Shenzhen, China, Apr. 10, 2013 -- DK Photonics Technology Co., Limited, one of the leading companies in designing and manufacturing of high quality optical passive components mainly for telecommunication, fiber sensor and fiber laser applications, announced that the Company got a successful show on OFC2013 in Anaheim. DK Photonics took two weeks to attend OFC2013 in Anaheim and visit customers in USA, and then came back last Monday with successful achievement.

On the exhibition, DK Photonics showed our traditional products such as PLC Splitter, WDM, FWDM, CWDM, DWDM, OADM, Optical Circulator, Optical Isolator, PM Circulator, PM Isolator, Fused Coupler, Fused WDM, Polarization Maintaining Components, Pump Combiner, High power Isolator, Patch Cord. At the same time, we took some innovative products such as 2000nm passive components to the show.

Through the OFC2013, customers got a deeper understanding to DK Photonics, such as products portfolio, inner management system, quality system and some research plan. Here we hope to express our salute to customers and DK Photonics would continue to give customers’ good service and quality products.

About DK Photonics

DK Photonics Technology Co., Limited is one of the leading companies in designing and manufacturing of high quality optical passive components mainly for telecommunication, fiber sensor and fiber laser applications. Headquartered and factory are located in Shenzhen of China.

DK Photonics have a group who has more than eight years working experience experts in the field of optical components and optical fiber laser; they formed a professional high-end R&D team. Our products are widely used in modern fiber lasers, amplifier, telecommunications, scientific research and other high-tech field of optical communication.

For more information about DK Photonics, please visit: https://www.dkphotonics.com/ , or email: info@dkphotonics.com

 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.

 What is WDM?

In the same optical fiber at the same time can let two or more than two wavelength signal transmit and receive information through different optical channel, called wavelength division multiplex, referred to as WDM. Wavelength division multiplexing includes frequency division multiplexing and wavelength division multiplexing. Optical frequency division multiplexing (FDM) technology and optical wavelength division multiplexing (WDM) technology has no obvious difference, because the light is part of the electromagnetic wave, frequency and wavelength of light have a single correspondence. Usually also can understand so, optical frequency division multiplexing mean subdivision of optical frequency, very dense optical channel. Wavelength division multiplexing means divided frequency of light, light channel far apart, even in the optical fiber with different window.

The general application of division multiplexing wavelength is respectively using a wavelength division multiplexer and demultiplexer arranged at both ends of the optical fiber, coupling and separation of different wavelength. The main four types of WDM are fused biconical taper type, dielectric film type, FBG type and planar waveguide grating type .The main characteristic is the insertion loss and isolation. Usually, the optical link using wavelength division multiplexing equipment, increase the amount of optical link loss is called WDM insertion loss. When the wavelength transmission through the same optical fiber, the D-value between the splitter input mixed power and the output end of the fiber power is called isolation.. The following are characteristics and advantages of optical wavelength division multiplexing technical:

(1) Make full use of low loss band fiber, increase the transmission capacity of optical fiber, the physical limit of an optical fiber for transmitting information doubled to several times. At present, we only use the low loss optical fiber spectrum (1310nm-1550nm) a few, WDM can fully utilize the huge bandwidth of single-mode fiber is about 25THz, the transmission bandwidth is sufficient.

(2) There are ability to transmit  two or more than two asynchronous signal in the same optical fiber ,there are compatible for digital and analog signals, has nothing to do with the data rate and modulation mode, the middle line can be removed or added channel.

(3) About the optical fiber system that has built, especially early laying optical cable that core number not much, as long as the original system power is margin, we can increase the capacity; realize the transmission of multiple one-way or two-way signals without making big changes to the original system, so it has strong flexibility.

(4) Due to the large number of reducing use amount of the fiber, it can greatly reduce the construction cost, because the fiber quantity is less, when a fault occurs, the recovery is also fast and convenient.

(5) Sharing of active optical devices, the cost of transmission of multiple signals or increase new business will reduce.

