Fusion splicing of PM fibres.pdf

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 15, NO. 1, JANUARY 1997 125

Automated Fusion-Splicing of PolarizationMaintaining Fibers

Wenxin Zheng, Member, IEEE

Abstract An advanced splicing technique for polarizationmaintaining (PM) fibers has been derived based on the polariza-tion observation by lens-effect-tracing (POL) method. With thistechnique, azimuthal alignment on common types of PM fiberscan be automatically performed in a passive way by an automatedfusion splicer. Because the method permits an accurate estimationof the splices extinction ratio before and after actual splicing,the quality of the splice can be estimated without having to makean active measurement. The experiment results illustrated in thepaper show a mean extinction ratio 32.2 with 2.73 dB standarddeviation and mean difference between measured and estimatedextinction ratio 0.18 dB, respectively, for six different types ofPM fibers.

I. INTRODUCTION

IN recent years, polarization maintaining optical fibers (PM-fibers) have become of great interest for the constructionof interferometric sensors and the newly emerging coherent

communication systems. The fast progress in PM-fiber devel-

opment for optic gyroscope [1], for communication systems

with erbium-doped fiber amplifiers [2], and especially for

decreasing the manufacturing cost (e.g., 1 USD per meter, [3])

has brought a bright future for wide applications of PM-fibers.

The polarization maintenance in an optical fiber is achieved

by introducing an azimuthal asymmetry in the fiber structure,

suppressing the cross coupling of power between the twoperpendicularly polarized modes. Such asymmetry can be in

the shape of the structure, creating a geometrical birefringence,

or in the internal physical stresses of the structure, a stress-

induced birefringence. Geometrical birefringence can be intro-

duced by, e.g., constructing an elliptically-shaped core region

of significantly higher index than the surrounding cladding [see

Fig. 1(a)]. Stress-induced birefringence is imparted by stress

applying parts (SAPs) that cause an anisotropic refractive

index difference in a circular core fiber that limits the coupling

of the two polarization modes. The most popular variations

of this type include: bow-tie fiber [see Fig. 1(b)], elliptically-

shaped stress jacket around a circular core [see Fig. 1(c)], and

PANDA fiber [see Fig. 1(d)].In a system, the polarization maintaining properties of the

PM-fibers can only be fully exploited if the polarization can

also be maintained across the splices. Although extensive

research has been performed on fusion splicing techniques for

many types of axial symmetric fibers (cf., e.g., [4] and [5]),

the splicing of PM-fibers demands extra effort to overcome the

Manuscript received November 17, 1995; revised September 5,1996.The author is with Ericsson Cables AB, Network Products Division, 172 87

Sundbyberg, Sweden.Publisher Item Identifier S 0733-8724(97)00706-8.

difficulty of correct rotational alignment as well as the mea-

surement of the splices extinction ratio. These requirements

are in addition to alignment in the , and directions, which

is the only criteria for splicing standard fibers.

There are two main approaches to align PM-fibers az-

imuthally: active alignment and passive alignment methods.

With the active method, a light source, a splicer with

rotators, and a detector should be used. They are often placed

in two or three locations when used in the field or factory.

A beam of linearly polarized light is launched into the PM-

fiber while transmission or reflection of the polarized light is

measured simultaneously (a typical setup is shown in Fig. 2).Good alignment can be achieved by maximizing the extinction

ratio at the output of the fiber while rotating one fiber with

respect to the other at the splicing point [6], [7]. Other

proposed alignment techniques, specifically intended for fusion

splicing, involve monitoring the power reflected from the

fiber end-face [8], measuring polarization dispersion with short

pulse modulation [9], or using a Michelson interferometer and

a broadband source [10], etc. With the active method, it is

often difficult to get rid of those bulk opto-electronic devices,

delicate optics, and a tedious and time-consuming process [11].

Moreover, for splicing PM-fiber pigtails of some devices, it is

extremely difficult to use the active alignment method.

The passive azimuthal alignment method can also be re-ferred as one point method. All alignment is done locally at

the splice point with the help of digital imaging techniques.

