The nanosatellite optical downlink experiment (NODE) implements a free-space optical communications (lasercom) capability on a CubeSat platform that can support low earth orbit (LEO) to ground downlink rates>10 Mbps. A primary goal of NODE is to leverage commercially available technologies to provide a scalable and cost-effective alternative to radio-frequency-based communications. The NODE transmitter uses a 200-mW 1550-nm master-oscillator power-amplifier design using power-efficient M-ary pulse position modulation. To facilitate pointing the 0.12-deg downlink beam, NODE augments spacecraft body pointing with a microelectromechanical fast steering mirror (FSM) and uses an 850-nm uplink beacon to an onboard CCD camera. The 30-cm aperture ground telescope uses an infrared camera and FSM for tracking to an avalanche photodiode detector-based receiver. Here, we describe our approach to transition prototype transmitter and receiver designs to a full end-to-end CubeSat-scale system. This includes link budget refinement, drive electronics miniaturization, packaging reduction, improvements to pointing and attitude estimation, implementation of modulation, coding, and interleaving, and ground station receiver design. We capture trades and technology development needs and outline plans for integrated system ground testing.

InP/InGaAs/InP heterostructures were selectively grown in an array of 60 nm holes on a 90 nm pitch by MOCVD. The pattern was defined photolithographically directly in hydrogen silsesquioxane, a spin-on-glass, obviating the need for pattern transfer processes.

Epitaxial diamond with remarkably low p-type doping (1x10(14)-1x10(17) cm(-3)) and exceptionally low compensation similar to1x10(13) cm(-3), has enabled the demonstration of a optically-switched conduction modulation of the epitaxial layer. Charge exchange between the diamond substrate and the epitaxial layer makes it possible to modulate the conductivity of the epitaxial layer. Incandescent light will make the lightly p-doped epitaxial layer insulating and ultraviolet radiation will make the layer conductive again. Once the layer conductivity has been established it will remain in the same electrical state for days, if kept in the dark. (C) 2004 American Institute of Physics.

Phase-encoded optical sampling allows radio-frequency and microwave signals to be directly down-converted and digitized with high linearity and greater than 60-dB (10-effective-bit) signal-to-noise ratio. Wide-band electrical signals can be processed using relatively low optical sampling rates provided that the instantaneous signal bandwidth is less than the Nyquist sampling bandwidth. We demonstrate the capabilities of this technique by using a 60-MS/s system to down-sample two different FM chirp signals: 1) a baseband (0-250 MHz) linear-chirp waveform and 2) a nonlinear-chirp waveform having a 10-GHz center frequency and a frequency excursion of 1 GHz. We characterize the frequency response of the technique and quantify the analog bandwidth limitation due to the optical pulse width: The 3-dB bandwidth imposed by a 30-ps sampling pulse is shown to be 10.4 GHz. We also investigate the impact of the pulse width on the linearity of the phase-encoded optical sampling technique when it is used to sample high-frequency signals.

The general configuration of the proposed high-speed ADC was described. A train of optical pulses samples the analog signal applied to the optical modulator. A dual-output Mach-Zehnder electro-optic modulator is used to implement phase-encoded optical sampling. Output circuits detect and integrate the modulated optical pulses. The integrated level is held until electronic digitizer converts the signal into digital form. The circuit then resets and waits for the next input pulse. The output circuits include amplifiers before the digitizers.

The agreement between the measured nonlinear response and the absorption saturation simulation for different absorber thickness provides strong evidence of an absorption-saturation photodiode nonlinearity. We have improved the linearity of a photonic ADC by developing large-area, thick photodiodes.

Summary from only given. Photodetector nonlinearities can limit the performance of high-dynamic-range analog optical systems. For systems that use short optical pulses to process or transmit analog information, this limitation is magnified due to the higher peak intensity at the photodetector input. In this paper, we examine the trade-off between signal-to-noise ratio (SNR) and linearity in optically sampled analog-to-digital converters (ADCs) due to saturation of the photodetector responsivity.

Phase-encoded optical sampling allows the processing of RF and microwave signals with high linearity and >60-dB signal-to-noise ratio in an optically sampled digital receiver. The wide analog-input bandwidth (>1 GHz) enables novel processing capabilities as demonstrated for a 250-MHz-bandwidth chirped signal undersampled at 60 MS/s.

We have demonstrated a new technique to characterize wideband nonlinear waveforms at microwave carrier frequencies. The analog sampling bandwidth can be increased with wider bandwidth sampling modulators and shorter optical pulses. The instantaneous Nyquist bandwidth can be extended using time-interleaving techniques.

