There has been a critical shift in the warfighting paradigm. Information superiority is now the critical capability for battlespace dominance. Photonics research enables future information superiority and hence battlespace dominance.
HgCdTe has emerged as an important electronic material because of its IRFPA applications. Technologies for growing the material are advanced and current sources for the material are more readily available than in the past. This brings an advantage to the manufacturing other types of HgCdTe devices. PHEMTs are attractive as applications of high-speed devices. In this paper, a model for PHEMT devices by using Hg1-xCdxTe as device materials is presented. High digital performance of the device is expected because electron mobility of the material is very high at low temperatures.
A mode-locked erbium-ytterbium fiber laser operating at 1550 nm using multiple quantum well (MQW) saturable absorbers was developed. The laser was constructed in a Fabry-Perot configuration using a fiber Bragg grating as a front reflector and a fiber chirped Bragg grating output as a back reflector of the laser cavity. The laser can either produce Q-switched or CW passively mode-locked pulse trains by simply changing the location of the saturable absorber with respect to a focal plane. The pulse train of laser Q- switching operates at an 85-KHz repetition rate when the position of the absorber is near the focus. Peak power of the Q-switching pulses is about thirty times higher than for the CW mode locking which occurs when the absorber is placed exactly at the focal plane. The CW passively mode-locked pulse trains have a 19-MHz repetition rate with 2.6-mW average output power.
Type-II interband cascade (IC) lasers take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions for serially connected active regions. Here, we describe recent advances in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.6 - 4 micrometers ; these semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Low threshold current densities (e.g., approximately 56 /A/cm2 at 80 K) and power efficiency exceeding 9% were observed from a mesa- stripe laser in cw operation. Also, these lasers were able to operate at temperatures up to 250 K in pulsed mode and 127 K in cw mode. We observed from several devices at temperatures above 80 K, slope efficiencies exceeding 1 W/A/facet, corresponding to a differential external quantum efficiency exceeding 600%. A peak optical output power of approximately 6 W/facet was observed from a type-II IC laser at 80 K.
We demonstrate generation of gigahertz to terahertz optical subcarrier radio frequencies in semiconductor optical amplifiers. The circuit arrangement consists of a laser diode riven below its lasing threshold to generate spontaneous emission spectrum. The spontaneous emissions are passed in a saturation driven semiconductor optical amplifier with low-end reflectivity. A fraction of the output signal emerging from the amplifier is fed back into the input of the amplifier. By appropriately arranging the phases of the input and the feedback signals, beat frequencies up to 3.75 THz were generated.
A novel photonic approach to analog-to-digital (A/D) conversion based on temporal and spatial oversampling techniques in conjunction with a smart pixel hardware implementation of a neural algorithm is described. In this approach, the input signal is first sampled at a rate higher than that required by the Nyquist criterion and then presented spatially as the input to the 2D error diffusion neural network consisting of M X N pixels. The neural network processes the input oversampled analog image and produces an M X N pixel binary output image which is an optimum representation of the input analog signal. Upon convergence, the neural network minimizes an energy function representing the frequency-weighted squared error between the input analog image and the output halftoned image. Decimation and low-pass filtering techniques, common to oversampling A/D converters, digitally sum and average the M X N pixel output binary image using high-speed digital electronic circuitry. By employing a 2D smart pixel neural approach to oversampling A/D conversion, each pixel constitutes a simple oversampling modulator thereby producing a distributed A/D architecture. Spectral noise shaping across the array diffuses quantization error thereby improving the signal-to-noise ratio performance. Here, each quantizer within the network is embedded in a fully- connected, distributed mesh feedback loop which spectrally shapes the overall quantization noise significantly reducing the effects of component mismatch typically associated with parallel or channelized A/D approaches. The 2D neural array provides higher aggregate bit rates which can extend the useful bandwidth of oversampling converters.
There has been much recent interest in the use of photonics for analog to digital conversion. It is anticipated that the use of photonic analog to digital converters (ADCs) will far surpass the performance of electronic ADCs in terms of both sampling speed and resolution. We have designed a novel photonic ADC module that incorporates the use of semiconductor linear absorbers to perform the data quantization at speeds up to 100 GS/s with 4 bits of resolution. The use of the passive materials in this flash photonic ADC architecture makes this module a candidate for insertion into future space-based platforms. Experimental characterization results will be presented for the semiconductor materials used in the data conversion process.
