We describe novel methods for waveform synthesis and detection relying on longitudinal spectral decomposition of subpicosecond optical pulses. Optical processing is performed in both all-fiber and mixed fiber/free-space systems. Demonstrated applications include ultrafast optical waveform synthesis, microwave spectrum analysis, and high-speed electrical arbitrary waveform generation. The techniques have the potential for time bandwidth products ≥104 due to exclusive reliance on time-domain processing. We introduce the principles of operation and subsequently support these with results from our experimental systems. Both theory and experiments suggest third order dispersion as the principle limitation to large time-bandwidth products. Chirped fiber Bragg gratings offer a route to increasing the number of resolvable spots for use in high speed signal processing applications.
Heavily doped active fibers based on the soft phosphate glass offer an attractive gain medium for compact and high-power laser oscillators. We report a passively modelocked fiber oscillator at 1.5μm based on such active fiber. The standing-wave laser cavity consists of a 20cm-long piece of the side-pumped active phosphate fiber which is heavily co-doped with Er and Yb ions, and a low-ratio fused coupler. The length of the all-fiber laser cavity is 65cm. The modelocked operation of the oscillator is started and sustained by a Semiconductor Saturable Absorber Mirror (SESAM), and no additional pulse narrowing mechanism is used. In order to avoid a premature over-saturation of the SESAM, the fiber end which is butt-coupled to the SESAM is adiabatically tapered which expands the propagating fiber mode and decreases the power density incident on the absorber substantially. The stable modelocked operation of the laser oscillator occurs in the range between 0.65W and 2.3W of the average output power, which is limited by the maximum available pump power at 975nm. The peak pulse power is limited by the saturated SESAM at ~450W, and the pulse width grows from 11psec to 35psec as the pump power is increased. At the pulse repetition rate of 160MHz, the pulse energy reaches 14.4nJ. Our laser oscillator combines the convenience of the all-fiber construction with the power performance previously achievable only with the modelocked bulk-optic laser oscillators or more complex systems involving fiber amplifiers.
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.
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.
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.