Optical circulators using beam angle turners

Details Optical circulators using beam angle turners Optical circulators using beam angle turners

1998-11-05

2001-01-16

Schuberg, Darren (Department: 2872)

Optical: systems and elements

Polarization without modulation

By relatively adjustable superimposed or in series polarizers

C359S490020, C359S456000, C359S484010

Reexamination Certificate

active

06175448

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to optical devices; more particularly, it relates to optical circulators.
2. Description of Related Art
An optical circulator is a nonreciprocal, typically three-port or four-port, device. Light entering the first port passes out the second port, but light entering the second port cannot pass back to the first port. Instead, it passes out of the third port. By installing an optical circulator at each end of a fiber link, an existing unidirectional fiber optic communication link can be quickly and economically converted to a bi-directional one. Such a modification results in a doubled bit carrying capacity. An optical circulator can also be used in applications such as wavelength division multiplexer (WDM), Erbium-doped fiber amplifier (EDFA), add-drop multiplexers, dispersion compensators and optical time domain reflectometers (OTDR's).
Optical circulators can be a key element in today's optical networks. However, they have not been widely adopted because of their high cost. A typical optical circulator usually comprises many optical elements and has a large optical footprint. Manufacturing of conventional optical circulators usually requires precise alignment of each optical element, leading to low yields and high production costs.
An early concept of a polarization independent optical circulator for telecommunication use was disclosed in Matsumoto, U.S. Pat. No. 4,272,159. This document, and all others referred to herein, are incorporated by reference as if reproduced fully herein.
Optical circulators have been described in patents, including the above-mentioned Matsumoto, U.S. Pat. No. 4,272,159; Emkey, U.S. Pat. No. 4,464,022; and Kuwahara, U.S. Pat. No. 4,650,289. However, these early optical circulators often suffer from high insertion loss and/or cross-talk that is unacceptable for many communication applications. Insertion loss may be defined as the difference between the power between light launched into the optical circulator and the power that exits the device. Insertion loss is largely due to coupling loss from fiber to fiber, absorption of light and to imperfect polarization separation. Cross-talk in an optical circulator refers to the amount of power emitted at an optical port to the receiver from light entering at an adjacent optical port from the transmitter. The conventional polarizing cubes used in these prior optical circulators often cause large insertion loss and cross-talk because of their low polarization extinction ratio.
Optical Circulators Using Beam Shifters
Koga, U.S. Pat. Nos. 5,204,771; 5,319,483 and Cheng U.S. Pat. Nos. 5,471,340; 5,574,596, disclose optical circulators using beam shifters. The beam path determining means of these patents shift a beam such that it possesses the same propagation direction but is spatially located in a different portion of the circulator. In this sense, the input beam to and output beam from the beam path determining means are parallel in propagation direction but are shifted in spatial location. A disadvantage of the Koga and Chen circulators is that the construction of these circulators demands precise fabrication of birefringent crystals and waveplates. These types of circulators are therefore often difficult and costly to make. The length of beam shifter in these circulators required to obtain adequate beam separation is also excessively large thus resulting in a large form factor.
Another drawback of the Cheng circulators is that polarization mode dispersion (“PMD”) in the circulators is not eliminated unless additional compensation crystals are introduced. Such additional crystals add cost and complexity. Polarization mode dispersion (PMD) is introduced in an optical component when signal energy at a given wavelength is resolved into two orthogonal polarization modes of slightly different propagation velocity or optical path. The resulting difference in propagation time between polarization modes is also called differential group delay. PMD causes a number of serious capacity impairments, including pulse broadening. In addition, alignment of this type of circulators depends on sub-micron precision positioning of single mode fibers. Therefore, manufacturing of PMD-corrected Cheng circulators is non-trivial.
FIGS.
1
A-B show respectively an isometric and a cross-sectional view of a walk-off crystal such as that employed in the Cheng and Koga references. Walk off crystals can be used either for splitting a natural light beam into orthogonally polarized rays, or for circulating light beams with orthogonal polarization components.
FIG. 1A
shows the later case in which a light beams
150
-
152
with orthogonal polarization states, circulate between respectively ports
106
-
104
and ports
102
-
106
of walk-off crystal
100
.
FIG. 1B
is a cross-sectional view at principal plane ABCD of the crystal
100
shown in FIG.
1
A. The optical axis
108
of the crystal is located in the principal plane and at an acute angle that is typically at around 45 degree with respect to the front surface of birefringent crystal, defined by the plane including AD. The polarization vector, i.e. electric field vector,
118
of ray
150
is normal to the principal section. Thus the propagation vector
124
and Poynting vector
126
for the ray
150
are substantially collinear and no walk-off is exhibited as the ray passes through the crystal to port
104
. The polarization, i.e. electric field vector,
116
of ray
152
is parallel to the principal section. Thus the propagation vector
120
and Poynting vector
122
for the ray
152
are not collinear and walk-off is exhibited as the ray passes through the crystal to port
106
. The complete explanation of this walk-off effect can be found using electromagnetic theory as embodied in Maxwell's equations. Further explanation, using Huygen's principle, may be found in Hecht, Optics 288 (1987) (2d ed. Addison-Wesley).
Optical Circulators Using Beam Benders
Pan et al., U.S. Pat. Nos. 5,682,446; 5,818,981; 5,689,367 and 5,689,593, describe another type of circulator in which optical ports, beam splitters and non-reciprocal rotators are radially arranged about a polarization sensitive prism pair and associated air gap. Circulation is achieved by polarization sensitive reflection or transmission of an incident light beam from or through the air gap defined between the prism pair as shown in FIG.
2
. The length of the beam splitters coupled with the radial arrangement of the ports makes for a circular form factor. The arrangement is bulky and expensive.
FIG. 2
shows an isometric view of a circulating element such as that employed in the Pan et al. references. Prism pair
208
,
212
defines an air gap
210
between internal faces
208
A,
212
A. The prism pair and air gap function as an optical circulator
200
by reflecting and transmitting orthogonally polarized light beams respectively
250
-
252
. Light beam
250
with polarization vector
222
propagates between ports
204
and
202
by entering prism
208
at a normal to face
208
C, by internally reflecting off face
208
A and air gap
210
within prism
208
and by exiting the prism
208
on a normal to face
208
B toward port
202
. Light beam
252
with polarization vector
220
propagates between ports
206
and
202
by entering prism
212
on a normal to face
212
B, by transmission through gap
210
into prism
208
on a normal to face
208
A and by exiting the prism
208
on a normal to
208
B toward port
202
.
In order to achieve this effect, i.e. polarization sensitive transmission and reflection, several requirements must be met. First, the prisms must have an optical axis. Second, the prisms
208
,
212
are separated by an air gap
210
defined between opposing interior faces
208
A,
212
A of respectively prisms
208
,
212
. The gap must be greater than the wavelength of the light being transmitted and the interior faces should be parallel. Third, ray
250
must intercept the gap at an angle of incidence greater than a c

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