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The Array of Low Energy X-ray Imaging Sensors (ALEXIS) satellite is Los Alamos’ pathfinding small space mission achieving low cost and rapid development time for its technology demonstration and science goals. The ALEXIS satellite contains the ALEXIS telescope array, which consists of six EUV/ultrasoft X-ray telescopes utilizing normal incidence multilayer mirrors, microchannel plate detectors, and thin UV rejecting filters. Each telescope is tuned to a relatively narrow bandpass centered at either 130, 171, or 186 angstroms. Each telescope has a 33° field-of-view, and a resolution of ~ 0.25°. With each 50 s rotation of the satellite, the telescopes scan most of the anti-solar hemisphere of the sky. The spacecraft is controlled exclusively from a ground station located at Los Alamos.This paper discusses the characteristics and performance of the ALEXIS telescopes and the results from the mission in spite of the damage incurred to the spacecraft at launch.

Careful measurements of the OIP3 in high power photodetectors show a resonance peak at a particular bias voltage and photocurrent combination. In fact, for a given photocurrent, there exists an optimum bias voltage that maximizes the OIP3. One can envision a DC control circuit that would be able to maintain the optimum OIP3 performance from the photodiode. By optimizing the bias voltage for a given photocurrent, a peak OIP3 of +65dBm was measured which is believed to be the highest reported value to date.

The small size, light weight and low cost are main advantages of analog fiber links over coax links. In particular, analog fiber link is examined for photonic antenna (see Fig. 1) operating at high center frequencies in which a high power photodiode is deployed close to the antenna for downlink broadcasting [1]. High power and high output third order intercept point (OIP3) are among critical photodiode requirements in such application.

We have described a high optical power transmitter suitable for applications in harsh environments that require data modulation rates of up to 10 Gbps and modulated optical output power up to 25 mW. The compact package format enables size reductions for free space laser communications and antenna remoting terminals. High efficiency operation at lower optical power will provide DC power savings for digital transmission.

Voltage-dependent responsivity nonlinearities are characterized experimentally and analytically for uni-traveling carrier directional coupled photodiodes (DCPD) with two types of design variations. OIP3 data follows the voltage-dependent responsivity predictions for both MMI width and PD width variations of the baseline DCPD. A maximum OIP3 of 39dBm was achieved for a device with a 3µm wide PD at 20mA and 4V.

A novel photodiode is presented using a directional coupler incorporated with a UTC style photodiode with 0.88 A/W responsivity and 35 dBm OIP3 at 25 mA. The device responsivity is characterized at various photocurrents up to 10 mA and the OIP3 is measured up to 25 mA and 10 GHz. Additionally, the device capacitance is measured and used to model the capacitance limited OIP3 of the device. The failure of the device was compared to a traditional waveguide photodiode showing burnout no longer occurs at the front of the device and demonstrated the potential of the new design to control the photocurrent density profile for a waveguide style photodiode.

Waveguide PIN photodiodes with different absorber thicknesses and lengths were fabricated and characterized for linearity. Device A has a thicker absorber and shorter length, resulting in a bandwidth of 20 GHz while device B reduces the absorber by half while maintaining the intrinsic layer thickness and almost doubles the length, resulting in a smaller optical overlap factor and a bandwidth of 10 GHz. Device B shows a significant enhancement in OIP3 with a record high maximum value for a PIN waveguide photodiode of 42.4 dBm at 28 mA and -4V bias compared to device A which has a maximum OIP3 of 32.7 dBm at 10 mA and -4V bias. The increased linearity in device B is attributed to the reduction in optical overlap factor and increase in device length resulting in an easing of the front facet photocurrent density and overall device heating. The DC saturation points are about 75 mA and160 mA for device A and B at -2V bias.

The generation of sub-millimeter waves and terahertz signals is a difficult challenge, and a few approaches are being studied by several groups. We are investigating the approach of heterodyning two optical signals in a uni-traveling-carrier photo detector, which has been identified as the device of choice for very high frequencies. In this paper we present a novel analytical model, which was developed as a design tool for the above device. The model was validated by designing, fabricating and testing a new 100 GHz photo detector. In addition, performance limits for the UTC traveling wave photo detector are calculated by the new model, and it is shown that this device has the potential of delivering more than 1 mW of power at frequencies approaching 1 THz.

One of the approaches used to generate millimeter and sub-MM wave signals is the heterodyning of two optical signals in a photo detector. The UTC-TW photo detector has been identified as the device of choice for very high frequencies. In this paper we present the design, fabrication and testing of a new UTC-TW device, which can operate at frequencies exceeding 100 GHz. The new device was designed by a novel analytical model, which was developed by the authors and reported previously. This device is intended to be the first one in a series of devices, which eventually approach 1 THz, as predicted by our model.

High speed photo detectors are very useful components for high capacity optical communication links, generation of MM and sub-MM wave signals and more. The most promising photo detector for speed is the UTC-TW PD, which was proposed a few years ago. Operation of such devices was reported at frequencies exceeding 300 GHz. Output power and efficiency improve as the optical power is increased, however, when the optical power exceeds the saturation level, substantial reduction is observed in bandwidth. In this paper we present experimental results demonstrating the bandwidth degradation at high optical power, and offer an explanation to this phenomenon, which is backed up by theoretical simulations.

An attitude reconstruction algorithm has been developed and tested for the ALEXIS spacecraft, whose dynamics include significant thermal/flexible motions due to structural damage that occurred during launch. The ALEXIS science mission needs the reconstructed attitude to do off-line mapping of soft x-ray photons from telescopes' image planes aboard the spinning spacecraft into inertial coordinates. The attitude reconstruction algorithm takes inputs from a fine sun sensor and a horizon crossing indicator and processes them using a square-root-information filter/smoother. The filter/ smoother model includes Euler dynamics and randomwalk torques to approximate thermal/flexible effects. The launch damage left a solar array paddle free to "flap" under the influence of thermal inputs, which causes spin rate and direction changes. Despite the complexity and uncertainty of the dynamic model, the algorithm achieves a 0.25° 1-cr accuracy over 30-min. periods of orbit night, when attitude data is unavailable.