May 1, 1990 / Vol. 15, No. 9 / OPTICS LETTERS 513

Performance of surface-plasma-wave fiber-optic polarizers

Michael N. Zervas and Ian P. Giles

Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WCIE 7fE, UK

Received June 29, 1989; accepted February 26, 1990

Variations in the TM-like polarization transmission characteristics and polarization extinction ratio of surface-plasma-wave fiber-optic polarizers owing to variations in the depth of polishing and the metal film thickness arepresented. The two effects have been isolated in carefully designed experiments and studied separately.

Surface-plasma-wave (SPW) fiber-optic polarizers,which utilize the SPW resonance as the polarization-selective mechanism, have recently received great at-tention because of their ease of manufacture and thelarge polarization extinction ratios (>50 dB) accompa-nied by small insertion losses (<0.5 dB) that theyachieve. Their operation is based primarily on theevanescent field interaction between the fundamentalHE,, fiber mode and the surface polaritonlike modesupported by a thin metal film that is brought intoclose proximity with the optical fiber. A thin metalfilm with negative-real-part dielectric constant sur-rounded by two semi-infinite dielectrics is known tosupport four different surface polaritonlike TM-po-larized modes (the magnetic field parallel to the metalsurface), namely, the symmetric and antisymmetricbound or leaky modes, which exhibit different phasevelocities and losses depending on the thickness of themetal and the refractive indices of the dielectrics.1-3

Two alternative configurations of a SPW fiber-opticpolarizer have been reported so far.4'5 In the first, adielectric buffer layer is formed between the polishedfiber surface and the thin metal film, and the trans-mission of the TM-like polarization varies with thebuffer-layer refractive index. The thicknesses of themetal film and the buffer layer are found to affect thetransmission characteristics of the device. This con-figuration offers the flexibility of selectively transmit-ting the TM-like or TE-like polarizations by a correctchoice of the buffer-layer refractive index. In thesecond configuration, the metal film is deposited di-rectly onto the polished fiber surface and is covered bya semi-infinite dielectric. When this configuration isused, the performance of the polarizer is enhanceddramatically if a bimetallic film is deposited instead.6The terms TM-like (TE-like) polarization are used todenote the hybrid HE,, fiber mode with the magnetic(electric) field parallel to the polished surface. Thetwo modes are orthogonal to each other.

In attenuated-total-reflection studies of the propa-gation characteristics of SPW's it has been shown thatthe SPW excitation efficiency depends critically onthe dielectric properties of the metal and the sur-rounding materials as well as on the prism-coupling-gap height.7'8 In previous research5 it has been im-plied that the performance of the SPW polarizer withthe metal film deposited directly onto the polishedsurface is determined entirely by the metal film thick-

ness. In this Letter, however, experimental evidenceis presented indicating that the overall performance ofthe device is critically dependent on the metal filmthickness as well as on the depth of polishing.

The device configuration is shown in Fig. 1. Tomaintain and ensure the state of polarization at theinteraction region, high-birefringent bow-tie fiberthat is single mode at 633 nm was used to build thedevice. The fiber was aligned with its slow axis per-pendicular to the glass substrate block by using anelasto-optic technique and then bonded into a 25-cm-radius curved slot in the substrate by means of astrong epoxy. The refractive indices of the core andcladding along the slow axis were 1.465 and 1.458,respectively, and the core diameter was 4.36 ,gm. Thesurface of the fiber was initially ground and then pol-ished to within a few micrometers of the core, whichresulted in an effective interaction length leff of ap-proximately 1.5 mm.9 Various liquids of known re-fractive indices (Cargile liquids) were regularly placedonto the polished surface, and the throughput trans-mission was monitored. The polishing continued un-til approximately 30 dB of the launched optical powerwas coupled out, which corresponds to a depth of pol-ishing of 1-2 /im.10'11 This procedure, however, re-sults in slightly different depths of polishing for differ-ent half-blocks, which are difficult to determine ex-perimentally. This causes no problem in theperformance of the polished couplers because there isstill freedom of lateral and longitudinal adjustmentfor the optimum coupling to be achieved. For theSPW fiber polarizers, however, the whole structure isfixed, and no adjustments are permitted; therefore thepolishing depth variations are expected to affect theperformance of the device.

