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Optical Isolators |
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1.INTRODUCTION |
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Connectors and other optical components are
present in the optical fiber transmission
line, and diverse return beams are generated
from the end faces of these components. Return
beams are known to have a destabilizing effect
on oscillation of the laser source and on
operation of the optical fiber amplifier,
thus resulting in poor transmission performance. |
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2.OPTICAL ISOLATORS |
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2-1.Polarization-Dependent Type |
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Figure 1 shoows the composition of a polarization-dependent
optical isolator.The optical isolator consists
of two polarization elements (a polarizer
and an analyzer having a 45°differential in the direction of their light
transmission axes), and of a 45° Faraday rotator interposed between the polarization
elements. A forward light passing the optical
isolator undergoes the following: (1) when
passing through the polarizer, the incident
light is transformed into a linearly polarized
light; (2) when passing through the Faraday
rotator, the polarization plane of the linearly
polarized light is rorared 45°; (3) this light passes through the analyzer
without loss since its polarization plane
is now in the same direction as the light
transmission axis of the analyzer, which
is tilted 45° from the polarizer in the direction
of Faraday rotation. In contrast, a backward light undergoes a slightly different process: (1) when passing through the analyzer, the vackward light is transformed into a linearly polarized light with a 45° tilt in the transmission axis; (2) when passing thruogh the Faraday rotater, the polarization plane of the backward light is rotated 45°in the same direction as the initial tilt; (3) this light is completely shut out by the polarizer because its polarization plane is now 90° away from the light transmission axis of the polarizer. ![]() Polarization-dependent isolator are primarily incorporated in semiconductor laser modules. Accordingly, miniaturization has been an important target. To realize this goal , PBS is being replaced by infrared polarization glass as a polarization element. Bi-substituted iron garnet films are used as Faraday rotators due to their low saturaton magnetization and large rotation capacity. Sm-Co type rare-earth magmets are employed to apply a designated magnetic filed to the Faraday rotator. The performance of optcal isolators is primarily evaluated by thier insertion losses and isolations, both of which are detetmined by the absorption losses end-face reflectances, and the extinction ratios of optical elements. For this reason, technologies to enhance and actualize the characteristics of optical elements become important. At present, we have adhesive-free and metal-joined type single-tier polarization-dependent isolators measuring 3 x 3 mm in dimension, 0.2 dB in insertion loss, and 40 dB in isolation (see Figure 2). ![]() |
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2-2.Pokarization-Independent Type |
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Figure 3 shows the composition of a polarization-independent
optical isolator. A 45° Faraday rotator is interposed between
two wedge-shaped birefringent plates, and
a lens is placed at both ends of the isolator
for junction with optical fibers. While polarization-dependent
optical isolators allow only the light polarized
in a specific direction, polarization-independent
isolators transmit all polarized light. Consequently,
these isolators are frequently used in optical
fiber amplifiers. First, a forward incident light is separated into ordinary and extraordinary rays by the No.1 birefringent plate. Second, the polarization planes of these rays are each rotate 45° by the Faraday rotator. Third, ordinary and extraordinary rays pass through the No.2 birefringent plate having such an optic axis that the relationbetween the two types of rays is maintained; consequently, both raysare refracted in an identical parallel direction when exiting from the No.2 birefringent plate. Fourth, these collomate beams are converged into the downstream optical fiber through a lens. ![]() On the other hand, a backward light incident
on the same optical isolator is separated
into ordinary and extraordinary rays whose
relation is reversed with that of a forward
light due to the non-reciprocality of the
Faraday rotation. Consequently, rays passing
through the No.1 birefringent plate do not
become parallel to each other, so they cannot
be converged into the upstream optical fiber. |
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2-3.Composite Type |
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An optical fiber amplifier is comprised of
Er-doped (rare earth added) fibers, a wavelength-division
multiplexer (WDM), a pumping diode laser,
a polarization-independent isolator, and
other passive components. Figure 5 illustrates
the general composition of an amplifier.
Previously, WDM and isolators were separately
incorporated in amplifiers, but recently
the sections enclosed in the broken lines
(1) to (3) have been increasongly modularized,
thus eliminating some of the lens systems
needed for the been further miniaturized
while preventing insertion loss from rising. The types of optical fiber amplifiers are determined by the directions of signal light and pump light inside Er-doped fibers. The directions are either (1) identical, (2) reverse, or (3) both. They are called, forword pumping, backward pumping and two-way pumping, respectively. Dwpending on which of these three types of amplifiers is necessary, either (1), (2) or both of the modules shown Figure 5 are selected. In addition, there is another module (3) that is combined with a bandpass filter (designed to shut out pump light from the isolator) in order to shut out return beams from the optical circuit. ![]() Figure 6 presents a composite type optical
isolator for forward pumping. This isolator
measures 25 x 30 x 8 mm in dimension,0.8
dB in insertion loss for both signal light
(wavelength of 1.55 um) and pump light, 40
dG in isolation, and 55 dB or more in reflection
attenuation. |
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3.Magnetic Garnet Crystal |
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3-1.YIG Single Crystal |
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A YIG single crystal is grown using the floating
zone (FZ) method. Figure 7 shows the deagram
of the FZ furnace used. Y2O3 and Fe2O3are mixed to suit the stoichiometric composition
of YIG, and then the mixture is sintered.
