1
introduction
Semiconductor
lasers, optical amplifiers and optical fiber lasers from the connector, fusion
point, filter the reflection light is very sensitive, and may cause performance
deterioration and even damaged, requiring a optical isolator to prevent the
reflection of light. The optical isolator
is permitted only light along one direction through and in the opposite
direction blocks light through the optical passive devices. In the optical
fiber communication, optical fiber reflection light through the optical
isolator can be a good isolation. In the fiber laser applications, optical
isolators are usually used in the optical path to avoid the light path of the
light source, the echo on the pumping source and other light emitting device
causes interference and damage. Isolators’s isolation represents the optical
isolator to echo the isolation (blocking) ability.
2
optical isolator principle
Optical
isolator using magnetic optical crystal Faraday effect ( also known as the
Faraday effect ). In 1845, Faraday first observed with optical material under
the action of magnetic field to make the material in the direction of
polarization rotation, therefore often called the Faraday effect. In Faraday
effect, the rotation of the polarization direction direction and magnetic
field, and the orientation of the light transmitting is independent of the
forward and reverse, and we usually in the index of refraction, reflection
phenomena seen in the reversibility of optical path difference. Along the
magnetic field direction of transmission line polarized, the polarization
direction rotating angle θand magnetic field strength of B and L is
proportional to the product of the length of the material, the proportion
coefficient is what we often say that the Wilde constant. Optical
isolator based on polarization characteristics can be divided into
polarization-independent and polarization dependent type. These two kinds of
isolators are used with the Faraday effect in magneto-optic crystal, Faraday
magnetic medium in 1~2μ m wavelength range usually adopts the optical loss low
yttrium iron garnet ( YIG ) single crystals. Model of input and output of the
fiber optical isolator has fairly good performance, the minimum insertion loss
of approximately 0.5 dB, the isolation of up to 35～ 60 dB, a maximum of 70
dB. The optical isolator using most
still is polarization independent type, its principle is shown in Figure 1,
using the forward and reverse transmission optical path is inconsistent, it is
this time signal transmission is not reversible, thereby forming isolation. The
typical structure of only four major components: the magnetic ring, a Faraday
rotator, two pieces of LiNbO3 wedge angle piece, with a pair of fiber
collimator, can be made into an in-line optical isolators.
Positive
transmission: the parallel light beam from the collimator, into the first wedge
angle piece P1, beam is divided into o light and e light, the polarization
direction perpendicular to the propagation direction, forming an included
angle. When they pass through 45o Faraday rotator, emitted by the o light and e
light polarizing surfaces of respective to the same direction of rotation 45o,
because the second wedge-shaped plate P2 crystal axis relative to the first
wedge angle piece is just in a 45o angle, so o light and e light is refracted
into a small space, synthesis. Parallel light, and then by another collimator
is coupled to the optical fiber core. In this case, the input optical power
only a very small fraction of outage, this loss is called isolator insertion
loss.
Reverse
transmission: when a beam of parallel light reverse transmission, first with a
P2 crystal, divided into the polarization direction and P1 crystal axis
respectively in 45o angle o light and E light. Due to the Faraday effect non
reciprocity, O Light and e light through the Faraday rotator, the polarization
direction to the same direction of rotation 45 °, so the original o light and e
light in the second wedge-shaped plate ( P1 ) later became e and O light.
Because the refractive index differences, the two light beam in the P1 no
longer possible synthesis of a parallel beam of light, but in different
directions to the refraction of light, e and o are further separated from a
larger perspective, even after a GRIN lens coupling, can not enter the fiber
core to, from and achieved reverse isolation purposes. The transmission loss is
bigger, this loss is called isolators isolation.
3
main technical parameters of optical isolator
The
optical isolator, the main technical indicators have insertion loss, reverse
isolation, return loss, polarization dependent loss, polarization mode
dispersion.
(1)
insertion loss ( Insertion Loss ): isolator core mainly comprises a Faraday
rotator and a two piece of LN wedge angle piece, a Faraday rotator extinction
ratio higher, lower reflectivity, absorption coefficient is smaller, insertion
loss is smaller, general Faraday rotator loss is about 0.02～ 0.06dB. Parallel light
pass through the isolator core, will be divided into o, e beams of parallel
light. Due to the inherent characteristics of birefringent crystals, O Light
and e light can not fully converge, which may cause additional insertion loss.
