In fiberoptic communications, light from a light source, such as a laser or a light emitting diode (LED), is modulated with a data stream at a high rate and transmitted through a length of an optical fiber between two or more geographical locations. The laser is usually coupled to the optical fiber using a lens.
One of the problems in fiberoptic communications is that optical feedback, or backreflection from the optical fiber to the laser light source affects the laser operation and gives rise to jitter in the timing of the rising and falling edges of the modulated optical signal. The effects of the optical feedback are most severe with single-mode lasers, for example 1310 nm vertical cavity surface emitting lasers (VCSELs) or distributed feedback (DFB) lasers, but are also significant with multimode lasers, for example 850 nm VCSELs or Fabry-Perot (FP) lasers. Therefore, there exists a need to reduce the optical feedback into single-mode lasers and multimode lasers due to reflection from optical fiber tips.
In U.S. Pat. Nos. 6,822,794 and 6,856,460 incorporated herein by reference, Coleman et al. disclose a diffractive optical element for launching light emitted by a laser into an optical fiber while reducing optical feedback into the laser. Referring to FIG. 1A, a light coupling system 10 of Coleman et al. is shown. The system 10 has a laser light source 11, a biconvex transfer lens 14 having a diffractive surface 15 and a refractive surface 16, and a multimode optical fiber 12. The diffractive surface 15 has a spatially non-uniform optical phase delay function represented by a vortex-like pattern shown in FIG. 1B. The vortex-like pattern is a combination of radially and axially symmetrical patterns and is constructed to create particular optical beam launch conditions at the multimode fiber 12, at which the center of the multimode optical fiber 12 is avoided. When the center of the multimode optical fiber 12 is avoided, a modal dispersion performance of the light coupling system 10 is improved. In addition, backreflection into the laser 11 is also reduced due to the presence of diffractive surface 15, which redirects the reflected light away from an aperture 17 of the light source 11.
In U.S. Pat. No. 6,807,336 incorporated herein by reference, van Haasteren discloses an optical lens having a surface mathematically described by a sum of conical, cone, and spiral components. The lens is suitable for launching light into a multimode optical fiber because the specific surface profile results in creating an annular light distribution pattern at the fiber tip, so that the center of the optical fiber can be avoided. When the fiber center is avoided, the modal dispersion is reduced. Optical feedback to the laser light source is also reduced.
One drawback of these prior art approaches is increased sensitivity to optical misalignment. The complex lenses of prior art create a relatively large spot at the fiber tip. Due to the large spot size, the focused laser beam can become clipped at the optical fiber resulting in power loss and reliability problems. Tight tolerances required for assembly of the laser-to-fiber couplers of the prior art result in high manufacturing costs.
A need therefore exists for a misalignment-tolerant optical subassembly for coupling light into an optical fiber, which has low levels of optical feedback into the light source.