Several approaches in this area are developed in combination

with automated fusion splicers, which are equipped with

a digital imaging system. The intensity profile analyzing

method with a side view image processing system is developed

for PANDA fiber (see [12] and [13]). An average angle

misalignment of 1.3 degree was reported. However, since

the method is strongly fiber design dependent, the algorithm

developed for one fiber type does not work for another

fiber type. It becomes unrealistic because of the frequent

changes and rapid developments in PM-fiber designs as wellas the increasing requirement of inter-connection between

different types of PM-fibers. As another alternative, the end-

view image processing technique (see [14], [15], and [16])

is also developed. With this technique, some fiber types

which could not be treated before are able to be azimuthally

aligned successfully. The main drawback of the end-view

technique is that it can only be employed before splicing.

Since the end surfaces disappear after splicing, this technique

can not be used to check the angular offset deviation caused

by imperfect cleaving angles during the fusion [17]. It is,

07338724/97$10.00 1997 IEEE


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ZHENG: AUTOMATED FUSION-SPLICING OF POLARIZATION MAINTAINING FIBERS 127

Fig. 3. Theoretical polarization extinction ratio (ER) degradation, D

, dueto the angle-offset between principal axes at the splice point B . The originalextinction ratio of the setup (without splice), measured at the point B , is usedas parameter to calculate the curves with (6)(8).

Fig. 4. Calculated final extinction ratio, C

, at the point C due to theangle-offset between principal axes of the splice at B . The original extinctionratio of the setup (without splice), measured at the point B , is used asparameter to obtain the curves with (7).

dB (7)

(8)

The degradation as a function of the angle-offset is

plotted in Fig. 3 for a set of values. It is clear from Fig. 3

that different quality of the measurement setup gives differentsensitivity to the angle-offset of the splice. One splice with 1

degree angle-offset in a system with 40 dB original extinction

ratio (ER) will cause a big degradation by 6 dB, while the

same splice in a system with 25 dB original ER makes almost

no difference to the system (about 0.4 dB degradation).

However, in most of practical applications the ER degra-dation is not the prime concern. Instead, the final extinction

ratio, , of the system after splicing measured at point

is often what people use to evaluate a PM-fiber splicer. To

find out an acceptable tolerance to the angle-offset of splices,

the relation between and is illustrated in Fig. 4. For

Fig. 5. Imperfectly cleaved fiber endfaces generate torsion at the momentwhen fibers touch in the splicing process.

Fig. 6. The setup for azimuthal position observation with the lens effecttracing technique.

instance, if the ER of a setup is measured 30 dB at the point

, and the specification of the whole system after splicing is

25 dB, from Fig. 4 one gets 2.5 degree angle-offset toleranceat the splice point. By assuming a perfect setup with infinite

extinction ratio, as people often do when they use the formula

(5) instead of (7), one may get a considerably large error in

either angle-offset calibration or ER estimation for

degree as shown in Fig. 4.

The extinction ratio degradation is generated not only by the

birefringent axis misalignment at the splice point, but also bythe transverse misalignment and the imperfect cleave-angles

of the fiber ends. The degradation from the transverse mis-

alignment is often neglectable comparing with that from the

birefringent axis misalignment as reported in [20]. However,

despite precise alignment of the birefringent axes before fusionsplicing, a large angular misalignment (more than 1 degree, cf.,

[17]) will occur during fusion, due to the torsion generated at

the moment when the imperfectly cleaved fiber endfaces with

a certain degree of inclination collide with each other in the

splicing process (illustrated in Fig. 5).

In general, a good PM-fiber splicer should eliminate the

extinction ratio degradation at the splice point as much as

possible, should locally estimate the final extinction ratio aftersplicing, without using any external measurement equipment

such as the light source, polarizer, and detector shown inFig. 2.

