Optical phase shifting, splitting and combining device

Details Optical phase shifting, splitting and combining device Optical phase shifting, splitting and combining device

2002-03-05

2004-08-24

Kim, Ellen E. (Department: 2874)

Optical waveguides

Polarization without modulation

C385S003000, C385S027000

Reexamination Certificate

active

06782147

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of optical devices and is particularly concerned with a polarization selective phase shifting, splitting and combining optical device.
BACKGROUND OF THE INVENTION
Optical fibers are becoming an increasingly popular data transmission medium since they are believed to be ideally suited to the requirements of high-speed communication networks. Some of the desirable characteristics of optical fiber networks are that they transmit signals over relatively long distances with relatively low signal losses and at relatively high rates. Fiber bandwidth is thus a critical characteristic of optical fiber networks since fiber bandwidth is directly related to the information carrying capacity of a fiber and limits the maximum rate at which information can be transmitted.
The search for higher bandwidth has resulted in the deployment of improved optical transmission systems. These high-speed networks have brought new challenges to the optical components industry. Optical phenomena, which have had negligible effects on system performance in the past, are now of utmost importance. Polarization is a common factor in a number of such phenomena that must be characterized if these high-bandwidth systems are to meet, and perhaps exceed, their potential.
As is well known in the art, the polarization of light is determined by the time course of the direction of the electric-field vector. For monochromatic light, the three components of the electric-field vector vary sinusoidally with time with amplitudes and phases that are generally different, so that at each position the endpoint of the vector moves in a plane and traces an ellipse. The plane, the orientation and the shape of the ellipse vary with position.
In paraxial optics, however, light propagates along directions that lie within a narrow cone centered about the optical axis. Waves are approximately transverse electromagnetic and the electric-field vector therefore lies approximately in the transverse plane. A polarized light signal can thus be divided between an x axis polarization component and an orthogonal y axis polarization component.
In instances wherein one of the polarization components is zero or wherein both polarization components are in phase, then the light signal is said to be linearly polarized and can be represented by a simple vector that has a given amplitude and a given angle relative to the reference axes. If the two polarization components are of same amplitude but out of phase relative to each other, then the polarization state is said to be circular. If the two polarization components are of different amplitude and phase, the polarization is said to be elliptical.
In modern fiber optic telecommunications, the polarization of the signal is typically used to help direct the signal along the fiber optic network. Network components or devices which function based upon the polarization of the light signal include polarization division multiplexers, polarizers, depolarizers, fiber optic polarization tunable filters, binary polarization switch/modulators, and many other polarization related fiber optic components. All of these devices require fiber optic variable polarization beam splitters and/or combiners that are adapted to either split a light signal into two orthogonal linearly polarized signals or to combine optical signals by reversing their paths in the device.
Polarization beam splitters and/or combiners are not only used as part of other optical components but are also used alone or in combination with other optical devices in a variety of situations. For example, polarization beam combiners may prove to be particularly useful in the context of signal amplification. Although modern fibers have very low losses per unit length, signal amplification is an important element of many optical information networks. Indeed, long fiber spans, for example, cables extending from one city to another, require periodic amplification of the transmitted signal to ensure accurate reception at the receiver.
Erbium doped fiber amplifiers have been developed to satisfy this need for signal amplification. Such amplifiers consist of a length of optical waveguide fiber, typically 5 to 30 meters of fiber, which has been doped with erbium. The quantum mechanical structure of erbium ions in a glass matrix allows for stimulated emission in the approximately 1520 to 1620 nanometer range, which is one of the ranges in which optical waveguide fibers composed of silica exhibit low loss. As a result of such stimulated emission, a weak input signal can achieve more than a hundred fold amplification as it passes through a fiber amplifier.
To achieve such stimulated emission, the erbium ions must be pumped into an excited electronic state. Such pumping can take place in various pump bands. Combining/splitting devices are an integral part of the amplification process being used along with semiconductor laser sources and wavelength multiplexing devices for generating a pumping signal.
One common method of producing a polarization splitter involves the use of a birefringent crystal. The splitter works by taking advantage of the anisotropic structure of this crystal; that is, the crystal does not have the same optical density for the two transverse propagation vectors.
When a randomly polarized signal is passed through a crystal of this kind the polarization is broken up into two components relative to the optical orientation of the crystal. Both beams will emerge linearly polarized, but with polarization orientations perpendicular to each other.
Only certain types of crystals will exhibit birefringent behavior. Crystals must have hexagonal, tetragonal, or trigonal lattice structures to allow the light to encounter an asymmetric structure. Some common materials with these characteristics are calcite (calcium carbonate), quartz, and tourmaline. There are many ways to make a beam splitter cube from these materials, the most common being slicing a rectangular prism of the material along a diagonal, and cementing it back together in a different orientation.
Some devices have gain widespread acceptance despite their numerous drawbacks. The Glan-Thompson polarizer, for example, includes a block of birefringent material cut into prisms and then cemented together. It reflects one polarization component at the cement interface and transmits the other. The device suffers from requiring a considerable amount of birefringent material, generally calcite, which is scarce and expensive. It is also unable to work with high-powered lasers and ultraviolet light, since the light destroys or clouds cement. Furthermore, this beam splitter, which makes use of the reflected polarization component, suffers from the added disadvantage that polarized beams exit the device at inconvenient angles, for example 45 degrees, when it is often useful that beams are parallel, orthogonal or otherwise oriented.
The Glan-Taylor polarizer, which is similar to the Glan-Thompson polarizer but uses an air space instead of cement to separate polarization components, can work with many light sources but suffers from reflection loss and ghosting caused by the air gap. The Wollaston, Rochon and Senarmont beam splitters, which separates polarization components by transmitting the components through an interface, permit optical contacting for use with most light sources, but produce beams which also exit at inconvenient angles, with one or both polarization components suffering from chromatism and distortion.
The double refracting element that produces parallel-polarized beams of light, achieves small beam separation and limited field. Also, since the beams may pass through a considerable amount of material before achieving useful separation, wavefront distortion can occur in the extraordinary beam due to imperfections in the crystal's structure. Beam separation can be further limited by the small size and high cost of suitable crystals.
Other types of known polarization beam splitters and combiners make use of semiconductors. These

Affiliated with

Also associated with

Rating

Optical phase shifting, splitting and combining device does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Optical phase shifting, splitting and combining device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical phase shifting, splitting and combining device will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3344056