The James Webb Space Telescope (JWST) is a space-based, infrared observatory designed to study the early stages of
galaxy formation in the Universe. It is currently scheduled to be launched in 2013 and will go into orbit about the
second Lagrange point of the Sun-Earth system and passively cooled to 30-50 K to enable astronomical observations
from 0.6 to 28 μm. The JWST observatory consists of three primary elements: the spacecraft, the optical telescope
element (OTE) and the integrated science instrument module (ISIM). The ISIM Element primarily consists of a
mechanical metering structure, three science instruments and a fine guidance sensor with significant scientific capability.
One of the critical opto-mechanical alignments for mission success is the co-registration of the OTE exit pupil with the
entrance pupils of the ISIM instruments. To verify that the ISIM Element will be properly aligned with the nominal
OTE exit pupil when the two elements come together, we have developed a cryogenic pupil measurement test
architecture to measure three of the most critical pupil degrees-of-freedom during optical testing of the ISIM Element.
The pupil measurement scheme makes use of: specularly reflective pupil alignment references located inside of the
JWST instruments; ground support equipment that contains a pupil imaging module; an OTE simulator; and pupil
viewing channels in two of the JWST flight instruments. Current modeling and analysis activities indicate this
measurement approach will be able to verify pupil shear to an accuracy of 0.5-1%.
In traditional Risley prism beam pointers, two wedge prisms are used to steer the optical path. This type of beam director is attractive for free space laser communication and beam scanning systems because the beam director is compact and conformal. Furthermore, moderately large apertures can be accommodated without significant weight or power consumption. However, this approach is not well suited to tracking systems, since while tracing a continuous, constant velocity path, control singularities can occur that require infinite rotational speeds of the prisms. This phenomenon is particularly evident at pointing angles near the system boresight. A third prism can be used to eliminate these singularities, but the system is then under-constrained and infinite solutions exist for the prism orientations. In this paper, a method of uniquely determining the proper orientations of three prisms in an optical beam pointer is presented. It is shown how the method can be used to optimize a system for optical tracking by minimizing the angular velocities required of the prisms. Also, a specific implementation of the method has been demonstrated in the laboratory. Smooth tracking of arbitrary target trajectories is demonstrated across the field of view of the system.
Ball Aerospace & Technologies Corp. (Ball Aerospace) has developed a Risley Beam Pointer (RBP) mechanism capable of high pointing accuracy and operational bandwidth. The prisms offer a wide field of regard (FOR) and can be manufactured and operated with diffraction-limited optical quality. The unit is capable of steering a 4-in. diameter beam over a 72° half angle cone with better than 100 μrad precision. Absolute accuracy of the beamsteering is in the range of 100 μrad to 1 mrad, depending on the thermal environment of the system. The system has demonstrated a control bandwidth of 23 Hz and better than 10 deg/sec of smooth target tracking anywhere within the FOR.
In this paper, the noise properties of vertical cavity semiconductor optical amplifiers (VCSOAs) operated in reflection mode are studied. Expressions for noise sources contributing to the total noise detected at amplifier output are derived, based on the photon statistics master equations. The noise figure, defined as the degradation of signal-to-noise ratio (SNR), is analyzed using the assumption that spontaneous emission-signal beat noise dominates. The analysis shows that the noise figure of reflection mode VCSOAs has the same values as that in transmission mode as long as amplifier gain is high (G>>1). Furthermore, simulations depict the dependence of noise figure on device parameters and bias conditions, as well as reveal the importance of the low reflectivity front mirror and the high reflectivity rear mirror for low noise operation. In addition, the noise figure analysis results are compared with experimental measurements, in which amplified spontaneous emission (ASE) power is measured by an optical spectrum analyzer and the noise figure is obtained from the ASE power and the amplifier gain. The measured data are in good agreement with the theoretical predictions.
Two different approaches are commonly used for Fabry-Perot Semiconductor Optical Amplifiers (FP SOAs) performance analysis: the Fabry-Perot resonator approach and rate equation approach. Compared with the Fabry-Perot resonator approach, the rate equation approach is more powerful because noise and mode-related performance analysis can be included. However, it has been shown that the results based on Fabry-perot approach contains multiplicative factor which arise from an explicit consideration of the resonator and those factors are missing in the rate equation approach. As a result, the existing rate equations provide a poor description of FP SOAs. Our analysis shows that this is due to the fact that the interference between the injected optical field and the intracavity optical field has not been taken into account properly. In this paper, a new photon density rate equation for Fabry-Perot semiconductor optical amplifiers is derived based on the electric field rate equation. By taking this interference into account, our derivation shows that the input coupling term in the photon density rate equation is a function of the top and bottom mirror reflectivity, as well as the bias condition. Optical gain predictions from this new photon density rate equation match well with experimental measurements.
Vertical cavity semiconductor optical amplifiers (VCSOAs) are attractive devices for use in coherent optical amplification, especially where 2-D amplifier arrays are required. However, the coherence preservation quality of a VCSOA depends strongly on the bias condition, resonant wavelength mismatch, and the optical input power level. We characterize the coherence degree of a VCSOA as a function of these parameters by measuring interference fringe visibility with an interferometer. The dominant factors influencing the contrast of the fringes are the ratio of coherent, stimulated emission photons to amplified spontaneous emission (ASE) photons, and the spectral distortion of the amplified signal. Mostly, the overall gain and the saturation characteristic of the amplifier determine the ratio of stimulated emission to ASE. The spectral distortion of the signal is due to the narrow gain window of the VCSOA, but the effect significantly degrades the visibility only for relatively large wavelength mismatch from the gain peak. Analytic expressions may be used to identify the optimal bias current and optical input power to maximize the amplifier gain and visibility of the interference.
Current biochip technologies typically rely on electrostatic or mechanical forces for the transport and sorting of biological samples such as single cells. In this paper we have investigated how optical pressure forces can be effectively used for the manipulation of cells and switching in a microfluidic system. By projecting the optical beams externally non-contact between the control devices and the sample chip is possible thus allowing the sample chips to be disposable which reduces the chance of cross-contamination. In one implementation we have shown that vertical cavity surface emitting laser (VCSEL) array devices used as parallel optical tweezer arrays can increase the parallelism of sample manipulation on a chip. We have demonstrated the use of a high-order Laguerre-Gaussian mode VCSEL for optical tweezing of polystyrene microspheres and live cells. We have also shown that optical pressure forces from higher- power sources can be used for the switching of particles within microfluidic channels. Both the attractive gradient force and the scattering force of a focused optical beam have been used to direct small particles flowing through junctions molded in PDMS. We believe that by integrating optical array devices for simultaneous detection and manipulation, highly parallel and low-cost analysis and sorting devices may be achieved.