III. POLARIZATION OBSERVATION BY

LENS-EFFECT-TRACING (POL) TECHNIQUE

When a fiber is illuminated from its side, due to the

circular cross section of the fiber, the fiber itself works as

a cylindrical lens (see Fig. 6). If an image is taken from the

other side of the fiber, by moving our observation plane, we

can find the maximum contrast of light intensity at the fiber


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128 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 15, NO. 1, JANUARY 1997

(a) (b)

(c) (d)

Fig. 7. Some examples of measured POL profiles for the four common types of PM-fibers shown in Fig. 1: (a) elliptic core fiber, (b) bow-tie fiber,(c) elliptic jacket fiber, and (d) Panda fiber.

center position. The measurement of the maximum contrast

is therefore possible without any interference from parameters

of the fiber design. When the PM-fiber is rotated, the heightof the light contrast changes at the focus point because the

refractive-indices of the PM-fibers are rotationally asymmetric.

However, the shapes of the light intensity profiles remain

similar (with only different value), and independent of the

azimuthal position and fiber type.

By tracing the contrast, , while rotating the fiber 360

degrees, a profile of the can be obtained in relation to

the rotation angle. The profile is termed POL (polarization

observation by lens-effect-tracing) profile, and the contrast, ,

is in turn termed POL value in this article. The unit of the

POL value is in gray scale. The range of the gray scale is

determined by the A/D (analog-to-digital signal) converter of

the charge-coupled camera and the algorithm for searchingmaximum between pixels. Since the refractive-index profiles

of all types of PM-fibers are always rotationally asymmetric,

the POL profile of a PM-fiber is never a straight line. The POL

profiles for four common types of PM-fibers are measured and

plotted in Fig. 7.

Since most of automated fusion splicers in the world are

already equipped with side view illumination and digital

imaging system, it is not difficult to make a modification to

take POL profiles. Two controllable rotators are needed to

rotate fibers at least 360 degrees. The observation plane of

the imaging system has to be moved from a position inside

the fibers to outside for obtaining maximum lens effect of the

fibers.

With the lens-effect-tracing method, the azimuthal positionof a PM-fiber is recognized only after fiber rotation. Before

rotation, a single image can be used to measure the POL value

but it does not aid in determining the azimuthal position.

Note that a common single-mode fiber (not PM) with normal

cladding circularity displays basically a horizontal line for

its POL profile and yields no information on its azimuthal

position.

IV. POL CORRELATION FOR ANGLE-OFFSET CALCULATION

With the help of the POL profiles, algorithms can be

developed to find out the polarization angle-offset of two

PM-fibers to be or being spliced. They constitute the funda-

mental procedure for PM-fiber principal axes alignment andprospective extinction ratio estimation.

To accomplish the PM-fiber azimuthal alignment and extinc-

tion ratio analyzes, two automated rotators, which are driven

by precision step motors and custom software, are installed

in an automatic fusion splicer equipped with a digital imageprocessing system. The splicer can rotate the PM-fibers to a

specific position and at a desired speed. The image of fibers

can be taken and the POL data of the two fibers can be obtained

in real time during the rotation. Rotating the two PM-fibers to

be spliced 360 degrees, and taking continuously POL samples

results in producing two POL profiles, which can be denoted


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(a)

(b)

Fig. 8. POL profiles of Panda coupler fiber (80 m) and computed directcorrelation coefficient profile with the cubic spline interpolation to the rightPOL array. The angular offset between the two fibers is found to be 8.4degree: (a) POL profiles of left (upper) and right fibers and (b) correlationcoefficient profile.

Besides the correlation methods, there are several other

methods to align the principal axes with the help of the POL

profiles. For instance, one can search the characteristic points

of the POL profiles, such as maximum, minimum, middle

points, etc., and then align those points together (see [22]). The

characteristic point method works for some types of PM-fiber

if the fiber is very well cleaned and the imaging system has a

very high signal-to-noise ratio. In other words, this method israther noise sensitive, since the alignment accuracy is totally

relying upon a few image points.

On the contrary, the correlation methods presented in this

paper is rather noise insensitive. The angle-offset is computed

from the two entire POL profiles. If there are corrupted

data of a few points, the maximum value of the correlation

will change. However, the location of the maximum point

(corresponding to the angle-offset or principal axis orientation)

will almost remain still. In Fig. 11(a), a measured data set of

original POL profile and three simulated profiles by adding to

the original 20% of white noise, noise and dust, and a sine

(a)

(b)

Fig. 9. POL profile of an elliptical cladding fiber (80 m) and the simulatedPOL profile. From the correlation profile in (b) the azimuthal position of thefiber is found to be 0 9.32 degree: (a) measured and simulated POL profilesand (b) correlation coefficient profile.

disturbance to imitate an eccentric coating, respectively, are

plotted. The four POL profiles are used to calculate correlation

with a POL profile measured on the other side of fiber. Even

though the calculated correlation profiles are very different

and the values of the maximum are far from each other, the

location of the maximums are the same. We obtain the same

angle-offset from the original data and the corrupted data.

