Optical coherence tomography (OCT) is the novel method for producing three-dimensional images of the relatively thin (up to 2 mm) layers of the biological tissues. OCT is based on the interferometry of low-coherence light. The important factors, characterizing OCT, are the measurement speed and the quality of the obtained images. Both of these two factors actually characterize the amount of new information generated during the measurement with OCT. Using the Shannon's information theory; the quantitative characteristics of the information efficiency of OCT are introduced. Information capacities of OCT realizations based on scanning low-coherence interferometry and on spectral interferometry are defined and compared. It is shown that, image quality being the same, information capacity of spectral interferometry exceeds that of scanning low-coherence interferometry by a factor equal to the number of resolvable elements in a single in-depth scan. The speed of measurement and the image quality characterizing two methods are also compared.
We describe several concepts for real time shaping and detection of femtosecond laser pulses using optical nonlinearities. Cascaded second order wave mixing is used for real-time conversion of spatial-domain images to ultrafast time-domain optical waveforms. We experimentally demonstrate a cascaded nonlinearity arrangement allowing generation of complex amplitude femtosecond waveforms with high fidelity and good conversion efficiency. Single-shot, phase-sensitive detection of femtosecond pulses is demonstrated using both nonlinear wave-mixing and 2-photon absorption in semiconductor detector arrays. Using commercial silicon charge-coupled device (CCD), the latter approach allows detection of broadband ultrashort signals in the important wavelength range around 1.5 microns without phase-matching limitations. Finally we describe an approach to characterization of the multimode fiber using ultrashort pulse interferometry.
We describe various optical techniques for processing and detection of femtosecond laser pulses. Photorefractive and cascaded second order nonlinear wave mixing are used for space-to-time conversion, transforming space domain information into ultrafast temporal waveforms. An inverse operation that transforms a femtosecond pulse sequence into a quasi-stationary spatial image is performed with spectral domain three wave-mixing. We also demonstrate single-shot phase sensitive femtosecond pulse detection with two-photon absorption in a conventional silicon detector array. This approach allows efficient detection of wide-bandwidth ultrafast signals in the wavelength range of 1-2 μm.
We demonstrate several nonlinear optical techniques that allow spatial-temporal processing of femtosecond laser pulses. Photorefractive and cascaded second order nonlinear wave mixing is used to convert space domain information into ultrafast temporal waveforms. Spectral domain three wave mixing allows time imaging of femtosecond signals as well as characterization of the signal complex amplitude. Femtosecond pulse interferometry is applied for spatial and temporal characterization of the multimode optical fiber.
Nonlinear optical processing techniques that produce space-time information processing are introduced and experimentally demonstrated. The basic concept of such space-time processors closely resembles conventional Fourier optical processors of the space domain. By using ultrafast short pulses and nonlinear optics, we can perform not only real-time optical information conversion between the space and time domains, but also the processing and imaging of temporal information.
Nonlinear optical processing techniques that produce space- time information processing are introduced and experimentally demonstrated. The basic concept of such space-time processors closely resembles conventional Fourier optical processors of the space domain. By using ultrafast short pulses and nonlinear optics, we can perform not only real-time optical information conversion between the space and time domains, but also the processing and imaging of temporal information.
In this paper, we show two optical storage and retrieval techniques: a technique to record/readout data in serial format with real time detection, and an orthogonal-code multiplexed recording/readout system with nonlinear gated detection. Both of these techniques are based on femtosecond optical short pulses. In the former storage and detection technique, a train of pulses is recorded via spectral holography into a photorefractive crystal at wavelength 460 nm and the recorded hologram is read at the wavelength 920 nm, allowing nonvolatile readout of information from the photorefractive crystal. For detection and demultiplexing of a femtosecond pulse sequence whose time duration is much longer than the pulse width, a new pulse correlation technique is developed that is capable of real-time conversion of a femtosecond pulse sequence into its spatial image. Our technique uses a grating at the entrance of the system, thus introducing a transverse time delay (TTD) into the transform-limited reference pulse. The shaped signal pulses and the TTD reference pulse are mixed in a nonlinear optical crystal, producing a second-harmonic field that carries the spatial image of the temporal shaped signal pulse. In the orthogonal-code multiplexed recording technique with spectral holography, a signal pulse that contains a 1-D spatial information is recorded with a unique spectral phase-coded reference pulse, and multiplexing is performed by orthogonal phase-coding of reference pulses. Information readout is performed employing a nonlinear time- grating technique with the use of wave mixing in nonlinear optical crystals. We present the basic principles and experimental results for those femtosecond optics systems.
