REFERENCES

1. Y.P. Liao, D.J. Chen, R.C. Lu, and W.S. Wang, Nickel-diffusedlithium niobate optical waveguide with process-dependent polar-

Ž .ization, IEEE Photon Technol Lett 8 1996 , 548]550.2. F. Laurell, J. Webjorn, G. Arvidsson, and J. Holmberg, Wet

etching of proton-exchanged lithium niobate}A novel process-Ž .ing technique, J Lightwave Technol 10 1992 , 1606]1609.

3. W.L. Chen, R.S. Cheng, J.H. Lee, and W.S. Wang, Lithiumniobate ridge waveguides by nickel diffusion and proton-ex-

Ž .changed wet-etching, IEEE Photon Technol Lett 7 1995 ,1318]1320.

4. R.S. Cheng, W.L. Cheng, and W.S. Wang, Mach]Zehnder modu-lators with lithium niobate ridge waveguides fabricated by pro-ton-exchange wet etch and nickel indiffusion, IEEE Photon

Ž .Technol Lett 7 1995 , 1282]1284.5. H.J. Lee and S.Y. Shin, Lithium niobate ridge waveguides fabri-

Ž .cated by wet etching, Electron Lett 31 1995 , 268]269.6. R.S. Cheng, T.J. Wang, and W.S. Wang, Wet-etched ridge wave-

Ž .guides in Y-cut lithium niobate, J Lightwave Technol 15 1997 ,1880]1887.

7. W.C. Chang, C.Y. Sue, H.C. Hou, S.J. Chang, and P.K. Wei, Anovel self-aligned fabrication process for nickel-indiffused lithium

Ž .niobate ridge optical waveguides, J Lightwave Technol 17 1999 .8. K. Noguchi, O. Mitomi, H. Miyazawa, and S. Seki, A broadband

Ti:LiNbO optical modulator with a ridge structure, J Lightwave3Ž .Technol 13 1995 , 1164]1168.

9. K. Noguchi, K. Kawano, T. Nozawa, and T. Suzuki, A Ti:LiNbO3optical intensity modulator with more than 20GHz bandwidth

Ž .and 5.2V driving voltage, IEEE Photon Technol Lett. 3 1991 ,333]335.

10. K. Noguchi, O. Mitomi, K. Kawano, and M. Yanagibashi,Highly-efficient 40-GHz bandwidth Ti:LiNbO optical modulator3

Ž .employing ridge structure, IEEE Photon Technol Lett 5 1993 ,52]54.

11. W.C. Chang, S.J. Chang, H.C. Hou, and C.Y. Sue, Measurementof coupling coefficients of Ni:LiNbO directional coupler, Mi-3

Ž .crowave Opt Technol Lett 19 1998 , 24]27.

Q 1999 John Wiley & Sons, Inc.CCC 0895-2477r99

HIGH-RESOLUTION LASER CHIRPMEASUREMENT USING QUADRATURESAMPLING TECHNIQUEMinho Song1 and Shizhuo Yin11 Department of Electrical EngineeringThe Pennsylvania State UniversityUniversity Park, Pennsylvania 16802

Recei ed 29 June 1999

ABSTRACT: A high-resolution laser chirp diagnosing technique is pro-posed and experimentally demonstrated by quadrature sampling of a

( )phase-modulated Mach]Zehnder MrZ interferometer. A light signalfrom an injection current-modulated DFB source passes through anMrZ interferometer whose output is sampled with quadrature time delay.Digital arctangent demodulation is used to extract the laser chirp infor-mation from the sampled data. To remo¨e the amplitude modulation( )AM effect, the intensities were di ided by the modulation signal. Usinga 30 cm path unbalanced MrZ interferometer, an 84 kHz readoutresolution in the 19 GHz range is demonstrated with frequency increaserdecrease discerning capability. Q 1999 John Wiley & Sons, Inc. Mi-crowave Opt Technol Lett 23: 335]337, 1999.

Key words: laser chirp; quadrature signal; Mach]Zehnder interferome-ter; frequency discriminator; amplitude modulation

Contract grant sponsor: Korea Research Foundation, Program Year 1997

1. INTRODUCTION

Ž .In optical frequency-domain reflectometry OFDR , the re-flections in waveguides can be located by computing theFourier transform of the interferogram that is generated bymodulating the injection current or the temperature of a

Ž . w xsemiconductor distributed feedback DFB laser 1]3 . Theuniformity with which the optical frequency changes asthe modulation is applied is a critical factor in determiningthe resolution and accuracy of the technique. However, thelaser frequency chirp is not a linear function of injectioncurrent or operating temperature; therefore, to obtain highresolution with this nonlinear chirping, use has been made of

w xan optical encoder 1 , active feedback control of modulationw xcurrent 2 , and flat sweep-rate range by diagnosing the

w xchirping behavior of the light source 3 . For successful imple-mentation of the techniques, a robust and compact design ofthe optical frequency discriminator, which has a fine fre-quency-reading capability in a wide range, is required.