The resultant sinter is set as a mother stick
on one shaft in an FZ furnace, while a YIG
seed crystal is set on the remaining shaft.
The sintered material of a prescribed formulation
is placed in the central area between the
mother stick and the seed crystal in order
to create the fluid needed to promote the
deposition of YIG single crystal. Light from
halogen lamps is focused on the central area,
shile the two shafts are rotated. The central
area, when heated in an oxygenic atmosphere,
forms a molten zone. Under this condition,
if the mother stick and the seed is moved
at a constant speed, if result in the movement
of the molten zone along the mother stick,
thus gowing single crystals from the YIG
sinter. Since the FZ method grows crystal from a mother stick that is suspended in the air, contamination is precluded and a high-purity crystal is cultivate. The FZ method produces ingots measuring 012 x 120 mm. The specular condition off the coating finish and the trecision of the disk thickness determine the characteristics of optical isolators. ![]() |
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3-2.Bi-Substituted Iron Garnet Thick Films |
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Bi-substitued iron garnet thick films are
grown by the liquid phase epitaxy (LPE) method.
Figure 8 illustrates the structure of an
LPE furnace. Crustal materials and a PbO-B2O3flux are heated and made molten in a platinum
crucible. Sigle crystal wafers, such as GdCa)2(GaMgZr)5O12, are soaked onthe molten surface while rotated,
which causes a Bi-substituted iron garnet
thick film to be grown on the wafers. Currently,
thick films measuring as much as 3 inches
in diameter can be grown. To obtain 45° Faraday rotators, these films must be ground to a certain thiclness, applied with anti-reflective coating, and then cut into 1-2 mm squares ti fit of the isolators. Having a greater Faraday rotation capacity than YIG single crystals, Bi-substituted iron garnet thicl films must be thinned in the order of 100 um, so higher-precision processing is required. ![]() |
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3-3.Comparison of YIG Single Crystals and Bi-substituted Iron Garnet Thick Films |
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YIG single crystals fare better than Bi-substituted
iron garnet thick films in temperature coefficient
and wavelength coefficient for Faraday rotation.
If these caracteristics are given higher
priority, YIG single crystals are selected
as Faraday rotators. On the other hand, the
advantages of Bi-substituted iron garnet
thick films include: (1) the time required
to grow crystal is greatly shortened,(2)
the length of rotators can be reduced to
one-fifth or even less due to increased rotation
capacity, and (3) the saturation magnetization
si low. These features lead to a price reduction and miniaturization of optical isolators; therefore, Bi-substituted iron garnet thick films are mostly used as polarization-dependent isolators incorporated in laser modules. Table 1 below compares the characteristics of YIG single crystals and Bi-substituted iron garnet thick films. |
Property | Wavelength(nm) | YIG single crystal | Bi-substituted iron garnetthick film |
Material | - | Y3Fe5O12 | (TbBi)3(FeAl)5O12 |
Saturation magnetization(mT) | - | 178 | 60 |
Insertion(dB) | 1310 | 0.1 | 0.1 |
Faraday rotation coefficient(deg./cm) | 1310 | 224 | -1570 |
1550 | 175 | -1060 | |
Faraday rotation temp. coefficient(deg./°C) | 1310 | 0.034 | 0.054 |
1550 | 0.042 | 0.062 | |
Faraday rotation wavekength coefficient(deg./nm) | 1310 | 0.056 | 0.089 |
1550 | 0.040 | 0.064 | |
Extinction fario(dB) | 1310 | >38 | >41 |
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4.Conclusion |
Ultra-speed and large-capacity optical fiber trunk systems are expanding as a result of the development of optical fiber amplifiers. In parallel, the demand for optical isolators is increasing. Demand is also expected to increase, as LAN and other subscriber optical fiber networks expand. It is therefore imperative that isolators and other optical compenents be further improved to achieve higher performance,smaller size, and lower price. Currently, however, a vital part of optical component production depends on human skill and know-how, so this poese limitations on the ability to meet callenge, therefore, is to develop new techonologies for the production of isolators and other optical components. |
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