Edward J. Vertatschitsch

High-precision fiber-optic position sensing using diode laser radar techniques

The analysis, design, and testing of a high-precision linear position sensor using diode laser radar techniques and fiber-optic signal distribution is described. A frequency-chirped, intensity-modulated semiconductor diode laser is used as the transmitter. Each sensor head consists of two reflectors -one moving and one fixed -in a differential ranging mode to cancel apparent range changes caused by temperature induced fiber length variations. The returned (round trip delayed) chirps are direct detected by a photodiode and then are mixed with the original (undelayed) chirp to produce a sum of beat frequencies, each proportional to the range of a reflector. Several sensors heads, located at different fiber distances, can be optically multiplexed by a single laser transmitter using a reflective or transmissive network. The performance of the laser radar position sensor is anal,rzed by first calculating the return signal-to-noise ratio (SNR). A Cramér-Rao lower bound is derived to relate the SNR, chirp bandwidth, and chirp duration to the root-mean-square (RMS) range error. The theoretical optimum performance of the experimental sensor system is determined. An experimental system was built that achieved 58 pm RMS range error using a 1 ms chirp duration with a processing time of 50 ps.

Ladar fiber optic sensor system for aircraft applications

A ladar fiber optic sensor (LFOS) for aircraft applications is described. Chirped intensity- modulated ranging is used to estimate linear position. LFOS technology offers several advantages over other fiber optic sensor techniques proposed for aircraft position sensing applications, including small and robust transducer heads, inherent multiplexing capability, and inherent fault isolation capability. LFOS sensors have been integrated inside a flight control surface hydraulic actuator and inside a pilots sidestick controller. Closed loop operation of the actuator using the LFOS sensor for position feedback was successfully demonstrated in the laboratory. The LFOS sensors in the sidestick controller were used as inputs to fly a flight simulator. The current LFOS interface electronics is contained on two VME circuit cards, with the capability to service four multiplexed sensors. Excellent performance has been achieved. Deviation from linearity over a 7-in. stroke is better than 0.05% of full scale. The RMS measurement noise is less than 15 microns for a 1 millisecond measurement interval.

Diode Laser Radar System Analysis And Design For High Precision Ranging

Laser radar (ladar) performance parameters such as range accuracy, estimation time and target range are analyzed as a function of the component parameters that comprise the system. Design tradeoffs and performance comparisons are evaluated for two popular ladar system designs: intensity modulated(IM)/direct detection(DD) and frequency modulated(FM)/coherent detection(CD). The comparison includes practical discussions of modulation, signal processing and component performance. The analysis begins by presenting accurate signal to noise ratio (SNR) equations that include the often omitted phase noise and intensity noise terms. Next, the SNR is related to the range accuracy, estimation time and modulation frequency or bandwidth via the Cramer-Rao lower bound (CRLB) for optimal processing. The results of the analysis are used to identify critical component design and performance issues for precision IM/DD and FM/CD ladar systems.

High-precision fiber-optic position sensor system using ladar techniques

Fly-by-light aircraft require high-precision, high-speed actuator position measurements. The cost and weight associated with shielding fly-by-wire systems from electromagnetic interference is the primary driver. Previous attempts have focused on digital and analog encoder technologies in which the sensor head varies the optical power.1 The result is a complex, bulky sensor head requiring one or more fibers (and associated connectors) between each remotely located sensor and the flight-control computer. Fly-by-light aircraft require high-precision, high-speed actuator position measurements. The cost and weight associated with shielding fly-by-wire systems from electromagnetic interference is the primary driver. Previous attempts have focused on digital and analog encoder technologies in which the sensor head varies the optical power.1 The result is a complex, bulky sensor head requiring one or more fibers (and associated connectors) between each remotely located sensor and the flight-control computer.

