Optical fiber collimator with long working distance and low insertion loss

A compact, low loss optical fiber collimator design consists of a lens, a glass wedge, and a single- or multi-fiber pigtail. The introduction of the glass wedge ensures minimum off-axis beam deflection and hence improves device reliability. By properly selecting the focusing lens, low insertion losses and long working distances in both reflection (for a multi-fiber collimator) and transmission are achieved. These collimators are critical to interferometer type devices and other micro-optical devices where uniform phase front and/or long path length are desired.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following the details of various preferred embodiments of the present invention are disclosed. The preferred embodiments are described with the aid of the accompanying drawings, wherein like reference numerals refer to like elements throughout. FIG. 3 is a diagram illustrating a single-fiber collimator 300 according to a first embodiment of the present invention. The collimator consists of a single-fiber pigtail, a glass wedge 325 and a collimating lens 330 . The fiber pigtail is made with a cylindrical ferrule 320 and an optical fiber 310 inserted in the center of the ferrule. The fiber and ferrule are bonded together with epoxy such as 353ND, manufactured by EPOXY TECHNOLOGIES. The fiber pigtail closely fits inside of a small glass tube 340 and is bonded to tube 340 using epoxy such as 353ND. The surface of the ferrule where optical fiber is terminated is polished to an angle (e.g., eight degrees) in order to reduce the back reflection (Return Loss). A glass wedge 325 with an uniform optical density, is placed immediately in front of the ferrule. The wedged surface of wedge 325 is polished to have a similar angle and the wedge is placed in a way to minimize beam displacement and deflection. A collimating lens 330 is placed at the opposite end of the collimator. Lens 330 is bonded to an outer tubing 350 . The distance between the fiber pigtail and the collection lens is adjusted such that a desired performance is obtained. This distance is then fixed by bonding corresponding parts of the collimator. According to one embodiment of the invention, the collimating lens 330 is an aspheric lens. All surfaces of optical components in the collimator are coated with anti-reflective coatings to minimize insertion loss of the collimator. The fiber end of the collimator is protected with epoxy 360 such as 353ND. FIG. 4 is a diagram illustrating a dual-fiber collimator 400 according to a second embodiment of the present invention. The collimator consists of a dual-fiber pigtail, a glass wedge 425 and a large collimating lens 430 . The fiber pigtail is made with a cylindrical ferrule 420 and two optical fibers 410 , 415 inserted near the center of the ferrule. The fibers and ferrule are bonded together with epoxy such as 353ND, manufactured by EPOXY TECHNOLOGIES. The fiber pigtail closely fits inside of a small glass tube 440 and is bonded to tube 440 using epoxy such as 353ND. The surface of the ferrule where optical fibers are terminated is polished to an angle (e.g., eight degrees) in order to reduce back reflection (Return Loss). A glass wedge 425 with a uniform optical density is placed immediately in front of the ferrule. The wedged surface of wedge 425 is polished to have a similar angle and the wedge is placed in a way to minimize beam displacement and deflection. A large collimating lens 430 is placed at the opposite end of the collimator. Lens 430 is bonded to an outer cylindrical package 450 . The distance between the fiber pigtail and the collimating lens is adjusted such that a desired performance is obtained. This distance is then fixed by bonding corresponding parts of the collimator. According to one embodiment of the invention, the collimating lens 430 is an aspheric lens. All surfaces of optical components in the collimator are coated with anti-reflective coatings to minimize insertion loss of the collimator. The fiber end of the collimator is protected with epoxy 460 such as 353ND. FIG. 5 is a diagram illustrating a modified dual-fiber pigtail 500 according to embodiments of the present invention. The fiber pigtail is made with a cylindrical ferrule 520 and two optical fibers 510 , 515 inserted near the center of the ferrule. The fibers and ferrule are bonded together with epoxy such as 353ND, manufactured by EPOXY TECHNOLOGIES. The fiber pigtail closely fits inside of a small glass tube 540 and is bonded to tube 540 using epoxy such as 353ND. The surface of the ferrule where optical fibers are terminated is polished to an angle (e.g., eight degrees) in order to reduce back reflection (Return Loss). A glass wedge 525 with a uniform optical density is placed immediately in front of the ferrule. The wedged surface of wedge 525 is polished to have a similar angle and the wedge is placed in a way to minimize beam displacement and deflection. All surfaces of optical components in the modified pigtail are coated with anti-reflective coatings to minimize insertion loss associated with the pigtail. The fiber end of the pigtail is protected with epoxy 460 such as 353ND. Several optical fiber collimators were assembled in accordance with the present invention. The typical reflection insertion losses of dual-fiber collimators are about 0.25 dB at a working distance of 1 cm. With a pair of randomly selected collimators placed at 41 cm from each other, the observed insertion loss is 0.65 dB.