|
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.
|