Thin metal film DielectricL - th I__ }__ d

f ocuti< Glass Block >

Fig. 1. SPW fiber-optic polarizer configuration.

0146-9592/90/090513-03$2.00/0 © 1990 Optical Society of America

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514 OPTICS LETTERS / Vol. 15, No. 9 / May 1, 1990

The aluminum films were thermally deposited ontothe polished fiber surfaces in a vacuum chamber.Aluminum has a bulk dielectric permittivity of -29.8- ill.6 at 633 nm and, in a thin-film configuration,constitutes a surface plasmon waveguide. The dis-tance of the metal film from the fiber core d is of theorder of 1-2 gim, and the device can be viewed as astrongly coupled two-waveguide system. The metal-to-core separation and the evanescent field overlap-ping vary along the interaction length L. TM-likepolarized light launched initially into the fiber is cou-pled into SPW's as it enters the interaction region andis redistributed between the thin metal film and thefiber. The relative amount of optical power remain-ing in the fiber at the end of the interaction length is,therefore, determined by the phase velocity of theSPW, the minimum metal-to-core separation d, andthe interaction length L.

For a given substrate, both the thickness of themetal film and the refractive index of the dielectricsuperstrate determine the typel'2 and the phase veloci-ty3 of the propagating SPW mode and, therefore, areexpected to affect the transmission characteristics ofthe device. Since the two waveguides are stronglycoupled, small variations of the metal-to-core separa-tion d are expected to affect the response of the deviceconsiderably.

A He-Ne laser (633-nm) output was intensity mod-ulated, passed through a 50-dB bulk crystal polarizer,and launched into one axis of the fiber by means of a20X microscope objective. At the output end, a simi-lar 50-dB analyzer was used, and the light was focusedonto a photodetector whose output was monitoredwith an oscilloscope and a lock-in amplifier. The po-larizer was oriented parallel to the axis of the fiber ofinterest, and the analyzer was used to measure thepolarization cross coupling and consequently the accu-racy of on-axis launching. The procedure was repeat-ed until the smallest possible polarization cross cou-pling was achieved, typically -30 dB. Once the axis ofthe fiber of interest was located, the analyzer wasaligned parallel to it and the intensity was measured.

Cargile liquids of various refractive indices wereplaced on top of the thin aluminum film, and thetransmission of the device was monitored with thelock-in amplifier. The total optical power launchedinto the system was measured by cleaving the fiberapproximately 50 cm from the input end face, and itwas used to normalize the transmission of the device.The experiment was performed for TM-like and TE-like polarizations.

To separate the effects of the polishing depth d andthe metal film thickness h on the transmission charac-teristics of plasmon wave polarizers, two different setsof experiments were carried out, one with a constantmetal film thickness and one with a constant polishingdepth.

For the experiment with a constant metal filmthickness, two half-blocks polished to differentdepths, designated HB1 and HB2, were placed next toeach other in the vacuum chamber to ensure identicaldeposition conditions and thin metal films of exactlythe same thicknesses. The film thickness was mea-sured to be 12 + 1 nm. The TM-like polarization

transmission of both half-blocks was measured for dif-ferent dielectric superstrates of various refractive in-dices; the results are summarized in Fig. 2. Theyclearly show different transmission characteristics,which are attributed to the difference in the depth ofpolishing. The polishing depths were slightly differ-ent in the range of 1-2 gim. The fact that the responseof HB2 is slightly broader than that of HB1 impliesthat d2 > d1 .

For the experiment with a constant polishing depth,the thin aluminum film was etched away from thepolished surface of half-block HB1 and a 16 + 1 nmaluminum film was redeposited. The TM-like andTE-like polarization transmissions of the new devicewere measured, and the thin aluminum film wasetched away once again and the half-block was coveredwith a 25 ± 1 nm aluminum film. The experimentalresults for TM-like polarization are summarized inFig. 3. The aluminum etching solution (90% phos-phoric acid, 10% nitric acid) was washed out as soon asthe metal film was removed completely, leaving thedepth of polishing unaffected. Therefore the markedchange in the transmission of the device was attribut-ed exclusively to the metal film thickness variation.