Signal processing results of a surveillance radar height-finding experiment

Many radar systems need to be able to accurately determine a targets height above terrain. In airborne early warning (AEW) systems, the radar antenna is limited in its vertical aperture, thus producing a broad beamwidth in elevation. This reduces its curacy for height fmding. An alternate method of height fmding is to measure the delay of a ground-bounce (multipath) signal from the target, referenced to the direct line of sight path. This has been demonstrated successfully with AEW radars which operate over water in certain situations, but has had limited success over land. The problem has been the lack of an accurate, robust signal processing algorithm for determining the time delay between closely spaced direct and ground bounce returns. Our basic objective was to make credible the premise that an airborne AEW radar can accurately determine height of targets by processing the delayed echo due to the multipath ground bounce. Current AEW systems use this technique, but in a limited way -usually only over water, where the surface reflection is strong and predictable and when it is well separated from the direct reflection. It has been recognized for some time that land also can cause multipath reflections, but due to its irregular nature, this technique has not been exploited thoroughly. Therefore, our objectives were to show height finding can be done accurately when the direct pulse overlaps the specular return (over water or land). The signal processing problem is essentially one of performing time-of-arrival estimation of two or more pulse returns; the direct plus one or more ground bounce echoes. For typical AEW scenarios, the echoes may overlap the direct return and are usually of lower amplitude. Therefore, the algorithm must make use of the maximum information content of the signal and should have as low a threshold signal to noise ratio as possible in order to apply in the greatest number of conditions. This problem will also occur in laser radar systems, and has an exact analog for the phased-array direcfion-of-arrival estimation. The signal processing results of this paper can be applied to pulsed or frequency modulated continuous wave radar/ladar systems. We assume that the target has been detected and that it is of interest to estimate the targets height above terrain. This processing would occur at baseband using a complex demodulated channel. In general, some form of averaging is often used to enhance the SNR of small signals, such as Doppler processing (the technique is described later in this paper) and the algorithms are shown to be compatible with these operations. The algorithm could be used in some situations for enhanced detection as well. Using knowledge of the terrain, the surveillance aircraft altitude and the time delay estimates of the echoes, we can infer target height. Concurrent with the basic objective, we gathered an extensive, fairly high-resolution data set of the surface reflection of various terrain types at two radar hand frequencies: 430 MHz (UHF) and 1255 MHz (L-band). Data were to be collected for each polarization: horizonial, vertical, and cross-polarization. The main terrain of interest was rugged terrain; the predominant view being that the echo data taken in very rugged terrain would be the most difficult to extract. Calibration data were collected over water, and much data of fairly flat, predictable terrain were also taken.

Standard fly-by-light sensor interface using ladar fiber optic sensor technology

Current fly-by-light (FBL) sensors represent a proliferation of different unique electro-optic interfaces and transducers. Development of standard electro-optic interfaces for diverse measurements (position, pressure, temperature, speed, etc.) offers potential to improve affordability of FBL sensor systems. Ladar fiber-optic sensor (LFOS) is a promising sensor technology that has demonstrated such a capability. A position transducer, temperature transducer, rotary speed transducer, liquid level transducer, and switch have all been demonstrated as plug compatible. In addition to providing a standard common interface, LFOS technology also offers the benefits of small and robust transducers, inherent multiplexing capability, and inherent fault detection and isolation capability. Current versions of the LFOS electro-optic interface consist of two VME circuit cards that are capable of interrogating and processing four multiplexed sensors. LFOS has been demonstrated in several flight and propulsion control laboratory testbeds.

Multiplexed ladar fiber optic sensor system

Practical considerations in the design of an optically multiplexed ladar fiber-optic linear position sensing system are discussed including network architecture, bus fiber count, fault location, sensor separation and network efficiency. The results of a six channel multiplexing experiment using a single laser diode are presented.

Multiplexed ladar fiber-optic sensor system

Abstract Practical considerations in the design of an optically multiplexed ladar fiber-optic linear-position sensing system are discussed, including network architecture, bus fiber count, fault location, sensor separation, and network efficiency. The results of a six-channel multiplexing experiment using a single laser diode are presented.