In this paper, we present a laser chirp measurementŽ .technique that uses a fiber Mach]Zehnder MrZ wave-

length discriminator with quadrature sampling. Quadraturesignal processing is used to obtain high resolution in a widefrequency range, and to discern the direction of the fre-quency increaserdecrease.

2. QUADRATURE SAMPLING OF M ///// Z INTERFEROMETER

Optical interferometers such as the Mach]Zehnder orMichelson are very effective optical frequency discriminatorsby converting optical frequency variations into intensity varia-tions that can be measured directly. However, the periodicityof their transfer functions causes difficulties, such as a trade-off between sensitivity and input range, signal fading, lack ofinput direction-discerning capability, and so forth. To addressthese problems, quadrature signal processing can be used. Inthe technique, two signals that have 908 phase difference areprocessed by arctangent phase demodulation, extracting thephase information that is linear against the input opticalfrequency variation. Recently, we developed a novel tech-nique that imposes a quadrature phase difference betweentwo sampled intensities, and in this paper, we use the tech-nique for high-resolution DFB laser chirping diagnoses.

The configuration of the proposed system is shown inFigure 1. Light from an injection current-modulated DFBsource is coupled into a fiber MrZ interferometer, and asinusoidal phase modulation signal is supplied to a fiberstretcher that is placed in one of the interferometer arms.Then, the output intensity is sampled twice in a phase modu-lation cycle using data acquisition electronics. This two-pointsampling repeats in every cycle. To place the first sampling inthe same phase of each cycle, a sync-out square wave, whichis synchronized to the modulation function, is fed to the dataacquisition board as a reference signal. The first sampling isactivated when the sync-out rises, and the second one followsafter a time delay D t. The sampled intensities can be approxi-mated as follows:

Ž . w Ž . Ž .xI n ( cos sin v t q 2np q f Dn ,1, 2 1, 2

n s 0, 1, 2, . . .

where v is the path-length modulation frequency, t and t1 2are the times when the samplings occur for the first time, andŽ .f Dn is the phase term that is modulated by the optical

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 23, No. 6, December 20 1999 335

Figure 1 System configuration

frequency deviation Dn . When sin v t y sin v t s pr2 is1 2Ž .satisfied, the quadrature phase condition 908 is established

Ž .between I and I . This quadrature time delay D t s t y t1 2 2 1is determined by observing the X]Y plot of two data streamsuntil it becomes circular. By applying the arctangent function

Ž .and phase unwrapping to I rI , f Dn is extracted, which is1 2proportional to the laser chirp as

2p ndŽ .f Dn s D f

c

Ž .where nd is the optical path difference OPD of the interfer-ometer, Dn is the applied optical frequency deviation, and cis the light speed in vacuum.

3. EXPERIMENT AND RESULT

ŽThe optical path difference of MrZ was set to ; 44 cm 30.cm in fiber length , which corresponded to a differential time

delay of t s 1.46 ns for the light signal. With the OPD, the0intensity in the photodetector changed over one cycle by afrequency chirp of Dn s 1rt f 685 MHz. To demonstrate0

Žthe frequency chirp recovery, a DFB laser diode Lasertron,.1.54 mm, threshold current 21 mA was modulated by a 50 Hz

triangular wave. A 17.6 mA pk]pk modulation was added tothe 38 mA bias current. The fiber stretcher was driven by a30 kHz, 1 V pk]pk sine wave, and the quadrature time delayD t for the modulation signal was determined to be 11 ms.

Ž .Figure 2 a is one of the sampled interferograms, which has aŽ .triangular envelope from the amplitude modulation AM of

the applied injection current modulation. This waveform withan asymmetric shape in the transverse direction cannot beprocessed by arctangent demodulation. As displayed in Fig-

Ž .ure 2 c , the transverse asymmetry moves the center of theX]Y plot, resulting in errors in arctangent phase recovery. Tocompensate for this AM effect, the sampled signals weredivided by the triangular modulation signal, and the compen-

Ž .sated interferogram and X]Y plot are shown in Figure 2 bŽ .and d , respectively. The transverse asymmetry was elimi-

nated in the divided interferogram, and the center of X]Yplot was maintained in the same position throughout thecurrent modulation. Figure 3 shows the demodulated fre-quency chirp and its derivative, i.e., the frequency sweep rate.

