Fundamentals of Si Photonics (1)

Properties and Classification of Light

To better understand the various technologies used in photonics packaging, it is necessary to examine the fundamental physical properties of light. Light is an electromagnetic wave, consisting of oscillating electric (E) and magnetic (B) fields that are perpendicular to each other and the direction of propagation. These fields generate each other and propagate through space together. As a result of this oscillation and propagation, light exhibits a wave-like nature with a characteristic wavelength. Depending on the wavelength, light can be classified into various types, including visible light, infrared, and ultraviolet radiation, each with distinct properties.

Light always travels in a straight line in a homogeneous medium known as rectilinearity. When light hits the boundary of another medium, it reflects at an angle equal to the angle of incidence, a phenomenon known as reflection. If light transitions into a different medium, it bends at the boundary, which is called refraction. Light has the property of polarisation, vibrating in a specific direction, and it exhibits diffraction when it passes the edges of obstacles, spreading out. Additionally, when two or more beams of light meet, they can interfere with each other, reinforcing or cancelling each other out, known as interference. Light also carries energy that can be transferred to other objects and exhibits both wave-like and particle-like properties. Optimal photonic packaging can be constructed by appropriately utilising light properties, such as rectilinearity, reflection, refraction, and polarization.

The wavelength of light is a crucial factor in photonic packaging. Depending on its wavelength, light can be categorized as shown in the table below.

Types of Light	Wavelength	Energy level	Characteristic	Applications
Radio waves	>30mm	Lowest energy	Suitable for communication and broadcasting	Radio, TV, Mobile communications
Microwaves	1 ~ 30 mm	Higher energy than radio waves	Suitable for communication and heating	Microwave, Satellite communications
Ultraviolet	700 nm ~ 1mm	Lower energy than visible light	Heat Energy Emission	Heater, night vision goggles
Visible light	400 ~ 700 nm	Intermediate energy	Indicate color	Sunlight, Fluorescent light
Infrared	10  ~ 400 nm	Higher energy than visible light	Sterilization, Vitamin D production	Solar radiation, UV lamp
X-rays	0.01 ~ 10 nm	Very high energy	Material Penetration, Imaging	Medical use, Airport Screening
Gamma-rays	<0.01 nm	Highest energy	High-energy radiation	Emission from radioactive materials, cancer treatment

Among the wavelengths of light divided as above, the wavelength range used for optical communication is from 1260 nm to 1625 nm in the Near Infrared region.

The most common wavelengths used in optical communications are listed below 850 nm, 1310 nm, and 1550 nm.

The 850 nm wavelength corresponds to a frequency of approximately 350 THz and is generally well-suited for inexpensive silicon-based optical fibers, exhibiting relatively low transmission losses. The 850 nm wavelength is extensively used in data centres and high-speed networks. Specifically, it is well-suited for 10Gbps Ethernet communications and is primarily employed in local area networks (LANs), offering high bandwidth over short distances. The fiber optic cables used are typically multimode fibers, capable of transmitting multiple modes of light simultaneously, enabling high data transmission rates over short distances.

1310 nm (O-band) is a wavelength commonly used in optical communication along with the 1550 nm (C-band) wavelength. It has lower dispersion compared to 1550 nm, making it advantageous for medium-distance communication, and the production cost of optical fibers at 1310 nm is lower than that at 1550 nm, providing an economic benefit. However, the loss in optical fibers at 1310 nm is higher than at 1550 nm, making it unsuitable for long-distance transmission, and it can be more sensitive to temperature changes, which may affect signal quality.

The primary applications include LAN (Local Area Network), MAN (Metropolitan Area Network), data centres, cable TV systems, and industrial networks, primarily used in communication systems over relatively short distances. Additionally, it offers low latency and high bandwidth, making it suitable for high-speed data communication.

The 1550 nm (C-band) wavelength is the most widely used in optical communications due to its significantly lower attenuation in fiber compared to 1310 nm, making it suitable for long-haul transmission over 100 km. Additionally, its wide bandwidth allows for the transmission of more data. However, the dispersion, or spreading of the signal, is higher than that of 1310 nm, which can lead to signal distortion over long distances. Moreover, the production cost of fiber optic cables for 1550 nm is higher than that for 1310 nm.

Its primary applications include long-haul communication systems such as submarine cables for international networks. The 1550 nm wavelength is extensively used in DWDM systems to enable wavelength division multiplexing, allowing multiple channels to be transmitted simultaneously over a single fiber. Additionally, it is a prevalent choice for data centers that require the high-speed transmission of large volumes of data.

Wavelength	850 nm	1310 nm	1550 nm
Light dispersion	High	Low	Very low
Light loss	High	Medium	Very low
Fiber Optic Price	Low	Medium	High
Applications	Short-range communications (LAN, intra-building networks), data centres, multi-mode fiber	Medium-distance communications, data centre CATV, industrial networks	Long-distance communications, DWDM, data centres

Other wavelength regions have the following applications:

The ultraviolet (UV) range, from 100nm to 400nm, has high energy and interacts strongly with matter. Its applications include sterilization lamps, fluorescence analysis equipment, and chemical analysis equipment due to its ability to sterilize and detect fluorescent substances. Visible light, ranging from 400nm to 700nm, is detectable by the human eye and is used in smartphone camera image sensors, luminance sensors and more.

The near-infrared (NIR) range, from 700nm to 1000nm, has similar properties to visible light but with higher penetration. It is used in remote temperature measurement, biometric recognition, night vision goggles, fingerprint sensors, and medical imaging equipment.

Short-wave infrared (SWIR) light, ranging from 1000nm to 3000nm, penetrates extremely well, allowing it to detect objects through fog or smoke. It is used in industrial inspection and military surveillance equipment.

Mid-wave infrared (MWIR) light, ranging from 3000 nm to 8000 nm, can detect thermal radiation and create thermal images. It is used in thermal imaging cameras and night vision goggles.

Long-wave infrared (LWIR) light, ranging from 8000nm to 15000nm, can also detect thermal radiation to measure object temperatures and is used in temperature sensors and thermal imaging cameras.

As shown, light with different wavelengths has various characteristics, and by applying the appropriate wavelength range for each application, it can be used in a wide variety of fields.