Optical device

An optical device comprising an optical system constituted by a combination of an output optical fiber, an input optical fiber and a gradient index rod lens, said output optical fiber and said input optical fiber being arranged so that optical axes of said two optical fibers are parallel to each other with a distance of W, said gradient index rod lens having a length of Z on its optical axis and having a radially gradient index distribution represented by a formula:

• n(r)2=n02·{1−(g·r)2+h4(g·r)4+h6(g·r)6h8(g·r)8+ . . . }
• in which r is a radial distance measured from the optical axis of said rod lens, no is the refractive index of said rod lens on the optical axis of said rod lens, g is a gradient index distribution coefficient of second order, and h4, h6, h8 . . . are gradient index distribution coefficients of fourth sixth, eighth . . . order respectively,
• wherein:
• said optical axes of said two optical fibers are disposed in parallel to said optical axis of said rod lens so that, after a luminous flux output from said output optical fiber is input to said rod lens through a first end surface of said rod lens and converted into an approximately parallel luminous flux in the inside of said rod lens, at least one part of said parallel luminous flux is reflected by a reflecting unit disposed on a second end surface of said rod lens opposite to said first end surface of said rod lens, converted into a convergent luminous flux again in the inside of said rod lens, and input to said input optical fiber;
• and
• said optical device satisfies a condition:
• W·g·(0.25−Z/P)2≦6×10−5
• in which P is a periodic length of said rod lens (equivalent to 2π/g).