The electro-optic modulator is used to modulate a beam of light. Laser-controlled modulators are capable of operating in the gigahertz range, and modulation may be imposed on the amplitude, phase, or polarization of the beam. The component that makes this possible incorporates an element that exhibits an electro-optic effect, a change in its optical properties in response to an electric field that varies slowly compared with the frequency of light. This effect then adds information to a light-wave carrier. Applications include analog and digital signal processing, information processing, optical computing and remote sensing. In all cases, the electro-optic effect enables modulation at high frequencies.
Electro-optic modulators are frequently built around a Pockels cell, basically a crystal whose refractive index is modified by an electric field. The degree of modification varies with the field strength. Described in 1906 by Friedrich Pockels, the effect can be observed in lithium niobite (LiNbO3) and other crystalline materials, as well as in organic polymers. This effect can shift light frequency, amplitude and polarization, and these effects can be exploited separately or in combination. The modulator can operate in the gigahertz range, immune to noise and EMI.
The change in the index of refraction of lithium-niobate crystals is proportional to the externally-applied electric field. But the effect is small even in crystals that exhibit high electro-optic coefficients, usually less than 1%. That amount, however, is enough to shift the phase and amplitude of the optical beam for typical applications.
Pockels cells employing organic polymers have the fastest response range. Polymers, as the name implies, are made up of many repeated subunits. Both naturally-occurring and synthetic polymers can form semi-crystalline materials, so it is not surprising that they have come to constitute the defining element in some electro-optic modulators.
Organic polymers are also widely used as matrices in the gain media of solid-state dye-doped polymer lasers. These organic lasers exhibit a narrow bandwidth, useful in spectroscopy and electronic instrumentation. In electro-optic modulators, their refractive index varies with change of temperature or applied voltage. For this reason, exacting applications require temperature control.
Pockels cells are half-wave plates, meaning they shift the polarization direction of light that is linearly polarized. In a Pockels-based electro-optic modulator, the voltage necessary to induce a phase change equal to π can run over a thousand volts (the cell basically behaves like a capacitor), requiring a high-voltage amplifier. It is possible to switch a voltage of this magnitude within a few nanoseconds, permitting electro-optic modulators to operate as high-speed switches.
Electro-optic modulators can’t implement frequency modulation. A Pockels cell can implement amplitude modulation by modifying the polarization state of the light and using a polarizer for subsequently converting this into a change in transmitted optical amplitude and power. An alternative approach to amplitude modulation uses an electro-optic phase modulator in one arm of an interferometer.
Some applications require a purely sinusoidal modulation with constant frequency. Such applications generally use a modulator other than a Pockels cell. The approach frequently used for this type of modulation is an electrically (not mechanically) resonant electro-optic modulator. The resonance comes from a resonant LC circuit. The input voltage of the device can then be substantially less than the voltage across the electrodes of the Pockels cell. The LC circuit generally has a high Q, implying a narrow bandwidth for the strong resonant enhancement. The disadvantage of this approach is that a change in resonance frequency requires swapping out at least one circuit 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.
For particularly high modulation bandwidths in the gigahertz region, integrated optical traveling-wave modulators are often used. The travelling wave structure, as opposed to a lumped electrode, refers to the fact that the electrode is designed as a transmission line matched to the microwave modulating input signal with the modulating signal fed colinearly with the propagating optical wave. An electronic 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 efficient modulation is possible even for frequencies which are so high that the electrode length corresponds to several wavelengths of the microwave.
There have been efforts to integrate electro-optic modulators within a single IC. Advantages include low power consumption, high speed due to reduced frequency-limiting inductance, and the possibility of enormous cost savings.
Besides the electro-optic modulator, there is an acoustic-optic modulator, also known as a Bragg cell. It uses a piezoelectric transducer attached to a material such as glass. An oscillating electric signal makes the transducer vibrate which creates sound waves in the glass material. These vibrations can be thought of as moving periodic planes of expansion and compression that change the material’s index of refraction. Incoming light scatters off the resulting periodic index modulation. Because the amount of light diffracted depends upon the intensity of the sound, the intensity of the light is modulated. The frequency of the diffracted light can vary from 27 MHz to 1 GHz.
The Bragg cell gets its name because the interference resembles that from Bragg diffraction. Bragg’s law gives the angles for coherent and incoherent scattering from a crystal lattice. The scattering arises when neutron waves from the atomic nuclei or a coherent spin interaction with an unpaired electron reradiates the waves with the same frequency but blurred slightly. These re-emitted wave fields interfere with each other either constructively or destructively (overlapping waves either add up together to produce stronger peaks or are subtracted from each other to some degree), producing a diffraction pattern on a detector or film. The resulting wave interference pattern is the basis of diffraction analysis.
Also, transverse and longitudinal acoustic waves change the polarization of light waves, which undergo a phase shift as in an electro-optic modulator.
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