This needs to be done when you create the Box2DLight's Light object instead of inside the Box2D code you posted.

You can fix this by setting the [setSoftnessLength(float)][1] on the Light object.

For example:

 m_Light = new PointLight(...);
 m_Light.setSoftnessLength(1.5f);


 [1]: https://github.com/libgdx/box2dlights/blob/master/src/box2dLight/Light.java#L298