# Technological implementation Fabry-Pérot cavity in Ti: LiNbO3 waveguide by FIB

Kamal Ghoumid, Richard Ferriere, Badr-Eddine Benkelfat, Slimane Mekaoui, Chafia Benmouhoub, Tijani Gharbi

► To cite this version:

Kamal Ghoumid, Richard Ferriere, Badr-Eddine Benkelfat, Slimane Mekaoui, Chafia Benmouhoub, et al.. Technological implementation Fabry-Pérot cavity in Ti: LiNbO3 waveguide by FIB. IEEE Photonics Technology Letters, 2012, 24 (4), pp.231-233. 10.1109/LPT.2011.2176536. hal-00662688

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# Technological Implementation Fabry-Pérot Cavity
in Ti:LiNbO₃ Waveguide by FIB

Kamal Ghoumid, Richard Ferrière, Badr-Eddine Benkelfat, Senior Member, IEEE,
Slimane Mekaoui, Chafia Benmouhoub, and Tijani Gharbi

***Abstract***—The realization of a Fabry-Pérot (F-P) cavity based on a Ti:LiNbO₃ optical waveguide obtained by using a concept which consists of milling the Bragg gratings (BGs) in the cavity by the focused ion beam technique is shown in this letter. This F-P cavity is formed by two BGs which represent the two reflecting mirrors of the interferometer. The length of the cavity is defined by the distance between the two BGs. Thirty-one peaks and a free spectral range (FSR) = 1.75 nm have been experimentally obtained in the range of wavelengths [1505–1548] nm. The experimental results are in accordance with those of theory. The fabrication process, the experimental results, as well as improvements to increase finesse F and FSR are discussed.

***Index Terms***—Bragg scattering, Fabry-Pérot (F-P) cavity, focused ion beam, wavelength filtering devices.

## I. INTRODUCTION

SINGLE and multiple Fabry-Pérot interferometers have been used as narrowband optical filters for a long time. They have a variety of applications in Telecommunication networks such as in employing wavelength division multiplexing (WDM). Meanwhile, they are used as basis elements to build single-mode lasers, stable frequency references and Dichroic filters which are widely used in optical equipment such as light sources, cameras and astronomical equipment [1], [2].

Many techniques are being used such as thin film multilayer reflectors [3], fiber BGs (photoinduced FBGs) [4], metallic mirrors [5]... to realize two spatially reflectors required by this Fabry-Pérot cavity probe. Devices with outstanding performances can be derived in the case of optical fiber applications, if coatings are deposited directly on the fiber ends. Nevertheless, many difficulties can rise up mainly like alignment and packaging. Thus, the use of optical collimation

is required generally with bulk glass elements using dielectric coatings. But, this operation has the disadvantage of increasing the optical losses [6]. In this letter and for the first time, the realization of the F-P cavity on lithium niobate (LN, LiNbO₃) material by FIB is proposed and discussed.

Indeed, the choice of this material is due to what LN is extensively used in integrated optics, owing to its large electro-optical, nonlinear and piezoelectric coefficients. In spite of these advantages, it can be noticed that the micromachining of LN represents a challenging task, due to the well known resistance of the material to standard machining methods like dry or wet etching [7], [8].

Different approaches have been conducted to achieve nanos- tructures with depths larger than 1µm. Fields of engineering which use wet etching represent a possible way to fabricate submicrometric holes [9]. However, this method is limited to Z-cut substrates. An alternative method relies on ultrafast laser machining [10]. Yet its application requires the laser spot to be focused near the diffraction limits. The method based on proton exchange followed by Reactive Ion Etching (RIE) or Inductively Coupled-Plasma-RIE (ICP-RIE) has been shown to be applicable [11], [12]. This latter technique was also used for the realization of a F-P intensity modulator with integrated BGs in Ti:LiNbO₃, where BGs are produced in an amorphous Si overlay film, the component was more than 7 mm.

Reconfigurable filters are key elements for WDM tech- nologies dedicated to dynamic metro and access networks. Monolithically integrating such as a function on LiNbO₃, a well established material system for electrooptic modulation, would bring additional functionality, efficiency and increased power budget.

