FREE-SPACE BROADBAND PHASE MODULATORS
Traveling-Wave electro-optic Phase modulators (TWP) are devices used in e.g. in quantum science to manipulate the phase of light in order to generate sidebands that can be dynamically tuned over a wide range of frequencies. The basic principle behind electro-optic phase modulation is the electro-optic effect, which is the change in refractive index of a material in response to an applied electric field. This effect allows the phase of light passing through the material to be altered by varying the voltage applied to the modulator.
Broadband phase modulators typically use crystals with high electro-optic coefficients, such as lithium niobate (LiNbO₃). These crystals are known for their strong electro-optic response and wide bandwidth capabilities. The modulator consists of a traveling wave type structure around the crystal that guides the RF-wave and light collinearly (phase matched) through the device.
BROADBAND PHASE MODULATORS - bulk
TYPICAL APPLICATIONS:
- laser frequency stabilisation (e.g. Sideband Offset-Lock - SOL)
- laser cooling and trapping of neutral atoms/ions/molecules
- laser frequency dynamic control (e.g. Master-Slave Lock)
- multi-mode laser generation
- quantum state manipulation
- spectroscopy
Altered Light Property: Phase
An important and easy to manipulate property of laser light that is commonly used for signal modulation is the phase. In general, optical radiation such as laser light is associated with electromagnetic waves which can be characterised with an amplitude and a phase. The phase determines in which part of an oscillation cycle the electric field is. Light where the optical phase evolves systematically and predictably in time possesses a high temporal coherence.
Coupling: Traveling Wave
QUBIGs traveling wave phase modulators consist of an electro-optic crystal that is coupled to a high frequency electrode circuitry via an RF input and a 50Ohms terminated output connector. This circuit is constructed such that the coupled microwave on its way from the input to the modulator output is phase matched with the light that is traveling through the crystal. An impedance matching network transforms the reactive crystal load to a 50Ohms input to allow for easy matching to standard RF drivers and function generators. Note: As the electro-optic effect is generally weak and no resonant enhancement implemented broadband RF sources with high output power might be required to achieve a significant effect.
Effect on the Laser Light: Sideband Generation
Resonant phase modulators are used to vary the phase of an optical laser beam. The induced sinusoidal phase variation f(t)=β*sin(Ω*t) at the modulation frequency Ω and peak phase change (β) generates frequency sidebands at multiples of Ω about the central cw optical frequency, ω. The spectrum of a sinusoidally phase-modulated electric field after passing through the modulator is given by Bessel functions:
The amplitude in the m-th sideband at ω+m*Ω is proportional to Jm(β), where Jm is the m-th order Bessel function of the first kind. The amount of energy transferred from the fundamental J0(β) to the m-th sideband is proportional to the square of the electric field amplitude | Jm(β)|2.
QUBIG’s traveling wave phase modulators are ideal for high optical power applications with VIS laser light that require GHz modulation bandwidth.
Four considerations prior operation
- The polarisation of the laser light must be linear and parallely aligned with the white markers above the aperture.
- The modulation efficiency depends on the propagation direction. Make sure the laser beam travels in the same direction as the RF wave.
- The output port needs to be terminated with a properly chosen RF attenuator (50Ohm) that can resist the applied RF power.
- The return loss (S11) strongly fluctuates over the specified frequency range and reaches values up to -6dB. It is therefore advisable to protect the RF driver/amplifier with a circulator
