Photoinduced grating filters in GeO 2 thin-film waveguides

Zhong-Yi Yin, Paul E. Jessop, and Brian K. Garside

Narrowband grating filters were fabricated in GeO2 thin-film optical waveguides by optically inducedchanges in the refractive index. Intense counterpropagating laser beams inside the waveguide resulted indistributed feedback reflectors having a peak efficiency of 40%. The spectral response of the filters wasmeasured by thermal tuning of the center frequency and agreed with the results of a coupled-mode analysis.The filter bandwidths were <0.01 nm.

1. IntroductionThe formation of Bragg diffraction gratings in thin-

film optical waveguides has important applications inintegrated optics. These include devices such as beamsplitters, modulators, and narrowband reflection filters.Gratings can be formed either by a corrugation in thewaveguide thickness1 or by a spatial modulation of therefractive index of the guiding layer. In materials whichexhibit the photorefractive effect2 it is possible to usetwo interfering laser beams to optically induce a re-fractive-index grating. This technique has been usedquite successfully in the case of LiNbO3 waveguides. 3

In this paper we report the fabrication of optically in-duced narrowband grating filters in amorphous GeO2thin-film waveguides.

There is growing interest in the use of GeO2 as wellas glasses4 with a high GeO2 content for optical wave-guides. These materials are potentially superior tosilica-based glasses for operation at long wavelengths.Devyatykh et al.5 fabricated GeO2 glass optical fiberswhich had low Rayleigh scattering losses and zero ma-terial dispersion at a wavelength of 1.74 um. Recently,thin-film optical waveguides of amorphous GeO2 onglass or fused quartz substrates have been fabricatedand shown to have propagation losses <0.7 dB/cm at awavelength of 633 nm.6 The possibility of observingphotosensitivity in thin films of GeO2 was suggested bythe observation of this effect in single-mode optical fi-bers of 4% GeO2-doped SiO2.' In these experimentstwo counterpropagating waves in the fiber opticallyinduced a spatial modulation of the fiber refractive

The authors are with McMaster University, Department of Engi-neering Physics, Hamilton, Ontario L8S 4M1.

Received 16 July 1983.0003-6935/83/244088-05$01.00/0.(© 1983 Optical Society of America.

index and formed narrowband distributed feedbackreflectors. Since there was every indication that theGeO 2 dopant was responsible for the photosensitivityof the glass fibers, it was to be expected that pure GeO2films would be even more strongly photosensitive.

I1. ExperimentsThe GeO2 thin-film waveguides were deposited by rf

reactive sputtering from a GeO2 target onto fused quartzsubstrates. Details of the fabrication procedure werereported previously.6 A reduction in propagation lossesfrom those reported in Ref. 6 was obtained when fusedquartz substrates were substituted for glass and thesputtering deposition rate reduced from 12.7 to 5 A/min.Figure 1 shows the measured attenuation at four dif-ferent wavelengths. The waveguides had a thicknessof -3000 A, which ensured that only a single TE modecould propagate.

Figure 2 illustrates the experimental procedure thatwas used to fabricate the grating filters. The beamfrom a single-mode argon-ion laser operating on the514.5-nm line was coupled into the waveguide using ahigh-efficiency prism coupler.9 The beam was linearlypolarized in the plane of the film and, therefore, coupledonly to the TEo waveguide mode. A second prismcoupler was used to couple the beam out of the film withessentially 100% efficiency. The beam path length inthe film ranged from 0.6 to -4 cm. A mirror was posi-tioned so that the exit beam was retroreflected andpartially coupled back into the waveguide. The twocounterpropagating waves were superimposed as pre-cisely as possible. Also, to have small beam cross sec-tions and to match the phase fronts of the two beams,the mirror was located quite close to (<1 cm) the outputcoupler, and the beam focus was positioned at the mir-ror rather than in the film. A beam splitter was placedbetween the argon laser and the input coupler so thata portion of the reflected beam could be monitored witha photodiode.

4088 APPLIED OPTICS / Vol. 22, No. 24 / 15 December 1983

z 'x

,I.-

a)4

0

i s

0 I

40 500 550 600 650OPTICAL WAVELENGTH nm)

Fig. 1. Waveguide attenuation vs optical wavelength. Films weredeposited onto fused quartz substrates at a rate of 5 A/min.

Fig. 2. Schematic of the apparatus for fabricating waveguidefilters.

z0viU

W

a:U.Wa:W

W

TIME (min)

Fig. 3. Relative reflected intensity vs exposure time.

