New integrated-opticsinterferometer in planar technology
Isabelle Schanen Duport, Pierre Benech, and Roger Rimet
Glass ion exchange is an attractive method for fabricating integrated optical components. We investi-gate the feasibility of making a single-mode glass ion-exchanged interferometer designed especially toobtain an interference pattern. The design of the interferometer is based on the use of taperedwaveguides to obtain a collimated beam. This interferometer could be used as a chemical or biologicalsensor.
Key words: Integrated optics, interferometer, ion exchange in glass.
1. Introduction
Interferometers are important elements in opticalsystems. They are usually used in sensor applica-tions since they can detect the phase variation of theoptical beam and then measure very small variations(approximately 10-6) in the optical index.
Different techniques are currently used to build aninterferometer in bulk optics. The Michelson andthe Mach-Zehnder interferometers are the well-known examples that have found many applications.The main practical problem in the construction ofsuch interferometers is the alignment of the differentcomponents (lenses, mirrors, beam splitters, etc).This problem can be easily solved with integrated-optics technology. With this technology the compo-nent alignment is automatically achieved by the maskdesign. The only alignment required is between theoptical source and the interferometer.
Different interferometer configurations exist inintegrated optics. One configuration is used in theMach-Zehnder interferometer", 2 shown in Fig. 1.With this type of interferometer first the source beamis divided by a beam splitter. This function is accom-plished in integrated optics with a Y junction thatseparates the light incident into two beams of equalintensities. A second Yjunction recombines the two
The authors are with the Laboratoire d'ElectromagndtismeMicro-ondes et Opto6lectronique (Unit6 de Recherche Associee auCentre de la Recherche Scientifique 833), Ecole Nationale Su-perieure d'Electronique et de Radioalectricit6, 23 avenue desMartyrs, BP 257, 38016 Grenoble Cedex, France.
Received 7 July 1993; revised manuscript received 6 December1993.
beams, and interference is observed at the output ofthe interferometer. In fact the interferometer causesvariations in the output intensity, which depends onthe relative phase and the relative intensity of the twobeams. Such an interferometer is used in somechemical and biological sensor.3-6 If one arm iscovered with a sensing material for a certain length,one can detect a variation in the output intensity thatresults from a variation in the optical path differencebetween the two arms. One cannot determinewhether these intensity variations correspond to anincrease or a decrease in the optical path of thesensing arm with respect to the optical path of thereference arm. Moreover the intensity variationsmay also be a result of instability of the optical source.So there is not always sufficient information from theoutput of this type of interferometer to interpretwithout ambiguity.
To overcome these problems, we propose a newtype of integrated-optics interferometer that permitsone to obtain more information. This device enablesone to obtain an interference pattern at the output,and then the path difference between the two armsleads to a displacement of the interference fringes.The direction of this displacement depends on thealgebraic value of the optical path difference betweenthe two arms of the interferometer.
The technology used to realize this interferometeris ion exchange in glass.7 8 This technique is attrac-tive for fabricating low-loss passive integrated opticaldevices, especially when long components (measuringa few centimeters) are required. Because they areeasy to process and are compatible with optical fibers,glass waveguides are suitable for low-cost integrated-optics components.
2. Fabrication of Integrated Optical Devices in Glass
The technology used to create the integrated-opticsinterferometer is the ion exchange on a glass substrate.This method involves exchanging monovalent cat-ions, such as Ag+ or K+, with the Na+ present in theglass in the form of Na2O. This exchange modifieslocally the chemical composition of the glass and thusits refractive index. Figure 2 shows the main stepsfor fabricating a channel waveguide in a glass wafer.
The B270 (Desag) glass is used because it permitsthe guides to be rapidly and easily produced. First, athin aluminum layer is evaporated onto the surface ofthe wafer. Then, using the standard technique ofphotolithography (resist spinning, backing, exposure,development, etching), one can transfer the mask ofthe device onto the wafer surface. The width of thechannel on the mask is 2 tim. After chemical etch-ing is performed, the width of the channel on thealuminum mask is 2.5 0.5 tim. The next step isthe ion-exchange process performed in a pure KNO3melt at a temperature of 450 C. Producing a low-loss single-mode straight waveguide at a wavelengthof 0.78 tim requires a diffusion time of 20 min.
* cleaning
* evaporation
4- glass substrate
thin filml aluminum (200 nm).- glass substrate
z20x or 50x robjective micropositioning plate
Fig. 3. Experimental setup used to characterize the output nearfield.
Planar waveguides produced with the same method(20 min in a pure KNO3 melt at 450 C) are alsofabricated and characterized by the M-line technique.We find that these planar waveguides are single modeat 0.78 m with an effective refractive index of1.5234. The refractive index of the glass substrate is1.5207.