In experiment, the correlation methods are found to be

more accurate and more robust than other characteristic-pointmethods.

V. EXPERIMENT RESULTS

When the angular offset between the fibers is known, as

well as the azimuthal orientation of each fiber, the fiber can be

rotated to any position with the controllable rotators to obtain

the desired angular offset and orientation. For example, if the

same type of PM-fibers are to be spliced, the angular offset

would be set to 0 degrees. If a stress-induced birefringence

PM-fiber is to be spliced to a geometrical birefringence PM-

fiber (cf., Fig. 7), the angular offset would be set to 90 degrees.


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(a)

(b)

Fig. 10. POL profile of an elliptical core fiber (125 m) and the simulatedPOL profile. From the correlation profile in (b) the azimuthal position of thefiber is found to be 0 4.84 degree: (a) measured and simulated POL profilesand (b) correlation coefficient profile.

If a depolarizer is to be made, the angular offset can be set

to 45 degrees.

Although the angular offset analysis and the rotational align-

ment can be accomplished with high accuracy (theoretically

to about 0.1 degree) during a normal splicing procedure,

the accurately aligned angular offset cannot be maintained

without change because of the imperfect cleaving angles of

the fiber ends. At the moment when the two fibers touchduring the splicing procedure, the imperfect cleaving angles

generate a torsion that twists the fibers. This torsion produces

an undesired deviation in the angular offset, as illustrated

previously in Fig. 5. In practice, the larger the cleaving angle,

the greater the possibility of having an angular offset deviation

after splicing.

To avoid system inaccuracy resulting from an angular offset

deviation, two important factors must be evaluated. First, the

cleaving angle must be checked before splicing. Second, the

angular offset must be measured after splicing, i.e., to estimate

the extinction ratio degradation due to the splice. The term

(a)

(b)

Fig. 11. (a) A measured data set of original POL profile and three simulatedprofiles by adding 20% of white noise, 20% noise plus dust, and a sinedisturbance to imitate an eccentric coating, respectively. (b) The computedcorrelation profiles with the four POL profiles shown in (a).

estimation is used instead of measurement, because we

want to distinguish the extinction ratio measurement done

passively by the splicer and that done actively by an optical

setup in Fig. 2.

After splicing, the angular offset can be determined by

first re-rotating the spliced fibers 360 degree and measuringPOL profiles. Then, direct or indirect correlation methods are

used to calculate the angular offset. When the angular offset

is established, the extinction ratio is estimated with (7)

and (8). In order to make an accurate estimation from (8), a

knowledge of the original extinction ratio of the system, ,

is necessary. However, in the case of lacking the knowledge,

a default value of (e.g., 40 dB) can be employed to get a

reasonably good ER estimation.

With an automatic splicer shown in Fig. 12, equipped with

the method presented in this paper, PM-fiber splicing is as easy

as ordinary fiber splicing. No optical equipment is necessary


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Fig. 12. Fully automated PM splicer equipped with software for aligning andsplicing all types of PM-fiber and estimating the extinction ratio after splicing.

Fig. 13. Measured extinction ratio distribution of splices of elliptical corePM-fiber (125 m).

for performing active measurement. No fiber-type selection

is necessary to make the splicer work. No extra training is

necessary to qualify the operators.However, in order to verify both theory and machine, ex-

tensive tests have been performed to actively measure splices

with a setup of 38.5 dB original extinction ratio (cf., Fig. 24).

In Figs. 1316, the measured extinction ratios are plotted for

splices of 4 major types of PM-fibers. Examples of splice loss

of PM-fibers are shown in Figs. 17 and 18. In Figs. 19 and

20, the estimated extinction ratios are compared with those

measured extinction ratios for splices with intentionally setangle-offset from 0 to 22 degrees.