The existing mismatch between the bandwidth capacity of optical fiber and electronic devices, can be used to increase the speed, provide security and reliability in the transmission and distribution of information. To implement these applications, all-optical multiplexer performing space-to-time (i.e., parallel-to-serial) transformation at the transmitter and demultiplexer performing time-to-space (i.e., serial-to-parallel) transformation at the receiver will need to be constructed. For efficient bandwidth utilization, these processors need to be operated at rates determined by the bandwidth of the optical pulses. Ultrashort pulse laser technology has recently experienced significant advances, producing high peak power waveforms of optical radiation in the femtosecond duration range. These ultrafast waveforms can be synthesized and processed in the temporal frequency domain by spatially dispersing the frequency components in a spectral processing device (SPD) and performing operations on the spectrally decomposed wave (SDW). Space-to-time multiplexing via waveform synthesis using SDW filtering has been demonstrated with prefabricated masks, spatial light modulators and holograms. These filters are limited in their adaptability rate -a new filter can be implemented only as fast as the modulator response time or recording time ofa new hologram - typicallywell over a microsecond. To fulfill our goal of real-time SDW processing, we utilize a nonlinear wave mixing process based on four-wave mixing via cascaded second-order nonlinearities (CSN) in a 2)medium performed inside the SPD. The CSN arrangement consists of a frequency-up conversion process followed by a frequency-down conversion process satisfying the type-Il non-collinear phase matching condition. Our experiments are concerned with ultrafast information exchange between spatially parallel signals and higher bandwidth temporal signals. For the waveform synthesis experiment, we introduce two spatial information modulated waves carried by quasi-monochromatic light and a SDW of a ultrashort femtosecond pulse. The four wave mixing process produces a SDW that is a product of three waveforms: a spatial Fourier Transform (FT) of the two spatial information carrying waves and the SDW (i.e., temporal FT) of a femtosecond laser pulse. The spatial-temporal information exchange (i.e., the generated SDW) results in a synthesized waveform that is a time-scaled version of the spatial image, performed on a single shot basis with femtosecond-rate response time due to the fast nonlinearity. The inverse time-to-space transformation for detection of femtosecond pulse sequences is achieved using nonlinear three-wave mixing in a crystal. The two input waves are the SDW of a sequence of ultrashort pulses that need to be detected and a reference pulse. The nonlinear interaction between the two SDW's results in generating a quasimonochromatic second harmonic wave. The frequency ofthe second harmonic fields is twice the center frequency ofthe incident fields. The generated second harmonic fields contain spatial frequencies determined by the time delay between the reference pulse and the pulses in the signal. Thus a 1-D spatial FT of the second harmonic field produces a l-D spatial image equivalent to the temporal cross-correlation between the reference and the signal pulses. With short pulses, the spatial image has one-to-one correspondence with the signal pulse, implementing the desired time-to-space demultiplexing at femtosecond rates.
We report a particular implementation of a quantum cryptographic device operating at 1540 nm wavelength and involving interference between phase-modulated sidebands produced by a pair of phase modulators in the transmitting and receiving modules. The principle of operation is described in terms of both classical and quantum optics. The method has been demonstrated experimentally using a strong- attenuated semiconductor laser diode. Single photon interference has been obtained with a fringe visibility greater than 90%, including that the system can be used for quantum key distribution.
KEYWORDS: Data processing, Optical signal processing, Data conversion, Telecommunications, Femtosecond phenomena, Laser applications, Nonlinear dynamics, Signal processing, Computing systems
Optical information processing, traditionally employed in the spatial domain, has been experiencing a renaissance with femtosecond laser pulse technology. Temporal optical information can now be manipulated via linear and nonlinear processes, and stored and retrieved, by converting optical signals between the spatial and temporal domains. In this manuscript, we review the state-of-the-art in the spatio-temporal optical signal processing techniques for information data coding, data conversion, signal recording, as well as signal characterization. Applications of these techniques for future computing, communication, storage, and signal processing systems are discussed.