The experimental setup used to characterize thefabricated channel waveguides is shown in Fig. 3.The output intensity is analyzed with an IR TVcamera. By analyzing the field of the straight wave-guide with a mask opening of 2.5 + 0.5 tim, weverified that it was single mode. The profile of theoptical field is shown in Fig. 4. We measured themode width in the y direction to be 10.6 tim at 1/e2 ofthe maximum intensity.
3. Interferometer Design
The interferometer pattern is shown in Fig. 5. As inthe Mach-Zehnder interferometer the source beam is
(a)
-resist spinning<. photoresist
(0.5 Rm)
_44i44 +
10(
c 40
% C
UV light
photomask
* development ofthe photoresist
* aluminumetching
= r -aluminumr_ _mask
100
>, 80Cal 60- 40
20
C
* elimination | _ '_ 'of the resist I
-diffusion I pure KNO3 salt meltL i ~ ~ ~ ~ ~ ~ ~ ~ ~ - -- - - -
* aluminummask removal
Fig. 2.
waveguides
Ion exchange on a glass substrate.
I kI
-10 -5 0 5 1
Oy(>im)
-I _ _ -
-10 -5 0Oy (m)(b)
0
5 10
Fig. 4. (a) Output intensity of the input straight waveguide. (b)Profile of the intensity in the x and y directions.
first coupled to a single-mode waveguide and thendivided into two beams at the Y junction. Eachbeam propagates in a symmetrical curved single-mode waveguide and then in a tapered section thattransforms the confined beam into a collimated beamwhen the taper geometry is chosen wisely. Thetapers have been positioned with a separation at eo(eo 20 tim) to eliminate the coupling phenomenabetween the field of each taper during the propaga-tion in the tapered waveguides. The angle of inclina-tion between the axis of the two tapered waveguides is,y. The collimated beams propagate in a planarwaveguide, and an interference pattern can be ob-tained at the output of the structure.
Before analyzing the interference pattern, we de-scribe the geometry and the characteristics of eachcomponent of the interferometer.
Several authors have analyzed the Y junction as anoptical power divider.9 We decided to use a Y junc-tion with curved waveguides to reduce the length ofthe device. The beam propagation method' 0 (BPM)is used for designing the junction. The angle be-tween the two arms is made as small as possible (lessthan 1) to reduce radiation losses. Moreover thecurved waveguides (with an S shape) should notinduce excess radiation losses, so the curvature ra-dius of these bend waveguides must be carefullychosen." "2 The fabrication technology (20 min in apure KNO3 melt at 450 0C) induces a variation in theeffective refractive index of 2.7 x 10-3; so the radiusof curvature was fixed at 3 cm to avoid significantradiation losses. In these conditions the angle be-tween the two arms of the Y junction is 0.940 and theexcess losses in the junction (calculated by the BPM)are negligible.
The tapered waveguides (Fig. 6) transform theconfined field of the single-mode waveguide into a
collimated beam.13 To produce collimated beamswith a small divergence, the lowest-order local nor-mal mode of the structure should propagate throughthe taper without a cumulative power transfer tohigher-order local normal modes. If at the output ofthe taper we have a maximum coupling of power tothe local fundamental mode, the output field hasnearly a plane wave front. There have been a num-ber of numerical investigations of mode conversion inlinear tapers with the BPM. The results are asfollows: Tapered waveguides with small deviationangles are more suitable for obtaining maximumcoupling of the fundamental local mode at the taperoutput. The geometrical parameters for producingthe linear tapering in the interferometric device areas follows:
The output near field [Fig. 7(a)] measured with theexperimental setup shown in Fig. 3 and the far fieldhave been characterized.'4 We measured a width inthe y direction at 1/e2 of the maximum intensity of35.7 ± 0.2 tim and an angular divergence in theplanar waveguide of 0.84 ± 0.05°. A comparisonbetween the measured and the theoretical near fieldis shown in Fig. 7(b). The theoretical field corre-sponds to the fundamental mode of a straight wave-guide with a width of 40 tim, which corresponds tothe output width of the tapered waveguide and anincrease in the refractive index of 0.0027.
After propagation in the tapered waveguides, theoptical beams propagate in a planar waveguide, sothat the fields are confined only in the x direction.In the y direction the field diverges freely and itsangular divergence is 0.84 ± 0.050. The interferencepattern obtained at the output is analyzed and ob-served in the X-Y plane where the output fieldsproceeding from each arm overlap exactly.
(a)
Linear taper - Angle 0.50Tapered section0 deviation angle
input straightwaveguide do
single mode at output straightwaveguide d,multimode at X
Fig. 6. Tapered waveguide: The linear taper (deviation angle 0)joins two waveguides of different sizes, the input straight wave-guide (do) and the output straight waveguide (di).
