The paper "Advanced Optical Modulation Techniques" is a wonderful example of a literature review on technology. There has been a significant development in the field of optical modulation techniques in recent years . Prior to the current shift towards the advanced optical modulation technologies, the Wavelength-Division Multiplexed (WDM) optical transmission techniques were widely used from as early as the 1990s . Later on, the technological advancements in the modulation techniques necessitated an improvement in spectral efficiencies, which is what set the journey on the advancement in the optical modulation techniques leading to such high-speed fiber-optic techniques as the Quaternary Phase Shift Keying (QPSK), Binary Phase Shift Keying (BPSK), Differential Phase Shift Keying (DPSK), optical Quadrature Amplitude Modulation (m-QAM), and Differential Quadrature Phase Shift Keying (DQPSK) in which both phase and amplitude could be modulated .
This paper, therefore, performs a literature review of the advanced optical modulation techniques. Differential Phase Shift Keying is a technique that is well known for its lack of absolute carrier phase reference . Typically, DPSK is characterized by the fact that the phase reference as the transmitted signal.
For that reason, DPSK may be said to be a phase modulation technique in which the conveyance of data is carried out through the inverted phase of the carrier wave . The very first step in DPSK entails encoding the data in question differentially prior to employing a Phase Modulator (PM) (also used is the Mach-Zehnder Modulator, MZM) to modulate the encoded data onto the optical carrier . Behind the scenes, the MZM acts by converting the initial optical phase into a 180-degree phase shift. Most instances involving the encoding of the encoded data onto the optical carrier are characterized by the use of MZM typically because of its superb tolerance of chromatic dispersion . Practically, it is not possible to perform demodulation of DPSK, at least directly .
In order to counter this property, therefore, a Delay Interferometer (DI) is usually placed along the path of light at the receiver. This helps in the conversion of the differential phase modulation into intensity modulation . Alternatively, the MZM produces intensity modulation by retaining the constructive or destructive interference courtesy of the input electrical voltage.
Most advanced optical modulation techniques are known for their excellent performance in modulation . Additionally, they can independently modulate the optical path’ s phase as well as intensity. Consequently, therefore, almost all advanced optical modulation techniques are based on MSMs . According to , the process of conversion of phase modulation into intensity modulation is achieved through the destructive interference of the two optical fields (usually at the destructive end), and constructive interference of the two optical fields in the event that there is a variation of phase between subsequent bits.
 points out that the delay interferometer along the optical route conserves energy within it, and for that reason, the constructive end generates a data pattern that is logical upside down . The greatest disadvantage of DPSK is the fact that it performs poorly beyond certain levels of data rates . It is practically impossible, for example, for DPSK to function effectively at a data rate of, say, 50 GB. That is where the use of Differential Quadrature Phase Shift Keying (DQPSK) comes in handy. In optical systems, DQPSK accomplishes this by employing a multi-level technology, usually characterized by several bits per symbol .
Due to this characteristic, the symbol rate for DQPSK is generally quite low . The transmitter of a DQPSK takes advantage of the bi-phase modulators with some phase shift to ensure a 900 shift of the output especially when the two outputs come together. In comparison to the DPSK, the DQPSK’ s receiver sensitivity is far much lower. On the flip side, though, the DQPSK is known for its excellent tolerance to chromatic dispersion as well as outstanding spectral efficiency .