A 3-Vppd 730-mW Linear Driver IC Using InP HBTs for Advanced Optical Modulations

This paper presents a 3-Vppd 730-mW linear optical modulator driver IC for advanced modulations suitable for optical communications systems operating at 100-Gb/b/ch and beyond. The driver was designed and fabricated using 0.5-um-emitter InP HBT technology, which yields a peak ft of 290 GHz, a peak f_max of 320 GHz, and BVceo of approximately 4 V. The linear driver has a two-stage lumped configuration comprising a variable-gain amplifier (VGA) and an output cascode amplifier. It provides linear differential output amplitude of up to 3 Vppd and consumes only 730 mW. It has a large bandwidth of 37.8 GHz and a maximum differential gain of 16.2 dB with 6-dB variable gain range. Total harmonic distortion (THD) is as low as 1.2% at 1 GHz and better than 3.8% up to 16 GHz. The driver provides very clear binary and multilevel output and can be applied to optical transmissions of 100-Gb/s/ch and beyond.

I. Introduction

Data traffic in optical communications networks is continuously increasing due to the spread of broadband applications. Digital coherent optical transmission schemes with advanced modulation formats, such as quadrature phase shift keying (QPSK) and Mary quadrature amplitude modulation (M-QAM), are promising techniques for constructing wave-length-division-multiplexing (WDM) optical communications systems operating at 100-Gb/s/ch and beyond [1] [2]. For such systems, a high-throughput digital signal processor (DSP) [3], high-speed digital-to-analog converters (DACs) [4] [5], and broadband linear modulator drivers are key electronic components for generating complex modulated signals, as shown in Fig. 1. The optical I/Q modulator, which is composed of two Mach-Zehnder modulators (MZMs), is also an important optical device for modulating the amplitude and phase of the light independently. InP-based MZMs are now attracting attention because they have smaller chip size and lower driving voltage than widespread-use LiNb0₃-based MZMs. A recently reported InP-based MZM has a half-wavelength voltage (V) of 3 V [6]. For 16-QAM transmission, the optical I1Q modulator is driven by four-level electrical signals, which are generated and amplified by the DSP, DACs, and linear drivers. Actually, an MZM has a non-linear electrical-to-optical (E/O) response, which can be fit to a sinusoidal curve. In order to operate in the linear region and to suppress the distortion, the driving voltage should be 50–60% of 2V for QAM transmission, as shown in Fig. 2 [2]. Therefore, approximately 3Vppd linear output is required for InP-MZM drivers. Very recently, a 2.4-Vppd 600-mW InP-MZM driver IC has been reported [7] [8]. However, it is a limiting driver and supports only QPSK modulation. To our knowledge, a linear InP-MZM driver that can support QAM has not been reported to date. Fig. 1. DSP-based transmitter for optical communications systems. Fig. 2. 16-QAM modulation using optical I1Q modulator. (a) Configuration of optical I1Q modulator. (b) E/O transfer of MZM.