We demonstrate a novel single shot autocorrelation technique for characterization of ultrashort pulses. Unlike existing single shot autocorrelation techniques, our new technique is capable of characterizing optical pulses over a femtosecond to picosecond pulse-width range. Our technique uses a grating at the entrance of the system, introducing a Transverse-Time- Delay (TTD) into the reference pulse. The pulse front in the resulting field is decoupled from the wave front. The signal pulse to be characterized and the TTD reference pulse are mixed in a nonlinear optical crystal, producing a second harmonic field whose transverse spatial extent is proportional to the signal pulse width. Since our technique allows for decoupling of the time delay from the propagation direction (unlike the commercial single shot autocorrelators), we can select the angle between the intersecting pulses to satisfy the phase matching conditions, achieving best efficiency while setting the resolution independently in the orthogonal direction. In addition, by controlling the slope of the TTD, the system can adapt to a wide range of input pulse widths. In this paper we will present the basic principles as well as experimental results for this new autocorrelation technique.
Optical signal processing, traditionally employed in the spatial domain, has been experiencing a renaissance with femtosecond laser pulse technology. Temporal optical information can now be manipulated via linear and nonlinear processes, and stored and retrieved, by converting optical signals between the spatial and temporal domains. In this manuscript, we review the state-of-the-art in the spatio- temporal optical signal processing techniques for information data coding, data conversion, signal recording, as well as signal characterization. Applications of these techniques for future computing, communication, storage, and signal processing systems are discussed.
Utilization of ultrahigh bandwidth available in optical fiber networks will require development of fast and efficient parallel-to-serial and serial-to-parallel all- optical multiplexing techniques. Such multiplexers are also useful for interfacing to optical storage devices. In this presentation we will review the application of space/time optical processing with femtosecond laser pulse to implement such multiplexers. We will focus on a novel real time optical space-time processor based on 3-wave mixing in a nonlinear optical crystal. This processor allows conversion of temporal signal sequence to a 1D spatial image, thereby realizing a serial-to-parallel multiplexer. The processor is also used to generate a wigner distribution function, which allows to determine both amplitude and phase of ultrashort temporal signals.
A realistic quantum cryptographic system must function in the presence of noise and channel loss inevitable in any practical transmission. We examine the effects of these channel limitations on the security and throughput of a class of quantum cryptographic protocols known as four-state, or BB84. Provable unconditional security against eavesdropping, which is the principal feature of quantum cryptography, can be achieved despite minor channel defects, albeit at a reduced transmission throughput. We present a semi-empirical relation between the fully-secure throughput and the loss and noise levels in the channel. According to this relation, a particular implementation of BB84, based on the frequency-division multiplexing scheme and utilizing commercially available detectors, can reach throughputs as high as 104 - 105 secure bits per second over a practical channel of reasonable quality.
KEYWORDS: Visualization, Diagnostics, Switching, Radiology, Switches, Data archive systems, Radio optics, Sensors, Medical imaging, Magnetic resonance imaging
The next generation of distributed imaging and visualization environments for diagnostic radiography and C4I will require the delivery of a guaranteed quality-of-service by a ultra-high bit rate network. Two aspects of the quality-of-service, the link bit rate and the round-trip packet latency, can be met through the use of transparent third-generation photonic networks. These networks can be implemented using ultra-short optical pulses in conjunction with spectral-domain processing to construct links. These links are combined with transparent photonic packet switches to form the network switching fabric. The quality-of-service is guaranteed by using virtual circuit-switching.
Photorefractive volume holography for processing ultrashort optical pulses carrying spatial, temporal, and spatio-temporal optical information is introduced. These new holographic methods can process 4-dimensional information that in addition to the 3 spatial coordinates also include the temporal evolution of optical signals on nanosecond to femtosecond scale. Photorefractive volume holographic materials provide the medium necessary for recording and reconstruction in real-time. Applications of direct time domain and spectral domain holography for image processing, temporal matched filtering, optical pulse shaping, 3-D optical storage, and optical interconnects are discussed. Furthermore, the combined space-time holographic processing that allows the conversion between spatial and temporal optical information carrying channels is introduced. This method is used to demonstrate experimentally parallel-to-serial and serial-to-parallel data conversion for 1-D images and image-format data transmission. This holographic processor provides the advantages of self- referenced signal transmission and self-compensation for optical dispersion induced by the holographic materials, communication channel, as well as other optical components. Finally, future research directions for optical information processing with complex spatio-temporal signals are identified and discussed.
