Integrated fiber Michelson interferometerbased on poled hollow twin-core fiber
Zhihai Liu,* Fusen Bo, Lei Wang, Fengjun Tian, and Libo YuanPhotonics Research Center, School of Science, Harbin Engineering University, Harbin 150001, China
Since Myers et al. demonstrated that thermal polingcould produce large second-order nonlinearities in fusedsilica [1], many researchers had made efforts to fabricatefunctional devices with poled silica fibers which havebeen used to realize electro-optic modulators for commu-nication applications [2], fiber optic voltage sensors forpower industry [3] and tunable visible sources frequencydoubling fiber lasers [4]. Poled fiber is an intrinsic phasemodulating device, and it is usually used to constructelectro-optic intensity modulators by constructing Mach–Zehnder or other types of interferometers [5,6]. Variouspoled fibers, such as D fibers [5,7], single-hole [8] fiber,and twin-hole fiber [9], have been employed for compos-ing poled fiber interferometers. The electrodes in thepoled fibers were obtained by side polishing fibers andsputtering metal films on the fiber surface, inserting thinmetal wires into the holes of the single- or twin-hole fi-bers [10], or introducing metal wires into a fiber duringits fabrication process [11]. Generally, a fiber Mach–Zehnder interferometer is composed of two independentarms, one of which contains the electro-optic device;therefore such an interferometer is sensitive to environ-mental fluctuations such as temperature and mechanicalvibration. To improve stability of the interferometer,Fokine set up an integrated Mach–Zehnder interferom-eter based on a twin-core fiber with two internal electro-des, and an optical switch was demonstrated [12].However, this kind of poled twin-core fiber with internalelectrodes can only be used independently, and it is dif-ficult to fusion splice to conventional single-mode fibers.In this Letter, we propose an integrated fiber Michel-
son interferometer based on a poled hollow twin-core fi-ber, which can be used as an electro-optic modulator byintroducing second-order nonlinearity in the twin-core fi-ber. The hollow twin-core fiber can be connected to com-mercial single-mode fiber with low coupling loss by usingthe “fusion-and-tapering” method. By inserting a piece ofwire into the hollow of the twin-core fiber and depositinga thin gold film on the cladding surface near the poledcore, the core between the internal electrode and thegold film can be thermal poled, which forms a tunableintegrated fiber Michelson interferometer. This compactand simple configuration can effectively compensate the
effects of temperature fluctuation and improve the stabi-lity of poled twin-core-fiber-based optical devices.
The end view of the hollow twin-core fiber is presentedin Fig. 1. The outer diameter of the fiber is 125 μm. Thediameter of the hollow is about 62 μm to facilitate insert-ing the inner poling electrode. The shape of the fibercores is an ellipse with an 8 μm major axis and a 4 μmminor axis. The two elliptical cores are symmetricallysuspended in the inner wall of the cladding. To ensurethat the twin-core fiber is a single-mode fiber at thewavelength of 1310 nm, we designed a thin cladding(about 3 μm) between the hollow and the cores.
The poling electrodes were fabricated as follows: a15 cm long protective plastic coating was first strippedoff from one end of the twin-core fiber. Then an 11 cmtungsten wire of diameter 45 μm was inserted into thehollow of the twin-core fiber, serving as the poling anode.The cathode was achieved by sputtering a 2 μm thick goldfilm on the cladding surface near the poling core. Thelength of the gold film was 9 cm, and it was about 2 cmaway from the fiber end to avoid air breakdown. Thecross-sectional view of the twin-core fiber with polingelectrodes is illustrated in the enlarged figure in Fig. 2,while its photograph is shown in Fig. 3.
The fusion-and-tapering technique was chosen to cou-ple light from a single-mode fiber into the twin-core fiber[9]. First, a piece of standard single-mode fiber wasspliced with the twin-core fiber by using a fiber fusionsplicer (type KL-260B), and then the splice point of the
Fig. 1. (Color online) End view of the hollow twin-core fiber(Media 1).
