We demonstrate 200-fs switching and 1.5-Tb/s demultiplexing with a polarization-discriminating, symmetric Mach-Zehnder (PD-SMZ) all-optical switch. This switch is based on a highly efficient but slowly relaxing band-filling effect resonantly excited in a passive InGaAsP bulk waveguide. By using a mechanism that cancels out the effect of the slow relaxation, ultrafast switching is realized. The applicability of this mechanism to optical demultiplexing of well over 1 Tb/s is experimentally demonstrated for the first time. High-repetition operation at 10 GHz with the same nonlinearity used for the present ultrafast switching is also verified.

Ultrafast (200-fs switching, 1.5-Tb/s demultiplexing) and high-repetition (10 GHz) operations of a polarization-discriminating symmetric Mach-Zehnder all-optical switch

We propose and demonstrate a novel scheme to reduce the recovery time of optical nonlinearity in a semiconductor laser optical amplifier driven under a loss condition for the signal light pulse. Additional light, which is set at a transparency wavelength in the active layer, promotes stimulated recombination of excess carriers induced by the absorption of the signal light. This scheme excludes any additional carrier transport mechanism and nonradiative recombination and hence generation of heat. The principle of operation is experimentally verified by measuring time-domain transmission variance using a pump–probe method. A drastic reduction of the excess-carrier lifetime to less than 70 ps was confirmed.

Mechanism for reducing recovery time of optical nonlinearity in semiconductor laser amplifier

We present Symmetric Mach–Zehnder (SMZ) type all-optical switches: an SMZ all-optical switch, a polarization-discriminating SMZ (PD-SMZ) all-optical switch, and a delayed interference signal wavelength converter (DISC). These switches are capable of ultrafast, low control power, and low chirp switching, which is not restricted by slow relaxation of highly efficient nonlinearities. High repetition operation unrestricted by slow relaxation is also possible for these switches. This is because of a push–pull modulation scheme or sometimes called a differential phase modulation scheme. These three devices are similar, but different in some important aspects, thus a comparison is made among the three. Then semiconductor optical amplifiers (SOAs) are discussed as a nonlinear phase shifter for these devices. Then, ultrafast all-optical signal processing using SOA based SMZs is demonstrated. Error-free demultiplexing from 168 to 10.5 Gbit/s is presented, in which a hybrid-integrated SMZ (HI-SMZ) is used as a demultiplexer. In pulse regeneration experiment, the signal pulses at 84 Gbit/s are regenerated by the PD-SMZ and the regenerated pulses are demultiplexed to 10.5 Gbit/s by the HI-SMZ to verify error-free operation. The retiming capability of this scheme is quantitatively demonstrated. Also presented is error-free all-optical wavelength conversion at 168 Gbit/s using the DISC. These results represent the fastest error-free operations reported to date in each category.

Ultrafast all-optical signal processing with Symmetric Mach–Zehnder type all-optical switches

Different optical signal regeneration techniques are reviewed. Fibre or semiconductor based devices are addressed and compared, and 2R or 3R schemes are discussed. We briefly review 2R (reshaping repeater) and 3R (retiming reshaping repeater) optical regeneration schemes, based on different technologies which could be promising for high speed optical signal processing, and also try to compare these new optical approaches with well known optoelectronic schemes.

All-optical regeneration

We propose a novel all-optical gate switch based on a frequency shift accompanied by the semiconductor band-filling effect. This all-optical switch is very simple in structure, yet is capable of ultra-high-speed and high-efficiency switching. The switching characteristics are numerically investigated with a rate equation model for nonlinear phase shift. Then, the principle of operation is experimentally verified by measuring the spectral and time-domain properties of the switch. Its switching time is nearly the same as the duration (600 fs in the present experiment) of a control pulse.

Ultrafast all-optical gate switch based on frequency shift accompanied by semiconductor band-filling effect

The high‐repetition switching capabilities of a recently developed symmetric Mach–Zehnder (SMZ) all‐optical switch are experimentally investigated. Using a series of four control pulses at intervals of 100 or 25 ps, nearly full switching is achieved at the corresponding intervals. The results experimentally show that the repetition rate, as well as the switching speed, of the SMZ all‐optical switch is not restricted by the slow relaxation of the utilized high‐efficiency band‐filling effect.

S. Nakamura

Papers

High‐repetition operation of a symmetric Mach–Zehnder all‐optical switch