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.
4 key technologies of high power isolator
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.
An