Optical Modulator Driver Amplifiers and Semiconductor Materials
Optical communications use an optical modulator to impose an (electrical) signal on continuous-wave (CW) light to vary the power and phase of the light and create an optical signal. Since the optical modulator can change the power and phase of the light using applied voltage, it converts an electrical signal to an optical signal. Figure 1 shows the block diagram of the optical signal transmitter used by an optical modulator. The signal-free CW light emitted from the semi-conductor laser is input to the optical modulator where the electrical signal from the driver amplifier converts the CW light to an optical signal. Since a high-voltage electrical signal is required to drive the modulator, a high-output driver amplifier is used to drive the optical modulator. The latest international communication standards are gradually increasing communication speeds, which requires driver amplifiers with both high output and fast operation. For example, the new IEEE 400GBASE-DR4 standard requires a driver amplifier operation frequency of at least 40 GHz and an output voltage of about 1 to 3 Vpp.
Fig. 1 Optical Signal Tx Block Diagram
The most commonly used semiconductor material is silicon, which is easily formed into large-scale integrated circuits (LSI) and is low cost. However, because it is difficult to achieve both high operation speed and high output using silicon, it is not appropriate for use as a driver amplifier semiconductor material. Generally, driver amplifiers are compound semiconductors fabricated using two or more elements. Since compound semiconductors have high electron mobility and a large bandgap, they are ideal for use in driver amplifiers requiring both high speed and high output. Additionally, another advantage is the ability to implement fast transistors, such as high electron mobility transistors (HEMT) and heterojunction bipolar transistors (HBT), by fabricating layers of compound semiconductor materials with different combinations of elements and composition ratios. Typical compound semiconductor materials are gallium arsenide (GaAs), indium phosphide (InP), silicon germanium (SiGe), gallium nitride (GaN), etc.
The example in Figure 2 shows the segregation of semiconductor materials used in amplifiers based on operation frequency (x-axis) and output power (y-axis). The Ethernet standards covering optical communications regulate speeds of 100 and 400 Gbps and are now examining 800 Gbps, but the semiconductor materials that are suitable for fabricating driver amplifiers supporting these communications standards are only GaAs, InP, and SiGe enclosed by the red line at the bottom right of the graph. Each material has its own unique characteristics. SiGe ingots can be manufactured with a large diameter at low-cost using general silicon semiconductor processes, but it is difficult to achieve high output with this material. GaAs is used widely in systems requiring high output and high speeds, but does not easily support faster future communication speeds requiring operation frequencies of 70 GHz or more used by 800-Gbps Ethernet. Transistors fabricated from InP are faster than GaAs, making InP an ideal semiconductor material for leading-edge optical driver amplifiers supporting future high-speed communications. Two types of transistor can be fabricated using InP: InP HEMT and InP HBT. Both types have superior characteristics based on the material properties. Whereas InP HEMT requires high-level technologies to assure good withstand voltage characteristics, the double-hetero structure of InP HBT features good withstand voltage characteristics.
Anritsu Devices has leveraged its long experience in InP HBT technology to offer driver amplifiers using this process for optical modulators.
Fig. 2 Segregation of semiconductor materials according to operation frequency and output power
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