Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With waveguide or long line
Reexamination Certificate
1998-07-24
2001-04-10
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With waveguide or long line
C324S754120, C324S601000
Reexamination Certificate
active
06215295
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to photonic devices or systems capable of detecting or measuring radio frequency (RF) or other rapidly time varying electromagnetic fields, and to methods of calibrating such probes.
BACKGROUND OF THE INVENTION
Photonic fiber-optic probes or sensors are useful in the measurement of RF fields because they minimally perturb the fields being measured, have low volume and weight, extensive bandwidths, and provide minimal thermal expansion of the signal transmission path, which facilitates reliable phase measurements. Prior fiber-optic probes have required both an expensive (usually high-power, low noise, and expensive-to-fiber-couple-to) laser and a separate sensing transducer. The transducer is typically an electro-optic modulator, for probes which probe electric fields, or a Faraday-effect modulator for probes which probe magnetic fields. These prior probes can be subject to calibration drifts due, for example, to temperature effects or fiber bending, because they include no means of checking or monitoring the sensitivity of the probe to RF fields while in operation. In order to avoid the use of field-perturbing metal wires or large and perturbing batteries, which must be replaced or recharged, these prior probes, further, have not been able to take advantage of modulators or directly-modulated lasers which require electrical biases (with one exception: U.S. Pat. No. 5,389,782 which employs an electro-optic modulator and an optically-powered amplifier). These prior-art probes further typically require expensive polarization-maintaining fibers which can also cause calibration drifts due to polarization cross-talk.
A prior voltage probe, as opposed to a field probe of the type shown in U.S. Pat. Nos. 5,583,444 and 5,703,491 used a directly-modulated laser in the probe head. This probe was limited to the capacitive pick up of voltages generated in devices on a surface (integrated circuits in particular) and to embodiments utilizing a constant current source to bias the lasers or which included placing and positioning means. No antenna capable of picking up free-space RF fields was involved—only voltage “detectors” capable of capacitively coupling to voltages relative to ground generated on a surface. The bias power was not provided optically, so metal conductors (which would grossly perturb free-space RF fields) were required. Further, because a constant current source was used, variations in temperature could cause significant changes in probe sensitivity by changing the slope of the laser power vs. laser input current curve.
A fiber-optic link (U.S. Pat. No. 5,739,938), which is not an RF field probe, but which could potentially be used as such if connected to an appropriate antenna, used a directly-modulated laser and an optically-supplied DC power source. The transmitter (main part of the probe head in a probe configuration) of this link was limited, however, by the inclusion of a PIN photodiode in the “laser module” together with a laser power regulator in the transmitter portion in all embodiments. These parts require a relatively large amount of power. The inclusion of this circuitry in the transmitter increases its size, weight, and electrical power consumption. Moreover, since this power was supplied by inefficient optical means (normally involving the loss of around 150% of the power consumed in the optical-to-electrical power converter alone), the total power consumption is very high. Moreover, much of this power must be dissipated within the transmitter itself, leading to thermal management complexities. The increased transmitter size, weight, and power consumption are problems in probes used for measurement purpose and, especially, in applications such as phased-array radars, where a very large number of such transmitters are involved. In addition, while the PIN photodiode and power regulator are used to keep the light power generated by the laser constant, there is no provision made to compensate for changes in the slope of the laser power vs. laser input current curve or in fiber coupling or transmission losses-the parameters on which the probe or link sensitivity is dependent. These parameters can change with environmental effects such as temperature independently of laser power. Furthermore, the many different parts, including the PIN photodiode, the power regulation circuit, and parts used for fiber coupling, are exposed to these same environmental effects possibly including, in addition to temperature, vibration, dust, humidity, and electromagnetic noise. This link is also limited to transmitters which incorporate an RF amplifier, which adds further complexity, volume, weight, and inefficient power consumption, as well as further susceptibility to various environmental effects, and is not needed in many probe applications.
SUMMARY OF THE INVENTION
Therefore, it is a general object of the invention to provide an improved RF field probe head of small size which does not significantly affect the field being measured.
Another more specific object of the invention is to provide an RF-field probe using a laser, with or without an RF amplifier, in the probe head which is modulated in response to RF fields, thus eliminating the need for both an expensive high-power, low-noise, high-stability laser at the receiver end along with an expensive sensing transducer, such as an electro-optic modulator, in the probe head.
A further object of the invention is to provide an RF-field probe head in which electrical biases are supplied optically, eliminating the need for field-perturbing metal conductors or large, field-perturbing batteries, which must be replaced or recharged.
Yet another object of the invention is to provide an RF-field probe and fiber-optic link using a directly-modulated laser, with or without an RF amplifier in the transmitter, which is optically powered, but which does not require a PIN diode or a power regulator in the transmitter, thus allowing minimum volume, weight, and power-consumption, along with maximum power efficiency.
It is also an object of the invention to provide an RF probe and fiber-optic link for which the amplitude calibration can be monitored and, in some cases, corrected for with minimal components and circuitry in the probe head, and which employs components and circuitry which are readily protected from or compensated for environmental effects such as temperature, vibration, dust, humidity, and electromagnetic noise, and which uses the actual system photodetector in providing this monitoring and/or correction.
Another object of the invention is to provide an RF-field probe using an inexpensive, easy-to-couple-to, low-threshold current, efficient vertical-cavity-surface-emitting laser (VCSEL), with or without an RF amplifier, in the transmitter.
A further object of the invention is to provide an RF-field probe which uses an inexpensive, easy-to-couple-to, low-drive-voltage reflective multiple-quantum-well modulator.
An additional object of the invention is to provide an RF-field probe which can be used to detect and measure either electric or magnetic fields over a wide frequency range, depending upon the particular embodiment, from near DC to well over 18 Ghz.
Another object of the invention is to provide an RF-field probe which is also capable of detecting or measuring all three directional components of an RF field.
Yet another object of the invention is to provide an RF-field probe which is simultaneously able to approximately determine the ambient temperature at the probe head.
It is also an object of the invention to provide an RF-field probe which does not require expensive, polarization-sensitive polarization-maintaining fibers.
A further object of the invention is to provide an RF-field probe which does not incorporate an RF amplifier, which adds complexity, volume, weight, inefficient power consumption, and further susceptibility to various environmental effects, in the probe head for probing applications where the added sensitivity provided by such an amplifier is no
Deb Anjan K
Metjahic Safet
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