Electrical Engineering
      and Computer Sciences

Electrical Engineering and Computer Sciences


UC Berkeley


2008 Research Summary

THz-Bandwidth Fast Light in Semiconductor Optical Amplifiers

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Forrest Grant Sedgwick, Bala Pesala, Alexander Uskov and Constance Chang-Hasnain

The ability to delay or advance optical pulses enables key functionalities like packet synchronization and buffering in future all-optical packet switched networks [1]. Slow and fast light techniques realize large time shifts in compact devices. Current research aims to increase two performance metrics: the bandwidth and the delay or advance normalized to the input pulse width (also called delay-bandwidth product, DBP, or advance-bandwidth product, ABP). Our work [2] has focused on the use of the ultrafast intraband effects in semiconductor optical amplifiers (SOAs) known as spectral hole burning and carrier heating to produce ABP of 2.5 for 700 fs pulses at a 1.55 µm wavelength and at room temperature. This is a record high ABP and bandwidth for semiconductor-based slow or fast light devices. The advance of the pulse is controlled by the SOA bias current. More recently, we have demonstrated an increase of ABP to more than 3 by a novel technique we call time-wavelength division multiplexing (TWDM). A fast 700 fs pulse is chirped out to 2.68 ps, such that different wavelength components of the pulse enter the SOA at different times. The output pulse is then recompressed and ABP increases by a factor of 170%.

Figure 1
Figure 1: Experimental setup to measure advance and ABP as a function of SOA bias current. A mode-locked fiber laser produces nearly transform-limited pulses of variable with duration ranging from 700 fs to 2.8 ps. Temporal shift of the pulses is measured via cross correlation. Grating-based chirpers at the SOA input and output can be switched into and out of the optical path. Grating-based chirpers preserve the pulse bandwidth and stretch or compress the pulse in time. The input grating delays red components while the output grating is opposite.

Figure 2
Figure 2: Normalized cross-correlation traces as SOA bias is swept from near transparency (dotted) to high gain (solid). Figure 2a shows unchirped 700 fs input pulse and Figure 2b shows the TWDM pulse. Between Figure 2a and Figure 2b the ABP increases by a factor of 170%.

C. J. Chang-Hasnain, P. C. Ku, J. Kim, and S. L. Chuang, "Variable Optical Buffer Using Slow Light in Semiconductor Nanostructures," Proc. IEEE 9, 2003, pp. 1884-1897.
F. G. Sedgwick, B. Pesala, J.-Y. Lin, W.-S. Ko, X. Zhao, and C. J. Chang-Hasnain, "THz-Bandwidth Tunable Slow Light in Semiconductor Optical Amplifiers," Opt. Express 15, 2007, pp. 747-753.