1. Field of the Invention
The invention relates to a motion monitor based on detection of modal noise produced by minute motions in a single- or multimode-optical fiber illuminated by a coherent or partially coherent light source, and more particularly to such a motion monitor for detecting body motions due to breaths, heartbeats, and determining breathing rate and heart rate.
2. Discussion of Background
A significant cause of death in infants (birth to about 2 years) is "Crib Death" or Sudden Infant Death Syndrome (SIDS). Medical authorities generally agree that some infants simply stop breathing during sleep (apnea) or their heart rates fall dangerously low (bradycardia) and that death from these conditions can be prevented if the condition is detected and help is provided within a short time (one or two minutes) by trained personnel or parents.
Apnea monitors already exist, but their cost creates an affordability problem that limits their use. Also, the existing monitors tend to be unreliable, typically having high false positive rates. Also, existing monitors are obtrusive and difficult to use because the subject must wear a belt or other device connected to the monitor. This further limits their use.
Of interest in the patent literature are U.S. Pat. No. 4,297,684 to Butter, U.S. Pat. No. 4,843,233 to Jeunhomme and U.S. Pat. No. 4,854,706 to Claus et al.
Butter discloses a fiber optic intruder alarm system for protecting the perimeter of an area utilizing a multimode optic fiber as a deformable sensing element, wherein a length of multimode optic fiber is buried in the ground, coherent light is directed through the buried optic fiber and the speckled output light pattern of the fiber is rectified, integrated and threshold detected to determine the presence of an intruder.
Jeunhomme discloses a fiber optic vibration detecting device, but with no means for quantifying the output of the optic fiber, and suggests that observed vibrations can be used to trigger an alarm and/or lighting in the case of a breathing deficiency of a monitored patient.
Claus et al disclose a modal domain optical fiber sensor for vibration monitoring and as a mechanical motion sensor. Claus et al teach filtering the speckle pattern at the output of an optical fiber to a single lobe, termed a single speckle or a single correlation cell elsewhere in the scientific literature, and finds application to monitoring force on mechanical structures, such as a cantilever beam.
However, in the prior art, how to reliably derive information relating to breaths, heartbeats, breathing rate, or heart rate of a monitored person is not well documented, and a need exists for such a monitor which is both inexpensive and reliable.