Optical circuits introduce a groundbreaking concept that utilizes light, specifically photons, rather than electrons, as seen in traditional electronic circuits. While we can perceive photons as light, electricity remains invisible to the naked eye.

What exactly is light? How do we define optics and photonics? Light is a common energy form, both naturally occurring and artificially generated. Scientifically, light is recognized as an electromagnetic (EM) wave within the visible spectrum, specifically ranging from 400nm to 700nm in wavelength. Throughout history, from Maxwell's equations to quantum mechanics and the photoelectric effect, light has continually astonished humanity.

The field dedicated to studying visible light and its properties across different mediums is known as optics. Optics is divided into two main branches: ray optics, which treats light as a particle, and wave optics, which considers its wave-like nature. Each branch offers distinct learning experiences.

Photonics, a relatively new area of physics, exists separately from optics. The term "photonics" is derived from the Greek word "phos," meaning light, and was coined alongside "photon." Emerging in the 1960s with the advent of the laser, photonics revolutionized the study of light, focusing on its generation, manipulation, and sensing.

Where do optical circuits fit into this framework? They occupy a space at the intersection of optics and electronics, closely related to photonics, and are vital in photonics education. As photonics becomes increasingly integral to electronics, a new field called optoelectronics has emerged.

Often referred to as photonic integrated circuits (PIC), optical circuits resemble the integrated circuits (ICs) familiar in electronics. PICs are primarily constructed from indium phosphide (InP) and lithium niobate (LiNbO3), rather than the more common silicon and germanium.

Photolithography is the process employed to fabricate PICs on thin wafers, using ultraviolet (UV) light, similar to how printed circuit boards are produced. After surface preparation, photomasks are utilized to imprint patterns, guiding the light's path.

Numerous devices contribute to the construction of optical circuits, akin to electronic components like diodes and transistors. Light sources play a crucial role, producing visible electromagnetic waves. There are two categories of light sources: point sources, which emit spherical wavefronts, and line sources, which emit cylindrical wavefronts.

Key light sources include light-emitting diodes (LEDs) and lasers (light amplification by stimulated emission of radiation). LEDs are semiconductor devices emitting light when electrically activated, while lasers, such as ruby, helium-neon, and semiconductor lasers, operate by exciting particles that release energy in the visible spectrum. This process continues as long as energy is supplied.

Power splitters, also known as beam splitters, are essential passive devices in optical circuits that divide a beam into multiple weaker beams. They come in two types: fused biconical taper (FBT) and planar lightwave circuit (PLC), with the splitting ratio determining how the beam is divided.

Waveguides are crucial for guiding light waves, typically featuring a high refractive index to facilitate bending. They include planar waveguides, which are flat surfaces, and channel waveguides, which provide defined paths and help minimize propagation loss. An excellent example of a waveguide is an optical fiber cable.

Photodetectors, on the other hand, convert photons into voltage or current, allowing the detection of light intensity and presence. Common photodetectors include photodiodes, phototransistors, avalanche photodiodes, solar cells, and light-dependent resistors (LDR). These devices have applications in photometry, radiometry, LiFi, optical communication, and LIDAR.

Optical filters are passive devices allowing only certain wavelengths to pass. They can be classified into absorptive and dichroic filters. Absorptive filters absorb specific wavelengths, while dichroic filters reflect unwanted wavelengths based on their thickness and the angle of incidence. Filters can also be categorized as longpass, shortpass, or bandpass, each permitting specific ranges of wavelengths.

Optical transistors function similarly to traditional electronic transistors, acting as light valves or optical signal amplifiers.

In optical communications, signal amplification is vital to counteract energy loss during light transmission. Amplifiers, such as erbium-doped amplifiers used in fiber optic communications, strengthen signals without converting them to electrical signals.

Key parameters associated with optical amplifiers include amplification factor (gain), power efficiency, saturation energy, energy storage time, bandwidth, and noise.

While electronic circuits revolutionized the 20th century, the 21st century will likely see a significant integration of photonics and electronics. This synergy allows for the use of photons in information transmission, a feat previously unattainable, and the lighting industry now heavily relies on electronic technologies.

