High-frequency fused high-voltage arc tapering process power supply for of fiber

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34 CHINA WELDING Vo1.22 No.2 June 2013High-frequencyfusedhigh·voltage arc power supply fortapering process of fiberYang Wei，Yang Tianbo，Wang Shufeng and ring Wen杨 威，杨天博，王树峰，荆 文Abstract The peormance ofthe opticalfiber coupler prepared withfused biconical taper method is determined by the qualityoffused stretching.Besides，the stable high-temperature heat source the esential guaranteeforfused stretching process to gosmoothly.The way 0厂using AC(alternating current)arc as the heat source is proposed in this article. addition，the paperalso introduces high-frequency high-voltage power supply with the characteristics of constant current and restricted voltage，which is composed ofpreceding voltage regulation part and following inverter。

Key words fiber coupler，fused tapering，arc heating，power supply0 IntroductionOptical fiber coupler is a kind of device that makesthe optical signal couple in the special coupling region andthen redistribute.It can transmit stably two orthogonal 1in-early polarized lights with low excess loss and polarizedcrosstalk，and has become an essential component of hi ghperformance FOG(fiber optic gyroscope)and fiber-optichydrophone[ -。

Optical fiber coupler is mainly made with fused bi-c0nical taper method[ -. Two or more naked optical fi。

bets without coating are fused at high temperature and arestretched in the opposite direction at the same time.Heat-ing and stretching are controled according to the propor。

tion between stretching length and spliting ratio which isobtained by monitoring the optical power coupling CHIVe。

Finally，the fibers are fused to gain bipyramid in the heat-ing region， and optical transverse coupling is realized。

There are many deficiencies remaining in the process ofoptical fiber coupler manufacturing，including low yieldand lack of consistency，mainly caused by high require-ment of precision machining。

The performance of coupler is determined by thequality of fused stretching[ -. While the stretch process-Yang Wei，School of Electrical Engineering and Automation，of Astronautics，Harbin Institute of Technology，Harbin，150006。

163414. Jing Wen，Harbin Welding Institute，Harbin，150028。

ing，the optical fiber can be melted by local high tempera-ture around，and be stretched by force.Since the diameterof optical fibers ranges in micron dimension，the surround-ing heat distribution is required to be controllable and uni-foril in order to avoid the external interferenoe。

The stable high temperature source is a critical guar-antee of the tapering fused process[ -. Curently.thereare mainly two kinds of heat source in practice.The firstis gas flame，such as oxyhydrogen flame and methaneflame.It is liable to reach a high temperature safely andreliably.However，it also has some deficiencies which re-strain high machining precision，such as non-uniform ten。

perature，external airflow vibrating interference to the ta-pering and combustion product disturbance to the stretc-hing.The second is electric heating.Using temperatureclose.1oop control，temperature drift is suppressed and u·niformity is improved obviously，and it is a feasible solu-tion for airflow vibrating interference and combustion prod-uct disturbance.Unfortunately，the components must bereplaced frequently because of their short life. In addi-tion.both of the methods mentioned require built-in tern-perature sensor in the heating region。

A heating method using AC(alternating current)arcHarbin Institute of Technology ，Harbin，150006. Yang Tianbo，SchoolWang Shung，The 2 Production Factory，Daqing Oilfield，Daqing，Yang Wei，Corresponding author，E-mail：yangv### hit.edu.anHigh-equency high-voltage arc power supply for fused tapering process of fiber 35generated by high·-frequency high-voltage power supply isproposed in the paper.Arc temperature is proportional toarc current，which can be effectively controlled. Usingcurent control，the method provides high precision of con-trol and little external interference， so the arc betweenspherical tip electrodes is stable and uniform in surround-ing temperature.In addition，the method has no need oftemperature sensor。

1 Power supply structure.1 0veral structureAs shown in Fig.1，the high-frequency high-voltageACAC square·-wave arc power supply consists of two-·stage cir·-cuit，including the preceding voltage regulation circuit andthe following inverter circuit.The control functions of cir-cuit include the controlling of frequency and amplitude ofthe AC square-wave，implementing constant-curent andrestricted-voltage control，switch of two control mode men-tioned above and communicating with the host computer。

In addition，the detection circuit of voltage and curent，protective circuit and host computer interface circuit arealso involved。

The single-stage inverter voltage amplitude is con·stant，so output curent can only be regulated by the dutyFig.1 Overal structure of power supplycycle.Therefore，when output curent is low and the dutycycle is short，current instantaneous value is zero for along time. So arc shape stability is affected， seriouswhen，arc extingu ishes.For this reason，single-stage cir-cuit is not applied，and two-stage circuit structure is de-signed to avoid problem mentioned.The preceding circuitis responsible for adjusting arc current by regulating theamplitude of AC square-wave.And the folowing one is re-sponsible for modulating frequency to output an ACsquare-wave with constant duty cycle(about 50%)。

