What Modulation Method Is Used For Optical Fibers? Three Technical Explanation
With the rapid development of 5G, cloud computing, and big data centers, fiber optic communications have become a core supporting technology for modern networks. So, how do fiber optic signals transmit efficiently? The answer lies in modulation technology. Modulation not only determines the transmission rate but also affects transmission distance and system cost. So, what modulation methods are used in fiber optics? Are the modulation methods used in different scenarios the same? This article will provide an in-depth analysis of common fiber optic modulation methods, their advantages and disadvantages, typical applications, and the products required.
What Is Fiber Optic Modulation?
In optical fiber communication, optical fiber modulation is the process of “loading data into optical signals”. Light itself is a single waveform and cannot directly carry complex information. Therefore, certain characteristics of light (such as brightness and vibration state) need to be adjusted through technology to correspond to the “0” and “1” in digital signals – just like pressing the “encoding key” on the light, and then the receiving end can “decode” it through the device to obtain the information we need. The main functions of modulation include:
- Boost transmission rate: More bits can be sent over the same bandwidth using sophisticated modulation techniques.
- Boost anti-interference capability: The signal-to-noise ratio can be greatly increased by using polarization multiplexing and phase modulation.
- Increasing transmission distance: Coherent detection technologies and high-order modulation are essential for long-distance backbone networks.
Common Modulation Methods For Optical Fibers
Having grasped the basic concepts, let’s take a closer look at what the commonly used optical fiber modulation methods are at present? Where are they respectively used and what devices do they need to be paired with?
1.Intensity Modulation (IM-DD)
Principle: The binary signals “0” and “1” are represented by adjusting the light intensity (changing between bright and dark).
Advantages: Simple implementation, low cost, and low power consumption
Disadvantages: Limited transmission rate, weak anti-interference ability, and not suitable for ultra-long distances and ultra-high speeds
Typical application scenarios
- Short-distance link: Connection between racks in data centers
- Access network: FTTH (Fiber to the Home), LAN (Local Area Network)
Required products and components
Lasers (LD) : DFB lasers (commonly used in 10G links), VCSEL lasers (850nm, commonly used in multimode fibers)
Direct modulation driver
Receiving end: PIN photodiode
Common optical modules
SFP, SFP+ modules (1G/10G short range)
QSFP+ module (40G short range)
2. Phase Modulation (PSK, including QPSK)
Principle: Digital information is represented by changing the phase Angle of the optical carrier. QPSK can transmit two bits in each symbol, improving spectral efficiency.
Advantages: Strong anti-noise performance, suitable for long-distance and high-capacity transmission
Disadvantages: The system is complex, requires coherent detection, and has a high cost
Typical application scenarios
- Long-distance backbone transmission
- Submarine optical cable system
- High-speed OTN network
Required products and components: External Modulator (e.g., Mach-Zehnder Modulator (MZM) or Acousto-Optic Modulator (AOM)), narrow-linewidth laser (to ensure signal coherence), Coherent Receiver
Common optical modules
100G CFP, CFP2 coherent optical modules
200G/400G coherent transmission equipment
3. Orthogonal Amplitude Modulation (QAM, such as 16-QAM, DP-QAM)
Principle: By combining phase and amplitude modulation, more bits can be carried in a single symbol. For instance, 16-QAM can carry 4 bits.
Advantages: High spectrum utilization rate and support for ultra-high-speed transmission
Disadvantages: High requirements for optical signal-to-noise ratio and high system complexity
Typical application scenarios
- Data Center Interconnection (DCI
- Ultra-high-speed optical transmission of 400G/800G
- Core backbone network
Required products and components: IQ modulator (for achieving amplitude and phase compound modulation), high-speed DAC, ADC (in conjunction with DSP processing), coherent receiver, DSP chip (digital signal processing)
Common optical modules
400G QSFP-DD DCO coherent module
800G Coherent optical module (CFP2-DCO)
Supplementary: Polarization Multiplexing (DP-QPSK) and Future Trends
Polarization multiplexing doubles the capacity by using two orthogonally polarized lights
It is commonly found in 100G DP-QPSK and 400G DP-16QAM systems
Future trend: Adaptive modulation combined with AI
Comparison of Optical Fiber Modulation Methods
| Modulation method | Advantages | Disadvantage | Application scenarios |
| IM-DD | Low cost and easy to implement | Poor anti-interference performance and limited speed | Short-distance access networks and data centers |
| QPSK | Strong anti-interference ability and long-distance transmission | The cost is high and relevant detection is required | Backbone network, submarine optical cable |
| 16-QAM | High spectrum utilization rate | It has high requirements for signal-to-noise ratio and is complex | Core network, 400G system |
How To Choose?
In fact, the selection and transfer system does not require complex calculations. It is only necessary to grasp three core demands:
Look at the transmission distance: For short distances (within 10 kilometers, such as homes or communities), select intensity modulation. Phase modulation is selected for medium and long distances (over 100 kilometers, such as across provinces). For short distances but at ultra-high speeds (such as between data centers), amplitude-phase joint modulation is selected.
Check the transmission speed: For low speeds (within 10Gbps), use intensity modulation. Phase modulation is used for medium and high speeds (100Gbps-200Gbps). Ultra-high speed (above 400Gbps) uses amplitude-phase combined modulation.
Consider the budget: If your budget is limited and you are pursuing cost performance (such as installing a home broadband), choose a device with intensity modulation. When the budget is sufficient and stable long-distance transmission is required (such as in the backbone network of an operator), phase modulation should be selected. For those with a high budget and a pursuit of ultra-high speed (such as large data centers), amplitude-phase combined modulation should be chosen.
Conclusion
After reading this article, you will find that fiber optic modulation is not a “sophisticated technology”, but a practical solution centered on “how far, how fast and how economically it can be transmitted”.
What modulation method is used for optical fibers? The essence is all about choosing the right way and matching the equipment based on the demand.
- Short distance: IM-DD (Simple, low cost)
- Long-distance backbone: QPSK (Coherent Detection)
- Ultra-high speed (400G/800G) : 16-QAM or higher order modulation (combined with polarization multiplexing)
