Ethernet is the most widely used networking technology, especially in environments that require high reliability, large data transfer, and easy scalability—such as enterprises and educational institutions. According to the IEEE 802.3 standard, each coaxial cable segment in an Ethernet network must not exceed 500 meters. With the use of repeaters, the total network distance can be extended up to 2.8 kilometers. However, in scenarios where physical cabling is impractical or limited by geography, laser wireless communication offers a powerful alternative, enabling efficient data transmission beyond traditional Ethernet constraints.
Figure 6 illustrates a modulation drive circuit that uses the MAX3263 chip from Maxim, operating at 155 MHz, along with a laser diode (LD) equipped with an internal monitor diode. The MAX3263 provides temperature-compensated bias and reference voltages, Vref1 and Vref2, which are used to configure the high-speed modulation driver, laser, and monitor diode through resistors R25, R26, R27, and R28. The output current is controlled via an internal mirrored current source, which has a 2Vbe junction temperature drift. By setting the reference voltage to 2Vbe, this drift can be canceled out. Resistor R28 is chosen to adjust the laser’s quiescent bias current (Ibo), ensuring it is slightly below the laser's threshold current for optimal extinction ratio.
The internal monitor diode converts light intensity changes into a photocurrent (Ipin), generating a feedback current (Ibs). Using the formula Ibo = 40(Ib + Ibs), the system translates light intensity variations into a portion of the bias current, providing feedback to maintain stable optical output. The differential PECL signal RD is modulated using an internal high-speed buffer and a common-emitter differential output stage. The modulation current is determined by the current Im set via resistor R26. Selecting the appropriate value for R26 ensures sufficient optical power output while maintaining a good extinction ratio. Additionally, the OUT+ and OUT- terminals should remain above 2.2V to avoid laser saturation.
Figure 7 presents a receiver demodulation circuit based on the MAX3963 and MAX3964 chips, along with necessary peripheral components. The MAX3963, a low-noise 155 MHz chip, includes a transimpedance preamplifier and an inverting amplifier with an emitter-following output. It integrates a 22kΩ transimpedance gain to convert weak PIN photodiode currents into a differential voltage output. The MAX3964, operating at 266 MHz, serves as a post-amplification conditioning circuit. It contains four limiting amplifiers with full-wave logarithmic detectors to measure input signal power. These outputs are summed and filtered by capacitor C25.
Resistors R30 and R31, combined with an internal 1.2V reference and a comparator, form a threshold detection and noise suppression circuit. Setting R30 to 100kΩ and adjusting R31 with a 100kΩ potentiometer allows the threshold voltage (VTR) to range from 1.2V to 2.4V. When the input signal exceeds VTR, the system outputs a stable PECL-level signal. If the signal is below VTR, the OUT+ goes high, OUT- goes low, and all limiting amplifiers reject the input. The post-amplifier then generates a no-light alarm signal (LOS+) to indicate loss of signal.
Since both circuits in Figures 6 and 7 utilize components operating above 155 MHz, careful parameter selection and PCB layout are essential to ensure reliable performance. This setup forms a complete transceiver suitable for high-speed optical communication systems.
In the testing phase, the system was assembled and successfully tested for indoor Ethernet-based laser wireless communication without the need for an optical antenna. The system is currently being refined and is expected to be deployed in future optical network applications.
1.27mm Pitch
1.27mm Pitch
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