(2)
reverse isolation ( Isolation ): reverse isolation isolator is one of the most
important indicators, which characterizes the isolator on the reverse
transmission attenuation ability. Effect of isolator isolation of many factors
: 1 ) the isolation and polarizer from the Faraday rotator is related to the
distance; 2) isolation and optical element surface reflectance relationship.
Isolator optical element surface reflectance is bigger, the isolation degree is
worse. The practical technology that R must be less than 0.25%, to ensure the
isolation degree is greater than 40 dB; 3) isolation of polarimeter and wedge
angle, spacing. Double refraction crystal yttrium vanadate ( YVO4 ) of the
optical isolator, when the wedge angle of less than 2°, isolation with the
perspective of the increase, when the wedge angle is greater than 2°, change is
much smaller, approximately stable at about 43.8 dB. Optical isolation with the
increase of the distance between the change range is not big, because isolation
depends mainly on the reverse output light and the angle between the optical
axis; 4) isolation and crystal axis angular relationship relative. The two
polarizers and rotator crystal axis relative angle to the isolation effect is
maximum, when the angle is greater than the difference between the 0.3o
isolation will not be greater than 40 dB; 5) the two polarizer extinction
ratio, crystal thickness on isolation effect; 6) the influence of temperature
and magnet. In Faraday effect, Verdet constant is a function of temperature, so
the Faraday rotation angle will change with the temperature, and the
temperature will be on permanent magnet performance impact, so it is one of
important factors.
(3)
return loss ( Return Loss ): optical return loss refers to the positive
incident to the isolator optical power and along the input path to return to
the isolator input port of the optical power ratio, this is one of the
important indicators, because the echo intensity, isolation would be affected
by. Isolator echo loss by each element and the air refractive index mismatch
caused by the reflection. The generally planar element caused by echo return
loss is controlled in 14 dB, through antireflective film and surface polishing
can make the return loss reached more than 60 dB. Optical return loss mainly
from the collimated light path (i.e., collimator parts), through the
theoretical calculation when the slant angle 8 °, return loss is greater than
65 dB.
(4)
the polarization-dependent loss ( Polarization Dependent Loss, PDL ) :PDL and
insertion loss is different, it is a when the input light polarization state
changes and other parameters unchanged, the insertion loss of maximum
variation, is a measure of device insertion loss by effect of polarization
degree index. The polarization-independent optical isolator, the device has
some may cause polarization components, impossible to achieve PDL is zero, a
generally accepted PDL is less than 0.2 dB.
(5)
the polarization mode dispersion ( Polarization Mode Dispersion, PMD ) :PMD is
defined through the device of the signal light with different polarization
states of the phase delay between, in high speed optical communication system
is very important in PMD. In optical passive devices, different polarization
modes have different propagation paths and different propagation speed, produce
corresponding polarization mode dispersion. At the same time, because the light
source spectrum lines have a certain bandwidth, can also cause certain
dispersion. In a polarization-independent optical isolator, birefringent
crystal to produce two beams linearly polarized light in different phase
velocity and group velocity of transmission, which is PMD, its main source is
used for separation and convergence o light and e light of birefringent
crystal. It can be made of two linearly polarized optical path differenceΔ L
approximation. PMD is mainly affected by E and O optical refractive index
difference, therefore also has great relationship with wavelength.
Compared
with the common optical fiber communication system in the use of low power
optical isolator is compared, in the high power laser, optical isolator design
and production also exhibit differences, it is also in high power device is
designed to solve the main problems in the development of.
(1)
the optical element at a high power density laser radiation damage problems.
Not only is this problem in a high power optical isolator in existence, is the
other high power optical device design process is also to face. In order to
solve this problem, first of all need to products in the production and testing
process to ensure good environmental cleanliness and selects the damage
threshold of high optical device and optical thin films, of course it is cost
constraint. Because the air in the tiny particles if adhesion in optical
surface will greatly reduce the laser damage threshold of optical surface,
these tiny particles on laser absorption is relatively large, easily lead to
particle near the energy is concentrated, resulting in optical surface film
damage even surface damage, the element surface pitting and even small pit to
device failure. Secondly, because in most cases within the optical element
damage threshold than the surface laser damage threshold is much higher, so the
surface of the laser power density is determined by the whole device resisting
laser damage ability, especially in the pulse work situation is even more so.