1.2 Preceding voltage regulation circuitAfter rectifying circuit，DC bus vohage is regulatedby preceding circuit and delivered to following inverter。

Using step-up transformer， the following circuit isolatesoutput from input.As a result，non-isolated topology canbe applied as preceding circuit and BUCK converter isused as shown in Fig.2.The output voltage，which is pro-portional to the duty cycle，can be obtained by the dutycycle regulation of switch T1.The range of output voltageof preceding circuit is adjustable between 0 V and 300 V，so the peak--to·-peak value of square-wave can be rangedbetween 0 V and 300 V after inverting。

1.3 Following inverterThe latter circuit works as an inverter for the DC out。

put of the preceding circuit.Half-bridge converter is usedin the following circuit，since the full-bridge converter iswidely used in the high-power condition rather than thislight power supply.Compared with full-bridge converter，half-bridge converter requires only two switches and simpledriving.Furtherm ore，the voltage of the midpoint of twofilter capacitors in half-bridge circuit is floating，whichcan help to automatically balance the redundant energy in36 CHINA WELDING Vo1.22 No.2 June 2013ACPreceding Vohagedriving sampleFollowingdrivingCurentsampleFig.2 Power supply main circuitthe transformer and to enhance the capability of restrainingmagnetic bias.Special driving chip IR21 10，as shown inFig.3，which is high voltage，high speed power MOSFETdriver with independent high and low side referenced out-put channels，is used.The gate charge for the high sideMOSFET TI is provided by the bootstrap capacitor C2which is charged by the power supply VDD through thebootstrap diode D1 during the time when the low sideMOSFET T2 is on.So only a set of low side driving powersupply is needed。

DrvHDrvLVDDVSSFig.3 The following driving circuitThe frequency of the AC square-wave converted bythe inverter is adjustable between 20 kHz and 35 kHz。

With the effect of step-up transformer，the high-frequencyhigh-voltage AC square-wave，whose maximum peak-to-peak voltage reaches 20 kV.generates arc between elec-trodes。

2 Power supply control2.1 Output characteristicThe power supply output characteristic is constant-current and restricted-voltage.In order to stabilize the arccurrent，curent detection in the primary side of the step-up transformer is essential to achieve constant current con-trol with current regulator.Meanwhile，the maximum am-plitude of high AC voltage can be restricted by detectingoutput voltage of preceding voltage regulation circuit andcontrolling it with voltage regulator。

2.2 Arc ignition controlWhen distance between electrodes on the output sideis 10-12 mm，high-vohage alternating curent which hasa frequency of several tens kilohertz and a peak-to-peakvoltage of more than a dozen kilovoh，can ignite the arcdirectly. Manual ignition is also applicable by using ablade，which wil be removed rapidly after the ignition，toreduce the distance between the electrodes。

At the beginning of ignition，the voltage control loopgoes into operation and output voltage is restricted. Theoutput vohage，which is supposed to be far above the volt-age that maintains the arc after ignition，should be preseton the value that conduces to effortless ignition.After theignition，the curent control loop assumes contro1.Powersupply generates constant curent by hosting computercommand in order to keep arc temperature stable。

When the power supply works in unloaded state be-fore the ignition，there is no arc curent that is exportedfrom transformer secondary side.At the moment of igni-tion，if arc fails to form，there is also no arc current(transformer secondary curent).However，because ofthe influence of step·-up transformer distribution parame-ters，there is current in transformer primary side.Further-38 CHINA WELDING Vo1.22 No.2 June 2013The power supply has an adjustable output currentranging 5-20 mA and Fig.5 shows the arc states suppor-ted by different currents.Fig.6 shows experimental wave-forms including main body output voltage(transformer pri-mary voltage)and arc curent(transformer secondary cur。

4 ConclusionsAC arc is used as heat source for fused biconical ta-per.Compared with the gas flame and electric heatingmethod，AC arc has many advantages， such as highertemperature stability， less influence on the stretchingprocess and longer life. The two-stage structure， corn-posed with preceding voltage regulation part and followinginverter part，achieves the independent regulation of fre-quency and amplitude of square-wave for assurance of thearc stability.Because of characteristics of constant-curentand restricted-voltage，advantages of the convenience ofignition and arc curent stability，the power supply pro-motes higher machining precision and yield。

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