The initial alignment of the system was done with thelaser beam attenuated to a power of <1 mW. Thispower level gives an intensity in the film which is wellbelow that required to induce changes in the refractiveindex. When the attenuator was removed, the powercoupled into the film was typically 250 mW in a beam0.3 mm wide. This corresponds to an intensity of 300kW/cm 2 in the film. Approximately 50% of the powerthat was coupled out of the film was coupled back inafter the beam was retroreflected. This resulted in astanding wave intensity distribution within the wave-guide, which induced a spatial modulation of the re-fractive index due to the photorefractive effect. Oncethis index grating began to form, it acted as a resonantreflector to the incoming beam. The reflection effi-ciency was monitored by intermittently blocking thepath between the output coupler and the retroreflectingmirror so that the photodiode sensed only the light thatwas reflected by the grating.

Figure 3 shows the buildup of the reflected intensityof a typical grating filter as a function of exposure time.The intensity rises to a maximum value, and then it willslowly fall if the film remains exposed to the intensewriting beam. But the reflection efficiency will stay atits peak value for a much longer period of time if thelaser power is reduced to below 1 mW after the peak hasbeen reached. The retroreflector is essential to estab-lish the initial standing wave intensity distributionwithin the waveguide. However, once the reflectionefficiency of the grating has reached a few percent, themirror can be removed and the reflection will continueto grow. This is similar to the observations in opticalfibers where the effective length of the filters willshorten to much less than the physical length of thefiber after the growth has been initiated by Fresnel re-flection from the end of the fiber.8 For the thin-filmwaveguides the grating efficiency which can be achieveddoes not seem to depend on whether or not the retro-reflector is removed after the writing process hasbegun.

The resonant frequency of the grating filters can betuned over a limited range by varying the temperatureof the waveguides. The effective grating spacingchanges as the refractive index is thermally tuned.Thermal expansion of the quartz substrate will alsocontribute to the tuning, but this is much smaller thanthe refractive-index effect. The thermal tuning of thefilters is quite evident when the writing process is haltedby putting the attenuator into the argon laser beam.The waveguide cools down when the intense beam isattenuated, and the reflectivity drops to zero as the filtertunes away from the fixed-frequency laser. Warmingthe waveguide and substrate back up with an externalheater will bring the filter back into resonance with thelow-power laser. Figure 4 shows a tuning curve for oneof the filters. With the retroreflector removed, a seconddetector was used to monitor the exit beam so that boththe reflected and transmitted powers could be recorded.A wire-wound heater was contacted to the back side ofthe quartz substrate to control the temperature. Whena fixed heater voltage was switched on a time zero, the

15 December 1983 / Vol. 22, No. 24 / APPLIED OPTICS 4089

a, ,

)- '.0 _

I-z

0.6 -a

LU

tU 0.4LU

I -

.2

0 0 1 2 3 4

TIME (min)

Fig. 4. Thermal tuning curve of a reflection filter.4 apply to the same filter.

I-U,

z0

CnzX-

LU

t--jLU

Figures 3 and

temperature rose monotonically and swept the filterthrough resonance with the low-power probe laser. Itis possible to determine the absolute reflection effi-ciency directly from the intensity of the reflected beam.But this requires accurate knowledge of the detectorsensitivity, the prism coupling efficiency, and the lossesat the lenses and beam splitter. It is much simpler toinfer the reflection efficiency from the observed de-crease in the transmission of the filter. In Fig. 4 theobserved transmission decrease of 47% corresponds toa peak reflectivity of 41% for this film, which had anattenuation of 1.5 dB/cm and a path length of 6 mm.This is in agreement with the directly measured re-flectivity.

To determine the spectral width of a filter from atemperature tuning curve such as Fig. 4 it is necessaryto accurately record the film's temperature while it isbeing scanned. Then the recorded temperature in-tervals can be translated into equivalent wavelengthintervals according to

A\X = - T,ndT

a scanning of the interference fringes. The temperaturewas monitored with a small platinum resistance ther-mometer contacted directly to the film. The rate ofchange of the effective index with temperature wastaken to be

dn AN X0

dT T L(2)

where AN is the number of fringes scanned through inthe temperature interval AT. XO is the vacuum wave-length, 514 nm, and L is the path length inside thewaveguide. Thermally induced changes in the refrac-tive index of the coupling prisms could be neglectedbecause the two arms of the interferometer had verynearly equal path lengths within the prisms. Also, asa check against systematic errors in the measurement,different path lengths from L = 2 to L = 4 cm and dif-ferent ranges of AT were used. A value of dn/dT = 1.6(+0.2) X 10-5/C was obtained which, according to Eq.(1), gives an effective wavelength change of 0.054 A forevery 0C temperature change.

