Cartographer num_laser_scans problem!

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My test.lua:

include "map_builder.lua"
include "trajectory_builder.lua"

options = {
  map_builder = MAP_BUILDER,
  trajectory_builder = TRAJECTORY_BUILDER,
  map_frame = "map",
  tracking_frame = "base_link",
  published_frame = "base_link",
  odom_frame = "odom",
  provide_odom_frame = true,
  use_odometry = false,
  use_nav_sat = false,
  use_landmarks = false,
  publish_frame_projected_to_2d = false,
  num_laser_scans = 2,
  num_multi_echo_laser_scans = 0,
  lookup_transform_timeout_sec = 0.2,
  submap_publish_period_sec = 0.3,
  pose_publish_period_sec = 5e-3,
  num_subdivisions_per_laser_scan = 1,
  num_point_clouds = 0,
  lookup_transform_timeout_sec = 0.2,
  submap_publish_period_sec = 0.3,
  pose_publish_period_sec = 5e-3,
  trajectory_publish_period_sec = 30e-3,
  rangefinder_sampling_ratio = 1.,
  odometry_sampling_ratio = 1.,
  fixed_frame_pose_sampling_ratio = 1.,
  imu_sampling_ratio = 1.,
  landmarks_sampling_ratio = 1.,
}

MAP_BUILDER.use_trajectory_builder_2d = true
TRAJECTORY_BUILDER_2D.use_imu_data = false
TRAJECTORY_BUILDER_2D.num_accumulated_range_data = 2
TRAJECTORY_BUILDER_2D.use_online_correlative_scan_matching = true
POSE_GRAPH.optimization_problem.huber_scale = 1e2

return options

my test.launch:

<launch>
  <param name="/use_sim_time" value="false" />

  <node name="cartographer_node" pkg="cartographer_ros"
      type="cartographer_node" args="
          -configuration_directory $(find cartographer_ros)/configuration_files
          -configuration_basename demo_hokuyo.lua"
      output="screen">
    <remap from="scan_1" to="f_scan" />
    <remap from="scan_2" to="b_scan" />
  </node>

 <node name="rviz" pkg="rviz" type="rviz" required="true"
      args="-d $(find cartographer_ros)/configuration_files/demo_hokuyo.rviz" />
</launch>

My num_laser_scans = 2 and I change the topic scan_1 scan_2,but I can't subscribe the laser data.