The normalized transmission of the TE-like polar-ization was found to be qualitatively the same in all

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-20

-30

-40 L1.42 1.44 1.46 1.48 1.50

Superstrate Refractive Index

Fig. 2. Normalized transmission of TM-like polarizationversus the superstrate refractive index for different depthsof polishing. Aluminum film thickness, 12 nm.

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Superstrate Refractive Index

Fig. 3. Normalized transmission of TM-like polarizationversus the superstrate refractive index for the different alu-minum film thicknesses indicated on half-block HB1.

May 1, 1990 / Vol. 15, No. 9 / OPTICS LETTERS 515

a_~

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E cn

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Superstrate Refractive Index

Fig. 4. Normalized transmission of TE-like polarizationversus the superstrate refractive index for aluminum filmthickness of 12 nm on half-block HB1.

cases. A typical response for half-block HB1 with analuminum thickness of 12 + 1 nm is shown in Fig. 4.

The polarization extinction ratio of the device isobtained by subtracting the TE-like-polarization nor-malized transmission from its TM-like counterpart.The normalized transmission of the TE-like polariza-tion alone, on the other hand, gives the insertion lossesof the polarizer. From Fig. 4 it is deduced that theinsertion losses of the device are less than 0.1 dB and,therefore, that the polarization extinction ratio of eachpolarizer is given by its TM-like normalized transmis-sion.

Practical devices will have fixed metal film thick-ness and will be covered with a certain dielectric mate-rial of constant refractive index. Given the limitedrange of the available dielectric materials with refrac-tive indices of 1.5-1.6 at 633 nm, from the data in Figs.2 and 3 one can infer that the correct combination ofmetal film thickness and polishing depth should bechosen so that the maximum possible polarization ex-tinction ratio is attained.

In terms of the effect on coupling conditions, it hasbeen shown that the depth of polishing considerablyaffects the coupling coefficient.8' 10 On the otherhand, when high-birefringence fibers are used, thedepth of polishing has been shown to affect the effec-tive refractive index of the particular polarization in acomplicated way.12 "13 Therefore, depth-of-polishingvariations affect several parameters, which, in turn,affect the coupling conditions and the performance ofthe device.

The thin metal film supports four eigenmodeswhose excitation depends on their relative propaga-tion constants and losses. Recent analysis shows thatin this coupled structure several SPW modes can beexcited, depending on the thickness of the metal andthe refractive index of the dielectric superstrate.However, for a certain refractive-index range, only oneSPW mode is predominantly excited.

The first dip in the normalized transmission of theTM-like polarization is due to the excitation of thesymmetric-bound (Sb) SPW mode. This is the well-known long-range SPW mode, which exhibits ex-tremely low propagation loss in quasi-symmetric thin-metal-film structures.' The observed dips, corre-sponding to h = 12 and h = 16 nm, are sharp because

the Sb mode is above cutoff only for a narrow range ofsuperstrate refractive indices near the index of thefiber cladding (nci = 1.458). Outside this range the Sbmode is below cutoff and therefore is not excited. Onthe other hand, the Sb mode supported by the 25-nmaluminum film is above cutoff for much wider range ofsuperstrate refractive indices and exhibits higherpropagation losses. Consequently, the pertinenttransmission characteristics are expected to be muchbroader and shallower (cf. Fig. 3).

The second dip in the transmission of the TM-likepolarization, which is observed for superstrate refrac-tive indices higher than the effective index of the fiber(neff - 1.460), is due to excitation of the antisymmetricleaky SPW mode and is more pronounced for thinnermetals. In this range, the Sb mode is below cutoff forall film thicknesses and is not excited. A full theoreti-cal analysis of the effect of the metal film thicknessand depth of polishing on the performance of the de-vice is under investigation.

As a result of the two sets of experiments describedabove, the effects of depth of polishing and metal filmthickness on the transmission characteristics of theSPW fiber-optic polarizer have been isolated andstudied separately. It is explicitly shown that theperformance of the device is governed by both param-eters. Therefore, the design of high-quality SPW fi-ber-optic polarizers should take both effects into ac-count.

The research of Michael N. Zervas was supported bythe State Scholarships Foundation (IKY) of Greeceand the Bodossakis Foundation, Athens, Greece. Wethank A. A. Stride for the patient thin-metal-filmevaporation and Tony Kerr for the fiber polishing.

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