Ill. Results and DiscussionThe optical field inside the waveguide can be written

as the sum of the forward and reverse travelling TEOmodes:

E(z,t) = B exp az2- exp[i(t - oz)]

+ A exp (- exp[i(wt + floz)]. (3)

where /0 is the propagation constant, and ae is the at-tenuation coefficient of the waveguide. The intensityis, therefore,

IE(z)12= B2 exp(-az) + A2 exp(az) + 2AB cos(20oz). (4)

The third term in this expression shows a sinusoidalmodulation in the laser intensity along the length of thewaveguide. Note that it is independent of a so that,when the grating formation begins, the depth of themodulation is uniform along the waveguide path eventhough finite waveguide losses have been included.

(1)

provided that dn/dT, the thermal tuning rate of thewaveguides effective index of refraction, is known. Thisquantity was measured by the method illustrated in Fig.5. The input coupler split the incoming beam in two.One beam was internally reflected at the base of theinput coupling prism, while the other was coupled intothe waveguide and then out again. A mirror and beamsplitter were used to recombine these two beams in aMach-Zehnder interferometer geometry. A circularfringe pattern was clearly visible when the system wasproperly aligned. When the temperature of the wave-guide and substrate was varied, the optical path lengthof that arm of the interferometer changed, resulting in

Fringe patternon screen

Fig. 5. Schematic of the apparatus for measuring the thermal tuningrate of the effective index of refraction.

4090 APPLIED OPTICS / Vol. 22, No. 24 / 15 December 1983

1.0

I-C,,zwF 0.6-

U-J,, 0.4

wa->

Ju. 0.2

2 -. I O 0.1 0.2WAVELENGTH OFFSET )

0.4

Fig. 6. Experimentally observed reflection spectrum compared tothe theoretically predicted shape of a filter which has the same peak

efficiency.

The optically induced perturbation of the dielectricconstant is assumed to have the simple form

e(z) = c' + AE cos(2:0z), (5)

where ' is the unperturbed dielectric constant (' = n2),and AE is the amplitude of the index perturbation. Thetheory of mode coupling in a period waveguide10 canthen be used to model the grating filter. The reflec-tance, as a function of wavelength, filter length, and AEis given by

-K sinh(SL) 2I (a/2 + iA3) sinh(SL) + S cosh(SL) (6)

where S2 = K2 - (AO - i /2) 2 ,K /4nc Ac is the forward-to-backward

travelling wave coupling coefficient,AO =0 - 0o, and

3 is the thermally tunable propagation con-stant of the probe beam.

This is identical to the results in Ref. 10, except for theinclusion of the attenuation constant a.

Figure 6 shows a plot of reflectivity vs wavelength aspredicted by Eq. (6) as well as an experimental curve forone of the filters where the temperature was accuratelyrecorded during the thermal tuning. The experimentalwavelength axis was calibrated as described in Sec. II.For the theoretical curve a coupling coefficient of K =0.30 cm-1 was assumed which forces the two curves tocincide at AX = 0. There are no other adjustable pa-rameters in the model. For this particular example, thepeak reflectivity was 10%, the filter length was 1.34 cm,and a = 0.3 cm-1 . K = 0.30 implies that the index per-turbation was A = 1.5 X 10-5. This falls within therange of Ac values that were induced in the optical fi-

bers.8 However, in the fibers, where the GeO2 contentof the core was only 4%, the writing intensities were afactor of -5 greater for a comparable value of AE.

The highest reflectivity which could be achieved was40%. This is lower than was observed in optical fibers,where filters having reflectivities of nearly 100% andeffective lengths <1 cm have been observed. 8 Thedifference appears to be due mainly to the much higherwaveguide losses in the case of thin films, which resultsin substantial heating during the growth of a filter.Thermal tuning of the refractive index causes thestanding wave interference pattern to shift relative tothe growing refractive-index grating. This effect limitsthe ultimate efficiency and contributes to the erasureof a filter which remains exposed to the intense writingbeam after the peak reflectivity has been reached.However, the agreement between the measured reflec-tion curve and the predictions of the coupled modeanalysis are reasonably good in spite of these compli-cations.

Lower laser intensities reduce the waveguide heatingbut also slow down the growth of the filter. For exam-ple, at an intensity of 30 kW/cm 2 it took 36 min to reachpeak efficiency for a 2.0-cm long filter. The reflectivitywas only 5%, due partly to changes in the ambient roomtemperature over the prolonged growth period. Thebest results were achieved by using a more intense beam(e.g., 220 kW/cm2 ) and using a mechanical chopper toreduce its duty cycle to 25-50% and thereby reduce theheating. The filters having the spectral characteristicsshown in Figs. 4 and 6 were fabricated in this way, andthe growth times were 13 and 12 min., respectively.(The quoted times were not reduced by the chopperduty cycle of 36%.)

The efficiency was a sensitive function of the wave-guide quality. Filters could not be fabricated in filmshaving losses of 2.0 dB/cm or more at X = 514 nm. Itwas also noted that, in good quality films, prolongedexposure to an intense beam would increase the diffusescattering that was visible in the waveguide, and anygrating filters that were subsequently grown would havelow efficiency.

