# Adjustment of the coupling ratio by laser ablation of Ti: LiNbO3 directional couplers

Chii-Chang Chen, Henri Porte, Jean-Pierre Goedgebuer, Vincent Armbruster, Richard Ferrière

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Chii-Chang Chen, Henri Porte, Jean-Pierre Goedgebuer, Vincent Armbruster, Richard Ferrière. Adjustment of the coupling ratio by laser ablation of Ti: LiNbO3 directional couplers. Optics and Lasers in Engineering, 2002, 34 (6), pp.453-455. 10.1016/S0030-3992(02)00041-5. hal-00097338

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# Adjustment of the coupling ratio by laser ablation of Ti:LiNbO₃ directional couplers

Chii-Chang Chen<sup>a,b,*</sup>, Henri Porte<sup>a</sup>, Jean-Pierre Goedgebuer<sup>a</sup>, Vincent Armbruster<sup>a</sup>, Richard Ferriere<sup>a</sup>

<sup>a</sup>Laboratoire d'Optique P.M. Duffieux, UMR CNRS 6603, Institut des Microtechniques (IMFC), Université de Franche Comté, F-25030 Besançon Cedex, France

<sup>b</sup>Institute of Optical Sciences, National Central University, Chung-Li, 32054 Taiwan

The coupling ratio of directional couplers in Z-cut Y-propagation and X-cut Z-propagation LiNbO₃ crystals was adjusted by laser ablation. The coupling ratio change is more efficient for the TM-mode of Z-cut couplers. The method can be used to correct the uncertainty of the coupling ratio for directional couplers that occurs during fabrication process.

*Keywords:* Ablation; Directional couplers; Integrated optics

## 1. Introduction

Titanium-indiffused lithium niobate optical directional couplers are key integrated optics devices used as power dividers, wavelength filters, switches [1] and modulators [2]. Several parameters for designing directional coupler are required such as the geometrical dimensions of the titanium stripe and the condition of the diffusion. In principle, theoretical approaches based on the beam propagation method (BPM) [3] allow the coupling length to be evaluated from the knowledge of the fabrication parameters. However, it is difficult to fabricate accurately a directional coupler as it was originally designed. One of the most critical fabrication parameters which determines the coupling length is the waveguide spacing [4]. The latter requires severe control of the fabrication process to achieve a predictable coupling ratio.

A possible method to correct the defect is to change the propagation constant of the waveguides. This modification can be achieved by deposition of a high-index film at the surface of the waveguide [5] or by using a metallic film on the directional coupler with an accurate lithographic process

[6]. Another solution proposed by Bulmer et al. [7] consists in using laser ablation to overcome similar problems that occur in LiNbO₃ integrated interferometric modulators. Recently, we reported results obtained by using laser ablation in the fabrication process of a polarization-independent LiNbO₃ Mach–Zehnder modulator [8]. This method is more efficient than the photolithographic correction since the process is quicker and more accurate.

In this letter, we report the results of the coupling ratio adjustment of directional couplers using laser ablation. The basic principle consists in reducing the effective refractive index of the guided mode by using laser ablation in the coupled waveguides, thus resulting in an exponential decrease of the coupling length and modification of the light coupling coefficient [4]. This method has been applied to directional couplers integrated in Z-cut Y-propagation and in X-cut Z-propagation lithium niobate crystal.

## 2. Experimental procedure

The directional couplers were fabricated in a Z-cut Y-propagation and an X-cut Z-propagation LiNbO₃ crystal. The Ti stripes patterned on the LiNbO₃ crystal are 500-Å-thick, 6-µm-wide, 5-mm-long and 4-µm waveguide spacing for single mode operation at 1300 nm. The Ti stripes are diffused for 10 h at 1050°C. To achieve laser

<footnote>* Corresponding author. Institute of Optical Sciences, National Central University, Chung-Li 32054, Taiwan. Tel.: +886-3-422-7151x5343; fax: +886-3-425-8816.</footnote>

<footnote>E-mail address: chx@phy.ncu.edu.tw (C.-C. Chen).</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1. (a) Ablated holes on the surface of the directional coupler; (b) output intensity distribution for the TM-mode of a Z-cut Y-propagation directional coupler before laser ablation; (c) after 100-nm deep ablation over a length of 1240 µm; (d) 2480 µm; (e) 3720 µm; and (f) 4340 µm.</figcaption></figure>

ablation and to control dynamically the coupling length, we used a KrF (Krypton–fluorine) excimer laser emitting at 249-nm wavelength. The length of the each ablation spot on the waveguides of the directional couplers is 620 µm long. The ablation laser is applied on the waveguides to form one hole by the latter ones [see Fig. 1(a)]. The flux of the laser is adjusted at 4 J/cm² resulting in an ablation rate to be 100 nm/pulse on the surface of the waveguides. The detail experimental setup was described elsewhere [8]. The light of a 1300-nm laser diode is launched into one of the waveguides of the directional coupler. The polarization is oriented at 45° from the X-axis of the crystal in order to

<figure><img src="image_3.png" /><figcaption>Fig. 2. Coupling ratio versus ablation length: (a) for a Z-cut Y-propagation; and (b) for an X-cut Z-propagation directional coupler.</figcaption></figure>

excite simultaneously the TE- and TM-modes. Both modes of the light transmitted by the directional coupler are analyzed by a polarizer. The light coupling and the losses are calculated simultaneously by a computer from the light intensity distribution at the two outputs of the waveguides.

