Low cost fiber optic circulator

Details Low cost fiber optic circulator Low cost fiber optic circulator

2000-04-05

2001-09-11

Palmer, Phan T. H. (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling structure

C385S011000, C385S048000, C359S484010

Reexamination Certificate

active

06289156

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to optical devices and systems, and in a particular embodiment, provides a Mach-Zehnder interferometer based device which may be used as an optical circulator or isolator.
An optical isolator is a nonreciprocal device which allows the passage of light in only one direction. A signal transmitted in a forward direction through a first port of an optical isolator will be passed to a second optical port. However, optical signals traveling in a rearward direction through the second optical port are blocked by the optical isolator from reaching the first port. Such optical isolators have found a wide variety of uses in optical systems, particularly those using optical fibers.
An optical circulator is a nonreciprocal optical device related to the optical isolator. Optical circulators allow the passage of light from a first port to a second port, as in an optical isolator. However, rather than simply blocking signals traveling in a reverse direction into the second port, such signals are instead transmitted to a third port. Any two consecutive ports of an optical circulator are, in effect, an optical isolator since signals travel in only one direction between the ports.
Circulators will generally have three or more ports. Light transmitted into the first or second port of a three port circulator will be directed to the next higher number port. In a closed circulator, light transmitted into the third (or other highest number port) is passed to the first port. In an open three port circulator, light directed into the third port will be blocked by the circulator without transmitting the light to any other active port. Regardless, the function performed by the circulator is called a circulating operation.
Several types of optical circulators have been developed. The structure of a conventional optical circulator includes three basic components: polarization beam splitters (PBSs), nonreciprocal Faraday rotators, and half-wave plates. Each beam splitter typically includes at least one optical deflection element such as a prism. Assembly of these conventional circulators is fairly difficult, so that the cost of conventional circulators is quite high.
Much work has gone into improving the performance of optical circulators. While conventional circulators provide an isolation of about 30 dB, additional birefringent crystals may be employed to improve isolation to over 40 db. Exemplary birefringent enhanced optical circulators are commercially available from E-T
EK
D
YNAMICS,
I
NC.
of San Jose, Calif., and related devices may also be available from N
IPPON
T
ELEGRAPH AND
T
ELEPHONE
C
ORPORATION
of Japan, FDK A
MERICA,
I
NC.,
of California, and other sources. Generally, circulators which include both a conventional polarization beam splitter and birefringent crystals will have costs higher than those of a conventional circulator.
Optical circulators based on light path deflection of birefringent polarizers have also been proposed and implemented. These birefringent polarizer based structures have enhanced isolation performance, but often at a substantially higher cost. Moreover, optical circulators based on either polarization beam splitters or birefringent polarizers are susceptible to polarization mode dispersion (PMD) if there is a lack of symmetry between the optical paths of the separated beams. Such polarization mode dispersion can limit the signal transmission speed of an optical network, while the symmetrical circulator structures proposed to date are often very difficult to align and/or include highly specialized optical elements. Once again, exemplary birefringent polarizer based optical circulators are commercially available from E-T
EK
D
YNAMICS,
while competing structures may be available from N
IPPON
T
ELEGRAPH AND
T
ELEPHONE
C
ORPORATION
of Japan, JDS F
ITEL,
I
NC.,
of Canada, P
HOTONIC
T
ECHNOLOGIES
of Australia, and others.
The incremental improvements in high performance circulators have provided a variety of options for applications requiring high isolation with low insertion loss. Unfortunately, the cost of each circulator structure is often prohibitive for applications requiring numerous circulators. Moreover, there are applications for the optical circulating operation which do not require the performance of these costly structures. For example, in fiber optic networks, relatively low cost amplification is available to overcome a relatively large amount of insertion loss.
A recent paper published by T. Shintaku et al. of NTT O
PTO
-
ELECTRONICS
L
ABORATORIES
of Japan, describes a waveguide polarization-independent optical circulator based on a Mach-Zehnder interferometer. This structure combines two 45° Faraday rotators and two half-wave plates with a Mach-Zehnder interferometer structure. A Faraday rotator and a half-wave plate are aligned symmetrically along each leg of the interferometer, and the resulting circulator is described as providing an isolation of between 14.1 and 23.7 dB with an insertion loss of between 3.0 and 3.3 dB.
While the recently proposed Mach-Zehnder interferometer based optical circulator appears to provide a useful alternative to circulators based on conventional polarization beam splitters, birefringent crystal enhanced polarization beam splitters, and birefringent crystal polarizers, particularly when the cost of these structures is not justified. Nonetheless, it would be desirable to provide still further improvements in optical circulators, and in optical circulation methods. It would be particularly desirable to provide optical circulator structures having improved manufacturability and still lower cost, while maintaining acceptable isolation, insertion loss, polarization mode dispersion, and polarization dependent loss characteristics. It would further be desirable if these improvements were applicable to fiber based optical circulators, integrated optical element systems, table top optical networks, optical isolators, and the like.
SUMMARY OF THE INVENTION
The present invention provides improved optical devices, systems, and methods for selectively transmitting optical signals. The optical devices of the present invention are generally based on a Mach-Zehnder interferometer. Through accurate control of the phase relationship, the Mach-Zehnder interferometer allows optical signals to either be constructively combined (so as to enhance the transmitted signal strength), or destructively combined (so as to reduce or prevent transmission). Surprisingly, this beneficial phase relationship can be combined with a simple asymmetrical nonreciprocal structure positioned along one of the two legs of the Mach-Zehnder interferometer. Generally, two retarder plates will be positioned along one leg, while a light sensitive fiber disposed along the other leg can allow the optical path length to be adjusted so as to avoid polarization mode dispersion. As aligning retarder plates relative to each other is significantly easier than independently aligning each retarder plate within the surrounding Mach-Zehnder structure, the present invention provides significant fabrication advantages over known Mach-Zehnder interferometer based optical circulators.
In a first aspect, the present invention provides an optical device comprising a first optical element in a first optical path of a first optical signal. The first element directs a portion of the first signal along a first optical path leg, and a portion of the first signal along a second optical path leg. A second optical element is optically coupled to the first and second legs. The second element constructively combines the first signal portions to transmit a first signal along a second optical path. The second element also directs a portion of a second optical signal from the second path along the first leg, and a portion of the second signal along the second leg. A Faraday rotator is disposed along the first leg or the second leg. First and second retarder plates are disposed along the first leg. The retarder plates are arranged r

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