*2.1.2 VCSEL driver*

**Figure 2** shows the schematic diagram of a 10-Gb/s VCSEL driver that consists of a main driver, a pre-driver, an EQ, and an input buffer. The main driver operates with two current sources, i.e., the bias current (IBIAS) and the modulation current (IMOD). When M5N in the main driver is turned off, the current sum (IBIAS+IMOD) flows through the VCSEL diode. When M5N is on, only IBIAS is supplied to the VCSEL diode. The feedforward preemphasis is conducted by using a capacitor (CFF) to alleviate the distortion effects of the output waveforms from the bond-wire inductance and the parasitic capacitance of a VCSEL diode. Simulations confirm 19.2% faster rising time in the output waveforms.

Considering the device reliability of VCSEL diodes, it is not clever to keep IBIAS to flow continuously through the array chip because it will rise the device

**Figure 2.** *Schematic diagram of the VCSEL driver.*

### *Integrated Circuits/Microchips*

temperature and thus the slope efficiency and expectant life period of VCSEL diodes will be severely deteriorated. Hence, input data detection circuit can be employed to avoid the superfluous current flow by turning off VCSEL diodes when no input signal transitions occur. Only with the emergence of input data, IDD detects the data transition and generates an average DC voltage, i.e., 3.3 V in this work through an active low-pass filter (LPF).

Then, this DC voltage turns on the two current sources in the main driver. Certainly, the turn-on delay of the current sources should be shorter than the signal delay from the main driver input.

**Figure 3** shows the chip microphotograph of the 4-channel VCSEL driver array realized in a 0.13-μm CMOS process, where the chip core of each channel occupies the area of 350 <sup>250</sup> <sup>μ</sup>m<sup>2</sup> . Each channel dissipates 21.25 mA (max.), in which the main driver consumes 11.6 mA.

**Figure 4** demonstrates the optically measured output eye diagrams at 10 Gb/s with a 10-m POF connected to a digital communication analyzer (DCA Agilent 861150D), where it is clearly seen that there is no overlap area of 0.6-UI optical mask condition.

**Table 1** compares the performance of the 4-channel Tx array chip with prior arts, in which only this work provides the measured optical magnitude amplitude (OMA) with a 10-m attached.