In these figures, denotes the total splice number in the test

of the corresponding fiber type, STD stands for the standard

deviation and Diff for the difference between measured and

estimated extinction ratios, respectively. Some of the tests have

been done repeatedly with three different splicers.

More splicing results can be found in [21] about intercon-

nections between PM or PZ fibers of different types (such as

E-jacket to E-core, Bowtie to Panda, etc.), of different sizes (80

to 125 m cladding diameters), and of different strip lengths

(18 mm standard or 5 mm for high strength splicing).

Fig. 14. Measured extinction ratio distribution of splices of bow-tie PM-fiber(125 m).

Fig. 15. Measured extinction ratio distribution of splices of elliptical jacketPM-fiber (80 m).

Fig. 16. Measured extinction ratio distribution of splices of Panda PM-fiber(125 m).

VI. CONCLUDING REMARKS

A new method for passive PM-fiber azimuthal alignment

and extinction ratio estimation are developed and discussed in

the paper.

With a fully automated splicer, the PM-fiber splicing can be

completed without using any additional optical equipment. The

splicer with its automatic rotators and the software is enough to

do all the alignment, splicing, and extinction ratio estimation.


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Fig. 17. Measured splice loss for Panda PM-fiber (125 m).

Fig. 18. Measured splice loss for bow-tie PM-fiber (125 m).

Fig. 19. Comparison of measured and estimated extinction ratio for fivedifferent PM-fiber types. Low extinction ratio is obtained by intentionally

setting angle-offset to 222 degree.

The new method works automatically for all major types of

PM-fibers, such as Bow-tie, PANDA, elliptical jacket, elliptical

core fibers. It also operates with 80 micron and 125 micron

outer diameter fibers, PM coupler fibers, without changing any

calculation parameter.

The mean extinction ratio from the splice experiment is

about 32 dB for all fiber types with 2.74 dB standard deviation.

The mean difference between the estimated and measured

extinction ratio is about 0.18 dB with 1.16 dB standard

deviation.

Fig. 20. Comparison of measured and estimated extinction ratio for PandaPM-fiber. Low extinction ratio is obtained by intentionally setting angle-offsetto 022 degree.

The total working time, from loading the fibers to comple-

tion of the splice, takes about 2 min without including the

estimation step for the extinction ratio, and 2.6 min when an

estimation is included.The new technique is not only limited to PM-fiber splicing.

It can also serve as a flexible platform for developing more

sophisticated optical fiber systems with D-shaped [23], V-

shaped, twin-core [24], multi-core [25], and all other types

of axial asymmetrical optical fibers.

ACKNOWLEDGMENT

The author wishes to express his appreciation to O. Hulten

for his support throughout the research, to B. Sundstrom for

fruitful discussion, to M. Bengtsson for hardware and software

development of test setup, and to M. Berse for all splice

experiments.

REFERENCES

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[3] Y. Mitomi, B. Yoshida, and V. Martinelli, International cooperationbring fiber optic gyroscopes to market, Photon. Spectra, pp. 8896,July 1995.

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[6] R. O. Miles, A. Ceruzzi, and M. J. Marrone, Attaching single-modepolarization-preserving fiber to single-mode semiconductor lasers,

Appl. Opt., vol. 23, pp. 10961099, 1984.[7] M. Suzuki, Y. Kikuchi, T. Yamada, and O. Watanabe, Arc fusion

splicing machine for single polarization single mode fibers, in EuropeanConf. Optic. Commun. (ECOC83), 1983, p. 177.

[8] Y. Kato, Fusion splicing of polarization preserving fibers, Appl. Opt.,vol. 24, pp. 23462350, 1985.

[9] Y. Sasaki, N. Shibata, and J. Noda, Splicing of single-polarization fibersby an optical short pulse method, Electron. Lett., vol. 18, p. 997, 1982.

[10] K. Takada, K. Chida, and J. Noda, Precise method for angular align-ment of birefringent fibers based on an interferometric technique with abroadband source, Appl. Opt., vol. 26, no. 15, pp. 29792987, 1987.