Analog-to-digital (AID) converters with sampling rate in the gigabits/sec are desirable in many advanced communication systems including RADAR, microwave and cable links1, and for designing zero intermediate frequency receivers, as well as in the implementation ofhigh-speed test and measurement equipment. Today's electronic AID converters have limiting speeds in the megahertz range, whereas most advanced communication systems operate in the gigahertz range. Optical devices and systems can operate at much higher speeds, and therefore there is great interest to design and implement all-optical A/D converters. Several optical AID converter schemes, intended to circumvent the electronic bottleneck have been proposed2 . Unfortunately, such schemes require complex circuitry for the sampler and/or the quantizer, and often require high-power. As a result these schemes are unsuitable for large-scale system that require low-power consumption, or for systems with many transmit/receive units such as in RADAR or satellite communication systems. We present the design and demonstration of an all-optical A/D converter whose attractive features include 1) direct optical sampling of the input RF signal, 2) optical quantization scheme, 3) utilization of optical signal multiplexing to provide parallel procession of quantized signals at electronic speed, and 4) low-power consumption.
External-cavity, actively-modelocked semiconductor diode lasers (SDLs) have proven to be attractive candidates for forming the backbone of next-generation analog-to-digital converters (ADCs), which are currently being developed to sample signals at repetition rates exceeding several GHz with up to 12 bits of digital resolution. Modelocked SDLs are capable of producing waveform-sampling pulse trains with very low temporal jitter (phase noise) and very small fluctuations in pulse height (amplitude noise)--two basic conditions that must be met in order for high-speed ADCs to achieve projected design goals. Single-wavelength modelocked operation (at nominal repetition frequencies of 400 MHz) has produced pulse trains with very low amplitude noise (approximately 0.08%), and the implementation of a phase- locked-loop has been effective in reducing the system's low- frequency phase noise (RMS timing jitter for offset frequencies between 10 Hz and 10 kHz has been reduced from 240 fs to 27 fs).
Optical analog to digital conversion schemes require a sampling source of high repetition rate, low temporal jitter, low amplitude noise, and short pulse duration to achieve the desired sampling rate and number of bits of resolution. We report on the development of an actively mode-locked semiconductor external cavity laser system where the emission is comprised of multiple wavelengths nominally centered around 1.55 microns. Cavity design includes an intra-cavity grating to produce a spatially dispersed optical spectral filtering plane. Amplitude filtering in this spectral plane serves to flatten the effective gain and a rectangular aperture array selects those wavelengths which are allowed to lase. Modelocked at 311 MHz and producing 8 spectral lines, the laser provides a sampling rate of approximately 2.5 GHz. Temporal interleaving of the pulse train by factor 4 increases the sampling rate to 10 GHz.
Joint development work by DLR and LH Systems has produced a new camera concept called Airborne Digital Sensor which is using forward-, nadir- and backward-looking linear arrays on the focal plane. The camera system provides panchromatic and stereo information using three CCD lines and up to five more lines for multispectral imagery including two NIR channels. Each CCD array for panchromatic measurements has 24000 elements, resulting in a field of view of 64 degrees (across track FOV) by using a focal length of 62.5 mm. The sensitivity covers a dynamic range of 12 bit with a recording interval time of 1.2 ms per line. The performance of the camera allows a 3D and multispectral image with a ground sample distance of 25 cm for an area of 300 square miles within a flight time shorter than one hour.
Precise control of the dispersion within mode-locked laser cavities can lead to optical pulse compression and reduced timing jitter of mode-locked lasers. Two simple measurement techniques are used to provide a complete picture of the dispersion within an erbium doped mode-locked fiber laser cavity. We measured the optical dispersion of erbium-doped fiber, standard single mode fiber, and chirped Bragg gratings. We built a Michelson interferometer with a wideband LED source to measure the dispersion of fiber lengths of less than 1 meter. Next, we measured the dispersion of chirped Bragg gratings using a network analyzer and a tunable laser in a differential phase measurement technique.
Multiple triple quantum wells (TQWs) are used in the active region of an AlGaAs p-i-n diode for a reflection modulator operating above the GaAs band gap. Photocurrent spectra of the TQW-based diode show sharp absorption features that retain their spectral character with reverse bias and show large Stark shifts--comparable to those obtained from alternative active-layer designs. Some performance characteristics of an 810-nm reflection modulator using the TQW active-layer design are presented. We also describe time-resolved pump/probe measurements made on a series of TQW-based p-i-n diodes with differing p-layer conductivity and contrast results with the predictions of a simplified model of the carrier dynamics.
This paper presents a photonic architecture for independently steering the broadband nulls of linear and conformal phased array antennas. Analytic expressions quantifying bandwidth requirements are developed and simulation results demonstrating array performance are presented.
The fabrication and performance of a colliding pulse mode locked laser with an intracavity saturable absorber is described. The laser has a threshold current of 65 mA and differential efficiency of 0.04 mW/mA when coupled into a single mode fiber. Mode locked pulses with approximately 1 ps pulse width at approximately 10 GHz has been obtained.