Optical circuits boast several advantages, particularly in speed, as photons travel significantly faster than electrons (3 x 10^8 m/s compared to electron drift velocities in the order of 10^3 m/s). This speed contributes to the growing preference for optical fiber communications (OFCs).

• The switching speed in optical circuits is markedly quicker.
• Light is immune to electromagnetic interference, a common issue in electrical circuits.
• Optical circuits experience less energy loss due to heat and attenuation.
• Energy conversion between light and electronic signals is unnecessary in optical circuits, minimizing power loss.

However, there are drawbacks to consider:

• The manufacturing of optical components can be costly and reliant on electronic technologies.
• The fabrication and integration of devices on a large scale pose challenges.
• Rapid advancements in electronics outpace developments in photonics, particularly with the rise of CMOS technology.
• Optical components are sensitive to dirt and external contaminants.

Optical circuits extend beyond experimental applications, finding uses across various scientific and technological fields, from basic lighting to high-speed broadband networks powered by fiber optics.

Optical fiber communication (OFC) represents a remarkable advancement in optoelectronics and communication, enabling data transmission at gigabit speeds with greater reliability than traditional media. OFC is unaffected by electromagnetic interference due to its reliance on light.

OFC operates on the principle of total internal reflection, where light bends at the fiber's surface based on the refractive index. Single-mode fibers transmit a single signal, while multimode fibers can carry multiple signals simultaneously.

The components of OFC include:

• Transmitter
• Medium, including cable, core, cladding, and insulation
• Receiver

Historically, OFC converted electronic signals to light and back, which introduced energy losses. However, advances in PIC technology allow for entirely optical signal transmission, reducing losses significantly.

Despite its potential, optical fiber communication is not yet fully realized, with most OFC lying beneath oceans connecting continents. Contrary to popular belief, a significant portion of internet data travels through undersea cables rather than satellites.

Light Fidelity (LiFi) is an emerging technology akin to WiFi, enabling internet access via light. LiFi is expected to surpass traditional broadband speeds without the need for physical connections. However, it remains in the research phase and is not yet commercially viable.

Optical computing involves problem-solving through various methodologies. Traditional digital computers use bits, while quantum computers utilize qubits. Optical computers replace bits with photons, offering the potential for faster computation through optical transistors and amplifiers, promising high bandwidth and speed.

Key components of optical computing include:

• Optical processors developed on a nanoscale for signal processing
• Optical data transfer mediums like fiber optic cables
• Optical storage for data, such as CDs, DVDs, and Blu-ray discs

Future optical computers are expected to utilize optical chips and PICs instead of conventional electronic chips.

Wave division multiplexing (WDM) addresses the challenge of transmitting multiple signals simultaneously. In optics, this involves combining light of different wavelengths for transmission and subsequently separating them. WDM enhances cost efficiency and minimizes hardware waste, often using prisms for signal manipulation, with amplifiers ensuring signal strength.

In the biomedical field, optical circuits play a crucial role in sensor technology. Fiber optics are more flexible than traditional fibers, enabling sensors at their ends to detect various chemicals, enzymes, and biomolecules, as well as pH levels, temperature, and liquid refractive indices. Applications include glucose sensing, laminate cure analysis, protein analysis, drug identification, and heart rate monitoring.

Despite the numerous advantages, challenges remain for optical circuits. High costs associated with extracting rare materials for device production and fabrication are significant hurdles. Additionally, light's inherent weakness compared to electricity necessitates frequent signal amplification, leading to a need for more repeaters to prevent signal loss.

Current optical systems still rely on electronic circuits for functionality, with only a handful of photonic integrated circuits operating independently on light. Furthermore, integrating lasers on-chip poses challenges due to their high power consumption.

Optical circuits require numerous resonators to maintain signal energy levels, addressing the issue of energy loss.

Looking ahead, photonics and optoelectronics are poised to dominate this century. Just as laser technology has replaced traditional firearms in various military applications, there will likely be a swift shift toward optical alternatives in the market. The distinction between science and engineering continues to blur as technology shapes our world.

Emerging technologies like LiFi and optical computing are expected to gain traction, making these innovations accessible to consumers. Optical fibers may replace existing coaxial cables, and devices that partially rely on electronics could transition entirely to optical systems. Striving for affordability, the industry must also prioritize sustainability, ensuring that advancements are both recyclable and eco-friendly.