The picosecond-pulse source (psPS) is a multi-stage, high-rate source of picosecond pulses.  It is composed of a gain-switched laser followed by several compression stages.  In its present configuration, it produces 1-3 ps pulses at repetition rates from 1-5 GHz.

Stage 1:  Gain-Switched Laser Diode
Stage 1 of the psPS is a distributed-feedback (DFB) laser diode (LD).  The LD is gain-switched by a combination of direct-current (DC) bias and a high-power, radio-frequency (RF) sine wave.   This gain-switching produces a pulse that is much shorter than the RF period and that has a strong, negative chirp which we use to advantage.

Stage 2:  Linear Pulse Compresser
Stage 2 is a linear compressor which consists of a length of positive-dispersion fiber (PDF).  We use Corning dispersion-compensating fiber (DCF) as our PDF.  The large positive dispersion of the DCF compresses the negatively chirped pulse.  The compressed pulse is nearly transform-limited;  when the correct length of DCF is chosen, a lower bound is placed on the time-bandwidth product (TBP) only by any higher-order chirp imposed by the gain-switching and the higher-order dispersion of the fiber.

Stage 3:  Adiabatic Pulse Compresser
Stage 3 is an adiabatic compressor composed of an erbium-doped fiber amplifier (EDFA) followed by a length of Corning dispersion-decreasing fiber (DDF).  The DDF exhibits negative dispersion (supporting soliton propagation) which decreases in magnitude along its length.  The EDFA ensures that the pulse forms a fundamental soliton (soliton order = 1) as it enters the high-dispersion, input end of the DDF.  Because the soliton experiences decreasing dispersion as it propagates along this fiber, it must temporally shorten to maintain its soliton order.

Stage 4:  Artificial Saturable Absorber
Stage 4 is an artificial saturable absorber (ASA) created by adding a quaterwave (l/4) plate and a polarizer to the end of the DDF.  The ASA reduces the pedestal of the pulse by nonlinear polarization rotation (NPR) and polarization selection .  The high-intensity region of the pulse (the peak) experiences greater polarization rotation in the DDF than the low-power regions (the wings).  Subsequently, the l/4 plate and polarizer select the peak and reject the wings.  Note the following:  (a) DDF is not required for NPR;  any material with the appropriate Kerr nonlinearity is sufficient.  (b)  The ASA is not a pulse compressor;  it reduces the pulse width by rejecting some of the pulse energy. (c)  Since the ASA is lossy, it is sometimes removed when maximum power is more important than minimum width.  (d) The autocorrelation process exaggerates the wings of a pulse, so the actual pedestal is smaller than that shown in an autocorrelation trace.

Stage 5:  Postamplifier
Stage 5 is a postamplifier.  Certain applications require the pulse output from Stage 4 to be amplified before use; an EDFA is therefore added to the subsystem.  Stage 5 can also provide soliton-effect compression.
 

For information, contact Michael Gross at michael.gross@ece.gatech.edu.