### 3. Results and discussion

Fig. 1(b)–(f) show the evolution of the energy of the TM-mode at the outputs of a Z-cut and Y-propagation LiNbO₃ directional coupler when varying the length of the ablated region. The laser light is launched in the left waveguide. A significant coupling ratio change from 65% to 98% is obtained with a 4340-µm-long ablated section on the surface of the coupler. The change of the coupling ratio induced by different ablation length is shown in Fig. 2(a) and (b) for the directional couplers integrated in Z- and X-cut LiNbO₃ crystals, respectively. The coupling ratio for the TM mode of the Z-cut LiNbO₃ couplers is changed from 65% to 90% via 100% after 7-mm-long ablation process. In this case, the change of the coupling ratio after ablation process is 45%(=35% + 10%). The change of the coupling ratio for the TE-modes of the Z-cut LiNbO₃ couplers in 5% after 7-mm-long ablation process. The change of the coupling ratio for the TM- and the TE-modes of the X-cut LiNbO₃ couplers is 6% and 10% after 3-mm-long ablation process, respectively. The TM-mode of the Z-cut directional coupler shows more sensitivity to the ablation process on the waveguides of the directional couplers. This
can be explained by the fact that the spatial distribution of
the TM-mode for the Z-cut LiNbO₃ waveguides which is
related to the extraordinary refractive index is located near
the waveguide surface [9]. This behavior may help to fabri-
cate a polarization-independent directional coupler in Z-cut
LiNbO₃ by adjusting the coupling ratio in the TM-mode to
reach the TE-mode using ablation process.

For 1% change of the coupling ratio for the Z-cut LiNbO3
couplers (i.e. to change the coupling ratio from x% to x ±
1%), the loss induced by ablation process is 0.15 and 0.4 dB
for the TM- and TE-modes, respectively. For the 1% change
of the coupling ratio for the X-cut LiNbO3 couplers, the loss
induced by ablation process is 0.86 and 0.9 dB for the TM-
and TE-modes, respectively. These results show that the
coupling ratio of the Z-cut LiNbO3 directional couplers in
the TM-mode is suitable to be adjusted by ablation process
due to the higher sensitivity of the coupling ratio change
and lower loss to the ablation process.

4. Conclusion

We reported a method that allows dynamic correction of
defects in the coupling length introduced during the fabri-
cation process of directional couplers integrated in LiNbO3
crystal. This method was applied to directional couplers
integrated in X-cut and Z-propagation and in Z-cut and
Y-propagation LiNbO3 crystals to analyze the coupling ra-
tio change for the TM- and TE-modes. The results show
that the coupling ratio change is the most efficient for the
TM-mode of Z-cut Y-propagation directional couplers. The
loss induced by ablation process can be negligible for small
coupling ratio adjustment. A good stability of the processed
samples was observed.

**Acknowledgements**

This work was supported by the Centre National d'Etudes des Télécommunications (France Telecom Contract 956B024). We thank J.C. Renaud for the mask fabrication, and J. Vendeville for the thin film evaporation and for the reactive ion etching process.

**References**

[1] Kondo M, Ohta Y, Tanisawa Y, Aoyama T, Ishikawa R. Low-drive-voltage and low-loss polarisation-independent LiNbO3 optical waveguide switches. Electron Lett 1987;23(21):1167-9.

[2] Granestrand P, Thylen L, Stoltz B. Polarisation independent switch and polarisation splitter employing Δβ and Δκ modulation. Electron Lett 1988;24(18):1142-3.

[3] Koai KT, Liu Pao-Lo. Modeling of Ti:LiNbO₃ waveguide devices: part I-directional couplers. IEEE J Lightwave Technol 1989;7(3): 533-9.

[4] Noda J, Fukuma M, Mikami O. Design calculations for directional couplers fabricated by Ti-diffused LiNbO3 waveguides. Appl Opt 1981;20(13):2284-90.

[5] Mikami O, Noda J, Zembutsu S, Fukunishi S. Phase tuning in optical directional coupler by photostructural effect of chalcogenide glass film. Appl Phys Lett 1977;31(6):376-8.

[6] Mikami O, Noda J. Coupling length adjustment for an optical directional coupler by loading a metal film. Appl Phys Lett 1978;33(10):856-8.

[7] Bulmer CH, Burns WK, Greenblatt AS. Phase tuning by laser ablation of LiNbO3 interferometric modulators to optimum linearity. IEEE Photonics Technol Lett 1991;3(6):510-2.

[8] Chen CC, Porte H, Careno A, Geedgebuer JP, Armbruster V. Phase correction by laser ablation of a polarization independent LiNbO3 Mach-Zehnder modulator. IEEE Photonics Technol Lett 1997;9(10):1361-3.

[9] Porte H, Goedgebuer JP, Ferriere R. An LiNbO3 integrated coherence modulator. IEEE J Lightwave Technol 1992;10(6): 760-6.