How Does a 10-Gbps DFB Laser Transmitter Work? (Video)

In this video, we will discuss the structure and working characteristics of a 10-Gbps DFB laser transmitter. So let’s get started.

This figure shows the structure of a DFB laser with modulator.

The DFB laser is a buried structure that has an pn block laser.  The active layer is a five-layer MQW (Multiple Quantum Well) structure where well layers are InGaAs and the barrier layers are InGaAsP.

The modulator region has a high mesa ridge structure where an InGaAsP bulk with bandgap wavelength of 1.48um is used for the absorption layer. The front end surface of the modulator is treated with antireflection coating.

The optical coupling between the DFB laser and the modulator is based on the butt-joint method.

This laser has a laser region length of 350um, and the modulator region length is 200 um. They are separated by a 30um long electrode separation region.

The optical output power of the laser is about 4mW (+6 dBm) at a laser drive current of 100 mA, the light extinction efficiency is 10 dB/V, and the -3 dB frequency bandwidth is 16 GHz.

The driver IC drives the modulator at 10 Gbps with an output amplitude of 2V peak-to-peak.

The driver IC’s output waveform is close to rectangular. The driver IC consists of a two-stage differential amplification circuit. This amplification is linear, and there is limiting of amplitude in the differential amplification area in the second stage. This makes the amplitude of the output signals constant.

This figure shows an eye diagram of the output waveform of the driver IC at 10 Gbps operation. The test input signals have an amplitude of 0.8 Vpp. The output amplitude is 2.7 Vpp. For small-signal-frequency characteristics, the gain was 15 dB, and the -3 dB bandwidth is 9.5 GHz. Power consumption was about 1.8 W.

This figure shows the top-view structure of the optical transmitter module. This module encloses a DFB laser with modulator, the driver IC, a photodiode for power monitoring, a thermistor, a thermoelectric cooler to stabilize internal temperature, a terminating resistor, an optical coupling lens, and an optical isolator.

The optical coupling efficiency of the transmitter module is over 60%. The extinction ratio is 12 dB when the input amplitude of the driver IC is 0.6 V.

This figure shows the small-signal-frequency response characteristics and return loss values.

The –3 dB bandwidth is 9 GHz, and the return loss is 15 dB or more up to 12 GHz.

On a 10-Gbps transmission experiment using the module, the input electrical signals were 10-Gbps NRZ pseudorandom signals. The output from the module was amplified by an EDFA and then was input to an optical fiber line.

The link length was 80 km using 1.55um dispersion-shifted fiber. The total dispersion was 54.4 ps/nm where the operational wavelength was 1.554 um.

The far-end receiver converted the optical input signals to electrical signals. These electrical signals were passed through a timing extraction circuit and then were passed to a decision circuit.

This figure shows the eye diagram before and after transmission.

This figure shows the resulting Bit Error Rate (BER) characteristics versus incoming signal level at the receiver. For a BER of 1x10-9, the receive signal level was -30.7 dBm. There was no power penalty required and no error rate floor was evident, even after transmission.