A single sideband from only one modulator

Carrying electrical signals on light waves in optical fibers makes it easy to exploit more of the electromagnetic spectrum, especially in the microwave and millimeter-wave region, since even these high frequencies are tiny compared to optical frequencies. Fiber-optic techniques are attracting increasing interest because they enable many new applications such as broadband wireless communications and ‘remoting’ of an antenna at a distance from the receiver electronics.¹

For these applications, the electroabsorption modulator (EAM) stands out as a good candidate to modulate optical signals at frequencies up to the microwave or even millimeter-wave range, and many systems using EAM have been proposed.^2,3 However, an EAM normally produces optical double-sideband (ODSB) modulation. In dispersive fiber links, destructive interference between the sidebands above and below the carrier leads to very serious power fading of microwave and millimeter-wave signals and significantly limits the length of the links.

It is well known that optical single-sideband (OSSB) modulation can effectively combat such power fading, so many schemes have been proposed to realize OSSB using EAM. One possibility is to optically filter out one of the sidebands,^3,4 but this method is wavelength dependent. Another possibility resembles OSSB modulation using a dual-drive Mach–Zehnder modulator.^5,6 This scheme normally requires two EAMs, and hence increases the system cost. Moreover, if it is implemented with discrete components, the likely unequal length of the two branches will lead to serious coherent interference.

We propose a novel way to realize OSSB modulation using only one EAM. The scheme is based on cancellation between a nonlinear phase due to dispersion and a linear phase due to optical delay, inside a Sagnac loop. The technique requires no optical filters, so it is independent of the optical wavelength, and it does not suffer from coherent interference even when it is based on discrete components. Most of the components can be integrated through planar light wave circuit techniques, potentially resulting in a compact size.

Figure 1. Optical single-sideband (OSSB) modulation by a radio-frequency (RF) signal can be generated using a single electroabsorption modulator (EAM), by inserting it into a Sagnac interferometer. Differences between the arm lengths L₁ and L₂can be tuned with a tunable delay line.

The proposed scheme is shown in Figure 1. An EAM is put inside a Sagnac loop formed by a 2×2 optical coupler. The output of a laser enters the loop after passing through an optical circulator and the coupler, and is then modulated in the EAM by a driving signal. The EAM need not be positioned at the midpoint of the loop. The optical path difference from the EAM to the 2×2 coupler between the counterclockwise (CCW) direction and the clockwise (CW) direction can be tuned by a tunable delay line. A dispersive element is also inserted in the loop to introduce nonlinear phase shift in the two sidebands generated by the modulation of the EAM. The output of the Sagnac loop is directed to the output port of the circulator.

This configuration introduces two sources of phase shift between the two sidebands, which can partially cancel one of them. First, after modulation by the EAM, the path-length difference for CCW and CW transmission leads to a phase difference for the two sidebands between the light waves in the two directions. This linear phase difference is directly proportional to the modulating frequency and to the length difference.

Second, the dispersive element introduces an additional, nonlinear phase shift between the two sidebands. For the configuration shown in Figure 1, this shift affects the optical field travelling in the CW direction because the field passes through the dispersive element after the EAM modulation. This nonlinear phase shift is proportional to the dispersion value and the square of the modulating frequency. When the CW and CCW light waves combine at the output port of the circulator, the nonlinear phase can cancel the linear phase for one of the sidebands, resulting in its suppression, if the two phases satisfy a certain relationship.

Figure 2. The measured optical spectrum when both sidebands have equal intensity.

We demonstrated our scheme with a 2.7km-long dispersion-compensated fiber (DCF) inside the Sagnac loop and 10GHz modulating frequency. The linear phase can be tuned by varying the length of the tunable delay line, which changes the relationship between the linear and the nonlinear phase and thus the relative intensity of the two sidebands. Figure 2 shows the optical spectrum when both sidebands have equal intensity. Tuning the delay line changes the linear phase, and the ratio of the intensity between the two sidebands can be varied continuously. Figure 3 shows the case when the upper sideband is suppressed. The experimental results are very stable even without using a vibration-free table because the scheme is free from optical coherent interference.

Figure 3. The measured optical spectrum when the modulation scheme is used to suppress the upper sideband.

In summary, we have realized a scheme to achieve OSSB modulation using only one EAM, based on the cancellation between the nonlinear phase due to dispersion and the linear phase due to optical delay in a Sagnac loop. The scheme is wavelength independent and does not have the problem of optical coherent interference even when it is based on discrete components. Future research will explore the integration of these components on a compact planar light wave circuit.