Optical wavelength demultiplexer

Details Optical wavelength demultiplexer Optical wavelength demultiplexer

1998-11-24

2001-03-06

Nguyen, Thong (Department: 2872)

Optical waveguides

With optical coupler

Particular coupling structure

C385S024000, C385S037000, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06198864

ABSTRACT:

TECHNICAL FIELD
The invention relates generally to wavelength division multiplexed optical communication systems and more particularly to an optical wavelength division demultiplexer.
DESCRIPTION OF THE RELATED ART
In a wavelength division multiplexed (WDM) optical system, light from several lasers, each having a different central wavelength, is combined into a single beam that is introduced into an optical fiber. Each wavelength is associated with an independent data signal through the fiber. At the exit end of the fiber, a demultiplexer is used to separate the beam by wavelength into the independent signals. In this way, the data transmission capacity of a fiber is increased by a factor equal to the number of single-wavelength signals combined into a single fiber.
Many examples of prior art optical demultiplexers exist. One example of a bulk optical filter-based demultiplexer is disclosed in U.S. Pat. No. 5,808,763, entitled “Optical Demultiplexer,” issued to Duck et al. (hereinafter Duck). The optical demultiplexer of Duck receives collimated light into a glass block and directs the collimated light onto a single interference filter. The single interference filter has a wavelength filtering characteristic that is dependent upon the angle of incidence with which the collimated light impacts the filter. To manipulate the angle of incidence of the collimated light impacting the filter, a series of reflective surfaces are located opposite the filter and are angled such that the collimated light zigzags between the filter and the reflective surfaces within the glass block, reaching the filter each time at a different angle of incidence. The different angles of incidence are predetermined to enable separation of a multi-wavelength beam of light into its wavelength components. Although the demultiplexer of Duck works well for its intended purpose, the beam diameters involved with collimated light place physical constraints on the degree of miniaturization that can be achieved in a demultiplexer of this type. In addition, the reflective surfaces must be precisely angled to achieve light filtration at the desired wavelength.
In another known bulk optical filter-based demultiplexer, a compound objective lens collimates light from an optical fiber and then directs the light onto a succession of wavelength-specific optical filters at a particular angle. At each optical filter, light of one wavelength, or a group of wavelengths, is transmitted while light of the remaining wavelengths is reflected. The transmitted light from each optical filter is refocused by filter-specific compound objective lenses and coupled into outgoing fibers for subsequent use. The light reflected from each optical filter propagates back and forth between successive wavelength-specific optical filters in a zigzag fashion within the body of the demultiplexer. Although the demultiplexer works well for its intended purpose, the demultiplexer requires several discrete objective lenses which must be assembled and precisely aligned with respect to one another. In addition, as with the Duck demultiplexer, the use of collimated light limits the degree of miniaturization that can be achieved in a demultiplexer of this type.
Another example of a prior art demultiplexer is disclosed in U.S. Pat. No. 4,675,860, entitled “Compact Wavelength Multiplexer-Demultiplexer with Variable Filtration,” issued to Laude et al. (hereinafter Laude). Laude discloses a demultiplexer that utilizes a number of spherical interference filters that are arranged in series along an optical path of a beam of light that is emitted from an optical fiber. Each filter is selective to a particular wavelength and reflects light of the particular wavelength back to a wavelength-specific output fiber, while the light of other wavelengths is passed onto the next filter in the series. While the demultiplexer works well for its intended purpose, since the filters are located in series along the direction of light propagation, light of a wavelength that is not initially reflected by a first filter will pass through each filter twice. For example, in a three-channel demultiplexer, portions of the original light beam must pass forwardly and rearwardly through two filters. In addition, since the filters refocus the diverging light upon reflection, the curvature of the filters must be precisely formed. Further, because the filters are arranged in a series along the optical path, the filters must be bonded to a device that is formed by combining multiple separately fabricated parts.
In view of the size constraints involved with bulk optical multiplexers and the drawbacks involved with utilizing filters arranged in series along an optical path, what is needed is an optical demultiplexer that can be easily produced with greater miniaturization.
SUMMARY OF THE INVENTION
A demultiplexer in accordance with the invention includes an optically transparent structure that utilizes focusing relay mirrors to relay a multi-wavelength light beam among laterally arranged wavelength-specific interference filters, with each filter separating out a specific wavelength component from the multi-wavelength beam. The relay mirrors are focusing mirrors, so that the demultiplexer can be operated with a non-collimated light beam in a manner that controls the tendency of such a beam to have a large angle of divergence, while taking advantage of the small beam diameter in order to create a demultiplexer with greater miniaturization.
A preferred demultiplexer includes a main optical block, wavelength-specific interference filters coupled to the main optical block, and a series of relay mirrors formed within the main optical block to direct and focus light onto the interference filters. The preferred main optical block of the demultiplexer is composed of a monolithic optically transparent material, such as plastic or glass. Mechanical features at an input end of the main optical block align and register an optical fiber, so that a beam of light from the fiber enters the block through a flat input surface of the block. An objective mirror is integrated into another surface of the main optical block to receive the beam of multi-wavelength optical energy from the input fiber and to direct the beam to the first one of the interference filters. The objective mirror is preferably a convex surface relative to the exterior of the block and is shaped such that light that is incident on the first interference filter has the desired spatial and angular characteristics. The surface segment that forms the objective mirror is preferably coated so as to be internally reflective, however at sufficient angles of incidence, an uncoated mirror with total internal reflection can be used. In an alternative embodiment, an objective lens is integrated into the input surface to focus the incoming beam of light. The focused light is directed from the objective lens to the first filter by a flat mirror that is formed on a surface of the main optical block. In either embodiment, additional internally reflecting surfaces can be implemented to fold the incoming beam from the optical fiber, so that the necessary optical distance between the input fiber and the first interference filter is obtained in a relatively small space.
The main optical block also includes an output end. The output end of the main optical block preferably includes a flat output surface to which the interference filters are attached. In some cases, mechanical features can be integrated into the output end of the main optical block to aid in the alignment and registration of the interference filters.
The interference filters are wavelength-selective filters that are preferably connected to the output end of the MOB in a linear array with fixed center-to-center spacing. Each filter has high transmission and low reflection over a particular range of wavelengths and low transmission and high reflection over another range of wavelengths. The preferable transmission spectrum for WDM demultiplexer applications is a “flat top” shape in which very high and u

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