GB2391107A

GB2391107A - Integrated Laser Driver

Info

Publication number: GB2391107A
Application number: GB0206480A
Authority: GB
Inventors: Hiroshi Nakamura; Subrata Halder; Yisheng He
Original assignee: DenseLight Semiconductors Pte Ltd
Current assignee: DenseLight Semiconductors Pte Ltd
Priority date: 2002-03-19
Filing date: 2002-03-19
Publication date: 2004-01-28
Also published as: GB0206480D0

Abstract

An integrated laser module having a laser chip, a transmission line and a driver chip, the laser chip including a laser diode and the driver chip including an emitter follower amplifier, wherein an output of the emitter follower amplifier is coupled to the laser diode via a transmission line.

Description

2391 1 07

INTEGRATED LASER DRIVER

Introduction

The modulation of the laser output is imperative to transfer the input 5 electronic data onto the optical carrier for optical fiber transmission Generally, there are two methods of modulation: that is direct modulation by variation of the injection current to the laser diode, and modulation by an external modulator through the electroabsorption or electro-optic effect.

To drive a laser diode by the direct modulation method to a bit rate of 2. 5 10 Gb/s or 10 Gb/s, usually, the configuration shown in Figure 1 is used. In this Figure, only the essential parts of the driver-lC and the laser module are shown. As the RF impedance of the laser diode is very low (about 5Q or less), another resistor is normally connected in series with it inside the laser module to achieve impedance matching with the 50Q transmission line.

15 In some cases (especially for 2.5 Gb/s transmission), a 25Q impedance is used instead of the 50Q system. The merit of using an impedance of 25Q against a value of 50Q is that of low power consumption. An inductor for DC bias is often used to decrease the voltage of the driver-lC, as shown in Figure 2. However, because of the limited RF performance of the inductor, the application of this configuration to 20 higher transmission speeds like 10 Gb/s Is limited. The impedance of 25Q is almost the lowest limit of line impedance with good RF performance achievable with a long (more than a few cm) transmission line and connectors.

In the configuration used in prior art, the power consumption of the laser

driver IC is quite high. This is especially so for the case of 10 Gb/s transmission, 25 where, normally, a 50Q system is used and an inductor is not used. The reason for this high power consumption is mainly the high voltage drop in the resistor used for impedance matching. In the present invention, a new configuration of a low-power laser driver for high speed direct modulation is proposed.

30 Proposed New Idea In the proposed laser driver configuration for low impedance matching and low power consumption, the laser chip and driverlC chip are placed inside the laser module. Both chips are connected by a short transmission line with low impedance.

For example, for a separation of less than 0.5 mm between the two chips, the 35 impedance of the transmission line is around 10Q or less. Such a low impedance

( can be easily realized with a short line created on top of a commonly used substrate. The output of the dnver-lC comes from the emitter of the transistor, as shown in figure 3 Here, the current of final stage transistor is directly supplied to 5 the laser chip. In the figure, the preceding stages of the driver IC are also shown for better understanding, but the configuration in the preceding stages is not limited to the example shown.

The operation of the proposed laser driver module can be explained in the following. We assume that the laser bias current for high speed direct modulation is 10 between 20 mA to 60 mA (the average is 40 mA), and the voltage drop is 1.3 V for the 20 mA drive and 1.5V for 60mA drive. In this case, the differential resistance is 5Q (given by (1.5-1.3)/(0.06-0. 02)), and the RF impedance around the bias point is also about 5Q. This circuit is operational at a speed of 10 Gb/s at a supply voltage (via) of 3 V using a transistor with an fT of 70 GHz.

15 The current consumption for the preceding stage can be estimated to be about 20 mA. Hence, the total power consumption of the driver and laser chips is estimated to be 180 mW (given by 3 V x (20 mA + 40 mA)). This value of total power consumption for the entire laser module of driver IC and laser diode is very much lower than that achievable in the prior art configuration.

20 The laser driver and laser diode are connected by a short lowimpedance transmission line. This line impedance is close to the output impedance of the laser driver as well as the impedance of the laser diode. Consequently, no significant reflection occurs between the two chips, and thus a good waveform of the optical output is achieved.

25 In summary, the proposed invention leads to a much reduced total power

consumption of the laser driver and laser diode. When properly designed, the novel configuration can result in a reduction of the power consumption to one tenth of that realized in the prior art. This performance of low power consumption is extremely

beneficial for high density integration of laser modules, which is often required for 30 DWDM application.

Claims

( CLAIMS

1. An integrated laser module having a laser chip, a transmission line and a driver chip, the laser chip including a laser diode and the driver chip including 5 an emitter follower amplifier, wherein an output of the emitter follower amplifier is coupled to the laser diode via a transmission line.

2. A module according to claim 1, wherein the module draws less than 200mW in power.

3. A module according to claim 1 or claim 2, wherein the transmission 10 line has an impedance of less than or equal to 10 ohms.

4. A module according to claims 1, 2 or 3, wherein the impedance of the transmission line substantially equals the impedance of the laser chip, thereby reducing unwanted reflection.

5. A module according to any one of the preceding claims, wherein the 15 transmission line has a length of less than a millimetre.