How can I simulate a line laser?

edit

Well, the laser plane from the projected line you see in Gazebo when using the laser scanner plugin is not visible by cameras.

I am not sure if this will help the problem you described but it may help other people looking for a solution based on the title you gave. The best way I've found out to simulate a laser in Gazebo was to use the projector plugin. It basically projects any image you want, so if you want a laser line just draw a line and make sure the background of the rest of the image is transparent before using it. Below is a snippet you can use:

<gazebo reference="your_link">
    <projector name="projector_wg6802418">
        <pose>0 0 0 0 0 ${pi/2}</pose>
        <texture>your_img.png</texture>
        <fov>0.5</fov>
        <near_clip>1</near_clip>
        <far_clip>10</far_clip>
     </projector>
</gazebo>
<gazebo>
     <plugin name="projector" filename="libgazebo_ros_projector.so">
         <projector>projector_wg6802418</projector>
         <alwaysOn>true</alwaysOn>
         <updateRate>10.0</updateRate>
         <!-- Apparently the 2 lines below does not affect anything -->
         <textureName></textureName>
         <filterTextureName></filterTextureName>
         <!-- end -->
         <textureTopicName>/projector/image</textureTopicName>
         <projectorTopicName>/projector/projector</projectorTopicName>
     </plugin>
</gazebo>

One disadvantage of this plugin is that the projection trespasses any object (just as occurs with all lights in Gazebo, except for the directional one), so if you have multiple objects one in front of another, the projector will project the image in all of them.

Keep in mind that (and this is another common questioned problem) the plugin will not find your image if it is not in the gazebo media path, even if it the image is on the same folder of your urdf or you try to put the full image path. In this way, add the following lines to the package.xml of your catkin package:

<exec_depend>gazebo_ros</exec_depend>
<export>                            
    <gazebo_ros gazebo_media_path="path_to_your_image"/>
</export>

And just for the sake of documentation, if the plugin cannot find your image, it will show a black image with a diagonal line in yellow, so that you know what happened.