Optical communication is a contemporary communication method that utilizes light for information transmission. In fiber communication technology, this involves signal transmission through lightwave carriers, employing fiber optics and semiconductor lasers.1,2
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Key advantages of optical communication systems include high bandwidth, point-to-point communication, a large reuse factor, transmission security, and reduced weight, volume, electromagnetic interference, loss, and manufacturing costs.1
Fundamentals of Optical Communication
An optical communication system comprises a transmitter, an optical channel, and a receiver. The transmitter consists of a laser diode and a modulator; the optical channel comprises an optical amplifier, an optical filter, and optical fiber; and the receiver contains a photodiode and a detector.1
The laser diode generates single-frequency light with a defined wavelength and produces coherent light, making it more effective as an optical transmitter compared to inexpensive light-emitting diodes (LEDs).2
Moreover, the development of vertical cavity surface emitting laser (VCSEL) has further increased the advantages of laser diodes as VCSEL combines the effectiveness of previous laser diodes and the inexpensiveness of LEDs.2
In the modulator, the digital signal is modulated onto the optical carrier. This modulation can be achieved via a separate external modulator or by directly modulating the laser diode's forward current.1
Additionally, the waveguide, typically made of plastic fiber/silica glass, is used to guide the light wave. Two types of fiber are primarily used in optical communications: multi-mode fiber (MMF) and single-mode fiber (SMF). Fiber has a 125 μm cladding and a 5-10 μm core (SMF) or 50 μm core (MMF). The core possesses a slightly higher refractive index.1
Optical-to-electrical conversion takes place in the photodiode. The received signal's power is detected using an envelope detector, and the received instantaneous light power leads to a proportional electrical signal/photocurrent.1
Noise in an optical transmission system mainly originates from the optical amplifier and the electrical domain.1 Common amplifiers used in optical communication systems include erbium-doped waveguide amplifiers, semiconductor optical amplifiers, and Raman amplifiers.2
Role of New Technologies
In basic optical systems comprising an optical source, a detector, and various optical components—such as optical fibers and couplers—the electrical signal is first modulated onto the light source, encoding the optical carrier's phase, wavelength, or intensity.3
The modulated optical signal is coupled into an optical fiber and transmitted to the destination, where an optical receiver detects the signal and retrieves the electrical information encoded on the optical carrier.3
However, these optical communication systems are only suitable for short-distance and low-data rate optical transmission. Direct modulation on a semiconductor laser source has several limitations, including a poor extinction ratio and frequency chirp.3
Additionally, attenuation within the optical fiber restricts the reach between the transmitter and receiver. The transmission capacity of single-mode fibers (SMF) is also limited by the modulation speed of the transmitter and the electrical bandwidth of the receiver, despite SMFs having a low-loss window in the hundreds of terahertz.3
To address these limitations, newer optical communication systems have introduced several technologies, such as wavelength-division multiplexing (WDM) and single-frequency semiconductor lasers.3
For instance, in a WDM optical system with N wavelength channels, every information channel is modulated onto an optical carrier with a specific wavelength using an external electro-optic modulator.3 All optical carriers are combined into a single optical fiber using a wavelength-division multiplexer, and the combined optical signal's power is increased using an optical amplifier and sent to the transmission optical fiber.3
Throughout the transmission line, in-line optical amplifiers periodically amplify the optical signal to counteract fiber transmission loss. A wavelength-division demultiplexer is utilized at the destination to separate optical carriers at various wavelengths, and each wavelength channel is separately detected to recover the data carried on each channel.3
This WDM configuration fully utilizes the wide bandwidth of the optical fiber, and optical amplification technology significantly extends the overall reach of the system. The availability of high-quality, single-frequency tunable semiconductor lasers has also increased the popularity of coherent detection-based optical receivers.3
Free-Space Optical Communication
Free space optical communication (FSOC) enables point-to-point wireless optical transmission through unguided media, utilizing visible light (VL) and infrared (IR) bands. FSOC is a line-of-sight (LOS) technology that utilizes eye-safe laser beams to provide wireless communication of optical data in free space.4
