Fiber optic line magnetometer having thermal compensation

A fiber optic line magnetometer for detecting the magnetic field form by a train positioned within a predetermined block of a train track, the fiber optic line magnetometer comprising;

• a. means for generating a light source:
• b. reference arm means optically coupled to said light source, said reference arm means optically transmitting said light source over the length of said reference arm and outputting a first optical signal;
• c. sense arm means optically coupled to said light source, said sense arm means optically transmitting said light source over the length of said sense arm and outputting a second optical signal, said sense arm means varying optical transmission characteristics in response to the magnetic field of the train being applied thereto;
• said sense arm means and said reference arm means being positioned closely adjacent to each other, both said sense arm means and said reference arm means position substantially parallel to the train tracks over substantially the entire length of the predetermined block;
• and
• d. a processing means optically coupled to receive said first optical signal and said second optical signal, said processing means arranged to output a detection signal when a phase shift between said first optical signal and said second optical signal exceeds a predetermined threshold,
• wherein said sense arm means comprises;
• a. a first fiber optic cable for optically transmitting the light source over the length of the sense arm means;
• b. a plurality of magnetostrictive elements, each of said plurality of magnetostrictive elements varying in length as a function of the magnetic field applied thereto;
• and
• c. mechanical coupling means for mechanically coupling each of said plurality of magnetostrictive elements to said first fiber optic cable in a manner such that changes in the length of the magnetostrictive elements causes changes in the length of the first fiber optic cable, and
• wherein said plurality of magnetostrictive elements of said sense arm means have a first thermal expansion coefficient, and wherein said reference arm means comprises;
• a. a second fiber optic cable for optically transmitting the light source over the length of the reference arm means;
• b. a plurality of non-magnetostictive elements, each of said plurality of non-magnetostrictive elements having a second thermal expansion coefficient, said ratio of the length of each of said plurality of magnetostrictive elements to the length of each of said plurality of non-magnetostrictive elements equals said first thermal expansion coefficient divided by said second thermal expansion coefficient;
• and
• c. mechanical coupling means for mechanically coupling each of said plurality of non-magnetostrictive elements to said second fiber optic cable in a manner such that changes in the length of the non-magnetostrictive elements causes changes in the length of the second fiber optic cable.