The main aim of the present work is the proposal of an efficient technique to realize F-P cavities which rely on BGs etching by Focused Ion Beam on Ti:LiNbO₃ optical waveguide. Comparing with traditional methods, FIB milling has the inducement of a direct etching on the substrate without other additional technological steps. In our previous work, the feasibility of efficient BG reflectors with well reproducible periodicity has been proved by using this method [13], [14].

The first paragraph deals with the different technological steps applied to realize the F-P cavity, giving details on the fabrication of the optical waveguides and the FIB milling of BGs. The evaluation of the various types of optical losses, the measurements of the spectral response, the FSR and the Finesse (F) are presented in the second paragraph. Finally, the next improvements expected in order to increase the F and the

K. Ghoumid is with École Nationale des Sciences Appliquées d'Oujda, Oujda 60000, Morocco. He is also with Institut FEMTO-ST Département LOPMD, Université de Franche-Comté, Besançon 25030, France (e-mail: kghoumid@ensa.univ-oujda.ac.ma).

R. Ferrière, C. Benmouhoub, and T. Gharbi are with Institut FEMTO-ST Département LOPMD, Université de Franche-Comté, Besançon 25030, France (e-mail: richard.ferrière@univ-fcomte.fr; smekaoui@yahoo.fr; chafia.benmouhoub@univ-fcomte.fr; tijani.gharbi@univ-fcomte.fr).

B.-E. Benkelfat is with Institut TELECOM, TELECOM Sud-Paris, Evry 91011, France (e-mail: Badr-Eddine.Benkelfat@int-edu.eu).

S. Mekaoui is with L.C.P.T.S. Télécommunications, Faculté d'Électronique et d'Informatique, USTHB, Alger 16111, Algeria (e-mail: smekaoui@usthb.dz).
<figure><img src="image_2.png" /><figcaption>Fig. 1. Two BGs which constitute the F-P cavity and a zoom out of one part of BG1 and the waveguide Ti:LiNbO3. Total length of each BG is LBG1 = N1 Λ and LBG2 = N2 Λ. Ic is the cavity length represented by the distance between the BG1 and BG2.</figcaption></figure>

FSR are discussed. Further work is in progress to use the same
application in the field of electro-optics taking advantage of
the other properties of LN.

II. FABRICATION PROCESS

Ti:LiNbO₃ optical waveguides have been fabricated on a
0.5 mm thick X-cut Y-propagation LiNbO₃ chip. The single-
mode optical waveguides in the 1550 nm window have been
fabricated by standard diffusion of 80 nm thick and 7 µm
wide titanium layers at 1020°C for 9 h. These parameters
were chosen in order to obtain the optical mode core as
close as possible to the surface while keeping single mode
propagation at 1.55 µm. With such experimental parameters,
the maximum optical field was estimated to be at 2 µm from
the surface, compared to the 3 µm depth that would result from
a same Ti-diffusion process for Z-propagation (the mode in
Y-propagation is less in depth compared to Z-propagation for
an X-cut type LN). Figure 1 (top zoom) illustrates a 7 µm-wide
Ti:LiNbO₃ waveguide FIB image.

The next step was devoted to the direct FIB patterning of
the substrate. After the deposition of a thin Cr layer by RF
sputtering in order to avoid charging effects, the chip was
entered into the FIB vacuum chamber, and then exposed to
a Ga⁺ liquid metal ion source (LMIS) with ion acceleration
energy of 30 keV. This FIB is a dual beam Orsay Physics

<figure><img src="image_3.png" /><figcaption>Fig. 2. Experimental setup: source [1550–1560] nm, L₁, L₂, L₃, and L₄ are focusing lenses, P is a Glan polarizer, waveguide containing both BGs which constitute the F-P cavity and optical spectrum analyzer.</figcaption></figure>

Canion 31/LEO4400. The system used to command the beam deflection is a Raith Elphy Quantum 4.0. The pseudo-Gaussian shaped spot size was estimated to be 40 nm on the target. In order to avoid redeposition of the material during the milling, holes were formed by multiple successive exposures (loops). Thus FIB milling was performed on a hole-by-hole basis. This implies a long time process and eventual drift when fabricating very long BGs. Typically for a BG realization, the achievement of a hole with a width of 525 nm, a length of 12 µm and a depth of 3.2 µm requires 130 loops. The FIB parameters were as follows: the dwell time: 0.5 ms, the step size: 10 nm, the probe current was 300 pA and the total exposure time was typically 7 hours for the realization of one BG.

