Semicondutor optical amplifier suitable for coarse WDM communications and light amplification method

Abstract

An active layer contains quantum structures. The active layer amplifies light propagating therein while current is injected therein. Electrodes are provided for sections of the active layer sectionalized along a light propagation direction. The electrodes inject different currents into the sections. Current is supplied to the electrodes in such a manner that a first current density is set to one section of the active layer and a second current density is set to another section. The first current density is lower than that at a cross point and the second current density is higher than that at the cross point. The cross point is a cross point between gain coefficient curves at least two different transition wavelengths of the quantum structures. The curves are drawn in a graph showing a relation between a density of current injected into the active layer and a gain coefficient of the active layer.

1 . A light amplification method comprising the steps of: 
 (a) injecting current into a first region of an active layer containing quantum structures made of at least ones of quantum dots, quantum wires and quantum dashes, at a current density satisfying that a gain coefficient of the quantum structures at the longest transition wavelength of the quantum structures becomes larger than a gain coefficient at the second longest transition wavelength, and injecting current into a second region different from the first region at a current density satisfying that the gain coefficient of the quantum structures at the longest transition wavelength becomes smaller than the gain coefficient at the second longest transition wavelength; and    (b) amplifying a laser beam introduced into the active layer while the current is injected into the active layer.    
     
     
         2 . The light amplification method according to  claim 1 , wherein: 
 in said step (a), injecting current into at least one third region different from the first region at a current density same as the current density injected into the first region, and injecting current into at least one fourth region different from the second region at a current density same as the current density injected into the second region; and    in said step (b), amplifying the laser beam also in the third and fourth regions.