Ricardo C. Coutinho

Detection of Partially Coherent Optical Emission Sources

We present a simple theoretical model and compare it with experimental results for highly emissive sources having various coherence lengths in the presence of incoherent background illumination. Agreement between the experimental results and the theory is discussed.

Variable numerical-aperture temporal-coherence measurement of resonant-cavity LEDs

The first interferometric measurements of temporal-coherence length variation with numerical aperture (NA) are described for 650 nm, resonant-cavity light-emitting diodes (LEDs) agreeing with spectrally derived results. The interferometrically measured coherence length (22 /spl mu/m to 32 /spl mu/m) reduced by 37% for a 0.42 increase in NA. For a larger range of NA (0-1), this would give coherence lengths (10 /spl mu/m-40 /spl mu/m) lying in the gap between that of conventional LEDs (/spl sim/5 /spl mu/m) and superluminescent diodes (/spl sim/60 /spl mu/m).

Angular Emission Profiles and Coherence Length Measurements of Highly Efficient, Low Voltage Resonant Cavity Light Emitting Diodes Operating Around 650nm

We present results on Resonant Cavity Light Emitting Diodes (RCLEDs) emitting at 650 nm, which have high efficiencies and low voltages. In particular, we report on the angular properties of these devices, and highlight the observation that overall spectral linewidth increases with collection angle. This unusual property of RCLEDs is largely a consequence of employing a microcavity in the design. An additional contributing factor is the relative distribution of gain amongst the cavity modes (i.e. the level of tuning or detuning of the underlying emission, defined with respect to the longitudinal cavity mode). We have used measurement techniques which spectrally resolve angular radiation profiles to determine the (de)tuning directly. Moreover, these profiles demonstrate how the overall spectral linewidth increases with collection angle. To this end, we have developed a semi- empirical method for determining the overall linewidth as a function of emission numerical aperture (NA). A 4 nm detuned device has been investigated and linewidths have been found to increase from 3.1 nm to 13.6 nm over a range of NA approximately equals 0 to NA equals 1, an increase by a factor of around 4. Obviously, a variable linewidth also implies a variable coherence length with NA. Consequently, the coherence length was found to decrease from 30 micrometer to 9 micrometer over the same range. Independent coherence length measurements were carried out by direct interferometric measurements, and confirmed the expected trends.

High-dynamic-range imaging optical detectors

Imaging spectrometers allowing spatially resolved targets to be spectrally discriminated are valuable for remote sensing and defense applications. The drawback of such instruments is the need to quickly process very large amounts of data. In this paper we demonstrate two imaging systems which detect a dim target in a bright background, using the coherence contrast between them, generating much less data but only operating over a limited optical bandwidth. Both systems use a passband filter, a Michelson interferometer, coupling optics and a CCD camera. The first uses the interferometer in a spatial mode, by tilting one of the mirrors to create a set of line fringes on the CCD array. The visibility of these fringes is proportional to the degree of coherence. The interferogram is displayed spatially on the CCD array, as a function of the path differences. The second system uses the interferometer in a temporal mode. A coherent point target and an extended background are imaged through the interferometer onto the CCD array, and one of the interferometers mirrors is scanned longitudinally to vary the path difference in time. In both cases the coherent target is detected over a large dynamic range down to negative signal-to-background power ratios (in dB). The paper describes an averaging technique to improve the signal-to-noise ratio and correction techniques required to extract interferograms from the images. The spatial technique developed has the advantage of using no moving parts.

Detection of coherent light in an incoherent background [for IRST]

Detection of a laser emission in daylight clutter in infrared search and track systems is usually carried out by measuring the energy from the scene in the instrument field of view. When the emitter is at a considerable distance or the reflected sunlight is intense, the target cannot be detected by this method. This paper describes a more sensitive method based on detecting the coherence properties of the incident radiation. The system uses both optical and digital signal processing. The optical section is an interferometer to measure the self-coherence function of the incident radiation to give an interferogram, and uses a narrow band filter to produce the sine envelope. The photodetector only detects power, which is the square of the modulus of /spl gamma/. So the region between the main lobe and the first sidelobe of the envelope appears as a minimum rather than as a zero crossing where the envelope changes sign. The enclosed carrier undergoes a /spl pi/ phase shift at this point, which we use to define the point more accurately. The post detector section comprises signal amplification with a low pass filter, A/D conversion and a demodulation MATLAB algorithm, giving phase and instantaneous frequency signals as a function of optical path difference.

Theoretical modeling of a detection system based on optical coherence contrast

Optical detection systems usually rely on the intensity contrast (visible) or temperature difference (infrared) between target and background. Adding new dimensionality to the detection process is essential to enhance the sensitivity. This paper presents a novel theory for modeling the performance of an optical detection technique called Interferogram Phase Step Shift (IPSS), which relies on the coherence contrast between target and background to perform discrimination. The technique uses an interferometer to measure the self-coherence function of the input radiation, forming an interferogram, and an interference filter to produce an event marker (phase step) in it. The model predicts the displacement of the phase step in the interferogram, when a coherent target enters the system field of view, which is the kernel of the IPSS technique. The paper assesses the effects of the target to optical filter bandwidth ratio in the system responsivity, for optimization purposes, and models the experiments presented in a previous publication, predicting the experimental results theoretically to perform a comparison. It also includes the analytical derivation of the self-coherence functions of target and background as measured by the systems interferometer, and the computer modeling of the same self-coherence functions for an interference filter, with any arbitrary spectral response, considering the effects of the polarization of the light sources and optical components in the experiments. Finally, the theoretical curves for displacement vs. target-to-background power ratio, among others, are compared with the experimental results. Good agreement is demonstrated, and the causes of differences are discussed.

Coherence-based optical detection system's receiver operating characteristics (ROC) curves

High detection probabilities with low false alarm rates, even in the presence of as much as - 28 dB signal to clutter can be obtained, if use is made of the coherence information contained in the input signal. Such a detection system is described and its Receiver Operating Curves (ROC) characterized from a reduced experimental set of data. The system has an input filter with a rectangular passband, an interferometer and a photodetector. The filter gives rise to a sinc function shaped interferogram envelope in optical path difference. The position of the interferograms first null depends on the effective bandwidth of the scene in the instruments field of view. The detection system is sensitive to the degree of coherence of the scene. The amount of the shift of the position of the first null is found to be a highly sensitive measure of the presence of a very narrow-band target within the instruments input bandpass filter. An important characteristic of this system is that the shift is still detected even when an intense wideband incoherent source, which may be considered to be clutter, is introduced into the input. We measured the ROC curves for the optical detection system using a He-Ne laser narrowband coherent source target with a tungsten halogen bulb wideband incoherent background clutter. We show that the target source can be detected even when it is substantially weaker than the clutter at - 28 dB signal to clutter. We compare the predictions of our theoretical model with the experimental results and show good agreement.