IV. ConclusionsThe formation of Bragg gratings by means of the

photorefractive effect in GeO2 thin-film optical wave-guides has been demonstrated for the first time.Counterpropagating beams were used to optically in-duce narrowband reflection filters which exhibited peakreflection efficiencies of up to 40% and bandwidths<0.01 nm. The center frequencies of the filters couldbe thermally tuned over a limited range. We believethat the efficiency could be substantially increased byimproving the thermal stability during the fabricationprocess and possibly by using CO2 laser annealing orother means to reduce the waveguide losses.

15 December 1983 / Vol. 22, No. 24 / APPLIED OPTICS 4091

Photoinduced refractive-index gratings are poten-tially useful for a number of optical devices. Reflectionfilters are attractive for wavelength division multi-plexing applications in optical communications becausethey are both spectrally narrow and tunable. Gratingscan also be formed by having the two writing beamscross each other at an adjustable angle. In this waydevices such as optical switches and modulators can befabricated, and the wavelength of the probe beam whichis operated on by the grating device need not be thesame as that of the writing beam.

Zhong-Yi Yin is a visiting scholar from ZhongyuanResearch Institute of Electronics Technology,Zhumadian, Henan Province, China.

References1. D. C. Flanders, H. Kogelnik, R. V. Schmidt, and C. V. Shank,

Appl. Phys. Lett. 24, 194 (1974).2. A. M. Glass, Opt. Eng. 17, 470 (1978).3. W. S. Goruk, P. J. Vella, R. Normandin, and G. I. Stegeman, Appl.

Opt. 20, 4024 (1981).4. D. L. Wood, K. Nassau, and D. L. Chadwick, Appl. Opt. 21,4276

(1982).5. G. G. Devyatykh et al., Sov. J. Quantum Electron. 10, 900

(1980).6. Zhong-Yi Yin and B. K. Garside, Appl. Opt. 21, 4324 (1982).7. B. S. Kawasaki, K. 0. Hill, D. C. Johnson, and Y. Fujii, Opt. Lett.

3, 66 (1978).8. D. K. W. Lam and B. K. Garside, Appl. Opt. 20, 440 (1981).9. Zhong-Yi Yin and B. K. Garside, Appl. Opt. 22, 1023 (1983).

10. A. Yariv, IEEE J. Quantum Electron. QE-9, 919 (1973).

Patter continued from page 4055

Video target tracking and ranging systemA proposed microcomputer-controlled system computes the range and range

rate of a moving object being tracked by two TV cameras. The pan and tilt(azimuth and elevation) angles of each camera are controlled by correctionsignals derived from the camera video signals. The range and range rate of theobject are computed by triangulation from pan- and tilt-angle data and knowncamera coordinates.

The system could be useful for target ranging at distances up to -300 m insuch applications as vehicle collison avoidance, traffic monitoring, and sur-veillance. It might also substitute for short-range radar in situations wherethe radar signal could not be tolerated.

Figure 3 shows a block diagram of one proposed configuration. The two TVcameras are aimed at the object being tracked. They are mounted on pan andtilt units that are controlled by the remote-control unit. Camera aiming mightbe controlled manually in some applications; but in the configuration showna closed feedback loop keeps the cameras aimed at the moving object. Eachcamera digitally encodes its current angle settings into one TV frame line.

The two TV signals are fed to TV monitors and to video target trackers bythe video switching unit and the remote-control unit. The balanced-to-un-balanced amplifiers convert the video into the standard unbalanced formatrequired by the trackers.

The trackers do two things: They insert cursors into the video signals to markthe camera aim points on the monitor screens, and they develop-analog error-signal voltages that indicate the direction and magnitude of aiming errors inboth the horizontal and vertical directions. The error signals go to zero whenthe target edge being tracked coincides with the cursor location. The commandformatter converts the analog error signals into digital commands for the panand tilt units to aim the cameras directly at the object.

The line n decoder retrieves the pan- and tilt-angle data from the TV signalsand feeds them to the microcomputer system in which the triangulation cal-culations are performed. The range and range-rate results are displayed, eitheron the video monitors or on separate readout units. The range data could alsobe used to control directly other automatic-response systems.

This work was done by Larry A. Freedman of RCA Corp. for Johnson SpaceCenter. Refer to MSC-20098.

continued on page 4154

Fig. 3. A proposed target tracking and ranging system uses two automatic video target trackers to keep two TV cameras trained on the objectbeing tracked. A microcomputer calculates range and range-rate information by triangulation. The input data for this calculation are the

position coordinates of the two cameras and the pan and tilt aiming angles of the two cameras.

4092 APPLIED OPTICS / Vol. 22, No. 24 / 15 December 1983