Research On Integrated High DI/dt Laser Diode Driver For Optic-Fiber Communication

The increasing demand for high-speed transport of data has prospered optical communications, leading to extensive design of high-speed integrated circuit. In optical communication systems, a laser diode driver (LDD) is the key component of optic transmitter. The critical challenge arises from the laser diode driver, a circuit that must deliver tens or hundreds of milliamps of current in very short rise and fall time over a broad operation temperature. Moreover, since laser diodes may experience large voltage swings between on and off states, the driver design becomes more difficult as low power consumption impose lower supply voltages (such as single +3.3 V) especially when it is DC-coupled to the laser diode.To begin with, the design methodology of high dI/dt laser diode modulator is researched systematically, including the process technology selection, circuit structures design and circuit modules optimizing technique. Whereafter, based on the laser diode's life and temperature characteristics, the implementation schemes of the control of average laser power and extinction ratio are studied considerably. And two low power supply (+3.3 V), multiple data rates (155 Mbps/622 Mbps/1.25 Gbps), driving current up to 185 mA and integrated automatic power control (APC) and extinction ratio control dc-coupling LDDs are designed and fabricated.On the high speed circuit design, high integrated, adjustable and performance molectron ICs can be gained only by combined with the modern high performance process technology and optimizing circuit design. After detailed analysis, BiCMOS process is selected for developing the laser diode drivers for high data rates and great power applications.The structure of the high dI/dt laser diode driver is thoroughly researched too. By the deep analysis on theΔI noises, the interconnection between the high speed IC chips and the interface between the driver and the laser diode, the fully differential modulator structure compatible with Positive-reference Emitter-Coupled Logic (PECL) inputs that is one of the IEEE standards is proposed. Especially, a unique modulation output driver configuration which can maintain proper high-speed performance when the instantaneous output voltage is above 0.6 V, leading to be capable of DC-coupling a laser to the driver at +3.3 V supply voltage is presented.The high speed performance of the modulator is researched. Several kinds of wide-band circuit design method are proposed, including VBE compensation technique and"dynamic circuit"design technique. ECL circuit, current mode circuit design technique and low voltage swing design which have the characteristics of high speed are adopted. The VBE compensation circuit is used to compensate for variations in VBE over the wide operating temperature range, which can maintain the high speed performance of the modulator; and the dynamic circuit design is adopted to optimize the high speed performance of the modulator over the wide modulation current adjustable range.As the characteristics of semiconductor laser diode change over the time and temperature, extinction ratio and average power are the key parameters affecting the performance of an optical system in optical module design. This dissertation proposes a scheme which combines the automatic power control loop and temperature compensation for modulation current in order to maintain constant extinction ratio and average power. The modulation current with temperature compensation adjustments is to track the variation of the LD's current-to-light conversion efficiency. And the automatic power control loop can adjust the value of the bias current to track the LD's threshold current change over the time and temperature. This paper firstly proposes a novel and simple circuit called Temperature-Dependent Current Switch (TDCS) technique to implement temperature compensation for modulation current. This circuit generates a PTAT current by using the injecting base current of a bipolar transistor in saturation region, and alternates the amplifier feedback loop (close or not) to control the state of the current path.On the basis of above research, two integrated LDD ICs are designed and fabricated in Jazz 0.35μm BiCMOS process. Simulation results show that the ICs can operate at 1.25 Gbps data rates from a voltage supply of 3.3 V, and the ICs feature the functions of programmable temperature compensation for modulation current, and automatic laser power control with safety logic in order to limit the maximum bias current and an alarm function that indicates laser end of life as well as failure of the APC circuitry to maintain average optical power, and stopping laser transmissions by disabling the driver without interrupting data reception at the input. Primary test results show that clear eye-diagrams can be obtained at 155 Mbps, and the output optical power is constant, and the variation of extinction ratio is lower than 0.7 dB.