Electrically driven arrays of optical switches can be used to time demultiplex a train of high-repetition-rate optical pulses into parallel lower speed circuits. These demultiplexers are used to advantage in high-speed communication systems and in photonic analog-to-digital converters. High-speed operation of an optical demultiplexer is simplified, by the use of sinusoidal electrical drives, but suppression of crosstalk is difficult. Adequate suppression of crosstalk in these demultiplexers can be obtained through the use additional modulators. The key to our approach is the use of a combination of harmonically related sinusoidal signals applied to the additional modulators in order to select one out of every M pulses. Results with a 1:4 demultiplexed photonic analog-to-digital converter operating at 208 MS/s demonstrate the efficacy of the technique.

This letter describes a demonstration of a photonic analog-to-digital converter operating at 208 MS/s and utilizing phase-encoded optical sampling to achieve an 87-dB two-tone third-order intermodulation-free dynamic range. A pair of LiNbO/sub 3/ 1-to-4 optical time-division demultiplexers with >35-dB channel extinction distribute the 30-ps sampling pulses to an array of photonic integrate-and-reset circuits followed by 12-b 52-MS/s electronic quantizers. Interleaving spurs due to temporal crosstalk currently limit the overall spur-free dynamic range to 65 dB.

Optically sampled analog-to-digital converters (ADCs) combine optical sampling with electronic quantization to enhance the performance of electronic ADCs. In this paper, we review the prior and current work in this field, and then describe our efforts to develop and extend the bandwidth of a linearized sampling technique referred to as phase-encoded optical sampling. The technique uses a dual-output electrooptic sampling transducer to achieve both high linearity and 60-dB suppression of laser amplitude noise. The bandwidth of the technique is extended by optically distributing the post-sampling pulses to an array of time-interleaved electronic quantizers. We report on the performance of a 505-MS/s (megasample per second) optically sampled ADC that includes high-extinction LiNbO3 1-to-8 optical time-division demultiplexers. Initial characterization of the 505-MS/s system reveals a maximum signal-to-noise ratio of 51 dB (8.2 bits) and a spur-free dynamic range of 61 dB. The performance of the present system is limited by electronic quantizer noise, photodiode saturation, and preliminary calibration procedures. None of these fundamentally limit this sampling approach, which should enable multigigahertz converters with 12-b resolution. A signal-to-noise analysis of. the phase-encoded sampling technique shows good agreement with measured data from the 505-MS/s system.

Time interleaving of samples digitized by a parallel array of analog-to-digital (A/D) converters provides a means of increasing the sampling rate beyond that possible with a single A/D converter. For time-interleaved photonic. A/D converters, optical demultiplexers can be used to advantage. Both time-division and wavelength-division demultiplexers must yield low crosstalk between the parallel output channels in order to yield accurate A/D conversion, An analysis predicts the level and form of the resulting errors, The analytical results compare well with experiment.

We report residual phase-noise measurements of both actively mode-locked fiber and semiconductor lasers using sinusoidal mode-locking frequencies between 500 MHz and 5 GHz. The fiber laser is a polarization maintaining erbium-doped fiber ring laser that is harmonically mode-locked using a sinusoidal drive signal. The mode-locked semiconductor laser uses an external cavity configuration

The phase-noise characteristics of a harmonically mode-locked fiber laser are investigated with a new measurement technique called phase-encoded optical sampling. A polarization-maintaining ring laser is mode locked by use of the short-pulse electrical output of a resonant-tunneling diode oscillator, enabling it to produce 30-ps pulses at a 208-MHz repetition rate. The interferometric phase-encoded sampling technique provides 60-dB suppression of amplitude-jitter noise and allows supermode phase noise to be observed and quantified. The white-noise pulse-to-pulse timing jitter and the rms supermode timing jitter of the laser are measured to be less than 50 and 70 fs, respectively.

Summary form only given.Analog-to-digital converters face severe performance constraints on bandwidth and accuracy because of limitations imposed by the input track-and-hold circuits that perform the sampling function. Optical sampling offers a way to dramatically enhance both the timing precision and linearity of the sampling function. A/D converters utilizing optical sampling have the potential for operating at multi-GHz sampling rates with >12-bit precision. The primary limitation of previously reported optical-sampling systems has been the nonlinearity of the sampling device. Our use of phase-encoded optical sampling overcomes this limitation at the expense of added complexity. We describe a system that combines phase-encoded optical sampling with time-division demultiplexing to distribute the sampled data to an array of commercial quantizers.

A high-speed optical sampling system for electrical signals has been developed using a gain-switched diode laser and a dual-output Mach-Zehnder interferometer. The optical phase shift between the branches of the interferometer is highly linear in the applied electrical signal. The phase shift is encoded in the two outputs of the interferometer and is recovered through digital signal processing. Analog-to-digital (A/D) conversion with 78-dB spur-free dynamic range is demonstrated. Our phase-encoded sampling technique allows high-resolution (12-bit) conversion with high linearity at practical laser power levels.