Kazutaka Oba

Femtosecond single-shot correlation system: a time-domain approach

We introduce, analyze, and experimentally demonstrate what to the best of our knowledge is a new pulse correlation technique 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 (LiB(3)O(5)), thus producing a second-harmonic field that carries the spatial image of the temporal shaped signal pulse. We show that the time scaling of the system is set by the magnification of the anamorphic imaging system as well as by the grating frequency and that the time window of the system is set by the size of the grating aperture. Our experimental results show a time window of approximately 20 ps. We also show that the chirp information of the shaped pulse can be recovered by measurement of the spectrum of the resulting second-harmonic field.

Nonvolatile photorefractive spectral holography

We demonstrate nonvolatile storage of femtosecond pulses in a photorefractive LiNbO(3) crystal with recording and readout of spectral holograms at wavelengths of 460 and 920 nm, respectively. No degradation was observed after 24 h of continuous readout. We also show that we can realize the time-lens effect with our system by appropriately setting the ratio of the recording and the reconstruction wavelengths and the spectral resolution of the recording and the reconstruction processes.

Space-time processing with photorefractive volume holography

Photorefractive volume holography for processing ultrashort optical pulses carrying spatial, temporal, and spatiotemporal optical information is introduced. These new holographic methods can process temporal information, i.e., the temporal evolution of optical pulse signals, in addition to the usual spatial information. Photorefractive volume holographic materials provide the medium necessary for recording and reconstruction of such optical information in real time. Spatial and temporal holography with photorefractive volume holographic materials are introduced, compared, and discussed. The direct time-domain holography is shown to possess two disadvantages, low fringe contrast and limited recording time, which is overcome by using the method of spectral domain holography. Applications of direct time-domain and spectral-domain holography for image processing, temporal matched filtering, optical pulse shaping, three-dimensional (3-D) optical storage, and optical interconnects are discussed. Furthermore, the combined space-time holographic processing that allows the conversion between the spatial and the temporal optical information carrying channels is introduced. This method is used to demonstrate experimentally parallel-to-serial and serial-to-parallel data conversion for one-dimensional (1-D) images and image-format data transmission. The demonstrated holographic processors provide 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.

Nonlinear femtosecond information processing systems

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.

Femtosecond optics for optical data storage and detection

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.

Nonlinear Spatio-Temporal Information Processing with Femtosecond Laser Pulses

Information processing with femtosecond laser pulses for optical communications and storage applications will be described. Specifically, results on femtosecond pulse imaging using nonlinear optical wave mixing as well as femtosecond pulse storage in photorefractive materials will be presented.

Optical conversion between time and space domain parallelism

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 SDWs 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.

Nonlinear spatio-temporal processing

To meet the speed requirements of ultra-high bandwidth optical communications, devices need to be operated in real time, i.e., as fast as the time window of the time-multiplexed pulse packet. Nonlinear optical processes such as nondegenerate wave mixing can achieve such real-time operation. We describe our real-time spatio-temporal processing techniques by wave mixing; time-to-space conversion using three-wave mixing in a second-order nonlinear crystal and space-to-time conversion by a four-wave mixing arrangement employing cascaded second-order nonlinearities (CSN) for enhanced conversion efficiency. The ultrafast waveform imager performs serial-to-parallel demultiplexing of the shaped pulse train into parallel spatial channels for electronic detection. Our pulse image converter (PIC) system is capable of real-time conversion of a femtosecond pulse sequence into its spatial images. The approach employs spectral domain nonlinear 3-wave mixing in a LiB/sub 3/O/sub 5/ (LBO) crystal, where the spectral decomposition waves (SDW) of a shaped femtosecond pulse are mixed with those of a transform limited pulse to generate a quasi-monochromatic second harmonic field.

Nonlinear space-time information processing

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.

Single-shot femtosecond/picosecond-range autocorrelator using tilted pulse front

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.