This can be through optical transform method to make optical element surface
spot area expansion method to increase the damage threshold, such as expanded
core fiber and beam expanding lens optical method is the use of the principle
of work, or by changing the laser pulse stretching method to reduce the power
density of laser, laser energy in space and by avoiding time of concentration
can effectively improve product for resisting laser damage properties.
(2)
the high power device for thermal effects and thermal design. Because of the
high power device to work in a higher power, and low power devices compared,
easy fever, inevitably subjected to temperature rise, so the device performance
by the thermal characteristics and thermal design to compare the effects of
severe. Usually the optically active crystal optical rotation characteristic of
easily affected by temperature, if the device is operating due to the
absorption of laser energy accumulation and lead to internal temperature
appears bigger rise, will make the optically active crystal on light
polarization plane rotation angle deviations from normal values and lead to
significant performance loss, serious and even lead to damaged devices; in
addition, the permanent magnet at work under high temperature but also more
prone to field weakening and demagnetization phenomenon, appear even the
magnetic field of the irreversible loss, so the high temperature to the
permanent magnet steady work is negative; and, in case of high optical power,
optical element temperature will appear bigger rise, due to heat from the
inside to the conveying surface, its internal the temperature is above its
surface temperature, so that it will in the optical component internal
temperature gradient and thermal stress, causing the beam cross-sectional
internal center of the refractive index and the edge of the refractive index
change in different extent, appear thereby the refractive index difference,
also is the emergence of lens effect, it will change the beam propagation
characteristics, leading to beam quality drops badly, seriously affect the
normal work and even cause damage to device. Therefore, we must take effective
measures to reduce the absorption of laser radiation and effective. To reduce
the absorption of laser selected absorption coefficient smaller optical
materials, Ko Hikaru in the components inside the transmission distance,
reasonable structure design, effective heat dissipation requirements may arise
in heat accumulation place provides effective heat transfer path and heat
dissipation, according to the size of power can adopt a passive or active heat
radiation method. The million kilowatts level optical isolation design on the
use of the lath shape of the optically active crystal to improve device cooling
temperature control ability.
(3)
the magnetic field design for high power isolator. High power optical isolator
design another key is the magnetic field and magnet design and selection. In
general, the optical isolator is the use of magnetic rotation effect work, so
must the optically active crystal with proper magnetic field. In order to
energy saving and convenient use, generally by the strong permanent magnetic
material to produce a desired magnetic field, the magnetic field and magnet
selection and design is very important, on device performance and the cost of.
Under normal circumstances required in the optically active crystal space to
provide a strong homogeneous magnetic field, so it can reduce the optically
active crystal size, high ratio of performance to price, so the requirement in
without significantly increasing the device volume in the case of the design of
suitable magnet to obtain a strong homogeneous magnetic field. In specific
design, through the choice of magnetic strong magnets, and adopt suitable shape
and volume, to obtain the required magnetic field.
(4) High power optical isolator assembly
process. High power optical isolator can work stably for a long time in bad
environment, this device structure and assembly process raised very tall
requirement. Design of the structure and assembly technology can effectively
reduce the optical components of the internal stress, thereby improving the
product performance and stability, allows the device to long-term stable and
reliable work. Isolator structure design mainly need to solve two problems,
first is the optical components of the assembly, stable and reliable heat
dissipation requirements, can effectively control; second is firm and reliable
assembly of strong permanent magnet, with the magnet design and manufacturing
capabilities, devices may use more complex shape of the magnet pieces combined
to provide a strong homogeneous magnetic field, between the magnets and strong
magnetic requires the design of suitable assembly method and reliable assembly
magnet, and required in the assembly process causes no damage or the magnet
demagnetization. These need to be accumulated in practice and improve.
Above
only briefly in the high power optical isolator design process often encounter
some problems, along with applications to expand and deepen, may be needed for
isolator corresponding improvement or design to meet the technical and market
development, in this process may occur early in the design of possibly
unforeseen problems, this requires us to according to the specific
circumstances to provide the corresponding solutions, only in this way can we
continue to design excellent performance to meet the application needs of the
high power optical isolator.
