Ethernet remains 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 should not exceed 500 meters. With the use of repeaters, the total network distance can be extended up to 2.8 kilometers. However, in situations where physical cabling is impractical or limited by geography, laser wireless communication offers a powerful alternative, enabling seamless data transmission beyond traditional Ethernet constraints.
Figure 6 illustrates a modulation drive circuit based on the MAX3263 chip from Maxim, which operates at 155 MHz. This circuit controls a laser diode (LD) with an integrated monitor diode. The MAX3263 provides temperature-compensated bias and reference voltages (Vref1 and Vref2), which are adjusted via resistors R25, R26, R27, and R28 to set the internal high-speed modulation and laser drive circuits. The output current is controlled by an internal mirrored current source, which has a 2Vbe junction temperature drift. By setting the reference voltage at 2Vbe, this drift can be canceled out. Resistor R28 is used to adjust the laser's quiescent bias current (Ibo), ensuring it is slightly below the laser's threshold current for optimal extinction ratio.
The monitor diode inside the LD converts light intensity variations into a photocurrent (Ipin), generating a feedback current (Ibs). Using the equation Ibo = 40(Ib + Ibs), the light intensity changes are translated into part of the bias current, stabilizing the optical output. The differential PECL signal RD is modulated using an internal high-speed input buffer and a common-emitter differential output. The modulation current is determined by resistor R26, which is selected to ensure sufficient optical power and a good extinction ratio. Additionally, the OUT+ and OUT- terminals must remain above 2.2V to avoid laser saturation.
Figure 7 shows a receiver demodulation circuit using the MAX3963 and MAX3964 chips, both operating at high frequencies (155 MHz and 266 MHz, respectively). The MAX3963 includes a transimpedance preamplifier and an inverting amplifier with an emitter-following output, converting weak PIN detector currents into a differential voltage. The MAX3964 forms a post-amplification conditioning stage, featuring four limiting amplifiers with full-wave logarithmic detectors. These detectors sum their outputs and filter them through capacitor C25.
Resistors R30 and R31, along with the internal 1.2V reference and a comparator, create a threshold detection and noise suppression system. By adjusting R31 with a 100kΩ potentiometer, the threshold voltage (VTR) can vary between 1.2V and 2.4V. When the input signal exceeds VTR, a stable PECL signal is output. If the signal is below VTR, the output terminals switch to high/low levels, and all limiting amplifiers reject the signal, triggering a no-light alarm (LOS+) from the post-amplifier.
Since both the transmitter and receiver circuits in Figures 6 and 7 use components operating above 155 MHz, careful selection of parameters and PCB layout is essential. This design is well-suited for high-speed optical communication systems.
In a practical test, the system was assembled without an optical antenna, and an indoor Ethernet laser wireless communication experiment was successfully conducted. The system is currently being refined and is expected to be deployed in future optical networks.
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