An experimental study of the mode-locking process in erbium- doped fiber lasers (EDFLs) operating at 1.55 micrometer using multiple quantum well saturable absorbers is described. The self-starting passively mode-locked laser was constructed in a Fabry-Perot configuration using the saturable absorber as the back reflector of the cavity. Picosecond pulses that ranged from 3.1 to 38.8 ps were generated using a series of saturable absorbers. The pulse widths were dependent upon the optical properties of the saturable absorber used as the mode- locking element as well as the dispersive elements contained within the cavity. The output power of the EDFL varied from 0.2 to 6.7 mW and was also dependent upon the saturable absorber used in the cavity.
This paper presents experimental results of using a diamond shaped semiconductor optical amplifier as the optical gain element in a high power external cavity semiconductor laser. An average output power of 740 mW is demonstrated in continuous wave operation while 400 mW of average power is obtained in both passive and hybrid modelocked operation, with subsequent optical amplification in an identical semiconductor optical amplifier. The modelocked laser operates at a repetition rate of 1.062 GHz. Optical pulses are generated with a temporal duration of 5 psec, which implies a pulse energy of 377 pJ, and a peak power of 60 watts. Further reduction of the optical pulsewidth to 1.3 psec is also achieved by using dispersion compensation techniques. These results show the promise of novel semiconductor optical amplifier devices for use as gain elements in external cavity semiconductor lasers. The generated output pulse characteristics from modelocked operation is sufficient for use in novel 3-dimensional data storage applications, and in large scale commercial printing and marking applications.
High-frequency self-pulsing effects in combined index and gain coupled two-section DFB lasers in a single cavity laser are investigated by numerical simulations. The gain coupling increases the modulation index and frequency of the intensity oscillations. The results show that the signals of oscillation frequencies up to 200 GHz may be possible to generate using such devices.
In this letter, we report a novel multiwavelength picosecond pulse transmitter based on a modelocked semiconductor laser. By actively modelocking a single-grating-loaded external cavity semiconductor laser system, 20 wavelength channels are simultaneously generated with each wavelength transmitting 12 ps pulses at a 600 MHz rate. A conventional pulse interleaving configuration multiplexes the output optical pulse train by eight fold to give a final 5Gbit/sec pulse rate. The resulting geometry provides a very compact configuration to reach an aggregate data throughput of 100Gbit/sec.
Recent developments in nonlinear optical polymer materials and devices combined with epitaxial liftoff (ELO) and grafting of semiconductor materials are leading to dramatic new possibilities in devices for photonic signal processing. For example, the development of new device architectures is leading to electro-optic modulators that have halfwave voltages of approximately 1V. Applications include very large bandwidth (greater than 100 GHz) electro-optic modulators and high speed (less than 1 ns) switches for programmable optical delay lines for use in phased array systems. Also, with the increase in operating frequency and angular scan resolution, the delay length accuracy can reach magnitudes of micrometers for millimeter wave frequencies. With micro fabrication methods, integrated delay line/switch networks can achieve superior delay performance with a single integrated optic chip that is compact, light weight, and has low optical insertion loss. The use of ELO allows electronic device driver circuits to be integrated with the polymer chip to provide further miniaturization. Also, ELO methods can be used to fabricate very high speed metal-semiconductor-metal (MSM) photodetectors for optical signal detection and monitoring. Here ELO methods can find applications in the fabrication of multispectral detectors and focal plane arrays. Yet other applications include very high speed analog-to-digital converters.
Two-dimensional multiple beam steering in the Acoustically STeered and ROtated (ASTRO) true time delay generation architecture is described. The architecture is capable of generating 2D multiple beams without causing any extraneous beams. The system can be used as both transmitter and receiver modes simultaneously.
Tapped delay line is a fundamental building block for designing filters and signal processors, specially the adaptive ones. For an example, true time-delay lines are indispensable for electronically beam steered phased array antenna system with multiple nulling capability. Photonic implementation of the delay line with wide bandwidth approaching 100 GHz uses either variable length or variable group velocity as a function of wavelength. Fiber Bragg reflection grating plays an important role for the first case whereas highly dispersive fiber/waveguide or material are the ingredients for the second case. High dispersion is obtained near a resonance of the transmission or absorption curve of the fiber. The resonance properties can be enhanced by using nanoparticles with complex structures such as metal coated dielectric spheres or using so-called photonic bandgap materials. This paper reviews the status of photonic implementation of the delay lines using Bragg gratings, high dispersion fibers, bandgap engineered materials, among others. Theoretical and experimental results are presented for a high dispersion fiber achieved by resonances in a thin metallic film surrounding the fiber core.
