different wavelengths can be sent down the same fiber and they do not interfere with one another.
3. Optical signals do not degrade as fast as electrical ones.
4. The bandwidth of an optical cable is far greater than an electrical cable.

CHAPTER 4

Key Advantages and Disadvantages of Optoelectronics 

Without a doubt, optoelectronics is a vital technology that has enabled practically a seamless function of the information industry, and it has benefitted numerous sectors that are involved, such as medical, communications, aerospace, and defence.

However, like any technology, there are key advantages and disadvantages of optoelectronics that need to be considered.

CHAPTER 5

The Future of the Optoelectronics Industry

According to Market Insight Reports, the optoelectronics market is expected to grow at a CAGR of 10.25% over the forecast period of 2019 to 2024.

Optoelectronic devices make up a significant part of the global semiconductor market, and growth is being witnessed across a few areas, specifically...

• High demands for LEDs have become an industry standard for display technology in electronic devices. This standard is due to increased demand for better performance and higher resolution among consumers. 
• There is a growth in demand in the automotive industry thanks to the adoption of electric vehicles and autonomous vehicles, which is expected to boost the usage of optoelectronics devices, thus propelling the market.
• Advanced manufacturing and fabrication technologies are seeing growing consumption, which is driving the use of optoelectronic components in the industrial sector.

Furthermore, optoelectronics provides significant opportunities for R&D, and its effect can be seen in areas of performance improvement, cost reduction, and large volume manufacturing. [source]