The novel method for the imaging of highly scattering objects is proposed. The method is based on the measurements of two-frequency correlations of scattered radiation.
The possibilities are proposed of imaging of highly scattering objects based on the measurements of correlation characteristics of the spectrum of scattered radiation.
Recording and reconstruction of pico- and nanosecond pulses were accomplished with spectral holography. The developing of spectral holography in nanosecond time-domain was provided while using high-spectral-resolution equipment based on Fabry-Perot etalon with the side entrance/exit. The recognition of shaped nanosecond light pulses was also realized.
The results of the holographic recording and reconstruction of the time-domain profile of femtosecond laser pulses in the polymer volume medium named Reoxan are presented. The experimental data show the possibility of the effective spectral filtering to control the shape of ultrashort pulses.
The spectral interferometric method of the objects longitudinal spatial structure observation is developed. It is based on spectral distribution correlation analysis of the radiation scattered by diffusely scattering objects. For this reason the developed method is called a spectral vision.
Half-tone photography and computer-generated holography are good examples of binary encoding schemes for synthesizing pictures. The aim of this paper is to present two encoding techniques for synthesizing 1-D complex amplitude distributions, by the use of 2-D binary patterns and an optical processor. To this aim, in section 2, we discuss the basic theory. Then, in section 3, we developed the two binary enconding techniques, which are similar to the computer-generated holograms, of type I and II, presented by Brown and Lohmannl.
In order to calculate the characteristics of vibrational quantum beats we use the Heisenberg (time-domain) approach to the nonlinear properties of vibronic spectra. While using this approach, the intensity of emission may be expressed as the convolution-type integral that contains the nonlinear-response function. To evaluate this integral the Laplace method is applied. In so doing the value of the Stokes shift is used as a natural great parameter. This allowed the simple expressions describing the manifestations of vibrational quantum beats to be obtained.
The principles of holographic storage and the reconstruction of short light pulses based on spatial spectral decomposition of radiation are described. This method may be designated as time-domain Fourier transform holography or spectral holography. Also described are various transformations of optical time signals based on holographic spectral filtering and dynamic interaction of spectral decomposition waves in nonlinear media. A system of methods of time signal processing based on spectral holography is proposed. Among these methods are shaping optical pulses, space-time conversions of signals, matched filtering, and recognition oftime optical signals. In addition, the possibilities for realizing time-division multiplexing of data streams using dynamic spectral holography are shown.
Recently experiments have been conducted in which the femtosecond oscillations of light absorption by dye
molecules were observed. These oscillations were induced when the molecules were excited by femtosecond light
pulses. It was shown that the oscillation frequencies correspond well to those of molecular vibrations. For this
reason the oscillations observed were explained as the result of quantum beats of vibrational molecular states.1'2
As is already known the quantum beats can occur when several, at least two closely spaced quantum levels
are excited simultaneously. Such excitation should be performed by a pulse which is shorter than the period of
oscillations corresponding to the energetic distance between two levels. Then interference of the stationary wave
functions occurs. If the probabilities of optical transitions from the levels considered to some third level are different
the the interference of stationary states leads to the modulation of absorption or emission intensity.
The vibrational quantum beats can exist when the coherent superposition of several stationary vibrational states
is created. This actually means the oscillation in phase of a number of molecules, which can be observed in the
absorption or emission of light.
In this paper we should like to draw attention to the essential significance of whether the observation of vibrational
quantum beats is carried out with spectral selectivity or without it.
Holographic storage and reconstruction of nonstationary waves can be accomplished in spectrally nonselective media through the use of a three-dimensional recording medium, as well as by the two- and three-dimensional holography of waves that are produced via spectral decomposition of light pulses. Attention is given to the superfast scanning of light beams and the possibility of picosec-femtosec chronophotography. It is shown that the interactions of spectrally resolved ultrashort wave packets with monochromatic waves allows the formation of holographic analogs of moving optical elements, while using neither direct nor indirect macroscopic shifts of optical hardware.
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