(6)The active devices in the system have been substantially reduced, which improves the reliability of the system. At present, because of the light multi carrier division multiplexing of optical transmitter, optical receiver equipment's requirements higher, technology implementation has certain difficulty, also multiple core cable used in traditional broadcast television transmission business does not appear especially shortage, so the practical application of WDM is still not much. However, with the development of CATV integrated service development, the growing demand for network bandwidth, all kinds of selective service upgrade and network implementation economic cost considerations and so on, the characteristics and advantages of WDM in the CATV transmission system gradually emerged, showing broad application prospects, even influence the development pattern of CATV network.

What Is the Difference Between DWDM and CWDM Optical Technologies?

DWDM (dense wavelength division multiplexing) is undoubtedly the first choice technology in the field of fiber optic applications today, But the cause of high cost make many do not bounteous operators hesitating. Is there a lower cost for using the wavelength division multiplexing technology? In the face of this demand, CWDM (coarse wavelength division multiplexing) emerges as the times require.

CWDM, just as its name implies, is a dense wavelength division multiplexing next of kin, the difference between CWDM and DWDM mainly has two points: first, the CWDM carrier channel spacing is wider, therefore, light in a single fiber can reuse about 5 to 6 wavelengths, that is where the "dense" and "coarse" appellation come from; Two, CWDM modulation laser using uncooled laser, but DWDM is used in cooling laser. Cooling laser using temperature tuning, uncooled laser adopts electronic tuning. Because the range of temperature distribution is nonuniform in a very wide wavelength, so the temperature tuning is very difficult to realize, the cost is very high. CWDM avoids this problem, so it greatly reduces the cost; the whole CWDM system cost only 30% of DWDM.

CWDM provides a very high access bandwidth with a low cost, suitable for point to point, Ethernet, SONET rings and all kinds of popular network structure, especially suitable for short distance, high bandwidth, access point intensive, communication applications, such as network communication between the building or building. It is particularly worth mentioning is that CWDM with the use of PON (passive optical network).PON is a cheap, one-point to multi-point optical fiber communication mode, in combination with the CWDM, each individual wavelength channel can be used as virtual optical link of PON, Implementation of broadband data transmission between center node and multiple distributed nodes.

There are several companies are offering CWDM related products at present. However, CWDM is a product of cost and performance tradeoffs; inevitably there are some performance limitations. Industry experts point out, at present the CWDM have four following disadvantages: first, CWDM in a single fiber support multiplexing wavelength number is minor, leading to future expansion cost is high; second, multiplexing, multiplexing equipment cost should also be reduced, the device cannot be simply modified of DWDM corresponding equipment; third, CWDM does not apply to metropolitan area network, the distance between metropolitan area network nodes is short , the money that operators use in CWDM equipment expansion can be used to laying more fiber, and get better effect; fourth, CWDM has not yet formed standards.

What’s more, something about the WDM products.

(1)CWDM Mux/Demux module

 CWDM Mux and CWDM Demux are designed to multiplex multiple CWDM channels into one or two fibers. The core of CWDM Module application is the passive MUX DEMUX unit. The common configuration is 1×4, 1×8, 1×16 channels. Available in 19″ Rack Mount or LGX module package. Optional wide band port is available to multiplex with CWDM Channels wavelength.

(2)DWDM Mux/Demux Modules

DWDM Mux and DWDM DeMux are designed to multiplex multiple DWDM channels into one or two fibers. The common configuration is 4, 8, 16 and 40 channels. These modules passively multiplex the optical signal outputs from 4 or more electronic devices, send them over a single optical fiber and then de-multiplex the signals into separate, distinct signals for input into electronic devices at the other end of the fiber optic link

(3)Optical Splitter-- a important component in EPON network

Optical splitter in optical communication era is a component of EPON network construction, is a connection of OLT and ONU passive device.

Its function is to distribute the downlink data, and focus on the uplink data. Optical splitter has an upstream optical interface, a plurality of downlink optical interface. Optical signals from the upstream optical interface over was assigned to the downstream optical interface out all transmissions, optical signals from the downlink optical interface over being allocated to uplink optical interface out transmission only. The light intensity signal downlink optical interface of each can be same, can also be different.