Ultrafast pulses contain a wide bandwidth of several THz for a typical 100 femtosecond pulse duration. The successful use of this bandwidth is desirable for high throughput of coded data in fiber optic communications channels. The most common methods to achieve this include wavelength-division multiplexing (WDM), time-division multiplexing (TDM), and code-division multiple-access (CDMA). Unfortunately, no modulator is available that can operate at several THz. By analogy with function generators in electronics, the ability to arbitrarily manipulate ultrafast (femtosecond) pulses all optically is of great interest and practical importance in optical communications, for example for encoding and decoding in a CDMA system. One technique to shape the femtosecond pulses is to use a Fourier-domain pulse shaper to manipulate the pulse in the frequency domain to obtain the desired pulse characteristics, in which the optical frequency components of a femtosecond pulse are manipulated by an amplitude or phase mask in the Fourier domain. Placement of a dynamic holographic material in the Fourier plane enables higher functionality and nonlinear operations, such as reversal of a pulse in time using femtosecond spectral holography.
In our labs, we have demonstrated the potential use of photorefractive quantum wells (PRQWs)
as dynamic holographic media positioned in the Fourier plane of
a pulse shaper to realize real-time control over the femtosecond
pulses. PRQWs are the most sensitive nonlinear optical materials
that can give refractive index changes of several percent at an
intensity level of just 10 microwatts per squared centimeter.
We demonstrated all optical temporal pattern generation, encoding
and decoding, data conversions between the time- and the space-domain,
dispersion compensation, and temporal pattern processing such
as edge enhancement, time reversal, and correlation, using four-wave
mixing geometry in PRQWs.
