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With the changing environment in optical communication, it is important to choose the appropriate optical transmitter to enhance the performance of CATV networks. Shandong Wanshuo Optoelectronic Equipment Co., Ltd., is a professional company that combines R&D, production, and sales in the field of optical communication, providing external modulated optical transmitter and direct modulated solutions. Knowing the distinctions helps network designers configure their infrastructure to achieve the best signal quality, transmission distance, and system reliability.
Understanding Direct Modulated Transmitters (DML)
Direct modulated transmitters encode information onto the optical signal by directly modulating the injection current of a laser diode. When the modulating signal is high, current passes through the laser diode and it emits light; when the modulating signal is low, the laser does not emit light. This simple method is cost-effective and is commonly utilized for short-distance applications.
But direct modulation has an effect called "chirp," unwanted modulation of the frequency of the light which occurs along with changes in its intensity. Chirp interacts with the fiber dispersion, leading to signal degradation over distance. Besides, laser is not stable in modulation, which will influence output signal quality from nonlinearity. The drawbacks are important issues for CATV applications that demand high linearity.
The External Modulated Optical Transmitter Advantage
An external modulated optical transmitter is based on a totally different architecture. The laser emits a continuous wave (CW) output and the external modulator, usually a Mach-Zehnder interferometer or electro-absorption modulator, is used to modulate the signal onto the optical carrier.
This "separation" of light generation and modulation maintains the spectral purity and coherence of the laser. In external modulation, the laser is kept in a constant and stable state and the chirp that is problematic in direct modulation is avoided. The result is excellent TV signal quality and greatly reduced composite second order (CSO) and composite triple beat (CTB) distortions, which are important performance parameters for CATV transmission.
Performance Comparison: Distance and Signal Quality
The decision between these technologies may depend on the distance of the CATV networks. The external modulated transmitters are mainly used at the wavelengths of 1550 nm, and can transmit communications over a distance of up to 50 km without compromising signal integrity. This makes them ideal for metropolitan area networks and long-haul trunk applications.
The direct modulated transmitters are usually 1310 nm, and best suited for shorter distances where dispersion effects are more negligible. Although modern designs have been refined, DMLs are not as efficient as external modulation for analog video transmission over long fibers.
Application Scenarios and Selection Criteria
The choice should be based on a number of factors when making a CATV transmitter comparison:
Choose an external modulated optical transmitter when:
The transmission distance exceeds 20-30 km.
High channel count, high linearity requirements exist.
An architecture requires 1550 nm wavelength support.
The superior carrier to noise ratio (CNR) is necessary
Directly modulated transmitters are suitable for:
Cost-sensitive applications
Shorter distribution networks
1310 nm wavelength systems
Relaxed signal quality requirements.
Shandong Wanshuo's Manufacturing Excellence
Leveraging its advanced manufacturing, Shandong Wanshuo Optoelectronic Equipment Co., Ltd. is able to manufacture both types of transmitters of high quality. The company guarantees the reliability of the products by having 12 high-grade purification workshops and 3D SPI+3D AOI full-process testing systems, which is used in the high-end SMT production line. They have strict quality control from raw materials to finished products for its assembly and device production lines, enabling both mass production and flexible customization.
Conclusion
In today's CATV systems, both external modulated optical transmitter and direct modulated transmitter work. Direct modulation is suitable for smaller applications and external modulation provides greater performance in long-distance and quality demanding applications. As an independent R&D enterprise and an intelligent manufacturing company, Shandong Wanshuo Optoelectronic Equipment Co., Ltd. delivers reliable solutions to the entire optical communication field.