1UU
80
.t' 600
,3 40
20
0-40 -20 0
O (m)
ital modeI profile
20 40
(b)
Fig. 7. (a) Output intensity of the tapered waveguide. (b) Profile
If the angle of inclination y between the taperedwaveguides is small (some degrees), the intensityprofile of the theoretical interference pattern is givenby13
Ij 12 = 4IA12cos2(Y + ky sin y), (1)
where A 1 2 corresponds to the intensity profile of thefield proceeding from the taper after the propagationin the planar waveguide, up is the phase differencebetween the arms of the interferometer, and k =kon = 2rn/Xo is the wave number.
So the intensity-profile envelope of the interferencepattern depends only on the tapered field. If thisfield is the fundamental mode, that is, approximatelya Gaussian field, the field profile remains Gaussianduring the propagation and the interference patternhas a Gaussian envelope.
The distance between the two fringes (the maximaor minima of intensity) is
Tr XoAy = k s - 2s
k sn 2nO sin y(2)
The geometrical parameters of the characterizedinterferometer are as follows:
Total length of the interferometer, 3 cm.Length of the input straight waveguide, 10 253 m.Width of the input straight waveguide, 2.5 ±
0.5 m.Length of the planar waveguide, 1910 tim.Angular inclination between the tapers, 0.90°.
The theoretical interference pattern correspondingto this interferometer is shown in Fig. 8(c). Thedistance between two fringes is 16.3 tim, and thewidth at 1/e2 of the intensity profile envelope is64 m.
The interferometer was fabricated in the condi-tions described in Section 2. The experimental setupshown in Fig. 3 characterizes the fabricated structure.The output intensity [Fig. 8(a)] is analyzed with an IRTV camera. By analyzing the output intensity pro-file in the y direction, we can compare the experimen-tal interference pattern [Fig. 8(b)] and the theoreticalone [Fig. 8(c)]. The results show good agreement.We have measured the distance between the twofringes to be 17.0 tim (the theoretical value is 16.3tim) and the width of the intensity-profile envelope at1/e2 to the 69 m. (The theoretical value is 64 m.)The small difference between the theoretical and theexperimental values is due to the calibration of the IRTV camera and the approximations in the theoreticalanalysis.
5. Application
By inducing a path difference between the two armsof the interferometer, we can observe a displacement
0c:
(a)
Experimental interference pattern-Linear taper 0.5° 40 am-Angle of inclination 0.90°
80 - - -- - -
60
20
-50 -25 0 25 5Coy (>m)
(b)
Theoretical interference pattern-Linear taper 0.5' 40 lra-Angle of inclination 0.90°
100 - - -
80 _ __ iI |
60 -- - -
40 -- -
20 .-iv~~ ~~ to
-50 -25 0
oy (m)
(C)
25 50
Fig. 8. Analysis of the interference pattern.
of the interference fringes. Such an interferometercan be used as a chemical or a biological optical sensorwhen one of its arms is covered by a sensing thin film.The refractive index of the sensing material maychange when a chemical or biological species is de-tected. The reaction between the sensing materialand this species modifies the effective refractive indexof the sensing arm, producing the optical path be-tween the two arms. In these conditions the detec-tion is directly related to the displacement of theinterference fringes.
With suitable photodetection placed on the interfer-ence pattern, it is possible to measure the displace-ment of more than one tenth of the fringes. Thevariation of the effective refractive index between thetwo arms causing this displacement is 10-6 if thelength of the waveguide covered with the sensingmaterial is - 1 cm. 6
Fig. 9. Variation in the effective refractive index of the waveguidewith the refractive index of the superstrate (the sensing material):*, superstrate refractive index between 1.00 and 1.5237; El,superstrate refractive index between 1.523 and 1.5237.
The variation of the effective refractive index of thewaveguide with the refractive index of the superstate(sensing material) is shown in Fig. 9. This variationis more important when the refractive index of thesuperstrate is near the effective refractive index of thewaveguide.
6. Conclusion
We have proposed a new type of integrated-opticsinterferometer that permits an interference patternto be obtained. We note two advantages of thisinterferometer with respect to their Mach-Zehnderinterferometer. The first is that the algebraic valuethe sign of the optical path difference between the twoarms is indicated by the direction of the fringedisplacement. The second is that the detection isnot perturbed by variations in the intensity of thelaser source. Such an interferometer could be usedas a chemical or biological sensor. The sensitivity ofthis interferometer versus the superstrate index varia-tion is very high (greater than 10-6). To develop
applications to chemical or biological sensors, we needfurther study of the sensitive materials (refractiveindex, adherence to the glass substrate, etc.).
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