FSOC receivers contain telescopic lenses that collect light streams and transmit digital information at speeds in the gigabits per second range. The broad availability of the optical spectrum allows for the transmission of large volumes of data, making FSOC a suitable alternative to radio relay links, as light travels faster through air than through glass.4
Outdoor FSOC links primarily function in the ultraviolet (UV) and VL bands, while VL and IR bands are used for underwater and indoor applications. While radio frequency (RF) networks are highly susceptible to multipath fading, FSOC's operation at high frequencies and short wavelengths enhances photodetector resistance to multipath signal fluctuations.4
Digital signals, including images, videos, and internet data, are initially converted into light signals using an optical transmitter, followed by modulation using an appropriate scheme. The multiplexed signal then travels through the FSOC channel in the optical domain, where it is received by an optical photodetector. The transmitted signals are demultiplexed and sent to their destination after necessary electronic switching.4
Despite its advantages, FSOC networking faces several limitations. The reliability of FSOC links can significantly deteriorate due to atmospheric turbulence caused by heavy rain, snow, fog, clouds, and wind-induced pointing stability issues. Other limiting factors include the shadowing effect, background light radiation, and beam dispersion.4
Major Applications
Optical communication systems are used across various applications, including medical devices, telecommunications, and data centers.5-7 For instance, many telecommunications companies use optical fibers to transmit cable television signals, telephone signals, and Internet communication.5
Implantable medical devices (IMDs) have gained significant attention due to their ability to provide precise diagnostics and monitor chronic diseases. Enhancing implant-to-surface communication can improve their effectiveness, especially as this process often consumes considerable battery power.6
A paper published in IEEE Communications Magazine introduced a novel optical wireless communication method utilizing unsynchronized pulse interval modulation for implantable medical devices (IMDs). This approach addresses the need for ultra-low-power optical wireless communication and features a prototype designed for compact implementation.6
With the rapid growth of cloud computing, data center operations have surged, creating challenges in development. Efficient interconnection schemes are crucial to meet the increasing demand for communication bandwidth.7
Optical fibers offer significant advantages due to their high bandwidth, providing low-cost and low-loss solutions. Additionally, optical switching technology enhances transmission rates compared to traditional copper wire connections.7 Companies like ADTRAN Inc., Ciena Corporation, and Infinera Corporation are some of the major optical communication and networking equipment companies driving the market.
In summary, optical communication systems offer significant bandwidth and low loss, benefiting sectors such as telecommunications and medical devices. As technologies evolve, they will likely address the increasing demand for high-speed and reliable data transmission.
More from AZoOptics: How is Laser Communication Used in Space?
References and Further Reading
- Jasim, AA. (n.d.). Introduction to Optical Communication. [Online] Communication Engineering Dept Faculty of Electronics Engineering Ninevah University. Available at https://uoninevah.edu.iq/public/files/datafolder_9/_20191016_075734_870.pdf (Accessed on 06 October 2024)
- Amireh, Y. (2023) Optical Communication Systems. [Online] ResearchGate. https://www.researchgate.net/publication/367204394_Optical_Communication_Systems
- Hui, R., O’sullivan, M. (2023). Fundamentals of optical devices. Fiber-Optic Measurement Techniques (Second Edition). DOI: 10.1016/B978-0-323-90957-0.00002-3, https://www.sciencedirect.com/science/article/abs/pii/B9780323909570000023
- Jahid, A., Alsharif, MH., Hall, TJ. (2022). A contemporary survey on free space optical communication: Potentials, technical challenges, recent advances and research direction. Journal of Network and Computer Applications. DOI: 10.1016/j.jnca.2021.103311, https://www.sciencedirect.com/science/article/abs/pii/S108480452100299X
- Mane, S. (2023) Fiber Optics in Communication Networks: Trends, Challenges, and Future Directions. [Online] ResearchGate. https://www.researchgate.net/publication/372315210_Fiber_Optics_in_Communication_Networks_Trends_Challenges_and_Future_Directions
- Sohn, I., Jang, YH., Lee, SH. (2020). Ultra-low-power implantable medical devices: Optical wireless communication approach. IEEE Communications Magazine. DOI: 10.1109/MCOM.001.1900609, https://ieeexplore.ieee.org/abstract/document/9112747
- Hao, T. (2020). Research of Optical Interconnection Technology in Datacenter Networks. Journal of Physics: Conference Series. DOI 10.1088/1742-6596/1710/1/012001, https://iopscience.iop.org/article/10.1088/1742-6596/1710/1/012001
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