Basics of Electrooptic Modulator
2.4 Types of Electrooptic Modulator (EOM) .1 Phase Modulators
The simplest type of EOM is a phase modulator containing only a Pockels cell [12], where an electric field (applied to the crystal via electrodes) changes the phase delay of a laser beam sent through the crystal. The polarization of the input beam often has to be aligned with one of the optical axes of the crystal, so that the polarization state is not changed. Many applications require only a small (periodic or nonperiodic) phase modulation. Example; using EOM for monitoring and stabilizing a resonance frequency of an optical resonator. Resonant modulators are often used when a periodic modulation is sufficient, and make possible a large modulation depth with a moderate drive voltage.
2.4.2 Polarization Modulators
Depending on the type and orientation of the nonlinear crystal, and on the direction of the applied electric field, the phase delay can depend on the polarization direction. A Pockels cell can thus be seen as a voltage-controlled waveplate, and it can be used for modulating the
polarization state. For a linear input polarization (often oriented at 45° to the crystal axes), the output polarization will in general be elliptical, rather than simply a linear polarization state with a rotated direction.
2.4.3 Amplitude Modulators
Combined with other optical elements, in particular with polarizers, Pockels cells can be used for other kinds of modulation. In particular, an amplitude modulator is based on a Pockels cell for modifying the polarization state and a polarizer for subsequently converting this into a change in transmitted optical amplitude and power.
Fig. 2.5: Electrooptic amplitude modulator, containing a Pockels cell between two polarizers.
An alternative technical approach is to use an Electrooptic phase modulator in one arm of a Mach–Zehnder interferometer in order to obtain amplitude modulation. This principle is often used in integrated optics (for photonic integrated circuits), where the required phase stability is much more easily achieved than with bulk optical elements. Optical switches are modulators where the transmission is either switched on or off, rather than varied gradually. Such a switch can be used, e.g., as a pulse picker, selecting certain pulses from a train of ultrashort pulses, or in cavity-dumped lasers (with an EOM as cavity dumper) and regenerative amplifiers.
2.4.4 Thermally Compensated Devices
In configurations where the induced relative phase change between two polarization directions is used, thermal influences can be disturbing. Therefore EOM often contain two matched Pockels cells in a thermal configuration where the temperature dependence of the relative phase shift is largely canceled. There are also configurations with four crystals of exactly the same length,
canceling both birefringence effects and spatial walk-off. Various types of multi-crystal designs are used, depending on the material and the exact requirements.
2.4.5 Resonant Versus Broadband Devices
For some applications, a purely sinusoidal modulation with constant frequency is required. In that case, it is often beneficial to use an electrically (not mechanically) resonant EOM, containing a resonant LC circuit. The input voltage of the device can then be substantially lower than the voltage across the electrodes of the Pockels cell. A high ratio of these voltages requires a high Q factor of the LC circuit and reduces the bandwidth in which strong resonant enhancement can be achieved. The disadvantage of using a resonant device is that one loses flexibility, changing the resonance frequency requires the exchange of at least one electric component.
Broadband modulators are optimized for operation in a wide frequency range, which typically starts at zero frequency. A high modulation bandwidth typically requires a Pockels cell with a small electric capacitance, and excludes the exploitation of a resonance.
2.4.6 Traveling-Wave Modulators
For particularly high modulation bandwidths, in the GHz region, integrated optical traveling- wave modulators are often used. Here, the electric drive signal generates an electromagnetic wave (microwave) propagating along the electrodes in the direction of the optical beam. Ideally, the phase velocities of both waves are matched so that efficient modulation is possible even for frequencies which are so high that the electrode length corresponds to several wavelengths of the microwave.
2.4.7 Electro Absorption Modulator (EAM)
EAM is used as an optical modulator for use with the external modulation system. It‟s an optical modulator that utilizes the electroabsorption effect, i.e. optical absorption coefficient of a substance varies depending on the electric field applied to it. EAM utilizes a mechanism such that if a modulation signal voltage is applied to light propagating through a waveguide, the resulting electric field causes an electric absorption coefficient in a medium to change, thereby intercepting the light. Optical intensity of EAM can reduce the waveform chirping phenomenon, as compared with direct modulation system by the semiconductor laser diode; however, the
waveform chirping amount cannot be zero. Among RF optical modulators, optical EAM having a multiple quantum well is a device having high-frequency operating speed, low-power consumption and capability to be integrated with other devices.
Fig. 2.6: Schematic of Mach-Zehnder EOM.
Fig. 2.7: (a): Top-view of Mach-Zehnder interferometer Silicon Modulator, (b) Schematic cross-sectional view.
The electrooptic modulator, shown schematically in Fig. 2.6 consists of two features, namely, the strip-loaded, single-mode optical waveguide Mach-Zehnder interferometer, and the coplanar waveguide electrodes. Fig. 2.7 shows both the top view and the schematic cross-section view of Mach-Zehnder interferometer Silicon Modulator. Embedded on the two arms of MZ interferometer are silicon waveguide phase shifter sections, fabricated on SOI wafers.
V L
Pc Pi
S W S
Ohmic Contact n- - GaAs
n+ - GaAs
h d
Au Schottky barrier
Fig. 2.8: Lateral view of GaAs EOM.
Fig. 2.8 shows a sample of the lateral view of a GaAs based EOM. Thus optical EAM is used in the optical transmission technology for ROF link. In optical EAM, the amount of carriers comprise of pairs of electrons and holes generated by light absorption, which increases in accordance with incident light intensity. The electron and hole pairs form an internal electric field so as to cancel an externally applied electric field. The screening effect on the externally applied electric field increases with the intensity level of the incident light, and there is a correlation between the intensity level of the incident light and the change in the absorption coefficient.