This paper presents a novel architecture for independently steering broadband nulls for phased array antennas. Measurements taken over a ten percent fractional bandwidth on a three-element proof-of-concept system shows a null depth of better than 40 dB uniform across the entire band in this laboratory setup. The architecture presented is best suited for small antenna array applications, for example self-guided airborne munitions. In a fully integrated, optimized system, null depths of 50 - 70 dB or greater across a multi-gigahertz bandwidth are anticipated and the critical factors, which influence this performance, are examined.
A novel coupled well structure for implementing a quantum wire MODFET is proposed and analyzed. A thin barrier layer is placed adjacent to the standard modulation doped heterointerface, resulting in a coupled quantum well region. Varying the distance between the barrier and the interface provides a means of controlling the location and distribution of the two-dimensional electron gas. Further confinement of the carriers to one-dimension is obtained by methods known in the literature, such as mesa etching and regrowth. It has been found that the peak of the electron distribution for the first confined state, as measured from the modulation doped interface, changes dramatically depending on the location of the thin barrier. The peak can be shifted as much as 45 Angstrom for a change in the barrier location of only 20 Angstrom. Implications of this are included in the charge control model, from which the current-voltage (IdVd, Id-Vg) and transconductance (gm-Vg) characteristics are obtained. Additionally, the frequency responses (fT-Vg) of several variations of this device are presented.
A powerful pump in a nonlinear medium can affect a weak probe beam propagating through a medium. In these induced effects there is not an exchange of power or a shifting of wavelengths, as seen in parametric effects, but instead the index of refraction, (Delta) n, brought about by the pump affects the propagation of the probe. The weak probe can be steered, guided, shaped or focused due to the influence of the pump beam. All of these effects fall under the general heading of cross phase modulation. This work presents a primary set of experiments demonstrating a nonlinear optical effect which displays a saturation profile. The nonlinear media is zinc selenide that is cut to be 3 X 3 X 25 mm with all sides polished. The probe is a continuous wave HeNe laser operating at 632.8 nm. The pump is a single mode, linearly polarized beam from an Argon laser with a center wavelength of 500 nm.
A novel bacteriorhodopsin based photonic crossbar system for broadband communications is proposed. This free-space dynamically reconfigurable N X N crossbar switch utilizes an intelligent holographic system for routing and switching by dynamically reconfigurable gratings of bacteriorhodopsin, which has high write/read photocyclicity that is greater than 106. The major advantages of the system include large interconnectivity density, transparent data redistribution, and fiber optic bandwidth capacity. In addition, the switching device resolves optical-to-electronic and electronic-to- optical conversion bottlenecks and reduces signal-to-noise degradation which is due to the conversions. This crossbar design is completely free of internal blocking which is one of the major drawbacks of guided optical crossbars. The system takes advantage of the parallelism and multidimensionality inherent in optics and can be scaled to a large capacity of N X N, while it maintains a low weight and portability which are a projected requirement for future broadband communications.
Soliton propagation in a system with linear and nonlinear amplifiers and spectral filtering is explored. We discuss different types of solutions of the cubic and the quintic complex Ginzburg-Landau equation (CGLE), namely solutions with fixed amplitude and solutions with arbitrary amplitude. The conditions to achieve a stable soliton propagation are analyzed within the domain of validity of the soliton perturbation theory. We obtain also a boundary for the region in the parameter space at which stable pulselike solutions of the quintic CGLE exist. In addition, an expression for the minimum value of the peak amplitude of these solutions is found, which depends uniquely on the quotient between the linear excess gain and the quintic saturating gain term.
We describe an encrypted optical memory that uses double- random phase encoding at the input plane and the Fourier plane. This technique allows the images to be stored as independent white complex stationary processes. Experimental results and computer simulations are presented.
There is a need for devices which will allow integration of photonic/optical computing subsystems into electronic computing architectures. This presentation reviews the nonlinear interface optical switch (NIOS) concept and then describes a new effect, the erasable optical memory (EOM) effect. We evaluate an extension of the NIOS device to allow simultaneous optical/electronic, i.e. dual mode, switching of light utilizing the EOM effect. Specific devices involve the fabrication of thin film tungsten (VI) oxide (WO3) and tungsten (V) oxide (W2O5) on the hypotenuse of glass (BK-7), fused silica (SiO2) and zinc selenide (ZnSe) right angle prisms. Chemical reactions and temporal response tests were performed and are discussed.