ŽThe 19 GHz frequency excursion 54p of interferogram. Žphase variation was recovered successfully with 12 bit ana-

.log-to-digital converter resolution readout resolution in thep range, which is equivalent to 84 kHz of minimum readablefrequency deviation. From the curves, the direction of fre-quency increaserdecrease is clearly discerned, which is oneof the main advantages of quadrature signal processing. The

Ž .optimum FFT fast Fourier transform sampling range for

Ž . Ž .Figure 2 Sampled MrZ interferograms a , b and their X]YŽ . Ž .plots c , d . Counterpart quadrature interferograms are not shown.

Ž . Ž . Ž . Ž .a , c Uncompensated. b , d AM compensated

Figure 3 Demodulated optical frequency deviation and its sweep-rate curve

OFDR applications, which has minimal sweep-rate fluctua-tion, can be determined from flattest range in the frequencysweep-rate curve.

4. CONCLUSION

In conclusion, we have demonstrated the optical frequency-discriminating capability of a quadrature-sampled Mach]Zehnder interferometer. Quadrature signal processing en-abled high-resolution optical frequency encoding in an unlim-ited input frequency sweep range; therefore, this techniquecould be useful for wavelength-encoded fiber sensors such as

w xfiber Bragg grating sensors 5 as well as OFDR applications.Also, by implementing a high-speed MrZ modulator, thistechnique could be used for high-bit-rate time-resolved chirpdiagnosis in long-haul fiber communication systems without

w xrepeating measurements to separate the AM effect 6 .

REFERENCES

1. K. Takada, High-resolution OFDR with incorporated fiber-opticŽ .frequency encoder, IEEE Photon Technol Lett 4 1992 ,

1069]1072.

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 23, No. 6, December 20 1999336

2. K. Iiyama, L.T. Wang, and K. Hayashi, Linearizing optical fre-quency-sweep of a laser diode for FMCW reflectometry, J Light-

Ž .wave Technol 14 1996 , 173]178.3. R. Passy, N. Gisin, J.P. Weid, and H.H. Gilgen, Experimental and

theoretical investigations of coherent OFDR with semiconductorŽ .laser sources, J Lightwave Technol 12 1994 , 1622]1630.

4. Y.L. Lo, J.S. Sirkis, and C.C. Chang, Passive signal processing ofin-line fiber etalon sensors for high strain-rate loading, J Light-

Ž .wave Technol 15 1997 , 1579]1585.5. A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo,

C.G. Askins, M.A. Putnam, and E.J. Friebele, Fiber grating sen-Ž .sors, J Lightwave Technol 15 1997 , 1442]1462.

6. R.A. Saunders, J.P. King, and I. Hardcastle, Wideband chirpmeasurement technique for high bit rate sources, Electron Lett 30Ž .1994 , 1336]1338.

Q 1999 John Wiley & Sons, Inc.CCC 0895-2477r99

CLADDED OPTICAL GLASSWAVEGUIDE AS PLANAR POLARIZERZ. A. Ansari,1 R. N. Karekar,1 and R. C. Aiyer11 Department of PhysicsUniversity of PunePune 411 007, India

Recei ed 23 June 1999

ABSTRACT: Planar optical wa¨eguide polarizers for TE r TM modes0 0( q q)are fabricated on an asymmetric ion-exchanged glass Na ª K

wa¨eguide. The thickness of the exchanged layer is about 2 mm, achie edafter 2 h of the exchanged treatment, and is enough to excite a first-ordermode in the layer. Polarizers are fabricated by gi ing the cladding of¨arious materials on the guide surface. For TE polarization, metals0( ) y 5y¨e dielectric constant e¨aporated at 10 torr with a thickness ofabout 250 nm measured using the Tolansky technique are used as a clad.Metals like Al, Ag, Cu, Pb, and Au are chosen for study. For TM0

( )polarization, semiconducting oxide materials q¨e dielectric constantlike BaTiO , Al O , SnO , etc. are screen printed and used as a clad.3 2 3 2The thicknesses of the printed films are measured by a light sectionmicroscope with an accuracy of 0.5 mm, and are about 20 mm. Thesesamples are characterized using the prism-film coupling method. In thecase of metal clads, the TM mode has a higher loss than the TE mode0 0while passing through the clad. Hence, the length where the TM r TE0 0loss approaches almost zero is taken as the cutoff length. On the otherhand, for semiconducting clads, a slightly higher attenuation of the TE0mode is obser ed than the TM mode. Therefore, following the basic law0of physics, 1re times the fall of the intensity of TE is treated as the0cutoff condition. The cutoff lengths are obser ed experimentally as wellas calculated theoretically. It is seen that both are in good agreementwithin an error of 5]10%. The cutoff lengths of the metals are comparedwith the reported ¨alues, and are analyzed on the basis of material purityand conducti ity. Q 1999 John Wiley & Sons, Inc. Microwave OptTechnol Lett 23: 337]342, 1999.