A number of parameters, such as gain, modulation response, linewidth enhancement factor and relative intensity noise in modulation-doped InGaAsP QW laser emitting at 1.55 micrometers have been theoretically investigated. The results indicate that the relaxation oscillation frequency for p-type modulation doped QW laser is enhanced by a factor of more than 2 compared to that for undoped MQW lasers, the linewidth enhancement factor of p-type modulation doped QW laser is reduced to 1/5 of that of undoped MQW laser and the relative intensity noise is reduced by a factor of > 10 dB compared to that for undoped MQW lasers.
The noise associated with a number of communications receivers contains a significant multiplicative element. While it is well known that minimum bit error rate occurs for matched filter reception of signals corrupted by additive white gaussian noise (AWGN), the application of the matched filter algorithm developed for signals corrupted by AWGN to signals corrupted by multiplicative-noise is far from optimal. In this paper a new algorithm is developed which weights the two halves of a multiplicative-noise corrupted Manchester-encoded signal based on the statistics of the two halves. In cases where the multiplicative noise dominates the additive noise, a situation which can arise in many optical communications receivers, the difference in performance of the new algorithm to a matched filter algorithm can be quite dramatic. In simulations conducted, a typical APD receiver circuit with internal gain of 200 produced a Bit Error Rate better than 10-6 compared to the matched filter performance of only about 10-2. Potential optical communications applications include receivers based on avalanche photodiodes, photomultiplier tubes, and optically amplified receivers.
We suggest to use modulation of distributed feedback in the volume reflection type diffractive periodic structures for the fast all-optical spatial-temporal light modulators. Different types of periodic structures are discussed: cholesteric liquid crystals, static and dynamic reflection gratings in the photorefractive crystals and photopolymers.
We suggest a new concept of the transient functionally graded materials for power conversion from the infrared and/or optical radiation. By illuminating homogeneous material with space-time modulated radiation, an array of thermoelectric and photoelectric devices (thermoelements and photoelements) can be created. These arrays can generate electric current. The proposed TFGM concept is the extension of a known FGM method. It potentially has an advantage of higher conversion efficiency due to transient nature of power conversion. The theory of TFGM for thermoelectric and pyroelectric materials was developed. It describes the generation of both AC and DC electric currents, and the application of the developed theory for a self-powered vibration sensing is discussed also.
The previously demonstrated beam-forming PIPG (photo-induced plasma-grating) technology was used to design a beam- steering antenna operating at X-band and a 2D tracking antenna operating at W-band. Both antennas possess entire optoelectronics control.
Current trends in device miniaturization and integration, especially in the development of microwave monolithic integrated circuits, calls for flexible, arbitrarily shaped and curved interconnects. Standard dielectric waveguides and microstrip lines are subject to prohibitive losses and their functionality is limited because of their unflexible structures. The problem is addressed by confining the wave- guiding path in a substrate with a Photonic Band Gap structure in a manner that will result in the guided mode being localized within the band gap. Two devices implementing Photonic Band Structures for millimeter waves confinement are presented. The first waveguide is a linear defect in triangular lattice created in a silicon slab (TE mode). The structure consists of parallel air holes of circular cross sections. The silicon was laser drilled to create the 2D crystal. The second device consists of alumina rods arranged in a triangular lattice, surrounded by air and sandwiched between two parallel metal plates (TM mode). Electromagnetic wave (W-band) confinement was obtained in both devices for straight and bent waveguides. Three branch waveguides (intersecting line defects) was studied as well. Measurements confirmed the lowloss waveguide confinement property of the utilizing Photonic Band Gap structure. This structure can find applications in power combiner/splitter and other millimeter wave devices.
Commercial interest in wavelength division multi/demultiplexer (WDM) components and systems is rapidly increasing. WDMs provide a new dimension for solving capacity and flexibility problems in telecommunication networks. Key components in WDM systems are wavelength multiplexers and demultiplexers. Many different techniques for realization of multiplexers and demultiplexers have been reported. Commercially-available components are based on fiber-optic or micro-optic techniques or PHASAR-based devices. A low-cost wavelength division multiplexer/demultiplexer has been analyzed and demonstrated by using a novel silica waveguide concentrator, an aspheric lens and blazer grating for wavelength separation. Because the rate of core-to-cladding for single-mode fiber, the conventional collimating optics and a reflecting grating cannot be used for single-mode fiber arrays. A silica waveguide concentrator technology is used to match the grating dispersion for dense WDM and fiber size. We constructed a complete, 1 X 8 single-mode WDM using waveguide concentrator techniques. A detailed description, including operation and design, of this device is presented here. Initial test results demonstrated the low cost, low insertion loss and low channel crosstalk of the WDM. These results make these (de)multiplexers very attractive for telecommunications.