Future broadband fiber communication networks for commercial and/or military application will require tens to hundreds of communication channels, each channel transporting multigigabit data resulting in an aggregate throughput upwards of 100 Gb/s. Optical-based systems have the potential for realizing such networks because of the inherent speed of optical signals, and the large optical (30-terahertz) bandwidth that can be exploited for communication. Optical signal multiplexing in the spectral and temporal domains provides means of interconnecting and transporting large user data through a network. One approach to achieving multigigabit data transport via time division multiplexed requires the development of femtosecond mode-locked lasers or soliton lasers of high repetition rates. Such lasers are bulky making them impractical for systems applications. Furthermore their performance can be severely affected by optical nonlinearities. WDM offers another approach to multigigabit data transport. Typically WDM systems utilize a separate single frequency laser for each channel. Stabilizing and controlling each individual wavelength of a high density WDM system is difficult and costly. Furthermore cross-talk and cross-phase modulation limit the channel capacity in WDM systems. In this paper we propose a subcarrier multiplexing scheme as an alternative to multigigabit fiber optics data communication. In this approach, a comb of optical RF subcarriers are generated in a matched-pair of optical traveling-wave amplifiers (TWA). Each subcarrier serves as a transmission channel. The most important aspect of this approach is that optical processing of the subcarriers are done via the main optical carrier. Thus amplification of the main carrier translates into a direct amplification of the subcarriers. We present theoretical framework and simulation results for optical RF subcarrier generation.
An active double heterostructure diode structure is employed as a nonlinear medium to demonstrate soliton waveguiding effects. It was observed that the nonlinearities due to reverse bandfilling in active semiconductor amplifiers give rise to a spectral region where self-focusing takes place for photon energies corresponding to the peak of the gain. By monitoring the mode-profile at the output of the slab waveguide as a function of wavelength, a district narrowing of the output beam lateral dimension was observed and the beam profile appeared to stay stable for a range of input intensities. The slab waveguide was 650 micrometer long and each of the contact pads for electrical carrier injection was 60 micrometer wide. The experiments showed that the lateral dimension of the near field profile output beam changed from a FWHM width of 32 micrometer to 5.5 micrometer as the wavelength of the laser was tuned into the optimum range for self-focusing nonlinearities. This corresponds to a peak nonlinear coefficient of n2 equals 2.8 X 10-10 cm2/watt.
A new high throughput imaging spectrometer is described that utilizes a digitally switched polarizing interferometer to sample the complex autocorrelation of the input spectrum. Using a single filter stage and high speed digital liquid crystal switches, the spectrometer can provide rapid measurements of power spectra using a simple, easily driven, light efficient structure. The spectrometer is well suited to visible/near infrared operation, due to its use of path-length differences in crystals, rather than free space path length differences. The spectrometer provides accurate and stable steps in path length difference from zero to hundreds of waves.
Optical Networks Inc. has developed and demonstrated a set of space-flight worthy, 12-channel fiber optic transmitter and receiver modules capable of providing data bandwidths up to 1 Gbps. This is accomplished using radiation tolerant commercial off-the-shelf (COTS) optical and electronic components. The parallel spaceborne fiber optic data bus (SFODB) implementation uses ten out of the available twelve fiber optic channels in the parallel fiber optic transmitter (PFOTX) and the parallel fiber optic receiver (PFORX) to implement a byte wide ring bus. The two spare fibers are used to implement a 2 by 10 redundancy configuration. The components were developed under a NASA/GSFC Small Business Innovation Research (SBIR) program.
Most of region-based image segmentation approaches suffered from the problem of different thresholds selecting for different images. In this paper, an new adaptive image segmentation approach based on an encoder-segmented neural network (ESNN) is presented. The novel ESNN combines the advantages of self-organizing feature map (SOFM) and fuzzy c- means clustering (FCM) algorithm. Feature encoder implemented by SOFM for vector quantization using the competitive learning where the feature vectors can be encoded as the definite sequence by which the most of the available feature vectors can be extracted for the final segmentation using encoded feature-based fuzzy c-means (EFFCM) algorithm. Since the contribution of feature encoder, ESNN can reduce the complexion of computation when processing a large number of multi-spectral images. ESNN have been applied for brain MRI segmentation. Comparing with FCM algorithm, experimental results have shown ESNN method for segmentation makes better performance on computation and adaptability.
Future Air Force RF links will require the speed, cost, reliability, weight, and EMI-immunity advantages of a monolithically integrated optical-electronic technology. GaAsN is a direct band-gap compound semiconductor material which potentially can be lattice-matched to silicon. This would permit true monolithically integrated optical-electronic circuits to be fabricated. To date, films of GaAsN grown by MBE under most conditions exhibit segregation into coexisting GaAsN, GaN and GaAs phases. The composition, microstructure, and signatures of these films in X-ray diffraction, Auger, and TEM analyses are reported. The possible ordering of these films is analyzed.