Key words: optical wa¨eguide; cladding; polarization

INTRODUCTION

Passive optical devices and components play an importantrole in the construction of integrated optical circuitsrdevicesby cladding the guides with proper material. Metals arenormally used for providing electrodes in electro-optical de-

w xvices using waveguides 1]7 . The waveguide polarizer is oneof the most important components, due to the variety of

applications of polarized light in fiber optical communicationsw xand integrated optical systems 1]5 , including electro- and

w xmagneto-optic devices 1]8 . Recently, it has also gainedw ximportance in the field of waveguide sensors 8 .

In order to fulfill such requirements, various methods areused to obtain polarized light either in the TE or TM mode.Currently, the use of a cladded optical fiberrwaveguide isbecoming famous for obtaining a polarized mode of light.The polarized mode is obtained by choosing the proper cladon the guide surface, where the clad acts as an interactionregion to control the state of polarization. In metal-cladwaveguides, TM modes suffer larger attenuation along the

w xlength than TE modes 1]5 . The nature, structure, andgeometry of the films are a few of the factors that affect thepolarization. The clad length is one of such parameters thatfilter out one of the polarization states, and is thus known asthe cutoff length.

The cladded optical fiber and waveguides are also used todetect various physical and chemical parameters, viz. pres-

w xsure, displacement, material refractive index 8 , pH of thew x w xsolutions 1, 8 , concentration of gases 8 , etc. In this paper,

an attempt is made to evaluate the dielectric constant ofmetal and semiconducting oxide clads using the prism-filmcoupling method by obtaining the experimental cutoff lengthfor polarization. The semiconducting clads are first experi-mentally studied as a TM polarizer, although the theory for aTM mode is available.

EXPERIMENTATION AND ANALYSIS

Planar optical single-mode glass waveguides are fabricatedusing an ion-exchanged process by replacing Naq ions on asoda glass surface by Kq ions in molten KNO salt at 4008C3w x2, 8 . In order to achieve polarization, the metalrdielectricclad is selectively deposited in a wedge shape on the guidesurface, with a variation of length from 2 to 8 mm from

w xbottom to top 2, 8 ; see Figure 1. Metals are evaporated at apressure of 10y5 torr, having a thickness of about 250 nmw Ž .xsee Table 1 a measured using the Fizeau Fringes method

˚ w xwith an accuracy of "30 A 8, 9 . The semiconducting oxidesware screen printed, and have a thickness of about 20 mm see

Ž .x Ž .Table 1 b , which is higher than the guide thickness 2 mmŽ .and wavelength of light 632.8 nm used. The thicknesses of

the printed clads are measured using a light section micro-Ž .scope BK 70 = 50, VEB Carl Zeisco Jena, Germany with

w xan accuracy of 0.5 mm 8 . In both cases, care is taken thatthe clad thicknesses are higher than the penetration depth of

Ž .light HeNe, l s 632.8 nm in clad, to prevent direct contactof the evanescent field from the atmosphere. The cladded

Žguides are characterized using a prism n s 1.7, 10 mmp. Žbase -film coupling method and an HeNe laser l s 632.8

. w xnm, beam diameter s 0.1 mm, power 0.5 W 8 . An Siw xphotovoltaic is used as a detector at the output 8 . The cutoff

length for a particular polarization is experimentally obtainedby scanning the clad length along the wedge, vertically up-

w xward at each 0.1 mm 9 . An externally polarized sheet isadditionally used at the output to confirm the polarized mode

Ž .passing through the clad see Fig. 1 . Experimentally, it isobserved that a particular mode has higher absorption in theclad as compared to the other mode, viz. the TM mode0absorbs more in metal layer and the TE mode in dielectric0clads. In general, the length at which maximum absorption orthe minimum intensity ratio of the obtained two modes is

w xtaken as the cutoff length l 4]8 . Therefore, for metal clads,the maximum attenuation of TM is taken as the cutoff0

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 23, No. 6, December 20 1999 337