Both BGs have the same period Λ = 1.05 µm, a total number of periods N₁ = N₂ = 270 (the total length of each BG is LBG1 = LBG2 = 284 µm), their etched depths are respectively l₁ = 3.2 µm and l₂ = 3.3 µm and the cavity length represented by the distance between the two Bragg reflectors is lc = 540 µm.

Figure 1 shows an FIB image of the etched BG and an SEM
image of an F-P cavity obtained in Ti:LiNbO3 waveguide by
FIB milling.

It is worth noting that the FIB etching process introduces an
unavoidable roughness in the BGs surface profile. Furthermore
the BGs exhibit a conical etching shape which modifies the
expected spectral response. To increase the structure vertical-
ity, reactive gases can be added to the milling process, for
example by employing XeF₂ gas-assisted gallium ion-beam
etching [14].

III. RESULT AND DISCUSSION

We recall that the performances of these BGs are similar to
those cited above and had been evaluated in the reference [14].
These performances are of an order of about 99% in reflection
associated to bandwidths at mid-height and of about 50 nm.

To assess the spectral response of the BGs, an experimental
setup depicted on figure 2 was utilized. It consists in a source
whose the wavelength range is in [1500–1560] nm. L₁, L₂, L₃
and L₄ are focusing lenses, P is a Glan polarizer to select
the appropriate TM mode, and OSA is an optical spectrum
analyser.

At first, the optical characterization of the waveguides is per-
formed. The emphasis is put on the fact that there are several
<figure><img src="image_4.png" /><figcaption>Fig. 3. Fabry-Pérot cavity experimental measured transmittance versus wavelengths and displayed by the mean of the optical spectrum analyzer.</figcaption></figure>

different types of optical losses due to the process insertion, the propagation and to the BGs small non-homogeneities. In the absence of corrugations, the two first types of optical losses can be estimated to be approximately 4.5 dB. Transmissions through a BG and through a standard Ti:LiNbO₃ optical waveguide have been compared. The losses due to the milling technique have been estimated to be between 2 and 2.5 dB.

At the second step of the process, the optical characterization of the F-P cavity was achieved. The experimental results of the transmission through the F-P cavity versus wavelength are given in figure 3. This figure depicts the variations of a classical function of an F-P cavity. It can be observed that 31 peaks have been experimentally obtained in the wavelength range [1505–1548] nm with a reflectivity close to 97%. Each peak has a bandwidth at half maximum $\Delta\lambda = 0.19$ nm and the F-P cavity is defined by a finesse $F = 9$ and a Free Spectral Range $FSR = 1.75$ nm. The experimental results are in high accordance with the theoretical results found by simulations.

Furthermore, in order to increase the values of F and FSR, the reflectivity of the BGs has to be improved either by increasing the etching depth or by rising up the number of periods of each BG. However, the optical losses due to the corrugations might also be increased.

## IV. CONCLUSION

The realization of an F-P cavity based on Ti:LiNbO₃ optical waveguide, integrating two spatially separated BGs which are etched by FIB has been demonstrated. Reflectivity coefficient of about 97%, finesse close to 9 and a Free Spectral Range equal to 1.75 nm were obtained. These values can be improved

by making deeper etching or by using very long BG (by increasing the number periods). Work is in progress to take advantage of the electro-optical effect characteristics of lithium niobate, and to develop another method built on using the Vernier effect in order to set a cascade of two F-P cavities.

## ACKNOWLEDGMENT

The authors would like to thank R. Salut, B. Guichardaz, D. Bitschene, J.-Y. Rauch, and V. Pitrini for technical assistance, and all of the staff members of the MIMINTO Technology Centre, Institut FEMTO-ST, Besançon, France, for their valuable and helpful suggestions.

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