Laser system with reduced apparent speckle

Abstract

Laser systems with reduced apparent speckle are provided. The laser systems emit laser light having different mode structures that change within a time period of an integration period of an imaging system used to observe a field of view that is at least in part illuminated by the laser systems.

1 . A laser system comprising:
 a semiconductor laser adapted to emit a beam coherent light when supplied with an electrical current;   a driving circuit adapted to supply a first current to the semiconductor laser and to modulate the current supplied to semiconductor across a range of current levels within a determined integration time;   wherein the current is modulated so that semiconductor laser will emit light having a first transverse mode structure during a first portion of the range of current levels and a second transverse mode structure during a second portion of the range of current levels causing a shift in the position of a speckle pattern during the integration time that reduces the appearance of speckle.   
     
     
         2 . The system of  claim 1 , wherein a change in transverse mode structure takes the form of a change in the number of transverse modes in the laser beam. 
     
     
         3 . The system, of  claim 1 , wherein the change in transverse mode structure comprises a change in the relative portion of the overall intensity of a beam formed by individual ones of more than one simultaneously emitted transverse modes. 
     
     
         4 . The system, of  claim 1 , wherein the laser beam has an angular emission profile that is a function of the transverse mode structure and wherein the direction of higher intensity emissions in the angular emission profile change with the transverse mode structure. 
     
     
         5 . The system of  claim 1 , wherein the laser has a ridge width selected to provide transverse mode structures that are different when energized in at least two different ranges of current in order to provide the shift in transverse mode structure. 
     
     
         6 . The system of  claim 5 , wherein the ridge width is between about 1 and 2 wavelengths of a light emitted as a laser beam by the semiconductor laser. 
     
     
         7 . A method for operating a laser system:
 determining an integration time for an imaging system to be used with the laser system;   supplying a current to a semiconductor laser used in the laser system;   modulating the current supplied to the semiconductor laser across a range of current levels during the determined integration time   wherein the current is modulated so that the semiconductor laser will emit light having a first transverse mode structure during a first portion of the range of current levels and a second transverse mode structure during a second portion of the range of current levels causing a shift in the position of a speckle pattern during the integration time that reduces the appearance of speckle.   
     
     
         8 . The method of  claim 7 , wherein a change in transverse mode structure takes the form of a change in the number of transverse modes in the laser beam. 
     
     
         9 . The method of  claim 7 , wherein the change in transverse mode structure comprises a change in the relative portion of the overall intensity of a beam formed by individual ones of more than one simultaneously emitted transverse modes. 
     
     
         10 . The method of  claim 7 , wherein the laser beam has an angular emission profile that is a function of the transverse mode structure and wherein the direction of higher intensity emissions in the angular emission profile change with the transverse mode structure. 
     
     
         11 . The method of  claim 7 , wherein the semiconductor laser has a ridge width selected to provide transverse mode structures that are different when energized in at least two different ranges of current in order to provide the shift in transverse mode structure. 
     
     
         12 . The system of  claim 11 , wherein the ridge width is between about 1 and 2 wavelengths of the light emitted by the semiconductor laser.