2.5.1 Mach-Zehnder Modulator
A dual-drive MZMs consists of two PMods in both arms that can be operated independently, in contrast to single-drive MZMs. As depicted in Figure 2.8, the incoming light is split into two paths, both equipped with PMods controlled by an electrical drive signal (u1(t) or u2(t)). After acquiring some phase differences relative to each other, the
two optical fields are recombined. The interference varies from constructive to destructive, depending on the relative phase shift.
Generation of arbitrary QAM signals or arbitrary quadrature signals using a dual-drive MZM have been proposed by Ho et al [22], with a drive signal having two and three levels. Recently, a 16-QAM signal generation method was successfully demonstrated [23] at 50 Gbit/s (12.5 Gbaud ⇥ 4 bit), using a single dual-drive MZMs and 4-level electrical signals. This method requires the amplitude ratio of two QPSK signals to be adjusted.
E (t)
in E (t)out
u (t)2 u (t)
1
Figure 2.8: Optical dual-drive MZM modulator. Dashed lines are electrical signals.
2.5.2 IQ modulator
An IQMod is a structure usually composed of a PMod and two MZMs. As shown in Figure 2.9, the incoming light is equally split into two arms, the in-phase (I) and the
2.5. Electro-optical format conversion systems E (t) in u PM u (t) I u (t) Q E (t) out
Figure 2.9: Optical IQ modulator. Dashed lines are electrical signals.
quadrature (Q) arm. In both paths, a field amplitude modulation is performed by operating the MZMs in the push-pull mode at the minimum transmission point [24]. Moreover, a relative phase shift of ⇡/2 is adjusted in one arm, for instance by an additional PMod. This way, any constellation point can be reached in the complex IQ plane after recombining the light of both branches.
Seimetz et al [25] have investigated transmitters which require only binary electrical driving signals, using m consecutive PMods in serial configuration, where m is the number of bits per symbol. They have proposed and theoretically analyzed several possible structures of optical M-PSK and m-QAM using IQMods. Differences in the optical transmitter configuration and the electrical driving lead to different properties of the optical multi-level modulation signals [26, 27]. We present a few schemes.
One of the proposed transmitters is shown on Figure 2.10. After the first PMod (180 phase shift), a Differential Binary Phase Shift Keying (DBPSK) signal is obtained; after the second PMod (90 phase shift), a Differential Quadrature Phase Shift Keying (DQPSK) signal is obtained, and so on. Another configuration, proposed by the same authors, is a combination of an optical IQMod and consecutive PMods, shown on Figure 2.11. Here, the IQMod accomplishes a DQPSK modulation, and high-order DPSK formats can be generated by the consecutive PMods. In both schemes, the first MZM is used for RZ pulse carving. Star QAM signals with differentially encoded phases may also be generated by the same scheme as for DPSK transmitters. The DPSK transmitters only have to be extended by an additional MZM for intensity modulation, to be able to place symbols on different intensity rings [24].
Another transmitter also requiring only binary electrical driving signals for Square 16- QAM is composed of an optical IQMod followed by a DQPSK modulator. The latter can be implemented either with one more IQMod, or with two consecutive PMods, as depicted 23
Rogério Pais Dionísio 2. Optical Modulation and Format Conversion: State of the Art
Figure 2.10: DPSK transmitter with binary electrical driving signals, serial configuration [24].
Figure 2.11: Parallel DPSK transmitter with binary electrical driving signals [24].
in Figure 2.12. MZMs within the IQMod achieve modulation in intensity. This way, only positive values on the I and Q axis are addressed. As regards quare 16-QAM, the MZMs are driven by binary electrical signals, and a constellation composed of four symbols in the first quadrant is created. With two consecutive phase modulators which perform phase shifts of ⇡ and ⇡/2, respectively, the three other quadrants can be approached, thus creating a complete square QAM constellation [24]. Yu et al [28] also proposed the generation of single-carrier optical signal by format conversion from QPSK to 16-QAM using an IQMod followed by a single arm MZM. Even if the setup was experimentally demonstrated at 432 Gbit/s (without using OFDM or Optical Time Division Multiplexing (OTDM)), the method is limited by a special coding of the electrical binary inputs, so the data sequences cannot be directly used to drive the IQMod.
A different solution to generate QAM signals was proposed by Sakamoto et al [29], by using a multi-parallel MZM transmitter. By arranging two IQMod in parallel, a square 16-
2.5. Electro-optical format conversion systems
Figure 2.12: Optical Tandem-QPSK transmitter for Square 16-QAM [24].
QAM signal can be synthesized from two QPSK signals, as presented in Figure 2.13.
The square 16-QAM signal is generated by driving MZMs only with binary electrical signals, so that the transmitter is free from handling multi-level electrical driving signals. The same architecture was also used to generate Minimum Shift Keying (MSK) signals by Guo-Wei et al [30] and 8-PSK signals [29]. Moreover, m-QAM may be generated by increasing the number of MZMs in parallel. This configuration was called Electro-Optical Digital to
Figure 2.13: Quad-parallel MZM transmitter for generation of square 16-QAM signals [24]. 25
Rogério Pais Dionísio 2. Optical Modulation and Format Conversion: State of the Art Analog Converter (EO-DAC) [29] and will be discussed in the next section.
Secondini et al [31] have proposed an alternative modulation scheme for the generation of 16-QAM optical signals, making use of two-level electrical driving signals and minimum digital precoding. It requires two cascaded IQMods, without any relative phase-stability control.
Recent progress in electronic data processing enables software-defined optical transmission [32]. Freude et al have demonstrated multi-format QAM modulation formats and symbol rates set by software-controlled Field Programmable Gate Arrays (FPGAs), using a single IQMod, up to 168 Gbit/s.
2.5.3 EO-DAC
Another approach for the synthesis of optical multilevel signals involves the use of EO- DAC [29]. As illustrated in Figure 2.14, the EO-DAC consists of a Continuous Wave (CW) laser source, a multiparallel modulator, and electrical encoders for driving the modulator.
The multiparallel modulator employs numbers of Electro-optical (EO) modulators integrated in parallel. In the multiparallel modulator, the CW light is divided into n branches with a 1 ⇥ n coupler, and in each branch, the CW light divided is EO modulated by a binary data sequence. The amplitude and phase offset of the binary-modulated lightwaves are
CW PS ATT MZM 1 x N coupler N x 1 coupler 1 2 3 4 n-1 n
Binary data sequences Encoder
Figure 2.14: Electro-Optical DAC diagram. ATT: Attenuator; PS: Phase shifter; MZM: Mach- Zehnder Modulator; CW: Continuous Wave laser.