What Is an Optical Probe?
An optical probe is an advanced device that improves upon traditional electrolytic probes by excluding metal elements from the sensor part. They leverage electro-optical effects for precise measurements and evaluations. Key features include their non-invasive nature, which does not disturb electric fields or pick up noise. The sensor head is small, operates without a power supply, and can measure a wide range of information such as frequency, phase, and intensity. Moreover, they function without interference, even in the presence of surrounding metal.
Uses of Optical Probes
Optical probes find applications in devices like controllers, oscilloscopes, and endoscopes. They are versatile in measuring various data for investigation and analysis purposes through optical means. Applications include, but are not limited to:
1. Specific Absorption Rate (SAR) Measurement
SAR quantifies the energy absorbed by the human body when exposed to electromagnetic waves above 100 kHz. Measurements, based on standards, are performed on a phantom mirroring the human body’s electrical characteristics. This is crucial for assessing energy absorption in the body from proximity use of small wireless devices like smartphones.
2. Plasma Measurement
Optical probes are instrumental in measuring plasma, which forms by applying a strong electric field to a gas of charged particles. They enable local electric field measurements within the plasma.
3. Magnetic Resonance Imaging (MRI)
They are also used in MRI to measure human exposure to electromagnetic fields, ensuring accurate measurements under strong magnetic fields. Other applications include measuring pulsed and ultra-strong electric fields in EMC design and simulation model verification.
Principles of Optical Probes
1. The Pockels Effect
Optical probes incorporate EO crystals utilizing the Pockels effect, where the material’s refractive index changes with an external voltage, affecting the polarization rate. In the absence of an electric field, light maintains its polarization upon reflection from an EO crystal. However, the applied electric field alters the crystal’s refractive index, changing the incident light’s polarization state. An analyzer measuring the polarized light’s intensity provides a signal proportional to the electric field intensity.
2. Impact on the Electric Field to Be Measured
Unlike dipole antennas, optical probes use crystals in their sensors, thus not disturbing the measured electric field. Dipole antennas, which consist of two straight conductors and are made of metal, can scatter radio waves, causing disturbances. Optical probes avoid this issue, ensuring undisturbed electric field measurements.
Composition of Optical Probes
The optical probe’s tip comprises the EO crystal, optical fiber, ferrule, collimator lens, and dielectric reflective film. This assembly facilitates precise and noise-immune measurements.
1. EO Crystal
Typically 1 mm square, the EO crystal’s compact size allows for the measurement of minute areas, a significant advantage over the longer antenna lengths of field probes that use dipole antennas.
2. Optical Fiber
Connected to the EO crystal through a collimator lens and ferrule, the optical fiber ensures signal transmission without noise interference. It enables the conversion of voltage signals into optical signals for long-distance, frequency-independent transmission.
3. Dielectric Reflective Film
Attached to the EO crystal’s tip, this film reflects the linearly polarized light injected into the crystal. A photodiode converts the reflected light into an electrical signal, which is then amplified. The electrical signal’s output is proportional to the polarization change, allowing the calculation of the electric field’s intensity and phase with spectrum analyzers or similar instruments.