[11] N. Kashima, Passive Optical Components for Optical Fiber Transmis-sion. Dedham, MA: Artech House, 1995, ch. 9.


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[12] H. Taya, K. Ito, T. Yamada, and M. Yoshinuma, New splicing methodfor polarization maintaining single mode fibers, in Conf. Optic. FiberCommun. (OFC89), THJ2, 1989.

[13] H. Taya, K. Ito, T. Yamada, and M. Yoshinuma, Fusion splicer forpolarization maintaining single mode fiber, Fujikura Tech. Rev., pp.3136, 1990.

[14] K. Itoh, T. Yamada, T. Onodera, M. Yoshinuma, and Y. Kato, Appara-tus for fusion splicing a pair of polarization maintaining optical fibers,U.S. Pat. 5 147434, Sept. 15, 1992.

[15] , Apparatus for fusion splicing a pair of polarization maintaining

optical fibers, US Pat. 5 156663, Oct. 20, 1992.[16] Vytran Corporation, Automated filament fusion splicing, Data Sheetof Vytran Corporation, 1995.

[17] A. Ishikura, Y. Kato, T. Abe, and M. Miyauchi, Optimum fusion splicemethod for polarization-preserving fibers, Appl. Opt., vol. 25, no. 19,pp. 34553459, Oct. 1986.

[18] R. B. Dyott, Method of determining azimuthal position of transverseaxes of optical fibers with elliptical cores, U.S. Pat. 3 323225, June21, 1994.

[19] S. L. A. Carrara, Birefringent-fiber splice alignment, in SPIE FiberOptic Sensors IV, vol. 1267, 1990, pp. 2428.

[20] J. Noda, N. Shibata, T. Edahiro, and Y. Sasaki, Splicing of singlepolarization-maintaining fibers, J. Lightwave Technol., vol. LT-1, pp.6166, Mar. 1983.

[21] W. Zheng, Auto-aligning and splicing PM-fibers of different typeswith a passive method, in SPIE Int. Symp., Fiber Optic. Geros: 20th

Anniversary Conf., Denver, CO, Aug. 1996, vol. 2837. pp. 356367.[22] , Azimuthal alignment and extinction ratio estimation for PM-

fiber splicing, in Tech. Dig. OPTO94, Paris, France, May 1994, pp.402406.

[23] T. Conese, G. Barbarossa, and M. N. Armenise, Accurate loss analysisof single-mode fiber/D-fiber splice by vectorial finite-element method,

J. Lightwave Technol., vol. 7, pp. 523525, May 1995.

[24] W. Zheng and O. Hulten, Twin-core fiber aligning and splicing withimage processing and real time control techniques, in Tech. Dig.

IOOC95, Hong Kong, June 1995, pp. 2223.[25] G. LeNoane, D. Boscher, C. Botton, P. Grosso, I. Hardy, J. C. Bizeul,

and A. LeMeur, Bunched multicore fibers: A new key for the futureFTTH networks, in Tech. Dig. OPTO95, Paris, France, May 1995, pp.228233.

Wenxin Zheng (M95) was born in Beijing, China,in 1953. He received the M.S. degree in electricalengineering from the graduate school of the NorthChina Institute of Electric Power in 1982, andthe Ph.D. degree in electromagnetic theory fromthe Royal Institute of Technology in Stockholm,Sweden in 1989.

He is now a Senior Specialist in Ericsson CablesAB, Sweden. His primary field of interest concernscomputer solutions of electromagnetic wave anal-ysis problems. He has been involved in research

concerning applying finite element and finite difference methods in guided-wave problems, developing null-field method for direct and inverse scatteringand resonance problems for composite objects, promoting applications ofthe mode coupling theory in splice loss analysis, improving fiber splicingtechnology, etc. He has published more than 30 papers in the PROCEEDINGS

OF THE IEEE, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES,IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATIONS, IEEE JOURNAL OFLIGHTWAVE TECHNOLOGY, Radio Science, Computer Physics Communications,etc. He has 22 patents on fiber splicing and image processing techniques.

Dr. Zheng is a member of SPIE.

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Fusion splicing of PM fibres.pdf