fiber was heated and drawn to gradually form a taperedzone by using a hydrogen flame. The light from the single-mode fiber was coupled into the cores of the twin-corefiber when it transmitted through the taper zone. Theo-retically, the coupling efficiency between a single-coreand a twin-core fiber can reach more than 90% [13,14].While in the present Letter, the actual efficiency wasabout 79.4% (1 dB loss), owing to the disability of theflame and the refractive index difference between thesingle-mode fiber and the twin-core fiber. Light signalstransmitting along the twin-core fiber were reflectedby the end face of the twin-core fiber and recoupled intothe single-mode fiber through the taper zone. Therefore,an integrated in-fiber Michelson interferometer based onthe hollow twin-core fiber was constructed. The end faceof the twin-core fiber was coated with a gold film byusing the sputter-coating method, and the reflectivityat the end face was enhanced to 90%.According to the mature poling technique, one core of
the twin-core fiber in the prepared fiber Michelson inter-ferometer was thermally poled at 3000V, 280 °C for15 min. In this Letter, the actual poling length of thetwin-core fiber was about 9 cm. When the poling processwas finished, the integrated Michelson interferometerwas connected in the experimental setup, which isshown in Fig. 4.Light at the wavelength of 1310 nm was coupled into
the twin-core fiber at the taper zone. The two beamstransmitting along the twin-core fiber were reflectedby the end face of the twin-core fiber and recombinedat the taper zone. The interference signal was collectedby a photodetector and displayed on an oscilloscope. If adriving voltage was loaded between the anode and thecathode of the twin-core fiber, the phase of the opticalsignal traveling along the core between the electrodeswill change, owing to the electro-optic effect, and then
the intensity of the optical signal detected by thephotodetector will change.
To demonstrate the response of the integrated fiberMichelson interferometer to the electro-optic effect, avariable DC voltage was applied on the two electrodesof the twin-core fiber. According to the electro-optic ef-fect, the optical phase of light along the poled core, andtherefore the intensity of the interference signal, will varywith the modulation voltage. The interference signalintensity was detected by an optical power meter. Theexperimental results of the phase change versus modula-tion voltage are shown in Fig. 5. From this figure, it canbe deduced that the half-wave voltage of the poled fiberis 135V, and the effective second-order nonlinear coeffi-cient is χð2Þ ¼ 1:23 pm=V [10]. Such a relatively high pol-ing efficient is due to two reasons: one is that the fiberMichelson configuration can double the poling efficiencycompared to the Mach–Zehnder configuration with thesame poling length; the other is that the distance betweenthe anode and the cathode is small (∼30 μm), especiallythat the distance between the anode and the poled corecan be as small as 3 μm.
We also studied the responses of the fiber Michelsoninterferometer to sinusoidal electrical signals with differ-ent frequencies. The experimental results of 149 and1000Hz signals are shown in Fig. 6, in which the dashedcurves present the driving voltages applied on the poledelectrodes and the solid curves are the optical intensitiesof the interference signal collected by the photodetector.From Fig. 6, we can see that the interference signals havethe same frequencies as the applied sinusoidal signals.Therefore, the integrated Michelson interferometerbased on the poled hollow twin-core fiber can be usedas a fiber electro-optic modulator.
It has to be noted that such an integrated fiberMichelson interferometer has a broad bandwidthbecause the bandwidth is inversely proportional to the
Fig. 2. (Color online) Schematic of the integrated fiberMichelson interferometer based on poled hollow twin-corefiber (Media 2): SMF, single-mode fiber.
Fig. 3. Photograph of the twin-core fiber with poling elec-trodes (Media 3).
Fig. 4. (Color online) Experimental setup of the integratedfiber Michelson interferometer (Media 4).
Fig. 5. (Color online) Relationship between phase change andmodulation voltage (Media 5).
optical path difference (OPD) between the arms of theinterferometer, which is quite small in the twin-core-fiber-based device [15]. In addition, the Michelson inter-ferometer may have a better temperature stabilitycompared with fiber interferometers that are composedof two separated single-mode fibers. However, the outputwill be affected by the temperature variation, because thegeometrical parameters and the refractive indices be-tween the cores of the twin-core fiber cannot be exactlythe same. Mechanical stability is another property thatshould be mentioned inasmuch as a bend in the twin-corefiber will introduce an additional OPD in the fiber inter-ferometer [14]. To improve mechanical stability, the
twin-core fiber, including the tapered zone, should bestraightly packaged in a quartz or metal tube.
In summary, an integrated in-fiber Michelson interfe-rometer based on a poled hollow twin-core fiber was de-monstrated. By thermal poling a piece of one core of thetwin-core fiber, the Michelson interferometer can beused as a fiber electro-optic modulator. We observed thatthe fiber Michelson interferometer had a good responsein the frequency from 149 to 1000Hz. The half-wave vol-tage of the poled fiber is 135V, and the effective second-order nonlinear coefficient is χð2Þ ¼ 1:23 pm=V. By usingthe fusion-and-tapering method, it is convenient to con-nect the in-fiber Michelson interferometer to various op-tical fiber systems.
This work was supported by the FundamentalResearch Funds for the Central Universities and by theNational Natural Science Foundation of China (NSFC)under grant numbers 60807032 and 61077062 to HarbinEngineering University.
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Fig. 6. Experimental results of the fiber intensity modulatorunder different frequency of driving signals (a) 149Hz and(b) 1 kHz (Media 6 and Media 7).
